September 23, 2014, Tuesday, 265

KSEA 08 COH 1

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Conference Section

Plenary Poster Session for General Papers

Title


Achromatic Polarization Gratings for Efficient Polarization Handling with Broadband Light


Abstract


We introduce and experimentally demonstrate achromatic polarization gratings (PGs) that perform at high diffraction efficiency (>98%) over a broad spectral range using reactive and non-reactive liquid crystals (LCs). Conventional PGs are thin diffraction gratings with a periodically varying birefringence profile and manifest compelling properties: ~100% diffraction efficiency possible into the first-orders, highly polarization-sensitive first-orders, and no higher orders. It has long been recognized that these anisotropic diffraction elements are useful for polarimetry, beamsplitting, displays, and the like, where the efficient control of light polarization is in consideration. However, high-efficiency diffraction with broadband light is limited due to the wavelength sensitivity of the conventional PG. Here we report on the successful implementation of achromatic PGs (increasing the high diffraction bandwidth five-fold) through the innovative use of LC twist.

Our approach for achromatic PGs is based on the retardation-compensation structure, where two low-twist chiral PG layers are stacked on the top of each other with opposite twist senses. This double-twisted configuration leads to a significant improvement in bandwidth for high diffraction efficiency while maintaining all other unique advantageous properties of PGs. The theoretical diffraction behaviors of achromatic PGs have been predicted by the Jones matrix analysis and confirmed by numerical simulations using in-house optical simulator – WOLFSIM, which is freely available as an open-source software. The optimized grating parameters (thickness and twist angle) were found for the widest wavelength range for a high efficiency (i.e., >99%).

We successfully implemented achromatic PGs with polymer and switchable LC materials as transmissive and reflective optical elements, respectively. First, we demonstrate achromatic PGs as a reactive mesogen (RM) film. We have patterned the PG profile on the photo-alignment layer and then amplified the grating structure by coating with RM material containing a small amount of chiral dopants to implement a twist. The second RM layer containing chiral dopants with the opposite twist sense is coated onto the first RM layer. We achieved >98% efficiency over the most visible wavelengths (i.e., 450 nm to 650 nm) and <2% incoherent scattering above 400 nm. We also demonstrate a switchable achromatic PGs on reflective substrates. The use of reflective substrates generates the same achromatic effect as described above, but only a single twist is needed due to the double-pass of light through the LC twist upon reflection. In addition, and perhaps most significantly, an applied voltage modulates the diffracted light, proving a compelling electro-optical diffractive switch. We again achieved >98% efficiency over the most visible wavelength. Our achromatic LCPG samples also show a significant improvement of operational bandwidth over the conventional LCPG configuration.


Authors

COH+MJE


Comments

<MJE> Chulwoo, I added my comments above, mostly minor wording adjustments. I suggest you use the difference tool in the Wiki to see where. Over all, it appears good to me. We need to finish the manuscript about this!