Resonant Phonon Research Articles

Mode confinement enhanced by exploiting epsilon-near-zero supercoupling in a mimic transverse electric hybrid phonon polariton waveguide

New capabilities in mid- to long-wave infrared sensing and telecommunications require ultracompact waveguides that support long propagation lengths. Hybrid waveguides supporting the coupling between a dielectric strip-waveguide mode (long propagation) and a surface polariton mode (tightly confined mode) are promising candidates. Here, an infrared (λ0 = 10.6 μm) hybrid waveguide design is presented that achieves enhanced mode confinement but with minimal impact on propagation distances. Modal area confinement is enhanced by the integration of a thin layer of epsilon-near-zero material, aluminum nitride near the longitudinal optical phonon resonance, which supports supercoupling, a term that describes the effect of field enhancement caused by squeezing energy into arbitrary-sized regions. While SPhPs are inherently transverse magnetic modes, a transverse electric (TE) mode is sought to best exploit the ENZ supercoupling phenomenon. By adding a thin high index layer (GaAs) over the 4H-SiC substrate, a mimic TE mode is achieved. The epsilon-near-zero supercoupling-enhanced mimic TE hybrid SPhP waveguide presented exhibits modal confinement improvement by as much as a factor of 4 while maintaining more than 95% of the original propagation length.

Ultrasonic Gas Sensing by Two-Dimensional Surface Phononic Crystal Ring Resonators.

An acoustic ring resonator employing a two-dimensional surface phononic crystal is proposed for high-sensitivity detection in binary gas mixtures. Band analyses and frequency-domain simulations via the finite-element method reveal that a single band for spoof surface acoustic waves appears at ultrasonic frequencies around 58 kHz where modification of its dispersion due to varying gas composition results in a linear shift of the resonance frequency. The shift rate is -17.3 and 8.8 mHz/ppm for CO2 and CH4, respectively. The linear shift of resonance frequency is experimentally validated. In addition, the ring resonator can also be employed to track acoustic intensity variation with gas concentration, where exponentially decaying intensity for low concentrations leverages high-sensitivity operation.

Cyclotron–phonon resonance line-width in monolayer silicene

We theoretically observe the cyclotron–phonon resonance (CPR) in monolayer silicene by considering the magneto-optical absorption power (AP) of the system. The AP is calculated utilising the projection operator technique when a static magnetic field is subjected perpendicularly to the plane of the material and the electron–optical phonon scattering is involved. The absorption spectra show the resonance behaviour where the photon energies at resonant peaks satisfy the cyclotron–phonon (CPR) conditions in which the CPR energy depends linearly on the square root of the magnetic field strength. This dependence is similar to that in graphene and different from that in monolayer MoS2 and conventional low-dimensional semiconductors. The dependence of the full-width at half maximum (FWHM) on the magnetic field strength has the laws similar to those obtained for graphene and monolayer MoS2. However, the FWHM value in silicene is two times smaller than it is in graphene and has the same order as it does in monolayer MoS2. At relatively high temperature (T>300 K), the FWHM for phonon emission differs very little from that for phonon absorption and becomes identical in two cases when T>350 K.

Intersubband Transitions in Nonpolar GaN-based Resonant Phonon Depopulation Multiple-Quantum Wells for Terahertz Emissions

We investigate the polarization effect in intersubband transitions in polar and nonpolar GaN-based multiple-quantum well (MQW) structures for terahertz (THz) emissions by using systematic comparisons and design a nonpolar GaN/Al0.2Ga0.8N two-well-based MQW structure with an emitting photon of 7.27 THz (30.07 meV). Its lower energy separation (92.7 meV) matches the resonant phonon depopulation condition for better population inversion. It shows a lower threshold current density Jth at all temperatures (1.548 kA/cm2 at 90 K) and a higher output power of up to 86.1 mW at 5.8 K and 33.6 mW at 100 K. Our results for the polar GaN MQW are very close to the experimental data in the literature. We find that the Jth of the nonpolar GaN MQW increases more slowly than that of the polar GaN MQW as temperature increases, indicating the nonpolar GaN MQW may be a worth-trying direction for improving the operation temperature. These results can provide meaningful references for the design and fabrication of nonpolar GaN-based THz MQW or quantum cascade structures.

Optical rectification in a chalcopyrite AgGaSe2 crystal for broadband terahertz radiation generation

We report on optical rectification in a ⟨110⟩AgGaSe2 crystal that produces terahertz (THz) electric field pulses across the broad frequency range of 0.02–4 THz. Due to the large coherence length of the AgGaSe2 crystal (>4 times larger than that of CdSiP2 and ZnGeP2), in-phase THz radiation is produced over the entire crystal thickness of 485 μm. Furthermore, THz time-domain spectroscopy is used to determine the ordinary and extraordinary refractive indices and extinction coefficients of the uniaxial AgGaSe2 crystal across the broad spectral range of 0.5–2.9 THz, encompassing several phonon resonances.

Resonant phonon modes in fullerene functionalized graphene lead to large tunability of thermal conductivity without impacting the mechanical properties

We investigate the effects of fullerene functionalization on the thermal transport properties of graphene monolayers via atomistic simulations. Our systematic molecular dynamics simulations reveal that the thermal conductivity of pristine graphene can be lowered by more than an order of magnitude at room temperature (and as much as by ∼93% as compared to the thermal conductivity of pristine graphene) via the introduction of covalently bonded fullerenes on the surface of the graphene sheets. We demonstrate large tunability in the thermal conductivity by the inclusion of covalently bonded fullerene molecules at different periodic inclusions, and we attribute the large reduction in thermal conductivities to a combination of resonant phonon localization effects, leading to band anticrossings and vibrational scattering at the sp3 bonded carbon atoms. The torsional force exerted by the fullerene molecules on the graphene sheets and the number of covalent bonds formed between the two carbon allotropes is shown to significantly affect the heat flow across the hybrid structures, while the size of the fullerene molecules is shown to have a negligible effect on their thermal properties. Moreover, we show that even for a large surface coverage, the mechanical properties of these novel materials are uncompromised. Taken together, our work reveals a unique way to manipulate vibrational thermal transport without the introduction of lattice defects, which could potentially lead to high thermoelectric efficiencies in these materials.

Terahertz quantum cascade laser with non-resonant extraction

In order to improve the maximum operating temperature of THz quantum cascade lasers (QCLs) we propose to modify the three well resonant phonon design by increasing the energy of the separation between the bottom transition levels and the ground states, which is usually adjusted to the LO-phonon energy in GaAs. The fabricated devices based on this design with non-resonant depopulation of the bottom transition level operated in pulsed mode up to 190K, which is the best-reported performance for THz QCLs with Al0.15Ga0.85As barriers based on an Au-Au double-metal waveguide. The performed analysis showed that the achieved improvement could be explained by the proposed design modification.

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.

Sensitivity enhanced temperature sensor: one-port 2D surface phononic crystal resonator based on AlN/sapphire

According to the structure of a one-port surface acoustic wave (SAW) resonator, a one-port surface phononic crystal (SPC) resonator is designed. Both one-port SAW resonator and one-port SPC resonator are fabricated on AlN/Sapphire. Frequency responses and quality factors are studied. The one-port SAW resonator has a fundamental resonance mode at 693.07 MHz. Unlike the one-port SAW resonator, the one-port SPC resonator has two distinct resonance modes at 680.28 MHz and 698.91 MHz, respectively. From the simulation results by finite element method (FEM), these two resonance modes may come from the two cavities of the one-port SPC resonator. Frequency-versus-temperature behaviors of these two resonators are investigated. The results show that the frequencies of resonance modes of these two resonators are decreased linearly with the increased temperature. A new kind of temperature sensor is proposed by using frequency difference between these two resonance modes of the one-port SPC resonator. The eventual temperature coefficient of frequency (TCF) value of the temperature sensor is ppm °C−1 which is twice more than that of the one-port SAW resonator. All results prove that temperature sensitivity is enhanced by using two mode characters of one-port SPC resonator based on AlN/Sapphire.

Nonreciprocal control and cooling of phonon modes in an optomechanical system.

Mechanical resonators are important components of devices that range from gravitational wave detectors to cellular telephones. They serve as high-performance transducers, sensors and filters by offering low dissipation, tunable coupling to diverse physical systems, and compatibility with a wide range of frequencies, materials and fabrication processes. Systems of mechanical resonators typically obey reciprocity, which ensures that the phonon transmission coefficient between any two resonators is independent of the direction of transmission1,2. Reciprocity must be broken to realize devices (such as isolators and circulators) that provide one-way propagation of acoustic energy between resonators. Such devices are crucial for protecting active elements, mitigating noise and operating full-duplex transceivers. Until now, nonreciprocal phononic devices3-11 have not simultaneously combined the features necessary for robust operation: strong nonreciprocity, in situ tunability, compact integration and continuous operation. Furthermore, they have been applied only to coherent signals (rather than fluctuations or noise), and have been realized exclusively in travelling-wave systems (rather than resonators). Here we describe a scheme that uses the standard cavity-optomechanical interaction to produce robust nonreciprocal coupling between phononic resonators. This scheme provides about 30 decibels of isolation in continuous operation and can be tuned in situ simply via the phases of the drive tones applied to the cavity. In addition, by directly monitoring the dynamics of the resonators we show that this nonreciprocity can control thermal fluctuations, and that this control represents a way to cool phononic resonators.

Fano Resonance Based on Long Range Surface Phonon Resonance in the Mid-Infrared Region

Surface plasmon polaritons with noble metals lack the capability to restrain an optical field in the long wavelength region, while surface phonon polaritons with polar dielectric crystals have been regarded as a potential low-loss alternative. In this paper, long range surface phonon resonance (LRSPhR) is designed to operate in the mid-infrared (MIR) wavelength regime with a narrow resonance dip. Furthermore, by coupling a photonic waveguide with the LRSPhR, strong coupling between waveguide mode and long-range surface phonon polariton has been realized to form high Q Fano resonance. By employing such structure as the index sensor, it is shown that the bulk sensitivity and surface sensitivity can reach ~9000 and 266.9 RIU -1 , respectively, which shows a great enhancement comparing with the one based on merely LRSPhR. The proposed configuration can be a promising platform for MIR biochemical sensing.

Peak emission of terahertz waves from (110)-oriented ZnTe by interacting phase-matched phonon resonances

We demonstrate the measurement and assignment of phonon resonances responsible for observed peak emission of terahertz (THz) waves from (110)-ZnTe. We show that optical rectification in ZnTe using femtosecond pulses centered at 790 nm involves phase-matched fundamental and difference-phonon resonances at 1.63, 1.95, 2.25, and 2.55 THz, which produce a THz peak at 2.1 THz and damped oscillations for 6 ps after the main THz pulse. Phonon resonances in ZnTe have been measured and assigned by mathematical modeling and terahertz time-domain spectroscopy (THz-TDS) over the range 0.1–3 THz. The refractive index and optical absorption coefficient of ZnTe have also been extracted from the measured THz-TDS data. The results give greater insight into the dynamics of ZnTe for ultrafast optoelectronic systems.

Thermal Transport Engineering in Graphdiyne and Graphdiyne Nanoribbons.

Understanding the details of thermal transport in graphdiyne and its nanostructures would help to broaden their applications. On the basis of the molecular dynamics simulations and spectrally decomposed heat current analysis, we show that the high-frequency phonons in graphdiyne can be strongly hindered in nanoribbons because of the boundary scattering. The isotropic transport in graphdiyne can be switched to anisotropic along the armchair and zigzag directions. Adding side chains onto the nanoribbon edges further reduces the thermal conductivity (TC) along both armchair and zigzag directions thanks to the reduction of heat current carried by low-frequency modes, a mechanism that arises from the phonon resonances. The uniaxial tensile strain plays a different role in the TC of graphdiyne, armchair nanoribbons, and zigzag nanoribbons. Tensile strain causes the thermal conductivities of graphdiyne, and armchair nanoribbons increase first and then get reduced, whereas for zigzag nanoribbons, the TC decreases with strain first and reaches to a plateau. The different low-frequency phonon response on strain is the main reason for the different TC behavior. For graphdiyne and armchair nanoribbons, the low-frequency heat current is enhanced gradually first and then get reduced with the increase of strain, while that of zigzag nanoribbons decreases with strain and then increases slightly. The current studies could help us understand the phonon transport in graphdiyne and its nanoribbons, which is useful for their TC engineering.

One- and two-photon-induced cyclotron–phonon resonance in modified-Pöschl–Teller quantum well

We study the two-photon-induced cyclotron–phonon resonance (CPR) effect resulting from the interaction between electrons and phonons in a quantum well with the modified-Poschl–Teller (MPT) potential. The CPR is considered via the magneto-optical absorption coefficient (MOAC) and the full-width at half-maximum (FWHM). Numerical results are presented for a typical GaAs MPT quantum well. It is found that the quantum well parameters, such as well-depth and well-width, the magnetic field, and the temperature affect significantly the energy separation, the MOAC, and the FWHM. Obtained results agree well with previous theoretical and experimental results. Especially, we found two new rules of the well-depth and well-width-dependent FWHM, which should be tested by experimental works to examine their validities.

Ferromagnetic effects on helium-vacancy complex formation in BCC Fe

The substitution of helium atom inside BCC Fe results in (1) lattice distortion, (2) the resonance phonon modes, and (3) the scattering center, enhancing the anharmonicity of phonon and magnon. The ferromagnetic effects are analyzed by comparing results in the ferromagnetic and non-magnetic systems using spin-lattice dynamics simulations based on quantum fluctuation dissipation relation. The ferromagnetic effects are subtracted into static and dynamic contributions, where the former is found to be dominant. Neglecting ferromagnetic effects would bring with unexpected large error in the description of kinetics of helium in metals.

Phonon Spectrum and Resonance with Acoustic Wave of Lithium by Lattice Dynamics and the Modified Analytic Embedded-Atom Method

The phonon spectrum and resonance are studied by lattice dynamics and using the second nearest-neighbor (2NN) modified analytic embedded-atom method (MAEAM). Through deriving the expressions for atomic force constants, the phonon spectrum of lithium metal is calculated along three principal directions ([100], [110], [111]). The calculated and experimental values are in good agreement. On this basis, the phonon dispersion of acoustic wave is added to the phonon spectrum of lithium metal and the transverse wave and longitudinal wave of resonance frequencies are calculated. It is proposed that terahertz acoustic wave can be used to produce resonance with metal lithium electrodes, which would break the detrimental lithium dendrites during the charge process of Li batteries. The method can also be used to calculate the phonon spectrum and the resonance frequencies of other metal electrodes.

Effects of Microcavity Local Resonators on the Bandgap Characteristics of a Two-Dimensional Phononic Crystal Structure

The emergence of acoustic metamaterials generated a lot of attention in the study of low-frequency vibration, noise control and reduction in engineering applications. As a result, the elastic wave bandgap characteristics of a two-dimensional microcavity local resonator structure for two soft rubber materials was investigated using finite element methods (FEM). The transmission spectrum of the displacement eigenmodes of the bandgap edges relating to the lowest bandgap was calculated. The results showed that the phononic crystal structure without a microcavity local resonator plate has bandgap characteristics of elastic wave propagation in the high-frequency range between 2200~2400Hz. However, with the introduction of microcavity resonator plates in the phononic crystal structure low-frequency bandgaps are obtained in the region of 0~198Hz and 0~200Hz respectively. The low-frequency bandgaps appeared as a result of the microcavity local resonator plate which increased the path length through which the wave is transmitted. The phononic crystal microcavity local resonator plate structure has varying transmission loss characteristics of -65dB, -85dB, -100dB and -150dB in the low-frequency range depending on the number of local resonator plates introduced into the cell structure and density of the cell structure. The study provided a good demonstration of wave propagation in artificially engineered materials with critical emphasis on the effects of local resonators in a microcavity structure.

Full Counting Statistics of Superconductor-Quantum Dot-Superconductor Contacts in the Presence of Interactions

In this paper, we discuss the full counting statistics of superconducting quantum dot contacts. We discuss the effects both of phonon and onsite electronic interaction focusing on the experimentally most relevant case of strong onsite electronic interactions. We find that in general, the Josephson effect and multiple Andreev reflections in these systems are strongly suppressed due to the onsite interaction. However, in case resonant phonons are found, the effect of the onsite interaction can be overcome.

Resonance of terahertz phonons in an acoustic nanocavity

We show the resonant behavior of \ensuremath{\sim}0.5-THz longitudinal acoustic (LA) phonons in an acoustic nanocavity of thickness \ensuremath{\sim}28 nm sandwiched between two GaAs/AlAs superlattices (SLs). One of the SLs, upon excitation with an ultrafast optical pulse, acts as a source of coherent LA phonons. These phonons, generated outside the cavity, resonantly drive the cavity mode. The phonon dynamics can be modeled using a driven-damped harmonic oscillator. The confinement and enhancement of phonons in the acoustic nanocavity opens up possibilities in the field of nanophononics.

Extremely-huge wave localization in coupled multilayer heterogeneous phononic crystal resonators

A coupled heterogeneous structure consisting of multiple phononic crystal resonators (PCRs) is presented. Because of the acoustic coupling between PCRs, the extremely-huge wave localization effect is achieved in coupled multilayer heterogeneous phononic crystals, and the cavity pressure accordingly increases with the number of resonators. The numerical results obtained by finite element methods show that the proposed structure composed of five resonators exhibits ∼33.9 times larger maximum pressure amplification than a single conventional PCR with the same size. This proposed structure can be further used to enhance acoustic energy harvesting, acoustic sensing and sound concentration.