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We report lateral photoconductive properties of multilayer-stacked undoped InAs/GaAs quantum dots (QDs) for the application of photoconductive terahertz (THz) antenna devices that operate in a 1.5 μm-telecom-wavelength band. The excitation power-dependent photocurrent showed a high value without saturation under high excitation power for the excitation wavelength of 1460 nm. From the reflection pump-probe signal, a fast photocarrier lifetime was derived. These results, together with the low dark current characteristic, support the applicability of the multilayer-stacked undoped InAs/GaAs QDs to photoconductive THz antennas operating in a 1.5 μm-wavelength band.

GaAs/AlAs coupled multilayer cavity structures on high-index substrates have been proposed as novel terahertz emission devices. Two cavity modes with an optical frequency difference in the terahertz region are realized when two cavity layers are coupled by an intermediate distributed Bragg reflector multilayer. Optical responses to ultrashort laser pulses have been simulated using the transfer matrix method. Interference between the enhanced light fields of the cavity modes was demonstrated when they were simultaneously excited by 100 fs Gaussian pulses. Extremely strong sum-frequency generation was experimentally observed in the (113)B coupled multilayer cavity. We also found that the polarization control by wafer-bonding might be one of the best ways to generate terahertz difference-frequency signal of two modes.

A GaAs/AlAs multilayer cavity containing self-assembled InAs quantum dots (QDs) embedded in strain-relaxed In0.35Ga0.65As barriers has been studied for planar-type optical Kerr gate switches. The multilayer cavity structure with smooth GaAs/AlAs interfaces was successfully grown on a (001) GaAs substrate by molecular beam epitaxy despite the lattice strain relaxation induced in the half-wavelength ($\lambda/2$) cavity layer. Time-resolved optical measurements at the cavity mode ($\lambda = 1.46$ \mbox{$\mu$m}) were performed by a pump--probe method at room temperature. Although only two layers of the InAs QDs were inserted into the $\lambda/2$ cavity layer, a large transmission change caused by the absorption saturation in the resonant QDs was clearly observed. The temporal profile was dominated by a fast (${\sim}16$ ps) decay component, which comes from carrier relaxation into the nonradiative centers arising from the lattice strain relaxation. We have also shown that an ultrafast response time (${<}1$ ps) of the strongly enhanced optical Kerr signal is determined by the photon lifetime in the multilayer cavity.

Effectively atomically flat interfaces over a macroscopic area (“(411)A super-flat interfaces”) were successfully achieved in In0.53Ga0.47As/In0.52Al0.48As quantum wells (QWs) grown on (411)A InP substrates by molecular beam epitaxy (MBE) at a substrate temperature of 570°C and V/III=6. Surface morphology of the In0.53Ga0.47As/In0.52Al0.48As QWs was smooth and featureless, while a rough surface of those simultaneously grown on a (100) InP substrate was observed. Photoluminescence (PL) linewidths at 4.2 K from the (411)A QWs with well width of 0.6–12 nm were 20–30 % narrower than those grown on a (100) InP substrate and also they are almost as narrow as each of split PL peaks for those of growth-interrupted QWs on a (100) InP substrate. In the case of the (411)A QWs, only one PL peak with very narrow linewidth was observed from each QW over a large distance (7 mm) on a wafer.

Optical Kerr signals markedly enhanced by increasing quality ($Q$) factor in a GaAs/AlAs multilayer cavity have been demonstrated using time-resolved optical measurements. In the optical Kerr measurements, the spectral width of a laser pulse was well tuned to the peak of the cavity mode using the wavelength-restricted optical system. Although the $Q$ of a 30-pair cavity is only 1.6 times larger than that of a 26-pair cavity, the optical Kerr signal intensity of the 30-pair cavity is 10 times larger than that of the 26-pair cavity. We have revealed that the optical Kerr signal of the GaAs/AlAs multilayer cavity is markedly enhanced nearly proportional to the $Q^{4}$ as a result of the enhanced nonlinear phase shift owing to the strong internal optical field and the long photon lifetime of the cavity-mode light.

A strong sum frequency generation (SFG) has been demonstrated using two cavity modes realized in a GaAs/AlAs coupled multilayer cavity structure in which two GaAs half-wavelength cavity layers are coupled by an intermediate distributed Bragg reflector multilayer. The coupled multilayer cavity structure was grown on a (113)B GaAs substrate by molecular beam epitaxy. Two cavity modes with an optical frequency difference of 3.3 THz were clearly observed at 1514 and 1540 nm in the optical reflection spectrum. The room-temperature SFG and second-harmonic generation (SHG) signals were measured by the simultaneous excitation of two cavity modes using 100 fs laser pulses. The peak intensity of the SFG signal was more than 400 times larger than that of the SHG signal from a (113)B GaAs substrate alone, indicating that the effective second-order nonlinearity of GaAs was strongly enhanced by the cavity effect.

We measured the two-color laser oscillation from a GaAs/AlGaAs-coupled multilayer cavity at 18 degC-42 degC using current injection. We confirmed simultaneous lasing by detecting the sum frequency generation signal generated by the two-color laser light, and performed time-resolved measurement using a streak camera with a spectrometer. From the observed time transient of the spectra at various temperatures, it is clarified that the temperature change of the device, induced by current injection, modulates the effective cavity length. Therefore, the temperature control of the device is a key factor in stable two-color lasing and THz wave generation.

Room-temperature two-color lasing was demonstrated by current injection into a GaAs/AlGaAs coupled multilayer cavity for terahertz emitting devices utilizing its difference-frequency generation (DFG) inside the structure. We prepared two epitaxial wafers with (001) and (113)B orientations and they were directly bonded to form the coupled multilayer cavity. The (001) side cavity contains two types of InGaAs multiple quantum wells to realize optical gain for two-color lasing while a single GaAs layer of the (113)B side cavity is a nonlinear medium for efficient DFG. Lasing behavior was clearly observed for two modes under pulsed current conditions at room temperature. We found that the intensity relation between two-color lasings was dependent on the pulse duration because of the different temporal profiles of emission intensity. Simultaneous two-color lasing in the device was also confirmed by measuring the sum-frequency generation signal using a beta barium borate crystal.

We fabricated a coupled multilayer cavity with a GaAs/Ge/GaAs sublattice reversal structure for terahertz emission application. Sublattice reversal in GaAs/Ge/GaAs was confirmed by comparing the anisotropic etching profile of an epitaxial sample with those of reference (113)A and (113)B GaAs substrates. The interfaces of GaAs/Ge/GaAs were evaluated at the atomic level by scanning transmission electron microscopy (STEM) and energy-dispersive X-ray spectroscopy (EDX) mapping. Defect-free GaAs/Ge/GaAs heterostructures were observed in STEM images and the sublattice lattice was directly seen through atomic arrangements in EDX mapping. A GaAs/AlAs coupled multilayer cavity with a sublattice reversal structure was grown on the (113)B GaAs substrate after the confirmation of sublattice reversal. Smooth GaAs/AlAs interfaces were formed over the entire region of the coupled multilayer cavity structure both below and above the Ge layer. Two cavity modes with a frequency difference of 2.9 THz were clearly observed.

GaAs/Ge/GaAs heterostructures were grown on high-index (113)B GaAs substrates by molecular beam epitaxy. Sublattice reversal in GaAs/Ge/GaAs was identified by comparing the anisotropic etching profile of the epitaxial sample with that for reference (113)A and (113)B GaAs substrates. The shape of the resulting mesa for the lower GaAs layer was similar to that for the reference (113)B GaAs substrate, whereas that for the upper GaAs layer was similar to that for the reference (113)A GaAs substrate. An atomic-resolution analysis was also conducted by mapping using energy-dispersive X-ray spectroscopy, whereby the sublattice reversal was directly observed through the atomic arrangements.

Current-injection two-color lasing has been demonstrated using a GaAs/AlGaAs coupled multilayer cavity that is a good candidate for novel terahertz-emitting devices based on difference-frequency generation (DFG) inside the structure. The coupled cavity structure was fabricated by the direct wafer bonding of (001)- and (113)B-oriented epitaxial wafers for the efficient DFG of two modes in the (113)B side cavity, and two types of InGaAs multiple quantum wells (MQWs) were introduced only in the (001) side cavity as optical gain materials. The threshold behavior was clearly observed in the current-light output curve even at room temperature. Two-color lasing was successfully observed when the gain peaks of MQWs were considerably tuned to the cavity modes by the operating temperature.

We have investigated a GaAs/AlAs triple-coupled multilayer cavity structure with InAs quantum dots for an ultrafast wavelength conversion device. Three cavity modes with the resonance frequencies o1, o2, and o3 were used for efficient wavelength conversion via a four-wave mixing (FWM) process. Identical frequency separation between two adjacent modes (o1 [?] o2 = o2 [?] o3) was successfully realized using a controlled lateral thickness variation across the wafer. Time-resolved FWM signals from the triple-coupled multilayer cavity were measured using 100 fs laser pulses. The incident laser pulses were divided into two pulses and each of them was spectrally shaped individually so that the input and control pulses only covered the o1 and o2 modes, respectively. The wavelength-converted FWM signal with a frequency of o3 (= 2o2 [?] o1) was clearly observed when the sample was simultaneously irradiated with the input and control laser pulses.

Time-resolved measurements of sum-frequency generation (SFG) of two cavity modes, which were realized in a GaAs/AlAs coupled multilayer cavity structure grown on a (113)B-oriented GaAs substrate, were performed. Strongly enhanced SFG and second-harmonic generation (SHG) signals have been demonstrated by the simultaneous excitation of two cavity modes using 100 fs laser pulses. In the time-resolved measurements, the sample was irradiated by two 100 fs laser beams with a time delay and the SFG and SHG signals generated by combining two beams were detected using a small slit. The delay-time-dependent measurements have revealed that the SFG and SHG signals decay with the photon lifetime (0.6 ps) of the coupled multilayer cavity, and the oscillating behavior with the period (0.3 ps) corresponding to the optical frequency difference (3.3 THz) between two cavity modes is significant only for the SFG. The experimental results were well explained by the simulated light electric field inside the coupled cavity under two Gaussian pulse excitation.

Two-color surface-emitting lasers were demonstrated, employing a GaAs/AlGaAs coupled multilayer cavity composed of two cavity layers and three distributed Bragg reflector (DBR) multilayers. InGaAs multiple quantum wells (MQWs) with two different well widths were introduced only in the upper cavity, and sandwiched between p- and n-type DBRs. This current-injection type device exhibited two-color lasing in the near-infrared region under room temperature pulsed conditions. Two-color lasing was achieved when the lower cavity had an optimal thickness relative to the upper cavity thickness and the MQW emission properties.

We fabricated a two-color surface emitting device of a coupled cavity structure, which is applicable to terahertz light source. GaAs/AlGaAs vertical multilayer cavity structures were grown on (001) and (113)B GaAs substrates and the coupled multilayer cavity structure was fabricated by wafer bonding them. The top cavity contains self-assembled InAs quantum dots (QDs) as optical gain materials for two-color emission of cavity-mode lights. The bonding position was optimized for the equivalent intensity of two-color emission. We formed a current injection structure, and two-color emission was observed by current injection, although no lasing was observed.

We fabricated a photoconductive antenna structure utilizing Er-doped InAs quantum dot layers embedded in strain-relaxed In0.35Ga0.65As layers on a GaAs substrate. Mesa-shaped electrodes for the antenna structure were formed by photolithography and wet etching in order to suppress its dark current. We measured the photocurrent with the excitation of [?]1.5 um cw and femtosecond pulse lasers. Compared with the dark current, the photocurrent was clearly observed under both cw and pulse excitation conditions and almost linearly increased with increasing excitation power in a wide range of magnitudes from 10 W/cm2 to 10 MW/cm2 order.

GaAs/air multilayer cavity structure is expected as a novel planar-type nonliner optical device because of the strong enhancement of optical electric fileds in the cavity. The optical Kerr signal was estimated to be two order of magnitude larger than that from GaAs/AlAs multilayer cavity with the same $Q$-value by numerical simulation. We fabricated a GaAs/air cavity structure with 2-period distributed Bragg reflector (DBR) layers on each side of the $\lambda$-cavity layer by wet etching of sacrificial AlGaAs layer. Measured reflection spectra showed the expected structure was successfully fabricated in some parts.

A GaAs/AlAs multilayer cavity with a $\lambda/2$ AlAs cavity layer, which includes Er-doped InAs quantum dots (QDs) embedded in a thin strain-relaxed In0.45Ga0.55As barrier, was fabricated. Structural and optical properties were characterized by scanning electron microscopy and optical reflection measurements, respectively. We found that the cavity quality of the Er-doped QD cavity was improved by reducing the thickness of the strain-relaxed In0.45Ga0.55As barriers in the $\lambda/2$ AlAs cavity layer. Furthermore, time-resolved optical measurements were performed to study the relaxation time of the photogenerated carriers in the Er-doped QD cavity. A full width at half maximum of a 1 ps with a large reduction of slowly decaying carriers was obtained for the Er-doped QD cavity with a thin strain-relaxed In0.45Ga0.55As barrier.

Strong ultrafast four-wave mixing signals at ${\sim}1.5$ μm were demonstrated in a GaAs/AlAs multilayer cavity containing self-assembled InAs quantum dots (QDs) embedded in strain-relaxed In0.35Ga0.65As barriers. Time-resolved optical measurements using 100 fs pulses with 100 kHz repetition rate were carried out in the various excitation powers at room temperature. Strongly enhanced four-wave-mixing signals were observed for a cavity with two InAs QDs layers inserted in a half-wavelength ($\lambda/2$) cavity layer compared with that of a GaAs $\lambda/2$ cavity that had no QDs, in the whole range of excitation power (0.3--2 mW). For a low excitation power below 0.6 mW, the four-wave-mixing signals were about two orders of magnitude larger than that of the GaAs $\lambda/2$ cavity owing to the large nonlinearity of the InAs QDs.

Er-doped InAs quantum dots (QDs) embedded in strain-relaxed InGaAs barriers, which exhibit an extremely short carrier decay time of 3 ps due to the nonradiative process, are superior materials for ultrafast all-optical switches using a GaAs/AlAs multilayer cavity. The intensity of the nonlinear signal due to the absorption saturation in the 20-layer stack of the Er-doped QDs was increased by increasing the In composition in the strain-relaxed InGaAs barriers while keeping the extremely short decay time. The QD cavity structure, which consisted of GaAs/AlAs distributed Bragg reflector (DBR) multilayers and a half-wavelength cavity layer containing two layers of the Er-doped QDs was grown by molecular beam epitaxy. The transmission change signal was clearly observed in the time-resolved measurements at the cavity mode wavelength of 1.55 μm. The response time of 4 ps was observed for the Er-doped QD cavity, which was much shorter than that (12 ps) for the undoped QD cavity.

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.

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.

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.

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 (?/2) cavity layer is further enhanced by the strong cavity effect. Although only two layers of InAs QDs were inserted in the ?/2 layer, the optical Kerr signal intensity was 60 times larger than that of a GaAs ?/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.

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 (??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 (?/2) cavity layer. We propose a GaAs/AlAs multilayer structure with the ?/2 cavity layer containing InAs quantum dots embedded in strain-relaxed barriers for an ultrafast optical Kerr gate switch with extremely low switching energy.

In this paper, we report on the material and device characteristics of pseudomorphic In0.7Ga0.3As/In0.52Al0.48As high electron mobility transistors (HEMTs) grown on a (411)A-oriented InP substrate by molecular-beam epitaxy. The electron Hall mobility in the (411)A HEMT was 2.5 times higher at 15 K and 1.3 times higher even at room temperature than that in a HEMT grown on a conventional (100) InP substrate. The (411)A HEMTs with 50 nm gates provided excellent DC and RF device characteristics.The maximum transconductance was as high as 1.1 S/mm, and the cutoff frequency reached 355 GHz.

Pseudomorphic InxGa1-xAs/Al0.28Ga0.72As (x= 0.085-0.15) quantum wells (QWs) with well widths of 1.2, 2.4, 3.6, 4.8, 7.2 and 12 nm have been grown on (411)A and (100) GaAs substrates at a temperature (Ts) of 520°C by molecular beam epitaxy (MBE). The interface flatness of the QWs was characterized by photoluminescence (PL) at 4.2 K. PL linewidths of the narrow (411)A QWs (Lw= 2.4 nm) with x= 0.085 and 0.15 were 7.3 meV which is approximately 30-40% smaller than those of the (100) QWs, indicating that extremely flat interfaces over a macroscopic area [(411)A super-flat interfaces] have been realized in the pseudomorphic InxGa1-xAs/Al0.28Ga0.72As QWs (up to x= 0.15) grown on the (411)A GaAs substrates, similar to lattice-matched GaAs/AlxGa1-xAs QWs grown on (411)A GaAs substrate previously reported.

Effectively atomically flat interfaces over a macroscopic area (super-flat interfaces) have been achieved in pseudomorphic InxGa1-xAs/Al0.3Ga0.7As (x= 0.0, 0.04, 0.07) quantum wells (QWs) with well widths (Lw) 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 ×5 mm). The linewidths for narrow QWs (Lw= 2.4 nm) were 8.9 meV (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.