Subwavelength Confinement Research Articles

Ultra-compact integrated graphene plasmonic photodetector with bandwidth above 110 GHz

Abstract Graphene-based photodetectors, taking advantage of the high carrier mobility and broadband absorption in graphene, have recently seen rapid development. However, their performance with respect to responsivity and bandwidth is still limited by the weak light-graphene interaction and large resistance-capacitance product. Here, we demonstrate a waveguide-coupled integrated graphene plasmonic photodetector on a silicon-on-insulator platform. Benefiting from plasmon-enhanced graphene-light interaction and subwavelength confinement of the optical energy, a small-footprint graphene-plasmonic photodetector is achieved working at the telecommunication window, with a large a bandwidth beyond 110 GHz and a high intrinsic responsivity of 360 mA/W. Attributed to the unique electronic band structure of graphene and its ultra-broadband absorption, operational wavelength range extending beyond mid-infrared, and possibly further, can be anticipated. Our results show that the combination of graphene with plasmonic devices has great potential to realize ultra-compact, high-speed optoelectronic devices for graphene-based optical interconnects.

Ultra-deep sub-wavelength mode confinement in nano-scale graphene resonator-coupled waveguides.

Graphene is capable of supporting very slow waves due to sustaining surface plasmon polaritons (SPPs) at THz frequencies, whereas the metal counterpart can support such modes only at optical frequencies. In this paper, a graphene-based resonator-coupled waveguide supporting transverse-magnetic-polarized SPP modes is rigorously studied, which is capable of providing ultra-deep sub-wavelength mode confinement at the working frequency of 40THz. First, graphene is described both electronically and electromagnetically, as in these regards, graphene's quantum capacitance plays an important role, which is calculated via its DC characteristic. Since we aim to excite extremely slow waves in graphene waveguides, namely, SPP modes, it is necessary to contemplate a non-local conductivity model to characterize graphene. Furthermore, SPP modes create strong fields at the vicinity of a graphene strip in addition to high mode confinement, accentuating the importance of including nonlinear phenomena in characterizing the wave vector of SPP (WVP) modes. Furthermore, the WVP associated with a graphene waveguide is perturbed when placing another waveguide next to it. In this work, these phenomena are explored in detail to design a graphene-based resonator-coupled waveguide, which is superior to a single graphene-based waveguide in terms of confining propagating waves. Here, a comprehensive methodology is established for assessing miniaturized graphene devices, in which nonlinear, coupling, and spatial dispersion phenomena significantly affect their characteristics.

Cm-Level Photonic-Crystal-Like Subwavelength Waveguide Platform with High Integration Density

In this paper, the cm-level photonic-crystal-like subwavelength waveguide platform is developed and analyzed by using the finite-difference time-domain method. The configuration can be considered as a hybrid waveguide combining with the advantages of a metal-dielectric-metal waveguide and a photonic crystal waveguide. The symmetric and high reflection effect of metallic sidewall and the effect of the photonic crystal structure on the light-guiding mechanism and integration characteristics of the waveguide are systematically investigated. The results reveal that the cm-level photonic-crystal-like waveguide platform provides subwavelength confinement and very low propagation loss with the isolation more than 30 dB, which are promising for high-density photonic integration. The tradeoff between integration density and propagation loss is optimized. In addition, a T-shaped power splitter based on the waveguide platform is proposed. The excess loss of the T-shaped power splitter is less than 0.4 dB. A set of passive components can be exploited on the proposed cm-level photonic-crystal-like subwavelength waveguide platform in future work to constitute the large-scale integrated photonic systems.

Magnetoplasmonic isolators based on graphene waveguide ring resonators

Nanoscale optical isolators are crucial for nanophotonic, quantum-optical, and optoelectronic applications and have attracted considerable attention recently. The application of external magnetic fields is the standard way for creating nonreciprocity as the basis for optical isolation. However, the magneto-optical response is usually small, thus it should be enhanced to create strong enough nonreciprocity for high-contrast isolation. We demonstrate that a graphene waveguide ring resonator allows a nanoscale platform for a high-contrast optical isolator. Graphene provides strong subwavelength confinement and low loss, therefore large nonreciprocity can be realized for ultracompact high-contrast isolators by plasmon resonance enhancement combined with resonator resonance enhancement. The resonance frequency is widely tunable by controlling the gate voltages of the rolling graphene sheet of a graphene resonator. Manipulation of the waveguide-resonator coupling can be achieved by controlling the gate voltage of the planar graphene sheet. The concept of magnetically biased ultracompact graphene waveguide ring resonators for one-way plasmon flow allows one to tune the operation band over a one-octave-spanning broad frequency range, keeping the contrast higher than 99% and a moderate insertion loss lower than 50%. High performance is achievable by only adjusting both gate voltages of the planar and rolling graphene sheets.

Deep subwavelength confinement and threshold engineering in a coupled nanorods based spaser.

In recent years, extensive efforts have been made for design and fabrication of low threshold spasers or plasmonic nanolasers at a deep subwavelength scale. Plasmonic nanolasers with coupled-nanorods structure can realize this purpose due to energy concentration in nano size volumes and effective amplification mechanisms. In this study, a group of structures based on metallic and CdS coupled nanorods are designed and analyzed using the finite element method (FEM). By changing the lateral adjacent surfaces of the metal and semiconductor nanorods through utilizing regular polygons as the cross sections of the nanorods, different characteristics of the plasmonic nanolaser are investigated. Simulation results show that the mode area normalized by the diffraction limit area is as low as 0.0062 in the structures based on hexagonal metallic core with circular semiconductor nanorods while structures based on circular Ag core with hexagonal CdS nanorods can provide a low threshold gain as 1.310 μm-1. Also, it is shown that if ZnO be used as the semiconductor gain material instead of CdS, a normalized mode area of almost one tenth can be attained in a structure with dodecagonal metallic core and circular ZnO nanorods.

Brillouin optomechanics in nanophotonic structures

The interaction between light and mesoscopic mechanical degrees of freedom has been investigated under various perspectives, from spectroscopy in condensed matter, optical tweezer particle trapping, and long-haul optical fiber communication system penalties to gravitational-wave detector noise. In the context of integrated photonics, two topics with dissimilar origins—cavity optomechanics and guided wave Brillouin scattering—are rooted in the manipulation and control of the energy exchange between trapped light and mechanical modes. In this tutorial, we explore the impact of optical and mechanical subwavelength confinement on the interaction among these waves, coined as Brillouin optomechanics. At this spatial scale, optical and mechanical fields are fully vectorial and the common intuition that more intense fields lead to stronger interaction may fail. Here, we provide a thorough discussion on how the two major physical effects responsible for the Brillouin interaction—photoelastic and moving-boundary effects—interplay to foster exciting possibilities in this field. In order to stimulate beginners into this growing research field, this tutorial is accompanied by all the discussed simulation material based on a widespread commercial finite-element solver.

A planar strongly confined spoof surface plasmonic waveguide with compact cells

ABSTRACTA new planar waveguide structure, based on spoof surface plasmon polaritons (SSPPs), is proposed featuring compact size, low loss, and good subwavelength field confinement. Compared with the rectangular- and trapezoidal-type cells in the conventional spoof surface plasmonic waveguides (SSPWs), the proposed cell has 50.7% and 27.8% size reduction at the same cut-off frequency, respectively. Moreover, the simulated electrical field distributions show that there are good improvements in the field enhancement and subwavelength confinement. To further validate the transmission characteristics of the proposed waveguide, coplanar waveguide (CPW) and corresponding transition structures are used to get a high efficient mode conversion. A CPW-SSPW-CPW structure on a dielectric board is designed, fabricated, and measured in the microwave region. Good agreement is achieved between the simulated and measured results. The last results show that the proposed waveguide has low insertion loss and flat group delay in an ultra-wideband from 2.4 to 10.3 GHz.

Hybrid cavity-antenna systems for quantum optics outside the cryostat?

Abstract Hybrid cavity-antenna systems have been proposed to combine the sub-wavelength light confinement of plasmonic antennas with microcavity quality factors Q. Here, we examine what confinement and Q can be reached in these hybrid systems, and we address their merits for various applications in classical and quantum optics. Specifically, we investigate their applicability for quantum-optical applications at noncryogenic temperatures. To this end we first derive design rules for hybrid resonances from a simple analytical model. These rules are benchmarked against full-wave simulations of hybrids composed of state-of-the-art nanobeam cavities and plasmonic-dimer gap antennas. We find that hybrids can outperform the plasmonic and cavity constituents in terms of Purcell factor, and additionally offer freedom to reach any Q at a similar Purcell factor. We discuss how these metrics are highly advantageous for a high Purcell factor, yet weak-coupling applications, such as bright sources of indistinguishable single photons. The challenges for room-temperature strong coupling, however, are far more daunting: the extremely high dephasing of emitters implies that little benefit can be achieved from trading confinement against a higher Q, as done in hybrids. An attractive alternative could be strong coupling at liquid nitrogen temperature, where emitter dephasing is lower and this trade-off can alleviate the stringent fabrication demands required for antenna strong coupling. For few-emitter strong-coupling, high-speed and low-power coherent or incoherent light sources, particle sensing and vibrational spectroscopy, hybrids provide the unique benefit of very high local optical density of states, tight plasmonic confinement, yet microcavity Q.

Molecular Optomechanics in the Anharmonic Cavity-QED Regime Using Hybrid Metal–Dielectric Cavity Modes

Using carefully designed hybrid metal-dielectric resonators, we study molecular optomechanics in the strong coupling regime ($g_{\rm }^2/\omega_m {>} \kappa$), which manifests in anharmonic emission lines in the sideband-resolved region of the cavity-emitted spectrum ($\kappa{<}\omega_m$). This nonlinear optomechanical strong coupling regime is enabled through a metal-dielectric cavity system that yields not only deep sub-wavelength plasmonic confinement, but also dielectric-like confinement times that are more than two orders of magnitude larger than those from typical localized plasmon modes. These hybrid metal-dielectric cavity modes enable one to study new avenues of quantum plasmonics for single molecule Raman scattering.

Nanowire-type plasmonic waveguides as strong and tunable optical tweezers

A series of nanowire-type plasmonic waveguides are proposed. The mode properties of these waveguides and their dependences on various geometry parameters are studied. It is shown that they can generate deep subwavelength confinement and long-range propagation simultaneously. Moreover, the optical forces exerted on dielectric nanoparticles by these waveguides are calculated. It is found that the optical trapping forces are very strong, and that their distribution can be effectively regulated by certain geometry parameters. Using these features, strong and tunable near-field optical tweezers can be designed.

Polaritonics: from microcavities to sub-wavelength confinement

Abstract Following the initial success of cavity quantum electrodynamics in atomic systems, strong coupling between light and matter excitations is now achieved in several solid-state set-ups. In those systems, the possibility to engineer quantum emitters and resonators with very different characteristics has allowed access to novel nonlinear and non-perturbative phenomena of both fundamental and applied interest. In this article, we will review some advances in the field of solid-state cavity quantum electrodynamics, focussing on the scaling of the relevant figures of merit in the transition from microcavities to sub-wavelength confinement.

Achieving subwavelength field confinement in sub-terahertz regime by periodic metallo-dielectric waveguides.

In this paper, we report on a periodic metallo-dielectric structure that supports geometry-induced surface plasmons in the sub-terahertz regime. The proposed structure is made up of a dielectric-coated metallic grating sandwiched by parallel metal plates. Based on the modal analysis of 2D and 3D structures, the impact of a metal cladding and a customized dielectric coating on the dispersion relation and field distribution of the guided surface wave is investigated. It is found that modal field confinement is improved in the presence of a metal cladding without narrowing the operational bandwidth of the waveguide. Moreover, a customized subwavelength-sized dielectric coating based on high-resistivity silicon (HR-Si) can further improve the confinement. As a result, by incorporating both the HR-Si coating and the metal cladding in a conventional metallic grating, subwavelength field confinement is achieved over nearly a 2:1 frequency bandwidth. The achieved performance makes the realization of extremely-low radiation loss sharp bends possible. In particular, the achieved radiation loss is less than 0.5dB for a 90° bend of radius λ0/4 based on a waveguide cross-sectional dimension of almost λ0/10 where λ0 is the free-space wavelength at the maximum frequency of operation. The proposed waveguide is promising for the implementation of sub-terahertz guided-wave devices and circuits thanks to its outstanding field confinement and ruggedized and shielded structure.

Transient exciton-polariton dynamics in WSe2 by ultrafast near-field imaging.

Van der Waals (vdW) materials offer an exciting platform for strong light-matter interaction enabled by their polaritonic modes and the associated deep subwavelength light confinement. Semiconductor vdW materials such as WSe2 are of particular interest for photonic and quantum integrated technologies because they sustain visible-near-infrared (VIS-NIR) exciton-polariton (EP) modes at room temperature. Here, we develop a unique spatiotemporal imaging technique at the femtosecond-nanometric scale and observe the EP dynamics in WSe2 waveguides. Our method, based on a novel ultrafast broadband intrapulse pump-probe near-field imaging, allows direct visualization of EP formation and propagation in WSe2 showing, at room temperature, ultraslow EP with a group velocity of v g ~ 0.017c. Our imaging method paves the way for in situ ultrafast coherent control and extreme spatiotemporal imaging of condensed matter.

Enhanced near field focus steering of rectangular nanoslit metasurface structure

&lt;sec&gt; Surface plasmon polaritons (SPPs) are electromagnetic excitations propagating along the metal-dielectric interface. The SPPs excited by the metal micro/nano structures have the ability to manipulate the light on a subwavelength scale. The SPPs are of interest to researchers for its excellent subwavelength field confinement and local field enhancement. So far, the SPPs have found numerous applications in optical tweezers, biological sensors, and near-field holographic imaging, due to its subwavelength focusing. &lt;/sec&gt;&lt;sec&gt; In order to achieve enhanced near field subwavelength focusing, we propose a metasurface structure in this paper, which is composed of rectangular nanoslit circular arrays and multilayer annular slits. The function of the inner ring arrays is to excite SPPs and the outer ring slits is to enhance focusing. The electric field expression of SPP is studied analytically and theoretically, and then the principle of rectangular nanoslit to excite SPP and the inner ring array structure to generate central focusing are explained. The parameters of the structure are optimized, and the focusing characteristics of the metasurface structure under different polarization light are studied by using the finite difference time domain method. Furthermore, we explain the principle of the external structure enhancing focusing by introducing the theory of Fresnel zone plate and depth modulation. The analytical expressions and simulations show that when the incident polarized light has a wavelength of 980 nm, the focal spot having a full width at half maximum of about 650 nm, and the distribution of the coupled field can be approximately expressed by the first kind Bessel function. Compared with the former single circular array structure, the composite structure proposed in this paper has a good effect of both enhancing the central focusing and inhibiting the outer field divergence, and the center focal spot intensity is doubled. In addition, the electric field excited by the arbitrary linearly polarized light is also discussed, the electric field satisfies the form of the polarization angle sinusoidal function multiplied by a Bessel function. &lt;sec&gt; The research results of our study have some applications in subwavelength light modulation, near-field imaging, optical tweezers, and subwavelength scale optical information processing and so on. &lt;/sec&gt;&lt;/sec&gt;

Topological surface plasmon polaritons

The miniaturization of electromagnetic devices is a long-term theme for the development of modern technologies to achieve higher flexibilities, better performances, and higher density integration. Surface plasmon polaritons (SPPs) provide a powerful solution for reducing the size of integrated electromagnetic device due to its deep subwavelength confinement. However, materials or structures that support SPPs inevitably have impurities or structural defects, which leads to the loss of the propagating mode. In order to avoid scattering from impurities or defects, topological structures are introduced to address issues of discontinuities and have been proved to be an effective solution. In this paper, we first review the recent efforts devoted to SPPs based optical devices and those of artificial surface plasmon in terahertz/microwave band, and then summarize several important topological systems of SPPs. Finally, we present our perspectives on the future developments of this field.

High Gain and High-Efficiency Millimeter-Wave Antenna Based on Spoof Surface Plasmon Polaritons

Planar spoof surface plasmonic polaritons (SPPs) have attracted much attention due to the features of the subwavelength confinement and low transmission loss. The latter feature is highly desired in millimeter wave (mm-wave) and terahertz band. In this communication, a novel method is proposed to convert bounded spoof SPPs mode to radiation mode. An equivalent radiating aperture is formed by gradually decreasing the grooves of an odd-mode plasmonic structure. The mechanism of SPPs wave radiating out through the aperture is clarified by referring to a horn antenna. Furthermore, a spoof SPPs traveling-wave antenna prototype is designed and measured to validate the concept. Measured results show that the antenna has a high gain (~15 dB) and high radiating efficiency (~90%) at the mm-wave frequency. The measured narrow beamwidth in endfire direction indicates that it can be used to construct switched-beam arrays for 5G applications.

P‐Wave Superfluid Phases of Fermi Molecules in a Bilayer Lattice Array

AbstractThe superfluid phases in an ultracold gas of dipolar Fermi molecules lying in two parallel square lattices in 2D are investigated. As shown by a two‐body study, dipole moments oriented in opposite directions in each layer are the key ingredients in our mean‐field analysis from which unconventional superfluidity is predicted. The phase diagram summarizes our findings: stable and metastable superfluid phases appear as a function of both, the dipole–dipole interaction coupling parameter and filling factor. A first‐order phase transition, and thus a mixture of superfluid phases at different densities, is revealed from the coexistence curves in the metastable region. The model predicts that these superfluid phases can be observed experimentally at 10 nK in molecules of NaK confined in optical lattices of size nm. Other routes to reach higher temperatures require the use of subwavelength confinement technique.

Suppression of near-field coupling in plasmonic antennas on epsilon-near-zero substrates

Epsilon-near-zero (ENZ) media are an emerging class of nanophotonic materials that engender electromagnetic fields with small phase variation due to their approximately zero permittivity. These quasi-static fields facilitate several unique optical properties, such as subwavelength confinement, arbitrary wavefront control, and enhanced light–matter interactions, which make ENZ materials promising platforms for nanophotonic and plasmonic systems. Here, we report our analysis of single and dimer nanoantennas deposited on an aluminum-doped zinc oxide layer with an ENZ wavelength around 1.5 μm. Using near-field microscopy, far-field spectroscopy, finite-element numerical simulations, and a semi-analytic Fabry–Perot (FP) model, we show that single nanoantennas support highly dispersive plasmonic modes with less than unity effective mode index at wavelengths greater than the ENZ wavelength, which consequently fixes the resonance near the ENZ wavelength of the substrate. Furthermore, we observe a strong reduction in the near-field coupling between dimer nanoantennas via measurements of the resonance shift as a function of gap size. This reduction of near-field coupling allows one to design arrays of independently operating antennas with higher densities and thereby significantly improve the array characteristics, especially when targeting gradient metasurface implementations. Our results demonstrate the use of ENZ materials for increasing the versatility and functionality of plasmonic structures and provide foundational insight into this exotic material phenomenon.

Low-Dimensional Materials and State-of-the-Art Architectures for Infrared Photodetection.

Infrared photodetectors are gaining remarkable interest due to their widespread civil and military applications. Low-dimensional materials such as quantum dots, nanowires, and two-dimensional nanolayers are extensively employed for detecting ultraviolet to infrared lights. Moreover, in conjunction with plasmonic nanostructures and plasmonic waveguides, they exhibit appealing performance for practical applications, including sub-wavelength photon confinement, high response time, and functionalities. In this review, we have discussed recent advances and challenges in the prospective infrared photodetectors fabricated by low-dimensional nanostructured materials. In general, this review systematically summarizes the state-of-the-art device architectures, major developments, and future trends in infrared photodetection.

Metallic Slit–Loaded Ring Resonator–Based Plasmonic Demultiplexer with Large Crosstalk

Photonics provides a key solution to limit the myriads of challenges offered by existing silicon technology. Here, we propose a simple 2-channel Plasmonic demultiplexer with Metal-Insulator-Metal (MIM) waveguide geometry. The demultiplexer is capable to separate two fundamental optical windows (i.e., 1310 nm and 1550 nm). An FDTD (finite difference time domain) method is used to investigate the structure numerically. The optical properties of the demultiplexer are also investigated. The performance of the proposed demultiplexer is measured using crosstalk and quality factor. The minimum value of crosstalk is − 30 dB and maximum quality factor is 60. The maximum transmission is limited to 45% due to large metallic losses. The proposed demultiplexer is ultra-compact in size and is capable to provide sub-wavelength confinement. It is suited for WDM applications and can pave a way to design the photonic integrated circuits.