Fig. 1.

Experimental setup. An 800-nm laser: the tunable Ti-Sapphire laser (3900S, Spectra Physics); guiding objective: a long working-distance objective (Mututoyo Corp., Illinois, M Plan Apo $10\times$, 0.28 NA); illumination: collimated LED light source (Thorlabs M470L2-C1); imaging objective: Nikon LWD $20\times /0.40$ Ph1 ADL $00/1.2\mathrm{WD}3.1$ objective; IR filter: short-pass filter (Thorlabs FES0700, 700 nm cutoff); camera: high-speed CMOS camera (Photon Focus, Switzerland, MV1-D1312-160-CL-12).