Mid-infrared Assisted THz Phonon Amplification in a 2D Semiconductor for Room Temperature Detection
Efficient and selective excitation of lattice vibrations is central to controlling energy flow at the nanoscale, yet remains challenging under conventional optical excitation. Here, we introduce a mid-infrared-assisted phonon amplification approach, termed MIRAPA, that enables efficient energy injection directly into vibrational bonds. Using surface-enhanced resonant Raman scattering in few-layer $\mathrm{MoS_2}$, we exploit strong exciton--phonon coupling to monitor phonon populations. When mid-infrared (MIR) light is introduced, it couples directly to out-of-plane lattice vibrations, leading to room-temperature phonon amplification exceeding $80\%$. Crucially, MIRAPA bypasses electronic excitation pathways, allowing the MIR power density to be nearly $300\times$ lower than that required for visible excitation to achieve comparable enhancement. The resulting phonon modulation is robust, persisting over more than $2800$ on/off cycles and exceeding $15$ hours of continuous-wave laser illumination without degradation. Quantitative analysis yields an effective noise-equivalent power of approximately $0.3\,\mathrm{nW}/\sqrt{\mathrm{Hz}}$ for MIR detection, highlighting the sensitivity of the approach. By combining vibrational selectivity, low-power operation, and long-term stability, MIRAPA provides a robust platform for probing and amplifying phonons in two-dimensional semiconductors. These results open new opportunities for nanoscale vibrational sensing, mid-infrared detection, and phonon-based coherent devices, including routes toward phonon lasing.