Ryotaro Inoue

Development of Fiber-Coupled Compact Terahertz Time-Domain Spectroscopy Imaging Head

The development of a fiber-coupled compact terahertz time-domain spectroscopy (THz-TDS) imaging system is reported. We focus the THz waves by means of two silicon hyper-hemispherical lenses with no parabolic reflectors. The THz transmission image of an IC card clearly shows that the spatial resolution of the system is less than 0.75 mm, which is quantitatively consistent with the THz beam diameter at the focal point estimated by knife-edge measurement. The discrepancy between the effective frequency of THz broadband pulses and the frequency which gives the maximum dynamic range in the THz spectrum is considered to be due to the coherent high-frequency components of THz pulses.

Microwave and millimeter wave spectroscopy in the slightly hole-doped ladders of Sr14Cu24O41

We have measured the temperature and frequency dependence of the microwave and millimeter wave conductivity σ1(T,ω) along both the ladder (c-axis) and the leg (a-axis) directions in Sr14Cu24O41. Below a temperature T* ( ~ 170 K), we observed a stronger frequency dependence in σ1c(T,ω) than that in σ1a(T,ω), forming a small resonance peak developed between 30 GHz and 100 GHz. We also observed nonlinear dc conduction along the c-axis at rather low electric fields below T*. These results suggest some collective excitation contributes to the c-axis charge dynamics of the slightly hole-doped ladders of Sr14Cu24O41 below T*.

Angular dependence of terahertz emission from semiconductor surfaces photoexcited by femtosecond optical pulses

In this paper, the angular dependence of terahertz (THz) emission from semiconductor surfaces photoexcited by femtosecond optical pulses is reported. Time-domain waveforms of THz emission from (100) surfaces of semi-insulating gallium arsenide (si-GaAs) and p-type indium arsenide (p-InAs) are measured at various angles after careful suppression of the nonlinear optical rectification effect. THz emission angle-frequency patterns under focusing conditions of the excitation beam are regarded as radiation from an electric dipole moment located on the semiconductor surface. Based on the experimental results in the magnetic field parallel to the semiconductor surface, we discuss the ultrafast carrier dynamics on the surfaces of both semiconductors.

Data analysis of the extraction of dielectric properties from insulating substrates utilizing the evanescent perturbation method

The evanescent perturbation technique utilizing open-ended coaxial resonator probes was investigated as a nondestructive method for measuring the dielectric properties of insulating substrates in the microwave frequency region. As an investigative result, we have proposed a new formulaic method of data analysis by which the complex permittivity of samples, from changes in resonant frequency (f) and the quality factor of the resonance (Q), may be extracted in a concise and highly reproducible manner. The proposed formula has been developed based upon experimentation and detailed numerical studies of full-wave Maxwell equations coupled with physical observation and interpretation of experimental data. The new formula is applicable to both bulk and film samples with zero and finite tip-sample distances. The geometric factors derived were analyzed for variable parameters such as tip curvature, sample thickness, and tip-sample distance. Additionally, the calibration procedures necessary for experimental determination of these geometric factors were established.

Enhanced Photon Generation in a Nb/n-InGaAs/p-InP Superconductor/Semiconductor-Diode Light Emitting Device

Research Institute for Electronic Science, Hokkaido University, Sapporo 001-0021, Japan 2CREST, Japan Science and Technology Agency, Kawaguchi 332-0012, Japan 3Graduate School of Information Science Technology, Hokkaido University, Sapporo 060-0814, Japan 4Central Research Laboratory, Hamamatsu Photonics, Hamamatsu 434-8601, Japan 5NTT Basic Research Laboratory, Atsugi 243-0198, Japan 6Japan Science and Technology Agency, Kawaguchi 332-0012, Japan Department of Applied Physics, Tokyo University of Science, Tokyo 162-8601, Japan 8Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan (Dated: October 27, 2009)

Switchable diode-effect mechanism in ferroelectric BiFeO3 thin film capacitors

We investigate the mechanism of a switchable diode behavior observed in ferroelectric SrRuO3/BiFeO3 (BFO)/SrRuO3 capacitors. We experimentally demonstrate that the switchable diode effect observed in the capacitors is induced by the polarization reversal in the BFO film. The conductivity in an Ohmic region in different oxidation states provides direct evidence that electron hole acts as the majority carrier, delivering p-type conduction. Density functional theory (DFT) calculations show that the p-type conduction arises from an unoccupied gap state of Fe4+ in an FeO5 pyramid which is derived from Bi vacancy. Our experimental and DFT study leads to the conclusion that the switchable diode effect originates from an asymmetric band bending in the top and bottom depletion layers modulated by ferroelectric polarization and oxygen vacancies.

Laser Terahertz Emission Microscope

Developments of laser terahertz (THz) emission microscope (LTEM) systems are reviewed. Femtosecond lasers can excite the THz wave emission from various electronic materials, such as semiconductors, high-temperature superconductors, manganites, multiferroic oxides, etc., due to ultrafast current modulation. Limiting the topic to semiconductors, the current modulation is realized by acceleration or deceleration of photoexcited carriers due to the local electric field extrinsically or intrinsically induced at the laser illumination spot. Thus, LTEM has a potential to visualize the local electric field distribution and photoresponse without any contacts or damages. We have ever constructed prototype free-space type and scanning fiber-probe (SFP) type LTEM systems with transmission or reflection mode. The system performance of the SFP-LTEM has been greatly improved compared with that for the prototype one. The spatial resolution of the SFP-LTEM system has a minimum spatial resolution less than 3 mum , which is defined by the laser beam diameter. The compact SFP-LTEM system, in particular the reflection system, has the potential to be utilized for wide applications as well as various materials. In this review paper, we introduce the details of the LTEM systems and example applications for the evaluation of electric field distribution in integrated circuits and supercurrent distribution in high-temperature superconductors.

Cooperative effect of oxygen-vacancy-rich layer and ferroelectric polarization on photovoltaic properties in BiFeO3 thin film capacitors

Photovoltaic (PV) properties of ferroelectric SrRuO3/BiFeO3/SrRuO3 (SRO/BFO/SRO) epitaxial thin-film capacitors are investigated. The experimental results of the markedly reduced PV response caused by the doping of Mn provide evidence that the PV properties originate not from the bulk-derived PV effect but from an interfacial band bending. We show that the capacitors having a defective layer composed of oxygen vacancies at the SRO/BFO interface exhibit a markedly large photocurrent. Our study demonstrates that a cooperative effect between the interface charges arising from ferroelectric polarization and the oxygen-vacancy-rich layer enhances the PV response in capacitor form in the BFO system.

Giant photovoltaic effect of ferroelectric domain walls in perovskite single crystals.

The photovoltaic (PV) effect in polar materials offers great potential for light-energy conversion that generates a voltage beyond the bandgap limit of present semiconductor-based solar cells. Ferroelectrics have received renewed attention because of the ability to deliver a high voltage in the presence of ferroelastic domain walls (DWs). In recent years, there has been considerable debate over the impact of the DWs on the PV effects, owing to lack of information on the bulk PV tensor of host ferroelectrics. In this article, we provide the first direct evidence of an unusually large PV response induced by ferroelastic DWs—termed ‘DW’-PV effect. The precise estimation of the bulk PV tensor in single crystals of barium titanate enables us to quantify the giant PV effect driven by 90° DWs. We show that the DW-PV effect arises from an effective electric field consisting of a potential step and a local PV component in the 90° DW region. This work offers a starting point for further investigation into the DW-PV effect of alternative systems and opens a reliable route for enhancing the PV properties in ferroelectrics based on the engineering of domain structures in either bulk or thin-film form.

A Cooper-Pair Light-Emitting Diode: Temperature Dependence of Both Quantum Efficiency and Radiative Recombination Lifetime

A light-emitting diode (LED) in the optical-fiber communication band showed special features after replacing the n-type electrode with niobium (Nb) superconducting metal. Nb electrodes prepared on an InGaAs-based semiconductor surface formed a superconductor/semiconductor/superconductor junction, and the current-voltage characteristics exhibited both DC and AC Josephson junction properties. This was a result of the injection of electron Cooper-pairs into the n-InGaAs active layer of an LED. The drastic enhancement of the electroluminescence output observed below the Nb superconducting critical temperature, T c, demonstrates the active role of electron Cooper-pairs in radiative recombination. Lifetime measurements of this LED and accurate evaluation of the luminescence output made it possible to estimate the radiative recombination lifetimes. A theoretical formula derived for the Cooper-pair radiative recombination accurately describes both the measured steep reduction of the radiative recombination lifetime and the observed enhancement of the internal quantum efficiency below T c. This work will assist the development of interdisciplinary physics and new applications in superconductivity and optoelectronics.