Over the past three decades a new spectroscopic technique with unique possibilities has emerged. Based on coherent and time-resolved detection of the electric field of ultrashort radiation bursts in the far-infrared, this technique has become known as terahertz time-domain spectroscopy (THz-TDS). In this review article the authors describe the technique in its various implementations for static and time-resolved spectroscopy, and illustrate the performance of the technique with recent examples from solid-state physics and physical chemistry as well as aqueous chemistry. Examples from other fields of research, where THz spectroscopic techniques have proven to be useful research tools, and the potential for industrial applications of THz spectroscopic and imaging techniques are discussed.

Terahertz spectroscopy and imaging – Modern techniques and applications

Recent research and development has been incredibly successful at advancing the capabilities for vacuum electronic device (VED) sources of powerful terahertz (THz) and near-THz coherent radiation, both CW or average and pulsed. Currently, the VED source portfolio covers over 12 orders of magnitude in power (mW-to-GW) and two orders of magnitude in frequency (from ; 10 THz). Further advances are still possible and anticipated. They will be enabled by improved understanding of fundamental beam-wave interactions, electromagnetic mode competition and mode control, along with research and development of new materials, fabrication methods, cathodes, electron beam alignment and focusing, magnet technologies, THz metrology and advanced, broadband output radiation coupling techniques.

Vacuum Electronic High Power Terahertz Sources

Ultrafast charge and spin excitations in the elusive terahertz regime1,2 of the electromagnetic spectrum play a pivotal role in condensed matter3,4,5,6,7,8,9,10,11,12,13. The electric field of free-space terahertz pulses has provided a direct gateway to manipulating the motion of charges on the femtosecond timescale6,7,8,9. Here, we complement this process by showing that the magnetic component of intense terahertz transients enables ultrafast control of the spin degree of freedom. Single-cycle terahertz pulses switch on and off coherent spin waves in antiferromagnetic NiO at frequencies as high as 1 THz. An optical probe pulse with a duration of 8 fs follows the terahertz-induced magnetic dynamics directly in the time domain and verifies that the terahertz field addresses spins selectively by means of the Zeeman interaction. This concept provides a universal ultrafast means to control previously inaccessible magnetic excitations in the electronic ground state. Researchers report the direct observation of ultrafast magnetic dynamics using the magnetic component of highly intense terahertz wave pulses with a time resolution of 8 fs. This concept provides a universal ultrafast method of visualizing magnetic excitations in the electronic ground state.

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Coherent terahertz control of antiferromagnetic spin waves

The effect of a tiny gap in a metal substrate on incident terahertz radiation in the regime where the gap's dimensions are smaller than the metal's skin-depth are investigated. The results and theoretical analysis show that the gap acts as a capacitor charged by light-induced currents, and dramatically enhances the local electric field.

Terahertz field enhancement by a metallic nano slit operating beyond the skin-depth limit

In this paper, a review of fiber laser technology as relevant for applications in ultrafast optics is given. We discuss core enabling fiber technologies, such as fiber amplifiers, all-fiber dispersion control, and highly nonlinear and large-core fibers. We concentrate on systems built around passively mode-locked fiber lasers and fiber frequency combs, which are further amplified in large-core fiber amplifiers. Our review further encompasses coherent supercontinuum generation and techniques for absolute phase control of fiber lasers and amplifiers. Applications concerned with spectral generation in the range from the vacuum UV to the terahertz range are also described.

Ultrafast Fiber Laser Technology

An experiment-theory comparison is presented to demonstrate terahertz-induced extreme-nonlinear transients in a GaAs/AlGaAs quantum-well system. The terahertz-pump and optical-probe experiments show pronounced spectral modulations of the light- and heavy-hole excitonic resonances. Excellent agreement with the results of microscopic many-body calculations is obtained, identifying clear ponderomotive contributions and the generation of terahertz harmonics.

Interaction of strong single-cycle terahertz pulses with semiconductor quantum wells.

We developed a tabletop source of intense, narrow band terahertz pulses via type-II difference-frequency generation in ZnTe crystal using two linearly chirped and orthogonally polarized optical pulses. The pulse energy is in the range of 1–3 nJ depending on the variable pulse duration from 1 to 5 ps. The amplitude of electric field reaches ∼10 kV/cm. The central frequency of the spectrum is continuously tunable from 0.3 to 2.5 THz with the bandwidth at 0.2–0.5 THz.

Intense narrow band terahertz generation via type-II difference-frequency generation in ZnTe using chirped optical pulses

The authors demonstrate a flexible terahertz pulse-shaping technique, manipulating spatially dispersed multifrequency components generated by optical rectification in a fanned-out periodically poled lithium niobate crystal. Spatial masks of low pass, high pass, and double slit in front of the crystal manipulate the spatial pattern of the optical excitation beam on the crystal, which is mapped onto the intensity profile of the terahertz spectrum. The spatial dispersion of the terahertz spectrum is removed by the line-to-point imaging of a spherical mirror.

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Generation of arbitrary terahertz wave forms in fanned-out periodically poled lithium niobate

We report $^{31}\mathrm{P}$ (impurity) and $^{29}\mathrm{Si}$ (host) nuclear magnetic resonance (NMR) data for heavily doped Si:P over a wide temperature range $(100--500\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ for samples with nominal doping levels between $4\ifmmode\times\else\texttimes\fi{}{10}^{18}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and $8\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. The data include resonance shifts, linewidths, and spin-lattice relaxation rates for both host and impurity nuclei. The NMR parameters for $^{31}\mathrm{P}$ exhibit complex dependences on temperature and dopant concentration that are distinct from those of the host $^{29}\mathrm{Si}$ nuclei indicating that the hyperfine fields at the impurity are determined by local characteristics of the impurity electronic state. For nominal carrier concentrations up to about $1\ifmmode\times\else\texttimes\fi{}{10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$, the impurity NMR properties are inconsistent with expectation for fully ionized dopants with carriers exclusively occupying states in the conduction band. A simple impurity band model yields numerical simulations of the resonance shifts and relaxation rates that reproduce the qualitative features and are in semi-quantitative agreement with the $^{31}\mathrm{P}$ data. These results imply that free carriers in Si:P are in dynamic exchange with residual impurity states at concentrations as high as ${10}^{19}\phantom{\rule{0.3em}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ and at temperatures well above room temperature.

Temperature-dependent NMR study of the impurity state in heavily doped Si:P

We demonstrate an efficient room temperature source of narrow-bandwidth terahertz (THz) radiation using femtosecond pump pulses and periodic GaAs structure as a nonlinear material. In the past, several THz generation schemes exploited optical rectification in nonlinear crystals using femtosecond laser technology. Most of them generated single-cycle THz-pulses with broad bandwidth, using nonlinear crystals shorter than the phase-matching coherence length. Recently a novel technique to generate multi-cycle THz-pulses in the pre-engineered domain structure of periodically-poled lithium niobate (PPLN) crystals has been demonstrated. Quasi-phase matching (QPM) structures such as PPLN consist of a periodic system of domains of inverted crystal orientation. The sign of second order nonlinear polarization generated by femtosecond pulses is inverted at domain boundaries. If domain length is comparable with coherence length, QPM between THz-wave and nonlinear polarization extends the 
interaction length between THz and optical pulses. In the present 
work, using periodic GaAs structures we have achieved exceptionally high photon as well as energy conversion efficiency: 3% and 0.07% respectively. We have examined two different types of periodic QPM GaAs samples: diffusion-bonded GaAs wafers and all-epitaxially-grown orientation-patterned GaAs crystals with 3-10 mm thicknesses. The incident optical pulse energy was in the micro-Joule range and pulse duration was ~100 fsec. We measured spectral properties of THz radiation using Michelson interferometer and a bolometer. Narrow-bandwidth (~100GHz) THz output, tunable between 1 and 3 THz, was achieved. THz frequency was tuned either by tuning the light source wavelength between 2 and 4.4 microns, or by selecting GaAs samples with different QPM periods. Our theoretical analysis, based on known GaAs dispersion properties, shows good agreement between the measured and predicted THz frequencies.

J. R. Danielson

Papers

Interaction of strong single-cycle terahertz pulses with semiconductor quantum wells.

Intense narrow band terahertz generation via type-II difference-frequency generation in ZnTe using chirped optical pulses

Generation of arbitrary terahertz wave forms in fanned-out periodically poled lithium niobate

Temperature-dependent NMR study of the impurity state in heavily doped Si:P

Generation of multi-cycle THz-pulses via optical rectification in periodically inverted GaAs