High contrast switching of transmission due to electrohydrodynamic effect in stacked thin systems of liquid crystals.

We study the opportunity of using electrohydrodynamic instabilities in a nematic liquid crystal mixture with negative dielectric anisotropy for controlling laser beams. Switching between naturally transparent and diffuse light scattering states is achieved by application of low frequency, low amplitude voltages. The specifics of diffuse light scattering state depending on the orientation and thickness of the liquid crystal layer are revealed. The switching occurs on a milliseconds time scale. Combination of thin, flexible liquid crystal cells allows polarization independent, high contrast, fast switching in a broad band of visible wavelengths.

Broadband waveplate lenses.

We report on lenses that operate over the visible wavelength band from 450 nm to beyond 700 nm, and other lenses that operate over a wide region in the near-infrared from 650 nm to beyond 1000 nm. Lenses were recorded in liquid crystal polymer layers only a few micrometers thick, using laser-based photoalignment and UV photopolymerization. Waveplate lenses allowed focusing and defocusing laser beams depending on the sign of the circularity of laser beam polarization. Diffraction efficiency of recorded waveplate lenses was up to 90% and contrast ratio was up to 500:1.

Thin waveplate lenses of switchable focal length--new generation in optics.

We present new lenses - waveplate lenses created in liquid crystal and liquid crystal polymer materials. Using an electrically-switchable liquid-crystal half-wave retarder we realized switching between focused and defocused beams by the waveplate lens. A combination of two such lenses allowed the collimation of a laser beam as well as the change of focal length of optical system.

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.

Azobenzene liquid crystalline materials for efficient optical switching with pulsed and/or continuous wave laser beams.

This study compares optical switching capabilities of liquid crystal (LC) materials based on different classes of azobenzene dyes. LCs based on molecules containing benzene rings with nearly symmetrical pi-pi conjugation respond more efficiently to a cw beam than to a nanosecond laser pulse and maintain the changes induced by the beam for tens of hours. Using azo dye molecules containing two benzene rings with push-pull pi-pi conjugation we demonstrate high photosensitivity to both a cw beam as well as nanosecond laser pulse with only 1 s relaxation of light-induced changes in material properties. Even faster, 1 ms restoration time is obtained for azo dye molecules containing hetaryl (benzothiazole) ring with enhanced push-pull pi-pi conjugation. These materials respond most efficiently to pulsed excitation while discriminating cw radiation.

Optically switchable, rapidly relaxing cholesteric liquid crystal reflectors.

Reversible, fast, all-optical switching of the reflection of a cholesteric liquid crystal (CLC) is demonstrated in a formulation doped with push-pull azobenzene dyes. The reflection of the photosensitive CLC compositions is optically switched by exposure to 488 and 532 nm CW lasers as well as ns pulsed 532 nm irradiation. Laser-directed optical switching of the reflection of the CLC compositions occurs rapidly, within a few hundred milliseconds for the CW laser lines examined here. Also observed is optical switching on the order of tens of nanoseconds when the CLC is exposed to a single nanosecond pulse with 0.2 J/cm(2) energy density. The rapid cis-trans isomerization typical of push-pull azobenzene dye is used for the first time to rapidly restore the reflection of the CLC from a photoinduced isotropic state within seconds after cessation of light exposure.

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.

Azobenzene liquid crystal polymer-based membrane and cantilever optical systems.

Optically deformable membranes and cantilevers based on azobenzene liquid crystal polymer networks (azo-LCN) are demonstrated in the context of dynamic optical systems. Large modulations in laser beam propagation direction or amplitude directed by laser-induced changes in material shape are demonstrated. These macroscopic shape changes are induced by local changes to the liquid crystalline order induced by photoisomerization processes. We demonstrate herein a number of concepts including the focusing and defocusing action of an azo-LCN membrane, laser beam steering from a bimorph azo-LCN/metal cantilever, and surface initiated bending and blocking of a parallel propagating laser beam. High speed and large angle deformations of an incident or reference beam is demonstrated when coupled into suitable optical architectures. The concepts under discussion appear to be highly practical for a number of applications due to the significant nonlinearity and photosensitivity of azo-LCN materials.

Bidirectional photoresponse of surface pretreated azobenzene liquid crystal polymer networks.

We report on the photodriven, polarization-controlled response of UV-pretreated azobenzene-based liquid crystal polymer networks (azo- LCN) of polydomain orientation to higher wavelength CW argon-ion laser light (Ar(+)) of 457-514 nm. The significant absorbance of the azo-LCN cantilever in the UV is used to form an approximately 1 microm thick cis-isomer rich skin. Subsequent exposure to the Ar(+) laser drives a bidirectional bending process that is the result of two distinguishable photochemical processes. First, 457-514 nm laser light (regardless of polarization state) drives cis-trans photoisomerization of the UV-pretreated surface, restoring the order of the azobenzene liquid crystalline moieties. Mechanically, the cis-trans process results in an expansion on the exposed surface that forces the cantilever to undergo a rapid bend away from the laser source. Once a sufficient number of trans-azobenzene moieties are regenerated, continued Ar(+) illumination promotes both the trans-cis and cis-trans processes enabling trans-cis-trans reorientation. In this particular system and conditions, trans-cis-trans reorientation enables polarization controlled mechanical bending of different angles towards the Ar(+) source. Photomechanical responses of UV-pretreated azo-LCN demonstrate the viability of photogenerated effects in UV-rich environments such as space.

High-efficiency 1.5 microm thick optical axis grating and its use for laser beam combining.

We demonstrate an optical axis grating (OAG) recorded in a nematic liquid crystal that yields a higher than 80% diffraction efficiency and over 800:1 switching contrast between diffraction orders for a laser beam of a red wavelength in a material layer only 1.5 microm thick. The grating was used for combining two laser beams with high efficiency. These observations prove the feasibility of new generation high-efficiency diffractive optical components, which are most promising for infrared and high-power applications owing to their enhanced transparency and reduced thermal effects in thin material layers.

Polarization-controlled switching between diffraction orders in transverse-periodically aligned nematic liquid crystals.

Transverse-periodic-oriented nematic liquid crystals (LCs) are a special type of optical axis grating that are capable of very high efficiency diffraction (theoretically, 100%) in thin layers of materials with thickness comparable to the radiation wavelength. In particular, they fully diffract linearly polarized input beam into circularly polarized +1st and -1st diffraction orders. We experimentally demonstrate switching between diffraction orders of such gratings when the polarization of the incident beam changes from right-circular to left-circular and vice versa with the aid of an electrically controlled LC phase retarder. Such a setup in which the diffraction efficiency and direction are controlled externally, without application of an electric field to the transverse-periodic grating, provides additional control opportunities and does not compromise the quality of the grating. The grating used in the experiment was 1.5 microm thick and had a period of 4 microm. The contrast ratio of switching between the +1st and -1st orders was as high as 267:1 for a He-Ne laser beam with a switching time of 6.6 ms.

Self-activated liquid-crystal cells with photovoltaic substrates.

We show that photovoltaic fields are capable of efficiently reorienting liquid crystals, leading to new concepts of optically addressable light modulators. Using an arrangement consisting of a liquid-crystal layer between LiNbO3:Fe photovoltaic substrates, we observed spatial filtering due to self-phase modulation in a planar-oriented cell and nonlinear transmission between crossed polarizers in a twist-oriented cell. These processes do not require an external electric field. The substrates are arranged such that light propagates along the +c axis in each substrate, allowing a secondary process of power transfer to occur through contradirectional photorefractive two-beam coupling.

Polymer film with optically controlled form and actuation.

A low power laser beam is used to induce large and fast variations in the shape of a polymer film due to photoinduced contraction and expansion of the polymer film surface subject to the beam. The direction of the photoinduced bend or twist of the polymer can be reversed by changing the polarization of the beam. Thus the film orientation could be varied within +/-70 masculine. The phenomenon is a result of optically induced reorientation of azobenzene moieties in the polymer network.