Phonon Dynamics Research Articles

Ultrafast generation and detection of coherent acoustic phonons in SnS0.91Se0.09

The rapid growth of attention to tin sulfide (SnS) is due to its efficient application in the field of thermoelectricity, and its doping product SnS0.91Se0.09 can obtain a high average thermoelectric figure of merit (ZTave≈ 1.25). Although many efforts have been made to reveal the mechanisms associated with electron transport and heat conduction, the role of lattice vibration remains poorly understood. Here, we studied the coherent vibration dynamics in SnS0.91Se0.09 bulk single crystal by using the femtosecond two-color pump-probe optical reflectivity technique. The oscillation signal in the time domain of SnS0.91Se0.09 is considered to be an electron–phonon interaction. The coherent dynamics of longitudinal acoustic phonons in this coupling behavior is closely related to probe polarization and excitation power. The stability of oscillation frequency reveals the microscopic mechanism of phonon conduction, and further understands the lattice nonharmonic caused by electron–phonon interaction in light-induced.

Excitonic quenching of the oxygen-chain phonon in the photoinduced metal-to-insulator transition of photoexcited Sr0.95NbO3.37 studied by ultraviolet-resonance Raman scattering

The quasi-one-dimensional metal ${\mathrm{Sr}}_{0.95}{\mathrm{NbO}}_{3.37}$ shows a metal-to-insulator transition upon laser-induced pumping of the d-d exciton at 1.1 eV. The oxygen-chain-related phonon modes are highly susceptible to this metal-to-insulator transition due to their sensitivity to structural and electronic changes. We employ angle-dependent UV-resonance Raman spectroscopy to probe the nonequilibrium oxygen-chain-related phonon dynamics and the concomitant changes in the angle-dependent midinfrared reflectance. The latter shows the expected reduction of charge carrier density upon pumping, whereas the central chain mode at 680 $\mathrm{cm}{}^{\ensuremath{-}1}$ can be selectively quenched upon pumping the d-d excitons. We find that the underlying quenching mechanism for this phonon is electronically driven and related to the photo-induced bleaching of the charge-transfer transition. First-principles calculations show the connection between phonon dispersion and electronic band structure leading to the selective quenching of the oxygen-chain mode. Our results contribute to a deeper understanding of the interplay between lattice and charge degrees of freedom for exciton-based optoelectronics.

First-principles study of thermoelasticity and structural phase diagram of CaO

We present pressure $(P)$ and temperature $(T)$ variations of elastic and anisotropic properties, solid-solid (rocksalt, B1 to cesium chloride, B2) and solid-liquid structural phase transitions of CaO. We employed first-principles density functional theory supplemented with an anharmonic contribution to phonon dynamics. Good agreement is obtained for all properties up to the pressure and temperature range relevant to the Earth's mantle and outer core. Born elastic criteria are generalized for arbitrary stress to elaborate the structural phase diagram from a thermoelastic viewpoint. We propose a self-consistent computational scheme to incorporate the effect of thermal hysteresis into Lindemann's melting law. This improvised melting law exhibits quantitative agreement with reported findings for the high-$P$ melting curve. We find the triple point (i.e., the coexistence of B1, B2, and liquid phases) at 23 GPa and 4600 K. By examining the pressure-term included elastic properties, we can show that the solid-solid transition is mechanical in a pressure range of 0--200 GPa and temperature up to 3000 K. The softening of shear elastic constant ${C}_{44}$ drives the B1-B2 phase transition. This assertion is corroborated by examining the phonon dispersion curve and mode Gr\"uneisen parameter at different pressures for both B1 and B2 phases. The solid-liquid phase boundary can be treated accurately through the temperature-dependent thermodynamic Gr\"uneisen parameter. Furthermore, in this paper, we predict a negative melting slope $>140\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$ with a peak temperature of 7800 K, suggesting a smaller molar volume on the liquid side than that of the solid phase. This finding is supported by electronic band structure calculation. It is proposed that the hybridization of the empty $3d$ band of the cation with $s$ orbitals lowers the conduction band to cross the Fermi energy at the Brillouin zone center and leads the insulator-to-metal/semimetal phase transition $\ensuremath{\sim}200\phantom{\rule{0.28em}{0ex}}\mathrm{GPa}$. Different electronic states on solid and liquid sides make the liquid phase more compressible than the solid phase, eventually reducing the melting temperature with pressure.

Catching the Killer: Dynamic Disorder Design Rules for Small‐Molecule Organic Semiconductors

AbstractSmall‐molecule organic semiconductors are promising materials for applications ranging from solar cells to medical sensors. Their nearly infinite design space means that, in theory, it is possible to tailor the material to the exact specifications of a particular application. In reality; however, design rules to improve mobility (μ) remain elusive because of the complex calculations required to understand its limiters. Transient localization theory posits that charge carriers are slowed down by the collective phonon motions, called dynamic disorder, which localize charge carriers temporarily. Traditionally, a mode‐by‐mode analysis is performed to try and identify “killer” modes. Herein, the flaws are demonstrated with this analysis and present a streamlined simulation workflow that simplifies simulation of dynamic disorder and enables engineering of new molecular structures based on phonon dynamics. This workflow is combined with a novel visualization technique that enables per‐atom insights into how μ is limited. This workflow is applied and analyzed to a series of ‐acenes and a series of BTBT‐based molecules. Then design rules are identified in each series that identify possible ways to improve the μ beyond current experimental limits using phonon engineering.

Novel Ultrahard Extended Hexagonal C10, C14 and C18 Allotropes with Mixed sp2/sp3 Hybridizations: Crystal Chemistry and Ab Initio Investigations

Based on 4H, 6H and 8H diamond polytypes, novel extended lattice allotropes C10, C14 and C18 characterized by mixed sp3/sp2 carbon hybridizations were devised based on crystal chemistry rationale and first-principles calculations of the ground state structures and energy derived properties: mechanical, dynamic (phonons), and electronic band structure. The novel allotropes were found increasingly cohesive along the series, with cohesive energy values approaching those of diamond polytypes. Regarding mechanical properties, C10, C14, and C18 were found ultrahard with Vickers hardness slightly below that of diamond. All of them are dynamically stable, with positive phonon frequencies reaching maxima higher than in diamond due to the stretching modes of C=C=C linear units. The electronic band structures expectedly reveal the insulating character of all three diamond polytypes and the conductive character of the hybrid allotropes. From the analysis of the bands crossing the Fermi level, a nesting Fermi surface was identified, allowing us to predict potential superconductive properties.

Influence of point defects and multiscale pores on the different phonon transport regimes

A common strategy to tailor the thermal conductivity of a material is to introduce structural features that modulate phonon scattering, such as atomic-scale defects and nano- and macro-sized pores. However, particle-like and wave-like phonon transport and scattering during a crossover in thermal transport regimes is not well understood. Here, we perform a rigorous quantitative comparison of the thermal conductivity obtained from molecular dynamics simulations and phonon Boltzmann transport equations, taking graphene as an example. We observe a generally increasing trend in thermal conductivity when the pore size increases from point defect to nanopore, due to a transition from Rayleigh scattering to geometric scattering and reduced boundary density. The thermal conductivity further converges to the diffusive limit for macropores because of the dominant effect of phonon-phonon scattering over phonon-boundary scattering. Moreover, we identify a critical interpore distance for the crossover from dependent to independent phonon-pore scattering and a critical pore size for the crossover from point defect scattering to boundary scattering. This work provides a comprehensive understanding of phonon transport in materials containing defects and pores.

Critical Evaluation of the Photovoltaic Performance of (AgI)x(BiI3)y Thin Films from the Viewpoint of Ultrafast Spectroscopy and Photocurrent Experiments

Silver iodobismuthate thin films obtained from mixtures of AgI and BiI3 have been frequently suggested as promising alternatives for lead-based perovskite materials in photovoltaic applications. Here, we investigated (AgI)x(BiI3)y thin films with stoichiometric ratios (x/y) of 3:1, 2:1, 1:1, and 1:2 produced via a low-temperature (<100 °C) spin coating process from AgI/BiI3 solutions in dimethyl sulfoxide. Several critical observations on the basis of ultrafast broadband UV–vis transient absorption spectroscopy and photocurrent spectroscopy were made for these Ag–Bi–I materials, which will have a considerable impact on their photovoltaic performance: Their carrier recombination kinetics were independent of the initial carrier number density over the range of 1.3–6.6 × 1017 cm–3 and well approximated by a monoexponential decay with a rate constant krec in the range of 2.0–4.3 × 108 s–1, which is consistent with trap-mediated charge-carrier recombination. Moreover, pronounced coherent phonon dynamics was observed for all of these (AgI)x(BiI3)y compounds, thereby suggesting strong electron–phonon coupling, which will favor charge-carrier localization and nonradiative carrier recombination, in agreement with the virtually absent photoluminescence of these materials. In addition, Fourier-transform step-scan photocurrent spectroscopy (FTPS) on AgBi2I7 provided an Urbach energy of 70 meV and found indications for deep defects (0.6 eV below the band gap), which was also consistent with a trap-mediated recombination mechanism. A combination of all of these effects is likely responsible for the still quite low light-harvesting performance of this class of materials, which has been reported to show photovoltaic conversion efficiencies (PCEs) below 5%. Finally, for “AgI-rich” compounds (3:1, 2:1), we found a substantial separate contribution of carrier loss processes through ultrafast relaxation in AgI domains with time constants of 0.73 and 2.4 ps.

The Migdal effect in semiconductors for dark matter with masses below ∼ 100 MeV

Dark matter scattering off a nucleus has a small probability of inducing an observable ionization through the inelastic excitation of an electron, called the Migdal effect. We use an effective field theory to extend the computation of the Migdal effect in semiconductors to regions of small momentum transfer to the nucleus, where the final state of the nucleus is no longer well described by a plane wave. Our analytical result can be fully quantified by the measurable dynamic structure factor of the semiconductor, which accounts for the vibrational degrees of freedom (phonons) in a crystal. We show that, due to the sum rules obeyed by the structure factor, the inclusive Migdal rate and the shape of the electron recoil spectrum is well captured by approximating the nuclei in the crystal as free ions; however, the exclusive differential rate with respect to energy depositions to the crystal depends on the phonon dynamics encoded in the dynamic structure function of the specific material. Our results now allow the Migdal effect in semiconductors to be evaluated even for the lightest dark matter candidates (mχ ≳ 1 MeV) that can kinematically excite electrons.

Resonance-enhanced excitation and relaxation dynamics of coherent phonons in Fe1.14Te.

Lattice dynamics plays a significant role in manipulating the unique physical properties of materials. In this work, femtosecond transient optical spectroscopy is used to investigate the generation mechanism and relaxation dynamics of coherent phonons in Fe1.14Te-a parent compound of chalcogenide superconductors. The reflectivity time series consist of the exponential decay component due to hot carriers and damped oscillations caused by the A1g phonon vibration. The vibrational frequency and dephasing time of the A1g phonons are obtained as a function of temperature. With increasing temperature, the phonon frequency decreases and can be well described with the anharmonicity model. Dephasing time is independent of temperature, indicating that the phonon dephasing is dominated by phonon-defect scattering. The impulsive stimulated Raman scattering mechanism is responsible for the coherent phonon generation. Owing to the resonance Raman effect, the maximum photosusceptibility of the A1g phonons occurs at 1.590 eV, corresponding to an electronic transition in Fe1.14Te.

Tunable lattice dynamics and dielectric functions of two-dimensional Bi2O2Se: striking layer and temperature dependent effects.

Two-dimensional (2D) Bi2O2Se semiconductors with a narrow band gap and ultrahigh mobility have been regarded as an emerging candidate for optoelectronic devices, whereas the ambiguous phonon characteristics and optical properties still limit their future applications. Herein, high-quality centimeter-scale 2D Bi2O2Se films are successfully synthesized to disclose the lattice dynamics and dielectric functions under the control of thickness and temperature. It has been demonstrated that the stronger electrostatic Bi-Se interactions result in a stiffened phonon vibration of thicker Bi2O2Se layers. Three excitons (Ea, Eb, and Ec) exhibit significant red shifts with layer stacking. Interestingly, the dielectric properties in the visible-near infrared region (Ea and Eb) are dominated by the combined effect of the joint density of states and mass density, whereas the dielectric properties in the ultraviolet region (Ec) are dominated by the exciton effect. Furthermore, the temperature-sensitivity of the phonon frequency and exciton transition energies is revealed to be layer-dependent. In particular, the optical response of Eb excitons exhibits a prominent dependence on temperature, which indicates a promising optical modulation by temperature in the visible spectrum. This study enriches the knowledge about phonon dynamics and dielectric properties for 2D Bi2O2Se, which provides an essential reference for high-performance related optoelectronic devices.

Phonon dynamics in MoSi2N4: insights from DFT calculations.

We have reported the density functional theory investigations on the monolayered, 2 layered and bulk MoSi2N4 in three structural modifications called α1 [Y.-L. Hong, et al., Chemical Vapor Deposition of Layered Two-Dimensional MoSi2N4 Materials, Science, 2020, 369(6504), 670-674, DOI: 10.1126/science.abb7023], α2 and α3 [Y. Yin, Q. Gong, M. Yi and W. Guo, Emerging Versatile Two-Dimensional MoSi2N4 Family, Adv. Funct. Mater., 2023, 2214050, DOI: 10.1002/adfm.202214050]. We showed that in the case of monolayers the difference in total energies is less than 0.1 eV between α1 and α3 phases, and less than 0.2 eV between α1 and α2 geometries. The most energetically favorable layer stacking for the bulk structures of each phase was investigated. All considered modifications are dynamically stable from a single layer to a bulk structure in energetically favorable stacking. Raman spectra for the monolayered, 2 layered and bulk structures were simulated and the vibrational analysis was performed. The main difference in the obtained spectra is associated with the position of the strongest band which depends on the Mo-N bond length. According to the obtained data, we can conclude that the Raman line at 348 cm-1 in the experimental spectra of MoSi2N4 can have more complex explanation than just Γ-point Raman-active vibration as was discussed before in [Y.-L. Hong, et al., Chemical Vapor Deposition of Layered Two-Dimensional MoSi2N4 Materials, Science, 2020, 369(6504), 670-674, DOI: 10.1126/science.abb7023].

First principles study of 2D ring-Te and its electrical contact with a topological Dirac semimetal.

In recent years, researchers have manifested their interest in two-dimensional (2D) mono-elemental materials of group-VI elements because of their excellent optoelectronic, photovoltaic and thermoelectric properties. Despite the intensive recent research efforts, there is still a possibility of novel 2D allotropes of these elements due to their multivalency nature. Here, we have predicted a novel 2D allotrope of tellurium (ring-Te) using density functional theory. Its stability is confirmed by phonon and ab initio molecular dynamics simulations. Ring-Te has an indirect band gap of 0.69 eV (1.16 eV) at the PBE (HSE06) level of theories and undergoes an indirect-direct band gap transition under tensile strain. The higher carrier mobility of holes (∼103 cm2 V-1 s-1), good UV-visible light absorption ability and low exciton binding (∼0.35 eV) of ring-Te give rise to its potential applications in optoelectronic devices. Furthermore, the electrical contact of ring-Te with a topological Dirac semimetal (sq-Te) under the influence of an electric field shows that the Schottky barriers and contact types can undergo transition from p-type to n-type Schottky contact and then to ohmic contact at a higher electric field. Our study provides an insight into the physics of designing high-performance electrical coupled devices composed of 2D semiconductors and topological semimetals.

Strain-engineered thermophysical properties ranging from band-insulating to topological insulating phases in β-antimonene.

The use of strain in semiconductors allows extensive modification of their properties. Due to their robust mechanical strength and flexibility, atomically thin 2D materials are very well suited for strain engineering to extract exotic electronic and thermophysical properties. We investigated the structural, electronic, thermal, and vibrational characteristics along with the phonon and carrier dynamics of β-Sb elemental monolayers for achieving the band-insulating phase at no strain and topological insulating phase at ∼15% biaxial strain. A reduction in stiffness was noticed due to the weakening of the π and σ bonds with strain, leading to anharmonicity in the system. This was further reflected by the drop in lattice thermal conductivity (κl) from 4.5 to 3.1 W m-1 K-1 at 15% strain, i.e., in the topological phase. The appearance of helical edge states at 15% strain and meeting the Z2 invariant criterion confirm the non-trivial topological state. The significant contribution of the out-of-plane A1g vibrational mode was noticed in the topological phase compared with the band-insulating phase. Further, the observed larger reduction in hole lifetime could be attributed to strong scattering near the valence band edge. Importantly, the dominance of the out-of-plane optical modes contributes significantly along the band edges to the topological phase, which is primarily due to the reduced buckling height under strain. Thus, this work emphasizes the microscopic origin of the onset of the topological phase in strained β-Sb monolayers and provides strain-engineered structure-property correlations for better insights.

SiC3N3 monolayer as a universal anode for alkali metal-ion batteries.

Our density functional theory calculations show that silicon doping in g-CN (SiC3N3) can improve the electrochemical performance of g-CN as an anode of alkali metal-ion batteries and solve the problems of too high adsorption ability and migration energy barrier commonly found in porous carbon nitride. The stability of SiC3N3 was verified by molecular dynamics simulations and phonon spectroscopy. Elastic constant calculations revealed that the Si doping in g-CN can improve its mechanical properties. Specifically, Li/Na/K has a suitable adsorption capability (-0.71/-0.52/-0.98 eV) and a lower migration barrier (0.73/0.43/0.21 eV) on SiC3N3, where the barrier of a single Li-ion is the lowest among the doped porous carbon nitride materials studied so far. Moreover, SiC3N3 exhibits a high theoretical capacity (253/1512/1512 mA h g-1) and a low open-circuit voltage (0.48/0.18/0.31 V) for Li/Na/K ion batteries. Compared with B-doped g-CN previously studied, Si doping can more effectively improve the electronic conductivity of g-CN owing to greater charge transfer between Si and g-CN; the migration energy barrier of alkali metal ions on SiC3N3 is reduced more significantly due to its puckered structure instead of a planar structure; and the capacity of SiC3N3 is nearly doubled for alkali metal ion batteries because it has more feasible adsorption sites for alkali metals. These results suggest that Si-doped g-CN can be a universal anode material for alkali metal ion batteries.

Lattice Transformation from 2D to Quasi 1D and Phonon Properties of Exfoliated ZrS2 and ZrSe2.

Recent reports on thermal and thermoelectric properties of emerging 2D materials have shown promising results. Among these materials are Zirconium-based chalcogenides such as zirconium disulfide (ZrS2 ), zirconium diselenide (ZrSe2 ), zirconium trisulfide (ZrS3 ), and zirconium triselenide (ZrSe3 ). Here, the thermal properties of these materials are investigated using confocal Raman spectroscopy. Two different and distinctive Raman signatures of exfoliated ZrX2 (where X = S or Se) are observed. For 2D-ZrX2 , Raman modes are in alignment with those reported in literature. However, for quasi 1D-ZrX2 , Raman modes are identical to exfoliated ZrX3 nanosheets, indicating a major lattice transformation from 2D to quasi-1D. Raman temperature dependence for ZrX2 are also measured. Most Raman modes exhibit a linear downshift dependence with increasing temperature. However, for 2D-ZrS2 , a blueshift for A1g mode is detected with increasing temperature. Finally, phonon dynamics under optical heating for ZrX2 are measured. Based on these measurements, the calculated thermal conductivity and the interfacial thermal conductance indicate lower interfacial thermal conductance for quasi 1D-ZrX2 compared to 2D-ZrX2 , which can be attributed to the phonon confinement in 1D. The results demonstrate exceptional thermal properties for Zirconium-based materials, making them ideal for thermoelectric device applications and future thermal management strategies.

Phonon transport mechanism of HfO2 ultrathin film with temperature-correction full-band Monte Carlo simulation

Hafnium dioxide (HfO2) has been widely used in microelectronics nowadays and commonly withstands extremely high temperatures, so the investigation of its thermodynamic properties is particularly essential. This paper develops a temperature-correction full-band Monte Carlo (TFMC) method to investigate the HfO2 ultrathin film. The phonon dynamics parameters of HfO2 are calculated based on the first-principles method. TFMC can better describe the phonon cumulative distribution function in different temperatures by modifying the phonon relaxation time and heat capacity. The thermal conductivity of HfO2 ultrathin film is calculated based on the above method and is in good agreement with the literature. It is observed that the optical phonons in HfO2 ultrathin film are prominent in the phonon heat transport, which is quite different from the mechanism in common semiconductor materials. Combined with the full-band diffuse mismatch model, the Si-based HfO2 ultrathin film is studied. It is found that the existence of the interface with substrates improves the thermodynamic properties of the ultrathin film, which provides a reference for the selection of substrate materials.

Effect of oxygen defects on microstructure, optical and vibrational properties of ScN films deposited on MgO substrate from experiment and first principles

We report on the effects of oxygen impurities on the microstructure, surface morphology, optical absorption, and vibrational properties of scandium nitride thin films deposited onto magnesium oxide substrate by reactive magnetron sputtering and under ultrahigh vacuum conditions. Two distinct deposition temperatures of 700°C and 800°C were applied to adjust the level of oxygen contamination. The ScN/MgO(001) thin films have been characterized by various spectroscopic methods, including X-ray diffraction, X-ray photoelectron spectroscopy, atomic force microscopy, Raman spectroscopy, and optical microscopy. The ScN films deposited at the lower temperature of the MgO substrate show higher content of defects, stronger disorientation, a larger concentration of twin domains, and suppressed adatom mobility as compared to layers deposited at the higher temperature. An increase by 2% in the amount of accommodated oxygen results in the increase in the direct energy gap of the ScN films by about 0.13 eV, which follows from the increased concentration of the n-type charge carriers incorporated by oxygen impurities. The observed increase in the absorption edge, i.e., enlargement of the optical band gap can be accounted for the Burstein–Moss effect. Experimental Raman spectra reveal coupling between longitudinal optic phonons and electronic states, which can be described by the Fano-type function with the asymmetry parameter increasing with increased content of oxygen defects. The ab initio Raman spectra of ScN with concentrations of oxygen impurities corresponding to the respective experimental values, obtained within density functional theory, demonstrate that incorporated impurities affect significantly the phonon dynamics via modified force constants, including those at atoms beyond the nearest neighbors of defects. The simulated Raman spectra reflect major features of the experimentally determined spectral patterns enabling better insight into the origin of changes experienced by the oxygen-modified ScN/MgO(001) films.

First-principles ultrafast exciton dynamics and time-domain spectroscopies: Dark-exciton mediated valley depolarization in monolayer WSe2

Calculations combining first-principles electron-phonon ($e$-ph) interactions with the Boltzmann equation enable studies of ultrafast carrier and phonon dynamics. However, in materials with weak Coulomb screening, electrons and holes form bound excitons so their scattering processes become correlated, posing additional challenges for modeling nonequilibrium physics. Here we show calculations of ultrafast exciton dynamics and related time-domain spectroscopies using ab initio exciton-phonon (ex-ph) interactions together with an excitonic Boltzmann equation. Starting from the nonequilibrium exciton populations, we develop simulations of time-domain absorption and photoemission spectra that take into account electron-hole correlations. We use this method to study monolayer ${\mathrm{WSe}}_{2}$, where our calculations predict subpicosecond timescales for exciton relaxation and valley depolarization and reveal the key role of intermediate dark excitons. The approach introduced in this paper enables a quantitative description of nonequilibrium dynamics and ultrafast spectroscopies in materials with strongly bound excitons.

Pseudo-gap in RuGa3: A microscopic point of view

We report a detailed study of the nature of the RuGa3in-gap states by means of precision microscopic methods: nuclear quadrupole resonance (NQR), nuclear magnetic resonance (NMR), and pump-probe spectroscopy. We observe a pronounced splitting of 69Ga nuclear spin-lattice relaxation curves below ∼40 K and between 70 and 145 K, although the corresponding NQR lines remain narrow over the entire temperature range under study. The slow relaxing component behaves like a typical phonon-induced relaxation way, while the fast relaxing one demonstrates signatures of paramagnetic (below ∼40 K) and activation (between 70 and 145 K) mechanisms. Moreover, additional Ga’ and Ga’’ positions with anomalously low electric field gradient were revealed for the first time for IrIn3-type structure gallides, which stay almost unchanged, at least up to 77 K. The observed microscopic features are accompanied with in-gap states saturation or depleting, which is seen as the resistivity mechanism crossover at ∼180 K and electron localization below 145 K evidenced by pump-probe spectroscopy. Based on our experimental results, we associate this pseudo-gap like behavior with inhomogeneously distributed electronic density defects, which manifest at both micro- (nuclear spin and phonon dynamics) and macroscale (bulk transport properties such as resistivity).

Q-switching of an optical tweezer phonon laser

We theoretically investigate the active Q-switching of an optical tweezer phonon laser Pettit et al (2019 Nat. Photon. 13 402) operating in a coupled-mode configuration. One of the modes is lasing and outcouples to the second mode. The coupling is induced via asymmetric modulation of the trap potential in the transverse plane of the trapped nanoparticle. We show that a time-modulated coherent coupling between two transverse modes of oscillation of an optically levitated nanoparticle holds the key to coherent pulsed phonon transfer between them. Our analytical and numerical results on the position dynamics, phonon dynamics as well as second-order coherence confirms pulsed phonon lasing transfer between the transverse modes. Our work on Q-switched operation of the optical tweezer phonon laser enhances understanding of the analogies between optical and mechanical lasers, and is relevant to levitated phonon transport, acoustic imaging, sensing and information processing technologies.