Negative acoustic radiation force induced on an elastic sphere by laser irradiation

We propose an alternative strategy to modulate the acoustic radiation force (ARF) acting on an elastic sphere (ES) using laser irradiation. A mathematical model of the interaction of the acoustic plane wave with the laser-irradiated ES is developed to calculate the ARF acting on the ES. It is demonstrated that the action of this laser-induced photoacoustic effect on a polystyrene (PS) sphere could strongly affect the ARFs of acoustic plane waves that are incident on the PS sphere and negative ARFs could thus be realized. The variational ARF behavior observed is ascribed to competition between the photoacoustic effect and the acoustic scattering. Furthermore, we find that both the laser intensity and the phase difference between the incident laser beam and the acoustic plane wave could be used to modulate the ARF acting on the irradiated PS sphere, with the phase difference playing the dominant role. One effective method for control of the negative ARF involves changing the laser intensity with a suitable phase difference. The work presented here may lead to an alternative way to perform single-beam acoustic manipulation.

Figure 1

Schematic of an ES irradiated by both an acoustic plane wave and a laser beam.

Figure 2

(a) Analytical and numerical ARF function $Y_p$ curves of the passive PS sphere $\left(I_0=0\right)$ as a function of frequency. (b) Amplitude and phase lag of the photoacoustic pressure of a PS sphere in water as a function of frequency. (c) Analytical and numerical ARF function $Y_p$ curves of the irradiated PS sphere $\left(I_0=0.1\mathrm{mW}/\mu {\mathrm{m}}^2\right)$ as a function of frequency. Here, $\mathrm{\Delta }\phi$ is assumed to be equal to $\mathrm{\Delta }\delta$; This means that the photoacoustic signal is in phase with the incident acoustic plane wave.

Figure 3

Scattering and absorption coefficients of the (a) PS sphere $\left(I_0=0\right)$ and (b) the irradiated PS sphere $\left(I_0=0.1\mathrm{mW}/\mu {\mathrm{m}}^2\right)$ as functions of frequency. Variations of asymmetry factor ${\langle cos\theta \rangle }_s$ of (c) the passive PS sphere and (d) the irradiated PS sphere with the frequency.

Figure 4

Contour plots of the (a) $Y_p$, (b) $Q_{\mathrm{abs}}$, (c) $Q_{\mathrm{sca}}$, and (d) ${\langle cos\theta \rangle }_s$ of the irradiated PS sphere as functions of the laser intensity and frequency. Here, $\mathrm{\Delta }\phi$ is assumed to be equal to $\mathrm{\Delta }\delta$.

Figure 5

Contour plots of the (a) $Y_p$, (b) $Q_{\mathrm{abs}}$, (c) $Q_{\mathrm{sca}}$, and (d) ${\langle cos\theta \rangle }_s$ of the irradiated PS sphere as functions of the phase difference $\mathrm{\Delta }\phi$ and frequency. Here, $I_0=0.1\mathrm{mW}/\mu {\mathrm{m}}^2$.

Figure 6

Contour plots of the ARF function $Y_p$ of the irradiated PS sphere at frequencies of (a) 10 MHz and (b) 20 MHz as functions of the laser intensity $I_0$ and phase difference $\mathrm{\Delta }\phi$.