Efficient Terahertz Emission Research Articles

Theoretical Models for Performance Analysis of Spintronic THz Emitters

The terahertz (THz) region of the electromagnetic spectrum, spanning from 0.1 to 10 THz, offers unique opportunities for imaging, spectroscopy, and communication applications. However, the potential of THz technologies has been limited by the availability of efficient and versatile THz emitters. Spintronic THz emitters (STEs), leveraging the ultrafast dynamics of electron spins in magnetic materials, have emerged as a promising solution to this challenge. STEs offer significant advantages, including broad bandwidth, high power output, and room-temperature operation, positioning them at the forefront of THz technology development. Despite these advances, understanding the operational principles and improving the performance of STEs remain areas of active research. This review focuses on the theoretical models that describe the behavior of STEs, aiming to provide a comprehensive overview of the underlying physics and suggest directions for future enhancements. Through a detailed examination of these models, the review seeks to clarify the basics of the physics driving STE performance and highlight innovative strategies for their optimization and application expansion.

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.

Giant Photon‐Drag‐Induced Ultrafast Photocurrent in Diamond for Nonlinear Photonics

AbstractDiamond is emerging as an attractive third‐generation wide‐bandgap semiconductor for future on‐chip nonlinear photonics and quantum optics due to its unique thermal, optical, and mechanical properties. However, the light‐driven current under below‐band gap excitation from the second‐order nonlinear optical effect in diamond is still challenging. Herein, a giant second‐order nonlinear photocurrent is observed in the chemical vapor deposition (CVD) diamond by utilizing terahertz (THz) emission spectroscopy. This ultrafast photocurrent originates from the photon drag effect (PDE), during which the momentum transfer from the incident photons to the charge carriers at the rich grain boundaries of the CVD diamond after the exclusive subgap π–π* transition upon femtosecond laser excitation. Especially, the interplay between circular and linear PDE to the THz generation is clarified and distinguished under elliptically polarized light excitation. Furthermore, the picosecond ultrafast dynamics of these charge carriers are also verified by infrared spectroscopy. Owing to the giant photon‐drag‐induced ultrafast photocurrent, the CVD diamond presents the highest THz emission efficiency compared with the reported carbon allotropes, which expands the new functionality of diamond nonlinear photonics into on‐chip THz devices.

Highly Efficient Spintronic Terahertz Emitter Utilizing a Large Spin Hall Conductivity of Type-II Dirac Semimetal PtTe2.

The remarkable spin-charge interconversion ability of transition metal dichalcogenides (TMDs) makes them promising candidates for spintronic applications. Nevertheless, their potential as spintronic terahertz (THz) emitters (STEs) remains constrained mainly due to their sizable resistivity and low spin Hall conductivity (SHC), which consequently result in modest THz emission. In this work, the TMD PtTe2, a type-II Dirac semimetal is effectively utilized to develop efficient STEs. This high efficiency primarily results from the large SHC of PtTe2, stemming from its low resistivity and significant spin-to-charge conversion efficiency, attributed to surface states and the local Rashba effect in addition to the inverse spin Hall effect. Remarkably, the peak THz emission from PtTe2/Co-STE exceeds that of Pt/Co-STE by ∼15% and is nearly double that of a similarly thick Pt/Co-STE. The efficient THz emission in the PtTe2/Co heterostructure opens new possibilities for utilizing the semimetal TMDs for developing THz emitters.

Ultrafast terahertz spin and orbital transport in magnetic/nonmagnetic multilayer heterostructures and a perspective

Ultrafast optically excited ferromagnetic (FM)/nonmagnetic (NM) multilayer heterostructures have been demonstrated recently as efficient, high-power, and broadband sources of terahertz (THz) electromagnetic radiation. Since these spintronic THz emitters exploit the conversion from ultrafast spin to charge current, either in bulk or at the interface, the THz pulses inhere all the characteristics of the involved mechanisms and dynamics associated with spin-charge interconversion processes. Deconvolving the same requires meticulous and careful experimentation and analysis. In this article, we review the current state-of-the-art in this field and provide a perspective on the emerging phenomena, which are prospering as new research avenues and demonstrate application potential for futuristic THz technologies. In the process of developing efficient spintronic THz emitters by optimizing various conditions including those with material parameters and excitation light, it turns out that THz emission spectroscopy itself can be a unique experimental tool for probing microscopic dynamical magnetic and spintronic effects, induced by femtosecond laser pulse excitation, in a noncontact and noninvasive manner. Several breakthroughs can be listed from the literature in this regard from the last decade. Just recently, ultrafast orbitronics is another dimension that is taking shape and will impact the field immensely. A fair account to this topic is also presented in the article.

Significant efficiency increment of spintronic terahertz emitters by oxygen engineering

Spintronic terahertz (THz) emitters have been intensively explored as next-generation sources of THz waves due to their low-cost, nanometer thickness, and broadband spectra. Growing research works are focusing on how to improve the THz emission efficiency, mainly by using a larger spin-Hall angle heavy metal. Currently, the highest intensity spintronic THz emission was based on a CoFeB/Pt heterostructure. Here, we significantly improve the THz emission intensity of CoFeB/Pt by a factor up to 270% through simply incorporating oxygen atoms into the Pt layer. The oxidation of a Pt layer generates a large extrinsic spin Hall angle, which promotes the spin-to-charge conversion of PtOx. Furthermore, the oxygen incorporation also causes a finite oxidation of CoFeB near the interface. We revealed that the significantly enhanced THz emission of CoFeB/PtOx is contributed by both the bulk inverse spin Hall effect of PtOx and the interface effect. Finally, we demonstrated that the oxygen engineering procedure to improve the THz emission of spintronic THz emitters is a common phenomenon as verified in examples, including Co/PtOx, NiFe/PtOx, CoFeB/WOx, and CoFeB/TaOx heterostructures. These findings show that an oxidized heavy metal is a simple, low-cost, and effective route to enhance the spin-to-charge conversion and achieve intense THz pulses, which is promising especially for on-chip THz devices.

An interplay between optical rectification and transient photocurrent effect on THz pulse generation from bulk MoS2 layered crystal

Development of novel schemes for efficient terahertz (THz) generation from transition metal dichalcogenides are useful for realizing integrated THz devices based on them and also, understanding of the related fundamental processes from such studies will guide to suitable designs. Here, we report the THz emission efficiency of bulk MoS2 layered crystal at varying femtosecond excitation wavelengths, from 550 nm to the telecommunication wavelength of 1550 nm. By using both the below bandgap excitation at longer wavelengths and the above bandgap excitation at shorter wavelengths, we resolve THz emission contributions from resonant and non-resonant optical rectification (OR) processes, and the surface field induced transient photocurrent effect (TPE). A relatively much larger contribution to THz emission from the TPE than the resonant OR is measured for the above bandgap excitation. We have measured a clear difference between the resonant and nonresonant OR processes. The pure OR part is exclusively determined from detailed experiments using excitation intensity, polarization angle, and azimuthal angle dependent measurements. For the above bandgap excitation, the THz emission gets highly saturated with the increasing excitation intensity. Also, the value of the saturation intensity increases (decreases) with the excitation photon energy (wavelength). Interestingly, we find that the linear polarization angle and the azimuthal angle dependent THz signal due to resonant OR is π/2 phase offset relative to that due to the nonresonant OR.

Direct evidence of terahertz emission arising from anomalous Hall effect

A detailed understanding of the different mechanisms being responsible for terahertz (THz) emission in ferromagnetic (FM) materials will aid in designing efficient THz emitters. In this report, we present direct evidence of THz emission from single layer Co_{0.4}Fe_{0.4}B_{0.2} (CoFeB) FM thin films. The dominant mechanism being responsible for the THz emission is the anomalous Hall effect (AHE), which is an effect of a net backflow current in the FM layer created by the spin polarized current reflected at the interfaces of the FM layer. The THz emission from the AHE-based CoFeB emitter is optimized by varying its thickness, orientation, and pump fluence of the laser beam. Results from electrical transport measurements show that skew scattering of charge carriers is responsible for the THz emission in the CoFeB AHE-based THz emitter.

Optimum excitation wavelength and photon energy threshold for spintronic terahertz emission from Fe/Pt bilayer

SummaryTerahertz emission from ferromagnetic/non-magnetic spintronic heterostructures had been demonstrated as pump wavelength-independent. We report, however, the pump wavelength dependence of terahertz emission from an optimized Fe/Pt spintronic bilayer on MgO substrate. Maximum terahertz generation per total pump power was observed in the 1200- to 1800-nm pump wavelength range, and a marked decrease in the terahertz emission efficiency beyond 2500 nm (pump photon energies <0.5 eV) suggests a ∼0.35-eV threshold pump photon energy for effective spintronic terahertz emission. The inferred threshold is supported by previous theoretical results on the onset energy of significant spin-filtering at the Fe-Pt interface, and confirmed by Fe/Pt electronic structure calculations in this present work. The results of terahertz time-domain emission spectroscopy show the sensitivity of spintronic terahertz emission to both the optical absorptance of the heterostructure and the energy-dependent spin transport, as dictated by the properties of the metallic thin films.

Terahertz Emission Enhanced by a Laser Irradiating on a T-Type Target

The generation of high field terahertz emission based on the interaction between an ultra-intense laser and solid targets has been widely studied in recent years because of its wide potential applications in biological imaging and material science. Here, a novel scheme is proposed to enhance the terahertz emission, in which a linearly polarized laser pulse irradiates a T-type target including a longitudinal target followed by a transverse target. By using two-dimensional particle-in-cell simulations, we find that the electron beam, modulated by the direct laser acceleration via the interaction of the laser with the longitudinal solid target, plays a crucial role in enhancing the intensity of terahertz emission and controlling its spatial distribution. Compared with the single-layer target, the maximum radiated electromagnetic field’s intensity passing through the spatial probe point is enhanced by about one order of magnitude, corresponding to the terahertz emission power increasing by two orders of magnitude or so. In addition, the proposed scheme is robust with respect to the thickness and length of the target. Such a scheme may provide important theoretical and data support for the enhancement of terahertz emission efficiency based on the ultra-intense laser irradiation of solid targets.

Enhancement of terahertz radiation from a filament by using circularly polarized two-color laser fields

We investigate the terahertz radiation from a filament in two-color laser fields with various polarizations. It has been experimentally demonstrated that the efficiency of terahertz emission in co-rotating two-color circularly polarized laser fields is eight times as strong as that in linearly polarized two-color laser fields with parallel polarization. This enhancement is explained based on the polarization-dependent clamping intensity inside a filament and the dynamics of electrons ionizing from a single atom or molecule. Our finding provides a simple method to improve the THz intensity from an air filament in two-color fields.

Superposition of 2ω and Electrostatic Field Induced Terahertz Waveforms in DC-Biased Two-Color Filament

Increase in conversion efficiency from a femtosecond optical pump into broadband terahertz (THz) radiation is currently an essential issue since it boosts THz source performance for medicine and security applications. An air-plasma based THz radiation from a two-color femtosecond filament is the most efficient gas-based THz emitter, with a dipole local source having a maximum on the beam propagation axis. In this work, we show the novel advancement to THz yield increase with preservation of the forwardly directed dipole radiation. The two-color THz source can be enhanced if the filament plasma channel is placed into an external electrostatic field (DC bias), which is parallel to the second harmonic polarization direction. In the experiment, we produce a plasma channel from 800-nm, ∼50-fs, 2-mJ pulse with 200 μJ of 400-nm, ∼50-fs mixed with the pump, and allocate it between the electrodes carrying 7-kV/cm static field. Time-domain measurements and 3D+time simulations of THz waveforms from the two-color DC-biased filament show that the THz emission is the superposition of the THz waveforms generated in the 800+400-nm filament without a DC-bias and in the 800-nm (without 400-nm) plasma channel biased by 7-kV/cm static field. The additivity of the two local dipole THz sources is possible if the majority of free electrons are produced by the pump pulse.

Generation of terahertz transients from Co2Fe0.4Mn0.6Si -Heusler-alloy/normal-metal nanobilayers excited by femtosecond optical pulses

We generated pulses of electromagnetic radiation in the terahertz (THz) frequency range by optical excitation of ${\mathrm{Co}}_{2}{\mathrm{Fe}}_{0.4}{\mathrm{Mn}}_{0.6}\mathrm{Si}$ (CFMS)/normal-metal (NM) bilayer structures. The CFMS is a Heusler alloy showing a band gap in one spin channel and is therefore a half metal. We compared the THz emission efficiency in a systematic manner for four different CFMS/NM bilayers, where NM was either Pt, Ta, Cr, or Al. Our measurements show that the THz intensity is highest for a Pt capping. We also demonstrate the tunability of the THz amplitude by varying the magnetic field for all four bilayers. We attribute the THz generation to the inverse spin Hall effect. In order to investigate the role of the interface in THz generation, we measured the spin mixing conductance for each CFMS/NM bilayer using a ferromagnetic resonance method. We found that the spin-orbit coupling cannot completely describe the THz generation in the bilayers and that the spin transmission efficiency of the CFMS/NM interface and the spin diffusion length, as well as the oxidation of the NM layer, play crucial roles in the THz emission process.

Highly efficient terahertz emission from layered ZrTe5 crystal with strong anisotropy and high in-plane carrier mobility

Intense, polarization controllable, and single-cycle terahertz pulses as light source are important tools to investigate physical and biological properties of materials. In this paper, we have detected efficient terahertz emission from layered transition-metal pentatelluride ZrTe5 excited by femtosecond laser pulse due to its strong anisotropy and high mobility of electrons. The physical mechanism of terahertz generation from this material is studied and the absolute predominance of diffusion current induced by photo-Dember effect is proved. In addition, we have achieved the modulation of terahertz polarization by adjusting the polarization states of the pump pulse. The generated elliptically polarized THz pulse is crucial for the investigation of vibration modes of chiral biological molecules, such as proteins and DNAs. Our findings indicate that strong and polarization-controllable THz emission can be generated by ZrTe5.

Highly-efficient terahertz emission from hydrogen-bonded single molecular crystal 4-nitro-2,5-bis(phenylethynyl)aniline.

We report terahertz electromagnetic wave emission by optical rectification from hydrogen-bonded single molecular crystal 4-nitro-2,5-bis(phenylethynyl)aniline designed to be polar via the hydrogen bonding between nitroaniline cores. The terahertz emission efficiency is comparable to the representative inorganic terahertz emitter ZnTe. We show terahertz emission characteristics, optical spectrum, and theoretical molecular orbital calculations. Another three kinds of nitroaniline-based organic molecules are revealed to form polar crystal structure, and they have large hyperpolarizabilities and have potential for terahertz photonics.

Enhanced Terahertz Radiation by Efficient Spin-to-Charge Conversion in Rashba-Mediated Dirac Surface States.

The enhancement of terahertz (THz) radiation is of extreme significance for the realization of the THz probe and imaging. However, present THz technologies are far from being enough to realize high-performance and room-temperature THz sources. Fortunately, topological insulators (TIs), with spin-momentum-locked Dirac surface states, are expected to exhibit a high terahertz emission efficiency. In this work, the novel concept of a Rashba-state-enhanced spintronic THz emitter is demonstrated on the basis of ferromagnet/heavy metal/topological insulator (FM/HM/TI) heterostructure. We find that the THz emission intensity changes as a function of HM interlayer thickness, and a 1.98 times higher intensity compared to that of FM/TI can be achieved when a meticulously designed thickness of the HM layer is inserted. The improvement of terahertz radiation is ascribed to the additive effect of Rashba splitting and topological surface states at the HM/TI interface. These results offer new possibilities for realizing spintronic THz emitters in TI-based magnetic heterostructures.

Tunneling and multiphoton ionization of atoms in two-color linear and circular laser fields and possibility of coherent polarization control of terahertz waves

The tunneling and multiphoton ionization of atoms in an intense two-color linearly and circularly polarized laser fields are discussed in the Keldysh theory framework. We use the “imaginary-time” method, where tunneling of the photoelectron is described by the classical equations of motion but with purely imaginary “time.” Together with using of the saddle-point method, this allows to obtain the dependence of the total ionization rate and the net photoelectron current, generated due to the interaction of an intense two-color laser field with an atom, on the ratio of the second and fundamental harmonic amplitudes, their relative phase and an angle between harmonics. Application of the “imaginary-time” method also allows us to specify the parameters maximizing the net photocurrent and to determine the Coulomb correction to the ionization rate. We investigate the properties of polarization and spectral intensity of terahertz (THz) radiation and also the possibility of the coherent control of THz waves polarization in two-color scheme, through the relative phase between harmonics. We theoretically demonstrate that the amplification of THz radiation in the case of parallel co-rotating circular laser pulses is greater than for a combination of circularly and linearly polarized harmonics and provides the most promising conditions for increasing the efficiency of THz emission and coherent control of the THz beam polarization.

Laser-induced terahertz emission in Co2MnSi/Pt structure

In this study, the terahertz (THz) emission in a Co2MnSi/Pt structure was investigated. The THz emission intensity increased after annealing, and the optimized temperature was above 400 °C owing to the increase in B2 ordering. The THz emission intensity of Co2MnSi/Pt was 1.4–2.0 times larger than that of CoFe/Pt. The THz conductivity for Co2MnSi/Pt was smaller than that for CoFe/Pt. The ultrafast spin-current intensity was almost the same in both materials. A small conductivity and comparable ultrafast spin-current injection into the Pt layer are possible reasons for the efficient THz emission in Co2MnSi/Pt.

Laser pulse induced efficient terahertz emission from Co/Al heterostructures

Investigation on terahertz (THz) emission is important not only for fundamental research but also for practical application. A rapidly developing approach to generate THz emission is using a heterostructure composed of ferromagnetic metal (FM) and nonmagnetic metal (NM) which is pumped by femtosecond laser pulse. Previous works in this regard mainly focused on bilayer with heavy NM (such as Pt) with a large spin Hall angle. Here we present a comprehensive investigation on THz emission from Co/Al heterostructures stemming from the inverse spin Hall effect. It is surprising to find that although the spin Hall angle of Al is two orders of magnitude smaller than that of Pt, the measured THz signals are close to one-third of that of the typical Co/Pt heterostructures. To explore the underlying physics, theoretical models are employed to investigate the spin-related properties of Al. We obtain that the upper limit of the spin Hall angle of Al is 0.55% and the spin-diffusion length is 2.2 \ifmmode\pm\else\textpm\fi{} 0.2 nm; the diffusion length is comparable to that of Pt (2.3 \ifmmode\pm\else\textpm\fi{} 0.1 nm). This work shows that it is worth revisiting light metals for the spintronic THz emitter.

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.