Stretchable polymer optics embed carbon nanotubes for better focus

Stretchable polymer optics embed carbon nanotubes for better focus

Technology News |
By Julien Happich

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.

Fig. 1: When stretching the flexible substrate radially by s from (a) to (b), the radius of the nth zone increases from rn to r’n and the focal length changes from f to f′ by a factor of by s2.

Referring to competing research performed with less adaptive rigid materials such as black silicon for the opaque material instead of CNTs, Li notes: “The advantages of using CNTs are first, the porous character of the layer, which allows the PDMS to better penetrate in the CNTs. This is important for radial control when stretching the lens. Secondly, the fabrication of CNTs is more controllable than black silicon, which makes it a more reliable process for large scale device manufacturing. The fabrication of our lens (array), of which the key process is the growth of patterned CNTs, is fast and reliable. Moreover, as a commercially available CVD system is used, and a wafer scale process is developed, it is possible to scale up the area for large volume manufacturing of these devices”.

Li sees such flat optics find their way into miniaturized photonic chips, integrated optics, optical interconnects, beam focusing or mask-less lithography systems, but also for deflecting and collimating tasks in optical sensor systems or for optical data transfers.

“The low cost and scalable manufacturability of our device provides solutions for disposable microscopes, which are valuable for health diagnostics in developing countries” he wrote eeNews Europe.

Fig. 2: The main fabrication steps: (1) Ti/TiN (10nm/50nm) is sputtered on a silicon wafer to prevent diffusion of the catalyst into the substrate; (2) 1.4μm photoresist is coated and (3) patterned on the wafer; (4) Iron (5nm) is evaporated as catalyst, followed by (5) a lift-off process to define the CNT growth regions; (6) Vertically aligned CNT bundles (10μm in height) are grown by CVD, (7) PDMS is poured on the horizontal silicon wafer substrate with the defined CNT patterns. After degassing, the device is released by peeling off the PDMS layer together with the encapsulated CNT from the silicon substrate (8).
Fig. 3: (b) A fabricated device containing 2×2 lens units, with each unit 6×6mm2 in size. (b) An optical microscopy image of a single lens and (c) a SEM image of a single lens unit, with 7mm in focal length and an innermost zone diameter of 133μm, before PDMS infiltration. (e) shows the CNT inside the PDMS well aligned after the PDMS percolation forming the CNT/PDMS composite


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