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Opt Express. 2013 ;21(5):6391-400.
Hajireza P, Krause K, Brett M, Zemp R.
Department of Electrical and Computer Engineering, University of Alberta, Edmonton, Alberta T6G 2V4, Canada.
Abstract
In this paper a new class of optical Fabry-Perot-based ultrasound detectors using low acoustic impedance glancing angle deposited (GLAD) films is demonstrated. glancing angle deposited is a single-step physical vapor-deposition (PVD) technique used to fabricate porous nanostructured thin films. Using titanium dioxide (TiO(2)), a transparent semiconductor with a high refractive index (n = 2.4), the glancing angle deposited technique can be employed to fabricate samples with tailored nano-porosity, refractive index periodicities, and high Q-factor reflectance spectra. The average acoustic impedance of the porous films is lower than bulk materials which will improve acoustic coupling, especially for high acoustic frequencies. For this work, two filters with high reflection in the C-band range and high transparency in the visible range (~80%) using glancing angle deposited films were fabricated. A 23 µm Parylene C layer was sandwiched between these two glancing angle deposited films in order to form a glancing angle deposited Fabry Perot Interferometer (glancing angle deposited-FPI). A high speed tunable continuous wavelength C-band laser was focused at the FPI and the reflection was measured using a high speed photodiode. The ultrasound pressure modulated the optical thickness of the FPI and hence its reflectivity. The fabricated sensor was tested using a 10 MHz unfocused transducer. The ultrasound transducer was calibrated using a hydrophone. The minimum detectable acoustic pressure was measured as 80 ± 20 Pa and the -3dB bandwidth was measured to be 18 MHz. This ultra-sensitive sensor can be an alternative to piezoelectric ultrasound transducers for any techniques in which ultrasound waves need to be detected including ultrasonic and photoacoustic imaging modalities. We demonstrate our glancing angle deposited-FPI for photoacoustic signal detection in optical-resolution photoacoustic microscopy (OR-PAM). To the best of our knowledge, this is the first time that a FPI fabricated using the glancing angle deposited method has been used for ultra-sensitive ultrasound detection.
