Review of phonons in moiré superlattices

Moirépatterns in physics are interference fringes produced when a periodic template is stacked on another similar one with different displacement and twist angles.The phonon in two-dimensional(2D)material affected by moirépatterns in the lattice shows various novel physical phenomena,such as frequency shift,different linewidth,and mediation to the superconductivity.This review gives a brief overview of phonons in 2D moirésuperlattice.First,we introduce the theory of the moiréphonon modes based on a continuum approach using the elastic theory and discuss the effect of the moirépattern on phonons in 2D materials such as graphene and MoS_(2).Then,we discuss the electron-phonon coupling(EPC)modulated by moirépatterns,which can be detected by the spectroscopy methods.Furthermore,the phonon-mediated unconventional superconductivity in 2D moirésuperlattice is introduced.The theory of phonon-mediated superconductivity in moirésuperlattice sets up a general framework,which promises to predict the response of superconductivity to various perturbations,such as disorder,magnetic field,and electric displacement field.

Terahertz phononic crystal in plasmonic nanocavity

Interaction between photons and phonons in cavity optomechanical systems provides a new toolbox for quantum information technologies.A GaAs/AlAs pillar multi-optical mode microcavity optomechanical structure can obtain phonons with ultra-high frequency(~THz).However,the optical field cannot be effectively restricted when the diameter of the GaAs/AlAs pillar microcavity decreases below the diffraction limit of light.Here,we design a system that combines Ag nanocav-ity with GaAs/AlAs phononic superlattices,where phonons with the frequency of 4.2 THz can be confined in a pillar with~4 nm diameter.The Q_(c)/V reaches 0.22 nm^(-3),which is~80 times that of the photonic crystal(PhC)nanobeam and~100 times that of the hybrid point-defect PhC bowtie plasmonic nanocavity,where Q_(c) is optical quality factor and V is mode volume.The optome-chanical single-photon coupling strength can reach 12 MHz,which is an order of magnitude larger than that of the PhC nanobeam.In addition,the mechanical zero-point fluctuation amplitude is 85 fm and the efficient mass is 0.27 zg,which is much smaller than the PhC nanobeam.The phononic superlattice-Ag nanocavity optomechanical devices hold great potential for applications in the field of integrated quantum optomechanics,quantum information,and terahertz-light transducer.

Phonon-assisted upconversion photoluminescence of quantum emitters

Quantum emitters are widely used in quantum networks,quantum information processing,and quantum sensing due to their excellent optical properties.Compared with Stokes excitation,quantum emitters under anti-Stokes excitation exhibit better performance.In addition to laser cooling and nanoscale thermometry,anti-Stokes excitation can improve the coherence of single-photon sources for advanced quantum technologies.In this review,we follow the recent advances in phononassisted upconversion photoluminescence of quantum emitters and discuss the upconversion mechanisms,applications,and prospects for quantum emitters with anti-Stokes excitation.

Room-temperature phonon-coupled single-photon emission in hexagonal boron nitride

Photon-lattice(phonon)coupling is fundamental to light-matter interaction,particularly when it reaches the quantum limit of the phonon-coupled single-photon emission,which holds great potential for quantum manipulation and quantum information transduction.Here,we report single defect state-phonon coupling in hexagonal boron nitride(h BN)at room temperature.An ultrabroad spectrum of single-photon emissions can be achieved by selecting the excitation energies.Using photoluminescence excitation spectroscopy,we observe single-phonon-assisted resonance-enhanced single-photon emission,along with multiple phonon replicas that herald the creation of phonon Fock state.We also develop a transition model to gain insight into the physical process behind the single defect state-phonon coupling.Our work sets the stage for manipulating electron-phonon coupling state with single quantum-level precision at room temperature.

Detection of electron-phonon coupling in two-dimensional materials by light scattering

Electron-phonon coupling affects the properties of two-dimensional(2D)materials significantly,leading to a series of novel phenomena.Inelastic light scattering provides a powerful experimental tool to explore electron-phonon interaction in 2D materials.This review gives an overview of the basic theory and experimental advances of electron-phonon coupling in 2D materials detected by Raman and Brillouin scattering,respectively.In the Raman scattering part,we review Raman spectroscopy studies of electron-phonon coupling in graphene,transition metal disulfide compounds,van der Waals heterostructures,strongly correlated systems,and 2D magnetic materials.In the Brillouin scattering part,we extensively introduce Brillouin spectroscopy in non-van der Waals 2D structures,including temperature sensors for phonons and magnons,interfacial Dzyaloshinsky-Moriya interaction and spin torque in multilayer magnetic structures,as well as exciton-polariton in semiconductor quantum well.