Figure 1.

Photothermal and photothermal circular dichroism concept. (a) Scheme of the wide-field-heating photothermal detection of a chiral structure (illustrated as a hand) on a glass substrate. The heating beam is wide (green), and the probe beam (red) is focused to the diffraction limit. The heating beam intensity is modulated between on and off states at a frequency f_m. Part of the absorbed power will be released as heat to the environment, creating a thermal lens (in purple) around the absorbing object. The wavevector direction (k) for both beams is shown. (b) Scheme for photothermal circular dichroism, where we modulate the polarization state of the heating beam between left and right circularly polarized light (dark and light green, respectively). The thermal lens is also created in this case (in purple). The wavevector direction (k) for both beams is shown on the right. (c) Time evolution of the heating power for the intensity-modulated photothermal microscopy, following the intensity modulation pattern at f_m. (d) Time evolution of the heating power for the polarization-modulated photothermal microscopy. In this case, the heating power is constant and the only change is the polarization state. (e) Time evolution of the absorbed power by the nanostructure under study for the intensity-modulated photothermal case. Naturally, when the heating power is zero, the absorbed power is null. (f) Time evolution of the absorbed power by the nanostructure under study for the polarization-modulated photothermal case. As it is a chiral structure, the absorbed powers for LCP and RCP are different.