In semiconductors, the electronic drift velocity can easily exceed the speed of sound in the material at moderate biases. The accelerated electrons lead to the generation and amplification of acoustic phonons that travel in the direction of the carriers via a process that is analogous to the generation of Cherenkov radiation by accelerated charged particles. Here, we demonstrate, using a high-Q acoustic cavity, orders of magnitude enhancement in subterahertz phonon amplification. The 0.44 THz acoustic cavity structure enables a long phonon lifetime (τ ≈ 0.56 ns), and we achieve a remarkable phonon gain efficiency of $0.3\times {10}^7\mathrm{c}{\mathrm{m}}^{-1}{\mathrm{W}}^{-1}$. The study of high-frequency coherent acoustic phonons in semiconductor materials and heterostructures has led to significant advancements in various technological applications, including polaritonic devices, terahertz light modulation, phonon imaging, nondestructive testing, and ultrafast phononic circuits. These results further enhance our understanding of sound amplification in phononic structures, paving the way for future developments in phonon-based technologies.

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• Acoustic phonons
• Cherenkov radiation
• Electron-phonon coupling
• Ultrafast phenomena
• Superlattices
• Terahertz sources