Estimation of trap concentration in photorefractive crystals by a technique of adaptive holographic interferometry

12 GeO₂₀, Bi₁₂SiO₂₀, Bi₁₂TiO₂₀). Received: 17 June 1998/Revised version: 3 August 1998     

Holographic recording mechanisms of gratings in indium oxide films using 325 nm helium–cadmium laser irradiation

UV (325 nm) holographic recording of gratings in indium oxide films fabricated by reactive pulsed laser deposition has been investigated as a function of growth temperature, oxygen pressure and angle of incidence of the plasma plume on the substrate. The influence of the ambient environment (air or vacuum) and the film temperature during recording has also been studied. Large steady state refractive index changes up to 6×10^-3 were observed in layers grown at an oblique angle of 75°. About 77% of the magnitude of these changes residues after thermal annealing and is attributed to UV-induced permanent structural rearrangements. In contrast, refractive index changes in films grown at normal incidence were smaller in magnitude and completely reversible. Received: 30 July 2001 / Accepted: 20 November 2001 / Published online: 23 January 2002     

Two-photon recording of microholograms in photopolymer materials with new cationic thioxanthone photoinitiators

We have demonstrated two-photon induced recording of the microholograms at an arbitrary point within thick (∼100 μm) photopolymer material using photoinitiators on a basis of new cationic thioxanthone derivatives. Such material provides high values of refractive index change Δn = 4.8 × 10⁻³, and holographic recording sensitivity S = 1.2 cm/J. A nanosecond laser pulse at a wavelength of 532 nm was used for recording. For the selective on the depth reading of the microholograms the method of collinear heterodyning was applied.     

Indium oxide thin-film holographic recorders grown by excimer laser reactive sputtering

2 O₃) thin films on glass substrates is performed by pulsed laser ablation of a metallic indium target in an oxygen atmosphere. X-ray diffraction analysis verifies that a transition, from amorphous to polycrystalline film growth, occurs at a temperature of 150 °C. Films grown under optimized conditions exhibit optical transmission higher than 80% in the visible light. Ultraviolet radiation (λ= 325 nm) induced dynamic holographic recording in films deposited at specific temperature and oxygen pressure settings is also demonstrated. Received: 25 April 1997/Accepted: 27 May 1997     

Saturation effect in azobenzene polymers used for polarization holography

The saturation behavior of diffraction efficiency has been experimentally studied and theoretically modeled in azobenzene polyesters by recording polarization gratings with two beams of orthogonal circular polarization. The model is an extension of the phenomenological expression of Kakichashvili, derived from the rate equations for photoanisotropic materials, taking into account the effect of saturation with time and intensity. The saturation model of the polyester material can be used for the computer simulation of a polarization holographic data storage system under development. The simulation is based on fast Fourier transforms, taking the parameters of the recording material and the optical processes into account. Agreement between measured and simulated results proves the applicability of the saturation model as an efficient tool in the optimization of the system. Received: 16 September 2002 / Published online: 3 April 2003 RID="*" ID="*"Corresponding author. Fax: +36-1/463-4194, E-mail: pvarhe@math.bme.hu     

Holographic study of chain length in photopolymerizable compositions

We describe a study of the free-radical homopolymerization kinetics for a system based on acrylamide, triethanolamine and methylene blue by analyzing temporal variations of the diffraction efficiency. The modulation index is related to the component’s parameters, giving as a result a method that can be used to determine the chain length of the polymer and the kinetic rate constants. By using this method we have studied the effect of intensity on the kinetic parameters of the photopolymerization process. Received: 20 August 2001 / Published online: 7 February 2002     

Dynamic light beam deflection caused by space charge waves in photorefractive crystals

During holographic recording in photorefractive crystals (BSO, BGO, and BTO) by an oscillating interference pattern we observe a strong dynamic deflection of the laser beams reflected from the crystal’s surface. The theoretical treatment shows that this new effect is associated with a nonlinear interaction of space charge gratings resulting in a quasi-homogeneous oscillating space charge field which provides deformations of the crystal via the piezoelectric effect. Received: 3 August 1999 / Accepted 5 August 1999 / Published online: 8 September 1999     

New photopolymerizable holographic recording material based on polyvinylalcohol and 2-hydroxiethylmethacrylate (HEMA)

We have developed a new polyvinylalcohol-based photopolymeric holographic recording material. The recording is obtained by the copolymerization of acrylamide and 2-hydroxiethylmethacrylate (HEMA). Diffraction efficiencies near 70% are obtained with exposures of ≈65 mJ/cm² in materials of 110-μm thickness. The addition of HEMA increases the thickness of the film, which results in improved storage capacity for holographic optical storage. Received: 7 March 2002 / Published online: 2 May 2002     

Single laser exposure fabrication of diamond-like 3-dimensional photonic crystal microstructures using circularly polarized light

Phase tunable multi-level diffractive optical elements define an attractive approach for single laser exposure fabrication of 3-dimensional photonic crystal microstructures. The significant advantage of these multi-level diffractive optical elements over two-level diffractive optical elements is the flexibility of fabricating a wide range of 3-dimensional periodic structures by manipulating the relative phase of different diffracted beams. Here, phase tuning was applied to demonstrate fabrication of a hybrid 3-dimensional structure intermediate between previously reported diamond-like Woodpile-type structure of tetragonal symmetry and structure having body-centered-tetragonal lattice symmetry. Circularly polarized light was applied for the first time to balance the diffraction order efficiencies and improve the structural uniformity. Design guidelines are presented for generating diamond-like photonic crystal template that possesses complete photonic bandgap when inverted with high refractive index materials.     

Three-photon interference spectra in four-level atomic systems

We have considered the interference spectra that occur at the three-photon generated frequency arising from the interaction of three laser fields with a four-level atom, where two of the laser fields are on two-photon resonance with the three levels forming a “λ” scheme while the third laser operates between the second ground and the second excited state of the atom. At low intensities of all three laser fields, the overall intensity of the peak at the three-photon generated frequency, describing the spectrum of an electron in the second excited state, depends on the strength of the combined field of the two laser fields that are on two-photon resonance and it takes negative values. This indicates that light amplification without population inversion is likely to occur at the three-photon generated frequency. The combined field of the three laser fields induces multiphoton excitations near the three-photon generated frequency, whose peaks are characterized by linewidths which are much less than the natural linewidths of the atoms. These excitations describe absorption or stimulating emission processes depending on the values of the detunings of the laser fields. The derived results are graphically presented and discussed. Received: 24 January 2001 / Published online: 8 June 2001     

Holographic recording in amorphous side-chain polymers: a comparison of two different design philosophies

Recently, Bieringer et al. [1] reported on the optical properties of liquid-crystalline side-chain polymers. These photoaddressable systems showed light-induced modulations Δn of their refractive index of up to 0.06. We present similar materials and compare them with a novel ansatz reaching birefringence levels of up to 0.2 [2]. This was achieved by donor–acceptor substitutions of the chromophores and by introducing a new type of second side chain. We discuss the different photophysical properties of the two materials under the point of view of potential applications. Received: 16 November 1998 / Revised version: 18 December 1998 / Published online: 7 April 1999     

Fabrication of three-dimensional periodic microstructures by means of two-photon polymerization

Received: 7 October 1998     

Nonlinear interactions and scattering of space charge waves in sillenites

For the first time the effects of overall doubling, spatial doubling, and overall rectification of space charge waves in a photorefractive crystal (Bi₁₂GeO₂₀, BGO) have been detected and theoretically studied. The effects of overall doubling and overall rectification resemble second-harmonic generation and rectification of light in nonlinear optics. A good agreement between theory and experiment is found for the effect of spatial doubling, while for overall doubling and overall rectification effects only a qualitative agreement between theory and experiment has been established until now. Received: 13 July 2001 / Published online: 10 October 2001     

Quantum interference spectra at the difference-frequency generation in four-level atomic systems

Analytical expressions of the spectral functions have been derived for a number of induced excitations that occur near the difference-frequency generation in a four-level atomic system interacting with three laser fields. The spectral heights and subnatural line widths of the induced peaks depend on the intensities of the laser fields involved. The maximum heights of the induced peaks take positive, zero and negative values, indicating that the processes of absorption (attenuation), dark resonance and stimulated emission (amplification) are likely to occur near the difference-frequency generation, respectively. Received: 15 March 2002 / Revised version: 17 June 2002 / Published online: 8 January 2003 RID="*" ID="*"Corresponding author. Fax: +1-613-954-5242     

Optical holographic memory using angular-rotationally phase-coded multiplexing in a LiNbO3:Fe crystal

We propose a new holographic memory scheme based on random-phase-coded multiplexing in a photorefractive LiNbO₃:Fe crystal. Experimental results show that rotating a diffuser placed as a random-phase modulator in the path of the reference beam provides a simple yet effective method of increasing the holographic storage capabilities of the crystal. Combining this rotational multiplexing with angular multiplexing offers further data-storage possibilities. The advantage of using post-image versus pre-image phase modulation of the object beam is demonstrated. Received: 15 August 2000 / Published online: 10 January 2001     

Polarized-light-controlled holographic recording in an azobenzene-doped polymer film

A new polarized-light-controlled holographic recording based on the optical enhancement/restraint of self-diffraction has been demonstrated in an azobenzene-doped polymer film. It is found that a continuous variation of the polarization status as well as of the intensity of the pumping light results in a continuous variation of self-diffraction efficiency. The mechanism originates from the photo-induced anisotropy and polarization-dependent absorption. Both positive and negative replicas of an incident image were presented in real time by means of this incoherent–coherent optical conversion technique. Received: 30 March 2000 / Revised version: 28 September 2000 / Published online: 27 April 2001     

Photorefractive effects in long, narrow BSO crystals with applied electric field

In a photorefractive Bi₁₂SiO₂₀ crystal with high applied electric ac field of square-wave shape a fast two-wave coupling response (less than 1 s) and a slow hologram readout decay (minutes) was found for a wavelength of 633 nm. This can be explained by electron–hole transport with two trap levels. An intensity dependence of the slower complementary grating was found. Illuminating with the readout wave without applied electric field leads to a very slow grating decay (many hours). Received: 18 November 1998 / Revised version: 3 February 1999 / Published online: 7 April 1999     

Photorefractive properties of Cr-doped Sr0.61Ba0.39Nb2O6 related to crystal purity and doping concentration

The purity and the concentrations of the constituents Sr, Ba, Nb and Cr were determined in single crystals of chromium-doped Sr_0.61Ba_0.39Nb₂O₆ (SBN) by instrumental neutron activation analysis (INAA) and X-ray fluorescence analysis (XRF). Experiments with different Cr concentrations C_Cr reveal constant Sr and Ba concentrations as well as a decreasing Nb concentration with an increasing C_Cr. Therefore, Cr is incorporated at Nb lattice sites. The distribution coefficient of Cr is 1.2 between 100 and about 7000 ppm Cr and 1 above. From measurements of the holographic two-beam coupling gain Γ for different C_Cr, we deduced response times for the buildup of holographic gratings. The saturation value of Γ depends in a nonlinear manner on C_Cr and reaches its maximum at about 6000 ppm Cr. The inverse photorefractive response time of the grating erasure depends linearly on the erasure intensity for all doping concentrations. Thus a one-center model can be assumed for the charge transport in SBN:Cr. Received: 8 December 2000 / Revised version: 23 January 2001 / Published online: 21 March 2001