Tray Research Articles

Spectroscopic Evidence of Ultrafast Topological Phase Transition by Light-Driven Strain.

Enabling reversible control over the topological invariants, transitioning them from nontrivial to trivial states, has fundamental implications for quantum information processing and spintronics. It offers a promising avenue for establishing an efficient on/off switch mechanism for robust and dissipationless spin-currents. While mechanical strain has traditionally been advantageous for such manipulation of topological invariants, it often comes with the drawback of in-plane fractures, rendering it unsuitable for high-speed, time-dependent operations. This study employs ultrafast optical and THz spectroscopy to explore topological phase transitions induced by light-driven strain in Bi2Se3. Bi2Se3 requires substantial strain for Z2 switching. Our observations provide experimental evidence of ultrafast switching behavior, demonstrating a transition from a topological insulator with spin-momentum-locked surfaces to hybridized states and normal insulating phases under ambient conditions. Notably, applying light-induced strong out-of-plane strain effectively suppresses surface-bulk coupling, facilitating the differentiation of surface and bulk conductance even at room temperature─significantly surpassing the Debye temperature. We expect various time-dependent sequences of transient hybridization and manipulation of topological invariant through photoexcitation intensity adjustments. The sudden surface and bulk transport alterations near the transition point enable coherent conductance modulation at hypersound frequencies. Our findings on the potential of light-triggered ultrafast switching of topological invariants hold promise for high-speed topological switching and its related applications.

Experimental and numerical validation of tape-based metasurfaces in guiding high-frequency surface waves for efficient power transfer

We present an effective method for transmitting electromagnetic waves as surface waves with a tape-based metasurface design. This design incorporates silver square patches periodically patterned on an adhesive tape substrate. Specifically, our study proposes a strategy to enhance the efficiency of power transfer in high-frequency bands by guiding signals as surface waves rather than free-space waves. Both the numerical and experimental results validate the markedly enhanced efficiency in power transfer of high-frequency signals compared to that achieved with conventional methods, such as wireless power transfer and microstrips. Importantly, our metasurface design can be readily manufactured and tailored for various environments. Thus, our study contributes to designing power-efficient next-generation communication systems such as 6G and 7G, which leverage high-frequency signals in the millimeter-wave and terahertz bands.

The frequency spectrum and energy content in a pulse flux of terahertz radiation generated by a relativistic electron beam in a plasma column with different density distributions

This paper reports on the generation of a directed flux of electromagnetic radiation with an energy content of 10 J in the frequency range of 0.2—0.3 THz at a microsecond pulse duration in a beam-plasma system. The flux is generated when a relativistic electron beam (REB) pumps electron plasma waves in a magnetized plasma column. In the described experiments, this fundamentally new approach to generate terahertz radiation was carried out at the GOL-PET facility in the conditions of varying the beam current density and the plasma density in the appropriate ranges of 1—2 kA/cm2 and 1014—1015 cm−3. From the comparison of the flux energy spectrum measured experimentally in the frequency range 0.15—0.45 THz with the calculated one obtained using the previously proposed model of radiation generation in a beam–plasma system, it was shown that this process occurs through resonant pumping by REB of precisely the branch of upper-hybrid plasma waves. Mastering this new method to generate terahertz radiation opens the prospect of its use to obtain multi-megawatt radiation fluxes in the frequency range up to 1 terahertz and higher. For such a development approach the most promising beam for pumping plasma oscillations seems to be a kiloampere REB generated in a linear induction accelerator.

Spectrally narrowband simultaneous dual-wavelength emission from Y-branch DBR diode lasers at 785 nm

Y-branch distributed Bragg reflector (DBR) diode lasers with a stable narrowband emission in simultaneous dual-wavelength operation with spectral distances below 3.2 nm are presented. The Y-branch laser consists of two laser branches with different DBR gratings serving as wavelength-selective rear-side mirrors. Therefore, two emission wavelengths with a spectral distance defined by the DBR grating periods can be generated simultaneously. A Y-coupler combines the two ridge waveguide (RW) branches into a single straight output RW. Devices with a spectral distance of 0.6 nm and 2.0 nm emitting around 785 nm are manufactured. Selecting the operation parameters carefully, stable narrowband emission for both wavelengths is obtained. Resistors serving as heaters implemented next to the DBR gratings allow for wavelength adjustment and a tuning of the spectral distance. At an optical output power of 100 mW, the spectral distance can be shifted from 0 to 1.55 nm (0–0.76 THz) for the former device or from 1.00 to 3.15 nm (0.49–1.54 THz) for the latter device, respectively. This makes the Y-branch DBR diode laser particularly interesting for the generation of THz beat-note signals, needed to generate THz radiation via photo-mixing.

Generation of Circularly Polarized Strong-Field Terahertz Waves

Abstract Terahertz (THz) circular dichroism (TCD) spectroscopy has been widely used to study the chiral characteristics of biological macromolecules and various other materials. With the rapid development of strong-field THz generation and field-modulated techniques, the advancement of tunable strong-field TCD spectroscopy technology is of great significance. In this work, we built an integrated strong-field TCD spectroscopy system. We employed the tilted-pulse-front technique to generate linearly polarized strong-field THz radiation and a reflective metasurface for linear-to-circular conversion of the strong-field THz radiation. We obtained circularly polarized THz radiation with an ellipticity of over 0.9 in the frequency range from 0.38 THz to 0.61 THz, with a LTC conversion efficiency of over 90%. The peak electric field strength of the obtained strong-field circularly polarized THz radiation exceeded 100 kV/cm. This system is expected to be used to study the chiral characteristics of materials under strong-field conditions, as well as changes in the chiral characteristics under different field conditions.

Coherent high-power terahertz vortex generation with tunable orbital angular momentum based on transverse phase mask shaping

Abstract Terahertz (THz) radiation has become a significant tool in cutting-edge research due to its superior properties. THz vortices with tunable orbital angular momentum are particularly attractive to the scientific community due to their well-defined discrete azimuthal phase around the propagation axis. However, the generation of high-power THz radiation with orbital angular momentum remains a challenge for most existing technologies. In this paper, a new method for generating coherent high-power THz vortices with tunable orbital angular momentum and frequency is proposed by combining frequency beating and transverse phase mask shaping techniques. Theoretical analyses and numerical simulations indicate that this method can generate coherent THz vortices with peak powers in the tens of megawatts and tunable topological charge numbers.

A Non-Invasive and DNA-free Approach to Upregulate Mammalian Voltage-Gated Calcium Channels and Neuronal Calcium Signaling via Terahertz Stimulation.

Mammalian voltage-gated calcium channels (CaV) play critical roles in cardiac excitability, synaptic transmission, and gene transcription. Dysfunctions in CaV are implicated in a variety of cardiac and neurodevelopmental disorders. Current pharmacological approaches to enhance CaV activity are limited by off-target effects, drug metabolism issues, cytotoxicity, and imprecise modulation. Additionally, genetically-encoded channel activators and optogenetic tools are restricted by gene delivery challenges and biosafety concerns. Here a novel terahertz (THz) wave-based method to upregulate CaV1.2, a key subtype of CaV, and boost CaV1-mediated Ca2+ signaling in neurons without introducing exogenous DNA is presented. Using molecular dynamics simulations, it is shown that 42.5 THz (7.05µm, 1418cm-1) waves enhance Ca2+ conductance in CaV1.2 by resonating with the stretching mode of the -COO- group in the selectivity filter. Electrophysiological recordings and Ca2+ imaging confirm that these waves rapidly, reversibly, and non-thermally increase calcium influx of CaV1.2 in HEK293 cells and induce acute Ca2+ signals in neurons. Furthermore, this irradiation upregulates critical CaV1 signals, including CREB phosphorylation and c-Fos expression, in vitro and in vivo, without raising significant biosafety risks. This DNA-free, non-invasive approach offers a promising approach for modulating CaV gating and Ca2+ signaling and treating diseases characterized by deficits in CaV functions.

A Review of Thermal Detectors of THz Radiation Operated at Room Temperature

This article concerns optical detection issues in the terahertz (THz) range. This is a kind of guide to various types of uncooled thermal detectors in the most often applications. Particular attention is paid to the principle of their operation, technology, and practical features. In addition, some detection methods were also characterized by comparing their performances. The article ends with a performance summary of the selected THz thermal detectors.

Generation of a coherent longitudinal optical phonon plasmon coupled mode in an n-type InSb single crystal in the absence and/or strong suppression of the photo-Dember effect confirmed with the use of terahertz time-domain spectroscopy

We explored the feasibility of generating a coherent longitudinal optical (LO) phonon–plasmon coupled mode in an n-type InSb single crystal with the condition that the photo-Dember effect is strongly suppressed and/or eliminated. We systematically performed terahertz time-domain spectroscopic measurements of the n-type InSb single crystal and undoped InSb crystal. The terahertz electromagnetic wave originating from the photo-Dember effect was extinguished in the n-type InSb single crystal, while the clear signal from the photo-Dember effect was detected in the undoped InSb single crystal. Meanwhile we detected the clear signal of the terahertz wave from the coherent LO-phonon–plasmon coupled mode in the n-type InSb single crystal, indicating that the photo-Dember effect is not dominant in the generation of the coherent LO-phonon–plasmon coupled mode although the photo-Dember effect has been attributed to the dominant generation mechanism of the coherent LO-phonon–plasmon coupled mode so far. We explain the present phenomenon, taking account of the fact that the impulsive stimulated Raman scattering process has a possibility of generating coherent LO-phonon–plasmon coupled modes in light-absorbing solids [Nakamura et al., Phys. Rev. B 99, 180301(R) (2019)].

Strain-Induced Tunable and Field-Free Spintronic THz Emitters Based on Flexible Substrate Heterostructures.

Spintronic THz emitters have been widely studied due to their advantages of broadband frequency, high efficiency, and easy fabrication. The spintronic THz signal is proportional to the magnetization of the ferromagnetic (FM) layer and requires an external magnetic field to maximize the THz signal intensity. Recently, a field-free emitter was designed based on a CoFeB/IrMn3 heterostructure via the exchange bias between two films. However, field-free spintronic THz emitters based on a common FM/nonmagnetic metal structure are rare. Here, we fabricate a tunable and field-free THz emitter with giant THz emission modulation ability based on a polyimide/CoFeB/Pt heterostructure. The THz emission can be modulated by changing the curvature radius of the polyimide substrate. After the emitter is forward bent, the THz radiation without an external magnetic field is stronger than in the flat state with a field, which we attribute to in-plane magnetic anisotropy on the FM layer induced by the tensile strain. When we curve the emitter backward, the THz intensity decreases sharply. Moreover, the modulation (ΔS/Smax) of the THz wave from the forward-curved and backward-curved emitter is over 95%, and the device shows good cyclic repeatability. We provide a novel way to design field-free spintronic THz sources, and flexible devices are promising for application in THz devices.

Directional Coupling to a λ/5000 Nanowaveguide.

Silicon-based microdevices are considered promising candidates for consolidating several terahertz technologies into a common and practical platform. The practicality stems from the relatively low loss, device compactness, ease of fabrication, and wide range of available passive and active functionalities. Nevertheless, typical device footprints are limited by diffraction to several hundreds of micrometers, which hinders emerging nanoscale applications at terahertz frequencies. While metallic gap modes provide nanoscale terahertz confinement, efficiently coupling to them is difficult. Here, we present and experimentally demonstrate a strategy for efficiently interfacing subterahertz radiation (λ = 1 mm) to a waveguide formed by a nanogap, etched in a gold film, that is 200 nm (λ/5000) wide and up to 4.5 mm long. The design principle relies on phase matching dielectric and nanogap waveguide modes, resulting in efficient directional coupling between them when they are placed side-by-side. Broadband far-field terahertz transmission experiments through the dielectric waveguide reveal a transmission dip near the designed wavelength due to resonant coupling. Near-field measurements on the surface of the gold layer confirm that such a dip is accompanied by a transfer of power to the nanogap, with an estimated coupling efficiency of ∼10%. Our approach efficiently interfaces millimeter waves with nanoscale waveguides in a tailored and controllable manner, with important implications for on-chip nanospectroscopy, telecommunications, and quantum technologies.

Exploring the Impact of 3D Printing Parameters on the THz Optical Characteristics of COC Material.

In terahertz (THz) optical systems, polymer-based manufacturing processes are employed to ensure product quality and the material performance necessary for proper system maintenance. Therefore, the precise manufacturing of system components, such as optical elements, is crucial for the optimal functioning of the systems. In this study, the authors investigated the impact of various 3D printing parameters using fused deposition modeling (FDM) on the optical properties of manufactured structures within the THz radiation range. The measurements were conducted on 3D printed samples using highly transparent and biocompatible cyclic olefin copolymer (COC), which may find applications in THz passive optics for "in vivo" measurements. The results of this study indicate that certain printing parameters significantly affect the optical behavior of the fabricated structures. The improperly configured printing parameters result in the worsening of THz optical properties. This is proved through a significant change in the refractive index value and undesirable increase in the absorption coefficient value. Furthermore, such misconfigurations may lead to the occurrence of defects within the printed structures. Finally, the recommended printing parameters, which improve the optical performance of the manufactured structures are presented.

Possibility of the magnitude of the chiral vector having an effect on the terahertz radiation generation by hot electrons in zigzag carbon nanotubes driven by DC-AC fields

The possibility that the magnitude of a chiral or circumferential vector (Chv) influences the generation of terahertz radiations (T-rays) by hot electrons in zigzag carbon nanotubes (z-CNTs) driven by direct current (DC) and alternating current (AC) fields is taken into consideration. The study conducted a semiclassical theoretical analysis by using the Boltzmann kinetic equation to determine current density in relation to high AC field frequency (ω) and z-CNT circumference (|Chv|) when hot electrons were present. A numerical analysis was performed by varying the magnitude of the circumference of the tube at a fixed temperature and the strong hot electron injection rate at a predetermined value. When the circumference of the semiconducting zigzag carbon nanotubes (sz-CNTs) is increased, the current density's lowest and highest peaks are also enhanced (i.e., large amplitudes). This increases positive differential conductivity (PDC), necessitating enhanced domain suppression for higher potential generation of T-rays. In contrast, it was observed that reducing the tube's circumference in metallic zigzag carbon nanotubes (mz-CNTs) resulted in an increase in both the lowest and highest peaks of current density, which enhanced the PDC. These findings imply that larger circumferential sz-CNTs and smaller circumferential mz-CNTs may be more suitable candidates for a higher potential generation of T-rays with applications pertinent to modern science and technology.

Generation of terahertz radiation with a specified spectral profile from a mosaic combined organic crystal

We present a method for the generation of terahertz (THz) pulses with a high optical-to-THz conversion efficiency and a smooth spectrum. The method is based on the optical rectification of near-infrared femtosecond laser pulses with a wavelength of 1240 nm in a mosaic combined crystal, consisting of two pieces of nonlinear organic crystals, OH1 and DSTMS. The mosaic combined crystal produces a relatively smooth spectrum in the 0.3–4.5 THz range with no pronounced absorption dips and with a maximum in a spectral amplitude at about 2 THz. The peak field strength is 9.6 MV/cm for an effective crystal diameter of 4.3 mm.

Nanometric Ge Films for Ultrafast Modulation of THz Waves with Flexible Metasurface

AbstractHarnessing confined light on a subwavelength scale within Fano metasurfaces enhances light‐matter interaction on a flexible, low‐loss substrate. This approach enables the integration of ultra‐thin semiconducting films for designing low‐power, ultrafast switchable terahertz, and optical meta devices. Here, an ultra‐thin, ∼λ/6000 germanium overlayers of 25nm or 50nm is thermally evaporated onto a flexible Fano metallic metasurface to achieve ultrafast optical modulation of terahertz radiation. A remarkable 85% modulation depth with an ultrafast speed of 400 GHz is obtained in the resonant transmission. These ultrathin, flexible, and ultrafast functional metasurfaces pave the way for developing flexible terahertz active meta devices based on nanometric scale germanium films, offering the additional advantage of compatibility with complementary metal‐oxide‐semiconductor (CMOS) technology in microelectronics.

Tight focusing of fractional-order topological charge vector beams by cascading metamaterials and metalens

Vector beams have attracted widespread attention because of their unique optical properties; in particular, their combination with tight focusing can produce many interesting phenomena. The rise of 3D printing technology provides more possibilities for exploration. In this work, a cascading method involving a metamaterial and a metalens is used to generate a tightly focused field of vector beams in the terahertz band, which is prepared via 3D printing. As a proof-of-concept demonstration, a series of metamaterial modules capable of generating states of different orbital angular momentum are proposed by cascading with a metalens. The experimental results are in good agreement with the simulation results, fully verifying the feasibility of the scheme. The proposed design and fabrication strategy provides a new idea for the tight focusing of terahertz vector beams.

Scanless Spectral Imaging of Terahertz Vortex Beams Generated by High‐Resolution 3D‐Printed Spiral Phase Plates

Terahertz technology has experienced significant advances in the past years, leading to new applications in the fields of spectroscopy, imaging, and communications. This progress requires the development of dedicated optics to effectively direct, control and manipulate terahertz radiation. In this regard, 3D printing technologies have shown great potential, offering fast prototyping, high design flexibility, and good reproducibility. While traditional 3D printing techniques allow for the preparation of terahertz optical components operating at relatively low frequencies (<0.4 THz) due to their limited resolution, two‐photon polymerization lithography (TPL) exhibits high detail resolution and low surface roughness and can thus potentially enable the fabrication of high‐frequency terahertz devices. Here, as a proof of principle, spiral phase plates operating at 1 THz are designed and fabricated by means of TPL. Moreover, these samples are characterized via a rapid and scanless terahertz imaging technique customized to obtain a coherent hyperspectral analysis of the generated vortex beams at varying distances along propagation. Numerical simulations are also conducted for comparison with experiments, revealing a good agreement. Current limitations of the technique are found to be mainly related with terahertz loss in TPL polymers, and possible solutions are discussed.

Frequency-resolved measurement of two-color air plasma terahertz emission

We investigated the far-field terahertz beam profile generated from an air plasma induced by two-color femtosecond laser pulses. Under our experimental conditions (filament length shorter than the dephasing length between the two-color pulses), using electro-optic sampling in both ZnTe (0.2-2.2 THz) and GaP (0.4-6.8 THz) crystals, and ultra-broadband ABCD technique (1-17.5 THz), we determined that the THz beam exhibits a unimodal beam pattern below 4 THz and a conical one above 6 THz. This experimental finding is consistent with theoretical studies based on the unidirectional pulse propagation equation, which predict the transition of THz emission from a flat-top profile to a conical one due to the destructive interference of THz waves emitted from the plasma filament. Our results also underscore the importance of accounting for experimental artifacts, such as photo-excited losses in silicon resulting in on-axis THz absorption along with the influence of drilled mirrors, in characterizing the complex spatial and frequency-dependent behavior of two-color plasma-induced terahertz emission.

Terahertz Sensing with Extraordinary Optical Transmission Hole Arrays

Purpose: The purpose of this paper is to substantiate the enhanced performance of hyperbolic anisotropic (HA) meta-surfaces at anomalous extraordinary optical transmission (EOT) resonances. These resonances exhibit distinct transmission peaks highly sensitive to environmental changes, making them particularly valuable for sensing applications. By demonstrating improved transmission efficiency and sensitivity, this study aims to contribute to developing advanced sensing technologies that leverage the unique properties of HA meta-surfaces in detecting minute variations in their surroundings. Methodology: To verify the enhanced sensing performance of anomalous EOT resonances, this study employs a well-established experimental method involving depositing a thin analyte layer of varying thicknesses onto the meta-surface. The study aims to determine which resonance condition provides optimal sensitivity by comparing the performance between regular and anomalous EOT resonances. It explores the improved performance of HA meta-surfaces at anomalous EOT resonances, which occur under certain non-standard conditions, offering potentially superior sensing capabilities compared to regular EOT. Findings: The results indicate that when the analyte is applied to the non-patterned side of the metasurface, the anomalous EOT resonance achieves superior sensing performance. This enhanced sensitivity can be attributed to the unique interaction dynamics between the incident THz waves and the meta-surface structure under abnormal conditions. Unique Contribution to Theory, Policy and Practice: This phenomenon is beneficial in the terahertz (THz) range, making HA meta-surfaces ideal for thin-film label-free sensing applications. The EOT resonance occurs due to the interaction between incident light and the periodic structure of the holes, leading to enhanced light transmission at specific wavelengths. The findings highlight the potential of using anomalous EOT resonances in HA meta-surfaces to develop highly sensitive and efficient sensors for detecting changes in thin films, paving the way for advanced sensing technologies in various scientific and industrial applications.

Experimental Analysis of Terahertz Wave Scattering Characteristics of Simulated Lunar Regolith Surface

This study investigates terahertz (THz) wave scattering from a simulated lunar regolith surface, with a focus on the Brewster feature, backscattering, and bistatic scattering within the 325 to 500 GHz range. We employed a generalized power-law spectrum to characterize surface roughness and fabricated Gaussian correlated surfaces from Durable Resin V2 using 3D printing technology. The complex dielectric permittivity of these materials was determined through THz time-domain spectroscopy (THz-TDS). Our experimental setup comprised a vector network analyzer (VNA) equipped with dual waveguide frequency extenders for the WR-2.2 band, transmitter and receiver modules, polarizing components, and a scattering chamber. We systematically analyzed the effects of root-mean-square (RMS) height, correlation length, dielectric constant, frequency, polarization, and observation angle on THz scattering. The findings highlight the significant impact of surface roughness on the Brewster angle shift, backscattering, and bistatic scattering. These insights are crucial for refining theoretical models and developing algorithms to retrieve physical parameters for lunar and other celestial explorations.