Nonlinear THz Spectroscopy Research Articles

Single-cycle, 643 mW average power terahertz source based on tilted pulse front in lithium niobate.

We present the highest, to the best of our knowledge, average power from a laser-driven single-cycle THz source demonstrated so far, using optical rectification in the tilted pulse front geometry in cryogenically cooled lithium niobate, pumped by a commercially available 500 W ultrafast thin-disk ytterbium (Yb) amplifier. We study repetition rate-dependent effects in our setup at 100 and 40 kHz at this high average power, revealing different optimal fluence conditions for efficient conversion. The demonstrated sources with multi-100 mW average power at these high repetition rates combine high THz pulse energies and high repetition rate and are thus ideally suited for nonlinear THz spectroscopy experiments with significantly reduced measurement times. The presented result is a first benchmark for high average power THz time-domain spectroscopy systems for nonlinear spectroscopy, driven by very high average power ultrafast Yb lasers.

Spatiotemporal walk-off and improved focusing of plasma THz sources

High-field THz sources with peak field strengths exceeding MV/cm are essential for nonlinear THz spectroscopy and coherent control of matter on ultrafast time scales. Two-color femtosecond laser plasma sources employing long filamentation have been reported as providing single-cycle, >MV/cm fields, with multi-decade spanning bandwidth and polarization control, making them promising sources for such experiments. In this work, we report the observation of spatiotemporal spreading of the THz pulse when standard off-axis parabolic mirrors are used for collection and focusing of long filament plasma-based THz pulses. This produces a flying focus for THz light, with the axial focal region propagating at a velocity of 1/3 the speed of light. The THz emission is then subsequently spread over a temporal width of ∼10 ps, approximately 100 times the THz pulse duration detected by electro-optic sampling at any single point along the focus. The consequences of this non-ideal focusing are a potential and drastic overestimation of the peak THz electric field based on energy measurements, as well as significant phase noise arising from beam pointing fluctuations. We show that this spatiotemporal spreading can be minimized using a simple axicon lens that perfectly collimates the extended filament source, resulting in improved spatial and temporal focusing of the THz pulse.

Optimum Optical Designs for Diffraction-Limited Terahertz Spectroscopy and Imaging Systems Using Off-Axis Parabolic Mirrors

Off-axis parabolic mirrors (OAPMs) are widely used in the THz and mm-wave communities for spectroscopy and imaging applications, as a result of their broadband, low-loss operation and high numerical apertures. However, the aspherical shape of an OAPM creates significant geometric aberrations: these make achieving diffraction-limited performance a challenge, and lower the peak electric field strength in the focal plane. Here, we quantify the impact of geometric aberrations on the performance of the most widely used spectrometer designs, by using ray tracing and physical optics calculations to investigate whether diffraction-limited performance can be achieved in both the sample and the detector plane. We identify simple rules, based on marginal ray propagation, that allow spectrometers to be designed that are more robust to misalignment errors, and which have minimal aberrations for THz beams. For a given source, this allows the design of optical paths that give the smallest THz beam focal spot, with the highest THz electric field strength possible. This is desirable for improved THz imaging, for better signal-to-noise ratios in linear THz spectroscopy and optical-pump THz-probe spectroscopy, and to achieve higher electric field strengths in non-linear THz spectroscopy.

Wideband dispersion-free THz waveguide platform

We present a versatile THz waveguide platform for frequencies between 0.1 THz and 1.5 THz, designed to exhibit vacuum-like dispersion and electric as well as magnetic field enhancement. While linear THz spectroscopy benefits from the extended interaction length in combination with moderate losses, nonlinear THz spectroscopy profits from the field enhancement and zero dispersion, with the associated reshaping-free propagation of broadband single- to few-cycle THz pulses. Moreover, the vacuum-like dispersion allows for velocity matching in mixed THz and visible to infrared pump-probe experiments. The platform is based on the motif of a metallic double ridged waveguide. We experimentally characterize essential waveguide properties, for instance, propagation and bending losses, but also demonstrate a junction and an interferometer, essentially because those elements are prerequisites for THz waveform synthesis, and hence, for coherently controlled linear and nonlinear THz interactions.

Contribution of collective excitations to third harmonic generation in two-band superconductors: The case of MgB2

Multiband superconductors can host collective excitations with marked differences with respect to their single-band counterpart. We first study the spectrum of collective amplitude fluctuations in a clean two-band superconductor, showing that the spectral weight of the Higgs mode rapidly deviates from the naive extension of the single band case as the interband coupling is turned on. These results are then used to critically analyze the nonlinear optical response in ${\mathrm{MgB}}_{2}$, providing an explanation for the apparently contradictory results of recent experiments, pointing towards a selective relevance either of the Leggett mode or of the amplitude fluctuations at twice the lower gap. By using exact numerical simulations and realistic estimates of disorder we compute the relative contribution of the quasiparticle, amplitude, and phase fluctuations to the nonlinear optical response. We show that at low pumping frequency only the resonance at twice the smaller gap emerges, as due to the BCS response, while the Leggett mode dominates only in a narrow range of higher pumping frequencies matching its low-temperature value. Our findings provide a fresh perspective on the potential of nonlinear THz spectroscopy to detect collective modes in other multiband systems, such as iron-based superconductors.

3D-printed THz wave- and phaseplates.

Three-dimensional printing based on fused deposition modeling has been shown to provide a cost-efficient and time-saving tool for fabricating a variety of THz optics for a frequency range of <0.2 THz. By using a broadband THz source, with a useful spectral range from 0.08 THz to 1.5 THz, we show that 3D-printed waveplates operate well up to 0.6 THz and have bandwidths similar to commercial products. Specifically, we investigate quarter- and half-waveplates, q-plates, and spiral phaseplates. We demonstrate a route to achieve broadband performance, so that 3D-printed waveplates can also be used with broadband, few-cycle THz pulses, for instance, in nonlinear THz spectroscopy or other THz high field applications.

Integration of a rapid scanning technique into THz time-domain spectrometers for nonlinear THz spectroscopy measurements.

We have implemented a rapid scanning technique into THz time-domain spectrometers using an oscillating frictionless delay line, especially adapted for nonlinear THz experiments. Thereby we were able to increase the dynamic range of THz measurements in the frequency range from 40 to 200 cm-1 by up to 24 dB and reduce the scanning time by up to a factor of 200. We report here test measurements on TDS-setups at repetition rates of 80 MHz and 5 kHz. The dynamic range exceeds 64 dB, which allows to record even small changes in the THz absorption upon optical excitation by a THz probe, covering the frequency range of the intermolecular modes and the phonon bands. We demonstrate the potential of this technique for optical-pump THz-probe experiments using a 70 μm thick high-resistivity silicon, excited by 400 nm, ∼50 fs pulses as a sample.

Efficient semiconductor multicycle terahertz pulse source

Multicycle THz pulse generation by optical rectification in GaP semiconductor nonlinear material is investigated by numerical simulations. It is shown that GaP can be an efficient and versatile source with up to about 8% conversion efficiency and a tuning range from 0.1 THz to about 7 THz. Contact-grating technology for pulse-front tilt can ensure an excellent focusability and scaling the THz pulse energy beyond 1 mJ. Shapeable infrared pump pulses with a constant intensity-modulation period can be delivered for example by a flexible and efficient dual-chirped optical parametric amplifier. Potential applications include linear and nonlinear THz spectroscopy and THz-driven acceleration of electrons.

Generation and Detection of THz Pulses With a Bandwidth Extending Beyond 4 THz Using a Subpicosecond Yb-Doped Fiber Laser System

We experimentally demonstrate the generation and detection of broadband THz pulses with a bandwidth extending beyond 4 THz by using a 1.04- μm Yb-doped fiber laser with a pulse duration of 0.48 ps. Owing to higher order nonlinear processes (such as the cascaded χ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(2)</sup> process and χ <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(3)</sup> process) in LiNbO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> crystals, THz pulses can be emitted efficiently with a bandwidth extending beyond the boundaries of the spectral width of the excitation pulse. By compressing divided sampling pulses to as short as 80 fs, we detected the THz pulse in the full spectral range, which could not be resolved with the original pulse. The precise control of the nonlinear effects and the compression of the sampling pulses compensated for the disadvantage of using a 1- μm fiber laser. This work will contribute to the progress of nonlinear THz spectroscopy.

THz Switching and THz Nonlinear Spectroscopy Applications

Slits in thin metal sheets and split-ring resonators (SRR) featuring gaps on the micro- or nano-scale are shown to be a promising tool for THz switching or THz nonlinear spectroscopy applications. Both structures show strong field enhancement in the gap region due to light-induced current flows and capacitive charging across the gap. Whereas nano-slits allow for broadband enhancement the resonant behavior of the SRRs leads to narrowband amplification and results in field enhancement of tens of thousands. This property is particularly beneficial for the realization of nonlinear THz experiments.

Generation of high-power terahertz pulses by tilted-pulse-front excitation and their application possibilities

The principles and most-recent results of high-power THz generation through optical rectification using a tilted optical pulse front are described. Single-cycle THz pulses of multimicrojoule energies are generated at kHz repetition rates, and average THz power levels exceeding 1 mW can be generated at kHz-MHz repetition rates. Applications in nonlinear THz spectroscopy and THz coherent control are discussed.

Identifying the distinct phases of carrier transport in semiconductors with 10 fs resolution.

We study the very early processes of photogenerated carrier transport in GaAs and Si using THz nonlinear spectroscopy with 10 fs time resolution. This ultrahigh time resolution allows us to clearly separate the instantaneous creation of polarized electron-hole pairs and the subsequent carrier motion for the first time. In addition, our results give the first clear demonstration of the distinct phases of carrier transport; ballistic transport, electron velocity overshoot, and steady-state drift.