A cw difference-frequency spectrometer has been developed for the 2.2 to 4.2 μm infrared region by mixing argon and tunable dye lasers in LiNbO3. A resolution of 5 × 10−4 cm−1 with continuous scans up to 1 cm−1 and a scan precision better than 2 × 10−3 cm−1 have been achieved. Tunable ir powers of ~1 μW have been obtained, enabling sensitive absorption measurements and real-time spectroscopy. The range, resolution, speed, and sensitivity of this difference-frequency technique are demonstrated by absorption traces of water vapor, ammonia, methane, and nitrous oxide.

Doppler-limited molecular spectroscopy by difference-frequency mixing*

Tunable, CW, far-infrared radiation has been generated by nonlinear mixing of radiation from two CO2 lasers in a metal-insulator-metal (MIM) diode. The FIR difference-frequency power radiated from the MIM diode antenna to a calibrated indium antimonide bolometer. Two-tenths of a microwatt of FIR power was generated by 250 mW from each of the CO2 lasers. The combination of lines from a waveguide CO2 laser, with its larger tuning range, with lines from CO2, N2O, and CO2 isotopic lasers promises complete coverage of the entire far-infrared band from 100 to 5000 GHz (3-200 per cm) with stepwise-tunable CW radiation. To demonstrate the usefulness of the technique, the J = 4-5 line of CO was observed at 567 GHz.

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Tunable far-infrared spectroscopy

Terahertz (THz) radiation occupies part of the electromagnetic spectrum between the infrared and microwave bands. Until recently, technology at THz frequencies was under-developed compared to the rest of the electromagnetic spectrum, leaving a gap between millimeter waves and the far-infrared (FIR). In the past decade, interest in the THz gap has been increased by the development of ultrafast laser-based T-ray systems and their demonstration of diffraction-limited spatial resolution, picosecond temporal resolution, DC-THz spectral bandwidth and signal-to-noise ratios above 104. This chapter reviews the development, the state of the art and the applications of T-ray spectrometers. Continuous-wave (CW) THz-frequency sources and detectors are briefly introduced in comparison to ultrafast pulsed THz systems. An emphasis is placed on experimental applications of T-rays to sensing and imaging, with a view to the continuing advance of technologies and applications in the THz band.

T-Ray Sensing and Imaging

Terahertz (THz)-wave generation using difference-frequency mixing in an organic 4-N, N-dimethylamino-4′-N′-methyl-stilbazolium tosylate (DAST) crystal was investigated theoretically and experimentally. We developed a dual-wavelength optical parametric oscillator (OPO) with two KTiOPO4 crystals as the input light source. This oscillator has a tunable range from 1300–1450 nm and is pumped by a diode-pumped, frequency-doubled, Q-switched Nd:YAG laser with a pulse duration of 10 ns. Widely tunable THz waves ranging from 2 to 20 THz were generated from DAST crystals with 0.5- and 1-mm thickness using the OPO. THz output energies of 82 nJ (peak power of 10.3 W) at 11.6 THz and 110 nJ (peak power of 13.8 W) at 19 THz were obtained with a 1-mm-thick DAST crystal. Using a tunable THz-wave system consisting of the THz source and a pyroelectric detector, THz spectroscopic data for a polytetrafluoroethylene sheet were obtained and were comparable to data obtained from a conventional far-infrared spectrometer.

Widely tunable terahertz-wave generation in an organic crystal and its spectroscopic application

This article reviews the fundamental concepts, technology, and recent developments on the topic of THz photomixers made from low-temperature-grown GaAs, ErAs:GaAs, and ErAs:InGaAs. It is designed to be comprehensive and useful both to students and to engineers and scientists new to the THz field.

THz Generation by Photomixing in Ultrafast Photoconductors

Tunable narrow‐band cw generation of far‐infrared radiation has been achieved for the first time in the 70‐μm to 1‐mm wavelength region by noncollinear difference‐frequency mixing of two single‐mode CO2 lasers in GaAs at 80 K. Using a Schottky diode as a heterodyne harmonic mixer with a carcinotron local oscillator, the far‐infrared signal is shown to have a linewidth of less than 100 kHz and a fine tuning capability in excess of 50 MHz.

R. L. Aggarwal

Papers

cw generation of tunable narrow-band far-infrared radiation