THz Field Research Articles

Van der Waals Heteroepitaxial VO2/Mica Films with Extremely Low Optical Trigger Threshold and Large THz Field Modulation Depth

AbstractMaterials with low optical trigger threshold and large THz field modulation depth are highly desirable for THz switches and modulators. VO2, a classical strongly correlated oxide undergoing a sharp metal–insulator transition (MIT), is advantageous for its large THz field modulation depth by virtue of MIT. However, for optically triggered VO2‐based THz switches and modulators, high pump laser intensity is normally required to trigger the MIT, adding considerable costs to THz systems. Reducing optical trigger threshold of VO2 is of substantial interest in the THz science. Here, van der Waals (vdW) heteroepitaxial VO2/mica films are reported with extremely low optical trigger threshold and large THz field modulation depth. Such VO2 films can be used to fabricate low cost but high performance THz switches and modulators. The vdW‐bonded VO2 film–substrate interface allows for negligible disturbance to the VO2 lattice by the mica substrate and reduces thermal conduction to the substrate, boosting the MIT and thus, reducing the pump threshold.

A THz driven split-ring resonator based ultrafast relativistic electron streak camera

The use of sub-wavelength metal structures to locally enhance high frequency electromagnetic fields, generally known as plasmonics, enables breakthrough opportunities across diverse fields of research such as nonlinear optics, biosensing, photovoltaics and others. Here we study the application of sub-wavelength metallic resonators tuned in the THz frequency range for manipulation and diagnostics of relativistic electron beams. In this work, we report on the use of a double-sided split-ring structure driven by a near single cycle THz field generated by optical rectification to impart a time-dependent angular deviation (streak) on a 4.5 MeV electron beam. Electrons passing through the small gap reveal field enhancement factors larger than 10, in good agreement with finite difference time domain simulations. This work paves the way for further application of high frequency metallic structures in compact particle accelerators such as for THz-based relativistic electron streaking at fs and sub-fs temporal resolution.

Numerical investigations into a THz-driven dielectric accelerator with a Bragg reflector

Abstract Using THz pulses to generate an accelerating field in a dielectric loaded accelerator structure is a promising way to mitigate the disadvantages of RF and Laser accelerator structures. This paper proposes an energy efficient side-coupling dielectric single-grating structure accompanied with a multi-layer Bragg reflector that is designed for THz pulse operation. To show the comparative advantages of the new design, we also numerically investigate a single-side coupling THz-driven dual-grating structure which is composed of Silicon Dioxide (SiO2). Simulation results show that the reflector structure effectively manipulates the THz field compared to dual-grating structure, not only boosts the field strength in the accelerating channel, but also optimizes the field distribution and generates a periodic field reversal to achieve better phase synchronicity for relativistic particles, thereby increasing the accelerating gradient by more than 40%. It is also found that the energy-recycling capability of a Bragg reflector highly depends on the operating THz pulse duration and beam channel width. Choosing a longer pulse duration and setting a narrower beam channel width with an optimum pillar height can increase the single pulse efficiency by more than 70% compared to the bare dual-grating structure.

Terahertz Dynamic Aperture Imaging at Standoff Distances Using a Compressed Sensing Protocol

In this paper, results of a 0.35 THz dynamic aperture imaging approach are presented. The experiments use an optical modulation approach and a single pixel detector at a standoff imaging distance of $\approx$ 1 m. The optical modulation creates dynamic apertures of 5-cm diameter with $\approx$ 2000 individually controllable elements. An optical modulation approach is used here for the first time at a large far-field distance, for the investigation of various test targets in a field-of-view of 8 × 8 cm $^{2}$ . The results highlight the versatility of this modulation technique and show that this imaging paradigm is applicable even at large far-field distances. It proves the feasibility of this imaging approach for potential applications like standoff security imaging or far field THz microscopy.

Towards terahertz spin Hall nano-oscillator with synthesized anti-ferromagnets

Abstract We calculated the spectrum of synthetic antiferromagnets (SAF) based spin Hall nano-oscillators (SHNO) with micromagnetic simulations. It was found that, even without an applied field, the frequency of generated spin waves can reach up to THz range once both Ruderman–Kittel–Kasuya–Yosida (RKKY) interactions and DC current density being reasonably high enough. We have further investigated the origin of THz spectrum. Although the magnetization dynamics of SAF is driven by spin transfer torque (STT) via spin Hall effect (SHE) in heavy metal (HM). The RKKY interactions can obviously boost it by accelerating the wiggle of magnetization in SAF. We also found that the bottom ferromagnetic layer thickness in SAF should be thin and the total thickness of SAF should not exceed the spin diffusion length in order to maximize STT effect and achieve THz spectrum. Finally, we clearly demonstrated the nonreciprocity of generated THz spin wave by introducing interfacial Dzyaloshinskii-Moriya interaction (DMI) at normal metal/ferromagnet (NM/FM) interface. Our results will technically extend the application of spintronic devices into THz field.

Fast retrieval of temporal characteristics of FEL pulses using streaking by THz field.

A simple and very fast method for reconstruction of temporal structure of linearly polarized FEL pulses from the THz streaking spectra of photoelectrons is suggested. The method is based on a quantum mechanical approach within the strong-field approximation. The method is suitable for online retrieval of the temporal characteristics of the FEL pulses. It can be applied for any photon frequency in a broad range of FEL pulse duration with a proper selection of the streaking field. To enhance its accuracy, it is suggested to simultaneously analyze the streaking spectra for several emission angles.

Significant enhancement of THz detectivity by lowering ZnTe crystal temperature in electro-optic sampling

The temperature of the ZnTe detection crystal in a THz time-domain system was tuned continuously from 300 K to 1.5 K to minimize phonon absorption, thus enhance its detectivity. The THz peak amplitude was observed to increase more than 10 times. To explain this phenomenon, the THz transmission spectra of the ZnTe crystal at different temperatures were also measured, and the refractive index and absorption coefficient of the crystal were extracted. The experiments and theoretic analysis reveal that the significant detectivity enhancement is due to the reduced absorption of the THz field and phase matching change in the detection crystal with temperature decreasing, which makes the ZnTe crystal suitable for integrating with cryogenic applications.

The Effect of Crystal Contact Forces on the Protein Global Motions

Protein crystals are widely used for structural determination using X-ray crystallography and solid state NMR. X-ray free electron lasers have enabled direct dynamical structural movies of photo or chemically initiated biochemical reactions. However the perturbation of protein dynamics by the crystal contact forces has been persistently under debate. If crystal contact forces are sufficiently strong, the time resolved X-ray diffraction measurements will not accurately reflect the in vivo biological dynamics. Here we probe the effect of crystal contact forces on protein structural dynamics by directly measuring the collective intramolecular vibrations as a function of crystal symmetry for chicken egg white lysozyme (CEWL). These vibrations extend throughout the protein, involving coherent motion of entire domains. While the collective vibrational density of states is a broad continuous spectrum peaked near 100 cm−1, (3 THz), polarization dependent terahertz absorption isolates vibrations with displacements primarily along the polarization direction [1]. Triclinic, monoclinic, tetragonal and orthorhombic CEWL crystals are grown according to previous published protocols. X-ray measurements verify their crystal structure and quality before and after terahertz measurements. The anisotropic terahertz absorbance is measured using our new technique: ideal polarization varying anisotropic THz microscopy (IPV-ATM). This near field THz microscopy method allows rapid determination of the anisotropic THz absorption. By starting from measurements of the lowest symmetry, triclinic crystals, which have a single protein per unit cell and absolute alignment of the protein molecule array, we can predict the spectra for higher symmetry groups, neglecting crystal contact perturbations. We compare our predicted spectra to the measurements of the monoclinic, orthorhombic and tetragonal crystals, attaining a measure of the impact of the intermolecular crystal forces on the intramolecular vibrations. [1]G. Acbas, K. A. Niessen, E. H. Snell, and A. G. Markelz, Nat. Comm.(2014) https://doi.org/10.1038/ncomms4076.

Angular streaking of Auger-electrons by THz field

Rotational streaking by a circularly polarized THz field of Auger electrons generated by a short extreme ultraviolet (XUV) or x-ray pulse is theoretically investigated. The character of the streaking pattern depends on three main parameters: the duration of the XUV pulse, the Auger decay time-of-life and the period of the THz field. Different cases with various interrelations of these parameters are discussed. Examples of the patterns are calculated within the strong field approximation. Correspondence between the angular streaking of electrons in the processes of photoionization and Auger ionization is considered. Retrieval of the Auger decay parameters from the circular streaking spectrum is discussed.

Fano resonance and power output in a quantum-dot-embedded Aharonov-Bohm ring subjected to THz irradiation

In this paper, we theoretically investigate the Fano resonance and thermoelectric transport in a nanoscale Aharonov-Bohm interferometer consisting of an InAs quantum dot coupled to two electrodes under THz irradiation. In the non-linear regime, the charge current shows a symmetric line shape at asymmetric THz frequency even in the presence of the Fano effect. Under asymmetric THz irradiation, the heat current presents the cooling effect. Interestingly, the heat current will change its sign and the cooling effect appears in different VG region if we switch the THz frequency asymmetric ratio of the two electrodes. This characteristic provides a new way of manipulating thermoelectric devices. In addition, with increasing strength or frequency of the THz field, a restricted zone of the output power would appear around the region VG=0.0. These results have potential for the application of thermoelectric devices under THz irradiation.

Diffraction of electromagnetic waves on the resonance graphene metasurfaces

By using three independent numerical approaches to solve the 3D diffraction boundary problems the reflection, transmittance coefficients and losses of metasurfaces of graphene strip elements, depending on the frequency and values of the chemical potential were calculated in the terahertz frequency range. The results of numerical simulation of performances of tuned by the external bias electric field THz absorbers and THz polarizers based on the resonance metasurfaces of single square and two connected rectangular graphene strips in the unit cell placed on the dielectric or multilayer graphene-dielectric substrates were obtained.

Control of THz field waveform emitted from air plasma by chirping two-color laser pulses

Abstract Few-cycle terahertz pulses can be effectively generated in air plasma by mixing an ultrashort laser pulse with its second harmonic. The temporal overlap between these two pulses is critical, which can degrade with the mismatch of group velocities. However, the overlap between the fundamental and second harmonic can be controlled by pre-stretching the temporally separated pulses. In this paper, we study the role of chirp and the relative phase between the fundamental and its second harmonic in the formation of the terahertz waveform. We demonstrate experimentally and explain within the context of transient photocurrent model, that the combination of chirping and relative phase shifting can provide a powerful tool to control the waveform of the terahertz pulse.

Terahertz radiation generation by beating of two oblique arbitrary laser beam profiles in a collisional plasma

This paper presents a scheme to achieve THz radiation by the beating of two laser beams in a plasma. Lasers are obliquely incident on an underdense plasma with density ripples. Lasers having different frequencies and wave numbers but the same electric fields exert a ponderomotive force on the plasma electrons at the beating frequency which creates THz waves. The general formulas for the efficiency and THz field amplitude with arbitrary laser beam profiles (such as super-Gaussian and triangular) are derived where the effect of electron-neutral collisions is taken into account. The results show that the efficiency of THz radiation is sensitive to the angle of incidence, beam profile, collision frequency, and beating frequency. The most striking feature of this paper is that with obliquely incident laser beams, higher efficiency can be reached for collisionless and collisional plasma compared to normal incidence.

Ultrafast polarization switching of (BaSr)TiO3 thin film by a single-period terahertz pulse in a vicinity of phase transition

We report here an experimental and theoretical study on ultrafast switching of dielectric polarization in (Ba0.8Sr0.2)TiO3 thin films by a strong electric field of a nearly single-cycle THz pulse at different temperatures. Optical second harmonic generation (SHG) is used as a probe of the polarization in the terahertz pump-optical probe experiment. We describe an approach to interpretation of SHG response from the ferroelectric polydomain film excited by a short electric field pulse. We show that dielectric polarization induced by the THz field decreases while approaching the Curie temperature. Simulations with the help of the Duffing equation reproduce the experimental observations.

Surface phase-transition dynamics of ice probed by terahertz time-domain spectroscopy

In this paper, we present an investigation of the solid-liquid phase-transition process of ice based on terahertz time-domain spectroscopy (THz-TDS), which can precisely demonstrate the low-frequency vibrational and rotational modes in gaseous or condensed state materials. The intensity of the transmitted signal is used to characterize the interaction between the terahertz wave and the melting ice. The structure transition lead to the change of physical properties such as dielectric constant with the melting of the ice. When the temperature gradually rises, the H2O molecules on ice surface overcome the hydrogenbond interaction and start to rotate under the driving of THz field which enhancing the interaction between water molecules and incident terahertz radiation. As H2O is a polar molecule, a dipolar moment will be induced, which contributes to the dielectric polarization. An analysis of the intensities and dielectric constant of THz-TDS signal peaks reveals the dynamic and continuous melting process on the surface of ice at room temperature.

Exciton ionization by THz pulses in germanium

After optical pumping, strong single cycle THz pulses are used to probe and manipulate excitons in bulk germanium. For strong THz fields a significant broadening and bleaching of the 1s–2p THz absorption peak is observed. The experimental results are analyzed using a microscopic many-body theory attributing the observations to a shortening of the excitonic state lifetime and eventual exciton ionization. Simultaneously, the ac THz Stark effect leads to a shift in the 1s–2p transition energy.

Coherent and incoherent ultrafast magnetization dynamics in 3d ferromagnets driven by extreme terahertz fields

Ultrafast spin dynamics in magnetic materials is generally associated with ultrafast heating of the electronic system by a near infrared femtosecond laser pulse, thus offering only an indirect and nonselective access to the spin order. Here we explore spin dynamics in ferromagnets by means of extremely intense THz pulses, as at these low frequencies the magnetic field provides a direct and selective route to coherently control the magnetization. We find that, at low fields, the observed off-resonantly excited spin precession is phase locked to the THz magnetic field. At extreme THz fields, the coherent spin dynamics become convoluted with an ultrafast incoherent magnetic quenching due to the absorbed energy. This demagnetization takes place upon a single shot exposure. The magnetic properties are found to be permanently modified above a THz pump fluence of $\ensuremath{\approx}100\phantom{\rule{0.28em}{0ex}}\mathrm{mJ}/{\mathrm{cm}}^{2}$. We conclude that magnetization switching cannot be reached. Our atomistic spin-dynamics simulations excellently explain the measured magnetization response. We find that demagnetization driven by THz laser-field coupling to electron charges occurs, suggesting nonconducting materials for achieving coherent THz-magnetization reversal.

Electron energy analysis by phase-space shaping with THz field cycles.

Time-resolved electron energy analysis and loss spectroscopy can reveal a wealth of information about material properties and dynamical light-matter interactions. Here, we report an all-optical concept for measuring energy spectra of femtosecond electron pulses with sub-eV resolution. Laser-generated terahertz radiation is used to measure arrival time differences within electron pulses with few-femtosecond precision. Controlled dispersion and subsequent compression of the electron pulses provide almost any desired compromise of energy resolution, signal strength, and time resolution. A proof-of-concept experiment on aluminum reveals an energy resolution of <3.5 eV (rms) at 70-keV after a drift distance of only 0.5 m. Simulations of a two-stage scheme reveal that pre-stretched pulses can be used to achieve <10 meV resolution, independent of the source's initial energy spread and limited only by the achievable THz field strength and measuring time.

Extreme THz fields from two-color filamentation of midinfrared laser pulses

Nonlinear THz photonics is probably the last frontier of nonlinear optics. The strength of both the electric and the magnetic fields of these ultrashort low-frequency light bunches opens the way to exciting science and applications. Progress in the field though is slow because of the deficiency in suitable sources. Here we show that two-color filamentation of midinfrared $3.9\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{m}$ laser pulses allows one to generate single-cycle THz pulses with multimillijoule energies and record conversion efficiencies. Moreover, the focused THz peak electric and magnetic fields reach values of GV/cm and kT, respectively, exceeding by far any available quasi-dc field source today. These fields enable extreme field science, including into other, relativistic phenomena. In addition, we elucidate the origin of this high efficiency, which is made up of several factors, including a mechanism where the harmonics produced by the midinfrared pulses strongly contribute to the field symmetry breaking and enhance the THz generation.

A terahertz pump mega-electron-volt ultrafast electron diffraction probe apparatus at the SLAC Accelerator Structure Test Area facility

We describe a new experimental setup capable of measuring structural dynamics following intense terahertz excitation. This system, developed at the SLAC Accelerator Structure Test Area facility, uses a high-energy ultrafast laser to produce intense terahertz pulses and femtosecond electron bunches that are accelerated to mega-electron-volt kinetic energies. The focused terahertz pulses have electric fields in excess of 600 kV/cm, and the resulting structural dynamics can be followed by electron diffraction. We also present some examples demonstrating its implementation where interactions between the THz pulses and the electron bunch are used to characterize the spatial and temporal characteristics of the THz field.