A White Random Laser

Random laser with intrinsically uncomplicated fabrication processes, high spectral radiance, angle-free emission, and conformal onto freeform surfaces is in principle ideal for a variety of applications, ranging from lighting to identification systems. In this work, a white random laser (White-RL) with high-purity and high-stability is designed, fabricated, and demonstrated via the cost-effective materials (e.g., organic laser dyes) and simple methods (e.g., all-solution process and self-assembled structures). Notably, the wavelength, linewidth, and intensity of White-RL are nearly isotropic, nevertheless hard to be achieved in any conventional laser systems. Dynamically fine-tuning colour over a broad visible range is also feasible by on-chip integration of three free-standing monochromatic laser films with selective pumping scheme and appropriate colour balance. With these schematics, White-RL shows great potential and high application values in high-brightness illumination, full-field imaging, full-colour displays, visible-colour communications, and medical biosensing.


A: Low-magnification scanning electron microscopy (SEM) images of self-assembled S420 and DCJTB
To get a better realization of the large-scale constructions, Figure S1a shows the lowmagnification scanning electron microscopy (SEM) image of the self-assembled DCJTB nanostructures doped in red-MPF. Various particles with irregular shapes and sizes ranging from hundreds to thousands of nanometers are observed. Similarly, Figure S1b shows the lowmagnification SEM image of the self-assembled S420 nanostructures doped in blue-MPF.
Random nanostructures and cavities are observed. The length scale is around hundreds of nanometers to several micrometers. Accordingly, the emitted light is trapped within these disordered nanostructures and derives large optical gain through the recurrent scattering process, leading to the amplification of spontaneous emission and random lasing action.

B: The simulation result of electric field in the vicinity of Ag nanoparticles (NPs)
To increase the performance, Green-MPF doped with R6G is embedded with Ag NPs as plasmonic scattering centers. Importantly, Ag NPs embedded inside PVA dominate the random lasing action in two ways. First, they provide light scattering with high efficiency. Second, the excitation of localized surface plasmon resonance enhances the local electromagnetic field near Ag NPs, enabling for high optical gain within those areas. Here we simulated the distribution of electric field intensity (|E| 2 ) at the pumping wavelength of 266 nm as shown in Figure S2.
As expected, the distribution of |E| 2 around the Ag NPs can be realized as the plasmonic enhancement effect, leading to the additional scattered |E| 2 to excite the process of random lasing action.

D: High-resolution lasing spectra of RGB monochromatic polymer films (MPFs)
The classification of random lasers includes incoherent and coherent random lasers.
Incoherent random lasers are characterized by emission band narrowing and increased emission intensity above pumping threshold. On the other hand, coherent feedback random lasers are characterized by random sharp lasing peaks that emerge on top of the emission bands. The mechanisms for coherent and incoherent random lasers are different. Incoherent feedback random lasing can be described by light diffusion in disordered gain medium, while for coherent feedback random lasers, strong interference effects should be considered. Sharp lasing peaks appear due to randomly-formed cavities that provide multiple scattering of light, enabling for strong interference along the cavity paths.
Our proposed system can be identified as a coherent feedback random laser. However, it is quite hard to be observed in the low-resolution lasing spectra (grating 300 grooves/mm with spectral resolution 0.1 nm) since the lasing signals from different modes may be averaged out in low-resolution detection. Thus, we provided the high-resolution lasing spectra (grating 1200 grooves/mm with spectral resolution 0.025 nm) as shown in Figures S4a-c. It is found that all the spectra display similar lasing behavior. Below the threshold excitation intensity, the photoluminescence spectrum is a single broad spontaneous emission peak. While the pumping power increased just above the threshold, multiple narrow peaks emerged in the emission spectrum. At higher pumping power well above threshold, the discrete peaks became even 8 sharper and stronger than those observed under lower pumping power. The full width of half maximum (FWHM) versus the pumping energy density is also presented. With a more detailed examination, the position of the wavelength, number, and the intensity of discrete laser peaks with extremely narrow linewidth changed randomly. The above-mentioned results are typical and well-known characteristics of coherent feedback random lasing action. Moreover, Figures   S4d-f show the obvious narrowing of the full width at half maximum (FWHM) when the exciting energy surpasses the threshold, which is a fundamental property of laser action.
Coherent feedback random laser may provide more efficient and stronger laser emission since the strong scattering system can trap the coherent photons to form a closed loop in the disordered nanostructures. In this type of random lasing system, the interference effect occurs in a closed loop. Its typical characteristic is the laser spikes fluctuating within the spectral emission. Researchers now attempt to control the coherent random lasing modes. Modelocking and single-mode random lasers have been demonstrated by the pumping scheme, Raman gain, intentional defect sites, and bioinspired photonic structure. On the other hand, incoherent feedback random laser can take place under dexterous scattering system, where the photons can partially return to the gain media but not to the original position. Thus, there is no spatial resonance. Since the mean frequency of the random laser only depends on the frequency of the emission band of the gain media. Such relatively broad spectral characteristics (as much as few to tens nanometer wide) can find various applications. For example, with low temporal 9 coherence, it can truly reduce speckle noise, serving as a promising speckle-free laser-level light source for imaging or sub-micron optical lithography.

E: Calculation of chromaticity
To calculate the chromaticity of white-RL, the random lasing signals and spontaneous emission are decomposed using Lorentz fitting method as shown in Figure S5. Subsequently, the unnecessary influence from spontaneous emission is eliminated, and the pure RL emission, which is of our interests, is studied in Figures 5, 6, S6, S7, S10, and S11. As a result, the chromaticity of White-RL is calculated using these fitted spectra to emphasize the variation of pure RL. It is shown that the chromaticity of White-RL is very close to ideal white emission and nearly independent of observation angles.

H: Stability and deformability of MPF-based white random laser (White-RL)
The samples are exposed to the air for different days to test the stability of organic laser dye shown in Figure S9. Differences in intensities are hardly observed, showing that MPFbased White-RL has great stability under ambient condition, which is very important for practical applications. On the other hand, the lasing performance of the dyes may be weakened under exposure to high energy UV pulses pumping. In our sample synthesis, we do concern this issue. The poly (methyl methacrylate) (PMMA) or poly (vinyl alcohol) (PVA) thin film was spin-coated onto RGB monochromatic films to protect laser dyes from possible damage and thus prolong the lifetime of White-RL.
Moreover, deformability remains a key factor towards the development of portable and wearable optoelectronic devices. To demonstrate the deformability of MPF-based White-RL, lasing spectra of White-RL with and without external strain are performed and shown in Figure   S10 where the applied strain is defined by the change in device dimension with respect to its original size. There are little differences in the spectra when the sample is stretched under 30% of external strain. The bendability of the device is estimated by recording the device performance at a bending diameter of 10 cm as shown in Figure S11. The emission spectrum of the sample under bending is similar to that of non-bending one. Therefore, MPF-based White-RL exhibits high deformability, which is potentially useful for advanced optoelectronics.  NPs. Furthermore, we use this new fabrication process (adding R6G after precursor solution) on the green monochromatic polymer film synthesis to examine the lasing performance. In Figure S13b, the light-in-light-out curves indicate that there is no obvious threshold change compared with Figure 4d. Based on these results, we think that there is no dramatic morphology change of Ag nanoparticles under two different processing schemes.