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

Significance The photoacoustic effect refers to the generation of sound through a process of optical heat deposition followed by thermal expansion, resulting in a local pressure increase that produces outgoing acoustic waves. In the linear acoustic regime, a unique property of the photoacoustic effect in a geometry with symmetry in one dimension is that when the optical source moves at the speed of sound, the amplitude of the acoustic wave increases linearly in time without bound. Here, the application of this effect to trace gas detection is described, using an optical grating that moves at the sound speed inside of a resonator equipped with a resonant piezoelectric crystal detector, yielding detection limits in the parts-per-quadrillion range. 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.