Exploring dynamics in the far-infrared with terahertz spectroscopy.

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.

https://iopscience.iop.org/article/10.1088/0022-3727/47/37/374001/pdf

A review of terahertz sources

Three-dimensional topological insulators are fascinating materials with insulating bulk yet metallic surfaces that host highly mobile charge carriers with locked spin and momentum. Remarkably, surface currents with tunable direction and magnitude can be launched with tailored light beams. To better understand the underlying mechanisms, the current dynamics need to be resolved on the timescale of elementary scattering events (∼10 fs). Here, we excite and measure photocurrents in the model topological insulator Bi2Se3 with a time resolution of 20 fs by sampling the concomitantly emitted broadband terahertz (THz) electromagnetic field from 0.3 to 40 THz. Strikingly, the surface current response is dominated by an ultrafast charge transfer along the Se-Bi bonds. In contrast, photon-helicity-dependent photocurrents are found to be orders of magnitude smaller than expected from generation scenarios based on asymmetric depopulation of the Dirac cone. Our findings are of direct relevance for broadband optoelectronic devices based on topological-insulator surface currents.

Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3

We compare the observed strong saturation of the free-carrier absorption in $n$-type semiconductors at 300 K in the terahertz (THz) frequency range when single-cycle pulses with intensities up to $150\text{ }\text{MW}/{\text{cm}}^{2}$ are used. In the case of germanium, a small increase in the absorption occurs at intermediate THz pulse energies. The recovery of the free-carrier absorption was monitored by time-resolved THz pump--THz probe measurements. At short probe delay times, the frequency response of germanium cannot be fitted by the Drude model. We attribute these unique phenomena of Ge to dynamical overpopulation of the high mobility $\ensuremath{\Gamma}$ conduction-band valley.

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Observation of nonequilibrium carrier distribution in Ge, Si, and GaAs by terahertz pump-terahertz probe measurements

Generation and measurement of ultrashort, subpicosecond pulses of electromagnetic radiation with their characteristic Fourier spectra that reach far into terahertz (THz) frequency range has recently become a versatile tool of far-infrared spectroscopy and imaging. This technique, THz time-domain spectroscopy, in addition to a femtosecond pulse laser, requires semiconductor components manufactured from materials with a short photoexcited carrier lifetime, high carrier mobility and large dark resistivity. Here we will review the most important developments in the field of investigation of such materials. The main characteristics of low-temperature-grown or ion-implanted GaAs and semiconducting compounds sensitive in the wavelength ranges around 1??m and 1.5??m will be surveyed. The second part of the paper is devoted to the effect of surface emission of THz transients from semiconductors illuminated by femtosecond laser pulses. The main physical mechanisms leading to this emission as well as their manifestation in various crystals will be described.

https://iopscience.iop.org/article/10.1088/0022-3727/43/27/273001/pdf

Semiconductors for terahertz photonics applications

THz emission from semiconductor surfaces

We report on terahertz (THz) emission from a (111)-cut InAs crystal in the reflection and transmission directions, excited by femtosecond optical pulses in the direction of its surface normal. THz pulse amplitudes emitted from the crystal surface in this case were only ~20% smaller than for optimal photoexcitation at a 45° angle. This observation evidences that THz emission from InAs is caused by lateral photocurrent transients appearing due to a crystal anisotropy rather than directly by the photo-Dember effect, which creates fast changing electric polarization perpendicular to the surface. Such a simple geometry of the photoexcitation could greatly enhance the fields of surface THz emitter applications.

Terahertz radiation from an InAs surface due to lateral photocurrent transients.

Terahertz emission from the surfaces of narrow-gap semiconductors excited by femtosecond laser pulses was described in terms of a transient interband photoconductivity. It has been found that the nonparabolicity of the electron dispersion law as well as the optical alignment of the photoexcited carrier momenta result in anisotropic photocurrent with a component perpendicular to the surface dc electric field even in semiconductors with a cubic symmetry. This lateral transient photocurrent component is the strongest during the first few hundreds of femtoseconds after the photoexcitation and causes the emission of terahertz radiation pulses with an amplitude dependent on the angle between the optical field and the crystallographic axes. In the case of InAs the contribution of this component explains experimental results of both the azimuthal anisotropy of the emitted terahertz pulse amplitude and its dependence on the exciting photon energy.

Terahertz emission from cubic semiconductor induced by a transient anisotropic photocurrent

Polarized terahertz radiation generated from (112)-oriented InSb surfaces is investigated as a function of the sample azimuthal orientation under excitation from femtosecond Yb:KGW laser pulses The expressions describing the optical rectification and the surface electric-field-induced optical rectification in reflection from zinc-blende crystals, such as InSb, are calculated It is shown the contributions of both these effects should be taken into account when describing terahertz emission from InSb surfaces

Terahertz emission from femtosecond laser illuminated (112) surfaces of InSb

We have investigated azimuthal angle dependences of terahertz (THz) emission from differently oriented surfaces of variously doped Ge crystals illuminated by femtosecond Ti:sapphire laser pulses and we have shown the presence of a significant nonlinear optical contribution due to the electric field-induced optical rectification effect. Obtained results have led to the separation of relative contributions of photo-Dember and nonlinear optical effects and to the estimation of the value of the third-order susceptibility in Ge.

V. L. Malevich

Papers

THz emission from semiconductor surfaces

Terahertz radiation from an InAs surface due to lateral photocurrent transients.

Terahertz emission from cubic semiconductor induced by a transient anisotropic photocurrent

Terahertz emission from femtosecond laser illuminated (112) surfaces of InSb

Terahertz emission from femtosecond laser excited Ge surfaces due to the electrical field-induced optical rectification