A hybrid optical tweezers and photoacoustic microscopy system

Abstract

Photoacoustic microscopy (PAM) is a promising imaging modality that combines optical and ultrasound imaging. It takes advantage of high optical contrast and high ultrasonic spatial resolution. Conventionally, piezoelectric transducers are used to de tect photoacoustic waves. Here, it is aimed to utilize a particle trapped by an optical tweezers (OT) system as a sensor by tracking the change in its displacements caused by the photoacoustic radiation force. By this way, it is possible to put the detector as close as a few microns to the absorber which is not possible with any other currently existing detection mechanisms. An all-fiber integrated laser with custom-developed electronics and software is developed specifically for the hybrid PAM and OT system. The laser is home-built for maximum flexibility in adjustment of its parameters; pulse duration (5-10 ns), energy (up to 10 µJ) and repetition frequency (up to 1 MHz) in dependently from each other. It covers a broad spectral region from 450 to 1100 nm and can also emit wavelengths of 532, 355, and 266 nm. Photoacoustic radiation force caused by the change in the density of time-averaged kinetic and potential energy of the acoustic wave when encountered with a compressible or incompressible sphere is analyzed theoretically. Then, experiments are conducted on several particles and cells to manipulate them inside an absorbing medium. Apart from system development for diagnostics, to turn a hand on the treatment side, proton-induced acoustics is studied which has a potential to get real-time feedback for intratreatment adjustments and to reduce range uncertainty via high spatial resolution in ultrasound. In this regard, an analytic solution for the proton-induced acoustic wave is presented; then it is combined with an analytic approximation of the Bragg curve. The influence of the beam energy, pulse duration and beam diameter variation on the acoustic waveform are investigated.