Intense Terahertz Field Research Articles

Simultaneous Probing of Electron Density and Temperature Dynamics Inside Air Plasma with Intense Terahertz Pulses

AbstractAir plasma induced by ultrafast laser pulses is an extraordinary source of electromagnetic waves, emitting microwave, terahertz (THz) radiation, and cavityless lasing in the near‐infrared and visible ranges. The temporal dynamics of the electron density have been revealed by optical pump‐probe techniques, while the evolution of the electron temperature remains elusive due to a lack of suitable methods. Here, it is demonstrated that the intense THz‐field‐enhanced fluorescence emission from the excited molecules of nitrogen is a novel tool that allows to explore the complex dynamics of the plasma density and electron temperature simultaneously. Two relaxation times of electrons in air plasma are observed and interpreted as a competition between the excitation of a triplet state by laser or THz‐field‐heated electrons and the dissociative recombination of nitrogen molecular ions. Based on the theoretical simulations, the tens of picoseconds relaxation process is attributed to the ultrafast temperature decrease, while the longer relaxation in the range of hundreds of picoseconds is ascribed to the decay of electron density. The temporal relaxation of both the electron density and temperature revealed by applying an intense THz field provides further insights into the laser‐air plasma interaction and will benefit the engineering of this exceptional source.

Investigation of spectro-temporal high-order sideband generation from transition metal dichalcogenides [Invited

High-order sideband generation (HSG) in semiconductors under intense terahertz fields has been extensively studied, because it provides essential information for studying ultrafast dynamics in strong-field-dressed quantum materials. In particular, transition metal dichalcogenides (TMDCs), characterized by their unique band structures, provide an exemplary semiconductor system to explore the influence of material band structure on strong-field-induced modulation of HSG. In this work, we investigate the spectro-temporally resolved HSG from different bulk TMDC materials. Our results reveal distinct temporal HSG spectra, which can be attributed to the different absorption behaviors of these materials. Simulations based on the strong-field approximation and Floquet theory can well reproduce the experimental observations. Our work also delves into the spectro-temporal interference that emerges when neighboring harmonic orders overlap in the HSG spectrum. This work enhances our understanding of high-order sideband dynamics in strong-field-dressed semiconductors, offering insights for applications in spectrum- and phase-resolved ultrafast measurements.

Time-of-flight detection of terahertz phonon-polariton

A polariton is a fundamental quasiparticle that arises from strong light-matter interaction and as such has attracted wide scientific and practical interest. When light is strongly coupled to the crystal lattice, it gives rise to phonon-polaritons (PPs), which have been proven useful in the dynamical manipulation of quantum materials and the advancement of terahertz technologies. Yet, current detection and characterization methods of polaritons are still limited. Traditional techniques such as Raman or transient grating either rely on fine-tuning of external parameters or complex phase extraction techniques. To overcome these inherent limitations, we propose and demonstrate a technique based on a time-of-flight measurement of PPs. We resonantly launch broadband PPs with intense terahertz fields and measure the time-of-flight of each spectral component with time-resolved second harmonic generation. The time-of-flight information, combined with the PP attenuation, enables us to resolve the real and imaginary parts of the PP dispersion relation. We demonstrate this technique in the van der Waals magnets NiI2 and MnPS3 and reveal a hidden magnon-phonon interaction. We believe that this approach will unlock new opportunities for studying polaritons across diverse material systems and enhance our understanding of strong light-matter interaction.

Prospects for the Implementation of an Intense Source of Ultraviolet Radiation Based on a Gas-Discharge Plasma in a Quasi-Optical Cavity Excited by a Pulse of Terahertz Radiation

An electrodynamic system is described that provides the creation of an electromagnetic wave field of high intensity at a frequency of 1 THz due to a combination of accumulation in time and compression in space of a wave pulse coming from an electron cyclotron maser (gyrotron). This system is based on the use of a three-mirror cavity consisting of two focusing mirrors and one flat corrugated Bragg-type photonic structure providing coupling between the gyrotron wave pulse and the operating wave of the cavity. The aim of this work is to use a “spot” of the intense terahertz field inside the cavity to provide a point-like plasma discharge in a gas stream injected into this spot; such a discharge can be a source of extreme ultraviolet radiation.

Vibrational dynamics of hydrogen molecules under intense THz waves

The physical properties and dynamics of molecules can be studied by the interaction between electromagnetic field and molecular system. The continuous development of terahertz technology provides a terahertz source capable of generating a sub-picosecond directional intense electric field. The generated intense-field terahertz wave has the same electric field intensity as the molecular local electric field environment, and on a sub-picosecond time scale of the directional electric field there can happen many ultrafast physical and chemical reactions. At present, the interaction between terahertz waves and molecules is limited mainly to the resonance interaction, that is, the molecules transition at different vibrational levels, caused through dipole interaction. In this work, based on the density functional theory calculation and the finite difference time domain solution method of Schrödinger equation, the intense non-resonance effect of intense terahertz wave electric field on hydrogen molecules is studied. The results show that under the action of intense terahertz wave sub-picosecond directional intense electric field, hydrogen molecule will produce an induced dipole moment. This dipole interacts with the external terahertz field, resulting in the fluctuation of proton probability density distribution and the change of vibration energy level population. Based on the non-resonant interaction between non-polar diatomic molecule hydrogen and intense terahertz wave, a unique way of producing the interaction between electromagnetic waves and molecules is displayed in this work, which is a method of studying the dynamics of non-polar molecules and molecules with weak polarity in intense terahertz field.

Dynamical amplification of electric polarization through nonlinear phononics in 2D SnTe

Ultrafast optical control of ferroelectricity using intense terahertz fields has attracted significant interest. Here we show that the nonlinear interactions between two optical phonons in SnTe, a two-dimensional in-plane ferroelectric material, enables a dynamical amplification of the electric polarization within subpicoseconds time domain. Our first-principles time-dependent simulations show that the infrared-active out-of-plane phonon mode, pumped to nonlinear regimes, spontaneously generates in-plane motions, leading to rectified oscillations in the in-plane electric polarization. We suggest that this dynamical control of ferroelectric material, by nonlinear phonon excitation, can be utilized to achieve ultrafast control of the photovoltaic or other nonlinear optical responses.

Influence of terahertz field on optical absorption coefficients and refractive index changes in double semi-V-shaped quantum wells

In this paper, the refractive index changes (RICs) and optical absorption coefficients (OACs) related to an intense terahertz (THz) field and the barrier widths in double semi-V-shaped quantum wells (DSVQW) are investigated theoretically by Kramers–Henneberger approximation and the finite difference method. The numerical results show that the THz field significantly widens the DSVQW and lowers the potential barrier. The DSVQW eventually turn into triple graded quantum wells under an intense THz field. As a result, the THz field essentially changes the energy gaps and dipole matrix, which determines the strength of the OACs and RICs. Both the OACs and the RICs are nonmonotonic functions of the applied THz field and have maximum and minimum value when the THz quiver motion parameters are close to the width parameter of a potential barrier. Based on the above analysis, we conclude that a THz field can effectively improve OACs and reduce RICs. Besides, the THz quiver motion parameters needed to achieve peak positions of the OACs and minimum position of the RICs at different barrier widths are proposed, which provides the possibility of designing various infrared optics devices.

Supercontinuum generation in OHQ-N2S organic crystal driven by intense terahertz fields.

A laser supercontinuum is generated by cross-phase modulation (XPM) driven by an intense terahertz (THz) field in organic crystal OHQ-N2S. In this highly nonlinear medium, the THz electric field induces a time-varying optical phase modulation, which causes a spectacular spectral broadening and shifting of a co-propagating near-infrared laser pulse. The effect is enabled by the large electro-optic coefficient, the low absorption, and the good velocity matching between the laser and the THz pulse over the OHQ-N2S crystal thickness. The XPM occurs when the THz field is aligned along the polar axis of the OHQ-N2S. The results display a promising pathway for ultrafast control of the spectral and temporal properties of laser pulses using THz stimuli.

Terahertz-driven phonon upconversion in SrTiO3

Direct manipulation of the atomic lattice using intense long-wavelength laser pulses has become a viable approach to create new states of matter in complex materials. Conventionally, a high frequency vibrational mode is driven resonantly by a mid-infrared laser pulse and the lattice structure is modified through indirect coupling of this infrared-active phonon to other, lower frequency lattice modulations. Here, we drive the lowest frequency optical phonon in the prototypical transition metal oxide SrTiO3 well into the anharmonic regime with an intense terahertz field. We show that it is possible to transfer energy to higher frequency phonon modes through nonlinear coupling. Our observations are carried out by directly mapping the lattice response to the coherent drive field with femtosecond x-ray pulses, enabling direct visualization of the atomic displacements.

Optimal wavelength for two-color filamentation-induced terahertz sources.

We theoretically study the generation of terahertz (THz) radiation by two-color filamentation of ultrashort laser pulses with different wavelengths. We consider wavelengths in the range from 0.6 to 10.6 μm, thus covering the whole range of existing and future powerful laser sources in the near, mid and far-infrared. We show how different parameters of two-color filaments and generated THz pulses depend on the laser wavelength. We demonstrate that there is an optimal laser wavelength for two-color filamentation that provides the highest THz conversion efficiency and results in generation of extremely intense single cycle THz fields.

Ultrafast nonlinear travel of hot carriers driven by high-field terahertz pulse

We aim to generate high-intensity terahertz (THz) electric fields and study nonlinear phenomena in GaAs and graphene to investigate their applications. To obtain a high-efficiency intense THz field, we employ the tilted pump-pulse front technique using a LiNbO3 crystal. With this technique, we obtain a THz field strength of over 300 kV cm−1. We investigate the hot-carrier dynamics in n- and p-type GaAs driven by high-field THz pulses. Although both samples show similar carrier concentrations, the nonlinear THz responses show different trends. Owing to hot-carrier generation, intervalley scattering is dominant in n-type GaAs, and intervalence band scattering is the main cause in p-type GaAs. In addition, we study the hot-carrier dynamics in graphene with the grain-size dependency. Although graphene has the same Fermi level regardless of the grain size, the THz responses are different for large- and small-grained graphene: charged impurity scattering in large-grained graphene and defect scattering in small-grained graphene. From these results, our study provides insights into high-speed electronics applications.

Significant Volume Expansion as a Precursor to Ablation and Micropattern Formation in Phase Change Material Induced by Intense Terahertz Pulses

With rapid advances occurring in terahertz (THz) radiation generation techniques, the interaction between matter and intense THz fields has become an important research topic. Among different types of THz radiation sources, the free electron laser (FEL) is a promising experimental tool that is expected to pave the way for new forms of material processing, control of phase transitions, and serve as a test bench for extreme operating conditions in high-speed small-size electrical and magnetic devices through the exploitation of strong THz electrical and magnetic fields without the presence of interband electronic excitation. In the current work, we irradiated Ge2Sb2Te5 phase change memory material with intense THz pulse trains from an FEL and observed THz-induced surface changes due to damage as a precursor to ablation and the formation of fine surface undulations whose spatial period is comparable to or slightly smaller than the wavelength of the excitation THz pulses in the material. The formation of undulations as well as the fact that no significant thermal effect was observed below the volume expansion threshold suggests that THz-induced effects mainly contributed to the observed changes. To the best of our knowledge, this is the first experimental observation of THz-induced undulations (so-called “LIPSS”), which are of potential importance for laser material processing.

Tunable electromagnetically induced transparency from a superconducting terahertz metamaterial

We demonstrate in this paper the tunable electromagnetically induced transparency (EIT) made from a superconducting (SC) niobium nitride (NbN) film induced by an intense terahertz (THz) field. As the variation of the incident THz field alters the intrinsic ohmic loss of the SC NbN film, the field-dependent transmittance is observed. To elaborate the role of the bright and dark modes, a hybrid coupling model is introduced to fit the experimental transmission spectra and extract the characteristic parameters of each mode. It is shown that the resonator for the bright mode is altered greatly due to strong direct coupling to the incident intense THz field, whereas the dark mode resonator has little interaction with the incident THz field via a weak near-filed coupling to the bright-mode resonator. This implies that we can partially control a mode or a part of metamaterial by introducing the intense THz field, which offers an effective manner to selectively control the electromagnetic property of the metamaterial. This work may bring many potential applications for the tunable EIT-like metamaterial.

Distortion reduction by introducing an initial birefringence in intense terahertz time-domain spectroscopy system

We present numerical and experimental demonstrations to reduce the distortion induced by intense terahertz field in a cross-polarized terahertz time-domain spectroscopy system. A quarter-wave plate is used to increase the initial birefringence of the probe beam. With an optimized angle, the distortion can be removed effectively, while the signal-to-noise ratio of the system is also improved.

Control of terahertz nonlinear transmission with electrically gated graphene metadevices

Graphene, which is a two-dimensional crystal of carbon atoms arranged in a hexagonal lattice, has attracted a great amount of attention due to its outstanding mechanical, thermal and electronic properties. Moreover, graphene shows an exceptionally strong tunable light-matter interaction that depends on the Fermi level - a function of chemical doping and external gate voltage - and the electromagnetic resonance provided by intentionally engineered structures. In the optical regime, the nonlinearities of graphene originated from the Pauli blocking have already been exploited for mode-locking device applications in ultrafast laser technology, whereas nonlinearities in the terahertz regime, which arise from a reduction in conductivity due to carrier heating, have only recently been confirmed experimentally. Here, we investigated two key factors for controlling nonlinear interactions of graphene with an intense terahertz field. The induced transparencies of graphene can be controlled effectively by engineering meta-atoms and/or changing the number of charge carriers through electrical gating. Additionally, nonlinear phase changes of the transmitted terahertz field can be observed by introducing the resonances of the meta-atoms.

Nonlinear radiation response of n-doped indium antimonide and indium arsenide in intense terahertz field**Project supported by the National Natural Science Foundation of China (Grant Nos. 11574105 and 61177095), the Natural Science Foundation of Hubei Province, China (Grant Nos. 2012FFA074 and 2013BAA002), the Wuhan Municipal Applied Basic Research Project, China (Grant No. 20140101010009), and the Fundamental Research Funds for the Central

The nonlinear radiation responses of two different n-doped bulk semiconductors: indium antimonide (InSb) and indium arsenide (InAs) in an intense terahertz (THz) field are studied by using the method of ensemble Monte Carlo (EMC) at room temperature. The results show that the radiations of two materials generate about 2-THz periodic regular spectrum distributions under a high field of 100 kV/cm at 1-THz center frequency. The center frequencies are enhanced to about 7 THz in InSb, and only 5 THz in InAs, respectively. The electron valley occupancy and the percentage of new electrons excited by impact ionization are also calculated. We find that the band nonparabolicity and impact ionization promote the generation of nonlinear high frequency radiation, while intervalley scattering has the opposite effect. Moreover, the impact ionization dominates in InSb, while impact ionization and intervalley scattering work together in InAs. These characteristics have potential applications in up-convension of THz wave and THz nonlinear frequency multiplication field.

Anisotropic high-field terahertz response of free-standing carbon nanotubes

We demonstrate that unidirectionally aligned, free-standing multi-walled carbon nanotubes (CNTs) exhibit highly anisotropic linear and nonlinear terahertz (THz) responses. For the polarization parallel to the CNT axis, strong THz pulses induce nonlinear absorption in the quasi-one-dimensional conducting media, while no nonlinear effect is observed in the perpendicular polarization configuration. Time-resolved measurements of transmitted THz pulses and a theoretical analysis of the data reveal that intense THz fields enhance permittivity in carbon nanotubes by generating charge carriers.

Intense terahertz field effects on photoexcited carrier dynamics in gated graphene

We study nonlinear effects of intense terahertz (THz) field on photoexcited carrier dynamics in gated monolayer graphene. By employing optical-pump/intense-THz-probe spectroscopy on lightly doped graphene, we observe a crossover from negative to positive photo-induced THz differential transmission as the THz probe field is increased. We attribute this qualitative change in the response to a crossover from a regime where the photo-induced increase in the carrier density dominates the differential response to one where a THz-field-induced increase in the scattering rate dominates.

Scheme to extend the range of electro-optic sampling of an intense terahertz field

Generation of intense terahertz (THz) radiation requires an efficient method to determine the field intensity with a wide range of intensity response and accuracy. A new scheme is proposed to extend the limited response range of available time-domain spectroscopy measurement. The method applies two pieces of thin ZnTe crystals with a fixed angle between the (001) axes. At the optimized angle, the linear response range of the THz field can reach 31 MV/cm.

Impact ionization in high resistivity silicon induced by an intense terahertz field enhanced by an antenna array

We report on the observation of ultrafast impact ionization and carrier generation in high resistivity silicon induced by intense subpicosecond terahertz transients. Local terahertz peak electric fields of several MV cm−1 are obtained by field enhancement in the near field of a resonant metallic antenna array. The carrier multiplication is probed by the frequency shift of the resonance of the antenna array due to the change of the local refractive index of the substrate. Experimental results and simulations show that the carrier density in silicon increases by over seven orders of magnitude in the presence of an intense terahertz field. The enhancement of the resonance shift for illumination from the substrate side in comparison to illumination from the antenna side is consistent with our prediction that the back illumination is highly beneficial for a wide range of nonlinear processes.