Plasmon-phonon Polaritons Research Articles

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.

(Invited) Near-Field Optical Microscopy and Spectroscopy of Nanocarbon Hybrid Materials

Plasmonic properties of novel low-dimensional materials (LDMs) are promising for a range of biosensing and information technology applications. Since optical properties of LDMs can be modulated at the ultimate scale of (a few) nanometers, the characterization of these materials requires a similar spatial resolution, not available with regular tools. Scattering-type scanning near-field optical microscope (sSNOM) allowed us to reveal such plasmonic properties of LDMs. For example, both single-wavelength and hyperspectral sSNOM imaging of graphene enables study of phonon-plasmon polariton coupling in this atomically thin layer of carbon. Here we cover several hybrid structures based on nanocarbon materials, such as graphene and single-wall nanotubes. SVR acknowledges help of Mr. M. Blades (Lehigh University) and Dr. T. Gokus (Neaspec). This research was partially supported by the UNCG Faculty First Award, the NSF ECCS-1509786, and Penn State Quantum Characterization Infrastructure initiative.

Tunable multi-wavelength absorption in mid-IR region based on a hybrid patterned graphene-hBN structure.

In this paper, we present a patterned graphene-hBN metamaterial structure and theoretically demonstrate the tunable multi-wavelength absorption within the hybrid structure. The simulation results show that the hybrid plasmon-phonon polariton modes originate from the coupling between plasmon polaritons in graphene and phonons in hBN, which are responsible for the triple-band absorption. By varying the Fermi level of graphene patterns, the absorption peaks can be tuned dynamically and continuously, and the surface plasmon-phonon polariton modes in the proposed structure enable high absorption and wideband tunability. In addition, how different structural parameters affect the absorption spectra is discussed. This work provides us a new method for the control and enhancement of plasmon-phonon polariton interactions.

Surface Polaritons in Silicon-Doped Aluminum and Gallium Nitride Films

The reflection and attenuated total reflection spectra of aluminum and gallium nitride films doped with silicon on sapphire substrates with a buffer layer of aluminum nitride have been measured. In the spectra of attenuated total reflection, surface phonon and plasmon–phonon polaritons were observed. A high concentration of charge carriers in the gallium nitride film and their practical absence in the aluminum nitride film were experimentally observed.

Enhancement of the Electron–Phonon Scattering Induced by Intrinsic Surface Plasmon–Phonon Polaritons

We investigate light-matter coupling in metallic crystals where plasmons coexist with phonons exhibiting large oscillator strength. We demonstrate theoretically that this coexistence can lead to strong light-matter interactions without external resonators. When the frequencies of plasmons and phonons are comparable, hybridization of these collective matter modes occurs in the crystal. We show that the coupling of these modes to photonic degrees of freedom gives rise to intrinsic surface plasmon-phonon polaritons, which offer the unique possibility to control the phonon properties by tuning the electron density and the crystal thickness. In particular, dressed phonons with reduced frequency and large wave vectors arise in the case of quasi-2D crystals, which leads to large enhancements of the electron-phonon scattering in the vibrational ultrastrong coupling regime. This suggests that photons can play a key role in determining the quantum properties of certain materials. A non-perturbative self-consistent Hamiltonian method is presented that is valid for arbitrarily large coupling strengths.

Anisotropic Flow Control and Gate Modulation of Hybrid Phonon-Polaritons.

Light-matter interaction in two-dimensional photonic or phononicmaterials allows for the confinement and manipulation of free-space radiation at sub-wavelength scales. Most notably, the van der Waals heterostructure composed of graphene (G) and hexagonal boron nitride (hBN) provides for gate-tunable hybrid hyperbolic plasmon phonon-polaritons (HP3). Here, we present the anisotropic flow control and gate-voltage modulation of HP3 modes in G-hBN on an air-Au microstructured substrate. Using broadband infrared synchrotron radiation coupled to a scattering-type near-field optical microscope, we launch HP3 waves in both hBN Reststrahlen bands and observe directional propagation across in-plane heterointerfaces created at the air-Au junction. TheHP3 hybridization is modulated by varying the gate voltage between graphene and Au. This modifies the coupling of continuum graphene plasmons with the discrete hBN hyperbolic phonon polaritons, which is described by an extended Fano model. This work representsthe first demonstration of the control of polariton propagation, introducing a theoretical approach to describe the breakingof the reflection and transmission symmetry for HP3 modes. Our findings augment the degree of control of polaritons in G-hBN and related hyperbolic metamaterial nanostructures, bringing new opportunities for on-chip nano-optics communication and computing.

Interaction of surface plasmon–phonon polaritons with terahertz radiation in heavily doped GaAs epilayers

We report on experimental studies of the surface plasmon-phonon polariton excitations in heavily doped GaAs epitaxial layers. Reflection and emission of radiation in the frequency range of 2–19 THz were investigated for samples with surface-relief grating, as well as for samples with planar surface. The reflectivity spectrum for p-polarized radiation measured for the sample with the surface-relief grating demonstrates a set of resonances attributed to excitations of different surface plasmon-phonon polariton modes. The observed resonances lie beyond the limits of the Reststrahlen band. Terahertz radiation emission from the samples was studied in nonequilibrium conditions under the pulsed electric field excitation. Two contributions to the spectral density of the terahertz radiation have been revealed, the first being due to bulk plasmon–phonon polaritons (PPhPs), while the second originating from the surface PPhPs. A field dependence of the effective temperature of the bulk PPhPs has been established. Polarization dependence of the terahertz radiation related to surface PPhPs has been experimentally examined for the first time.

Polarization dependent plasmonic modes in elliptical graphene disk arrays.

Plasmonic modes at mid-infrared wavelengths in elliptical graphene disk arrays were studied. Theoretically, analytical expressions for the modes and their dependence on the size, Fermi energy and the permittivity of substrate materials of the ellipses were derived. Experimentally, the elliptical graphene disks were fabricated and their plasmonic modes were characterized with the polarization-resolved extinction spectra. Both experimental and analytical results show that two electrical dipole modes, whose dipole moments are orthogonal to each other and along the major and minor axis of the ellipse respectively, exist in the elliptical disks. By adjusting the polarization directions of the incident light, the two orthogonal plasmonic modes could be excited either together or separately, showing that the optical properties of elliptical graphene disks are highly polarization dependent. By using ultraviolet illumination to change the Fermi energy of the elliptical graphene disks, the two modes can be tuned dynamically. Moreover, the highly polarization dependent modes are able to couple with the surface phonons of the substrate, leading to polarized plasmon-phonon polaritons. Thus the elliptical graphene disks can provide more degrees of freedom to design the mid-infrared polarization-resolved photonic devices.

Поверхностные поляритоны в пленках нитридов алюминия и галлия, легированных кремнием

AbstractThe reflection and attenuated total reflection spectra of aluminum and gallium nitride films doped with silicon on sapphire substrates with a buffer layer of aluminum nitride have been measured. In the spectra of attenuated total reflection, surface phonon and plasmon–phonon polaritons were observed. A high concentration of charge carriers in the gallium nitride film and their practical absence in the aluminum nitride film were experimentally observed.

Drude and Kukharskii mobility of doped semiconductors extracted from Fourier-transform infrared ellipsometry spectra

The factorized plasmon-phonon polariton description of the infrared dielectric function is generalized to include an additional factor to account for the effects of interband electronic transitions. This new formalism is superior to the usual Drude–Lorentz summation of independent oscillators, especially in materials with large transverse-longitudinal optical phonon splittings, multiple infrared-active phonon modes, or high concentrations of free carriers, if a broadband description of the dielectric function from the far-infrared to the vacuum-ultraviolet spectral region is desired. After a careful comparison of both approaches, the factorized description is applied to the dielectric function of undoped and doped semiconductors (GaAs, GaSb, and InAs) and metal oxides from 0.03 to 9.0 eV. Specifically, the authors find that both descriptions of the far-infrared dielectric function yield the same carrier density and mobility, at least for a single species of carriers. To achieve valid results for moderately high doping concentrations, measurements to lower energies would be helpful.

Surface Polaritons in GaAs/CdTe/PbTe Multilayer Structures

Infrared spectra of reflection and attenuated total reflection of PbTe thin films deposited by molecular beam epitaxy on a GaAs/CdTe substrate have been measured. The high-frequency dielectric constant, frequencies and oscillator strengths of transverse optical phonons, plasma frequencies, and plasma collision frequencies have been determined for all layers using the dispersion analysis. Attenuated total reflection spectra obtained with silicon and diamond prisms demonstrate surface phonon and plasmon–phonon polaritons. The dispersion curves of surface polaritons have been obtained.

Tunable absorption as multi-wavelength at infrared on graphene/hBN/Al grating structure.

This work designs a graphene/hBN/Al grating anisotropic hybrid structure. Formed by strong coupling between plasmonic Magnetic polaritons (MPs) in the metal grating and phonon-plasmon polaritons, hybrid hyperbolic phonon-plasmon polaritons in the graphene/hBN film have been excited, resulting in three sharp, high absorption peaks, which are 0.75, 0.97 and 0.97, formed at 5.92 μm, 6.32 μm, and 7.64 μm respectively. The absorption mechanisms have been theoretically analyzed. Local electromagnetic field and power dissipation density are depicted for further elucidating the underlying mechanisms. The different structural parameters and chemical potential, which affect the absorption peak were discussed. These numerical results can provide potential application in the field of optical detection and optoelectronic.

Compact mid-infrared broadband absorber based on hBN/metal metasurface

We propose a route to obtain broadband absorption in the mid-infrared region by coupling plasmon and phonon polaritons in compact hBN/metal metasurface. Here we show, with the assistance of the dispersion relation contours, that hybrid phonon-plasmon polaritons modes can exist in the hBN/metal heterostructures. By exciting these modes, i.e., hyperbolic phonon-plasmon polaritons in the Reststrahlen band and surface phonon-plasmon polaritons out of the Reststrahlen band of hBN, broadband near-perfect absorption can be achieved when carving the hBN/metal mutilayer into sawtooth gratings. The thickness of the gratings is about λ0/13, which is an ultra-thin metasurface compared with previous works. The electric field and power dissipation density distributions are plotted to elucidate the mechanisms of the near-perfect absorption in different regimes. Furthermore, we present a detail analysis about the influence of changing the shape of gratings on the absorption performance and quantitatively evaluate the role hBN played in the absorber. The proposed route of designing broadband absorbers will benefit many practical applications, especially in the mid-infrared range.

Acoustically-driven surface and hyperbolic plasmon-phonon polaritons in graphene/h-BN heterostructures on piezoelectric substrates

Surface plasmon polaritons in graphene couple strongly to surface phonons in polar substrates leading to hybridized surface plasmon-phonon polaritons (SPPPs). We demonstrate that a surface acoustic wave (SAW) can be used to launch propagating SPPPs in graphene/h-BN heterostructures on a piezoelectric substrate like AlN, where the SAW-induced surface modulation acts as a dynamic diffraction grating. The efficiency of the light coupling is greatly enhanced by the introduction of the h-BN film as compared to the bare graphene/AlN system. The h-BN interlayer not only significantly changes the dispersion of the SPPPs but also enhances their lifetime. The strengthening of the SPPPs is shown to be related to both the higher carrier mobility induced in graphene and the coupling with h-BN and AlN surface phonons. In addition to surface phonons, hyperbolic phonons polaritons (HPPs) appear in the case of multilayer h-BN films leading to hybridized hyperbolic plasmon-phonon polaritons (HPPPs) that are also mediated by the SAW. These results pave the way for engineering SAW-based graphene/h-BN plasmonic devices and metamaterials covering the mid-IR to THz range.

Infrared Fingerprints of Natural 2D Talc and Plasmon–Phonon Coupling in Graphene–Talc Heterostructures

Two-dimensional (2D) materials occupy noteworthy place in nanophotonics providing for subwavelength light confinement and optical phenomena dissimilar to those of their bulk counterparts. In the mid-infrared, graphene-based heterostructures and van der Waals crystals of hexagonal boron nitride (hBN) overwhelmingly concentrate the attention by exhibiting real-space nano-optics from plasmons, phonon-polaritons and hybrid plasmon phonon-polaritons quasiparticles. Here we present the mid-infrared nanophotonics of talc, a natural atomically flat layered material, and graphene-talc (G-talc) heterostructures using broadband synchrotron infrared nano-spectroscopy. We achieve wavelength tuning of the talc resonances, assigned to in- and out-of-plane vibrations by changing the thickness of the crystals, which serves as its infrared fingerprints. Moreover, we encounter coupling of the graphene plasmons polaritons with surface optical phonons of talc. As in the case of the G-hBN heterostructures, this coupling configures hybrid surface plasmon phonon-polariton modes causing 30 % increase in intensity for the out-of-plane mode, blue-shift for the in-plane mode and we have succeeded in altering the amplitude of such hybridization by varying the gate voltage. Therefore, our results promote talc and G-talc heterostructures as appealing materials for nanophotonics, like hBN and G-hBN, with potential applications for controllably manipulating infrared electromagnetic radiation at the subdiffraction scale.

Experimental study of surface plasmon-phonon polaritons in GaAs-based microstructures

Optical properties of a heavily-doped GaAs epitaxial layer with a regular grating at its surface have been experimentally investigated in the terahertz spectral range. Reflectivity spectra for the layer with a profiled surface drastically differ from those for the as-grown epilayer with a planar surface. For s-polarized radiation, this difference is totally caused by the electromagnetic wave diffraction at the grating. For p-polarized radiation, additional resonant dips arise due to excitation of surface plasmon-phonon polaritons. Terahertz radiation emission under significant electron heating in an applied pulsed electric field has also been studied. Polarization measurements revealed pronounced peaks related to surface plasmon-phonon polariton resonances of the first and second order in the emission spectra.

Surface plasmon-phonon polaritons in GaAs

We investigate surface plasmon-phonon polaritons in n-doped GaAs with the aim to obtain terahertz radiation emission. Dispersion of the surface plasmon-phonon polaritons is theoretically simulated taking into account contributions of the lattice and electron subsystems to the dielectric permittivity of n-GaAs. It is shown that regular grating at outer surface of GaAs provides a possibility to convert surface plasmon-phonon polaritons into terahertz photons and vice versa.

Near-field heat transfer between graphene/hBN multilayers

We study the radiative heat transfer between multilayer structures made by a periodic repetition of a graphene sheet and a hexagonal boron nitride (hBN) slab. Surface plasmons in a monolayer graphene can couple with a hyperbolic phonon polaritons in a single hBN film to form hybrid polaritons that can assist photon tunneling. For periodic multilayer graphene/hBN structures, the stacked metallic/dielectric array can give rise to a further effective hyperbolic behavior, in addition to the intrinsic natural hyperbolic behavior of hBN. The effective hyperbolicity can enable more hyperbolic polaritons that enhance the photon tunneling and hence the near-field heat transfer. However, the hybrid polaritons on the surface, i.e. surface plasmon-phonon polaritons, dominate the near-field heat transfer between multilayer structures when the topmost layer is graphene. The effective hyperbolic regions can be well predicted by the effective medium theory (EMT), thought EMT fails to capture the hybrid surface polaritons and results in a heat transfer rate much lower compared to the exact calculation. The chemical potential of the graphene sheets can be tuned through electrical gating and results in an additional modulation of the heat transfer. We found that the near-field heat transfer between multilayer structure does not increase monotonously with the number of layer in the stack, which provides a way to control the heat transfer rate by the number of graphene layers in the multilayer structure. The results may benefit the applications of near-field energy harvesting and radiative cooling based on hybrid polaritons in two-dimensional materials.

Electrothermal Control of Graphene Plasmon-Phonon Polaritons.

Graphene plasmons are known to offer an unprecedented level of confinement and enhancement of electromagnetic field. They are hence amenable to interacting strongly with various other excitations (for example, phonons) in their surroundings and are an ideal platform to study the properties of hybrid optical modes. Conversely, the thermally induced motion of particles and quasiparticles can in turn interact with electronic degrees of freedom in graphene, including the collective plasmon modes via the Coulomb interaction, which opens up new pathways to manipulate and control the behavior of these modes. This study demonstrates tunable electrothermal control of coupling between graphene mid-infrared (mid-IR) plasmons and IR active optical phonons in silicon nitride. This study utilizes graphene nanoribbons functioning as both localized plasmonic resonators and local Joule heaters upon application of an external bias. In the latter role, they achieve up to ≈100 K of temperature variation within the device area. This study observes increased modal splitting of two plasmon-phonon polariton hybrid modes with temperature, which is a manifestation of increased plasmon-phonon coupling strength. Additionally, this study also reports on the existence of a thermally excited hybrid plasmon-phonon mode. This work can open the door for future optoelectronic devices such as electrically switchable graphene mid-infrared plasmon sources.

Strong coupling of phonon polaritons in h-BN nanowires with graphene plasmons

The phonon polaritons in hexagonal boron nitride (h-BN) can couple with graphene plasmons, forming new hybrid modes which are called plasmon-phonon polaritons. In this paper, we point out that such coupling can occur within the strong coupling regime, where the dispersion relations of the graphene plasmons and the phonon polaritons avoid crossing with each other. We have investigated graphene-wrapped h-BN nanowires, and found that such strong couplings exist between the graphene plasmons and the fundamental as well as the high-order phonon polariton modes, even if the graphene layer is far away from the h-BN. This is interesting because the high-order phonon polariton modes are mostly confined within the bulk h-BN with little fields outside the nanowire, i.e., the interactions of these modes with the graphene plasmons should be rather weak; however, our calculations disagree with such judgements. Due to the dispersive permittivity of h-BN, the graphene plasmons can couple simultaneously with two branches of phonon polariton modes under carefully chosen electron doping levels. Our investigations may open the way to explore the plasmon-phonon polaritons in the strong coupling regime.