Table of contents

Bibliometric data set the scene by illustrating the growth of terahertz work and the present interest in terahertz science and technology. After locating terahertz sources within the broader context of terahertz systems, an overview is given of the range of available sources, emphasizing recent developments. The focus then narrows to terahertz sources that rely on surface phenomena. Three are highlighted. Optical rectification, usually thought of as a bulk process, may in addition exhibit a surface contribution, which, in some cases, predominates. Transient surface currents, for convenience often separated into drift and diffusion currents, are well understood according to Monte Carlo modelling. Finally, terahertz surface emission by mechanical means—in the absence of photoexcitation—is described.

The photo-Dember effect is a source of pulsed THz emission following femtosecond pulsed optical excitation. The emission results from the ultrafast spatial separation of electron–hole pairs in carrier gradients due to their different diffusion coefficients. The associated time dependent polarization of the photo-carriers within the photo-Dember emitters is oriented perpendicular to the excited surface. THz emission under photo-excitation can also be due to the surface field effect or optical rectification. We review the studies that attempt to understand surge current and study the surface emission of different semiconductor materials. We also review the work that has been done on increasing the efficiency of THz surface emitters using plasmonic interactions and other resonant effects. We later focus on a new THz emission mechanism called lateral photo-Dember which proposes a scheme for generating strong carrier current parallel to the excited surface, using the Dember field as well as dipole quenching effect. Because the resulting currents are oriented parallel to the surface and the THz radiation is emitted collinearly to the optical excitation. Surprisingly, the lateral photo-Dember THz emitters provide a higher bandwidth than photoconductive emitters. The theory for the mechanism of emission is reviewed and the parameters that affect the performance of the lateral photo-Dember emitters, namely fluence and polarization, are summarized. Finally we review multiplexing geometries with periodically tailored photo-excited spatial carrier distributions that create phase coherent photo-Dember and surface field currents which enhance the THz emission. These multiple emitters can reach electric field amplitudes comparable to a high-efficiency externally biased photoconductive emitter.

When (nanostructured) metals, such as gold and silver, are illuminated with laser pulses having a duration in the femtosecond range, they can emit pulses of THz light. Most of these experiments have been performed using amplified lasers, giving rise to energy densities on the sample on the order of mJ cm⁻². The results of the different experiments are surprisingly inconsistent in both the measurements of the THz fluence as a function of laser fluence and in the interpretation of the results. This paper reviews the current state of affairs of this interesting topic and discusses some effects related to surface preparation that may influence the emission THz light on metals, particularly silver and copper. We also show results of measurements on nanostructured metals using unamplified laser pulses, which emphasize the role played by plasmons in the generation of THz light. When increasing the optical energy density on a specially nanostructured sample, we observe a transition from a 'classical' second-order non-linear optical process to a higher-order process as the source of the THz radiation. This supports recent results on a differently structured metal by Polyushkin et al (2014 Phys. Rev. B 89 125426), who also observe two different power regimes when decreasing the intensity coming from the high energy density side.

Research involving the tetrahertz (THz) part of the electromagnetic spectrum, commonly taken to be the region between 0.1 and 10 THz (3 mm to 30 µm), has seen rapid growth in recent years because of the importance of THz radiation as a low energy probe of the optical properties and dynamics of matter and partly because of emerging real world applications in areas as diverse as industrial quality control, biosensing and security screening. Despite a vigorous growth in THz technology, many components and processes taken for granted at higher and lower frequencies are still in an early stage of development. One example of this is the use of waveguides to transport or spatially confine radiation. In this review we summarize progress in developing THz waveguides, paying attention to the role that microstructuring on a sub-wavelength length scale can play in engineering new capability and the various trade-offs between loss, bandwidth, group velocity dispersion and spatial confinement.

Ultrafast photoconductivity and charge carrier transport in nanostructured semiconductors is poorly understood on the microscopic level in many systems. The terahertz spectroscopy constitutes a suitable method to probe the nanoscopic motion of charges with a sub-picosecond time resolution and without the need to deposit electrical contacts. However, straightforward fitting of the raw terahertz conductivity spectra by the Drude-Smith model, which is abundantly used in the literature, has not lead to a significant advance in an in-depth understanding of these phenomena. This is mainly because of the depolarization fields which build up in any inhomogeneous system. On the one hand, these fields reflect the sample morphology and our understanding of each particular system may provide new information about e.g. the nanostructure connectivity; on the other hand, the effect of unknown depolarization fields can hide or distort fingerprints of the nanoscopic transport. In this paper we provide a general analytical description of the photoconductivity and transient transmission spectra, where the effects of depolarization fields are systematically disentangled from the local carrier response function for both percolated and non-percolated samples. Application of our formula to the retrieval of the carrier response function may help significantly in uncovering the nature of charge carrier transport at the nanoscale level in quite arbitrary nanostructured systems.

Terahertz time-domain spectroscopy permits the coherent motion of charges to be examined in a diverse range of two-dimensional semiconductor heterostructures. Studies of the THz conductivity and magnetoconductivity of two-dimensional quantum systems are reviewed, including cyclotron resonance spectroscopy and the transverse conductivity in the Hall and quantum Hall regimes. Experiments are described that demonstrate quantum phenomena at THz frequencies, principally coherent control and enhanced light–matter coupling in electromagnetic cavities.

The laser terahertz (THz) emission microscope (LTEM) is a unique inspection tool which can directly two-dimensionally map the THz pulse emission from a variety of electric materials and devices. Thus, two-dimensional mapping directly yields various physical information by visualizing the distributions of electric field, supercurrent, ferroelectric domain structures, magnetic fluxes, etc. Based on techniques created for conventional LTEM, we have developed a more highly functional laser THz imaging system, including a scanning laser THz (near-field) imaging system having the performance of high-speed and high-resolution, a scanning probe LTEM coupled with an atomic force microscope for high-resolution imaging, and a dynamic THz emission microscope to investigate the ultrafast carrier dynamics two-dimensionally in the sample. These systems offer diverse characteristics, making them suitable for various applications. We take a look at these systems and some typical applications.

The terahertz (THz) frequency quantum cascade laser (QCL) is a compact source of THz radiation offering high power, high spectral purity and moderate tunability. As such, these sources are particularly suited to the application of THz frequency imaging across a range of disciplines, and have motivated significant research interest in this area over the past decade. In this paper we review the technological approaches to THz QCL-based imaging and the key advancements within this field. We discuss in detail a number of imaging approaches targeted to application areas including multiple-frequency transmission and diffuse reflection imaging for the spectral mapping of targets; as well as coherent approaches based on the self-mixing phenomenon in THz QCLs for long-range imaging, three-dimensional imaging, materials analysis, and high-resolution inverse synthetic aperture radar imaging.

Interest in biomedical terahertz research is growing rapidly and there are now several terahertz groups in Asia, Europe and the US investigating potential applications such as pharmaceutical quality control, protein characterization and cancer detection. This review article outlines the technological bottlenecks that have been overcome which have made biomedical terahertz research possible. Key research findings will be presented, and the limitations that remain and the research initiatives that strive to address them will also be discussed.