Helicity-dependent Photocurrent Research Articles

Helicity-dependent photocurrent of topological surface states in the intrinsic magnetic topological insulator MnBi2Te4

Helicity-dependent photocurrent (HDPC) of the topological surface states (TSSs) in the intrinsic magnetic topological insulator MnBi2Te4 is investigated. It is revealed that the HDPC is mainly contributed by the circular photogalvanic effect (CPGE) current when the incident plane is perpendicular to the connection of the two electrodes, while the circular photon drag effect plays the dominant role when the incident plane is parallel to the connection of the two electrodes. The CPGE current shows an odd function dependence on incident angles, which is consistent with the C3v symmetry group of the TSSs in MnBi2Te4. The amplitude of the CPGE current increases with the decrease in temperature, which can be attributed to the increase in mobility at low temperatures, confirmed by the transport measurements. Furthermore, we modulate the CPGE of MnBi2Te4 by applying top gate and source–drain voltages. Compared to Bi2Te3 of the same thickness, the CPGE current of MnBi2Te4 can be more effectively tuned by the top gate because the Fermi level of MnBi2Te4 can be effectively regulated by the top gate, and it is tuned across the Dirac point. This work suggests that the intrinsic magnetic topological insulator MnBi2Te4 is a good candidate for designing opto-spintronics devices.

Chirality manipulation of ultrafast phase switches in a correlated CDW-Weyl semimetal

Light engineering of correlated states in topological materials provides a new avenue of achieving exotic topological phases inaccessible by conventional tuning methods. Here we demonstrate a light control of correlation gaps in a model charge-density-wave (CDW) and polaron insulator (TaSe4)2I recently predicted to be an axion insulator. Our ultrafast terahertz photocurrent spectroscopy reveals a two-step, non-thermal melting of polarons and electronic CDW gap via the fluence dependence of a longitudinal circular photogalvanic current. This helicity-dependent photocurrent reveals continuous ultrafast phase switches from the polaronic state to the CDW (axion) phase, and finally to a hidden Weyl phase as the pump fluence increases. Additional distinctive attributes aligning with the light-induced switches include: the mode-selective coupling of coherent phonons to the polaron and CDW modulation, and the emergence of a non-thermal chiral photocurrent above the pump threshold of CDW-related phonons. The demonstrated ultrafast chirality control of correlated topological states here holds large potentials for realizing axion electrodynamics and advancing quantum-computing 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.

Gate voltage control of helicity-dependent photocurrent and polarization detection in (Bi1−xSbx)2Te3 topological insulator thin films

Optical helicity provides us with an effective means to control the helicity-dependent photocurrent in the spin-momentum-locked surface states of topological insulators (TIs). Also, the TIs show potential in polarization detection as an intrinsic solid-state optical chirality detector for easier integration and fabrication. However, the complex photoresponses with the circular photogalvanic effect, the linear photogalvanic effect, and the photon drag effect in the TIs prevent them from direct chirality detection of the elliptically polarized light. Here, by fitting with the theoretical models to the measured photocurrents, the microscopic origin of different components of the helicity-dependent photocurrent has been demonstrated. We show a comprehensive study of the helicity-dependent photocurrent in (Bi1−xSb x )2Te3 thin films of different thicknesses as a function of the light incident angle and the gate-tuned chemical potential. The observation of the light incident angle dependence of the helicity-dependent photocurrent provides us with a polarization detection strategy using a TI thin film without the use of any additional optical elements, and the detection accuracy can be enhanced by gate tuning. Additionally, the Stokes parameters can be extracted by arithmetic operation of photocurrents measured with different incident angles and gating voltages for complete characterization of the polarization states of a light beam. Using this means, we realize the polarization detection and the Stokes parameters analysis with a single device. Our work provides an alternative solution to develop miniaturized intrinsic polarization-sensitive photodetectors.

Anomalous Skew-Scattering Nonlinear Hall Effect and Chiral Photocurrents in PT-Symmetric Antiferromagnets.

Berry curvature and skew scattering play central roles in determining both the linear and nonlinear anomalous Hall effects. Yet in PT-symmetric antiferromagnetic metals, Hall effects from either intrinsic Berry curvature mediated anomalous velocity or the conventional skew-scattering process individually vanish. Here we reveal an unexpected nonlinear Hall effect that relies on both Berry curvature and skew-scattering working in cooperation. This anomalous skew-scattering nonlinear Hall effect (ASN) is PT even and dominates the low-frequency nonlinear Hall effect for PT-symmetric antiferromagnetic metals. Surprisingly, we find that in addition to its Hall response, ASN produces helicity dependent photocurrents, in contrast to other known PT-even nonlinearities in metals that are helicity blind. This characteristic enables us to isolate ASN and establishes new photocurrent tools to interrogate the antiferromagnetic order of PT-symmetric metals.

Electric Control of Helicity-Dependent Photocurrent and Surface Polarity Detection on Two-Dimensional Bi2O2Se Nanosheets.

Bismuth oxyselenide (Bi2O2Se) is a two-dimensional (2D) layered semiconductor material with high electron Hall mobility and excellent environmental stability as well as strong spin-orbit interaction (SOI), which has attracted intense attention for application in spintronic and spin optoelectronic devices. However, a comprehensive study of spin photocurrent and its microscopic origin in Bi2O2Se is still missing. Here, the helicity-dependent photocurrent (HDPC) was investigated in Bi2O2Se nanosheets. By analyzing the dependence of HDPC on the angle of incidence, we find that the HDPC originates from surface states with Cs symmetry in Bi2O2Se, which can be attributed to the circular photogalvanic effect (CPGE) and circular photon drag effect (CPDE). It is revealed that the HDPC current almost changes linearly with the source-drain voltage. Furthermore, we demonstrate effective tuning of HDPC in Bi2O2Se by ionic liquid gating, indicating that the spin splitting of the surface electronic structure is effectively tuned. By analyzing the gate voltage dependence of HDPC, we can unambiguously identify the surface polarity and the surface electronic structure of Bi2O2Se. The large HDPC in Bi2O2Se nanosheets and its efficient electrical tuning demonstrate that 2D Bi2O2Se nanosheets may provide a good platform for opto-spintronics devices.

Different spin relaxation properties observed in linearly and circularly polarized laser induced terahertz emission from a Bi/Co bilayer

Recently, helicity-dependent photocurrent was reported in Bi single thin fi lms. It is proposed that the origin of this photocurrent is the combination of photo-spin conversion and spin-charge conversion effects in Bi and efficient spin conversion in Bi is expected. In this study, we measured two types of terahertz (THz) emissions from Bi/Co bilayer films induced by spin current generation using laser-induced demagnetization of the Co layer and photo-spin conversion effect in the Bi layer to investigate the spin current induced by the two mechanisms simultaneously. We clearly observed diff erent Bi thickness dependence of peak intensity and that of bandwidth for THz spin current in two experiments, i.e., spin current induced by demagnetization of Co and that by photo-spin conversion in Bi. The different Bi thickness dependence of spin current intensity and bandwidth in two experiments is caused by different spin relaxation properties of optically excited spin currents in Bi layers.

Unveiling Weyl-related optical responses in semiconducting tellurium by mid-infrared circular photogalvanic effect

Elemental tellurium, conventionally recognized as a narrow bandgap semiconductor, has recently aroused research interests for exploiting Weyl physics. Chirality is a unique feature of Weyl cones and can support helicity-dependent photocurrent generation, known as circular photogalvanic effect. Here, we report circular photogalvanic effect with opposite signs at two different mid-infrared wavelengths which provides evidence of Weyl-related optical responses. These two different wavelengths correspond to two critical transitions relating to the bands of different Weyl cones and the sign of circular photogalvanic effect is determined by the chirality selection rules within certain Weyl cone and between two different Weyl cones. Further experimental evidences confirm the observed response is an intrinsic second-order process. With flexibly tunable bandgap and Fermi level, tellurium is established as an ideal semiconducting material to manipulate and explore chirality-related Weyl physics in both conduction and valence bands. These results are also directly applicable to helicity-sensitive optoelectronics devices.

Photothermal Control of Helicity-Dependent Current in Epitaxial Sb2Te2Se Topological Insulator Thin-Films at Ambient Temperature.

Optical control of helicity-dependent photocurrent in topological insulator (TI) Sb2Te2Se has been studied at room temperature on epitaxial thin-films grown by pulsed laser deposition (PLD). Comparison with a theoretical model, which fits the data very well, reveals different contributions to the measured photocurrent. Study of the dependence of photocurrent on the angle of incidence (wave-vector) of the excitation light with respect to the sample normal helps to identify the origin of different components of the photocurrent. Enhancement and inversion of the photocurrent in the presence of the photothermal gradient for light incident on two opposite edges of the sample occur due to selective spin-state excitation with two opposite circularly polarized lights in the presence of the unique spin-momentum locked surface states. These observations render the PLD-grown epitaxial TI thin-films promising for optoelectronic devices such as sensors, switches, and actuators whose response can be controlled by polarization as well as the angle of incidence of light under ambient conditions. The polarization response can also be tuned by the photothermal effect by suitably positioning the incident light beam on the device.

Impact of domain disorder on optoelectronic properties of layered semimetal MoTe2

We address the impact of crystal phase disorder on the generation of helicity-dependent photocurrents in layered MoTe2, which is one of the van der Waals materials to realize the topological type-II Weyl semimetal phase. Using scanning photocurrent microscopy, we spatially probe the phase transition and its hysteresis between the centrosymmetric, monoclinic 1Tʹ phase to the symmetry-broken, orthorhombic T phase as a function of temperature. We find a highly disordered photocurrent response in the intermediate temperature regime. Moreover, we demonstrate that helicity-dependent and ultrafast photocurrents in MoTe2 arise most likely from a local breaking of the electronic symmetries. Our results highlight the prospects of local domain morphologies and ultrafast relaxation dynamics on the optoelectronic properties of low-dimensional van der Waals circuits.

Modeling of the photocurrent induced by inverse spin Hall effect under local circularly polarized photoexcitation

The inverse spin Hall effect (ISHE) has been recently demonstrated through the photoinduced inverse spin Hall effect (PISHE), where a focused laser normal to the device plane generates photocurrent that depends on the helicity of the photoexcitation. Here, we have employed a finite element method to rigorously simulate the helicity-dependent photocurrent (HDPC) under local helical photoexcitation, taking into account the complications of minority carriers, metal contact junctions, and spin relaxation. We found that the ISHE-induced electromotive force is inversely proportional to the doping level of the material, caused by the diffusion and drift current balance. Furthermore, the HDPC near the metal contact can either increase or decrease, because of the competing mechanisms of fast carrier recombination and spin relaxation at the contact. These simulation results provide insightful visualization of charge, spin, electric field, and current density distributions and deeper understanding of photoinduced ISHE.

Interface-enhanced helicity dependent photocurrent in metal/semimetal bilayers

One of the hallmarks of light-spin interaction in solids is the appearance of photocurrent that depends on the light helicity. Recent studies have shown that helicity dependent photocurrent (HDP) emerges due to light induced spin current and the inverse spin Hall effect of semimetal thin films. We have studied HDP in metal/semimetal bilayers. Compared to Bi single layer films, we find the HDP is enhanced in metal/Bi bilayers. For the bilayers, the sign of HDP under back illumination reverses from that of front illumination. The back illumination photocurrent is the largest for Ag/Bi bilayers among the bilayers studied. Using a diffusive spin transport model, we show that the HDP sign reversal under back illumination is caused by spin absorption and spin to charge conversion at the interface. Such interfacial effects contribute to the HDP enhancement under front illumination for the bilayers when the Bi layer thickness is small. These results show that the HDP can be used to assess interface states with strong spin orbit coupling.

Nonlinear transport without spin-orbit coupling or warping in two-dimensional Dirac semimetals

It has recently been realized that the first-order moment of the Berry curvature, namely, the Berry curvature dipole (BCD), can give rise to nonlinear current in a wide variety of time-reversal invariant and non-centrosymmetric materials. While the BCD in two-dimensional Dirac systems is known to be finite only in the presence of either substantial spin-orbit coupling where low-energy Dirac quasiparticles form tilted cones or higher order warping of the Fermi surface, we argue that the low-energy Dirac quasiparticles arising from the merging of a pair of Dirac points without any tilt or warping of the Fermi surface can lead to a nonzero BCD. Remarkably, in such systems, the BCD is found to be independent of Dirac velocity as opposed to the Dirac dispersion with a tilt or warping effects. We further show that the proposed systems can naturally host helicity-dependent photocurrent due to their linear momentum-dependent Berry curvatures. Finally, we discuss an important byproduct of this work, i.e., nonlinear anomalous Nernst effect as a second-order thermal response.

Electrical detection of light helicity using a quantum-dot-based hybrid device at zero magnetic field

Photon helicity-dependent photocurrent is measured at zero magnetic field on a device based on an ensemble of (In,Ga)As/GaAs quantum dots that are embedded into a GaAs-based $p$-$i$-$n$ diode. This device can be also operated as a spin light-emitting diode to emit the light with a circular polarization up to 20% at zero magnetic field. The demonstration of the multifunctional capabilities of the device gives valuable insights into the spin relaxation of the electrons in the quantum dots and paves the way for future spin-optoelectronic applications.

Helicity-dependent photocurrents in the chiral Weyl semimetal RhSi.

Weyl semimetals are crystals in which electron bands cross at isolated points in momentum space. Associated with each crossing point (or Weyl node) is a topological invariant known as the Berry monopole charge. The circular photogalvanic effect (CPGE), whereby circular polarized light generates a helicity-dependent photocurrent, is a notable example of a macroscopic property that emerges directly from the topology of the Weyl semimetal band structure. Recently, it was predicted that the amplitude of the CPGE associated with optical transitions near a Weyl node is proportional to its monopole charge. In chiral Weyl systems, nodes of opposite charge are nondegenerate, opening a window of wavelengths where the CPGE resulting from uncompensated Berry charge can emerge. Here, we report measurements of CPGE in the chiral Weyl semimetal RhSi, revealing a CPGE response in an energy window that closes at 0.65 eV, in agreement with the predictions of density functional theory.

Scattering symmetry-breaking induced spin photocurrent from out-of-plane spin texture in a 3D topological insulator

We theoretically study helicity-dependent photocurrent in a three-dimensional topological insulator Bi2Te3 under elastic scattering of different symmetries. By exploring spin-selective optical transitions and symmetry-breaking scattering, we are able to address the out-of-plane spin texture of the topological helical surface states and to generate directional, spin-polarization tunable photocurrent that is otherwise forbidden for the original C3v symmetry of the surface. This can be achieved regardless of the Fermi level, even under the condition when the topological states are inaccessible in dark. This work paves the way to robustly explore the out-of-plane spin texture for harvesting opto-spintronic functionalities of topological insulators.

Circular photocurrent in the carbon nanowall film.

We report the helicity-dependent photocurrent in the carbon nanowall film synthesized on the silicon substrates by the chemical vapor deposition technique. The film is composed of multilayer graphene flakes grown along the substrate normal. We measured the transverse photocurrent generated in the film under irradiation with nanosecond laser pulses by depositing two conductive electrodes along the plane of incidence. The measurements were performed by using elliptically polarized fundamental, second-, third-, and fourth-harmonics beams of the Nd:YAG laser. We revealed that the shorter the excitation wavelength, the higher the magnitude of the helicity-dependent transverse photocurrent generated in the film. In particular, at wavelengths of 266 and 355 nm, the photocurrent strongly depends on the degree of the circular polarization of the laser beam while, at the wavelength of 1064 nm, the transverse photocurrent is almost helicity independent.

Control of Circular Photogalvanic Effect of Surface States in the Topological Insulator Bi2Te3 via Spin Injection.

The circular photogalvanic effect (CPGE) provides a method utilizing circularly polarized light to control spin photocurrent and will also lead to novel opto-spintronic devices. The CPGE of three-dimensional topological insulator Bi2Te3 with different substrates and thicknesses has been systematically investigated. It is found that the CPGE current can be dramatically tuned by adopting different substrates. The CPGE current of the Bi2Te3 films on Si substrates are more than two orders larger than that on SrTiO3 substrates when illuminated by 1064 nm light, which can be attributed to the modulation effect due to the spin injection from Si substrate to Bi2Te3 films, larger light absorption coefficient, and stronger inequivalence between the top and bottom surface states for Bi2Te3 films grown on Si substrates. The excitation power dependence of the CPGE current of Bi2Te3 films on Si substrates shows a saturation at high power especially for thicker samples, whereas that on SrTiO3 substrates almost linearly increases with excitation power. Temperature dependence of the CPGE current of Bi2Te3 films on Si substrates first increases and then decreases with decreasing temperature, whereas that on SrTiO3 substrates changes monotonously with temperature. These interesting phenomena of the CPGE current of Bi2Te3 films on Si substrates are related to the spin injection from Si substrates to Bi2Te3 films. Our work not only intrigues new physics but also provides a method to effectively manipulate the helicity-dependent photocurrent via spin injection.

Circular photogalvanic spectroscopy of Rashba splitting in 2D hybrid organic\u2013inorganic perovskite multiple quantum wells

The two-dimensional (2D) Ruddlesden−Popper organic-inorganic halide perovskites such as (2D)-phenethylammonium lead iodide (2D-PEPI) have layered structure that resembles multiple quantum wells (MQW). The heavy atoms in 2D-PEPI contribute a large spin-orbit coupling that influences the electronic band structure. Upon breaking the inversion symmetry, a spin splitting (‘Rashba splitting’) occurs in the electronic bands. We have studied the spin splitting in 2D-PEPI single crystals using the circular photogalvanic effect (CPGE). We confirm the existence of Rashba splitting at the electronic band extrema of 35±10 meV, and identify the main inversion symmetry breaking direction perpendicular to the MQW planes. The CPGE action spectrum above the bandgap reveals spin-polarized photocurrent generated by ultrafast relaxation of excited photocarriers separated in momentum space. Whereas the helicity dependent photocurrent with below-gap excitation is due to spin-galvanic effect of the ionized spin-polarized excitons, where spin polarization occurs in the spin-split bands due to asymmetric spin-flip.

Helicity-dependent THz emission induced by ultrafast spin photocurrent in nodal-line semimetal candidate Mg3Bi2

Helicity-dependent ultrafast spin current generated by circularly polarized photons in topological materials holds the crux to many technological improvements, such as quantum communications, on-chip communication processing and storage. Here, we present the manipulation of helicity-dependent terahertz emission generated in a nodal line semimetal candidate Mg3Bi2 by using photon polarization states. The terahertz emission is mainly ascribed to the helicity-dependent photocurrent that is originated from circular photogalvanic effects, and the helicity-independent photocurrent that is attributed to linear photogalvanic effect. Our work will inspire more explorations into novel nodal line semimetals and open up new opportunities for developing ultrafast optoelectronics in the topological system.