Nonlinear optical probe of tunable surface electrons on a topological insulator

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.

Manipulation of nonlinear optical responses in layered ferroelectric niobium oxide dihalides

Realization of highly tunable second-order nonlinear optical responses, e.g., second-harmonic generation and bulk photovoltaic effect, is critical for developing modern optical and optoelectronic devices. Recently, the van der Waals niobium oxide dihalides are discovered to exhibit unusually large second-harmonic generation. However, the physical origin and possible tunability of nonlinear optical responses in these materials remain to be unclear. In this article, we reveal that the large second-harmonic generation in NbOX2 (X = Cl, Br, and I) may be partially contributed by the large band nesting effect in different Brillouin zone. Interestingly, the NbOCl2 can exhibit dramatically different strain-dependent bulk photovoltaic effect under different polarized light, originating from the light-polarization-dependent orbital transitions. Importantly, we achieve a reversible ferroelectric-to-antiferroelectric phase transition in NbOCl2 and a reversible ferroelectric-to-paraelectric phase transition in NbOI2 under a certain region of external pressure, accompanied by the greatly tunable nonlinear optical responses but with different microscopic mechanisms. Our study establishes the interesting external-field tunability of NbOX2 for nonlinear optical device applications.

Anomalous Second Harmonic Generation from Atomically Thin MnBi2Te4

MnBi₂Te₄ is a van der Waals topological insulator with intrinsic intralayer ferromagnetic exchange and A-type antiferromagnetic interlayer coupling. Theoretically, it belongs to a class of structurally centrosymmetric crystals whose layered antiferromagnetic order breaks inversion symmetry for even layer numbers, making optical second harmonic generation (SHG) an ideal probe of the coupling between the crystal and magnetic structures. Here, we perform magnetic field and temperature-dependent SHG measurements on MnBi₂Te₄ flakes ranging from bulk to monolayer thickness. We find that the dominant SHG signal from MnBi₂Te₄ is unexpectedly unrelated to both magnetic state and layer number. We suggest that surface SHG is the likely source of the observed strong SHG, whose symmetry matches that of the MnBi₂Te₄-vacuum interface. Our results highlight the importance of considering the surface contribution to inversion symmetry-breaking in van der Waals centrosymmetric magnets.

Nonlinear nanoelectrodynamics of a Weyl metal

Chiral Weyl fermions with linear energy-momentum dispersion in the bulk accompanied by Fermi-arc states on the surfaces prompt a host of enticing optical effects. While new Weyl semimetal materials keep emerging, the available optical probes are limited. In particular, isolating bulk and surface electrodynamics in Weyl conductors remains a challenge. We devised an approach to the problem based on near-field photocurrent imaging at the nanoscale and applied this technique to a prototypical Weyl semimetal TaIrTe₄ As a first step, we visualized nano-photocurrent patterns in real space and demonstrated their connection to bulk nonlinear conductivity tensors through extensive modeling augmented with density functional theory calculations. Notably, our nanoscale probe gives access to not only the in-plane but also the out-of-plane electric fields so that it is feasible to interrogate all allowed nonlinear tensors including those that remained dormant in conventional far-field optics. Surface- and bulk-related nonlinear contributions are distinguished through their "symmetry fingerprints" in the photocurrent maps. Robust photocurrents also appear at mirror-symmetry breaking edges of TaIrTe₄ single crystals that we assign to nonlinear conductivity tensors forbidden in the bulk. Nano-photocurrent spectroscopy at the boundary reveals a strong resonance structure absent in the interior of the sample, providing evidence for elusive surface states.

All-Optical Probe of Three-Dimensional Topological Insulators Based on High-Harmonic Generation by Circularly Polarized Laser Fields

We report the observation of an anomalous nonlinear optical response of the prototypical three-dimensional topological insulator bismuth selenide through the process of high-order harmonic generation. We find that the generation efficiency increases as the laser polarization is changed from linear to elliptical, and it becomes maximum for circular polarization. With the aid of a microscopic theory and a detailed analysis of the measured spectra, we reveal that such anomalous enhancement encodes the characteristic topology of the band structure that originates from the interplay of strong spin-orbit coupling and time-reversal symmetry protection. The implications are in ultrafast probing of topological phase transitions, light-field driven dissipationless electronics, and quantum computation.

Spatial Control of Charge Doping in n-Type Topological Insulators

Spatially controlling the Fermi level of topological insulators and keeping their electronic states stable are indispensable processes to put this material into practical use for semiconductor spintronics devices. So far, however, such a method has not been established yet. Here we show a novel method for doping a hole into n-type topological insulators Bi₂X₃ (X= Se, Te) that overcomes the shortcomings of the previous reported methods. The key of this doping is to adsorb H₂O on Bi₂X₃ decorated with a small amount of carbon, and its trigger is the irradiation of a photon with sufficient energy to excite the core electrons of the outermost layer atoms. This method allows controlling the doping amount by the irradiation time and acts as photolithography. Such a tunable doping makes it possible to design the electronic states at the nanometer scale and, thus, paves a promising avenue toward the realization of novel spintronics devices based on topological insulators.

Table-top extreme ultraviolet second harmonic generation

The lack of available table-top extreme ultraviolet (XUV) sources with high enough fluxes and coherence properties has limited the availability of nonlinear XUV and x-ray spectroscopies to free-electron lasers (FELs). Here, we demonstrate second harmonic generation (SHG) on a table-top XUV source by observing SHG near the Ti M_2,3 edge with a high-harmonic seeded soft x-ray laser. Furthermore, this experiment represents the first SHG experiment in the XUV. First-principles electronic structure calculations suggest the surface specificity and separate the observed signal into its resonant and nonresonant contributions. The realization of XUV-SHG on a table-top source opens up more accessible opportunities for the study of element-specific dynamics in multicomponent systems where surface, interfacial, and bulk-phase asymmetries play a driving role.

A compact and stable incidence-plane-rotating second harmonics detector

We describe a compact and stable setup for detecting the optical second harmonics, in which the incident plane rotates with respect to the sample. The setup is composed of rotating Fresnel rhomb optics and a femtosecond ytterbium-doped fiber laser source operating at the repetition frequency of 10 MHz. The setup including the laser source occupies an area of 1 m² and is stable so that the intensity fluctuation of the laser harmonics can be less than 0.2% for 4 h. We present the isotropic harmonic signal of a gold mirror of 0.5 pW and demonstrate the integrity and sensitivity of the setup. We also show the polarization-dependent six-fold pattern of the harmonics of a few-layer WSe₂, from which we infer the degree of local-field effects. Finally, we describe the extensibility of the setup to investigate the samples in various conditions such as cryogenic, strained, ultrafast non-equilibrium, and high magnetic fields.

Rich information on 2D materials revealed by optical second harmonic generation

Two-dimensional (2D) materials have brought a spectacular revolution in fundamental research and industrial applications due to their unique physical properties of atomically thin thickness, strong light-matter interaction, unity valley polarization and enhanced many-body interactions. To fully explore their exotic physical properties and facilitate potential applications in electronics and optoelectronics, an effective and versatile characterization method is highly demanded. Among the many methods of characterization, optical second harmonic generation (SHG) has attracted broad attention because of its sensitivity, versatility and simplicity. The SHG technique is sufficiently sensitive at the atomic scale and therefore suitable for studies on 2D materials. More importantly, it has the capacity to acquire abundant information ranging from crystallographic, and electronic, to magnetic properties in various 2D materials due to its sensitivity to both spatial-inversion symmetry and time-reversal symmetry. These advantages accompanied by its characteristics of non-invasion and high throughput make SHG a powerful tool for 2D materials. This review summarizes recent experimental developments of SHG applications in 2D materials and also provides an outlook of potential prospects based on SHG.

Observation of chiral surface excitons in a topological insulator Bi2Se3

The protected electron states at the boundaries or on the surfaces of topological insulators (TIs) have been the subject of intense theoretical and experimental investigations. Such states are enforced by very strong spin-orbit interaction in solids composed of heavy elements. Here, we study the composite particles-chiral excitons-formed by the Coulomb attraction between electrons and holes residing on the surface of an archetypical 3D TI, [Formula: see text] Photoluminescence (PL) emission arising due to recombination of excitons in conventional semiconductors is usually unpolarized because of scattering by phonons and other degrees of freedom during exciton thermalization. On the contrary, we observe almost perfectly polarization-preserving PL emission from chiral excitons. We demonstrate that the chiral excitons can be optically oriented with circularly polarized light in a broad range of excitation energies, even when the latter deviate from the (apparent) optical band gap by hundreds of millielectronvolts, and that the orientation remains preserved even at room temperature. Based on the dependences of the PL spectra on the energy and polarization of incident photons, we propose that chiral excitons are made from massive holes and massless (Dirac) electrons, both with chiral spin textures enforced by strong spin-orbit coupling. A theoretical model based on this proposal describes quantitatively the experimental observations. The optical orientation of composite particles, the chiral excitons, emerges as a general result of strong spin-orbit coupling in a 2D electron system. Our findings can potentially expand applications of TIs in photonics and optoelectronics.

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.

An Ultrabroadband Mid-Infrared Pulsed Optical Switch Employing Solution-Processed Bismuth Oxyselenide

Pulsed lasers operating in the mid-infrared (3-25 µm) are increasingly becoming the light source of choice for a wide range of industrial and scientific applications such as spectroscopy, biomedical research, sensing, imaging, and communication. Up to now, one of the factors limiting the mid-infrared pulsed lasers is the lack of optical switch with a capability of pulse generation, especially for those with wideband response. Here, a semiconductor material of bismuth oxyselenide (Bi₂ O₂ Se) with a facile processibility, constituting an ultrabroadband saturable absorber for the mid-infrared (actually from the near-infrared to mid-infrared: 0.8-5.0 µm) is exhibited. Significantly, it is found that the optical response is associated with a strong nonlinear character, showing picosecond response time and response amplitude up to ≈330.1% at 5.0 µm. Combined with facile processibility and low cost, these solution-processed Bi₂ O₂ Se materials may offer a scalable and printable mid-infrared optical switch to open up the long-sought parameter space which is crucial for the exploitation of compact and high-performance mid-infrared pulsed laser sources.

Ultrafast Surface State Spin-Carrier Dynamics in the Topological Insulator Bi_{2}Te_{2}Se

Topological insulators are promising candidates for optically driven spintronic devices, because photoexcitation of spin polarized surface states is governed by angular momentum selection rules. We carry out femtosecond midinfrared spectroscopy on thin films of the topological insulator Bi_{2}Te_{2}Se, which has a higher surface state conductivity compared to conventionally studied Bi_{2}Se_{3} and Bi_{2}Te_{3}. Both charge and spin dynamics are probed utilizing circularly polarized light. With a sub-band-gap excitation, clear helicity-dependent dynamics is observed only in thin (<20  nm) flakes. On the other hand, such dependence is observed for both thin and thick flakes with above-band-gap excitation. The helicity dependence is attributed to asymmetric excitation of the Dirac-like surface states. The observed long-lasting asymmetry over 10 ps even at room temperature indicates low backscattering of surface state carriers which can be exploited for spintronic devices.

Multiphoton excitation and high-harmonics generation in topological insulator

Multiphoton interaction of coherent electromagnetic radiation with 2D metallic carriers confined on the surface of the 3D topological insulator is considered. A microscopic theory describing the nonlinear interaction of a strong wave and metallic carriers with many-body Coulomb interaction is developed. The set of integrodifferential equations for the interband polarization and carrier occupation distribution is solved numerically. Multiphoton excitation of Fermi-Dirac sea of 2D massless carriers is considered for a THz pump wave. It is shown that in the moderately strong pump wave field along with multiphoton interband/intraband transitions the intense radiation of high harmonics takes place.

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.

Controlling the growth of Bi(110) and Bi(111) films on an insulating substrate

We demonstrate the controlled growth of Bi(110) and Bi(111) films on an α-Al₂O₃(0001) substrate by surface x-ray diffraction and x-ray reflectivity using synchrotron radiation. At temperatures as low as 40 K, unanticipated pseudo-cubic Bi(110) films are grown with thicknesses ranging from a few to tens of nanometers. The roughness at the film-vacuum as well as the film-substrate interface, can be reduced by mild heating, where a crystallographic orientation transition of Bi(110) towards Bi(111) is observed at 400 K. From 450 K onwards high quality ultrasmooth Bi(111) films form. Growth around the transition temperature results in the growth of competing Bi(110) and Bi(111) domains.

Possible flat band bending of the Bi1.5Sb0.5Te1.7Se1.3 crystal cleaved in an ambient air probed by terahertz emission spectroscopy

We investigate an evolution of the surface electronic state of the Bi_1.5Sb_0.5Te_1.7Se_1.3 single crystal, which is one of the most bulk insulating topological insulators, by examining terahertz light emitted from the sample surface upon the illumination of the near-infrared femtosecond laser pulses. We find that the surface state with a flat band bending can appear in the course of the natural maturation process of the surface state in an ambient air. Furthermore, we demonstrate that the evolution of the surface electronic state can be accelerated, decelerated, or even stopped by controlling environmental conditions to contain different amount of H₂O, in particular.

Optically induced effective mass renormalization: the case of graphite image potential states

Many-body interactions with the underlying bulk electrons determine the properties of confined electronic states at the surface of a metal. Using momentum resolved nonlinear photoelectron spectroscopy we show that one can tailor these many-body interactions in graphite, leading to a strong renormalization of the dispersion and linewidth of the image potential state. These observations are interpreted in terms of a basic self-energy model, and may be considered as exemplary for optically induced many-body interactions.

Identification of photocurrents in topological insulators

Optical injection and detection of charge currents is an alternative to conventional transport and photoemission measurements, avoiding the necessity of invasive contact that may disturb the system being examined. This is a particular concern for analyzing the surface states of topological insulators. In this work one- and two-color sources of photocurrents are isolated and examined in epitaxial thin films of Bi₂Se₃. We demonstrate that optical excitation and terahertz detection simultaneously captures one- and two-color photocurrent contributions, which has not been required for other material systems. A method is devised to extract the two components, and in doing so each can be related to surface or bulk excitations through symmetry. The separation of such photocurrents in topological insulators opens a new avenue for studying these materials by all-optical methods.

Nonlinear optical observation of coherent acoustic Dirac plasmons in thin-film topological insulators

Low-energy collective electronic excitations exhibiting sound-like linear dispersion have been intensively studied both experimentally and theoretically for a long time. However, coherent acoustic plasmon modes appearing in time-domain measurements are rarely observed due to Landau damping by the single-particle continua. Here we report on the observation of coherent acoustic Dirac plasmon (CADP) modes excited in indirectly (electrostatically) opposite-surface coupled films of the topological insulator Bi₂Se₃. Using transient second-harmonic generation, a technique capable of independently monitoring the in-plane and out-of-plane electron dynamics in the films, the GHz-range oscillations were observed without corresponding oscillations in the transient reflectivity. These oscillations were assigned to the transverse magnetic and transverse electric guided CADP modes induced by the evanescent guided Lamb acoustic waves and remained Landau undamped due to fermion tunnelling between the opposite-surface Dirac states.

Zero-bias photocurrent in ferromagnetic topological insulator

Magnetic interactions in topological insulators cause essential modifications in the originally mass-less surface states. They offer a mass gap at the Dirac point and/or largely deform the energy dispersion, providing a new path towards exotic physics and applications to realize dissipation-less electronics. The nonequilibrium electron dynamics at these modified Dirac states unveil additional functions, such as highly efficient photon to spin-current conversion. Here we demonstrate the generation of large zero-bias photocurrent in magnetic topological insulator thin films on mid-infrared photoexcitation, pointing to the controllable band asymmetry in the momentum space. The photocurrent spectra with a maximal response to the intra-Dirac-band excitations can be a sensitive measure for the correlation between Dirac electrons and magnetic moments.

By magnetic-doping, the electronic band structure of a topological insulator can be significantly altered to yield functional behaviour. Here, the authors demonstrate a large photocurrent response, and its control, under zero-bias in an optimally-designed magnetically-doped topological insulator thin film.

Surface oxidation effect on the electrical behaviour of Bi2Te2Se nanoplatelets

Charge transport in topological insulators is notably influenced by moisture and air in the surrounding environment. At present, however, little is known about the detailed composition of the oxidized surface and its impact on the electrical characteristics of these materials. Here, we investigate the surface oxide formation on the topological insulator Bi2Te2Se (BTS) and how this affects its electrical behavior. While ambient exposure of BTS nanoplatelets predominantly creates surface hydroxyl groups, oxygen plasma treatment yields a compact, few-nanometer thick surface oxide layer. The plasma causes p-type doping, accompanied by a decrease of the effective platelet thickness, the interplay of which is manifested in a resistance maximum as a function of plasma treatment time. It is furthermore demonstrated that the structural integrity of the plasma-derived surface oxide is sufficient to enable its use as a gate insulator layer in combination with a top gate.

Direct measurement of proximity-induced magnetism at the interface between a topological insulator and a ferromagnet

When a topological insulator (TI) is in contact with a ferromagnet, both time-reversal and inversion symmetries are broken at the interface. An energy gap is formed at the TI surface, and its electrons gain a net magnetic moment through short-range exchange interactions. Magnetic TIs can host various exotic quantum phenomena, such as massive Dirac fermions, Majorana fermions, the quantum anomalous Hall effect and chiral edge currents along the domain boundaries. However, selective measurement of induced magnetism at the buried interface has remained a challenge. Using magnetic second-harmonic generation, we directly probe both the in-plane and out-of-plane magnetizations induced at the interface between the ferromagnetic insulator (FMI) EuS and the three-dimensional TI Bi2Se3. Our findings not only allow characterizing magnetism at the TI-FMI interface but also lay the groundwork for imaging magnetic domains and domain boundaries at the magnetic TI surfaces.

Enhancement of the Bulk Photovoltaic Effect in Topological Insulators

We investigate the shift current bulk photovoltaic response of materials close to a band inversion topological phase transition. We find that the bulk photocurrent reverses direction across the band inversion transition, and that its magnitude is enhanced in the vicinity of the phase transition. These results are demonstrated with first principles density functional theory calculations of BiTeI and CsPbI_{3} under hydrostatic pressure, and explained with an analytical model, suggesting that this phenomenon remains robust across disparate material systems.

Observing the interplay between surface and bulk optical nonlinearities in thin van der Waals crystals

Van der Waals materials, existing in a range of thicknesses from monolayer to bulk, allow for interplay between surface and bulk nonlinearities, which otherwise dominate only at atomically-thin or bulk extremes, respectively. Here, we observe an unexpected peak in intensity of the generated second harmonic signal versus the thickness of Indium Selenide crystals, in contrast to the quadratic increase expected from thin crystals. We explain this by interference effects between surface and bulk nonlinearities, which offer a new handle on engineering the nonlinear optical response of 2D materials and their heterostructures.

Anisotropic Terahertz Emission from Bi2Se3 Thin Films with Inclined Crystal Planes

We investigate the surface states of topological insulator (TI) Bi2Se3 thin films grown on Si nanocrystals and Al2O3 substrates by using terahertz (THz) emission spectroscopy. Compared to bulk crystalline Bi2Te2Se, film TIs exhibit distinct behaviors in the phase and amplitude of emitted THz radiation. In particular, Bi2Se3 grown on Al2O3 shows an anisotropic response with a strong modulation of the THz signal in its phase. From x-ray diffraction, we find that the crystal plane of the Bi2Se3 films is inclined with respect to the plane of the Al2O3 substrate by about 0.27°. This structural anisotropy affects the dynamics of photocarriers and hence leads to the observed anisotropic response in the THz emission. Such relevance demonstrates that THz emission spectroscopy can be a sensitive tool to investigate the fine details of the surface crystallography and electrostatics of thin film TIs.

High-speed measurement of rotational anisotropy nonlinear optical harmonic generation using position-sensitive detection

We present a method of performing high-speed rotational anisotropy nonlinear optical harmonic generation experiments at rotational frequencies of several hertz by projecting the harmonic light reflected at different angles from a sample onto a stationary position-sensitive detector. The high rotational speed of the technique, 10(3) to 10(4) times larger than existing methods, permits precise measurements of the crystallographic and electronic symmetries of samples by averaging over low frequency laser-power, beam-pointing, and pulse-width fluctuations. We demonstrate the sensitivity of our technique by resolving the bulk fourfold rotational symmetry of GaAs about its [001] axis using second-harmonic generation.

Manifestation of a Second Dirac Surface State and Bulk Bands in THz Radiation from Topological Insulators

Topological insulators (TIs) are interesting quantum matters that have a narrow bandgap for bulk and a Dirac-cone-like conducting surface state (SS). The recent discovered second Dirac surface state (SS) and bulk bands (BBs) located ~1.5 eV above the first SS are important for optical coupling in TIs. Here, we report on the time-domain measurements of THz radiation generated from TIs n-type Cu(0.02)Bi2Se3 and p-type Bi2Te3 single crystals by ultrafast optical pulse excitation. The observed polarity-reversal of the THz pulse originated from transient current is unusual, and cannot be reconciled with the photo-Dember effect. The second SS and BBs are found to be indispensable for the explanation of the unusual phenomenon. Thanks to the existence of the second SS and BBs, TIs manifest an effective wide band gap in THz generation. The present study demonstrates that time-domain THz spectroscopy provide rich information of the optical coupling and the electronic structure of TIs.

Effect of Surface States on Terahertz Emission from the Bi2Se3 Surface

Three-dimensional topological insulators are materials that behave as an insulator in the interior, but as a metal on the surface with Dirac surface states protected by the topological properties of the bulk wavefunctions. The newly discovered second surface state, located about 1.5 eV above the conduction band in Bi2Se3 allows direct photoexcitation of the surface electrons in n-doped samples with a Ti:sapphire femtosecond laser. We have observed efficient THz generation from the Bi2Se3 basal plane upon femtosecond optical excitation. By performing polarization-resolved studies on the emitted THz spectrum, two emission mechanisms have been identified, namely, emission generated from the transient photocurrent under the influence of the surface depletion field and from nonlinear optical rectification. The two types of emission are governed by distinct selection rules. And while the former is characterized by a narrow-band spectrum, the latter, involving almost instantaneous optical transitions, has a broad bandwidth and is enhanced by the presence of resonant transitions. These two emission mechanisms are further separated by their distinct doping dependence upon exposure to ambient air. With surface selectivity, THz emission spectroscopy thus provides a valuable spectroscopic tool for studies of the optical conductivity and dynamics of the surface state in centrosymmetric Bi2Se3.

Structural distortion-induced magnetoelastic locking in Sr(2)IrO(4) revealed through nonlinear optical harmonic generation

We report a global structural distortion in Sr_{2}IrO_{4} using spatially resolved optical second and third harmonic generation rotational anisotropy measurements. A symmetry lowering from an I4_{1}/acd to I4_{1}/a space group is observed both above and below the Néel temperature that arises from a staggered tetragonal distortion of the oxygen octahedra. By studying an effective superexchange Hamiltonian that accounts for this lowered symmetry, we find that perfect locking between the octahedral rotation and magnetic moment canting angles can persist even in the presence of large noncubic local distortions. Our results explain the origin of the forbidden Bragg peaks recently observed in neutron diffraction experiments and reconcile the observations of strong tetragonal distortion and perfect magnetoelastic locking in Sr_{2}IrO_{4}.

Thickness tunable quantum interference between surface phonon and Dirac plasmon states in thin films of the topological insulator Bi₂Se₃

We report on a >100-fold enhancement of Raman responses from Bi2Se3 thin films if laser photon energy switches from 2.33 eV (532 nm) to 1.58 eV (785 nm), which is due to direct optical coupling to Dirac surface states (SS) at the resonance energy of ∼1.5 eV (a thickness-independent enhancement) and due to nonlinearly excited Dirac plasmon (a thickness-dependent enhancement). Owing to the direct optical coupling, we observed an in-plane phonon mode of hexagonally arranged Se-atoms associated with a continuous network of Dirac SS. This mode revealed a Fano lineshape for films <15 nm thick, resulting from quantum interference between surface phonon and Dirac plasmon states.

A low temperature nonlinear optical rotational anisotropy spectrometer for the determination of crystallographic and electronic symmetries

Nonlinear optical generation from a crystalline material can reveal the symmetries of both its lattice structure and underlying ordered electronic phases and can therefore be exploited as a complementary technique to diffraction based scattering probes. Although this technique has been successfully used to study the lattice and magnetic structures of systems such as semiconductor surfaces, multiferroic crystals, magnetic thin films, and multilayers, challenging technical requirements have prevented its application to the plethora of complex electronic phases found in strongly correlated electron systems. These requirements include an ability to probe small bulk single crystals at the μm length scale, a need for sensitivity to the entire nonlinear optical susceptibility tensor, oblique light incidence reflection geometry, and incident light frequency tunability among others. These measurements are further complicated by the need for extreme sample environments such as ultra low temperatures, high magnetic fields, or high pressures. In this review we present a novel experimental construction using a rotating light scattering plane that meets all the aforementioned requirements. We demonstrate the efficacy of our scheme by making symmetry measurements on a μm scale facet of a small bulk single crystal of Sr2IrO4 using optical second and third harmonic generation.

Defects and strain enhancements of second-harmonic generation in Si/Ge superlattices

Starting from experimental findings and interface growth problems in Si/Ge superlattices, we have investigated through ab initio methods the concurrent and competitive behavior of strain and defects in the second-harmonic generation process. Interpreting the second-harmonic intensities as a function of the different nature and percentage of defects together with the strain induced at the interface between Si and Ge, we found a way to tune and enhance the second-harmonic generation response of these systems.

Identification of helicity-dependent photocurrents from topological surface states in Bi2Se3 gated by ionic liquid

Dirac-like surface states on surfaces of topological insulators have a chiral spin structure with spin locked to momentum, which is interesting in physics and may also have important applications in spintronics. In this work, by measuring the tunable helicity-dependent photocurrent (HDP), we present an identification of the HDP from the Dirac-like surface states at room temperature. It turns out that the total HDP has two components, one from the Dirac-like surface states, and the other from the surface accumulation layer. These two components have opposite directions. The clear gate tuning of the electron density as well as the HDP signal indicates that the surface band bending and resulted surface accumulation are successfully modulated by the applied ionic liquid gate, which provides a promising way to the study of the Dirac-like surface states and also potential applications in spintronic devices.

Snapshots of Dirac fermions near the Dirac point in topological insulators

The recent focus on topological insulators is due to the scientific interest in the new state of quantum matter as well as the technology potential for a new generation of THz optoelectronics, spintronics and quantum computations. It is important to elucidate the dynamics of the Dirac fermions in the topologically protected surface state. Hence we utilized a novel ultrafast optical pump mid-infrared probe to explore the dynamics of Dirac fermions near the Dirac point. The femtosecond snapshots of the relaxation process were revealed by the ultrafast optics. Specifically, the Dirac fermion-phonon coupling strength in the Dirac cone was found to increase from 0.08 to 0.19 while Dirac fermions were away from the Dirac point into higher energy states. Further, the energy-resolved transient reflectivity spectra disclosed the energy loss rate of Dirac fermions at room temperature was about 1 meV/ps. These results are crucial to the design of Dirac fermion devices.

Reading charge transport from the spin dynamics on the surface of a topological insulator

Resolving the conductance of the topological surface states (TSSs) from the bulk contribution has been a great challenge for studying the transport properties of topological insulators. By developing a nonperturbative diffusion equation that describes fully the spin-charge dynamics in the strong spin-orbit coupling regime, we present a proposal to read the charge transport information of TSSs from its spin dynamics which can be isolated from the bulk contribution by the time-resolved second harmonic generation pump-probe measurement. We demonstrate the qualitatively different Dyaknov-Perel spin relaxation behavior between the TSSs and the two-dimensional spin-orbit coupling electron gas. The decay time of both in-plane and out-of-plane spin polarization is naturally proved to be identical to the charge transport time. The out-of-plane spin dynamics is shown to be in the experimentally reachable regime of the femtosecond pump-probe spectroscopy and thereby we suggest experiments to detect the charge transport properties of the TSSs from their unique spin dynamics.

Gas doping on the topological insulator Bi2Se3 surface

Gas molecule doping on the topological insulator Bi(2)Se(3) surface with existing Se vacancies is investigated using first-principles calculations. Consistent with experiments, NO(2) and O(2) are found to occupy the Se vacancy sites, remove vacancy-doped electrons, and restore the band structure of a perfect surface. In contrast, NO and H(2) do not favor passivation of such vacancies. Interestingly we have revealed a NO(2) dissociation process that can well explain the speculative introduced "photon-doping" effect reported by recent experiments. Experimental strategies to validate this mechanism are presented. The choice and the effect of different passivators are discussed. This step paves the way for the usage of such materials in device applications utilizing robust topological surface states.

Robustness of topological order and formation of quantum well states in topological insulators exposed to ambient environment

The physical property investigation (like transport measurements) and ultimate application of the topological insulators usually involve surfaces that are exposed to ambient environment (1 atm and room temperature). One critical issue is how the topological surface state will behave under such ambient conditions. We report high resolution angle-resolved photoemission measurements to directly probe the surface state of the prototypical topological insulators, Bi(2)Se(3) and Bi(2)Te(3), upon exposing to various environments. We find that the topological order is robust even when the surface is exposed to air at room temperature. However, the surface state is strongly modified after such an exposure. Particularly, we have observed the formation of two-dimensional quantum well states near the exposed surface of the topological insulators. These findings provide key information in understanding the surface properties of the topological insulators under ambient environment and in engineering the topological surface state for applications.

Selective probing of photoinduced charge and spin dynamics in the bulk and surface of a topological insulator

Topological insulators possess completely different spin-orbit coupled bulk and surface electronic spectra that are each predicted to exhibit exotic responses to light. Here we report time-resolved fundamental and second harmonic optical pump-probe measurements on the topological insulator Bi(2)Se(3) to independently measure its photoinduced charge and spin dynamics with bulk and surface selectivity. Our results show that a transient net spin density can be optically induced in both the bulk and surface, which may drive spin transport in topological insulators. By utilizing a novel rotational anisotropy analysis we are able to separately resolve the spin depolarization, intraband cooling, and interband recombination processes following photoexcitation, which reveal that spin and charge degrees of freedom relax on very different time scales owing to strong spin-orbit coupling.