Hybrid Plasmon-phonon Research Articles

Plasmon-Phonon Hybridization in Doped Semiconductors from First Principles.

Although plasmons and phonons are the collective excitations that govern the low-energy physics of doped semiconductors, their nonadiabatic hybridization and mutual screening have not been studied from first principles. We achieve this goal by transforming the Dyson equation to the frequency-independent dynamical matrix of an equivalent damped oscillator. Calculations on doped GaAs and TiO_{2} agree well with available Raman data and await immediate experimental confirmation from infrared, neutron, electron-energy-loss, and angle-resolved photoemission spectroscopies.

EXCITATION OF WAKE SURFACE PLASMON-PHONON OSCILLATIONS BY A RELATIVISTIC ELECTRON BUNCH IN A POLAR SEMICONDUCTOR WAVEGUIDE

The process of surface wakefields excitation by the relativistic electron bunch in a polar semiconductor waveguide with a vacuum channel for electron bunch propagation is studied. The spectra and spatio-temporal structure of the excited surface wakefield in such system are investigated. It is shown that the excited full surface wakefield in the terahertz and infrared frequency ranges consists of the fields of LF and HF hybrid plasmon-phonon oscillations. The amplitudes of them are determined.

Nanoscale infrared imaging and spectroscopy of hot-electron plasmons in graphene

We report a nanoscale infrared (IR) imaging and spectroscopy study of hot-electron plasmons in graphene, which are excited by the sharp metallic probe of the scattering-type scanning near-field optical microscope (s-SNOM) illuminated with a mid-IR femtosecond pulsed laser. We found the average electron temperature $({T}_{e})$ can reach as high as $\ensuremath{\sim}1400\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ within the pulse duration, which can be controlled by tuning the laser power. With s-SNOM, we monitored both the plasmon interference fringes and the hybrid plasmon-phonon resonances of graphene. When graphene is heavily doped, a higher ${T}_{e}$ leads to a smaller plasmon wavelength and a weaker plasmon-phonon resonance intensity. At the charge neutrality point, on the other hand, the plasmon-phonon resonance intensity is enhanced when ${T}_{e}$ increases. With quantitative modeling and theoretical analysis, we concluded that the observed plasmonic responses of hot electrons are governed by the temperature dependencies of chemical potential, electron scattering, and thermal carrier generation. The competition of these factors leads to distinct ${T}_{e}$ dependence of graphene plasmons at various doping levels.

Second harmonic generation from grating-coupled hybrid plasmon–phonon polaritons

Polaritons can provide strong optical field enhancement allowing them to boost light–matter interaction. Here, we experimentally observe enhancement in mid-infrared second-harmonic generation (SHG) using grating-coupled surface phonon polaritons of the 6H-SiC surface. In our experiment, we measure the SHG along the polariton dispersion by changing the incidence angle of the excitation beam. We observe hybridization between the propagating surface phonon polaritons and localized plasmon resonances in the gold grating, evidenced by the modification of the polariton dispersion as we change the area ratio of grating and substrate. Design options for engineering the plasmon–phonon polariton hybridization are discussed. Overall, we find a rather low yield of polariton-enhanced SHG in this geometry compared to prism-coupling and nanostructures and discuss possible origins.

Electrically controlled molecular fingerprint retrieval with van der Waals metasurface

Polaritons in two-dimensional van der Waals (vdW) materials possess extreme light confinement, which have emerged as a potential platform for next-generation biosensing and infrared spectroscopy. Here, we propose an ultra-thin and electric tunable graphene/hexagonal boron nitride/graphene metasurface for detecting molecular fingerprints over a broad spectrum. The vdW metasurface supports hybrid plasmon–phonon polariton resonance with high-quality factor (Q > 120) and electrically controlled broadband spectra tunability from 6.5 to 7 μm. After coating a thin layer of bio-molecular (e.g., CBP) on top of the metasurface, the molecular absorption signatures can be readout at multiple spectral points and, thus, achieve broadband fingerprint retrieval of bio-molecules. Additionally, our electric tunable metasurface works as an integrated graphene-based field-effect transistor device, without the need of multiple resonance generators such as angle-resolved or pixelated dielectric metasurfaces for broadband spectra scanning, thereby paving the way for highly sensitive, miniaturized, and electrically addressed biosensing and infrared spectroscopy.

Stopping and image forces acting on a charged particle moving near a graphene- Al2O3 -graphene heterostructure

We study the impact of plasmon-phonon hybridization on the stopping and image forces acting on a charged particle moving parallel to a graphene-${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$-graphene sandwichlike composite system by considering a broad domain in the parameter space. The effective dielectric function of the system is obtained using two descriptions of the electronic response of doped graphene: an ab initio method based on the time-dependent density functional theory calculations and an analytical expression based on the massless Dirac fermion (MDF) approximation for graphene $\ensuremath{\pi}$ bands. It is found that the main discrepancies between the two methods come from the high-energy interband electron transitions, which are included in the ab initio method but not in the MDF method. Special attention is paid to the regime of low-particle speeds, where the MDF method compares well with the ab initio method, but the modeling is sensitive to the effects of finite temperature and the treatment of phenomenological damping. It is observed at low particle speeds that both forces exhibit an interesting interplay between the hybrid modes of the phononic type and the continuum of the intraband electron-hole excitations in graphene. Furthermore, the effects of the lowest-energy antisymmetric modes, which exhibit acoustic dispersions at long wavelengths in a system with equally doped graphene layers, are exposed in calculations of the stopping force on a co-moving pair of incident particles with opposite charges that are symmetrically positioned around the target system.

EXCITATION OF WAKE FIELDS BY A RELATIVISTIC ELECTRON BUNCH IN A POLAR SEMICONDUCTOR WAVEGUIDE

The process of wake fields excitation by a relativistic electron bunch in polar semiconductors is studied. Cylindrical semiconductor waveguide, in which relativistic electron bunch moves along axis, is considered. It is shown that the excited wake field in the terahertz and infrared frequency ranges consists of the field of longitudinal HF and LF hybrid plasmon-phonon oscillations and the field of the HF and LF transverse polaritons, which are a set of eigen electromagnetic waves of the polar semiconductor waveguide. The spatio-temporal structure of the total excited wake field is obtained, the intensity of the excited wake waves is determined.

Surface plasmons induce topological transition in graphene/α-MoO3 heterostructures

Polaritons in hyperbolic van der Waals materials—where principal axes have permittivities of opposite signs—are light-matter modes with unique properties and promising applications. Isofrequency contours of hyperbolic polaritons may undergo topological transitions from open hyperbolas to closed ellipse-like curves, prompting an abrupt change in physical properties. Electronically-tunable topological transitions are especially desirable for future integrated technologies but have yet to be demonstrated. In this work, we present a doping-induced topological transition effected by plasmon-phonon hybridization in graphene/α-MoO3 heterostructures. Scanning near-field optical microscopy was used to image hybrid polaritons in graphene/α-MoO3. We demonstrate the topological transition and characterize hybrid modes, which can be tuned from surface waves to bulk waveguide modes, traversing an exceptional point arising from the anisotropic plasmon-phonon coupling. Graphene/α-MoO3 heterostructures offer the possibility to explore dynamical topological transitions and directional coupling that could inspire new nanophotonic and quantum devices.

Actively tunable hybrid plasmon-phonon polariton modes in ferroelectric/graphene heterostructure systems at low-THz frequencies

Attributed to hyperbolic phonon polaritons (HPPs) of ferroelectric materials such as lithium tantalate (LiTaO3) and lithium niobate (LiNbO3) in low-THz frequencies (<20 THz), the ferroelectric/graphene heterostructure systems are put forward to obtain actively tunable hybrid plasmon-phonon polariton modes. When graphene layers are added in ferroelectric-based heterostructures, tunable hybrid plasmon-phonon polariton modes are obtained by the hybridization of ferroelectric phonon polaritons and graphene plasmons, and these characteristics also lead to dramatic regulation of spontaneous emission (SE) rates. Moreover, SE rates in ferroelectric/graphene heterostructure systems greatly rely on the ferroelectric thickness, graphene chemical potential and the emitter distance from the heterostructure. This work provides a meaningful instruction for actively tunable hybrid plasmon-phonon polariton modes and SE rate regulation in low-THz frequencies.

Plasmon-Phonon Coupling in Electrostatically Gated β-Ga2O3 Films with Mobility Exceeding 200 cm2 V-1 s-1.

Monoclinic β-Ga2O3, an ultra-wide bandgap semiconductor, has seen enormous activity in recent years. However, the fundamental study of the plasmon-phonon coupling that dictates electron transport properties has not been possible due to the difficulty in achieving higher carrier density (without introducing chemical disorder). Here, we report a highly reversible, electrostatic doping of β-Ga2O3 films with tunable carrier densities using ion-gel-gated electric double-layer transistor configuration. Combining temperature-dependent Hall effect measurements, transport modeling, and comprehensive mobility calculations using ab initio based electron-phonon scattering rates, we demonstrate an increase in the room-temperature mobility to 201 cm2 V-1 s-1 followed by a surprising decrease with an increasing carrier density due to the plasmon-phonon coupling. The modeling and experimental data further reveal an important "antiscreening" (of electron-phonon interaction) effect arising from dynamic screening from the hybrid plasmon-phonon modes. Our calculations show that a significantly higher room-temperature mobility of 300 cm2 V-1 s-1 is possible if high electron densities (>1020 cm-3) with plasmon energies surpassing the highest energy LO mode can be realized. As Ga2O3 and other polar semiconductors play an important role in several device applications, the fundamental understanding of the plasmon-phonon coupling can lead to the enhancement of mobility by harnessing the dynamic screening of the electron-phonon interactions.

Gate tunable light–matter interaction in natural biaxial hyperbolic van der Waals heterostructures

Abstract The recent discovery of natural biaxial hyperbolicity in van der Waals crystals, such as α-MoO3, has opened up new avenues for mid-IR nanophotonics due to their deep subwavelength phonon polaritons. However, a significant challenge is the lack of active tunability of these hyperbolic phonon polaritons. In this work, we investigate heterostructures of graphene and α-MoO3 for actively tunable hybrid plasmon phonon polariton modes via electrostatic gating in the mid-infrared spectral region. We observe a unique propagation direction dependent hybridization of graphene plasmon polaritons with hyperbolic phonon polaritons for experimentally feasible values of graphene chemical potential. We further report an application to tunable valley quantum interference in this system with a broad operational bandwidth due to the formation of these hybrid modes. This work presents a lithography-free alternative for actively tunable, anisotropic spontaneous emission enhancement using a sub-wavelength thick naturally biaxial hyperbolic material.

Tunable phonon-plasmon hybridization in α-MoO3-graphene based van der Waals heterostructures.

The plasmon-phonon hybridization behavior between anisotropic phonon polaritons (APhP) of orthorhombic phase Molybdenum Trioxide (α - MoO3) and the plasmon-polaritons of Graphene layer - forming a van der Waals (vdW) heterostructure is investigated theoretically in this paper. It is found that in-plane APhP shows strong interaction with graphene plasmons lying in their close vicinity, leading to large Rabi splitting. Anisotropic behavior of biaxial MoO3 shows the polarization-dependent response with strong anti-crossing behavior at 0.55 eV and 0.3 eV of graphene's Fermi potential for [100] and [001] crystalline directions, respectively. Numerical results reveal unusual electric field confinement for the two arms of enhanced hybrid modes: the first being confined in the graphene layer representing plasmonic-like behavior. The second shows volume confined zigzag pattern in hyperbolic MoO3. It is also found that the various plasmon-phonon hybridized modes could be wavelength tuned, simply by varying the Fermi potential of the graphene layer. The coupling response of the hybrid structure is studied analytically using the coupled oscillator model. Furthermore, we also infer upon the coupling strength and frequency splitting between the two layers with respect to their structural parameters and interlayer spacing. Our work will provide an insight into the active tunable property of hybrid van der Waals (vdW) structure for their potential application in sensors, detectors, directional spontaneous emission, as well as for the tunable control of the propagating polaritons in fields of flat dispersion where strong localization of photons can be achieved, popularly known as the flatband optics.

Exact dispersion relations for the hybrid plasmon-phonon modes in graphene on dielectric substrates with polar optical phonons.

Intrinsic optical phonons and extrinsic polar optical phonons (POPs) strongly affect the graphene surface plasmons. Specifically, extraneous POPs present on the surface of an underlying substrate change the behavior of the graphene's surface plasmons sharply due to the plasmon-phonon hybridization. Here, we report modeling of exact dispersion relations for graphene's surface plasmons affected by intrinsic optical phonons and extrinsic POPs of the surface of polar dielectric substrates with one or more vibrational frequencies. In doing so, we have employed random phase approximation with modified two-dimensional polarizability (2D-Π0). The adapted Π0 addresses limitations of the previously derived plasmons dispersion, obtained using classical two-dimensional polarizability. We show the new model overcomes the unsatisfying behavior of the plasmonic dispersion relation obtained by the classical 2D-Π0 at high-wavenumbers and its inability to indicate the starting point of the mode damping. Our new simple model eliminates the complexity of the other presented models in describing the surface plasmons' behavior, specifically at high wavenumbers. Besides, we use our dispersion model to learn about the plasmon content of the hybrid modes, which is a vital value to compute output current in plasmonic graphene-based devices. The coupled-mode lifetime due to the hybrid nature depends on both plasmon and phonon lifetimes. We capture this value here. There is an excellent agreement between our theoretical results and the experimental data reported earlier. They pave the way for the exact modeling of graphene plasmons on common polar substrates and bring in the closeness of the theoretical approaches and experimental results.

The Effects of Pseudomagnetic Fields on Plasmon–Phonon Hybridization in Supported Graphene Probed by a Moving Charged Particle

We analyze the effects of the strain-induced pseudomagnetic field on the subthreshold mechanism of hybridization taking place between the Dirac plasmon in graphene and the surface optical phonon modes in a nearby substrate. It is shown that the pseudomagnetic field exerts quite strong influence on the oscillatory pattern in the total potential in the plane of graphene, as well as on the stopping and the image forces on a charge, which moves parallel to the graphene at a speed below the Fermi velocity, specially for small graphene–substrate gap sizes. One may conclude that the subthreshold mechanism of the plasmon–phonon hybridization can be controlled by varying the pseudomagnetic field strength and the doping density in graphene.

Plasmon coupling in topological insulator multilayers

Topological insulators (TIs) house two-dimensional Dirac plasmons on their surfaces. In TI thin films, plasmons on the top and bottom surfaces couple electrostatically, giving rise to an acoustic and an optical plasmon mode. By extension, a superlattice comprising TI layers and trivially insulating layers could house multiple complex plasmon modes which could be used to create a new type of Dirac metamaterial. In this paper, we synthesize TI superlattices, fabricate them into stripe arrays, characterize their optical and plasmonic properties, and model the samples using transfer matrices. We excite plasmon modes that couple to the phonons in the superlattice, resulting in hybrid plasmon-phonon polaritons. These modes are modeled using an analytical Fano resonance model and the extracted resonant positions are reproduced by transfer matrix modeling. We also excite an epsilon near-zero mode in the top dielectric material ${({\mathrm{Bi}}_{0.5}{\mathrm{In}}_{0.5})}_{2}{\mathrm{Se}}_{3}$. Understanding the behavior of the polariton modes in this complex system will lend insight into the many-body interaction in low-dimensional systems.

Coupled plasmon–phonon modes in monolayer MoS2

We present a theoretical study of the plasmon–phonon coupling in a suspended monolayer MoS2 and a MoS2 substrate system using a diagrammatic self-consistent field theory. The four coupled plasmon–phonon modes and the four plasmon-surface phonon modes are observed due to the spin–orbit and electron-optic phonon interactions. The two of coupled plasmon–phonon and plasmon-surface phonon modes are optic-like and the other two are acoustic-like. The plasmon are strongly coupled with the optic-phonon in MoS2 and the surface optic-phonon in the substrates as the electron density or wave-vector increases. The strong plasmon–phonon coupling shows that the optoelectronic properties of monolayer MoS2 are evidently modulated by electron–phonon interactions. The hybrid plasmon–phonon polaritons can be achieved by strong light-mater interactions. This study is relevant to the application of MoS2 as novel plasmonic and nanophotonic devices.

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.

Observation of plasmon-phonons in a metamaterial superconductor using inelastic neutron scattering

Metamaterial approach is capable of drastically increasing the critical temperature, Tc, of composite metal-dielectric superconductors. Tripling of Tc was observed in bulk Al-Al2O3 core-shell metamaterials. A theoretical model based on the Maxwell-Garnett approximation provides a microscopic explanation of this effect in terms of electron-electron pairing mediated by a hybrid plasmon-phonon excitation. We report the first observation of this excitation in Al-Al2O3 core-shell metamaterials using inelastic neutron scattering. This result provides support for this novel mechanism of superconductivity in metamaterials and explains the 50 year old mystery of enhanced Tc in granular aluminium films.

Optimizing encapsulated graphene in hexagonal boron nitride toward low propagation loss and enhanced field confinement

In this paper, we propose and numerically demonstrate a new type of highly confined low-loss mid-infrared hybrid plasmon–phonon mode by incorporating graphene with hexagonal nitride (hBN) above a dielectric substrate. Here, we have conducted the plasmonic properties of two hybrid plasmon–phonon waveguides which consist of an hBN–graphene–hBN on substrate (hGhS) and a graphene–hBN–graphene–hBN on substrate (GhGhS). It is found that Fermi energy of graphene (Ef) and thickness of hBN (H) are two key parameters in this study. Comparing the modal properties of GhGhS and hGhS waveguides, it is found that the change in H and Ef results in the following benefits: (i) for an ultrathin hBN layer (H=1 nm), the Re(neff) of the GhGhS waveguide shows a ninefold enhancement as compared with the hGhS waveguide, leading to a strong field confinement of the plasmon–phonon edge mode in the GhGhS waveguide. However, by increasing H, low propagation loss, reasonable field confinement, and high optical performance can be realized simultaneously in the GhGhS waveguide. (ii) By increasing Ef from 0.2 to 1 eV, the propagation length (Lm) and figure of merit of the GhGhS waveguide show nearly threefold and twofold enhancement at 1550 cm−1 as compared with the hGhS waveguide, respectively, resulting in a larger Lm and a better optical performance. These results confirm that the GhGhS waveguide not only improves the balance between confinement and propagation loss but also provides strong coupling and hybridization of the plasmon–phonon mode of graphene with hBN.

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.