Time-resolved, pulsed terahertz spectroscopy has developed into a powerful tool to study charge carrier dynamics in semiconductors and semiconductor structures over the past decades. Covering the energy range from a few to about 100 meV, terahertz radiation is sensitive to the response of charge quasiparticles, e.g., free carriers, polarons, and excitons. The distinct spectral signatures of these different quasiparticles in the THz range allow their discrimination and characterization using pulsed THz radiation. This frequency region is also well suited for the study of phonon resonances and intraband transitions in low-dimensional systems. Moreover, using a pump-probe scheme, it is possible to monitor the nonequilibrium time evolution of carriers and low-energy excitations with sub-ps time resolution. Being an all-optical technique, terahertz time-domain spectroscopy is contact-free and noninvasive and hence suited to probe the conductivity of, particularly, nanostructured materials that are difficult or impossible to access with other methods. The latest developments in the application of terahertz time-domain spectroscopy to bulk and nanostructured semiconductors are reviewed.

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Carrier dynamics in semiconductors studied with time-resolved terahertz spectroscopy

Within the last several years, the field of terahertz science and technology has changed dramatically. Many new advances in the technology for generation, manipulation, and detection of terahertz radiation have revolutionized the field. Much of this interest has been inspired by the promise of valuable new applications for terahertz imaging and sensing. Among a long list of proposed uses, one finds compelling needs such as security screening and quality control, as well as whimsical notions such as counting the almonds in a bar of chocolate. This list has grown in parallel with the development of new technologies and new paradigms for imaging and sensing. Many of these proposed applications exploit the unique capabilities of terahertz radiation to penetrate common packaging materials and provide spectroscopic information about the materials within. Several of the techniques used for terahertz imaging have been borrowed from other, more well established fields such as x-ray computed tomography and synthetic aperture radar. Others have been developed exclusively for the terahertz field, and have no analogies in other portions of the spectrum. This review provides a comprehensive description of the various techniques which have been employed for terahertz image formation, as well as discussing numerous examples which illustrate the many exciting potential uses for these emerging technologies.

https://iopscience.iop.org/article/10.1088/0034-4885/70/8/R02/pdf

Imaging with terahertz radiation

Exploring dynamics in the far-infrared with terahertz spectroscopy.

We propose a simple model based on the Drude–Lorentz theory of carrier transport to account for the details of the ultrashort terahertz pulses radiated from small photoconductive semiconductor antennas. The dynamics of the bias field under the influence of the space-charge field from the accelerated carriers is included in the model. We consider in detail the optical system used to image the terahertz radiation onto the terahertz detector, and we calculate the frequency-dependent response of the detector. The proposed model is compared with several different experiments, each focusing on different parameters of the model. Agreement between experiment and model is found in all cases, supporting the validity of this simple and appropriate model.

Generation and detection of terahertz pulses from biased semiconductor antennas

Terahertz radiation was generated with several designs of photoconductive antennas (three dipoles, a bow tie, and a coplanar strip line) fabricated on low-temperature-grown (LT) GaAs and semi-insulating (SI) GaAs, and the emission properties of the photoconductive antennas were compared with each other. The radiation spectrum of each antenna was characterized with the photoconductive sampling technique. The total radiation power was also measured by a bolometer for comparison of the relative radiation power. The radiation spectra of the LT-GaAs–based and SI-GaAs–based photoconductive antennas of the same design showed no significant difference. The pump-power dependencies of the radiation power showed saturation for higher pump intensities, which was more serious in SI-GaAs-based antennas than in LT-GaAs-based antennas. We attributed the origin of the saturation to the field screening of the photocarriers.

Emission characteristics of photoconductive antennas based on low-temperature-grown GaAs and semi-insulating GaAs.

We present a theoretical and experimental investigation into the generation of subpicosecond pulses of terahertz radiation from large-aperture biased photoconductors with 1.5-eV photon excitation. A model that describes the far-field radiation from the optically excited, biased photoconductor is developed. The peak of the radiated electric field as well as waveforms are presented as a function of optical excitation fluence and pulse width. The dependence of the terahertz radiation from biased InP and GaAs emitters on the applied bias field and on incident optical fluence for bias fields as high as 12 kV/cm and for optical fluences of 0.01–1.0 mJ/cm2 is presented. For a given level of optical excitation the radiated electric field is predicted by theory to scale linearly with the applied bias field. This prediction is verified experimentally, with radiated-field strengths as high as 1.23 ± 0.13 kV/cm being demonstrated. The radiated electric field also exhibits the monotonic saturation behavior predicted by theory, and saturation fluences of 0.058 ± 0.015 and 0.018 ± 0.008 mJ/cm2 are obtained for InP and GaAs emitters, respectively.

Scaling of terahertz radiation from large-aperture biased photoconductors

A model of the electromagnetic radiation generated by the triggering of large-aperture biased photoconductors with ultrashort pulses is developed. In the far field, the radiated fluence and intensity, as well as waveforms of the radiated electric field, are presented as a function of optical excitation fluence.

Modeling of femtosecond electromagnetic pulses from large-aperture photoconductors.

We present, for biased InP emitters, the dependence of the generated terahertz radiation on bias field and optical fluence for optical fluences of 0.01–1.0 mJ/cm2 and bias fields as high as 12 kV/cm. The radiated electric field scales linearly with the bias field up to 12 kV/cm and exhibits monotonic saturation behavior, radiating half the maximum field at an excitation fluence of 0.058 mJ/cm2.

Scaling of terahertz radiation from large-aperture biased InP photoconductors.

We describe a synchronously pumped mode-locked dye laser that uses a novel combination of saturable absorber dyes (HITC-I and DTP) to yield satellite-free, 39-fs pulses at 815 nm

Generation of 39-fs pulses at 815 nm with a synchronously pumped mode-locked dye laser.

The generation of millimeter and sub-millimeter radiation with subpicosecond pulse durations using large-aperture biased photoconductors is being pursued experimentally by a number of groups.1-3 Theoretical modeling of the radiated electromagnetic pulse from these devices has been developed for the near field;1 however, to our knowledge a detailed analysis of the radiated electromagnetic pulse in the far field has not been performed. Far field calculations are important since most of the experimental data on sub-millimeter wave generation has been acquired in the far field. We develop here a simple model using the current surge picture2 to describe the far field radiation from a biased large-aperture photoconducting emitter triggered by an ultrashort optical pulse.

P. K. Benicewicz

Papers

Scaling of terahertz radiation from large-aperture biased photoconductors

Modeling of femtosecond electromagnetic pulses from large-aperture photoconductors.

Scaling of terahertz radiation from large-aperture biased InP photoconductors.

Generation of 39-fs pulses at 815 nm with a synchronously pumped mode-locked dye laser.

Modeling of femtosecond electromagnetic pylses from large-apertyre photocondyctors