Pronounced light-matter coupling in periodic semiconductor quantum wells

Abstract

The development of advanced technological methods for growth of semiconductors, such as molecular beam epitaxy, have allowed growth of layered semiconductor structures with precision to a single atomic layer. One important structure is the semiconductor quantum well, consisting of a thin layer of a smaller bandgap semiconductor grown between layers of thicker, larger bandgap semiconductor. Quantum wells are, for example, largely responsible for making the semiconductor laser a practical device. By increasing the binding energy of excitons (hydrogen-like, bound electron-hole pairs in semiconductors), and allowing them to couple to the continuum of vacuum photon modes, semiconductor quantum wells have made excitons the focus of numerous fundamental optical studies. Stacks of periodically grown quantum wells, grown far enough apart such that electronic tunneling between quantum wells is unimportant, can still be coupled by light. N light-coupled quantum wells have N exciton-light, or exciton-polariton, eigenmodes, each characterized by a distinct energy and radiative lifetime dependent on the periodicity of the quantum wells. By adjusting the periodicity of the quantum wells and the material parameters, engineering of the light-matter interaction of these one-dimensional mesoscopic crystals is possible. The interesting new properties of these structures open the possibility for new devices. Periodic multiple quantum wells with a period in the vicinity of half the exciton resonance wavelength are studied in linear measurements of reflection, transmission and absorption. The optical properties are dominated by the eigenmodes of the light-coupled quantum wells. At Bragg periodicity, where the oscillator strengths of all quantum well excitons are concentrated into one superradiant mode, a photonic band gap grows in amplitude and linearly in energy width with increasing number of quantum wells N. A corresponding N times increased radiative damping rate compared to a single quantum well is observed, originating from expulsion of the light character of the superradiant mode from the photonic bandgap structure. The slope of linewidth versus N gives the radiative linewidth of the exciton. For periods away from Bragg condition, all normal modes become optically active, and are observed in reflection and absorption experiments. Because light-coupling alters the photon density of states, formation of the N exciton-polariton eigenmodes is also evidenced in photoluminescence after nonresonant excitation into the free carrier continuum. The strongly modified light-matter interaction for photons in the photonic gap at Bragg periodicity is also manifest in the inhibited emission from the superradiant mode, a surprising result explained by a consideration of the linear properties. The temporal dynamics of Rayleigh scattering of a resonant excitation pulse from disordered semiconductor multiple quantum wells has many interesting aspects, and has recently been the subject of much debate. The effect of light-coupling on resonant Rayleigh scattering from periodic semiconductor multiple quantum well structures is investigated both experimentally and theoretically. Polaritonic effects are found to dominate the Rayleigh scattered light temporal dynamics due to the simultaneous coexistence of several eigenmodes with different energy and radiative decay times for a given periodicity. They give rise to polarization beating between modes and determine rise and decay times of the resonance Rayleigh scattered signals.