Radiative energy transfer assisted amplified spontaneous emission in asymmetric-coupled-waveguide structures

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.