Upconverted photoluminescence induced by radiative coupling between excitons

Abstract

We propose an unconventional scheme of photoluminescence in a semiconductor thin film, where the nonlocal correlation between an excitonic wave and a light wave prominently enhances the exciton-light coupling beyond the long-wavelength approximation (the so-called excitonic superradiance regime). On the basis of the developed method extending input-output theory, we elucidate atypical photoluminescence effects due to the strong wave-wave correlation. In particular, the upconverted photoluminescence based on the coherent superposition of excitons is found to be highly efficient, i.e., it can be realized by weak pumping without auxiliary systems such as cavities or photonic antennas. 71.36.+c, 42.50.Nn Photoluminescence (PL) is one of the fundamental phenomena arising from light-matter coupling, and has been extensively utilized for monitoring the structure of the electronic levels in materials [1]. PL spectroscopy provides the essential information of matter systems, such as the structure of the levels and oscillator strengths of electronic excited states after reaching quasiequilibrium. On the other hand, PL is also important for observing the interplay scheme between light and electronic systems. A representative example is PL reflecting the dispersion relation of polaritons and not the thermal equilibrium for bare excitons [2][3][4]. In order to access polaritons by photon emission, photons have to be emitted before the exciton-photon coupling is "dephased", or the excitations relax to quasi-equilibrium. Then, it is necessary to consider polariton accumulation at the bottleneck of the decay [2], or confine the electronic system in high-density photons [5][6][7][8].Recently, another scheme that allows PL to access exciton-light coupled modes has been demonstrated in simple semiconductor thin films [9]. When the film thickness is in the nano-to-bulk crossover regime (a few hundred nanometers), the exciton-light coupling is extremely enhanced by the nonlocal correlation between a light wave and an excitonic center-of-mass (c.m.) wave [10][11][12][13]. Note that the resultant exciton-light coupled state should be distinguished from polaritons because it exhibits an ultrafast radiative decay exceeding the polariton formation, and is in the so-called excitonic superradiance regime [14][15][16]. Such a strong coupling beyond the long-wavelength approximation (LWA) leads to unprecedented PL properties. Actually, photon emission simultaneously occurs from more than one exciton-light coupled mode including optically-forbidden ones [9].Here, we focus on a unique feature of the beyond the LWA regime; there exists strong coupling between the different c.m. states of excitons via radiation that causes quantum superposition of these states. The purpose of * Electronic address: matsuda@pe.osakafu-u.ac.jp † Electronic address: ishi@pe.osakafu-u.ac.jp this paper is to theoretically demonstrate that the control of the superposition provides unconventional and functional aspects of PL properties. In particular, remarkable up-...

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