Published papers

We present an experimental setup to dramatically reduce a measurement time for obtaining spatial distributions of terahertz electric-field (E-field) vectors. The method utilizes the electro-optic sampling, and we use a charge-coupled device to detect a spatial distribution of the probe beam polarization rotation by the E-field-induced Pockels effect in a < 110 >-oriented ZnTe crystal. A quick rotation of the ZnTe crystal allows analyzing the terahertz E-field direction at each image position, and the terahertz E-field vector mapping at a fixed position of an optical delay line is achieved within 21 ms. Video-rate mapping of terahertz E-field vectors is likely to be useful for achieving real-time sensing of terahertz vector beams, vector vortices, and surface topography. The method is also useful for a fast polarization analysis of terahertz beams. (C) 2014 AIP Publishing LLC.

We investigate the laser-induced spin precession dynamics in ferromagnetic Ni0.8Fe0.2 thin films by using a time-resolved magneto-optical Kerr effect (TR-MOKE) measurement with two pump pulses. By changing the time interval between the pump pulses, the amplitude of the TR-MOKE signal is strongly modulated according to the phase of the TR-MOKE signal at the time when the second pulse arrives. The change in the oscillation amplitude can be explained with the temporal evolution of the spin precession induced by laser heating, as shown with a numerical simulation. We conclude that the ultrafast control of coherent spin precession dynamics is possible by multiple pump pulses via thermal excitation.

© 2020 Author(s). By using a double-pulse excitation scheme, we demonstrate the optical control of higher-order spin waves in a ferromagnetic permalloy thin film. Three spin precession modes are observed in the film under single pulse excitation. Based on a theoretical calculation, these spin precession modes are assigned to the fundamental Kittel mode and the first- and second-order perpendicular standing spin-wave (PSSW) modes. In order to excite the first-order PSSW mode selectively, we use double-pulse excitation at 400 nm. We show that, by choosing an appropriate time interval between the two pump pulses, it is possible to obtain a relatively large amplitude of the first-order PSSW mode while the amplitudes of the other two modes are almost completely suppressed. By analyzing the spin dynamics, it is found that the spin precession motion under the double-pulse excitation condition can be explained by the interference between the spin waves that have been induced by the first and second pulses. Our finding indicates that the selective excitation of a spin precession mode with a high precession frequency in a ferromagnetic metal may be realized by a multiple-pump pulse excitation scheme with more than two pulses.

We introduce polarization-sensitive (PS) terahertz spectroscopy as a novel tool for non-destructive inspection of black rubber. The PS terahertz optical response of black rubber is mostly determined by the concentration and orientation of the carbon black fillers inside the sample. Therefore, PS terahertz spectroscopy allows access to filler information inside the material. In addition, we can quantitatively estimate the internal strain of the sample through the filler-orientation information. Our analysis model based on a mixture of the polymer matrix and fillers can be applied not only to black rubber but also general polymer composite materials. Recently, rubber and polymer composite materials are widely used in our daily life, so our novel nondestructive inspection method is useful to test such materials.

Interpretation of the vector waveform detected by using the electro-optic (EO) sampling method in the terahertz time-domain spectroscopy is presented, especially when there is a velocity difference between the elliptically-polarized terahertz pulse and the optical probe pulse in the EO crystal. The formulation can be used to interpret the time-domain vector waveform detected by the EO sampling method using the symmetry of the EO crystal without resorting to the polarizers.

The mechanisms of molecular beam epitaxy growth on the mesa patterned substrates which results in the formation of sharp ridge structure were investigated. A simple model, based oil the scanning electron microscope measurements, nicely explains the time dependent growth rates as well as the change of the facet shape during the formation of ridge structure originating from the strong Ga atoms migration. The estimated number of atoms crossing the boundary changes according to the alternating facet shape and this indicates the instability in the facet growth. As a result, the mechanism of mesa growth was clarified, which becomes self-regulatory while the facet shape and/or the boundary condition vary as the growth progresses. (C) 2001 Elsevier Science B.V. All rights reserved.

We demonstrate a photo-induced insulator-to-metal (I-M) transition in an organic linear chain compound (TMTSF)(2)PF6 [bis-(tetramethyl-tetraselenafulvalene)hexafluorophosphate] in the spin-density-wave (SOW) ground state by optical pump and terahertz probe experiments We observe the collapse of SOW gap in the optical conductivity spectrum at 3 ps after the photo-excitation indicating the ultrafast I-M transition in the SOW system Temporal evolution of the terahertz reflectivity change shows that the photo-induced metallic state sustains longer than 400 Ps (C) 2009 Elsevier B V All rights reserved

Photo-induced terahertz reflectivity change of the spin-density wave (SDW) gap in (TMTSF)(2)PF6 is observed. We find an ultrafast (similar to ps) increase of terahertz conductivity indicating a destabilization of the SDW states by a weak photo-excitation. (C) 2009 Optical Society of America

Development of a very compact terahertz time domain spectroscopy (THZ-TDS) imaging system with diffraction-limited spatial resolution is presented. The size of our device made of a cupper rod is 20 mm diameter times 61 mm length. Four parabolic-shaped concave mirrors dug in the rod effectively focus a THz beam on a sample, achieving similar to 0.45 effective numerical aperture.

We report on the formation and optical characteristics of ridge waveguide structures containing quantum wires (QWRs). To form a waveguide? we adopted digital alloys, i.e. short-period superlattices (SLs), rapid changes of As fluxes using valved cells, growth temperatures (T-s), and introduction of growth interruptions. Stimulated emission from these ridge QWR laser structures was observed at temperatures 4.7-290 K by optical pumping. (C) 1999 Elsevier Science B.V. All rights reserved.

We report the observation of stimulated emission in ridge quantum wire (QWR) structures at temperatures from 4.7 to 290 K. To examine the origin of the stimulated emission, the spatially and spectrally resolved microscopic images of the emission were measured. It was most likely attributed to the optical transition between the excited states in QWRs. (C) 1998 American Institute of Physics.

The formation mechanisms, structure and optoelectronic properties of Ga(In)As/(Al)GaAs quantum dot (QD) heterostructures grown by organometallic chemical vapor deposition on patterned (111)B GaAs substrates are reviewed. With this approach, it is possible to prepare high-quality semiconductor QDs whose position on a substrate is precisely controlled via a pre-growth photolithography step. The dots are formed at the center of an inverted, tetrahedral pyramid and are surrounded by distinct, low-dimensional barriers (quantum wires and quantum wells). Reproducible neutral and charged exciton states are observed in the QDs and are probed using microphotoluminescence and photon correlation spectroscopy. Single- and correlated-photon emissions are systematically detected from specific single and biexciton states. The emission spectra of light emitting diodes incorporating single pyramidal dots indicate the possibility of preferential injection of charge carriers via self-ordered wires connected to the dots. Finally, photonic crystal structures incorporating pyramidal QDs with controlled energy states are demonstrated. (C) 2004 Elsevier B.V. All rights reserved.

We report on the growth and optical properties of various configurations of sub-micron pitch dense arrays of pyramidal quantum dots (QDs) grown by organometallic chemical vapour deposition on patterned substrates. We show that the effective growth rate of these QDs is influenced by the ratio between the free {1 1 1}B area and {1 1 1}A exposed facets surrounding them. This provides a powerful technique for engineering the energy level structure of ordered QD arrays by means of geometrical patterning of the growth template. Such technique should be particularly useful for applications in photonic crystals incorporating QDs with tailored absorption and/or emission properties. (C) 2004 Elsevier B.V. All rights reserved.

We have investigated the electroabsorption of aligned micelle-wrapped single-walled carbon nanotubes induced by intense terahertz (THz) pulses by using THz-pump and optical-probe spectroscopy at room temperature. The electroabsorption signal is enhanced at exciton resonances near the optical communication wavelength. The signal showed a quadratic dependence on the THz electric field and instantaneously followed the THz oscillation (similar to 1 ps). The observed ultrafast electro-optic modulation with a THz bandwidth suggests the potential functionality of carbon nanotubes in high-speed electro-optic devices. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3470105]

We have investigated the spatial distribution of the polarization state of a terahertz electromagnetic wave focused by an off-axis parabolic mirror (OPM) in the focal plane. We employed polarization-resolved terahertz time-domain spectroscopy and found that a steep spatial variation in the polarization state appears slightly distant from the focus when a linearly polarized terahertz wave is focused. The spatial variation includes an abrupt change in the polarization state (states change between circular and linear polarizations) within a wavelength. The observed phenomena are confirmed by numerical calculations and are shown to be intrinsic to the reflection from the OPM. (C) 2016 The Japan Society of Applied Physics

© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement We have developed a terahertz time-domain polarimetry (THz-TDP) system by applying frequency modulation to electro-optic sampling detection in a nonlinear crystal. We characterized the precision of this system in determining the polarization angles to be 1.3° for fixed time delay, and 0.5° for complete time-domain waveform. Furthermore, we calculated the Jones matrix of the optical components used for beam propagation to calibrate the induced systematic error. The advantages of employing this calibration approach are demonstrated on a sapphire crystal investigated at different sample test positions in transmission configuration, and using high resistivity Si, AlN and quartz in reflection geometry. The new THz-TDP technique has the advantage of not using any external polarizers, and therefore is not constrained by their optical performance limitations, such as restricted bandwidths and frequency-dependent extinction ratio. Finally, the THz-TDP technique can be easily implemented on existing time-domain spectroscopy (TDS) systems.

© 2019 The Japan Society of Applied Physics. We develop a terahertz electric-field vector imaging system that uses the rotating polarizer technique. The imaging system is used to investigate the polarization-dependent optical response of a patterned indium tin oxide (ITO) film deposited on a glass substrate. While ITO is transparent for near-infrared probe light, it acts as a metal for terahertz light. Therefore, polarization-dependent diffraction can be observed at the edge of the ITO thin film, and only electric field components perpendicular to the edge boundary can propagate behind the ITO layer. By comparison with a numerical simulation, we reveal the polarization dependence of this so-called knife-edge diffraction.