Plasmonic Terahertz Research Articles

Terahertz field confinement and enhancement in various sub-wavelength structures

Terahertz plasmonics is one of the fascinating research fields that includes diverse applications in nonlinear terahertz optics and nano-optics, as well as studies about strong localization and enhancement of millimeter waves. Recent advanced lithography techniques enable one to reach the confinement scale of electromagnetic waves down to subnanometer, which corresponds to wavelength/10 000 000. In particular, the strong electric-field enhancement in metal gaps can produce atomically strong field exceeding ∼10 MV/cm in a picosecond time scale. In this article, we review the strong localization and enhancement of electromagnetic waves in terahertz frequency ranges achieved by subwavelength metal structures. Based on understanding the underlying capacitive coupling and antenna effects, we focus on the field confinement and enhancement of various metal structures such as slit, slot antenna, and dipole antenna. We also give a brief introduction about near-field detection methods.

Multipolar terahertz absorption spectroscopy ignited by graphene plasmons

Terahertz absorption spectroscopy plays a key role in physical, chemical and biological systems as a powerful tool to identify molecular species through their rotational spectrum fingerprint. Owing to the sub-nanometer scale of molecules, radiation-matter coupling is typically dominated by dipolar interaction. Here we show that multipolar rotational spectroscopy of molecules in proximity of localized graphene structures can be accessed through the extraordinary enhancement of their multipolar transitions provided by terahertz plasmons. In particular, specializing our calculations to homonuclear diatomic molecules, we demonstrate that a micron-sized graphene ring with a nano-hole at the core combines a strong near-field enhancement and an inherently pronounced field localization enabling the enhancement of the dipole-forbidden terahertz absorption cross-section of {{rm{H}}}_{2}^{+} by 8 orders of magnitude. Our results shed light on the strong potential offered by nano-structured graphene as a robust and electrically tunable platform for multipolar terahertz absorption spectroscopy at the nanoscale.

Terahertz plasmonics: The rise of toroidal metadevices towards immunobiosensings

This work reviews fundamentals and the recent state-of-art achievements in the field of plasmonic biosensing based terahertz (THz) spectroscopy. Being nonpoisonous and nondestructive to the human tissues, THz signals offer promising, cost-effective, and real-time biodevices for practical pharmacological applications such as enzyme reaction analysis. Rapid developments in the field of THz plasmonics biosensors and immunosensors have brought many methodologies to employ the resonant subwavelength structures operating based on the fundamental physics of multipoles and asymmetric lineshape resonances. In the ongoing hunt for new and advanced THz plasmonic biosensors, the toroidal metasensors have emerged as excellent alternates and are introduced to be a very promising technology for THz immunosensing applications. Here, we provide examples of recently proposed THz plasmonic metasensors for the detection of thin films, chemical and biological substances. This review allows to compare the performance of various biosensing tools based on THz plasmonic approach and to understand the strategic role of toroidal metasensors in highly accurate and sensitive biosensors instrumentation. The possibility of using THz plasmonic biosensors based on toroidal technology in modern medical and clinical practices has been briefly discussed.

Emission of terahertz plasmons from driven electrons in grated graphene.

Terahertz plasmon emission is the key to getting terahertz radiation, which has resulted in numerous studies on it. In this paper, we present the results of a theoretical investigation of terahertz plasmon emission by drifting electrons in a grated graphene system driven by an electric field by applying the Boltzmann's equilibrium equation method. The results show that plasmon frequencies from terahertz to infrared are generated by drifting electrons through the interaction between plasmons and electrons. Obvious increase of the plasmon emission strength with the driving electric field can be seen when the electric field is more than a certain strength (e.g. 1.0 kV/cm). The effects of electron density and the grating period on the emission strength of plasmons were also investigated. It was found that terahertz plasmons can be obtained by applying a grating with appropriate period. The plasmon frequencies can be tuned using either the driving electric field or the electron density controlled by the gate voltage or the grating parameters. This work may help to gain insight into graphene plasmonics and be pertinent to the application of graphene-based structures as electrically tunable terahertz plasmonic devices.

Dynamic Terahertz Plasmonics Enabled by Phase‐Change Materials

AbstractPhase‐change phenomena have been an attractive research theme for decades due to the dynamic transition of material properties providing extraordinary capabilities for versatile optical device applications. Even at the terahertz (THz) frequency regime, phase‐change materials (PCMs) promote the development of dynamic devices, especially when combined with a plasmonic approach delivering strong field enhancement and localization. According to the design of plasmonic metamaterials or hybrid composites, PCMs can actively modulate the electromagnetic properties of THz waves through thermal, electrical, and optical means. In turn, THz waves can affect the PCM properties in the nonlinear regime due to the intense field strength enhancement by plasmonic structures. Here, a few types of PCMs demonstrating promising potential in THz plasmonic applications are introduced. Starting from the best‐known transition metal oxide, vanadium dioxide (VO2), which possesses an insulator‐to‐metal phase transition near room temperature, superconductors, chalcogenides, ferroelectrics, liquid crystals, and liquid metals are covered along with their phase‐change properties and the control mechanisms infused with THz plasmonic applications. The corresponding recent progress presenting how PCMs combined with plasmonic structures can demonstrate effective THz modulation is reviewed. This general overview may provide a better understanding of dynamic THz plasmonics and new ideas for future THz technology.

A Theoretical Treatment of THz Resonances in Semiconductor GaAs p-n Junctions.

Semiconductor heterostructures are suitable for the design and fabrication of terahertz (THz) plasmonic devices, due to their matching carrier densities. The classical dispersion relations in the current literature are derived for metal plasmonic materials, such as gold and silver, for which a homogeneous dielectric function is valid. Penetration of the electric fields into semiconductors induces locally varying charge densities and a spatially varying dielectric function is expected. While such an occurrence renders tunable THz plasmonics a possibility, it is crucial to understand the conditions under which propagating resonant conditions for the carriers occur, upon incidence of an electromagnetic radiation. In this manuscript, we derive a dispersion relation for a p–n heterojunction and apply the methodology to a GaAs p–n junction, a material of interest for optoelectronic devices. Considering symmetrically doped p- and n-type regions with equal width, the effect of certain parameters (such as doping and voltage bias) on the dispersion curve of the p–n heterojunction were investigated. Keeping in sight the different effective masses and mobilities of the carriers, we were able to obtain the conditions that yield identical dielectric functions for the p- and n-regions. Our results indicated that the p–n GaAs system can sustain propagating resonances and can be used as a layered plasmonic waveguide. The conditions under which this is feasible fall in the frequency region between the transverse optical phonon resonance of GaAs and the traditional cut-off frequency of the diode waveguide. In addition, our results indicated when the excitation was slightly above the phonon resonance frequency, the plasmon propagation attained low-loss characteristics. We also showed that the existence or nonexistence of the depletion zone between the p- and n- interfaces allowed certain plasmon modes to propagate, while others decayed rapidly, pointing out the possibility for a design of selective filters.

Terahertz Plasmon Resonances in Two-Dimensional Electron Systems: Modeling Approaches

Known approaches to modeling terahertz plasmons in two-dimensional electron systems may differ significantly in their assumptions. There has, however, been little effort to analyze the differences between and application of different models to the same sets of structures. This paper discusses, develops, and compares several different theoretical approaches—namely, an effective-medium approximation, a transmission-line model, modal analysis, and full-wave simulations. In particular, we present a transmission-line model that takes into account the dielectric-air surrounding a two-dimensional system. Using modal analysis, we also solve analytically the problem of plasmon reflection and transmission when plasmons are incident on a junction between gated and ungated two-dimensional waveguides. Comparing the predictions made by the models for several structures, we find good agreement between full-wave simulations and both analytical and numerical modal analysis. The results of the effective-medium approximation and the transmission-line model also agree with each other, but differ quantitatively from those of the full-wave simulations and modal analysis. We attribute the differences to the phases of the plasmon reflection and transmission coefficients obtained with the different approaches. Our analytical expressions for the plasmon transmission and reflection coefficients represent a simple, yet accurate way to model plasmons in two-dimensional systems comprising both gated and ungated sections.

Structure Metallic Surface for Terahertz Plasmonics

Plasmonics is the field of study of the interaction between incident light and electrons in metals. It is used widely for developing nanophotonic devices. The structured metallic surface such as metamaterials can be used to produce spoof surface plasmons at any frequencies with the dimensions of unit cell less than the incident wavelength. Terahertz plasmonics is attracted to the field of research since it is used for sensing biological components even in a weak environment. The issue with planar metamaterials is a lower quality factor value. Several methods have been adopted for obtaining high Q-value in metamaterials. Among them, Fano- and Toroidal-based metamaterials offer high Q-factor and string localized field enhancement. This article discusses the importance and developments in the field of high-Q terahertz metamaterial for plasmonics applications. The nonlinear responses of terahertz metamaterial under high-intense THz pulses are also discussed.

Tight-Binding Terahertz Plasmons in Chemical-Vapor-Deposited Graphene

Transistor structures comprising graphene and sub-wavelength metal gratings hold a great promise for plasmon-enhanced terahertz detection. Despite considerable theoretical effort, little experimental evidence for terahertz plasmons in such structures was found so far. Here, we report an experimental study of plasmons in graphene-insulator-grating structures using Fourier transform spectroscopy in 5-10 THz range. The plasmon resonance is clearly visible above the Drude absorption background even in chemical vapor deposited (CVD) graphene with low carrier mobility $\sim 10^3$ cm$^2$/(V s). We argue that plasmon lifetime is weakly sensistive to scattering by grain boundaries and macoscopic defects which limits the mobility of CVD samples. Upon placing the grating in close proximity to graphene, the plasmon field becomes tightly bound below the metal stripes, while the resonant frequency is determined by the stripe width but not by grating period. Our results open the prospects of large-area commercially available graphene for resonant terahertz detectors.

Quasi-static Analysis Based on an Equivalent Circuit Model for a CMOS Terahertz Plasmon Detector in the Subthreshold Region.

An analytic method for a complementary metal-oxide-semiconductor (CMOS) terahertz plasmon detector operating in the subthreshold region is presented using the equivalent circuit model. With respect to design optimization of the detector, the signal transmission from the antenna port to the output of the detector is described by using the proposed circuit model, which does not include a complicated physical operating principle and mathematical expressions. Characteristics from the antenna port to the input gate node of the detector are analyzed through the superposition method by using the characteristic impedance of transmission lines. The superposition method shows that the effect of interconnection lines at the input is simplified with the optimum bias point. The characteristics of the plasmon detection are expressed by using small-signal analysis of the single transistor at the sub-threshold operation. The results of the small-signal analysis show that the unity gain preamplifier located between the detector core and the main amplifier can improve the detection performances such as the voltage responsivity and the noise equivalent power. The measurement results using the fabricated CMOS plasmon detector at 200 GHz suggest that the unity gain preamplifier improves the detector performances, which are the same results as we received from the proposed analytic method.

Continuously Frequency-Tuneable Plasmonic Structures for Terahertz Bio-sensing and Spectroscopy

Plasmon-based devices are powerful for use in highly sensitive evanescent-field detection and analysis, but they exhibit the problem of limited frequency tunability for fixed structures. This feature causes problems in the multi-frequency investigations required for materials characterization, bio-related research, etc. Here, we propose and fabricate a spiral-shaped plasmonic structure that enables a continuous frequency-tuneable evanescent-field concentration in the terahertz (THz) region with simple operation. The device also increases the electric field intensity at the subwavelength aperture, thus significantly amplifying the transmission. Highly tuneable transmission bands are observed by simply rotating the spiral plasmonic structure, which are in good agreement with the behaviour expected from electromagnetic simulation. Medical examinations are performed by measuring the interactions between the frequency-tuneable plasmons and bio-samples, which enables observing distinct tissue-dependent transmission spectra and images. The developed device simultaneously offers the advantages of both plasmonic devices and frequency-tuneable devices, which can increase the availability and versatility of evanescent-field THz sensing and analysis. The mechanism presented will shed light on THz plasmonics and motivate the implementation of a variety of applications based on plasmon-mediated THz technologies.

An Electrically Tunable Terahertz Plasmonic Device Based on Shape Memory Alloys and Liquid Metals

AbstractAdv. Optical Mater. 2018, 6, 1700684The above article, first published online on 4 January 2018 on Wiley Online Library (https://onlinelibrary.wiley.com) has been retracted in agreement with the Editor‐in‐Chief and the publisher, Wiley.The retraction has been agreed following an internal investigation by the Research Integrity Office at the University of Utah who concluded that Figure 1d was falsified by the corresponding author, Dr. Ajay Nahata. The University of Utah confirmed that the other co‐authors were unaware of the falsification of the figure, and sole responsibility for this lies with the corresponding author.ReferenceH. Zhou, T. Zhang, S. Guruswamy, A. Nahata, Adv. Optical Mater. 2018, 6, 1700684, https://doi.org/10.1002/adom.201700684.

Линейная теория усиления терагерцевых плазменных волн в двух параллельных слоях графена

Линейная теория усиления терагерцевых плазменных волн в двух параллельных слоях графена

Gated graphene island-enabled tunable charge transfer plasmon terahertz metamodulator.

Graphene-enhanced optoelectronic terahertz (THz) signal processing offers an exquisite potential for tailoring extreme-subwavelength platforms to develop tunable and highly-responsive photonic tools. In this study, we propose a hybrid graphene island-mediated THz metadevice to support tunable charge transfer plasmon (CTP) resonances. We show that bias variations in the gated graphene significantly change the metadevice transmittance at the CTP frequency, while the capacitive dipolar mode remains unchanged. Our numerical and experimental studies show that tuning the conductivity of the graphene islands between a cluster of metallic blocks provides an active and exotic control over the charge transition across the assembly. To experimentally prove the viability of our concept in a practical photonic application, we utilized the presented tunable system as a high modulation-depth THz modulator. This enabled us to facilitate a THz modulation speed of 19 μs and 21 μs for rising and falling durations, respectively, with a modulation depth of 72%.

P-Diamond as candidate for plasmonic terahertz and far infrared applications

High values of the hole mobility, low contact resistance, and high hole sheet densities in diamond two-dimensional hole gas make p-diamond field effect transistors superb candidates for implementing high temperature plasmonic sub-terahertz, terahertz, and far infrared devices. Our calculations show that p-diamond sub-THz transistors are viable contenders, especially for applications in the 200 to 600 GHz atmospheric window which are of special interest for the beyond 5 G sub-THz communications.

Plasmonic HEMT Terahertz Transmitter based on the Dyakonov-Shur Instability: Performance Analysis and Impact of Nonideal Boundaries

Terahertz-band (0.1---10 THz) wireless technology is envisioned as key to future communication networks. For the time being, the lack of compact THz transmitters, on-chip and at room temperature, has hampered the use of the THz band. In this paper the performance of an on-chip THz source based on the Dyakonov-Shur (DS) plasma instability is investigated analytically and with powerful numerics, revealing the importance of the termination impedances on the power of THz signals generated and radiated by a plasmonic transmitter. These results provide guidance for successful experimental realization of DS-based plasmonic THz transmitters.

Observation of terahertz plasmon and plasmon-polariton splitting in a grating-coupled AlGaN/GaN heterostructure.

Plasmon in two-dimensional electron gas (2DEG) has long been considered as a promising active medium for terahertz emitters and detectors. However, the efficiency of terahertz plasmonic devices is severely limited by the high damping rate of plasma wave in solid state. In addition to the enhancement of plasmon lifetime by using 2DEGs with higher carrier mobility, engineering on the boundary condition and electromagnetic environment of plasmon cavity helps to preserve the plasmon states. Here we report on terahertz reflection spectroscopy of plasmon states in a grating-coupled AlGaN/GaN-2DEG plasmonic device at 7 K in equilibrium with ambient blackbody irradiation. Localized plasmon states and plasmon-polariton states were observed when the core plasmonic device is integrated with a silicon lens and when it is embedded in a terahertz Fabry-Pérot cavity, respectively. Simulation results including the reflection spectra and total reflection power agree well with the measured results. The Rabi splitting is found to be inversely proportional to the resonance frequency, and follows a linear relation with the square root of the sheet electron density. A normalized coupling ratio, ΩRω0≈0.13, is achieved between the Rabi splitting ΩR and the resonance frequency ω0. The coupling ratio could be further increased to allow for ultrastrong coupling between terahertz photons and plasmons.

Electrical Tunability of Terahertz Amplification in a Periodic Plasmon Graphene Structure with Charge-Carrier Injection

The dependence of the plasmon terahertz resonant frequency in the generation mode on the quasi-Fermi energy in the active (ungated) region of graphene with an inverse charge carrier population and on the Fermi energy in the gated p- and n-type regions in a periodic p–i–n structure based on graphene with injection pumping is theoretically investigated. It is shown that electrically frequency-tunable nanoscale plasmon graphene amplifiers and generators operating in a broad terahertz frequency range at room temperature can be designed.

Four Terahertz Plasmon Modes of Hole Gas in Monolayer MoS2

We investigate on the terahertz (THz) plasmons of hole gas in monolayer MoS2 in the presence of spin-orbit interactions (SOIs) under the random phase approximation. The study shows that four THz plasmon modes can be achieved via intra- and inter-spin-subband transitions in p-doped monolayer MoS2. The two plasmons induced by intra-spin-subband transitions are acoustic-like, while the other two plasmon modes via inter-spin-subband transitions are optic-like. The frequencies of the four plasmons strongly depend on the hole density and the two plasmon modes induced by inter-spin-subband transitions are sensitive on the spin polarizability. Our theoretical results reveal that the plasmons in monolayer MoS2 can be applied to realize THz plasmonic devices, such as THz source emitters.

Gain of terahertz plasmons in a double-layer graphene

Amplification of terahertz plasmons in a pair of parallel active graphene monolayers is studied theoretically. It is shown that symmetric mode increment of plasmons in the double-layer graphene may be greater than that in a single graphene layer by approximately two times of magnitude.