Paper is such a versatile material that it never ceases to amaze. It was not until recently that its potential in developing low-cost microfluidic diagnostic technologies started being utilized . In yet another intriguing demonstration of the use of paper, a group at Istituto Nanoscienze, CNR in Italy realized lasing with paper as the substrate . Paper, with its complex network of randomly distributed fibers, may seem like an odd choice with which to realize a laser, which traditionally required a gain medium of controlled purity, periodicity, size, concentration, and shape. However, the incipient technique of random lasing , which employs scattering processes of light for optical gain and doesn’t rely on external feedback to achieve above unity gain, can be implemented on a low-ordered substrate, such as paper. When imbibed with a fluorescent lasing dye like Rhodamine B (RhB), paper provides a randomly distributed network of scatterers (fibers) in an optical gain medium (dye) required for random lasing. The emission spectrum of a paper-based random laser was found to be dependent on laser-dye characteristics, microfluidic channel dimensions, and the shape, pore-size, local refractive index, and functionalization of the substrate (Fig 1). With a tunable spectral response and susceptibility to a variety of channel properties, random lasers may find application as an optical transducer or sensor in biosensing and diagnostics. The disadvantage of random lasing is loss of coherence in light output and the requirement of optical pumping. The understanding of physics behind this observation is still in its incipient stage and further work in lasing in paper is warranted.
Figure 1: Methods and results of paper-based laser. (a) The microfluidic circuit is realized lithographically on a single layer chromatography paper and is filled by capillary driven laser dye (RhB). Inset shows true colors of channel wetting. (b) shows variation in emission peak intensity as a function of optical pumping energy for channels of different widths compared to native non-patterned paper. (c-d) demonstrates variation in emission characteristics with functionalization of surface with high refractive index TiO2 and channel shape respectively. (e) shows emission spectra for 100μm wide channel and in the inset the emission spectra of pure RhB in ethylene glycol. Pictures adapted without permission from .
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Gottardo, Stefano, et al. "Resonance-driven random lasing." Nature Photonics 2.7 (2008): 429-432.
Author: Rahil Jain is a graduate student in the Electrical Engineering department at UW, Seattle. His work in the Lutz Lab focusses on developing microfluidic technologies for application in low-cost diagnostics.