Resonant Terahertz Research Articles

Plasmons in AlGaN/GaN grating-gate structure probing with 300 K background illumination

We show that terahertz plasmons in AlGaN/GaN grating-gate structures efficiently modulate the reflection of room temperature thermal radiation, leading to spectra that are in agreement with the measurements of plasmon absorption using high-power external sources. For typical samples of a few square millimeters in size, the reflected radiation intensity is relatively weak, and measurements need the use of gate voltage plasmon modulation and lock-in detection techniques. We show that unintentional use of lock-in techniques may lead to artifacts and demonstrate what kind of special precautions need to be taken into account. We show that drain voltage modulation also leads to modulation of the reflected thermal radiation by plasmons. Our results are of key importance for the research on new resonant plasmon-based terahertz sources because of the always present superposition of electrically excited terahertz emission and background radiation reflected from the structures.

Sensitive detection and evaluation of ultrafine dust particles with a resonant terahertz metasurface [Invited

We demonstrate efficient and highly sensitive detection methods for ultrafine dust and introduce a controllable evaluation way. Using the nanogaps of terahertz resonant metasurfaces where the terahertz field is greatly enhanced by the squeezed mode volume, the ultrafine dust particles were efficiently detected. The measured signal changes of the resonance can be modified in their spectral shape by the deposited particle concentrations with their effectively changed optical properties. Various resonant metasurfaces were compared and evaluated in terms of their geometrical design, relative gap size to the particle size, and particle concentration. Positioning ultrafine particles into the small nano gaps via the Polydimethylsiloxane film sweeping technique results in further significant changes in measured terahertz optical signal. The proposed method for ultrafine dust detection by photonic metasurface is promising as it guides advanced stages of ultrasensitive terahertz molecule sensors even at the real-world environmentally hazardous particulates both in qualitative and quantitative manners.

Enhanced crystallinity, optical conductivity and terahertz cutoff frequency of stacked layers of FeSe2 by Al nanosheets

Herein a 250 nm thick two stacked layers of FeSe2 (abbreviated as FF) thin films and FF stacks comprising aluminum nanosheets of thicknesses of 50 nm (FAF) are studied. Amorphous FeSe2 thin films are deposited using thermal evaporation technique under a vacuum pressure of 10−5 mbar. It is observed that insertion of Al nanosheets induced the evolution of orthorhombic phase of FeSe2. nanowire and elongated rectangular grains are also formed. Optically Al nanosheets enhanced the light absorption by 4.1 times and redshifted the indirect/direct energy band gaps from 2.80/2.49 eV to 2.42/1.85 eV, respectively. It also improved the optical conductivity and terahertz cutoff frequency as well. In addition the optical conductivity parameters of FF and FAF terahertz resonators are determined by the Drude-Lorentz approach. It is observed that in the infrared range of light Al nanosheets improved the drift mobility of FF from 2.06 cm2/Vs to 12.55 cm2/Vs. The hole density is reduced and the scattering time constant at femtosecond level is increased. FF and FAF terahertz resonators performed as optical filters suitable for terahertz technology applications with terahertz cutoff frequency varying in the range of 17.2–300 THz.

High Q and sub-wavelength THz electric field confinement in ultrastrongly coupled THz resonators

The control of light–matter coupling at the single electron level is currently a subject of growing interest for the development of novel quantum devices and for studies and applications of quantum electrodynamics. In the terahertz (THz) spectral range, this raises the particular and difficult challenge of building electromagnetic resonators that can conciliate low mode volume and high quality factor. Here, we report on hybrid THz cavities based on ultrastrong coupling between a Tamm cavity and an LC circuit metamaterial and show that they can combine high quality factors of up to Q=37 with a deep-subwavelength mode volume of V=3.2×10−4λ3. Our theoretical and experimental analysis of the coupled mode properties reveals that, in general, the ultrastrong coupling between a metamaterial and a Fabry–Perot cavity is an effective tool to almost completely suppress radiative losses and, thus, ultimately limit the total losses to the losses in the metallic layer. These Tamm cavity-LC metamaterial coupled resonators open a route toward the development of single photon THz emitters and detectors and to the exploration of ultrastrong THz light–matter coupling with a high degree of coherence in the few to single electron limit.

Probing the Néel-Type Antiferromagnetic Order and Coherent Magnon-Exciton Coupling in Van Der Waals VPS3.

2D van der Waals (vdW) antiferromagnets have received intensive attention due to their terahertz resonance, multilevel magnetic-order states, and ultrafast spin dynamics. However, accurately identifying their magnetic configuration still remains a challenge owing to the lack of net magnetization and insensitivity to external fields. In this work, the Néel-type antiferromagnetic (AFM) order in 2D antiferromagnet VPS3 with the out-of-plane anisotropy, which is demonstrated by the temperature-dependent spin-phonon coupling and second-harmonic generation (SHG), is experimentally probed. This long-range AFM order even persists at the ultrathin limit. Furthermore, strong interlayer exciton-magnon coupling (EMC) upon the Néel-type AFM order is detected based on the monolayer WSe2 /VPS3 heterostructure, which induces an enhanced excitonic state and further certifies the Néel-type AFM order of VPS3 . The discovery provides optical routes as the novel platform to study 2D antiferromagnets and promotes their potential applications in magneto-optics and opto-spintronic devices.

Detection of Benzoic Acid Additive Based on a Terahertz Metasurface Sensor

Benzoic acid and its derivative benzoate are widely utilized as food preservatives. This paper presents a highly sensitive method for detecting benzoic acid using a cross-shaped aluminum resonator integrated on top of a polyimide substrate. The resonance dip for benzoic acid was designed to be at 0.93 THz. The terahertz (THz) transmission spectrum was simulated using a designed structure with varying thicknesses of benzoic acid. Analysis of the spectral response of the THz resonance dips to the thickness of the benzoic acid layer shows that the amplitude of the resonance dips generally increases with an increase in the benzoic acid concentration. The spectral frequency shifts and transmission intensities both demonstrate high sensitivity of the metasurface sensor to trace benzoic acid thicknesses. The sensor exhibits a sensitivity of approximately 0.247 THz/RIU (Refractive Index Unit) and FOM (figure of merit) of 3.927/RIU. The THz metasurface sensor is a fast and accurate sensing tool that can detect trace substances, making it crucial in the field of food safety.

High-Q transmission characteristics in terahertz guided-mode magnetic resonance system

Introduction: Guided mode resonance is generated by coupling wave diffractions with the waveguided mode. The guided mode resonances provide narrow-linewidth and resonance intensity for high quality factor (Q-factor) optical resonators.Methods: we demonstrate the high-Q guided mode resonances propagating on a low-loss, terahertz guided-mode magnetic resonance system, which are periodic square lattices of U-shaped split ring resonators (SRRs) on quartz substrates.Results: By choosing a judicious array period, two distinct frequency guided mode resonances and a magnetic dipole resonance with high Q-factor are observed. The interaction of the two resonances at similar frequencies produces a total transmission peak.Discussion: The dependences of the magnetic dipole resonance on the lattice period and structural parameters are investigated and discussed. The frequency difference between these two guided mode resonances widens with increasing Lattice period. The sharp spectral feature of each resonance results in the abrupt degradation of the spectral edge transmission. The proposed scheme is promising for efficient THz sensing, THz switching, and slow-light devices.

In-plane hyperbolic polariton tuners in terahertz and long-wave infrared regimes

One of the main bottlenecks in the development of terahertz (THz) and long-wave infrared (LWIR) technologies is the limited intrinsic response of traditional materials. Hyperbolic phonon polaritons (HPhPs) of van der Waals semiconductors couple strongly with THz and LWIR radiation. However, the mismatch of photon − polariton momentum makes far-field excitation of HPhPs challenging. Here, we propose an In-Plane Hyperbolic Polariton Tuner that is based on patterning van der Waals semiconductors, here α-MoO3, into ribbon arrays. We demonstrate that such tuners respond directly to far-field excitation and give rise to LWIR and THz resonances with high quality factors up to 300, which are strongly dependent on in-plane hyperbolic polariton of the patterned α-MoO3. We further show that with this tuner, intensity regulation of reflected and transmitted electromagnetic waves, as well as their wavelength and polarization selection can be achieved. Our results can help the development of THz and LWIR miniaturized devices.

Dual resonance based on quasi-bound states in continuum in the all-dielectric terahertz metasurface and its application in sensing

In recent years, the concept of bound states in continuum (BIC) in metasurfaces has become a topic of interest. BIC involves the physical mechanics of wave optics, metamaterial sensors and other high-quality factor (Q-factor) applications. In this paper, the basic concept of BIC is introduced, followed by a brief description of the progression of the research regarding the application of BIC in terahertz (THz) metasurfaces. Subsequently, an all-dielectric metasurface consisting of tetramer clusters with dual terahertz resonances is proposed. The materials used for the substrate and the tetramer cluster are quartz and silicon, respectively. Quasi-BIC works by varying the cylinder radius on the metasurface structure. The transmission spectra are analyzed under different structural and material parameters with control variables. When the metasurface is applied as a refractive-index sensor, the Q-factor is 2303.77 and the figure of merit (FOM) is 7022.90, which are calculated using the asymmetric parameter k = 0.8 μm. Additionally, when the incident wave is x- or y-polarized, resonance peaks with the same frequencies can be obtained, and the overall trend of the transmission spectrum remains the same.

Hot-electron resonant terahertz bolometric detection in the graphene/black-AsP field-effect transistors with a floating gate

We evaluate the terahertz (THz) detectors based on field-effect transistor (FET) with the graphene channel (GC) and a floating metal gate (MG) separated from the GC by a black-phosphorus (b-P) or black-arsenic (b-As) barrier layer. The operation of these GC-FETs is associated with the heating of the two-dimensional electron gas in the GC by impinging THz radiation leading to thermionic emission of the hot electrons from the GC to the MG. This results in the variation of the floating gate potential, which affects the source–drain current. At the THz radiation frequencies close to the plasmonic resonance frequencies in the gated GC, the variation of the source–drain current and, hence, the detector responsivity can be resonantly large.

On‐chip terahertz whispering gallery mode resonator with high‐Q based on multislot waveguide

AbstractA whispering gallery mode resonator (WGMR) with a high‐quality factor and large coupling spectrum width can enhance the interaction of matter and lightwave by several orders of magnitude in a large bandwidth range. The performance of the resonator determines the accuracy and range of terahertz (THz) sensing. However, the quality factors of the reported THz WGMRs are highly limited by the high absorption loss of materials within THz spectral range. The coupling spectrum width is usually limited by the coupling structure. In this paper, an on‐chip THz whispering gallery mode resonator based on multislot waveguide and gradual coupling structure is proposed. By increasing the evanescent field of guided wave mode, the absorption loss in the resonator is significantly reduced. Through gradual coupling structure, resonance curves are more uniform within a wide spectrum. The simulation results show that the structure can improve the instinct quality factor of the THz resonator to the order of when the bending radius is as small as 2mm. At the same time, sensitivity is increased by more than six times and uniform resonance curves are achieved from THz.

Excitation of third harmonic terahertz surface plasmon over a rippled graphene-free space interface

We examined the third harmonic (TH) terahertz (THz) surface plasmon waves (SPWs) generation over a graphene surface deposited on a rippled dielectric substrate (SiO2) by obliquely incidenting a p-polarized laser beam on a graphene surface with frequency ω0 and wave numbers k0x and k0z at an incident angle θ. Laser beam exerts ponderomotive force on free electrons of the graphene surface and produce nonlinear second harmonic (SH) oscillatory velocity v→2ω0 at frequency 2ω0. SH oscillatory velocity couples with perturbed ripple electron density to produce TH nonlinear current density J→3ω0nl at frequency 3ω0. Ripple wave number (q) provides an extra wave number to develop resonant TH THz SPWs at frequency 3ω0 and wave number kz=3k0z+q. The required ripple wave number for the excitation of THz TH SPWs is lower for the higher Fermi energy of the graphene sheet as compared to lower Fermi energy. Graphene sheet’s Fermi energy and angle of incidence laser beam significantly modify the amplitude of TH THz SPWs.

Epigenetic modification of gene expression in cancer cells by terahertz demethylation

Terahertz (THz) radiation can affect the degree of DNA methylation, the spectral characteristics of which exist in the terahertz region. DNA methylation is an epigenetic modification in which a methyl (CH3) group is attached to cytosine, a nucleobase in human DNA. Appropriately controlled DNA methylation leads to proper regulation of gene expression. However, abnormal gene expression that departs from controlled genetic transcription through aberrant DNA methylation may occur in cancer or other diseases. In this study, we demonstrate the modification of gene expression in cells by THz demethylation using resonant THz radiation. Using an enzyme-linked immunosorbent assay, we observed changes in the degree of global DNA methylation in the SK-MEL-3 melanoma cell line under irradiation with 1.6-THz radiation with limited spectral bandwidth. Resonant THz radiation demethylated living melanoma cells by 19%, with no significant occurrence of apurinic/apyrimidinic sites, and the demethylation ratio was linearly proportional to the power of THz radiation. THz demethylation downregulates FOS, JUN, and CXCL8 genes, which are involved in cancer and apoptosis pathways. Our results show that THz demethylation has the potential to be a gene expression modifier with promising applications in cancer treatment.

Laser‐Printed Terahertz Plasmonic Phase‐Change Metasurfaces

AbstractChalcogenide phase‐change materials (PCMs) have offered an appealing material solution by acting as a switchable dielectric layer to tune the electromagnetic properties of terahertz metamaterials and metasurfaces. Here, this work demonstrates large‐scale and lithography‐free manufacturing of all‐PCM terahertz metasurfaces based on direct laser switching of crystalline micro‐domains in a thin film with high switching ratio of the emerging plasmonic PCM, In3SbTe2 (IST). The fabricated high‐quality IST metasurfaces achieve efficient plasmonic resonances and a large modulation depth with ultrafast response (full width at half maxima of the modulation time ≈1.6 ps) in a deep‐subwavelength switching volume. For the dynamic evolution of terahertz resonance modes, theoretical modeling reveals a delicate interplay between amorphous and crystalline IST due to the bonding‐structure‐induced different carrier lifetimes and spatially localized electric fields. These studies open new avenues for realizing all‐PCM terahertz ultrafast nanophotonics.

Room-Temperature CMOS Monolithic Resonant Triple-Band Terahertz Thermal Detector

Multiband terahertz (THz) detectors show great application potential in imaging, spectroscopy, and sensing fields. Thermal detectors have become a promising choice because they could sense THz radiations on the whole spectrum. This paper demonstrates the operation principle, module designs with in-depth theoretical analysis, and experimental validation of a room-temperature CMOS monolithic resonant triple-band THz thermal detector. The detector, which consists of a compact triple-band octagonal ring antenna and a sensitive proportional to absolute temperature (PTAT) sensor, has virtues of room-temperature operation, low cost, easy integration, and mass production. Good experimental results are obtained at 0.91 THz, 2.58 THz, and 4.2 THz with maximum responsivities of 32.6 V/W, 43.2 V/W, and 40 V/W, respectively, as well as NEPs of 1.28 μW/Hz0.5, 2.19 μW/Hz0.5, and 2.37 μW/Hz0.5, respectively, providing great potential for multiband THz sensing and imaging systems.

GeTe2 Phase Change Material for Terahertz Devices with Reconfigurable Functionalities Using Optical Activation.

The phenomenon of phase change transition has been a fascinating research subject over decades due to a possibility of dynamically controlled materials properties, allowing the creation of optical devices with unique features. The present paper unravels the optical characteristics and terahertz (THz) dielectric permittivity of a novel phase change material (PCM), GeTe2, prepared by pulsed laser deposition (PLD) and their remarkable contrast in crystalline and amorphous states, in particular, a difference of 7 orders of magnitude in conductivity. The THz spectra were analyzed using the harmonic oscillator and Drude term. Using GeTe2 PLD films, we designed and prepared a THz metasurface in the form of periodic structure and revealed a possibility of tuning the THz resonance either by a thermal control or light-induced crystallization response, thus achieving the dynamic and tunable functionality of the metastructure. We propose controlling the state of metasurface by observing the intensity characteristics of the Raman peak of 155 cm-1. Density functional theory (DFT) modeling demonstrates that in the process of crystallization the mode intensity of 155 cm-1 assigned to Te-Te stretching in amorphous chain fragments decreases and disappears at full crystallization.

Dynamic Modulation of Terahertz Resonance Modes by Embedded Vanadium Dioxide Metasurfaces

Dynamic Modulation of Terahertz Resonance Modes by Embedded Vanadium Dioxide Metasurfaces

Subwavelength Terahertz Resonance Imaging (STRING) for Molecular Fingerprinting.

Subwavelength terahertz (THz) imaging methods are highly desirable for biochemical sensing as well as materials sciences, yet sensitive spectral fingerprinting is still challenging in the frequency domain due to weak light-matter interactions. Here, we demonstrate subwavelength THz resonance imaging (STRING) that overcomes this limitation to achieve ultrasensitive molecular fingerprinting. STRING combines individual ring-shaped coaxial single resonators with near-field spectroscopy, yielding considerable sensitivity gains from both local field enhancement and the near-field effect. As an initial demonstration, we obtained spectral fingerprints from isomers of α-lactose and maltose monohydrates, achieving sensitivity that was enhanced by up to 10 orders of magnitude compared to far-field THz measurements with pelletized samples. Our results show that the STRING platform could enable the development of THz spectroscopy as a practical and sensitive tool for the fingerprinting and spectral imaging of molecules and nanoparticles.

Interaction of obliquely incident lasers with anharmonic CNTs acting as dipole antenna to generate resonant THz radiation

The present paper explores the terahertz (THz) generation by the interaction of obliquely incident laser beams with the array of vertically aligned anharmonic carbon nanotubes (CNTs) acting as dipole antennas. In the scheme, anharmonicity arises due to the nonlinear variation of restoration force on the various electrons of CNTs and it plays a key role in the enhancement of THz generation. The anharmonic CNTs help in broadening the resonance peak, which paves the way for the enhancement of the normalized THz amplitude. The laser beams incident obliquely on the close-packed array of vertically aligned anharmonic CNTs grown over the glass substrate and set oscillations in the CNTs so that each CNT act as the oscillatory dipole to generate THz radiation. The THz electric field shows enhancement at surface plasmon resonance frequency , where is the characteristic parameter of CNTs, is the relative permittivity of lattice and is the plasma frequency. This scheme is quite suitable to generate THz radiations in the milliwatt range of optimized values of the laser and CNTs parameters. We also explore the impact of polarization, S-parameter, critical angle, and length-matching effects of CNT antennas on the THz generation.

Molecular orientation echoes via concerted terahertz and near-IR excitations.

A new and efficient method for orientation echo spectroscopy is presented and realized experimentally. The excitation scheme utilizes concerted rotational excitations by both ultrashort terahertz and near-IR pulses and its all-optical detection is enabled by the molecular orientation-induced second harmonic method [J. Phys. Chem. A126, 3732 (2022)10.1021/acs.jpca.2c03237]. This method provides practical means for orientation echo spectroscopy of gas phase molecules and highlights the intriguing underlying physics of coherent rotational dynamics induced by judiciously-orchestrated interactions with both resonant (terahertz) and nonresonant (NIR) fields.