Surface Phonon Polariton Modes Research Articles

Enhancing mid-infrared surface phonon polariton modes: optimization of structural parameters in Nb-doped SrTiO3 with hole array structures for advanced infrared sensors

This study utilized the finite difference time domain method to investigate the mid infrared surface phonon polaritons and localized surface phonon resonances in undoped and niobium (Nb)-doped SrTiO3 (STO) with planar and holes array structures. Research has shown that Nb-doped STO operates in the Reststrahlen band of 8.06–18.48 µm, providing a wider spectral response than undoped STO (12.58–18.26 µm) and effectively covering the atmospheric window of long wave infrared. This indicates that the increase in virtual permittivity has the least impact on spectral broadening, indicating that the new infrared sensor technology has broad prospects. The optimization of structural parameters, including the period, filling factor, and depth of STO holes array, as well as the response to changes in incident light angle, is crucial for guiding the design of high-performance optoelectronic devices. In addition, this study explored the excitation of four resonant modes within a holes array and analyzed their relationship with array parameters to enhance the design of optoelectronic applications.

Lifetime and Molecular Coupling in Surface Phonon Polariton Resonators.

Surface phonon polariton (SPhP) modes in polar semiconductors offer a low-loss platform for infrared nanophotonics and sensing. However, the efficient design of polariton-enhanced sensors requires a quantitative understanding of how to engineer the frequency and lifetime of SPhPs in nanophotonic structures. Here, we study organ-pipe resonances in 4H-SiC trenches as a prototype system for infrared sensing. We use a transmission line framework that accounts for the field distribution within the trench, accurately predicting mode frequency and lifetime when compared against finite element method (FEM) electromagnetic calculations. Accounting for the electric field profile across the gap is critical in our model to accurately predict mode frequencies, quality factor (Q factor), and reflectance, outperforming previous circuit models developed in the literature. Beyond structural simulation, our model can provide insights into the frequency ranges in the Reststrahlen band where enhanced sensor activity should be present. The radiative lifetime is significantly enlarged close to the longitudinal optic phonon, restricting sensor efficiency at this wavelength range. This pushes the optimal frequency for sensing closer to the center of the Reststrahlen band than might be naively expected. This model ultimately demonstrates the primary challenge of designing SPhP-based sensors: only a relatively narrow region of the Reststrahlen band offers efficient sensing, guiding future designs for infrared spectroscopy.

Ghost surface polaritons in naturally uniaxial hyperbolic materials

We investigated ghost surface phonon polaritons localized in a naturally uniaxial hyperbolic material, where the longitudinal axis (optical axis) is at any angle with respect to the surface plane. Two ghost surface phonon polaritons (GSPs) were found in the two Reststrahlen frequency bands, respectively. They are two TM surface waves, and their electromagnetic fields not only attenuate but also oscillate with the distance away from the surface. Unlike conventional surface phonon polaritons, GSPs are virtual surface phonon-polariton modes without electrostatic limit. The numerical simulations of the GSP dispersion and polarization properties were based on the hexagonal boron nitride, and the numerical simulation of the attenuated total reflection (ATR) experiment demonstrates the existence of the GSPs and prove their excitability and observability.

Transition from Phononic to Geometrical Mie Modes Measured in Single Subwavelength Polar Dielectric Spheres

Spherical dielectric resonators are highly attractive for light manipulation, thanks to their intrinsic electric and magnetic resonances. Here, we present measurements of the mid-infrared far-field thermal radiation of single subwavelength dielectric spheres deposited on a gold substrate, of radii ranging from 1 to 2.5 μm, which agree quantitatively with simulated absorption cross sections. For SiO2 microspheres, we evidence the excitation of both surface phonon-polariton (SPhP) modes and geometrical electric and magnetic Mie modes. The transition from a phonon-mode-dominated to a Mie-mode-dominated emission spectrum is observed, with a threshold radius of ∼1.5 μm. We also show that the presence of the metallic substrate augments the computed spheres absorption cross-section due to increased local field enhancement, arising from the near-field interaction of the spheres oscillating charges with their image in the metallic mirror. In contrast, measurements of single subwavelength SPhP-inactive PTFE spheres reveal that the mid-infrared response of such lossy spheres is dominated by their bulk absorption. Our results demonstrate how engineering the geometrical and dielectric properties of subwavelength scatterers can enable the control of thermal emission near room temperature, with exciting perspectives for applications such as radiative cooling.

Surface Phonon Polariton Modes in Suspended Monolayer Hexagonal Boron Nitrides

Dispersion properties and characteristics of transverse magnetic (TM) and transverse electric (TE) surface phonon polaritons (SPhP) in suspended monolayer hexagonal boron nitrides (hBN) was studied extensively. The analytical results show that the hBN based TM (TE) phonon polaritons exist in restsrahlen bands when imaginary surface conductivity is positive. The effective mode index of TM phonon polaritons is much higher than that of TE phonon polaritons with respective values of ~3000 and ~1.0002 which makes TM SPhP more promising in the practical realization. In addition, the propagation length of TE SPhP is less lossy and surpass that of TM SPhP by factor of 104. This study compares these important properties and sheds more insight into their applications in optical communications, photonics and optoelectronics devices.

Photonic Thermal Rectification with Composite MetamaterialsSupported by the National Natural Science Foundation of China (Grant Nos. 11774252 and 92050104), the Qing Lan Project, and the PAPD of Jiangsu Higher Education Institutions.

We demonstrate strong photonic thermal rectification effect between polar dielectrics plate and the composite metamaterials containing nonspherical polar dielectric nanoparticles with small volume fractions. Thermal rectification efficiency is found to be adjusted by the volume fractions and the nanoparticles' shape, and it can be as large as 80% when the polar dielectric nanoparticles are spherical in shape and are in the dilute limit with the volume fraction f = 0.01. Physically, there exists strong electromagnetic coupling between the surface phonon polariton mode of polar dielectrics plate and the localized surface phonon polariton mode around polar dielectric nanoparticles. The results provide alternative new freedom for regulating energy flow and heat rectification efficiency in the near field, and may be helpful for design of multiparameter adjustable thermal diodes.

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.

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.

High Q-factor controllable phononic modes in hybrid phononic–dielectric structures

Phonon polariton resonances in the mid-infrared spectral range demonstrate properties superior to noble metal-based plasmonics, owing to smaller dissipative loss and better field confinement. However, a conventional way to excite the localized phonon resonance involves ion etching, which reduces the attainable quality factors (Q-factors) of the resonators. We show that by introducing a deep subwavelength layer of dielectric gratings on a phononic substrate, localized dipolar resonance and higher order modes with high Q-factors 96 and 195, respectively, can be excited. We further demonstrate, via experiments and simulations, that the resonant wavelength and field confinement can be controlled by coupling the localized hybrid mode with propagating surface phonon-polaritons. We also observed for the first time the coupling between a localized dipolar mode and a propagating higher-order surface phonon-polariton mode. The results will be useful in designing on-chip, low-loss, and highly integrated phononic devices in the infrared spectral domain.

Hybrid slot-waveguide fed antenna using hexagonal boron nitride D'yakonov polaritons.

Polaritonic slot waveguides have been explored as a means of manipulating nanoscale fields to compete in the race for the sub-diffractional confinement of light. Hexagonal boron nitride (h-BN), when incorporated into hyperbolic-insulator-hyperbolic (HIH) configurations, is a strong contender, with its naturally occurring anisotropy allowing it to strongly confine and enhance local fields. However, while the volumetric phonon polaritons of h-BN have been widely used for these means, its hyperbolic surface phonon polaritons (HSPhPs) or D'yakonov polaritons contain untapped potential and are widely unused. In this paper, we qualitatively discuss the hybridization of fundamental hyperbolic surface phonon polariton modes in an HIH slot waveguide. The resulting symmetric dark, or lower mode, is then used to design a patch antenn, which shows possibilities for applying the familiar microstrip transmission-line approach of antenna design to this HSPhP antenna.

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.

Excitation of long-wavelength surface optical vibrational modes in films, cubes and film/cube composite system using an atom-sized electron beam.

Using spatially resolved Electron Energy-Loss Spectroscopy, we investigate the excitation of long-wavelength surface optical vibrational modes in elementary types of nanostructures: an amorphous SiO2 slab, an MgO cube, and in the composite cube/slab system. We find rich sets of optical vibrational modes strongly constrained by the nanoscale size and geometry. For slabs, we find two surface resonances resulting from the excitation of surface phonon polariton modes. For cubes, we obtain three main highly localized corner, edge, and face resonances. The response of those surface phonon resonances can be described in terms of eigenmodes of the cube and we show that the corresponding mode pattern is recovered in the spatially resolved EELS maps. For the composite cube/substrate system we find that interactions between the two basic structures are weak, producing minor spectral shifts and intensity variations (transparency behaviour), particularly for the MgO-derived modes.

Harmonicity and anharmonicity of phonon and surface phonon-polariton in high symmetry directions in wurtzite AlN

We report on the potential of self-nucleated AlN single crystals as tunable near-field infrared sources. A self-nucleated AlN crystal was grown with appropriate care to ensure minimal contact with crucible walls or other crystals. The grown crystal exhibits natural AlN growth characteristics with several well-developed facets of different orientations. The characteristics of surface-phonon-polariton (SPhP) modes on the developed crystal facets have been investigated. Reflectivity spectra were recorded from five facets of different orientations. The measured spectra were analyzed by a model taking into account the dependence of harmonicity and anharmonicity of the excited zone center optical phonons on the surface orientation. Consequently, the dielectric properties that determine the condition of existence, dispersion relations, and lifetimes of the SPhP modes were accurately retrieved. The dielectric functions were determined as a function of the angle of incidence and used to compute the characteristics of the SPhP modes on each of the measured facets. We found that facets of different orientations exhibit SPhP modes of different frequencies and lifetimes, which makes the investigated self-nucleated crystal potential candidates for tunable near-field infrared sources.

Optical characterization of polar HfO2 nanoparticles in the mid- and far-infrared

Monoclinic HfO2 nanoparticles with nominal diameters of 9, 30, and 45 nm are characterized using transmission and reflection spectroscopy from the mid- to far-infrared. Phonon modes are identified in the measured spectra and agree with previously reported values in HfO2 thin films and density functional perturbation theory calculations. An anomaly in both reflection and transmission is observed at 556 cm−1 that is not attributed to the optical phonon modes. Numerical models predict a localized surface phonon polariton mode near this frequency, and we attribute the anomaly to coupling to this surface mode. The results of this work suggest that HfO2 nanoparticles could enable engineered optical properties in new mid- and far-infrared materials and devices.

Collective near-field thermal emission from polaritonic nanoparticle arrays

The spectral characteristics of near-field thermal emission from nanoparticle arrays are explained by comparison to the dispersions for propagating modes. Using the coupled dipole model, we analytically calculate the spectral emission from single particles, chains, planes, and 3D arrays of SiO2 and SiC. We show that the differences in their spectra are due to the existence or absence of propagating surface phonon polariton modes and that the emission is dominated by these modes when they are present. This work paves the way for understanding and control of near-field radiation in nanofluids, nanoparticle beds, and certain metamaterials.

Effects of charges on the localized surface phonon polaritons in dielectric nanoparticles

Recently, there has been a great deal of interest in studying the surface phonon polariton (SPhP) modes in polar dielectrics due to their potential applications in the mid-infrared (MIR) spectral range. However, the effects of charges on the SPhP modes have not been discussed up to now. In fact, exotic electrons and electron–phonon (EPh) interactions widely exist in many nanostructures in practice. Here, we theoretically investigate effects of electrons and EPh interactions on the localized SPhPs in polar dielectric nanoparticles within the framework of Mie theory. Our results show that electrons have different effects on the localized SPhP modes in the nanostructures with different electron affinity. For particles with negative electron affinity, such as the MgO case, exotic electrons only lead to blueshifts of the SPhPs resonances with little effect on the magnitudes of resonance absorptions and electric near-field enhancements. While for particles with positive electron affinity, such as the Al2O3 case, except for blueshifts of the resonances, EPh interactions decrease the resonance absorptions largely, and almost annihilate multiple resonance modes into only one in multi-sphere nanostructures. Also, EPh terms reduce the electric near-fields dramatically to values far less than that of particles with no exotic electrons. Our results are helpful to the applications of polar dielectrics in nanoscale within the MIR spectral range.

Excitation of surface phonon polariton modes in gold gratings with silicon carbide substrate and their potential sensing applications

We demonstrate the excitation of surface phonon polaritons (SPhPs) in the mid-infrared (mid-IR) Reststrahlen band (10.288μm-12.563μm) on patterned surfaces with silicon carbide (SiC) substrate and gold (Au) gratings. The very large negative permittivity of Au limits its applications in the mid-IR range, and to couple incident light to SPhPs modes, their momentum mismatch can be compensated by patterning Au grating onto the surface of SiC substrate. Samples were fabricated and characterized experimentally by Fourier transform infrared reflection (FTIR) spectroscopy. The optical properties were also simulated by the rigorous coupled wave analysis (RCWA) method. Reflection dips are observed for light polarized vertical to the grating lines (TM-polarized), which are attributed to the coupling of electromagnetic (EM) waves into the SPhP modes. In addition, we present small-volume index sensing with analyte specificity based on mid-IR SPhPs in the fabricated configuration.

Strong Coupling of Rydberg Atoms and Surface Phonon Polaritons on Piezoelectric Superlattices.

We propose a hybrid quantum system where the strong coupling regime can be achieved between a Rydberg atomic ensemble and propagating surface phonon polaritons on a piezoelectric superlattice. By exploiting the large electric dipole moment and long lifetime of Rydberg atoms as well as tightly confined surface phonon polariton modes, it is possible to achieve a coupling constant far exceeding the relevant decay rates. The frequency of the surface mode can be selected so that it is resonant with a Rydberg transition by engineering the piezoelectric superlattice. We describe a way to observe the Rabi splitting associated with the strong coupling regime under realistic experimental conditions. The system can be viewed as a new type of optomechanical system.

Hybrid Optical–Thermal Antennas for Enhanced Light Focusing and Local Temperature Control

Metal nanoantennas supporting localized surface plasmon resonances have become an indispensable tool in bio(chemical) sensing and nanoscale imaging applications. The high plasmon-enhanced electric field intensity in the visible or near-IR range that enables the above applications may also cause local heating of nanoantennas. We present a design of hybrid optical-thermal antennas that simultaneously enable intensity enhancement at the operating wavelength in the visible and nanoscale local temperature control. We demonstrate a possibility to reduce the hybrid antenna operating temperature via enhanced infrared thermal emission. We predict via rigorous numerical modeling that hybrid optical-thermal antennas that support high-quality-factor photonic-plasmonic modes enable up to two orders of magnitude enhancement of localized electric fields and of the optical power absorbed in the nanoscale metal volume. At the same time, the hybrid antenna temperature can be lowered by several hundred degrees with respect to its all-metal counterpart under continuous irradiance of 104-105 W/m2. The temperature reduction effect is attributed to the enhanced radiative cooling, which is mediated by the thermally-excited localized surface phonon polariton modes. We further show that temperature reduction under even higher irradiances can be achieved by a combination of enhanced radiative and convective cooling in hybrid antennas. Finally, we demonstrate how hybrid optical-thermal antennas can be used to achieve strong localized heating of nanoparticles while keeping the rest of the optical chip at low temperature.