Yuta Inose

Random laser action in GaN nanocolumns

We report observations of random laser action in self-organized GaN nanocolumns. We have measured three samples with different filling fractions and investigated the dependence of the lasing property on the random configuration of nanocolumns. Numerical calculations based on a finite-difference time-domain method have also been performed and the comparison with the experimental results shows a clear relationship between the strength of light localization and the occurrence of random laser action.

Carrier-density dependence of photoluminescence from localized states in InGaN/GaN quantum wells in nanocolumns and a thin film

We have measured and analyzed the carrier-density dependence of photoluminescence (PL) spectra and the PL efficiency of InGaN/GaN multiple quantum wells in nanocolumns and in a thin film over a wide excitation range. The localized states parameters, such as the tailing parameter, density and size of the localized states, and the mobility edge density are estimated. The spectral change and reduction of PL efficiency are explained by filling of the localized states and population into the extended states around the mobility edge density. We have also found that the nanocolumns have a narrower distribution of the localized states and a higher PL efficiency than those of the film sample although the In composition of the nanocolumns is higher than that of the film.

Near-field optical imaging of light localization in GaN nanocolumn system

The first near-field optical imaging of light localization in a GaN nanocolumn system was performed. The sample used was a randomly arranged GaN nanocolumn with high aspect ratio. We attached an InGaN single quantum well at the apex of each GaN nanocolumn as an illuminant antenna and observed luminescence from the illuminant using an aperture-type scanning near-field optical microscope. By this technique, we directly obtained optical images of luminescence and its spatial distribution for the GaN nanocolumn system. These images, along with histogram analysis, excitation wavelength dependence, and numerical calculations, offer evidence of Anderson localization of light.

Fundamental optical properties of InN grown by epitaxial lateral overgrowth method

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.

Anderson localization of light in two-dimensional random arrays of semiconductor nanocolumns

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.

Anderson localization of light in a random configuration of nanocolumns

We show pseudogap maps of Anderson localization of light adopting the parameters of self-organized nanocolumn samples, which consist of random arrays of parallel nanosized columnar semiconductor crystals. The maps indicate the parametric dependence of the localization effect. To obtain the maps, we simulated light propagation in open random media using the two-dimensional finite-difference time-domain method and analyzed the simulation results by Fourier transformation. We found that the shape of the pseudogaps is close to the one of bandgaps in photonic crystals. We conclude that strong localization of light occurs because of interference by average Bragg diffraction, not strong Mie resonant peaks.

Second harmonic generation from photonic structured GaN nanowalls

We observed large enhancement of reflected second harmonic generation (SHG) using the one-dimensional photonic effect in regularly arranged InGaN/GaN single-quantum-well nanowalls. Using the effect when both fundamental and SH resonate with the photonic mode, we obtained enhancement of about 40 times compared with conditions far from resonance.

Randomness dependence and generation mechanism of stimulated emission in regularly arranged InGaN/GaN nanocolumns

We report the dependence of randomness in sample configuration on stimulated emission phenomena in two-dimensional nanocolumn arrays. From the wavelength selectivity of the photoluminescence, we found that the stimulated emission is apparently related to a distributed feedback mechanism. By comparing the emission behavior between two regularly arranged InGaN/GaN nanocolumn samples with different degrees of randomness, we found that localization effects become prominent if the sample array has any randomness, even in an almost perfect sample. Several modes are localized at different areas in the nanostructures and partially overlap in space, and they compete with each other, especially in a slightly imperfect sample. In addition to the randomness dependence, by observing the wave number space images of the photoluminescence, we confirmed that the stimulated emission phenomena in the crystal arrays are generated by Bragg diffraction at photonic band edges, though the modes have some degree of variability via the sample randomness.

Anderson localization of light in two-dimensional random media

The possibility of Anderson localization of light in random dielectric systems has been discussed over the last three decades [1]. However, theoretical and experimental studies for the light localization in random media are much more difficult than those in photonic crystals. Therefore, observation of the light localization has been realized only recently [2]. As a result, it is fair to say the wave localization phenomenon is not well understood.

Analysis of Anderson localization of light in GaN nanocolumns

In disordered materials, the combination of multiple light scattering and optical interference induces the localization of light. This phenomenon is called Anderson localization which is widely observed in electron systems with random potentials. Experimental studies of Anderson localization of light have been performed over the last three decades [1]. However, most of those studies attempted to secure the evidence of light localization by macroscopic observation of the light scattered by disordered materials. Recently, we proposed the direct observation of light localization using GaN nanocolumn samples by near-field scanning optical microscopy (NSOM), and presented the evidence of the light localization by a histogram analysis of NSOM image and by showing the wavelength dependence of the position of the localized spots in NSOM images [2,3]. We also observed random lasing in GaN nanocolumn samples and discussed the lasing property from a viewpoint of Anderson localization [4]. In this study, we extracted the two-dimensional (2D) spatial dependence from the NSOM image to estimate the localization length in GaN nanocolumns.