Acousto-Optic Modulation of Water in a Microfluidic Channel Using Planar Fresnel Type GHz Ultrasonic Transducer

Acousto-optic modulation at GHz frequencies is demonstrated in water in a microfluidic channel. The photoelastic effect in water is induced by a silicon based GHz bulk acoustic wave aluminum nitride transducer placed in a Fresnel lens configuration. The planar GHz ultrasonic transducer is fabricated with a CMOS compatible process with no thin-film release step enabling easier integration with CMOS and microfluidics. A UHF vibrometer which is sensitive to changes in the refractive index along the laser path is used to measure the peak surface displacement. The peak refractive index change was determined to be O(10-6), when 1.08GHz RF drive voltages between 1Vp and 5Vp were applied to the focusing transducer. Peak phase modulation of 6mrad was determined from the experimental results for 5Vp RF drive signal. The total modulator area of such a system is 0.086 mm2. This result provides a framework to implement CMOS integrated acousto-optic modulator arrays.