Intense Terahertz Pulses Research Articles

Phase-controlled, second-harmonic-optimized terahertz pulse generation in nitrogen by infrared two-color laser pulses

Broadband terahertz radiation can be efficiently produced by mixing laser pulses of different colors in the mid-infrared (MIR) and longwave-infrared (LWIR) spectral region. In this paper, we report on a numerical investigation of ultrashort terahertz pulse generation from plasmas created in nitrogen gas by two-color laser pulses with the fundamental laser pulse wavelength between 2.15 and 15.15 µm, in order to explore the efficiency of the terahertz pulse generation process. The results show that the electron acceleration efficiency increases monotonically with the fundamental laser pulse wavelength. The most intense terahertz pulse generation is observed at 12.30 µm with four optical-cycle laser pulses with 2.5 GW peak power. The results show that the terahertz pulse generation with a MIR laser is one order of magnitude and with a LWIR laser is two orders of magnitude more efficient than the terahertz pulse generation with Ti:Sapphire lasers using the exact same pulse parameters. The terahertz pulse generation efficiency is also known to be very sensitive to the relative phase between the components of the two-color laser pulses. One of the most useful tools to control the relative phase and optimize the terahertz pulse intensity is thin dielectric plates. It has been shown that alkaline halides and alkaline earth halides have suitable optical properties for the relative phase control for efficient terahertz pulse generation in the MIR spectral range.

Second-harmonic generation in zinc blende crystals under combined action of femtosecond optical and strong terahertz fields

The influence of a short intense (with an electric field strength up to 250 kV cm−1) terahertz (THz) pulse on the generation of second harmonic (SH) of Ti : sapphire laser radiation in crystals of zinc blende type (InAs and GaAs), characterised by nonzero bulk quadratic susceptibility, is investigated. It is experimentally shown for InAs(100) that, in the case of s-polarised first and second harmonics, an application of s-polarised THz field changes significantly the SH signal. The THz field-induced azimuthal dependence of the SH signal is in good agreement with the results of theoretical calculation within a phenomenological approach. The dependence of the SH signal on the delay time between the optical and THz pulses is investigated. This dependence for the GaAs crystal repeats the envelope of the THz pulse intensity, whereas in the case of InAs crystal there is a significant discrepancy, caused by the nonlinear dynamics of strong THz field in InAs.

Reconfiguration of magnetic domain structures of ErFeO3 by intense terahertz free electron laser pulses

Understanding the interaction between intense terahertz (THz) electromagnetic fields and spin systems has been gaining importance in modern spintronics research as a unique pathway to realize ultrafast macroscopic magnetization control. In this work, we used intense THz pulses with pulse energies in the order of 10 mJ/pulse generated from the terahertz free electron laser (THz-FEL) to irradiate the ferromagnetic domains of ErFeO3 single crystal. It was found that the domain shape can be locally reconfigured by irradiating the THz − FEL pulses near the domain boundary. Observed domain reconfiguration mechanism can be phenomenologically understood by the combination of depinning effect and the entropic force due to local thermal gradient exerted by terahertz irradiation. Our finding opens up a new possibility of realizing thermal-spin effects at THz frequency ranges by using THz-FEL pulses.

Coherent band-edge oscillations and dynamic longitudinal-optical phonon mode splitting as evidence for polarons in perovskites

The coherence of collective modes, such as phonons, and their modulation of the electronic states are long sought in complex systems, which is a cross-cutting issue in photovoltaics and quantum electronics. In photovoltaic cells and lasers based on metal halide perovskites, the presence of polaronic coupling, i.e., photocarriers dressed by the macroscopic motion of charged lattice, assisted by terahertz (THz) longitudinal optical (LO) phonons, has been intensely studied yet still debated. This may be key for explaining the remarkable properties of the perovskite materials, e.g., defect tolerance, long charge lifetimes and diffusion length. Here we use the intense single-cycle THz pulse with the peak electric field up to $E_{THz}=$1000\,kV/cm to drive coherent band-edge oscillations at room temperature in CH$_3$NH$_3$PbI$_3$. We reveal the oscillatory behavior dominantly to a specific quantized lattice vibration mode at $\omega_{\mathrm{LO}}\sim$4 THz, being both dipole and momentum forbidden. THz-driven coherent dynamics exhibits distinguishing features: the room temperature coherent oscillations at $\omega_{\mathrm{LO}}$ longer than 1 ps in both single crystals and thin films; the {\em mode-selective} modulation of different band edge states assisted by electron-phonon ($e$-$ph$) interaction; {\em dynamic mode splitting} controlled by temperature due to entropy and anharmonicity of organic cations. Our results demonstrate intense THz-driven coherent band-edge modulation as a powerful probe of electron-lattice coupling phenomena and provide compelling implications for polaron correlations in perovskites.

Terahertz quantum plasmonics at nanoscales and angstrom scales

Abstract Through the manipulation of metallic structures, light–matter interaction can enter into the realm of quantum mechanics. For example, intense terahertz pulses illuminating a metallic nanotip can promote terahertz field–driven electron tunneling to generate enormous electron emission currents in a subpicosecond time scale. By decreasing the dimension of the metallic structures down to the nanoscale and angstrom scale, one can obtain a strong field enhancement of the incoming terahertz field to achieve atomic field strength of the order of V/nm, driving electrons in the metal into tunneling regime by overcoming the potential barrier. Therefore, designing and optimizing the metal structure for high field enhancement are an essential step for studying the quantum phenomena with terahertz light. In this review, we present several types of metallic structures that can enhance the coupling of incoming terahertz pulses with the metals, leading to a strong modification of the potential barriers by the terahertz electric fields. Extreme nonlinear responses are expected, providing opportunities for the terahertz light for the strong light–matter interaction. Starting from a brief review about the terahertz field enhancement on the metallic structures, a few examples including metallic tips, dipole antenna, and metal nanogaps are introduced for boosting the quantum phenomena. The emerging techniques to control the electron tunneling driven by the terahertz pulse have a direct impact on the ultrafast science and on the realization of next-generation quantum devices.

Experimental system for studying bioeffects of intense terahertz pulses with electric field strength up to 3.5 MV/cm

Terahertz (THz) waves can influence a diverse range of cellular processes. The use of high-power THz sources in biological studies may lead to major advances in understanding biological systems and help to determine safe exposure levels for existing THz technologies. We are devoted to the development of an experimental system for irradiating cells with intense broadband THz pulses. Subpicosecond pulses of THz radiation with intensities of 32 GW / cm2 and electric field strength up to 3.5 MV/cm are obtained by optical rectification, using an OH1 organic crystal, of near-infrared femtosecond pulses generated by a Cr:forsterite laser. The system has been developed to allow cells to be kept in suitable conditions for long-term exposure and to be irradiated with THz pulses in single-point mode as well as in scanning mode. The transmission in the THz region of various plastic dishes for cell culture is estimated.

Spectral dependencies of terahertz emission from femtosecond laser excited surfaces of germanium crystals

Pulsed terahertz emission excitation spectra from germanium crystals are being presented. The most intense terahertz pulses from germanium crystals are emitted at quanta energies coinciding with technologically significant telecommunication wavelengths. The terahertz generation mechanisms are an interplay of the photocurrent surge in the surface electric field and the photo-Dember effect. Remarkably, the terahertz emission is also observed at quanta energies below the direct bandgap of this material even when photoexcited at a surface normal. This is the result of a broken symmetry of effective electron mass in the L valleys.

Terahertz Optomagnetism: Nonlinear THz Excitation of GHz Spin Waves in Antiferromagnetic FeBO_{3}.

A nearly single cycle intense terahertz (THz) pulse with peak electric and magnetic fields of 0.5 MV/cm and 0.16T, respectively, excites both modes of spin resonances in the weak antiferromagnet FeBO_{3}. The high frequency quasiantiferromagnetic mode is excited resonantly and its amplitude scales linearly with the strength of the THz magnetic field, whereas the low frequency quasiferromagnetic mode is excited via a nonlinear mechanism that scales quadratically with the strength of the THz electric field and can be regarded as a THz inverse Cotton-Mouton effect.THz optomagnetism is shown to be more energy efficient than similar effects reported previously for the near-infrared spectral range.

Impact of damping on the superconducting gap dynamics induced by intense terahertz pulses

We investigate the interplay between coherent gap dynamics and damping in superconductors taken out of equilibrium by strong optical pulses with subgap terahertz frequencies. A semiphenomenological formalism is developed to include the damping within the electronic subsystem that arises from effects beyond Bardeen-Cooper-Schrieffer mean-field theory, such as interactions between Bogoliubov quasiparticles and decay of the Higgs mode. These processes, conveniently expressed as longitudinal ${T}_{1}$ and transverse ${T}_{2}$ relaxation times in the standard pseudospin language for superconductors, cause the gap amplitude to be suppressed after the pulse is turned off, but before the timescale where thermalization occurs due to coupling to the lattice. We show that our model quantitatively captures the experimental gap dynamics reported here of NbN and ${\mathrm{Nb}}_{3}\mathrm{Sn}$ through the picosecond timescale.

Intense terahertz pulse-induced impact ionization and electron dynamics in InAs

Electron dynamics and impact ionization are investigated by Monte Carlo method in n-type InAs exposed to intense ultrashort terahertz pulse at 300 K temperature. The sequence of hot electron transfer between the Γ and upper L and X valleys is established: hot electrons of the Γ valley transfer firstly to the L valley, and then they are scattered to the X valley. Electron population in the valleys and characteristic time of their redistribution between the Γ, L and X valleys is investigated in strong electric fields. It is found that the threshold electric field of impact ionization increases with shorter THz pulses. The process of impact ionization is shown to be the dominant energy loss mechanism of hot electrons having energy larger than the threshold energy of impact ionization. Good agreement between the results of calculation and available experimental data is demonstrated.

Review of Intense Terahertz Radiation from Relativistic Laser-Produced Plasmas

Recent years have witnessed a rapid development of sources for intense terahertz (THz) pulses and an increasing number of their applications. The advent of laser pulses with intensities in excess of $10^{18}$ W/cm2 offers an immense opportunity for generating extremely energetic THz pulses from laser-produced plasmas. This paper will review recent progress in the study of THz radiation generated from relativistically intense laser–plasma interactions, mainly focusing on the underlying THz generation models under different laser and plasma parameters, albeit still not yet fully explored. Various THz generation scenarios are discussed associated with laser-excited waves in plasmas, the transport and emission of laser-accelerated electrons, and structured targets.

Anisotropic ultrafast optical response of terahertz pumped graphene

We have measured the ultrafast anisotropic optical response of highly doped graphene to an intense single cycle terahertz pulse. The time profile of the terahertz-induced anisotropy signal at 800 nm has minima and maxima repeating those of the pump terahertz electric field modulus. It grows with increasing carrier density and demonstrates a specific nonlinear dependence on the electric field strength. To describe the signal, we have developed a theoretical model that is based on the energy and momentum balance equations and takes into account optical phonons of graphene and the substrate. According to the theory, the anisotropic response is caused by the displacement of the electronic momentum distribution from zero momentum induced by the pump electric field in combination with polarization dependence of the matrix elements of interband optical transitions.

Temporal and spectral fingerprints of ultrafast all-coherent spin switching.

Future information technology demands ever-faster, low-loss quantum control. Intense light fields have facilitated milestones along this way, including the induction of novel states of matter1-3, ballistic acceleration of electrons4-7 and coherent flipping of the valley pseudospin8. These dynamics leave unique 'fingerprints', such as characteristic bandgaps or high-order harmonic radiation. The fastest and least dissipative way of switching the technologically most important quantum attribute-the spin-between two states separated by a potential barrier is to trigger an all-coherent precession. Experimental and theoretical studies withpicosecond electric and magnetic fields have suggested this possibility9-11, yet observing the actual spindynamics has remained out of reach. Here we show that terahertz electromagnetic pulses allow coherent steering of spins over a potential barrier, and we report the corresponding temporal and spectral fingerprints. This goal is achieved by coupling spins in antiferromagnetic TmFeO3 (thulium orthoferrite) with the locally enhanced terahertz electric field of custom-tailored antennas. Within their duration of one picosecond, the intense terahertz pulses abruptly change the magnetic anisotropy and trigger a large-amplitude ballistic spin motion. A characteristic phase flip, an asymmetric splitting of the collective spinresonance and a long-lived offset of the Faraday signal are hallmarks of coherent spin switching into adjacent potential minima, in agreement with numerical simulations. The switchable states can be selected by an external magnetic bias. The low dissipation and the antenna's subwavelength spatial definition could facilitate scalable spin devices operating at terahertz rates.

Overcoming the thermal regime for the electric-field driven Mott transition in vanadium sesquioxide

The complex interplay among electronic, magnetic and lattice degrees of freedom in Mott-Hubbard materials leads to different types of insulator-to-metal transitions (IMT) which can be triggered by temperature, pressure, light irradiation and electric field. However, several questions remain open concerning the quantum or thermal nature of electric field-driven transition process. Here, using intense terahertz pulses, we reveal the emergence of an instantaneous purely-electronic IMT in the Mott-Hubbard vanadium sequioxide (V2O3) prototype material. While fast electronics allow thermal-driven transition involving Joule heating, which takes place after tens of picoseconds, terahertz electric field is able to induce a sub-picosecond electronic switching. We provide a comprehensive study of the THz induced Mott transition, showing a crossover from a fast quantum dynamics to a slower thermal dissipative evolution for increasing temperature. Strong-field terahertz-driven electronic transition paves the way to ultrafast electronic switches and high-harmonic generation in correlated systems.

Direct excitation of molecular vibrational motion by intense broadband THz pulses

We theoretically investigate molecular vibrational dynamics driven by intense and broadband terahertz (THz) pulses. The light–nuclei interaction is derived from a fully quantum-mechanical minimal coupling Hamiltonian to analyse the direct excitation of vibrational modes by the THz pulses. We show that for diatomic molecules a ‘vibrational moment’ can be defined in a manner similar to the dipole moment and that it has non-zero values only for heteronuclear diatomic molecules. Taking ultra-cold heteronuclear diatomic molecules as examples, we show that vibrational-mode excitation by THz pulse irradiation occurs in an almost stepwise manner, especially for low vibrational modes. A coherent control method using a frequency-controlled THz pulse sequence is also proposed.

Observation of crossover from intraband to interband nonlinear terahertz optics.

In this Letter, we investigate the nonlinear effects of extremely intense few-cycle terahertz (THz) pulses (generated from the organic crystal 4-NN, NN-dimethylamino-4'4'-N'N'-methyl-stilbazolium 2, 4, 6 trimethylbenzenesulfonate, with peak electrical fields of a few MV/cm) on the carrier dynamics in n-doped semiconductor thin film In0.53Ga0.47As. By performing open-aperture Z-scan measurements and recording the transmitted THz energy through semiconductor sample, we observed a strong THz absorption bleaching effect at high fields, followed by an absorption enhancement at even higher fields. We attribute our observations to a crossover from pure intraband carrier dynamics to an interplay between intraband carrier heating and interband carrier generations.

Proposal for CEP measurement based on terahertz air photonics

Single-shot carrier envelope phase (CEP) measurement is a challenge in the research field of ultrafast optics. We theoretically investigate how an intense terahertz pulse modulates second harmonic emission (SH) from a gas plasma induced by a few-cycle laser pulse (FCL). Results show that the modulation quantity of SH intensity has a cosinoidal dependence on the CEP of FCL pulses, based on which we propose a low energy, all-optical method for single-shot CEP measurements via using a known intense terahertz pulse. Moreover, we propose an experimental realization.

Nonlinear transfer of an intense few-cycle terahertz pulse through opaque n -doped Si

Intense few cycle terahertz pulses exhibit complex non-linear behavior under interaction with heavily n-doped Si. Fast increase in the transmission of a 700 fs pulse (central frequency 1.5 THz) through the Si sample (low field transmission of 0:02 %) saturates at 8 % for the external field of 5 MV/cm and then drops twofold at 20 MV/cm. An electro-optical sampling measurements revealed formation of a single cycle terahertz pulse at this field due to formation of a thin ionized layer by the first intense oscillation of the terahertz field.

Zener Tunneling Breakdown in Phase-Change Materials Revealed by Intense Terahertz Pulses.

We have systematically investigated the spatial and temporal dynamics of crystallization that occur in the phase-change material Ge_{2}Sb_{2}Te_{5} upon irradiation with an intense terahertz (THz) pulse. THz-pump-optical-probe spectroscopy revealed that Zener tunneling induces a nonlinear increase in the conductivity of the crystalline phase. This fact causes the large enhancement of electric field associated with the THz pulses only at the edge of the crystallized area. The electric field concentrating in this area causes a temperature increase via Joule heating, which in turn leads to nanometer-scale crystal growth parallel to the field and the formation of filamentary conductive domains across the sample.

Terahertz microscopy assisted by semiconductor nonlinearities.

Terahertz (THz) imaging is currently based on linear effects, but there is great interest on how nonlinear effects induced by terahertz radiation could be exploited to provide extra information that is unobtainable by conventional imaging schemes. In particular, at field strengths on the order of 100 kV cm-1 to 1 MV cm-1, transmission properties inside semiconductor materials are largely affected at the picosecond time-scale, which raise the prospect of interesting nonlinear imaging applications at THz frequencies. Here, we experimentally investigate a method to map the two-dimensional nonlinear near-field distribution of an intense THz pulse passing through a thin film-doped semiconductor. By inserting a metamaterial structure between the electro-optic sensor and the doped film, the nonlinear near-field dynamics shows a different and enhanced contrast of the sample when compared to its linear counterpart.