This work reports progress towards demonstrated Raman-based optical refrigeration. Previously  we introduced Raman refrigeration and its required photonic structures. Building on the previous, the mechanisms and photonic structures are detailed and simplified proof-of-concept devices described. Three wavelength bands, two illuminated and one dark, are considered. The width of each of these bands is predicated upon the magnitude of the Raman shift of the active layer. Optimally, the first illuminated band is approximately one Raman shift (wavelength) in width. This illuminated band is capped above (at shorter wavelengths) by a dark band. At longer wavelengths a second illuminated band again one Raman shift in width is reflected by photonic structuring that forbids light propagation. The dark bands provide an exhaust through which up energy (anti-Stokes) shifted light can be emitted thereby carrying away heat. The Photonic structure prohibits the propagation of a band of long wavelength light thereby both blocking Stoke’s shifted illuminated band light and reflecting incident light having these wavelengths. The naturally occurring solar spectrum with its light and dark bands caused by atmospheric absorption is a good match to diamond-based Raman refrigeration. Diamond also has extremely low absorption and large Raman cross-section especially in small grain form. Proof-of-concept devices employing simple one-dimensional photonic structures are the focus of present experimental effort. The prospect of broadband refrigeration remains a delicate balance requiring limited absorption and increased Raman cross section through phonon engineering of the Raman active layer.
Spectral management and diffusive light transport enables plants to both survive on extremely low irradiance and also to survive long enough under extreme temperature and pressure to leave imprints in super-heated impact glasses. These properties are related to structures wherein a low density of light absorbent particles are embedded in a light scattering and spectral selective reflective matrix. This marvelous diffusive light engineering has wide-ranging applications based on bio-mimicry. Where, environmentally sensitive radiative-to-non-radiative lifetime ratios increase the photon flux to the chlorophyll molecules best positioned for favorable photochemistry and for preservation under extreme conditions. The embedding of absorptive particles within a transparent scattering matrix has far reaching intriguing applications. Included is the extreme heating of light absorbent particles within a relatively cold matrix. Interestingly, the hot absorbent particle-cold matrix condition is critical for the efficient extreme heating of small particles. Here the potential of non-equilibrium passive diffusive light collection will consider be explored using one of the most challenging application extreme particle heating for controlled nuclear fusion.
Described are the prospects for broadband optical refrigeration based on Raman scattering of incoherent light. Laser pumped rare earth fluorescence has been demonstrated and commercial applications are sure to follow. Broadband refrigeration requires strong Raman scattering and large Raman shift. Also required are spectral management and photonic patterning to offset the unfavorable anti-Stoke’s to Stoke’s shift ratio. Materials such as diamond, silicon, and a number of molecular systems are ideal and have low absorption. Optics splits the broadband spectrum into light and dark bands with width corresponding to the Raman shift. Broadband spectrums where photon flux decreases with increasing photon energy are ideal. By tailoring the incoming spectrum, by utilizing extremely transparent strong Raman shift materials and by photonic inhibition of Stoke’s shifted light the prospect become feasible. The Raman optical cross- section increases with decreasing particle size (until the particle become too small to support the Raman-phonons). Where conservation of phonon states in these truncated Brillioun-zone particles requires an increased density (number/cm3) of the allowed-states to compensate for states lost to particle size. Nonetheless, the anti-Stokes to Stokes ratio is approximately one-to-two at laboratory temperature. Thin film deposited diamond is an excellent candidate for refrigeration applications due to its high transparency small grain size and its large Raman magnitude and large shift. Simple one-dimensional photonic structures selectively inhibit the Stoke’s shifted light making refrigeration possible.
Control of refractive index in amorphous silicon materials is investigated. Elementary waveguide structures were prepared on two micron thick amorphous silicon by photon lithographic patterning of a silver masking layer. Hydrogen was implanted at fluence of ~5×1017 cm2 for three energies, 50, 100 and 175 KeV yielding a total does of ~1.5×1018 cm2 consistent with a 10% increase in atoms due to the hydrogen addition. The optical properties of the implanted and non-implanted regions were probed as a function of low temperature annealing. The optical band gap shift to higher energy was consistent with hydrogen addition. Some darkening, absorption increase, were noted on the implanted regions. However, low temperature annealing is known to remove dangling bond damage in amorphous silicon. Prospects of utilizing these waveguides to probe light induced optical changes in amorphous silicon is described as well as the prospects of more advanced devices.
In recent years the prospect of engineering an integrated photonic technology based on amorphous silicon-based has focused efforts on providing a unified understanding of the optical properties of this material. From a optical properties prospective the science of amorphous silicon is most transparent from a nano-crystalline material framework. Of particular interest for photonic engineering is the tunable range of the refractive index in amorphous silicon, the fast and slow light induced changes in epsilon 1 and 2, the means by which to deposit films of sufficient thickness and smoothness for the photonic application and the relationships among deposition conditions, material properties, and in particular the optical parameters. The present work reviews some of the previous work and examines the experimental and theoretical basis for the fast light induced refractive index change with the hope of providing the insight needed for device engineering. This work suggests several novel designs for light amorphous silicon based light valves and other devices.
In recent years the prospect of engineering an integrated photonic technology based on amorphous siliconbased has focused efforts on providing a unified understanding of the optical properties of this material. From a optical properties prospective the science of amorphous silicon is most transparent from a nanocrystalline material framework. Of particular interest for photonic engineering is the tunable range of the refractive index in amorphous silicon, the fast and slow light induced changes in epsilon 1 and 2, the means by which to deposit films of sufficient thickness and smoothness for the photonic application and the relationships among deposition conditions, material properties, and in particular the optical parameters. The present work reviews some of the previous work and examines the experimental and theoretical basis for the fast light induced refractive index change with the hope of providing the insight needed for device engineering. This work suggests several novel designs for light amorphous silicon based light valves and other devices.
The design latitude for photonic engineering in amorphous silicon-based materials is great because of the very high solubility limits for impurities in the amorphous phase and the large change in refractive index that accompanies impurity infusion. Our recent experimental work found both the classical dynamic light induced refractive index changes and a rich set of light induced changes in the optical constants of amorphous silicon materials not found in classical systems. Included in the changes unique to amorphous silicon are slow light induced structure changes triggered by an above gap illumination induced defect's effect on the relaxation of surrounding structure. There are also very fast refractive index changes associated with above gap illumination which our recent work reports are not be associated with heating nor is it directly related to the slow change. Additionally, it has long been known that amorphous silicon has a strong electro-absorption response near the band edge. The fast changes and the electro- absorption are explained in terms of a simple tetrahedral bonded silicon model in which the electron coherence length is limited and the optical transitions are indirect. This model provides a framework for the development of a photo- active integrated photonic technology based on amorphous silicon.
The prospects for a thin film amorphous silicon based integrated photonic technology spanning materials, devices, and physics are described. Impurity implantation is an effective technique for the preparation of permanent refractive index patterning due to the very high solubility limits of the amorphous phase. Methods of preparing films of the requisite thickness and smoothness for photonic application have been identified. Other experiments suggest that there is a light induced refractive index change of sufficient magnitude for patterning light adaptive and/or light defined optical elements. Two light induced refractive index changes, one fast and one slow, were observed in amorphous silicon materials. These changes were observed over temperatures ranging from room temperature to 250 degree(s)C and do not appear to diminish with increasing temperature over this range. Simulations were used to elucidate the physics of light induced change. Several classes of thin film devices were developed which span a wide range of functionality.
The prospect of hydrogenated amorphous silicon based photonic thin film material and devices is introduced. The hydrogen content of hydrogenated amorphous silicon controls its refractive index. Hydrogen content and therefore the refractive index patterning techniques and possibilities are described. For example, regions of a growth surface exposed to a hydrogen radical (and/or ion) flux have increased optical band gap and decreased refractive index. By careful implementation of hydrogen control the preparation of 3-D photonic crystal films on a wide variety of substrates including single crystal silicon and flexible polymer becomes possible. The size scales on which it is possible to pattern the hydrogen content are appropriate for the preparation of photonic crystal films and bulk materials designed to interact with the infrared, visible light, or micro-wave electro- magnetic spectrums. The optical band gap of amorphous silicon depends on specific hydrogenated structures. The relatively independent patterning of the band gap and refractive index makes possible an extensive array of optical devices.