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Recently, metal-halide perovskite semiconductors have been extensively studied because of their potential use in solar-cell, photodetector, light-emitting-diode, and laser applications. In particular, perovskite nanocrystals (NCs) are gaining more and more attention because their optical spectra can be manipulated by changing the NC size and the halogen composition [1]. In contrast to the conventionally structured NCs such as colloidal CdSe NCs, high-quality perovskite NCs with unique cubic shapes and large sizes can be fabricated. However, their intrinsic photocarrier recombination dynamics have not yet been understood. Especially, the recombination dynamics of exciton complexes including biexcitons and charged excitons (trions) are complicated due to the fast Auger recombination [2]. Thus, a combination of different optical techniques for ensemble and single NC samples should be performed for a deep understanding of multiexciton dynamics in perovskites.

Time-resolved photoluminescence (PL) is very useful for analyzing carrier dynamics and electrical properties of subcells which cannot be contacted directly. We investigate the excitation spot-size dependence of the PL decay time constants which are observed at high excitation powers. The excitation spot-size dependence is approximately linear and almost the same for both time constants. The origin of the spot-size dependence and the method for obtaining the subcell carrier-collection efficiencies are discussed.

We have systematically investigated the time-evolution of transient electron transport in semiconductor superlattices (SLs) under dc electric fields for various initial electron distributions created by femtosecond laser pulses. It is found that the terahertz (THz) waveforms are very insensitive to the pump photon energy and that the transient current starts from its maximum as damped cosω B t. We will show that the unique electron distribution under WS localization of electron wave functions in the translational symmetry is essential for the observed phase of the photoinduced current. Furthermore, as a first step toward realization of Bloch oscillators, a preliminary result on the effects of planar photonic crystal electrode on the THz emission will be briefly presented.

Gain characteristics of optically-excited semiconductor quantum wires are studied theoretically and experimentally at various carrier densities to clarify the correlation effects in electron-hole plasma. The profiles of gain spectra are significantly modified by the Coulomb interactions, by which serious suppression of peak gain occurs at high carrier densities.

CW single-mode lasing from 30 K to 70 K with the lowest threshold current of 0.27 mA has been demonstrated in 20-period T-shaped quantum-wire laser diodes with parallel p and n doping layers.

Variable-density one-dimensional electron gas was realized in a T-shaped quantum wire with an FET gate. We achieved photoluminescence-excitation measurements on a single wire and observed spectral evolution from degenerate to non-degenerate one-dimensional electron gas.