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We propose quantum receivers for 3- and 4-ary phase-shift-keyed (PSK) coherent state signals to overcome the standard quantum limit (SQL). Our receiver, consisting of a displacement operation and on-off detectors with or without feedforward, provides an error probability performance beyond the SQL. We show feedforward operations can tolerate the requirement for the detector specifications.

Optical properties of InN grown by the epitaxial lateral overgrowth (ELO) method have been studied using photoluminescence (PL) and excitation-correlation (EC) measurements. The PL spectrum is analyzed by free-electron recombination band (FERB) model, which shows that the ELO sample has a very low background carrier concentration (n=5.5*1016[cm−3]). EC measurements show that the dependences of the band gap renormalization and Auger effect on the carrier concentrations are similar in spite of the different physical origins.

We conducted numerical and experimental studies on Anderson localization of light in two-dimensional random systems of semiconductor columns. We investigated finite size scaling of the localization effect as a function of localization length and system size from the calculation. We therefore obtained a localization parameter map. We also report observations of random lasing in the semiconductor samples. We show that the occurrence of the laser action have a strong relationship to the localization length by comparisons of the map with the experimental results.

We observed coherent phonon oscillations in zinc blende and wurtzite wide‐gap semiconductors (ZnTe, ZnS, ZnO). We clarified the different relaxation mechanism of the coherent oscillations between LO phonon in zinc blendes and non polar mode in ZnO. Resonant effect of coherent phonon generation is also studied.

We perform a transient four‐wave mixing experiment to investigate the initial dynamics of the time evolution of exciton coherence in highly‐stacked InAs quantum dots. For short time delays, an oscillatory behavior with a period of 31 ps is observed only in the weak excitation regime. The excitation density and polarization dependence of this oscillation cannot be explained by the usual quantum beat phenomena such as biexcitonic and bright excitonic beats.

We measure the polarization dependence of four‐wave mixing (FWM) signals in highly‐stacked InAs quantum dots on an InP(311)B substrate. Dephasing time T2 and FWM signal intensity show large anisotropy for the crystal axes. The measured T2 is determined only by the exciton population lifetime, and thus the polarization dependence of T2 reflects the difference of the transition dipole moments related to bright exciton states with orthogonally linear polarizations.

We have investigated the detailed ultrafast electronic relaxation dynamics and coherent nonlinearity at optical communication wavelength in GaN/AlN multiple quantum wells (MQWs), using spectral‐integrated and — resolved two‐color pump‐probe techniques in a wide energy range. First, we found that the ISBT relaxation dynamics consists of the ultrafast (∼140 fs) intersubband relaxation and slow (∼1.3 ps) carrier cooling process. Second, we have observed coherent spectrum oscillations, which show that the ISBT dephasing time is more than 100 fs at a room temperature. Finally, we have observed the polarization‐dependent absorption in waveguide structures.

We have investigated the relaxation process of photo‐excited carriers in anatase TiO2 films, in which the concentration of the oxygen vacancy is controlled by annealing in ultra high vacuum. A broad luminescence band peaked at 2.32 eV is ascribed to a recombination of self‐trapped excitons (STEs). The luminescence decay curve was composed of several time constants corresponding to the direct and indirect formation of STEs. We have confirmed that the longer annealing, i.e. higher concentration of the oxygen vacancy, leads to an increase in the indirect STEs.

We have investigated the 2s‐2p splitting of excitons in an organic‐inorganic hybrid quantum well material, (C4H9NH3)2PbBr4, by measuring one‐photon and two‐photon excitation spectra. Generally, energy level of 2s is lower than that of 2p for usual Wannier excitons because of the polaron effect. For the hybrid system, however, we have found that the energy level of 2p is lower by 20 meV than that of 2s. This large and opposite splitting of 2s‐2p excitons can be explained by considering image charge effect.