It is well known that in standard diffaction experiments only the amplitudes of structural Fourier components are recovered but phase information is lost. This problem is known as the ’phase problem’ in crystallography. In this contribution, we point out how the phase problem of diffraction can be solved in some particular cases by employing multi-wave interference. In the experimental situation described here, we were able to determine the form of the refractive-index profile of a 1-D nanocomposite holographic grating by using a multi-wave coupling analysis of the measured angular dependence of the diffraction efficiencies for a number of diffraction orders.
We investigated recording and readout of transmission gratings in composites of poly(ethylene glycol) dimethacrylate (PEGDMA) and ionic liquids (IL) in detail. Gratings were recorded using a two-wave mixing technique for different grating periods, exposures and a series of film thicknesses. The recording kinetics as well as the post-exposure behavior of the gratings were studied by diffraction experiments. We found that - depending on the parameters - different grating types (pure phase or mixed) are generated, and at elevated thicknesses strong light-induced scattering develops. Gratings with thicknesses up to 85 micrometers are of the required quality with excellent optical properties, thicker gratings exhibit strong detrimental light-induced scattering. The obtained results are particularly valuable when considering PEGDMA-ionic liquid composites for applications as e.g., holographic storage materials or as neutron optic diffractive elements.
The origin of optical diffraction in holographic polymer-dispersed liquid crystal (H-PDLC) transmission gratings was
investigated by optical two beam-coupling analysis based on the linear phase-shift technique. Gratings with the pitch of 1
micrometer and the thickness of 50 micrometers were fabricated from a UV curable mixture of commercially available
constituents. Photopolymerization in the interference field of two laser beams produces not only a periodic variation of
the refractive index, but also a periodic modulation of optical extinction due to light scattering. Both of them contribute
to the diffraction efficiency of the gratings. The magnitudes and relative phases of the two contributions were measured
as a function of a recording time of the grating and as a function of an applied external electric field. During the initial
stage of the grating formation phase modulation is predominant, while at longer exposures both contributions have the
same order of magnitude. They are phase shifted with respect to each other for around π/2. Application of an external
electric field causes a strong decrease of the amplitude modulation, while phase modulation is much less perturbed.
Photopolymerization-induced phase separation of the constituent components in holographic polymer-dispersed liquid
crystals (H-PDLCs) causes a huge variation of the refractive index for light as well as for neutrons. We demonstrated
that H-PDLCs with the thickness of only 30 micrometers act as extremely efficient gratings for neutrons. The lightinduced
refractive-index modulation for neutrons of about 10<sup>-6</sup> was observed, which is nearly two orders of magnitude
larger than found in the best photo-neutron-refractive materials probed up to now. This makes H-PDLCs very promising
candidate for fabricating neutron-optical devices.
The activation energy of thermal fixing is determined in congruent and nearly stoichiometric lithium niobate crystals doped with manganese or iron, respectively. Three different techniques were employed: two-wave mixing, holographic scattering and DC conductivity measurements. A comparison between the three techniques is made and the possible reasons for the discrepancy in the values of the activation energy are discussed. Holographic techniques have the advantage of being contactless methods by which problems coming from electrodes effects are ignored. The holographic scattering technique is much simpler than two-wave mixing technique and gives the same results at high density of the compensating ions. At low free ions concentration it is an ideally sensitive technique to detect the possible dependence of the compensation time constant on the spatial frequency and to determine the concentration of free ions that are responsible for thermal fixing.
The thermal decay of holographic gratings recorded using the conventional two-wave mixing technique has been studied in congruent and nearly stoichiometric LiNbO<SUB>3</SUB> crystal doped with Mn. The activation energies of this process have been determined in the 70-130 degrees C range for congruent and 20-80 degrees C range for nearly stoichiometric crystals, the obtained values being 1.06 +/- 0.03 and 1.10 +/- 0.03 eV, respectively. The kinetics of the OH absorption spectrum has also been studied in undoped nearly stoichiometric LiNbO<SUB>3</SUB> between 40-120 degrees C. The time dependence of the band intensities can be characterized by exponential time constants obeying the Arrhenius-law. The average activation energy, E<SUB>a</SUB> equals 1.1 +/- 0.1 eV is in good agreement with those obtained from the thermal decay indicating that the hologram fixing process in nearly stoichiometric LiNbO<SUB>3</SUB> is governed by proton migration.