Spherical lenses can be prone to optical aberrations and astigmatism that cause unwanted diffraction in imaging optics. A research team at Swinburne University of Technology say particular kinds of flat lenses can avoid these effects, and could open up new applications that traditional lenses are unsuited for.
Metasurfaces and metamaterials are crucial to these flat lens structures, and their complex and costly manufacturing processes have constrained commercial viability. The Swinburne research team present a possible solution, and demonstrate how the action of a femtosecond laser on the surface of a metal can produce local surface structures that make the material act as a flat lens. Their work is published in Light Science & Applications, a Nature scholarly journal.
In particular their project showed that the process could be applied to a 2D metal monolayer to produce an ultrathin flat lens, a potentially straightforward route to achieving the phase and amplitude modulations needed for lens behavior in these thin metal layers.
The project’s raw materials were transition metal dichalcogenides (TMDCs), where transition metal species are paired with sulphur, selenium or tellurium. These are known to be promising candidates for next-generation nanometric optoelectronic devices due to their strong light-matter interactions, but using them in ultrathin forms has so far produced only low-resolution lenses.
The new manufacturing processes designed by the team focussed an 800-nanometre, 100-femtosecond laser onto single-crystal monolayers of TMDC compounds, with the first target being a WSe2 (Tungsten diselenide) monolayer around 7 angstroms thick. A computer-controlled mobile stage allowed the laser to trace a pattern of concentric rings, and so create the same pattern on the metal of nanostructures 20 nanometres high and 50 to 150 nm wide. Having proven the principle, the team believe this approach could enable minaturisation of optical devices through the use of 2D materials, along with potential applications as camera lenses.