Optical ultrasound detection techniques. (a) Interferometric methods: (i) Michelson, (ii) Mach–Zehnder, (iii) doppler-based sensing, (iv) resonator-based sensing, and (b) refractometric: (i) intensity-sensitive detection of refractive index, (ii) single-beam deflectometry, (iii) phase-sensitive ultrasound detection. AL: acoustic lens, US: ultrasound, BS: beam splitter, D: detector, DM: demodulator, LA: laser, R: reflector, US: ultrasound, CMOS: complementary metal-oxide-semiconductor, FP: Fourier plane, L: lens, P: prism, PD: photodiode, QPD: quadrant photodiode, SB: Schlieren beam, SF: spatial filter. Reprinted with permission from [42].