Researchers at Beijing University of Technology achieved direct writing of MPCs by simply exposing a film of colloidal gold nanoparticles (NPs) to the interference pattern of a single laser pulse in the ultraviolet (UV). The method is defined as "interference ablation" and, according to the developers, is the simplest and cheapest method so far for fabricating MPCs with an area of the order of millimeters or even centimeters. Based on these results, the technique could pave the way for true mass fabrication of MPC devices.

Laser patterning

A UV laser pulse at 266 nm with a pulse length of about 6 ns is split into two beams before they are overlapped onto a thin film of spin-coated gold nanoparticles that have been synthesized chemically in a colloidal solution. The gold nanoparticles covered with ligands are evaporated instantly and completely at the bright fringes of the interference pattern of the two UV laser beams.

In this way, grating structures consisting of colloidal gold nanoparticles are produced, corresponding to the dark fringes of the interference pattern. A subsequent annealing process performed at about 300–400 °C leads to sublimation of the ligands and the complete melting of the gold nanoparticles. The molten gold nanoparticles are fused into solid entities along the grating lines and waveguide MPCs are formed on top of a glass substrate with the indium tin oxide (ITO) layer functioning as the waveguide.

The period of the MPCs is about 300 nm in the scanning electron microscopic images shown left (for more details, click on thumbnail) and may be tuned over a large range from less than 200 nm to several microns simply by changing the separation angle between the two UV laser beams.

More complex structures

A variety of two-dimensional MPCs can be produced by performing a number of exposures and rotating the sample (at a designated angle about a centre axis parallel to the normal of the substrate) in between each laser pulse. The right panel in the above picture shows square-lattice structures produced by a double-exposure procedure. Here the sample was rotated by 90° before the second exposure takes place.

The researchers reported their results in the journal Nanotechnology.