Takahiro Kitada

GaAs/AlAs coupled multilayer cavity structures for terahertz emission devices

GaAs/AlAs coupled multilayer cavity structures are proposed as terahertz emission devices. Two cavity modes with an optical frequency difference in the terahertz region can be realized when two cavity layers are coupled by an intermediate distributed Bragg reflector multilayer. Interference between the enhanced light fields of the cavity modes has been demonstrated by the simultaneous injection of two cavity-mode lights using an ultrashort pulse laser. Such coupled multilayer cavity structures are promising for use as compact and room temperature operable terahertz emission devices based on difference frequency generation by the cavity-mode lights.

A GaAs/AlAs Multilayer Cavity with Self-Assembled InAs Quantum Dots Embedded in Strain-Relaxed Barriers for Ultrafast All-Optical Switching Applications

An optical Kerr signal has been simulated for GaAs/AlAs multilayer cavity structures by using the self-consistent transfer matrix method. Enhancement of the Kerr signal intensity was clearly demonstrated for the cavity mode (λ~1504 nm) owing to the strong optical field in the multilayer cavity. The Kerr signal intensity can be further enhanced with the use of a higher nonlinear refractive index only for the half-wavelength (λ/2) cavity layer. We propose a GaAs/AlAs multilayer structure with the λ/2 cavity layer containing InAs quantum dots embedded in strain-relaxed barriers for an ultrafast optical Kerr gate switch with extremely low switching energy.

Enhanced Optical Kerr Signal of GaAs/AlAs Multilayer Cavity with InAs Quantum Dots Embedded in Strain-Relaxed Barriers

Using time-resolved optical measurements, a strong, ultrafast optical Kerr signal was demonstrated in a GaAs/AlAs multilayer cavity containing self-assembled InAs quantum dots (QDs) embedded in strain-relaxed InGaAs barrier layers. The large optical nonlinearity of the resonant InAs QDs in the half wavelength (λ/2) cavity layer is further enhanced by the strong cavity effect. Although only two layers of InAs QDs were inserted in the λ/2 layer, the optical Kerr signal intensity was 60 times larger than that of a GaAs λ/2 cavity. The response time of the optical Kerr signal was less than 1 ps, which was much faster than the decay time (15 ps) of optically generated carriers in the InAs QDs.

Optical Anisotropy of Strongly Enhanced Sum Frequency Generation in (113)B GaAs/AlAs Coupled Multilayer Cavity

We have investigated optical anisotropy of sum frequency generation (SFG) of two cavity modes realized in a (113)B GaAs/AlAs coupled multilayer cavity. The SFG signals were measured by simultaneous excitation of two cavity modes with an optical frequency difference of 3.3 THz using 100-fs laser pulses. The peak intensity of the SFG signal was more than 400 times larger than that of the second-harmonic generation signal from the (113)B GaAs bulk substrate. Large optical anisotropy depending on the polarization angle of the excitation laser was observed in the strongly enhanced SFG in the (113)B coupled cavity sample. The observed anisotropic behavior was well explained by the in-plane anisotropy of the second-order nonlinear polarization on the (113)B GaAs substrate.

Super-flat interfaces in pseudomorphic In0.72Ga0.28As/In0.52Al0.48As quantum wells grown on (411)A InP substrates by molecular beam epitaxy

We investigated flatness of heterointerfaces in pseudomorphic In0.72Ga0.28As/In0.52Al0.48As quantum wells (QWs) with well widths Lw of 1.3, 1.7, and 3.4 nm grown on (411)A InP substrates by molecular beam epitaxy (MBE). Full width at half maximum of a photoluminescence (12 K) peak from the (411)A QW with Lw=1.3 nm was as narrow as 20.7 meV, which is 28% smaller than that (28.9 meV) of the corresponding QW grown on a conventional (100) InP substrate. This result indicates that effectively atomically flat interfaces [(411)A super-flat interfaces] were successfully formed, for the first time, in pseudomorphic In0.72Ga0.28As/In0.52Al0.48As QWs grown on the (411)A InP substrate by MBE.

Marked Enhancement of Optical Kerr Signal in Proportion to Fourth Power of Quality Factor of a GaAs/AlAs Multilayer Cavity

The enhanced optical Kerr effect in a GaAs/AlAs multilayer cavity has been studied by numerical simulation using the self-consistent transfer matrix method. The simulated Kerr signal intensity of the cavity mode (λ~1504 nm) markedly increases with increasing multilayer period because of the high quality factor (Q) of the multilayer cavity. The simulation results reveal that Kerr signal intensity increases in proportion to Q4, which is well explained by the enhanced nonlinear phase shift owing to (1) the large refractive index change induced by the strong internal optical field of the pump light and (2) the long photon lifetime of the probe light in the multilayer cavity.

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.

Enhanced Two-Photon Absorption in a GaAs/AlAs Multilayer Cavity

A strong enhancement of two-photon absorption (TPA) has been demonstrated in a 30-period GaAs/AlAs multilayer with a GaAs half-wavelength (λ/2) cavity layer. In order to measure the time-resolved TPA signals, we successfully developed a selective etching process using an etch-stopper structure consisting of 5-nm-thick AlAs and 200-nm-thick Al0.3Ga0.7As double layers for removing the GaAs substrate. An analysis of the observed TPA results reveals that the average light intensity of the multilayer cavity is 41 times larger than that of a GaAs bulk sample. The obtained enhancement factor is in good agreement with the simulated value (45 times larger), which is determined from the simulated light intensity distribution by the conventional transfer matrix method.

Extremely flat interfaces in InxGa1-xAs/Al0.3Ga0.7As quantum wells grown on (411)A GaAs substrates by molecular beam epitaxy

Effectively atomically flat interfaces over a macroscopic area (super-flat interfaces) have been achieved in pseudomorphic In x Ga 1-x As/Al 0.3 Ga 0.7 As (x = 0.0, 0.04, 0.07) quantum wells (QWs) with well widths (L w ) of 1.2-11.8 nm grown on (411)A GaAs substrates at 520°C by molecular beam epitaxy (MBE). A single, sharp photoluminescence (PL) peak was observed for each QW over the large area of the wafer (8 mm x 5 mm). The linewidths for narrow QWs (L w = 2.4nm) were 8.9meV (x = 0.04) and 9.9 meV (x = 0.07) at 4.2 K, which were about 30% smaller than those of QWs simultaneously grown on conventional (100) 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.