In the two-dimensional random system composed of a disordered array of a dielectric cylindrical column ensemble,
Anderson localization of light is possible. We show localization parameter maps for the light localization adopting
parameters of gallium nitride nanocolumn samples, which consist of random arrays of parallel nanosized columnar
semiconductor crystals. The maps indicate parametric dependence of the localization characteristics on the light
frequency, the radius of the columns, and the filling fraction of the columns. To obtain the maps, we have simulated
temporal light diffusion in random media using the two-dimensional finite-difference time-domain method and analyzed
the simulation results by Fourier transformation. We conclude that the main mechanism for localization varies
continuously with the column filling fraction from Mie resonance of single column to Bragg-like diffraction of the
The relaxation dynamics of photo-excited carriers of indium nitride (InN) films and nanocolumns were examined using
degenerate pump-probe measurements at room temperature. We measured two InN films and nanocolumns with
different background carrier densities, and performed numerical calculations incorporating band-filling and bandgap-renormalization
effects, as well as LO phonon scattering. We found that the intrinsic relaxation properties of InN can be
understood by considering the density of states and electron occupation number of the conduction band. It was also
revealed that the decay dynamics of InN are not affected by the carrier recombination time under the appropriate
conditions. In addition, we examined the differences in carrier relaxation properties between films and nanocolumns.
The population and coherent dynamics of excitons in InAs quantum dots were investigated using transient pump-probe and four-wave mixing spectroscopies in the telecommunications wavelength range. The sample
was fabricated on an InP(311)B substrate using strain compensation to control the emission wavelength. This technique also enabled us to stack over a hundred QD layers, which resulted in a significant enhancement of nonlinear signals. By controlling the polarization directions of incident pulses, we precisely estimated the radiative and non-radiative lifetimes, the transition dipole moment, and the dephasing time while taking into account their anisotropic properties. The measured radiative lifetime and dephasing time shows large anisotropies with respect to the crystal axes, which results from the anisotropic nature of the transition dipole moment. The
anisotropy is larger than that for InAs quantum dots on a GaAs(100) substrate, which seems to reflect a lack of symmetry on an (311)B substrate. A quantitative comparison of these anisotropies demonstrates that nonradiative population relaxation and pure dephasing are quite small in our QDs.
We investigate the dephasing of excitons in InAs self-assembled
quantum dots by using a transient four-wave-mixing technique. A used sample is specially designed to compensate the strain. We observe long-lived coherence of excitons at 5 K which corresponds to the dephasing time longer than a nanosecond, where the photon energy of
the excitation pulse is 0.874~eV. We find that a pure dephasing due to exciton-phonon interactions dominates in the exciton dephasing
rather than the population decay and the exciton-exciton interaction
in the weak excitation region, by analyzing the population lifetime and the polarization-dependent dephasing time.
We have investigated the ultrafast coherent dynamics of the intersubband transition in GaN/AlN multiple quantum wells, using a spectral resolved two-color pump-probe technique. We have found a significant spectrum change as a function of the delaytime τ. For negative delaytimes corresponding to τ < 0, coherent spectrum oscillations have been observed. At τ ~ 0, asymmetric dispersion has been observed. For positive delaytimes, τ > 0, no measurable spectrum change has been observed. From the analysis of these results, we have estimated that intersubband transitions dephasing time is more than 100 fs measured at room temperature.
We have investigated the ultrafast relaxation dynamics of intersubband transition (ISBT) in GaN/AlN, using a two-color pump-probe technique, in a wide energy range around the optical communication wavelength. We suggest that the origin of the signal depends on the relation between the pump and probe pulse energies. We have observed an ultrafast induced absorption signal and a slow negative component which are due to the absorption of electrons during intra-subband scattering and a carrier cooling process with a hot-phonon effect, respectively. Moreover, we clarify the origin of the inhomogeneous broadening width of the ISBT and of the intrinsic absorption width of ISBT from the detailed analyses of the result. We have reproduced the relaxation dynamics by a numerical calculation to confirm this interpretation of ISBT relaxation dynamics.