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 CO 2 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. (1) is that in a one-dimensional geometry, an optical beam moving in an absorbing medium at the sound speed generates a traveling compressive wave whose amplitude, in the linear acoustics limit, increases in direct proportion to the irradiation timewithout bound. Here, we report the application of this principle for trace detection of gases in a scheme where a pair of frequency-shifted laser beams produced by two acousto-optic modulators operating at slightly different frequencies are combined in space to produce a moving optical grating tuned to an absorption of an infrared active gas. The angle between the two beams is adjusted so that the fringe spacing of the grating Λ along with the frequency difference in the two beams Ω obeys the rule ΩΛ = 2πc; that is, the grating is tuned so that it moves at the sound speed with each antinode in the optical beam pattern moving synchronously with its photoacoustically generated compressive wave. It is shown here that, when a moving grating is produced in a cavity with two parallel reflecting surfaces resembling a laser resonator, two resonance conditions exist for generation of the photoacoustic effect-the first one when the grating motional speed matches the sound speed and the second one when twice the length of the cavity is an integral number of wavelengths of the sound wave. In addition, the method makes use of a resonant piezoelectric crystal as one of the reflecting surfaces of the detection cell so that the effects of three matched resonances are used to advantage to produce high sensitivity.The theory of operation of the detector is based on solution of the wave equation for pressure for a moving optical grating in a resonator. The wave equation for the photoacoustic effect in an inviscid fluid when heat conduction effects can be ignored (1, 5) is given bywhere c is the sound speed, t is the time, β is the coefficient of thermal expansion, CP is the specific heat capacity, and H is the optical energy deposited per unit volume and time. Consider a pair of phase-coherent, frequency-shifted optical beams directed at an angle to...

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“…There has been a focus recently to improve the transducing mechanism for PA imaging, involving the use of piezoelectric transducers [ 60 ], quartz based transducers [ 75 , 76 ], and MC-based transducers [ 57 , 74 ]. These have found use as ultrasensitive gas detectors [ 57 , 77 ] because of their high sensitivity (in the parts per trillion range). As the transducer is one of the limiting factors in the effectiveness of PA imaging, and with the development of high sensitivity microelectromechanical systems (MEMS) as transducers, it is likely that transducer improvements will continue to be an area of interest for the foreseeable future.…”

Correlated emission of X-ray and sound from water film irradiated by femtosecond laser pulses

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Quartz-Enhanced Photoacoustic Detection of Ethane in the Near-IR Exploiting a Highly Performant Spectrophone

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“…Laser based spectroscopic techniques guarantee a high detection selectivity because of the narrow linewidths characterizing the laser source employed in the visible to the terahertz range, capable of exciting the target molecules while not being absorbed by the gas matrix [11][12][13][14]. The spectral purity of the excitation source, combined with wavelength/amplitude modulation approaches and the efficiency of the optical detection techniques result in reaching detection sensitivity levels as low as parts per quadrillion [15][16][17].…”