Reflective Polarization Gratings
As opposed to their transmissive counterparts, reflective LCPGs can be implemented on a reflective substrate such as aluminum or silicon. Both the diffracted orders and the incident beam are on the same side of the grating as shown in Fig. 2. Since light passes through the LC structure almost twice as much as it does in the transmissive case, the grating thickness (d) can be reduced by half for the same optical retardation. This mode allows for the fabrication of LCPGs on silicon backplanes. Liquid Crystal on Silicon (LCOS) technology allows for high resolution and low cost. One important application involving this principle is discussed under the section Polarization-Independent LC Microdisplays.
Motivation for Approach
When compared with transmissive LCPGs, fabrication in the reflective mode is not straightforward, especially on LCOS backplanes. The diffraction from the backplane pixel structure destroys the holographic patterning process. To circumvent this, we have developed two techniques, both of which have been shown to yield high quality holograms on both ordinary aluminum mirrors and silicon backplanes.
In most switchable LC based devices, the switching speed is inversely proportional to the square of the cell thickness (d). Since reflective LCPGs d is half that for the transmissive ones, their switching speeds are 4 times faster, making a field sequential color system more feasible. Moreover in the case of LCPGs, the minimum attainable grating period scales linearly with d. So larger diffraction angles can be attained with the reflective mode, that leads to simplified optics for applications such as microdisplays. Fig. 3 shows some of the results from reflective gratings manufactured here at OLEG. Note the high-efficiency with unpolarized LED light sources in part (a) and the reduced sub-ms switching times at modest votltages in part (b).
1. RK Komanduri, C Oh, and MJ Escuti, "Polarization-Independent Liquid Crystal Microdisplays," SID Symposium Digest 2008 (Details:TBA),
2. RK Komanduri, WM Jones, C Oh, and MJ Escuti, "Polarization-Independent Modulation for Projection Displays Using Small-Period LC Polarization Gratings," Journal of the Society for Information Display, vol. 15, no. 8, pp. 589-594, 2007.
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