Energy transfer and amplified spontaneous emission in temperature-controlled random scattering media

Hydroxypropyl cellulose (HPC) in aqueous solution forms nanoparticles and becomes highly scattering above its lower critical solution temperature (LCST approximately 41 degrees C). Enhancement of energy transfer (ET) and amplified spontaneous emission (ASE) have been observed in turbid HPC solutions containing Rhodamine 6G (RG) as an energy donor and Kiton Red 620 (KR) as an acceptor. A detailed analysis of self-absorption, absorption saturation, and multiple scattering effects has revealed the importance of photon diffusion in shutting down the intensity leakage. A 5-fold enhancement of ET in the turbid condition is estimated. Possible factors crucial for ET and ASE in random media are discussed, such as the donor-to-acceptor ASE energy pumping, the optical path elongation by multiple scattering, and the formation of "light pipes" in the near-field of the Mie scattering. The temperature-dependent colloidal formation is found to successfully control optical processes via multiple scattering with a sharp threshold and abrupt emergence of dense scatterers.