NCTU Nanophotonics

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Microcavity polariton lasers

Semiconductor microcavities (MCs) are powerful in controlling interaction between light and matter. In a semiconductor MC, strong interaction between excitons and photons would produce new admixed quasiparticles, called exciton-polaritons. Polaritons have a very light effective mass (typically 10^-8 times the hydrogen atom), have controllable energy-momentum dispersion curves, and follow bosonic statistics at low densities. As a result, a wealth of intriguing phenomena have been observed and extensively studied in the polariton system, including dynamical polariton Bose-Einstein condensation (BEC), polariton lasing and polariton parametric amplification. The bosonic nature of polaritons allows them to condense in the state with the lowest kinetic energy. The radiative decay of polaritons from the condensate creates laser-like coherent light, termed polariton lasers. Electronic population inversion is not necessary in such polariton lasers, as compared to conventional lasers, leading to ultra-low threshold coherent light sources. Furthermore, the extremely small effective mass of polaritons enables polariton condensation at higher critical temperatures compared to atomic system such as Sodium gas (Tc=2μK), which is crucial for practical applications. Polariton lasers have been demonstrated in GaAs and CdTe MCs, albeit only at cryogenic temperatures due to the small exciton binding energies in these materials. In contrast, wide-bandgap semiconductor materials have larger exciton binding energies, and have emerged as candidates for high temperature polariton lasers. We have demonstrated strong coupling and bottleneck effect in ZnO based hybrid microcavity at RT under optical pumping. In addition, we recently demonstrated first GaN-based exciton-polariton light emitters by current injection.