Fig. 1: Resonant NIR laser heating for light induced crystallization.

a Light absorption of NIR (1435 nm) by the first overtone of the O-H stretch vibration of water results in local heating. b Optical setup in which NIR laser light is focused on the bottom (ITO coated) glass substrate of a reaction cell filled with the precursor solution to locally induce crystallization. Using white light, the process is monitored in-situ. c Local heating by NIR light (red line) increases the local supersaturation of salt with retrograde solubility MX (dashed black line) to induce crystallization (blue rhombohedron). d Interfacial temperature measurement for a glass substrate, obtained by monitoring the dual-band emission of HPTS, showing the solution temperature in the NIR focal point as function of the incident NIR laser power. e Solution temperature upon modulation of the NIR power from 0 to 180 mW NIR irradiation over time for the beam center (red dots) and bulk solution measured 150 μm away from the NIR focal point (blue dots). The NIR was turned on and off manually after 10 and 110 s respectively. f Temperature map around the NIR focal point (indicated by red dashed circle) with 180 mW light intensity (pixel size 0.375 μm). g Schematic showing that moving the NIR focal spot (red) results in nucleation and growth of a line of crystals with retrograde solubility on an ITO coated glass substrate. h In-situ optical microscopy time-lapse showing that moving the NIR laser spot induces local precipitation of CaCO₃. i Backscatter SEM image of a NIR laser-written substrate of calcite crystals following a user-defined pattern, with red inset. j Showing agglomerates of CaCO₃ organized in intersecting lines on an ITO-coated substrate.