Breaking the Absorption Limit
Graphene and other ultrathin, electrically conductive films are useful for absorbing light in compact optoelectronic devices. But conventional wisdom suggested that these materials could absorb no more than 50% of incoming light, restricting device efficiency. Now Jie Luo at Soochow University in China and his colleagues have shown that these films can instead reach an absorption limit of 82.8% for light arriving at a grazing angle [1]. In addition to potentially improving device efficiency, this finding advances physicists’ understanding of matter–light interactions at scales much smaller than the light’s wavelength.
The previously assumed 50% absorption limit arises from a fundamental physical constraint: When light hits an ultrathin film and is partly reflected and partly transmitted, the total electric field parallel to and directly above the film must be equal to the field parallel to and directly below the film. In a uniform environment, this constraint forces a fixed relationship between the amount of light that is reflected and the amount that is transmitted. And in turn, it places an upper bound of 50% on absorption. This limit was generally regarded to be independent of the light’s frequency, polarization, and initial direction. Moreover, for grazing light, absorption was thought to be negligible.
Luo and his colleagues studied the scenario of grazing light theoretically and found that, in this scenario, the electric-field constraint does not impose the fixed reflection–transmission relationship. This difference enables the absorption limit to surpass 50% across a broad frequency range and to reach a universal peak of 82.8% at a frequency dependent on the film’s composition. The researchers confirmed these results experimentally using terahertz-frequency light and ultrathin silicon films. They found that the absorption limit exceeded 50% between 0.03 and 3 THz and hit the peak at roughly 0.3 THz.
–Ryan Wilkinson
Ryan Wilkinson is a Corresponding Editor for Physics Magazine based in Durham, UK.
References
- Y. Liu et al., “Breaking the intrinsic absorption limit for arbitrarily thin conductive films at grazing incidence,” Phys. Rev. Lett. 136, 046902 (2026).