Impact of fundamental thermodynamic fluctuations on light propagating in photonic waveguides made of amorphous materials

To improve the detection limit of optical sensors it is of paramount importance to understand light-matter interaction processes at a fundamental level. At room temperature, the ultimate detection limit is governed by the dynamic nature of the elementary constituents of matter, also known as fundamental thermodynamic fluctuations. However, such thermodynamic fluctuations are often overlooked in many optical systems. In particular, their effect on the properties of light that propagates in amorphous materials is unclear. Here, we unveil for the first time a model that predicts the impact of the fundamental thermodynamic fluctuations on the modification of the optical spectrum of light propagating in optical waveguides, especially at high difference frequencies between the newly generated spectral components and the laser input signal. A salient feature of our approach is to consider a mean relaxation time of the spontaneous random heat flux in the medium, which leads to a spatial correlation of the thermo-refractive noise. As a result, our model allows us to explain the origin and specificities of the background that is observed in the Raman optical spectra of silicon nitride waveguides.