Theoretical description of thermal lens spectrometry in micro space

For thermal lens spectrometry in micro space, temperature distributions in the sample under continuous-wave modulated excitations of either laser or incoherent light source (ILS) are for the first time deduced with respective considerations of different experimental parameters, such as flow of the sample, excitation beam divergence, effects of top/bottom layers adjacent to the sample, and sidewall effects. On the basis of the temperature distributions, thermal lens (TL) models are built based on the Fresnel diffraction theory. Numerical simulations of the influence of the above factors on the TL signal are made for different detection schemes in search of higher detection sensitivity. We were able to predict the optimum probe beam offset as a function of the flow of the sample, the optimum detection scheme to minimize the effects of fluctuation of the flow velocity and/or excitation beam radius, the optimum excitation beam radius for a given sample length, and propose a three-layer system in an ILS-excited thermal lens microscope for sensitivity enhancement of up to 10 times.