Probing non-radiative quantum relaxation in fluorophores using an optical fiber Bragg grating photothermal sensor

While optical fiber Bragg gratings (FBGs) have been exploited in the field of sensing, their potential for investigating quantum processes of photon-molecule interactions remains unexplored. Here, we experimentally demonstrated a method of probing photon-induced non-radiative thermal relaxation in fluorophores using the FBG technique. In response to various excitation wavelengths of photons, the FBG with fluorescent dye, Rhodamine B, present on its cladding, exhibits distinct Bragg wavelength shifts, reflecting the level of vibronic transitions and the absorption characteristics of the fluorophore based on non-radiative thermal release. The photoexcitation intensity-dependent response demonstrates that the FBG technique can probe localized photothermal relaxation at the micron-scale with LED intensity below 5 mW cm^-2. Moreover, the modulation of the observed split in the Bragg wavelength spectrum provides further insights into photothermal localization in addition to yielding photothermal information. This approach of realizing photon-molecule interaction makes fiber Bragg grating-based quantum phenomena sensing accessible and can be extended for spectroscopy, biosensing, and quantum applications.