Surface Phonon Polaritons Research Articles

Ultrasensitive Terahertz Imaging Sensors Based on the Strong Coupling of Surface Phonon Polariton and Graphene Surface Plasmon Polariton

Surface plasmon polaritons (SPPs) with noble metals lose the capacity to confine the optical field in the long wavelength region, while surface phonon polaritons (SPhPs) in polar dielectrics show greater ability to achieve high local field, sub-diffraction confinement, and low optical losses in the mid-IR to THz spectral ranges. We propose a hybrid structure based on SPhPs mode with CsI and SPPs mode with graphene in THz spectral ranges. The characteristics of the coupling of these two modes have been analyzed and strong coupling has been confirmed theoretically. The results are applied to an intensity-interrogated terahertz sensor; the highest imaging sensitivity as high as 954 RIU−1 and a large figure of merit of 74 740 RIU−1 have been demonstrated. The results could find potential applications in gas detection in the THz band.

Oriented Asymmetric Wave Propagation and Refraction Bending in Hyperbolic Media

Crystal quartz is a well-known anisotropic medium with optically active phonons in the THz region where hyperbolic phonon-polaritons can be excited. Here, we use this material to illustrate how the behavior of bulk and surface hyperbolic polaritons can be drastically modified by changing the orientation of the crystal's anisotropy axis with respect to its surface. We demonstrate, both theoretically and experimentally, phenomena associated with the orientation of hyperbolic media. We show the consequences of slight changes in the crystal's orientation in various ways, from the creation of hyperbolic surface phonon-polaritons to the demonstration of oriented asymmetric transmission of radiation passing through a hyperbolic medium.

Apparent Spectral Shift of Thermally Generated Surface Phonon-Polariton Resonance Mediated by a Nonresonant Film

Near-field radiative heat transfer between dissimilar materials is important in thermal management and energy-harvesting, but an in-depth physical analysis is needed. Using fluctuational electrodynamics, this study shows that the spectral redshift or blueshift of thermally generated surface phonon-polaritons (SPhPs) can be mediated by a nonresonant layer. The apparent shift is due to multiple reflections within the subwavelength gap separating resonant and nonresonant layers, which generate gap modes and decrease thermal emission around the SPhP resonance---valuable insight for applications based on near-field thermal radiation, such as thermophotovoltaics and thermal rectification.

Surface-phonon-polariton-mediated photon response of terahertz quantum-well infrared photodetectors

A surface-phonon-polariton (SPHP)-mediated photon response near the longitudinal optical (LO) phonon frequency of GaAs is investigated in a terahertz GaAs/AlGaAs quantum-well infrared photodetector, integrated with a one-dimensional metal grating. For a contrast device without a grating coupler, no SPHP-related signature is found in the photocurrent spectrum, because the incident radiation from free space cannot excite the SPHP, due to the mismatch of momentum. The intensity of the electric field component along the growth direction of the device absorption layer is numerically calculated. The results show that the photocurrent response peaks near the LO phonon frequency band (8.8–9.0 THz) are attributed to the local field enhancement induced by SPHP. Our results are useful to realize high performance SPHP-mediated terahertz photodetectors and other related terahertz devices.

Stored electromagnetic energy and radiated power by spherical thermal micro/nano-emitters: Radiation Q-factor

We examine theoretically (based on fluctuational electrodynamics) the stored electromagnetic energy and radiated power by a spherical thermal emitter. Moreover, we propose a measure for the radiation quality Q-factor for a polychromatic source such as a thermal emitter. We consider that the thermal emitter is made of tungsten and silicon carbide and we study in detail how the aforementioned radiative quantities depend on size and temperature of the antenna. Particularly, we obtain the emissivity (ratio of spectral radiated power by emitter and that by an ideal blackbody), and the spectral density of stored electromagnetic energy normalized with respect to that of a blackbody. For a SiC-emitter, the emissivity and the normalized spectral stored density energy exhibit multiple narrow peaks associated with surface phonon-polariton and whispering gallery mode resonances; while for a W-antenna, high ohmic losses yield broad spectra of the aforementioned quantities (only an interband resonance appears). These spectra depend strongly on particle size; the corresponding unnormalized spectra are perturbed by the temperature. There is an optimal size of the antenna for which the emissivity is maximal; the aforementioned materials yield an emissivity larger than one. The radiation Q-factor decreases as particle radius and temperature increase, implying that the radiation efficiency of a thermal emitter increases as these parameters grow. The Q-factor for a W-antenna is smaller than that for a SiC-antenna (same size and temperature) for nano-scale particles, whereas, for micrometric particles, the opposite happens over a certain temperature range. Our work might have implications for infrared sources and thermophotovoltaics.

Near-field radiative heat transfer enhancement using natural hyperbolic material

Hyperbolic materials have been newly discovered that can enhance near-field radiative transfer. Multilayer and three-dimensional hyperbolic materials require challenging fabrication techniques. From a practical point of view, natural hyperbolic materials serve better in applications requiring near-field radiative transfer enhancement. In this study, we investigate enhancement of near-field radiative transfer using natural hyperbolic material – calcite, and compare with that from (also natural) polar dielectric materials, SiC, where phonon polaritons are responsible for enhanced near-field heat transfer. Spectral analyses of enhanced near-field radiative transfer from hyperbolic modes in hyperbolic materials and surface phonon polaritons in polar dielectric materials are carried out and compared, which determine the origin and amount of near-field radiative transfer enhancement in these materials.

Controlling nanoscale air-gaps for critically coupled surface polaritons by means of non-invasive white-light interferometry

We report an experimental method to control large-area air-gaps in the nanometer range for evanescent coupling of light to surface waves such as surface plasmon polaritons or surface phonon polaritons. With the help of spectrally resolved white-light interferometry, we are able to stabilize and tune the gap with nanometer precision and high parallelism. Our technique is non-invasive and leaves the coupling area unobstructed, and the setup delivers reference-free real-time readout up to a distance of 150 μm between the coupling prism and the sample. Furthermore, we demonstrate the application to prism coupled surface polariton excitation. The active gap control is used to determine the dispersion of a critically coupled surface phonon polariton over a wide spectral range in the mid infrared region.

Ghost surface phononic polaritons in ionic-crystal metamaterial

Ghost electromagnetic waves were recently predicted in anisotropic transparent materials by E. Narimanov. In this paper, three ghost surface phonon polaritons (GSPhPs) are predicted at the single oblique surface of layered ionic-crystal metamaterial (ICMM). They propagate along the surface, but their electromagnetic fields in the ICMM sinusoidally oscillate and exponentially attenuate with the distance from the surface. The three GSPhPs are mutually different in character. An additional unique GSPhP is found on the boundary between the effective metal and hyperbolic regions of the ICMM, and its characteristics are as follows: (1) its energy can only conduct outside the ICMM; (2) its wave vector and electric field possess the same direction transverse to the material optical axis. The conditions for the existence of the GSPhPs are discussed. The numerical results were obtained based on ZnS/SiO2 metamaterials.

Sub-wavelength field enhancement in the mid-IR: photonics versus plasmonics versus phononics.

The ability to concentrate the electrical field into sub-wavelength volumes is a key benefit sought and, to a certain degree, found within the discipline of plasmonics. This ability is restricted only by the ohmic loss in noble metals and, recently, in the infrared region, metals are beginning to face a challenge from emerging alternative media: phononic (i.e., relying on surface phonon polaritons) and photonic (i.e., relying on high refractive index) all-dielectric structures, and highly doped semiconductors, all of them having smaller intrinsic loss than metals. In this Letter, we compare the degree of enhancement and its spectral selectivity for different media and confirm that, while one can obtain sharper resonant features with all-dielectric structures, the magnitude of the field enhancement is invariably higher with metals such as gold and copper, primarily due to a higher density of electrons. On the whole, depending on the application, metals and dielectrics have their own unique advantages.

Tailoring far-infrared surface plasmon polaritons of a single-layer graphene using plasmon-phonon hybridization in graphene-LiF heterostructures

Being one-atom thick and tunable simultaneously, graphene plays the revolutionizing role in many areas. The focus of this paper is to investigate the modal characteristics of surface waves in structures with graphene in the far-infrared (far-IR) region. We discuss the effects exerted by substrate permittivity on propagation and localization characteristics of surface-plasmon-polaritons (SPPs) in single-layer graphene and theoretically investigate characteristics of the hybridized surface-phonon-plasmon-polaritons (SPPPs) in graphene/LiF/glass heterostructures. First, it is shown how high permittivity of substrate may improve characteristics of graphene SPPs. Next, the possibility of optimization for surface-phonon-polaritons (SPhPs) in waveguides based on LiF, a polar dielectric with a wide polaritonic gap (Reststrahlen band) and a wide range of permittivity variation, is demonstrated. Combining graphene and LiF in one heterostructure allows to keep the advantages of both, yielding tunable hybridized SPPPs which can be either forwardly or backwardly propagating. Owing to high permittivity of LiF below the gap, an almost 3.2-fold enhancement in the figure of merit (FoM), ratio of normalized propagation length to localization length of the modes, can be obtained for SPPPs at 5–9 THz, as compared with SPPs of graphene on conventional glass substrate. The enhancement is efficiently tunable by varying the chemical potential of graphene. SPPPs with characteristics which strongly differ inside and around the polaritonic gap are found.

Tunable and angle-independent thermal emitter based on surface phonon polariton mode in the mid-infrared range

A tunable and angle-independent thermal emitter which consists of silicon carbide (SiC) gratings incorporated with Ge2Sb2Te5 (GST) phase change materials (PCMs) in the mid-infrared (MIR) range has been proposed and investigated. The reflectivity under different conditions is simulated by the rigorous coupled wave analysis (RCWA) method. It is shown that a large shift of reflection dip is attainable within the Reststrahlen band of SiC (10.288 μm-12.563 μm) for transverse magnetic (TM) polarization light, when GST is changed between the crystalline and amorphous states. The deep reflection dips appeared for the two states of GST are attributed to the excitation of surface phonon plaritions (SPhPs). In addition, the incident angle independence of wavelength for both states of GST is obtained, which is a major feature of the tunable thermal emitter.

Hybrid Quantum System with Nitrogen-Vacancy Centers in Diamond Coupled to Surface-Phonon Polaritons in Piezomagnetic Superlattices

We investigate a hybrid quantum system where an ensemble of nitrogen-vacancy (NV) centers in diamond is interfaced with a piezomagnetic superlattice that supports surface phonon polaritons (SPhPs). We show that the strong magnetic coupling between the collective spin waves in the NV spin ensemble and the quantized SPhPs can be realized, thanks to the subwavelength nature of the SPhPs and relatively long spin coherence times. The magnon-polariton coupling allows different modes of the SPhPs to be mapped and orthogonally stored in different spatial modes of excitation in the solid medium. Because of its easy implementation and high tunability, the proposed hybrid structure with NV spins and piezoactive superlattices could be used for quantum memory and quantum computation.

Theoretical analysis of graphene plasmon cavities

Unprecedented physical properties make graphene a very promising plasmonic candidate material from the terahertz to mid-infrared frequencies. However, a theoretical model for graphene plasmon cavities has not been fully established. In this work, we analyse the graphene surface plasmon polaritons supported on silicon carbide or metallic cavities. We derive an analytical expression for the dispersion relation of graphene plasmon waves in a multilayer system, providing a useful tool to illustrate the dependence on the cavity's geometric properties. The results show that the analytical description can precisely predict the excitation of cavity graphene plasmon waves. Complete absorption can be achieved under certain parameters, which can be predicted precisely using two Fabry–Pérot models. We further show that the interaction of surface plasmon polaritons with surface phonon polaritons can be used to tune the cavity resonances. The tunability of the Fermi energy and the geometric parameters of the cavities make for flexible system design. High enhancement (∼4000) and extraordinary compression (∼λ0/400) of graphene plasmon waves are also realized under certain conditions. These findings make this an ideal setup for molecular sensing, pushing the potential for enhanced, broadly tunable spectroscopy into the mid-infrared, while also offering distinct advantages for integrated optics.

Cavity-mediated interactions between the guided mode resonance and surface phonon polaritons in a hybrid photonic–phononic system

A strong mode coupling between the guided mode resonance (GMR) and surface phonon polaritons (SPhPs) is numerically demonstrated. The photonic–phononic mode coupling can be mediated by a homogeneous spacer which works as a Fabry–Pérot (FP) cavity, and its strength, manifested by mode splitting and bending of dispersion curves, can be tuned flexibly by adjusting the thickness of the FP cavity. This mode-coupling effect can be used to control the near-field localization sites in this hybrid structure, which provide useful implications for manipulating infrared electromagnetic near fields in nanophotonics devices.

The near-field radiative heat transfer between graphene/SiC/hBN multilayer structures

We propose a multilayer structure alternately stacked by graphene, silicon carbide (SiC) film, and hexagonal boron nitride (hBN) bulk to theoretically study the near-field radiative heat transfer. Compared with blackbody result, the heat transfer coefficient (HTC) can be significantly enhanced due to the effect of surface plasmons (SPs), surface phonon polaritons (SPhPs), and hyperbolic modes (HMs) supported by graphene, SiC, and hBN, respectively. HTC between the proposed structures with thin SiC can even be larger than that of graphene-covered hBN. Since SPs could be coupled both with HMs supported by the hBN and with SPhPs supported by SiC, HTC can be flexibly modulated by chemical potential of graphene. In addition, HTC of another structure which is composed with graphene, hBN film, and SiC bulk in order is also investigated in detail. The impacts of the thickness of components and vacuum gap on HTCs for both configurations are also studied. The results in this study are helpful to control near-field radiative heat transfer.

RETRACTED ARTICLE: Singular Resonance in Fluctuation-Induced Electromagnetic Phenomena at the Rotation of a Nanoparticle near the Surface of a Condensed Medium

It is shown that in fluctuation-electromagnetic phenomena (Casimir force, Casimir friction, radiative heat generation) for a spherical nanoparticle with a radius $R$ rotating near a surface a singular resonance can occur, near which fluctuation-electromagnetic effects are strongly enhanced even in the presence of dissipation in the system. The resonance takes place at the particle-surface separation $ d <d_0= R[3/4\varepsilon_1''(\omega_1)\varepsilon_2''(\omega_2)]^{1/3}$ (where $\varepsilon_i''(\omega_i)$ is the imaginary part of the dielectric function of a particle or a medium at the surface plasmon or phonon polariton frequency $\omega_i$), when the rotation frequency $\Omega$ coincides with the poles in the photon generation rate at $\Omega\approx \omega_1 + \omega_2$. These poles arise due to the multiple scattering of electromagnetic waves between the particle and the surface under the conditions of the anomalous Doppler effect and they exist even in the presence of dissipation. For $ d <d_0$ in the dependence on the particle rotation frequency the Casimir force can change sign, i.e. the attraction of the particle to the surface is replaced by the repulsion. The obtained results can be important for nanotechnology.

Strong Coupling of Epsilon-Near-Zero Phonon Polaritons in Polar Dielectric Heterostructures.

We report the first observation of epsilon-near-zero (ENZ) phonon polaritons in an ultrathin AlN film fully hybridized with surface phonon polaritons (SPhP) supported by the adjacent SiC substrate. Employing a strong coupling model for the analysis of the dispersion and electric field distribution in these hybridized modes, we show that they share the most prominent features of the two precursor modes. The novel ENZ-SPhP coupled polaritons with a highly propagative character and deeply subwavelength light confinement can be utilized as building blocks for future infrared and terahertz nanophotonic integration and communication devices.

Thermometry with Subnanometer Resolution in the Electron Microscope Using the Principle of Detailed Balancing.

We measure phonon energy gain and loss down to 20 meV in a single nanostructure using an atom-wide monochromatic electron beam. We show that the bulk and surface, energy loss and energy gain processes obey the principle of detailed balancing in nanostructured systems at thermal equilibrium. By plotting the logarithm of the ratio of the loss and gain bulk/surface scattering as a function of the excitation energy, we find a linear behavior, expected from detailed balance arguments. Since that universal linearity scales with the inverse of the nanosystem temperature only, we can measure the temperature of the probed object with precision down to about 1 K without reference to the nanomaterial. We also show that subnanometer spatial resolution (down to ∼2 Å) can be obtained using highly localized acoustic phonon scattering. The surface phonon polariton signal can also be used to measure the temperature near the nanostructure surfaces, but with unavoidable averaging over several nanometers. Comparison between transmission and aloof probe configurations suggests that our method exhibits noninvasive characteristics. Our work demonstrates the validity of the principle of detailed balancing within nanoscale materials at thermal equilibrium, and it describes a transparent method to measure nanoscale temperature, thus representing an advance in the development of a noninvasive method for measurements with angstrom resolution.

Tomographic and multimodal scattering-type scanning near-field optical microscopy with peak force tapping mode

Scattering-type scanning near-field optical microscopy (s-SNOM) enables nanoscale spectroscopic imaging and has been instrumental for many nano-photonic discoveries and in situ studies. However, conventional s-SNOM techniques with atomic force microscopy tapping mode operation and lock-in detections do not provide direct tomographic information with explicit tip−sample distance. Here, we present a non-traditional s-SNOM technique, named peak force scattering-type scanning near-field optical microscopy (PF-SNOM), by combination of peak force tapping mode and time-gated light detection. PF-SNOM enables direct sectioning of vertical near-field signals from a sample surface for both three-dimensional near-field imaging and spectroscopic analysis. Tip-induced relaxation of surface phonon polaritons are revealed and modeled by considering tip damping. PF-SNOM also delivers a spatial resolution of 5 nm and can simultaneously measure mechanical and electrical properties together with optical near-field signals. PF-SNOM is expected to facilitate three-dimensional nanoscale near-field characterizations and correlative in situ investigations on light-induced mechanical and electrical effects.

Far Infrared Synchrotron Near-Field Nanoimaging and Nanospectroscopy

Scattering scanning near-field optical microscopy (s-SNOM) has emerged as a powerful imaging and spectroscopic tool for investigating nanoscale heterogeneities in biology, quantum matter, and electronic and photonic devices. However, many materials are defined by a wide range of fundamental molecular and quantum states at far-infrared (FIR) resonant frequencies currently not accessible by s-SNOM. Here we show ultrabroadband FIR s-SNOM nanoimaging and spectroscopy by combining synchrotron infrared radiation with a novel fast and low-noise copper-doped germanium (Ge:Cu) photoconductive detector. This approach of FIR synchrotron infrared nanospectroscopy (SINS) extends the wavelength range of s-SNOM to 31 μm (320 cm–1, 9.7 THz), exceeding conventional limits by an octave to lower energies. We demonstrate this new nanospectroscopic window by measuring elementary excitations of exemplary functional materials, including surface phonon polariton waves and optical phonons in oxides and layered ultrathin van der W...