Fdtd analysis of nanoscale temperature distribution induced by near-Field photothermal effect

We have developed a novel nanoscale patterning method of self-assembled monolayer (SAM) using near-field light. This method utilizes the thermal desorption of constituent molecules of a SAM (e.g. the desorption temperature of Octadecanethiol on Au is 130̃230 °C) through the irradiation with near-field light, which can make noncontact and noncontaminating patterning of the SAM at nanoscale. In this paper, the near-field photothermal effect is numerically analyzed by the finite difference time domain (FDTD) method, and the electromagnetic field intensity and temperature distributions are estimated. The sample consists of Au thin film as a bonding layer with thiolated molecules of SAM, Ti thin film as an adhesion layer for Au, and SiO2 substrate. In the analysis, the shape of the near-field optical fiber probe and the thickness of the thin film layer are considered. In the case of the thick Au layer with a double-tapered near-field optical fiber probe, the temperature of the fiber-tip becomes higher than that of Au surface. The strong heating of the probe tip causes a fatal damage of the coating metal of the fiber, therefore it is difficult to couple the high intensity laser into the near-field optical fiber probe in order to reach the desorption temperature. On the other hand, the desorption temperature can be achieved with the 10 nm-thick Au thin film. Moreover, in order to gain high optical intensity enhancements, the triple-tapered near-field optical fiber probe is utilized. Our simulations confirm extremely high temperature distribution on the sample surface by using the triple-tapered near-field optical fiber probe with 10 nm-thick Au thin film layer on 50 nm-thick Ti membrane.