复旦大学：《光子学器件与工艺 Photonics Devices and Technology》文献资料_Optical microcavities

insight review articles Optical microcavities Kerry J Vahala California Institute of Technology. Mail Stop 128-95,Pasadena. California 91125.USA(e-mail: vahalaecaltechedu) Optical microcavities confine light to small volumes by resonant recirculation Devices based on optical microcavities are already indispensable for a wide range of applications and studies. For example, microcavities made of active lll-V semiconductor materials control laser emission spectra to enable long-distance transmission of data over optical fibres; they also ens spot-size laser read/write beams in CD and DVD players. In quantum optical devices, microcavities ironment whe atoms or quantum dots to emit spontaneous photons in a desired direction or can provide an enviro here dissipative mechanisms such as spontaneous emission are overcome so that quantum entanglement of radiation and matter is possible Applications of these remarkable devices are as diverse as their geometrical and resonant properties ke its acoustic analogue the tuning fork, the optical microcavity (or microresonator) has a size-dependent resonant frequency spectrum. Microscale volume ensures that resonant throughout this spectrum than they are in a corresponding macroscale resonator. An ideal cavity would confine light indefinitely(that is, without loss)and would have resonant frequencies at precise values. Deviation from this ideal condition is described by the cavity Q factor(which is proportional to the confinement time in units of the optical period). Q factor and microcavity volume(V figure prominently in applications of these devices, and a summary of values typical for the devices discussed in this review is given in Table 1. In addition, representative xamples of the three methods of confinement employed in microcavities are provided in Figs 1-3(refs 1-7) In this review, I consider four applications of optical microcavities: strong-coupling cavity quantum electro- dynamics(QED), enhancement and suppression ofsponta- neousemission, novelsources, and dynamic filtersin optical communication. These areas are just four ofseveral possible, d many topics, such as soliton effects, chaosandeffects in quantum-well microcavities", will not be reviewed Figure 1 Micropost (or micropillar) cavities have played amajor rolein ecause of space limitations. Also, I will not review micro- recent applications of the Purcelleffect to triggered, single-photon sources. cavity types in commercial semiconductor lasers because Theyoffer small caviy volume and relatively high @, have an emission patter xtensive texts and treaties on this subject already exist 23. that is well suited for coupling and manipulation of emitted photons"for Even for the four applications discussed, there are by neces- example, with optical fibres) and can incorporate quasi-atomic,quantum dots ity omissions. Cavity QED, for example, is a vast topic, and as emitters In the rendering, Bragg mirrors at the output(upper stack near lections have been made on the basis of their importance arrow) and below provide one dimension of cavily confinement, whereasair-. and for the interesting design limits they illustrate. I will pro- dielectric guiding provides lateral (i-the-plane)confinement.A single vide a brief introduction to each area, then describe a few quantun dot is shown spontaneously emitting a photon that is subsequently representative applications, their microcavity requirements, directed via the Purcell ffect through the cavity top. The inset shows a and the state-of-the-art for these devices, before outlining scanning electron micrograph of such amicropost cavity used inrecent the challenges for the future triggered single-photon source experiments. Inset micrograph courtesy ofY. Yamamoto"(Stanford University, CA Strong-coupling cavity QED Anelectron transitions within an atom from an excited state to a ground state, emitting a photon into the continuum of cavity states(states that are not factorable into cavity and radiationmodes". This irreversible behaviour is an example atom components). If the probe frequency ismaintained at of weak coupling. If the same atom is inserted into a micro- the cavity's original resonant frequency, then the entry of a cavity and if thestrong-coupling conditions described in single atom into the cavity can block transmission"(that is, x I are satisfied, then the atom can interact coherently the probe isreflected) In state-of-the-art systems, the result witha cavity mode forameaningfultime This can take place ing extreme sensitivity of transmission to the atoms posi even when the mode is initially in its vacuum state(ground tion within the cavity is used to ascertain atomic centre-of- state or state of lowest energy). Use of a weak optical probe mass motion-", such as the orbital motion of ultracold reveals that the cavitys transmission spectrum issplit by the atoms entrained by their interaction with the cavity presence of the atom into two distinct peaks, which corre- mode(Fig 4). In addition, such a cavity containing cold pond to eigen frequencies of the quantum entangled atom- atoms exhibits extreme dispersive properties, which have NatuRevoL42414august2003www.nature-e2009NaturePublishingGroup 839

insight review articles NATURE | VOL 424 | 14 AUGUST 2003 | www.nature.com/com 839 L ike its acoustic analogue the tuning fork, the optical microcavity (or microresonator) has a size-dependent resonant frequency spectrum. Microscale volume ensures that resonant frequencies are more sparsely distributed throughout this spectrum than they are in a corresponding ‘macroscale’ resonator. An ideal cavity would confine light indefinitely (that is, without loss) and would have resonant frequencies at precise values. Deviation from this ideal condition is described by the cavity Q factor (which is proportional to the confinement time in units of the optical period). Q factor and microcavity volume (V) figure prominently in applications of these devices, and a summary of values typical for the devices discussed in this review is given in Table 1. In addition, representative examples of the three methods of confinement employed in microcavities are provided in Figs 1–3 (refs 1–7). In this review, I consider four applications of optical microcavities: strong-coupling cavity quantum electro￾dynamics(QED), enhancement and suppression of sponta￾neous emission, novel sources, and dynamic filters in optical communication. These areas are just four of several possible, and many topics, such as soliton effects8,9, chaos10 and effects in quantum-well microcavities11, will not be reviewed because of space limitations. Also, I will not review micro￾cavity types in commercial semiconductor lasers because extensive texts and treaties on this subject already exist12,13. Even for the four applications discussed, there are by neces￾sity omissions. Cavity QED, for example, is a vast topic, and selections have been made on the basis of their importance and for the interesting design limits they illustrate. I will pro￾vide a brief introduction to each area, then describe a few representative applications, their microcavity requirements, and the state-of-the-art for these devices, before outlining the challenges for the future. Strong-coupling cavity QED An electron transitions within an atom from an excited state to a ground state, emitting a photon into the continuum of radiation modes14. This irreversible behaviour is an example of weak coupling. If the same atom is inserted into a micro￾cavity and if the strong-coupling conditions15–17 described in Box 1 are satisfied, then the atom can interact coherently with a cavity mode for a meaningful time. This can take place even when the mode is initially in its vacuum state (ground state or state of lowest energy). Use of a weak optical probe reveals that the cavity’s transmission spectrum is split by the presence of the atom into two distinct peaks, which corre￾spond to eigen frequencies of the quantum entangled atom￾cavity states (states that are not factorable into cavity and atom components)18. If the probe frequency is maintained at the cavity’s original resonant frequency, then the entry of a single atom into the cavity can block transmission19 (that is, the probe is reflected). In state-of-the-art systems, the result￾ing extreme sensitivity of transmission to the atom’s posi￾tion within the cavity is used to ascertain atomic centre-of￾mass motion20–22, such as the orbital motion of ultracold atoms entrained by their interaction with the cavity mode21,22 (Fig. 4). In addition, such a cavity containing cold atoms exhibits extreme dispersive properties, which have Optical microcavities Kerry J. Vahala California Institute of Technology, Mail Stop 128-95, Pasadena, California 91125, USA (e-mail: vahala@caltech.edu) Optical microcavities confine light to small volumes by resonant recirculation. Devices based on optical microcavities are already indispensable for a wide range of applications and studies. For example, microcavities made of active III–V semiconductor materials control laser emission spectra to enable long-distance transmission of data over optical fibres; they also ensure narrow spot-size laser read/write beams in CD and DVD players. In quantum optical devices, microcavities can coax atoms or quantum dots to emit spontaneous photons in a desired direction or can provide an environment where dissipative mechanisms such as spontaneous emission are overcome so that quantum entanglement of radiation and matter is possible. Applications of these remarkable devices are as diverse as their geometrical and resonant properties. Photon Quantum dot Figure 1 Micropost (or micropillar) cavities1,2 have played a major role in recent applications of the Purcell effect to triggered, single-photon sources. They offer small cavity volume and relatively high Q, have an emission pattern that is well suited for coupling and manipulation of emitted photons56 (for example, with optical fibres) and can incorporate quasi-atomic, quantum dots as emitters. In the rendering, Bragg mirrors3 at the output (upper stack near arrow) and below provide one dimension of cavity confinement, whereas air￾dielectric guiding provides lateral (in-the-plane) confinement. A single quantum dot is shown spontaneously emitting a photon that is subsequently directed via the Purcell effect through the cavity top. The inset shows a scanning electron micrograph of such a micropost cavity used in recent triggered single-photon source experiments. Inset micrograph courtesy of Y. Yamamoto76 (Stanford University, CA). © 2003 Nature PublishingGroup

insight review articles native measure of cavity perfection that does not include propaga Silica torold tion effects within the cavity as does the Q factor, of 1.9 x 10"(ref. 26) has been obtained using these mirrors. Optimization of n, and N however, involves joint optimization of the mode volume and finess (or Q). So, for example, the results cited above used resonator with mode volume v=1.69×10pm3 and finesse of48×10°. a detailed review of the technological limits imposed by mirror technology in In addition to ultrahigh-finesse Fabry-Perot microcavities, the whispering gallery modes of silica and quartz microspheres have ed considerable attention27-293434 pering gallery res- onators are typically dielectric spherical structures in which waves are confined by continuous total internal reflection Silica micros- pheres, which are robust ultrahigh-Q microresonators, were first studied by Braginsky and Ilchencko" Spheres feature an atomic-like mode spectrum in which high t number(principal angularindex or optical mode) and low radial number modes execute orbits near the sphere s surface"(Fig. 5). Excellent surface finish is crucial for maxi- mizing Q, and the formation ofspheres throughsurface tension(that IS in red,is coupled from a fibre-taper waveguide and subsequently guided within and(with only a few nanometres or less of surface roughness 7).The along the periphery of the microtoroid in a whispering gallery mode, which is named bulk optical loss fromsilica is also exceptionally low and record Qfac- after its acoustic equivalent.Whispering gallery microcavities can be found in several tors22of8 x 10(and finesse2of 23X 106)have been obtained For geometries including spheres(see Fig. 5), disks (see Fig, 6) and rings (inset to Fig. 6). these measurements, dependence of Qon sphere diameter is consis- Inset: A scanning electron micrograph of a microtoroid resonator consisting of a thin tent with Qbeing limited by losses of surface roughness2".Also, atime lica layer upon a silicon post and substrate. The device has a diameter of 120 um dependency for the measured Was observed andis believedtoresult and exhibits a Factor in excess of 100 million. The smooth exterior toroid suface is from water adsorption and formation of OH groups at the sphere's the result of the toroid going through a molten state during its fabrication Inset surface2. For diameters below 20 um in silica spheres, radiation micrograph courtesy of D Armani leakage becomes a significant factor in determining Q. The lowest eI are minimal yol- ume, equatorial ring orbits(see Fig. 5)and are best suited recently been observed. These measurements, as well as tions of cavity QED to quantum information studies, have P QED. Experimental work has demonstrated strong coupl system", and recent modelling shows that substantial been reviewedelsewhere 24 ments in strong coupling are possible using spheres with reduced Efforts toincrease strong-coupling effects(as measured diameters ions in saturation photon number, n, and critical atom number, N, Microcavities based on photonic crystals(Fig 3)can provide tie (see Box d)have provided much impetus for researchinto ultrahigh- extremely smallmode volumes,, and donor-mode cavity geometries lowered from near unity levels in the earliest demonstrations of sin- have been modelled witha neutralatom suspended within the hole gle-atom Rabisplitting at optical frequencies to recent levels Strong couplings theoretically feasible; however, at present, Values of n, =2.82 x 10 and N=6.1x 10(refs 21,31). Microcavities in in fabricated structures are well below the theoretical optima these experiments feature Fabry-Perot-style resonators (an optical For the purposes of optical probing/output-coupling resonator in whichfeedbackis accomplished using twomirrors)with Fabry-Perot cavities enable direct 'endfire' coupling along the ultrahigh reflectance mirror technology. A cavity finesse, an alter- axis. Whispering gallery modes, however, must be phase matched Table 1 The microcavities are organized by column according to Fabry-Perd Whispering gallery Photonic crystal that are somewhat co been taken from the following cited references. Uppe ionosphere microtoroid. n is the material refractive L7,000 and the second for a llk-V semiconductor F:4.8×105 .8×109 :108 e2003NaturepUblishingGroupNatUrevOl42414august2003www.nature.com/nature

native measure of cavity perfection that does not include propaga￾tion effects within the cavity as does the Q factor, of 1.92106 (ref.26) has been obtained using these mirrors. Optimization of no and No, however, involves joint optimization of the mode volume and finesse (or Q). So, for example, the results cited above used a resonator with mode volume V=1.692103 µm3 and finesse of 4.8×105 . A detailed review of the technological limits imposed by mirror technology in optimizing Fabry–Perot microcavities for strong-coupling studies has recently been performed32. In addition to ultrahigh-finesse Fabry–Perot microcavities, the whispering gallery modes of silica and quartz microspheres have received considerable attention27–29,33,34. Whispering gallery res￾onators are typically dielectric spherical structures in which waves are confined by ‘continuous total internal reflection’. Silica micros￾pheres, which are robust ultrahigh-Q microresonators, were first studied by Braginsky and Ilchencko35. Spheres feature an atomic-like mode spectrum in which high , number (principal angular index or optical mode) and low radial number modes execute orbits near the sphere’s surface33 (Fig. 5). Excellent surface finish is crucial for maxi￾mizing Q, and the formation of spheres through surface tension (that is, as a molten droplet) provides a near atomically smooth surface (with only a few nanometres or less of surface roughness28,29). The bulk optical loss from silica is also exceptionally low and record Qfac￾tors28,29 of 82109 (and finesse29 of 2.32106 ) have been obtained. For these measurements, dependence of Q on sphere diameter is consis￾tent with Qbeing limited by losses of surface roughness29. Also, a time dependency for the measured Qwas observed and is believed to result from water adsorption and formation of OH groups at the sphere’s surface28,34. For diameters below 20 mm in silica spheres, radiation leakage becomes a significant factor in determining Q31. The lowest order radial modes (in terms of nodes) with m=,31 are minimal vol￾ume, equatorial ring orbits (see Fig. 5) and are best suited for cavity QED. Experimental work has demonstrated strong coupling in this system34, and recent modelling31 shows that substantial improve￾ments in strong coupling are possible using spheres with reduced diameters. Microcavities based on photonic crystals (Fig. 3) can provide extremely small mode volumes7 , and donor-mode cavity geometries (in which a small additional hole is drilled within the design of Fig. 3) have been modelled with a neutral atom suspended within the hole30. Strong coupling is theoretically feasible; however, at present, Qvalues in fabricated structures are well below the theoretical optima. For the purposes of optical probing/output-coupling, Fabry–Perot cavities enable direct ‘endfire’ coupling along the cavity axis. Whispering gallery modes, however, must be phase matched36, insight review articles 840 NATURE | VOL 424 | 14 AUGUST 2003 | www.nature.com/nature recently been observed23. These measurements, as well as applica￾tions of cavity QED to quantum information studies, have recently been reviewed elsewhere24,25. Efforts to increase strong-coupling effects (as measured by reduc￾tions in saturation photon number, no, and critical atom number, No (see Box 1)) have provided much impetus for research into ultrahigh￾Q, small-volume resonant cavities17,26–30. Both no and No have been lowered from near unity levels in the earliest demonstrations of sin￾gle-atom vacuum Rabi splitting at optical frequencies to recent levels of no42.82210–4 and No46.1210–3 (refs 21,31). Microcavities in these experiments feature Fabry–Perot-style resonators (an optical resonator in which feedback is accomplished using two mirrors)with ultrahigh reflectance mirror technology26. A cavity finesse, an alter￾Fibre-taper waveguide 42.5 µm Optical wave Silicon post Silica toroid Figure 2 Rendering of an ultrahigh-Q microtoroid resonator6 . An optical wave, shown in red, is coupled from a fibre-taper waveguide and subsequently guided within and along the periphery of the microtoroid in a whispering gallery mode, which is named after its acoustic equivalent5 . Whispering gallery microcavities can be found in several geometries including spheres (see Fig. 5), disks (see Fig. 6) and rings (inset to Fig. 6). Inset: A scanning electron micrograph of a microtoroid resonator consisting of a thin silica layer upon a silicon post and substrate. The device has a diameter of 120 µm and exhibits a Q factor in excess of 100 million. The smooth exterior toroid surface is the result of the toroid going through a molten state during its fabrication. Inset micrograph courtesy of D. Armani6 . Table 1 The microcavities are organized by column according to the confinement method used and by row according to high Q and ultrahigh Q. Small mode volume and ultrasmall mode volume are other possible classifications that are somewhat complementary to this scheme. Representative, measured Qs and Vs are given and have been taken from the following cited references. Upper row: micropost48, microdisk52, semiconductor103, polymer104 add/drop filter, photonic crystal cavity62. Lower row: Fabry-Perot bulk optical cavity21,31, microsphere29, microtoroid6 . n is the material refractive index, and, V, if not indicated, was not available. Microsphere volume V was inferred using the diameter noted in the cited reference and finesse (F) is given for the ultrahigh-Q Fabry–Perot as opposed to Q. Two Q values are cited for the add/drop filter: one for a polymer design, QPoly, and the second for a III–V semiconductor design, QIII–V. Fabry–Perot Whispering gallery Photonic crystal High Q Ultrahigh Q Q: 2,000 V: 5 (λ/n)3 F: 4.8×105 V: 1,690 µm3 Q: 8×109 V: 3,000 µm3 Q: 108 Q: 12,000 V: 6 (λ/n)3 QIII–V: 7,000 QPoly: 1.3x105 Q: 13,000 V: 1.2 (λ/n)3 © 2003 Nature PublishingGroup

insight review articles Box 1 Strong coupling An atom, initially in the excited state of a dipole transition, enters a 1992, unity n, and N, were demonstrated at optical frequencies lossless microcavity of volume V. A single mode of the cavity, in its Recently, state-of-the-art, strongly coupled systems have produced ound state, is resonant with the transition The atom andvacuum n, and N, values that are much less than unity, reflecting a field'couple, which results in a quantum of energy shifting back and fundamental Rabi dynamic that exists over many cycles. For reviews forth between the atom and the mode at the vacuum Rabi frequency of strong coupling and its application to cavity QED see refs The interaction strength of atom and cavity mode is linear in the field 17.25,46.120 and hence smaller cavity volumes concentrate the vacuum field of the mode, producing larger Rabi frequencies. This fundamental dynamic of the atom-field system is reversible as long as the system is isolated. In reality the cavity will have a finite photon lifetime (finite g) that will limit, perhaps even prevent, Rabi oscillations by allowing the energy to ak irreversibly into the continuum. Likewise, the atomic transition will ouple to continuum radiation modes and thereby experience spontaneous decay of its population as well as polarization dephasing. Strongly coupled systems are those in which the Rabi ynamic can exist, even if only briefly, despite the reality of dissipation Strong coupling occurs when the atom-field coupling strength, g (which is half the Rabi frequency), is faster than any underlying ⊙m(⊙ dissipative rate and larger than 1/ where T is the interaction time". Under conditions of strong coupling, weak optical probing near the icrocavity resonant frequency reveals two spectral transmission peaks(where only one existed before) giving the energies of new eig states, which are now entangled states of the atom and cavity field Given modal and atomic dissipation, the degree to which the atom and cavity mode are strongly coupled can be quantified by defining a saturation photon number(nd and critical atom number(No.These quantities are, by necessity, functions of parameters describing both the reversible dynamic and the various decay rates". Box 1 Figure Vacuum Rabi oscillation. An excited atom is introduced into the cavity no ox -ov xto (top)and undergoes vacuum Rabi oscillation mediated by the atom-field coupling strength, g resulting in one quantum being added to the mode(shown in red) where(ya, y are the atomic dissipation rates(population relaxation (bottom). Dissipation mechanisms are also illustrated. yis the atomic damping rate and atomic dephasing, respectively) and t is the cavity lifetime. In and t is the cavity lifetime. which is typically achieved using total internal reflection from the tional. Reviews of this mechanism both at optical and at microwave back face ofa prism. Other g methods have also been demon- frequencies appearinref 4(chapter 2)andre. 46 strated. Formany applications of cavity QED, such asin quantum The ability to create InAs quantum dots that have excellent lumi- onators will figure prominently. A fibre-optic cable is considered a point in the recent history of the Purcell effect. As noted by Gerard likely medium over which to transport quantum information". In these structures can act as a local probe of the field within a Ill-V thisregard, fibre-optic tapers(see Fig. 5)provide ultralow loss, direct microcavity and can also efficiently capture and then confine elec- oupling toultrahigh-Qspheres"and have been proposed asa means trons and holes, making them less susceptible to the semiconductor to couple quantum states to or from resonator onto a fibre".Also, surface effects that occur when cavity dimension decreases. Quan- the recent demonstration of a fibre-taper-coupled ultrahigh-q tum dots are also well suited to the study of the Purcell effect insemi- microtoroid-on-a-chip'(see Fig. 2)enables integration of wafer- conductors, because, unlike bulk or quantum-well media, individua based functions withultralow-loss fibre-coupled quantum devices. quantum dots exhibit a relatively narrow spectral lineshape that fits within a high-Q microcavity mode Enhancement and suppression of spontaneous emission Using quantum-dot-loaded micropost.(or micropillar)cavi- Veak coupling results when dissipation overwhelms the fundamen- ties, such as the one shown in Fig. 1, with Qs of 2,000(1 um diameter tal Rabi dynamic. The control of spontaneous emission through the posts), Gerard and coworkers showed a five-fold Purcell enhance- Purcell" thisregime has become an important applicationof ment. Spontaneous emission suppression was later verified in microcavities. Because all cavities exhibit loss at some level, cavity similar structures using a metallic sidewall coating to exclude trans- modes are, more rigorously, quasi modes, and, in the strictest sense, verse continuum modes". Purcell enhancement of emission from a the spectral-lineshape function of a'discrete' mode is proportional single quantum dot within a micropost was later demonstrated. In d references therein). Applications of the Purcell effect make use of dots support modes with Qs in the range of 10,000-17,0002,an this duality by, on the one hand, using local enhancement of the con- Purcell enhancement has been measuredup to 15-foldin the tinuum DM function to influence spontaneous emission, while, on structures. Measured Purcell factors are influenced by radialspatial et of the averaging emble mission process(see Box 2 for further discussion of this and the and estimated Purcell factors based on Q measurement and cavity omplementary effect, spontaneous-emission suppression). Thus, volume estimates are typically much higher. Using Q measure- atomic decay rates are not only speeded-up but are also made direc- ment alone, a Purcell factor of 32 for microposts and a factor of NatuRevOl42414AugUst2003www.nature.com/nature e 2003 Nature Publishing Group

insight review articles NATURE | VOL 424 | 14 AUGUST 2003 | www.nature.com/nature 841 which is typically achieved using total internal reflection from the back face of a prism . Other coupling methods have also been demon￾strated37–40. For many applications of cavity QED, such as in quantum information studies, parasitic loss in coupling to and from microres￾onators will figure prominently. A fibre-optic cable is considered a likely medium over which to transport quantum information41. In this regard, fibre-optic tapers (see Fig. 5) provide ultralow loss, direct coupling to ultrahigh-Qspheres42 and have been proposed as a means to couple quantum states to or from a resonator onto a fibre42,43. Also, the recent demonstration of a fibre-taper-coupled ultrahigh-Q microtoroid-on-a-chip6 (see Fig. 2) enables integration of wafer￾based functions with ultralow-loss fibre-coupled quantum devices. Enhancement and suppression of spontaneous emission Weak coupling results when dissipation overwhelms the fundamen￾tal Rabi dynamic. The control of spontaneous emission through the Purcell effect44 in this regime has become an important application of microcavities45. Because all cavities exhibit loss at some level, cavity modes are, more rigorously, quasi modes, and, in the strictest sense, the spectral–lineshape function of a ‘discrete’ mode is proportional to a continuum density of modes (DM) function (ref. 4, chapter 1, and references therein). Applications of the Purcell effect make use of this duality by, on the one hand, using local enhancement of the con￾tinuum DM function to influence spontaneous emission, while, on the other, using the quasi mode of the resonator as the target of the emission process (see Box 2 for further discussion of this and the complementary effect, spontaneous-emission suppression). Thus, atomic decay rates are not only speeded-up but are also made direc￾tional. Reviews of this mechanism both at optical and at microwave frequencies appear in ref. 4 (chapter 2) and ref. 46. The ability to create InAs quantum dots that have excellent lumi￾nescent properties47 within III–V semiconductors was a turning point in the recent history of the Purcell effect. As noted by Gerard1 , these structures can act as a local probe of the field within a III–V microcavity and can also efficiently capture and then confine elec￾trons and holes, making them less susceptible to the semiconductor surface effects that occur when cavity dimension decreases. Quan￾tum dots are also well suited to the study of the Purcell effect in semi￾conductors, because, unlike bulk or quantum-well media, individual quantum dots exhibit a relatively narrow spectral lineshape that fits within a high-Q microcavity mode48,49. Using quantum-dot-loaded micropost1,3 (or micropillar) cavi￾ties, such as the one shown in Fig. 1, with Qs of 2,000 (1 mm diameter posts), Gerard and coworkers showed a five-fold Purcell enhance￾ment48,50. Spontaneous emission suppression was later verified in similar structures using a metallic sidewall coating to exclude trans￾verse continuum modes51. Purcell enhancement of emission from a single quantum dot within a micropost was later demonstrated2 . In addition to microposts, microdisk cavities incorporating quantum dots support modes with Qs in the range of 10,000–17,00052,53, and Purcell enhancement has been measured54 up to 15-fold55 in these structures. Measured Purcell factors are influenced by radial spatial averaging, which occurs over an ensemble of resonant emitters48,56, and estimated Purcell factors based on Q measurement and cavity volume estimates are typically much higher. Using Q measure￾ment alone, a Purcell factor of 32 for microposts50 and a factor of An atom, initially in the excited state of a dipole transition, enters a lossless microcavity of volume V. A single mode of the cavity, in its ground state, is resonant with the transition. The atom and ‘vacuum field’ couple, which results in a quantum of energy shifting back and forth between the atom and the mode at the vacuum Rabi frequency. The interaction strength of atom and cavity mode is linear in the field and hence smaller cavity volumes concentrate the vacuum field of the mode, producing larger Rabi frequencies. This fundamental dynamic of the atom–field system is reversible as long as the system is isolated. In reality the cavity will have a finite photon lifetime (finite Q) that will limit, perhaps even prevent, Rabi oscillations by allowing the energy to leak irreversibly into the continuum. Likewise, the atomic transition will couple to continuum radiation modes and thereby experience spontaneous decay of its population as well as polarization dephasing. Strongly coupled systems are those in which the Rabi dynamic can exist, even if only briefly, despite the reality of dissipation. Strong coupling occurs when the atom–field coupling strength, g (which is half the Rabi frequency), is faster than any underlying dissipative rate and larger than 1/T where T is the interaction time17. Under conditions of strong coupling, weak optical probing near the microcavity resonant frequency reveals two spectral transmission peaks (where only one existed before) giving the energies of new eigen states, which are now entangled states of the atom and cavity field. Given modal and atomic dissipation, the degree to which the atom and cavity mode are strongly coupled can be quantified by defining a saturation photon number (no) and critical atom number (No). These quantities are, by necessity, functions of parameters describing both the reversible dynamic and the various decay rates17, no ]} g g ⊥g 2 || }]V No ] }τ g g ⊥ }2]}Q V } where (g||, γ⊥) are the atomic dissipation rates (population relaxation and atomic dephasing, respectively) and τ is the cavity lifetime. In 1992, unity no and No were demonstrated at optical frequencies18. Recently, state-of-the-art, strongly coupled systems have produced no and No values that are much less than unity, reflecting a fundamental Rabi dynamic that exists over many cycles. For reviews of strong coupling and its application to cavity QED see refs 17,25,46,120. Box 1 Figure Vacuum Rabi oscillation. An excited atom is introduced into the cavity (top) and undergoes vacuum Rabi oscillation mediated by the atom-field coupling strength, g, resulting in one quantum being added to the mode (shown in red) (bottom). Dissipation mechanisms are also illustrated. γ is the atomic damping rate and τ is the cavity lifetime. Box 1 Strong coupling g γ γ γ γ τ–1 τ–1 τ–1 © 2003 Nature PublishingGroup

insight review articles Box 2 Purcell effect A two level system will decay spontaneously by interaction with a presented on the basis of calculation of the continuum mode density cuum continuum at a rate proportional to the spectral density of (ref. 4, chapters 1 and 2 and references therein) odes per volume evaluated at the transition frequency Within a Design of microcavities for observation of the Purcell effect must avity, the density of modes is modified and large swings in its take account of the corresponding atomic (or atom-like)transition mplitude can occur From the viewpoint of cavity modes(which in characteristics. Use of a small microcavity volume is important he presence of dissipation must be viewed as quasi modes (ref. 4 because enhancements driven by manipulation of Q alone are limited chapter 1), the maximal density of modes occurs at the quasi-mode by the spectral width of the transition. Likewise, all other things being sonant frequencies and can greatly exceed the corresponding free- equal, narrow, atomic transitions can be Purcell-enhanced more as a ace density. Historically, Purcellarrived at this conclusion by higher O becomes possible. It is for this reason that individual ting that a single (quasi) mode occupies a spectral bandwidth wo quantum dots, with their relatively narrow transition widths(compared ithin a cavity of volume V Normalizing a resulting cavity-enhance th bulk semiconductors), are playing a significant role in this field" ode density per unit volume to the mode density of free space (see Fig. 1) ields the Purcell 'spontaneous emission enhancement factor 46. ccount for emission within dielectrics. An atom whose transition falls within the mode linewidth will experience an enhancement to its spontaneous decay rate given by the Purcell factor. More significantly ecause the enhancement comes about from coupling to only those ontinuum modes that make up the corresponding quasi-mode of the resonator, the spontaneous emission is directed to this quasi 米樂 mode and has great utility with regard to coupling spontaneous power. In spectral locations that are intermediate to modal frequencies, the density of the modes can fall well below the density in free space. With proper cavity design and for operation at these off- Box 2 Figure Purcell enhancement of spontaneous emission. Weak coupling to a esonance frequencies, spontaneous decay can be suppressed cavity mode will enhance the spontaneous rate of emission by increasing the local Rigorous developments of Purcells physical model have been density of modes (right) compared with their density in free space(left) 190 for 2 um diameter microdisks have been inferred. Q factors as single cavity mode(a necessity for efficient coupling to high as 10,000 with corresponding mode volumes of 1.6(/n) have Instead, the Purcell effect is applied to improve cou- been predicted to exist in optimized micropost cavities. A post microdisk and micropost-based devices-have beer diameter of 0.5 um with oved Q factor of 4,800 yields a Pur d. Significantly, asingle photon source thatis anelectrical- cell factor of 14756 ngle quantum dot has recently been demonstrated".An Since Yablonovitch first proposed using a photonic crystal forspon taneous emission suppression much attention has been directed to photonic bandgap microcavities. As noted earlier, photonic-crystal defect microcavities can provide extremely small mode volumes, and large theoretical Q values have been predicted for certain designs Recently, a @ 4,000 for an H2 (seven holes removed to form the hexa gon)defect cavity and a Qof 13,000 fora donor-mode cavity(calcu- lated mode volume of 1.2 (n))were reported. Purcell enhancement hasalso beenstudied inthis systemus. Controlling the emission of single photons has been a priority for quantum encryption systems. Single-photon es, which are required in these systems, are a recent application of the Purcell effect in uantum-dotmicrocavities(see Fig. 1). Quantumdotsare quasi-atom- ic systems and hence share many properties with atoms. For example mission from a single atom or molecule and from quantum dots Defect region exhibits non-classical photon anti-bunching behaviour, because, upo mission, anintervalmustpassinorder for the atomtobere-excited and Figure 3 Cross-sectional llustration of a photonic crystal defect microcavity laser. The toemitaphoton,5.This behaviourin quantum dots has beenadapted microcavity is formed by dry etching a hexagonal array of holes and subsequent selective to generate triggeredsingle photons5-7. Leading up to thisapplication, etch of an interior region, creating a thin membrane. One hole sleft unetched creating triggered single-photon emission using photo-pumped, single-mole- defect in the array and therefore a defect mode in the optical spectrum. The mode cule systems was demonstrated"273. However, quantum-dot single- (lustrated in green) is confined to the interior of the aray by Bragg reflection in the plar photon sources, which are compact and potentially electrically and conventional waveguiding in the wertical direction. Also, shown in pink are quantum pumped, are very appealing for many of thesamereasons thatsemicon- wells that upon photo pumping provide the amplification necessary for laser oscilation ever, the usefulemissioninthese newquantumsourcesisaspontaneous Micrograph is courtesy of 0. Painter and A. Scherer (Caltech. c icrocaitylaser. ductor lasers are so compelling in communications. Unlike lasers, how- Inset: Scanning electron micrograph of a photonic crystal defect photon; therefore, stimulated emission cannot be relied upon to direct 842 e2003NaturepUblishingGroupNatUrevOl42414august2003www.nature.com/nature

power into a single cavity mode (a necessity for efficient coupling to optical fibres). Instead, the Purcell effect is applied to improve cou￾pling55. Both microdisk68 and micropost-based devices74–76 have been demonstrated. Significantly, a single photon source that is an electrical￾ly pumped single quantum dot has recently been demonstrated77. An insight review articles 842 NATURE | VOL 424 | 14 AUGUST 2003 | www.nature.com/nature 190 for 2 mm diameter microdisks52 have been inferred. Q factors as high as 10,000 with corresponding mode volumes of 1.6 (l/n) 3 have been predicted to exist in optimized micropost cavities57. A post diameter of 0.5 mm with an improved Q factor of 4,800 yields a Pur￾cell factor of 14756,57. Since Yablonovitch first proposed using a photonic crystal for spon￾taneous emission suppression58, much attention has been directed to photonic bandgap microcavities59,60. As noted earlier, photonic-crystal defect microcavities can provide extremely small mode volumes7 , and large theoretical Q values have been predicted for certain designs30,61. Recently, a Qof 4,000 for an H2 (seven holes removed to form the hexa￾gon) defect cavity59 and a Qof 13,00062 for a donor-mode cavity (calcu￾lated mode volume of 1.2 (λ/n) 3 ) were reported. Purcell enhancement has also been studied in this system60,63. Controlling the emission of single photons has been a priority for quantum encryption systems64. Single-photon sources, which are required in these systems, are a recent application of the Purcell effect in quantum-dot microcavities (see Fig. 1). Quantum dots are quasi-atom￾ic systems and hence share many properties with atoms. For example, emission from a single atom or molecule and from quantum dots67 exhibits non-classical photon anti-bunching behaviour, because, upon emission, an interval must pass in order for the atom to be re-excited and to emit a photon65,66. This behaviour in quantum dots has been adapted to generate triggered single photons68–71. Leading up to this application, triggered single-photon emission using photo-pumped, single-mole￾cule systems was demonstrated72,73. However, quantum-dot single￾photon sources, which are compact and potentially electrically pumped, are very appealing for many of the same reasons that semicon￾ductor lasers are so compelling in communications. Unlike lasers, how￾ever, the useful emission in these new quantum sources is a spontaneous photon; therefore, stimulated emission cannot be relied upon to direct A two level system will decay spontaneously by interaction with a vacuum continuum at a rate proportional to the spectral density of modes per volume evaluated at the transition frequency. Within a cavity, the density of modes is modified and large swings in its amplitude can occur. From the viewpoint of cavity modes (which in the presence of dissipation must be viewed as quasi modes (ref. 4, chapter 1)), the maximal density of modes occurs at the quasi-mode resonant frequencies and can greatly exceed the corresponding free￾space density. Historically, Purcell44 arrived at this conclusion by noting that a single (quasi) mode occupies a spectral bandwidth n/Q within a cavity of volume V. Normalizing a resulting cavity-enhanced mode density per unit volume to the mode density of free space yields the ‘Purcell’ spontaneous emission enhancement factor44,46. P 4 }4 3 p }2 1 } l n }2 3 } Q V } where refractive index, n, is a modern addition to this expression to account for emission within dielectrics55. An atom whose transition falls within the mode linewidth will experience an enhancement to its spontaneous decay rate given by the Purcell factor. More significantly, because the enhancement comes about from coupling to only those continuum modes that make up the corresponding quasi-mode of the resonator, the spontaneous emission is directed to this quasi mode88 and has great utility with regard to coupling spontaneous power. In spectral locations that are intermediate to modal resonance frequencies, the density of the modes can fall well below the density in free space. With proper cavity design and for operation at these off￾resonance frequencies, spontaneous decay can be suppressed4,46,58. Rigorous developments of Purcell’s physical model have been presented on the basis of calculation of the continuum mode density (ref. 4, chapters 1 and 2 and references therein). Design of microcavities for observation of the Purcell effect must take account of the corresponding atomic (or atom-like) transition characteristics. Use of a small microcavity volume is important because enhancements driven by manipulation of Q alone are limited by the spectral width of the transition. Likewise, all other things being equal, narrow, atomic transitions can be Purcell-enhanced more as a higher Q becomes possible. It is for this reason that individual quantum dots, with their relatively narrow transition widths (compared with bulk semiconductors), are playing a significant role in this field49 (see Fig. 1). Box 2 Figure Purcell enhancement of spontaneous emission. Weak coupling to a cavity mode will enhance the spontaneous rate of emission by increasing the local density of modes (right) compared with their density in free space (left). Box 2 Purcell effect Etched holes Defect region Figure 3 Cross-sectional illustration of a photonic crystal defect microcavity laser. The microcavity is formed by dry etching a hexagonal array of holes and subsequent selective etch of an interior region, creating a thin membrane. One hole is left unetched creating a ‘defect’ in the array and therefore a defect mode in the optical spectrum. The mode (illustrated in green) is confined to the interior of the array by Bragg reflection in the plane and conventional waveguiding in the vertical direction. Also, shown in pink are quantum wells that upon photo pumping provide the amplification necessary for laser oscillation7 . Inset: Scanning electron micrograph of a photonic crystal defect microcavity laser. Micrograph is courtesy of O. Painter and A. Scherer (Caltech, CA). © 2003 Nature PublishingGroup

insight review articles exceeds 1 GWatt/cm' with less than 1 mW of coupled input power Observation of stimulated raman scattering ulti-order Stoke emission" stimulated Brillouin scattering and many other nonlin ear effects were first studied in microdroplets by Chang and by campillo(ref. 4, chapter 5, and references therein). The Kerr effect hasalso been observed by Treussart et al. inultrahigh-Qsilicamicros pheres" at microwatt input power levels. More recently, efficient solid-state Raman laser sources using fibre-coupled" ultrahigh-Q microspheres have been demonstrated, and they produced record low-threshold pump powers of 65 uw(ref. 43)(see Fig. 5).Raman sources can be used to extend the wavelength range of conventional lasers into difficult-to-access bands Dynamic filters in optical communications the past decade, wavelength division multiplexed (WDM) ightwave systems have been deployed in long-distance transmis ion to take better advantage of the vast bandwidth available in an Ultracold oviding addi length, used as a dynamic parameter, can enhance network perfor nance". With attention turned towards future system needs, there has been interest in devices that enable new filtering and switching functions Figure 4 If the coupling energy hg in a strongly coupled systemexceeds the thermal a microcavity filter that has received considerable atter IS 4-ergy of the atom, then the atomic centre of mass motion will be altered by interaction one that enables resonant transfer of optical power between two ith the vacuum cavity mode. In recent experiments 1, ultracold atoms have been waveguides. In its simplest form, it consists of a single whispering produced to satisfy this condition, resulting in atomic motion that is entrained for gallery microresonatorsandwiched between twosingle-mode wave- stantial periods of time by cavity QED coupling In this figure, an ultracold atom is ides(Fig. 6). As a passive filter, this structure can per entrained in anorbital motion before escaping Because the coupling energy depends on called channel add/drop in which a single channel is'dropped on the amplitude of the vacuum cavity field near the atom, optical transmission probing with high extinction from a first waveguide and coupled with low of the cavity during the atomic entrainment acts as an ultrasensitive measure of atomic loss to a second waveguide. In practice, this coupling should take location. Figure used with permission of H. J. Kimble(Caltech. CA) place with high selectivity because other wavelength channels will be present on the input waveguide. Other versions of this device(that are not based on a microresonator) are also being investigated- ortant next step will be to combine this feature with a microcavity however, the microresonatorversion is attractive because of its small ped forsignificant Purcellenhancement. size and potential for high-density integration on a wafer. If the whispering gallery resonator is made active by addition of an electri- Novel sources cally controllable refractive index, then the add/drop function is The pursuit of efficient and compact laser sources that offer dynamic and provides control of the resonant wavelength for tun- enhanced functionality or that provide insight into microcavity ing, ultrafast modulationorswitching. Alternatively, introduc- physics has inspired a large body of microcavity research. Lasing has ing a controlled loss within the resonator enables switching-off of een demonstrated in droplets"7, silica, 0 and polystyrene the coupling05-107. Arrays of these devices on a common sub- heres", semiconductor microdisks 828, micropillars(vertical cavities)and photonic crystal cavities. Small cavity volumes and high Q have allowed the production of submicrowatt optical pum thresholds inmicrospheres and microamp-scale current thresholds Pump wave in semiconductor lasers. With the advent of multi-wavelength communications systems tunable and compact sources have taken whispering n anincreased importance. In addition, interestin the Purcell effect gallery orbit and more efficient lasers has focused attention on threshold control and also on the concept of threshold. Lasers" that operate like micro-masers have also been studied A development of practical importance is the use of lateral oxida f=m mode tion in vertical cavity lasers". These lasers have a lateral oxide aper Emission wave ture that is normal to the cavity axis, which creates lateral mode con- finement and concentrates pumping current at the optical gain region, thereby making the device very efficient Cavity enhance- Silica ment effects have also been observed in versions of these devices con- taining quantum dots Sources that use a nonlinearly stimulated process to achieve laser ction represent another class of device. Resonant recirculation of Figure 5 lustration of a silica microsphere whispering gallery resonator. The green arbit weak input signals within ultrahigh-Q, small-mode-volume res- is a (=m mode entrained at the spheres surface. Also shown is a fibre taper waveguide onators will produce enormous modal field intensities and thereby used for power coupling to and from the resonator. h the figure, a blue pump wave lower the threshold for nonlinear phenomena". For a given coupled, induces a circulating intensity within the sphere that is sufficient toinduce laser power, Pi the circulating intensity within the resonator is oscillation (green emission wave) Inset: Photomicrograph showing a doped microsphere byl-Pin(/2rn)(Q/W)where nis the groupindex ForacavityQ (the glass sphere contains arare earth). The green emission in this case traces the pump million and a mode volume of 500 um(both obtainable in wave whispering gallery orbit. The inset micrograph was provided by M.Cal spheres roughly 40 um in diameter.)the circulating intensity NatuRevOl42414AugUst2003www.nature.com/nature e 2003 Nature Publishing Group 843

insight review articles NATURE | VOL 424 | 14 AUGUST 2003 | www.nature.com/nature 843 important next step will be to combine this feature with a microcavity equipped for significant Purcell enhancement. Novel sources The pursuit of efficient and compact laser sources that offer enhanced functionality or that provide insight into microcavity physics has inspired a large body of microcavity research. Lasing has been demonstrated in droplets78,79, silica38,80 and polystyrene spheres81, semiconductor microdisks52,82,83, micropillars3 (vertical cavities84) and photonic crystal cavities7 . Small cavity volumes and high Q have allowed the production of submicrowatt optical pump thresholds85 in microspheres and microamp-scale current thresholds in semiconductor lasers86,87. With the advent of multi-wavelength communications systems12, tunable and compact sources have taken on an increased importance. In addition, interest in the Purcell effect and more efficient lasers has focused attention on threshold control88 and also on the concept of threshold89. Lasers90 that operate like micro-masers15,46 have also been studied. A development of practical importance is the use of lateral oxida￾tion in vertical cavity lasers84. These lasers have a lateral oxide aper￾ture that is normal to the cavity axis, which creates lateral mode con￾finement and concentrates pumping current at the optical gain region, thereby making the device very efficient86,87. Cavity enhance￾ment effects have also been observed in versions of these devices con￾taining quantum dots91. Sources that use a nonlinearly stimulated process to achieve laser action represent another class of device. Resonant recirculation of weak input signals within ultrahigh-Q, small-mode–volume res￾onators will produce enormous modal field intensities and thereby lower the threshold for nonlinear phenomena35. For a given coupled, input power, Pin, the circulating intensity within the resonator is given by I=Pin(λ/2πn)(Q/V) where nis the group index. For a cavity Q of 100 million and a mode volume of 500 mm3 (both obtainable in spheres roughly 40 mm in diameter31,43) the circulating intensity exceeds 1 GWatt/cm2 with less than 1 mW of coupled input power. Observation of stimulated Raman scattering92,93, multi-order Stokes emission94, stimulated Brillouin scattering95 and many other nonlin￾ear effects were first studied in microdroplets by Chang92 and by Campillo (ref. 4, chapter 5, and references therein). The Kerr effect has also been observed by Treussart et al. in ultrahigh-Qsilica micros￾pheres96 at microwatt input power levels. More recently, efficient solid-state Raman laser sources using fibre-coupled42 ultrahigh-Q microspheres have been demonstrated, and they produced record￾low-threshold pump powers of 65 mW (ref. 43) (see Fig. 5). Raman sources can be used to extend the wavelength range of conventional lasers into difficult-to-access bands. Dynamic filters in optical communications During the past decade, wavelength division multiplexed (WDM) lightwave systems have been deployed in long-distance transmis￾sion to take better advantage of the vast bandwidth available in an optical fibre97. Beyond providing additional bandwidth, wave￾length, used as a dynamic parameter, can enhance network perfor￾mance98. With attention turned towards future system needs, there has been interest in devices that enable new filtering and switching functions99. A microcavity filter that has received considerable attention is one that enables resonant transfer of optical power between two waveguides. In its simplest form, it consists of a single whispering gallery microresonator sandwiched between two single-mode wave￾guides (Fig. 6). As a passive filter, this structure can perform a func￾tion called channel add/drop in which a single channel is ‘dropped’ with high extinction from a first waveguide and coupled with low loss to a second waveguide. In practice, this coupling should take place with high selectivity because other wavelength channels will be present on the input waveguide. Other versions of this device (that are not based on a microresonator) are also being investigated99–101; however, the microresonator version is attractive because of its small size and potential for high-density integration on a wafer102. If the whispering gallery resonator is made active by addition of an electri￾cally controllable refractive index, then the add/drop function is dynamic and provides control of the resonant wavelength for tun￾ing103, ultrafast modulation104 or switching. Alternatively, introduc￾ing a controlled loss within the resonator enables switching-off of the coupling105–107. Arrays of these devices on a common sub￾Mirror surface Probe laser Cavity mode Ultracold atom Figure 4 If the coupling energy ùg in a strongly coupled system exceeds the thermal energy of the atom, then the atomic centre of mass motion will be altered by interaction with the vacuum cavity mode. In recent experiments21,22, ultracold atoms have been produced to satisfy this condition, resulting in atomic motion that is entrained for substantial periods of time by cavity QED coupling. In this figure, an ultracold atom is entrained in an orbital motion before escaping. Because the coupling energy depends on the amplitude of the vacuum cavity field near the atom, optical transmission probing of the cavity during the atomic entrainment acts as an ultrasensitive measure of atomic location. Figure used with permission of H. J. Kimble (Caltech, CA). Emission wave Pump wave Fibre-taper waveguide Silica microsphere =m mode Pump wave whispering gallery orbit Figure 5 Illustration of a silica microsphere whispering gallery resonator. The green orbit is a ,=m mode entrained at the sphere’s surface. Also shown is a fibre taper waveguide used for power coupling to and from the resonator. In the figure, a blue pump wave induces a circulating intensity within the sphere that is sufficient to induce laser oscillation (green emission wave). Inset: Photomicrograph showing a doped microsphere (the glass sphere contains a rare earth). The green emission in this case traces the pump wave whispering gallery orbit. The inset micrograph was provided by M. Cai. © 2003 Nature PublishingGroup

insight review articles UV trimming of device refractive index could be required to offset fabrication-induced variances in critical resonator dimension Waveguide Future directions We have reviewed four broad application areas of optical microcavi ties and highlighted several microcavity designs foreach(see Table 1). Impressive results have been achieved in all areas. Substantial, addi- tional gains are possible in quantum optical applications with con- tinued improvement in microfabrication techniques and with plementation of new low-loss designs. Triggered, single photon sources will benefit from higher Purcell factors for improved fibre coupling, and miniaturization to the submicrometre scale of cavity QED devices(using either strong or weak coupling)is feasible. Also, the emergence of new ultrahigh-Q, wafer-based geometries should provide a platform for strong-coupling studies that combine both laboratory-on-chip functionsand efficient coupling to optical fibres. Technological applications such as the dynamic add/drop device will provide better control and reproducibility of filter characteristics in Waveguides designs that are increasingly complex. One other area that deserves special note is that of biological and chemical sensing Optical sensors that use evanescent field coupling Figure 6 lustration of a microcavity add/drop filter in which two buried waveguides have been developed however, high-Qoptical microcavities, as shown in brown) are vertically coupled to a disk whisper ing gallery resonator. This structure is fabricated by first a sensor transducer, offer the potential to greatly enhance detection ting the waveguides through a process of lithography sensitivity. Recently, sensors based on both monolithic and and etching and then wafer bonding an initially mechanically separate, second wafer microsphere whispering gallery transducers have been demon- ontaining layers that ultimately become the microresonator. After wafer bonding, the strated. It seems likely that this will become animportant application substrate isremoved from the second wafer, and lithography and etching are applied to area for these devices. Likewise, the broad technological impact that create the microdisk. Critical gaps that control field-coupling strengths between resonant devices have had at acoustic, radio and microwave frequen waveguides and the microresonator are thereby determined by thin-film crystal growth cies suggests that many other applications for these devices will methods as opposed to lithography and etch ing as is the case in side-coupled emerge in the optical domai structures. As shown, a red input channel is resonant with a mode of the disk and is doi: 10. 103w/nature 01939 ubsequently coupled through to the second waveguide. A blue channel that is non- L. Gerard, L M etal. Quantum bowes as active probes for photonic microstructures: The pillar coupling to the input waveguide along the indicated direction of signal flow. This device dot. Phys Status Solidi 178, 341-344(2000). can be made dynamic by indusion of electrically controllable loss or refractive layers 3JewellLJ. L ef al Lasing characteristics of GaAs microresonators. Appl. Plys Left 5L 1400-1402 of waveguide loading, Q,Furthermore, efficient coupling requires waveguide loading to &Armani, D, K.Kippenberg T.L. Spillane. M& Wahala K.I. Ultra- high-Q toroid microcavity c be balanced, such that field coupling between the disk and each waveguide is nearly ual. O, is then determined by the requisite information bandwidth, B, with B<w/0 Painter, O ef al Two-dimensional photonic band-gap defect mode laser Scierce 281, 1819-1821 required for proper information transfer. This schematic is based on one by PD Taranenko, V.B. &Weiss, C. O Spatial solitons in semiconductor microresonators. IEEEL Select Top Dapkus. The inset is a photomicrograph of a ring-resonator add/drop filter provided by Quantun Elect& Brent Little( Little Optics). The device shown is a second order design containing a pair of coupled ring resonators. The waveguides (top and bottom) feature vertical coupling IL. Khitrova G. Gibbs. H.M.hahnke, E, Kira. M& Kod,sW Nonlinear optics of normal-mode. to the resonators, providing excellent coupling control. strate,or interconnected by a fibre could one day be used to 14. Yariv. A Quanturm Electroulios (wiley. New York, 190). tegrated Ciruit(Wiley.New York, 1995) perform complex functions within the context of WDM 5. Berman, P.R. (ed )Cavity Quantun Electrodynamics(Academic Press, New York, 1993) Many versions of this basic design have been demonstrated, 16. Haroche, S Entanglement, mesoscopic superpositions with atoms and including monolithic Ill-V, silicaand polymer-based devices 17. Kimble, H I Strong interactions of single ato rrays have also been fabricated to explore issues associated with fab- 127-137(1998) photons in cavity QED. Physica Scripta T76. lenge for fabrication of these devices is ensuring control and repro- 1s optical avity, Plys. Re:Left11历/图m时m甲mmm ducibility of waveguide-to-resonator coupling andresonator dimen- atoms falling through high-finesse optical cavity. Opt. Let. 1, 1393-1395(1996) pling is to use so-called vertical coupling structures (coupling App. Pbs. B60233-2537(1995). regions that are defined primarily by layer thickness as opposed to atom-cavity microscope challenge for fabrication thespectralsha lape and 22Pinkse, P.W.H. Fischer.T.Maunz,P&Rempe,GTrapping anatom withsingle photons.Nature filter roll-off characteristics of these devices. Spectral response func- 23. Shimizu, Y ef al Control of light pulse propagation withonly a few cold atoms in a hig -finesse ions with roll-offs that are steeper than are possible using a simple microcavity. Phys. Rev. Lent. 89, 233001(2002) Lorentzian(single pole)filter resp re necessary for concatena 24 Mabuchi, H. Doherty, A C Cavity quantum electrodynamics: Coherence in context. Scierce 9 ion of components in networks". Designs for such multi-pole fil- 25. Raimond. 1. M, Brune, M. Haroche, S& Colloquium: Manipulating quantum entanglement witl ters based on coupled resonators"have been proposed, and several devices have been demonstrated1. Active frequency controlor26 pe, G,Thompson,里上地且山出 R. Measurement of ultralow lasses in an opt 844 e2003NaturepUblishingGroupNatUreIvOl42414august2003www.nature.com/nature

insight review articles 844 NATURE | VOL 424 | 14 AUGUST 2003 | www.nature.com/nature strate108,109 or interconnected by a fibre could one day be used to perform complex functions within the context of WDM. Many versions of this basic design have been demonstrated, including monolithic III–V110, silica111 and polymer-based devices104. Arrays have also been fabricated to explore issues associated with fab￾rication tolerance and post-process trimming102. The major chal￾lenge for fabrication of these devices is ensuring control and repro￾ducibility of waveguide-to-resonator coupling and resonator dimen￾sions. One approach to obtaining better fabrication control of cou￾pling is to use so-called vertical coupling structures (coupling regions that are defined primarily by layer thickness as opposed to lithography)104,106,111. Another challenge for fabrication concerns the spectral shape and filter roll-off characteristics of these devices. Spectral response func￾tions with roll-offs that are steeper than are possible using a simple Lorentzian (single pole) filter response are necessary for concatena￾tion of components in networks112. Designs for such multi-pole fil￾ters based on coupled resonators113 have been proposed, and several devices have been demonstrated114,115. Active frequency control103 or UV trimming of device refractive index102 could be required to offset fabrication-induced variances in critical resonator dimensions. Future directions We have reviewed four broad application areas of optical microcavi￾ties and highlighted several microcavity designs for each (see Table 1). Impressive results have been achieved in all areas. Substantial, addi￾tional gains are possible in quantum optical applications with con￾tinued improvement in microfabrication techniques and with implementation of new low-loss designs. Triggered, single photon sources will benefit from higher Purcell factors for improved fibre coupling, and miniaturization to the submicrometre scale of cavity QED devices (using either strong or weak coupling) is feasible. Also, the emergence of new ultrahigh-Q, wafer-based geometries should provide a platform for strong-coupling studies that combine both laboratory-on-chip functions and efficient coupling to optical fibres. Technological applications such as the dynamic add/drop device will provide better control and reproducibility of filter characteristics in designs that are increasingly complex. One other area that deserves special note is that of biological and chemical sensing. Optical sensors that use evanescent field coupling have been developed116,117 ; however, high-Q optical microcavities, as a sensor transducer, offer the potential to greatly enhance detection sensitivity39. Recently, sensors based on both monolithic118 and microsphere119 whispering gallery transducers have been demon￾strated. It seems likely that this will become an important application area for these devices. Likewise, the broad technological impact that resonant devices have had at acoustic, radio and microwave frequen￾cies suggests that many other applications for these devices will emerge in the optical domain. ■ doi:10.1038/nature01939 1. Gerard, J. M. et al. Quantum boxes as active probes for photonic microstructures: The pillar microcavity case. Appl. Phys. Lett. 69, 449–451 (1996). 2. Solomon, G. S., Pelton, M. & Yamamoto, Y. Modification of spontaneous emission of a single quantum dot. Phys. Status Solidi 178, 341–344 (2000). 3. Jewell, J. L. et al. Lasing characteristics of GaAs microresonators. Appl. Phys. Lett. 54, 1400–1402 (1989). 4. Chang, R. K. (ed.) Optical Processes in Microcavities (World Scientific, Singapore, 1996). 5. 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Microdisk resonator Waveguide Waveguides Waveguide Coupled ring resonators Mode 80 µm Figure 6 Illustration of a microcavity add/drop filter in which two buried waveguides (shown in brown) are vertically coupled to a disk whispering gallery resonator. This structure is fabricated by first creating the waveguides through a process of lithography and etching and then wafer bonding an initially mechanically separate, second wafer containing layers that ultimately become the microresonator. After wafer bonding, the substrate is removed from the second wafer, and lithography and etching are applied to create the microdisk. Critical gaps that control field-coupling strengths between waveguides and the microresonator are thereby determined by thin-film crystal growth methods as opposed to lithography and etching as is the case in side-coupled structures. As shown, a red input channel is resonant with a mode of the disk and is subsequently coupled through to the second waveguide. A blue channel that is non￾resonant is also shown. The channel add function is not illustrated; however, it results from coupling power into the other port of the drop waveguide with subsequent resonant coupling to the input waveguide along the indicated direction of signal flow. This device can be made dynamic by inclusion of electrically controllable loss or refractive layers within the resonator. Low-loss coupling and high drop extinction require the microdisk to have an intrinsic Q factor, Qo, that is substantially larger than its Q factor under conditions of waveguide loading, QL. Furthermore, efficient coupling requires waveguide loading to be balanced, such that field coupling between the disk and each waveguide is nearly equal. QL is then determined by the requisite information bandwidth, B, with B<n/QL required for proper information transfer. This schematic is based on one by P. D. Dapkus103. The inset is a photomicrograph of a ring-resonator add/drop filter provided by Brent Little (Little Optics). The device shown is a ‘second order’ design containing a pair of coupled ring resonators. The waveguides (top and bottom) feature ‘vertical coupling’ to the resonators, providing excellent coupling control. © 2003 Nature PublishingGroup

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