THz Emission Research Articles

Nanostructured Spintronic Emitters for Polarization-Textured and Chiral Broadband THz Fields

Spintronic THz emitters (STEs) rely on spin current acceleration or decay, resulting in a burst of broadband THz emission, offering a new route to THz optics. Here, we demonstrate novel metastructures of STEs for molding the vectorial focal distribution of the emitted THz radiations. The metastructures also allow controlling the characteristics of local fields. Performing combined micromagnetic-electromagnetic simulations, we demonstrate the generation of broadband THz fields with designed vectorial, chiral, magnetic, or topological features and discuss the potential of these fields for studying magnetic, multiferroic, and chiral structures. The STEs metastructure is shown to produce higher-order electric and magnetic multipoles as well as localized, longitudinal, broadband magnetoelectric THz pulses. Furthermore, subwavelength features of the near fields at a distance less than 10 μm can be tuned by steering the magnetization of the metastructure. The results point to a new type of engineered STEs for the generation of structured broadband THz fields, with potential applications in the fields of optoelectronics, optics, and ultrafast magnetism.

Boosting photoconductive large-area THz emitter via optical light confinement behind a highly refractive sapphire-fiber lens.

We report on a sapphire-fiber-based lens that can be used to enhance the emitted THz power of a large-area photoconductive antenna (PCA). Using numerical simulations, we demonstrate that the lens provides a spatial redistribution of the photocarriers density in the PCA's gap. By optimizing the diameter of the sapphire-fiber, one could reach efficient confinement of the photocarriers in the vicinity of the PCA electrodes with a 10-μm gap size for a 220-μm-thick sapphire-fiber. This allows enhancing the coupling of the incident electromagnetic waves at the interface between the sapphire fiber and the semiconductor with the antenna terminals by ∼40 times for a single PCA element, as well as boosting the total efficiency of the large-area PCA-emitter up to ∼7-10 times. To validate our approach, we propose a step-by-step process that can be used for the precise and controllable placement of the sapphire-fiber on the surface of a single PCA.

On the effect of stimulated raman scattering on THz radiation in the laser plasma interactions

In present study, two-dimensional Particle-In-Cell (PIC) simulations are used to investigate the interaction of ultrashort high-power laser pulses with an under-dense plasma medium. The results of the numerical simulations are followed by some mathematical procedures to obtain the terahertz electric field and the growth rates of the Stimulated Raman Forward (RFS) and Backward Scattering (RBS). The influences of incident laser pulse intensity and initial plasma density are examined on RFS and RBS instabilities and their consequent effects on the characteristics of THz radiation generation. It is seen that at the lower initial plasma densities i.e. 0.0001nc, the larger laser intensities will lead to more broadband and intense THz signals. Where nc=1.74 × 1027m−3 is known as critical plasma density corresponding to the laser pulse frequency. On the other hand, at the higher plasma densities, i.e., 0.0026nc, due to the instabilities growth, the effects of higher laser pulse intensities on the THz emission are insignificant. While for all the considered plasma densities and incident laser intensities, RBS inevitably affects the THz generation, RFS starts to grow for the initial plasma densities of n0=0.0026nc at the incident laser intensities higher than 1 × 1015Wcm−2. Hence, a higher bound is established on the pump intensity which is based on the RFS growth initiation. Thus, to achieve a stable THz signal for the initial plasma density of n0=0.0026nc (4.5 × 1018 cm−3), the incident laser intensity needs to be less than 1 × 1016Wcm−2.

Particle-in-cell simulations of THz emission from plasma by oblique collision of two-electron beams

We studied the THz radiation generated by a beam-plasma system using two-dimensional (2D) particle-in-cell (PIC) simulations. The Langmuir waves excited by two counterpropagating electron beams, via two-stream instability, collide with each other at an oblique angle, which forms a high beam-density modulation near the collision region, where both beam electrons become trapped. As a result, spatially localized Langmuir wave packets with large longitudinal-electric field amplitudes are formed, which give rise to bursts of electromagnetic radiation. Our 2D PIC simulations of the two thin, low-density, asymmetric, electron beams colliding obliquely show that a strong THz emission is obtained at the second harmonic of the plasma frequency (f = 1.0 THz), with a narrow spectral width (∼0.80%) in vacuum and significantly higher efficiency than the head-on-collision case. The efficiency of power conversion from electron beams to THz radiation measured in vacuum reaches around ∼0.0289, for a continuous injection of beams into the plasma, making it suitable for applications requiring high-power narrow-band THz radiation sources.

THz generation by two-color laser air plasma coupled to antiresonance hollow-core sapphire waveguides: THz-wave delivery and angular distribution management.

In this paper, hollow-core antiresonance sapphire waveguides were applied to guide the THz radiation emitted by the two-color laser air plasma, as well as to manage the THz source angular distribution. For this aim, three distinct waveguides were developed. Each of them is based on a cylindrical sapphire tube, either suspended in free space or coated by a polymer. The waveguides were first studied numerically, using the finite-difference eigenmode method, and experimentally, using the in-house THz pulsed spectrometer. The observed data uncovered the antiresonance regime of their operation, as well as their ability to guide broadband THz pulses over tens of centimeters with a high optical performance. The waveguides were then used to couple and guide (over the considerable distance) of THz radiation from the in-house two-color laser air plasma emitter, that exploits the mJ-energy-level femtosecond pulses of a Ti-sapphire laser. Small dispersion of a THz pulse and low-to-moderate propagation loss in the developed waveguide were observed, along with a considerable narrowing of the THz radiation angular distribution after passing the waveguide. Our findings revealed that such technologically-reliable hollow-core sapphire waveguides can boost the performance of laser air plasma-based THz emitters and make them more suitable for applications in the vigorously-explored THz sensing and exposure technologies.

Composition-dependent ultrafast THz emission of spintronic CoFe/Pt thin films

We measure the transient THz emission of ultrathin spintronic bilayers composed of a cobalt–iron alloy (CoxFe1−x) and platinum (Pt), depending on the Co content 0 ≤ x ≤ 1. We find that the THz emission amplitude changes only slightly over the entire concentration range, indicating that Fe and Co contribute similarly to the THz generation process. Additionally, we fabricate multilayer stacks and find that the THz amplitude is increased by up to 36% compared to a single bilayer CoFe/Pt spintronic emitter.

THz emission with X-mode laser pulses

THz emission with X-mode laser pulses

Spintronic emitters for super-resolution in THz-spectral imaging

We investigate local THz field generation using spintronic THz emitters to enhance the resolution for micrometer-sized imaging. Far-field imaging with wavelengths above 100 μm limits the resolution to this order of magnitude. By using optical laser pulses as a pump, THz field generation can be confined to the area of laser beam focusing. The divergence of the generated THz beam due to laser beam focusing requires the imaged object to be close to the generation spot at a distance below the THz field wavelength. We generate THz-radiation by fs-laser pulses in CoFeB/Pt heterostructures, based on spin currents, and detect them by commercial low-temperature grown-GaAs (LT-GaAs) Auston switches. The spatial resolution of THz radiation is determined by applying a 2D scanning technique with motorized stages allowing step sizes in the sub-micrometer range. Within the near-field limit, we achieve spatial resolution in the dimensions of the laser spot size on the micrometer scale. For this purpose, a gold test pattern is evaporated on the spintronic emitter separated by a 300 nm SiO2 spacer layer. Moving these structures with respect to the femtosecond laser spot, which generates THz radiation, allows for resolution determination. The knife-edge method yields a full-width half-maximum beam diameter of 4.9±0.4 μm at 1 THz. The possibility to deposit spintronic emitter heterostructures on simple glass substrates makes them attractive candidates for near-field imaging in many imaging applications.

Field-enhancing tapered planarized waveguides for THz quantum cascade laser frequency combs

We present a new planarized waveguide geometry for THz quantum cascade laser frequency combs with improved waveguide losses, RF and thermal dissipation properties. Ridge devices display broadband free-running comb states, and the THz emission can be further broadened by RF injection. Tapered waveguide devices feature a strong field-enhancement effect, which results in an improved comb performance. This includes free-running comb states with strong single beatnotes up to nearly -30 dBm at 90 K, almost three orders of magnitude stronger than for ridge devices. Improved comb operation is maintained also for high operating temperatures, up to 115 K.

A Numerical Study on an Enhanced Antenna-Coupled Plasmonic Piezotronic Pulsed Gan-Based Thz Emitter

A Numerical Study on an Enhanced Antenna-Coupled Plasmonic Piezotronic Pulsed Gan-Based Thz Emitter

Laser pulse energy localization in a photoconductive THz emitter via sapphire fibers

We report a seminal approach for localization of photocarriers in a photoconductive antenna (PCA)-emitter via a focusing element comprising the sapphire fiber. Using numerical simulation, we showed that at a certain ratio between the fiber diameter and antenna gap size (d/g~ 22.5), one can attain a ~ 35-fold enhancement of laser irradiation in the vicinity of the PCA electrodes. This provides the formation of subwavelength electromagnetic wave caustics located at the edges of the PCA electrodes, which potentially promotes an increase in the optical-to-terahertz conversion efficiency. Keywords: terahertz frequency, terahertz emitters, semiconductors, photoconductive antenna, IR radiation.

Nanoplasmonic‐Enhanced Spintronic Terahertz Emission (Adv. Mater. Interfaces 2/2022)

Nanoplasmonic-Enhanced Spintronic Terahertz Emission In article number 2101296, Shaojie Liu, Yong Xu, Xiaojun Wu, and co-workers demonstrate that nanoplasmonic resonance can effectively enhance the spintronic THz radiation from W/CoFeB/Pt heterostructures by 140%. The enhanced THz emission properties are systematically investigated through varying the pumping laser incidence direction, polarization, and the gold nanorods dimension parameters. Theoretical interpretations combined with Drude–Lorentz model and numerical simulations semi-quantitatively reproduce the experimental results.

Efficiency of Photoconductive Terahertz Generation in Nitrogen-Doped Diamonds

The efficiency of the generation of terahertz radiation from nitrogen-doped (∼0.1–100 ppm) diamonds was investigated. The synthetic polycrystalline and monocrystalline diamond substrates were pumped by a 400 nm femtosecond laser and tested for the photoconductive emitter operation. The dependency of the emitted THz power on the intensity of the optical excitation was measured. The nitrogen concentrations of the diamonds involved were measured from the optical absorbance, which was found to crucially depend on the synthesis technique. The observed correlation between the doping level and the level of the performance of diamond-based antennas demonstrates the prospects of doped diamond as a material for highly efficient large-aperture photoconductive antennas.

GaAsBi: From Molecular Beam Epitaxy Growth to Devices

GaAsBi has been researched as a candidate material for optoelectronic devices for around two decades. Bi‐induced localized states induce a rapid rising of the valence band edge through a band anticrossing interaction, which has a profound effect on the bandgap and the spin–orbit splitting. The band engineering possible, even with just a few percent bismuth, makes GaAsBi an attractive material for THz emitters, telecommunication lasers, and low noise photodetectors, among other devices. There has been substantial progress in some of these areas; however, progress toward many of the potential applications of GaAsBi has been hindered by device quality issues, brought about by the low substrate temperatures necessary for the growth of GaAsBi with sufficiently large Bi fractions. This review, presents an overview of the applications for which GaAsBi has been advocated and the key results in these areas. The molecular beam epitaxy growth and postgrowth processing of GaAsBi are then explored as well as the novel techniques that have been suggested to improve material quality.

Effect of laser intensity redistribution on semiconductor plasma based THz emission

THz emission has been investigated using laser beat wave interaction with density rippled semiconductor plasmas. Two lasers with frequency ω1 and ω2 co-propagate through density rippled semiconductor plasma having wave vector α and beat laser wave imparts ponderomotive force on electrons that produce nonlinear current and allows the THz emission. Gaussian, ring shape and hollow Gaussian field envelopes have been considered introducing laser intensity distribution along the polarization direction. Redistribution of laser intensity due to varied spatial profile changes the dynamics of plasma electrons and improves the ponderomotive force. Enhancement in the doping level in semiconductor also acts as a tuning parameter for the THz emission. Combined effect of laser intensity redistribution and amplitude of density modulation (nα/n0) enhances the amplitude of the THz field. Conversion Efficiency of the order ≈ 10−6 is estimated for hollow Gaussian field envelope and density modulation of 20% in semiconductor plasma.

Transfer-matrix description of heterostructured spintronics terahertz emitters

In this work we developed an extension to the transfer-matrix method (TMM) to include a current source term, with the aim of describing both the transmission and emission of THz pulses in spintronics THz emitters. The TMM with source is derived from the Maxwell equations with a volume free current term. This extension to the TMM allows for the study of realistic spintronics THz emitters as multilayers of different materials with different thicknesses. We use this to describe the drop rate of the THz emission amplitude at higher heavy metal layer thicknesses, and use it as a chance to remind that is not related and does not provide information about the spin diffusion length. It is instead the effect of the increase in the parasitic absorption of the generated radiation by the conducting parts of the spintronics THz emitter itself.

Ultra-narrowband and highly-directional THz thermal emitters based on the bound state in the continuum

AbstractThe development of novel and cost-effective THz emitters, with properties superior to current THz sources, is an active and important field of research. In this work, we propose and numerically demonstrate a simple yet effective approach of realizing terahertz sources working in continuous-wave form, by incorporating the new physics of bound state in the continuum (BIC) into thermal emitters. By deliberately designing the structure of slotted disk array made of high-resistivity silicon on top of a low index dielectric buffer layer supported by a conducting substrate, a quasi-BIC mode with ultra-high quality factor (∼104) can be supported. Our results reveal that the structure can operate as an efficient terahertz thermal emitter with near-unity emissivity and ultranarrow bandwidth. For example, an emitter working at 1.3914 THz with an ultranarrow linewidth less than 130 MHz, which is roughly 4 orders of magnitude smaller than that obtained from a metallic metamaterial-based thermal emitter, is shown. In addition to its high monochromaticity, this novel emitter has additional important advantages including high directionality and linear polarization, which makes it a promising candidate as the new generation of THz sources. It holds a great potential for practical applications where high spectral resolving capability is required.

Organic crystal-based THz source for complex refractive index measurements of window materials using single-shot THz spectroscopy

We employed N-benzyl-2-methyl-4-nitroaniline (BNA) crystals bonded on substrates of different thermal conductivity to generate THz radiation by pumping with 800 nm laser pulses. Crystals bonded on sapphire substrate provided four times more THz yield than glass substrate. A pyrodetector and a single-shot electro-optic (EO) diagnostic were employed for measuring the energy and temporal characterisation of the THz pulse. Systematic studies were carried out for the selection of a suitable EO crystal, which allowed accurate determination of the emitted THz spectrum from both substrates. Subsequently, the THz source and single-shot electro-optic detection scheme were employed to measure the complex refractive index of window materials in the THz range.

Tunable broadband THz emission from air plasma pumped by femtosecond pulses composed of a fundamental frequency with its detuned second harmonic

We demonstrate both theoretically and experimentally that the broadband THz pulses emitted by air plasma can be tuned in the spectrum domain by employing pump pulses composed of a fundamental femtosecond pulse and its detuned second harmonic (SH). Based on the plasma photocurrent model, our numerical simulations show that the central frequency of the broadband THz emission can be shifted from ∼4 THz to much higher frequency (∼38 THz) by red-shifting the second harmonic pulse. The relative phase of the two optical frequencies is found to present a critical role on the yield and spectrum of the THz emission. Experimentally, we obtained detuned second harmonic of the fundamental 800 nm femtosecond pulse by tilting the second harmonic generation (SHG) nonlinear crystal and achieved a tuning range from 391 nm to 403 nm. The central frequency of the corresponding THz emission was observed to shift from 4 THz to higher frequency side, in agreement with the simulation. This method therefore provides a simple mean to tune the spectrum of the broadband THz obtained from air plasma. We believe that even larger tunability can be achieved with a wavelength tunable Optical Parametric Amplification (OPA) driving laser.

Optical damage limit of efficient spintronic THz emitters

SummaryTHz pulses are generated from femtosecond pulse-excited ferromagnetic/nonmagnetic spintronic heterostructures via inverse spin Hall effect. The highest possible THz signal strength from spintronic THz emitters is limited by the optical damage threshold of the corresponding heterostructures at the excitation wavelength. For the thickness-optimized spintronic heterostructure, the THz generation efficiency does not saturate with the excitation fluence even up till the damage threshold. Bilayer (Fe, CoFeB)/(Pt, Ta)-based ferromagnetic/nonmagnetic (FM/NM) spintronic heterostructures have been studied for an optimized performance for THz generation when pumped by sub-50 fs amplified laser pulses at 800 nm. Among them, CoFeB/Pt is the best combination for an efficient THz source. The optimized FM/NM spintronic heterostructure having α-phase Ta as the nonmagnetic layer shows the highest damage threshold as compared to those with Pt, irrespective of their generation efficiency. The damage threshold of the Fe/Ta heterostructure on a quartz substrate is ∼85 GW/cm2.