Terahertz Pulse Emission Research Articles

Determination of the terahertz pulse emitting dipole orientation by terahertz emission measurements

Terahertz pulse emission from the photoexcited semiconductor surface heavily depends on the orientation of the photo-induced electric dipole. Two methods to determine the orientation of the terahertz pulse emitting dipole have been demonstrated. Method I relies on the measurement of THz emission dependencies on the angle of incidence in the transmission geometry, while method II is based on the measurements of THz emission in the reflection geometry. Theoretical reasoning for both of these methods has been presented. Both of these methods have been tested with a semi-insulating GaAs substrate using an external magnetic field in order to change the tilt angle of the electric dipole. The investigation presented in this work leads to an assumption that the magnetic field induced change in the dipole tilt angle is proportional to the mobility of electrons. Thus, the suggested method may become a promising tool for testing the quality of substrates and epitaxial layers.

Terahertz pulse emission from GaInAsBi

Quaternary GaInAsBi alloy epitaxial layers were grown on InP substrates with 6% Bi. It was found that the thick layers remain fully strained. The measured carrier lifetimes were of the order of a few picoseconds. The terahertz (THz) emission was investigated using a GaInAsBi layer as an unbiased surface emitter and as a substrate for photoconductive antenna. It was observed that fabricated THz emitters were sensitive to the optical pulses with wavelengths longer than 2 μm. The demonstrated spectral characteristics of THz pulses obtained when using an Er-doped fiber laser for photoexcitation were comparable with those observed in other emitters used for THz-time-domain spectroscopy systems.

Terahertz generation measurements of multilayered GeTe-Sb2Te3 phase change materials.

Multilayered structures of GeTe and Sb2Te3 phase change material, also referred to as interfacial phase change memory (iPCM), provide superior performance for nonvolatile electrical memory technology in which the atomically controlled structure plays an important role in memory operation. Here, we report on terahertz (THz) wave generation measurements. Three- and 20-layer iPCM samples were irradiated with a femtosecond laser, and the generated THz radiation was observed. The emitted THz pulse was found to be always p polarized independent of the polarization of the excitation pulse. Based on the polarization dependence as well as the flip of the THz field from photoexcited Sb2Te3 and Bi2Te3, the THz emission process can be attributed to the surge current flow due to the built-in surface depletion layer formed in p-type semiconducting iPCM materials.

Ultrafast Zero-Bias Surface Photocurrent in Germanium Selenide: Promise for Terahertz Devices and Photovoltaics.

Theory predicts that a large spontaneous electric polarization and concomitant inversion symmetry breaking in GeSe monolayers result in a strong shift current in response to their excitation in the visible range. Shift current is a coherent displacement of electron density on the order of a lattice constant upon above-bandgap photoexcitation. A second-order nonlinear effect, it is forbidden by the inversion symmetry in the bulk GeSe crystals. Here, we use terahertz (THz) emission spectroscopy to demonstrate that ultrafast photoexcitation with wavelengths straddling both edges of the visible spectrum, 400 and 800 nm, launches a shift current in the surface layer of a bulk GeSe crystal, where the inversion symmetry is broken. The direction of the surface shift current determined from the observed polarity of the emitted THz pulses depends only on the orientation of the sample and not on the linear polarization direction of the excitation. Strong absorption by the low-frequency infrared-active phonons in the bulk of GeSe limits the bandwidth and the amplitude of the emitted THz pulses. We predict that reducing GeSe thickness to a monolayer or a few layers will result in a highly efficient broadband THz emission. Experimental demonstration of THz emission by the surface shift current in bulk GeSe crystals puts this 2D material forward as a candidate for next-generation shift current photovoltaics, nonlinear photonic devices, and THz sources.

Highly Efficient Spintronic Terahertz Emitter Enabled by Metal–Dielectric Photonic Crystal

AbstractSpintronic terahertz (THz) emitter provides the advantages such as apparently broader spectrum, significantly lower cost, and more flexibility compared with the commercial THz emitters, and thus attracts great interest recently. In past few years, efforts have been made in optimizing the material composition and structure geometry, and the conversion efficiency has been improved close to that of ZnTe crystal. One of the drawbacks of the current designs is the rather limited laser absorption—more than 50% energy is wasted and the conversion efficiency is thus limited. Here, a novel device that fully utilizes the laser intensity and significantly improves the conversion efficiency is theoretically proposed and experimentally demonstrated. The device, which consists of a metal–dielectric photonic crystal structure, utilizes the interference between the multiple scattering waves to simultaneously suppress the reflection and transmission of the laser, and to reshape the laser field distributions. The experimentally detected laser absorption and THz generation show one‐to‐one correspondence with the theoretical calculations. The strongest THz pulse emission that presents a 1.7 times improvement compared to the currently designed spintronic emitter is achieved. This work opens a new pathway to improve the performance of spintronic THz emitter from the perspective of optics.

Direct Measurements of Terahertz Meta-atoms with Near-Field Emission of Terahertz Waves

We present the direct measurements of terahertz meta-atoms, an elementary unit of metamaterials, by using locally generated terahertz waves in the near-field region. In contrast to a conventional far-field terahertz spectroscopy or imaging, our technique features the localized emission of coherent terahertz pulses on a sub-wavelength scale, which has a potential for visualizing details of dynamics of each meta-atom. The obtained data show the near-field coupling among the meta-atoms and the impact of the electric field distribution from the excited meta-atom to neighbor meta-atoms. The observable LC resonance response is enhanced with an increase of numbers of meta-atoms. Furthermore, our approach also has a potential for visualizing the individual mode of meta-atom at different terahertz irradiation spots. These data can help us to understand the important role of the meta-atom in metamaterials and develop the novel terahertz components and devices such as active terahertz metamaterial and compact, high-sensitive bio-sensor devices.

Ultrabroadband single-cycle terahertz pulses with peak fields of 300 kV cm−1 from a metallic spintronic emitter

We explore the capabilities of metallic spintronic thin-film stacks as a source of intense and broadband terahertz electromagnetic fields. For this purpose, we excite a W/CoFeB/Pt trilayer (thickness of 5.6 nm) on a large-area glass substrate (diameter of 7.5 cm) by a femtosecond laser pulse (energy 5.5 mJ, duration 40 fs, and wavelength 800 nm). After focusing, the emitted terahertz pulse is measured to have a duration of 230 fs, a peak field of 300 kV cm−1, and an energy of 5 nJ. In particular, the waveform exhibits a gapless spectrum extending from 1 to 10 THz at 10% of its amplitude maximum, thereby facilitating nonlinear control over matter in this difficult-to-reach frequency range on the sub-picosecond time scale.

Controllable Terahertz Radiation from a Linear-Dipole Array Formed by a Two-Color Laser Filament in Air.

We demonstrate effective control on the carrier-envelope phase and angular distribution as well as the peak intensity of a nearly single-cycle terahertz pulse emitted from a laser filament formed by two-color, the fundamental and the corresponding second harmonics, femtosecond laser pulses propagating in air. Experimentally, such control has been performed by varying the filament length and the initial phase difference between the two-color laser components. A linear-dipole-array model, including the descriptions of both the generation (via laser field ionization) and propagation of the emitted terahertz pulse, is proposed to present a quantitative interpretation of the observations. Our results contribute to the understanding of terahertz generation in a femtosecond laser filament and suggest a practical way to control the electric field of a terahertz pulse for potential applications.

THz pulse emission from InAs-based epitaxial structures grown on InP substrates

Undoped InAs and InAs p-n junction epitaxial layers were grown on (100)-cut InP substrates with molecular beam epitaxy. The lattice difference between the substrate and the InAs layers was matched with a graded AlInAs buffer layer. The alloy composition, structural characteristics and carrier mobility of the structures were determined from the high-resolution x-ray diffraction, atomic force microscopy and Hall-effect measurements, respectively. The optical parameters of the layers were characterized by the emission of terahertz (THz) pulses when the samples were illuminated with femtosecond laser pulses. It has been found that the built-in electric field in the p-n junction enhances the THz emission. Registering THz signals in the quasi-reflection direction, the p-n junction emits more intense radiation in comparison to an undoped bulk InAs. At excitation wavelengths >1.8 μm the InAs p-n junction provides stronger THz pulses than those from (111)-cut p-InAs, the best surface THz emitter known to date. The epitaxial layers were also exposed to a constant magnetic field from neodymium permanent magnets, which further enhances THz emission and allows registering THz radiation in the line-of-sight terahertz time-domain-spectroscopy geometry.

From near IR to terahertz photon emission in the LaF3 crystals heavily doped by Nd3+; the use of the Dicke and the Purcell effects

We discuss the novel idea for direct terahertz pulse emission as a result of implementation of Purcell and Dicke effects simultaneously in the Nd3+ heavily doped LaF3 crystals after femtosecond near IR laser excitation with high power density at transitions between crystal-field (Stark) levels of the Nd3+ manifolds of the appropriate energy.

Study of photoexcited-carrier dynamics in GaAs photoconductive switches using dynamic terahertz emission microscopy

We propose dynamic terahertz (THz) emission microscopy (DTEM) to visualize temporal–spatial dynamics of photoexcited carriers in electronic materials. DTEM utilizes THz pulses emitted from a sample by probe pulses irradiated after pump pulse irradiation to perform time-resolved two-dimensional mapping of the THz pulse emission, reflecting various carrier dynamics. Using this microscopy, we investigated carrier dynamics in the gap region of low-temperature-grown GaAs and semi-insulating GaAs photoconductive switches of the identical-dipole type. The observed DTEM images are well explained by the change in the electric potential distribution between the electrodes caused by the screening effect of the photoexcited electron-hole pairs.

Emission of strong Terahertz pulses from laser wakefields in weakly coupled plasma

The present paper discusses the laser plasma interaction for the wakefield excitation and the role of external magnetic field for the emission of Terahertz radiation in a collisional plasma. Flat top lasers are shown to be more appropriate than the conventional Gaussian lasers for the effective excitation of wakefields and hence, the generation of strong Terahertz radiation through the transverse component of wakefield.

Influences of different gases on the terahertz radiation based on the application of two-color laser pulses

In this work, using a two-dimensional Particle In Cell-Monte Carlo Collision simulation method, a comparative study is performed on the influences of different types of atomic and molecular gases at various background gas pressures on the generation of broadband and intense Terahertz (THz) radiation via the application of two-color laser pulses. These two modes are focused into Argon (Ar), Xenon (Xe), Nitrogen (N2), Oxygen (O2), and air as the background gaseous media and the plasma channel is created. It is observed that the THz radiation emission dramatically changes due to the propagation effects. A wider THz pulse is emitted from the formed plasma channel at the higher gas pressures. The significant effects of the propagation features of the emitted THz pulse on its energy at the longer lengths of the plasma channel are observed.

Emission of terahertz pulses from vanadium dioxide films undergoing metal–insulator phase transition

This paper describes research on the optics of functional materials, which can change their dielectric properties according to their function. Vanadium dioxide is a good example of such a material where the insulator-to-metal phase transition offers the possibility to control dielectric properties and to use them as a triggering element for photonic applications in the wide spectral range from optical to terahertz frequencies. We observed emission of terahertz (THz) radiation from VO2 films in insulating and conductive phase states under femtosecond pulse irradiation. We found that the efficiency of THz emission increases up to 30 times after the insulator-to-metal phase transition. This process occurs in thin films while it is fundamentally forbidden in the bulk material, and polarization analysis of the emitted radiation reveals the crucial importance of interface contributions. The properties of the THz radiation emitted by VO2 are determined by displacement photocurrents at the VO2–air and VO2–substrate interfaces induced by the incident laser light. In each phase state the contributions of the two boundaries are different. Properties of the effective dielectric susceptibility χ(2) tensor for the insulating phase were defined. In demonstrating the conversion of optical into THz radiation in VO2 films, we found that fundamental symmetry restrictions are not applicable to problems of nonlinear optics of thin films.

Investigation of Terahertz Emission from BiVO4/Au Thin Film Interface

We demonstrate emission of terahertz pulses from a BiVO4/Au thin film interface, illuminated with femtosecond laser pulses. Based on the experimental observations, we propose that the most likely cause of the THz emission is the Photo-Dember effect caused by the standing wave intensity distribution formed at the BiVO4/Au interfaces.

Thickness dependent terahertz emission from cobalt thin films.

When cobalt thin films are illuminated with femtosecond laser pulses, we observe the emission of terahertz pulses. For a cobalt film thickness less than about 40 nm, the THz electric field direction rotates when the sample is rotated about the surface normal. This azimuthal angle-dependent emission is consistent with the assumption that laser-induced changes in an in-plane magnetization are responsible for the emission. For thicker layers, however, we observe the development of an azimuthal angle-independent contribution to the THz emission which we attribute to laser-induced changes in an out-of-plane magnetization component. We show that the relative contribution of this component grows when the cobalt film thickness increases. Our observations are supported by magnetic force microscopy measurements which show that for film thicknesses below 40 nm, the magnetization is predominantly in-plane whereas for thicknesses larger than 40 nm, an out-of-plane magnetization component develops.

Enhanced Terahertz Emission from Schottky Junctions Using Plasmonic Nanostructures

We present measurements of the enhanced emission of terahertz pulses after the optical excitation of grating-coupled near-IR surface plasmons at the interface of gold and cuprous oxide, using femtosecond laser pulses. Terahertz emission is the result of the acceleration of charge carriers optically excited in the Schottky depletion field of the metal/semiconductor interface. The enhancement is a direct consequence of the localized nature of the surface plasmon field, which is strongest near the nanostructured metal surface where the Schottky electric field is strongest too. Surface plasmon excitation is confirmed by reflection spectroscopy of gratings with different periods, by varying the azimuthal angle of the grating, and by calculations of the plasmon frequencies and fields. We observe a terahertz field enhancement factor of 5.8 when compared to the emission from a flat sample. This corresponds to a THz power-enhancement factor of 34. Our results show that for THz emission from these metal/semiconduct...

Physical mechanisms of terahertz pulse emission from photoexcited surfaces of tellurium crystals

We report on terahertz (THz) emission from tellurium crystal surfaces excited by femtosecond optical pulses. Measurements were performed on three differently cut Te samples and with different wavelength optical excitation pulses. THz pulse amplitude dependences on the azimuthal angle measured at various excitation wavelengths have evidenced that three different mechanisms are responsible for THz generation in tellurium: second order nonlinear optical rectification effect, dominating at lower excitation photon energies, as well as transverse and ordinary photo-Dember effects, which emerge at energies larger than 0.9 eV. The shapes of the azimuthal angle dependences were also explained by theoretical model.

Emission of terahertz pulses from nanostructured metal surfaces

When (nanostructured) metals, such as gold and silver, are illuminated with laser pulses having a duration in the femtosecond range, they can emit pulses of THz light. Most of these experiments have been performed using amplified lasers, giving rise to energy densities on the sample on the order of mJ cm−2. The results of the different experiments are surprisingly inconsistent in both the measurements of the THz fluence as a function of laser fluence and in the interpretation of the results. This paper reviews the current state of affairs of this interesting topic and discusses some effects related to surface preparation that may influence the emission THz light on metals, particularly silver and copper. We also show results of measurements on nanostructured metals using unamplified laser pulses, which emphasize the role played by plasmons in the generation of THz light. When increasing the optical energy density on a specially nanostructured sample, we observe a transition from a ‘classical’ second-order non-linear optical process to a higher-order process as the source of the THz radiation. This supports recent results on a differently structured metal by Polyushkin et al (2014 Phys. Rev. B 89 125426), who also observe two different power regimes when decreasing the intensity coming from the high energy density side.

Scanning laser THz imaging system

The laser terahertz (THz) emission microscope (LTEM) is a unique inspection tool which can directly two-dimensionally map the THz pulse emission from a variety of electric materials and devices. Thus, two-dimensional mapping directly yields various physical information by visualizing the distributions of electric field, supercurrent, ferroelectric domain structures, magnetic fluxes, etc. Based on techniques created for conventional LTEM, we have developed a more highly functional laser THz imaging system, including a scanning laser THz (near-field) imaging system having the performance of high-speed and high-resolution, a scanning probe LTEM coupled with an atomic force microscope for high-resolution imaging, and a dynamic THz emission microscope to investigate the ultrafast carrier dynamics two-dimensionally in the sample. These systems offer diverse characteristics, making them suitable for various applications. We take a look at these systems and some typical applications.