Ultrafast surface carrier dynamics in the topological insulator Bi₂Te₃

Surface terminations control charge transfer from bulk to surface states in topological insulators

Topological insulators (TI) hold significant potential for various electronic and optoelectronic devices that rely on the Dirac surface state (DSS), including spintronic and thermoelectric devices, as well as terahertz detectors. The behavior of electrons within the DSS plays a pivotal role in the performance of such devices. It is expected that DSS appear on a surface of three dimensional(3D) TI by mechanical exfoliation. However, it is not always the case that the surface terminating atomic configuration and corresponding band structures are homogeneous. In order to investigate the impact of surface terminating atomic configurations on electron dynamics, we meticulously examined the electron dynamics at the exfoliated surface of a crystalline 3D TI (Bi2 Se3 ) with time, space, and energy resolutions. Based on our comprehensive band structure calculations, we found that on one of the Se-terminated surfaces, DSS is located within the bulk band gap, with no other surface states manifesting within this region. On this particular surface, photoexcited electrons within the conduction band effectively relax towards DSS and tend to linger at the Dirac point for extended periods of time. It is worth emphasizing that these distinct characteristics of DSS are exclusively observed on this particular surface.

Enhanced Photocharacteristics by Fermi Level Modulating in Sb2 Te3 /Bi2 Se3 Topological Insulator p-n Junction

Topological insulators have recently received attention in optoelectronic devices because of their high mobility and broadband absorption resulting from their topological surface states. In particular, theoretical and experimental studies have emerged that can improve the spin generation efficiency in a topological insulator-based p-n junction structure called a TPNJ, drawing attention in optospintronics. Recently, research on implementing the TPNJ structure is conducted; however, studies on the device characteristics of the TPNJ structure are still insufficient. In this study, the TPNJ structure is effectively implemented without intermixing by controlling the annealing temperature, and the photocharacteristics appearing in the TPNJ structure are investigated using a cross-pattern that can compare the characteristics in a single device. Enhanced photo characteristics are observed for the TPNJ structure. An optical pump Terahertz probe and a physical property measurement system are used to confirm the cause of improved photoresponsivity. Consequently, the photocharacteristics are improved owing to the change in the absorption mechanism and surface transport channel caused by the Fermi level shift in the TPNJ structure.

Study on Bulk-Surface Transport Separation and Dielectric Polarization of Topological Insulator Bi1.2Sb0.8Te0.4Se2.6

This study successfully fabricated the quaternary topological insulator thin films of Bi1.2Sb0.8Te0.4Se2.6 (BSTS) with a thickness of 25 nm, improving the intrinsic defects in binary topological materials through doping methods and achieving the separation of transport characteristics between the bulk and surface of topological insulator materials by utilizing a comprehensive Physical Properties Measurement System (PPMS) and Terahertz Time-Domain Spectroscopy (THz-TDS) to extract electronic transport information for both bulk and surface states. Additionally, the dielectric polarization behavior of BSTS in the low-frequency (10-107 Hz) and high-frequency (0.5-2.0 THz) ranges was investigated. These research findings provide crucial experimental groundwork and theoretical guidance for the development of novel low-energy electronic devices, spintronic devices, and quantum computing technology based on topological insulators.

Nonadiabatic Molecular Dynamics with Non-Condon Effect of Charge Carrier Dynamics

Nonradiative multiphonon transitions play a crucial role in understanding charge carrier dynamics. To capture the non-Condon effect in nonadiabatic molecular dynamics (NA-MD), we develop a simple and accurate method to calculate noncrossing and crossing k-point NA coupling in momentum space on an equal footing and implement it with a trajectory surface hopping algorithm. Multiple k-point MD trajectories can provide sufficient nonzero momentum multiphonons coupled to electrons, and the momentum conservation is maintained during nonvertical electron transition. The simulations of indirect bandgap transition in silicon and intra- and intervalley transitions in graphene show that incorporation of the non-Condon effect is needed to correctly depict these types of charge dynamics. In particular, a hidden process is responsible for the delayed nonradiative electron-hole recombination in silicon: the thermal-assisted rapid trapping of an excited electron at the conduction band minimum by a long-lived higher energy state through a nonvertical transition extends charge carrier lifetime, approaching 1 ns, which is about 1.5 times slower than the direct bandgap recombination. For graphene, intervalley scattering takes place within about 225 fs, which can occur only when the intravalley relaxation proceeds to about 50 fs to gain enough phonon momentum. The intra- and intervalley scattering constitute energy relaxation, which completes within sub-500 fs. All the simulated time scales are in excellent agreement with experiments. The study establishes the underlying mechanisms for a long-lived charge carrier in silicon and valley scattering in graphene and underscores the robustness of the non-Condon approximation NA-MD method, which is suitable for rigid, soft, and large defective systems.

Electron- versus Spin-Phonon Coupling Governs the Temperature-Dependent Carrier Dynamics in the Topological Insulator Bi2Te3

Ultrafast charge and spin dynamics have immense effects on the applications of topological insulators (TIs). By performing spin-adiabatic nonadiabatic molecular dynamics simulations in the presence of electron-phonon (e-ph) and spin-phonon couplings, we investigate temperature-dependent intra- and interband charge and spin relaxation dynamics via the bulk and surface paths in the three-dimensional TI Bi2Te3. The e-ph coupling dominates charge relaxation in the bulk path, and the relaxation rate is positively correlated with temperature due to the large energy gaps and weak spin polarization. Conversely, the relaxation dynamics exhibits an opposite temperature dependence in the surface path because of electron re-excitation and spin mismatching induced by spin-phonon coupling, which arises from small energy gaps and strong spin polarization. The two mechanisms rationalize the charge carriers being long-lived in the bulk and surface phases at low and room temperature, respectively. Additionally, strong thermal fluctuations of the topological states' magnetic moments destroy the spin-momentum locking and trigger backscattering at room temperature.

Optical bulk-boundary dichotomy in a quantum spin Hall insulator

The bulk-boundary correspondence is a critical concept in topological quantum materials. For instance, a quantum spin Hall insulator features a bulk insulating gap with gapless helical boundary states protected by the underlying Z₂ topology. However, the bulk-boundary dichotomy and distinction are rarely explored in optical experiments, which can provide unique information about topological charge carriers beyond transport and electronic spectroscopy techniques. Here, we utilize mid-infrared absorption micro-spectroscopy and pump-probe micro-spectroscopy to elucidate the bulk-boundary optical responses of Bi₄Br₄, a recently discovered room-temperature quantum spin Hall insulator. Benefiting from the low energy of infrared photons and the high spatial resolution, we unambiguously resolve a strong absorption from the boundary states while the bulk absorption is suppressed by its insulating gap. Moreover, the boundary absorption exhibits strong polarization anisotropy, consistent with the one-dimensional nature of the topological boundary states. Our infrared pump-probe microscopy further measures a substantially increased carrier lifetime for the boundary states, which reaches one nanosecond scale. The nanosecond lifetime is about one to two orders longer than that of most topological materials and can be attributed to the linear dispersion nature of the helical boundary states. Our findings demonstrate the optical bulk-boundary dichotomy in a topological material and provide a proof-of-principal methodology for studying topological optoelectronics.

Van der Waals Engineering of Ultrafast Carrier Dynamics in Magnetic Heterostructures

Heterostructures composed of the intrinsic magnetic topological insulator MnBi₂Te₄ and its nonmagnetic counterpart Bi₂Te₃ host distinct surface electronic band structures depending on the stacking order and exposed termination. Here, we probe the ultrafast dynamical response of MnBi₂Te₄ and MnBi₄Te₇ following near-infrared optical excitation using time- and angle-resolved photoemission spectroscopy and disentangle surface from bulk dynamics based on density functional theory slab calculations of the surface-projected electronic structure. We gain access to the out-of-equilibrium charge carrier populations of both MnBi₂Te₄ and Bi₂Te₃ surface terminations of MnBi₄Te₇, revealing an instantaneous occupation of states associated with the Bi₂Te₃ surface layer followed by carrier extraction into the adjacent MnBi₂Te₄ layers with a laser fluence-tunable delay of up to 350 fs. The ensuing thermal relaxation processes are driven by phonon scattering with significantly slower relaxation times in the magnetic MnBi₂Te₄ septuple layers. The observed competition between interlayer charge transfer and intralayer phonon scattering demonstrates a method to control ultrafast charge transfer processes in MnBi₂Te₄-based van der Waals compounds.

Optical manipulation of Rashba-split 2-dimensional electron gas

In spintronics, the two main approaches to actively control the electrons’ spin involve static magnetic or electric fields. An alternative avenue relies on the use of optical fields to generate spin currents, which can bolster spin-device performance, allowing for faster and more efficient logic. To date, research has mainly focused on the optical injection of spin currents through the photogalvanic effect, and little is known about the direct optical control of the intrinsic spin-splitting. To explore the optical manipulation of a material’s spin properties, we consider the Rashba effect. Using time- and angle-resolved photoemission spectroscopy (TR-ARPES), we demonstrate that an optical excitation can tune the Rashba-induced spin splitting of a two-dimensional electron gas at the surface of Bi₂Se₃. We establish that light-induced photovoltage and charge carrier redistribution - which in concert modulate the Rashba spin-orbit coupling strength on a sub-picosecond timescale - can offer an unprecedented platform for achieving optically-driven spin logic devices.

The major challenge for the development of spin based information processing is to obtain efficient ways of controlling spin. Here, Michiardi et al show that the Rashba spin-splitting at the surface of Bi₂Se₃ topological insulator can be controlled via optical pulses on picosecond timescales.

Construction of Bi2O2Se/Bi2Se3 Van Der Waals Heterostructures for Self-Powered and Broadband Photodetectors

Due to its superior carrier mobility and high air stability, the emerging two-dimensional (2D) layered bismuth oxyselenide (Bi₂O₂Se) nanosheets have attracted extensive attention, showing great potential for applications in the electronic and optoelectronic fields. However, a high mobility easily leads to a high dark current, seriously restricting optoelectronic applications, especially in the field of photodetectors. In this paper, we report a high-quality Van der Waals (vdWs) Bi₂O₂Se/Bi₂Se₃ heterostructure on a fluorophlogopite substrate, exhibiting excellent photodiode characteristics. By means of the effective separation of photogenerated electrons and holes by a junction barrier at the interface, the current on/off ratio is up to about 3 × 10³ under 532 nm laser illumination with zero bias. In addition, the photodetector not only achieves a fast response speed of 41 ms but also has a broadband photoresponse from 532 to 1450 nm (visible-NIR). Additionally, the responsivity can reach 0.29 A/W, and the external quantum efficiency exceeds 69% when the device operates in the reverse bias condition. The results indicate that the Bi₂O₂Se/Bi₂Se₃ vdWs heterostructure has great potential for self-powered, broadband, and fast photodetection applications.

Bi2Te3/Graphene Heterostructure as the Saturable Absorber for ~1.0 μm Passively Q-switched Solid State Pulsed Laser

Due to the tunable nonlinear optical properties of the Bi2Te3/graphene heterostructure, stable solid state pulsed lasers based on the Bi2Te3/graphene saturable absorber have attracted intensive attention. In this work, the Bi2Te3/graphene heterostructure with good nonlinear absorption characteristics was synthesized by a self-assembly solvothermal route, and the optical saturable absorption properties of the saturable absorber were investigated. Owing to the large modulation depth of Bi2Te3 nanosheets and the high thermal conductivity of graphene, the Bi2Te3/graphene heterostructure saturable absorber shown good nonlinear saturable absorber performance and contributed the improved passively Q-switched Yb3+: GdAl3(BO3)4 pulsed laser when compared with that of the pure Bi2Te3 based Yb3+: GdAl3(BO3)4 laser, no matter pulse width or pulse energy. Our work demonstrates that the Bi2Te3/graphene heterostructure was a promising saturable absorber in ~1 μm solid-state pulsed lasers.

Light-Tunable Charge Density Wave Orders in MoTe_{2} and WTe_{2} Single Layers

By using constrained density functional theory modeling, we demonstrate that ultrafast optical pumping unveils hidden charge orders in group VI monolayer transition metal ditellurides. We show that irradiation of the insulating 2H phases stabilizes multiple transient charge density wave orders with light-tunable distortion, periodicity, electronic structure, and band gap. Moreover, optical pumping of the semimetallic 1T^{'} phases generates a transient charge ordered metallic phase composed of 2D diamond clusters. For each transient phase we identify the critical fluence at which it is observed and the specific optical and Raman fingerprints to directly compare with future ultrafast pump-probe experiments. Our work demonstrates that it is possible to stabilize charge density waves even in insulating 2D transition metal dichalcogenides by ultrafast irradiation.

Extremely low-energy ARPES of quantum well states in cubic-GaN/AlN and GaAs/AlGaAs heterostructures

Quantum well (QW) heterostructures have been extensively used for the realization of a wide range of optical and electronic devices. Exploiting their potential for further improvement and development requires a fundamental understanding of their electronic structure. So far, the most commonly used experimental techniques for this purpose have been all-optical spectroscopy methods that, however, are generally averaging in momentum space. Additional information can be gained by angle-resolved photoelectron spectroscopy (ARPES), which measures the electronic structure with momentum resolution. Here we report on the use of extremely low-energy ARPES (photon energy ~ 7 eV) to increase depth sensitivity and access buried QW states, located at 3 nm and 6 nm below the surface of cubic-GaN/AlN and GaAs/AlGaAs heterostructures, respectively. We find that the QW states in cubic-GaN/AlN can indeed be observed, but not their energy dispersion, because of the high surface roughness. The GaAs/AlGaAs QW states, on the other hand, are buried too deep to be detected by extremely low-energy ARPES. Since the sample surface is much flatter, the ARPES spectra of the GaAs/AlGaAs show distinct features in momentum space, which can be reconducted to the band structure of the topmost surface layer of the QW structure. Our results provide important information about the samples’ properties required to perform extremely low-energy ARPES experiments on electronic states buried in semiconductor heterostructures.

Morphology evolution and enhanced broadband photoresponse behavior of two-dimensional Bi2Te3nanosheets

Bismuth telluride (Bi₂Te₃), as an emerging two-dimensional (2D) material, has attracted extensive attention from scientific researchers due to its excellent optoelectronic, thermoelectric properties and topological structure. However, the application research of Bi₂Te₃mainly focuses on thermoelectric devices, while the research on optoelectronic devices is scarce. In this work, the morphology evolution and growth mechanism of 2D Bi₂Te₃nanosheets with a thickness of 12 ± 3 nm were systematically studied by solvothermal method. Then, the Bi₂Te₃nanosheets were annealed at 350 °C for 1 h and applied to self-powered photoelectrochemical-type broadband photodetectors. Compared with the as-synthesized Bi₂Te₃photodetector, the photocurrent of the photodetector based on the annealed Bi₂Te₃is significantly enhanced, especially enhanced by 18.3 times under near-infrared light illumination. Furthermore, the performance of annealed Bi₂Te₃photodetector was systematically studied. The research results show that the photodetector not only has a broadband response from ultraviolet to near-infrared (365-850 nm) under zero bias voltage, but also obtains the highest responsivity of 6.6 mA W^-1under green light with an incident power of 10 mW cm^-2. The corresponding rise time and decay time are 17 ms and 20 ms, respectively. These findings indicate that annealed Bi₂Te₃nanosheets have great potential to be used as self-powered high-speed broadband photodetectors with high responsivity.

Germanium Nanosheets with Dirac Characteristics as a Saturable Absorber for Ultrafast Pulse Generation

Bulk germanium as a group-IV photonic material has been widely studied due to its relatively large refractive index and broadband and low propagation loss from near-infrared to mid-infrared. Inspired by the research of graphene, the 2D counterpart of bulk germanium, germanene, has been discovered and the characteristics of Dirac electrons have been observed. However, the optical properties of germanene still remain elusive. In this work, several layers of germanene are prepared with Dirac electronic characteristics and its morphology, band structure, carrier dynamics, and nonlinear optical properties are systematically investigated. It is surprisingly found that germanene has a fast carrier-relaxation time comparable to that of graphene and a relatively large nonlinear absorption coefficient, which is an order of magnitude higher than that of graphene in the near-infrared wavelength range. Based on these findings, germanene is applied as a new saturable absorber to construct an ultrafast mode-locked laser, and sub-picosecond pulse generation in the telecommunication band is realized. The results suggest that germanene can be used as a new type of group-IV material for various nonlinear optics and photonic applications.

Hot carriers in graphene - fundamentals and applications

Hot charge carriers in graphene exhibit fascinating physical phenomena, whose understanding has improved greatly over the past decade. They have distinctly different physical properties compared to, for example, hot carriers in conventional metals. This is predominantly the result of graphene's linear energy-momentum dispersion, its phonon properties, its all-interface character, and the tunability of its carrier density down to very small values, and from electron- to hole-doping. Since a few years, we have witnessed an increasing interest in technological applications enabled by hot carriers in graphene. Of particular interest are optical and optoelectronic applications, where hot carriers are used to detect (photodetection), convert (nonlinear photonics), or emit (luminescence) light. Graphene-enabled systems in these application areas could find widespread use and have a disruptive impact, for example in the field of data communication, high-frequency electronics, and industrial quality control. The aim of this review is to provide an overview of the most relevant physics and working principles that are relevant for applications exploiting hot carriers in graphene.

Dirac surface plasmons in photoexcited bismuth telluride nanowires: optical pump-terahertz probe spectroscopy

Collective excitation of Dirac plasmons in graphene and topological insulators has opened new possibilities of tunable plasmonic materials ranging from THz to mid-infrared regions. Using time resolved Optical Pump-Terahertz Probe (OPTP) spectroscopy, we demonstrate the presence of plasmonic oscillations in bismuth telluride nanowires (Bi₂Te₃ NWs) after photoexcitation using an 800 nm pump pulse. In the frequency domain, the differential conductivity (Δσ = σ_{pump on}-σ_{pump off}) spectrum shows a Lorentzian response where the resonance frequency (ω_p), attributed to surface plasmon oscillations, shifts with photogenerated carrier density (n) as . This dependence establishes the absorption of THz radiation by the Dirac surface plasmon oscillations of the charge carriers in the Topological Surface States (TSS) of Bi₂Te₃ NWs. Moreover, we obtain a modulation depth, tunable by pump fluence, of ∼40% over the spectral range of 0.5 to 2.5 THz. In addition, the time evolution of Δσ(t) represents a long relaxation channel lasting for more than 50 ps. We model the decay dynamics of Δσ(t) using coupled second order rate equations, highlighting the contributions from surface recombination as well as from trap mediated relaxation channels of the photoinjected carriers.

Ultrafast evolution of bulk, surface and surface resonance states in photoexcited [Formula: see text]

We use circular dichroism (CD) in time- and angle-resolved photoemission spectroscopy (trARPES) to measure the femtosecond charge dynamics in the topological insulator (TI) [Formula: see text]. We detect clear CD signatures from topological surface states (TSS) and surface resonance (SR) states. In time-resolved measurements, independently from the pump polarization or intensity, the CD shows a dynamics which provides access to the unexplored electronic evolution in unoccupied states of [Formula: see text]. In particular, we are able to disentangle the unpolarized electron dynamics in the bulk states from the spin-textured TSS and SR states on the femtosecond timescale. Our study demonstrates that photoexcitation mainly involves the bulk states and is followed by sub-picosecond transport to the surface. This provides essential details on intra- and interband scattering in the relaxation process of TSS and SR states. Our results reveal the significant role of SRs in the subtle ultrafast interaction between bulk and surface states of TIs.

MXenes: Synthesis, Optical Properties, and Applications in Ultrafast Photonics

Recently, 2D materials are in great demand for various applications such as optical devices, supercapacitors, sensors, and biomedicine. MXenes as a kind of novel 2D material have attracted considerable research interest due to their outstanding mechanical, thermal, electrical, and optical properties. Especially, the excellent nonlinear optical response enables them to be potential candidates for the applications in ultrafast photonics. Here, a review of MXenes synthesis, optical properties, and applications in ultrafast lasers is presented. First, aqueous acid etching and chemical vapor deposition methods for preparing MXenes are introduced, in which the storage stability and challenges of the existing synthesis techniques are also discussed. Then, the optical properties of MXenes are discussed specifically, including plasmonic properties, optical detection, photothermal effects, and ultrafast dynamics. Furthermore, the typical ultrafast pulsed lasers enabled by MXene-based saturable absorbers operated at different wavelength regions are summarized. Finally, a summary and outlook on the development of MXenes is presented in the perspectives section.

Role of carrier-transfer in the optical nonlinearity of graphene/Bi2Te3 heterojunctions

Two-dimensional (2D) topological insulators (TIs) have attracted a lot of attention owing to their striking optical nonlinearity. However, the ultra-low saturable intensity (SI) of TIs resulting from the bulk conduction band limits their applications, such as in mode-locking solid-state lasers. In this work, through fabricating a graphene/Bi₂Te₃ heterojunction which combines monolayer graphene and a Bi₂Te₃ nanoplate, the optical nonlinearities are analyzed. Moreover, the thickness-dependent characteristics are also investigated by varying the thickness of the Bi₂Te₃ when synthesizing the heterojunctions. Furthermore, with the aid of the estimated junction electron escape time, a model of the photo-excited carrier-transfer mechanism is proposed and used to describe the phenomena of depression of ultra-low saturable absorption (SA) from the Bi₂Te₃ bulk band. The increased modulation depth of the graphene/Bi₂Te₃ heterojunction can accordingly be realized in more detail. In addition, a Q-switched solid-state laser operating at 1064 nm with heterojunction saturable absorbers is built up and characterized for validating the proposed model. The laser performance with varied Bi₂Te₃ thickness, such as pulse duration and repetition rate, agrees quite well with our proposed model. Our work demonstrates the functionality of optical nonlinear engineering by tuning the thickness of the graphene/Bi₂Te₃ heterojunction and demonstrates its potential for applications.

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 C_3v 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.

Ultrafast and broadband optical nonlinearity in aluminum doped zinc oxide colloidal nanocrystals

Heavily doped oxide semiconductors can be tailored for widespread application in near-infrared (NIR) and mid-infrared (mid-IR) wavelength ranges because of both functional and fabrication advantages. Here, the ultrafast and broadband nonlinear saturable absorption of Al-doped zinc oxide nanocrystals (AZO NCs) is investigated by using the Z-scan technique and the pump-probe technique. The nonlinear absorption coefficient is as high as -1.90 × 10³ cm GW^-1 in the wide infrared (IR) wavelength range (from 800 to 3000 nm). Furthermore, a maximum optically induced refractive index of -1.85 × 10^-1 cm² GW^-1 in the dielectric region and 2.09 × 10^-1 cm² GW^-1 in the metallic region leads to an ultrafast nonlinear optical response (less than 350 femtoseconds). Mode-locked fiber lasers at 1064 nm and 1550 nm as well as Q-switched fiber lasers near 2000 nm and 3000 nm prove the use of employing AZO NCs as a broadband and ultrafast nonlinear optical device, which provides a valuable strategy and intuition for the development of nanomaterial-based photonic and optoelectronic devices in the NIR and mid-IR wavelength ranges.

High performance broadband bismuth telluride tetradymite topological insulator photodiode

A small bulk gap and the presence of Dirac electrons due to conductive surface states make tetradymite topological insulators promising candidates for optoelectronic devices. In this work, we demonstrate a highly responsive Bi₂Te₃-Si heterostructure photodiode. The thermally evaporated Bi₂Te₃ film, exhibiting a nanocrystalline nature, shows p-type doping behavior due to bismuth vacancies. As a result of the work function difference between Bi₂Te₃ and p-type Si, charge transfer occurs and a Schottky barrier is formed. Using the thermionic emission model, the barrier height (Φ_B) is extracted to be ∼0.405 eV. For minimizing the effect of extrinsic defects, the photodiodes were capped with graphene or Si₃N₄. Since graphene acts as an efficient photoexcited carrier collector, the graphene capped device outperforms the Si₃N₄ capped device. The higher quality Bi₂Te₃ nanocrystalline film of the Si₃N₄ capped photodiode contributes to a one-order-of-magnitude improvement in responsivity at 1550 nm wavelength, as compared to the graphene capped photodiode. The Si₃N₄ capped photodiode shows photoresponse even at zero bias for 1550 nm wavelength. Built-in potential due to charge transfer at the interface of Bi₂Te₃ and Si capped with a graphene electrode exhibits the highest responsivity (8.9 A W^-1). Broadband photodetection is observed in both types of photodiodes.

Emerging 2D materials beyond graphene for ultrashort pulse generation in fiber lasers

Ultrafast fiber lasers have significant applications in ultra-precision manufacturing, medical diagnostics, medical treatment, precision measurement and astronomical detection, owing to their ultra-short pulse width and ultra-high peak-power. Since graphene was first explored as an optical saturable absorber for passively mode-locked lasers in 2009, many other 2D materials beyond graphene, including phosphorene, antimonene, bismuthene, transition metal dichalcogenides (TMDs), topological insulators (TIs), metal-organic frameworks (MOFs) and MXenes, have been successively explored, resulting in rapid development of novel 2D materials-based saturable absorbers. Herein, we review the latest progress of the emerging 2D materials beyond graphene for passively mode-locked fiber laser application. These 2D materials are classified into mono-elemental, dual-elemental and multi-elemental 2D materials. The atomic structure, band structure, nonlinear optical properties, and preparation methods of 2D materials are summarized. Diverse integration strategies for applying 2D materials into fiber laser systems are introduced, and the mode-locking performance of the 2D materials-based fiber lasers working at 1-3 μm are discussed. Finally, the perspectives and challenges facing 2D materials-based mode-locked fiber lasers are highlighted.

Ultra-sensitive graphene-bismuth telluride nano-wire hybrids for infrared detection

The myriad technological applications of infrared radiation sensors make the search for ultra-sensitive detectors extremely crucial. Materials such as bismuth telluride (Bi2Te3), having a small bulk band gap of 0.17 eV, are ideal infrared detectors. However, due to the high recombination rate of photo-generated charge carriers in the bulk, the electrical response under optical illumination is typically very weak in these materials. We have circumnavigated this by sensitizing graphene with Bi2Te3 nano-wires. These hybrid devices show an ultra-high sensitivity of ∼106 A W-1, under incident electromagnetic radiation from 940 nm to 1720 nm. The theoretical limit of the noise equivalent power and specific detectivity in these devices are ∼10-18 W Hz-1/2 and ∼1011 Jones respectively, which are comparable to those of some of the best known detectors.

Broadband polarization-insensitive saturable absorption of Fe2O3 nanoparticles

The synthesis and functionalization of transition-metal oxides are one of the most active research areas in advanced materials. As a typical transition-metal oxide, iron oxide has been widely used in lithium-ion batteries, gas sensors, and for water treatment. Herein, we synthesized Fe₂O₃ nanoparticles by a co-precipitation method that is inexpensive and non-toxic. The Fe₂O₃ nanoparticles exhibited broadband saturable absorption. Furthermore, thin Fe₂O₃ polyvinyl alcohol films were prepared to realize Q-switched operations in a ytterbium-doped fibre laser, an erbium-doped fibre laser, and a thulium-doped fibre laser. Attributed to the polarization-insensitive feature of the saturable absorber, Q-switched cylindrical vector beams were also generated based on mode coupling and selection in two-mode fibre lasers. Such Fe₂O₃ nanoparticles show great promise for use in Q-switching applications of infrared fibre lasers and cylindrical vector lasers.

Optical control of spin-polarized photocurrent in topological insulator thin films

Dirac electrons in topological insulators (TIs) provide one possible avenue to achieve control of photocurrents and spin currents without the need to apply external fields by utilizing characteristic spin-momentum locking. However, for TI crystals with electrodes it is actually difficult to characterize the net flow of spin-polarized photocurrents because of the coexistence of surface carriers and bulk carriers generated by optical excitations. We demonstrate here that the net flow directions of spin-polarized photocurrents in TI polycrystalline thin films without electrodes can be precisely and intentionally controlled by the polarization of the excitation pulse alone, which is characterized by performing time-domain terahertz (THz) wave measurements and time-resolved magneto-optical Kerr rotation measurements that are non-contact methods. We show that the amplitudes of s-polarized THz waves radiated from photocurrents under right- and left-circularly polarized excitations are inverted relative to one another. Moreover, we observe the inversion of time-resolved magneto-optical Kerr rotation signals between the two excitations. Our results will open the way as innovative methods to control spin-polarized electrons in optoelectronic and spintronic TI devices without the need to apply external fields.

A broadband, self-biased photodiode based on antimony telluride (Sb2Te3) nanocrystals/silicon heterostructures

Low bulk band gaps and conductive surface electronic states of tetradymite topological insulators (TTI) make them potential candidates for next generation ultra-broadband photodevices. Here, we demonstrate a broadband and self-biased photodiode based on a Sb2Te3-Si heterostructure. A low-cost thermal evaporation technique was employed to fabricate the photodiode. The self-biased nature of the photodiode was due to the built-in potential at the Sb2Te3-Si interface. Upon characterizing the Sb2Te3 nanocrystalline film via AFM, SEM, EDX, and XPS it was found that the film exhibited p-type behavior due to antimony vacancies or antisites. The fabricated photodiode showed an excellent rectification ratio of 3388 with n-Si confirming a robust Schottky barrier at the interface and a well-defined photocurrent upon illumination. Due to the p-type behavior of the Sb2Te3 nanocrystalline film, a rectification ratio of only 0.38 was observed with p-Si. The barrier at the interface also increases the carrier lifetimes, thereby eliminating one of the biggest drawbacks of ultrafast carrier recombination times in TTI as a photodetection material. Moreover, the photodiode exhibited excellent Ion/Ioff of three orders of magnitude under the self-biased conditions, and photocurrents ranging from 520 nm to 980 nm wavelengths were observed.

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.

Terahertz emission from metal nanoparticle array

We demonstrate theoretically that ultrafast heating of metal nanoparticles by the laser pulse should lead to the generation of coherent terahertz (THz) radiation during the heat redistribution process. It is shown that after the femtosecond laser pulse action, the time-dependent gradient of the electronic temperature induces low-frequency particle polarization with the characteristic timescale of about fractions of a picosecond. In the case of the directed metallic pattern, the THz pulse waveform can be controlled by changing the geometry of the particle. The proposed THz generation mechanism can be the basis for interpretation of recent experiments with metallic nanoparticles and nanostructures.

Prolonged duration of nonequilibrated Dirac fermions in neutral topological insulators

Topological insulators (TIs) possess spin-polarized Dirac fermions on their surface but their unique properties are often masked by residual carriers in the bulk. Recently, (Sb_1−xBi_x)₂Te₃ was introduced as a non-metallic TI whose carrier type can be tuned from n to p across the charge neutrality point. By using time- and angle-resolved photoemission spectroscopy, we investigate the ultrafast carrier dynamics in the series of (Sb_1−xBi_x)₂Te₃. The Dirac electronic recovery of ∼10 ps at most in the bulk-metallic regime elongated to >400 ps when the charge neutrality point was approached. The prolonged nonequilibration is attributed to the closeness of the Fermi level to the Dirac point and to the high insulation of the bulk. We also discuss the feasibility of observing excitonic instability of (Sb_1−xBi_x)₂Te₃.

Quantum size effect on charges and phonons ultrafast dynamics in atomically controlled nanolayers of topological insulators Bi2Te3

Heralded as one of the key elements for next generation spintronics devices, topological insulators (TIs) are now step by step envisioned as nanodevices like charge-to-spin current conversion or as Dirac fermions based nanometer Schottky diode for example. However, reduced to few nanometers, TIs layers exhibit a profound modification of the electronic structure and the consequence of this quantum size effect on the fundamental carriers and phonons ultrafast dynamics has been poorly investigated so far. Here, thanks to a complete study of a set of high quality molecular beam epitaxy grown nanolayers, we report the existence of a critical thickness of around ~6 nm, below which a spectacular reduction of the carrier relaxation time by a factor of ten is found in comparison to bulk Bi2 Te3 In addition, we also evidence an A1g optical phonon mode softening together with the appearance of a thickness dependence of the photoinduced coherent acoustic phonons signals. This drastic evolution of the carriers and phonons dynamics might be due an important electron-phonon coupling evolution due to the quantum confinement. These properties have to be taken into account for future TIs-based spintronic devices.

Exploiting nonlinear properties of pure and Sn-doped Bi2Te2Se for passive Q-switching of all-polarization maintaining ytterbium- and erbium-doped fiber lasers

Due to their broadband nonlinear optical properties, low-dimensional materials are widely used for pulse generation in fiber and solid-state lasers. Here we demonstrate novel materials, Bi₂Te₂Se (BTS) and Sn-doped Bi₂Te₂Se (BSTS), which can be used as a universal saturable absorbers for distinct spectral regimes. The material was mechanically exfoliated from a bulk single-crystal and deposited onto a side-polished fiber. We have performed characterization of the fabricated devices and employed them in polarization-maintaining ytterbium- and erbium-doped fiber lasers. This enabled us to obtain self-starting passively Q-switched regime at 1 µm and 1.56 µm. The oscillators emitted stable, linearly polarized radiation with the highest single pulse energy approaching 692 nJ. Both lasers are characterized by the best performance observed in all-polarization maintaining Q-switched fiber lasers with recently investigated new saturable absorbers, which was enabled by a very high damage threshold of the devices. This demonstrates the great potential of the investigated materials for the ultrafast photonics community.

Ultrafast spin-polarized electron dynamics in the unoccupied topological surface state of Bi2Se3

The three-dimensional topological insulator Bi₂Se₃ presents two cone-like dispersive topological surface states centered at the [Formula: see text] point. One of them is unoccupied in equilibrium conditions and located 1.8 eV above the other one lying close to the Fermi level. In this work we employ time- and angle-resolved photoemission spectroscopy with circularly polarized pump photons to selectively track the spin dynamics of the empty topological states. We observe that spin-polarized electrons flow along the topological cone and recombine towards the unpolarized bulk states on a timescale of few tens of femtoseconds. This provides direct evidence of the capability to trigger a spin current with circularly polarized light.

Thickness-dependent carrier and phonon dynamics of topological insulator Bi2Te3 thin films

As a new quantum state of matter, topological insulators offer a new platform for exploring new physics, giving rise to fascinating new phenomena and new devices. Lots of novel physical properties of topological insulators have been studied extensively and are attributed to the unique electron-phonon interactions at the surface. Although electron behavior in topological insulators has been studied in detail, electron-phonon interactions at the surface of topological insulators are less understood. In this work, using optical pump-optical probe technology, we performed transient absorbance measurement on Bi₂Te₃ thin films to study the dynamics of its hot carrier relaxation process and coherent phonon behavior. The excitation and dynamics of phonon modes are observed with a response dependent on the thickness of the samples. The thickness-dependent characteristic time, amplitude and frequency of the damped oscillating signals are acquired by fitting the signal profiles. The results clearly indicate that the electron-hole recombination process gradually become dominant with the increasing thickness which is consistent with our theoretical calculation. In addition, a frequency modulation phenomenon on the high-frequency oscillation signals induced by coherent optical phonons is observed.

Theory of Inverse Edelstein Effect of The Surface States of A Topological Insulator

The surface states of three-dimensional topological insulators possess the unique property of spin-momentum interlocking. This property gives rise to the interesting inverse Edelstein effect (IEE), in which an applied spin bias μ is converted to a measurable charge voltage difference V. We develop a semiclassical theory for the IEE of the surface states of Bi2Se3 thin films, which is applicable from the ballistic regime to diffusive regime. We find that the efficiency of the spin-charge conversion, defined as γ = V/μ, exhibits a universal dependence on the ratio between sample size and electron mean free path. The efficiency increases from γ = π/4 in the ballistic limit to γ = π in the diffusive limit, suggesting that sufficient strength of impurity scattering is favorable for the IEE.

Energy dissipation from a correlated system driven out of equilibrium

In complex materials various interactions have important roles in determining electronic properties. Angle-resolved photoelectron spectroscopy (ARPES) is used to study these processes by resolving the complex single-particle self-energy and quantifying how quantum interactions modify bare electronic states. However, ambiguities in the measurement of the real part of the self-energy and an intrinsic inability to disentangle various contributions to the imaginary part of the self-energy can leave the implications of such measurements open to debate. Here we employ a combined theoretical and experimental treatment of femtosecond time-resolved ARPES (tr-ARPES) show how population dynamics measured using tr-ARPES can be used to separate electron–boson interactions from electron–electron interactions. We demonstrate a quantitative analysis of a well-defined electron–boson interaction in the unoccupied spectrum of the cuprate Bi₂Sr₂CaCu₂O_8+x characterized by an excited population decay time that maps directly to a discrete component of the equilibrium self-energy not readily isolated by static ARPES experiments.

Differentiation of quantum interactions in correlated materials is ambiguous in measurements of the single particle self-energy. Here, Rameau et al. employ a combined theoretical and experimental time domain treatment to separate electron-boson interactions from electron-electron interactions in Bi₂Sr₂CaCu₂O_8+x.

Enhancement of carrier lifetime by spin-orbit coupling in a topological insulator of an Sb2Te3 thin film

Electrons and phonons in chalcogenide-based materials are important factors in the performance of optical data-storage media and thermoelectric devices. However, the fundamental kinetics of carriers in chalcogenide materials remains controversial, and active debate continues over the mechanism responsible for carrier relaxation. In this study, we used optical-pump terahertz-probe spectroscopy, which permits the relationship between structural phase transition and optical property transitions to be examined, to investigate the ultrafast carrier dynamics in a multilayered [Sb(3 Å)/Te(9 Å)]_n thin film during the transition from the disordered to crystalline phase. Using terahertz time-domain spectroscopy and a contact-free optical technique, we demonstrated that the optical conductance and carrier concentration vary as functions of annealing temperature. Moreover, we observed that the topological surface state (TSS) affects the enhancement of the carrier lifetime, which is closely related to the degree of spin-orbit coupling (SOC). The combination of the optical technique and proposed carrier relaxation mechanism provides a powerful tool for monitoring TSS and SOC. Consequently, it was determined that the response of the disordered phase is dominated by an electron-phonon coupling effect, while that of the crystalline structure is controlled by a Dirac surface state and SOC effects. These results are important for understanding the fundamental physics of phase change materials and for optimizing and designing materials with better performance in optoelectronic devices.

High-quality and Large-size Topological Insulator Bi2Te3-Gold Saturable Absorber Mirror for Mode-Locking Fiber Laser

A novel high-quality, large-size, reflection-type topological insulator Bi₂Te₃-Gold (BG) film-based nonlinear optical modulator has been successfully fabricated as a two-dimensional saturable absorber mirror (SAM) by pulsed laser deposition (PLD). This BG-SAM possesses saturation fluence of 108.3 μJ/cm², modulation depth (ΔR) of 6.5%, non-saturable loss of 38.4%, high damage threshold above 1.354 mJ/cm² and excellent uniformity providing for the generation of passive mode-locked (ML) pulses for erbium-doped fiber lasers (EDFLs) on a large sample area. Under 124 mW 976 nm pumping, We obtained 452-fs continuous-wave ML pulses with pulse energy of 91 pJ and full width at half-maximum (FWHM) of 6.72-nm from this EDFL. The results clearly evidence that the PLD is an efficient method for fabricating BG-SAM that is suitable for a compact ultrafast ML fiber laser system.

Photonics and optoelectronics of two-dimensional materials beyond graphene

Apart from conventional materials, the study of two-dimensional (2D) materials has emerged as a significant field of study for a variety of applications. Graphene-like 2D materials are important elements of potential optoelectronics applications due to their exceptional electronic and optical properties. The processing of these materials towards the realization of devices has been one of the main motivations for the recent development of photonics and optoelectronics. The recent progress in photonic devices based on graphene-like 2D materials, especially topological insulators (TIs) and transition metal dichalcogenides (TMDs) with the methodology level discussions from the viewpoint of state-of-the-art designs in device geometry and materials are detailed in this review. We have started the article with an overview of the electronic properties and continued by highlighting their linear and nonlinear optical properties. The production of TIs and TMDs by different methods is detailed. The following main applications focused towards device fabrication are elaborated: (1) photodetectors, (2) photovoltaic devices, (3) light-emitting devices, (4) flexible devices and (5) laser applications. The possibility of employing these 2D materials in different fields is also suggested based on their properties in the prospective part. This review will not only greatly complement the detailed knowledge of the device physics of these materials, but also provide contemporary perception for the researchers who wish to consider these materials for various applications by following the path of graphene.

Spin-polarized surface resonances accompanying topological surface state formation

Topological insulators host spin-polarized surface states born out of the energetic inversion of bulk bands driven by the spin-orbit interaction. Here we discover previously unidentified consequences of band-inversion on the surface electronic structure of the topological insulator Bi₂Se₃. By performing simultaneous spin, time, and angle-resolved photoemission spectroscopy, we map the spin-polarized unoccupied electronic structure and identify a surface resonance which is distinct from the topological surface state, yet shares a similar spin-orbital texture with opposite orientation. Its momentum dependence and spin texture imply an intimate connection with the topological surface state. Calculations show these two distinct states can emerge from trivial Rashba-like states that change topology through the spin-orbit-induced band inversion. This work thus provides a compelling view of the coevolution of surface states through a topological phase transition, enabled by the unique capability of directly measuring the spin-polarized unoccupied band structure.

The spin-orbit interaction is central to the defining characteristics of topological insulators. Here, Jozwiak et al. report a spin-polarized unoccupied surface resonance coevolving with topological surface states from a pair of Rashba-like states through spin-orbit induced band inversion.

Manipulating the Topological Interface by Molecular Adsorbates: Adsorption of Co-Phthalocyanine on Bi2Se3

Topological insulators are a promising class of materials for applications in the field of spintronics. New perspectives in this field can arise from interfacing metal-organic molecules with the topological insulator spin-momentum locked surface states, which can be perturbed enhancing or suppressing spintronics-relevant properties such as spin coherence. Here we show results from an angle-resolved photemission spectroscopy (ARPES) and scanning tunnelling microscopy (STM) study of the prototypical cobalt phthalocyanine (CoPc)/Bi2Se3 interface. We demonstrate that that the hybrid interface can act on the topological protection of the surface and bury the Dirac cone below the first quintuple layer.

High-Responsivity, High-Detectivity, Ultrafast Topological Insulator Bi2Se3/Silicon Heterostructure Broadband Photodetectors

As an exotic state of quantum matter, topological insulators have promising applications in new-generation electronic and optoelectronic devices. The realization of these applications relies critically on the preparation and properties understanding of high-quality topological insulators, which however are mainly fabricated by high-cost methods like molecular beam epitaxy. We here report the successful preparation of high-quality topological insulator Bi2Se3/Si heterostructure having an atomically abrupt interface by van der Waals epitaxy growth of Bi2Se3 films on Si wafer. A simple, low-cost physical vapor deposition (PVD) method was employed to achieve the growth of the Bi2Se3 films. The Bi2Se3/Si heterostructure exhibited excellent diode characteristics with a pronounced photoresponse under light illumination. The built-in potential at the Bi2Se3/Si interface greatly facilitated the separation and transport of photogenerated carriers, enabling the photodetector to have a high light responsivity of 24.28 A W(-1), a high detectivity of 4.39 × 10(12) Jones (Jones = cm Hz(1/2) W(-1)), and a fast response speed of aproximately microseconds. These device parameters represent the highest values for topological insulator-based photodetectors. Additionally, the photodetector possessed broadband detection ranging from ultraviolet to optical telecommunication wavelengths. Given the simple device architecture and compatibility with silicon technology, the topological insulator Bi2Se3/Si heterostructure holds great promise for high-performance electronic and optoelectronic applications.

Electronic structure and relaxation dynamics in a superconducting topological material

Topological superconductors host new states of quantum matter which show a pairing gap in the bulk and gapless surface states providing a platform to realize Majorana fermions. Recently, alkaline-earth metal Sr intercalated Bi2Se3 has been reported to show superconductivity with a Tc ~ 3 K and a large shielding fraction. Here we report systematic normal state electronic structure studies of Sr0.06Bi2Se3 (Tc ~ 2.5 K) by performing photoemission spectroscopy. Using angle-resolved photoemission spectroscopy (ARPES), we observe a quantum well confined two-dimensional (2D) state coexisting with a topological surface state in Sr0.06Bi2Se3. Furthermore, our time-resolved ARPES reveals the relaxation dynamics showing different decay mechanism between the excited topological surface states and the two-dimensional states. Our experimental observation is understood by considering the intra-band scattering for topological surface states and an additional electron phonon scattering for the 2D states, which is responsible for the superconductivity. Our first-principles calculations agree with the more effective scattering and a shorter lifetime of the 2D states. Our results will be helpful in understanding low temperature superconducting states of these topological materials.

Generation of Transient Photocurrents in the Topological Surface State of Sb_{2}Te_{3} by Direct Optical Excitation with Midinfrared Pulses

We combine tunable midinfrared (mid-IR) pump pulses with time- and angle-resolved two-photon photoemission to study ultrafast photoexcitation of the topological surface state (TSS) of Sb_{2}Te_{3}. It is revealed that mid-IR pulses permit a direct excitation from the occupied to the unoccupied part of the TSS across the Dirac point. The novel optical coupling induces asymmetric transient populations of the TSS at ±k_{∥}, which reflects a macroscopic photoexcited electric surface current. By observing the decay of the asymmetric population, we directly investigate the dynamics of the long-lived photocurrent in the time domain. Our discovery promises important advantages of photoexcitation by mid-IR pulses for spintronic applications.

Unraveling Photoinduced Spin Dynamics in the Topological Insulator Bi(2)Se(3)

We report on a time-resolved ultrafast optical spectroscopy study of the topological insulator Bi_{2}Se_{3}. We unravel that a net spin polarization cannot only be generated using circularly polarized light via interband transitions between topological surface states (SSs), but also via transitions between SSs and bulk states. Our experiment demonstrates that tuning photon energy or temperature can essentially allow for photoexcitation of spin-polarized electrons to unoccupied topological SSs with two distinct spin relaxation times (∼25 and ∼300  fs), depending on the coupling between SSs and bulk states. The intrinsic mechanism leading to such distinctive spin dynamics is the scattering in SSs and bulk states which is dominated by E_{g}^{2} and A_{1g}^{1} phonon modes, respectively. These findings are suggestive of novel ways to manipulate the photoinduced coherent spins in topological insulators.

Optically detecting the edge-state of a three-dimensional topological insulator under ambient conditions by ultrafast infrared photoluminescence spectroscopy

Ultrafast infrared photoluminescence spectroscopy was applied to a three-dimensional topological insulator TlBiSe2 under ambient conditions. The dynamics of the luminescence exhibited bulk-insulating and gapless characteristics bounded by the bulk band gap energy. The existence of the topologically protected surface state and the picosecond-order relaxation time of the surface carriers, which was distinguishable from the bulk response, were observed. Our results provide a practical method applicable to topological insulators under ambient conditions for device applications.

Ultrasensitive nonlinear absorption response of large-size topological insulator and application in low-threshold bulk pulsed lasers

Dirac-like topological insulators have attracted strong interest in optoelectronic application because of their unusual and startling properties. Here we report for the first time that the pure topological insulator Bi₂Te₃ exhibited a naturally ultrasensitive nonlinear absorption response to photoexcitation. The Bi₂Te₃ sheets with lateral size up to a few micrometers showed extremely low saturation absorption intensities of only 1.1 W/cm² at 1.0 and 1.3 μm, respectively. Benefiting from this sensitive response, a Q-switching pulsed laser was achieved in a 1.0 μm Nd:YVO₄ laser where the threshold absorbed pump power was only 31 mW. This is the lowest threshold in Q-switched solid-state bulk lasers to the best of our knowledge. A pulse duration of 97 ns was observed with an average power of 26.1 mW. A Q-switched laser at 1.3 μm was also realized with a pulse duration as short as 93 ns. Moreover, the mode locking operation was demonstrated. These results strongly exhibit that Bi₂Te₃ is a promising optical device for constructing broadband, miniature and integrated high-energy pulsed laser systems with low power consumption. Our work clearly points out a significantly potential avenue for the development of two-dimensional-material-based broadband ultrasensitive photodetector and other optoelectronic devices.

Gigantic surface lifetime of an intrinsic topological insulator

The interaction between light and novel two-dimensional electronic states holds promise to realize new fundamental physics and optical devices. Here, we use pump-probe photoemission spectroscopy to study the optically excited Dirac surface states in the bulk-insulating topological insulator Bi_{2}Te_{2}Se and reveal optical properties that are in sharp contrast to those of bulk-metallic topological insulators. We observe a gigantic optical lifetime exceeding 4  μs (1  μs=10^{-6}  s) for the surface states in Bi_{2}Te_{2}Se, whereas the lifetime in most topological insulators, such as Bi_{2}Se_{3}, has been limited to a few picoseconds (1  ps=10^{-12}  s). Moreover, we discover a surface photovoltage, a shift of the chemical potential of the Dirac surface states, as large as 100 mV. Our results demonstrate a rare platform to study charge excitation and relaxation in energy and momentum space in a two-dimensional system.

Ultrafast saturable absorption in topological insulator Bi₂SeTe₂ nanosheets

Topological insulator (TI) Bi2SeTe2 nanosheets with very regular hexagonal morphology were synthesized by a hydrothermal route. Open aperture (OA) z-scan method was performed to measure the saturable absorption (SA) characteristics of the as-prepared TI Bi2SeTe2 nanosheets. The measured modulation depth, saturation intensity and nonsaturable loss of the sample were 61.9%, 4.46 GW/cm2 and 4.5% respectively. An ultrafast intraband scattering time of ~50 fs was obtained through simulating the SA curve, which indicates the TI Bi2SeTe2 nanosheets may be a good candidate for mode-locking material.