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Wireless transmission of QPSK signals at 2 THz has been demonstrated using modulator-based optical combs for the transmitter and hot electron bolometer mixers combined with phase-locked terahertz quantum cascade lasers for the receiver.

We are developing a waveguide-type hot electron bolometer mixer (HEBM) at 2 THz. A membrane-type Ni-NbN HEBM device on a Si3N4 substrate was fabricated in a clean room facility of our laboratory. The membrane bridge with a size of W55 μm x L820 μm x t1 μm was supported by a Si frame with a size of 3 mm by 3 mm and a thickness of 200 μm. The device was installed in a waveguide-type mixer mount with a WR-0.51 waveguide and a diagonal horn. The measured receiver noise temperature and IF gain bandwidth are ~1,690 K in DSB and ~4 GHz which must be improved by getting the HEBM device closer to the design. We are also trying to fabricate a mixer block with a corrugated horn at 2 THz.

We are developing a hot electron bolometer mixer (HEBM) at 2 THz for the atmospheric observations. We demonstrated an integrated HEB mixer in a cryostat with a cryogenic local oscillator (LO). An alignment of the LO into an HEB mixer was made using a cryogenic rotator. We have detected a beat note using this receiver system at 2 THz.

We proposed a terahertz (THz) communication system operating in the 3 THz range using an optical-comb-based THz source and a heterodyne receiver system using a hot electron bolometer mixer (HEBM) along with a quantum cascade laser (QCL) operating at 3 THz for a local oscillator. The THz source is composed of a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG), and a uni-traveling-carrier photodiode (UTC-PD). By using a phase-locked loop (PLL) system, the oscillation frequency of the QCL was highly stabilized. By using this system, generation and detection of coherent THz carriers in the 3 THz range have been demonstrated, which had a phase noise of -70 dBc/Hz at an offset frequency of 100 kHz.

Simultaneous stabilization of two quantum cascade lasers operated at 2 and 3 THz has been demonstrated by using a multi-frequency THz source based on Mach-Zehnder-modulator-based flat comb generator (MZ-FCG) combined with a programmable optical filter.

We have developed a phase-locking system of a terahertz quantum cascade laser (THz-QCL) using a hot electron bolometer mixer (HEBM) and a terahertz reference such as an amplifier/multiplier chain (AMC) or an optical-comb generator followed by a UTC-PD. We have demonstrated simultaneous phase-locking of two THz-QCLs. The phase-locked THz-QCLs can be used as local oscillators (LOs) of an HEBM. This technique is useful for atmospheric and astronomical observations.

A novel technique that is capable of discriminating absorption variations by measuring angular distribution of diffuse reflected light is presented for semi-infinite two-layered isotropic diffusive media. By using Monte-Carlo simulation, it was found that, in principle, the technique is useful to discriminate absorption variations caused in individual layers. This capability was validated in terms of partial path length as well as spatial sensitivity distribution.

We have proposed using nano-antennas to improve the response performance of infrared detectors. Prototypes of mid-infrared superconducting hot electron bolometer formed by a twin-slot-antenna with an NbN strip were fabricated. By the irradiation of mid-infrared (λ=4.89 μm) pulsed light, clear polarization dependency of responses was observed. When the HEB was biased close to the critical current, the detector output synchronized with the trigger signal was observed. The output waveforms were composed of voltage pulse trains, and the half width of the pulse was evaluated about 0.3 nsec.

We have developed a superconducting low noise heterodyne receiver at 3-THz band based on a hot electron bolometer mixer (HEBM) with a phase-locked THz Quantum Cascade Laser (THz-QCL) as a local oscillator for atmospheric or astronomical observations. We have demonstrated gas cell measurement of emission line spectra of methanol [CH 3 OH] at 3.7-THz band. We have also measured methanol line at 2.07-THz band.

NICT has commenced development of an accurate and stable frequency standard for THz frequency band. We succeeded in phase-locking a single-mode 3THz quantum cascade laser (THz-QCL) to a THz reference. Using the phase-locked THz-QCL and a hot electron bolometer mixer, a THz wave generated by photomixing was evaluated. The result showed that the phase-locked THz-QCL and the optically generated THz wave had the same sability as the microwave references at the 10−15 level and also that imposed noises during heterodyne mixing were negligibly small.

Generation of cw-THz waves with high frequency accuracy was demonstrated by using a Mach-Zehnder-modulator-based flat comb generator (MZ-FCG). A combination of the MZ-FCG and a highly nonlinear dispersion shifted fiber generated broadband optical combs, and cw-THz waves in the range of 3 THz were generated by photonic down-conversion of two-tone signals extracted from the broadband combs.

We are developing a low noise superconducting heterodyne receiver based on a hot electron bolometer mixer (HEBM) and a THz Quantum Cascade Laser (THz-QCL) as a local oscillator at 3THz for atmospheric and astronomical observations. The best value of the uncorrected receiver noise temperature of 1,200 K (DSB) which corresponds to 8 times quantum limit was achieved. We demonstrated phase-locking of a 3 THz-QCL using the HEBM and evaluated its performance.

We characterized a 3THz continuous wave generated by photomixing two optical modes of a broadband optical frequency comb. This optically generated THz wave and the output beam of a 3THz quantum cascade laser phase-locked to a THz reference were coupled onto a hot electron bolometer mixer operated at 4K and the beat signal was precisely evaluated. This direct comparison at 3THz reveals that the optically generated 3THz wave has fractional frequency instability of 4×10−15 at 100sec averaging time and the power spectral density of −10 dBc/Hz at 1Hz Fourier frequency.

We have developed a phase-locking system of a 3.1THz QCL (Quantum Cascade Laser) using a low noise hot electron bolometer mixer (HEBM) and a THz reference. The THz reference was generated by photomixing two optical modes of a frequency comb. The THz-QCL and HEBM devices are fabricated in our laboratory. A line width of the phase-locked QCL of narrower than 1Hz was achieved.

To improve the response performance of superconducting infrared detectors, we propose an nano slot antenna with a NbN load element. Results of simulation and experiment were showed qualitative agreement.

A Low noise superconducting heterodyne receiver at 3THz is developing for atmospheric or astronomical observations. A NbN HEB(Hot Electron Bolometer) with quasi optical planar (spiral or log-periodic) antenna and a THz-QCL(Quantum Cascade Laser) which is used as a local oscillator are fabricated in our laboratory. The receiver noise temperature in double sideband of ∼2800K which is around 19 times of quantum limit is measured.