A better limiter for laser light

Hank Hogan, hank.hogan@photonics.com

Fifty years after the birth of the laser, there may be better protection against the light fantastic, reports a team of researchers. They have found that nanographene, a form of carbon that comes in single atomic layers, might be the right choice for an optical limiter, a device that restricts the transmission of light.

With 10 times the absorption coefficient of carbon black, nanographene did a better job at preventing light overload than other materials. The researchers also found that nanographene did an excellent job of limiting light, even when it was embedded in a polymer gel. Team leader Wei Zhao said this result is exciting because it could lead to recoverable and reusable devices if self-healing polymers are used.

For limiting laser light, graphene nanoparticles (GS) in water or polyvinyl alcohol (PVA) work equally well. This performance could lead to reusable and recoverable devices, if a self-healing polymer is used. Reprinted from the Journal of Physical Chemistry.

“These findings can be used for eye and sensor protection from powerful lasers, for ultrafast optical switching devices and for ultrafast photonics such as a saturable absorber,” he said.

In addition to Zhao, a professor of chemistry at the University of Arkansas, others on the research team were from the University of New Orleans and Monash University in Melbourne, Australia. The lead author of a Journal of Physical Chemistry paper on this work that was published online on July 7, 2010, is Little Rock Central High School student Boshan Zhao, son of Wei Zhao.

In the study, the group compared the optical limiting properties of nanographene to carbon black suspensions, the benchmark material for broadband optical limiters. Such suspensions, unfortunately, do not work well for short pulses, such as those in the picosecond range. The carbon also can clump up over time, leading to a loss in performance.

Suspension of graphene nanoparticles (GS) proved better at limiting laser light than gold-graphene nanoparticles (Au-GS), carbon black (CB) or fullerenes (C60). Insets show structure of nanographene (A), DNA-coated nanographene (B), gold-nanographene nanoparticles (C) and fullerene (D). Reprinted from the Journal of Physical Chemistry. Images courtesy of Wei Zhao, University of Arkansas.

The researchers also looked at the performance of nanographene in relation to solutions of buckminsterfullerene, a spherical fullerene with 60 carbon atoms. These fullerene solutions are the best-known optical limiters at 532 nm. However, the material is easily damaged by laser light and does not offer broadband performance.

For the tests, the group used solutions of nanographene, nanographene oxide, a mixture of nanographene and gold nanoparticles, carbon black and buckminsterfullerene. The nanographene sheets measured about 200 x 300 nm, with a height of about 0.9 nm. The researchers investigated the optical performance of the nano-graphene for different-size sheets and for when it was dissolved in a variety of solvents, including organic ones and a polyvinyl alcohol gel.

They used a Continuum Nd:YAG laser to generate 8-ns pulses at 1064 nm. With these pulses they created visible-infrared laser beams using LaserVision converters and were able to adjust their wavelength from 532 nm to 5 µm. To measure transmittance, they collected intensity data before and after the beam passed through the sample using a pyroelectric detector from Coherent-Molectron. This setup allowed them to plot transmittance versus incident fluence at various wavelengths.

The data showed that the nanographene did as good a job as the fullerenes at 532 nm and better than the carbon black across a broadband spectrum. Some of the results were expected, but some were not, Zhao said. Among the unexpected findings, he listed the lack of photobleaching of nanographene oxide in aqueous solution.

Zhao noted that the next step in the investigation will be further tailoring of the material. This tweaking will be based on measurements of the third-order optical nonlinearity of nanographene and its derivatives. Those results will pave the way for practical applications.

In describing what the researchers ultimately hope to achieve, Zhao said, “We are looking for low-cost broadband optical limiting materials for device applications.”