Surface Plasmon-phonon Polaritons Research Articles

Coupling of Surface Plasmon Polaritons and Hyperbolic Phonon Polaritons on the Near-Field Radiative Heat Transfer Between Multilayer Graphene/hBN Structures

ABSTRACT Near-field radiative heat transfer (NFRHT) between multilayer graphene/hBN heterostructures has been demonstrated to exceed the blackbody limit due to the coupling mechanism of surface plasmon polaritons and hyperbolic phonon polaritons, opening the door to applications in thermal management, thermophotovoltaics, and nanoscale metrology. Recent studies have shown that adding vacuum layers within multilayer structures can effectively promote surface modes and thus enhance NFRHT. However, the influence of vacuum layers on NFRHT between multilayer graphene/hBN heterostructures has not been investigated. Moreover, the influence of vacuum layers on coupled resonance modes excited in multilayer structures is worth discussing. In this work, we study the NFRHT based on multilayer graphene/vacuum/hBN/vacuum structures. The results show that as the gap distance increases from 20 nm to 100 nm, the NFRHT of three-cell and six-cell configurations is enhanced, while that of unit-cell configuration is suppressed. The potential mechanism can be identified as the excitation of surface plasmon-phonon polaritons (SPPPs) and hyperbolic plasmon-phonon polaritons (HPPPs) in multilayer structures. The enhancement factor of the six-cell configurations is up to 4.82 when the gap distance is 80 nm. Moreover, the influences of the chemical potential of graphene and the layer thickness on the NFRHT are discussed. The interesting results in this work indicate the perspectives for future near-field research involving coupling of SPPPs and HPPPs, and shed new light on high-performance devices introducing vacuum layers based on near-field radiative heat transfer.

Actively tuning anisotropic light–matter interaction in biaxial hyperbolic material α-MoO3 using phase change material VO2 and graphene

Anisotropic hyperbolic phonon polaritons (PhPs) in natural biaxial hyperbolic material α-MoO3 has opened up new avenues for mid-infrared nanophotonics, while active tunability of α-MoO3 PhPs is still an urgent problem necessarily to be solved. In this study, we present a theoretical demonstration of actively tuning α-MoO3 PhPs using phase change material VO2 and graphene. It is observed that α-MoO3 PhPs are greatly dependent on the propagation plane angle of PhPs. The insulator-to-metal phase transition of VO2 has a significant effect on the hybridization PhPs of the α-MoO3/VO2 structure and allows to obtain actively tunable α-MoO3 PhPs, which is especially obvious when the propagation plane angle of PhPs is 90°. Moreover, when graphene surface plasmon sources are placed at the top or bottom of α-MoO3 in α-MoO3/VO2 structure, tunable coupled hyperbolic plasmon–phonon polaritons inside its Reststrahlen bands (RBs) and surface plasmon–phonon polaritons outside its RBs can be achieved. In addition, the above-mentioned α-MoO3-based structures also lead to actively tunable anisotropic spontaneous emission (SE) enhancement. This study may be beneficial for realization of active tunability of both PhPs and SE of α-MoO3, and facilitate a deeper understanding of the mechanisms of anisotropic light–matter interaction in α-MoO3 using functional materials.

Compensating losses in polariton propagation with synthesized complex frequency excitation.

Surface plasmon polaritons and phonon polaritons offer a means of surpassing the diffraction limit of conventional optics and facilitate efficient energy storage, local field enhancement and highsensitivity sensing, benefiting from their subwavelength confinement of light. Unfortunately, losses severely limit the propagation decay length, thus restricting the practical use of polaritons. While optimizing the fabrication technique can help circumvent the scattering loss of imperfect structures, the intrinsic absorption channel leading to heat production cannot be eliminated. Here, we utilize synthetic optical excitation of complex frequency with virtual gain, synthesized by combining the measurements made at multiple real frequencies, to compensate losses in the propagations of phonon polaritons with dramatically enhanced propagation distance. The concept of synthetic complex frequency excitation represents a viable solution to the loss problem for various applications including photonic circuits, waveguiding and plasmonic/phononic structured illumination microscopy.

Dynamic construction of refractive index-dependent vibrations using surface plasmon-phonon polaritons

One of the fundamental hurdles in infrared spectroscopy is the failure of molecular identification when their infrared vibrational fingerprints overlap. Refractive index (RI) is another intrinsic property of molecules associated with electronic polarizability, but with limited contribution to molecular identification in mixed environments currently. Here, we investigate the coupling mode of localized surface plasmon and surface phonon polaritons for vibrational de-overlapping. The coupling mode is sensitive to the molecular refractive index, attributed to the RI-induced vibrational variations of surface phonon polaritons (SPhP) within the Reststrahlen band, referred to as RI-dependent SPhP vibrations. The RI-dependent SPhP vibrations are linked to molecular RI features. According to the deep-learning-augmented demonstration of bond-breaking-bond-making dynamic profiling in biological reaction, we substantiate that the RI-dependent SPhP vibrations effectively disentangle overlapping vibrational modes, achieving a 92% identification accuracy even for the strongly overlapping vibrational modes in the reaction. Our findings offer insights into the realm of light-matter interaction and provide a valuable toolkit for biomedicine applications.

Significant Enhancement of Near-Field Radiative Heat Transfer by Misaligned Bilayer Heterostructure of Graphene-Covered Gratings

Abstract The near-field radiative heat transfer of heterostructure consisting of SiC gratings and graphene is investigated in this work. The rigorous coupled-wave analysis is employed to calculate the spectral heat flux. Nevertheless, monolayer heterostructure and nonmisaligned bilayer heterostructure consistently suffer from a lack of spectral heat flux. In this work, we investigate the prominent effect of misaligned bilayer heterostructure in enhancing near-field radiative heat transfer by plotting energy transmission coefficients and electromagnetic fields. The results show that when the misalignment reaches half a period, the bilayer heterostructure exhibits optimal performance with a total heat flux of 3.5 × 104 W/m2. Besides the well-known coupled surface phonon polaritons supported by SiC gratings, the surface plasmon polaritons supported by graphene dominate the enhancement of heat flux from 0.01 × 1014 rad/s to 1.5 × 1014 rad/s. Due to the spatial misalignment of the upper and lower gratings, the lower layer graphene surface plasmon polaritons are intensified, compensating for the lack of spectral heat flux. Meanwhile, the graphene surface plasmon polaritons and SiC surface phonon polaritons can be hybridized to form surface plasmon-phonon polaritons. In addition, the dynamic modulation of near-field radiative heat transfer in the misalignment state is achieved by manipulating the Fermi level of graphene. We finally show that the superiority of misaligned heterostructure is robust with respect to the frequency shift in the phonon band, providing an effective way to improve the near-field radiative heat transfer in different configuration.

Directive and coherent thermal emission of hybrid surface plasmon-phonon polaritons in n-GaN gratings of linear and radial shapes

Beaming and coherent thermal emission of the hybrid surface plasmon phonon polaritons (SPPhPs) was numerically and experimentally investigated employing the n-GaN surface relief gratings (SRGs) shaped in a linear and radial geometry. The polariton propagation losses were minimized numerically with the help of a rigorous coupled wave analysis model, while the spatial and temporal quality of selected mode radiation in a normal direction was maximized by fixing the grating period value at 17.5 µm and varying the grating filling factor from 75% to 25%. A set of optimal design linear and radial geometry SRG samples were fabricated in order to validate the emission characteristics of hybrid SPPhPs found by numerical modeling. We demonstrated that both efficient emission and beaming are possible to achieve through the excitation and interference of the same number but opposite sign hybrid polariton modes in n-GaN SRG.

Near-field radiative heat transfer in three-body system based on topological insulator Bi2Se3

This study investigates the near-field radiative heat flux in a three-body system made of Bi2Se3, a topological insulator that supports hyperbolic phonon polaritons (HPPs) in the terahertz frequency range. We demonstrate that the near-field radiative heat flux in the three-body system made of Bi2Se3 is remarkably enhanced compared to the two-body counterpart. The thickness of the intermediate body or the chemical potential of Bi2Se3 can be used to control the radiative heat flux. Our numerical results reveal that the heat flux initially increases and then decreases with the thickness of the intermediate body, indicating that an optimal heat flux can be obtained by selecting an appropriate chemical potential and intermediate body thickness. Moreover, increasing the chemical potential of Bi2Se3 shifts the hybrid surface plasmon phonon polaritons, which arise from the coupling of surface plasmons and HPPs, toward higher frequencies. These findings have important implications for developing thermal devices such as thermal transistors and offer a promising approach to modulating near-field radiative heat transfer and thermal management.

Surface-acoustic-wave-driven graphene plasmonic sensor for fingerprinting ultrathin biolayers down to the monolayer limit

Surface plasmon polaritons in graphene can enhance the performance of mid-infrared spectroscopy, which is key for the study of both the composition and the conformation of organic molecules via their vibrational resonances. In this paper, a plasmonic biosensor using a graphene-based van der Waals heterostructure on a piezoelectric substrate is theoretically demonstrated, where far-field light is coupled to surface plasmon-phonon polaritons (SPPPs) through a surface acoustic wave (SAW). The SAW creates an electrically-controlled virtual diffraction grating, suppressing the need for patterning the 2D materials, that limits the polariton lifetime, and enabling differential measurement schemes, which increase the signal-to-noise ratio and allow a quick commutation between reference and sample signals. A transfer matrix method has been used for simulating the SPPPs propagating in the system, which are electrically tuned to interact with the vibrational resonances of the analytes. Furthermore, the analysis of the sensor response with a coupled oscillators model has proven its capability of fingerprinting ultrathin biolayers, even when the interaction is too weak to induce a Fano interference pattern, with a sensitivity down to the monolayer limit, as tested with a protein bilayer or a peptide monolayer. The proposed device paves the way for the development of advanced SAW-assisted lab-on-chip systems combining the existing SAW-mediated physical sensing and microfluidic functionalities with the chemical fingerprinting capability of this novel SAW-driven plasmonic approach.

Giant and controllable Goos–Hänchen shift of a reflective beam off a hyperbolic metasurface of polar crystals

We conduct a theoretical analysis of the massive and tunable Goos–Hänchen (GH) shift on a polar crystal covered with periodical black phosphorus (BP)-patches in the THz range. The surface plasmon phonon polaritons (SPPPs), which are coupled by the surface phonon polaritons (SPhPs) and surface plasmon polaritons (SPPs), can greatly increase GH shifts. Based on the in-plane anisotropy of BP, two typical metasurface models are designed and investigated. An enormous GH shift of about −7565.58λ 0 is achieved by adjusting the physical parameters of the BP-patches. In the designed metasurface structure, the maximum sensitivity accompanying large GH shifts can reach about 6.43 × 108 λ 0/RIU, which is extremely sensitive to the size, carrier density, and layer number of BP. Compared with a traditional surface plasmon resonance sensor, the sensitivity is increased by at least two orders of magnitude. We believe that investigating metasurface-based SPPPs sensors could lead to high-sensitivity biochemical detection applications.

Active tuning hBN phonon polaritons and spontaneous emission rates based on VO<sub>2</sub> and graphene

<sec>Active tunability of phonon dispersion and spontaneous emission (SE) are still open problems due to their exciting potential applications. In view of the fact that polaritons are very sensitive to the dielectric environment, in this study, with the help of the differences in optical property between the phase change material vanadium dioxide (VO<sub>2</sub>) during the phase transition from the insulating state to metallic state and the tunable surface plasmon polaritons (SPPs) in graphene, a heterostructure composed of hyperbolic material hexagonal boron nitride (hBN) and graphene and VO<sub>2</sub> is proposed to investigate the active tunability of hBN phonon polaritons (PhPs). In order to illustrate the underlying physical mechanism of the above heterostructures, the dispersion distributions of the above heterostructures are calculated and represented by the imaginary part of the p-polarized Fresnel reflection coefficient of the heterostructure, meanwhile the dispersion relation of the hBN/VO<sub>2</sub> heterostructure in hyperbolic region is verified by the quasi-static approximation method.</sec><sec>Results indicate that the active tunability of hBN PhPs inside type-I and type-II hyperbolic bands can be achieved by controlling VO<sub>2</sub> phase transition in hBN/VO<sub>2</sub> heterostructure. The PhP dispersion change of the hBN/VO<sub>2</sub> heterostructure is mainly caused by the change of the VO<sub>2</sub> dielectric function when VO<sub>2</sub> substrate changes from the insulating state into metallic state, which affects the total Fresnel reflection coefficient of the heterostructure, finally resulting in the PhP dispersion change of hBN/VO<sub>2</sub> heterostructure. When graphene is introduced into the hBN/VO<sub>2</sub> heterostructure, coupled hyperbolic plasmon-phonon polaritons (HPPPs) are obtained within type-I and type-II hyperbolic band of hBN, while the surface plasmon-phonon polaritons (SPPPs) are generated outside its hyperbolic bands. Moreover, comparative analysis of SE rates is presented for a quantum emitter positioned with the hBN/VO<sub>2</sub> and graphene/hBN/VO<sub>2</sub> heterostructure, revealing that the SE rates of these heterostructures can be modulated by the passive means including changing the hBN thickness and distance between the dipole emitter and the proposed heterostructure, and also by the active means including tuning VO<sub>2</sub> phase states and graphene chemical potential without changing structural configurations. This study provides a theoretical guidance in realizing active tunability of both phonon dispersion and SE rate of the natural or artificial anisotropic optical materials by using functional materials including phase change materials and graphene.</sec>

Enhanced near-field thermal radiation between black phosphorus with high electron density by BP/hBN heterostructures

ABSTRACT The black phosphorus (BP)/hexagonal boron nitride (hBN) Van der Waals heterostructure has great potential application in BP-based devices due to its higher stability than monolayer BP film. The near-field thermal radiation (NFTR) between two identical BP/hBN heterostructures with high electron density is studied to promote the application of BP-based devices. The BP/hBN heterostructures show a higher heat transfer coefficient (HTC) at a 10 nm gap distance compared to BP films with electron density n larger than 1 × 1013 cm−2, due to the coupling of surface plasmon polaritons (SPPs) and hyperbolic phonon polaritons (HPPs). Especially at n ≥ 3 × 1013 cm−2, the improvement is more than 100%. The SPPs and the HPPs couple into surface plasmon-phonon polaritons (SPPPs) out of the hyperbolic region and hyperbolic plasmon-phonon polaritons (HPPPs) inside. The SPPPs can achieve photon tunneling in the broader wavevector region than SPPs, making most of the contribution to heat transfer. The influences of the thickness of the hBN sheet and gap distance are also discussed. This scheme only effectively enhances NFTR for BP with high electron density at small nanogaps. After structural optimization, h = 2 nm is the optimal thickness for BP/hBN configuration with low electron density, such as n = 1 × 1013 cm−2. In contrast, large thickness h = 500 nm is optimal for BP with high electron density. At high electron density, a thick hBN sheet is prominent in enhancing the SPPPs in the frequencies below the type-I region and the HPPPs inside the type-II region. We further propose BP/hBN/BP heterostructures and find that HTC is further enhanced by 4.4% ~ 30.8% due to the more robust surface modes. Our work may contribute to developing BP-based near-field thermal devices and promote understanding the NFTR mechanism of BP/hBN heterostructure.

Coupled surface plasmon–phonon polariton nanocavity arrays for enhanced mid-infrared absorption

Abstract Resonant optical cavities are essential components in mid-infrared applications. However, typical film-type cavities require multilayer stacks with a micron-thick spacer due to mid-infrared wavelengths, and their performance is limited by narrow frequency tunability and angular sensitivity. We propose and experimentally demonstrate the subwavelength-scale (≈λ 0/150) resonant nanocavity arrays that enhance the absorption spectrum of the device in the mid-infrared (10–12 microns) via excitation of coupled surface plasmon–phonon polaritons. The proposed metal–insulator–polar dielectric (gold–silicon–silicon carbide) structure supports a guided mode of the coupled surface polaritons in the lateral direction while vertically confining the mid-infrared wave within the 80 nm thick dielectric spacer. In particular, the subwavelength-scale (≈λ 0/10) gratings are imposed to form Fabry–Pérot cavity arrays displaying angle-insensitive and frequency-tunable absorption of up to 80% of the optical power in the mid-infrared. Our work should benefit diverse mid-infrared applications and novel designs of polariton-based photonic devices.

Enhancement of Casimir Friction between Graphene-Covered Topological Insulator.

Casimir friction is theoretically studied between graphene-covered undoped bismuth selenide (Bi2Se3) in detail. In the graphene/Bi2Se3 composite structure, the coupling of the hyperbolic phonon polaritons supported by Bi2Se3 with the surface plasmons supported by graphene can lead to the hybrid surface plasmon–phonon polaritons (SPPPs). Compared with that between undoped Bi2Se3, Casimir friction can be enhanced by more than one order of magnitude due to the contribution of SPPPs. It is found that the chemical potential that can be used to modulate the optical characteristic of SPPPs plays an important role in Casimir friction. In addition, the Casimir friction between doped Bi2Se3 is also studied. The friction coefficient between doped Bi2Se3 can even be larger than that between graphene-covered undoped Bi2Se3 for suitable chemical potential due to the contribution of unusual electron surface states. The results obtained in this work are not only beneficial to the study of Casimir frictions but also extend the research ranges of topological insulators.

Tunable coupling of terahertz Dirac plasmons and phonons in transition-metal dichalcogenide-based van der Waals heterostructures

Dirac plasmons in graphene hybridize with phonons of transition metal dichalcogenides (TMDs) when the materials are combined in so-called van der Waals heterostructures (vdWh), thus forming surface plasmon-phonon polaritons (SPPPs). The extend to which these modes are coupled depends on the TMD composition and structure, but also on the plasmons’ properties. By performing realistic simulations that account for the contribution of each layer of the vdWh separately, we calculate how the strength of plasmon-phonon coupling depends on the number and composition of TMD layers, on the graphene Fermi energy and the specific phonon mode. From this, we present a semiclassical theory that is capable of capturing all relevant characteristics of the SPPPs. We find that it is possible to realize both strong and ultra-strong coupling regimes by tuning graphene’s Fermi energy and changing TMD layer number.

Hyperbolic plasmon–phonon dispersion and tunable spontaneous emission enhancement in Ge2Sb2Te5-based multilayer graphene and hBN system

Active tunability of phonon dispersion and spontaneous emission (SE) still remain open owing to its exciting application potential. In this study, multilayer graphene and hexagonal boron nitride (hBN) combined with the Ge2Sb2Te5 (GST) functional substrate are proposed to obtain tunable hyperbolic plasmon–phonon dispersion and SE. Results indicate that GST phase change has a profound impact on hyperbolic phonon polaritons in the hBN layer and can obtain an actively tunable SE. Moreover, when graphene is introduced into the hBN/GST system, hyperbolic plasmon–phonon polaritons inside the two reststrahlen (RS) bands of hBN and surface plasmon–phonon polaritons outside the two RS bands can be achieved, both of which can be actively tuned by adjusting the GST phase state and graphene chemical potential. In addition, the coupled polariton branches between hBN phonon polaritons and graphene plasmons become stronger and corresponding SE enhancement regions obviously broaden with increasing the graphene/hBN layer numbers for the fixed thickness multilayer graphene/hBN on GST substrate. These characteristics may provide an available route to realize active tunability of both phonon dispersion and SE enhancement of natural or artificial hyperbolic materials.

Tunable propagation of surface plasmon-phonon polaritons in graphene-hBN metamaterials

We present a detailed study on the tunable propagation derived from the coupled surface plasmon–phonon polaritons (SPPPs) with the effective medium theory (EMT) in graphene-hBN metamaterials (MMs). Four reststrahlen bands (RBs) are observed, two of which mainly come from the hBN and the others originate from the effect of the graphene. The RBs frequency windows can be adjusted by the chemical potential and the filling ratio. An epsilon-near-zero-and-pole(εNZP)hyperbolic metamaterial (HMM) is detected at the precise frequency, chemical potential and filling ratio where the HMM undergoes a completely inversion of anisotropy. We derive the relative dispersion relation and demonstrate that the propagation of SPPP modes can be regulated by modifying the chemical potential. In addition, the tunability of the graphene-hBN MMs also can be improved by changing the thicknesses of the hBN and the number of graphene sheets. The energy-flux density in the graphene-hBN MM seriously deviates from its wave vector and can be localized at a certain depth. Besides, the ghost SPPP modes with the oscillation-attenuation character are observed at some special conditions through checking the distribution of electric fields. The attenuation total reflection (ATR) measurement is established to examine these SPPP modes. The numerical results show that the observation of each SPPP modes requires different conditions dictated by material parameters and the polarization direction of the incident light.

Tunable far-infrared hyperbolic metamaterial based on graphene-polar dielectric

Abstract In this work, we propose a tunable hyperbolic graphene-polar dielectric metamaterial in the far-infrared range. Here, we employ LiF as a strongly dispersive photonic material which can support surface plasmon polariton within its polaritonic gap. The proposed metamaterial has been analyzed using effective medium approximation (EMA) and transfer matrix method (TMM). It is found that the characterization of the hyperbolic behavior and reflection coefficient of the proposed metamaterial can be effectively controlled by Fermi energy, number of graphene layers and thickness of LiF. According to the results, the hyperbolic bandwidth broadening is possible by increasing Fermi energy, number of graphene layers and decreasing LiF thickness. Furthermore, we have shown fully controllable reflective filtering property of the proposed metamaterial using EMA and TMM. It can be clearly visualized that for thick LiF, effective medium approximation results in complete validation in calculation of the reflection coefficient. This is demonstrating by comparison between the reflection coefficient of the proposed metamaterial calculated using EMA and TMM. We believe that the proposed graphene-polar dielectric metamaterial offers a potential strategy for fine tunning the hybridized surface plasmon phonon polariton in the far-infrared region towards high performance, tunable and active metamaterials.

Near-Field Electroluminescent Refrigeration System Consisting of Two Graphene Schottky Diodes

Abstract It has been shown that the performance of a thermal radiative device, such as a thermophotovoltaic (TPV) and an electroluminescent (EL) refrigerator, can be significantly enhanced when the vacuum gap between a reservoir and a semiconductor diode becomes nanoscale. Recently, several studies have reported the integration of a TPV and a light emitting diode (LED) in one near-field thermal radiative device to improve the operation efficiency. However, surface polaritons were hardly exploited in previous research because bare semiconductor diodes were used. In this paper, we propose a TPV-LED integrated near-field EL refrigeration system consisting of two graphene-semiconductor Schottky diodes. A substantial refrigeration rate (101.9 kW/m2) is achieved owing to the coupling of surface plasmon-phonon polaritons excited by a symmetric configuration of graphene-polar materials. Moreover, the cooling coefficient of performance (COP) of the system can be enhanced up to 2.65 times by recycling the electrical power generated in the TPV cell. The cooling performance is further investigated in relation to design parameters, namely the doping concentration of Si and insulator thickness.

Investigation of n-type gallium nitride grating for applications in coherent thermal sources

In this study, polarized reflection and emission spectroscopy was employed in order to describe experimentally and numerically the coherent behavior of surface plasmon phonon polaritons in shallow n-type gallium nitride grating. For the grating with a 1 μm depth and an 11 μm period, polariton coherence was observed measuring the reduction of the linewidth and the redshift of the resonance position in both reflection and emission spectra with the increase in the incident angle. Maximum coherence conditions were found at a frequency of 562 cm−1 and an angle of 37°, demonstrating the coherence length value up to 724 μm (41λ).

Controllable and Highly Propagative Hybrid Surface Plasmon–Phonon Polariton in a CdZnO-Based Two-Interface System

The development of new nanophotonic devices requires the understanding and modulation of the propagating surface plasmon and phonon modes arising in plasmonic and polar dielectric materials, respectively. Here we explore the CdZnO alloy as a plasmonic material, with a tunable plasma frequency and reduced losses compared to pure CdO. By means of attenuated total reflectance, we experimentally observe the hybridization of the surface plasmon polariton (SPP) with the surface phonon polariton (SPhP) in the air-CdZnO-sapphire three-layer system. We show how through the precise control of the CdZnO thickness, the resonance frequencies of the hybrid surface plasmon-phonon polariton (SPPP) are tuned in the mid-infrared, and the nature of the hybrid mode turns from a plasmon-like behavior in the thicker films to a phonon-like behavior in the thinnest films. The presence of sapphire phonons not only allows the hybrid mode to be formed, but also improves its characteristics with respect to the bare SPP. The reduced damping of the phonon oscillators allows to reduce the losses of the hybrid mode, enhancing the propagation length above 500 microns, one order of magnitude larger than that of typical SPPs, clearing the path for its application on emerging devices such as plasmonic waveguides.