Fig. 1. Nonlinear absorption of gold nanoparticles. (a) Coefficient of nonlinear absorption κ against the two-photon absorption coefficient β and the peak intensity of excitation field Ipeak. The white solid line marks the boundary between the SA and RSA regimes. (b) Change in the coefficient of nonlinear absorption κ as a function of Ipeak when β=29cm/GW. The black dashed line in panel (b) is the dividing line between the SA and RSA regimes. The diameter of the gold nanoparticle is 60 nm; the medium in which the particle was immersed is water.

Fig. 2. Nonlinear polarizability and extinction cross section of gold nanoparticles. (a) Peak intensity of the focused excitation field as a function of averaged incident power Pave ranging from 0 to 2.0 W. (b) Real and imaginary parts of the nonlinear polarizability α change with increasing excitation intensity. (c) Extinction cross section σext varies with excitation intensity; a maximum value occurs for SA and RSA. Dashed lines in panels (b) and (c) mark threshold values between SA and RSA.

Fig. 3. Nonlinear optical tweezers within the RSA regime. (a) and (b) Schematics of the trapping states of gold nanoparticles within the early and deep RSA regimes. (c) and (d) Recorded particle trajectories obtained from experiments operating in the early and deep RSA regimes, respectively.

Fig. 4. Switching of the nonlinear optical tweezers from the early to the deep RSA regime. (a)–(c) Radial optical force and (d)–(f) trapping potential distributions in the focal plane acting on the gold nanoparticle, for averaged incident power Pave=0.7W, 1.15 W, and 1.54 W, corresponding to respective points D, E, and F labeled in Fig. 2. (g) and (h) Experimental screenshots of gold nanoparticles trapped under incident powers of 1.15 and 1.54 W, respectively. Because of the distinctive trapping potential formed within the deep RSA regime, a gold nanoparticle is constrained stably at the center, while another performs an outer circumgyration (Video 1, MP4, 888 KB [URL: https://doi.org/10.1117/1.AP.5.4.046006.s1]; Video 2, MP4, 200 KB [URL: https://doi.org/10.1117/1.AP.5.4.046006.s2]).