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

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-[Formula: see text], piezoelectric crystal with a resonance on the order of [Formula: see text] 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.

Keywords: moving grating; photoacoustics; piezocrystal; resonator; trace detection.

Fig. 1.

Schematic of the experimental apparatus used for high-frequency photoacoustic trace detection. The two light paths from the acousto-optic light modulators (AOM) are matched to ensure that the two $10.6$-$\mu$m laser beams are phase coherent. The beams intersect in the acoustic cell (AC) with the power monitored by a thermal detector (TD). The intersection angle can be tuned by Mirror $1$ (M$1$), which is mounted on a motorized precision rotation stage. The reference to the lock-in amplifier is generated by feeding the outputs from the two function generators (FG) to a mixer and using a low-pass filter (LPF) so that only the difference frequency from the mixer is sent to the lock-in amplifier.

Fig. S1.

Transducer voltage output vs. incremental cell length. The cell length was continuously increased using a micrometer attached to a motorized rotation stage.

Fig. 2.

Lock-in amplifier output vs. mole fraction for three gases in N₂ and for SF₆ in Ar (top curve, SF₆ I). The detection limit for SF₆ in ultrahigh-purity Ar is $750$ parts per quadrillion using the $\alpha$-BiB₃O₆ crystal and $10$ parts per trillion for SF₆ in N₂ using the LiNbO₃ transducer.

Fig. 3.

Voltage output in arbitrary units (a.u.) vs. grating frequency for the $\alpha$-BiB₃O₆ crystal. $\alpha$-BiB₃O₆was chosen for the experiments because of its combination of a high quality factor and large value of the charge per unit force applied.

Fig. S2.

Averaged noise spectrum of the detector measured with a cell filled with pure Ar and with a laser power of approximately $1$ W.