Inverse Altermagnetic Spin Splitting Effect‐Induced Terahertz Emission in RuO2

Spin Hall effect and its reciprocal, the inverse spin Hall effect (ISHE), provide the means for conversion between spin and charge currents [1, 2]. In particular, the ISHE transforms a pure spin current into a transverse charge current due to spin-orbit coupling. Recently, the decisive role of the ISHE effect on extending the field of spintronics into the terahertz (THz) regime was revealed [3, 4]. Here, we demonstrate the generation of pulsed broadband terahertz radiation utilizing the inverse spin hall effect in Fe/Pt bilayers on MgO and sapphire substrates. The emitter was optimized with respect to layer thickness, growth parameters, substrates and geometrical arrangement. The experimentally determined optimum layer thicknesses of Fe (2 nm) / Pt (3 nm) were in qualitative agreement with simulations of the spin current induced in the ferromagnetic layer [5]. Our model takes into account generation of spin polarization, spin diffusion and accumulation in Fe and Pt and electrical as well as optical properties of the bilayer samples. Using the device in a counterintuitive orientation a Si lens was attached to increase the collection efficiency of the emitter. The optimized emitter provided a bandwidth of up to 8 THz which was mainly limited by the low-temperature-grown GaAs (LT-GaAS) photoconductive antenna used as detector and the pulse length of the pump laser. The THz pulse length was as short as 220 fs for a sub 100 fs pulse length of the 800 nm pump laser. Average pump powers as low as 25 mW (at a repetition rate of 75 MHz) have been used for terahertz generation. This and the general performance make the spintronic terahertz emitter compatible with established emitters based on optical rectification in nonlinear crystals. Furthermore, we correlate the interface structural properties with the THz-E-field amplitude and the bandwidth of the THz radiation. By allowing the Pt layers to grow along different crystallographic directions on top of Fe we modify the THz emission characteristics of the optimized emitters. We present a theoretical model which correlates the loss of energy of the hot electrons and the electron-phonon/defect scattering lifetime at the Fe/Pt interface with the ISHE current that causes the THz emission. By taking into account the response function of the THz detector we describe the influence of the crystal structure of Pt onto the THz signal shape and spectrum. The demonstration of the role of the individual layer thicknesses and of the interface quality in the THz emission spectra paves the way for more efficient spintronic THz emitters.

In this work, we reveal unconventional, polarization-dependent anisotropic terahertz (THz) emission from a sputtered Co/Mo bilayer attributed to competing mechanisms of THz generation. Unlike conventional spintronic THz emitters that operate solely via the inverse spin Hall effect (ISHE), the present system generates THz radiation through the combined action of ISHE and second-order nonlinear-optical processes. THz magnetometry shows pronounced asymmetries in the THz hysteresis loops compared with planar magneto-optical Kerr effect magnetometry. Systematic experiments on azimuthal rotation of the sample and controlled variation of the pump pulse polarization (linear and elliptical) enable a full decomposition of the THz field. Roughly 70% of the amplitude is an odd-in-field contribution generated by the ISHE in Mo, whereas the remaining 30% is an even-in-field component whose angular behavior points to a second-order interface nonlinearity governed by a frozen in-plane interfacial magnetization <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msup> <mml:mi>M</mml:mi> <mml:mi>′</mml:mi> </mml:msup> </mml:mrow> </mml:math> that breaks inversion symmetry. These results demonstrate that the Co/Mo interface functions as a THz emitter whose amplitude is strongly sensitive to both the orientation of magnetization and the polarization of the incident light. The interplay between interfacial spin–orbit coupling and broken inversion symmetry gives rise to a distinct emission mechanism, which in turn results in the observed behavior not commonly found in conventional spintronic emitters. The symmetry-resolved separation of ISHE and optical rectification mechanisms provides clear design rules for tailoring interface-driven THz emitters based on ultrathin heterostructures.

We report on terahertz (THz) emission from a single layer ferromagnet which involves the generation of backflow nonthermal charge current from the ferromagnet/dielectric interface by femtosecond laser excitation and subsequent conversion of the charge current to a transverse transient charge current via the anomalous Hall effect (AHE), thereby generating the THz radiation. The THz emission can be either enhanced or suppressed, or even the polarity can be reversed by introducing a magnetization gradient in the thickness direction of the ferromagnet. The emission can also be enhanced using a synthetic antiferromagnet (SAF) via the same AHE mechanism. In addition, we demonstrate by both simulation and experiment that the THz emission provides a powerful tool to probe the magnetization reversal process of individual ferromagnetic layers in the SAF structure – an important building block for all types of spintronic devices.

The 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.

We observed efficient terahertz (THz) emission from sol-gel grown Cu2ZnSnSe4 (CZTSe) thin films using THz time domain spectroscopy technique. The THz emission bandwidth exceeds 2 THz with a dynamic range of 20 dB in the amplitude spectrum. The THz emission amplitude from CZTSe is found to be independent of external magnetic fields. Comparing the polarity of THz emission waveforms of CZTSe and GaAs, we suggest that the acceleration of photo-carriers in the surface accumulation layer of CZTSe is the dominant mechanism of radiation emission. Optical excitation fluence dependence measurements show that the saturation fluence of the CZTSe thin film reaches 1.48 μJ/cm2.

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.

Enhanced terahertz emission from GaAs substrates deposited with aluminum nitride films caused by high interface electric fields

Terahertz (THz) emission from spintronic THz emitters (STEs) has been extensively studied, both for its potential application in THz technologies and as a tool for probing spin dynamics, conductivity, and crystal anisotropy via time-domain THz spectroscopy. However, substrate birefringence can modify the emitted THz polarization, distorting the measured spectra and leading to potential misinterpretation of the results. While some aspects of substrate birefringence on THz emission have been studied in the time domain, its effects on THz spectra have not been discussed in detail. In the THz emission spectrum from STEs, we observe distinct spectral signatures that depend on sample orientation. To understand these signatures, we use STEs grown on c-cut sapphire and systematically vary the orientation of a (100) rutile TiO2 window in both pump-through and THz-through geometries to identify the birefringence-induced effects. These findings demonstrate that substrate birefringence plays a critical role in the emitted THz spectrum and must be carefully considered in the analysis of time-domain THz spectroscopy data. We also show that artifacts from smaller birefringence can have a more misleading effect on the THz spectra.

Spintronic terahertz (THz) emitters [1]–[2], consisting of ultrathin ferromagnetic and non-ferromagnetic metallic layers, have advantages over conventional THz emitters, such as photoconductive antennas (PCAs). For example, the emission bandwidth is much broader than a typical PCA [2]. A broad range of optical pump wavelength from UV to near IR region, including the communication wavelength <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$(\sim 1.5 \mu \mathrm{m})$</tex> is usable [3]–[4]. However, the efficiency of spintronic emitters is still low compared to PCAs. Therefore, improvement of the THz emission efficiency of spintronic emitters is the most important technical issue for development of practical THz spintronic emitters. Fe/Pt heterostructure [5] is one of the most efficient spintronic THz emitters among those reported so far. In this work, the authors report their investigations on this type of THz spintronic emitter. Firstly, the optimization of the layer structure and thickness [5] is discussed. The saturation and damage threshold properties with the optical pump power, which are important for the pump power scalability, are reported. We have investigated the pump wavelength dependence of THz emission from the spintronic device from 400 nm to 2700 nm, and found that the THz emission efficiency (THz emission/pump power) gradually decrease from 1500 nm to the threshold wavelength around 2800 nm. In addition, a diabolo-shaped Fe/Pt antenna is fabricated for enhancement of THz emission from the spintronic device, and its THz emission properties are evaluated.

Terahertz (THz) spectroscopy offers unique insights into the electronic and vibrational degrees of freedom of materials. Because of the long wavelength of THz portion of the electromagnetic spectrum, it can be challenging to generate and detect THz radiation on nanometer scales. In this talk I will describe our efforts to develop a platform for generation and detection of broadband terahertz (~10 THz bandwidth) emission using 10-nm-scale LaAlO3/SrTiO3 nanostructures created by conductive atomic force microscope (c-AFM) lithography. The technique relies on the large inherent third-order optical susceptibility of SrTiO3. Inversion symmetry is locally broken at this interface at nanoscale junctions, enabling local optical rectification of light from ultrashort near-infrared optical pulses that is electrically detected by the same junction1. This system is capable of generating and detecting broadband THz emission on a scale set by the junction, which can be as small as 10 nanometers2. I will describe how these devices are created and used to probe nanoscale objects (e.g., Au nanorods3) that can be placed and co-located with these sensitive sources and detectors of THz emission. Our current focus involves graphene nanostructures that are integrated with the LaAlO3/SrTiO3 system, but a host of other materials can in principle be integrated with this platform. 1. Irvin, P.; Ma, Y. J.; Bogorin, D. F.; Cen, C.; Bark, C. W.; Folkman, C. M.; Eom, C. B.; Levy, J., Rewritable nanoscale oxide photodetector. Nat Photonics 2010, 4 (12), 849-852. 2. Ma, Y.; Huang, M.; Ryu, S.; Bark, C. W.; Eom, C.-B.; Irvin, P.; Levy, J., Broadband Terahertz generation and detection at 10 nm scale. Nano Lett 2013, 13 (6), 2884-2888. 3. Jnawali, G.; Chen, L.; Huang, M.; Lee, H.; Ryu, S.; Podkaminer, J. P.; Eom, C.-B.; Irvin, P.; Levy, J., Photoconductive response of a single Au nanorod coupled to LaAlO$_3$/SrTiO$_3$ nanowires. Applied Physics Letters 2015, 106 (21), 211101.

The generation of terahertz (THz) emission from a dc-biased two-color femtosecond laser-induced filament in air was systematically investigated. A polarization analysis demonstrated that the THz emission could be the sum of two components: one generated by two-color laser-induced filamentation and the second induced by the external dc electric field. The first component is mostly from four-wave mixing process and a transient transverse electric current under the action of the external dc field could be responsible for the second THz emission.

In this work, the flnite-difierence time-domain (FDTD) technique is used to study novel terahertz (THz) emitter structures. The proposed THz antenna requires less pump power of the femtosecond laser pulse whilst provides higher THz output power. This is achieved by the enhancement of the localized electric fleld in the THz emitter This electric fleld enhancement is found to have two origins: One is owing to the enhancement of the static electric fleld of the bias voltage, and the other is the enhancement of the electric fleld of the incoming femtolaser pulse. The latter enhancement is caused by the interaction of the pump laser and the surface plasmon resonance at the conical gold structure of the photoconductive emitter. This new terahertz emitter could lead to new applications where high-power and broadband terahertz sources are needed.

Terahertz (THz) emission from a single-color ultraviolet (UV) (248 nm) filament in air is studied experimentally and compared to that of an infrared (IR) (744 nm) filament for similar pulse durations, focusing conditions, and excess of pulse power over the critical power for self-focusing. An angular distribution of the THz emission for both UV and IR filaments is conical with the close cone angle. In contrast, the THz radiation energy and spectrum differ significantly. The energy of THz emission from the UV filament is 1–2 orders of magnitude lower than the THz yield from the IR filament. The THz spectrum of the UV filament is shifted to the low-frequency range and is narrower compared to the spectrum of THz emission from the IR filament. We explain qualitatively the difference in THz yields and spectra by lower intensity and plasma density in the UV filament.

Terahertz (THz) emission by difference frequency generation (DFG) technique has attracted vast interests recently. Quantum dot (QD) system is proposed to be the best candidate for THz emission by DFG technique due to its large optical nonlinearity. For efficient QD THz emission, the QD transition energies must lie within the THz range (0.4-41 meV). However, QD transition energies generally fall outside the critical energy difference, i.e. more than 41 meV. In this work, we investigated the effects of QD structures and growth conditions on the energy states. Consequently, we determined the required QD structures and growth conditions needed to obtain energy difference in the THz range.

Intense terahertz (THz) emission in high quality GaAs film upon ultrafast excitation is demonstrated. Results showed that high quality GaAs grown by molecular beam epitaxy with a thin n-doped buffer can rival existing intense bare semiconductor THz surface emitters. The incorporation of a 0.2 µm n-GaAs buffer proved effective in enhancing the THz emission of GaAs by 281% and 295% in reflection and transmission THz time-domain excitation geometries, respectively. The GaAs film was of high crystallinity with or without the n-doped buffer layer as confirmed from X-ray diffraction and Raman scattering. The similar crystalline quality of the two samples was further exemplified by their comparable built-in field strength as measured by photoreflectance spectroscopy. The distinguishable difference in GaAs with and without the doped buffer was observed via low temperature photoluminescence (PL) spectroscopy. The GaAs film with the n-doped buffer exhibited intense GaAs PL while the GaAs film without the n-doped buffer exhibited prominent carbon impurity-related PL. THz enhancement was inferred to be due to the decrease in shallow defects in GaAs with n-doped buffer.