The photoacoustic effect takes place when a beam of light is absorbed by a gas, liquid or solid causing it to expand, a mechanical motion that results in the launching of a sound wave. Detectors exploit this effect by zapping a material with a laser tuned to a wavelength that is absorbed by the molecule of interest.
In a typical photoacoustic experiment, the laser beam is switched on and off at a frequency that can be detected by a sensitive microphone to listen for any sound waves produced. Different molecules absorb light at different frequencies, so by adjusting the frequency of the laser, it’s possible to fine-tune a detector for specific substances.
But the smaller the concentration of the target substance, the quieter the signal. Academic researchers from the U.S. and China devised a new photoacoustic technique, based on three different resonances, that boosts the signal and enables measurement down to the parts-per-quadrillion level.
Instead of a single laser beam, two beams are combined at a specific frequency and angle. The joining of the beams creates a grating—a pattern of interference between the two. When the laser frequencies are tuned just right, the grating travels in a detection cell at the speed of sound, creating an amplification effect at each of the peaks in the grating.
The second resonance is produced by a piezoelectric crystal used in the experiment, which vibrates precisely at the frequency of the combined laser beams. The small compressive forces in the pressure waves gradually induce motion in a crystal in much the same way that small, repeated pushes of a playground swing eventually lead to a large amplitude motion of the swing.
The third resonance is generated by adjusting the length of the cavity in which the crystal is mounted so that it resonates when an integral number of half wavelengths of the sound exactly match the cavity length. The output of the crystal, which is piezoelectric so that it generates a voltage proportional to its oscillatory motion, is sent to amplifiers and sensitive electronic devices to record the acoustic signal.
The technique was demonstrated to detect the gas sulfur hexafluoride in amounts down to parts per quadrillion. The researchers from Brown University, RI, and Shandong University, China, expect the technique to be useful in developing detectors that are sensitive to very low pollutant gas concentrations, or for detecting molecules that have weak absorptions that make them inherently difficult to detect.