Exploring dynamics in the far-infrared with terahertz spectroscopy.

The THz-radiation power from bulk InAs irradiated with femtosecond optical pulses is significantly enhanced and reaches 650 μW in a 1.7-T magnetic field with 1.5-W excitation power. The THz-radiation power is related almost quadratically both to the magnetic field and excitation laser power. We have also found that the power of the THz-radiation from an InAs sample in a magnetic field is over one order of magnitude higher than that from GaAs. Additionally, a dramatic change of ellipticity is observed, and the spectra of the horizontal and vertical polarization components are found to differ.

High average-power THz radiation from femtosecond laser-irradiated InAs in a magnetic field and its elliptical polarization characteristics

Spectroscopies using terahertz (THz) radiation excited by ultrashort laser pulses have been rapidly developing recently. In this paper, the principles of various types of THz time domain spectroscopies (THz TDSs), i.e. transmission-, reflection-and ellipsometry-type THz TDSs, and their applications to the characterization of semiconductors are described. In addition to the THz TDS using a femtosecond laser, a sub-THz TDS system using a cheap and compact continuous multimode laser diode is also described.

https://iopscience.iop.org/article/10.1088/0957-0233/13/11/309/pdf

Spectroscopy by pulsed terahertz radiation

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

A comparative study of magnetic-field-enhanced THz generation in semiconductor surfaces of InSb, InAs, InP, GaAs, and GaSb is reported. Applying an external magnetic field, the power of the generated THz radiation is increased for all examined semiconductor materials. The use of time-resolved measurements of the THz waveform allows to distinguish between the fraction of radiation originating from the surface depletion field and the fraction that is additionally generated by the magnetic field. It turns out that the power enhancement factor due to the magnetic field is inversely proportional to the effective electron mass.

Magnetic-field-enhanced generation of terahertz radiation in semiconductor surfaces

We demonstrate the use of a magnetic field to switch and to control the direction and polarization of a THz beam radiated from a semiconductor emitter.

Magnetic switching of THz beams

With illumination by femtosecond laser pulses at an oblique incident angle, a semiconductor sample emits subpicosecond submillimeter-wave radiation1 This phenomenon was interpreted as radiation from accelerated photocarriers in the surface depletion field1 and the dependence of the radiation on crystal rotation symmetry was explained as strong evidence of X (3) related optical rectification2 More recently, quantum well structures have been used to limit the carrier transport This allows examination of the mechanism of optical rectification as well as the study of electron coherent oscillations in a double-well potential3 To distinguish between the contribution from carrier transport and from optical rectification is crucial for a physical understanding of the origin of the radiation, as well as for realizing an intense THz source application

Terahertz Radiation from Electro-Optic Crystals

We provide recent experimental evidences of THz radiation from semiconductor generated by photocarrier transport and by terahertz optical rectification.

L. E. Kingsley

Papers

Magnetic switching of THz beams

Terahertz Radiation from Electro-Optic Crystals

THz Beam Emission by Carrier Transport or by Optical Rectification