Satoshi Tsuzuki

Terahertz emission from GaAs-AlGaAs core-shell nanowires on Si (100) substrate: Effects of applied magnetic field and excitation wavelength

Terahertz (THz) emission from GaAs-AlGaAs core-shell nanowires (CSNW) on silicon (100) substrates was investigated using THz time-domain spectroscopy. The applied magnetic field polarity dependence strongly suggests that THz emission originated from photo-carriers in the CSNWs. Optical excitation of the GaAs-AlGaAs core-shell yielded a wider THz emission bandwidth compared with that of just the GaAs core material. This result is currently attributed to faster carrier lifetimes in the AlGaAs shell. The THz emission spectral data are supported by time-resolved photoluminescence studies.

A matter of symmetry: terahertz polarization detection properties of a multi-contact photoconductive antenna evaluated by a response matrix analysis

While terahertz time domain spectroscopy (THz-TDS) is a well-established technique, polarization sensitive measurements are challenging due to the need of broadband polarization devices. Here, we characterize our recently introduced multi-contact photoconductive detector antenna with a response matrix analysis. We show that the lead lines attached to electrodes reduce the antenna symmetry and thereby influence the properties of the response matrices. With a wire grid polarizer, we simulate a sample influencing the polarization angle and the intensity of the incident THz pulse. Evaluating the measurements with the response matrix analysis, our results show a well agreement of the adjusted and measured polarization angles and intensities over a frequency range from 0.25 to 0.8 THz.

Highly sensitive electro-optic sampling of terahertz waves using field enhancement in a tapered waveguide structure

We report on increasing the sensitivity of electro-optic sampling detection of terahertz pulses by using the effect of terahertz field enhancement in a tapered metallic waveguide. A thin Si prism, placed in the narrower section of the waveguide, is used to synchronize the enhanced terahertz field with the probe laser pulse propagating in a LiNbO3 crystal attached to the obliquely cut end of the waveguide. A 40-µm-thick MgO-doped layer on the surface of LiNbO3 is used for guiding the probe pulse. The developed structure demonstrates a 20-fold higher detection sensitivity than a no-waveguide, Si-prism-coupled LiNbO3 crystal.

Influence of OH-group concentration on optical properties of silica glass in terahertz frequency region

We evaluated the optical properties of various types of silica glass with different OH-group concentrations by using terahertz (THz) time-domain spectroscopy. The results show that the absorption coefficient in the THz frequency region varied with the OH-group concentration and that the variation depended only on the OH-group concentration, irrespective of the glass-manufacturing process. On the other hand, the refractive index did not depend on the OH-group concentration. Thus, the THz optical properties of silica glass can be predicted based on the OH-group concentration in the glass, and vice versa.

Superfocusing effect of V-groove metallic structure for terahertz wave

Focusing THz waves into a sub-wavelength area is a crucial issue for THz spectroscopy of minute samples and also for high-spatial resolution THz-imaging. Recently, super-focusing phenomena of THz waves, or more generally, those of electromagnetic waves are under intense investigation with theoretical and experimental studies using tapered metallic waveguiding structures. In this report we present our experimental study on a super-focusing effect of a V-groove metallic structure for THz pulsed waves with an opening angle about 10 degrees. Without coupling substrate lens, a 17% amplitude transmission of THz broadband waves (0∼2.5 THz) through a 2-μm bottom-width was observed without significant waveform distortion and phase delay.

Techniques of non-collinear electro-optic sampling for efficient detection of pulsed terahertz radiation

Cherenkov radiation mechanism is an established technique to achieve phase matching between ultrashort optical pulses and terahertz (THz) waves having a large collinear velocity mismatch, in a nonlinear optical material such as LiNbO3 (LN). Phase matching is achieved with the optical and THz pulses propagating at angle with respect to each other. Recently, we have experimentally demonstrated that Cherenkov phase matching mechanism can also be used for efficient electro-optics (EO) sampling of broadband THz pulses [1]. In the detection case, the phase matching is achieved between an optical and THz pulse propagating non-collinearly at the Cherenkov phase-matching angle θC, satisfying the following equation: equation Here, ngLN is the group index of the EO crystal at the sampling optical wavelength and nTHzLN is the refractive index of the EO crystal in the THz frequency region. An advantage of the non-collinear Cherenkov phase matching is that we can find a corresponding Cherenkov phase matching angle, θC, for any electro-optic crystal at a given optical sampling wavelength. When the EO crystal has a much larger refractive index in THz frequency region compared to that in optical region, a coupling prism is used as illustrated in Fig. 1. Moreover, using a low-loss coupling prism can also reduce absorption in the EO crystal. Silicon is an ideal coupling prism material owing to low absorption losses and is non-dispersive in the THz frequency region. From Snells law, the incident angle α of THz wave with respect to the prism- EO crystal interface is given as follows: equation Equation (2) leads to the relation, sin β = cosθC. Therefore, Eq. (1) reduces to the following equation for the apex angle of the coupling prism (α) at the Cherenkov phase-matching condition given by the ratio of the group index of the EO crystal at the sampling optical wavelength ngLN, and the refractive index of Si, nTHzSi, in the THz frequency region: equation.