Emitted Terahertz Radiation Research Articles

Multi-plane imaging based on cascade spintronic terahertz emitters with curved substrates.

As a novel terahertz (THz) source, a spintronic THz emitter (STE) has become a research hot topic recently due to its ultra-broadband emission, powerful scalability, simple fabrication, and ultrawide pump-wavelength range. To optimize the performance of a STE, its spintronic heterostructure has been extensively investigated and its accessories have been also appropriately improved. In this work, a curved substrate of a STE was proposed and utilized to achieve the modulation of the THz wave front as a new degree of freedom. A STE with a neutral-meniscus substrate was designed and fabricated to attain the focusing function of the emitted THz radiation. Coaxial THz bi-focus with a non-overlapping spatio-temporal distribution were effectively generated and applied in multi-plane imaging by properly using two cascade STEs. Amplitude- and phase-type objects consisting of bilayer structures were measured by the scheme. The focused and defocused regions of the samples were distinguished and analyzed on different cross sections. Furthermore, a STE with a spiral stair substrate was manufactured in this way and the generation of a THz vortex beam was fulfilled. The convenient approach offered more possibilities for developing THz optospintronic devices.

Magnetically enhanced THz generation by self-focusing laser in VA-MCNTs

Magnetically enhanced terahertz (THz) radiations are generated on account of the self-focusing of the laser beam in the bunch of anharmonic Vertically Aligned Metallic Carbon Nanotubes (VA-MCNTs) embedded on the non-conductive sapphire or silicon on sapphire (SOS) substrate. The high-power Gaussian laser beam gets self-focused in the bunch of VA-MCNTs as the initial power of the propagating beam is greater than its critical power. The resulting laser beam interacts with the bunch of VA-MCNTs and as a result, the electrons of MCNTs experience a nonlinear ponderomotive force to show oscillatory behavior with resonant nonlinear transverse velocity. It produces the nonlinear current which drives the THz radiation generation. Enhanced THz generation is noticed in the regions where self-focusing becomes stronger. We have observed that an applied magnetic field, anharmonic behavior of MCNTs, self-focusing, and dimensions of MCNTs also pave the way for the enhancement of the normalized THz amplitude. The anisotropic behavior of the dielectric tensor in the presence of an externally applied static magnetic field also helps to enhance the THz amplitude. The results shown (by the beautiful graphs and well supported by the numerical simulation) in the present scheme indicate that the bunch of VA-MCNTs can play a diverse and significant role in the important applications of THz medical photonics by varying the values of various parameters. The emitted THz radiation has the ability to detect changes in the DNA of human beings because the frequency of the emitted radiation is observed to lie in the frequency region of molecular spectra of DNA and the corresponding energy of THz radiation is not high enough to damage the DNA by ionization.

Ben Ofori-Okai

Ben Ofori-Okai, a spectroscopist at the SLAC National Accelerator Laboratory, has some big questions about what’s happening at the center of planets. The secret lives of high-pressure and high-temperature environments hold the key to scientists’ long-standing questions about the evolution of our universe and even the elusive chemistry within nuclear reactors. To explore these exotic conditions, Ofori-Okai is taking spectroscopy to the extreme. Ofori-Okai specializes in a field called terahertz spectroscopy, which measures the vibrational energy of molecules as they interact with and emit terahertz radiation. This region of the electromagnetic spectrum, which lies roughly between infrared and microwave wavelengths, captures distinct vibrational modes of molecules that other spectroscopic measurements miss. For example, chemists can use terahertz spectra to reveal the chemical composition of molecular mixtures that might be indistinguishable by nuclear magnetic resonance or infrared spectroscopy, Ofori-Okai says. Yet chemists have been slow to adopt terahertz spectroscopy because “it’s

Quantum classical approach to spin and charge pumping and the ensuing radiation in terahertz spintronics: Example of the ultrafast light-driven Weyl antiferromagnet Mn3Sn

The interaction of a femtosecond laser pulse with magnetic materials has been intensely studied for more than two decades in order to understand ultrafast demagnetization in single magnetic layers or terahertz emission from their bilayers with nonmagnetic spin-orbit (SO) materials. However, in contrast to well-understood spin and charge pumping by dynamical magnetization in spintronic systems driven by microwaves or current injection, analogous processes in light-driven magnets and radiation emitted by them remain largely unexplained due to the multiscale nature of the problem. Here we develop a multiscale quantum-classical formalism---where conduction electrons are described by quantum master equation (QME) of the Lindblad type, classical dynamics of local magnetization is described by the Landau-Lifshitz-Gilbert (LLG) equation, and incoming light is described by classical vector potential, while outgoing electromagnetic radiation is computed using the Jefimenko equations for retarded electric and magnetic fields---and apply it to a bilayer of antiferromagnetic Weyl semimetal ${\mathrm{Mn}}_{3}\mathrm{Sn}$, hosting noncollinear local magnetization, and SO-coupled nonmagnetic material. Our $\mathrm{QME}+\mathrm{LLG}+$Jefimenko scheme makes it possible to understand how a femtosecond laser pulse directly generates spin and charge pumping and electromagnetic radiation by the ${\mathrm{Mn}}_{3}\mathrm{Sn}$ layer, including both odd and even high harmonics (of the pulse center frequency) up to order $n\ensuremath{\le}7$. The directly pumped spin current then exerts spin torque on local magnetization whose dynamics, in turn, pumps additional spin and charge currents radiating in the terahertz range. By switching on and off LLG dynamics and SO couplings, we unravel which microscopic mechanism contributes the most to emitted terahertz radiation---charge pumping by local magnetization of ${\mathrm{Mn}}_{3}\mathrm{Sn}$ in the presence of its own SO coupling is far more important than standardly assumed (for other types of magnetic layers) spin pumping and subsequent spin-to-charge conversion within the adjacent nonmagnetic SO-coupled material.

Multi-millijoule terahertz emission from laser-wakefield-accelerated electrons

High-power terahertz radiation was observed to be emitted from a gas jet irradiated by 100-terawatt-class laser pulses in the laser-wakefield acceleration of electrons. The emitted terahertz radiation was characterized in terms of its spectrum, polarization, and energy dependence on the accompanying electron bunch energy and charge under various gas target conditions. With a nitrogen target, more than 4 mJ of energy was produced at <10 THz with a laser-to-terahertz conversion efficiency of ~0.15%. Such strong terahertz radiation is hypothesized to be produced from plasma electrons accelerated by the ponderomotive force of the laser and the plasma wakefields on the time scale of the laser pulse duration and plasma period. This model is examined with analytic calculations and particle-in-cell simulations to better understand the generation mechanism of high-energy terahertz radiation in laser-wakefield acceleration.

Terahertz Pulse Generation from GaAs Metasurfaces.

Ultrafast optical excitation of select materials gives rise to the generation of broadband terahertz (THz) pulses. This effect has enabled the field of THz time-domain spectroscopy and led to the discovery of many physical mechanisms behind THz generation. However, only a few materials possess the required properties to generate THz radiation efficiently. Optical metasurfaces can relax stringent material requirements by shifting the focus onto the engineering of local electromagnetic fields to boost THz generation. Here we demonstrate the generation of THz pulses in a 160 nm thick nanostructured GaAs metasurface. Despite the drastically reduced volume, the metasurface emits THz radiation with efficiency comparable to that of a thick GaAs crystal. We reveal that along with classical second-order volume nonlinearity, an additional mechanism contributes strongly to THz generation in the metasurface, which we attribute to surface nonlinearity. Our results lay the foundation for engineering of semiconductor metasurfaces for efficient and versatile THz radiation emitters.

Spintronic terahertz emitters exploiting uniaxial magnetic anisotropy for field-free emission and polarization control

We explore the terahertz (THz) emission from CoFeB/Pt spintronic structures in the below-magnetic-saturation regime and reveal an orientation dependence in the emission, arising from in-plane uniaxial magnetic anisotropy (UMA) in the ferromagnetic layer. Maximizing the UMA during the film deposition process and aligning the applied magnetic field with the easy axis of the structure allow the THz emission to reach saturation under weaker applied fields. In addition, the THz emission amplitude remains at saturation levels when the applied field is removed. The development of CoFeB/Pt spintronic structures that can emit broadband THz pulses without the need for an applied magnetic field is beneficial to THz magneto-optical spectroscopy and facilitates the production of large-area spintronic emitters. Furthermore, by aligning the applied field along the hard axis of the structure, the linear polarization plane of the emitted THz radiation can be manipulated by changing the magnitude of the applied field. We, therefore, demonstrate THz polarization control without the need for mechanical rotation of external magnets.

Frequency tuning for broadband terahertz emission from two-color laser-induced air plasma

Effective manipulation of broadband terahertz emission, especially on spectrum tuning, is of great importance for many applications. We demonstrate a method to realize frequency tuning of terahertz emissions from two-color laser-induced air plasmas. The terahertz central frequency is switched from 0.56 to 0.82 THz by changing the polarization state of the fundamental wave with a quarter-wave plate. Based on numerical simulation, it is found that this frequency tuning is due to the birefringence effect induced by the fundamental wave on the second harmonic inside the filament, which leads to a discrepancy on the polarization chirality of the two-color laser components. Two-color lasers with opposite chirality will emit terahertz radiation with higher central frequency compared to two-color lasers with the same chirality at moderate laser intensity.

Terahertz wave amplification by a laser-modulated relativistic electron beam

Generation of tunable terahertz radiation based on the interaction of a frequency-chirped low power terahertz wave with a relativistic electron beam is proposed. The electron beam is first modulated temporally due to the ponderomotive force induced by the beat wave of two copropagating intense laser beams of slightly different frequencies. Then the electron beam interacts with a low-amplitude driver wave at terahertz frequency with a linear frequency chirping. A nonlinear transverse current density is driven that emits terahertz radiation at an up-shifted frequency with a relatively enhanced amplitude. The modulation process is analyzed theoretically by a complete set of fluid equations and considering the effects of both the electrons space-charge potential and the beat wave ponderomotive force. The initial density and energy of the electron beam are both optimized in order to improve the terahertz amplification process. The effect of the driver's frequency chirping, as well as the effects of the initial electron beam density and energy on the emitted terahertz radiation are investigated numerically. It is shown that the frequency chirping of the driver wave plays a key role in terahertz amplification. It is shown that there is an optimum chirp value for which the terahertz radiation is amplified significantly compared with the case without chirping. Moreover, transverse effects of the laser beams and the driver wave on the emitted terahertz radiation are also investigated.

Hat-Top Beams for Generating Tunable THz Radiations Using a Medium of Conducting Nanocylinders

There are a large number of studies for terahertz (THz) radiation generation, but tunable THz sources are still a challenge since it is difficult to tune frequency, focus and intensity of the radiation simultaneously. The present work proposes the THz generation by the interaction of two hat-top laser beams with a host medium of argon gas containing graphite nanocylinders, as these beams result in highly nonlinear interaction because of a smooth dip in their peak intensity and a fast rise and fall in the overall intensity pattern. Such an interaction produces nonlinear current (6.7 × 108 A/m2) because of the electron cloud of the nanocylinders, which can be modulated by the laser and medium properties for realizing tunable THz radiation. The orientation of basal planes of nanocylinders is shown to be important for this mechanism, though it may be challenging for the experimentalists. The resonant excitation takes place when the plasmon frequency matches the beating frequency of the laser beams, and in the proposed mechanism one can have longitudinal surface plasmon resonance (~12 THz) and transverse surface plasmon resonance (~40 THz), leading to frequency-tunable THz radiation. The role of height and inter particle distance between the adjacent nanocylinders on the THz field amplitude and the efficiency of the mechanism is uncovered by controlling the aspect ratio in the nanocylinders. For example, reducing the inter particle distance from 180 nm to 60 nm leads to the enhancement of THz field from 1 × 108 V/m to 5.48 × 108 V/m. The profile of the emitted THz radiation is investigated in detail under the effect of various parameters in order to prove the practicality of the proposal. The proposed design and mechanism would be attractive for electromagnetic and communication societies which are dealing with millimeter-waves and THz components in addition to its medical application.

Terahertz pulse generation by multi-color laser fields with linear versus circular polarization.

We study the influence of the polarization state of multi-color femtosecond laser pulses ionizing air or noble gases on the emitted terahertz radiation. A local-current model and plane wave evaluations predict a cross-over in the THz energy yields with increasing number of pump harmonics, for which circular laser polarization is more efficient for a few harmonics, and linear polarization is favorable for more than six pump colors. Comprehensive 3D numerical simulations of gas jet experiments confirm this property for singly and multiply ionized gases. Rotation of the THz polarization ellipse in the case of circular laser polarization is explained by phase shifts that may alter the phase angle between the harmonics.

Giant Electrical Modulation of Terahertz Emission in Pb(Mg1/3Nb2/3)0.7Ti0.3O3/Co−Fe−B/Pt Structure

Spintronic terahertz (THz) emitters based on ferromagnetic (FM)-nonmagnetic (NM) heterostructures are the focus of many experimental endeavors for THz sources. However, existing spintronic THz sources control emission by changing the external magnetic field, which is inconvenient for compact integration. Therefore, various modulated mechanisms are in high demand. Here, we present an electric-field-controlled spintronic THz emitter based on the ferroelectric (FE)-FM/NM structure that can emit THz radiation with voltage-modulated amplitude and polarization angle. The modulation depth of the THz amplitude is up to 69%, and the maximum rotation of the THz polarization angle is 28\ifmmode^\circ\else\textdegree\fi{}. In addition, after removing the electric field, two different THz-emission states can remain unchanged, demonstrating nonvolatile behavior. Modulation is attributed to magnetoelectric coupling between the FM and FE layers, which modifies magnetization of the FM layer by the electric-field-induced strain in the FE layer, and thus, modulates THz emission. Furthermore, the possibility of a programmable array-type THz source with varied bias voltage is theoretically demonstrated, allowing a spatially modulated THz light to be generated directly without the use of additional spatial light modulators. We believe that this electric-field-controlled THz emitter has the potential to be used in next-generation on-chip THz sources, THz memory devices, and THz ghost imaging.

Spintronic terahertz emitters: Status and prospects from a materials perspective

Spintronic terahertz (THz) emitters, consisting of ferromagnetic (FM)/non-magnetic (NM) thin films, have demonstrated remarkable potential for use in THz time-domain spectroscopy and its exploitation in scientific and industrial applications. Since the discovery that novel FM/NM heterostructures can be utilized as sources of THz radiation, researchers have endeavored to find the optimum combination of materials to produce idealized spintronic emitters capable of generating pulses of THz radiation over a large spectral bandwidth. In the last decade, researchers have investigated the influence of a wide range of material properties, including the choice of materials and thicknesses of the layers, the quality of the FM/NM interface, and the stack geometry upon the emission of THz radiation. It has been found that particular combinations of these properties have greatly improved the amplitude and bandwidth of the emitted THz pulse. Significantly, studying the material properties of spintronic THz emitters has increased the understanding of the spin-to-charge current conversion processes involved in the generation of THz radiation. Ultimately, this has facilitated the development of spintronic heterostructures that can emit THz radiation without the application of an external magnetic field. In this review, we present a comprehensive overview of the experimental and theoretical findings that have led to the development of spintronic THz emitters, which hold promise for use in a wide range of THz applications. We summarize the current understanding of the mechanisms that contribute to the emission of THz radiation from the spintronic heterostructures and explore how the material properties contribute to the emission process.

Dissipative Hot-Spot-Enabled Shock and Bounce Dynamics via Terahertz Quantum Quenches in Helical Edge States.

We study quantum quenches of helical liquids with spin-flip inelastic scattering. Counterpropagating charge packets in helical edges can be created by an ultrashort electric pulse applied across a 2D topological insulator. Localized "hot spots" that form due to scattering enable two types of strongly nonlinear wave dynamics. First, propagating packets develop self-focusing shock fronts. Second, colliding packets with opposite charge can exhibit near-perfect retroreflection, despite strong dissipation. This leads to frequency doubling that could be detected experimentally from emitted terahertz radiation.

Terahertz Photoconductivity Spectra of Electrodeposited Thin Bi Films.

Electron dynamics in the polycrystalline bismuth films were investigated by measuring emitted terahertz (THz) radiation pulses after their photoexcitation by tunable wavelength femtosecond duration optical pulses. Bi films were grown on metallic Au, Pt, and Ag substrates by the electrodeposition method with the Triton X-100 electrolyte additive, which allowed us to obtain more uniform films with consistent grain sizes on any substrate. It was shown that THz pulses are generated due to the spatial separation of photoexcited electrons and holes diffusing from the illuminated surface at different rates. The THz photoconductivity spectra analysis has led to a conclusion that the thermalization of more mobile carriers (electrons) is dominated by the carrier–carrier scattering rather than by their interaction with the lattice.

Terahertz pulse emission from photoexcited bulk crystals of transition metal dichalcogenides

Multilayer MoS2, MoSe2, and WSe2 crystals were excited with femtosecond optical pulses of various wavelengths. The emitted terahertz (THz) radiation pulses were found to be the most intense at photon energies coinciding with the direct energy gap at the K points of the Brillouin zone of corresponding materials. It was shown that the transition metal dichalcogenides of different dopant type emit THz pulses of opposite polarity, suggesting that the surface field plays a role in THz emission. Nevertheless, the optical pump–THz probe experiments contradicted THz emission observations, demonstrating a decrease in photoconductivity at higher energy quanta. The proposed THz pulse generation mechanism, therefore, assumes the accumulation of excitonic electric dipoles during optical pulse absorption and a fast build-up of the internal electric field inside the samples. This assumption was supported by double optical pulse investigations.

Tunable elliptically polarized Hermite-Gaussian terahertz radiation driven by two-color twisted laser pulses.

We propose a scheme for tunable elliptically polarized terahertz (THz) radiation by two-color linearly polarized Laguerre-Gaussian lasers irradiating gas plasmas. Three-dimensional particle-in-cell simulations show that the field strength of THz radiation can achieve MV/cm-scale, and the radiation frequency is determined by the plasma frequency and the electron cyclotron frequency. The emitted THz radiation is Hermite-Gaussian (HG) with a broadband waveform which can be attributed to the axial magnetic fields induced by the twisted drive pulses. Meanwhile, the ellipticity of the emitted THz wave can be effectively tuned by changing the laser intensities and the extra relative phase of the two driving lasers. Thus our scheme provides an efficient and practical approach to acquire tunable HG THz radiation with elliptical polarization, which may own some novel and unique application prospects in various areas.

Terahertz pulse generation by two-color laser fields with circular polarization

We study the influence of the polarization states of ionizing femtosecond two-color pulses on the emitted terahertz radiation in gases. A local-current model and plane-wave evaluations justify the previously-reported impact on the THz energy yield and a (almost) linearly-polarized THz field when using circularly-polarized laser harmonics. For such pump pulses, the THz yield is independent of the relative phase between the two colors. When the pump pulses have same helicity, the increase in the THz yield is associated with longer ionization sequences and higher electron transverse momenta acquired in the driving field. Reversely, for two color pulses with opposite helicity, the dramatic loss of THz power comes from destructive interferences driven by the highly symmetric response of the photocurrents lined up on the third harmonic of the fundamental pulse. While our experiments confirm an increased THz yield for circularly-polarized pumps of same helicity, surprisingly, the emitted THz radiation is not linearly-polarized. This effect is explained by means of comprehensive 3D numerical simulations highlighting the role of the spatial alignment and non-collinear propagation of the two colors.

Terahertz generation by beating of two chirped pulse lasers in spatially periodic density plasma

The present paper reports on the combined influence of the chirp characteristics of pump pulses and spatially periodic density (density ripple) plasma, on terahertz (THz) radiation generation, by beating of two chirped pulses. The beating lasers exert a nonlinear ponderomotive force along the z-direction. A self-consistent field is generated due to the nonlinear oscillations of plasma electrons; as a result of these linear and nonlinear forces the plasma electrons attain an oscillatory velocity that couples with the density ripple to generate a stronger transient transverse current, driving THz radiation. The importance of chirp parameter, amplitude and periodicity of density structure are discussed for emitted THz radiation. Our numerical simulations disclose that the variation of the chirp frequency parameter and ripple amplitude have considerable roles in improving the nonlinear oscillating current. By optimizing the chirp parameter and amplitude of the density ripple, a notable change in the magnitude of the terahertz field amplitude is found. The present paper maybe useful for broadband THz pulses, for use in plasma diagnostics and time-domain spectroscopy.

Terahertz Emission Functionality of High‐Temperature Superconductors and Similar Complex Systems

AbstractDuring the past two decades, terahertz (THz) science and technology have rapidly expanded in all fundamental and applied aspects of physical, chemical, and biological sciences. In physical sciences, these developments have been twofolds: i) the use of THz technology in understanding the complexities of materials and ii) the exploration of new THz functionalities of emerging materials for further advancement of THz technology. Here, the THz emission spectroscopy and the ultrafast functionality of three pillars of electronic and magnetic condensed matter systems that emerged in the following order: high‐temperature superconductors, colossal magnetoresistive manganites, and magnetoelectric multiferroics are reviewed. Emission functionality refers to the emitted THz radiation that carries the information of the underlying functional property of the material such that the overall effect evolves as the THz decoding of the information in a noninvasive manner and on an ultrafast timescale.