Polarization Holography research addresses the following hypothesis: We can record phase profiles as diffractive Polarization Holograms via polarization holography. We utilized various holography setups to fabricate the diffractive polarization holograms, and the same setup can be used to fabricate various diffractive polarization holograms showing various diffraction properties. The fabrication results in high efficiency (93% ∼ 99%).

Here we address additional question: What novel techniques can enable easier fabrication of
diffractive polarization holograms? Compared to conventional holography setup, our novel approaches provide easier way to tune periods with smaller working space (more than 5 times less). Moreover, new fabrication method, proximate lithography called Risley-Mask also has order of magnitude ability for tuning periods of diffractive polarization holograms with extremely small working space.
The primary scientific contribution within this work is demonstration of various
diffractive polarization holograms showing advanced phase profiles. We also derived governing theory for diffractive polarization holograms fabrication, and evaluated several polarization holography setups.

Fabrication of Liquid Crystal Polarization Gratings
We have successfully fabricated polarization gratings (PGs) that manifest ~100% efficiency, high polarization sensitivity, and low incoherent scattering by adopting polarization holography and photo-alignment techniques. Holography is a photographic technique to create an interference pattern using multiple beams of coherent light. While most conventional holography uses an intensity modulation, polarization holography involves a modulation of the polarization state that interference of light with different polarization. The spatially varying birefringence of the PG can be interpreted as the interference of two beams with orthogonal (left- and right-handed) circular polarization.

We utilize a linear photopolymerizable polymer (LPP) that presents a strong orienta- tional photo-chemical reaction in response to the local direction of linearly polarized UV light. When the LPP is exposed with a linearly polarized UV light, it manifests intermolecular reaction and anisotropic molecular configuration. This alignment technique allows to generate high resolution azimuthal LC director patterns. A proper choice of the exposure fluence is n
ecessary to obtain the strong alignment condition that is generally dependent on the surface pattern and LC material properties.

Polarization grati
ngs (PGs) locally modify the polarization state of transmitted light, which is achieved by spatially varying birefringence and/or dichroism. Among many possible types and creation methods, in this work, we utilize the “circular PGs” created by polarization holography and recorded on polymer or LC materials. Next pictures show the diffraction property of a PG fabricated on a transmissive substrate. When the incident light is broadband and unpolarized, the PG makes three diffraction orders with chromatic dispersion on the two first orders. The structure of a PG is comprised of an in-plane, uniaxial birefringence that varies with position.

Advanced Polarization Holography

Optical diffractive elements based on spatial control of the phase have been studied but previous studies were limited by the fact there was no practical way to realize and control the spatially varying polarization elements. We suggest the effective method to generate diffractive polarization holograms that can modulate various changes of local polarization utilizing advanced holography technique and liquid crystal materials. We fabricated and demonstrated diffractive polarization holograms as liquid crystal diffractive optics working as polarization-sensitive diffractive gratings, optics and azimuthal waveplates with excellent optical quality that manifest nearly 100% efficiency, polarization-selectivity, and fast electro-optical switching (an order of milliseconds).

Diffractive polarization holograms, including polarization gratings, can be fabricated by photo-alignment technique and polarization holography. The polarization holography produces spatially varying linear polarization fields with a constant intensity using two laser-beams interference of orthogonal circular polarization. Conventional holography recording setups require comparably large area and are not commercially viable for creating gradual changes of the linear polarization fields, which are required to fabricate very large grating periods for the polarization gratings. Moreover, there was no practical way to realize and control the change of the linear polarization fields that leads a freedom of diffraction on the optical elements.

In this work, we introduced and demonstrated new polarization holography techniques that create scalable period polarization holograms and the changed polarization fields that produce various types of
diffractive polarization holograms. We showed a novel method that offers proximate lithography with easily tunable-periods, and demonstrated multi-axis polarization gratings that make multiple diffraction at different azimuthal angles.

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