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.
[1] 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,[2][3] equal to the quality factor or
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.
[4][5] 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.
[1] 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.
[10] The rate at which lasing in such a cavity can be modulated depends on the relaxation frequency of the resonator described by equation 1.
However, thermal effects practically limit the modulation frequency to around 20 GHz, making this approach is inefficient.
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.
This structure having periodic changes in refractive index on the order of the length of the optical wavelength satisfies Bragg reflection conditions in the
[6] However, because of the diffraction of waves propagating inside this structure, radiation energy does escape the cavity within the photonic crystal slab plane.
[1] Beside those conventional resonators, they are some examples of rewritable and/or movable cavities, which are accomplished by a micro infiltration system [13] and by a manipulation of single nanoparticles inside photonic crystals.
[14][15] Metals can also be an effective way to confine light in structures equal to or smaller than the optical wavelength.
[18] This effect was originally observed in radio and microwave engineering, where metal antennas and waveguides may be hundreds of times smaller than the free-space wavelength.
In the same way, visible light can be constricted to the nano level with metal structures which form channels, tips, gaps, etc.
Gold is also a convenient choice for nanofabrication because of its unreactivity and ease of use with chemical vapour deposition.
[7] Planar nanocavities are commonly used for thin film interference, which occurs when incident light waves reflected by the upper and lower boundaries of a thin film interfere with one another forming a new wave.
An example of this is the colorful patterns produced by thin layers of oil on a surface.
As expressed in the equation 3, the optical path length difference (OPD) can be related to wavelengths which constructively interfere in the thin film.
At optical frequency, metals behave much less like ideal conductors, and also exhibit plasmon-related effects like kinetic inductance and surface plasmon resonance.
[20] A nantenna is a nanoscopic rectifying antenna, a technology being developed to convert light into electric power.
[20] It has been suggested that nanophotonic resonators be used on multi core chips to both decrease size and boost efficiency.
[21] This is done by creating arrays of nanophotonic optical ring resonators that can transmit specific wavelengths of light between each other.
[22] Researchers have developed planar nanocavities that can reach 90% peak absorption using interference effects.
This result is useful in that there are numerous applications that can benefit from these findings, specifically in energy conversion [7]
