Modulating an electron beam with a frequency-beating laser enables a free-electron laser to generate high-power, narrowband terahertz pulses that can be continuously tuned from 7.8 to 30.8 terahertz.

This study investigates the formation of two types of laser-induced periodic surface structures (LIPSS) on amorphous Ge2Sb2Te5 (GST) using terahertz free-electron laser (THz-FEL) irradiation. The THz pulses used in this study consist of a macro pulse containing 150 micro pulses, each with a duration of 2 ps. The central wavelength λ of the THz radiation is 75 μm. By exposing GST to THz-FEL pulses at fluences up to 35 J/cm2, two distinct periodic structures were observed: low spatial frequency LIPSS (LSFL) and high spatial frequency LIPSS (HSFL). The LSFL formed parallel to the laser's polarization direction with a period of approximately λ/4.5. The HSFL, with a period λ/18, formed perpendicular to the polarization near the ablation edges. The periods of the generated HSFL and LSFL are 4 μm and 16 μm, respectively, and can be observed with an optical microscope. The period of the LSFL is generally consistent with λ/n (where n is the material’s refractive index), which is the approximate value expected based on previous reports in the optical region. This work contributes to a deeper understanding of LIPSS formation mechanisms under long-wavelength terahertz irradiation and suggests pathways for fine-tuned surface structuring applications using THz-FEL.

It has been shown that optical devices that use vanadium dioxide (VO2) metastructure allow us to flexibly modulate properties like the phase of the reflected or transmitted electromagnetic waves over a relatively wide range. VO2 is a strongly correlated material and exhibits an insulator-to-metal transition that can be induced by external stimuli such as heating. During an insulator-to-metal transition, metallic domains emerge and grow, and therefore the metallic domains in a VO2 layer behave like a tunable electrode. In this study, we found that the use of such a VO2 layer in a silicon-based diode results in an enhanced response to terahertz light that can be controlled via the metastructure. This device can make use of nonlinear conduction induced by the avalanche effect in silicon (Si) under terahertz pulse irradiation. We fabricated a VO2 thin-film with submicrometer metallic domains on a Si substrate and evaluated the change in the current through Si as the phase transition in VO2 progresses via heating. Near the phase-transition temperature, a relatively strong signal was observed even at low terahertz pulse energies. This phenomenon is attributed to the electric-field enhancement resulting from the formation of a network of metallic domains that contain nanogaps and thus enhances impact ionization in Si. The results suggest that devices with such VO2 layers inherently incorporate a certain amount of tunable electrical capacitance and inductance, which can provide novel device functionalities.

The efficient nonlinear optical effect due to free carrier in the infrared region and terahertz (THz) region has been studied in narrow band gap semiconductor. The widely accepted mechanism has been attributed to the momentum-dependent effective mass of the free carrier [1]. This mechanism leads to a real value of the nonlinear susceptibility. Later, several authors extended the calculation and showed that the nonlinear susceptibility is a complex number [2,3], in which the momentum-dependent scattering time contributes to the imaginary part.

Our study focused on the formation of laser-induced periodic surface structures (LIPSS) on Ge 2 Sb 2 Te 5 a phase change recording material, using a terahertz (THz) free electron laser with a fluence up to 35 J/cm 2 . Two types of LIPSS emerged: one running parallel and the other perpendicular to the direction of laser polarization. The spacings of the former and of the latter corresponded to approximately 1/4 wavelength and 1/18 wavelength, respectively.

We report on the third harmonic generation (THG) in InSb semiconductor irradiated by a terahertz (THz) free electron laser (FEL). The conversion of 4 THz (wavelength 70 µm) FEL outputs into its third harmonic 12 THz was observed. We found that by tuning the sample temperature to 360 K, high conversion efficiency up to 1% can be obtained and is the highest in the THz and FIR regions below 10 THz. We also discuss the observed intensity dependence of the THG with the nonlinear order lower than 3 when the pumping intensity was high.

Terahertz (THz) technology has recently seen significant advancements and applications in several fields, including ultrafast electric field measurement[1], semiconductor diagnostics[2], and magnetic material control[3]. Ablation phenomena in the terahertz range have been relatively underexplored due to the limited availability of powerful THz sources. However, recent progress in laser technology has opened up opportunities for the development of high-intensity THz sources, thereby facilitating investigations into ablation processes [4] and nonlinear effects [5]. These advancements have also made it possible to generate high-intensity pulses capable of inducing laser induced periodic surface structures (LIPSS) on materials, offering a promising avenue for surface nanostructuring. In this study, we investigated LIPPS formation on Ge2Sb2Te5 (GST), a material renowned for its phase-change recording properties [6], by the irradiation of a high-intensity THz free electron laser (FEL). We observed the emergence of two types of LIPSS through laser ablation. These LIPSS can be categorized as either low spatial frequency LIPSS (LSFL) or high spatial frequency LIPSS (HSFL) depending on their periodicity.

The nonlinear susceptibility in the terahertz region is expected to have a non-negligible imaginary part originating from the momentum-dependent scattering time of free carriers, but it has been scarcely reported. By utilizing an intense 4 THz beam from a terahertz free electron laser, we investigated the azimuth angle dependence of the third harmonic generation (THG) from semiconductors. The observed angular anisotropy of THG revealed the contribution of the imaginary part of the nonlinear susceptibility originating from the momentum-scattering time relation in addition to its real part originating from the band nonparabolicity. The results provide a deeper understanding of nonlinear optics in the terahertz region.

We demonstrate THz-induced martensitic transformation. We irradiated the intense THz pulses resonant for the lowest optical phonon on the surface of the partially stabilized zirconia plate and found that the clear evidence of martensitic transformation from tetragonal to monoclinic phase. We calculated the phonon dispersion in tetragonal zirconia and found the effective channel for the trigger of the transformation. Since the THz pulse excitation allows specific local shear deformation beyond thermal equilibrium, it will open new scheme of nonlinear phononics in condensed matters.

Martensitic crystal structures are usually obtained by rapid thermal quenching of certain alloys, which induces stress and subsequent shear deformation. Here, we demonstrate that it is also possible to intentionally excite a suitable transverse acoustic phonon mode to induce a local shear deformation. We irradiate the surface of a partially stabilized zirconia plate with intense terahertz pulses and verify martensitic transformation from the tetragonal to the monoclinic phases by Raman spectroscopy and the observed destructive spallation of the zirconia microcrystals. We calculate the phonon modes in tetragonal zirconia and determine the decay channel that triggers the transformation. The phonon mode required for the martensitic transformation can be excited via the Klemens process. Since terahertz pulses can induce a specific local shear deformation beyond thermal equilibrium, they can be used to elucidate phase transformation mechanisms with approaches based on nonlinear phononics.

Single-pulse extraction from a terahertz (THz) pulse train is demonstrated for a free-electron laser oscillator using the laser-activated semiconductor switching technique with gallium arsenide (GaAs). Using a GaAs wafer as the switching substrate and a titanium sapphire laser (Ti:sapphire laser), a single THz pulse can be extracted with a high contrast from the pulse train at intervals of 37 ns in a simple experimental setup due to a short decay time of the electron-hole plasma. For a single THz pulse with a duration of a few to a few tens of picoseconds and a nominal wavelength of 70 & mu;m, we achieve an extracted single pulse energy of 76 & mu;J.

This study presents the third harmonic generation (THG) in two-dimensional MoS 2 deposited on Si/SiO 2 substrate. Intense terahertz pulses from a free-electron laser were used as pump. The pump central frequency is 3.5 THz, and the THG peak is observed around 10 THz. The THG intensity dependence on excitation density follows the $I_{3 \omega} \propto I_{\omega}^{3}$ relation which further confirms the THG.

Laser-induced ablation of Ge2Sb2Te2 using terahertz vortex beam was investigated. Intense terahertz pulses from a free-electron laser were made to pass through a Tsurupica spiral phase plate to generate the vortex beam. Laser-induced periodic surface structures (LIPSS) were formed with a period quarter of the wavelength and an orientation parallel to the beam polarization. Due to the singularity of the vortex beam, LIPSS formation was not observed at the center of the irradiated spot.

We have successfully generated a highly efficient terahertz (THz) vortex beam using a THz free-electron laser incident to a spiral phase plate (SPP). The singularity at the center is apparently observed in the image of the transmitted beam after the SPP. We demonstrate the THz vortex beam-induced laser ablation on phase change materials, Ge2Sb2Te5 and GeTe/Sb2Te3. The ablation marks show an annular spatial form and modulated laser-induced periodic surface structures (LIPSS) pattern. These observations can be attributed to the torque of the THz vortex.

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 ErFeO 3 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.

We demonstrate photoacoustic wave propagation with a plane wavefront in liquid water using a terahertz (THz) laser pulse. The THz light can effectively generate the photoacoustic wave in water because of strong absorption via a stretching vibration mode of the hydrogen bonding network. The excitation of a large-area water surface irradiated by loosely focused THz light produces a plane photoacoustic wave. The photoacoustic wave generation and plane wave propagation are observed using a system with a THz free-electron laser and shadowgraph imaging. The plane photoacoustic wave is generated by incident THz light with a small radiant exposure of <; 1 mJ/cm 2 and delivered 600 times deeper than the penetration depth of THz light for water.

We demonstrate photoacoustic wave propagation with a plane wavefront in liquid water using a terahertz (THz) laser pulse. The THz light can effectively generate the photoacoustic wave in water because of strong absorption via a stretching vibration mode of the hydrogen bonding network. The excitation of a large-area water surface irradiated by loosely focused THz light produces a plane photoacoustic wave. This is in contrast with conventional methods using absorbers or plasma generation using near-infrared laser light. The photoacoustic wave generation and plane wave propagation are observed using a system with a THz free-electron laser and shadowgraph imaging. The plane photoacoustic wave is generated by incident THz light with a small radiant exposure of <1 mJ/cm(2) and delivered 600 times deeper than the penetration depth of THz light for water. The THz-light-induced plane photoacoustic wave offers great advantages to non-invasive operations for industrial and biological applications as demonstrated in our previous report (Yamazaki et al. in Sci Rep 10:9008, 2020).

The effect of terahertz (THz) radiation on deep tissues of human body has been considered negligible due to strong absorption by water molecules. However, we observed that the energy of THz pulses transmits a millimeter thick in the aqueous solution, possibly as a shockwave, and demolishes actin filaments. Collapse of actin filament induced by THz irradiation was also observed in the living cells under an aqueous medium. We also confirmed that the viability of the cell was not affected under the exposure of THz pulses. The potential of THz waves as an invasive method to alter protein structure in the living cells is demonstrated.