Illuminated materials bend, curl and move

12 Jan 2004

Materials that bend, curl up or contract on exposure to light might sound far-fetched, but that is exactly what many scientists are busy trying to develop. Rob van den Berg describes the science and applications of optical actuation.

From Opto & Laser Europe January 2004

Anyone who has ever operated an infrared remote control to switch on a TV or open a garage door knows intuitively that light can activate a response from a device. But imagine if instead of triggering an electrical circuit, a light beam could directly induce movement, such as bending. The possibilities for a so-called "optical actuator" are tantalizing - micromachines that are kicked into life and controlled by pulses of light rather electrical signals.

As microelectromechanical machines (MEMS) shrink in size, supplying electrical power via wires becomes increasingly troublesome. And there's no doubt that in the future the accurate control of atomic and molecular structures will require some novel solutions. Many scientists believe that optical actuation could be the answer because it does not require wiring and is immune to power failures.

Generating motion Although it is well known that light can indirectly generate motion - through heating or the redistribution of electrical charge, for example - these are often slow processes that require high light intensities.

As a result, research groups all over the world are looking for materials that change shape when illuminated. It is an area of study in which chemistry, physics and materials science come together. In the last few years, processes have been discovered that cause changes in the length and volume of an illuminated material. Examples include phase transitions, internal restructuring (isomerization) in polymers, and photostriction (a combination of the photovoltaic and piezoelectric effect).

Photostriction was first developed for optical-actuation purposes by Kenji Uchino, professor of electrical engineering at Penn State University in the US. "In the early nineties we were approached by a Japanese electrical firm asking us to develop a novel kind of switch for use in large generating plants," said Uchino. "Frequent electrical storms cause arcing of existing switches, inadvertently shutting down entire plants."

A light-activated switch would be the perfect solution. Uchino started work on the idea and discovered a ceramic material PLZT (made of lead, lanthanum, zirconium and titanium) that had some useful properties: when ultraviolet light was shone on the material, a large electric voltage was generated, triggering a piezoelectric effect that altered the material's shape.

"The incoming photons excite electrons that are orbiting tungsten atoms, and, due to the asymmetrical structure of the PLZT crystal, a charge imbalance is created," said Uchino. "This results in a high electric voltage and expansion of the material. The strain induced may be as much as 0.1%."

Unfortunately, it took several minutes for the material to return to its original state once the light was turned off. To speed up the relaxation time, Uchino pasted two PLZT plates together so that each side would give a shape change in the opposite direction. This reduced the relaxation time to a few seconds and Uchino was able to make a device that flexed from side to side. However, faster relaxation speeds - needed in real-life applications - proved difficult to achieve, so Uchino's research on photostriction was consigned to being "a hobby".

For Stephen Elliott of Cambridge University, UK, his work on semiconducting glass alloys is far from being a hobby. In fact he intends to set up his own spin-off company to exploit an optomechanical effect that he first reported in 1997.

On irradiation with polarized light, chalcogenide-based materials show a contraction that is parallel to the direction of the light's polarization, and an expansion that is the perpendicular to it. These physical changes, which are reversible, occur on a nanometre scale.

In an upcoming paper in IEE Proceedings: Science Measurement and Technology, Elliott reports on research into thin films made from various combinations of arsenic, selenide and sulphur. His team coated silicon microcantilevers with a 1 µm thick layer of these elements and studied the cantilever deflection as a function of various optical parameters, including wavelength, light intensity and polarization.

"The effect is strongest for wavelengths that are [approximately equal to that] of the bandgap, so by changing the composition of the coating we can tune the wavelength selectivity," commented Elliott. "Also, the optomechanical response can be precisely controlled by varying the angle between the electric-field vector of the polarized light and the cantilever axis." A variety of lasers were used for illumination, including a 10mW, HeNe laser and a 1W ring dye laser at wavelengths of between 560 and 615nm. The maximum deflection for a 380µm cantilever was found to be 6.5µm.

The mechanism involved in the transformation of light energy into mechanical motion is, to a large extent, still a mystery. X-ray analysis, which can detect subtle changes in atomic bond lengths and bond angles, suggests that stress is being induced inside the glass on illumination.

Elliott has driven his cantilevers with light modulated at a few tens of kilohertz, and, although the absolute magnitude of the deflection decreases, it seems that the response time is less than 100µs.

"That would make the material suitable for switching applications, for instance in telecommunications and in environments with flammable gases, where you would not want to use a high-voltage to drive a piezoelectric transducer," he commented.

Moving mirrors Elliott's research group is not the only one developing light-activated cantilevers. In November 2003 a Japanese collaboration reported that it had grown miniature hinged mirrors from layers of different compound semiconductors (AlGaAs, InGaAs and GaAs). Shining 488 nm light from a Argon-ion laser onto the 666 nm thick mirror causes it to pivot on a hinge that is just 144 nm thick. The maximum angle of deflection was 0.5º for an optical power density of 450 mW/cm².

This research is being carried out by a team from the Adaptive Communications Laboratory (ATR) near Kyoto, Konan University, and Osaka City University. "Optical actuation is a promising, non-contact method. You don't need to add electrodes to a device and you can even use light as an actuation method in harsh environments," said Jose Zarnardi from ATR. "The advantage of our devices is that they are made of GaAs-based materials, so they are promising for integration into optical systems."

As for other research in the field, Heino Finkelmann of the University of Freiburg, Germany, was the first to use thin polymer films for optomechanical actuation.

Finkelmann's group specializes in phase transitions in elastomeric liquid-crystalline polymers - chain-like molecules that are thousands of subunits in length from which hang long, rigid, "pendant" molecules. Finkelmann has shown that by choosing the chemical composition of these polymer chains he can make them react to light or heat in a specific way.

The polymer chains are fastened together in an irregular network with so-called crosslinker molecules. Stretching the material aligns the pendants, and by adding more crosslinkers the stretched nematic state is maintained after tension is released. Heat disorders the pendants and causes the material to contract along the original axis of tension. Cooling enables the pendants to realign and the material to re-expand.

Finkelmann had the idea of adding a photosensitive crosslinker, which undergoes an isomerization when irradiated with ultraviolet light at 365 nm. "This creates a kink in the polymer chain that reduces order in the material and thus leads to an overall contraction," he said. His prototype shortens by about 20% in one dimension when illuminated with ultraviolet light, but calculations suggest that shape changes of 400% should be possible.

Tomiki Ikeda's group at the Tokyo Institute of Technology in Japan has gone one step further. It uses an isomerization of an azobenzene in a liquid-crystal network. However, when illuminating a thin film of this material using 366 nm ultraviolet light, the film bends in the direction of the polarization of the light. Green light at a wavelength of more than 540 nm is used to take the film back to its stretched state (Nature 11 September 2003).

The bending mechanism relies on isomerization in specific domains where the light-sensitive azomolecules are appropriately aligned. The effect only occurs in the first 10 to 15 µm of the film - the optical penetration depth - but this upper part drags and bends the whole film. In his paper, Ikeda writes that this striking opto-mechanical effect "results from a photoselective volume contraction and may be useful in the development of high-speed actuators for microscale and nanoscale applications, for example in optical microtweezers or for driving micromachines in medicine without the aid of batteries."

Chemical reaction A final chemical reaction that can be exploited in an optical actuator is the ring opening of spiropyran. By incorporating this molecule in a diluted form in a polymer film, Thanassa Athanassiou of the Foundation for Research and Technology, Heraklion, Crete, has created a material that can act as an optical actuator.

Athanassiou explains how irradiation using green light causes a chemical reaction and a deformation of the polymer film. "The reaction from spiropyran to merocyanin causes strain in the surrounding polymer matrix," she explained. "The polymer has a low glass transition temperature, so the green laser pulses change its viscosity and allow the merocyanin molecules to aggregate. The polymer chains are forced to adapt to this new situation."

The green laser pulses also reverse the reaction, returning the film to its original position. Currently Athanassiou is setting up a micro-optical mechanical system in which this optical actuator is replacing an electrical switch. Her biggest challenge yet is to find polymer materials for her actuators that are compatible with the standard photoresists used for patterning microscopic structures. Athanassiou also envisions microfluidic systems (a lab on a chip) in which an optical switch is used to stop or divert a fluid flow, and smart surfaces that, by a flick of the laser switch, turn from hydrophobic to hydrophilic.