Raman Scattering in Resonant Cavities

Abstract

The modification of the optical properties of matter due to photon confinement in optical microcavities has been an active field of research in the last ten years. This review addresses the problem of Raman scattering in these optically confining structures. Two completely different regimes exist and will be discussed here. Firstly, a situation in which the action of the microcavity is basically to enhance, and to spatially and spectrally confine, the photon field, but otherwise the light-matter interaction process remains unaltered. This is the case when the laser and scattered photon energies are well below those of the excitonic transitions in the structure, or when the coupling between the latter and the cavity mode can be treated in a “weak coupling” approximation. This regime will be labeled here as “optical resonant Raman scattering”. And secondly, a “strong-coupling” regime in which exciton and cavity-photon modes cannot be treated separately, leading to coupled excitations, so-called cavity polaritons. In this second case, when the laser or scattered photons are tuned to the excitonic energies, and thus an electronic resonant Raman-scattering process is achieved, the Raman-scattering process has to be described in a fundamentally different way. This regime will be labeled as “cavity-polariton-mediated Raman scattering”.

The Chapter will begin, after a brief historical introduction, with a review of the fundamental properties of optical microcavities, and of the different strategies implemented to achieve double optical resonant Raman scattering in planar microcavities. A section will be then devoted to experimental results that highlight the different characteristics and potentialities of Raman scattering under optical confinement, including, in some detail, an analysis of the performance of these structures for Raman amplification. A subsequent section will present a series of research efforts devoted to the study of nanostructure phonon physics that rely on microcavities for Raman enhancement. In particular, emphasis will be given to recent investigations on acoustic cavities that parallel their optical counterparts but that, instead, confine hypersound in the GHz–THz range. The Chapter will then turn to a quite different topic, that of cavity-polariton-mediated scattering. The theory developed in the 1970s for bulk materials will be briefly introduced, and its modifications to account for photon confinement in planar structures will be addressed. Finally, a series of experiments that demonstrate the involvement of polaritons in the inelastic scattering of light in strongly coupled cavities will be reviewed. The Chapter will end with some conclusions and prospects for future developments on this subject.