Bright-field thermal imaging at the microscopic level was proved to be possible by quantitatively measuring the transmitted phase with a Quadriwave Lateral Shearing Interferometry (QWLSI). This approach, introduced by G. Baffou in collaboration with PHASICS SA , is able to measure temperature fields from temperature-induced refractive index changes of the medium surrounding heat sources. These heat sources are generated by a continuous plane laser wave source illuminating nano-particles at their plasmonic resonance. Heat maps and temperature fields are measured in parallel thanks to a QWLSI SID4BIO camera, developed by PHASICS. The quantitative phase signal is deconvolved to obtain the temperature variation and heat map values. With this set-up, a resolution of 1°C has been achieved.
We present here a highly-resolution thermal imaging system applied to the detection and characterization of isolated single nano-particles by measuring their cross-section size. Because single nano-particles induce low temperature signals, we implement a noise-reduction modality based on synchronous detection. To do so, we use a modulated laser source locked-in with the SID4Bio camera. Since the heating process is modulated in time, this configuration improves the signal-to-noise ratio of the temperature measurement as we get rid of most of low-frequency noise which is a main contribution of the noise signal.
The carried out experimental measurements presented will be analyzed along with the theoretical calculations introduced in .
 Baffou, G., Bon, P., Savatier, J., Polleux, J., Zhu, M., Merlin, M., ... & Monneret, S. (2012). Thermal imaging of nanostructures by quantitative optical phase analysis. ACS nano, 6(3), 2452-2458.