V. V. Lazarev

Mirrorless lasing from nematic liquid crystals in the plane waveguide geometry without refractive index or gain modulation

Lasing from dye doped liquid crystals was observed in the nematic and isotropic phases in the plane waveguide geometry without mirrors and special modulation of the gain or refraction index by a holographic or other technique. The light was generated in a liquid crystal layer confined by two glasses with semitransparent electrodes, which formed an optical waveguide. A strong light amplification occurred along the length of a narrow stripe produced by the pump beam with the necessary feedback provided by the walls of the waveguide in the direction perpendicular to the stripe. The conditions for lasing were fulfilled for the lowest group velocity waveguide modes bouncing at the angles very close to the total reflection angle at the waveguide-glass interface. These modes leaked into the glasses were guided therein at very grazing angles and left the cell from its edges.

Simple voltage tunable liquid crystal laser

Lasing has been observed in a dye-doped liquid crystal placed between one substrate with a large transparent electrode and the other with a system of parallel nontransparent electrode stripes of 15μm period. The stripes operate as a shadow mask providing the spatial modulation of the pump beam and material gain and, with the voltage applied, it additionally creates a Bragg resonator structure due to the spatial modulation of refraction. With that simple cell, a voltage induced tuning of the spectral positions of laser lines over 25nm has been demonstrated.

Lasing in cholesteric liquid crystal cells: Competition of Bragg and leaky modes

Measurements of the angular dependency of luminescence and lasing spectra were carried out for a planar layer of a dye-doped cholesteric liquid crystal (CLC). In addition to the known Bragg emission mode propagating along the cell normal, we observed that leaky modes also depend on the pump intensity and on the dimension and shape of the transverse cross section of the excitation beam. The leaky modes are valuable as a specific lasing regime but impose serious limitation on the energy of the Bragg lasing modes. This has to be taken into consideration when designing high energy lasing devices based on CLC.

Photonics of liquid-crystal structures: A review

The original results of studies of the electro-optical and laser effects which have been performed at the Laboratory of Liquid Crystals of the Institute of Crystallography, Russian Academy of Sciences, over the last few years are reviewed. Cholesteric liquid crystals as vivid representatives of photonic structures and their behavior in an electric field are considered in detail. The formation of higher harmonics in the periodic distribution of the director field in a helical liquid crystal structure and, correspondingly, the new (anharmonic) mode of electro-optical effects are discussed. Another group of studies is devoted to bistable light switching by an electric field in chiral nematics. Polarization diffraction gratings controlled by an electric field are also considered. The results of studies devoted to microlasers on various photonic structures with cholesteric and nematic liquid crystals are considered in detail. Particular attention is given to the new regime: leaky-mode lasing. Designs of liquid crystal light amplifiers and their polarization, field, and spectral characteristics are considered in the last section.

Planar amplifier for a microlaser on a cholesteric liquid crystal

A planar device is described, which combines an oscillator (microlaser) based on a dye doped cholesteric liquid crystal and an amplifier made of another laser dye solution in glycerin. Both the oscillator and amplifier are pumped longitudinally with the same pump beam. With that simple 4mm thick structure, the oscillator beam was amplified seven times.

Quasi-in-plane leaky modes in lasing cholesteric liquid crystal cells

Using a planar cell consisting of a prism and a flat glass, we have carried out precise measurements of the angles at which quasi-in-plane leaky (QIPL) laser modes propagate within a thin layer of a cholesteric liquid crystal (CLC) doped with a laser dye. Both the prism and the glass have refraction indices higher than relevant CLC indices, therefore, the waveguide effect in CLC is excluded. For this type of almost thresholdless lasing neither mirrors nor distributed feedback is necessary. The modes are generated in the amplifying CLC layer due to strong Fresnel reflections from the glass boundaries at propagation angles very close to 90° with respect to the cell normal. In the experiment, two modes polarized differently (s- and p-) have been found outgoing from the prism. Using equivalency between optical properties of a CLC and optically negative nematic liquid crystal at the propagation angles close to 90°, the analytical approach known for a uniform nematic liquid crystal was used for calculations of ...

Electro-optical switching of a thin antiferroelectric liquid crystal film in the planar geometry with evaporated top electrode

An antiferroelectric liquid crystal phase formed by bent-shape molecules was investigated in a geometry typical of solid-state technology. A thin Langmuir film of the compound was transferred onto a conductive glass and supplied with a top electrode evaporated in vacuum. Upon heating the sandwich keeps its integrity even in a high-temperature B2 phase which, despite strong confinement, manifests antiferroelectric and electrooptic properties similar to those in bulk samples. This approach fills the gap between investigations of solid and liquid crystal ferroelectrics and open new ways for studying dielectric, ferro- and antiferroelectric properties of thin and ultrathin films of liquid crystals.

Electroabsorption and oppositely directed built-in fields in a photovoltaic organic heterostructure

The directions and intensities of local electric fields spontaneously built into organic nanoscale structures of Schottky-diode type, indium tin oxide (ITO)-CuPc-Al and ITO-C60-Al, with donor and acceptor layers of copper phthalocyanine (CuPc) and fullerene (C60) have been investigated using an improved spectral electroabsorption technique. It is established that, in the absence of external field and illumination, the built-in fields in the bulk of these structures are directed differently: from Al to ITO in the case of CuPc and from ITO to Al in the case of C60. The best studied photovoltaic heterostructure ITO-CuPc-C60-Al contains simultaneously strong built-in fields in CuPc and C60 layers, with strengths of about 15 and −22 V/μm, respectively. A high (on the order of 10−3 C/m2) positive space charge arises at the donor-acceptor interface, and the oppositely directed fields may either increase or reduce the efficiency of light-energy converters, depending on the heterostructure parameters.

Strong amplification of an electric field and electro-optical response in ultrathin heterostructures ferroelectric-linear dielectric

An electroded heterostructure consisting of a dye layer sandwiched between two polymer ferroelectric layers is discussed. The dye layer plays a role of the probe of the electric field measured by an electroabsorption technique. Using this new method the electric field in ferroelectric and dielectric layers can be measured separately. When an a.c. voltage is applied to the heterostructure, the electric field in the dye layer increases 2.2 times (up to 0.55 GV/m), whereas the field in the ferroelectric decreases 2 times with respect to the average field in the entire structure. Moreover, the dye layer sandwiched between the ferroelectric layers may stand without breakdown the fields 5–7 times higher than a neat reference dye layer confined between metal electrodes. Therefore, the performance of electro-optical, electromechanical and other field controlled devices may be improved considerably when their functional materials are placed between ferroelectrics layers.

ELECTRIC FIELD TUNING A SPECTRUM OF NEMATIC LIQUID CRYSTAL LASING WITH THE USE OF A PERIODIC SHADOW MASK

Lasing has been observed in the dye-doped isotropic and nematic phases of liquid crystals in a plane layer placed between one substrate with a uniform transparent electrode and another substrate with a system of parallel chromium (nontransparent) electrodes. The latter has a periodicity of 15 μm and plays a double role: (i) in both the phases it operates as a shadow mask providing a spatial modulation of the pump beam and consequently, a gain of the material and (ii) in the nematic phase it additionally creates a Bragg resonator structure with spatial modulation of the refraction index when an electric voltage is applied across the transparent and non-transparent electrodes. The resonator operates at high order Bragg resonance modes of numbers m = 71–79 for three lasing dyes studied. With that simple cell, a voltage induced tuning of the spectral positions of lasing lines over 25 nm has been demonstrated without using holographic or other complicated techniques. The mechanism of the observed phenomenon is discussed using modeling of the liquid crystal reorientation and optical properties in the spatially periodic electric field.