Influence of thermal conductivity on photothermal lens spectroscopy

Highlights: • Sensitivity of thermal lens microscopy doesn't necessarily show a 1/κ dependence. • Ratio of thermal diffusion length to excitation radius is key to κ-dependent signal. • Detection of ensemble sample shows a different κ-dependence from that of nanoparticle. • Volumetric heat capacity is also a key parameter impacting the photothermal signal. -- Abstract: Thermal conductivity (κ) is a key factor influencing the sensitivity of photothermal techniques. In thermal lens spectroscopy (TLS), photothermal signal was previously regarded as being proportional to 1/κ. Here, I show that the 1/κ dependence of photothermal signal is only valid on condition that the thermal diffusion length (L_D) exceeds quintuple the excitation beam radius (a_e) for an ensemble sample or twice the probe beam radius (r_ps) for a nanoparticle. When L_D is smaller than 0.5a_e for ensemble sample (or r_ps/6 for nanoparticle), the signal reaches a plateau (or starts to decrease), totally deviating from the 1/κ dependence. It was experimentally demonstrated that at high modulation frequencies, the signal enhancement induced by organic solvent can be twice smaller than that predicted previously. This work illuminates the impact of κ on TLS, and provides a guidance on selecting optimal TLS parameters for sensitive nonfluorescent target detection.