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/naturenative 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