Andris Ozols

Holographic recording and photocontraction of amorphous As2S3 films by 488.0 nm and 514.5 nm laser light illumination

Abstract Holographic gratings were recorded into as-deposited As 2 S 3 films using 488.0 nm or 514.5 nm lines from the Ar + -laser. Gratings for the two recording wavelengths were systematically compared. Photostructural changes were found to depend on the light intensity, with laser induced heating proven to have no role in the grating formation. Changes in film thickness were observed on the exposed grating areas. These changes suggest surface contraction of 20% from the total film thickness. These contractions may be caused by SS bondbreaking and subsequent crosslinking with the AsS network.

Synthesis and properties of 1,3-dioxo-1H-inden-2(3H)-ylidene fragment and (3-(dicyanomethylene)-5,5-dimethylcyclohex-1-enyl)vinyl fragment containing derivatives of azobenzene for holographic recording materials

New glassy 1,3-dioxo-1H-inden-2(3H)-ylidene fragment und (3-(dicyanomethylene)-5,5-dimethylcyclohex-1-enyl)vinyl fragment containing push-pull type derivatives of azobenzene able to create thin layers have been synthesized. Thin films of synthesized glasses for holographic recording were prepared using spin coating technique from saturated chloroform solution. Holographic grating recording in films of 6a-b, 7 and 12 has been experimentally studied at 633 and 532 nm in both transmission and reflection modes with p-p recording beam polarizations. The film 12 was found to be the most efficient at both wavelengths in transmission mode exhibiting the maximum self-diffraction efficiency of 9.9% at 633 nm, and 15.3% at 532 nm. The film of 6a was the most efficient in reflection mode with the maximum selfdiffraction efficiency of about 3%.

Holographic recording of surface relief gratings in stilbene azobenzene derivatives at 633 nm

Holographic recording in stilbene azobenzene derivatives by He-Ne 633 nm laser light has been experimentally studied. It was found that surface relief gratings (SRG) can be recorded by red light. Usually shorter wavelengths are used to induce the trans-cis photo-isomerization in organic materials. SRG with 2 μm period and an amplitude of 130 nm have been recorded with 0.88 W/cm2 light in about 20 minutes in amorphous films of 3-(4-(bis(2-(trityloxy)ethyl)amino)phenyl)-2-(4-(2-bromo-4-nitrophenyl)diazenyl)phenyl)acrylonitrile spin-coated on glass substrates. Self-diffraction efficiency up to 17.4% and specific recording energy down to 114 J/(cm2%) were measured. The recorded SRG were stable as proved by subsequent AFM measurements. The photo-induced changes in absorption spectra did not reveal noticeable signs of trans-cis transformations. Rather, spectrally uniform bleaching of the films took place. We conclude that a photothermally stimulated photo-destruction of chromophores is responsible for the SRG recording. The recording of stable SRG in the stilbene azobenzene derivatives we studied is accompanied by the recording of relaxing volume-phase gratings due to the photo-orientation of chromophores by the linearly polarized recording light. It should also be noted that holographic recording efficiency in stilbene azobenzene derivatives exhibit an unusual non-monotonic sample storage-time dependence presumably caused by the peculiarities of structural relaxation of the films.

Hologram recording in azobenzene oligomers

Elementary hologram (holographic grating) recording and their coherent optical erasure have been experimentally studied in azobenzene oligomer (ABO) layers differing by their chemical composition, matrices and by the connection type of azobenzene chromophores to the matrix (dispersed or covalently bound). The best holographic parameters (7.9% diffraction efficiency and 86 J/cm2 specific recording energy) were achieved in the samples with covalent bonding to the matrix. Vector recording is also possible. Recording is unstable and reversible. The coherent optical erasure studies have shown its efficiency dependencies on the initial diffraction efficiency, erasing beam intensity and grating period which are different for three groups of ABO samples. The conclusion is made that recording is due to the photoinduced alignment of the azobenzene chromophores followed by refractive index changes. These are the first results and further studies are in progress.

Photoinduced anisotropy and holographic recording in amorphous chalcogenides

The effect of photoinduced anisotropy and its application to vector hologram recording is reviewed focusing on amorphous chalcogenides. Vector holographic grating recording in amorphous As-S-Se(a-As-S-Se) films is experimentally studied and analyzed in comparison with scalar recording. It is holographically established that a linearly polarized 632.8 nm light produces photoinduced anisotropy and the chalcogen related D+, D- center reorientation and generation mechanism is proposed. It is used to explain the observed peculiarities of vector recording in comparison with scalar recording based on photoinduced structural changes: much lower diffraction efficiency (4 X 10-3% versus 4%), much larger specific recording energy [6.4 kJ/(cm2%) versus 20 J/(cm2%)], difference in spatial frequency response, instability (vector hologram lifetime of about two days versus practically permanent scalar holograms), the absence of hologram self-enhancement (present in scalar recording), near perfect reversibility. It is also experimentally found that vector holograms in a-As-S-Se films indeed reconstruct the signal wave polarization but only in the minus first diffraction order. It is also shown that photoinduced anisotropy also contributes to the scalar hologram recording in amorphous chalcogenides stimulating it by means of subbandgap readout light and enabling a subbandgap recording.

Effect of Light Polarization on Holographic Recording in Glassy Azocompounds and Chalcogenides

Light polarization effects on a holographic grating recording in a glassy chalcogenide a-As40S15Se45 film has been experimentally studied and compared with previously studied glassy molecular azobenzene film 8a at 633, using s − s,p − p, CE-1 and CE-2 circular-elliptic recording-beam polarizations (differing by light electric field rotation directions). The azocompound exhibited much higher self-diffraction efficiency (SDE) and diffraction efficiency whereas chalcogenide was more sensitive. Their recording efficiency polarization dependences also were different. SDE up to 45% was achieved in 8a with p − p and up to 2.6% in a-As40S15Se45 with CE-2 polarized recording beams. The polarization changes in the diffraction process were studied as well in these and other materials (11, 16, 19 and a-As2S3 film, LiTaO3:Fe crystal). It was found that light polarization changes in the process of diffraction from gratings recorded vectorially by s−p polarizations depended on chemical composition, wavelength, and exposure time. Vector gratings with SDE up to 25% were recorded in 8a, rotating a linear polarization by 90°. No light polarization changes were found in azobenzene 19 and chalcogenide films and in LiTaO3:Fe crystal, thus showing a vector recording of scalar holograms. The recording mechanisms in azocompounds and chalcogenides are discussed and compared.

Thermal, glass-forming, nonlinear optical and holographic properties of push-pull type azochromophores with triphenyl moieties containing isophorene and pyranylidene fragments

Molecular organic compounds with electron donating fragment bounded through π-conjugated system with electron acceptor fragment, as well as with incorporated triphenyl groups in their molecules show potential for creating cheap and simple solution processable materials with nonlinear optical properties. Additional insertion of azobenzene fragment in their structures makes them also possible to form holographic volume and surface relief gratings (SRG) after exposure to laser radiation, which could be useful for holographic data storage. For these purposes polymers are generally used. However, their application is complicated and challenging task as in every attempt to obtain the same polymer it will have different physical properties. On the other hand, the synthetic procedure of molecular glasses is more simple as their structure and physical properties are strongly defined. Unfortunately, there is still no clear relation between compound organic structures and their thermal, glass-forming and optical properties. In order to investigate the above mentioned regularities, we have synthesized and investigated ten molecular glassy organic compounds with three different fragments as main backbones of the molecules: indene-1,3-dione (WE-1, WE-2, WE-3), isophorene (IWK-1D, IWK-2M, IWK-2D) and pyranylidene (DWK-2TB, ZWK-2TB, JWK-2TB, ZWK-3AZO). Compounds containing isophorene fragment in their molecules had the highest NLO efficiencies (d33 up to 125.7 pm/V for IWK-2D) and also were the most effective holographic data storage compounds with holographic self diffraction efficiency 13% and holographic diffraction efficiency 20%, also for IWK-2D, but their thermal stability (Td from 288°C to 295°C) and glass transition (Tg from 90°C to 105°C) values were just average. Pyranylidene type compounds had the highest thermal stability and highest glass transition (Tg from 115°C to 180°C). But their ability to form and maintain amorphous structure were low and they had average NLO efficiencies (d33 up to 66.2 pm/V for ZWK-2TB) and average holographic self diffraction efficiency 2% and holographic diffraction efficiency 8% for ZWK-3AZO. The molecules with just azobenzene fragment and indene-1,3-dione as electron acceptor has the lowest thermal (Td from 250°C to 282°C, Tg from 70°C to 98°C) and also the lowest holographic properties with holographic diffraction and self diffraction efficiencies at 4% for WE-1 and lower for other compounds. Nevertheless, some of the investigated molecular glasses show potential as multifunctional optical materials.

Effects of light polarization and crystal orientation on the holographic recording efficiency in doped LiNbO 3 crystals

The holographic recording efficiency in doped LiNbO3 crystals has been studied both experimentally and theoretically depending on the type of dope, on the recording geometry and on the polarizations of the recording and readout light.. The studied crystals can be arranged in the following order by their efficiency: LiNbO3:Cu, LiNbO3:Fe (yet having a smaller thickness and donor concentration than LiNbO3:Cu), LiNbO3:Fe+Cu, LiNbO3:Fe+Ti, LiNbO3:Ti. It was found that the recording geometry with the holographic grating vector along the optical axis is much more efficient than in the perpendicular configuration.This fact is in accordance with the photorefraction theory based on photogalvanic and linear electro-optic effects (PGE-LEO theory). Other recording mechanisms are active, too, but much less eficient. The recording efficiency polarization dependence is mainly determined by dopes.It is different in the cases of Fe and Cu impurities. It also follows from our studies that photoconductivity along the optical axis is much larger than in the perpendicular direction.

Relaxation effect of stilbene azobenzene derivatives on their holographic properties

The material relaxation effect on holographic properties of stilbene azobenzene derivatives in the form of glassy films has been experimentally studied. Holographic grating recording with the period of 2 μm was made by a He-Ne laser at 633 nm in the self-diffraction mode. The readout was made simultaneously in order to follow the fast self-diffraction efficiency changes. The existence of the optimal material storage time (6-51 day) is established enabling the most efficient recording. Material relaxation amplitude and the holographic recording efficiency increased when the chromophore concentration was increased, especially above the threshold of about 70 mass %. It is also found that 633 nm recording due to the modulation of refraction and absorption indices is accompanied by the formation of surface relief grating. The conclusion is made that holographic recording in stilbene azobenzene derivatives at 633 nm is due to the chromophore reorientation by linearly polarized light possibly including trans-cis-trans transformations.

Nanosecond and picosecond pulse transmission in optical fibres

The transmission light intensity dependences of different silica glass fibres have been experimentally studied when nanosecond pulse trains were incident. A pronounced 13-42% transmission increase was found in contrast to the picosecond pulse irradiation when a monotonic transmission decrease took place. The photoinduced transmission increase is explained in terms of photothermally increased numerical aperture, the photothermal lens effect, and the photoinduced saturation of a weak impurity absorption. The obtained results can be applied in high power applications of fibres such as WDM fibre communication systems.