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Carbon nanotubes enable laser protection applications

New materials exhibit a broadband optical limiting response that can be used to protect human eyes, optical elements, and sensors from intense laser pulses.
21 June 2007, SPIE Newsroom. DOI: 10.1117/2.1200706.0750

Aside from their numerous civilian applications, lasers have evolved into numerous modern battlefield weapons. Some are designed to dazzle or permanently disable humans by blinding, while others are used to destroy optical sensors, missiles, and other targets. In the past decade, significant research effort has been invested into optical limiting materials and processes in an attempt to achieve some measure of protection from such laser beams.

Optical limiters are devices that significantly attenuate high intensity laser light while allowing the transmission of ambient light. In this context, fullerenes, carbon nanotubes (CNTs), porphyrins, phthalocyanines, and other materials have been widely studied to develop practical passive optical limiters that can protect against laser radiation exposure. An example is provided in Figure 1.1–5 However, the preparation of the nonlinear and optically active materials required for such applications still represents a significant challenge.

Figure 1. The response of an optical limiter. An ideal material limits the output energy to some specified value. Suitable materials (in black circle) include phthalocyanines, porphyrins, carbon nanotubes, and fullerenes.

CNTs exhibit strong optical limiting responses at 532nm and 1064nm.2–6 The responses of CNT suspensions are dominated by nonlinear scattering resulting from thermally induced solvent bubble formation and sublimation of the nanotubes. As for solubilized CNTs, these limit optically through nonlinear mechanisms while exhibiting significant concentration-dependent optical limiting responses. Dissolving CNTs in water and organic solvents however, is still challenging. To address this problem, one strategy has been to obtain soluble organic and polymeric derivatives through chemical reactions. In polymer/CNT composite materials, besides nonlinear scattering, other mechanisms such as nonlinear absorption and electronic absorption may contribute to the optical limiting efficiency. Further improvements could be achieved in the composites, and in the dispersion and alignment properties of the CNTs of the polymer matrix, e.g. by varying both nanostructured guest and polymer host, and through use of ex situ alignment methods. This would provide a larger nonlinear absorption coefficient and a smaller threshold for optical limiting.

In our work, we seek to design and synthesize novel CNT-based organic or polymeric optical limiters. We use, for example, charge-transfer supramolecular materials, phthalocyanines and porphyrins covalently or non-covalently grafted to CNTs to synthesize several classes of new materials. We also determine the relationship between their structural parameters and optical limiting response while seeking to optimally combine materials and devices.

Our goal is to fabricate functional elements capable of performing advanced optoelectronic functions in photonic and optical limiting systems. To achieve simultaneous protection against both pulsed and continuous wave (cw) or quasi-cw lasers,5 broadband optical limiting chromophores need to be designed for multimechanistic function. This includes the following requirements: reverse saturable absorbing (RSA) behavior at the high-energy end of the visible; two-photon or multiphoton absorption behavior at the low-energy end of the visible; and nonlinear scattering behavior in the visible near-infrared region.

Recently, we synthesized a new series of soluble axially substituted or bridged gallium phthalo- and naphthalocyanine compounds with excellent optical limiting potential. Of interest is that their limiting response can be tailored over a very broad range due to the high architectural flexibility of the phthalocyanine structure.3

As for CNTs and their soluble derivatives,6 theoretical studies suggest that future work should be focused on in-depth studies of solution and solid state systems to provide a detailed understanding of the factors affecting the optical limiting response.

The authors acknowledge financial support from the following programs: NSFC (20676034), ECUST (YJ0142124), China/Ireland STCRF (CI-2004-06), SRF for ROCS, NCET (050413), Shanghai Shuguang Project (05SG35) and STCSM (05XD14004).

Yu Chen, Ying Liu, Nan He 
Department of Chemistry
East China University of Science and Technology
Shanghai, China

Yu Chen is currently a full professor at the East China University of Science and Technology in Shanghai. He has published 120 articles, of which more than 90 have appeared in international peer-reviewed journals. His main research interests are focused on the preparation of organic and polymeric functional materials designed for nonlinear optics, and molecular electronic devices.

Ying Liu was born in Handan, China, in 1982. She received her BS degree from Hebei University of Technology in 2005, and then moved to the East China University of Science and Technology to pursue her studies under the direction of professor Yu Chen. She has published seven papers in international journals.

Nan He was born in Haerbin, China, in 1984. After receiving her BS degree from the East China University of Science and Technology in 2006, she joined professor Yu Chen's group to pursue her PhD studies. Her recent research interests are focused on the design, synthesis and application of organic and polymeric optoelectronic materials.