Thermal lens model of Sb thin film in super-resolution near-field structure

Using a time-resolved dual-beam mode-mismatched thermal lens method, the refractive index change of the Sb thin film with a thickness of 20 nm at the wavelength of 632.8 nm with temperature is measured. According to the results measured and the radial distribution law of the refractive index within the aperture, by considering the aperture as a thermal lens and setting the thickness of Sb thin film being 20 nm, recording or readout laser power 10 mW, the radius of the aperture 200 nm, and laser irradiation time on the Sb thin film 20 ns, the focal length of thermal lens of Sb thin film is 38.5 nm, correspondingly, the focused spot size through the thermal lens can be reduced to about 80 nm, at the same time, the intensity can be approximately increased to 20 times as compared to that of the primary focused spot by lens.