Fig. 1. (a) The chemical structure of a monolayer graphene oxide adapted from the Lerf-Klinowski model¹⁸. (b) GO flakes prepared in water solution exhibit brown color before photoreduction and transform into dark color after photoreduction.

Fig. 2. (a) GO exhibits a broadband fluorescence emission ranging from 550 nm to 750 nm. rGO displays similar fluorescence emission but with a reduced intensity¹⁴. (b) Dispersion relations of refractive indices and extinction coefficients of GO and rGO films²⁶. (c) Raman spectra of GO before and after reduction exhibit prominent peaks at D (1354 cm^-1) and G (1599 cm^-1) bands¹⁴. XPS spectra of GO before (d) and after reduction (e)¹⁵.

Fig. 3. (a) Schematic illustration of photoreduction and patterning of GO through direct laser writing techniques. Optical microscopic images of patterned rGO samples by femtosecond-pulsed laser beams with a high repetition rate of 80 MHz (b)⁶ and highly spatially confined reduction by single femtosecond pulses (c).

Fig. 4. (a) Retrieved fluorescence image of two patterns recorded through two-photon photoreduction in two layers in GO-polymer sample separated by a spacing of 20 μm. The scale bar is 10 μm¹⁴. (b) A complex grayscale photograph patterned through LightScribe methods³⁵.

Fig. 5. (a) Schematic illustration of the laser-patterned rGO planar lens¹³. (b) Topographic profile of the rGO planar lens prepared by direct laser writing methods¹³. (c) Simulated and experimental results of the intensity distributions of the focal field in the lateral and axial directions¹³.

Fig. 6. (a) Schematic illustration of subwavelength scale and continuously tunable phase modulation in rGO holograms for full-color 3D displays achieved by exquisitely controllable photoreduction¹⁵. (b) Photograph of a rGO hologram recorded in a GO-dispersed sample¹⁵. (c) The optical image of the rGO holograms generated by laser reduction¹⁴. (d) CCD-captured images by focusing at different depths of reconstructed 3D objects, two teapots¹⁵. (e) Reconstructed color images of two balloons by rGO-polymer holograms through wavelength multiplexing¹⁵.