Strong Terahertz Emission Research Articles

Topologically influenced terahertz emission in Co2MnGa with a large anomalous Hall effect

The terahertz (THz) spectral zone is one of the most exciting but least explored domains of the electromagnetic spectrum. To extend the applicability of THz waves, the present objective is to develop an efficient, compact, durable, and low-cost THz emitter source. A spintronic THz emitter consisting of a ferromagnetic/nonmagnetic bilayer heterostructure is a promising innovation that can provide an alternative solution/replacement for conventional THz emitters. To further develop these spin-based THz emitters, we demonstrate an efficient and strong THz emission from a single layer of Co2MnGa with a large anomalous Hall effect (AHE) influenced by its Weyl semimetallic nature. Strong correlations among the THz emission, AHE, and chemical ordering of the full Heusler crystal structures for Co2MnGa are shown. Based on proper structural and chemical design, the topological nature of this material facilitates systematic optimization. Our initial findings provide a new design concept for the topological influences on spin-based THz emitters, and these emitters are expected to facilitate the further development of the intriguing Weyl physics.

Probing photocarrier dynamics in a Bi2Te3–Te eutectic p–n junction with a laser terahertz emission microscope

Bismuth telluride (Bi2Te3)-based heterostructures have attracted considerable attention owing to their interesting anisotropic properties and expected higher thermoelectric performance. Therefore, exploring the nature of the carrier dynamics in these heterostructures has been an important subject in the design and optimization of advanced materials. In the present study, hot carrier injection and its subsequent spatiotemporal behavior in a multilayered crystalline Bi2Te3–Tellurium (Te) eutectic composite were studied using a laser terahertz (THz) emission microscopy (LTEM). The THz emission electric fields at the Bi2Te3–Te interface were polarized perpendicular to the interface. The polarities of these waveforms reveal the direction of the electric field between the Bi2Te3 and Te regions, indicating the carrier types of these components and the p–n junction formed at the interface. In addition, in the Te region, a strong THz emission with an electric field polarized parallel to the interface was observed. This unique THz emission can be qualitatively explained through hot photocarrier anisotropic transport by considering the effective mass of electrons and holes. LTEM clarified the local carrier dynamics in the microstructures and revealed the potential distribution and anisotropic transport properties. These findings contribute to the exploration of eutectic heterostructures as new functional materials and provide new avenues for cutting-edge thermoelectric and photovoltaic devices.

Terahertz Emission on the Surface of a van der Waals Magnet CrSiTe3

AbstractThe van der Waals magnet CrSiTe3 (CST) has captured immense interest because it is capable of retaining the long‐range ferromagnetic order even in its monolayer form, thus offering potential use in spintronic devices. Bulk CST crystal has inversion symmetry that is broken on the crystal surface. Here, by employing ultrafast terahertz (THz) emission spectroscopy and time resolved THz spectroscopy, the THz emission of the CST crystal is investigated, which shows a strong THz emission from the crystal surface under femtosecond (fs) pulse excitation at 800 nm. Theoretical analysis based on space symmetry of CST suggests the dominant role of shift current occurring on the surface with a thickness of a few quintuple layers in producing the THz emission, consistent with the experimental observation that the emitted THz amplitude strongly depends on the azimuthal and pumping polarization angles. This study offers a new efficient THz emitter as well as a better understanding of the nonlinear optical response of CST. It hopefully will open a window toward the investigation on the nonlinear optical response in the mono/few‐layer van der Waals crystals with low‐dimensional magnetism.

Strong terahertz emission from copper oxides/silver micro thin film deposited on nanoparticles aggregation substrate

Terahertz emission properties of oxidized silver/copper micro thin film were investigated. Enhanced terahertz emission was observed from the oxidized micro thin film deposited on nanoparticles aggregation substrate due to the various directions of transient current surge. The amplitude of terahertz emission from the film was comparable to that from traditional non-external gallium arsenide. The sample, firstly, was synthesized by hydrothermal method to fabricate nanoparticles aggregation substrate. Next, the etched sample was treated by physical vapor deposition to grow silver/copper micro thin film and oxidized at 200 °C atmosphere environment. The space charge region was the dominant area for terahertz emission, built-in field at depletion layer played a key role in terahertz emission, and the contribution arising from optical rectification and photogalvanic was negligible due to the centrosymmetry structure; high band gap, low carriers concentration and mobility leads to the photon drag effect was not obvious; Because the differences of mobility and mass between hole and electron were not distinct, the Dember effect was also ineffective. The intensity of the terahertz emission was related to the thickness of the silver film because the silver film has a close relationship with the oxidation rate of the copper layer, leading to a dramatic influence on band gap structure, which produced a direct response on optical absorption at red zone. Compared with the planar substrate, the radiation of the nanoparticles substrate is 1.2 times that of the planar substrate, and the enhancement of the terahertz emission was ascribed to the various vibrated directions of photo-generated carriers. Our results could provide a potential portable terahertz emitter, which is economical, simple processing and operatable at room temperature.

Strong terahertz emission by optical rectification of shaped laser pulse in transversely magnetized plasma

This communication presents a theoretical model for efficient terahertz (THz) radiation generation by the optical rectification of shaped laser pulse in transversely magnetised ripple density plasma. The laser beam imparts a nonlinear ponderomotive force to the electron and this force exerts a nonlinear velocity component in both transverse and axial directions which have spectral components in the THz range. These velocity components couple with the pre-existing density ripple and give rise to a strong nonlinear current density which drives the THz wave in the plasma. The THz yield increases with the increasing strength of the background magnetic field and the sensitivity depends on the ripple wave number. The emitted power is directly proportional to the square of the amplitude of the density ripple. For exact phase matching condition, the normalised power of the generated THz wave can be achieved of the order of 10−4.

Strong terahertz emission from electromagnetic diffusion near cutoff in plasma

A new mechanism for electromagnetic emission in the terahertz (THz) frequency regime from laser-plasma interactions is described. A localized and long-lasting transverse current is produced by two counter-propagating short laser pulses in weakly magnetized plasma. We show that the electromagnetic wave radiating from this current source, even though its frequency is close to cut-off of the ambient plasma, grows and diffuses towards the plasma-vacuum boundary, emitting a strong monochromatic THz wave. With driving laser pulses of moderate power, the THz wave has a field strength of tens of MV m−1, a frequency of a few THz and a quasi-continuous power that exceeds all previous monochromatic THz sources. The novelty of the mechanism lies in a diffusing electromagnetic wave close to cut-off, which is modelled by a continuously driven complex diffusion equation.

Wafer-bonded coupled multilayer cavity with InAs quantum dots for two-color emission

A GaAs/AlAs coupled multilayer cavity structure with InAs quantum dots (QDs) was fabricated by wafer-bonding of two cavity structures grown individually. The wafer-bonding technique is important to control the spatial distribution of nonlinear polarization for strong terahertz emission by the differential frequency generation of the two cavity modes of the coupled cavity. Three layers of self-assembled InAs QDs were inserted in a cavity grown on a (001) GaAs substrate as optical gain materials for two-color emission of the cavity mode lights. The other cavity with a GaAs cavity layer was grown on a (113)B GaAs substrate. Two-color emissions with a 3.8 THz frequency difference were successfully observed from the wafer-bonded coupled cavity by cw optical pumping at room temperature.

Strong Terahertz Emission and Its Origin from Catalyst-Free InAs Nanowire Arrays

The unique features of nanowires (NW), such as the high aspect ratio and extensive surface area, are expected to play a key role in the development of very efficient semiconductor surface emitters in the terahertz (THz) spectral range. Here, we report on optically excited THz emission from catalyst-free grown arrays of intrinsically n-type InAs NWs using THz time-domain spectroscopy. Depending on the aspect ratio, the THz emission efficiency of the n-type InAs NWs is found to be up to ∼3 times stronger than that of bulk p-type InAs, known as currently the most efficient semiconductor-based THz surface emitter. Characteristic differences from bulk p-type InAs are particularly revealed from excitation wavelength-dependent measurements, showing monotonously increasing THz pulse amplitude in the NW arrays with increasing photon energy. Further polarization-dependent and two-color pump-probe experiments elucidate the physical mechanism of the THz emission: In contrast to bulk p-type InAs, where the anisotropic photoconductivity in the surface electric field is the dominant cause for THz pulse generation, the origin of the intrinsic THz emission in the NWs is based on the photo-Dember effect. The strong THz emission from high aspect ratio NW arrays further suggests an improved out-coupling of the radiation, while further enhancements in efficiency using core-shell NW geometries are discussed.

Quantum theory of terahertz emission due to ultrashort pulse ionization of gases

A microscopic model is developed to analyze terahertz (THz) emission after ultrashort one- and two-color laser-pulse excitations of an atomic gas. Optical Bloch equations are derived to describe the pulse-induced ionization in the many-atom system including the Coulombic scattering of the ionized electrons. The model captures the continuous transition between the tunneling and the multiphoton ionization regimes. Numerical evaluations for a wide range of pulse configurations identify optimized excitation conditions for strong THz emission.

Surface plasmon-enhanced terahertz emission from a hemicyanine self-assembled monolayer

Emission of terahertz radiation is observed when surface plasmons are excited on a thin film of gold, in the Kretschmann geometry. When a hemicyanine-terminated alkanethiol self-assembled monolayer of thickness 1.2 nm is deposited on the gold film, stronger terahertz emission is observed. Our experimental results confirm that enhanced terahertz emission is possible from planar gold surfaces when surface plasmons are excited.

Ionization currents and terahertz emission from the interaction of few-cycle laser pulses with gas targets

Based on a theoretical model and numerical simulations, the ionization currents and subsequent terahertz (THz) emission induced by the interaction of a few-cycle laser pulses with He gas targets are studied. It is shown that owing to the large transverse current generated by field ionization with few-cycle laser pulses, strong THz emission can be generated. The change of the carrier phase of the few-cycle laser pulses leads to the variation of the ionization currents. Correspondingly, the THz emission amplitude shows the characteristic as a periodic function of the carrier phase, which is also confirmed by one-dimensional particle-in-cell simulations. For a given carrier phase, the THz emission amplitude is not proportional to the laser amplitude. It shows at least two peaks at certain laser amplitudes. When the gas density profile is not uniform, the emission amplitude has a similar dependence on laser amplitude and carrier envelope phase, but the THz pulse duration and spectrum are quite different.

Studies on the optimization of terahertz emission based on the field ionization current model

Based on the theoretical model of field ionization current for terahertz (THz) emission in laser-gas interaction, the parameter optimization for strong THz emission via the two-color laser scheme is explored. It is found from numerical calculation that the ionization current is due mainly to the first-order ionization process and the contribution from high-order ionization is ignorable. In order to produce stronger THz emission, the ionization current can be enhanced by adjusting the amplitude ratio between the fundamental and its second harmonic laser pulses as well as their relative phase difference. The ionization current can also be increased by use of laser pulses at mid-infrared and by changing the gas species. Under the same laser pulse and gas density conditions, the ionization current from the Helium gas is almost twice that from the Neon gas.

Terahertz Emission from GaAs Films on Si(100) and Si(111) Substrates Grown by Molecular Beam Epitaxy

We report on the terahertz emission from femtosecond-laser-irradiated GaAs layers grown on Si(100) and Si(111) substrates. The results show that the terahertz emission from GaAs on Si is stronger than that of a semi-insulating bulk GaAs crystal. This increase is attributed to the strain field at the GaAs/Si interface. In the GaAs of the Si(100) sample, the stronger terahertz emission is observed compared with GaAs on Si(111). Moreover, the effect of changing the doping type of the Si substrate from n-type to semi-insulating was also studied and it was found that the terahertz emission intensity of GaAs on semi-insulating Si(100) is stronger than that of GaAs on n-type Si(100). Finally, strong terahertz emission from GaAs on semi-insulating Si(100) was observed not only in the reflection geometry but also in the transmission geometry. These results hold promise for new applications of terahertz optoelectronics.

Strong terahertz emission from (100) p-type InAs

Terahertz emission has been observed from (100) Zn-acceptor-doped InAs under illumination by fs pulses of near-infrared radiation. Turning the crystal about the surface normal produces two maxima per rotation, whether the angle of incidence is 45° or 75°, in contrast to (111) p-InAs, where three maxima per rotation have been reported. The emitted terahertz power has a quadratic variation with the pump power and decreases with increasing temperature in the range 20–300K. This behavior is consistent with a photocurrent surge being the dominant terahertz generating mechanism at low excitation fluences. The p-type InAs generates about two orders of magnitude more power than the standard unbiased terahertz emitter, 1mm thick ZnTe.