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1. Acoustic and optical phonon frequencies and acoustic phonon velocities in Si-doped AlN thin films

   https://doi.org/10.1063/5.0233163  

   Wright, Dylan; Mudiyanselage, Dinusha Herath; Guzman, Erick; Fu, Xuke; Teeter, Jordan; Da, Bingcheng; Kargar, Fariborz; Fu, Houqiang; Balandin, Alexander A (September 2024, Applied Physics Letters)

   We report the results of the study of the acoustic and optical phonons in Si-doped AlN thin films grown by metal–organic chemical vapor deposition on sapphire substrates. The Brillouin–Mandelstam and Raman light scattering spectroscopies were used to measure the acoustic and optical phonon frequencies close to the Brillouin zone center. The optical phonon frequencies reveal non-monotonic changes, reflective of the variations in the thin film strain and dislocation densities with the addition of Si dopant atoms. The acoustic phonon velocity decreases monotonically with increasing Si dopant concentration, reducing by ∼300 m/s at the doping level of 3 × 1019 cm−3. The knowledge of the acoustic phonon velocities can be used for the optimization of the ultra-wide bandgap semiconductor heterostructures and for minimizing the thermal boundary resistance of high-power devices. 

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2. Optomechanical Quantum Entanglement Mediated by Acoustic Phonon Fields

   https://doi.org/10.1103/PhysRevLett.129.203604  

   Xu, Qidong; Blencowe, M. P. (November 2022, Physical Review Letters)
3. Engineering Electron–Phonon Coupling of Quantum Defects to a Semiconfocal Acoustic Resonator

   https://doi.org/10.1021/acs.nanolett.9b02430  

   Chen, Huiyao; Opondo, Noah F.; Jiang, Boyang; MacQuarrie, Evan R.; Daveau, Raphaël S.; Bhave, Sunil A.; Fuchs, Gregory D. (September 2019, Nano Letters)
4. Direct Imaging of Localized Anisotropic Acoustic-Phonon Dynamics in MoS ₂

   https://doi.org/10.1017/S1431927619011371  

   Zhang, Yichao; Flannigan, David J. (August 2019, Microscopy and Microanalysis)
5. Distinguishing optical and acoustic phonon temperatures of supported 2D materials by nanosecond time-resolved Raman scattering

   https://doi.org/10.1364/OL.532999  

   Rahbar, Mahya; Al_Keyyam, Ibrahim; Liu, Jing; Wang, Xinwei (August 2024, Optics Letters)

   Upon laser irradiation, 2D materials experience a cascading energy transfer from electrons to optical phonons (OPs) and then to acoustic phonons (APs), resulting in a significant thermal non-equilibrium among energy carriers. This non-equilibrium presents challenges for Raman-based thermal characterization, as Raman scattering measures only OP temperature rise, while APs are the primary energy carriers. Despite recent efforts to address this issue, OP–AP thermal non-equilibrium in supported 2D materials remains poorly resolved. Here, we develop a method to distinguish the OP and AP temperature rises based on their different temporal thermal responses under laser irradiation: the OP–AP temperature difference responds almost immediately (∼a few to tens of ps), while the AP temperature rise takes longer to establish (∼tens of ns). Using energy transport-state resolved Raman, we probe the transient thermal response of Si-supported nm-thick MoS₂from 20 to 100 ns. We find that the OP–AP temperature difference exceeds 120% of the AP temperature rise under ∼0.439 µm radius laser heating. The intrinsic interfacial thermal conductance of the samples, based on the true AP temperature rise, varies from 0.199 to 1.46 MW·m⁻²·K⁻¹, showing an increasing trend with sample thickness. 

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