Feb 15, 2011
Near-field thermal transport in laser-irradiated nanotips
The interaction of a scanning probe microscope (SPM) tip with laser radiation has motivated considerable developments in nanoscale material characterization, surface structuring and surface imaging. However, the extreme focus of a laser beam in a near-field region (<10 nm) could impose a heating effect both in the SPM tip and on the substrate during characterization/structuring. In-depth knowledge of the thermal behaviour of the SPM tip under laser irradiation will help establish a cornerstone for understanding numerous physical phenomena involved in surface nanostructuring and characterization using laser-assisted SPM.
Researchers led by Prof. Xinwei Wang from Iowa State University, US, have reported a systematic study of the highly enhanced optical field inside and outside an SPM tip upon nanosecond laser irradiation. The work examines the dynamic response (thermal evolution and distribution) of the tip thanks to the use of combined full-field optical modelling and high-fidelity modelling techniques.
The team's results show how the thermal response of the tip is affected by the radius of curvature, tip aspect ratio and polarization angle of the incident laser. The group's Poynting vector analysis has identified a very interesting phenomenon concerning the focusing of near-field light by the SPM tip: when a laser interacts with a metallic tip, the emission is bent around the tip and concentrated under the apex, where extremely high field enhancement appears. This phenomenon is more like a liquid flow being forced or squeezed through a narrow channel.
The thermal transport inside the nanoscale tungsten tips due to absorption of incident laser radiation revealed a small fraction of light penetrating the tip. As the polarization angle or apex radius increases, the peak apex temperature decreases. The peak apex temperature goes down as the half taper angle increases, even though the mean laser intensity inside the tip increases, revealing the very strong effect of the taper angle on thermal transport.
The researchers presented their results in the journal Nanotechnology.
About the author
The study was conducted by researchers from the Micro/Nanoscale Thermal Science Laboratory at Iowa State University, US. The work is funded by the National Science Foundation to study the near-field SPM tip heating and SPM-based surface imaging. Xiangwen Chen is a PhD student in mechanical engineering. Prof. Xinwei Wang is the director of the Micro/Nanoscale Thermal Science Laboratory.