Bumpy molecular structure improves hyperpolarisability of optical materials

Materials World magazine
1 Feb 2007
Scientist writing formulae

The incorporation of ‘speed bumps’ in optical molecules (chromophores) may lead to the enhanced performance of optical technologies, according to researchers at Washington State University (WSU), USA, the University of Leuven, Belgium, and the Chinese Academy of Science in China.

Mark Kuzyk, Professor of Physics at WSU, explains how since optical devices became prominent in the 1970s, researchers have attempted to improve how the materials used interact with light (optical non-linearity/intrinsic hyperpolarisability).

In 1999, he discovered that, theoretically, the fundamental limit of intrinsic hyperpolarisability stands at 1.0. But the best molecules available today – such as lithium niobate inorganic crystals that are presently used in electrooptical switching – fall far short of this limit. They have 30 times less ‘optical brawn’ than is thought possible, which limits the scope for creating all-optical data processing networks.

Kuzyk says, ‘When [the intrinsic hyperpolarisability] of a material is small, high light intensities are required to make a device work. For example, for all-optical switching in a telecom system, you might need a high power US$50,000 laser system that is the size of a PC tower. By increasing non-linearity by a factor of 30, the device can be powered with a laser diode, making all kinds of consumer products possible.

Through computer modeling techniques that vary the shape of the molecules while calculating the resulting change in intrinsic hyperpolarisability, the team at WSU found that a bumpy structure improved performance. ‘The paradigm for making optical molecules has [traditionally] been to separate an electron donating group from an electron acceptor with a bridge that would allow the electron to pass freely.’ Explains Kuzyk. Previous efforts have therefore focused on ‘smoothing out’ the bridge to help this flow.

However, ‘due to quantum mechanics, an electron acts as a wave [and] can be viewed as being in lots of places at the same time. So [when spread out], it can interfere with itself, causing hyperpolarisability to fall short of its potential. By inserting “speed bumps”, [the electrons] bunch up, preventing [this]’, he adds.

New molecules synthesised by Yuxia Zhao and his group at the Chinese Academy of Sciences put these theoretical guidelines into practice. ‘Our discovery [of Zhao’s work] was the result of serendipity,’ says Kuzyk.

Professor Koen Clays at the University of Leuven, a co adviser of a PhD candidate with Kuzyk, was using hyper-Rayleigh scattering to characterise Zhao’s molecules at the time. It was found that they are 50% more efficient at converting light energy to a useable form than any previously tested – although they still do not reach the fundamental limit of optical nonlinearity that Kuzyk believes possible.

‘But we are going in the right direction with the idea of modulated conjugation (bumpiness),’he insists. The molecules devised in China only have one ‘speed bump’, so the next step is to create long chains of molecules with multiple undulations. ‘The calculations show that the more bumps the better. One can then use nanotechnology to combine these super molecules to make the ultimate material.

Kuzyk envisages that eventually the science will crossover to improve the capability of a range of devices, including photodynamic cancer therapies, optical switches and internet connections, and that the new molecules will probably be embedded into a clear polymer for such applications. Polymers, unlike crystalline materials, have the advantage that they can be formed into thin films and fibres. Moreover, Kuzyk explains, ‘Polymers doped with organic molecules have nonlinearities that are much higher than inorganic crystals with better dielectric properties.