High refractive index polymers (HRIPs) have attracted much attention with the increased use of organic light-emitting diodes (OLEDs), antireflective coatings, near to eye (NTE) displays, etc. Typical refractive indices (RIs) required for these applications are about 1.7 or greater. It is also common for HRIPs to undergo nano-replication to form gratings in various products. UV cured HRIPs are preferred for their high production speeds and ability to be nano-replicated in situ. Different patterns and features are used to efficiently couple light into and out of light guides. In addition, high refractive index materials are known to improve in-coupling efficiencies. Another feature of the nano-replicated materials is an ability to capture and emit light from various angles and wavelengths to improve the field of view for an electronic device such as virtual reality glasses. Device success depends on the efficiency of the high refractive index polymer to promote light coupling into the light guide and reduce scattering that allows wide angle field of views (FOVs).
High refractive index polymers are desired because they allow (1) improved total internal reflectance of light passing through a waveguide, thereby reducing light loss; and (2) improved angles at which light can enter and exit the light guide (i.e., a higher refractive index produces a larger field of view). FIG. 2 is a graph demonstrating the importance of refractive index and the effect that it has on field of view as a function of coefficient R (the ratio of the tangent of the largest diffracted angle of the longest wavelength to the tangent of the smallest diffracted angle of the shortest wavelength). The coefficient R is a measure of the maximum ratio of the step lengths of the beams between total internal reflections. Material with an R value of 5 or greater will have trouble extracting the longer wavelengths out of the guide material. As shown in FIG. 2, there are many types of polymers that can exhibit improved refractive index values, including polycarbonates and polyimides (not shown).
However, most of these materials do not possess the ability to be replicated at nano-scale dimensions required to make gratings for waveguides. Lower viscosity material is desired to facilitate shaping of a desired grating profile that is cured in place. UV cured (meth) acrylics, vinyls and epoxies are suitable for gratings that can be mechanically replicated. Additionally, some applications require use of resin material that is mechanically stable (high Tg) above 50° C. or even 100° C., while still achieving refractive indices of 1.7 or greater. Known materials have been selected to achieve either high RI (with low Tg) or high Tg (with low RI). The literature has focused on developing high RI materials with no emphasis on mechanical stability at high temperatures. Current product needs dictate that high RI materials are warranted, but thermal stability, mechanical stability, high Abbe number, low haze and high transmittance must also be designed into the finished product.
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