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 843insight 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