A temperature-controlled stage was integrated with an inverted microscope (TE2000-U, Nikon Eclipse), as shown in Fig. 2B. Laser illumination was directed toward the sample using an optical fiber, a set of mirrors, and a filter. Several laser wavelengths were used: single-mode 10-μm fiber, tunable from 1536 to 1569 nm (2 W, 1540 nm; model KPS-BT2-TFL-1550-20-FA, Keopsys); single-mode 10-μm fiber (5 W, 1060 nm with a spectral range of ±0.5 nm; model KPS-BT2-YFL-1060-050-LC-COL, Keopsys); and multimode 100-μm fiber (5 W, 980 nm with a spectral range of ±3 nm; model SDL-822, Spectra Diode Labs).

Laser intensity calculations. For a Gaussian beam with an optical power P and Gaussian beam radius ω, the peak intensity (on the beam axis) is given by IP=P12πω2 (51). In our system, ω was measured to be 1.5, 0.865, and 1.145 mm for the 1540-, 1060-, and 980-nm laser beams.

The 1540- and 1060-nm lasers were ice selective, meaning that the absorption coefficients of the ice were higher than those of the water: α (water, 1540 nm) = 12 cm−1 (27), α (ice, 1540 nm) = 34.7 cm−1 (28), α (water, 1060 nm) = 0.149 cm−1 (27), and α (ice, 1060 nm) = 0.243 cm−1 (28). The 980-nm laser was water selective: α (water, 980 nm) = 0.482 cm−1 (27) and α (ice, 980 nm) = 0.139 cm−1 (28).

Experiments conducted using halogen illumination used the halogen lamp of the microscope (100-W lamp; model LHS-H100P-1, Osram halogen 12 V). Experimental data were collected using a charge-coupled device camera (DMK 23G274 GigE camera, Imaging Source).

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