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

We present a theory of the magnetic field enhancement of terahertz (THz) emission from photogenerated carriers in the surface depletion region of a semiconductor. A combination of the Drude-Lorentz model for the carrier dynamics with an appropriate solution of the radiation problem is sufficient to explain the strong B -field enhancement in THz radiation that has been observed experimentally. The effect arises primarily from the increased radiation efficiency of transient currents flowing in the plane of the surface. The model provides quantitative agreement with experiment for the pronounced angular dependence of the enhancement and predicts the correct trend for the enhancement in a variety of materials.

Origin of magnetic field enhancement in the generation of terahertz radiation from semiconductor surfaces

We provide a review of the different schemes for generating terahertz (THz) radiation using photoconductive emitters excited by femtosecond lasers. The discussion is concerned principally with large-area emitters because of their relative simplicity, both in fabrication and analysis, and their capabilities for high-power THz generation. In addition to discussing the principal characteristics of these emit- ters, we present a simple, but unified description of their behavior. The principal ingredients in this description are a Drude-Lorentz model for the carrier dynamics combined with a suitable solution of the radiation problem for a current sheet. This formalism permits one to examine the nature of the generated THz waveforms in the near and far field, as well as to consider the effect of the material properties and excitation conditions on the THz emission process. Within this picture, we describe the origin of the strong enhancement in THz emission that is observed upon appli- cation of an external magnetic field to emitters relying on transient photocurrents flowing perpendicular to the surface. Saturation processes that limit the efficiency of THz emission are also an important feature of these devices. These effects are dis- cussed in terms of perturbation of the bias field induced by space-charge screening and the THz radiation field.

Terahertz Radiation from Semiconductors

Generation of subpicosecond terahertz pulses is observed when graphite surfaces are illuminated with femtosecond near-infrared laser pulses. The nonlinear optical generation of THz pulses from graphite is unexpected since, in principle, the material possesses a centre of inversion symmetry. Experiments with highly oriented pyrolytic graphite crystals suggest that the THz radiation is generated by a transient photocurrent in a direction normal to the graphene planes, along the c-axis of the crystal. This is supported by magnetic-field induced changes in the THz electric-field polarization, and consequently, the direction of the photocurrent. We show that other forms of graphite, such as a pencil drawing on paper, are also capable of emitting THz pulses.

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Terahertz generation from graphite.

The crystallographic orientation dependence of the far‐infrared (FIR) light generated at the (001) surface of a zincblende semiconductor is shown to derive principally from bulk difference‐frequency mixing. A strong modulation is observed for 1‐GW/cm2 pulses on InP, which demonstrates that the radiated FIR wave produced by bulk optical rectification is comparable to that generated by the transport of photoinjected carriers. Using the bulk rectification light as a clock, we show that more than 95% of the light produced from an InP (111) crystal by 100‐fs, 100‐μJ pulses is generated in a time shorter than the excitation pulse.

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Short terahertz pulses from semiconductor surfaces: The importance of bulk difference‐frequency mixing

We report measurement of the influence of electric and magnetic fields on optically induced THz electromagnetic radiation from semiconductors at normal incidence. The measurements show that electric and magnetic fields strongly influence the amplitude and phase of transient carrier‐generated THz radiation, but do not affect THz optical rectification generated by the bulk second order nonlinear susceptibility, for static electric fields up to 104 V/cm.

L. E. Kingsley

Papers

Influence of electric and magnetic fields on THz radiation