Surface Phonon Polaritons Research Articles

Plasmonically Enhanced Thermal Radiation by Means of Surface Phonon Polaritons

Radiative heat transfer at the nanoscale has the potential to produce thermal emission with both spectral feature and intensity different from the bulk materials. Specifically, the idea of coupling the energy states of photons and phonons using surface phonon polaritons (SPhPs) blurs the boundary between light and heat, opening a path to the manipulation of heat transfer through photonic engineering and breaking the classical limit established by Planck's law. SPhPs can generate a strong energy confinement effect near the surface; however, this effect is only valid within a narrow spectral regime, the so-called reststrahlen band. In this study, we employ a hybrid structure to broaden the effective energy regime for thermal emission by coupling SPhPs and surface plasmon polaritons (SPPs). We report remarkably enhanced thermal emission of a hybrid structure made of a ${\mathrm{Si}\mathrm{O}}_{2}$ nanoribbon decorated with Au nanoparticles by a factor of 4 over that of a bare ${\mathrm{Si}\mathrm{O}}_{2}$ nanoribbon. We introduce an experimental technique to quantify the emissivity from the complex nanostructured surface using frequency-dependent heat-transfer measurement. Not only does it enable us to detect feeble emission from a single nanoemitter, but it also differentiates two distinct heat-conducting processes of conduction and radiation, simultaneously. Our work may offer insights to engineer far-field thermal emission using hybrid structures combining the surface plasma and phonon polaritons.

Impact of phonon nonlocality on nanogap and nanolayer polar resonators

Polar dielectric nanoresonators can support hybrid photon-phonon modes termed surface phonon polaritons with length scales below the diffraction limit. In the deep subwavelength regime the optical response of these systems was shown to diverge from that predicted through a standard dielectric description. Recently, we developed an analytical, dielectric approach and applied it to spheres and planar heterostructures, reproducing anomalous features observed in experiment and microscopic calculations. In this Rapid Communication we develop tools to describe the nonlocal response of polar nanoresonators of arbitrary symmetry, and use them to investigate systems with nanogaps and nanolayers of practical technological relevance. We demonstrate that the available field enhancement is strongly reduced, as the electromagnetic energy leaks away from the hot spots, while phononic resonances are shifted by resonator effects.

Energy Plasmon Modes in Metamaterial-filled Double-layer Graphene-wrapped Cylindrical Waveguides

The characteristics of hybrid surface plasmon modes in a double-layer graphene cylindrical waveguide filled with metamaterials are theoretically investigated. The dispersion relation for different configurations of metamaterials (double positive (DPS)-graphene-double negative (DNG)-graphene-DPS and DNG-graphene-DPS-graphene-DNG) is derived by solving Maxwell’s equations for cylindrical symmetry and implementing impedance boundary conditions at the interface. The Kubo formalism is used for the modeling of the electromagnetic response of graphene. The influence of the interlayer distance of graphene, chemical potential of graphene, and permittivity of interlayer metamaterials on the dispersion curve, effective mode index, propagation length, and phase velocity are presented. It is observed that the existence of double layers of graphene along with metamaterials provides better control and tuning of the propagation of the surface waves. The coupled surface plasmon polariton (SPP) and surface phonon polariton (SPhP) in the optical regime are observed due to the coupling of electromagnetic waves between graphene layers. The forward- and backward-propagating surface waves for the fundamental mode are studied for different waveguide configurations. The active trapping of light and tunable fast and ultraslow plasmon modes for both forward and backward propagation modes for the proposed design may enable the realization of potential optoelectronic applications, such as modulators, fast forward-wave amplifiers, frequency selectors, back wave contra couplers, thermal sensors, circular polarizers, slow light devices, infrared (IR) and molecular spectroscopy, on-chip buffers, communication switches, and energy devices.

Design of Sub wavelength-Grating-Coupled Fano Resonance Sensor in Mid-infrared

By introducing the sub-wavelength grating (SWG) waveguide in the long-range surface phonon resonance (LRSPhR) device, a mid-infrared Fano resonance is formed due to the coupling between surface phonon polariton and Bloch mode. By taking advantage of strong light-matter interaction in the SWG, such Fano resonance is expected to offer improved sensing performance. Based on the rigorous coupled-wave analysis (RCWA) method, the index sensitivity and figure of merit of such a sensor reach 7496 RIU−1 and 46,432, respectively, which is 6 times compared with the conventional waveguide-coupled LRSPhR. The proposed SWG-coupled Fano resonance can be a promising platform for mid-infrared biochemical sensing.

Effect of surface phonon polariton in unimplanted and oxygen implanted GaN layers

This paper presents the investigation of Surface Phonon Polariton (SPP), Interface Phonon Polariton (IPP), and resonant absorption at 1118−1129 cm−1 using Infra-Red Attenuated Total Reflection (IR-ATR) technique on an unimplanted and oxygen ion-implanted GaN layers on sapphire substrate using fluences 5 × 1014/cm2 and 5 × 1015/cm2 with (∼120 keV) energy. A strong absorption dip due to SPP mode at 689.2 cm-1 for the GaN layer and IPP mode at 773 cm−1 for GaN and sapphire substrate interface and resonant absorption mode at 1129 cm−1 are observed experimentally. As implanted dose increases SPP mode and resonant absorption mode are red-shifted, while IPP mode remains unchanged. The SPP mode is red-shifted around 3.3 cm−1 and 5 cm−1 in the implanted samples due to lattice damage and oxygen incorporation in the GaN layer. In this article SPP absorption mode broadening (10 cm−1 to 29 cm−1), peak shift, and other parameters (IPP, damping terms, resonance absorption dip frequency) for unimplanted and implanted GaN samples have been investigated and reported. The surface phonon dispersion and reflectivity equations are used to simulate experimental results for IR-ATR reflectance curve SPP, IPP mode, and resonance absorption for GaN samples are found to be in good agreement.

Enhanced thermal conduction by surface phonon-polaritons.

Improving heat dissipation in increasingly miniature microelectronic devices is a serious challenge, as the thermal conduction in nanostructures is markedly reduced by increasingly frequent scattering of phonons on the surface. However, the surface could become an additional heat dissipation channel if phonons couple with photons forming hybrid surface quasiparticles called surface phonon-polaritons (SPhPs). Here, we experimentally demonstrate the formation of SPhPs on the surface of SiN nanomembranes and subsequent enhancement of heat conduction. Our measurements show that the in-plane thermal conductivity of membranes thinner than 50 nm doubles up as the temperature rises from 300 to 800 kelvin, while thicker membranes show a monotonic decrease. Our theoretical analysis shows that these thickness and temperature dependencies are fingerprints of SPhP contribution to heat conduction. The demonstrated thermal transport by SPhPs can be useful as a previously unidentified channel of heat dissipation in a variety of fields including microelectronics and silicon photonics.

Near-field radiative heat transfer between uniaxial hyperbolic media: Role of volume and surface phonon polaritons

It has been found that near-field radiative heat transfer (NFRHT) between hyperbolic media, such as hexagonal boron nitride (hBN), can be significantly enhanced due to excitation of hyperbolic volume phonon polaritons (HVPPs) and hyperbolic surface phonon polaritons (HSPPs). However, the role of HVPPs and HSPPs in type I and type II hyperbolic bands has rarely been discussed in depth. In this paper, we investigate the NFRHT between two hBN slabs considering the role of HVPPs and HSPPs. The theoretical analysis and numerical results show that when the optical axis is perpendicular to the material surface, the enhanced NFRHT is attributed to the excitation of HVPPs in both type I and type II hyperbolic bands, while HSPPs cannot be excited in this case. When the optical axis is parallel to the material surface, only HVPPs can be excited in type I hyperbolic band, while both HVPPs and HSPPs can be excited in type II hyperbolic band. In addition, the effect of the slab thickness on the dispersion relations of HVPPs and HSPPs, and on the enhancement of NFRHT is also studied. We believe the results in this paper could help to elucidate the mechanisms and manipulation of enhanced NFRHT between uniaxial hyperbolic materials.

Probing and steering bulk and surface phonon polaritons in uniaxial materials using fast electrons: Hexagonal boron nitride

We theoretically describe how fast electrons couple to polaritonic modes in uniaxial materials by analyzing the electron energy loss (EEL) spectra. We show that in the case of an uniaxial medium with hyperbolic dispersion, bulk and surface modes can be excited by a fast electron traveling through the volume or along an infinite interface between the material and vacuum. Interestingly, and in contrast to the excitations in isotropic materials, bulk modes can be excited by fast electrons traveling outside the uniaxial medium. We demonstrate our findings with the representative uniaxial material hexagonal boron nitride. We show that the excitation of bulk and surface phonon polariton modes is strongly related to the electron velocity and highly dependent on the angle between the electron beam trajectory and the optical axis of the material. Our work provides a systematic study for understanding bulk and surface polaritons excited by a fast electron beam in hyperbolic materials and sets a way to steer and control the propagation of the polaritonic waves by changing the electron velocity and its direction.

Magnetoplasmon-surface phonon polaritons’ coupling effects in radiative heat transfer

In this work, the coupling of magnetoplasmon polaritons (MPP) to surface phonon polaritons (SPhPs) in near-field radiative heat transfer is theoretically investigated. The system is composed of two parallel graphene-coated SiO2 substrates. By applying an external magnetic field, the separated branches of MPPs can couple with SPhPs to form tunable modes. The behavior remolds the energy transport of the system. The relative thermal magnetoresistance ratio can reach values of up to 160% for a magnetic field of 8 T. In addition, the thermal stealthy for the coated graphene is realized by tuning the intensity of fields. This work has substantial importance to graphene-based magneto-optical devices.

Control of full absorption with functional materials and silicon carbide gratings

We have proposed a novel silicon carbide (SiC) grating with vanadium dioxide (VO2) composite. Polarization-dependent surface phonon polaritons (SPhPs) can be excited. The device switches on/off light by switching the VO2 between its metallic and semiconductor states. Coupled wave analysis (RCWA) method is used to simulate the optical responses under different conditions. By introducing the structure of VO2 as an active material, this switchable, tunable, wavelength-selective structure can pave the way toward the ultimate control of thermal absorption/emissivity in the atmospheric window (8–14 μm).

Thermophysical study of surface phonon polaritons in multilayer systems for heat dissipation

Heat dissipation is an important concern for micro devices which could be overheated in their working process. In the present work, aiming to find new methods for heat dissipation, the thermophysical properties induced by surface phonon polaritons in multilayer systems were probed. The specific heat capacity, expansion coefficient, thermal conductivities along in-plane and also cross-plane directions in a multilayer system which is composed of SiC films separated by vacuum gaps were all calculated and discussed. Results turn out that: The polariton-induced specific heat is much smaller than that contributed by phonons because of the large difference of the density of states. The thermal conductivity along in-plane direction could be as high as about 104 W m−1 K−1 for the SiC film with a thickness of 0.1 mm, which is almost 1012 times larger than that along cross-plane direction. This large difference of thermal conductivities suggests a potential application of the multilayer systems for heat dissipation and also a long-distance thermal energy transport. Additionally, the expansion coefficient caused by polaritons is several orders of magnitude lower than that caused by phonons, which will benefit the method by utilizing polaritons to enhance heat transfer.

Vortices Formed by Two Counter-Propagating Waves for Surface Phonon Polaritons with Material Losses

We examine the surface phonon polaritons established on resonance between lossy silicon carbide and vacuum over the Reststrahlen frequency range. To this goal, we investigate two waves counter-propagating along a planar material interface in terms of the orbital and spin parts of the Poynting vector. Vortices of the spin part are thus found to be generated near the collision point of the two waves. In addition, vortex pairs of opposite circulations are identified near the material interface. The locality of the electric permittivity plays an essential role in enabling our simple analysis by neglecting the spatial propagations of phonons while keeping the material damping. The resulting dynamics of the total displacement field turns out to be loss-free in the middle of the lossy dynamics of the other field variables, thus exhibiting a superfluid-like feature in an otherwise normal fluid.

Ultrafast Optical Generation of Coherent Bright and Dark Surface Phonon Polaritons in Nanowires

The coherence of polaritons plays a fundamental role in numerous recent experimental observations, from strong coupling to Bose-Einstein condensation, but accessing the coherence of polaritons is a difficult task due to the high energy of plasmons and excitons. However, surface phonon polaritons offer similar promises at much lower energy. Here, we demonstrate the possibility to use visible ultrafast lasers to generate and time resolve coherent surface phonon polaritons in nanowires. We show that these modes are generated through the ultrafast screening of the surface depletion field. By comparing experiments and FDTD simulations, we show that this method allows the simultaneous generation of dark and bright mode. Our observations open the way to deeper investigations of the role of coherence in the rich polariton physics.

High Surface Phonon-Polariton in-Plane Thermal Conductance along Coupled Films.

Surface phonon-polaritons (SPhPs) are evanescent electromagnetic waves that can propagate distances orders of magnitude longer than the typical mean free paths of phonons and electrons. Therefore, they are expected to be powerful heat carriers capable of significantly enhancing the in-plane thermal conductance of polar nanostructures. In this work, we show that a SiO/Si (10 m thick)/SiO layered structure efficiently enhances the SPhP heat transport, such that its in-plane thermal conductance is ten times higher than the corresponding one of a single SiO film, due to the coupling of SPhPs propagating along both of its polar SiO nanolayers. The obtained results thus show that the proposed three-layer structure can outperform the in-plane thermal performance of a single suspended film while improving significantly its mechanical stability.

Geometric phase of a two-level atom near a dissipative and dispersive dielectric slab

In this paper, we study the geometric phase of a two-level atom near a dielectric slab. For this purpose, by applying the Von-Neumann equation, we obtain eigenvalues and eigenvectors of the reduced density operator of the atomic system. Then, we obtain the Lamb shift and transition rates of the atomic system in term of the electromagnetic Green tensor. Finally, by calculating the electromagnetic Green tensor of the system and making use of the kinematic approach, we study the geometric phase of the atomic system near the dielectric slab. We show that the geometric phase can be used as a sensitive probe to the surface-phonon polariton waves.

Near-Field Spectroscopy of Cylindrical Phonon-Polariton Antennas.

Surface phonon polaritons (SPhPs) are hybrid light-matter states in which light strongly couples to lattice vibrations inside the Reststrahlen band of polar dielectrics at mid-infrared frequencies. Antennas supporting localized surface phonon polaritons (LSPhPs) easily outperform their plasmonic counterparts operating in the visible or near-infrared in terms of field enhancement and confinement thanks to the inherently slower phonon-phonon scattering processes governing SPhP decay. In particular, LSPhP antennas have attracted considerable interest for thermal management at the nanoscale, where the emission strongly diverts from the usual far-field blackbody radiation due to the presence of evanescent waves at the surface. However, far-field measurements cannot shed light on the behavior of antennas in the near-field region. To overcome this limitation, we employ scattering-scanning near-field optical microscopy (sSNOM) to unveil the spectral near-field response of 3C-SiC antenna arrays. We present a detailed description of the behavior of the antenna resonances by comparing far-field and near-field spectra and demonstrate the existence of a mode with no net dipole moment, absent in the far-field spectra, but of importance for applications that exploit the heightened electromagnetic near fields. Furthermore, we investigate the perturbation in the antenna response induced by the presence of the AFM tip, which can be further extended toward situations where for example strong IR emitters couple to LSPhP modes.

Predicting Multi-Order Magnetic Polaritons Resonance in SiC Slit Arrays by Mutual Inductor–Inductor–Capacitor Circuit Model

Abstract Magnetic polariton (MP) that couples electromagnetic waves with magnetic excitation can be predicted by equivalent inductor–capacitor (LC) circuit model. However, when the resonance frequencies of MP and surface phonon polariton (SPhP) is close, the absorption and transmission peaks predicted by LC circuit model are far different from solving electromagnetic field calculation results. In this work, absorption and transmission enhancements with a SiC slit array are theoretically demonstrated within the SiC phonon absorption band with finite difference time-domain (FDTD) method. The interactions between SPhP and MP are confirmed by electromagnetic field distributions. Mutual inductor–inductor–capacitor (MLC) circuit model is used to predict the multiorder MP resonance conditions, and the coupling between MP and SPhP is treated as a mutual inductor in MLC model. The geometric effects of SiC slit arrays are investigated and MLC circuit model works well. This study may contribute to the design and prediction of thermal radiative properties and micro-/nanostructure metamaterials thermal radiative properties database building.

Near-Field Radiative Heat Transfer Between Two α-MoO3 Biaxial Crystals

Abstract The near-field radiative heat transfer (NFRHT) between two semi-infinite α-MoO3 biaxial crystals is investigated numerically based on the fluctuation–dissipation theorem combined with the modified 4 × 4 transfer matrix method in this paper. In the calculations, the near-field radiative heat flux (NFRHF) along each of the crystalline directions of α-MoO3 is obtained by controlling the orientation of the biaxial crystals. The results show that much larger heat flux than that between two semi-infinite hexagonal boron nitride can be achieved in the near-field regime, and the maximum heat flux is along the [001] crystalline direction. The mechanisms for the large radiative heat flux are explained as due to existence of hyperbolic phonon polaritons (HPPs) inside α-MoO3 and excitation of hyperbolic surface phonon polaritons (HSPhPs) at the vacuum/α-MoO3 interfaces. The effect of relative rotation between the emitter and the receiver on the heat flux is also investigated. It is found that the heat flux varies significantly with the relative rotation angle. The modulation contrast can be as large as two when the heat flux is along the [010] direction. We attribute the large modulation contrast mainly to the misalignment of HSPhPs and HPPs between the emitter and the receiver. Hence, the results obtained in this work may provide a promising way for manipulating near-field radiative heat transfer between anisotropic materials.

High-Q dark hyperbolic phonon-polaritons in hexagonal boron nitride nanostructures.

The anisotropy of hexagonal boron nitride (hBN) gives rise to hyperbolic phonon-polaritons (HPhPs), notable for their volumetric frequency-dependent propagation and strong confinement. For frustum (truncated nanocone) structures, theory predicts five, high-order HPhPs, sets, but only one set was observed previously with far-field reflectance and scattering-type scanning near-field optical microscopy. In contrast, the photothermal induced resonance (PTIR) technique has recently permitted sampling of the full HPhP dispersion and observing such elusive predicted modes; however, the mechanism underlying PTIR sensitivity to these weakly-scattering modes, while critical to their understanding, has not yet been clarified. Here, by comparing conventional contact- and newly developed tapping-mode PTIR, we show that the PTIR sensitivity to those weakly-scattering, high-Q (up to ≈280) modes is, contrary to a previous hypothesis, unrelated to the probe operation (contact or tapping) and is instead linked to PTIR ability to detect tip-launched dark, volumetrically-confined polaritons, rather than nanostructure-launched HPhPs modes observed by other techniques. Furthermore, we show that in contrast with plasmons and surface phonon-polaritons, whose Q-factors and optical cross-sections are typically degraded by the proximity of other nanostructures, the high-Q HPhP resonances are preserved even in high-density hBN frustum arrays, which is useful in sensing and quantum emission applications.

Resonant nanostructures for highly confined and ultra-sensitive surface phonon-polaritons

Plasmonics on metal-dielectric interfaces was widely seen as the main route for miniaturization of components and interconnect of photonic circuits. However recently, ultra-confined surface phonon-polaritonics in high-index chalcogenide films of nanometric thickness has emerged as an important alternative to plasmonics. Here, using mid-IR near-field imaging we demonstrate tunable surface phonon-polaritons in CMOS-compatible interfaces of few-nm thick germanium on silicon carbide. We show that Ge-SiC resonators with nanoscale footprint can support sheet and edge surface modes excited at the free space wavelength hundred times larger than their physical dimensions. Owing to the surface nature of the modes, the sensitivity of real-space polaritonic patterns provides pathway for local detection of the interface composition change at sub-nanometer level. Such deeply subwavelength resonators are of interest for high-density optoelectronic applications, filters, dispersion control and optical delay devices.