Athermally photoreduced GOs

We record holographically correlated refractive-index modulation in the GO-dispersed photopolymers through the area-by-area digitalization by multilevel multifocal arrays created by the Debye diffraction method31, as shown in Fig. 1a, while the intensity of each focal spot produced by this method and hence the refractive-index modulation in the rGO composite can be finely tuned in terms of the correlation. By removing the undesired reduction associated with the diffusion of accumulative heating, the athermal photoreduction through a single fs pulse is confined to a diffraction-limited region of each focal spot. Consequently, increasing the numerical aperture (NA) of the objective used for parallel digitalization can lead to a decreased size of each focal spot down to a subwavelength scale of the reconstruction beam, and hence an increased viewing angle, as illustrated in Fig. 1b. Moreover, the spectrally flat refractive index of the rGO composite in the visible range32,33 makes it ideal for multicolour holography. A wavelength-multiplexed phase hologram can be used for colour images, where light waves at three wavelengths are incident simultaneously in a tilted configuration to synthesize corresponding colour components (Fig. 1c).

Figure 1: rGO holograms by a single femtosecond pulse for 3D images with wide viewing angles and colour images. (a) Schematic illustration of the optical digitalization of refractive-index/phase modulation by the athermal photoreduction using a single fs pulse. The subwavelength-scale phase modulation from 0 to π is finely tuned by the intensity of a fs beam. The area-by-area parallel digitalization is achieved through an objective capable of generating MMAs with variant intensities in each focal spot corresponding to the phase correlation. (b) Scheme of wide-angle 3D images by confining the photoreduction at a subwavelength scale through increasing the NA of the parallel digitalization objective. (c) Reconstruction of colour objects through the wavelength-multiplexed phase hologram recorded in GO polymers. Full size image

The photoreduction of as prepared GO polymers by a single fs pulse (Methods) into rGO polymers was verified by micro-Raman spectroscopy and X-ray photoemission spectroscopy in Fig. 2. The accumulative heating can be excluded from the experiment since a single pulse is employed. The athermal nature of the observed photoreduction was confirmed by monitoring the temperature increment in the focal region using fluorescent CdSe spherical nanoparticles as nanothermometers34,35. The inset of Fig. 2a shows the experimental set-up for the temperature measurement by monitoring the spectral shift of the fluorescence peak of CdSe nanoparticles (Methods). Indeed, no notable spectral shift of CdSe nanoparticles (Fig. 2a) and hence no temperature increment (inset of Fig. 2a) were observed at the entire range of the pulse fluence used for the photoreduction. Figure 2b depicts the Raman spectra of rGOs at different fluences of the single pulse irradiation. The characteristic D- and G-bands centred at 1,348 and 1,578 cm−1, respectively, are broad and accompanied by a peak at 1,050 cm−1, which is associated with the vibration bands of carbon atoms at the presence of oxygen-containing groups36. The strength of the peak at 1,050 cm−1 is decreased as the pulse fluence increases, indicating a deoxygenation process. The intensity ratio between D- and G- bands remains almost constant during the photoreduction accompanied by the rising of the 2D bands (Supplementary Fig. 1). The observed photoreduction can be possibly attributed to the photoionization after absorbing the pulse (Supplementary Discussion).

Figure 2: Athermal photoreduction of GOs by a single fs pulse. (a) Spectral shift of the two-photon fluorescence of CdSe nanoparticles irradiated by single fs pulses at a variety of pulse energy densities. The dashed line indicates the peak position of the CdSe fluorescence spectra. The insets show the experimental configuration for the focal temperature measurement and extracted temperature increment. (b) Raman spectra of the rGO polymers as a function of the fs pulse fluence. Three dashed lines indicate the D-bands, G-bands and 1,050 cm−1, respectively. X-ray photoemission spectroscopy spectra of GOs before (c) and after (d) photoreduction by a single fs pulse at the pulse fluence of 2 nJ cm−2. Insets are the scanning electron microscope (SEM) images of GOs before (c) and after (d) the photoreduction. Full size image

The strength of reduction can be controlled by the fluence of the single fs pulse, which can be quantified by X-ray photoemission spectroscopy. Figure 2c,d shows the photoemission spectra of the carbon 1s of GOs before and after the photoreduction. The deoxygenation through the athermal photoreduction process is evident by a drastic decrease of the C–O–C (286 eV), C=O (287 eV) and OH–C=O (288.5 eV) peaks, meaning a restoration of the pure C–C/C=C bond from 28 to 54%. The deoxygenation was accompanied by a morphology change from the disordered manner in GO stacks to micro-sized rGO flakes (insets of Fig. 2c,d). In contrast, quasi-continuous pulsed irradiation operated at a laser repetition rate of 80 MHz induces significant accumulative heating and a pronounced temperature increment in the focal plane (Supplementary Figs 2 and 3). The deoxygenation by the constant thermal treatment changed the morphology to micrometre-long rGO wrinkles (Supplementary Fig. 4).

Reduced graphene-oxide holograms for 3D images

The athermally digitalized photoreduction of GOs in photopolymers enables one to judiciously control the refractive index of rGOs as verified by the diffraction experiment in Fig. 3, which is crucial for the subsequent recording of the correlated phase modulation in individual pixels for rGO holograms. Figure 3a–c depicts the measured phase accumulation when the reconstruction beam propagates through the rGO polymer irradiated by the single fs pulse (Methods). The wide-field image of one typical example of the phase grating composed of arrays of refractive-index pixels through the photoreduction and its diffraction image are shown in Fig. 3a,b, respectively. Intensifying the strength of the photoreduction by increasing the pulse fluence at each focal spot leads to an exponential increase in the strength of the phase modulation, which is a basis to finely digitalize the localized phase modulation by judiciously configuring the exposing intensity (Fig. 3c). The large dynamic range of the phase modulation to π opens the possibility of multilevel rGO holograms with a high diffraction efficiency (Methods). Figure 3d shows examples of microscopic images of 4-, 8- and 16-level digitalized rGO holograms. Increased first-order diffraction efficiency was observed as increasing the levels of modulations. A diffraction efficiency of 15.88% was achieved at a 16-level modulation.

Figure 3: Localized phase modulation produced by the refractive-index change of GOs through the tunable extent of the photoreduction. (a) Wide-field image of one typical phase grating composed of arrays of refractive-index modulation pixels through the athermal photoreduction with a parallel digitalization objective. Scale bar, 6 μm. (b) Example of diffraction images of ±1st and 0th orders. (c) Phase modulation strength as a function of the pulse fluence of the fs beam. The circles are experimental data and the blue curve is the guide for eyes. The colour levels indicate digitalized phase modulations by judicious control of the intensity of the fs beam. (d) Microscopic images of a section of a rGO hologram through 4-, 8- and 16-level digitalized photoreduction processes of GO polymers and their statistics of strengths of randomly selected refractive-index pixels. The table shows the comparison of the first-order diffraction efficiency of rGO holograms with different levels of modulation. Full size image

A computed-generated 3D cubic was obtained by the point source method37 and fast digitalized into a phase profile through translating the GO polymer sample with respect to the focal plane of the parallel digitalization objective (Supplementary Fig. 5; Supplementary Movie 1). Since the athermal photoreduction can be confined within the diffraction-limited focal voxel of the multilevel multifocal array, we can increase the NA of the parallel digitalization objective to reduce the constitutive pixel size below the wavelength of the beam employed for the image reconstruction, which is impossible when a thermal reduction induced by quasi-continuous pulsed irradiations is employed (Supplementary Figs 2 and 3). Examples of microscopic images of a section of the holograms recorded by different NA are shown in Fig. 4a–d. Figure 4e depicts that the viewing angle is drastically increased by reducing the constitutive pixel size, which is reasonably consistent with the calculation (Methods). In particular, when the pixel size is reduced to 0.55 μm, the viewing angle can be increased up to 52 degrees (inset of Fig. 4e; Supplementary Movie 2), which is one order of magnitude larger than that of metamaterial4,5,6 or carbon nanotube7 holograms with a similar image size. For comparison, a commercially available liquid crystal-based spatial light modulator (SLM) (1,920 × 1,024 pixels and pixel size of 8 μm) with a viewing angle of ∼5 degrees is also shown. Since the space–bandwidth product38 of a hologram is approximately proportional to the product between the number of constitutive pixels in each direction, the rGO hologram exhibits the remarkably extended space–bandwidth product through the parallel digitalization, by one order of magnitude larger than that of metasurface- and metamaterial-based write-once holograms4,5,6. Thus, the reconstruction of 3D images with high-resolution full-depth perceptions is feasible (Methods). Figure 4f shows images of reconstructed objects, two teapots floating above the rGO hologram, captured at different depths (Supplementary Movie 3). The rGO holograms can also be up-scalable for practical applications with sufficient high resolution (Supplementary Fig. 6).

Figure 4: rGO holograms for 3D colour images. Typical examples of microscopic images of sections of rGO holograms produced by objectives with different values of the NA. Scale bars, 5 μm. The insets show examples of intensity cross-sections of constitutive pixels with an effective size of 2 μm (a), 1 μm (b), 0.75 μm (c) and 0.55 μm (d). (e) Viewing angle as a function of the size of the constitutive pixel. The circles are experimental data and the blue curve presents calculation results. The inset shows reconstructed images of a rGO hologram with a pixel size of 0.55 μm captured at different viewing angles. The liquid crystal-based spatial light modulator (SLM, indicated by the arrow) with a pixel size of 8 μm has a limited viewing angle of ∼5 degrees. (f) Images of reconstructed 3D objects, two teapots, captured at different depths. (g) Reconstruction of colour objects, two balloons through a wavelength-multiplexed phase hologram recorded in GO polymers. Full size image

In addition, the spectrally flat refractive-index modulation of the photoreduction process (Supplementary Fig. 7) enables its application in colour 3D images. Full-colour images can be synthesized by a wavelength-multiplexed phase hologram with angular offsets at corresponding wavelengths. For reconstruction, three laser beams at the wavelengths of 405, 532 and 632 nm, respectively, were employed in a tilted configuration (Fig. 1c). The white colour balance was obtained by carefully controlling the powers of the three constitutive laser beams. Reconstruction of full-colour objects, two balloons, can be seen vitally from the multiplexed rGO hologram (Fig. 4g; Supplementary Movie 4).

To date, the reconstruction of vectorial or polarization-dependent wavefronts, termed as vectorial holographic images where polarization orientations on a 3D wavefront are spatially variant, still remains elusive for two reasons. First of all, a large modulation strength over π in constitutive pixels of a pure phase hologram to generate the constructive or deconstructive interference of diffracted fields is crucial to realize spatially varying polarization orientations through the vectorial diffraction39,40. Meanwhile, the isotropic or polarization-insensitive refractive-index modulation in each pixel is essential to record polarization-multiplexed phase holograms without any distortion of the vectorial field distribution. In this context, rGO composites by the athermal photoreduction provide the perfect material platform to fulfil the requirement for the reconstruction of vectorial wavefronts. Such reconstructed vectorial wavefronts carrying the polarization-sensitive information of objects can be distinctively discerned by rotating the polarization angle of the analyzer (Supplementary Fig. 8). As an example, Fig. 5a,b shows the captured images of the reconstructed vectorial wavefront of two human figures with left and right parts clearly discerned at the vertical and horizontal polarization angles, respectively. The two images carried by the wavefront are smoothly transitioned from one to the other when rotating the polarization alignment of the analyzer (Supplementary Movie 5). Figure 5c–e shows that the reconstructed 3D vectorial wavefront, two kangaroos with different polarization orientations, can be discerned at corresponding polarization angles or viewed simultaneously.