Shine a flashlight onto a sheet of metal or plastic, and substantial mechanical movement usually does not happen. But what if light could make that sheet bend or vibrate almost instantly in response? Researchers such as Kui Yao, a Distinguished Principal Scientist at the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE), are now exploring that unusual phenomenon—known as photostriction—in advanced materials for new kinds of remotely operated devices and signal transmitters.
“Photostriction refers to the generation of mechanical strain in a ferroelectric material under light irradiation, distinct from normal thermal expansion,” explained Yao. “Unlike the converse piezoelectric effect, which relies on an electric field applied to the material, photostriction doesn’t need electrical wires, electrodes or electric poling to induce that strain.”
From speakers and sensors to microelectromechanical systems (MEMS), many devices today rely on piezoelectricity to trigger tiny movements for the key functional mechanisms. Although photostriction has been known for decades as a potential alternative, it has remained difficult to use in practical devices as its mechanical effects are relatively slow. In many ferroelectric materials, light-induced motion can take seconds to build up, limiting applications that need a more hair-trigger response.
However, at least one photostrictive material might just make that bar. In a recent study, Yao, A*STAR IMRE PhD scholar Hui Li and A*STAR IMRE colleagues, together with researchers from Nanyang Technological University, Singapore, uncovered unusually rapid mechanical responses from scandium-doped aluminium nitride (AlScN) thin films, as measured by modulated lasers.
“We observed an outstandingly large photostrictive strain rate of up to 3.62 s⁻¹ in these thin films, which is at least two orders of magnitude faster than previously reported photostrictive systems using continuously modulated lasers,” said first author and A*STAR Graduate Scholar, Hui Li.
While aluminium nitride-based thin films are already widely used in electronics and MEMS devices for their excellent electrical and thermal properties, AlScN’s exceptional speed was linked to a highly textured nanoscale columnar grain structure. These slim nanoscale domains shortened the distance travelled by light-excited charge carriers, reducing delays caused by charge buildup.
“When illuminated with pulsed light that resonated with AlScN’s structure, our analyses found these charge carriers rapidly separated into slim nanoscale domains, generating photovoltage that coupled with the local converse piezoelectric effect,” explained Yao. “The strains from individual domains then combine constructively to produce an overall fast photostrictive response.”
The team carefully ruled out the normal effects of heat: both simulations and experimental measurements showed that thermal expansion made negligible contributions to the observed deformation. They confirmed that the effect stemmed from genuine ferroelectric photostriction through its characteristic dependence on polarisation angles.
Yao noted that while the light-induced strain observed in AlScN remains weaker than electrically-driven actuation, its speed as obtained here has the potential to open a new pathway for light-driven actuation and acoustic wave excitation. “This could enable future acoustic micro-actuators and non-contact optomechanical systems,” Yao added.
The A*STAR-affiliated researchers contributing to this research are from the A*STAR Institute of Materials Research and Engineering (A*STAR IMRE).
