Satoru S. Kano

Determination of quadratic nonlinear optical coefficient of AlxGa1−xAs system by the method of reflected second harmonics

The quadratic nonlinear optical coefficient in the AlGaAs system has been systematically measured as a function of the alloy composition at the fundamental wavelength of 1.064 μm by the method of reflected harmonics. The harmonic waves from the thin‐film samples are analyzed considering various interferences including multiple reflections. The experimental results show a reduction of the magnitude of the second‐harmonic coefficient with increasing Al content in the AlGaAs system. This tendency is consistent with Miller’s rule.

The effect of electric field on the excitonic states in coupled quantum well structures

We have calculated by a variational procedure the excitonic states in symmetric coupled quantum well structures of GaAs/AlGaAs in the presence of an external electric field perpendicular to the interfacial plane using a nonseparable, nonspherical hydrogenic trial wavefunction. The exciton binding energies have been calculated as a function of external electric field for various well and barrier widths. The nonspherical trial wavefunction leads to a better estimate of the exciton binding energy, especially in high electric field, than that calculated with a spherical one. The calculations show that the binding energy is strongly dependent on the external electric fields: the variation (increase or decrease) is related to the spatial distribution of the wavefunctions. The results of the calculation are consistent with our experimental results.

New method for determining the nonlinear optical coefficients of thin films

A new scheme for characterizing the quadratic optical nonlinearity of thin films grown on opaque substrates is proposed and demonstrated. This involved the measurement, as a function of the film thickness, of second‐harmonic waves reflected from a film surface. The d36 coefficient of a ZnSe‐on‐GaAs film is estimated by this method to be 33±7 pm/V at the fundamental wavelength of 1.06 μm, which agrees reasonably well with the known value for the bulk crystal.

Two-dimensional exciton dynamics in InGaAs/GaAs quantum wells

We have investigated the exciton dynamics in InGaAs/GaAs single quantum wells using time‐resolved photoluminescence spectroscopy. The temperature dependence of the decay time shows that the scattering rate of two‐dimensional excitons is reduced in the narrower wells compared with that of AlGaAs/GaAs quantum wells. The results suggest the dominant contribution of the alloy disorder scattering to the exciton scattering processes over the interface roughness scattering.

Oscillator Strength of Excitons in InGaAs/GaAs Quantum Wells

The oscillator strength of excitons in InGaAs/GaAs single quantum wells is studied for the first time by reflectance measurements using a Fourier transform spectrometer. Decrease of the oscillator strength of the zero phonon line at higher temperatures is found and analyzed using the temperature dependence of a modified Debye-Waller factor with an averaged phonon mode. For a sample of L =140 A, the oscillator strength is estimated to be 5.1×10 -4 A -2 . The well width dependence of the oscillator strength is found to reach a maximum at a critical well width. We also observe anomalies in the oscillator strength and linewidth at temperatures lower than 50 K.

Fast Lateral Transport of Excitons in a GaAs/AlGaAs Quantum Well

We have studied the lateral transport of excitons in a GaAs/AlGaAs single quantum well by the exciton time-of-flight technique. The lateral motion of excitons has been measured as a function of temperature (4 K to 40 K) and excitation density (≥108 cm-2). Our analysis shows that the exciton diffusivity at temperatures below 10 K becomes considerably enhanced, implying extremely long (several micrometers) exciton mean free paths. This behavior is discussed in terms of several scattering mechanisms including interaction with non-equilibrium phonons emitted during energy relaxation of carriers.

Nonlinear Optical Coefficient of AlAs Thin Film on GaAs Substrate

We report the first measurement of the nonlinear optical coefficient, d14, at the fundamental wavelength of 1.06 µm, of AlAs grown by molecular beam epitaxy. The d coefficient is deduced from the intensities of second-harmonic waves reflected from an AlAs film grown on GaAs. Multiple reflections in the film and second-harmonic generation in the substrate are taken into account. The ratio |d14(AlAs)/d14(GaAs)| is found to be 0.23±0.06.

In-plane transport of excitons in quantum well structures

Abstract We have investigated the in-plane transport of photoexcited carriers and excitons in Al0.3Ga0.7As/GaAs quantum well structures as a function of excitation photon energy and temperature at low carrier density (108 cm-2). We have discovered a strong correlation between the diffusivity and the excitation photon energy; the exciton diffusivity becomes quenched as the excitation photon energy decreases closer to the heavy-hole exciton state. We have also found significant enhancement of diffusivity when a light-hole exciton state is resonantly excited. We attribute the enhancement to the difference between exciton scattering and free carrier scattering.

Direct observation of exciton localization in a GaAs/AlGaAs quantum well

We present the direct observation of exciton localization in a GaAs/AlGaAs quantum well. We have observed the two‐component exponential decay of photoluminescence from heavy‐hole excitons when the excitation density is very low. We have confirmed by measuring the lateral spatial motion of excitons that the fast component is attributable to the radiative recombination of free excitons while the slowly decaying component is due to the localized excitons.

Linear and Nonlinear Optics in Substituted Polysilanes

The linear and nonlinear optical properties of polysilanes, and in particular poly(di-n-hexylsilane), have been investigated by vacuum UV spectroscopy and by two-photon induced processes. The electronic structure of the polymer inferred from these measurements is consistent with models which view the polymer as a one-dimensional semiconductor quantum wire. The exciton related optical nonlinearities are also large enough to allow polysilanes to be considered for various nonlinear switching applications.