Photoacoustic spectroscopy employs acoustic resonators for signal amplification. Resonators are usually closed, however, in some applications, open resonators are preferred. The opening deteriorates the photoacoustic signal hence reducing the sensitivity of the photoacoustic measurement. We present two new approaches for simulating the photoacoustic signal in open resonators using finite element modelling. The approaches are based on the amplitude mode expansion model and the viscothermal model with the opening modelled using perfectly matched layers and the boundary element method respectively. Additionally, the performance of the viscothermal model using perfectly matched layers for simulating open resonators is extended to the ultrasound region. The simulation results are verified by comparing them to photoacoustic measurements. The approaches provide an accurate basis for designing and optimizing open resonators with high sensitivity.

T-cell resonators have been used lately for non-invasive blood glucose measurements for photoacoustic spectroscopy on skin samples. A resonator has a significant role in determining the strength of the measured signal and the overall sensitivity of the sensor. Here we present results of the measurement of the photoacoustic signal of such a T-cell resonator. The signal is also modelled using the amplitude mode expansion method, which is based on eigenmode expansion and the introduction of losses in the form of loss factors. The measurement reproduced almost all the calculated resonances from the numerical models with fairly good agreement. The cause of the differences between the measured and the simulated resonances are explained. In addition, the amplitude mode expansion simulation model is established as a faster and computationally less demanding photoacoustic simulation alternative to the viscothermal model. The resonance frequencies from the two models differ by less than 1.8%. It is noted that the relative height of the amplitudes from the two models depends on the location of the antinodes within the different parts of the resonator. The amplitude mode expansion model provides a quick simulation tool for the optimization and design of macro resonators.

Photoacoustic (PA) measurements with open resonators usually provide poor detection sensitivity due to signal leakage at the resonator opening. We have recently demonstrated three different approaches for modelling the photoacoustic signal of open resonators. In this work, one of the approaches is applied for the optimization of the geometry of the T-shaped resonator for improved signal strength and thus sensitivity. The results from the numerical optimization show an increase in the photoacoustic signal by a factor of approximately 7.23. They are confirmed using numerical methods other than the one applied for the optimization and by experimental measurement. The measurement shows an increase in the photoacoustic signal by a factor of approximately 2.34.