Optoacoustic sensor based on self-assembled arrays of polystyrene microspheres

Optical techniques are a promising technology for high frequency ultrasound imaging arrays. We are exploring an optoacoustic structure based on self-assembled dielectric microspheres. Polystyrene microspheres are readily available in sub-micron diameters, making monolayer arrays potentially attractive for realizing extremely thin optoacoustic sensors. A monolayer of 0.35 um diameter polystyrene microspheres are formed by self-assembly on a water surface. This layer is transferred to a glass substrate and coated with a 40 nm thick gold film to enhance the optical reflectivity. This structure is attached to the side of a water tank containing a 85 MHz spherically focused f/2 lithium niobate transducer. The probe laser is a temperature controlled Fabry-Perot AlGaAs diode laser with a wavelength of 785 nm. The recorded optical signal and pulse echo data have peak frequencies of 95 MHz and 85 MHz, respectively. The corresponding -6 dB bandwidths are 90 MHz and 70 MHz, respectively. These encouraging results suggest the potential of optoacoustic detection by self-assembled arrays of polystyrene microspheres.