We demonstrate the efficient amplified spontaneous emission (ASE) of rhodamine 640 in asymmetric-coupled-waveguides. In these structures, two active waveguides, one doped with [2-[2-[4-(dimethylamino)phenyl]ethenyl]-6-methyl-4H-pyran-4-ylidene]-propanedinitrile (DCM) and the other with rhodamine 640, are coupled by a passive layer of polyvinyl alcohol. Under a suitable pumping wavelength of 500 nm, the DCM waveguiding layer acts as the donor and rhodamine waveguiding layer acts as the acceptor. Time-resolved fluorescence measurements are employed to confirm that radiative energy transfer is the responsible mechanism for ASE when the waveguiding layers are separated by a thickness of less than 50 nm. In this case, the ASE threshold was found to be reduced by a factor of 16, associated with an intensity enhancement of 56 times, compared to that of the rhodamine waveguiding layer. Next, we investigate the ASE properties of asymmetric-coupled-waveguides as a function of coupling layer thickness. The thickness-dependent coupling constant is simulated using the coupled-mode theory, and its behavior is applied to understand the experimental results.
We demonstrate the role of coupling coefficients in asymmetric coupled waveguides towards delivering a low threshold amplified spontaneous emission (ASE). A theoretical investigation in four different coupled waveguides possessing varying order of symmetry that arises from the bounding media refractive indices shows that the coupling coefficient carries the asymmetry in accordance with the nature of symmetry of the investigating structure. That is, a symmetric coupled waveguide is characterized with identical and asymmetric coupled waveguides, with, unlike coupling coefficients. The effect of unequal coefficients designating the electromagnetic energy transfer from one waveguide to the other and vice versa are experimentally demonstrated on the ASE of the acceptor (Rhodamine 640) by flipping the positions of the donor [[2-[2-[4-(dimethylamino) phenyl] ethenyl]-6-methyl-4H-pyran-4-ylidene]-propanedinitrile (DCM) and acceptor molecules that exist in the opposite waveguides of an asymmetric coupled waveguide. We also correlate the value of the coupling coefficient with the observed threshold of the acceptor ASE towards the optimization of a coupled waveguide as an efficient optical amplifier.
We demonstrate efficient amplified spontaneous emission (ASE) of an acceptor from a sophisticated on-chip dynamic droplet Förster resonance energy transfer (FRET) system. The effect of changing the FRET efficiency and the pump wavelength absorption by the donor molecules is studied using two FRET pairs having dissimilar donors [Rhodamine 6G (R6G) and Rhodamine B (RB)] but a common acceptor [Nile blue (NB)], in microdroplets of different sizes and shapes ranging from spherical to squashed cylindrical. We show that the threshold of acceptor ASE depends on the extent of resonance of pump wavelength with the absorption maximum of the donor molecule, whereas this has no effect on the FRET efficiency, in agreement with the existing finding that FRET efficiency is independent of the excitation wavelength. We also demonstrate a tunability of 18 nm in the acceptor ASE by tweaking the size and shape of the generated droplets from spherical to squashed cylindrical. The reason for tunability is attributed to a change in the effective concentration of the dye molecules in different droplets.
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