Milliwatt terahertz harmonic generation from topological insulator metamaterials

Terahertz frequency electronics and photonics: materials and devices

The terahertz frequency region of the electromagnetic spectrum sits at the interface of electronics and optics, lying between the microwave and infrared (IR) spectral regions. Although there are significant challenges to access, understand and exploit this distinctive region of the spectrum, there are immense benefits in its exploration for both discovery- and challenge-led research, from fundamental studies of laser operation through to the development of new spectroscopy instrumentation. The last 25 years has witnessed remarkable efforts to advance the field of terahertz science and engineering, and this is the subject of this article. Advances in the growth of precisely layered semiconductor materials have enabled a number of new terahertz device technologies, including high-performance quantum cascade lasers (QCLs) and quantum well photodetectors. Recent advances have included the use of thin magnetic films for efficient terahertz generation. We also review the increasing interest in contemporary two-dimensional (2D) materials for terahertz optoelectronic devices. New materials including graphene, topological insulators, transition metal dichalcogenides and novel semi-metals have shown promise as highly efficient terahertz radiation detectors and modulators. Finally, we summarize the challenges which still exist in the field of terahertz electronics and photonics, and how new materials and new device technologies might meet these challenges.This article is part of the theme issue 'Science into the next millennium: 25 years on'.

Nanoscale ultrafast dynamics in Bi2Te3 thin film by terahertz scanning near-field nanoscopy

Ultrafast laser interactions with topological insulators (TIs) have garnered tremendous interest for understanding light-matter interactions and developing optoelectronic devices across visible to terahertz (THz) regions owning to their high carrier mobility and sensitivity to electric fields. In particular, within the THz regime, TIs hold considerable promise to realize advanced emitters, modulators, and detectors because of their fascinating ultrafast THz dynamics. However, a detailed understanding of TIs' THz dynamics necessitates access to both nanoscale and femtosecond timescale. By utilizing THz scattering-type scanning near-field optical microscopy, we investigated THz time-domain spectroscopy, optical-pump THz probe, and THz emission in TI thin films at nanoscale. We analyzed their static scattering characteristics, morphology, and observed THz emission and photocarrier dynamics lasting approximately 10 ps, with dependencies of thickness and power. Our findings reveal the nanoscale ultrafast dynamics of TIs and demonstrate THz s-SNOM's potential to enhance compact THz device development.

Spin-orbit interaction driven terahertz nonlinear dynamics in transition metals

The interplay of electronic charge, spin, and orbital currents, coherently driven by picosecond long oscillations of light fields in spin-orbit coupled systems, is the foundation of emerging terahertz lightwave spintronics and orbitronics. The essential rules for how terahertz fields interact with these systems in a nonlinear way are still not understood. In this work, we demonstrate a universally applicable electronic nonlinearity originating from spin-orbit interactions in conducting materials, wherein the interplay of light-induced spin and orbital textures manifests. We utilized terahertz harmonic generation spectroscopy to investigate the nonlinear dynamics over picosecond timescales in various transition metal films. We found that the terahertz harmonic generation efficiency scales with the spin Hall conductivity in the studied films, while the phase takes two possible values (shifted by π), depending on the d-shell filling. These findings elucidate the fundamental mechanisms governing non-equilibrium spin and orbital polarization dynamics at terahertz frequencies, which is relevant for potential applications of terahertz spin- and orbital-based devices.

Strategies to enhance THz harmonic generation combining multilayered, gated, and metamaterial-based architectures

Graphene has unique properties paving the way for groundbreaking future applications. Its large optical nonlinearity and ease of integration in devices notably makes it an ideal candidate to become a key component for all-optical switching and frequency conversion applications. In the terahertz (THz) region, various approaches have been independently demonstrated to optimize the nonlinear effects in graphene, addressing a critical limitation arising from the atomically thin interaction length. Here, we demonstrate sample architectures that combine strategies to enhance THz nonlinearities in graphene-based structures. We achieve this by increasing the interaction length through a multilayered design, controlling carrier density with an electrical gate, and modulating the THz field spatial distribution with a metallic metasurface substrate. Our study specifically investigates third harmonic generation (THG) using a table-top high-field THz source. We measure THG enhancement factors exceeding thirty and propose architectures capable of achieving a two-order-of-magnitude increase. These findings underscore the potential of engineered graphene-based structures in advancing THz frequency conversion technologies for signal processing and wireless communication applications.

FPGA-based measurements of the relative arrival time of a high-repetition rate, quasi-cw fourth generation light source

In this paper, we demonstrate the successful implementation of reconfigurable field-programmable gate array technology into a pulse-resolved data acquisition system to achieve a femtosecond temporal resolution in ultrafast pump-probe experiments in real-time at large scale facilities. As proof of concept, electro-optic sampling of terahertz waveforms radiated by a superradiant emitter of a quasi-cw accelerator operating at a 50 kHz repetition rate and probed by an external laser system is performed. Options for up-scaling the developed technique to a MHz range of repetition rates are discussed.

Complex anisotropy ratio of third harmonic generation in a semiconductor using a terahertz free electron laser

The nonlinear susceptibility in the terahertz region is expected to have a non-negligible imaginary part originating from the momentum-dependent scattering time of free carriers, but it has been scarcely reported. By utilizing an intense 4 THz beam from a terahertz free electron laser, we investigated the azimuth angle dependence of the third harmonic generation (THG) from semiconductors. The observed angular anisotropy of THG revealed the contribution of the imaginary part of the nonlinear susceptibility originating from the momentum-scattering time relation in addition to its real part originating from the band nonparabolicity. The results provide a deeper understanding of nonlinear optics in the terahertz region.

Terahertz magneto-optical sampling in quartz glass

In this Letter, we demonstrate terahertz (THz) magnetic field detection in fused silica with sensitivity that can be easily controlled by sample tilting (for both amplitude and polarization). The proposed technique remains in the linear regime at magnetic fields exceeding 0.3 T (0.9 MV/cm of equivalent electric field) and allows the use of low-cost amorphous materials. Furthermore, the demonstrated effects should be present in a wide variety of materials used as substrates in different THz-pump laser-probe experiments and need to be considered in order to disentangle different contributions to the measured signals.

Nonlinear Optical Properties of 2D Materials and their Applications

2D materials are a subject of intense research in recent years owing to their exclusive photoelectric properties. With giant nonlinear susceptibility and perfect phase matching, 2D materials have marvelous nonlinear light-matter interactions. The nonlinear optical properties of 2D materials are of great significance to the design and analysis of applied materials and functional devices. Here, the fundamental of nonlinear optics (NLO) for 2D materials is introduced, and the methods for characterizing and measuring second-order and third-order nonlinear susceptibility of 2D materials are reviewed. Furthermore, the theoretical and experimental values of second-order susceptibility χ(2) and third-order susceptibility χ(3) are tabulated. Several applications and possible future research directions of second-harmonic generation (SHG) and third-harmonic generation (THG) for 2D materials are presented.

Emerging probing perspective of two-dimensional materials physics: terahertz emission spectroscopy

Terahertz (THz) emission spectroscopy (TES) has emerged as a highly effective and versatile technique for investigating the photoelectric properties of diverse materials and nonlinear physical processes in the past few decades. Concurrently, research on two-dimensional (2D) materials has experienced substantial growth due to their atomically thin structures, exceptional mechanical and optoelectronic properties, and the potential for applications in flexible electronics, sensing, and nanoelectronics. Specifically, these materials offer advantages such as tunable bandgap, high carrier mobility, wideband optical absorption, and relatively short carrier lifetime. By applying TES to investigate the 2D materials, their interfaces and heterostructures, rich information about the interplay among photons, charges, phonons and spins can be unfolded, which provides fundamental understanding for future applications. Thus it is timely to review the nonlinear processes underlying THz emission in 2D materials including optical rectification, photon-drag, high-order harmonic generation and spin-to-charge conversion, showcasing the rich diversity of the TES employed to unravel the complex nature of these materials. Typical applications based on THz emissions, such as THz lasers, ultrafast imaging and biosensors, are also discussed. Step further, we analyzed the unique advantages of spintronic terahertz emitters and the future technological advancements in the development of new THz generation mechanisms leading to advanced THz sources characterized by wide bandwidth, high power and integration, suitable for industrial and commercial applications. The continuous advancement and integration of TES with the study of 2D materials and heterostructures promise to revolutionize research in different areas, including basic materials physics, novel optoelectronic devices, and chips for post-Moore's era.

Compact terahertz harmonic generation in the Reststrahlenband using a graphene-embedded metallic split ring resonator array

Harmonic generation is a result of a strong non-linear interaction between light and matter. It is a key technology for optics, as it allows the conversion of optical signals to higher frequencies. Owing to its intrinsically large and electrically tunable non-linear optical response, graphene has been used for high harmonic generation but, until now, only at frequencies < 2 THz, and with high-power ultrafast table-top lasers or accelerator-based structures. Here, we demonstrate third harmonic generation at 9.63 THz by optically pumping single-layer graphene, coupled to a circular split ring resonator (CSRR) array, with a 3.21 THz frequency quantum cascade laser (QCL). Combined with the high graphene nonlinearity, the mode confinement provided by the optically-pumped CSRR enhances the pump power density as well as that at the third harmonic, permitting harmonic generation. This approach enables potential access to a frequency range (6-12 THz) where compact sources remain difficult to obtain, owing to the Reststrahlenband of typical III-V semiconductors.

Third harmonic generation due to free carrier in InSb using a terahertz free electron laser

We report on the third harmonic generation (THG) in InSb semiconductor irradiated by a terahertz (THz) free electron laser (FEL). The conversion of 4 THz (wavelength 70 µm) FEL outputs into its third harmonic 12 THz was observed. We found that by tuning the sample temperature to 360 K, high conversion efficiency up to 1% can be obtained and is the highest in the THz and FIR regions below 10 THz. We also discuss the observed intensity dependence of the THG with the nonlinear order lower than 3 when the pumping intensity was high.

Local measurement of terahertz field-induced second harmonic generation in plasma filaments

The concept of Terahertz Field-Induced Second Harmonic (TFISH) Generation is revisited to introduce a single-shot detection scheme based on third order nonlinearities. Focused specifically on the further development of THz plasma-based sources, we begin our research by reimagining the TFISH system to serve as a direct plasma diagnostic. In this work, an optical probe beam is used to mix directly with the strong ponderomotive current associated with laser-induced ionization. A four-wave mixing (FWM) process then generates a strong second-harmonic optical wave because of the mixing of the probe beam with the nonlinear current components oscillating at THz frequencies. The observed conversion efficiency is high enough that for the first time, the TFISH signal appears visible to the human eye. We perform spectral, spatial, and temporal analysis on the detected second-harmonic frequency and show its direct relationship to the nonlinear current. Further, a method to detect incoherent and coherent THz inside plasma filaments is devised using spatio-temporal couplings. The single-shot detection configurations are theoretically described using a combination of expanded FWM models with Kostenbauder and Gaussian Q-matrices. We show that the retrieved temporal traces for THz radiation from single- and two-color laser-induced air-plasma sources match theoretical descriptions very well. High temporal resolution is shown with a detection bandwidth limited only by the spatial extent of the probe laser beam. Large detection bandwidth and temporal characterization is shown for THz radiation confined to under-dense plasma filaments induced by < 100 fs lasers below the relativistic intensity limit.

Ultrafast Tunable Terahertz-to-Visible Light Conversion through Thermal Radiation from Graphene Metamaterials

Several technologies, including photodetection, imaging, and data communication, could greatly benefit from the availability of fast and controllable conversion of terahertz (THz) light to visible light. Here, we demonstrate that the exceptional properties and dynamics of electronic heat in graphene allow for a THz-to-visible conversion, which is switchable at a sub-nanosecond time scale. We show a tunable on/off ratio of more than 30 for the emitted visible light, achieved through electrical gating using a gate voltage on the order of 1 V. We also demonstrate that a grating-graphene metamaterial leads to an increase in THz-induced emitted power in the visible range by 2 orders of magnitude. The experimental results are in agreement with a thermodynamic model that describes blackbody radiation from the electron system heated through intraband Drude absorption of THz light. These results provide a promising route toward novel functionalities of optoelectronic technologies in the THz regime.

Generation of Tunable Terahertz Waves from Tailored Versatile Spintronic Meta-Antenna Arrays

Broadband spintronic terahertz (THz) radiation can be efficiently generated by spin-to-charge current conversion in a ferromagnetic/nonmagnetic heterostructure. There had been many studies on realizing the enhancement or the modulation of spintronic terahertz waves. However, reported devices so far focus on implementing certain specific modulation methods, either related to the spintronic stacks or related to the metamaterial structures. In this study, a set of femtosecond laser-driven versatile spintronic terahertz devices are proposed by integrating meta-antenna structures with W/CoFeB/Pt nanolayer stacks. These monolithic integrated devices exhibit spintronic terahertz wave emission, spectral modulation, and polarization manipulation simultaneously. The terahertz pulses are generated within the ferromagnetic heterostructure interfaces and transmitted along the metallic structures, leading to the modulation of the spintronic terahertz waves. Results have shown that the center-frequency shift is up to 140 GHz and the value of ellipticity can reach 0.6, demonstrating a set of integrated and efficient spintronic terahertz devices to modulate the emitted wave. In addition, compared with the slotline antenna, the maximum peak value of the bandpass band is enhanced up to 1.63 times by carefully designing the metamaterial structure. The spintronic meta-antenna array proposed here paves an integrated way for the manipulation of spintronic terahertz optoelectronic devices.

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.

Topological Insulator Films for Terahertz Photonics

We discuss experimental and theoretical studies of the generation of the third terahertz (THz) frequency harmonic in thin films of Bi₂Se₃ and Bi_2-xSb_xTe_3-ySe_y (BSTS) topological insulators (TIs) and the generation of THz radiation in photoconductive antennas based on the TI films. The experimental results, supported by the developed kinetic theory of third harmonic generation, show that the frequency conversion in TIs is highly efficient because of the linear energy spectrum of the surface carriers and fast energy dissipation. In particular, the dependence of the third harmonic field on the pump field remains cubic up to the pump fields of 100 kV/cm. The generation of THz radiation in TI-based antennas is obtained and described for the pump, with the energy of photons corresponding to the electron transitions to higher conduction bands. Our findings open up possibilities for advancing TI-based films into THz photonics as efficient THz wave generators and frequency converters.