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A nanophotonic resonator or nanocavity is an optical cavity which is on the order of tens to hundreds of nanometers in size. Optical cavities are a major component of all lasers, they are responsible for providing amplification of a light source via positive feedback, a process known as amplified spontaneous emission or ASE. Nanophotonic resonators offer inherently higher light energy confinement than ordinary cavities, which means stronger light-material interactions, and therefore lower lasing threshold provided the quality factor of the resonator is high.〔Akahane, Y., Asano, T., 5, B. S., & Noda, S. (2003). High-Q photonic nanocavity in a two-dimensional photonic crystal. Nature, 425(6961), 944-947.〕 Nanophotonic resonators can be made with photonic crystals, silicon, diamond, or metals such as gold. For a laser in a nanocavity, spontaneous emission (SE) from the gain medium is enhanced by the Purcell effect,〔Altug, H., Englund, D., & Vučković, J. (2006). Ultrafast photonic crystal nanocavity laser. Nature Physics, 2(7), 484-488.〕〔Purcell, E. Spontaneous emission probabilities at radio frequencies. Phys. Rev. 69, 681 (1946).〕 equal to the quality factor or Q-factor of the cavity divided by the effective mode field volume, F = Q/Vmode. Therefore, reducing the volume of an optical cavity can dramatically increase this factor, which can have the effect of decreasing the input power threshold for lasing.〔Painter, O. et al. Two-dimensional photonic band-gap defect mode laser. Science 284, 1819–1821 (1999).〕〔Loncar, M., Yoshie, T., Scherer, A., Gogna, P. & Qiu, Y. Low-threshold photonic crystal laser. Appl. Phys. Lett. 81, 2680–2682 (2002).〕 This also means that the response time of spontaneous emission from a gain medium in a nanocavity also decreases, the result being that the laser may reach lasing steady state picoseconds after it starts being pumped. A laser formed in a nanocavity therefore may be modulated via its pump source at very high speeds. Spontaneous emission rate increases of over 70 times modern semiconductor laser devices have been demonstrated, with theoretical laser modulation speeds exceeding 100 GHz, an order of magnitude higher than modern semiconductor lasers, and higher than most digital oscilloscopes.〔Altug, H., Englund, D., & Vučković, J. (2006). Ultrafast photonic crystal nanocavity laser. Nature Physics, 2(7), 484-488.〕 Nanophotonic resonators have also been applied to create nanoscale filters 〔Noda, S., Chutinan, A. & Imada, M. Trapping and emission of photons by a single defect in a photonic bandgap structure. Nature 407, 608–610 (2000).〕〔Song, B. S., Noda, S. & Asano, T. Photonic devices based on in-plane hetero photonic crystals. Science 300, 1537 (2003).〕 and photonic chips 〔Noda, S., Chutinan, A. & Imada, M. Trapping and emission of photons by a single defect in a photonic bandgap structure. Nature 407, 608–610 (2000).〕 ==Differences From Classical Cavities== For cavities much larger than the wavelength of the light they contain, cavities with very high Q factors have already been realized (~125,000,000).〔Armani, D. K., Kippenberg, T. J., Spillane, S. M. & Vahala, K. J. Ultra-high-Q toroid microcavity on a chip. Nature 421, 925–928 (2003).〕 However, high Q cavities on the order of the same size as the optical wavelength have been difficult to produce due to the inverse relationship between radiation losses and cavity size.〔Akahane, Y., Asano, T., 5, B. S., & Noda, S. (2003). High-Q photonic nanocavity in a two-dimensional photonic crystal. Nature, 425(6961), 944-947.〕 When dealing with a cavity much larger than the optical wavelength, it is simple to design interfaces such that light ray paths fulfill total internal reflection conditions or Bragg reflection conditions. For light confined within much smaller cavities near the size of the optical wavelength, deviations from ray optics approximations become severe and it becomes infeasible, if not impossible to design a cavity which fulfills optimum reflection conditions for all three spatial components of the propagating light wave vectors.〔Akahane, Y., Asano, T., 5, B. S., & Noda, S. (2003). High-Q photonic nanocavity in a two-dimensional photonic crystal. Nature, 425(6961), 944-947.〕〔Bayn, I., & Salzman, J. (2008). Ultra high-Q photonic crystal nanocavity design: The effect of a low-ε slab material. Optics express, 16(7), 4972-4980. 〕 In a laser, the gain medium emits light randomly in all directions. With a classical cavity, the number of photons which are coupled into a single cavity mode relative to the total number of photons spontaneously emitted photons is relatively low because of the geometric inefficiency of the cavity, described by the Purcell factor Q/Vmode.〔Coldren, L. A. & Corzine, S. W. Diode Lasers and Photonic Integrated Circuits (Wiley, New York, 1995). 〕 The rate at which lasing in such a cavity can be modulated depends on the relaxation frequency of the resonator described by equation 1. R2 = (avgP0)/τp + β/(τpτr0/F) + (βN0)/((τr0/F)P0)(1/τtotal - 1/(τr0/F)) (1) Where τr0 is the intrinsic carrier radiative lifetime of the bulk material, a is the differential gain, vg is the group velocity, τp = Q/ωL is the photon lifetime, ωL is the lasing frequency, β is the spontaneous emission coupling factor which is enhanced by the Purcell effect, and 1 /τtotal = F/τr0 +1/τnr where τnr is the non-radiative lifetime. In the case of minimal Purcell effect in a classical cavity with small F = Q/Vmode, only the first term of equation 1 is considered, and the only way to increase modulation frequency is to increase photon density P0 by increasing the pumping power. However, thermal effects practically limit the modulation frequency to around 20 GHz, making this approach is inefficient.〔Altug, H., Englund, D., & Vučković, J. (2006). Ultrafast photonic crystal nanocavity laser. Nature Physics, 2(7), 484-488.〕〔Lear, K. L. et al. Small and large signal modulation of 850 nm oxide-confined vertical-cavity surface-emitting lasers. Advances in Vertical Cavity Surface Emitting Lasers in Trends in Optics and Photonics Series 15, 69–74 (1997).〕 In nanoscale photonic resonators with high Q, the effective mode volume Vmode is inherently very small resulting in high F and β, and terms 2 and 3 in equation 1 are no longer negligible. Consequently nanocavities are fundamentally better suited to efficiently produce spontaneous emission and amplified spontaneous emission light modulated at frequencies much higher than 20 GHz without negative thermal effects.〔Altug, H., Englund, D., & Vučković, J. (2006). Ultrafast photonic crystal nanocavity laser. Nature Physics, 2(7), 484-488.〕〔Yamamoto, Y., Machida, S. & Bjork, G. Microcavity semiconductor laser with enhanced spontaneous emission. Phys. Rev. A 44, 657–668 (1991).〕 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Nanophotonic resonator」の詳細全文を読む スポンサード リンク
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