We discuss the growth and shape stabilization of small objects made of smectic and nematic liquid crystals (NLCs) in aqueous surfactant solutions. When dispersed or put in contact with an aqueous solution of CTAB, smectic A liquid crystals spontaneously grow into fibers of very uniform diameter and good optical quality, which makes them appropriate for light guiding applications. However, it is difficult to control the growth of smectic A fibers and attempts to stabilize them by photo polymerization fail to produce good quality structures for optical application. We discuss a novel method for self-shaping of nematic liquid crystal droplets into various LC fibers. The method is based on the use of two surfactants: one is dissolved in the LC and the other in the aqueous phase. By changing the temperature, the surface of the droplet increases at a fixed volume of the LC, which triggers the transformation of a droplet into fibers. This is a novel mechanism of LC droplet shape transformation, where the surface of the LC interface is controlled by the temperature and concentration of two surfactants.
We discuss the electric field tuning of ferroelectric liquid crystal microlasers. The microlasers were made of 90:10 wt % mixture of CE3 and CE14 ferroelectric liquid crystals (FLCs), which was doped with ~0.1% Pyrromethene 580 fluorescent dye. The ferroelectric Sm C* phase was observed between 42°C and 74°C. The droplets were embedded into the CYTOP CTX-809A, a polymer with low electric conductivity and high viscosity. Within the temperature range 42-60°C the droplets obtained good homeotropic structure with the perpendicular anchoring of the molecules to the surface of the droplets. When the droplets were illuminated with a 532 nm pulsed laser light, Whispering Gallery Mode lasing was observed. The application of a low frequency electric field induced a red-shift of the WGM resonance peaks. The shift was reversible and had a quadratic dependency on the electric field. The observed tuning range was 4.5 nm for 2 V/μm applied electric field. The observed behaviour is explained by the soliton-like deformation of the helical ferroelectric Sm C* structure in an external electric field.
We demonstrate non-contact temperature measurement with 0.1 K precision at distances of several meters using omnidirectional laser emission from dye-doped cholesteric liquid crystal droplets freely floating in a fluid medium. Upon the excitation with a pulsed laser the liquid crystal droplet emits laser light due to 3D Bragg lasing in all directions. The spectral position of the lasing is highly dependent on temperature, which enables remote and contact-less temperature measurement with high precision. Both laser excitation and collection of light emitted by microlasers is performed through a 20 cm aperture optics at a distance of up to several meters. The optical excitation volume, where the droplets are excited and emit the laser light, is approx. 10 cubic millimeters. The measurement is performed with sub-second speed when several droplets pass through the excitation volume due to their thermal motion. Since the method is based solely on measuring the spectral position of a single and strong laser line, it is quite insensitive to scattering, absorption and background signals, such as auto-fluorescence. This enables a wide use in science and industry, with a detection range exceeding tens of meters.
We review recent experiments on the fast and ultrafast all-optical control of light in bulk nematic and smectic-A liquid
crystals. Ultrafast optical control at sub-picosecond time scalecan be achieved via the optical Kerr response of a nematic
liquid crystal. We show that the refractive index changes are of the order of 10-4 in 5CB nematic liquid crystal and can be
optically induced by applying 100 fs pulses of 4 mJ/cm2 fluence. We discuss stimulated emission depletion of
fluorescence in a smectic-A liquid crystal and demonstrate nanosecond light control of fluorescent pulse shaping. Both
methods could be applied to control light by light in future photonic devices based on liquid crystals.
This article presents the design of a miniature detection system and its associated signal processing electronics, which can detect and selectively recognize vapor traces of different materials in the air – including explosives. It is based on the array of surface-functionalized COMB capacitive sensors and extremely low noise, analog, integrated electronic circuit, hardwired digital signal processing hardware and additional software running on a PC. The instrument is sensitive and selective, consumes a minimum amount of energy, is very small (few mm3) and cheap to produce in large quantities, and is insensitive to mechanical influences. Using an electronic detection system built of low noise analog front-end and hard-wired digital signal processing, it is possible to detect less than 0.3ppt of TNT molecules in the atmosphere (3 TNT molecules in 1013 molecules of the air) at 25°C on a 1 Hz bandwidth using very small volume and approx. 10 mA current from a 5V supply voltage. The sensors are implemented in a modified MEMS process and analog electronics in 0.18 um CMOS technology.
We demonstrate a new sort of optical fibers, which are self-assembled from a smectic-A liquid crystal. When this liquid crystal is put in contact with water solution of surfactant CTAB, microfibers start spontaneously growing at the liquid crystal-water interface. The fibers are of very uniform diameter and can be several hundreds of micrometers long. They all have a line topological defect in the core of the fiber with a local optical axis pointing from the defect core towards the surface. The ends of the fiber are of perfect spherical shape. By doping the fibers with a fluorescent dye, we demonstrate guiding of light along the fiber. When the fiber is illuminated with pulsed light, which is absorbed by the dye, we observe Whispering Gallery Mode (WGM) lasing in a plane perpendicular to the fiber. The smectic-A fibers are soft and flexible and can be manipulated with laser tweezers demonstrating a promising approach for the realization of soft matter photonic circuits.
Manipulation and transport of microparticles and even fluorescent molecules by thermally induced gradient of the order parameter is demonstrated in the nematic liquid crystal. IR light absorption of a focused beam of the laser tweezers is used to heat locally a thin layer of the nematic liquid crystal by several degrees, thus creating a spatial gradient of temperature of the nematic liquid crystal over tens of micrometers. It is observed that a colloidal particle with dipolar symmetry of the director configuration is attracted into the hot spot of the tweezers. The strength of trapping potential increases linearly with particle radius, which indicates that the trapping is due to elastic energy of the distorted nematic liquid crystal around the particle. By using fluorescent molecules instead of colloidal particles, we observed that this thermal trapping of colloidal particles is efficient down to the nanoscale, as fluorescent molecules are also attracted to the hotter regions of the liquid crystal. This effect is absent in the isotropic phase.
When liquid crystals are dispersed in an immiscible fluid, microdroplets of liquid crystal are spontaneously formed in a fraction of a second. They have optically anisotropic internal structure, which is determined by the ordering of liquid crystal molecules at the interface. Spherical droplets of a nematic liquid crystal can function as whispering-gallery-mode microresonators with an unprecedented width of wavelength tunability by an electric field. WGM pulsed lasing in dyedoped nematic microdroplets is sensitive to strain, temperature and presence of molecules that change molecular orientation at the interface. Omnidirectional 3D lasing was demonstrated in droplets of chiral nematic liquid crystals that form 3D Bragg-onion resonators. We present recent progress in this field, including electric tuning of 3D lasing from chiral nematic droplets and self-assembly of ferroelectric smectic-C* microdroplets with the onion-Bragg structure. We show that anisotropic fibres could be self-assembled from smectic liquid crystals.
In this work we show resonant transfer of light from a planar polymer waveguide into a high index solid microsphere
(BaTiO3) or nematic liquid crystal microdroplet. BaTiO3 spheres were deposited on the waveguide
surface either in dry form or as dispersion in pure water. On the other hand nematic liquid crystal (NLC)
droplets were dispersed in a 10 mM sodium dodecyl sulfate (SDS) in water that promoted perpendicular surface
anchoring of 5CB and therefore radial droplet configuration. Planar waveguides were produced by spinning a
high refractive index polymer (1.68 at 632 nm) onto a soda lime glass. We used two different sources of light,
either 671 nm diode laser or the supercontinuum (SC) laser for the mode launching into the thin film waveguide
using a prism film coupler. The resonant tunneling of light from the waveguide into the high index spheres and
LC microcavities was observed in the case of SC illumination, because the spectrum of light radiated from the
both microcavities clearly showed whispering gallery modes.
In this work we show that nematic liquid-crystal droplets can be used as low-loss and highly tunable whisperinggallery
mode (WGM) optical microcavities. They are spontaneously formed by mixing the liquid crystal with an
immiscible liquid. The optical modes can be tuned either by applying an electric field, changing the temperature
or by mechanical deformation. The tuning range for the electric field is as high as 20 nm at 2.6 V/μm for a ~ 600
nm WGM in 17 μm diameter droplets. Tuning is fast and almost linear with the applied voltage. In the case of
the temperature tuning, we can shift the modes by more than 15 nm at a temperature change of 30 K. Further,
we can also apply mechanical deformation to a free standing film of PDMS polymer containing the liquid crystal
droplets. At 15% strain the mode shift is more than 30 nm. In all the three cases the tuning exceeds the free
spectral range of the resonators and is completely reversible.
The interactions between different types of colloidal particles are measured and analyzed. We use these interactions to
build different self-assembled microstructures, such as dimers, chains, wires, crystals and superstructures. In the
experiments we have used different size, different symmetry of colloids (elastic dipoles and quadrupoles) and different
way of colloidal binding (via localized defects and via entangled defects). We use optical tweezers for directed selfassembly
of colloidal particles. Special attention is devoted to the hierarchical superstructures of large and small
particles. We show that smaller, submicron colloidal particles are trapped into the topological defect rings or loops,
twisting around larger colloidal particles, which are sources of strong nematic deformations. Various possible
applications are discussed, especially in photonics and metamaterials.
We describe and analyze experiments, where optical manipulation of small colloidal particles in the nematic liquid
crystal (NLC) was used to create artificial colloidal structures, such as 1D chains and 2D colloidal crystals, and
superstructures of different types of colloids. In all cases, the colloidal particles are strongly bound to each other, with a
typical pair interaction energy of several 1000 kBT per 1μm size particle. There are two distinct mechanisms of colloidal
binding in a spatially homogeneous NLC: (i) binding via spatially localized topological (point) defects, and (ii) binding
via entangled topological defects, where the defect line winds around and wraps several colloidal particles.
Colloidal structures assembled in confined nematic liquid crystals are examined. Theoretical predictions based on
Landau-type approaches are complemented with the latest studies of laser assisted colloidal assembling. Effective
colloidal interactions are particularly sensitive to the confinement and external fields. Their complexity leads to
numerous stable or metastable colloidal superstructures not present in isotropic solvents. Particularly important are
colloidal structures coupled by entangled disclinations. Such a string-like coupling is very robust and opens new routes
to assemble new photonic materials.
The ability to generate regular spatial arrangements of particles on different length scales is one of the central issues of
the "bottom-up" approach in nanotechnology. Current techniques rely on single atom or molecule manipulation by the
STM, colloidal particle manipulation by laser or optoelectronic tweezers, microfluidics, optofluidics, micromanipulation
and classical lithography. Of particular interest is self-assembly, where the pre-determined spatial arrangements of
particles, such as 3D photonic crystals, could be realized spontaneously. Dispersions of particles in liquid crystals show
several novel classes of anisotropic forces between inclusions, which result in an amazing diversity of self-assembled
patterns, such as linear chains and 2D photonic crystals of microspheres. The forces between the particles in nematic
colloids are extremely strong and long-range, resulting in several thousand times stronger binding compared to the
binding in water based colloids. The mechanisms of self-assembly in nematic colloids are discussed, showing this is a
novel paradigm in colloidal science, which can lead to new approaches in colloidal self-assembly for photonic devices.
We describe and analyze laser trapping of small colloidal particles in a nematic liquid crystal, where the index of refraction of colloidal particles is smaller compared to the indices of the liquid crystal. Two mechanisms are identified that are responsible for this anomalous trapping: (i) below the optical Freedericksz transition, the trapping is due to the anisotropic dielectric interaction of the polarized light with the inhomogeneous director field around the colloidal particle, (ii) above the optical Freedericksz transition, the optical trapping is accompanied by the elasticity-mediated interaction between the optically distorted region of a liquid crystal and the particle. In majority of the experiments, the trapping above the Freedericksz transition is highly anisotropic. Qualitative agreement is found with a numerical analysis, considering nematic director elastic distortion, dielectric director-light field coupling and optical repulsion due to low refraction index colloid in a high index surroundings.
We report on an observation of attractive gradient force on dielectric particles suspended in a medium with a higher refractive index. This unexpected phenomenon was observed with micron sized silica spheres (n=1.37) suspended in optically anisotropic nematic liquid crystal (no=1.5 and ne=1.7). The newly discovered interaction has a range an order of magnituded bigger than the laser beam waist diameter. Above transition temperature, where nematic order of a liquid crystal is lost, the gradient force becomes repulsive and it's range is reduced to expected values. We attribute an anomalous gradient force in nematic phase to two phenomena: particle dressing with a liquid crystal molecules resulting in a colloid with a higher effective index of refraction than surroundings and laser field induced distortino of nematically ordered liquid crystal molecules.
We have studied the order parameter dynamics close to the SmA-SmC*A phase transition in homeotropic cells of 4-(1-ethylheptyloxycarbonyl) phenyl-4'-alkylcarbonyloxy biphenyl-4-carboxylate by photon correlation spectroscopy. The order parameter fluctuations in the antiferroelectric SmC*A phase can be decomposed into the fluctuations of the phase and the amplitude of the molecular tilt angle. Considering the unit cell to consist out of two adjacent layers, one can describe these fluctuations with two ferroelectric modes and two antiferroelectric modes. Using photon correlation spectroscopy we measured both ferroelectric phase modes in the backward scattering geometry. This is the first simultaneous observation of these modes. The temperature dependence of the relaxation rates of these modes gives the coupling coefficients of the ferroelectric and antiferroelectric order parameters, whereas the dispersion relation leads to the values of the diffusivity coefficients for antiferroelectric and ferroelectric phase modes in the SmC*A phase.