Cavity-Enhanced Excitation of Molecules with Continuous-wave Lasers^†

The precise excitation of molecular vibrational states is critical for advancing chemical dynamics, precision spectroscopy, and trace gas sensing. This objective, however, is often hindered by the weak oscillator strengths of ro-vibrational transitions, which render conventional continuous-wave (cw) lasers ineffective due to their limited power. This fundamental challenge is overcome by cavity-enhanced excitation (CEE), a technique that locks a cw laser to a high-finesse optical cavity. This configuration amplifies the intra-cavity light intensity by several orders of magnitude while preserving a narrow spectral linewidth. The resulting synergy enables highly efficient, state-selective population transfer and high-resolution spectroscopy previously considered impractical. This review elucidates the core technique of laser-cavity locking and highlights its applications, notably in the quantitative detection of trace isotopes and the investigation of highly excited vibrational states with kilohertz-level accuracy.