Photoacoustic trace detection of gases at the parts-per-quadrillion level with a moving optical grating

Author:

Xiong Lian,Bai Wenyu,Chen Feifei,Zhao Xian,Yu Fapeng,Diebold Gerald J.

Abstract

The amplitude of the photoacoustic effect for an optical source moving at the sound speed in a one-dimensional geometry increases linearly in time without bound in the linear acoustic regime. Here, use of this principle is described for trace detection of gases, using two frequency-shifted beams from a CO2 laser directed at an angle to each other to give optical fringes that move at the sound speed in a cavity with a longitudinal resonance. The photoacoustic signal is detected with a high-Q, piezoelectric crystal with a resonance on the order of 443 kHz. The photoacoustic cell has a design analogous to a hemispherical laser resonator and can be adjusted to have a longitudinal resonance to match that of the detector crystal. The grating frequency, the length of the resonator, and the crystal must all have matched frequencies; thus, three resonances are used to advantage to produce sensitivity that extends to the parts-per-quadrillion level.

Funder

U.S. Department of Energy

Publisher

Proceedings of the National Academy of Sciences

Subject

Multidisciplinary

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