Sarik R. Nersisyan

The Promise of Diffractive Waveplates

Diffractive waveplates exhibit the high diffraction efficiency of Bragg gratings in micron-thick material layers.

Fabrication of liquid crystal polymer axial waveplates for UV-IR wavelengths

We show the opportunity of fabricating axially symmetric waveplates fine tuned to a desired wavelength. High quality waveplates are obtained using liquid crystal polymer layers on photoaligning substrates extending their functional range from UV to IR wavelengths. We characterize the effect of the waveplate on laser beams showing formation of a doughnut beam with over 240 times attenuation of intensity on the axis. We pay attention that the power density is strongly reduced on the doughnut ring as well and use this opportunity for taking charge coupled devices (CCDs) out of a deep saturation regime. Strong deformation of the beam profile is observed when the vortex axis is shifted towards the periferies of the beam. We demonstrate feasibility of using this phenomenon for shaping the profile of light beams with a set of waveplates.

Polarization insensitive imaging through polarization gratings

Liquid crystal polarization gratings exhibit high diffraction efficiency (approximately 100%) in thin material layers comparable to the radiation wavelength. We demonstrate that they can be combined for polarization-insensitive imaging and optical switching applications. A pair of closely spaced, parallel oriented, cycloidal polarization gratings is capable of canceling the diffractive property of an individual grating. As a result, the phase of the beam is not distorted, and holographic images can be formed through them. An anti-parallel arrangement results in a broader effective diffraction band and doubles the diffraction angle. Broadband diffraction spanning from 480 nm to beyond 900 nm wavelengths has been obtained for a pair of gratings with 500 nm and 633 nm peak diffraction wavelengths. Liquid crystal polymer cycloidal gratings were used in the study showing 98% diffraction efficiency over a large area, and allowed for the use of laser beams expanded to 25 mm. The characteristics of combined cycloidal gratings were tested with laser beams at both UV and red wavelengths.

Characterization of optically imprinted polarization gratings

We provide detailed description and characterization of specifics of the imprinting technique for fabrication of large-area and high-efficiency liquid crystal polymer polarization gratings. We show that the quality of polarization gratings imprinted with linear polarized light is as high as that of gratings obtained in the holographic process, while exhibiting twice larger diffraction angle. The cycloidal polarization pattern used for imprinting is obtained from a master polarization grating, and the importance of fine tuning of its peak diffraction wavelength to the wavelength of imprinting radiation is emphasized. Tuning of the peak diffraction wavelength of imprinted polarization gratings from UV to near IR was realized with the aid of multilayer structures. Since the imprinting process does not involve a holographic setup, it is insensitive to ambient conditions and vibrations and provides an opportunity for large scale production of polarization gratings.

Transparent thin film polarizing and optical control systems

We show that a diffractive waveplate can be combined with a phase retardation film for fully converting light of arbitrary polarization state into a polarized light. Incorporating a photonic bandgap layer into a system of such polarizers that unify different polarization states in the input light into a single polarization state at its output, rather than absorbing or reflecting half of it, we developed and demonstrated a polarization-independent optical controller capable of switching between transmittive and reflective states. The transition between those states is smoothly controlled with low-voltage and low-power sources. Using versatile fabrication methods, this “universally polarizing optical controller” can be integrated into a thin package compatible with a variety of display, spatial light modulation, optical communication, imaging and other photonics systems.

The principles of laser beam control with polarization gratings introduced as diffractive waveplates

The development history of polarization gratings (PGs), with origins in holography and Bragg gratings, accentuated and reinforced their perception as gratings. We highlight their nature as waveplates - diffractive waveplates (DWs) - and stress their family connection to vector vortex waveplates. This approach provides a straightforward understanding of the unusual properties of PGs, such as nearly 100% diffraction in thin material layers, the presence of only one diffraction order for a circularly polarized beam, wide diffraction bandwidth and the possibility of achromatic behavior. With technology being ripe for applications such as beam steering, and optical switching, we characterize the resistance of DWs to optical radiation, the effects of temperature and deformations. We also show that the boundary effects in the manufacturing process make it necessary to use substrates larger than the desired aperture of the DW. The multicomponent systems are discussed for developing normally transmissive switchable imaging systems, beam scanning, and achromatic diffraction.

Optical switching of liquid-crystal polarization gratings with nanosecond pulses.

Optical switching with single nanosecond laser pulses of 532 nm wavelength is reported using high-efficiency optical axis gratings made with a nematic liquid crystal doped with an azobenzene dye. The azobenzene dye we have synthesized exhibits enhanced photosensitivity at green wavelengths, allowing for low threshold switching (approximately 10 mJ/cm(2)). The dye is also characterized by fast relaxation of isomers, allowing for restoration of the diffractive properties of the grating within approximately 100 ms. Change of the diffraction efficiency of the zeroth-order beam from 10% to 70% is observed in a micrometer-thick material layer.

All-optical diffractive/transmissive switch based on coupled cycloidal diffractive waveplates

Pairs of cycloidal diffractive waveplates can be used to doubly diffract or collinearly propagate laser radiation of the appropriate wavelength. The use of a dynamic phase retarder placed in between the pair can be utilized to switch between the two optical states. We present results from the implementation of an azo-based retarder whose optical properties can be modulated using light itself. We show fast and efficient switching between the two states for both CW and single nanosecond laser pulses of green radiation. Contrasts greater than 100:1 were achieved. The temporal response as a function of light intensity is presented and the optical switching is shown to be polarization independent.

Axial polarizers based on dichroic liquid crystals

Polarizers capable of producing linearly polarized beams with axial (radial and azimuthal) symmetry have been fabricated with the aid of a dichroic liquid crystal. Photoalignment was achieved using a printing technique to reduce the UV exposure time required for production of axially aligning substrates from 1 h, typical for direct writing techniques, to 10 min. The polarizing features of axial polarizers and their pairs are characterized and their differences outlined. We demonstrate that the transmission switching contrast of an axial polarizer/analyzer pair, comprised of an electrically controlled liquid crystal cell, is comparable to conventional systems with linear polarizers. The opportunities for using axial polarizers for polarization imaging, sensor protection, and nonlinear optics are discussed. Particularly, we show that the technology could reduce the fluence of a laser beam on an optical sensor without affecting imaging.

Diffractive waveplate arrays [Invited]

Diffractive waveplate technology presents an opportunity for designing arrays of all types of optical components. We present here different architectures of arrays of waveplate lenses and vector vortex waveplates. Due to the continuous nature of diffractive waveplate coatings and the high spatial resolution of the technology, the sizes of array elements can span from micrometers to tens of millimeters. Both fixed and electrically switchable arrays are discussed. Arrays of diffractive waveplates present new challenges and opportunities for digital light polarization holography for applications in polarizer-free displays, smart windows, optical communications, beam shaping, and other photonics technologies.