The high coherence of laser light sources is a key to the application of diffractive optics usable in holographic AR/VR displays. This can be combined with switchable diffractive elements, which are advantageous for several optical functions used in immersive holographic displays such as shutters, polarization filters, for rapid beam deflection and selection. We demonstrate a compact, effective and robust diffraction wide-angle switchable beam-deflecting device based on circular polarization gratings possessing Bragg-performances (Bragg-PG) and a polarization switch. Such grating/polarization switch pair may, for instance, be a discrete switchable deflection element or as a switching element for pre-deflection with field lenses for application in holographic AR/VR displays. Micrometer-thick circular polarization gratings characterized by high diffraction efficiency (DE > 95%), large diffraction angles (< 30°) and wide angular and wavelength acceptance were developed. In the presented embodiment, the output signal is controlled between the zero- and first-diffraction orders by the handedness of circular polarization of the incident light. Forming a stack of two such oppositely aligned gratings can double the deflection angle. These gratings are the result of a two-step photochemichal/thermal processing procedure of a photocrosslinkable liquid crystalline polymer (LCP). The holographic patterning provides a high spatial resolution (period < 700 nm) and the arbitrary orientation of the LC director as well as high optical quality and thermal and chemical stability of the final gratings. Highly efficient (diffraction efficiency, DE > 95% in the vis spectral range) and stable symmetric and slanted circular Bragg polarization gratings were fabricated using the developed material and processing technique. The high usable diffraction angles combined with high DE make the Bragg-PG attractive for HMD AR/VR applications because of the system inherent short focus lengths and large numerical apertures needed to meet the low space budget in HMD and other optical systems.
We present a device for tunable spatial polarization shaping, based on a red light photo-addressable cell. Such a cell compartment is based on a bisazobenzene containing photoaligning layer and a rubbed PI aligning and is filled with the LC mixture E5. Switchable spatial addressing patterns are generated by a 200 channel micro optical addressing unit based on a red VCSEL array (λ = 650 nm) and diffractive beam shapers.
Photoluminescence enhancement of CdSe/CdS/ZnS QDs by localized surface plasmon resonance of large Au-NPs has
been investigated. The photoluminescence of the QDs with an emission wavelength at 620 nm in a PMMA matrix is
enhanced by immobilized Au-NPs. By considering the lifetime and excitation dependent photoluminescence we realized
that the emission and excitation rate enhancements both contributed to the total photoluminescence enhancement. PL
measurements were carried out for different sizes of Au-NPs to find out their influences on the emission of QDs. The
largest enhancement is achieved by applying 80 nm Au-NPs. Silanization method gives us the opportunity easily to
prepare samples with different concentrations of Au-NPs. It is revealed that increasing the concentration of the Au-NPs
layer provides higher scattering cross section which contributes in PL enhancement.
Since organic laser materials offer broad optical gain spectra they are predestined for the realization of widely
tunable laser sources. Here we report on a compact organic laser device that allows for voltage controlled
continuously wavelength tuning in the visible range of the spectrum by external deformation. The device
consists of an elastomeric distributed feedback (DFB) laser and an electro-active elastomer actuator also
known as artificial muscle. Second order DFB lasing is realized by a grating line structured elastomer
substrate covered with a thin layer of dye doped polymer. To enable wavelength tuning the elastomer laser is
placed at the center of the electro-active elastomer actuator. Chosen design of the actuator gives rise to
homogeneous compression at this position. The voltage induced deformation of the artificial muscle is
transferred to the elastomer laser and results in a decrease of grating period. This leads to an emission
wavelength shift of the elastomer laser. The increase of actuation voltage to 3.25 kV decreased the emission
wavelength from 604 nm to 557 nm, a change of 47 nm or 7.8%.
Holographic surface relief gratings written in azobenzene containing films were studied for the use as masters for
polymeric thin film distributed feedback (DFB) lasers. Light induced mass transport driven by E-Z isomerization in
azobenzene containing materials have shown to be attractive for all optical and one-step fabrication of periodic surface
structures with varying parameters for different optical applications. Based on new azobenzene materials and their
holographic processing deep surface relief gratings were generated with grating pitches in the range of 400 nm as
resonant structures for second order DFB lasers emitting in the VIS range. Nanoimprint techniques enabled multiple
duplications of azobenzene master gratings in UV adhesives. The replicas were coated via spin casting with thin films of
red light emitting polymer layers to form DFB thin film lasers. These active layers are guest-host-systems consisting of
an UV-light absorbing conjugated polymer as host transferring its excitation via Förster resonant energy transfer to a red
emitting conjugated polymer. Simple adjustment of grating depth via controlling of illumination time allowed it to
investigate the influence of the corrugation depth and thereby the coupling of laser light and grating on the lasing
behavior of second order DFB lasers in the red region. For this purpose multiple surface structures with different
corrugation depths of up to 130 nm were generated holographically, duplicated and coated.
We present here two approaches for the fabrication of 2D and 3D optical structures. The first one is a step-by-step
fabrication process of 3D structures using thin relief gratings (stacks of thin 1D or 2D gratings). Azobenzene containing
materials for the surface relief inscription have been used in the step-by-step procedure, where after holographic
inscription of desired relief structure and coverage with spacer layer another correlated relief structure has been written
in the next active layer etc. The method provides full flexibility of the structure type and parameters including different
gratings in different layers (hierarchical structures). A technique to produce hexagonal relief gratings of enlarged
diameter which can be used for layer-by-layer photonic structures is developed. The second approach is a multi-beam
holographic recording using special phase masks. Such mask consists of three phase gratings arranged in plane
equilateral triangle geometry with gratings vectors at 120° to each other. A simple method of fabrication of well-adjusted
mask with rather high diffraction efficiency is developed. Hexagonal 2D surface relief and 3D volume phase structures
were fabricated by a single laser beam exposure using UV or visible wavelengths (depending on the material) through
the mask. Azobenzene containing materials and photopolymers, including new specially designed one, were used as
holographic materials.
An innovative approach for voltage-tunable optical gratings based on dielectric elastomer actuators (DEAs) using electro active polymers is presented. Sinusoidal surface gratings, holographically written into azobenzene containing films, are transferred via nanoimprinting to DEAs of different carrier materials. We demonstrate that the surface relief deformation depends on the mechanical and geometrical properties of the actuators. The tested DEAs were made using commercially available elastomers, including a tri-block copolymer poly-styrene-ethylene-butadiene-styrene (SEBS), a silicone polydimethylsiloxane rubber (PDMS) and commonly used polyacrylic glue. The polyacrylic glue is ready to use, whereas the SEBS and the PDMS precursors have to be processed into thin films via different casting methods. The DEA material was pre-stretched, fixed to a stiff frame and coated with stretchable electrodes in appropriate designs. Since the actuation strain of the DEA depends strongly upon the conditions such as material properties, pre-stretch and geometry, the desired voltage-controllable deformations can be optimized during manufacturing of the DEA and also in the choice of materials in the grating transfer process. A full characterization of the grating deformation includes measurements of the grating pitch and depth modulation, plus the change of the diffraction angle and efficiency. The structural surface distortion was characterized by measuring the shape of the transmitted and diffracted laser beam with a beam profiling system while applying an electro-mechanical stress to the grating. Such surface distortions may lead to decreasing diffraction efficiency and lower beam quality. With properly chosen manufacturing parameters, we found a period shift of up to 9 % in a grating with 1 μm pitch. To describe the optical behavior, a model based on independently measured material parameters is presented.
Dielectric elastomer actuators (DEA) of poly-styrene-ethylene-butadiene-styrene (SEBS) and commonly used VHB4910
tape were studied for voltage tunable optical transmission gratings. A new geometry is proposed, in which the grating is
placed in an area without electrodes, permitting for light transmission through the device. Experiments were performed
to implement surface relief gratings on DEA films from pattern masters made from holographic recorded gratings. Since
the actuation strain of the DEA depends strongly on the boundary conditions, the desired voltage-controllable
deformation of the grating can be achieved by choosing suitable manufacturing parameters. Conditions were found
permitting a shift of up to 9 % in a 1 μm grating. A model based on independently measured material parameters is
shown to describe the optical behavior.
We present here novel easily processible organic-inorganic nanocomposites suitable for holographic fabrication of
diffraction optical elements (DOE). The nanocomposites are based on photocurable acrylate monomers and inorganic
nanoparticles (NP). The compatibility of inorganic NP with monomers was achieved by capping the NP surface with
proper organic shells. Surface modification allows to introduce up to 50wt.% of inorganic NP in organic media.
Depending on the NP nature (metal oxides, phosphates, semiconductors, noble metals) and their properties, the materials
for both efficient DOE and multifunctional elements can be designed. Organic-inorganic composites prepared have been
successfully used for the effective inscription of periodic volume refractive index structures using the holographic
photopolymerization method. The nanocomposite preparation procedure, their properties and optical performance of
holographic gratings are reported. The use of functional NP makes it possible to obtain effective holographic gratings
having additional physical properties such as light-emission or NLO. Some examples of such functional polymer-NP
structures and their possible application fields are presented. The combination of easy photo-patterning of soft organic
compounds with physical properties of inorganic materials in new nanocomposites and the flexibility of the holographic
patterning method allow the fabrication of mono- and multifunctional one- and multi-dimensional passive or active
optical and photonic elements.
Light-induced mass transport in azobenzene functionalized polymers has been used for generation of surface relief
gratings (SRG) for different optical applications. The effect of grating formation has been ascribed to the light-induced
motion of the azobenzene chromophores involving the covalently bound polymer chains. We propose a concept of
supramolecular materials for the effective all-optical generation of surface relief structures and optical anisotropy. The
materials are based on the non-covalent interactions between charged photochromic azobenzene units and oppositely
charged polymer matrix, for example polyelectrolytes including charged alkoxysilanes. This new supramolecular
approach opens a new way for the simple, cost effective and environment friendly preparation from building blocks of a
variety of materials for the effective formation of SRG. Up to 1.65 μm deep relief gratings were inscribed onto a few
micrometers thick films of these materials. The high thermal stability of the induced structures has been explained in
terms of the network of oppositely charged ions inherent to the materials. Also 2D-structures, for example square and
hexagonal gratings, were inscribed by the successive recordings. The latter possibility was also used to generate gratings
with non-sinusoidal profiles by Fourier transform technique. A new technique to control the grating profile has been
developed based on the real-time process of grating formation in these materials. The gratings with sow-tooth like
profiles were induced by this method. The diffraction efficiencies up to 60 % in one diffraction order were achieved.
A series of new polyesterimides (PEIs) containing photochromic azobenzene moieties have been synthesized. For this purpose the following diesterdianhydrides with azobenzene groups have been prepared: 2,2'-{4-[(4-nitrophenyl)-azo} phenyl]iminobis(ethyl benzene-1,2-dicarboxylic acid anhydride-4-carboxylate), 2,2'-{4-[(4-methylphenyl)-azo}phenyl] iminobis(ethyl benzene-1,2-dicarboxylic acid anhydride-4-carboxylate) and 2,2'-{4-[(4- fluorophenyl)-azo}phenyl] iminobis(ethyl benzene-1,2-dicarboxylic acid anhydride-4-carboxylate). The polyesterimides have been obtained from diesterdianhydrides and 1,3-phenylenediamine using high-temperature polycondensation. The resulting polyimides are characterized as having good solubility in some solvents, high glass transition temperatures (168-210°C) and good film forming properties. The irradiation with linearly polarized light causes a reorientation of the azobenzene groups by angular-selective E/Z photoisomerization. In this way optical anisotropy is induced in the initially isotropic films casting from solution of the amorphous polymers. The photoinduced dichroism in the films has been measured to investigate the effect of azobenzene substitution.
A new type of volume holographic gratings based on polymer-liquid crystal composite named POLIPHEM (POlymer LIquid Crystal Polymer Holograms Electrically Manageable) is presented. The new composite material in combination with the proper holographic fabrication results in switchable holographic gratings with high diffractive and electro-optical parameters. Periodic structures consisting of alternating polymeric and LC-rich regions with aligned mono-domain morphology of the LC are formed due to photopolymerisation and phase separation of the initially homogenous film of photo-curable monomers and liquid crystals under illumination with an interference field (λrec=364 nm) at room temperature. Compared to typical holographic polymer-dispersed liquid crystals (H-PDLCs) POLIPHEM films are characterized by the absence of light-scattering, strong anisotropy after holographic exposure, low driving voltages and fast electro-optic time-response. The kinetics of the holographic recording under different irradiation intensities were investigated at λtest=632.8 nm, the microstructure and the electro-optical response of POLIPHEM transmission gratings have been analyzed. POLIPHEMs were realized in the pitch range of 0.28-6 μm. The diffraction efficiency of more than 96% was achieved for p-polarized probe light (for λtest=632.8 nm). Possible mechanism of POLIPHEM formation is discussed briefly.
The defined preparation of anisotropic films of functionalized polymers is important for different fields of optical applications. A promising way to create such films is based on the interaction of photochromic polymers with linearly polarized light. Different mechanisms are known to generate anisotropy. The best studied process is the photoorientation in the steady state of the E/Z photoisomerization of azobenzene moieties. But azobenzene based systems have some disadvantages. For this reason the light-induced orientation of photochromic liquid crystalline polymers without azobenzene moieties is presented. In the case of the studied photochromic LC polymers the light-induced anisotropy is used as a starting point for the thermal development of the order by a thermal post-orientation step based on the self-organization of LC polymers. This two-step orientation process combining a photoinduced and a subsequent thermal ordering process is a very promising way to orient efficiently LC polymers containing photochromic moieties. The light induced and the thermal developed order of four different polymers containing a mesogenic, a photochromic and a fluorescent stilbene side group were compared with respect to the composition of the polymers. This orientation procedure results in films with anisotropic fluorescence properties in the case of two of the investigated polymers.
The three-dimensional structure of thin anisotropic polymer films is of great importance for many applications, such as display technology and optical data storage. We developed different methods for the accurate determination of the in-plane and the out-of-plane anisotropy of thin polymer films. The anisotropic properties are generated or modified by irradiation with polarized light at room temperature and subsequent thermal development of the liquid crystalline polymers. For the characterisation of thin films different ellipsometric methods and absorbance measurements were developed and for thick films the prism coupling method was used. The three-dimensional structure of the films and the resulting optical properties are strongly dependent on thickness and preparation of the films. The photoinduced order is generated by the photoorientation of azobenzene containing side chain polymers. This order is changed by annealing of the films in the liquid crystalline phase of the polymers. For thinner spin coated films the order after annealing is almost a prolate in-plane order. This should be caused by the initial in-plane order induced by the spin coating process. In the case of thicker films the order after annealing is homeotropic due to the interfacial order. Under certain conditions even an oblate order was found after annealing.
Liquid crystalline polymethacrylates with benzanilide and photochromic azobenzene side groups and a related terpolymer containing an additional side group with a phenylethynyl substituted anthracene chromophore were oriented by the irradiation with linearly polarized light. The orientation of both polymers were compared irradiating with polarized visible or, alternatively, UV light. Caused by the co-operativity of the photoorientation process, the light-induced orientation of the azobenzene groups is connected to the alignment of the non-photochromic side groups below the glass transition temperature. The light-induced order generated in the glassy state was significantly amplified by the subsequent annealing of the irradiated films at temperatures in the mesophases. Factors of amplification of about 30 were found in the case of both polymers. The photo-induction process and its amplification by thermotropic self-organization were investigated in dependence on the polymer composition, the irradiation dose and the wavelength of the incident light with respect to the absorption of the dye and its limited photo-stability. The required dose or the irradiation time, respectively, were significantly reduced by the optimization of the light-induced and thermal processing. In this way, dichroic films of co- and terpolymers were created. However, the green fluorescence of the anthracene chromophore is effectively quenched by the azobenzene side group within the film.
UV light influence on the surface potential φs of poly(vinylcinnamate) (PVCN) layers widely used for liquid crystal (LC) photoalignment is investigated. In case when PVCN film is spin coated on the ITO electrodes the surface potential changes under irradiation in both PVCN aligning film and ITO coating. The measured increment of the surface potential changes under irradiation in both PVCN aligning film and ITO coating. The measured increment of the surface potential Δφs=400 mV is mainly cuased by the photoconductivity of ITO electrodes, whereas the part caused by PVCN substrate is at least one order of magnitude lower. The relaxation time of Δφs for the ITO-PVCN coatings was estimated to be about 1 h. The induced potential may substantially modify dielectric properties of the aligned LC layers and its anchoring characteristics when the freshly irradiated substrates are used. On the contrary, this influence is weak when the substrates are irradiated several hours before the use for LC alignment.
Two amorphous side-chain copolymethacrylates containing 60% azobenzene chromophores (PII, PIV) were irradiated homogeneously with a linear polarized Ar+ ion laser beam at 488 nm. From polarized UV/VIS spectra the maximum of the photoinduced dichroism was calculated (dPII=0.25, dPIV=0.35). No intensity dependence of this values was notified. PIV was irradiated under holographic conditions with a grating period of 30 μm. Two types of polarization gratings, circular orthogonal and linear orthogonal ones were inscribed. Using a microscope spectrometer polarized UV/VIS spectra were measured within the grating period with a lateral resolution of about 2 μm. The induced local dichroism was measured and the different modulation of the molecular orientation in the recorded gratings was confirmed by this direct experiment. PII was used to monitor the grows up stages of the surface relief grating, starting from the molecular photoorientation until the surface deformation. Parallel to the online observed diffraction efficiency, the offline surface profile measurements were done by AFM. The details of surface topography and the reversibility of the process are discussed and an interesting possibility of the surface structuring is demonstrated.
The irradiation with linearly polarized light generates optical anisotropy in isotropic films of photochromic amorphous and liquid crystalline polymers due to photoorientation. The photochemically induced orientation of the photochromic side groups causes a co-operative orientation of the non-photochromic parts of the polymers such as the non-photochromic side groups. The orientational order photogenerated in the glassy state acts as an initializing force to align LC polymers by thermotropic self-organization on annealing above Tg. The combination of both ordering principles can result in a significant amplification of the light-induced in-plane anisotropy or, results alternatively in a homeotropic alignment of the side groups. The dependence of the photoorientation process and its thermal amplification on the liquid crystallinity, aggregation and the irradiation conditions is discussed.
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