The integration of CNTs, grown perpendicular to the optical plane, ensures a very high optical absorption (a transmittance of only 0.06% in the visible light spectrum versus the 93.9% transmittance of the PDMS membrane), hence a very good contrast to spatially modulate the intensity distribution of the light passing through the diffraction patterns of the binary Fresnel lens.
The carbon nanotubes are grown in concentric rings only 10μm high, before being percolated with polydimethylsiloxane and being peeled off with the stretchable membrane. This makes such diffractive optics only a few tens of micron thick.
In their paper "Stretchable Binary Fresnel Lens for Focus Tuning", the researchers report their results using lenses that are 6×6mm square with a focal length tuneable from 7mm to about 9mm by radially stretching the lens a few percent of elongation. Although they experienced some distortion due to the clamping system they used to stretch the lens, an integrated circular actuator could certainly solve the issue, providing a uniform radial deformation.
Lead author Xueming Li is keen to highlight that although the paper only mentions a 7mm focal length, the researchers worked on other focal lengths.
"Using the same concept and fabrication process, we fabricated lenses with focal lengths ranging from 100µm to 20mm, so to match application specific demands. For instance, a focal length ranging from 5 µm to 8.5mm can be used in microscopy to investigate fluorescence signals. Lens arrays with focal length ranging from 2mm to 20mm can be used for compound eye applications and point of care devices" Li wrote eeNews Europe.
"Due to the controllable variable focal length, this configuration can also be applied to multi-focus contact lens applications" he added.