Terahertz Pulse Emission Research Articles

Interfacial Spintronic THz Emission

AbstractThe broken inversion symmetry at the ferromagnet (FM)/heavy‐metal (HM) interface leads to spin‐dependent degeneracy of the energy band, forming spin‐polarized surface states. As a result, the interface serves as an effective medium for converting spin accumulation into 2D charge current through the inverse Rashba–Edelstein effect. Exploring and assessing this spin‐to‐charge conversion (SCC) phenomenon at the FM/HM interface can offer a promising avenue to surpass the presumed limits of SCC in bulk HM layers. Spintronic heterostructures are utilized as a platform to measure the SCC experienced by photoexcited spin currents. Therefore, FM/HM heterostructures emitting terahertz electric field upon illumination by femtosecond laser pulses enable quantitative measure of the ultrafast SCC process. This results demonstrate a robust interfacial spin‐to‐charge conversion (iSCC) within a synthetic antiferromagnetic heterostructure, specifically for the NiFe/Ru/NiFe configuration, by isolating the SCC contribution originating from the interface and the bulk heavy‐metal (HM). Through the measurements of the emitted terahertz pulse, the iSCC at the NiFe/Ru interface is identified to be ≈27% of the strength as compared to SCC from the highest spin‐Hall conducting heavy‐metal, Pt. The results thus highlight the significance of interfacial engineering as a promising pathway for achieving efficient ultrafast spintronic devices.

Spintronic terahertz emitters with integrated metallic terahertz cavities

Abstract Spintronic terahertz emitters (STEs), based on optical excitation of nanometer thick ferromagnetic/heavy metal (FM/HM) heterojunctions, have become important sources for the generation of terahertz (THz) pulses. However, the efficiency of the optical-to-THz conversion remains limited. Although optical techniques have been developed to enhance the optical absorption, no investigations have studied the application of THz cavities. Here, to enhance the THz efficiency of STEs in a selected THz spectral range, FM/HM structures are realized on ultra-thin sapphire layers with metallic mirrors to create λ/4 THz resonant cavities. THz emission time domain spectroscopy of these STE/sapphire/mirror heterostructures, with sapphire thicknesses ranging from 110 µm to 25 µm, shows enhancement of the emitted THz field that fits the λ/4 cavity resonance with up to a doubling of the field in the spectrum, and in agreement with temporal simulations of the emitted THz pulse. By taking advantage of birefringent materials, we further show the potential of control of the polarization state of the emitted THz pulse. This work shows the potential of enhancing and engineering THz emission from STEs using THz cavities that can be controlled over a broad spectral range, which can be easily combined with optical cavities.

Emission of terahertz pulses from near-critical plasma slab under action of p-polarized laser radiation

The theory of the terahertz (THz) waves emission from a near-critical plasma slab under the action of the focused p-polarized laser pulse is developed. The spectral, angular and energy characteristics of the THz signal are studied as functions of the focusing degree and the incidence angle of laser radiation, as well as the density and thickness of the plasma slab. It is shown that the extremely strong increase in the energy of the THz signal (up to millijoule level) and conversion rate (up to 10 %) occurs at the almost normal incidence of the ultra-short, tightly focused p-polarized laser pulse on the thin plasma slab with the near-critical density and rare electron collisions.

Controlling the time shape of terahertz pulses from two-color plasma by combining wavelength dispersion and laser chirp

Using a two-color plasma in air or gas allows for the generation of ultra-broadband and highly intense terahertz (THz) pulses. Precisely controlling the shape of THz pulses, particularly the carrier envelope phase, is crucial for a wide range of technological and scientific applications. Recent research has revealed that the phase difference between the fundamental laser wave and its second harmonic influences the shape of the emitted THz pulse. However, the combined impact of laser chirp and the air-dispersion-induced phase difference between the two colors has not been explored in detail. This study aims to fill this gap. The results show that both of these factors have a significant impact on the THz waveform and emitted pulse energy. Positive and negative chirps lead to distinct effects on the pulse shape. A positively chirped pump laser produces a THz waveform with a negative monopolar shape, while a negatively chirped pump laser results in a positive THz pulse shape. Additionally, our findings indicate that the relationship between THz pulse energy and chirp changes with variations in the specific phase difference between the first and second harmonics. These results underscore the importance of carefully tuning both parameters to achieve maximum generation efficiency for a given waveform.

Femtosecond Laser-Induced Emission of Coherent Terahertz Pulses from Ruthenium Thin Films.

We demonstrate emission of electromagnetic pulses with frequencies in the terahertz (THz) range from ruthenium thin films through a second-order nonlinear optical process. Ruthenium deposited on different substrates showed different THz emission properties. We provide evidence that for Ru on glass above a certain power threshold, laser-induced oxidation occurs, resulting in an increased slope of the linear dependence of the THz electric field amplitude on pump power. The THz electric field is mainly polarized parallel to the sample surface, pointing in the same direction everywhere. In contrast to Ru on glass, the electric field amplitude of the THz pulses emitted by Ru on sapphire and on CaF2 shows a simple single linear dependence on pump power, and it is polarized orthogonal to the sample surface. In this case, thermal oxidation in an oven enhances the emission and introduces an additional polarization component along the sample surface. This component also points in the same direction everywhere on the surface, similar to the as-deposited Ru on glass. Although the precise THz generation mechanism remains an open question, our results show a strong correlation between the emission strength and the degree of oxidation. Furthermore, the results highlight the importance of the interfaces, i.e., both the choice of the substrate and the chemical composition of the top surface in THz emission experiments. Knowledge of the state of the sample surface is therefore crucial for the interpretation of THz emission experiments from (nonmagnetic) metal surfaces.

Layer‐controlled nonlinear terahertz valleytronics in two‐dimensional semimetal and semiconductor PtSe2

AbstractPlatinum diselenide () is a promising two‐dimensional (2D) material for the terahertz (THz) range as, unlike other transition metal dichalcogenides (TMDs), its bandgap can be uniquely tuned from a semiconductor in the near‐infrared to a semimetal with the number of atomic layers. This gives the material unique THz photonic properties that can be layer‐engineered. Here, we demonstrate that a controlled THz nonlinearity—tuned from monolayer to bulk —can be realized in wafer size polycrystalline through the generation of ultrafast photocurrents and the engineering of the bandstructure valleys. This is combined with the layer interaction with the substrate for a broken material centrosymmetry, permitting a second order nonlinearity. Further, we show layer dependent circular dichroism, where the sign of the ultrafast currents and hence the phase of the emitted THz pulse can be controlled through the excitation of different bandstructure valleys. In particular, we show that a semimetal has a strong dichroism that is absent in the monolayer and few layer semiconducting limit. The microscopic origins of this TMD bandstructure engineering are highlighted through detailed DFT simulations, and shows the circular dichroism can be controlled when becomes a semimetal and when the K‐valleys can be excited. As well as showing that is a promising material for THz generation through layer controlled optical nonlinearities, this work opens up a new class of circular dichroism materials beyond the monolayer limit that has been the case of traditional TMDs, and impacting a range of domains from THz valleytronics, THz spintronics to harmonic generation.image

Charge Dynamics in Spintronic Terahertz Emitters

An important issue for spintronic terahertz emitters is the analysis of spin-current generation and spin-to-charge conversion based on the emitted terahertz signal. This study shows that for such analysis it is mandatory to understand and account for local charging phenomena, which occur due to the limited area in which ultrafast currents are generated. Charge backflow strongly modifies the ensuing charge-current profile. This result is crucial for correctly interpreting the profile of the emitted terahertz pulse.

Nonlinear Optical Properties in an Epitaxial YbFe2O4 Film Probed by Second Harmonic and Terahertz Generation.

An epitaxial film of YbFe2O4, a candidate for oxide electronic ferroelectrics, was fabricated on yttrium-stabilized zirconia (YSZ) substrate by magnetron sputtering technique. For the film, second harmonic generation (SHG), and a terahertz radiation signal were observed at room temperature, confirming a polar structure of the film. The azimuth angle dependence of SHG shows four leaves-like profiles and is almost identical to that in a bulk single crystal. Based on tensor analyses of the SHG profiles, we could reveal the polarization structure and the relationship between the film structure of YbFe2O4 and the crystal axes of the YSZ substrate. The observed terahertz pulse showed anisotropic polarization dependence consistent with the SHG measurement, and the intensity of the emitted terahertz pulse reached about 9.2% of that emitted from ZnTe, a typical nonlinear crystal, implying that YbFe2O4 can be applied as a terahertz wave generator in which the direction of the electric field can be easily switched.

Efficient and temperature-independent terahertz emission from CoFeB/NiCu heterostructures

Spintronic terahertz (THz) emission has been investigated in ferromagnetic (FM) metal/nonmagnetic (NM) metal, FM/semiconductor, and FM/topological insulator systems. However, THz emission in the heterojunction of FM and $3d$ transition light metal is rarely reported. Large spin Hall angles have been observed in a light metal alloy of the $3d$ transition metal NiCu, allowing for the emission of high-performance THz waves without the use of heavy metal. Here, we present a comprehensive study of THz emission from a CoFeB/NiCu heterostructure, with peak intensity of emitted THz pulses up to half that of CoFeB/Pt. The THz radiation from CoFeB/NiCu is shown to be generated by the spin-to-charge conversion in NiCu via the inverse spin Hall effect. The evolution of the THz signal of $\mathrm{CoFeB}/{\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$ across ${\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$ Curie temperature is also explored, and it is discovered that the THz emission signal in this sample is temperature independent and unaffected by the magnetic state of ${\mathrm{Ni}}_{70}{\mathrm{Cu}}_{30}$. Our findings highlight the great potential of light metal alloys containing $3d$ transition elements as spintronic THz emitters. Moreover, it has been demonstrated that THz emission spectroscopy is a helpful tool for investigating inverse spin Hall effect or anomalous Hall effect in ferromagnets.

Field-free spintronic terahertz emitters based on IrMn/Fe/Pt exchage bias heterostructures

Spintronic terahertz (THz) emitter has more advantages such as lower cost, broader spectrum and easier operation than the commercial THz emitters, and thus has become a focus of research towards the next-generation THz source. However, in such a spintronic THz emitter, an external magnetic field is technologically required to align the orientation of the magnetization, which is detrimental for practical applications. Here, a spintronic terahertz emitter based on IrMn/Fe/Pt exchange bias structure is presented. By means of ultrafast spin injection on Fe/Pt interface followed by the spin-to-charge conversion in Pt, plus the effective magnetic field originating from the IrMn/Fe interface, the THz pulse with considerable intensity can be generated in such a structure without the assistance of external field. Besides, the remanent magnetization for thin Fe layer is enhanced by inserting an ultrathin Cu layer between the IrMn surface and the Fe surface, which is beneficial to the field-free THz emission. The range of obtained dynamic THz spectrum exceeds 60 dB and the positive saturation field can reach up to ~ –10 mT by optimizing the multilayer thickness, meeting the standard for commercial application. By rotating the sample, it is found that the polarization direction of the generated THz wave circulates simultaneously and keeps perpendicular to the direction of exchange bias field in the film plane. Moreover, we design a spin valve THz emitter based on the structure of IrMn/Fe/Pt/Fe by adding a free ferromagnetic Fe layer into the exchange bias multilayers. The emitted THz pulse amplitude is larger for the antiparallel alignment of the Fe layers at zero field than for the parallel alignment or exchange bias structure. The present work shows that the spin terahertz emitter based on IrMn/Fe/Pt exchange bias structure can produce the considerable terahertz signals without external field. Furthermore, the polarization direction of the emitted THz signal can be easily manipulated by rotating the sample. Because of this series of advantages, such exchange bias heterostructures are expected to play an important role in designing the next-generation THz source.

Forward terahertz wave generation from liquid gallium in the non-relativistic regime

We characterize a terahertz (THz) source based on plasma in liquid gallium. The dependence of the emitted THz pulse energy on second-order phase, pump pulse energy, and polarization of the short laser pulse is demonstrated. Our study suggests that the THz emission mechanism is due to the ponderomotive force and is aided by a direct-field driven term. The proposed source and accompanying generation mechanism are studied under a non-relativistic regime ( 1 0 15 < I < 1 0 18 W / c m 2 ) for forward directed THz under a single pump excitation scheme.

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.

Terahertz Pulse Emission from Semiconductor Heterostructures Caused by Ballistic Photocurrents.

Terahertz radiation pulses emitted after exciting semiconductor heterostructures by femtosecond optical pulses were used to determine the electron energy band offsets between different constituent materials. It has been shown that when the photon energy is sufficient enough to excite electrons in the narrower bandgap layer with an energy greater than the conduction band offset, the terahertz pulse changes its polarity. Theoretical analysis performed both analytically and by numerical Monte Carlo simulation has shown that the polarity inversion is caused by the electrons that are excited in the narrow bandgap layer with energies sufficient to surmount the band offset with the wide bandgap substrate. This effect is used to evaluate the energy band offsets in GaInAs/InP and GaInAsBi/InP heterostructures.

High-field THz pulses from a GaAs photoconductive emitter for non-linear THz studies.

We report the emission of high-field terahertz pulses from a GaAs large-area photoconductive emitter pumped with a Ti:Sapphire amplifier laser system at 800 nm wavelength and 1 kHz repetition rate. The maximum estimated terahertz electric field at the focus is ≳230 kV/cm. We also demonstrate the capability of the terahertz field to cause a non-linear effect, which usually requires high-field terahertz pulses generated through optical rectification or an air plasma. A significant drop in the optical conductivity of optically pumped GaAs due to Γ-L inter-valley scattering of free electrons caused by the strong THz field is found.

Terahertz emission at a p-polarized laser radiation action on plasma

Terahertz (THz) pulse emission is theoretically investigated when a p-polarized laser radiation falls on semi-bounded plasma. A significant increase in the energy of terahertz radiation is predicted when a laser pulse is incident at the angle of total reflection on the plasma with a density less than the critical value. It is shown that the THz signal amplification becomes especially strong when the ultra-short laser pulse falls at a small angle on the near critical plasma under conditions of rare electron collisions.

Terahertz photoconductive waveguide emitter with excitation by a tilted optical pulse front.

We explore the tilted-pulse-front excitation technique to control the superradiant emission of terahertz (THz) pulses from large-area photonconductive semiconductor switches. Two cases are studied. First, a photoconductive antenna emitting into free space, where the propagation direction of the optically generated THz beam is controlled by the choice of the tilt angle of the pump pulse front. Second, a THz waveguide structure with an integrated photoconductive window for the generation of THz radiation, where the injection of the THz radiation into a waveguide mode is optimized by the pulse front tilt. By providing long interaction lengths, such a waveguide-based optical-pump/THz-probe set-up may provide a new platform for the study of diverse short-lived optically induced excitations.

Scaling up and parametric characterization of two-color air plasma terahertz source

The plasma produced by frequency mixing of an ultrafast pulsed laser and its second-harmonic field in air is a promising, table-top source of terahertz radiation. To maximize the generation efficiency of such a source, multi-parameter optimization is essential. A parametric study involving various external controls is presented here. Starting from the optimization of phase and polarization of the second-harmonic field, the effect of laser intensity on the terahertz flux is reported. Moreover, the effect of external focusing conditions, such as focal length of the lens and effect of laser beam aperture, is explored in detail. Contrary to the previous reports, we find that with the increase of laser intensity, terahertz (THz) flux increases with more conversion efficiency; this is attributed to the multiple ionization of the air, leading to stacked release of contributory electrons. Exploration of beam convergence reveals that relatively weaker focusing leads to increased terahertz production compared to sharp focusing conditions. Moreover, temporal characterization of the emitted terahertz pulse conducted using electro-optic sampling provides pulse duration of 368 fs with corresponding frequency spectra up to 3.6 THz. The maximum electric field of the terahertz pulse is estimated to be 1.12 MV cm−1 in our experimental conditions. Results of 2D particle-in-cell simulations are also presented with the objective of validating the role of transient photocurrent mechanism in THz generation from two-color laser-induced gas plasma. We believe the broadband, bright THz source presented here will be useful in the imaging of bigger samples as well as nonlinear spectroscopy.

Терагерцовое излучение фотопроводящих антенн на основе сверхрешеточных структур \LT-GaAs/GaAs:Si\

A multilayer structure based on low-temperature LT-GaAs with crystallographic orientation (111) A is proposed for the fabrication of terahertz (THz) photoconductive antennas. The structures contain active layers of undoped LT-GaAs and high-temperature GaAs:Si layers doped with Si acceptors. At an optical pump power of 19 mW and a bias voltage of 30 V, a photoconductive antenna on an optimized {LT-GaAs / GaAs: Si} (111) A structure emitted THz pulses with an average power of 2.3 μW at a pulse repetition rate of 80 MHz, the conversion efficiency was 1.2 ∙ 10-4. It is shown that the dependence of the integrated power of THz antenna pulses based on {LT-GaAs / GaAs: Si} (111) A structures on the applied voltage is superlinear, and on the optical pump power it has the form of a saturation curve. The possibility of practical application of the obtained antennas for the tasks of terahertz spectroscopy of biological solutions is shown.

Helicity-dependent terahertz photocurrent and phonon dynamics in hybrid metal halide perovskites.

We report the discovery of helicity-dependent ultrafast photocurrent generation in organic-inorganic perovskite by measuring terahertz (THz) electric field emissions in the time-domain. We find signatures of the circular photogalvanic effect (CPGE) where right circularly polarized light and left circularly polarized light lead to different photocurrent generation. The direction of photocurrent is also resolved by measuring the polarization of the emitted THz pulses. Furthermore, we observe distinct wavelength-dependent, coherent phonon dynamics using THz pump-induced differential reflectivity, indicative of multiple exciton resonances. Both the CPGE and exciton fine structure, together with theoretical simulations, provide compelling and complementary evidence for the existence of Rashba-type bands in perovskite.

Controlling terahertz radiation with subwavelength blocky patterned CoFeB/Pt heterostructures

We report the broadband emission of terahertz (THz) pulse in metallic patterned ferromagnetic heterostructures CoFeB/Pt based on inverse spin-Hall effect, by illuminating a train of linearly polarized 120-fs-wide laser pulses at 800 nm. The spatial-temporal distribution of charge currents by changing the length of the subwavelength rectangular metal blocks allows for not only effectively controlling the magnitude, but subtly tuning the center frequency and bandwidth of the emitted THz pulses. Our results will open new avenues for the study of modulated spintronic-based THz emitters.