Photothermal Detection of Nanoscopic Liquid Crystalline Nematic-Isotropic Phase Transition

Various techniques have been established to image tiny nonfluorescent nanoparticles with optical far-field microscopy; we implore one of these techniques, photothermal heterodyne imaging (PHI), to detect the nematic-to-isotropic phase transition of 5CB at the nanoscale. The technique involves heating the nanoparticle with a modulated resonant laser which creates a index of refraction profile that diffracts a nonresonant probing laser to create a measurable signal. The heating beam creates a temperature profile that is maximum at the nanoparticle surface and drops quickly as the energy is dissipated into the liquid crystal. At a high enough power, the liquid crystal surrounding the nanoparticle transitions to the isotropic phase, causing a change in the photothermal signal. Interestingly, due to the large index of refraction variation during the liquid crystalline phase change, we find a signal to noise enhancement of 7-fold.