Sho Katoh

Terahertz Waveforms Generated by Second-Order Nonlinear Polarization in GaAs/AlAs Coupled Multilayer Cavities Using Ultrashort Laser Pulses

Temporal terahertz waveforms generated from GaAs/AlAs coupled multilayer cavity structures were simulated and compared with experimental results. Femtosecond laser pulses covering two cavity-mode frequencies were used for the difference frequency generation (DFG) in the terahertz region. The Fourier components dependent on the frequency and spatial position were determined for the second-order nonlinear polarization induced by a 100-fs Gaussian pulse injection. When the temporal waveform was simulated using the Fourier components, the oscillating behavior due to the efficient DFG of the two cavity modes was clearly observed after the initial ultrafast response near the incident surface. Assuming the exponential decay of signal sensitivity in the high-frequency region, the simulated results were consistent with the experimentally measured ones for coupled cavity structures grown on (113)B GaAs substrates.

Terahertz emission from a GaAs/AlAs coupled multilayer cavity with nonlinear optical susceptibility inversion

Terahertz wave generation was demonstrated by the difference frequency generation of two cavity modes in a polarization-controlled GaAs/AlAs coupled multilayer cavity. Inversion of the second-order nonlinear optical susceptibility was achieved by face-to-face bonding of the two halves of a coupled cavity structure grown on a (113)B-oriented GaAs substrate. Signal enhancement due to this inversion was observed through the comparison of inverted and normal coupled cavity samples in both simulated and experimentally observed terahertz waveforms produced by simultaneous excitation of two cavity modes using femtosecond laser pulses.

Terahertz Radiation from a (113)B GaAs/AlAs Coupled Multilayer Cavity Generated by Ultrashort Laser Pulse Excitation

Terahertz (THz) radiation was demonstrated using a GaAs/AlAs coupled multilayer cavity grown on a (113)B GaAs substrate. Two cavity modes realized in the high-reflection band were simultaneously excited using ultrashort laser pulses for the difference frequency generation (DFG). Oscillations with a period of 0.3 ps were clearly observed in the temporal waveforms of time-resolved THz measurements. The oscillation period well agreed with the difference frequency between the two cavity modes (3.3 THz). We also measured the THz waveforms depending on the polarization direction of the excitation laser pulses, and the anisotropic signal amplitudes also agreed with the calculated anisotropy of the second-order nonlinear polarization on the (113)B GaAs substrate.

Generation of Terahertz Radiation from Two Cavity Modes of a GaAs/AlAs Coupled Multilayer Cavity

We observed terahertz (THz) radiation using difference frequency generation (DFG) of two cavity modes in a (113)B GaAs/AlAs coupled multilayer cavity. 100 fs laser pulses were used to simultaneously excite two cavity modes, and oscillations with a period of 0.45 ps were clearly observed in the temporal waveform of THz time-domain measurements. The oscillation period agrees well with the difference frequency of the two cavity modes (2.2 THz). This clearly indicates that the observed signal originates from THz radiation generated by DFG of the two cavity modes in the GaAs/AlAs coupled multilayer cavity.

Effect of cavity-layer thicknesses on two-color emission in coupled multilayer cavities with InAs quantum dots

A GaAs/AlAs coupled multilayer cavity structure was grown on a (001) GaAs substrate. The top cavity contains self-assembled InAs quantum dots (QDs) as optical gain materials for two-color emission of cavity-mode light. The bottom cavity layer was grown with lateral thickness variation in the wafer to investigate the effects of the thickness difference between the two cavity layers quantitatively. The frequency difference was minimum, and the intensity ratio of the two-color emission was unity when the optical thicknesses of the two cavity layers were the same. The emission intensity ratio was explained in terms of the electric fields at the top cavity region containing the QDs.

Interface roughness characterization by electron mobility of pseudomorphic In0.74Ga0.26As∕In0.52Al0.48As modulation-doped quantum wells grown on (411)A InP substrates by molecular beam epitaxy

Interface roughness of pseudomorphic In0.74Ga0.26As∕In0.52Al0.48As modulation-doped quantum wells (MD-QWs) grown on the (411)A and (100) InP substrates by molecular beam epitaxy was characterized by sheet electron concentration (Ns) dependence of two-dimensional electron gas (2DEG) mobility by applying gate bias at 15K. The (411)A MD-QW with well width of 4nm (8nm) showed value of 1.8–2.1 (1.5–1.6) times higher 2DEG mobility at 15K comparing to the corresponding (100) sample in the range of Ns (1.2–2.3×1012cm−2), which results from reduced interface roughness scattering due to the super-flat (411)A InGaAs∕InAlAs interfaces. By analyzing Ns dependence of the 2DEG mobility, we deduced wave-number dependence of the Fourier components of the lateral well-width fluctuation (Δq0) arising from the interface roughness in the range of q0=0.55–0.7nm−1. Values of ∣Δq0∣2 of the (411)A InGaAs∕InAlAs interface were about half of those of the (100) interface in the whole range of q0=0.55–0.7nm−1.

Wafer-bonded coupled multilayer cavity with InAs quantum dots for two-color emission

A GaAs/AlAs coupled multilayer cavity structure with InAs quantum dots (QDs) was fabricated by wafer-bonding of two cavity structures grown individually. The wafer-bonding technique is important to control the spatial distribution of nonlinear polarization for strong terahertz emission by the differential frequency generation of the two cavity modes of the coupled cavity. Three layers of self-assembled InAs QDs were inserted in a cavity grown on a (001) GaAs substrate as optical gain materials for two-color emission of the cavity mode lights. The other cavity with a GaAs cavity layer was grown on a (113)B GaAs substrate. Two-color emissions with a 3.8 THz frequency difference were successfully observed from the wafer-bonded coupled cavity by cw optical pumping at room temperature.