Shunsuke Kono

Study of terahertz radiation from InAs and InSb

Terahertz radiation from InSb and InAS, which are typical narrow band-gap semiconductors, was investigated using time-resolved THz emission measurements. When we compared between the polarity of the THz waveforms of these narrow band-gap semiconductors with that of InP, which is a wide bandgap semiconductor, we concluded that the ultrafast buildup of the photo-Dember field is the main mechanism for the emission of THz radiation in both InAs and InSb. The emission efficiency of InSb is approximately one-hundredth of that of InAs, although the electron mobility in InSb is higher than in InAs. Wavelength-dependent measurements implied that the anomalously low THz emission efficiency of InSb might be due to a reduction in transient mobility resulting from the scattering of electrons into the low-mobility L valley.

A direct comparison between terahertz time-domain spectroscopy and far-infrared Fourier transform spectroscopy

We perform a comprehensive comparison between terahertz (THz) time-domain spectroscopy and conventional far-infrared Fourier transform spectroscopy, including radiation source, detector, signal to noise ratio, bandwidth, availability, applications, and their own uniqueness. In terms of signal to noise ratio, THz time-domain spectroscopy is advantageous at low frequencies under 3 THz, while Fourier transform spectroscopy works better at frequencies above 5 THz. In addition, we provide a detailed discussion of the unique features of THz time-domain spectroscopy and its application to dynamic and time-resolved processes.

Mode identification of high-quality-factor single-defect nanocavities in quantum dot-embedded photonic crystals

We investigate the quality (Q) factor and the mode dispersion of single-defect nanocavities based on a triangular-lattice GaAs photonic-crystal (PC) membrane, which contain InAs quantum dots (QDs) as a broadband emitter. To obtain a high Q factor for the dipole mode, we modulate the radii and positions of the air holes surrounding the nanocavity while keeping sixfold symmetry. A maximum Q of 17u2009000 is experimentally demonstrated with a mode volume of V=0.39(λ∕n)3. We obtain a Q∕V of 44u2009000(n∕λ)3, one of the highest values ever reported with QD-embedded PC nanocavities. We also observe ten cavity modes within the first photonic band gap for the modulated structure. Their dispersion and polarization properties agree well with the numerical results.

Gallium Nitride-based Semiconductor Optical Amplifiers

GaN-based material can potentially cover a wide spectral emission range, and laser di‐ odes emitting in the UV, violet, blue, green, and red wavelengths have already been dem‐ onstrated and/or commercialized. GaN-based semiconductor optical amplifiers (SOAs) have the ability to boost the output power of laser diodes and thus are candidates for a broad variety of potential uses. Applications that utilize short wavelength, ultrafast puls‐ es, including microprocessing, orthoptics, and next-generation optical storage can most benefit from GaN-based SOAs since current ultrafast pulse sources rely on large, expen‐ sive solid-state lasers. GaN-based SOAs can generate high-energy, high peak power opti‐ cal pulses when used in conjunction with mode-locked laser diodes. In this chapter, the basic characteristics of these devices are discussed, concentrating on pulse amplification. Early experimental work, as well as the latest results, is presented, and improvements in the SOA design allowing the generation of higher optical pulse energy are discussed.

Optical spectroscopy of charge-tunable quantum dots emitting at 1.2 μm

In this study, we investigated the optical characteristics of single charge-tunable InAs QDs emitting at 1.2 μm, whose size is about twice those discussed in the previous reports. For probing electronic shell structures, large QDs are favorable in terms of deep confinement potentials and weak Coulomb interaction.

Coherent control of exciton in a single InAs/GaAs quantum dot

In this presentation, we observed the coherent optical nonlinear interaction of exciton in a single InAs/GaAs self-assembled QD by using coherent control technique at the wavelength 1127 nm, 4.2 K. Due to the large diameter of InAs QDs in the longer wavelength range, dipole moment of confined excitons is predicted to be large so that coherent optical interaction such as Rabi oscillation is expected to be observed. Large dipole moment is one of the most important criteria to realize strong interaction between photons and excitons in QDs.

Time-resolved photoluminescence measurement of exciton and biexciton in an InAs/GaAs single quantum dot

Dynamical behaviors of the exciton and biexciton luminescence at the wavelength of 1.18 μm from an InAs/GaAs single quantum dot are studied at 4.3 K. The rate equation analysis on the luminescence decays shows that the biexciton lifetime is longer than the exciton lifetime. The estimated lifetimes reflect molecular nature of the biexciton in the single quantum dot. Single photon sources working at the communication wavelength are quite desired for the practical applications of quantum information processing typically represented by quantum key distribution. In such a situation, the spectroscopic studies on single quantum dots having the resonant wavelength longer than 1 μm have recently attracted much attention [1,2]. Apart from these practical interests, InAs self-assembled quantum dots having the longer resonant wavelength will show interesting properties, such as large binding energy of biexciton, large energy separation between the confinement levels, due to strong confinement effects [3]. In this presentation, we report on the time-dependence of the luminescence peaks attributed to the exciton and biexciton in an InAs/GaAs single quantum dot at the wavelength of 1.18 μm by using time-correlated photon counting technique. We analyzed the time dependence of the luminescence decays of the exciton and biexciton by rate equations describing the cascade process from the biexciton to exciton after the biexciton radiative recombination. As a result of the rate equation analysis, we found that the biexciton radiative lifetime, τB, is longer than the exciton lifetime, τX. The ratio τX/τB is expected to be two when the constituent excitons of a biexciton are independent. The deviation of the ratio from two for the excitons in low dimensional structures was theoretically predicted by Citrin and Takagahara as a result of the quantum confinement and dielectric confinement [4,5]. Bacher et al. reported the ratio, τX/τB, close to one by the time-resolved measurement on the exciton and biexciton luminescence in single CdSe/ZnSe quantum dots [6]. Santori et al. reported the ratio τX/τB ~ 1.5 with InAs/GaAs single quantum dots whose resonant wavelength was about 0.88 μm [7]. Our estimated ratio τX/τB even less than one is explained with the quantum confinement effects dependent on the quantum dot dimension. We used the InAs quantum dots grown on (001) GaAs surface by molecular beam epitaxy at 590 °C. The average diameter and height of the quantum dots were estimated to be 49 nm and 13 nm respectively by using an atomic force microscope. The dot density was about 10 dots/cm. In order to reduce the number of the quantum dots in the focus area of a microscope, we fabricated micro apertures on the 100-nm-thick Au layer on the sample surfaces by using electron beam lithography and lift-off technique. The sample was cooled down to 4.3 K in a conduction type microscope cryostat mounted on a piezo-actuated translation stage. Figure 1(a) shows luminescence spectra observed from a 0.6-μm-diameter aperture under band-to-band excitation using a He-Ne laser. At weak excitation, shown in the lowest trace in Fig. 1(a), a single peak labeled X was observed at 1.0483 eV. Another peak labeled B appeared at 1.0471 eV when the excitation intensity was increased. The integrated intensities of these peaks were plotted as a function of the excitation laser intensity in Fig. 1(b). At low excitation power, the intensity of the peak X is proportional to the excitation laser intensity, while that of the peak B is proportional to the square of the excitation laser intensity. According to this excitation intensity dependence, the peak X and B are attributed to the exciton and biexciton recombination respectively. The binding energy of the biexciton was estimated to be 1.2 meV. The time-dependences of these luminescence JTuH3-4

Ultrabroadband detection of THz radiation by LT-GaAs photoconductive antenna gated with 15-fs laser pulses

Ultrabroadband detection of terahertz radiation is demonstrated with a lowtemperature- grown GaAs photoconductive antenna gated with 15-fs laser pulses. The observed spectra extends up to 20 THz and shows an absorption band between 7-10 THz due to the phonon resonance in GaAs substrate of the detector.