Credit: © 2007 AIP

Laser irradiation is being studied as a way to pattern nanoscale features on the surfaces of semiconductors and even harder materials such as diamond-like carbon (DLC). Processing solid surfaces with this ablation technique enables nanostructures as small as 30 nm to be defined, which is less than the diffraction limit of light. There still remains some doubt, however, concerning the physical mechanism of laser nanostructuring.

Now, Godai Miyajia and Kenzo Miyazaki of Kyoto University, Japan have studied the effect of low-power femtosecond laser pulses on DLC films and found that nanostructures were formed on ridges in a direction perpendicular to the laser polarization1. A 900-nm-thick film of DLC was deposited onto a silicon substrate that had been previously patterned with stripe-like mesa structures using electron-beam lithography. Ablation of the raised DLC stripes was carried out with a linearly polarized Ti:sapphire laser with a wavelength of 800 nm, pulse width of 100 fs and pulse frequency of 10 Hz.

It is observed that nanostructures begin to form on top of the stripes — where the curvature is the greatest — in a direction perpendicular to the laser polarization. These results imply that locally enhanced electric fields initiate formation of the nanoscale features on the ridges.