Plasmon-phonon Coupling Research Articles

Enhanced phonon-plasmon interaction in film-coupled dimer nanoridges mediated by surface acoustic waves

The interaction between phonons and localized plasmons in coupled nanoparticles can be exploited both for modulating the scattered electromagnetic field and the understanding of the mechanical vibrations at nanoscale. In this paper, we demonstrate by numerical analysis an enhanced optomechanical interaction in a film-coupled gold dimer nanoparticles mediated by surface acoustic waves. Two gold nanoridges are placed atop a multilayer structure consisting of a thin dielectric spacer covering a gold film layer on a silicon dioxide substrate. Numerical simulations of the optical properties reveal the existence of three surface localized plasmons in the infrared range with enhanced scattering and narrower linewidths than with a single nanoridge. The physical origin of such modes as well as their tunability as function of key geometrical parameters are successfully captured with a simple model based on effective Metal-Insulator-Metal (MIM)-like plasmonic cavity. We calculate the optomechanic coupling rates between the GHz localized mechanical modes and plasmonic modes of the dimer, finding that the strongest coupling is observed for the in-phase compressional mode followed by the out-of-phase flexural mode. Both such modes can be excited by launching a surface acoustic wave (Sezawa wave) at the inlet in front of the dimer structure. It is also found that the flexural mode which is inactive optomechanically in case of a monomer becomes active due to dimer coupling, with a significant phonon-plasmon coupling rate. The findings in this work may facilitate design of new optomechanical components monitored with fast coherent acoustics, leading to new generation of light acousto-optic modulators where strong optomechanical interactions are required.

Tunable coupling of terahertz Dirac plasmons and phonons in transition-metal dichalcogenide-based van der Waals heterostructures

Dirac plasmons in graphene hybridize with phonons of transition metal dichalcogenides (TMDs) when the materials are combined in so-called van der Waals heterostructures (vdWh), thus forming surface plasmon-phonon polaritons (SPPPs). The extend to which these modes are coupled depends on the TMD composition and structure, but also on the plasmons’ properties. By performing realistic simulations that account for the contribution of each layer of the vdWh separately, we calculate how the strength of plasmon-phonon coupling depends on the number and composition of TMD layers, on the graphene Fermi energy and the specific phonon mode. From this, we present a semiclassical theory that is capable of capturing all relevant characteristics of the SPPPs. We find that it is possible to realize both strong and ultra-strong coupling regimes by tuning graphene’s Fermi energy and changing TMD layer number.

Comparison between optical and electrophysical data on free electron concentration in n-InAs samples

A theoretical model has been developed for determining free electron concentration in n-InAs from characteristic points in far infrared region of reflection spectra. We show that when determining free electron concentration one should take into account the plasmon-phonon coupling, otherwise free electron concentration will be overestimated. We have calculated electron concentration, Nopt, as a function of characteristic wave number, ν+, which is described by a third order polynomial. Twenty one n-InAs samples (5-doped with tin and 16-doped with sulfur) have been tested at room temperature for electron concentration using two methods, i.e., the conventional four-probe (Van der Pau) method (NHall) and the optical method developed by us (Nopt). The reflective surfaces of investigated samples were processed either with chemical-mechanical polishing or treating with short-grained abrasive powder. It was shown that for all the investigated samples the condition Nopt > NHall was relevant. The difference between optical and electrophysical electron concentration values has been shown to be greater in case of chemically polished reflective surface of the sample and smaller in case of abrasive-treated one. The experimental results have been compared with the same data previously obtained for n-GaAs samples. Qualitative model has been suggested to explain obtained experimental data.

(Invited) Vertically Stacked Junction Devices Fabricated Using Single-Crystal Graphene on SiC Substrate

Graphene is a potential post-silicon nanomaterial because of its tremendous properties. Single-crystal graphene epitaxially grown on a SiC substrate is the most promising candidate for future electron devices. Single-crystal graphene with a 100 mm2 size has been successfully fabricated using a high-temperature rapid thermal annealing system [1]. In this study, two types of stacked graphene devices using graphene on SiC are demonstrated. The first device is a bi-crystal system consisting of two graphene samples as shown in Fig. 1 (a). These samples are directly bonded in a face-to-face manner. Contact pins are attached to the graphene surface for ohmic contact. The electrical characteristics of the stacked graphene junction were ohmic at a low bias voltage below 20 V. The linear characteristic at a low bias voltage is attributed to the linear dispersion relation in the band structure of graphene, Dirac cone. At a high bias voltage, the conductance of the graphene junction drastically increased. This nonlinear behavior originates from a semi-metallic property far from the Dirac point. Thermal imaging results indicated that electrical power was directly converted to infrared radiation. The radiation efficiency was estimated to be approximately 10%, which was high compared to that of a previously reported graphene blackbody emitter. The infrared emission spectrum measured using FT-IR was blackbody-like [2]. The peak wavelength of the measured spectrum was almost constant at 10.2 µm with changing applied power. The estimated junction voltage was approximately 5 V at a bias voltage of 80 V. Hot electrons were injected to the graphene layer. The results suggested that infrared emissions from the graphene–graphene stacked junction could be attributed to a nonthermal emission mechanism, such as phonon–plasmon coupling. The other vertically stacked graphene junction is a tunneling diode as shown in Fig. 1 (b). An insulative layer is sandwiched between two graphene samples. A structured water layer [3] formed by deionized water treatment on the epitaxial graphene sample acts as a tunneling barrier. Nonlinear I–V characteristics were observed. The dynamic range of the current change reached the sixth order of magnitude. The estimated thickness of the structured water layer was approximately 0.5 nm. As the tunneling barrier was sufficiently thin, a direct tunneling phenomenon could be observed in a low bias regime. In a high bias regime, the Fowler–Nordheim tunneling phenomenon was observed. A sandwich structure of a vertically stacked graphene junction with a sub-nm-thick structured water layer was constructed through simple water treatment and a direct bonding technique [4]. [1] T. Aritsuki, et al., Jpn. J. Appl. Phys. 55 (2016) 06GF03. [2] N. Murakami, et al., Jpn. J. Appl. Phys. 60 (2021) SCCD01. [3] M. Kitaoka, et al., Jpn. J. Appl. Phys. 56 (2017) 085102. [4] J. Du, et al., Jpn. J. Appl. Phys. 58 (2019) SDDE01. Figure 1

Comparison between results of optical and electrical measurements of free electron concentration in n-InAs specimens

A theoretical model has been developed for determining the free electron concentration in n-InAs specimens from characteristic points in far IR reflection spectra. We show that this determination requires plasmon-phonon coupling be taken into account, otherwise the measured electron concentration proves to be overestimated. A correlation between the electron concentration Nopt and the characteristic wavenumber ν+ has been calculated and proves to be well fit by a third order polynomial. The test specimens have been obtained by tin or sulfur doping of indium arsenide. The electron concentration in the specimens has been measured at room temperature using two methods: the optical method developed by the Authors (Nopt) and the conventional four-probe Hall method (the Van der Pau method, NHall). The reflecting surfaces of the specimens have been chemically polished or fine abrasive ground. The condition Nopt > NHall has been shown to hold for all the test specimens. The difference between the optical and the Hall electron concentrations is greater for specimens having polished reflecting surfaces. The experimental data have been compared with earlier data for n-GaAs. A qualitative model explaining the experimental data has been suggested.

Extraction of carrier concentration and mobility of ZnO by mid-infrared reflectance spectroscopy

As a representative of the third-generation semiconductors, ZnO has great development potential in the fields of optics and electricity, so the measurement on its electrical properties is quite important. Here, we try to extract the carrier concentration and mobility of ZnO by mid-infrared reflectance spectroscopy. Considering that the longitudinal optical phonon-plasmon coupling (LOPC) mode formed by the coupling of ZnO optical phonons and free carriers depends on carrier concentration and mobility, we conduct a test on the MIR spectra (400-1600 cm−1) of ZnO crystals with different carrier concentration (1018-1015 cm−3). By fitting the strong reflection peak shape corresponding to phonon-polaritons formed by the coupling of its LOPC and infrared light through m-CDF model, the carrier concentration and mobility of these samples are obtained, which are consistent with those got from the Hall-effect measurement.

Graphene-based mid-infrared photodetectors using metamaterials and related concepts

Graphene, a semi-metal with a gapless band structure, has been used in mid-infrared (MIR) photodetectors (PDs) for some time. However, these detectors often suffer from low responsivity due to the intrinsically low absorption and ultra-short carrier lifetime in graphene, large dark current, and low detectivity due to the semi-metallic nature of graphene. Over the past decade, much effort has been devoted to addressing these issues. A variety of metamaterials and related concepts has been employed to improve the detector responsivity by enhancing the graphene absorption and/or the carrier collection efficiency. Here, we provide an overview of the graphene MIR PDs with and without the use of approaches for responsivity enhancement. We focus our attention on the state-of-the-art graphene MIR PDs whose performance is improved by employing metamaterials and related concepts, including band structure engineering, the photogating effect, integration with plasmonic nanostructures and waveguides, the use of asymmetric plasmons, coupled plasmon–phonon polaritons, and small-twist-angle bilayer graphene. We conclude by providing possible directions for further performance improvement of graphene MIR PDs and a discussion on future applications of these detectors.

Imaging Strongly Coupled Plasmon–Phonon Modes in Mid-Infrared Double Antennas

Strong-coupling effects alter the polaritonic states of metal–dielectric nanostructures and, therefore, can be used to tailor the response of infrared (IR) photonic elements. We report on the devel...

Comparison of the results of optical and electrophysical measurements of free electron density in n-GaAs samples doped with tellurium

A theoretical model has been developed that allows one to determine free electron density in n-GaAs from the characteristic points on far-infrared reflection spectra. It was shown that, in this case, it is necessary to take into account the plasmon-phonon coupling (otherwise, the electron density is overestimated). The calculated dependence of electron density, Nopt, on the characteristic wave number, ν+, which is described by a second degree polynomial, has been obtained.Twenty-five tellurium-doped gallium arsenide samples were used to measure the electron density in two ways: according to traditional four-contact Hall method (Van der Pauw method) and using the optical method we developed (measurements were carried out at room temperature). Based on the experimental results, the dependence was constructed of the electron density values obtained from the Hall data, NHall, on the electron density obtained by the optical method, Nopt. It is shown that this dependence is described by linear function. It is established that the data of optical and electrophysical measurements coincide if the electron density is Neq = 1.07 ⋅ 1018 cm-3, for lower values of the Hall density NHall < Nopt, and for large values NHall > Nopt. A qualitative model is proposed to explain the results. It has been suggested that tellurium atoms bind to vacancies of arsenic into complexes, as a result of which the electron density decreases. On the surface of the crystal, the concentration of arsenic vacancies is lower and, therefore, the condition Nopt > NHall should be satisfied. As the doping level increases, more and more tellurium atoms remain electrically active, so electron density in the volume begins to prevail over the surface one. However, with a further increase in the doping level, the ratio NHall/Nopt again decreases, tending to unity. This, probably, is due to the fact that the rate of decomposition of the complexes “tellurium atom + arsenic vacancy” decreases with increasing doping level.

Optical Characterization of the Structural and Electrical Properties of the n-GaP Nanolayers Grown on Conductive (001) n-GaP Substrates

The results of investigations of the structural and electrical properties of a homoepitaxial nanoscale (001) n-GaP layer of 70 nm in thickness grown by metalorganic vapor phase epitaxy on a conductive highly doped substrate of an n-GaP crystal oriented along (001) axis are reported. In the Raman spectrum of such an n-GaP nanolayer in the n-GaP/(001) n-GaP sample, as compared to the spectrum of the high-ohmic crystalline (001) si-GaP sample, we have managed to discover two quite narrow lines attributed to vibrations of transverse TO(Γ)– phonons and high-frequency longitudinal coupled plasmon–phonon LO(Γ)+ vibrations. It is established that the spectral parameters of the LO(Γ)+ vibrations in both the n-GaP nanolayer and the (001) n-GaP substrate are significantly different from each other and from the spectral parameters of the line of longitudinal optical LO(Γ)– phonons. Analysis of the revealed strict quantitative features of the spectral parameters has made it possible to obtain valuable data on the degree of perfection of the crystal structure for the nanoscale homoepitaxial (001) n-GaP layer. In addition, it is shown that numerical calculations based on the microscopic model of light scattering by LO(Γ)+ vibrations, caused by the mechanisms of the deformation potential and electro-optical scattering, has enabled noncontact and nondestructive determination of the concentration n and mobility μ of free charge carriers in the nanoscale-thick homoepitaxial layer and the conductive highly doped substrate. The obtained values have proved to be as follows: the concentration nhepi = (3.25 ± 0.1) × 1017 cm–3 for and the mobility μhepi = (40.0 ± 0.1) cm2 V–1 s–1 for the homoepitaxial (001) n-GaP layer, against nsubs = (2.52 ± 0.1) × 1017 cm–3 and μsubs = (51.0 ± 0.1) cm2 V–1 s–1 for the (001) n-GaP substrate of the n-GaP/(001) n-GaP sample.

Ultrastrong plasmon–phonon coupling via epsilon-near-zero nanocavities

Vibrational ultrastrong coupling (USC), where the light-matter coupling strength is comparable to the vibrational frequency of molecules, presents new opportunities to probe the interactions of molecules with zero-point fluctuations, harness cavity-enhanced chemical reactions, and develop novel devices in the mid-infrared regime. Here we use epsilon-near-zero nanocavities filled with a model polar medium (SiO$_2$) to demonstrate USC between phonons and gap plasmons. We present classical and quantum mechanical models to quantitatively describe the observed plasmon-phonon USC phenomena and demonstrate a splitting of up to 50% of the resonant frequency. Our wafer-scale nanocavity platform will enable a broad range of vibrational transitions to be harnessed for USC applications.

Investigation of THz Radiation from Longitudinal Optical Phonon- Plasmon Coupling in p-i-n Diodes

Background: Terahertz radiation (THz) in infrared domain at room temperature has many applications in science and technology, especially in the technology of analyzing and processing image. This paper introduces a relatively simple method to investigate the Terahertz radiation from the coupling of Longitudinal Optical (LO) phonon and coherent plasmon in p-i-n diode structure. The frequency spectra are found from the Fast Fourier Transform (FFT) of the voltage between two neighboring points in the insulating region of this diode. Numerical calculations have been applied for GaAs semiconductor device with the photo-excited carrier density ranging from 17 1.0×10 cm-3 to 18 3.0×10 cm-3 and the insulating layer size of 500 nm. Methods: In order to study of the coupling of LO phonon in p-i-n diode based on the EMC simulations, the equation of density vibration is solved simultaneously with the simulation of carrier dynamics, this is performed with spatial resolution of 1A° and time resolution of 0.20 fs . We solve the Poisson’s equation to derive the potentials along the x-axis in both cases with and without taking into account the coupling. The frequency spectrum derived from the FFT of the voltage of two layers separated by a distance 10 nm in insulating layer (i). Results: The frequency spectrum derived from the Fourier transform of the voltage between two neighboring points with and without LO phonon–plasmon coupling is shown. We can easily observe the existence of the modes which are close to the frequency values of bulk semiconductor. It should be noted that, our calculations are reasonable agreement with experiments measured by the Ibanez et al in Phys. Rev. B 69 (7), 075314 (2004). Conclusion: In this paper, we present a relatively simple approach to investigate the THz radiation from the coupling of LO phonon-plasmon in p-i-n diode structure by taking the FFT of the voltage of two neighboring points in insulating semiconductor region. The voltage is calculated through the electric potentials which relate to the charge density via Poisson’s equation. Numerical calculations have been performed for GaAs semiconductor device with carrier density ranging from 17 1.0×10 cm-3 to 18 3.0×10 cm-3. Our simulations calculations show that with appropriate photoexcited carrier density, two strong coupling LO phonon-plasmon coherent modes are appear.

Probing the structure and composition of van der Waals heterostructures using the nonlocality of Dirac plasmons in the terahertz regime

Dirac plasmons in graphene are very sensitive to the dielectric properties of the environment. We show that this can be used to probe the structure and composition of van der Waals heterostructures (vdWh) put underneath a single graphene layer. In order to do so, we assess vdWh composed of hexagonal boron nitride and different types of transition metal dichalcogenides (TMDs). By performing realistic simulations that account for the contribution of each layer of the vdWh separately and including the importance of the substrate phonons, we show that one can achieve single-layer resolution by investigating the nonlocal nature of the Dirac plasmon-polaritons. The composition of the vdWh stack can be inferred from the plasmon-phonon coupling once it is composed by more than two TMD layers. Furthermore, we show that the bulk character of TMD stacks for plasmonic screening properties in the terahertz regime is reached only beyond 100 layers.

Adjusting transmissivity based on grapheme–h-BN–graphene heterostructure as a tunable phonon–plasmon coupling system in mid-infrared band

Phonon polaritons in van der Waals materials show the strong electric confinement and low energy loss. Combining with the materials hexagonal boron nitride, hyperbolic phonon polaritons can be excited in two RS bands, which changes a lot in electric field distribution and transmission characteristics. In order to increase the tunability of hyperbolic phonon polaritons, a structure based on graphene/h-BN heterostructure has been formed in previous studies. In this paper, the electric field distribution and transmissivity characteristics are calculated and discussed based on grapheme–h-BN–graphene heterostructure. The results illustrate the excellent transmissivity tunability in 780 cm−1 incident wavenumber of type-I band and poor transmissivity tunability in 1368 cm−1 of type-II band. In addition, superior field-enhanced characteristic (115.76% transmissivity in 6 V external voltage) is shown in the structure. These excellent properties make such structure a promising component in future optoelectronic devices design.

Plasmon-phonon coupling between mid-infrared chiral metasurfaces and molecular vibrations.

Plasmon-phonon coupling between metamaterials and molecular vibrations provides a new path for studying mid-infrared light-matter interactions and molecular detection. So far, the coupling between the plasmonic resonances of metamaterials and the phonon vibrational modes of molecules has been realized under linearly polarized light. Here, mid-infrared chiral plasmonic metasurfaces with high circular dichroism (CD) in absorption over 0.65 in the frequency range of 50 to 60 THz are demonstrated to strongly interact with the phonon vibrational resonance of polymethyl methacrylate (PMMA) molecules at 52 THz, under both left-handed and right-handed circularly polarized (LCP and RCP) light. The mode splitting features in the absorption spectra of the coupled metasurface-PMMA systems under both circular polarizations are studied in PMMA layers with different thicknesses. The relation between the mode splitting gap and the PMMA thickness is also revealed. The demonstrated results can be applied in areas of chiral molecular sensing, thermal emission, and thermal energy harvesting.

Impact of Synthesis Technique on Lanthanum Doped Copper Oxide Nanocrystals, an Alternative Counter Electrode for Pt-Free Dye-Sensitized Solar Cells.

The ascendancy of a counter electrode (C.E) in a dye-sensitized solar cell (DSSC) has an unrivaled impact on the performance of the dye-sensitized solar cell, where the Pt-based C.E has set up a phenomenal benchmark against its counterparts. Due to the high cost of such noble metals, an operative and a much cost effective replacement were greatly in demand to gratify this need. To address this issue, monoclinic lanthanum doped copper oxide (CuO) nanoparticles were prepared by solution combustion and co-precipitation techniques respectively as they possess highly superior optoelectronic, catalytic and charge transfer properties. The effect of incorporating lanthanum (La) ions and the variation of synthesis technique on their structural, optical, photo-thermal and morphological properties are empirically investigated by XRD, FTIR, TEM, UV-Vis, Raman and Photoacoustic Spectroscopy (PAS). The X-ray diffraction pattern revealed the formation of La-doped CuO nanoparticles. The finger-print vibrational modes of the as-synthesized samples are confirmed by the FTIR spectra. The required optical properties of the as-synthesized nanocrystals were confirmed using UV-Vis absorption spectra, from which the bandgap was determined by Kubelka-Munk plot. The Raman modes were determined using Raman spectra, further a multi-phonon band was revealed, which was formed due to the plasmon-phonon coupling. The photothermal phenomenon was revealed using the photoacoustic spectra. The morphological investigation by TEM found the change in the morphology of the material as the synthesis route is varied. The power conversion efficiency results unveiled that the combustion-derived C.E posted a promising efficiency (η) of 0.20% and the co-precipitation derived C.E posted an efficiency of 0.02%. Overall results suggest that the combustion derived La doped CuO nanostructures exhibited substantive properties with deeper implication and also stood out to be a viable, cost-effective and self-exemplifying replacement for Pt as C.E in DSSC's.

Low-loss two-dimensional plasmon modes in antimonene

The effects of spin-orbit (SOC) and electron-phonon coupling on the collective excitation of doped monolayer Sb$_2$ are investigated using density functional and many-body perturbation theories. The spin-orbit coupling is exclusively important for the monolayer Sb$_2$ and it leads to the reconstruction of the electronic band structure. In particular, plasmon modes of monolayer Sb$_2$ are quite sensitive to the SOC and are characterized by very low damping rates owing to small electron-phonon scatterings. Our results show plasmons in antimonene are significantly less damped compared to monolayer graphene when plasmon energies are $\hbar \omega> 0.2$ eV due to smaller plasmon-phonon coupling in the former material.

Surface Acoustic Waves-Localized Plasmon Interaction in Pillared Phononic Crystals

We study numerically the interaction of surface acoustic waves propagating at the surface of a multilayer structure (the so-called Sezawa waves) with localized surface plasmons at the bottom of metallic pillars deposited on the substrate. The pillars are made of gold and the multilayer structure is constituted by a layer of gold on a silicon substrate and covered by a thin spacer of dielectric material. We are interested in localized plasmons, which are the analog of metal-dielectric-metal surface plasmons. The physical characteristics of these plasmons, such as their eigenvalues and absorption or reflection spectra, are modulated by surface acoustic waves due to deformation of the structure in the vicinity of the pillars. The surface waves are generated in the area in front of the pillars deposited on the substrate. Two types of acoustic modes are envisioned to interact with the localized plasmons, namely, confined modes associated with the local resonances of the pillars and the propagating Sezawa modes that deform the structure in the vicinity of the surface. We discuss the efficiency of phonon-plasmon coupling for both types of modes and select those that can be useful for an experimental realization based on this platform. Also, we show that, besides the plasmon frequency, the phonon-plasmon coupling magnitude strength is sensitive to the distance between the pillars, which reveals the role of interactions between the pillars on the coupling. The geometrical parameters are chosen such that the acoustic waves are in the sub-GHz range and the plasmons are around the telecommunication wavelength of 1.55 mu m. This work may help with the design of optomechanical devices that can be controlled by fast coherent acoustics and facilitate applications involving enhanced sound-light interactions, such as light modulation.

Tailored Nanoscale Plasmon-Enhanced Vibrational Electron Spectroscopy.

Atomic vibrations and phonons are an excellent source of information on nanomaterials that we can access through a variety of methods including Raman scattering, infrared spectroscopy, and electron energy-loss spectroscopy (EELS). In the presence of a plasmon local field, vibrations are strongly modified and, in particular, their dipolar strengths are highly enhanced, thus rendering Raman scattering and infrared spectroscopy extremely sensitive techniques. Here, we experimentally demonstrate that the interaction between a relativistic electron and vibrational modes in nanostructures is fundamentally modified in the presence of plasmons. We finely tune the energy of surface plasmons in metallic nanowires in the vicinity of hexagonal boron nitride, making it possible to monitor and disentangle both strong phonon–plasmon coupling and plasmon-driven phonon enhancement at the nanometer scale. Because of the near-field character of the electron beam–phonon interaction, optically inactive phonon modes are also observed. Besides increasing our understanding of phonon physics, our results hold great potential for investigating sensing mechanisms and chemistry in complex nanomaterials down to the molecular level.

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.