Gate-Variable Mid-Infrared Optical Transitions in a (Bi1-xSbx)2Te3 Topological Insulator

Topological Insulator Metamaterials

Confinement of electromagnetic fields at the subwavelength scale via metamaterial paradigms is an established method to engineer light-matter interaction in most common material systems, from insulators to semiconductors and from metals to superconductors. In recent years, this approach has been extended to the realm of topological materials, providing a new avenue to access nontrivial features of their electronic band structure. In this review, we survey various topological material classes from a photonics standpoint, including crystal growth and lithographic structuring methods. We discuss how exotic electronic features such as spin-selective Dirac plasmon polaritons in topological insulators or hyperbolic plasmon polaritons in Weyl semimetals may give rise to unconventional magneto-optic, nonlinear, and circular photogalvanic effects in metamaterials across the visible to infrared spectrum. Finally, we dwell on how these effects may be dynamically controlled by applying external perturbations in the form of electric and magnetic fields or ultrafast optical pulses. Through these examples and future perspectives, we argue that topological insulator, semimetal and superconductor metamaterials are unique systems to bridge the missing links between nanophotonic, electronic, and spintronic technologies.

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

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

Electrically Tunable All-PCM Visible Plasmonics

The realization of electrically tunable plasmonic resonances in the ultraviolet (UV) to visible spectral band is particularly important for active nanophotonic device applications. However, the plasmonic resonances in the UV to visible wavelength range cannot be tuned due to the lack of tunable plasmonic materials. Here, we experimentally demonstrate tunable plasmonic resonances at visible wavelengths using a chalcogenide semiconductor alloy such as antimony telluride (Sb₂Te₃), by switching the structural phase of Sb₂Te₃ from amorphous to crystalline. We demonstrate the excitation of a propagating surface plasmon with a high plasmonic figure of merit in both amorphous and crystalline phases of Sb₂Te₃ thin films. We show polarization-dependent and -independent plasmonic resonances by fabricating one and two-dimensional periodic nanostructures in Sb₂Te₃ thin films, respectively. Moreover, we demonstrate electrically tunable plasmonic resonances using a microheater integrated with the Sb₂Te₃/Si device. The developed electrically tunable Sb₂Te₃-based plasmonic devices could find applications in the development of active color filters.

Graphene-tuned EIT-like effect in photonic multilayers for actively controlled light absorption of topological insulators

As newly emerging nanomaterials, topological insulators with unique conducting surface states that are protected by time-reversal symmetry present excellent prospects in electronics and photonics. The active control of light absorption in topological insulators are essential for the achievement of novel optoelectronic devices. Herein, we investigate the controllable light absorption of topological insulators in Tamm plasmon multilayer systems composed of a Bi_1.5Sb_0.5Te_1.8Se_1.2 (BSTS) film and a dielectric Bragg mirror with a graphene-involved defect layer. The results show that an ultranarrow electromagnetically induced transparency (EIT)-like window can be generated in the broad absorption spectrum. Based on the EIT-like effect, the Tamm plasmon enhanced light absorption of topological insulators can be dynamically tuned by adjusting the gate voltage on graphene in the defect layer. These results will pave a new avenue for the realization of topological insulator-based active optoelectronic functionalities, for instance light modulation and switching.

STM and STS studies of topological n-type (Bi, In)2(Te, Se, S)3thermoelectrics

In topological n-type thermoelectrics based on Bi2Te3with atomic substitutions Bi → In, Te → Se, S, the morphology and the surface states of Dirac fermions on the interlayer (0001) surface of van der Waals were studied by scanning tunneling microscopy and spectroscopy (STM/STS) techniques. By the STM method, the dark and light spots on the surface were found, which intensities depend on the composition and thermoelectric properties of solid solutions such as the Seebeck coefficient and thermoelectric power factor. The observed surface morphology features in the solid solutions are explained by distortions of surface electronic states originated from atomic substitutions, the influence of doping impurity, and formation mainly of substitutional impurity defects in thermoelectrics. The dips associated with substitutional impurities and antisite defects were found from the analysis of the height profiles obtained on the (0001) surface. Fast Fourier transform of the morphology STM images of the (0001) surface were used to obtain the interference patterns of the quasiparticles excitation caused by surface electrons scattering by defects. The Dirac point energy and its fluctuations, peak energies of surface defects, the positions of the valence and conduction band edges, and the energy gap were determined from an analysis of tunneling spectra. A correlation between the parameters of surface states of Dirac fermions and thermoelectric properties was found. Thus, a contribution of the fermions surface states increases with rise of the surface concentration in solid solutions with high power factor, and the largest concentration value was observed in the Bi1.98In0.02Te2.85Se0.15composition. The dependences of Fermi energy on the wave vector for different solid solutions are described by a set of Dirac cone sections located within the limits of the fluctuations of the Dirac point energy that explained by weak changes of the Fermi velocity for studied atomic substitutions in sublattices of bismuth telluride.

Femtosecond time-evolution of mid-infrared spectral line shapes of Dirac fermions in topological insulators

Mid-infrared (MIR) light sources have much potential in the study of Dirac-fermions (DFs) in graphene and topological insulators (TIs) because they have a low photon energy. However, the topological surface state transitions (SSTs) in Dirac cones are veiled by the free carrier absorption (FCA) with same spectral line shape that is always seen in static MIR spectra. Therefore, it is difficult to distinguish the SST from the FCA, especially in TIs. Here, we disclose the abnormal MIR spectrum feature of transient reflectivity changes (ΔR/R) for the non-equilibrium states in TIs, and further distinguish FCA and spin-momentum locked SST using time-resolved and linearly polarized ultra-broadband MIR spectroscopy with no environmental perturbation. Although both effects produce similar features in the reflection spectra, they produce completely different variations in the ΔR/R to show their intrinsic ultrafast dynamics.

Infrared dielectric metamaterials from high refractive index chalcogenides

High-index dielectric materials are in great demand for nanophotonic devices and applications, from ultrathin optical elements to metal-free sub-diffraction light confinement and waveguiding. Here we show that chalcogenide topological insulators are particularly apt candidates for dielectric nanophotonics architectures in the infrared spectral range, by reporting metamaterial resonances in chalcogenide crystals sustained well inside the mid-infrared, choosing Bi2Te3 as case study within this family of materials. Strong resonant modulation of the incident electromagnetic field is achieved thanks to the exceptionally high refractive index ranging between 7 and 8 throughout the 2-10 μm region. Analysis of the complex mode structure in the metamaterial allude to the excitation of circular surface currents which could open pathways for enhanced light-matter interaction and low-loss plasmonic configurations by coupling to the spin-polarized topological surface carriers, thereby providing new opportunities to combine dielectric, plasmonic and magnetic metamaterials in a single platform.

Stoichiometric Engineering of Chalcogenide Semiconductor Alloys for Nanophotonic Applications

A variety of alternative plasmonic and dielectric material platforms-among them nitrides, semiconductors, and conductive oxides-have come to prominence in recent years as means to address the shortcomings of noble metals (including Joule losses, cost, and passive character) in certain nanophotonic and optical-frequency metamaterial applications. Here, it is shown that chalcogenide semiconductor alloys offer a uniquely broad pallet of optical properties, complementary to those of existing material platforms, which can be controlled by stoichiometric design. Using combinatorial high-throughput techniques, the extraordinary epsilon-near-zero, plasmonic, and low/high-index characteristics of Bi:Sb:Te alloys are explored. Depending upon composition they can, for example, have plasmonic figures of merit higher than conductive oxides and nitrides across the entire UV-NIR range, and higher than gold below 550 nm; present dielectric figures of merit better than conductive oxides at near-infrared telecommunications wavelengths; and exhibit record-breaking refractive indices as low as 0.7 and as high as 11.5.

Mid-to-far infrared tunable perfect absorption by a sub - λ/100 nanofilm in a fractal phasor resonant cavity

Integrating an absorbing thin film into a resonant cavity is the most practical way to achieve perfect absorption of light at a selected wavelength in the mid-to-far infrared, as required to target blackbody radiation or molecular fingerprints. The cavity is designed to resonate and enable perfect absorption in the film at the chosen wavelength λ. However, in current state-of-the-art designs, a still large absorbing film thickness (∼λ/50) is needed and tuning the perfect absorption wavelength over a broad range requires changing the cavity materials. Here, we introduce a new resonant cavity concept to achieve perfect absorption of infrared light in much thinner and thus, really nanoscale films, with a broad wavelength tenability by using a single set of cavity materials. It requires a nanofilm with giant refractive index and small extinction coefficient (found in emerging semi-metals, semi-conductors and topological insulators) backed by a transparent spacer and a metal mirror. The nanofilm acts both as absorber and multiple reflector for the internal cavity waves, which after escaping follow a fractal phasor trajectory. This enables a totally destructive optical interference for a nanofilm thickness more than 2 orders of magnitude smaller than λ. With this remarkable effect, we demonstrate angle-insensitive perfect absorption in sub - λ/100 bismuth nanofilms, at a wavelength tunable from 3 to 20 μm.

Compositionally controlled plasmonics in amorphous semiconductor metasurfaces

Amorphous bismuth telluride (Bi:Te) provides a composition-dependent, CMOS-compatible alternative material platform for plasmonics in the ultraviolet-visible spectral range. Thin films of the chalcogenide semiconductor are found, using high-throughput physical vapor deposition and characterization techniques, to exhibit a plasmonic response (a negative value of the real part of relative permittivity) over a band of wavelengths extending from ~250 nm to between 530 and 978 nm, depending on alloy composition (Bi:Te at% ratio). The plasmonic response is illustrated via the fabrication of subwavelength period nano-grating metasurfaces, which present strong, period-dependent plasmonic absorption resonances in the visible range, manifested in the perceived color of the nanostructured domains in reflection.

Nanomaterial-Based Plasmon-Enhanced Infrared Spectroscopy

Surface-enhanced infrared absorption (SEIRA) has attracted increasing attention due to the potential of infrared spectroscopy in applications such as molecular trace sensing of solids, polymers, and proteins, specifically fueled by recent substantial developments in infrared plasmonic materials and engineered nanostructures. Here, the significant progress achieved in the past decades is reviewed, along with the current state of the art of SEIRA. In particular, the plasmonic properties of a variety of nanomaterials are discussed (e.g., metals, semiconductors, and graphene) along with their use in the design of efficient SEIRA configurations. To conclude, perspectives on potential applications, including single-molecule detection and in vivo bioassays, are presented.

2D library beyond graphene and transition metal dichalcogenides: a focus on photodetection

Two-dimensional materials beyond graphene and TMDs can be promising candidates for wide-spectra photodetection.