A typical step in the investigation of the potentiality of a polymer as a gain medium for lasing is the investigation of the presence of Amplified Spontaneous Emission (ASE) in waveguides. ASE is characterized by a directional emission, a strong intensity, and a narrow emission spectrum, leading to quasi-monochromatic emission, and thus good color quality. Organic polymers are soluble in various common organic solvents, thus allowing an easy realization of uniform waveguides by using simple depositions techniques, such as spin coating or drop casting, from the solution phase. Unfortunately any given solvent will bring into solution many different organic polymers, thus the task to realize multicolor ASE emitters, and eventually multicolor lasers, in a single device made of multiple layers of polymers with different ASE wavelengths is absolutely not trivial, as the deposition of a top layer on an existing polymer film typically leads to the dissolution of the bottom layer.
In this Optical Materials Express paper, K. L. Chan and co-authors present a smart approach to generate white light by combining the simultaneous ASE of two different polymers, one emitting in the blue and one in the green, deposited in a single multilayered device. The issue of solubility of the bottom layer during the deposition of the top layer is overcome by realizing two independent waveguides, one for each of the two polymers, on silica substrates, and then sticking them together with a 60 micron thick layer of an optically clear adhesive (OCA). The low refractive index of the OCA layer allows the preservation of waveguiding in both polymers, while the high thickness of the OCA layer also leads to a spatially well-separated output of the two waveguides, thus allowing a spatially separated collection of the blue and the green ASE with an optical fiber placed close to the waveguide lateral edge. A movement of the optical fiber changes the fraction of light collected from the two waveguides, making it possible to generate light with continuously tunable color ranging from the blue to the green, passing very close to pure white.
This paper presents an interesting proof-of-concept of the possibility to generate chromatically tunable light from multilayers of organic polymers by means of ASE. It therefore represents an important first step toward the generation of white organic laser light sources.
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