Terahertz Surface Plasmon Polaritons Research Articles

A sensing method based on InSb grating coupled terahertz surface plasmon polariton resonance

A grating-coupled terahertz (THz) surface plasmon polariton (SPP) resonant biochemical sensing structure is designed with simulation, which can be easily prepared by etching a submillimeter grating on the surface of indium antimonide (InSb) substrate. The simulation results based on the phase matching equation show that when the TM-polarized broadband terahertz collimated beam is incident on the InSb grating at a 30° angle, the low-frequency SPP and high-frequency SPP with opposite propagation directions can be simultaneously excited by the –1st and +1st order diffraction beams of the grating, respectively. Since the low-frequency SPP is easy to accurately measure with a commercial THz time-domain spectroscopy devices, the dependence of the resonance characteristics and sensing characteristics of low-frequency SPP on the grating structure parameters is systematically simulated in this paper. The simulation results show that the refractive-index sensitivity of the InSb grating-coupled THz-SPP resonant sensor chip decreases with the increase of the grating period, and is 1.05 THz/RIU at a grating period of 120 μm and an incident angle of 30°. Under these conditions, the sensor chip cannot make a detectable response to the monolayer adsorption of biomolecules, because the evanescent field penetration depth of the low-frequency SPP is much greater than the biomolecular size, resulting in insufficient field-biomolecular interaction at the surface. In order to detect biomolecules, a sensitivity enhancement method based on porous thin films is proposed and analyzed with simulation. The porous films enable not only to enrich biomolecules, but also to extend the interaction between THz-SPP and biomolecules from the molecular size to the entire film thickness, thereby improving the sensitivity of the sensor to biomolecular adsorption. Taking tyrosine adsorption as an example, the simulation results show that when the InSb grating is covered with a porous polymethyl methacrylate (PMMA) film with a thickness of 120 μm and a porosity of 0.4, the sensor sensitivity to tyrosine adsorption is 0.39 THz/unit volume fraction.

Evaluation of the Efficiency of Generation of Terahertz Surface Plasmon Polaritons by the End-Fire Coupling Technique

One of the most important problems in the plasmonics of the terahertz (THz) range, which is actively developing now, is the efficient generation of surface plasmon polaritons (SPPs). The simplest and most promising technological technique of photon excitation of THz SPPs is through diffraction of radiation on the edge of the conducting surface of the sample (the end-fire coupling technique). In this paper, we experimentally evaluated the efficiency of the generation of monochromatic THz SPPs (λ0 = 141 μm) by this method with a sample in the form of a cylindrical segment, the convex surface of which has a gold layer coated by zinc sulfide (ZnS) with thickness d = 0–2 µm. Such configuration of the surface supporting the SPPs not only shields the detector from parasitic bulk waves arising during diffraction but also enables one to change the distribution of the SPP field in the air by varying the coating layer thickness d. On an uncoated gold surface, the SPP generation efficiency was η ≈ 20%. In the presence of a ZnS layer on the gold, the SPP generation efficiency gradually increased with d, reached the maximum (ηmax ≈ 60%) at d ≈ 1 μm, and then gradually decreased. Theoretical analysis showed that the efficiency of the SPP generation can be raised up to 80% due to the selection of an optimal SPP field profile via variation of the thickness of the dielectric layer on the metal surface, as well as with optimal incidence of the focused radiation on the edge of the sample.

Obtaining the Effective Dielectric Permittivity of a Conducting Surface in the Terahertz Range via the Characteristics of Surface Plasmon Polaritons

With the intensive development of data transmitting and processing devices in the terahertz (THz) frequency range, an important part of which are integrated plasmonic components and communication lines, it becomes necessary to measure correctly the optical constants of their conductive surfaces. In this paper, we describe a reliable method for determining the effective permittivity εm of a metal surface from the measured characteristics (refractive and absorption indices) of THz surface plasmon polaritons (SPPs). The novelty of the method is the conduction of measurements on a metal surface with a dielectric layer of subwavelength thickness, suppressing the radiative losses of SPPs, which are not taken into account by the SPP dispersion equation. The method is tested on a number of flat “gold sputtering–zinc sulfide layer–air” structures with the use of the THz radiation (λ0 = 141 μm) from the Novosibirsk free electron laser (NovoFEL). The SPP characteristics are determined from interferograms measured with a plasmon Michelson interferometer. It is found that the method allows a significant increase in the accuracy of the εm in comparison with measurements on the same metal surface without a dielectric layer.

Focusing enhancement of terahertz surface plasmon polaritons

Subwavelength-sized metallic structures exhibit extraordinary responses to electromagnetic waves due to their geometry, generating surface plasmon polaritons (SPPs) at the metal–dielectric interface. The development and application of terahertz (THz) science, which is an emerging science, can be advanced by studying SPPs in the THz band. Because the field strength of SPPs gradually weakens in the propagation direction, a coherent phase-matched THz SPP enhancement structure was designed. Here, we demonstrated an asymmetrical circular groove structure. And using this structure, THz SPPs can be excited and can be focused and enhanced under the illumination of linearly polarized THz pulses, generating a subwavelength-sized focal spot. These functions are achieved via phase matching. Furthermore, the original structure was modified by controlling the propagation phase of the THz SPPs and a periodic enhancement structure was designed. Compared to the original structure, the periodic enhancement structure has a stronger focusing effect on THz SPPs. Moreover, we imaged the THz SPPs by using a THz real-time near-field spectral imaging system and verified the feasibility of the designed structure by comparing with the simulation results. The study results are of great significance to the application of THz SPPs and the fabrication of related devices.

Coherent surface plasmon polariton amplification via free-electron pumping

Surface plasmonics with its unique confinement of light1,2 is expected to be a cornerstone for future compact radiation sources and integrated photonics devices. The energy transfer between light and matter is a defining aspect that underlies recent studies on optical surface-wave-mediated spontaneous emissions3-5. However, coherent stimulated emission of free electrons, which is essential for free-electron light sources, and its dynamical amplification process remain to be disclosed in a clear, unambiguous and calibrated manner. Here we present the coherent amplification of terahertz surface plasmon polaritons via free-electron-stimulated emission: a femtosecond optical pulse creates an in-phase free-electron pulse with an initial terahertz surface wave, and their ensuing interactions intensify the terahertz surface wave coherently. The underlying dynamics of the amplification, including a twofold redshift in the radiation frequency over a one-millimetre interaction length, are resolved as electromagnetic-field-profile evolutions using an optical pump-probe method. By extending the approach to a properly phase-matched electron bunch, our theoretical analysis predicts a super-radiant surface-wave growth, which lays the ground for a stimulated surface-wave light source and may facilitate capable means for matter manipulation, especially in the terahertz band.

Propagation of Terahertz Surface Plasmon Polaritons in a Dielectric Fiber with a Metal Wire Core

The influence of the conductivity and radius of metal wires embedded into the dielectric fiber on the velocity and attenuation length of terahertz surface plasmon polaritons has been theoretically investigated. It was shown that the phase velocities and attenuation lengths increase with increasing conductivity and radius of the wire. With increasing frequency, the velocity of surface plasmon waves increases and the propagation length decreases. The effect of the dielectric coating on the propagation velocity of surface electromagnetic waves is analyzed. It is shown that the coating leads to a decrease in the phase velocity and an increase in the propagation length of surface plasmon waves.

Enhanced confinement of infrared surface plasmon polaritons in borophene waveguides

In this paper, a sub-wavelength terahertz plasmonic waveguide based on a borophene–insulator–metal (BIM) structure is investigated by using the 2D finite-difference time-domain method. It is proved there is a plasmonic mode of high confinement and low propagation loss in an optimized wavelength range. A dynamically adjustable filter, formed by introducing a borophene nanoribbon into the BIM waveguide, is computationally and theoretically explored, which can achieve tailoring terahertz surface plasmon polaritons in deep-subwavelength scale. These results can further provide a broader path for dynamic regulation of light transmission in the borophene-based waveguide and the development of adjustable borophene-based optoelectronic filtering.

Terahertz surface plasmon polaritons travelling on laser-induced porous graphene

Surface plasmon polaritons (SPPs) have shown huge application potentials in photonic on-chip devices, sensing, imaging, and metamaterials. However, in the terahertz (THz) regime, metals used in the visible and near infrared frequencies cannot support SPPs due to their near-zero skin depth. At present, feasible methods mainly include exploiting spoof SPPs through complex structural design on metals or using semiconductors. In this work, we experimentally demonstrate that porous graphene induced by laser beams with low fabrication cost can support SPPs in the THz regime with good performance. Using a classical structure of the semicircular slit, a super-resolution focus with a size of ∼0.43λ is characterized by a THz-SPPs imaging system. Furthermore, by changing the fabrication parameters of the laser, the propagation loss of SPPs is found to be effectively controlled. This method for controllably excited THz-SPPs on laser-induced porous graphene is of great significance for the design and wide-range applications of more compact THz on-chip devices.

Enhanced sensing of terahertz surface plasmon polaritons in graphene/J-aggregate coupler using FDTD method

J-aggregates nanostructure proposed for realization of plasmonic manufactories like switchable localized surface resonances (LSR) due to their optical nonlinearities' properties. J-aggregates to enhancement of light-matter interaction is applied. In this regard, we propose a novel graphene/J-aggregate plasmonics coupler with subwavelength structure at telecommunication range. Utilizing device geometries and optical properties optimized for enhanced light-matter interactions lets us to harness the propagation of TM (TE) modes without the change of configuration. Here, the coupling parameters have been numerically and investigated using the finite-difference time-domain (FDTD) method. The results show that the coupling condition can be tuned by optical properties of graphene in turn leads to a tunable plasmonics coupler. It is shown that, by considering magnetic field of 0.01 T and graphene chemical potential of mc = 0.45 eV, the TM mode, and magnetic field of 0.02 T and graphene chemical potential of mc = 0.4 eV, the TE mode can be propagated in the selected ports. With the above behavior, the proposed structure may potentially be applied in telecommunication links.

Adiabatic following of terahertz surface plasmon-polaritons coupler via two waveguides structure

Most recently, two remarkable papers (Huang et al., 2019, 2020) proposed broadband completely transferred terahertz (THz) surface plasmon-polaritons (SPPs) waveguide coupler based on coherent quantum control — Stimulated Raman adiabatic passage (STIRAP). However, these THz SPPs waveguide couplers based on the three-level quantum control technology require three SPPs waveguides, which are relatively difficult to manufacture. Hence, this paper proposes a novel broadband completely transmitted THz waveguide SPPs coupler adopting two-state adiabatic following technology. The novel coupler requires only two waveguides, which greatly reduces the manufacturing difficulty of the THz SPPs waveguide coupler. It is the first time to introduce the detuning parameter into the coupled equation of the SPPs waveguide. We believe that this discovery will further broaden the research domain of THz communication.

Terahertz surface plasmon polariton resonances and microparticle sensing in bulk Dirac semimetal with spatially perturbed geometries

Bulk Dirac semimetals (BDSs) are a three-dimensional counterpart of graphene and exhibit rich plasmonic response at terahertz (THz) frequencies. In this work, we investigate THz surface plasmon polariton (SPP) resonances in BDS systems with spatially perturbed geometries using Kretschmann–Raether and Otto configurations. A THz SPP conversion efficiency of around 91% is witnessed in a straight BDS slab with the Otto configuration. The sustainment of THz SPP resonances for microbending and macrobending BDS geometries reveals the confinement capabilities of the BDS system as a potential sensing element. For example, microparticle sensing is demonstrated numerically based on THz SPP resonance in a parabolically perturbed BDS system. A reflection amplitude change of more than 75% is observed for microplastic-like spherical particles stuck on the surface of a parabolic BDS in comparison with an empty parabolic BDS configuration. We anticipate that the demonstrated THz SPP resonances and sensing in BDS systems may enable plasmonic devices for environmental monitoring and biological sensing.

Broadband terahertz surface plasmon-polaritons beam splitter

In this paper, we propose a novel design of broadband terahertz (THz) surface plasmon-polaritons (SPPs) beam splitter, which obtains 50%-50% energy splitting on input and output SPPs waveguides. The conventional THz SPPs beam splitter is a component of the two parallel THz SPPs waveguides coupler, which can only operate at the single frequency. To make the device more universal, we employ the quantum engineering technique, called fractional stimulated Raman adiabatic passage (f-STIRAP) to design the 50%-50% beam splitter with good performance on broadband.

Terahertz surface plasmons propagation in semiconducting parallel plates waveguide configuration

In this study we theoretically and numerically demonstrated a 3-D plasmonic terahertz waveguide made up of two parallel plates of semiconductor sandwiching a thin region of vacuum/air. The semiconductor can reveal conductive properties at the THz regime, making it an appropriate replacement for structured metallic surfaces in plasmonic devices. The proposed waveguide configuration not only offers an extra degree of freedom to tailor the propagation of THz wave but also promises more confinement compared to the planar waveguide configuration. The dispersion relations of the terahertz surface plasmon polaritons as they propagate in the proposed configuration is calculated using the Drude model. It is observed that the dispersion behavior of the modes is altered by the carrier densities of the plates and the separation between them. We also numerically analyze the terahertz surface waves propagation in the proposed geometry and examine terahertz surface modes behavior in conjunction with the theory. For the tapered configuration, we observed that the transmission amplitude increases as the tapering angle increases for the same input width. The present study could be significant in the construction of terahertz active and passive devices that may utilize semiconductors instead of metals.

Study on THz SPPs excitation and field distributions of graphene microribbons

Surface plasmon polaritons have wide applications in sensing, biasing, absorption, and beam splitting, etc. because they can break diffraction limits and have strong field localization. So far, most of the research on terahertz surface plasmon polaritons is in the far-field spectrum, and exploring near-field properties is further needed. In this paper, based on the graphene strip structure, the excitation and field distribution characteristics of terahertz surface plasmon polaritons were studied. Specifically, the graphene microribbons that could excite surface plasmon polaritons by terahertz waves were designed, and the field distribution was calculated. Then, we experimentally fabricated the designed structure and measured its surface plasmon polaritons by terahertz near-field microscope. Finally, the effects of resonance detuning on surface plasmon polaritons field distribution were investigated. Our results provide a research basis for the application of graphene surface plasmon polaritons components in terahertz detection, biomedicine, safety detection, and high data rate communication.

Design of Compact Terahertz Surface Plasmon Polaritons Devices

A slot-antenna-based device which can stimulate terahertz surface plasmon polaritons (SPPs) and control the wave front of the SPPs simultaneously has been designed and fabricated. The device is constituted of H-shaped-slot antennas. By adjusting the geometric parameters of antennas, the grouped structure can provide a linear phase gradient which will improve the conversion efficiency of the terahertz propagating wave into the surface wave. The phase of the converted surface wave can be adjusted by changing the order of the slot antennas in each group, such the wave front of the surface wave can be modulated simultaneously. A terahertz SPPs lens is designed and fabricated based on this approach. Experimental results demonstrate the validity of the proposed method. This approach can significantly simplify the design and fabrication of most integrated SPPs devices and may pave the avenue for developing complex SPPs devices.

Terahertz Spoof Surface Plasmonic Logic Gates.

SummaryLogic gates are important components in integrated photonic circuitry. Here, a series of logic gates to achieve fundamental logic operations based on linear interference in spoof surface plasmon polariton waveguides are demonstrated at terahertz frequencies. A metasurface-based plasmonic source is adopted to couple free-space terahertz radiation into surface waves, followed by a funnel-shaped metasurface to efficiently couple the surface waves to the waveguides built on a domino structure. A single Mach-Zehnder waveguide interferometer can work as logic gates for four logic functions: AND, NOT, OR, and XOR. By cascading two such interferometers, NAND and NOR operations can also be achieved. Experimental investigations are supported by numerical simulations, and good agreement is obtained. The logic gates have compact sizes and high intensity contrasts for the output “1” and “0” states. More complicated functions can be envisioned and will be of great value for future terahertz integrated computing.

Quantum Engineering Enables Broadband and Robust Terahertz Surface Plasmon-Polaritons Coupler

The in-plane terahertz (THz) surface plasmon polaritons (SPPs) coupler is a key element of THz information transmission and processing. However, existing parallel coupler based on two SPPs waveguides is not robust against perturbations of geometric parameters (arising due to fabrication imperfections) and are limited by the single frequency of operation. In this work, we propose a robust and broadband integrated THz coupler based on the in-plane SPPs waveguides, conducted with the quantum engineering – Stimulated Raman Adiabatic Passage (STIRAP). Our coupler consists of two asymmetric specific curved corrugated metallic structures working as the input and output SPPs waveguides, and one straight corrugated metallic structure functioning as the middle SPPs waveguide. From the theoretical and simulated results, we demonstrate that the SPPs can be efficiently transferred from the input to the output waveguides. Our device is robust against the perturbations of geometric parameters, and meanwhile it manifests broadband performance (from 0.5 THz to 0.8 THz) with a high transmission rate over 70 ${\%}$ . This work represents a step forward for significant improvements in THz sensing, THz imaging, on-chip THz signal processing, and other next-generation THz photonic device.

Graphene hybrid waveguide stimulation using a photoconductive terahertz generator.

Typically, terahertz (THz) surface plasmon polariton (SPP) excitation involves phase-matching engineering and THz plane-wave generation. This requires antennas, lenses, and other optical phase-matching devices. Herein, we demonstrate a novel, to the best of our knowledge, method to excite THz SPPs in graphene directly by using an 800 nm optical pump and a photoconductive source. We miniaturize the SPP excitation setup by eliminating the plane-wave generator and the need for mode matching between the plane wave and the waveguide, thereby improving the power efficiency of THz SPP excitation; an average SPP power of 0.6 mW is obtained for an optical pump power of 25 mW.

Over-the-Horizon Terahertz Surface Plasmon Locator

The article demonstrates the possibility of creating an over-the-horizon locator with an information carrier of surface plasmon polaritons (SPPs), a kind of surface electromagnetic waves, of the terahertz (THz) spectral range. This possibility is based on the macroscopic propagation length (decimeters) of THz SPPs, as well as on their ability to propagate along convex surfaces of metals. Two schemes of location measurements on gold convex samples using THz SPPs were developed and tested for SPPs generated by free-electron laser radiation with a wavelength of 130 μm. It was found that the intensity of SPPs reflected by an object placed on a conducting surface about 4 cm beyond the horizon line depended on the height of the object above the surface, in accordance with the distribution of the SPP field in the air (a few millimeters or fractions of a millimeter). The distance, orientation, coordinates, and height of an object placed beyond the horizon line can be determined using the SPP locator, which can be applied for localization of hard-to-reach objects on a convex surface.

In-plane interferometry of terahertz surface plasmon polaritons

The paper is devoted to the development of interferometric schemes for determining the complex refractive index κ = κ′+i·κ″ κ of surface plasmon-polaritons (SPPs) of the terahertz (THz) spectral range. As the value of κ depends on the dielectric constant εm of the metal surface guiding the SPPs it can be used for determining ϵm in the far infrared what other optical methods fail to do due to high reflectivity of metals. We discuss two types of THz SPP interferometers in which the interference pattern is formed as a result of the interaction of SPP beams themselves rather than bulk waves produced by these beams. The first type of interferometers are static devices that enable one to investigate fast processes on the metal surface, while the second type are dynamic ones that make it possible to realize Fourier spectroscopy of the metal surface and its transition layer at THz frequencies. Devices of the both types produce interferograms enabling one to determine the real and the imaginary parts of κ. The results of experiments on the interaction of THz SPPs with flat mirrors and beam – splitting plates, the key elements of the THz SPP interferometers, are presented.