Masahiko Tani

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.

Generation of coherent terahertz radiation by photomixing in dipole photoconductive antennas

We demonstrate the generation of continuous-wave terahertz radiation at frequencies up to 3.5 THz by photomixing in LT-GaAs photoconductors with printed dipole antennas. The expected resonance peak was clearly observed in the radiation spectrum of a 50 μm dipole antenna, and the radiation property was discussed in terms of the antenna impedance. The spectral bandwidth was greater than the electrical bandwidth of the photoconductor determined by the carrier lifetime of the material. The quality of the resonance, however, was not as high as expected because of the imperfect geometry and ohmic loss of the antenna.

Noninvasive Inspection of C-4 Explosive in Mails by Terahertz Time-Domain Spectroscopy

We have measured the refractive index, extinction coefficient, and absorption coefficient of the explosive C-4 by THz time-domain spectroscopy and examined its applicability to detecting C-4 in mails. The explosive C-4, which is used in almost all mail-bomb terrorist activities, shows six THz bands in the frequency region from 5 to 90 cm-1 (150 GHz–2.7 THz). The 26.9 cm-1 band is very strong and can be used as a fingerprint of C-4. We confirmed that the THz absorption spectrum of C-4 hidden inside an envelope coincides essentially with that of C-4 itself. We also examined the applicability of the THz reflection spectroscopy to C-4 detection by calculating the reflectance spectra. Our results demonstrate that the THz techniques are very promising for counterterrorism against C-4 mail bombs.

Detection of up to 20 THz with a low-temperature-grown GaAs photoconductive antenna gated with 15 fs light pulses

We report on the ultrabroadband coherent detection of radiation in wavelengths spanning from far to midinfrared with a low-temperature-grown GaAs photoconductive dipole antenna gated with 15 fs light pulses. The detected spectral frequency exceeds 20 THz.

Ultrashort Electromagnetic Pulse Radiation from YBCO Thin Films Excited by Femtosecond Optical Pulse

We have observed ultrashort electromagnetic pulse radiation from YBa2Cu3O7-δ thin-film dipole antennas. The supercurrent transient is created by the excitation of the supercarriers into quasiparticles with a femtosecond laser pulse, and freely propagated electromagnetic pulses are measured and characterized. A pulse with 0.5 ps full width at half-maximum was obtained, containing frequency components up to 2.0 THz. A femtosecond time-resolved characterization of the spectra revealed that they strongly depend on the excitation conditions, and the quasiparticle recombination time becomes longer with increase in the excitation intensity. It is also observed that the radiation power increases in proportion to the square of both the bias current and the laser power in the region of weak excitation, which is consistent with the classical theory based on a two-fluid model. In the region of strong excitation, deviation from the classical theory was observed.

Carrier Dynamics and Terahertz Radiation in Photoconductive Antennas

In this paper we report the calculations of terahertz radiation from biased photoconductive antennas. The calculations are based on the Drude-Lorentz theory of carrier transport in semiconductors. The calculation model takes into account the interaction between electrons and holes, trapping of carriers in mid-gap states, scattering of carriers and dynamical space-charge effects. Our calculation results indicate that, when a biased photoconductive antenna is pumped by femtosecond laser pulses to generate THz radiation, a major portion of the radiation results from the ultrafast change of the carrier density. The results also show that the local electrical field oscillates and induces electromagnetic radiation at high carrier generation density. When a photoconductive antenna is used as a detector, the detected THz signal deviates considerably from the THz pulse incident upon the photoconductive antenna.

Ultrabroadband photoconductive detection: Comparison with free-space electro-optic sampling

The spectral response of a low-temperature-grown GaAs photoconductive (PC) antenna is compared with that of the electro-optic sampling technique for the frequency region up to 40 THz. The spectral response of the PC antenna for broadband detection is found to be determined by the temporal profile of the number of photocarriers injected by ultrashort optical gate pulses.

Terahertz imaging of silicon wafers

Silicon samples with and without implanted layers have been imaged with a standard time-domain terahertz (THz) imaging system. The carrier concentration and mobility of the substrate have been extracted from the frequency dependence of the THz transmittance using a simple model based on the Drude approximation. The carrier concentration of implanted layers could be determined simply from the relative amplitude of the main THz pluse with a spatial resolution of ≈1 mm. Both substrates and thin layers of a semiconductor were characterized with the same THz system.

Ultrafast Photoconductive Detectors Based on Semi-Insulating GaAs and InP.

Photoconductive dipole antennas fabricated on semi-insulating (SI) GaAs and SI-InP were used to detect terahertz (THz) pulses. The responses of these long-carrier-lifetime photoconductive detectors were compared to that of the photoconductive antenna fabricated on a low-temperature grown GaAs (LT-GaAs) with a subpicosecond carrier lifetime. The SI-InP-based photoconductive detector showed a higher responsivity and a better signal-to-noise ratio (SNR) than the LT-GaAs-based photoconductive detector at low gating laser powers. The SI-GaAs-based detector, however, showed a responsivity comparable to that of the LT-GaAs photoconductive detector only at very weak gating laser power, and the SNR of the SI-GaAs-based detector was poor for overall gating laser powers due to the high background noise originating from a large amount of stray photocurrent.

The complex refractive indices of the liquid crystal mixture E7 in the terahertz frequency range

We have used terahertz time-domain spectroscopy to investigate the complex optical constants and birefringence of a widely used liquid crystal mixture E7 in both nematic and isotropic phases (26°C–70°C). The extinction coefficient of E7 at room temperature is less than 0.035 and without sharp absorption features in the frequency range of 0.2–2.0 THz. The extraordinary (ne) and ordinary (no) indices of refraction at 26°C are 1.690–1.704 and 1.557–1.581, respectively, giving rise to a birefringence of 0.130–0.148 in this frequency range. The temperature-dependent (26°C–70°C) order parameter extracted from the birefringence data agrees with that in the visible region quite well. Further, the temperature gradients of the terahertz optical constants of E7 are also determined. The optical constants of E7 in the terahertz or sub-millimeter wave range are found to deviate significantly from values predicated by the usual extended Cauchy equations used in the visible and near-infrared.