11Metal nanoclusters consist of a few to few hundreds of atoms, and exhibit attractive molecular 12 properties such as ultrasmall size, discrete energy levels and strong fluorescence. Although 13 patterning of these clusters down to the microscale or nanoscale could lead to 14 applications such as high-density data storage, it has been reported only for inorganic 15matrices. Here we demonstrate the first submicron-scale mask-free patterning of fluorescent 16 silver nanoclusters in an organic matrix. The nanoclusters were produced by direct 17 laser writing in poly(methacrylic acid) thin films, and exhibit a broadband emission at visible 18 wavelengths with photostability that is superior to Rhodamine 6G dye. This fabrication 19 method could open new opportunities for applications in nanophotonics like imaging, 20 labeling, and metal ion sensing. We foresee that this method can be further applied to prepare 21 other metal nanoclusters embedded in compositionally different polymer matrices. 22 KEYWORDS 24Optical lithography, metal nanoclusters, photoluminescence, photobleaching, photostability, 25 polymer 26

Noble metal nanoclusters are ultrasmall nanomaterials with tunable properties and huge application potential; however, retaining their enhanced functionality is difficult as they readily lose their properties without stabilization. Here, we demonstrate a facile synthesis of highly photostable silver nanoclusters in a polymer thin film using visible light photoreduction. Furthermore, the different stages of the nanocluster formation are investigated in detail using absorption and fluorescence spectroscopy, fluorescence microscopy, and atomic force microscopy. A cost-effective fabrication of photostable micron-sized fluorescent silver nanocluster barcode is demonstrated in silver-impregnated polymer films using a low-power continuous-wave laser diode. It is shown that a laser power of as low as 0.75 mW is enough to write fluorescent structures, corresponding to the specifications of a commercially available laser pointer. The as-formed nanocluster-containing microstructures can be useful in direct labeling applications such as authenticity marking and fluorescent labeling.

Metal nanoclusters (NCs) are nanomaterials of size of less than 2 nm that exhibit a set of unique physical, chemical, optical, and electronic properties. Because of recent interest in NCs, a great deal of effort is being made to develop synthetic routes that allow control over the NC size, shape, geometry, and properties. Direct laser writing is one of the few synthesis methods that allow the generation of photostable NCs with high quantum yield in a highly controlled fashion. A key advantage of laserwritten NCs is the ability to create easy-to-use solid-state devices for a range of applications. This review will present necessary background and recent advances in laser writing of silver NCs and their applications in different solid-state matrixes such as glass, zeolites, and polymer substrate. This topic will be of interest to researchers in the fields of materials science, optics and photonics, chemistry, and biomedical sciences.

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We demonstrate the capability of polarized third-harmonic generation (THG) for high contrast imaging of three-dimensional microstructures fabricated by photopolymerization. Using circular polarization of fundamental light, background-free optically-sectioned THG images were obtained from laser-written photopolymerized microstructures. The technique has great potential for simple and noninvasive characterization of photopolymerized devices, which typically show poor contrast in conventional light microscopy.