Hiroyuki Bando

Ultrafast and Wideband Response in Optical Nonlinearity of Molecular-Beam-Epitaxy-Grown GaAs

In order to obtain ultrafast responses with no additional slow component, we have investigated optical nonlinearity in molecular-beam-epitaxy(MBE)-grown GaAs at various excitation wavelengths ?. The slow response component decreases in magnitude and almost disappears when undoped GaAs is excited deep inside the energy band of GaAs. For beryllium-doped low-temperature-grown GaAs, fast responses of less than 1 ps response time with a negligible slow component are obtained from the bandedge to 230 meV or more above it. Measurement of saturation energy density Is of the optical absorption shows that the increase in Is at ?=780 nm as compared to Is at the bandedge is only by a factor of 3?4 for undoped GaAs and it is very small for beryllium-doped low-temperature MBE-grown GaAs.

Measurement of third-order nonlinear susceptibility tensor in InP using extended Z-scan technique with elliptical polarization

The elliptical polarization dependence of the two-photon absorption coefficient β in InP has been measured by the extended Z-scan technique for thick materials in the wavelength range from 1640 to 1800 nm. The analytical formula of the Z-scan technique has been extended with consideration of multiple reflections. The Z-scan results have been fitted very well by the formula and β has been evaluated accurately. The three independent elements of the third-order nonlinear susceptibility tensor in InP have also been determined accurately from the elliptical polarization dependence of β.

Measurement of polarization dependence of two-photon absorption co-efficient β in InP using extended Z-scan technique for thick materials

Two-photon absorption coefficient β in InP has been measured in the wavelength range of 1640 to 1800 nm by means of the Z-scan technique in relatively thick films. The values of β for the polarization direction parallel to [110] are evaluated to be 22.33 ± 0.05, 15.40 ± 0.07 and 4.34 ± 0.07 cm/GW at wavelengths of 1640, 1700 and 1800 nm, respectively, from the fitting to the equation including the spatial and temporal profiles of the focused Gaussian beam. The polarization dependence of β has been also measured. The dependence has been able to be expressed very well by the expression of β with the imaginary part of the third-order nonlinear susceptibility tensor χ(3). These results also indicate that our derived equation of the transmittance is appropriate for the analysis of the two-photon absorption in the thick materials.

Absorption Saturation Energy Density of InGaAs-InAlAs Multiple Quantum Well under Tensile and Compressive Strain

Optical absorption saturation density Is was measured for InxGa1-xAs-In0.52Al0.48As multiple quantum well structures grown on InP (100) substrates by molecular beam epitaxy. Indium composition x was varied from 0.32 to 0.70 so that the quantum well layer was under tensile strain (x 0.53). Optical measurement was carried out using femtosecond light pulses from the optical parametric amplifier of a mode-locked Ti-sapphire laser amplifier. The density Is in a sample changed as a function of photon energy and exhibited a minimum value at the band-edge exciton energy. This minimum Is showed the smallest value at x=0.46 (under tensile strain) of all the samples with different x. This was in marked contrast to the results reported in the literature. Degenerate pump-probe measurement was also carried out, and the results showed that the relative transmission change ΔT/T measured at zero delay between the pump and probe pulses exhibited the largest value at x=0.46, confirming the result of Is measurement.

Absorption saturation intensity of InGaAs-InAlAs MQW under tensile and compressive strain

Optical absorption saturation intensity Is was measured by using femtosecond light pulse for In/sub x/Ga/sub 1-x/As-In/sub 0.52/Al/sub 0.48/As MQW grown on InP (100) substrates. Is showed minimum at x=0.46 (under tensile strain). This was in marked contrast to the previous results.