Terahertz Photonic Devices Research Articles

Terahertz photon to dc current conversion via magnetic excitations of multiferroics

Direct conversion from terahertz photon to charge current is a key phenomenon for terahertz photonics. Quantum geometrical description of optical processes in crystalline solids predicts existence of field-unbiased dc photocurrent arising from terahertz-light generation of magnetic excitations in multiferroics, potentially leading to fast and energy-efficient terahertz devices. Here, we demonstrate the dc charge current generation from terahertz magnetic excitations in multiferroic perovskite manganites with spin-driven ferroelectricity, while keeping an insulating state with no free carrier. It is also revealed that electromagnon, which ranges sub-terahertz to 2 THz, as well as antiferromagnetic resonance shows the giant conversion efficiency. Polar asymmetry induced by the cycloidal spin order gives rise to this terahertz-photon-induced dc photocurrent, and no external magnetic and electric bias field are required for this conversion process. The observed phenomena are beyond the conventional photovoltaics in semi-classical regime and demonstrate the essential role of quantum geometrical aspect in low-energy optical processes. Our finding establishes a paradigm of terahertz photovoltaic phenomena, paving a way for terahertz photonic devices and energy harvesting.

Switchable and multifunctional terahertz photonic devices based on VO2-assisted phase encoded metasurfaces

Metasurfaces, especially coded ones, have gained significant attention for their excellent ability to control electromagnetic (EM) waves, opening up possibilities for a new generation of miniaturized devices. However, integrating multiple functionalities into a single metasurface while endowing tunable performance remains a challenge. In this study, we propose a strategy to design switchable and multifunctional terahertz (THz) photonic devices using complementary Gold-VO2 split resonant ring (GV-SRR) and complementary Gold-VO2 split resonant oblong (GV-SRO) structures. The designed devices can convert right-handed circularly polarized (RCP) light into left-handed circularly polarized (LCP) light in three frequency bands, corresponding to center frequencies fre.1 = 0.34 THz (narrowband), fre.2 = 0.50 THz (narrowband), and fre.3 = 0.80 THz (broadband). The phases of the reflected LCP wave can be independently modulated across different frequency bands by rotating the in-plane orientation of GV-SRR and GV-SRO separately. In addition, the overall phase reconstruction can be achieved through the state change of VO2. By arranging GV-SRR and GV-SRO in a defined encoding form, the engineered photonic devices can exhibit distinct and switchable capabilities in modulating EM wavefronts across various frequency ranges. As proof-of-concept examples, we have designed three encoded photonic devices, namely, a dynamic beam splitter (DBS), an achromatic and zoom metalens (AZM), and a switchable device between metalens and focusing orbital angular momentum (OAM) generator (MFOAM). These devices have potential applications in wireless communication, AR holographic displays, and information processing.

Terahertz Metasurface Modulators Based on Photosensitive Silicon

AbstractMetasurfaces solve the lack of materials in the terahertz (THz) band and control precisely the amplitude, phase, polarization, and transmission characteristics of THz waves, providing an effective way to realize THz functional devices. This article focuses on the design of THz metasurface modulators with a unit structure consisting of metal square rings, including resonance frequency, phase, and amplitude modulators. By embedding photosensitive semiconductor silicon (Si) in the unit structure, the unit structure is built from meta‐atom to molecularization model under the optical pumping condition, and the resonance frequencies are switched between high and low frequencies. The resonance frequency switchable characteristic is demonstrated using the equivalent LC oscillation circuit model, and the theoretical calculation results agree well with the simulations. Through theoretical calculations, the modulators achieve ultrafast switching times of less than 0.141 ps by the optical pumps, which have significant advantages in ultrafast THz modulators. By continuing to change the embedded position of the silicon in the unit structure, not only is a wide range of THz phase modulation achieved, but also multilevel modulation of the phase is realized. It is found that there is a strong relationship between the modulation depth and phase variation of THz waves, and a reasonable analysis is given. Further the amplitude modulator with a larger modulation depth (MD) is developed, and when the conductivity of photosensitive semiconductor silicon (σSi) reaches 2.5 × 106 S m−1, return loss (RL) is ≈0 dB, and the maximum MD reaches ≈100%; in order to gain insight into the nature of modulation, the modulation mechanism of THz waves under optical pumping conditions is analyzed. In addition, graphene‐based THz metasurface amplitude modulators are designed. When the depth of amplitude modulation is achieved by bias voltage modulation of the Fermi energy level of graphene, the maximum modulation amplitude is 23.42 dB, with a minimal modulation accuracy of 0.05 THz eV−1. In the article, the designed modulators have extremely excellent modulation performance. It has great potential applications in silicon‐based THz photonic devices, ultrahigh frequency electronic devices, high sensitivity sensors, and high‐precision imaging.

Double Narrowband Induced Perfect Absorption Photonic Sensor Based on Graphene–Dielectric–Gold Hybrid Metamaterial

Double narrowband induced perfect absorption in the terahertz region is achieved in a graphene–dielectric–gold hybrid metamaterial, whose physical mechanism is analyzed using the coupled-mode theory (CMT), which agreed well with the finite-difference time-domain (FDTD) simulation. This study found that the Fermi level of graphene can be adjusted to improve the absorptivity when the refractive index (RI) nd of the chosen dielectric cannot achieve a good absorption effect. In addition, the blue shift of absorption spectrum can be used in the design of dual-frequency electro-optical switches, of which the modulation degree of amplitude (MDA) can reach as high as 94.05% and 93.41%, indicating that this is a very promising electro-optical switch. Most significantly, the RI sensing performance is investigated, which shows an ultra-high absorption sensitivity SA = 4.4°/RIU, wavelength sensitivity Sλ = 9.8°/RIU, and phase shift sensitivity Sφ = 2691°/RIU. At last, an interesting finding is that the two peaks (R1 and R2) of plasmon-induced absorption (PIA) show different polarization characteristics (insensitive or sensitive) to the incident light angle; this polarization-sensitive is particularly important for the PIT/PIA-based optical polarizers. Undoubtedly, this paper is of great significance to the research and design of terahertz photonic devices and sensors.

On/Off Switching of Valley Topological Edge States in the Terahertz Region

Topological photonic devices provide a powerful platform to control the flow of light because their topologically protected edge states can be immune to backscattering and be robust to impurities, disorders, and defects. As one of the topological photonic systems, valley photonic crystals have attracted great attention because they can achieve a large bandgap and be easily combined with modern micro-nano fabrication technology. Terahertz photonic devices are of enormous interest in next-generation communications. Here, we design a valley photonic crystal consisting of a triangular lattice with three metal rod scatterers. By rotating the three metal rods in opposite directions to break the inversion symmetry, we achieve a bandgap width of up to 70% and observe the transport of the edge states in the terahertz region. By replacing part of the structure with phase change material VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , on/off switching of the terahertz photonic topological edge states is realized, and the attenuation can be also modeled by adjusting the size of the VO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> region. The proposed regime has potential applications in sensing and 6G communications.

Enhanced Terahertz Generation From the Lithium Niobate Metasurface

The lithium niobate (LiNbO3) metasurface is an attractive platform for nonlinear frequency conversion due to its excellent nonlinearity, high damage threshold, and strong enhancement of the driving field. Here, we demonstrated the nonlinear metasurface for terahertz (THz) generation from LiNbO3on an insulator. The THz electric field from the LiNbO3metasurface of 300 nm thickness is enhanced by more than one order of magnitude compared to the unstructured sample. The enhanced terahertz electric field is very selective in the excitation wavelength due to its resonant feature and is highly anisotropic with respect to the excitation polarization. The polarization direction of the THz electric field can be controlled by rotating the optical axis of the LiNbO3metasurface. By combining the nanofabrication technology of the LiNbO3metasurface and ultrafast nonlinear optics, our work paves the way for the design of new compact terahertz photonic devices that integrate THz emitters into LiNbO3-based chips with multifaceted capabilities.

Enhancement of Terahertz Radiation by Surface Plasmons Based on CdTe Thin Films.

Terahertz (THz) time-domain spectroscopy (TDS) is a powerful tool used to characterize the surface/interface of materials, and semiconductor/metal interfaces can generate THz emission through ultrafast optical excitation, which can be further improved through the optical excitation of surface plasmons. Here, we assembled cadmium telluride (CdTe) on an AuAg alloy (Au25Ag75, wt.%) substrate and obtained five times stronger THz emission compared with silicon substrate, and found that the enhancement can be tuned by controlling the thickness of the semiconductor materials and plasmonic metal substrates. We believe that our results not only promote the development of THz emission enhancement, but also provide a straightforward way of producing small, thin, and more efficient terahertz photonic devices.

300-GHz Intermodulation/Noise Characterization Enabled by a Single THz Photonics Source

In this letter, we present the first use of a terahertz (THz) photonic-driven source toward active circuit characterization. We show that a single source can be used for intermodulation points and noise figure measurement. The method is applied at 300 GHz with a HEMT technology-based low noise amplifier (LNA). We show that the third and fifth intermodulation behavior at 300 GHz can be extracted taking advantage of the linearity of the THz photonic devices. Here, the millimeter-wave/THz source is an unitraveling carrier photodiode (UTC-PD) used either as a signal generator with two-tones or as a noise source.

Multilayer Graphene Terahertz Plasmonic Structures for Enhanced Frequency Tuning Range

Graphene plasmonics has recently found a variety of applications in terahertz photonic devices. High spatial confinement and large frequency tunability are two key advantages of graphene plasmonics. Nevertheless, the frequency tuning range of plasmonic devices employing single-layer graphene is ultimately limited by its carrier density tuning range. Here, we demonstrate that the frequency tuning range of graphene-based plasmonic devices can be further extended by employing multilayer graphene structures. Both our experimental investigations and theoretical calculations show that the frequency tuning range of gate-controlled graphene plasmonic resonators can be significantly enhanced by employing two or three layers of stacked graphene, which is a result of the carrier distributions in multiple layers leading to higher total optical conductivity. However, contrary to the previous prediction, stacking even more graphene layers yields little additional benefit, as the interlayer charge screening effect leads to insignificant gate-induced carrier density in additional graphene layers. Our findings provide new insights for designing and optimizing graphene-based plasmonic structures for various photonic device applications, such as modulators, sensors, and detectors.

Simulation of Multilayer Graphene–Dielectric Metamaterial by Implementing SBC Model of Graphene in the HIE-FDTD Method

An improved FDTD method is introduced to propose a new numerical simulation approach for a multilayer graphene–dielectric metamaterial (MGDM) structure, which is a promising effective medium with tunable constitutive parameters in terahertz photonic devices. We applied the hybrid implicit–explicit FDTD (HIE-FDTD) method, which is a popular method for analyzing dimensionally non-uniform structures with fine features in one or two directions to model the MGDM structure. Considering the ignorable atomic thickness of graphene layers in the structure, we increased the fine dimension of the mesh to the scale of the dielectric layer using the surface boundary condition (SBC) method for modeling the graphene sheets; this means that we integrated the SBC model of graphene in the HIE-FDTD method. Therefore, the thickness limitations of both graphene and dielectric layers can be eliminated in the computational resources. Accordingly, we implemented the introduced method in a proposed tunable graded index (GRIN) lens structure based on the MGDM and achieved valid results.

Solution-Processed Lead Iodide for Ultrafast All-Optical Switching of Terahertz Photonic Devices.

Solution-processed lead iodide (PbI2 ) governs the charge transport characteristics in the hybrid metal halide perovskites. Besides being a precursor in enhancing the performance of perovskite solar cells, PbI2 alone offers remarkable optical and ultrasensitive photoresponsive properties that remain largely unexplored. Here, the photophysics and the ultrafast carrier dynamics of the solution processed PbI2 thin film is probed experimentally. A PbI2 integrated metamaterial photonic device with switchable picosecond time response at extremely low photoexcitation fluences is demonstrated. Further, findings show strongly confined terahertz field induced tailoring of sensitivity and switching time of the metamaterial resonances for different thicknesses of PbI2 thin film. The approach has two far reaching consequences: the first lead-iodide-based ultrafast photonic device and resonantly confined electromagnetic field tailored transient nonequilibrium dynamics of PbI2 which could also be applied to a broad range of semiconductors for designing on-chip, ultrafast, all-optical switchable photonic devices.

Color‐Sensitive Ultrafast Optical Modulation and Switching of Terahertz Plasmonic Devices

Abstract2D micro‐nanostructured metal films with hole arrays show promising features such as the extraordinary transmission of light. Such systems are interesting in the field of subwavelength photonics and nonlinear optics due to their high field confinement in addition to their inherent spectral scalability and frequency selective response. Several active schemes to control the extraordinary transmission are recently demonstrated. However, these dynamic devices do not reveal any obvious color‐dependent modulation of the resonant transmission behavior. Here, color‐sensitive ultrafast modulation of extraordinary resonant transmission of terahertz (THz) waves through 2D metallic hole arrays is demonstrated. Pumping the silicon beneath the metallic array with light of different colors and identical fluences exhibit significantly different ultrafast switching dynamics and modulation. The color‐dependent sensitivity and control of THz waves at an ultrafast timescale provide an extra degree of freedom that opens up new opportunities for future applications in active subwavelength optics, optoelectronics, and all‐optical switching of THz photonic devices.

Preface to Special Topic: Frontiers on THz photonic devices

Terahertz (THz) photonic devices are now exploiting emerging materials systems, while novel device designs utilise plasmonic effects, nanophotinics, and metamaterials. The scope of this special topic highlights and reviews the recent cutting-edge THz photonic devices which have been revolutionised from the advances in the above research areas.

Boosting the terahertz nonlinearity of graphene by orientation disorder

The conical band structure is the cornerstone of graphene’s ultra-broadband optical conductivity. For practical use of graphene in nonlinear photonics, however, substantial increases of the light–matter interaction strength will be required while preserving the promising features of monolayers, as the interaction of light with a single atomic layer is limited due to the extremely short interaction length and low density of state, particularly for the long-wavelength region. Here, we report that this demand can be fulfilled by random stacking of high-quality large-area monolayer graphene up to a requested number of layers, which leads to the electronic interaction between layers being effectively switched off due to turbostratic disorder. The nonlinear characteristics of randomly stacked multilayer graphene (RSMG), which originates from a thermo-modulational feedback mechanism through ultrafast free-carrier heating and temperature-dependent carrier-phonon collisions, show clear improvements in the terahertz (THz) regime with increasing layer numbers, whereas as-grown multilayer graphene (AGMG) exhibits limited behaviors due to strong interlayer coupling. This controllable nonlinearity enhancement provides an ideal prerequisite for developing efficient graphene-based THz photonic devices.

Bias field tailored plasmonic nano-electrode for high-power terahertz photonic devices.

Photoconductive antennas with nano-structured electrodes and which show significantly improved performances have been proposed to satisfy the demand for compact and efficient terahertz (THz) sources. Plasmonic field enhancement was previously considered the dominant mechanism accounting for the improvements in the underlying physics. However, we discovered that the role of plasmonic field enhancement is limited and near-field distribution of bias field should be considered as well. In this paper, we clearly show that the locally enhanced bias field due to the size effect is much more important than the plasmonic enhanced absorption in the nano-structured electrodes for the THz emitters. Consequently, an improved nano-electrode design is presented by tailoring bias field distribution and plasmonic enhancement. Our findings will pave the way for new perspectives in the design and analysis of plasmonic nano-structures for more efficient THz photonic devices.

Plasmon enhanced terahertz emission from single layer graphene.

We show that surface plasmons, excited with femtosecond laser pulses on continuous or discontinuous gold substrates, strongly enhance the generation and emission of ultrashort, broadband terahertz pulses from single layer graphene. Without surface plasmon excitation, for graphene on glass, 'nonresonant laser-pulse-induced photon drag currents' appear to be responsible for the relatively weak emission of both s- and p-polarized terahertz pulses. For graphene on a discontinuous layer of gold, only the emission of the p-polarized terahertz electric field is enhanced, whereas the s-polarized component remains largely unaffected, suggesting the presence of an additional terahertz generation mechanism. We argue that in the latter case, 'surface-plasmon-enhanced optical rectification', made possible by the lack of inversion symmetry at the graphene on gold surface, is responsible for the strongly enhanced emission. The enhancement occurs because the electric field of surface plasmons is localized and enhanced where the graphene is located: at the surface of the metal. We believe that our results point the way to small, thin, and more efficient terahertz photonic devices.

Gate-tunable nearly total absorption in graphene with resonant metal back reflector

The gate-tunable absorption of graphene layers with a resonant metal back reflector (RMBF) is theoretically investigated. We demonstrate that the absorption of graphene with RMBF can vary from nearly negligible to nearly total by tuning the external gate voltage within the terahertz (THz) spectra range. Total THz absorption is less affected by the incident angle of THz beams. This peculiar nearly total THz absorption can be attributed to the Fabry-Perot cavity effect, which enhances the absorption and reduces the reflection of graphene. The absorption spectra of the graphene-RMBF structure can also be tailored in bandwidth and center frequency by changing the thickness and dielectric constant of the spacer layer. These findings can lead to the development of tunable THz photonic devices and have potential applications in studies on the ultrafast dynamics of Dirac fermions in graphene.

The Fano Resonance in Symmetry Broken Terahertz Metamaterials

The spectral characteristic of a Fano resonance is a distinct and unique asymmetric line shape. It arises from the destructive interference between a bright continuum mode and a discrete dark mode. Metamaterials and plasmonics have facilitated the observation of Fano resonances in the realm of classical electrodynamics and have spurred intense interest to study and to exploit their peculiarities due their low loss and very sharp resonant features. Here, we survey the excitation of Fano resonance at terahertz frequencies by using asymmetric metamaterial structures. In particular, we discuss the high Q of Fano resonances, their sensitivity to the asymmetry parameter and potential applications. We believe that the manipulation of terahertz radiation by exploiting Fano resonances in terahertz metamaterials could usher in next-generation terahertz photonic devices for delay, storage, filtering, sensing, and nonlinear applications.

A review of terahertz plasmonics in subwavelength holes on conducting films

In this paper, we present a review of experimental studies of terahertz plasmonic transmission properties through subwavelength holes patterned in conducting films. The frequency-dependent transmission spectrum reveals resonant behavior with an anomalously high peak transmission which is mediated by the excitation of surface plasmon polaritons. We show how terahertz time-domain spectroscopy has been utilized to determine the resonant transmission effects of hole shape, dielectric properties of materials, and thickness of the arrays. Enhanced terahertz transmission was also observed through a single hole, accompanied by annular periodic corrugations. In addition to metals films, we review films comprised of highly doped semiconductors and superconductors. We finally review various modulation schemes to actively control or manipulate the resonant terahertz transmission using external stimuli such as thermal, optical, and electrical fields. This body of work is used to provide perspective on how manipulation of terahertz radiation via surface plasmon polaritons could affect next-generation terahertz photonic devices.

Terahertz photonic states in semiconductor–graphene cylinder structures

We propose a semiconductor-graphene cylinder that can serve as a terahertz (THz) photonic crystal. In such a structure, graphene plays a role in achieving a strong mismatch of the dielectric constant at the semiconductor-graphene interface due to its two-dimensional nature and relatively low value of the dielectric constant. We find that when the radius of the outer semiconductor layer is about ρ(1)~100 μm, the frequencies of the photonic modes are within the THz bandwidth and they can be efficiently tuned via varying ρ(1). Furthermore, the dispersion relation of the photonic modes shows that a semiconductor-graphene cylinder is of excellent light transport properties, which can be utilized for the THz waveguide. This study is pertinent to the application of graphene as THz photonic devices.