Resonant Terahertz Research Articles

Reconfigurable multiport switch in coupled mode devices

We consider a coupled mode system where the effective propagation constants of localized modes are amenable to modulation. Starting from an unmodulated system where power transfer is heavily suppressed, we demonstrate that small, periodic modulation of the propagation constants enhance power transfer using a slowly varying envelope approximation for the field mode amplitudes. We calculate an approximate modulation frequency enabling complete transfer between otherwise negligibly coupled elements. The ability to control these modulations by electrical or thermal effects allows for reconfigurable multiport switching. We use an array of coupled silica waveguides and the thermo-optic effect to test our predictions in the telecom C-band. However, this requires a refractive index modulation with period of the order 10−3 m and yields total power transfer with a propagation distance of the order of 10−1 m, which might make it unattractive for integrated photonic applications. Nevertheless, our results are valid for devices described by an equivalent coupled mode matrix for space or time propagation; for example, arrays of microring or terahertz resonators, microwave cavities, radio frequency antennas, or RLC circuits.

Dielectric tetrahedrons as terahertz resonators switched from perfect absorber to reflector

Tetrahedrons are basic building blocks in natural and artificial materials, while the terahertz response of micro tetrahedrons has been little explored. Here we fabricate subwavelength ceramic tetrahedrons for use in the terahertz frequency range, and find that the three-dimensional geometry significantly affects their terahertz properties. The transmission spectra are independent of the orientation of the tetrahedrons, while the first magnetic resonance disappears in the reflection spectra when an upright tetrahedron is flipped upside down on the metallic substrate, which changes it from a perfect absorber to a perfect reflector. This is attributed to the destructive interference between two magnetic dipoles induced respectively by the incident and the reflected wave. The study brings new insights in the materials design with 3D building blocks to realize more interesting and exotic terahertz properties.

THz power optimization and analysis of plasmonic unbiased photoconductive THz emitters coupled to a spiral-like dipole antenna

We propose a new spiral-like dipole antenna-coupled and unbiased terahertz (THz) photomixer with plasmonic unsymmetrical interdigitated electrodes (UIEs). Each electrode pair is made of subwavelength metal–semiconductor–metal (MSM) structures constituting heterogeneous Schottky contacts. From the optical view and analysis of the electrical transport in the semiconductor, by applying a systematic procedure to achieve optimum design, it is possible to benefit from advantages of having high THz photocurrent derived from the plasmonic enhanced optical absorption and, hence, photogeneration as well as severe built-in Schottky field in the photoconductor. Furthermore, from the electromagnetic view, by introducing an output resonant THz antenna having great impedance matching with the photomixer device, the efficiency of free-space coupling has significantly been enhanced. By exploiting all aspects of design, it has been provided to achieve the THz power of 409 µW from the spiral-like dipole antenna coupled to the proposed plasmonic UIEs with 10 × 10 µ m 2 active area and pitch of Λ = 395 n m . This is more than 1 order of magnitude larger THz power than the highest THz power radiated from the same area of a conventional dipole antenna-coupled unbiased array of the near-field emitters. Also, this improvement is more than 3 orders of magnitude higher THz power as compared with the power radiated from a non-plasmonic far-field operating antenna-less THz chip of the same size.

CMOS fully integrated terahertz thermal detector using metamaterial absorber and proportional-to-absolute temperature sensor

Terahertz (THz) detectors have drawn much attention and have been widely applied in imaging, spectroscopy, and sensing fields. An uncooled monolithic resonant THz thermal detector implemented in a standard 55-nm CMOS process is presented. The integration of a frequency selective metamaterial (MM) absorber coupled with an optimized proportional-to-absolute temperature (PTAT) sensor leads to an approach toward uncooled, compact, low-cost, easy-integration, and mass-production THz detectors. The theoretical analysis, design considerations, and characteristic measurements are demonstrated in detail. The proposed thermal detector is designed to resonate at 2.58 THz for the accessible THz source and the natural atmospheric window. The MM absorber achieves near-perfect absorptivity of 98.6%, and the optimized PTAT sensor obtains a high-temperature sensitivity of 10.11 mV / ° C. The calculated responsivity is 49.81 V / W at 2.58 THz with a calculated noise-equivalent power (NEP) of 0.78 μW / Hz0.5 at a modulation frequency of 3 Hz. Relatively better experimental results are obtained at 2.58 THz with a maximum responsivity of 48.3 V / W and a minimum NEP of 1.06 μW / Hz0.5.

Room-Temperature Amplification of Terahertz Radiation by Grating-Gate Graphene Structures

We report on experimental studies of terahertz (THz) radiation transmission through grating-gate graphene-channel transistor nanostructures and demonstrate room temperature THz radiation amplification stimulated by current-driven plasmon excitations. Specifically, with increase of the direct current (dc) under periodic charge density modulation, we observe a strong red shift of the resonant THz plasmon absorption, its complete bleaching, followed by the amplification and blue shift of the resonant plasmon frequency. Our results are, to the best of our knowledge, the first experimental observation of energy transfer from dc current to plasmons leading to THz amplification. We present a simple model allowing for the phenomenological description of the observed amplification phenomena. This model shows that in the presence of dc current the radiation-induced correction to dissipation is sensitive to the phase shift between THz oscillations of carrier density and drift velocity, and with increase of the current becomes negative, leading to amplification. The experimental results of this work as all obtained at room temperature, pave the way towards the new 2D plasmons based, voltage tuneable THz radiation amplifiers.

Terahertz faraday rotation of magneto-optical films enhanced by helical metasurface

The Faraday rotation effect of both the La: yttrium iron garnet (YIG) film and the YIG metasurface were experimentally and numerically investigated in the terahertz (THz) region. A THz magneto-optic polarization measurement system was used to observe the transmission, resonance, and magneto-optical effect of the La:YIG film and YIG metasurface. The THz artificial chirality and resonance localization of the helical metasurface generate the superchiral THz field, which enhances the THz magneto-optical effect of the YIG film. The results show that the Faraday effect of the YIG metasurface is about three times that of the pure YIG film, whose differential rotation angle increases from 8° to over 24°. This work achieves more sensitively active polarization control of THz waves, which is of great significance for THz polarization conversion, sensing, and non-reciprocal transmission.

Symmetry-broken silicon disk array as an efficient terahertz switch working with ultra-low optical pump power**Project supported by the National Key R&D Program of China (Grant No. 2017YFA0701005) and the National Natural Science Foundation of China (Grant Nos. 11974221, 91750201, 61927813, and 61775229). Z. Han also acknowledges the support from the Taishan Scholar Program of Shandong Province, China (Grant No. tsqn201909079)

The advancement of terahertz technology in recent years and its applications in various fields lead to an urgent need for functional terahertz components, among which a terahertz switch is one example of the most importance because it provides an effective interface between terahertz signals and information in another physical quantity. To date many types of terahertz switches have been investigated mainly in the form of metamaterials made from metallic structures and optically-active medium. However, these reported terahertz switches usually suffer from an inferior performance, e.g., requiring a high pump laser power density due to a low quality factor of the metallic metamaterial resonances. In this paper, we report and numerically investigate a symmetry-broken silicon disk based terahertz resonator array which exhibits one resonance with ultrahigh quality factor for normal incidence of the terahertz radiations. This resonance, which can never be excited for regular circular Si disks, can help to realize a superior terahertz switch with which only an ultra-low optical pump power density is required to modify the free carrier concentration in Si and its refractive index in the terahertz band. Our findings demonstrate that to realize a high terahertz transmittance change from 0 to above 50%, the required optical pump power density is more than 3 orders of magnitude smaller than that required for a split-ring resonator (SRR) based terahertz switch reported in the literature.

Terahertz polarization conversion and sensing with double-layer chiral metasurface**Project supported by the National Key Research and Development Program of China (Grant No. 2017YFA0701000), the National Natural Science Foundation of China (Grant Nos. 61971242, 61831012, and 61671491), the Natural Science Foundation of Tianjin City, China (Grant No. 19JCYBJC16600), and the Young Elite Scientists Sponsorship Program by Tianjin, China (Grant No. TJSQNTJ-2017-12).

The terahertz (THz) resonance, chirality, and polarization conversion properties of a double-layer chiral metasurface have been experimentally investigated by THz time domain spectroscopy system and polarization detection method. The special symmetric geometry of each unit cell with its adjacent cells makes a strong chiral electromagnetic response in this metasurface, which leads to a strong polarization conversion effect. Moreover, compared with the traditional THz transmission resonance sensing for film thickness, the polarization sensing characterized by polarization elliptical angle (PEA) and polarization rotation angle (PRA) shows a better Q factor and figure of merit (FoM). The results show that the Q factors of the PEA and PRA reach 43.8 and 49.1 when the interval film is 20 μm, while the Q factor of THz resonance sensing is only 10.6. And these PEA and PRA can play a complementary role to obtain a double-parameter sensing method with a higher FoM, over 4 times than that of resonance sensing. This chiral metasurface and its polarization sensing method provide new ideas for the development of high-efficiency THz polarization manipulation, and open a window to the high sensitive sensing by using THz polarization spectroscopy.

Energy transfer within the hydrogen bonding network of water following resonant terahertz excitation.

Energy dissipation in water is very fast and more efficient than in many other liquids. This behavior is commonly attributed to the intermolecular interactions associated with hydrogen bonding. Here, we investigate the dynamic energy flow in the hydrogen bond network of liquid water by a pump-probe experiment. We resonantly excite intermolecular degrees of freedom with ultrashort single-cycle terahertz pulses and monitor its Raman response. By using ultrathin sample cell windows, a background-free bipolar signal whose tail relaxes monoexponentially is obtained. The relaxation is attributed to the molecular translational motions, using complementary experiments, force field, and ab initio molecular dynamics simulations. They reveal an initial coupling of the terahertz electric field to the molecular rotational degrees of freedom whose energy is rapidly transferred, within the excitation pulse duration, to the restricted translational motion of neighboring molecules. This rapid energy transfer may be rationalized by the strong anharmonicity of the intermolecular interactions.

Grating Metamaterials Based on CdTe/CdMgTe Quantum Wells as Terahertz Detectors for High Magnetic Field Applications

The cyclotron and magnetoplasmon resonances were studied at 2 K in grating metamaterials fabricated on wafers with one or two modulation doped CdTe/CdMgTe quantum wells. The gratings (with the period varied between 2 μ m and 8 μ m) were prepared with an electron beam lithography either by etching or by evaporation of Au. The gratings were studied with an atomic force microscope which revealed a correlation between the depth and width of etched grooves at a constant time of etching. The sharpest resonances observed are due to excitation of magnetoplasmon in the case of Au gratings on a wafer with one quantum well. Etched samples with two quantum wells showed the strongest tuneability of magnetoplasmon resonances with the period of the grating and illumination with white light. We showed that the samples studied can be used as resonant or quasi-resonant terahertz detectors tuneable with magnetic field and white light.

Imaging in Chronic Wound Diagnostics.

Significance: Chronic wounds affect millions of patients worldwide, placing a huge burden on health care resources. Although significant progress has been made in the development of wound treatments, very few advances have been made in wound diagnosis. Recent Advances: Standard imaging methods like computed tomography, single-photon emission computed tomography, magnetic resonance imaging, terahertz imaging, and ultrasound imaging have been widely employed in wound diagnostics. A number of noninvasive optical imaging modalities like optical coherence tomography, near-infrared spectroscopy, laser Doppler imaging, spatial frequency domain imaging, digital camera imaging, and thermal and fluorescence imaging have emerged over the years. Critical Issues: While standard diagnostic wound imaging modalities provide valuable information, they cannot account for dynamic changes in the wound environment. In addition, they lack the capability to predict the healing outcome. Thus, there remains a pressing need for more efficient methods that can not only indicate the current state of the wound but also help determine whether the wound is on track to heal normally. Future Directions: Many imaging probes have been fabricated and shown to provide real-time assessment of tissue microenvironment and inflammatory responses in vivo. These probes have been demonstrated to noninvasively detect various changes in the wound environment, which include tissue pH, reactive oxygen species, fibrin deposition, matrix metalloproteinase production, and macrophage accumulation. This review summarizes the creation of these probes and their potential implications in wound monitoring.

Noninvasive Near‐Field Spectroscopy of Single Subwavelength Complementary Resonators

AbstractSubwavelength metallic resonators provide a route to achieving strong light–matter coupling by means of tight confinement of resonant electromagnetic fields. Investigation of such resonators however often presents experimental difficulties, particularly at terahertz (THz) frequencies. A single subwavelength resonator interacts weakly with THz beams, making it difficult to probe it using far‐field methods, whereas arrays of resonators exhibit inter‐resonator coupling, which affect the resonator spectral signature and field confinement. Here, traditional far‐field THz spectroscopy is systematically compared with aperture‐type THz near‐field microscopy for investigating complementary THz resonators. While the far‐field method proves impractical for measuring single resonators, the near‐field technique gives high signal‐to‐noise spectral information, only achievable in the far‐field with resonator arrays. At the same time, the near‐field technique allows to analyze single resonators without significant interaction with the near‐field probe. Furthermore, the near‐field technique allows highly confined fields and surface waves to be mapped in space and time. This information gives invaluable insight into spectral response in resonator arrays. This near‐field microscopy and spectroscopy method enables investigations of strong light–matter coupling at THz frequencies in the single‐resonator regime.

Graphene assisted terahertz metamaterials for sensitive bio-sensing

We report that single-stranded deoxyribonucleic acids (ssDNAs) at very low concentrations can be detected using graphene-combined nano-slot-based terahertz (THz) resonance. A combination of the resonant structure and tuned electro-optical properties of graphene can provide unprecedentedly sensitive biomolecule sensing even using very low energy THz photons, overcoming the huge scale difference of 10,000:1 between the wavelength and the size of the ssDNAs. Ultrahigh sensitivity is obtained by the significant increase in the absorption cross-section of the graphene sheet with the targeted biomolecules, induced by strong THz field enhancement at the resonance frequency inside the slots. Clearly distinguishable THz optical signals were observed between different species of ssDNAs even at the nano-mole level and analyzed quantitatively in terms of the electro-optical properties of the suspended graphene layer modified by the attached ssDNAs without any molecular-specific labeling for the THz regime. Quantitative analysis of ssDNA molecule adsorption was carried based on the change in conductivity using a theoretical THz transmission model.

Tunable multi-resonance of terahertz metamaterial using split-disk resonators

We present three tunable multi-resonance of terahertz (THz) metamaterials. They are composed of single-, dual-, and triple-split-disk resonators (SDRs) on Si substrates, which are denoted as SDR-1, SDR-2, and SDR-3, respectively. They exhibit extraordinary electromagnetic characteristics. SDR-1 exhibits polarization-dependence owing to the asymmetrical SDR structure. To increase the flexibility and applicability of SDR configuration, SDR-2 and SDR-3 are presented to modify the distances between the SDR layers. By moving the top SDR layer of SDR-2, a controllable resonance with a 0.32 THz shifting and tunable free spectrum range (FSR) of 0.15 THz at transverse magnetic mode is achieved, while an electromagnetically induced transparency-like effect appears at the transverse electric mode. The spectral bandwidth of SDR-3 can be tuned to 0.10 THz, and the resonant intensity becomes controllable by moving the middle SDR layer of SDR-3. Furthermore, by moving the top SDR layer of SDR-3, the tuning ranges of resonance, FSR, and bandwidth of SDR-3 are 0.23 THz, 0.20 THz, and 0.08 THz, respectively. Such designs of SDR configurations provide a high-efficient THz resonator in the THz-wave applications such as filters, switches, polarizers, sensors, imaging, and so on.

The Simulation of a Surface Plasmon Resonance Metallic Grating for Maximizing THz Sensitivity in Refractive Index Sensor Application

Nowadays, the simplicity of both designing and fabrication process of a terahertz (THz) resonator-based sensing technique leads to its ongoing development. The consumable THz resonator needs to be easily integrated into an existing terahertz time domain spectroscopy (THz TDS) measurement system. It should also be able to be fabricated in a mass scale with a low production cost. In this work, a metal-coated surface plasmon resonance- (SPR-) based sensor is simulated and designed as a low-cost refractive index sensor utilizing rigorous coupled wave analysis (RCWA). To demonstrate our methodology, we design a gold-coated grating with a polydimethylsiloxane (PDMS) as a substrate, in order to perform quantitative analysis of gasoline-toluene mixture composition, which has a refraction index variation of 0.1 at THz frequency. The grating period is tuned such that its surface plasmon resonance (SPR) frequency matches with the peak frequency of the THz TDS system. Moreover, other grating parameters, i.e., a filling factor and a grating depth, are optimized to increase the sensor sensitivity and sharpen the resonance dip. High sensitivity up to 500 GHz/RIU with a refractive index resolution up to 0.01 is numerically revealed. The H-field of the designed grating is then evaluated to indicate a strong SPR excitation. The well-developed designed grating introduces a promising, low-cost, and easily fabricated THz refractive index sensor.

Uncooled CMOS Integrated Triple-Band Terahertz Thermal Detector Comprising of Metamaterial Absorber and PTAT Sensor

This paper presents an uncooled monolithic resonant triple-band terahertz (THz) thermal detector using a 55 nm complementary metal oxide semiconductor (CMOS) process. The novel integration of a compact triple-band metamaterial (MM) absorber coupled with a sensitive proportional to absolute temperature (PTAT) sensor leads to a compact, uncooled, cost-effective, easy-integration, and mass-production CMOS integrated triple-band THz thermal detector. The principles of operation and physical modeling such as selective absorption of electromagnetic energy, thermal energy conversion, and electrical response are demonstrated. The triple-band MM absorber achieves high absorption magnitudes of 90.4%, 96.8%, and 98.8% at 0.91, 2.58, and 4.3 THz respectively. The optimized PTAT sensor shows a high temperature coefficient of voltage of 10.11 mV/°C. Simulated responsivities are 37.28 V/W at 0.91 THz, 49.9 V/W at 2.58 THz, and 61.11 V/W at 4.3 THz. The proposed triple-band THz thermal detector shows calculated noise equivalence powers (NEPs) of 1.07, 1.6, and 1.31 μW/Hz 0.5 at 0.91, 2.58, and 4.3 THz respectively. Relatively better experimental results are obtained at 0.91 THz and 2.58 THz due to the scarcity of THz sources. The proposed detector achieves maximum responsivities of 33.4 V/W and 47.9 V/W, while minimum NEPs of 1.49 μW/Hz 0.5 at 0.91 THz with a modulation frequency of 2 Hz, and 1.88 μW/Hz 0.5 at 2.58 THz with a modulation frequency of 0.5 Hz.

Resonant THz transmission through asymmetric aperture array with polarization controlled resonant peaks and Q-factors

In this article, we conduct a study on terahertz (THz) transmission through a periodic array of asymmetric apertures to demonstrate polarization controlled dual transmission windows. Specifically, we show that resonance peaks and Q-factors of these two transmitted bands are controllable via switching the incoming THz light from one polarization state to the orthogonal state. We investigate the origin and dispersion of two transmission peaks using a thorough study of numerical simulation. In our study, we show that depending on the polarization state, Q-factor can be changed from 3 to 100. Such a large variation of resonance quality can be utilized for a variety of applications like high resolution sensing and data communication.

Role of Plasmon Modes on the Optical Reflectivity of Graphene-Metallic Structures: A Theoretical Approach

In recent years, the tunable plasmon modes in the terahertz region of a multilayer graphene structure interacting with a metallic film substrate have attracted significant interest motivated by the graphene´s unique optical and electronic properties and the possibility to enhance light-matter interaction. In this work, the plasmon waves in graphene layered systems on a conducting thin film are investigated, the hybrid graphene-metal metamaterialis surrounded by two semi-infinite materials with different dielectric constants ε1andε2, respectively. The dispersion relations of electronic collective excitations are calculated by the zeros of an effective dielectric constant obtained from a recursive relation for the amplitudes associated with the electric field between graphene layers in the metamaterial. Long-range Coulomb interactions based on the hybrid layered graphene-metal structure lead new set spectra of collective excitations. At long wavelength (q®0) the optical modes (w~q1/2)depend on the two-dimensional carrier density, the metallic thickness, the metallic substrate plasmon frequency, the number of the graphene layers and the dielectric constants in which the hybrid graphene-metal structure is embedded. This latter plays an important role in a wide range of applications such as a surface plasmon resonance biological sensors and terahertz surface plasmons in optically pumped graphene metamaterials.

Efficient Control of THz Transmission of PEDOT:PSS with Resonant Nano-Metamaterials

Nano-metamaterials designed to operate at a certain resonance frequency enhance the magnitude of terahertz (THz) wave transmission by three orders of magnitude or even more. In this pursuit, controlling magnitude of resonant transmission and tuning the resonance frequency is increasingly important for application in low power THz electronics and devices. THz optical properties of chemically doped poly(3,4‐ethylenedioxythiophene):poly(4‐styrenesulfonate) (PEDOT:PSS) have been studied, however its effect on the THz transmission properties in combination with nano-metamaterials have not yet been demonstrated. Here we demonstrate the efficient control over resonant THz transmission and tuning of resonance frequency of different nano-metamaterials using PEDOT:PSS, without any toxic chemical doping. By ease of simple solution processing with single step and drop-casting 10 μL aqueous solution of PEDOT:PSS on different nano-metamaterials with varied concentrations, we were able to dynamically control the THz transmission along with resonance frequency. This dynamic control of transmission and shift in resonance frequency can be attributed to improved conductivity of PEDOT:PSS and its interaction with strongly localized THz field of the metamaterial.

Reconfigurable double C-shape metamaterial (DCM) for a terahertz resonator

We present a reconfigurable terahertz (THz) resonator using double C-shape metamaterials (DCM) that can be used as filter and single-/dual-resonance switch. By changing the position of the C-shape metamaterial along x-axis and y-axis directions, the resonances can be modulated from single-resonance to dual-resonance in TE mode and the corresponding free spectrum range (FSR) can be changed from 0.19 THz to 0.09 THz. These results indicate the proposed DCM can be used as a single-/dual-resonance switch and polarization switch. To increase the tunability, flexibility, and applicability of DCM, the resonant frequency could be tuned by changing the gap between DCM with the dual-layer. The resonances are blue-shifted 0.04 THz from 0.22 THz to 0.26 THz (1st resonance) and 0.11 THz from 0.36 THz to 0.47 THz (2nd resonance) in TE mode. The relationship of resonance and gap variation is quite stable and linear. This design of DCM provides a potential possibility of feasible opto-electronics applications in the THz frequency range.