Nelson V. Tabiryan

Orientational Optical Nonlinearity of Liquid Crystals

Abstract As a result of development of optics during many centuries the concept was formed that a medium can strongly influence on the light propagation, but the light itself cannot have backward influence on the medium. That concept was refuted only with appearing of coherent and high power radiation of lasers, see, e.g., Nobel Lecture by N. Bloembergen in 1981. A new field in physics–nonlinear optics—formed as a result. The main part of laser investigations concerns with nonlinear optical phenomena, nowadays. First of all these phenomena are of extraordinary interest and beauty and provide for new methods of physical investigations. Secondly, the knowledge of laws governing the interaction of a powerful light with a medium is necessary for the correct utilization of laser beams. Finally, a great number of devices of up-to-date laser optics is totally based on utilization of nonlinear optical effects; generation of higher harmonics of light, stimulated scattering of light, optical phase conjugation, self...

A high frequency photodriven polymer oscillator

High frequency and large amplitude oscillations are driven by laser exposure in cantilevers made from a photosensitive liquid crystal polymer.

Polarization-controlled, photodriven bending in monodomain liquid crystal elastomer cantilevers

We report on the fast and optically controlled angular bending of cantilevers made from liquid crystal polymer networks functionalized with azobenzene moieties (azo-LCN). For potential applications such as adaptive optics, photoresponsive cantilevers should rapidly deform to controlled angles while maintaining a modulus that can accomplish appreciable mechanical work. This work demonstrates cantilevers made of monodomain azo-LCN containing pendant (side chain) azobenzene mesogens with a storage modulus of 1.4 GPa bend 85° in less than 300 ms upon exposure to 442 nm irradiation. Moreover, the bending angle of these monodomain azo-LCN cantilevers can be controlled by the polarization angle of the source relative to the long-axis of the cantilever. The bending performance (deformation angle and speed) of this monodomain system is compared to a polydomain analogue. The impact of azobenzene concentration, laser intensity, and thickness on these parameters is also presented.

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.

Liquid crystalline polymer cantilever oscillators fueled by light

We report on the laser and sunlight driven, fast and large oscillation of cantilevers composed of photoresponsive liquid crystal polymer materials. The oscillation frequency, driven with a focused 100 mW laser of multiple wavelengths (457, 488, 514 nm), is as high as 270 Hz and is shown to be strongly correlated to the physical dimensions of the cantilever. The experimental frequency response is accurately described by the calculated natural resonant frequency for a non-damped cantilever. To further understand the conversion efficiency of light energy to mechanical work in the system, the oscillatory behavior of a 2.7 mm × 0.7 mm × 0.04 mm cantilever was examined at pressures ranging from 1 atm to 0.03 atm. A large increase in amplitude from 110° at STP to 250° at low pressure was observed. A first approximation of the system efficiency was calculated at 0.1%. The large increase in amplitude at low pressure indicates strong hydrodynamic loss and thus, the material efficiency is potentially much greater. Using a simple optical setup, oscillatory behavior was also demonstrated using sunlight. This work indicates the potential for remotely triggered photoactuation of photoresponsive polymer cantilevers from long distances with lasers or focused sunlight.

Photogenerating work from polymers

The ability to control the creation of mechanical work remotely, with high speed and spatial precision, over long distances, offers many intriguing possibilities. Recent developments in photoresponsive polymers and nanocomposite concepts are at the heart of these future devices. Whether driving direct conformational changes, initiating reversible chemical reactions to release stored strain, or converting a photon to a local temperature increase, combinations of photoactive units, nanoparticles, ordered mesophases, and polymeric networks are providing an expansive array of photoresponsive polymer options for mechanical devices. Framing the typically geometry-specific observations into an applied engineering vocabulary will ultimately define the role of these materials in future actuator applications, ranging from microfluidic valves in medical devices to optically controlled mirrors in displays.

Spatial soliton all-optical logic gates

We demonstrate some basic all-optical (electrically unbiased) logic gates in azobenzene liquid crystalline cells, exploiting their large nonlinearity for light localization and the trans-cis photoisomerization for all-optical external control. Spatial solitons were excited at microwatt power levels at 632.8 nm, whereas gating and switching were achieved with milliwatt beams at 409 nm

Photomechanical mechanism and structure-property considerations in the generation of photomechanical work in glassy, azobenzene liquid crystal polymer networks

Azobenzene-functionalized polymeric materials have proven capable of shape adaptive responses when irradiated with light. This work focuses on isolating the fundamental differences between the photogenerated mechanical output of glassy, polydomain azobenzene liquid crystal polymer networks (azo-LCN) upon exposure to either UV and blue-green irradiation. Profound differences in the fundamental photochemical mechanism are identified through spectroscopic examination of representative materials before and after irradiation with UV or blue-green light. The photomechanical response is further elucidated in structure-property examination to ascertain the role of crosslink density, azobenzene concentration, and azobenzene connectivity (crosslinked or pendant) on the photomechanical output.

The Promise of Diffractive Waveplates

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

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.