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Physical Review Letters

Observation of Superradiance by Nonlocal Wave Coupling of Light and Excitons in CuCl Thin Films

Masayoshi Ichimiya, Masaaki Ashida, Hideki Yasuda, Hajime Ishihara, Tadashi Itoh

Published Dec 14, 2009• 45 citations
Publisher

Abstract

We report the observation of a remarkably strong coupling between light and a multinode-type exciton. The observed radiative decay time reaches the order of 100 fs, which is much faster than the dephasing process of nonradiative scattering. In this high-speed superradiance, the light wave and the excitonic wave in a high-quality thin film form a harmonized wave-wave coupling over a range of multiple wavelengths. This mechanism contradicts the conventional physical description of light-matter interaction based on the long-wavelength approximation.

DOI
10.1103/physrevlett.103.257401

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We report the observation of a remarkably strong coupling between light and a multinode-type exciton. The observed radiative decay time reaches the order of 100 fs, which is much faster than the dephasing process of nonradiative scattering. In this high-speed superradiance, the light wave and the excitonic wave in a high-quality thin film form a harmonized wave-wave coupling over a range of multiple wavelengths. This mechanism contradicts the conventional physical description of light-matter interaction based on the long-wavelength approximation.
We demonstrate ultrafast superradiance showing 10-fs decay due to the nonlocal interaction between light waves and exciton waves in CuCl mesoscopic thin films. This time constant is comparable with the thermal dephasing even at room temperature. Actually, we observed photoluminescence signal from one confined mode of translational motion of the exciton surviving at room temperature. In addition, temperature dependence of the photoluminescence spectra with multiple peaks was reproduced well by theoretical calculation. This finding opens a new avenue of “thermal free photonics”, where the exciton coherence is utilized at higher temperature.
We propose an unconventional scheme of photoluminescence in a semiconductor thin film, where the nonlocal correlation between an excitonic wave and light wave prominently enhances the interaction between different excitonic states via radiation 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 quantum 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.
Time- and spectrally resolved reflectivity measurements on multiple-quantum-well structures excited by ultrashort laser pulses are presented. In Bragg structures, where the interwell distance amounts to half the optical wavelength at the exciton resonance, superradiant coupling of the polarization dynamics within different quantum wells leads to an enhancement of the reflectivity strength and linewidth. We show that the superradiance is not limited to the heavy-hole exciton but can be observed even on excited and light-hole exciton states as well as on correlated electron-hole pair states. These coupling effects grow with the number of quantum wells. For our samples containing 60 and 30 quantum wells, pronounced superradiant enhanced reflectivity is present even at high excitation intensities and high temperatures, showing the insensitivity of radiative coupling with respect to incoherent scattering processes. Our findings are in good agreement with solutions of the semiconductor Maxwell-Bloch equations.