Fig. 1. (Color online) Schematic diagram of the femtosecond laser writing system.

Fig. 2. (Color online) (a) PL spectra of pristine and laser-irradiated PMMA (single fs). The PL data were acquired using 355, 532 and 633 nm lasers. (b) SEM image of dots modified by threshold fs laser power at the surface. (c, d) Optical and confocal fluorescence microscopy images of the dots beneath the 30 μm surface. The scale bar is 8 μm. Here, 14 dots can be observed when the measurement length is 8 μm, suggesting that the dot spacing is ~600 nm.

Fig. 3. (Color online) (a) Absorption spectrum of pristine PMMA, the chemical structure of which is shown in the inset. (b) The 3D fluorescence structure of the array with fixed writing power (40 nJ) inside PMMA separated by 10 μm, which was read out by confocal fluorescence microscopy. (c) Confocal fluorescence microscopy image of the fs laser-modified Emblem of Nanjing Normal University and Southeast University, The Great Wall and Albert Einstein in the 1st, 2nd, 3rd, and 4th layers, respectively, scale bar of which is 30 μm. Specific patterns can be stored in PMMA with a layer separated by 10 μm.

Fig. 4. (Color online) (a, c) Confocal fluorescence microscopy images of the dots (1st and 50th layers) written by a fs laser with a single pulse, scale bar: 20 μm. (b, d) show the fluorescence intensity change according to pulse energy evolution, which are acquired from the 1st and 50th layers, respectively. The threshold power is 40 nJ for the 1st layer and 114 nJ for the 50th layer.

Fig. 5. (Color online) Gray level assignment based on fluorescence distinguish. It should be noted that both (a) the fresh sample and (b) the aged sample can be of capability of 8 gray level assignment.