A technique is suggested to measure a threshold of two-photon initiated photopolymerisation involving Z-scan of a thin
film of sensitive material along the focusing axis of the laser beam. The condition of reaching the threshold when
gradually increasing the light intensity by moving the film towards the focal spot of the beam is defined as that with
minimal intensity at which polymerization occurs. The occurrence of the polymerization is detected by interferometric
effect inside the transmitted beam itself, which is due to interference of the wave going through the polymerization area
and the wave going around it. The technique is demonstrated for measurements employing Nd:YAG laser in nanosecond
regime with fundamental frequency 1064 nm and its harmonic of 532 nm, as well as with pumped by its third harmonic
optical parametric oscillator. Threshold data are presented for particular systems, indicating threshold of 5 GW/cm<sup>2</sup> for a
system based on Rose Bengal exposed by 1064 nm nanosecond-pulsed radiation and 0.05 GW/cm<sup>2</sup> for Darocur initiators
exposed to 532 nm.
Significant enhancement of the thermal stability of the hologram recorded on photochromic materials had been achieved
via covalent bonding of the photochromic dye to the polymer matrix as compared to the host-guest systems. One time
partial reduction of the hologram's initial diffraction efficiency due to thermal exposure was observed for both -
polymers with attached dye as well as host-guest materials. Such one time reduction is interpreted to be due to the
thermal relaxation of the polymer network induced with photochromic transition in the dye molecule. A gradual
hologram erasure at elevated temperatures was observed for the host-guest system, which is assumed to be due to dye's
diffusion between highly lit and dark areas. At temperatures ca. 100 °C there was no detectable diffusion type
degradation of the hologram after 6 hours of exposure to elevated temperature. Same heating of the hologram in
photochromic host-guest polymer led to a full hologram erasure within 40 minutes. In both cases the one time reduction
of the diffraction efficiency (DE) due to matrix relaxation and heat adjustment had comparable value of about 0.5 of
hologram's initial DE.
Volume holographic properties of diarylethene doped polymers are reported for the first time. Resolution of more than
1500 lines/mm with ▵n about 10<sup>-4</sup> is demonstrated on a sample 1 mm thick. Angular selectivity contains two
contributions - the wide one from the amplitude holographic component, and the sharp one from the phase counterpart.
The half-width of the amplitude component is about order of magnitude larger than that of the phase component. High
fatigue resistance of the dye allows multiple record/erase cycles without degradation. Thermal stability of the dye
allows for durability of the recording material at the temperatures above 115 °C. Holograms while stable at room
temperature exhibit thermal degradation at elevated temperatures even below 115 °C. This gives potential to implement
on that material multiple use holographic thermal sensors.
It is demonstrated experimentally that an efficient alignment of a reactive liquid crystal (LC) mesogen is achievable by employing a relief diffraction grating having a pitch smaller than the correlation length of the LC mesogen. Full alignment of the reactive LC mesogen using a 500 lines/mm diffraction grating is confirmed by polarization microscopy. A grating of 200 lines/mm exhibited partial alignment in the deepest regions of the groove. Alignment was not maintained in areas closer to the relief profile peaks of the periodic structure (which caused the spatial modulation of the alignment to occur). UV exposure of the aligned layer allowed to permanently fix the alignment due to polymerization of the unsaturated acrylate bonds. We estimate the correlation length of the reactive mesogen from this experiment to be 5 micrometers <(kappa) <2 micrometers .
Reactive liquid crystal (LC) mesogen (trade mark C6M of Merck) is shown to possess efficient relief formation properties. Photosensitive formulation was made by mixing of the reactive LC mesogen C6M with 2% of photoinitiator (Irgacure 651). Photosensitive layer (thickness 70 micrometers ) was obtained by melting the formulation in the gap between two glass substrates. One of the substrates contained a photosensitive silver-halide layer, in which the amplitude mask was recorded with the resolution Fequals40 lines/mm. The amplitude function of the mask represented a phase function of the desired CO<SUB>2</SUB> laser focusing element, designed for wavelength 10.6 micrometers and 45 degrees incidence . Exposure of the reactive mesogen layer through the mask was carried out at elevated temperature of Tequals85 degrees C by UV source (wavelength 365 nm). Immediately after exposure a substrate with photomask was removed and the relief structure in the exposed layer was measured to be ca. hequals1 micrometers deep. After that a dark self-development was allowed resulting in the increase of relief depth h up to 15 micrometers within several hours. The desired hequals7.5 micrometers needed for the most efficient performance of CO<SUB>2</SUB> laser focusing element at 10.6 micrometers and 45 degrees incidence, was stabilized by interrupting self-development with uniform UV exposure of the recorded structure.
Cationic-induced two-photon photopolymerization is demonstrated at 710 nm, using an isopropylthioxanthone/diarylidonium salt initiating system for the cationic polymerization of an epoxide. The polymerization threshold J<SUB>2th</SUB> is found to be approximately 1 GW/cm<SUP>2</SUP>, with a dynamic range of > 100, i.e. the material can be fully polymerized at intensities > 100 times the threshold level without damage. The polymerization rate R is found to be proportional to the m equals 1.7 power of the intensity, or R equals [C (J-J<SUB>2th</SUB>)]<SUP>m</SUP> equals [C (J-J<SUB>2th</SUB>)]<SUP>1.7</SUP>, which implies a significantly stronger localization of the photochemical response than that of free radical photoinitiators. R and J<SUB>2th</SUB> significantly improve when the concentration z of the initiator (onium salt) increases.
It is demonstrated that threshold reduction of two-photon polymerization is achievable by means traditionally employed for sensitivity enhancement for single photon photoinitiation, such as heavy atom enhancement or intersystem crossing, electron donor agent, concentration increase of initiator. It is shown that measured threshold is in reverse proportion to square root of initiator concentration, whereas observed length of induction period exhibits reverse proportionality to the square of light intensity. Overall, experimentally observed threshold values of two-photon induced photopolymerization are effected by all intermediate stages of energy transformation in the photochemical sequences leading to photoinitiation, in particular inter-system crossing of excited initiating molecules as well as by monomer reactivity.
H-PDLC materials are introduced with sensitivity in the 800- 850 nm region. Materials presented are based on acrylate monomers mixed with liquid crystalline (LC) compounds and sensitized to near IR. This allows highly efficient in-situ recording of holographic optical elements by diode lasers. Various LC compounds have been tested, in particular: E7, 5CB, MBBA. The following parameters of the H-PDLC materials and the recorded gratings were investigated: sensitivity of the materials in the range 800-850 nm, diffraction efficiency in a wide range of wavelengths and photochemical stability of the recorded gratings. The sensitivity of the materials with LCs E7 and 5CB was found to be ca. 300-500 mJ/cm<SUP>2</SUP> to wavelengths 834 and 850 nm. Employment of MBBA led to drop in sensitivity of about an order of magnitude. High diffraction efficiency was demonstrated. Particularly interesting are the following properties of our materials: (1) Control of the dispersion of the diffraction efficiency for p-polarization is shown to be possible by changing the monomer content of the formulations. (2) Recorded gratings exhibit excellent photochemical stability. (3) Switching of the gratings is in principle possible and currently worked on.
Holographic polymer dispersed liquid crystal (H-PDLC) material was recently sensitized that is sensitive in the near infrared (800 nm to 855 nm). The compound was based on mixture of an acrylate monomer and liquid crystals (e.g., E7 and MBBA were investigated). In the present work, the angular sensitivity of gratings, recorded in these H-PDLC films, was investigated for various wavelengths. The holographic recording was realized by a diode laser (834 nm), which produced 1000 lines/mm spatial frequency of intensity grating. Significant broadening in the angular selectivity for p-polarization was observed for the formulation based on E7. The half-height width (HHW) of the angular selectivity was approximately 7 degrees (for 850 nm) and 10 degrees (for 670 nm). However, for s-polarization, the same sample has an HHW of about 2 degrees for both. We believe that this behavior is the indicator of a spatial modulation in the shape of the liquid crystals (LC) micro droplets. Droplet's shape should change spatially from elliptical to spherical correlated with (but not necessarily the same) spatial frequency of the light grating. For MBBA based samples, there is almost no polarization dependence and the HHW of the angular selectivity is about 2 degrees. Thus, the shape of the micro droplets is strongly influenced by the chemical composition of the compound used. It has been demonstrated that angular selectivity for H-PDLC compound is strongly polarization dependent. This indicates that the forms and sizes of micro droplets could be strongly modulated along the gradient of light interference patterns.
A new technological approach using dry photopolymer recording material makes it possible to fabricate the relief focusing elements for IR radiation. The formation of the relief structure by self-development takes place in the dark, subsequent to the illumination through the computer generated amplitude mask. No wet processing is required. These diffractive elements exhibit low surface scattering. Dye sensitized polymerization of acrylamide has been found to produce nonlinearity of the relief recording. A diffraction efficiency of about 55% has been achieved for CO2 laser radiation.
This work is devoted to the development of diffractive and quasidiffractive optical elements characterized by the microrelief height ranging from several microns up to 180 (mu) k. The technology of the synthesis is based on a novel physical phenomenon of the substance mass- transfer taking place in liquid photopolymerizable compositions (LPC). Using the proposed method, we fabricated and tested the focusators of laser radiation with a microrelief height of 7.5 (mu) k and the raster elements with quasidiffractive microlenses for the white light.
A new technological approach makes possible the fabrication of relief computer-generated focusing elements for infrared radiation by using a dry photopolymer recording material. A photopolymer material, was used to make a computer-generated focusing element. Its performance is described when it is used with a CO<SUB>2</SUB> laser and solar illumination. Results describing the spectral frequencies measured are presented here.
A review of original theoretical and experimental investigations of volume holographic recording on liquid photopolymerizable layers is presented. The experimental results that confirm the thermodynamic model of hologram formation in the layers of question are summarized. Also considered are the actions of intensity gradient of interference field on
Light scattering elements formation are studied in the liquid photopolymerizable layers. Under recording, two of different photopolymerizable layers were replaced one by one, the first layer providing the scattering structure formation and the second one the noise hologram recording with its amplification. The recorded efficiency was found to depend on the delay value in the second layer exposure beginning as it related to the first. The effects of scattering indicate tuning under scattering wave formation and wave formation suppression--both by composition content changes. The light scattering elements obtained were found to provide the light scattering in the range of about 25 degree(s) with the efficiency not less than 90%.
The formation mechanisms of relief holograms in liquid photopolymerizable layers are analyzed and noted to involve monomer diffusion from darker to lighter areas. The relief depth obtained increases monotonically with initial layer thickness. After gelation in highlighted areas, the mobility of polymer molecules becomes limited by network bounds and monomer-polymer interdiffusion ceases, so that only monomer diffusion into the gel can restore thermodynamic equilibrium.
A thermodynamic analysis is conducted for volume-hologram formation processes in the layer formed by a two-component, liquid photopolymerizable composition-contained polymerizable monomer and solvent. The formation of both photochemically stable and unstable holograms by a diffusive recording mechanism can be accounted for in terms of the microheterogeneous character of photopolymerization. The principles of recording medium compositions' formulation are presented.