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Description
Photoacoustic (PA) imaging combines optical absorption contrast with the high imaging depth of ultrasonic techniques. It has been well established for the investigation of biological tissue at shallow (millimeters) depths. Imaging multi-centimeter depths in heterogeneous tissue is more challenging. For example, strong ultrasonic attenuation in bone tissue hinders applications such as human brain imaging—driving the need for more sensitive ultrasound detectors.We have developed ultrasensitive plano-concave Fabry-Pérot (FP) sensors for deep PA imaging. The sensor geometry allows mode matching to the interrogation laser beams. In combination with soft, thick and impedance matched spacers, this results in high optical Q-factors and a high, broadband acoustic sensitivity.The sensors detect sub-Pascal pressure amplitudes over a bandwidth of more than 1 MHz. While single sensors have been used for PA imaging, parallelised detection using a sensor array would require separate interrogation lasers for each sensor element, which is economically infeasible. Also, if a single laser was used to interrogate multiple sensors, the fabrication tolerances would be in the sub-nanometer range for a millimeter-sized spacer to ensure sufficient spectral overlap of the resonances.In this study, we demonstrate a method for simultaneous photothermal tuning of multiple plano-concave FP sensors using a single tunable interrogation laser for synchronization and interrogation, enabling simultaneous PA measurements. This method makes use of resonance-dependent photothermal heating induced by the interrogation laser, which affects the optical path length and therefore the resonance wavelength. We outline a general approach for designing synchronizable ultrasensitive FP sensors by controlling their thermal properties and the resulting photothermal shift.