Elastomeric photonic crystals for optoacoustic detection of high frequency ultrasound

Optical techniques are a promising technology for high frequency ultrasound imaging arrays. We have previously demonstrated an extremely thin etalon sensor based on a self-assembled monolayer of polystyrene microspheres. Increased sensitivity is possible with a stronger optical resonance. We are therefore exploring self-assembled multi-layers of microspheres (photonic crystals) as optoacoustic sensors. The proposed sensing mechanism is the modulation of the microsphere spacing due to the incident ultrasound pressure. A 2% aqueous suspension of 320 nm diameter polystyrene microspheres is placed on a glass cover slip. The colloidal suspension is then completely covered with silicone fluid. The fluid controls water evaporation to produce a more uniform crystallization of the microspheres. The entire sample is maintained at 60 degrees Celsius for one day. The crystallized microspheres are then infused with a polydimethylsiloxane (PDMS) matrix and cured. The orderly arrangement produces strong optical diffraction at a resonant wavelength determined by the spacing and refractive index of the microspheres and the refractive index of the surrounding matrix material. This “photonic sheet” is attached to the side of a water tank containing a 25 MHz spherically focused f/2 transducer. The probe laser is a temperature controlled Fabry-Perot AlGaAs diode laser with a wavelength of 780 nm. The photonic crystal and pulse echo data both have peak frequencies of 25 MHz. The corresponding -6 dB bandwidths are 16 MHz and 11 MHz, respectively. The signal-to-noise ratio of the optical signal is 34 dB over a 50 MHz bandwidth. Further experimental investigations (e.g. reducing cracks) are under way to improve photonic crystal quality. Elastomeric photonic crystals can be coated on curved surfaces as well, making it possible to make focused sensors. We believe these encouraging results suggest the potential of optoacoustic detection by elastomeric photonic crystals.