Optical Limiting in Metal Nanowires Studied

Daniel S. Burgess

Optical limiters transmit low-intensity but block high-intensity light, making them suitable for use in applications such as the protection of sensors and the human eye from laser radiation. In the pursuit of alternative material for such devices, scientists at National University of Singapore and at the Institute of Chemical and Engineering Sciences, also in Singapore, have characterized the optical limiting properties of various metal nanowires.

Metal nanowires of uniform dimensions with promising optical limiting thresholds are produced using an Al₂O₃ template with a series of regular pores. Scanning electron microscope images show a batch of nickel nanowires after the template has been partially (a) and completely removed (b). Courtesy of National University of Singapore.

The work is inspired by the discovery that carbon nanotubes are promising for optical limiting. The problem with nanotubes is that they have been difficult to produce economically in volume with uniform lengths and diameters.

The researchers, however, have developed a relatively inexpensive means of fabricating metal nanowires with consistent dimensions. A two-step anodization process yields a template in Al₂O₃ featuring a series of regular nanopores into which the metal may be electrochemically deposited. Following deposition, the Al₂O₃ is dissolved with a sodium hydroxide solution, freeing the nanowires.

To investigate the optical limiting properties of such structures, the scientists produced batches of nickel, palladium, platinum, silver, copper and cobalt nanowires 50 nm in diameter and 30 μm in length, which they suspended in distilled water in a quartz cell. They performed fluence-dependent transmission experiments at 532 and 1064 nm using a Spectra-Physics Q-switched Nd:YAG laser, and low-intensity spectral transmission measurements with a Shimadzu spectrophotometer.

They found that the transmittance of the suspended materials was 80 percent for incident fluences of less than 0.1 J/cm². Above this, the transmittance fell with rising fluence. At 532 nm, the optical limiting threshold — the point at which the transmittance declines to half of the normalized linear transmittance — of the nickel, palladium, platinum and silver nanowires was similar to that of single- and multiwalled carbon nanotubes, at roughly 1 J/cm². The limiting threshold of all the metal nanowires was significantly higher, at 1064 nm, but the nickel, palladium, platinum and silver structures still were similar to carbon nanotubes, at roughly 10 J/cm².

The long-term stability of the suspensions currently is an issue, and the scientists are working to discover how the size of the nanowires and the choice of liquid for the suspension play a role. Beyond optical limiting, they hope to use the nanowire fabrication process in the study of plasmonic structures.

Applied Physics Letters, May 29, 2006, 223106.