We developed a novel dopamine sensing and measurement technique based on aggregation of gold nanoparticles in random lasers. Dopamine combined with copper ions triggers the aggregation of gold nanoparticles and thus affects the performance of random lasers. Dopamine sensing can be achieved using four parameters which are sensitive to the presence of dopamine, that is emission peak shift, emission linewidth, signal-to-noise ratio (peak emission intensity/noise) and random lasing threshold. The dopamine is most sensitively detected by a change in the emission linewidth with a limit of detection of 1 x 10(-7) M, as well as by an increase in the lasing threshold. The dopamine concentration from 1 x 10-7 M to 1 x 10(-2) M can be determined by calibrating with the laser threshold. (C) 2015 Optical Society of America

obtained, and the energy transfer parameters, including the radiative and non-radiative energy transfer rate constants (KR and KNR), are investigated using Stern-Volmer analysis. The analysis indicates that radiative energy transfer is the dominant energy transfer mechanism in this system. We demonstrate long-wavelength operation (> 700 nm) of random dye lasers (using a methylene blue dye) with the addition of rhodamine 6G and titania, enabled by radiative and non-radiative energy transfer. The pump energy is efficiently absorbed and transferred to the acceptors, to support lasing in random dye lasers in the near infrared. The optimum random laser performance with the highest emission intensity and the lowest lasing threshold was achieved for a concentration of methylene blue as the acceptor equal to 6x the concentration of rhodamine 6G (donor). Excessive levels of methylene blue increased the lasing threshold and broadened the methylene blue emission linewidth due to dye quenching from re-absorption. This is due to competition between the donor emission and energy transfer and between absorption loss and fluorescence quenching. The radiative and non-radiative energy transfer is analyzed as a function of the acceptor concentration and pump energy density, with consideration of the spectral overlap. The dependence of the radiative and nonradiative transfer efficiency on the acceptor concentration is

We demonstrate improved characteristics in Rhodamine dye random lasers with the addition of gold nanoparticles. As a result of the strong plasmonic enhancement induced by gold nanoparticles, Rhodamine 640/gold random lasers have less than half the lasing threshold compared with Rhodamine 640/alumina random lasers in the weakly scattering regime for 10(-3) M dye concentration. The optimum concentration of gold nanoparticles occurs at similar to 8 x 10(10) cm(-3), close to the transition between the weakly scattering and diffusive regimes. Rhodamine 640 has a better performance compared with Rhodamine 6G which is attributed to the greater spectral overlap of the Rhodamine 6G fluorescence spectrum with the plasmon resonance of gold, leading to an increased energy transfer and fluorescence quenching for Rhodamine 6G by gold. We also observe the contrasting trends of lasing threshold between random dye lasers incorporating dielectric and metal nanoparticles in the diffusive scattering regime. The effects of gold nanoparticles in random dye lasers are discussed in the context of the tradeoff between local field enhancement and fluorescence quenching.

We investigated the spectral and coherence signatures of threshold in random lasers with incoherent feedback consisting of alumina colloidal nanoparticles suspended in rhodamine 6G methanol solution under nanosecond-pulsewidth pumping, based on measurement of temporal and spatial coherence properties and comparison with emission spectra. Feedback in this random laser was provided by multiple scattering from the alumina particles, and the effects of particle concentration and scattering length were studied for the weakly scattering and diffusive scattering regimes. At threshold, in each regime, the visibility of the interference fringes jumped abruptly, coinciding with a substantial increase in peak emission intensity and decrease in the linewidth of a single dominant emission peak.