Light-Matter Interactions in Photonic Crystals

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We propose to study novel and fundamental light-matter interactions in photonic crystals and related nano-structures. The novelty of these materials results from their ability to trap light in unique ways and to engineer ultra-strong coupling between photons and resonant electronic excitations in matter. We propose to elucidate the practical applications of these materials to solar energy harvesting, optical information processing, novel light sources, and optical bio-sensing. We will work closely with leading experimental and nano-fabrication groups to realize the predictions of our theoretical and numerical modeling. We propose to develop thin-film photonic crystal architectures for light trapping, absorption and solar spectral reshaping in next generation photovoltaics. Our effort will focus on the computational design, synthesis, and optical testing of photonic architectures that alter fundamental photon-matter interactions at sub-wavelength scales. The designed architectures offer new paradigms to control, trap, and convert solar energy and to compress the frequency bandwidth of sunlight within the solar cell. The underlying physics of our enhanced light trapping and solar absorption is the through the coupling of sunlight to slow-light modes of the photonic crystal that propagate in directions transverse to the thin-film surface. This corresponds to a spectral range where the photonic density of states is enhanced relative to that of a homogeneous material. Our approach involves the integration of graded anti-reflection architectures with photonic crystal based light-trapping and absorption. This enables light absorption in thin-film (< 500 nanometer) solar cells to rival that of thick (~300 micron) cells and to surpass previously suggested statistical ray trapping limits. Strong light concentration by the photonic crystal, augmented by plasmonic resonances near the back mirror, at frequencies below the solar cell electronic band gap enable nonlinear up-conversion of photons. This spectral compression of the solar spectrum paves the way to solar-to-electricity power conversion efficiency by single junction silicon solar cells exceeding 30%. The role of random structural disorder in the photonic crystal solar cells and its influence on overall solar power conversion efficiency will be investigated. We propose to study a new route to the realization of excitonic coherence and Bose-Einstein condensation at or near room temperature in GaAs multiple quantum well (QW) structures. This involves a new type of quantum cavity enabled by a 3D photonic band gap (PBG) material. The essential scientific advance in this proposal rests on two unique and fundamental properties of PBG materials: (i) By inhibiting radiative recombination of excitons over a specific range of frequencies, it is possible to extend the exciton lifetime well beyond the time scale required to establish thermodynamic equilibrium in a trapped exciton gas. (ii) 3D PBG materials enable stronger, sub-wavelength confinement and focusing of light (without loss), thereby leading to much stronger exciton-photon coupling than achievable with previously studied 1D optical cavities. We will study the nature of quantum many-body correlations in the excitonic Bose condensate and consider the nature of laser-like light emission as the condensate decays radiatively. We propose to design structures that enable ultra-strong coupling between excitons and photons such that equilibrium Bose condensation temperature approaches room temperature. We will work closely with fabrication groups that can synthesize our designed photonic crystal architectures and with experimental optics groups with experience in characterizing excitonic condensates.