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Experimental setup. The pulsed laser (532 nm, repetition frequency 10 Hz, and fluence ), whose spot is shaped by a SLM in amplitude configuration (by using the two crossed polarizers P1 and P2), pumps a single titanium dioxide cluster (diameter between 5 and 12 μm). The RL emission is collected by a microscope objective (OBJ) to be imaged by using a beamsplitter (BS) in two different image planes. In one of them lies a fiber controlled by translators with nanometric resolution that allows the measurement of the spatio-spectral map. In this way it is possible to scan a magnified (50×) image of the sample and measure the spectra emitted from a single point. The fibre core (50 μm in diameter) collects spectra originating in an area of 1 μm of diameter of the sample. The other light path allows imaging the sample on a CCD.
(a) A cluster and surrounding pumped area for . (b)-(e) Images of the random lasing intensity emitted from the cluster for different values of for the cluster C1.
(a) Spectrum from the cluster C3 obtained by pumping with ; (b)-(d) spatial distribution of intensity for the three modes indicated in (a).
(a)-(c) Represent the spatial intensity distribution for the most intense mode of cluster C3 when pumped with , , and , respectively, while panel (d) reports the spatial distribution of the intensity (all wavelengths summed) below lasing threshold providing the shape of the cluster.
Localization length versus angular spread of pump for samples C3 (a) and C6 (b). Left insets show the shape of the most intense mode (M1) for , while right insets show the samples shape.
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