Ultrafast photocurrents at the surface of the three-dimensional topological insulator Bi2Se3

Terahertz manipulation of nonlinear optical response in topological material PtBi2

The substantial nonlinear optical response observed in numerous topological materials renders them well-suited for optic and photonic applications, underscoring the critical need to devise effective strategies for manipulating their nonlinearity to enhance their versatility across different applications. In this Letter, we focus on the second-harmonic generation (SHG) response of the topological material PtBi2 under intense terahertz (THz) pulses and unveil the transient nonlinearity manipulated by a THz electric field. Our findings demonstrate that upon excitation of an intense THz pulse, there emerges a substantial enhancement in the SHG signal of PtBi2, which is predominantly attributed to the linear term involving the THz field, i.e., χ(2)χ(3)ETHz. We also clearly observe the transient change in the nonlinear coefficients, which could arise from the excitation of the bands with linear dispersion via the intense THz pulse. These findings bear significant implications for achieving ultrafast modulation of nonlinearity in topological materials, thereby opening avenues for advanced applications in this field.

Dual polarization-enabled ultrafast bulk photovoltaic response in van der Waals heterostructures

The bulk photovoltaic effect (BPVE) originating from spontaneous charge polarizations can reach high conversion efficiency exceeding the Shockley-Queisser limit. Emerging van der Waals (vdW) heterostructures provide the ideal platform for BPVE due to interfacial interactions naturally breaking the crystal symmetries of the individual constituents and thus inducing charge polarizations. Here, we show an approach to obtain ultrafast BPVE by taking advantage of dual interfacial polarizations in vdW heterostructures. While the in-plane polarization gives rise to the BPVE in the overlayer, the charge carrier transfer assisted by the out-of-plane polarization further accelerates the interlayer electronic transport and enhances the BPVE. We illustrate the concept in MoS2/black phosphorus heterostructures, where the experimentally observed intrinsic BPVE response time achieves 26 ps, orders of magnitude faster than that of conventional non-centrosymmetric materials. Moreover, the heterostructure device possesses an extrinsic response time of approximately 2.2 ns and a bulk photovoltaic coefficient of 0.6 V-1, which is among the highest values for vdW BPV devices reported so far. Our study thus points to an effective way of designing ultrafast BPVE for high-speed photodetection.

Temperature-Dependent Spin-to-Charge Conversion and Efficient Manipulation of Elliptical THz Waves in Bi2Te3/TbFeCo Heterostructures

Topological insulators (TIs) with spin-momentum-locked surface states and considerable spin-to-charge conversion (SCC) efficiency are ideal substitutes for the nonmagnetic layer in the traditional ferromagnetic/nonmagnetic (FM/NM) spintronic terahertz (THz) emitters. Here, the TI/ferrimagnetic structure as an effective polarization tunable THz source is verified by terahertz emission spectroscopy. The emitted THz electric field can be separated into two THz components utilizing their opposite symmetry on pump polarization and the magnetic field. TI not only emits a THz electric field via the linear photogalvanic effect (LPGE) but also serves as the medium of SCC via the inverse Edelstein effect (IEE) in the heterostructure. In addition, the amplitude and polarity of the SCC component can be efficiently manipulated by temperature in our ferrimagnetic TbFeCo layer compared with Co or Fe. Once these two THz components are delicately set orthogonally, an elliptical THz wave is generated by the intrinsic phase difference at the THz frequency range. The feasible control of its polarization and chirality is demonstrated by three means: pump polarization, magnetic field, and temperature. These appealing observations may pave the way for the development of elliptical THz wave emitters and polarization-sensitive THz spectroscopy.

Defect-induced helicity dependent terahertz emission in Dirac semimetal PtTe2 thin films

Nonlinear transport enabled by symmetry breaking in quantum materials has aroused considerable interest in condensed matter physics and interdisciplinary electronics. However, achieving a nonlinear optical response in centrosymmetric Dirac semimetals via defect engineering has remained a challenge. Here, we observe the helicity dependent terahertz emission in Dirac semimetal PtTe2 thin films via the circular photogalvanic effect under normal incidence. This is activated by a controllable out-of-plane Te-vacancy defect gradient, which we unambiguously evidence with electron ptychography. The defect gradient lowers the symmetry, which not only induces the band spin splitting but also generates the giant Berry curvature dipole responsible for the circular photogalvanic effect. We demonstrate that the THz emission can be manipulated by the Te-vacancy defect concentration. Furthermore, the temperature evolution of the THz emission features a minimum in the THz amplitude due to carrier compensation. Our work provides a universal strategy for symmetry breaking in centrosymmetric Dirac materials for efficient nonlinear transport.

Exploring the Potentials of Chitin and Chitosan-Based Bioinks for 3D-Printing of Flexible Electronics: The Future of Sustainable Bioelectronics

Chitin and chitosan-based bioink for 3D-printed flexible electronics have tremendous potential for innovation in healthcare, agriculture, the environment, and industry. This biomaterial is suitable for 3D printing because it is highly stretchable, super-flexible, affordable, ultrathin, and lightweight. Owing to its ease of use, on-demand manufacturing, accurate and regulated deposition, and versatility with flexible and soft functional materials, 3D printing has revolutionized free-form construction and end-user customization. This study examined the potential of employing chitin and chitosan-based bioinks to build 3D-printed flexible electronic devices and optimize bioink formulation, printing parameters, and postprocessing processes to improve mechanical and electrical properties. The exploration of 3D-printed chitin and chitosan-based flexible bioelectronics will open new avenues for new flexible materials for numerous industrial applications.

Emergence of threefold symmetric helical photocurrents in epitaxial low twinned Bi2Se3

We present evidence of a strong circular photon drag effect (PDE) in topological insulators (TIs) through the observation of helicity-dependent topological photocurrents with threefold rotational symmetry using THz spectroscopy in epitaxially-grown Bi2Se3 with reduced crystallographic twinning. We establish how twinned domains introduce competing nonlinear optical (NLO) responses inherent to the crystal structure that obscure geometry-sensitive optical processes through the introduction of a spurious mirror symmetry. Minimizing the twinning defect reveals strong NLO response currents whose magnitude and direction depend on the alignment of the excitation to the crystal axes and follow the threefold rotational symmetry of the crystal. Notably, photocurrents arising from helical light reverse direction for left/right circular polarizations and maintain a strong azimuthal dependence-a result uniquely attributable to the circular PDE, where the photon momentum acts as an applied in-plane field stationary in the laboratory frame. Our results demonstrate new levels of control over the magnitude and direction of photocurrents in TIs and that the study of single-domain films is crucial to reveal hidden phenomena that couple topological order and crystal symmetries.

A versatile model with three-dimensional triangular lattice for unconventional transport and various topological effects

The finite Berry curvature in topological materials can induce many subtle phenomena, such as the anomalous Hall effect (AHE), spin Hall effect (SHE), anomalous Nernst effect (ANE), non-linear Hall effect (NLHE) and bulk photovoltaic effects. To explore these novel physics as well as their connection and coupling, a precise and effective model should be developed. Here, we propose such a versatile model-a 3D triangular lattice with alternating hopping parameters, which can yield various topological phases, including kagome bands, triply degenerate fermions, double Weyl semimetals and so on. We reveal that this special lattice can present unconventional transport due to its unique topological surface states and the aforementioned topological phenomena, such as AHE, ANE, NLHE and the topological photocurrent effect. In addition, we also provide a number of material candidates that have been synthesized experimentally with this lattice, and discuss two materials, including a non-magnetic triangular system for SHE, NLHE and the shift current, and a ferromagnetic triangular lattice for AHE and ANE. Our work provides an excellent platform, including both the model and materials, for the study of Berry-curvature-related physics.

Time-domain observation of ballistic orbital-angular-momentum currents with giant relaxation length in tungsten

The emerging field of orbitronics exploits the electron orbital momentum L. Compared to spin-polarized electrons, L may allow the transfer of magnetic information with considerably higher density over longer distances in more materials. However, direct experimental observation of L currents, their extended propagation lengths and their conversion into charge currents has remained challenging. Here, we optically trigger ultrafast angular-momentum transport in Ni|W|SiO2 thin-film stacks. The resulting terahertz charge-current bursts exhibit a marked delay and width that grow linearly with the W thickness. We consistently ascribe these observations to a ballistic L current from Ni through W with a giant decay length (~80 nm) and low velocity (~0.1 nm fs-1). At the W/SiO2 interface, the L flow is efficiently converted into a charge current by the inverse orbital Rashba-Edelstein effect, consistent with ab initio calculations. Our findings establish orbitronic materials with long-distance ballistic L transport as possible candidates for future ultrafast devices and an approach to discriminate Hall-like and Rashba-Edelstein-like conversion processes.

Nanoscale mapping of edge-state conductivity and charge-trap activity in topological insulators

We report the nanoscale mapping of topological edge-state conductivity and the effects of charge-traps on conductivity in a Bi₂Se₃ multilayer film under ambient conditions. In this strategy, we applied an electric field perpendicular to the surface plane of Bi₂Se₃via a conducting probe to directly map the charge-trap densities and conductivities with a nanoscale resolution. The results showed that edge regions had one-dimensional characteristics with higher conductivities (two orders) and lower charge-trap densities (four orders) than those of flat surface regions where their conductivities and charge-traps were dominated by bulk effects. Additionally, edges showed an enhanced conductivity with an elevated electric field, possibly due to the creation of new topological states by stronger spin-Hall effects. Importantly, we observed ultra-high photoconductivity predominantly on edge regions compared with that of flat surface regions, which was attributed to the excitation of edge-state carriers by light. Since our method provides an important insight into the charge transport in topological insulators, it could be a significant advancement in the development of error-tolerant topotronic devices.

Exchange-Driven Intermixing of Bulk and Topological Surface States by Chiral Excitons in Bi_{2}Se_{3}

Topological surface states (TSS) in the prototypical topological insulator (TI) Bi_{2}Se_{3} are frequently characterized using optical probes, but electron-hole interactions and their effect on surface localization and optical response of the TSS remain unexplored. Here, we use ab initio calculations to understand excitonic effects in the bulk and surface of Bi_{2}Se_{3}. We identify multiple series of chiral excitons that exhibit both bulk and TSS character, due to exchange-driven mixing. Our results address fundamental questions about the degree to which electron-hole interactions can relax the topological protection of surface states and dipole selection rules for circularly polarized light in TIs by elucidating the complex intermixture of bulk and surface states excited in optical measurements and their coupling to light.

Polarization-controlled tunable directional spin-driven photocurrents in a magnetic metamaterial with threefold rotational symmetry

Future spintronics and quantum technologies will require a portfolio of techniques for manipulating electron spins in functional nanodevices. Especially, the establishment of the methods to control spin current is the key ingredient essential for the transfer and processing of information, enabling faster and low-energy operation. However, a universal method for manipulating spin currents with full-directional controllability and tunable magnitude has not been established. Here we show that an artificial material called a magnetic metamaterial (MM), which possesses a novel spintronic functionality not exhibited by the original substance, generates photo-driven ultrafast spin currents at room temperature via the magneto-photogalvanic effect. By tuning the polarization state of the excitation light, these spin currents can be directed with tunable magnitude along an arbitrary direction in the two-dimensional plane of the MM. This new concept may guide the design and creation of artificially engineered opto-spintronic functionalities beyond the limitations of conventional material science.

By carefully structuring and patterning a material, it is possible to introduce emergent properties that would otherwise not exist. These metamaterials have allowed the development of a wide variety of new optical properties. Here, Matsubara et al present a magnetic metamaterial, where spin-currents can be directed by tuning the polarization of the incident light.

Terahertz wave emission from the trigonal layered PtBi2

In this work, broadband terahertz (THz) wave emissions have been detected from the trigonal layered PtBi₂ on the excitation of the femtosecond laser pulses. Such THz generation is found to arise from the dominated linear photogalvanic effect, which is further discovered to strongly depend on the unique electronic structures of PtBi₂. Furthermore, an effective nonlinear susceptibility of PtBi₂ is also obtained and is nearly two orders of magnitude larger than that of the traditional nonlinear crystal for THz generation.

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Terahertz (THz) wave emission, topological semimetal, ultrafast photocurrent

Photovoltaic effect by soft phonon excitation

SignificanceThe quantum-mechanical geometric phase of electrons provides various phenomena such as the dissipationless photocurrent generation through the shift current mechanism. So far, the photocurrent generations are limited to above or near the band-gap photon energy, which contradicts the increasing demand of the low-energy photonic functionality. We demonstrate the photocurrent through the optical phonon excitations in ferroelectric BaTiO3 by using the terahertz light with photon energy far below the band gap. This photocurrent without electron-hole pair generation is never explained by the semiclassical treatment of electrons and only arises from the quantum-mechanical geometric phase. The observed photon-to-current conversion efficiency is as large as that for electronic excitation, which can be well accounted for by newly developed theoretical formulation of shift current.

Ultrafast photothermoelectric effect in Dirac semimetallic Cd3As2 revealed by terahertz emission

The thermoelectric effects of topological semimetals have attracted tremendous research interest because many topological semimetals are excellent thermoelectric materials and thermoelectricity serves as one of their most important potential applications. In this work, we reveal the transient photothermoelectric response of Dirac semimetallic Cd₃As₂, namely the photo-Seebeck effect and photo-Nernst effect, by studying the terahertz (THz) emission from the transient photocurrent induced by these effects. Our excitation polarization and power dependence confirm that the observed THz emission is due to photothermoelectric effect instead of other nonlinear optical effect. Furthermore, when a weak magnetic field (~0.4 T) is applied, the response clearly indicates an order of magnitude enhancement on transient photothermoelectric current generation compared to the photo-Seebeck effect. Such enhancement supports an ambipolar transport nature of the photo-Nernst current generation in Cd₃As₂. These results highlight the enhancement of thermoelectric performance can be achieved in topological Dirac semimetals based on the Nernst effect, and our transient studies pave the way for thermoelectric devices applicable for high field circumstance when nonequilibrium state matters. The large THz emission due to highly efficient photothermoelectric conversion is comparable to conventional semiconductors through optical rectification and photo-Dember effect.

Many topological semimetals are excellent thermoelectric materials, but previous studies were limited to steady-state properties. Here, the authors observe a transient thermoelectric response in Cd₃As₂ by detecting the resulting THz emission, with an enhanced response when a small magnetic field is applied.

Generation and Control of Terahertz Spin Currents in Topology-Induced 2D Ferromagnetic Fe3 GeTe2 |Bi2 Te3 Heterostructures

Future information technologies for low-dissipation quantum computation, high-speed storage, and on-chip communication applications require the development of atomically thin, ultracompact, and ultrafast spintronic devices in which information is encoded, stored, and processed using electron spin. Exploring low-dimensional magnetic materials, designing novel heterostructures, and generating and controlling ultrafast electron spin in 2D magnetism at room temperature, preferably in the unprecedented terahertz (THz) regime, is in high demand. Using THz emission spectroscopy driven by femtosecond laser pulses, optical THz spin-current bursts at room temperature in the 2D van der Waals ferromagnetic Fe₃ GeTe₂ (FGT) integrated with Bi₂ Te₃ as a topological insulator are successfully realized. The symmetry of the THz radiation is effectively controlled by the optical pumping incidence and external magnetic field directions, indicating that the THz generation mechanism is the inverse Edelstein effect contributed spin-to-charge conversion. Thickness-, temperature-, and structure-dependent nontrivial THz transients reveal that topology-enhanced interlayer exchange coupling increases the FGT Curie temperature to room temperature, which provides an effective approach for engineering THz spin-current pulses. These results contribute to the goal of all-optical generation, manipulation, and detection of ultrafast THz spin currents in room-temperature 2D magnetism, accelerating the development of atomically thin high-speed spintronic devices.

Tunable Quantum Tunneling through a Graphene/Bi2Se3 Heterointerface for the Hybrid Photodetection Mechanism

Graphene-based van der Waals heterostructures are promising building blocks for broadband photodetection because of the gapless nature of graphene. However, their performance is mostly limited by the inevitable trade-off between low dark current and photocurrent generation. Here, we demonstrate a hybrid photodetection mode based on the photogating effect coupled with the photovoltaic effect via tunable quantum tunneling through the unique graphene/Bi₂Se₃ heterointerface. The tunneling junction formed between the semimetallic graphene and the topologically insulating Bi₂Se₃ exhibits asymmetric rectifying and hysteretic current-voltage characteristics, which significantly suppresses the dark current and enhances the photocurrent. The photocurrent-to-dark current ratio increases by about a factor of 10 with the electrical tuning of tunneling resistance for efficient light detection covering the major photonic spectral band from the visible to the mid-infrared ranges. Our findings provide a novel concept of using tunable quantum tunneling for highly sensitive broadband photodetection in mixed-dimensional van der Waals heterostructures.

Enhanced terahertz emission from mushroom-shaped InAs nanowire network induced by linear and nonlinear optical effects

The development of powerful terahertz (THz) emitters is the cornerstone for future THz applications, such as communication, medical biology, non-destructive inspection, and scientific research. Here, we report the THz emission properties and mechanisms of mushroom-shaped InAs nanowire (NW) network using linearly polarized laser excitation. By investigating the dependence of THz signal to the incidence pump light properties (e.g. incident angle, direction, fluence, and polarization angle), we conclude that the THz wave emission from the InAs NW network is induced by the combination of linear and nonlinear optical effects. The former is a transient photocurrent accelerated by the photo-Dember field, while the latter is related to the resonant optical rectification effect. Moreover, thep-polarized THz wave emission component is governed by the linear optical effect with a proportion of ∼85% and the nonlinear optical effect of ∼15%. In comparison, thes-polarized THz wave emission component is mainly decided by the nonlinear optical effect. The THz emission is speculated to be enhanced by the localized surface plasmon resonance absorption of the In droplets on top of the NWs. This work verifies the nonlinear optical mechanism in the THz generation of semiconductor NWs and provides an enlightening reference for the structural design of powerful and flexible THz surface and interface emitters in transmission geometry.

Superluminal-like magnon propagation in antiferromagnetic NiO at nanoscale distances

Magnon-mediated angular-momentum flow in antiferromagnets may become a design element for energy-efficient, low-dissipation and high-speed spintronic devices^1,2. Owing to their low energy dissipation, antiferromagnetic magnons can propagate over micrometre distances³. However, direct observation of their high-speed propagation has been elusive due to the lack of sufficiently fast probes². Here we measure the antiferromagnetic magnon propagation in the time domain at the nanoscale (≤50 nm) with optical-driven terahertz emission. In non-magnetic-Bi₂Te₃/antiferromagnetic-insulator-NiO/ferromagnetic-Co trilayers, we observe a magnon velocity of ~650 km s^-1 in the NiO layer. This velocity far exceeds previous estimations of the maximum magnon group velocity of ~40 km s^-1, which were based on the magnon dispersion measurements of NiO using inelastic neutron scattering^4,5. Our theory suggests that for magnon propagation at the nanoscale, a finite damping makes the dispersion anomalous for small magnon wavenumbers and yields a superluminal-like magnon velocity. Given the generality of finite dissipation in materials, our results strengthen the prospects of ultrafast nanodevices using antiferromagnetic magnons.

Quantitative terahertz emission nanoscopy with multiresonant near-field probes

By sampling terahertz waveforms emitted from InAs surfaces, we reveal how the entire, realistic geometry of typical near-field probes drastically impacts the broadband electromagnetic fields. In the time domain, these modifications manifest as a shift in the carrier-envelope phase and emergence of a replica pulse with a time delay dictated by the length of the cantilever. This interpretation is fully corroborated by quantitative simulations of terahertz emission nanoscopy based on the finite element method. Our approach provides a solid theoretical framework for quantitative nanospectroscopy and sets the stage for a reliable description of subcycle, near-field microscopy at terahertz frequencies.

Terahertz emission from ultrathin bismuth layers

Thinner than 10 nm layers of bismuth (Bi) were grown on (111) Si substrates by molecular beam epitaxy. Terahertz (THz) radiation pulses from these layers excited by tunable wavelength femtosecond optical pulses were measured. THz emission sets on when the photon energy exceeds 0.45 eV, which was explained by the semimetal-to-semiconductor transition at this Bi layer thickness. A THz signal has both isotropic and anisotropic components that could be caused by the lack of balance of lateral photocurrent components and the shift currents, respectively.

Enhanced Spin-to-Charge Conversion Efficiency in Ultrathin Bi2Se3 Observed by Spintronic Terahertz Spectroscopy

Owing to their remarkable spin-charge conversion (SCC) efficiency, topological insulators (TIs) are the most attractive candidates for spin-orbit torque generators. The simple method of enhancing SCC efficiency is to reduce the thickness of TI films to minimize the trivial bulk contribution. However, when the thickness reaches the ultrathin regime, the SCC efficiency decreases owing to intersurface hybridization. To overcome these contrary effects, we induced dehybridization of the ultrathin TI film by breaking the inversion symmetry between surfaces. For the TI film grown on an oxygen-deficient transition-metal oxide, the unbonded transition-metal d-orbitals affected only the bottom surface, resulting in asymmetric surface band structures. Spintronic terahertz emission spectroscopy, an emerging tool for investigating the SCC characteristics, revealed that the resulting SCC efficiency in symmetry-broken ultrathin Bi₂Se₃ was enhanced by up to ∼2.4 times.

Tunable non-integer high-harmonic generation in a topological insulator

When intense lightwaves accelerate electrons through a solid, the emerging high-order harmonic (HH) radiation offers key insights into the material^1-11. Sub-optical-cycle dynamics-such as dynamical Bloch oscillations^2-5, quasiparticle collisions^6,12, valley pseudospin switching¹³ and heating of Dirac gases¹⁰-leave fingerprints in the HH spectra of conventional solids. Topologically non-trivial matter^14,15 with invariants that are robust against imperfections has been predicted to support unconventional HH generation^16-20. Here we experimentally demonstrate HH generation in a three-dimensional topological insulator-bismuth telluride. The frequency of the terahertz driving field sharply discriminates between HH generation from the bulk and from the topological surface, where the unique combination of long scattering times owing to spin-momentum locking¹⁷ and the quasi-relativistic dispersion enables unusually efficient HH generation. Intriguingly, all observed orders can be continuously shifted to arbitrary non-integer multiples of the driving frequency by varying the carrier-envelope phase of the driving field-in line with quantum theory. The anomalous Berry curvature warranted by the non-trivial topology enforces meandering ballistic trajectories of the Dirac fermions, causing a hallmark polarization pattern of the HH emission. Our study provides a platform to explore topology and relativistic quantum physics in strong-field control, and could lead to non-dissipative topological electronics at infrared frequencies.

Terahertz Spin-to-Charge Conversion by Interfacial Skew Scattering in Metallic Bilayers

The efficient conversion of spin to charge transport and vice versa is of major relevance for the detection and generation of spin currents in spin-based electronics. Interfaces of heterostructures are known to have a marked impact on this process. Here, terahertz (THz) emission spectroscopy is used to study ultrafast spin-to-charge-current conversion (S2C) in about 50 prototypical F|N bilayers consisting of a ferromagnetic layer F (e.g., Ni₈₁ Fe₁₉ , Co, or Fe) and a nonmagnetic layer N with strong (Pt) or weak (Cu and Al) spin-orbit coupling. Varying the structure of the F/N interface leads to a drastic change in the amplitude and even inversion of the polarity of the THz charge current. Remarkably, when N is a material with small spin Hall angle, a dominant interface contribution to the ultrafast charge current is found. Its magnitude amounts to as much as about 20% of that found in the F|Pt reference sample. Symmetry arguments and first-principles calculations strongly suggest that the interfacial S2C arises from skew scattering of spin-polarized electrons at interface imperfections. The results highlight the potential of skew scattering for interfacial S2C and propose a promising route to enhanced S2C by tailored interfaces at all frequencies from DC to terahertz.

A light-induced phononic symmetry switch and giant dissipationless topological photocurrent in ZrTe5

Dissipationless currents from topologically protected states are promising for disorder-tolerant electronics and quantum computation. Here, we photogenerate giant anisotropic terahertz nonlinear currents with vanishing scattering, driven by laser-induced coherent phonons of broken inversion symmetry in a centrosymmetric Dirac material ZrTe₅. Our work suggests that this phononic terahertz symmetry switching leads to formation of Weyl points, whose chirality manifests in a transverse, helicity-dependent current, orthogonal to the dynamical inversion symmetry breaking axis, via circular photogalvanic effect. The temperature-dependent topological photocurrent exhibits several distinct features: Berry curvature dominance, particle-hole reversal near conical points and chirality protection that is responsible for an exceptional ballistic transport length of ~10 μm. These results, together with first-principles modelling, indicate two pairs of Weyl points dynamically created by B_1u phonons of broken inversion symmetry. Such phononic terahertz control breaks ground for coherent manipulation of Weyl nodes and robust quantum transport without application of static electric or magnetic fields.

Reversible engineering of topological insulator surface state conductivity through optical excitation

Despite the broadband response, limited optical absorption at a particular wavelength hinders the development of optoelectronics based on Dirac fermions. Heterostructures of graphene and various semiconductors have been explored for this purpose, while non-ideal interfaces often limit the performance. The topological insulator (TI) is a natural hybrid system, with the surface states hosting high-mobility Dirac fermions and the small-bandgap semiconducting bulk state strongly absorbing light. In this work, we show a large photocurrent response from a field effect transistor device based on intrinsic TI Sn-Bi_1.1Sb_0.9Te₂S (Sn-BSTS). The photocurrent response is non-volatile and sensitively depends on the initial Fermi energy of the surface state, and it can be erased by controlling the gate voltage. Our observations can be explained with a remote photo-doping mechanism, in which the light excites the defects in the bulk and frees the localized carriers to the surface state. This photodoping modulates the surface state conductivity without compromising the mobility, and it also significantly modify the quantum Hall effect of the surface state. Our work thus illustrates a route to reversibly manipulate the surface states through optical excitation, shedding light into utilizing topological surface states for quantum optoelectronics.

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.

Giant topological longitudinal circular photo-galvanic effect in the chiral multifold semimetal CoSi

The absence of mirror symmetry, or chirality, is behind striking natural phenomena found in systems as diverse as DNA and crystalline solids. A remarkable example occurs when chiral semimetals with topologically protected band degeneracies are illuminated with circularly polarized light. Under the right conditions, the part of the generated photocurrent that switches sign upon reversal of the light's polarization, known as the circular photo-galvanic effect, is predicted to depend only on fundamental constants. The conditions to observe quantization are non-universal, and depend on material parameters and the incident frequency. In this work, we perform terahertz emission spectroscopy with tunable photon energy from 0.2 -1.1 eV in the chiral topological semimetal CoSi. We identify a large longitudinal photocurrent peaked at 0.4 eV reaching  ~550 μ A/V², which is much larger than the photocurrent in any chiral crystal reported in the literature. Using first-principles calculations we establish that the peak originates only from topological band crossings, reaching 3.3 ± 0.3 in units of the quantization constant. Our calculations indicate that the quantized circular photo-galvanic effect is within reach in CoSi upon doping and increase of the hot-carrier lifetime. The large photo-conductivity suggests that topological semimetals could potentially be used as novel mid-infrared detectors.

Defect tolerant zero-bias topological photocurrent in a ferroelectric semiconductor

Lattice defect is a major cause of energy dissipation in conventional electric current due to the drift and diffusion motions of electrons. Different nature of current emerges when noncentrosymmetric materials are excited by light. This current, called the shift current, originates from the change in the Berry connection of electrons' wave functions during the interband optical transition. Here, we demonstrate the defect tolerance of shift current using single crystals of ferroelectric semiconductor antimony sulfoiodide (SbSI). Although the dark conductance spreads over several orders of magnitude in each crystal due to the difference in the density of defect levels, the observed shift current converges to an identical value. We also reveal that the shift current is scarcely disturbed by the surface defects while they drastically suppress the conventional photocurrent. The defect tolerance is a manifestation of the topological nature of shift current, which will be a crucial advantage in optoelectronic applications.

Enhanced Shift Currents in Monolayer 2D GeS and SnS by Strain-Induced Band Gap Engineering

Group IV monochalcogenides exhibit spontaneous polarization and ferroelectricity, which are important in photovoltaic materials. Since strain engineering plays an important role in ferroelectricity, in the present work, the effect of equibiaxial strain on the band structure and shift currents in monolayer two-dimensional (2D) GeS and SnS has systematically been investigated using the first-principles calculations. The conduction bands of those materials are more responsive to strain than the valence bands. Increased equibiaxial compressive strain leads to a drastic reduction in the band gap and finally the occurrence of phase transition from semiconductor to metal at strains of -15 and -14% for GeS and SnS, respectively. On the other hand, tensile equibiaxial strain increases the band gap slightly. Similarly, increased equibiaxial compressive strain leads to a steady almost four times increase in the shift currents at a strain of -12% with direction change occurring at -8% strain. However, at phase transition from semiconductor to metal, the shift currents of the two materials completely vanish. Equibiaxial tensile strain also leads to increased shift currents. For SnS, shift currents do not change direction, just as the case of GeS at low strain; however, at a strain of +8% and beyond, direction reversal of shift currents beyond the band gap in GeS occur.

Enhanced Photoinduced Carrier Generation Efficiency through Surface Band Bending in Topological Insulator Bi2Se3 Thin Films by the Oxidized Layer

Topological insulators (TIs) have become popular in the field of optoelectronic devices because of their broadband and high-sensitivity properties, which are attributed to the narrow band gap of the bulk state and high mobility of the Dirac surface state. Although perfectly grown TIs are known to exhibit strong stability against oxidation, in most cases, the existence of vacancy defects in TIs reacts to air and the characteristics of TIs is affected by oxidation. Therefore, changes in the band structure and electrical characteristics by oxidation should be considered. A significant change occurs because of the oxidation; however, the dependence of the photoresponse of TIs on oxidation has not been studied in detail. In this study, the photoresponsivity of oxidized Bi₂Se₃ films is enhanced, rather than degraded, after oxidation in air for 24 h, resulting in a maximum responsivity of 140 mA W^-1. This responsivity is substantially higher than previously reported values for Bi₂Se₃. Furthermore, a change in the photoresponse time of Bi₂Se₃ due to air exposure is systematically observed. Based on variations in the Fermi level and work function, using photoelectron spectroscopy, it is confirmed that the responsivity is improved from the junction effect of the Bi-based surface oxidized layer.

Chiral terahertz wave emission from the Weyl semimetal TaAs

Weyl semimetals host chiral fermions with distinct chiralities and spin textures. Optical excitations involving those chiral fermions can induce exotic carrier responses, and in turn lead to novel optical phenomena. Here, we discover strong coherent terahertz emission from Weyl semimetal TaAs, which is demonstrated as a unique broadband source of the chiral terahertz wave. The polarization control of the THz emission is achieved by tuning photoexcitation of ultrafast photocurrents via the photogalvanic effect. In the near-infrared regime, the photon-energy dependent nonthermal current due to the predominant circular photogalvanic effect can be attributed to the radical change of the band velocities when the chiral Weyl fermions are excited during selective optical transitions between the tilted anisotropic Weyl cones and the massive bulk bands. Our findings provide a design concept for creating chiral photon sources using quantum materials and open up new opportunities for developing ultrafast opto-electronics using Weyl physics.

Temperature and excitation wavelength dependence of circular and linear photogalvanic effect in a three dimensional topological insulator Bi2Se3

The circular (CPGE) and linear photogalvanic effect (LPGE) of a three-dimensional topological insulator Bi₂Se₃ thin film of seven quintuple layers excited by near-infrared (1064 nm) and mid-infrared (10.6 [Formula: see text]m) radiations have been investigated. The comparison of the CPGE current measured parallel and perpendicular to the incident plane, together with the comparison of the CPGE current under front and back illuminations, indicates that the CPGE under front illumination of 1064 nm light is dominated by the top surface states of the Bi₂Se₃ thin film. The CPGE current excited by 10.6 [Formula: see text]m light is about one order larger than that excited by 1064 nm light, which may be attributed to the smaller cancelation effect of the CPGE generated in the two-dimensional electron gas when excited by 10.6 [Formula: see text]m light. Under the excitation of 1064 nm light, the LPGE current is dominated by the component which shows an even parity of incident angles, while the LPGE current excited by 10.6 [Formula: see text]m light is mainly contributed by the component which is an odd parity of incident angles. Both of the CPGE and LPGE currents excited by 1064 nm decrease with increasing temperature, which may be owing to the decrease of the momentum relaxation time and the stronger electron-electron scattering with increasing temperature, respectively.

Band-Resolved Imaging of Photocurrent in a Topological Insulator

We study the microscopic origins of photocurrent generation in the topological insulator Bi_{2}Se_{3} via time- and angle-resolved photoemission spectroscopy. We image the unoccupied band structure as it evolves following a circularly polarized optical excitation and observe an asymmetric electron population in momentum space, which is the spectroscopic signature of a photocurrent. By analyzing the rise times of the population we identify which occupied and unoccupied electronic states are coupled by the optical excitation. We conclude that photocurrents can only be excited via resonant optical transitions coupling to spin-orbital textured states. Our work provides a microscopic understanding of how to control photocurrents in systems with spin-orbit coupling and broken inversion symmetry.

Terahertz surface and interface emission spectroscopy for advanced materials

Surfaces and interfaces are of particular importance for optoelectronic and photonic materials as they are involved in many physical and chemical processes such as carrier dynamics, charge transfer, chemical bonding, transformation reactions and so on. Terahertz (THz) emission spectroscopy provides a sensitive and nondestructive method for surface or interface analysis of advanced materials ranging from graphene to transition metal dichalcogenides, topological insulators, hybrid perovskites, and mixed-dimensional heterostructures based on 2D materials. In this review paper, we start with the THz radiation mechanisms under ultrafast laser excitation. Then we concentrate on the recent progresses of THz emission spectroscopy on the surface and interface properties of advanced materials, including transient surface photocurrents, surface nonlinear polarization, surface states, interface potential, and gas molecule adsorption/desorption processes. This novel spectroscopic method can not only promote the development of new and compact THz sources, but also provide a nondestructive optical method for surface and interface characterization of photocurrent and nonlinear polarization dynamics of materials.

Ultrafast Zero-Bias Surface Photocurrent in Germanium Selenide: Promise for Terahertz Devices and Photovoltaics

Theory predicts that a large spontaneous electric polarization and concomitant inversion symmetry breaking in GeSe monolayers result in a strong shift current in response to their excitation in the visible range. Shift current is a coherent displacement of electron density on the order of a lattice constant upon above-bandgap photoexcitation. A second-order nonlinear effect, it is forbidden by the inversion symmetry in the bulk GeSe crystals. Here, we use terahertz (THz) emission spectroscopy to demonstrate that ultrafast photoexcitation with wavelengths straddling both edges of the visible spectrum, 400 and 800 nm, launches a shift current in the surface layer of a bulk GeSe crystal, where the inversion symmetry is broken. The direction of the surface shift current determined from the observed polarity of the emitted THz pulses depends only on the orientation of the sample and not on the linear polarization direction of the excitation. Strong absorption by the low-frequency infrared-active phonons in the bulk of GeSe limits the bandwidth and the amplitude of the emitted THz pulses. We predict that reducing GeSe thickness to a monolayer or a few layers will result in a highly efficient broadband THz emission. Experimental demonstration of THz emission by the surface shift current in bulk GeSe crystals puts this 2D material forward as a candidate for next-generation shift current photovoltaics, nonlinear photonic devices, and THz sources.

Spectral dynamics of shift current in ferroelectric semiconductor SbSI

Photoexcitation in solids brings about transitions of electrons/holes between different electronic bands. If the solid lacks an inversion symmetry, these electronic transitions support spontaneous photocurrent due to the geometric phase of the constituting electronic bands: the Berry connection. This photocurrent, termed shift current, is expected to emerge on the timescale of primary photoexcitation process. We observe ultrafast evolution of the shift current in a prototypical ferroelectric semiconductor antimony sulfur iodide (SbSI) by detecting emitted terahertz electromagnetic waves. By sweeping the excitation photon energy across the bandgap, ultrafast electron dynamics as a source of terahertz emission abruptly changes its nature, reflecting a contribution of Berry connection on interband optical transition. The shift excitation carries a net charge flow and is followed by a swing over of the electron cloud on a subpicosecond timescale. Understanding these substantive characters of the shift current with the help of first-principles calculation will pave the way for its application to ultrafast sensors and solar cells.

Nanoscale Near-Field Tomography of Surface States on (Bi0.5Sb0.5)2Te3

Three-dimensional topological insulators (TIs) have attracted tremendous interest for their possibility to host massless Dirac Fermions in topologically protected surface states (TSSs), which may enable new kinds of high-speed electronics. However, recent reports have outlined the importance of band bending effects within these materials, which results in an additional two-dimensional electron gas (2DEG) with finite mass at the surface. TI surfaces are also known to be highly inhomogeneous on the nanoscale, which is masked in conventional far-field studies. Here, we use near-field microscopy in the mid-infrared spectral range to probe the local surface properties of custom-tailored (Bi_0.5Sb_0.5)₂Te₃ structures with nanometer precision in all three spatial dimensions. Applying nanotomography and nanospectroscopy, we reveal a few-nanometer-thick layer of high surface conductivity and retrieve its local dielectric function without assuming any model for the spectral response. This allows us to directly distinguish between different types of surface states. An intersubband transition within the massive 2DEG formed by quantum confinement in the bent conduction band manifests itself as a sharp, surface-bound, Lorentzian-shaped resonance. An additional broadband background in the imaginary part of the dielectric function may be caused by the TSS. Tracing the intersubband resonance with nanometer spatial precision, we observe changes of its frequency, likely originating from local variations of doping or/and the mixing ratio between Bi and Sb. Our results highlight the importance of studying the surfaces of these novel materials on the nanoscale to directly access the local optical and electronic properties via the dielectric function.

Recent Progress and Future Prospects of 2D-Based Photodetectors

Conventional semiconductors such as silicon- and indium gallium arsenide (InGaAs)-based photodetectors have encountered a bottleneck in modern electronics and photonics in terms of spectral coverage, low resolution, nontransparency, nonflexibility, and complementary metal-oxide-semiconductor (CMOS) incompatibility. New emerging two-dimensional (2D) materials such as graphene, transition metal dichalcogenides (TMDs), and their hybrid systems thereof, however, can circumvent all these issues benefitting from mechanically flexibility, extraordinary electronic and optical properties, as well as wafer-scale production and integration. Heterojunction-based photodiodes based on 2D materials offer ultrafast and broadband response from the visible to far-infrared range. Phototransistors based on 2D hybrid systems combined with other material platforms such as quantum dots, perovskites, organic materials, or plasmonic nanostructures yield ultrasensitive and broadband light-detection capabilities. Notably the facile integration of 2D photodetectors on silicon photonics or CMOS platforms paves the way toward high-performance, low-cost, broadband sensing and imaging modalities.

Ultrafast Spin-to-Charge Conversion at the Surface of Topological Insulator Thin Films

Strong spin-orbit coupling, resulting in the formation of spin-momentum-locked surface states, endows topological insulators with superior spin-to-charge conversion characteristics, though the dynamics that govern it have remained elusive. Here, an all-optical method is presented, which enables unprecedented tracking of the ultrafast dynamics of spin-to-charge conversion in a prototypical topological insulator Bi₂ Se₃ /ferromagnetic Co heterostructure, down to the sub-picosecond timescale. Compared to pure Bi₂ Se₃ or Co, a giant terahertz emission is observed in the heterostructure that originates from spin-to-charge conversion, in which the topological surface states play a crucial role. A 0.12 ps timescale is identified that sets a technological speed limit of spin-to-charge conversion processes in topological insulators. In addition, it is shown that the spin-to-charge conversion efficiency is temperature independent in Bi₂ Se₃ as expected from the nature of the surface states, paving the way for designing next-generation high-speed optospintronic devices based on topological insulators at room temperature.

The Extremely Enhanced Photocurrent Response in Topological Insulator Nanosheets with High Conductance

The photocurrent was performed in topological insulator nanosheets with different conductances. The higher photocurrent is observed in the nanosheet with higher conductance. The responsivity is proportional to the nanosheet conductance over two orders. The responsivity is independent of the light power intensity in vacuum, but responsivity drastically decreases at low power intensity in air. The ratio of the responsivity in air to that in vacuum is negatively proportional to the the inverse of the light power intensity. These behaviors are understood as the statistical photocurrent in a system with blocked molecules. The time constant decreases as the thickness increases. A longer time constant is observed in lower atmosphere pressure.

Bi2Se3 Sensitized TiO2 Nanotube Films for Photogenerated Cathodic Protection of 304 Stainless Steel Under Visible Light

Titanium dioxide (TiO₂) nanotube arrays coupled with a narrow gap semiconductor-bismuth selenide (Bi₂Se₃)-exhibited remarkable enhancement in the photocathodic protection property for 304 stainless steel under visible light. Bi₂Se₃/TiO₂ nanocomposites were successfully synthesized using a simple two-step method, including an electrochemical anodization method for preparing pure TiO₂ and a chemical bath deposition method for synthesizing Bi₂Se₃ nanoflowers. The morphology and structure of the composite films were studied by scanning electron microscopy, energy dispersion spectroscopy, X-ray photoelectron spectroscopy and X-ray diffraction. In addition, the influence of the Bi₂Se₃ content on the photoelectrochemical and photocathodic protection properties of the composite films was also studied. The photocurrent density of the Bi₂Se₃/TiO₂ nanocomposites was significantly higher than that of pure TiO₂ under visible light. The sensitizer Bi₂Se₃ enhanced the efficient separation of the photogenerated electron-hole pairs and the photocathodic protection properties of TiO₂. Under visible light illumination, Bi₂Se₃/TiO₂ nanocomposites synthesized by the chemical bath deposition method with Bi³⁺ (0.5 mmol/L) exhibited the optimal photogenerated cathodic protection performance for 304 stainless steel.

Revealing Optical Transitions and Carrier Recombination Dynamics within the Bulk Band Structure of Bi2Se3

Bismuth selenide (Bi₂Se₃) is a prototypical 3D topological insulator whose Dirac surface states have been extensively studied theoretically and experimentally. Surprisingly little, however, is known about the energetics and dynamics of electrons and holes within the bulk band structure of the semiconductor. We use mid-infrared femtosecond transient reflectance measurements on a single nanoflake to study the ultrafast thermalization and recombination dynamics of photoexcited electrons and holes within the extended bulk band structure over a wide energy range (0.3 to 1.2 eV). Theoretical modeling of the reflectivity spectral line shapes at 10 K demonstrates that the electrons and holes are photoexcited within a dense and cold electron gas with a Fermi level positioned well above the bottom of the lowest conduction band. Direct optical transitions from the first and the second spin-orbit split valence bands to the Fermi level above the lowest conduction band minimum are identified. The photoexcited carriers thermalize rapidly to the lattice temperature within a couple of picoseconds due to optical phonon emission and scattering with the cold electron gas. The minority carrier holes recombine with the dense electron gas within 150 ps at 10 K and 50 ps at 300 K. Such knowledge of interaction of electrons and holes within the bulk band structure provides a foundation for understanding how such states interact dynamically with the topologically protected Dirac surface states.

Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27 fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current j_s arises on the same ~100 fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal–insulator interface. Analytical modeling shows that the electrons’ dynamics are almost instantaneously imprinted onto j_s because their spins have a correlation time of only ~4 fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.

Probing spin pumping in the terahertz regime allows one to reveal its initial elementary steps. Here, the authors show that the formation of the spin Seebeck current in YIG/Pt critically relies on hot thermalized metal electrons because they impinge on the metal-insulator interface with maximum noise.

A cascading nonlinear magneto-optical effect in topological insulators

Topological insulators (TIs) are characterized by possessing metallic (gapless) surface states and a finite band-gap state in the bulk. As the thickness of a TI layer decreases down to a few nanometers, hybridization between the top and bottom surfaces takes place due to quantum tunneling, consequently at a critical thickness a crossover from a 3D-TI to a 2D insulator occurs. Although such a crossover is generally accessible by scanning tunneling microscopy, or by angle-resolved photoemission spectroscopy, such measurements require clean surfaces. Here, we demonstrate that a cascading nonlinear magneto-optical effect induced via strong spin-orbit coupling can examine such crossovers. The helicity dependence of the time-resolved Kerr rotation exhibits a robust change in periodicity at a critical thickness, from which it is possible to predict the formation of a Dirac cone in a film several quintuple layers thick. This method enables prediction of a Dirac cone using a fundamental nonlinear optical effect that can be applied to a wide range of TIs and related 2D materials.

Inverse spin Hall effect induced by linearly polarized light in the topological insulator Bi2Se3

The inverse spin Hall effect (ISHE) induced by the normal incidence of linearly-polarized infrared radiation has been observed in the topological insulator Bi₂Se₃. A model has been proposed to explain the phenomenon, and the spin transverse force has been determined by the model fitting. The anomalous linear photogalvanic effect (ALPGE) is also observed, and the photoinduced momentum anisotropy is extracted. Furthermore, the ISHE and ALPGE are investigated at different temperatures between 77 and 300 K, and the temperature dependence of the spin transverse force and photoinduced momentum anisotropy are obtained. This study suggests a new way to investigate the inverse spin Hall effect via linearly polarized light even at room temperature.

Spin Hall photoconductance in a three-dimensional topological insulator at room temperature

Three-dimensional topological insulators are a class of Dirac materials, wherein strong spin-orbit coupling leads to two-dimensional surface states. The latter feature spin-momentum locking, i.e., each momentum vector is associated with a spin locked perpendicularly to it in the surface plane. While the principal spin generation capability of topological insulators is well established, comparatively little is known about the interaction of the spins with external stimuli like polarized light. We observe a helical, bias-dependent photoconductance at the lateral edges of topological Bi₂Te₂Se platelets for perpendicular incidence of light. The same edges exhibit also a finite bias-dependent Kerr angle, indicative of spin accumulation induced by a transversal spin Hall effect in the bulk states of the Bi₂Te₂Se platelets. A symmetry analysis shows that the helical photoconductance is distinct to common longitudinal photoconductance and photocurrent phenomena, but consistent with optically injected spins being transported in the side facets of the platelets.

While the spin generation in topological insulators is well studied, little is known about the interaction of the spins with external stimuli. Here, Seifert et al. observe a helical, bias-dependent photoconductance at the lateral edges of topological Bi₂Te₂Se platelets for perpendicular incidence of light, distinct to common longitudinal photoconductance phenomena.