Sub-Micron Diffractive Optical Elements Facilitated by Intrinsic Deswelling of Auxetic Liquid Crystal Elastomers

Diffractive optical elements (DOEs) enable precise control over the direction and filtering of light, making them common components in spectrometers, waveguides, and sensors. There is great interest in tunable and sub-micron diffractive optical elements in flexible photonics and for responsive structural colors. Here this study presents sub-micron tunable diffraction gratings produced by patterning a liquid crystal elastomer (LCE). The intrinsic anisotropic deswelling of the liquid crystal elastomer enables sub-micron (707 nm) pitch structures to be produced from a micron-scale (1040 nm) surface relief grating. Using atomic force microscopy (AFM) and diffraction measurements, a thermal pitch tunability is demonstrated of +212 nm (+31%) or -322 nm (-33%) over a temperature range of 215 degrees C depending on grating orientation. A mechanical pitch tunability is demonstrate of +1110 nm by applying strains of up to 157% to the liquid crystal elastomer. The height of the diffraction grating is preserved over strain due to the negative Poisson-ratio, or "auxetic", behavior exhibited by this chosen family of the liquid crystal elastomers. This report opens the possibility of using LCEs to facilitate flexible sub-micron diffractive optical elements, with a high degree of tunability for sensing and structural color applications. This article presents sub-micron pitch diffractive optical elements produced via the anisotropic deswelling of liquid crystal elastomers (LCEs). A pitch tunability of +31% or -33% is achieved via heating (depending on grating orientation). A pitch tunability of 167% is achieved via straining. A growth in surface feature height with strain is observed confirming a negative Poisson-ratio on nanometer length-scales. image