Yozo Shimada

Bound-to-continuum intersubband photoconductivity of self-assembled InAs quantum dots in modulation-doped heterostructures

We have designed and fabricated a quantum dot infrared photodetector which utilizes the lateral transport of photoexcited carriers in the modulation-doped AlGaAs/GaAs two-dimensional (2D) channels. A broad photocurrent signal has been observed in the photon energy range of 100–300 meV due to the bound-to-continuum intersubband absorption of normal incidence radiation in the self-assembled InAs quantum dots. A peak responsivity was as high as 4.7 A/W. The high responsivity is realized mainly by a high mobility and a long lifetime of photoexcited carriers in the modulation-doped 2D channels. Furthermore, it is found that the observed photosensitivity survives up to 190 K.

Transient carrier velocities in bulk GaAs: Quantitative comparison between terahertz data and ensemble Monte Carlo calculations

Transient carrier velocities determined from terahertz radiation emitted from photoexcited bulk GaAs have been compared with ensemble Monte Carlo calculations. It is found that, if actual experimental conditions (sample geometry, photoexcitation condition, etc.) are properly taken into account, Monte Carlo calculations give a very good description of the transient carrier velocities determined from THz measurements. Although the THz signal is dominated by electron transport, contribution of holes becomes appreciable when the electric field is higher than 20 kV/cm.

Time-resolved terahertz emission spectroscopy of wide miniband GaAs/AlGaAs superlattices

We have investigated terahertz (THz) emission due to dynamical electron transport in wide miniband GaAs/Al0.3Ga0.7As superlattices by time-resolved THz emission spectroscopy. Quasiautocorrelation measurements were performed on the emitted THz electromagnetic waves by using a bolometer as a wideband detector. The bias-field dependence of the emitted THz radiation intensity clearly showed a crossover from the miniband transport to the formation of Wannier–Stark ladder. In the Wannier–Stark regime, a few cycle Bloch oscillations were observed even at 300 K in a superlattice whose miniband width is greater than the longitudinal optical phonon energy. Furthermore, it has been found that clear Bloch oscillations are visible only when the bottom of the miniband is photoexcited.

Modulation-doped quantum dot infrared photodetectors using self-assembled InAs quantum dots

Abstract We have designed and fabricated a new quantum dot infrared photodetector which utilizes lateral transport of photoexcited carriers in the modulation-doped AlGaAs/GaAs two-dimensional (2D) channels. A broad photocurrent signal has been observed in the photon energy range of 100–300 meV due to bound-to-continuum intersubband absorption of normal incidence radiation in the self-assembled InAs quantum dots. The peak responsivity was as high as 2.3 A/W. The high responsivity is realized mainly by a high mobility and a long lifetime of photoexcited carriers in the modulation-doped 2D channels.

Dephasing mechanisms of Bloch oscillations in GaAs/Al0.3Ga0.7As superlattices investigated by time-resolved terahertz spectroscopy

We systematically investigated the relaxation time of Bloch oscillations (BOs) in GaAs/Al0.3Ga0.7As superlattices as functions of the bias electric field, temperature, and structural parameters using the time-resolved terahertz spectroscopy. It was found that the relaxation time dramatically increases by ∼80% and a clear Wannier–Stark ladder shows up when the bias field exceeds ∼9 kV/cm. The dominant dephasing mechanism of BOs was identified to be interface roughness scattering (alloy disorder scattering) below (above) the critical bias electric field. From the temperature dependence, it was also found that phonon scattering plays a rather minor role in the dephasing process.

Terahertz emission due to interminiband resonant Zener tunneling in wide-miniband GaAs/Al0.3Ga0.7As superlattices

We have investigated terahertz (THz) emission induced by high-field electron transport in biased wide-miniband GaAs/Al0.3Ga0.7As superlattices. With increasing bias electric fields, two distinct regimes are observed in the bias-field dependence of the emitted THz intensity. These two regimes are attributed to the intraminiband transport and interminiband Zener tunneling regimes, respectively. In the Zener tunneling regime, quasiperiodic structures are observed in the bias field dependence of the emitted THz intensity and are identified to be due to the resonant Zener tunneling between the Wannier–Stark ladders associated with the ground and excited minibands.

Optical and Transport Properties of Single Quantum Well Infrared Photodetectors

We have studied the optical and transport properties of AlGaAs/GaAs single quantum well infrared photodetectors (SQWIPs). The SQWIP shows a narrowband photocurrent at around 9.2 µm, due to intersubband transition in the steady state characteristics, measured using a Fourier transform infrared spectrometer. The bias voltage dependence of the magnitude and the spectral shape of the observed steady state photocurrent indicates that the photocurrent is strongly affected by the tunneling escape process. Furthermore, a transient photoresponse of the SQWIP has been studied using free electron laser (FEL) pulses. A comparison of the steady state photocurrent and the transient photoresponse shows that the charging current induced by the band bending effect contributes to an increased photo sensitivity.

Calorimetric measurement of absolute terahertz power at the sub-microwatt level

We developed a calorimeter for determining absolute terahertz (THz) power. The calorimeter is based on a DC substitution method using an isothermal temperature-control technique. A neutral-density optical filter glass was used as a volume absorber, and its THz absorption was evaluated by a time-domain spectrometer. Highly sensitive measurement of the absolute THz power was experimentally achieved in the range from the submicrowatt to microwatt level at room temperature. Power measurement of a continuous-wave THz source using a photomixer was demonstrated. This calorimeter is expected to demonstrate the traceability of THz power at a submicrowatt level.

Validation of Power Linearity in Terahertz Time-Domain Spectroscopy Using a Programmable Step Attenuator

We have constructed a programmable terahertz step attenuator by stacking metallized-film attenuators (MFAs) of different transmittances. Sputter deposition of Inconel alloy on thin polyester films is used to fabricate the MFAs. Good repeatability and flatness of the step attenuator are confirmed in transmittance measurements by a terahertz time-domain spectroscopy (THz-TDS) system. A method to evaluate the linearity of the THz-TDS system is proposed by using the step attenuator over a wide dynamic range. Calibration curves are analyzed by measuring the transmittance of maltose in different concentrations, and the validity of the proposed method is discussed.

Power Linearity Measurement in Terahertz Time-Domain Spectroscopy Using Metalized Film Attenuators

We describe a method for evaluating the power linearity over a wide dynamic range of a terahertz time-domain spectroscopy (THz-TDS) system. The dynamic range is achieved by means of metalized film attenuators (MFAs). The evaluation was based on repeated measurements of transmittance in a particular sample at different incident power levels. We apply the method to both focused- and collimated-beam systems and show that the method can be applied to reflectance measurements as well.