Norihiko Sekine

At the Dawn of a New Era in Terahertz Technology

The National Institute of Information and Communications Technology (NICT, Japan) started the Terahertz Project in April 2006. Its fundamental purpose in the next five years is to enable a nationwide technical infrastructure to be created for diverse applications of terahertz technology. The technical infrastructure includes the development of semiconductor devices such as terahertz quantum cascade lasers, terahertz-range quantum well photodetectors, and high-precision tunable continuous wave sources. It also includes pulsed terahertz measurement systems, modeling and measurement of atmospheric propagation, and the establishment of a framework to construct a materials database in the terahertz range including standardization of the measurement protocol. These are common technical infrastructure even in any terahertz systems. In this article, we report the current status of developments in these fields such as terahertz quantum cascade lasers (THz-QCLs) (with peak power of 30 mW, 3.1 THz), terahertz-range quantum well photodetectors (THz-QWPs) (tuned at 3 THz) an ultrawideband terahertz time domain spectroscopy (THz-TDS) system (with measurement range of from 0.1 to 15 THz), an example of a database for materials of fine art, and results obtained from measuring atmospheric propagation.

Detection of terahertz radiation from quantum cascade laser, using vanadium oxide microbolometer focal plane arrays

The authors tried real-time imaging of THz radiation from Quantum Cascade Laser (QCL), using vanadium oxide (VOx) microbolometer focal plane arrays (FPAs) of 320x240 with pitches of 37 μm and 23.5 μm as well as 640x480 with 23.5μm pitch. The QCL has such parameters as 3.1 THz emission frequency (97μm in wavelength), 300-400 nsec pulse width, 1.07 msec repetition period, 30 mW peak intensity, 15K operation temperature. The THz radiation from QCL is collimated by off-axis parabola (OAP) and focused on FPA by another OAP. The 10 μm range infrared radiation from scene is blocked by sapphire disk or metal mesh filter. Noise Equivalent Power (NEP) at 3.1 THz is estimated to be 200~400 pW.

Nonequilibrium Green’s function calculation for four-level scheme terahertz quantum cascade lasers

We have calculated the performance of a recently proposed four-level scheme terahertz quantum cascade laser (4L terahertz-QCL) with the nonequilibrium Green’s function method. The calculation result for 40 K showed that the 4L QCL has a larger terahertz gain than the conventional resonant phonon QCL. This is because a large number of electrons accumulate in the upper lasing level and contribute to lasing in the new scheme. When the temperature is increased, the advantage of gain decreases due to thermally activated phonon scattering.

Observation of terahertz radiation from higher-order two-dimensional plasmon modes in GaAs/AlGaAs single quantum wells

We have observed terahertz (THz) radiation from higher-order two-dimensional (2D) plasmon modes in GaAs/AlGaAs single quantum wells by time-resolved THz emission spectroscopy. Up to fifth-order plasmon modes are clearly resolved and the observed mode frequencies are in excellent agreement with theory. Relaxation times of the 2D plasmon modes are found to be almost independent of the plasmon wave vector accessible in the present experiment.

Development of Terahertz focal plane arrays and handy camera

Uncooled Terahertz (THz) focal plane array (FPA), 320x240 format-23.5 μm pitch, and THz imager were developed. There are two types of THz-FPAs, i.e., broad-band type and narrow-band type. Since broad-band type THz-FPA was developed, a couple of modifications have been made to improve Noise Equivalent Power. The narrow-band type THz-FPA has such a new structure that Si cover is put above thermal isolation structure of broad-band type THz-FPA at a distance of half of wavelength of interest. Measurements on responsivities of narrow-band type FPAs show enhancement by a factor of ca. 3. Lock-in imaging technique has been developed, which increases signal-to-noise ratio as a function of square root of the number of frames of integration. Both passive and active THz imaging experiments were finally described.

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.

Intensity modulation of terahertz quantum cascade lasers under external light injection

We investigated the light-current characteristics of terahertz (THz) quantum cascade lasers under external light injection, which excites interband transitions in the active materials. It was found that the amount of reduction in the THz power was constant for all injection currents above threshold, and the dependence of the reduction amount on the wavelength of the external light was observed to show a resonancelike feature. The dominant intensity modulation mechanism was found to be the loss change caused by interband transitions in the active region. Further, the effective coupling efficiency plays an important role in the intensity modulation.

Experimental and Theoretical Investigation of the Dependence of Oscillation Characteristics on Structure of Integrated Slot Antennas in Sub-terahertz and Terahertz Oscillating Resonant Tunneling Diodes

We report experimental and theoretical work on the dependence of oscillation frequency and output power on the offset of a feeding point and the aperture width of a slot antenna in sub-terahertz (THz) and THz resonant tunneling diode (RTD) oscillators. Theoretical analysis shows that the oscillation frequency increases with the offset of the RTD from the center of the slot antenna. Output power also increases with the offset up to the point of the best impedance matching. In the experiment, an oscillation frequency of 504 GHz without offset increases to 593 GHz with an offset of 15 µm (60% of half the antenna length) from the antenna center for a 50-µm-long antenna. The experimental frequencies are in good agreement with the calculation. For the dependence on the width of the slot antenna, a slight increase in frequency is expected theoretically, although the output power is almost invariant. The oscillation frequency increases experimentally to about 345 GHz with a width of 2 µm from about 330 GHz with a width of 4 µm, in reasonable agreement with theory. The increase in oscillation frequency without using short antennas, which results in a decrease in output power, is shown by these experimental and theoretical results.

Ultrahigh Relative Refractive Index Contrast GaAs Nanowire Waveguides

An effective and practical fabrication procedure toward compound semiconductor nanowire waveguide on insulator is developed. GaAs nanowire waveguides with a cross-section of 400×400 nm2, bend radius of 10 µm, and a high relative refractive index contrast, Δ, of 40% were successfully fabricated for the first time. The propagation loss at 1.55 µm was measured to be approximately 4.5 dB/mm, and the bend loss was less than 0.8 dB per 90° bend. The losses were considered to be dominated by light scattering at the rough sidewalls.

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.