Spatially Graded Nanostructured Chiral Films as Tunable Circular Polarizers

In this paper, we introduce a simple single-step method for creating spatially graded helical nanostructured thin films. The films mimic some of the interesting polarization and coloration properties found in nature and enhance the application prospects for helically structured thin films. Our helical nanostructures, fabricated using a variant of the glancing angle deposited technique (GLAD), are spatially graded with thickness gradations are predicted by simulation and verified by scanning electron microscopy (SEM). The resultant films act as Bragg multilayers and can be employed as optical filters which not only preferentially transmit one handedness of circularly polarized light, but also allow for spatially determined frequency tunability. Through spectroscopic measurements, we demonstrate that when appropriate deposition conditions are chosen these nanostructures exhibit strong polarization selectively, concurrent with excellent frequency tunability. The preferentially transmitted peak wavelength can be changed from approximately 620 to 690 nm by translating the film over a spatial distance of 30 mm. These graded nanostructures can be incorporated into photonic and sensing devices. The graded helical nanostructures may also be useful for providing a graded scaffolding to support liquid crystals.