Photoacoustic Doppler Effect

The Photoacoustic Doppler Effect is one specific type of Doppler effect which occurs when an intensely modulated light wave induces a photoacoustic wave on moving particles with a specific frequency. The observed  frequency shift is a good indicator of the velocity of the illuminated moving particles. A potential biomedical application is measuring blood flow.
Specifically, when an intensity modulated light wave is exerted  on a localized medium, the resulting heat can induce an alternating and localized pressure change. This periodic pressure change generates an acoustic wave with a specific frequency. Among various factors that determine this frequency, the velocity of the heated area and thus the moving particles in this area can induce a frequency shift proportional to the relative motion. Thus, from the perspective of an observer, the observed frequency shift can be used to derive the velocity of illuminated moving particles.
The following equation is used to find the frequency shift of the photoacoustic doppler effect: `f_(PAD)=-f_0v/c_0cosalpha+f_0v/c_alphacostheta`, where:

• `f_0` = frequency
• `v` = velocity
• `c_0` = speed of light in the medium
• `alpha` = angle between the velocity and the photon density wave propagation direction
• `c_alpha` = speed of sound (constant)
• `theta` = angle between the velocity and the ultrasonic wave propagation direction

References

Wikipedia (https://en.wikipedia.org/wiki/Photoacoustic_Doppler_effect)