THz Pulses Research Articles

High-field THz pulses from a GaAs photoconductive emitter for non-linear THz studies.

We report the emission of high-field terahertz pulses from a GaAs large-area photoconductive emitter pumped with a Ti:Sapphire amplifier laser system at 800 nm wavelength and 1 kHz repetition rate. The maximum estimated terahertz electric field at the focus is ≳230 kV/cm. We also demonstrate the capability of the terahertz field to cause a non-linear effect, which usually requires high-field terahertz pulses generated through optical rectification or an air plasma. A significant drop in the optical conductivity of optically pumped GaAs due to Γ-L inter-valley scattering of free electrons caused by the strong THz field is found.

Analysis of THz generation using the tilted-pulse-front geometry in the limit of small pulse energies and beam sizes.

Optical rectification in lithium niobate using the tilted-pulse-front geometry is one of the most commonly used techniques for efficient generation of energetic single-cycle THz pulses and the details of this generation scheme are well understood for high pulse energy driving lasers, such as mJ-class, kHz-repetition rate Ti:Sa amplifier systems. However, as modern Yb-based laser systems with ever increasing repetition rate become available, other excitation regimes become relevant. In particular, the use of more moderate pulse energies (in the few µJ to multi-10 µJ regime), available nowadays by laser systems with MHz repetition rates, have never been thoroughly explored. As increasing the repetition rate of THz sources for spectroscopy becomes more relevant in the community, we present a thorough numerical analysis of this regime using a 2+1-D numerical model. Our work allows us to confirm experimental trends observed in this unusual excitation regime and shows that the conversion efficiency is naturally limited by the small pump beam sizes as a consequence of spatial walk-off between the pump and THz beams. Based on our findings, we discuss strategies to overcome the current limitations, which will pave the way for powerful THz sources approaching the watt level with multi-MHz repetition rates.

Nonlinear electric conductivity and THz-induced charge transport in graphene

Based on the quantum master equation approach, the nonlinear electric conductivity of graphene is investigated under static electric fields for various chemical potential shifts. The simulation results show that, as the field strength increases, the effective conductivity is firstly suppressed, reflecting the depletion of effective carriers due to the large displacement in the Brillouin zone caused by the strong field. Then, as the field strength exceeds 1 MV m−1, the effective conductivity increases, overcoming the carrier depletion via the Landau–Zener tunneling process. Based on the nonlinear behavior of the conductivity, the charge transport induced by few-cycle THz pulses is studied to elucidate the ultrafast control of electric current in matter.

Erratum: "A broad range frequency measurement method for continuous and pulsed THz waves" [Rev. Sci. Instrum. 91, 014710 (2020)

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Observation of “Frozen‐Phase” Propagation of THz Pulses in a Dispersive Optical System

AbstractDispersion occurs naturally in many wave systems. So far, a variety of techniques have been proposed and demonstrated to compensate the second‐order (group‐velocity) dispersion, such as dispersion management and nonlinear soliton formation, yet it is commonly believed that the first‐order dispersion (FOD) is less amenable and cannot be eliminated. In this work, it is shown that a phase‐invariant propagation of THz pulses, namely the “frozen‐phase” propagation, can be realized by a complete cancellation of the FOD, achieved by implementing a synchronized THz moving source with a locked initial phase. The dynamic generation and propagation process of the THz pulses is examined directly by a time‐resolved imaging system. To the best of authors' knowledge, this represents the first demonstration of the “frozen‐phase” propagation of a light pulse in dispersive optical systems.

Terahertz radiation may not alter rat basophilic leukemia cell membrane permeability to propidium iodide

Terahertz radiation (THz) technology is fast-growing, with applications in sensing, security, monitoring and pharmaceutical industries. Since it is non-invasive, THz has been used as a diagnostic and therapeutic tool in medicine but its specific effects on biological systems is still largely under-studied. THz has been used to image tissues and cells mainly because it allows for identification of morphological features without the need for fluorescent or radioactive labels, but the potential effects of high intensities of THz radiation are currently not well understood. One of the hypotheses that has been proposed for possible effects of THz on living cells, is that it disrupts the cell membrane and induces increased permeability. To test this hypothesis we exposed a rat basophilic leukemia cell line (RBL-2H3) to non-thermal intense THz radiation (duration, dose, etc) and observed the internalization of propidium iodide, a fluorescent intercalating agent that binds to DNA. We did not observe any changes in RBL-2H3 fluorescence following exposure to these intense THz pulses suggesting that exposure of RBL-2H3 to THz radiation may not increase their membrane permeability. These experiments were preliminary and further optimization and analysis is required before we can make definitive conclusions. However, our preliminary observations have set a baseline of RBL-2H3 internalization of propidium iodide, show that it is possible to expose RBL-2H3 cells to THz radiation using our configuration, and set the stage for future experiments.

Ultrafast Terahertz Nanoseismology of GaInN/GaN Multiple Quantum Wells

AbstractTerahertz (THz) emission spectroscopy and microscopy are applied to investigate the electron and lattice dynamics of Ga0.8In0.2N/GaN multiple quantum wells (MQWs). The THz emission consists of three distinct, differently timed signals, whose physical mechanisms are attributed to i) laser‐induced ultrafast dynamical screening of built‐in bias electric field in MQWs followed by ii) capacitive charge oscillation of the excited carriers and iii) the coherent acoustic phonon (CAP)‐driven polarization surge at the discontinuity between the GaN capping layer and air. These multifunctional optical responses show strong dependence on the quantum well width and photon energies. The temporal separation between the first and third THz pulses corresponds to the propagation of the CAP across the GaN capping layer of the MQW structure, whose thickness can thus be determined with 10 nm precision.

Terahertz Generation by Optical Rectification in Sugar Crystal

We report on THz pulse generation through optical rectification of femtosecond laser pulses in sugar crystals. The sugar crystals were grown from an aqueous solution of caster cane sugar bought in a local grocery. Sugar exhibits rather good nonlinear coefficients ( $d_{23}\simeq 56$ pm/V), but the optical-THz conversion efficiency remains quite weak because of the lack of phase-matching, when pumped by a laser beam of 0.8-μm central wavelength. Nevertheless, sugar availability at a low price makes it a relevant material for trial experiments.

THz birefringence inside femtosecond laser filament in air

Large birefringence of THz wave inside a femtosecond laser filament in air has been found in this work. By varying the laser pulse energy, the filament length is changed, inducing variation in the phase delay between two perpendicularly polarized THz pulses. The birefringence could be explained by the delayed rotational molecular wave packet of air molecules inside the femtosecond laser-induced filament. The newly discovered THz birefringence is essentially attributed to the strong confinement of the THz pulse inside the filament. This phenomenon has opened a new way to freely control the polarization of THz pulses emitted from an optical filament and significantly benefits polarization-sensitive THz spectroscopy and control of matter.

Switchable generation of azimuthally- and radially-polarized terahertz beams from a spintronic terahertz emitter.

We propose and demonstrate a method of generating two fundamental terahertz cylindrical vector beams (THz-CVBs), namely the azimuthally- and radially-polarized THz pulses, from a spintronic THz emitter. We begin by presenting that the spintronic emitter generates the HE21 mode, a quadrupole like polarization distribution, when placed between two magnets with opposing polarity. By providing an appropriate mode conversion using a triangular Si prism, we show both from experiment and numerical calculation that we obtain azimuthal and radial THz vector beams. The proposed method facilitates the access of CVBs and paves the way toward sophisticated polarization control in the THz regime.

Optimization of Electrooptical Sampling of THz Pulse With a Chirped Broadband Probe Pulse in Thick EO Crystal

Electrooptic (EO) sampling is a method in THz time-domain spectroscopy. In previous studies, a thick EO crystal with a broadband probe pulse is used to meet the requirements of THz detection with a large time window and high-precision frequency spectrum. However, the broadband probe pulses are significantly broadened in the time domain during the transmission in thick EO crystals due to group velocity dispersion, which distorts the EO signal. In this article, a negative prechirp of broadband (10 fs in full-width at half-maximum) laser pulse is utilized to compensate the dispersion and enhances the detected signals. In the experiments, we use an interdigital photoconductive antenna to generate typical THz pulses in the region of 0.1-3 THz. The measurement results confirm that the prechirp of -2100 fs <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> is more effective to optimize the detected signals in ZnTe crystal with a thickness of 3 mm and enhances the EO signal by 71%, and the frequency redshift caused by the filter effect of the electrooptical response function is decreased. Meanwhile, the experimental results show that the enhancements of different prechirps depend on the frequency components of THz pulses and the frequency-dependent optimization of EO sampling is effective, which is consistent with the simulation results. This technique is helpful to optimize the EO sampling in thick ZnTe crystals to obtain the large time window and higher signal-to-noise ratio of EO sampling.

Enantioselective orientation of chiral molecules induced by terahertz pulses with twisted polarization

Chirality and chiral molecules are key elements in modern chemical and biochemical industries. Individual addressing and the eventual separation of chiral enantiomers have been and still are important elusive tasks in molecular physics and chemistry, and a variety of methods have been introduced over the years to achieve these goals. Here, we theoretically demonstrate that a pair of cross-polarized THz pulses interacting with chiral molecules through their permanent dipole moments induces in these molecules an enantioselective orientation. This orientation persists for a long time, exceeding the duration of the THz pulses by several orders of magnitude, and its dependency on temperature and pulses' parameters is investigated. This persistent orientation may enhance the deflection of the molecules in inhomogeneous electromagnetic fields, potentially leading to viable separation techniques.Received 16 December 2020Accepted 24 February 2021DOI:https://doi.org/10.1103/PhysRevResearch.3.013249Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article's title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasAtomic & molecular processes in external fieldsCoherent controlPhysical SystemsMoleculesPropertiesElectric momentTechniquesSchroedinger equationAtomic, Molecular & Optical

Variable Repetition Rate THz Source for Ultrafast Scanning Tunneling Microscopy.

Broadband THz pulses enable ultrafast electronic transport experiments on the nanoscale by coupling THz electric fields into the devices with antennas, asperities, or scanning probe tips. Here, we design a versatile THz source optimized for driving the highly resistive tunnel junction of a scanning tunneling microscope. The source uses optical rectification in lithium niobate to generate arbitrary THz pulse trains with freely adjustable repetition rates between 0.5 and 41 MHz. These induce subpicosecond voltage transients in the tunnel junction with peak amplitudes between 0.1 and 12 V, achieving a conversion efficiency of 0.4 V/(kV/cm) from far-field THz peak electric field strength to peak junction voltage in the STM. Tunnel currents in the quantum limit of less than one electron per THz pulse are readily detected at multi-MHz repetition rates. The ability to tune between high pulse energy and high signal fidelity makes this THz source design effective for exploration of ultrafast and atomic-scale electron dynamics.

Time-resolved measurements of light–current characteristic and mode competition in pulsed THz quantum cascade laser

Quantum cascade lasers (QCL) are widely adopted as prominent and easy-to-use solid-state sources of terahertz radiation. Yet some applications require generation and detection of very sharp and narrow terahertz-range pulses with a specific spectral composition. We have studied time-resolved light-current (L–I) characteristics of multimode THz QCL operated with a fast ramp of the injection current. Detection of THz pulses was carried out using an NbN superconducting hot-electron bolometer with the time constant of the order of 1 ns while the laser bias current was swept during a single driving pulse. A nonmonotonic behavior of the L–I characteristic with several visually separated subpeaks was found. This behavior is associated with the mode competition in THz QCL cavity, which we confirm by L–I measurements with use of an external Fabry–Perot interferometer for a discrete mode selection. We also have demonstrated the possibility to control the L–I shape with suppression of one of the subpeaks by simply adjusting the off-axis parabolic mirror for optimal optical alignment for one of the laser modes. The developed technique paves the way for rapid characterization of pulsed THz QCLs for further studies of the possibilities of using this approach in remote sensing.

High-efficiency terahertz wave generation combined with optimized cascaded difference frequency generation and optical parametric oscillator

We propose a high-efficiency scheme to generate THz waves combined with optimized cascaded difference frequency generation (OCDFG) and optical parametric oscillator (OPO) consisting of a periodic poled lithium niobate (PPLN) crystal or an aperiodic poled lithium niobate (APPLN) crystal at 100 K. The phase mismatches of the OCDFG are decreased in cascaded Stokes processes and increased in cascaded anti-Stokes processes respectively by setting desirable poling periods along the APPLN crystal length. By setting the reflectance of the OPO cavity mirrors, pump energies are transferred to the cascaded Stokes waves rather than the cascaded anti-Stokes waves. As a result, the cascaded Stokes processes are greatly enhanced and the cascaded anti-Stokes processes are greatly weakened simultaneously through the combination of OCDFG and OPO. Theoretical calculations show that optical-to-THz energy conversion efficiencies in excess of 40 % can be achieved when the pump intensity is 10 MW/cm2 at 100 K. The scheme can be applied to the generation of pulsed, quasi-continuous or continuous THz waves.

Design of a THz-driven compact relativistic electron source

A THz-pulse-driven compact, < 150 mm in total length, two-stage electron accelerator setup was designed. It uses 2 × 2 pairs of nearly counter-propagating focused THz pulses. The effects of the initial bunch charge and the propagation direction of the THz pulses on the energy of the accelerated electrons were investigated by numerical simulations. Generation of 8 fC electron bunches with up to 340 keV energy; only 2.0% energy spread and compressed on-target duration of 200 fs is predicted using single-cycle low-frequency THz pulses with less than 4.5 mJ total energy.

Homodyne Solid-State Biased Coherent Detection of Ultra-Broadband Terahertz Pulses with Static Electric Fields.

We present an innovative implementation of the solid-state-biased coherent detection (SSBCD) technique, which we have recently introduced for the reconstruction of both amplitude and phase of ultra-broadband terahertz pulses. In our previous works, the SSBCD method has been operated via a heterodyne scheme, which involves demanding square-wave voltage amplifiers, phase-locked to the THz pulse train, as well as an electronic circuit for the demodulation of the readout signal. Here, we demonstrate that the SSBCD technique can be operated via a very simple homodyne scheme, exploiting plain static bias voltages. We show that the homodyne SSBCD signal turns into a bipolar transient when the static field overcomes the THz field strength, without the requirement of an additional demodulating circuit. Moreover, we introduce a differential configuration, which extends the applicability of the homodyne scheme to higher THz field strengths, also leading a two-fold improvement of the dynamic range compared to the heterodyne counterpart. Finally, we demonstrate that, by reversing the sign of the static voltage, it is possible to directly retrieve the absolute THz pulse polarity. The homodyne configuration makes the SSBCD technique of much easier access, leading to a vast range of field-resolved applications.

Population difference gratings created on vibrational transitions by nonoverlapping subcycle THz pulses

We study theoretically a possibility of creation and ultrafast control (erasing, spatial frequency multiplication) of population density gratings in a multi-level resonant medium having a resonance transition frequency in the THz range. These gratings are produced by subcycle THz pulses coherently interacting with a nonlinear medium, without any need for pulses to overlap, thereby utilizing an indirect pulse interaction via an induced coherent polarization grating. High values of dipole moments of the transitions in the THz range facilitate low field strength of the needed THz excitation. Our results clearly show this possibility in multi-level resonant media. Our theoretical approach is based on an approximate analytical solution of time-dependent Schrödinger equation (TDSE) using perturbation theory. Remarkably, as we show here, quasi-unipolar subcycle pulses allow more efficient excitation of higher quantum levels, leading to gratings with a stronger modulation depth. Numerical simulations, performed for THz resonances of the H_20 molecule using Bloch equations for density matrix elements, are in agreement with analytical results in the perturbative regime. In the strong-field non-perturbative regime, the spatial shape of the gratings becomes non-harmonic. A possibility of THz radiation control using such gratings is discussed. The predicted phenomena open novel avenues in THz spectroscopy of molecules with unipolar and quasi-unipolar THz light bursts and allow for better control of ultra-short THz pulses.

Vacuum post-acceleration of relativistic electrons by combinations of THz electromagnetic pulses and constant magnetic fields

The potential for post-acceleration of relativistic electrons in vacuum with the help of monocyclic THz electromagnetic pulses augmented with constant magnetic fields is explored. The directions of the electromagnetic wave propagation and electron injection are completely or closely aligned to enhance the time of energy transfer from the field to the electron. Monocyclic THz pulses efficiently capture and drive fast electrons until they are released due to field-induced transverse displacement or overtaken by the electromagnetic radiation. While the unidirectional motion of the electromagnetic wave and the relativistic electrons provides for longer overall capture of the particles by the field, protracted drift of an electron across the accelerating part of the monocyclic THz pulse may be achieved by calibrating the additional magnetic field. Simulations demonstrate that, under optimized parameters, the eventual electron energy gains may reach the magnitudes ranging from several to tens of electron rest energies depending on the injection energy. The suggested scheme of post-acceleration of relativistic electrons by combinations of comparatively moderate-intensity THz electromagnetic pulses and constant magnetic fields may thereby provide a viable alternative to the conventional laser accelerator designs employing extremely powerful laser systems that generate relativistically intense electromagnetic envelopes.

Ultrafast Neuromorphic Dynamics Using Hidden Phases in the Prototype of Relaxor Ferroelectrics.

Materials possessing multiple states are promising to emulate synaptic and neuronic behaviors. Their operation frequency, typically in or below the GHz range, however, limits the speed of neuromorphic computing. Ultrafast THz electric field excitation has been employed to induce nonequilibrium states of matter, called hidden phases in oxides. One may wonder if there are systems for which THz pulses can generate neuronic and synaptic behavior, via the creation of hidden phases. Using atomistic simulations, we discover that relaxor ferroelectrics can emulate all the key neuronic and memristive synaptic features. Their occurrence originates from the activation of many hidden phases of polarization order, resulting from the response of nanoregions to THz pulses. Such phases further possess different dielectric constants, which is also promising for memcapacitor devices.