Nanophotonic resonator

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. Nanophotonic resonators can be made with photonic crystals, silicon, diamond, or metals such as gold. 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. 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, 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. 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. Nanophotonic resonators have also been applied to create nanoscale filters and photonic chips 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). 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. 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. 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. 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. 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. Nanocavities made from photonic crystals are typically implemented in a photonic crystal slab structure. Such a slab will generally have a periodic lattice structure of physical holes in the material. For light propagating within the slab, a reflective interface is formed at these holes due to the periodic differences in refractive index in the structure. A common photonic crystal nanocavity design shown is essentially a photonic crystal with an intentional defect (holes missing). This structure having periodic changes in refractive index on the order of the length of the optical wavelength satisfies Bragg reflection conditions in the y and z directions for a particular wavelength range, and the slab boundaries in the x direction create another reflective boundary due to oblique reflection at dielectric boundaries. This results in theoretically perfect wave confinement in the y and z directions along the axis of a lattice row, and good confinement along the x direction. Since this confinement effect along the y and z directions (directions of the crystal lattice) is only for a range of frequencies, it has been referred to as a photonic bandgap, since there is a discrete set of photon energies which cannot propagate in the lattice directions in the material. However, because of the diffraction of waves propagating inside this structure, radiation energy does escape the cavity within the photonic crystal slab plane. The lattice spacing can be tuned to produce optimal boundary conditions of the standing wave inside the cavity to produce minimal loss and highest Q.