Large Propagation Length Research Articles

Graphene based waveguide fed hybrid plasmonic terahertz patch antenna

Abstract The terahertz (THz) technology has fascinated lot of attention due to its enormous potential for a wide range of applications in the public, private and enterprise domains. This research work demonstrates the design structure and analysis of graphene based waveguide fed hybrid plasmonic THz patch antenna (HPTPA) constructed at around 3 THz. Hybrid plasmonic THz waveguide (HPTW) as a feeding line for the proposed THz patch antenna can increases antenna efficiency. The graphene is sandwiched between gallium arsenide (GaAs) and silver (Ag) to confine THz waves efficiently. Using mode analysis in finite element method for the proposed HPTW the propagation length, effective refractive index has been thoroughly examined. Based on the finite element method (FEM) approach, the results of the designed graphene-based waveguide fed HPTPA shows a high effective refractive index of 2.9, large propagation length of 230 µm, gain of 2.29 dBi, bandwidth of 200 GHz and efficiency of 85 % has obtained. The proposed graphene-based waveguide fed HPTPA could be beneficial to enable several photonic integrated circuit applications in next-generation wireless communication.

Numerical Analysis of a GaAs-Based Hybrid Plasmonic Waveguide with Nanoscale Optical Confinement and Low Losses

A numerical analysis of a hybrid plasmonic waveguide (HPW) for deep subwavelength optical confinement and long-range propagation with low loss is presented here. Two types of material platforms, namely, Si/SiO2/Au and GaAs/SiO2/Ag, were analyzed to optimize the HP waveguide. The mode character, an important and crucial design parameter for HP waveguides, was calculated based on the coupled mode theory, providing the coupling strength between the SPP and optical mode. As for the Si/SiO2/Au HP waveguide, the coupling strength varied from 0.47 to 0.60 with a mode area ranging from 0.0002/μm2 to 0.001/μm2 and mode character near to the SPP mode character (i.e., |a+(tsi, w, td)|= 0.47). While for the GaAs/SiO2/Ag HP waveguide, the coupling strength varied from 0.57 to 0.69 with a mode area ranging from 0.0002/μm2 to 0.0004/μm2 and mode character to SPP approached as |a+(tGaAs, w, td)|= 0.49. Finally, a finite element method (FEM) model was used to investigate the modal properties. The simulation analysis shows that at td = 10 nm the GaAs/SiO2/Ag waveguide gives 50% larger propagation length (205 μm), ten times smaller mode area (0.0002) with 60% lower modal propagation loss (0.021 db/μm), and 20% stronger coupling strength (0.62) with HPP mode character as 0.48 as compared to Si/SiO2/Au.

Phonon Polaritons Assisted Extraordinary Transmission in α-MoO-Silver Grating and Its Application in Switching

Sub-wavelength metal grating structures exhibit extraordinary transmission (EOT) due to constructive interference of surface plasmon polaritons (SPPs) tunneling. Phonon polaritons (PhPs) having several advantages over metal-supported plasmons, such as zero ohmic loss and larger propagation lengths, can further enhance the transmission. We propose the design of silver gratings filled with anisotropic Molybdenum Trioxide (α - MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) to exhibit EOT by utilizing PhPs. We also numerically demonstrate the tunability (amplitude) of EOT by introducing a layer of graphene nanoribbon in the proposed device and show its application as a mid-infrared switch with bandwidth of 650 nm. α-MoO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> /silver grating produces normalized transmission as high as ∼5.5 for [100] and ∼6 for [001] crystalline directions. This extraordinary effect is attributed to the formation of (i) hybrid modes, specifically cavity modes coupled with SPPs outside the reststrahlen band (RB) and (ii) cavity modes coupled with PhPs inside the RB. The reported work can be employed in diverse applications including molecular sensing, space communications, remote controlling and, imaging.

On the sensing performance enhancement in SPR-based Biosensor using specific two-dimensional materials (Borophene and Antimonene)

Antimonene-borophene based surface plasmon resonance (SPR) biosensor for sensitive detection is proposed, simulated, and analyzed. To the best of our knowledge, the antimonene-borophene combination has been studied in the plasmonic sensors for the first time. The proposed sensor utilizes the anisotropy property of black phosphorus (BP)/borophene between graphene/antimonene and gold layer along with the high adsorption efficiency/delocalized orbitals of graphene/antimonene for sensitivity improvement. The thickness of gold layer has been optimized for the rotation angle of the proposed biosensor around the z -axis (in-plane). The sensitivity of proposed sensor structure is as high as 206.26°/RIU. Further, extremely fine detection limit (4.84 × 10 −6 RIU), large propagation length (3220 nm) and large penetration depth (76.73 nm) are achieved for the proposed sensor. The obtained values of above parameters are considerably better than the reported sensors in the literature. • Au-borophene-antimonene based sensor chip exhibits high sensitivity with a detection limit of 4.84×10 -6 RIU. • Au-borophene-antimonene biosensor has higher penetration depth of 76.73nm. • The combination of Au-borophene-antimonene is superior to ultrasensitive detection of biomolecules with finer limit of detection.

Millimeter-Wave-to-Terahertz Superconducting Plasmonic Waveguides for Integrated Nanophotonics at Cryogenic Temperatures.

Plasmonics, as a rapidly growing research field, provides new pathways to guide and modulate highly confined light in the microwave-to-optical range of frequencies. We demonstrated a plasmonic slot waveguide, at the nanometer scale, based on the high-transition-temperature (Tc) superconductor Bi2Sr2CaCu2O8+δ (BSCCO), to facilitate the manifestation of chip-scale millimeter wave (mm-wave)-to-terahertz (THz) integrated circuitry operating at cryogenic temperatures. We investigated the effect of geometrical parameters on the modal characteristics of the BSCCO plasmonic slot waveguide between 100 and 800 GHz. In addition, we investigated the thermal sensing of the modal characteristics of the nanoscale superconducting slot waveguide and showed that, at a lower frequency, the fundamental mode of the waveguide had a larger propagation length, a lower effective refractive index, and a strongly localized modal energy. Moreover, we found that our device offered a larger SPP propagation length and higher field confinement than the gold plasmonic waveguides at broad temperature ranges below BSCCO’s Tc. The proposed device can provide a new route toward realizing cryogenic low-loss photonic integrated circuitry at the nanoscale.

Spin-wave localization and guiding by magnon band structure engineering in yttrium iron garnet

In spintronics, the propagation of spin-wave excitations in magnetically ordered materials can also be used to transport and process information. One of the most popular materials in this regard is the ferrimagnetic insulator yttrium iron garnet due its exceptionally small spin-wave damping parameter. While the small relaxation rate allows for large propagation length of magnetic excitations, it also leads to nonlocality of the magnetic properties. By imaging spin waves, their band structure is mapped with high-frequency resolution using a magneto-optic super-Nyquist sampling technique. In doing so, wave-vector selection is shown to suppress dispersion effects to a large extent, allowing for local measurements of spin relaxation. Moreover, we demonstrate even higher control of magnon propagation by employing the wave-vector selectivity near an avoided crossing of different spin-wave modes where the group velocity approaches zero. Here the local engineering of the dispersion allows us to construct magnonic waveguides, and at the same time it reveals the local relaxation properties.

Propagation length enhancement in a magneto optic plasmonic Mach–Zehnder isolator using graphene

In this study, a magneto optic plasmonic Mach–Zehnder isolator is designed and simulated in an Insulator–Graphene–Insulator configuration including one layer of graphene. The proposed waveguide takes advantages of propagating graphene’s surface plasmon polaritons (SPPs) at the interface of the graphene and dielectrics. Under special circumstances, in which the imaginary part of the graphene’s conductivity is negative, it behaves as a metal. Therefore, graphene is applied as the conductor layer. In addition, due to the magneto optic effect, nonreciprocal phase shift occurs in the magneto optic branch of the Mach–Zehnder structure. Also, two Yttrium Iron Garnet layers are deployed to create magneto-optic effect. In order to simulate our proposed device, finite difference time domain and mode solutions approaches are utilized. In this regard, propagating constant is extracted through the effective index method. Our proposed device provides considerably larger propagation length in comparison with the conventional plasmonic isolators, in which noble metals such as gold have been developed for SPPs propagation.

Feasibility of surface plasmon lasing in HgTe quantum wells with population inversion

Surface plasmon lasing in semiconductor gain media at far-infrared frequencies requires simultaneously long non-radiative recombination times and large plasmon propagation length. In this paper, we show that these conditions are realized in mercury-telluride quantum wells (HgTe QWs) near the topological transition. We derive the conditions of surface plasmon amplification in HgTe QWs with interband population inversion. To this end, we calculate the spatially-dispersive high-frequency conductivity of pumped HgTe QWs taking into account their realistic band structure, and compare the interband gain with Drude absorption and collisionless Landau damping. An extra necessary condition of plasmon lasing is revealed, namely, the non-equilibrium carrier density should be high enough to make the plasmon spectrum overlap with the frequency domain of interband excitations. The latter condition limits the processes of both stimulated and spontaneous plasmon emission at low temperatures, and should have a strong impact on the recombination kinetics of HgTe QWs at low temperatures.

Mode analysis of a confocal elliptical dielectric nanowire coated with double-layer graphene

Mode properties of a confocal elliptical dielectric nanowire coated with double-layer graphene are analyzed by using the separation of variables method (SVM). The graphene surface plasmon modes could be tuned by changing the working wavelength, the distance between semi-major axes, the Fermi energy level of graphene and the geometry parameters. The results show that the semi-analytical results of SVM are completely consistent with the numerical simulation results of finite element method (FEM). Compared with the elliptical dielectric nanowire coated with single-layer graphene, the novel waveguide structure has stronger mode confinement, lower propagation loss, larger propagation length, wider adjustable working wavelength and wider adjustable Fermi energy level of graphene. The waveguide may be exploited for many applications such as plasmon resonances, detectors and microstrip antennas.

Open-Type SPP Waveguide with Ultrahigh Bandwidth up to 3.5 THz

Due to losses in metals, the propagation length of the surface plasmon-polariton (SPP) waves on metal surfaces is small. This severely limits development of numerous applications of the SPP optics: in the near-infrared spectral region propagation length of SPP waves is no longer than 200 μm as for plane SPP waves and for all types of SPP waveguides. In this work, we show that the focusing of SPPs allows for the first time realizing open-type waveguide for SPP waves characterized by long distance of SPP effective propagation length up to 1 mm at a wavelength of 780 nm. We show that focused SPP waves in such a waveguide can be effectively excited by a 16 fs laser as well as be amplitude modulated within a bandwidth about 3.5 THz. The fast dynamics of the focused SPP waves is limited by the SPP group velocity dispersion. The large effective propagation length of the SPPs and its ultra-high bandwidth open up new possibilities for using focused SPPs in different areas of plasmonics and photonics.

Backward Phase Matching for Second Harmonic Generation in Negative-Index Conformal Surface Plasmonic Metamaterials.

Backward phase matching, which describes counterpropagating fundamental and harmonic waves in a negative‐index medium, is one of the most intriguing phenomena in nonlinear metamaterials. Predicted theoretically decades ago, however, it is still a challenging task to be applied for efficient second harmonic (SH) generation in a nonlinear metamaterial with ultrathin geometry and ultralow loss. Here, a negative‐index spoof plasmonic metamaterial is reported, which is composed of an ultrathin symmetrical corrugated metallic strips loaded with nonlinear active devices. The simulated and measured power spectra and surface near‐field distributions show that a peak SH signal can be generated at the backward phase‐matched frequency point in a 120° curved surface with high efficiency, thanks to the ultrathin flexible geometry, significant confinement effect, and large propagation length of the spoof surface plasmons. The results open new technological challenges from nano‐ and micro‐nonlinear photonics to science and engineering of compact, broadband, and efficient frequency‐mixing metamaterials and electromagnetic devices.

TM01mode in a cylindrical hybrid plasmonic waveguide with large propagation length

This study reports on a cylindrical hybrid plasmonic waveguide (CHPW) consisting of a high-index dielectric core, a sandwiched low-index dielectric layer, and a metal cladding. The CHPW supports the TM01 mode with a radially polarized transverse component of the electric field. Optical fields can be significantly enhanced in the sandwiched low-index dielectric region with nanoscale thickness down to 5nm, and tight mode confinement with the same order of the normalized mode area compared with that of the plasmonic waveguide is achieved. Moreover, the mode propagation loss is well compensated by adjusting dimensions of the waveguide to obtain a longer propagation distance. The calculated figure of merit reaches a value several times greater than that in the similarly reported structure. The results indicate that this novel type of hybrid structure can support the limited propagation of a radially polarized mode with good confinement and low loss, consummate the whole system of manipulating the cylindrical vector beams, and show great potential of applications for various integrated nano-photonic devices.

Subwavelength dielectric nanorod chains for energy transfer in the visible range.

We report a new type of energy transfer device, formed by a dielectric nanorod array embedded in a silver slab. Such dielectric chain structures allow surface plasmon wave guiding with large propagation length and highly suppressed crosstalk between adjacent transmission channels. The simulation results show that our proposed design can be used to enhance the energy transfer along the waveguide-like dielectric nanorod chains via coupled plasmons, where the energy spreading is effectively suppressed, and superior imaging properties in terms of resolution and energy transfer distance can be achieved.

Tunable coupling of a hybrid plasmonic waveguide consisting of two identical dielectric cylinders and a silver film☆☆Project supported by the National Natural Science Foundation of China (Grant No. 11647021) and the Fundamental Research Funds for the Central Universities of China (Grant No. ZY1531).

The propagation length of surface plasmon polaritons (SPPs) is intrinsically limited by the metallic ohmic loss that is enhanced by the strongly confined electromagnetic field. In this paper, we propose a new class of hybrid plasmonic waveguides (HPWs) that can support long-range SPP propagation while keeping subwavelength optical field confinement. It is shown that the coupling between the waveguides can be well tuned by simply varying the structural parameters. Compared with conventional HPWs, a larger propagation length as well as a better optical field confinement can be simultaneously realized. The proposed structure with better optical performance can be useful for future photonic device design and optical integration research.

Simulation of optical propagation based on wave optics for phase retrieval in shadowgraph of ultrasonic field

A shadowgraph method has been proposed for fast and noninterference measurement of ultrasonic pressure field. However, special care has been needed in choosing an appropriate optical propagation length to obtain a satisfactory signal-to-noise ratio of the measurement while avoiding error from a geometrical optics approximation. In this study, we propose a new numerical method replacing the geometrical optics approximation to retrieve the optical phase for the measurement. Optical intensity distribution obtained from the numerical simulation of optical propagation based on the Huygens–Fresnel principle agreed well with the measurement at a relatively large optical propagation length at which the geometrical optics approximation failed. Optical phase retrieval from the simulated optical intensity distribution by the proposed method was then tested. The range of optical propagation for successful phase retrieval was extended a few times by the proposed method compared with the geometrical optics approximation.

Ultra-Low-Loss High-Contrast Gratings Based Spoof Surface Plasmonic Waveguide

Large metallic losses and short propagation lengths associated with surface plasmons (SPs) have long been considered as the obstacles which severely limit the practical applications of surface plasmonic waveguides. In this paper, we introduce the concept of dielectric spoof SPs (SSPs) and show that subwavelength high-contrast gratings (HCGs) offer a route to effectively suppress the losses and hence dramatically increase the propagation length of surface electromagnetic waves. We experimentally realized a wideband ultra-low-loss high-confinement plasmonic waveguide constructed by a high refractive-index dielectric array with deep-subwavelength periodicity on a metal substrate. Simulation and measurement results on the near-field distributions and S-parameters at microwave frequencies provide explicit evidences of strong field localization and show excellent transmission efficiency of HCGs-based SSPs across a broad frequency band. More importantly, the propagation length of the HCGs-based SSPs is proved to be at least more than one order of magnitude larger than that of metallic gratings-based SSPs at the same or even higher level of field confinement. Thus, the SSPs as experimentally realized in this paper hold great promise for numerous practical applications in ultra-low-loss and long-range transmission SP devices and circuits and may open up new vistas in SP optics.

Topological mode switching in a graphene doublet with exceptional points

We investigate the switching between two surface plasmon polariton modes in a non-Hermitian system composed of a pair of graphene sheets using topological operations. The topological operation is implemented by suitably designing the waveguide geometry and the chemical potential of graphene, which is equivalent to changing the system parameters along a closed loop in the parameter space. Efficient and robust mode switching takes place as the loop encloses an exceptional point and the wave experiences a sufficiently large propagation length. Moreover, we show this mode switching is chiral, in the sense that the output modes are different for choosing different loop directions. The study provides a promising approach to robust mode switching on a deep-subwavelength scale.

Hybrid Airy plasmons with dynamically steerable trajectories.

With their intriguing diffraction-free, self-accelerating, and self-healing properties, Airy plasmons show promise for use in the trapping, transporting, and sorting of micro-objects, imaging, and chip scale signal processing. However, high dissipative loss and lack of dynamical steerability restrict the implementation of Airy plasmons in these applications. Here we reveal hybrid Airy plasmons for the first time by taking a hybrid graphene-based plasmonic waveguide in the terahertz (THz) domain as an example. Due to coupling between optical modes and plasmonic modes, the hybrid Airy plasmons can have large propagation lengths and effective transverse deflections, where the transverse waveguide confinements are governed by the hybrid modes with moderate quality factors. Meanwhile, the propagation trajectories of the hybrid Airy plasmons are dynamically steerable by changing the chemical potential of graphene. These hybrid Airy plasmons may promote the further discovery of non-diffracting beams along with the emerging developments of optical tweezers and tractor beams.

A THz semiconductor hybrid plasmonic waveguide with fabrication-error tolerance

In this letter, a novel waveguide based on semiconductor THz hybrid surface plasmon polariton (STHSPP) is proposed and numerically analyzed. The structure under study can confine light in the ultradeep-subwavelength region (ranging from λ2/360 to λ2/156) with a large propagation length ranging from 374 to 506 µm. Compared with a conventional hybrid SPP (HSPP) waveguide without a ridge, our proposed structure with the same propagation length has a much higher mode confinement with a one order of magnitude smaller normalized mode area.

The Tunability of Surface Plasmon Polaritons in Graphene Waveguide Structures

The tunability of propagation properties of surface plasmon polariton (SPP) modes in a waveguide formed by two parallel graphene layers separated by a dielectric layer is studied. For this purpose, the dispersion equation of the structure is numerically solved and the effects of applied bias voltage, the role of effective structural parameters, and electron–phonon scattering rate on the propagation of symmetric and antisymmetric SPP waves are investigated. The results of calculations show that considering the electron–phonon scattering rate as a function of Fermi energy and temperature leads to a considerable decrease in the propagation length of SPPs. As the main result of this work, tuning the propagation characteristics of SPPs is possible by varying any of the parameters such as applied voltage, thickness of insulating layer between two graphene layers and permittivities of dielectric layers, and finally the temperature. It is found that antisymmetric mode benefits from a larger propagation length in comparison with that of the symmetric mode.