Topological optimization method “MMA-BP” for photoacoustic resonator and implementations in ppt-level gas sensor using miniaturized vase-type photoacoustic cells

A topological optimization method for photoacoustic resonators is proposed as the Method of Moving Asymptotes with Bernstein Polynomials (MMA-BP). This method is demonstrated in a ppt-level ultra-sensitive photoacoustic spectroscopy gas sensor using miniaturized vase-type photoacoustic cell (V-PAC). The V-PAC has a volume of merely 5 mL and achieves a minimum detection limit of 281 ppt for C₂H₂ with an integration time of 768 s, corresponding to a normalized noise equivalent absorption of 4.46 × 10⁻⁹∙cm⁻¹∙Hz⁻^1/2 with a single optical path. It represents an improvement of approximately 14 times beyond that of using the conventional T-type PAC. We experimentally investigated consequent frequency shifts within conventional PACs with thin resonator tubes, and determined the influence of losses on the resonant frequency shift. The geometric contraction effect in vase-type PAC is also investigated. The proposed topological MMA-BP method and implementations provide a universal approach to establish optimized PAC structures for photoacoustic gas detection.