Photoinduced effects

The materials

The hybrid sol-gel materials are potentially interesting for various applications, according to the organic molecules that one can insert into the vitreous matrix. We have studied the introduction into the glass matrix of the Disperse Red 1 azo-dye (DR1) and other functionalising molecules as carbazole group and trinitrofluorenone TNF. DR1, thanks to its photoisomerization capabilities, introduces photoanisotropy, whereas carbazole units and TNF may be responsible for photorefractive behaviour.

Between many applications possible of these materials, the main fields are on the optical data storage and the creation of diffractive elements and of polarization sensitive holograms.

Photoinduced birefingence

One important property of the DR1 molecule is its possibility to change isomeric form when exposed to light in the visible absorption band, which has a peak at about 480 nm. Initially, the TRANS molecules are oriented randomly. If the incident radiation is linearly polarized along the molecular axis, the isomerization to the CIS form is extremely probable. If theta is the angle between light polarization and molecular axis, the probability of absorption is proportional to cos(theta)^2. The CIS form is unstable and after a characteristic time (depending on the surroundings of the molecule) it decays to the TRANS form. When the CIS molecules decay to the TRANS form, the TRANS orientation is roughly random. The light is absorbed again and creates new CIS molecules.. and so on. The only TRANS molecules which have high probability to survive to the isomerisation are those oriented perpendicularly to the direction of the EM field, so they accumulate in the plain defined by this condition.

In our lab, we apply the pump-probe technique to measurements of photoinduced birefringence.

The so-called "Pump-probe technique" is widely use. One light beam (pump, 480 nm) induces some changes in the optical properties of a sample and second beam (probe, 633 nm), which passes trough the sample, can be characterized in intensity and polarization. The changes produced by the pump are revealed by the probe and carry information about the optical properties of sample (absorption, birefringence, dichroism, ...). One can study also the dynamic behavior of the sample during the pumping phase or the relaxation phase (after the pump is turned off). It's obvious but not trivial that the probe light should not influence the sample itself (non-actinic beam).

The figure below shows our optical bench for the measurements of photoinduced birefringence. The sample is a silica/hybrid thin (about 10^2 nm) film, doped with a synthetic dye, Disperse Red 1 (DR1). The film is placed between two crossed polarizers. The polarization axis of each polarizers form 45° with the vertical direction. If a birefringent medium is placed between the polarizers, the light of the HeNe laser is polarized by the first polarizer, then is modified in his polarization by the medium, passes through the second polarizers and finally reaches the photodiode and the computerized acquisition system. If the medium is not birefringent, no light reaches the photodiode, because the two polarizers are crossed.

Our thin films become birefringent when illuminated with a linearly polarized light beam, with wavelength of about 480 nm. The 488 line of a Ar+ laser can be then used. The figure shows the measured intensity signal during a pump on-off cicle. The build-up and relaxation dynamics can be studied through many different techniqes, and provide a lot of useful information about the sample.

Polariscopic analysis of photoinduced birefringence

A wide-field polariscopic technique can been used to track the photoinduced birefringence obtained in a pump-probe experiment on photoanisotropic systems. The technique requires pump and probe beams expansion and 2D acquisition of the irradiance distribution and allows to evaluate simultaneously the time- and irradiance- dependence of the photoinduced birefringence. The figure below shows the distribution of retardation (birefringence) obtained in a sol-gel film after pumping with a 488 nm expanded beam.

Photoinduced dichroism

The photoinduced anisotropy results in dichroism too, in addition to birefringence. To measure dichroism, the anisotropy is induced by a pump probe at 488 nm and the absorption is measured by a 100 times weaker probe beam at 488 mn.

Photoinduced gratings and holography

When the interference pattern obtained by the superposition of two monochromatic and coherent light beams creates a diffraction grating for a third light beam, the phenomenon is known as "holography". The most suitable light is laser light. At a very elementary level, the interference pattern can be created by the superposition of two linear polarized plane beams (R1 and R2), as shown in figure below.

In this case, the interference fringes are simply brigth lines. If a photoanisotropic thin film is placed in the interference region of two plane monochromatic laser beams, the interference pattern becames a diffraction grating on the film surface. A third laser beam (R3) is diffracted. If R3=R1 and R2 is turned off, then R2 is "reconstructed" as a diffracted beam of R1. Even if R2 has not a simply plane parallel wavefront, but a more complicated one, after the creation of the diffraction grating (now called hologram), R1 is completely reconstructed.

If an interference pattern is projected on the sample, instead of a uniform radiation, a birefringence (and dichroism!) grating is recorded, which have interesting properties, as its diffractive properties change with the writing polarization configurations. When an interference pattern is recorded on a material, an hologram is obtained.

In a "conventional" hologram, where only the intensity of the interference pattern is recorded, only an index or absorption grating may be formed. In any case at every diffraction order the polarization is the same as that of the incoming probe beam. In a polarization hologram, in addition to the intensity and phase of the object beam, the polarization state is also recorded. This implies that any reference beam may reconstruct also the polarization state of the object. The reconstruction is more complete and efficient with respect to the conventional holograms. The polarization holograms show interesting properties. They can control, switch, analyse the polarisation state of a probe beam.

Holography is interesting for data or image storage, image filtering. Recently, polarization holography (vectorial holography) is investigated in polymeric materials for complete image reconstruction and for polarization controlling and detecting devices. Recently we have demonstrated the possibility of recording polarization sensitive holographic gratings in sol-gel hybrid systems. As far as we know this is the first report for a sol-gel (silica based) non polymeric system.

Photorefractivity

The dynamical holography is based essentially on the photorefractive effect.

Photorefractive crystals are by far the most efficient nonlinear optical media for wave mixing and phase conjugation with relatively low intensity requirements. In addition, holograms can be recorded and erased in these media. These properties can be technologically applied in optical information processing, optical interconnection and neural networks.

Publications

1. I. G. MARINO, D. BERSANI, P.P. LOTTICI, L.TOSINI and A. MONTENERO, Raman investigation of protonation of DR1 molecules in silica or ORMOSILs matrices by sol gel, J. Raman Spectrosc. 31, 555-558 (2000)
2. I. G. MARINO, D. BERSANI, P.P. LOTTICI Photoinduced birefringence in DR1 doped sol-gel silica and ORMOSILs thin films, Optical Materials 15, 175-180 (2000)
3. I. G. MARINO, D. BERSANI, P.P. LOTTICI Holographic gratings in DR1 doped sol-gel silica and ORMOSILs thin films, Optical Materials 15, 279-284 (2001)
4. G. GNAPPI, A. MONTENERO, L. TOSINI, D. BERSANI, P.P. LOTTICI, I.G. MARINO, Synthesis and characterization of compounds for non-linear optics: silica based matrices doped with organic dyes, Mater. Engineering 12, 329-340 (2001)
5. I.-G.MARINO, A. FERRARI, D. BERSANI, P.P. LOTTICI, A. MONTENERO, G. GNAPPI, Photoinduced effects in hydrid DR1-doped sol-gel silica-titania thin films 19th International Congress on Glass, Edinburgh, United Kingdom, vol. 2 (2001), 51-52
6. R. RASCHELLA', I-G. MARINO, P.P. LOTTICI, D. BERSANI, A. LORENZI, A. MONTENERO, Holographic gratings in hybrid sol-gel films, Proceedings of SPIE-The International Society for Optical Engineering (Advanced Optical Devices, Technologies, and Medical Applications) 5123, 117-124 (2003)
7. D. BERSANI, R. RASCHELLA', I-G. MARINO, P.P. LOTTICI, A. LORENZI, G. GNAPPI, A. MONTENERO, Hybrid sol-gel films for holographic applications, in " Emerging fields in sol-gel Science and Technology Materials ", Tessy Maria Lopez edt., ISBN: 1402074581, Kluwer Academic Publisher (2003) pag. 307-316
8. R. RASCHELLA', I-G. MARINO, P.P. LOTTICI, D. BERSANI Polarization holographic gratings in hybrid sol-gel films doped with Disperse Red 1 , Optics Letters 28, 2240-2242 (2003)
9. R. RASCHELLA', I-G. MARINO, P.P. LOTTICI, D. BERSANI, A. LORENZI, A. MONTENERO, Photorefractive gratings in DR1-doped hybrid sol-gel films, Optical Materials 25, 419-423 (2004)
10. D. BERSANI, P.P. LOTTICI, G. ANTONIOLI, E. CAMPANI, A. CASOLI, C. VIOLANTE, Pigments and binders in the wall-paintings of Santa Maria della Steccata in Parma (Italy): the ultimate technique of Parmigianino, J.Raman Spectrosc. 35, 694-703 (2004)
11. R. RASCHELLA', I-G. MARINO, P.P. LOTTICI, D. BERSANI, A. LORENZI, A. MONTENERO, Silica based photorefractive sol-gel films for holography, J. Non-Cryst. Sol. 345-346, 428-432 (2004)
12. I-G. MARINO, R. RASCHELLA', P.P. LOTTICI, D. BERSANI, C. RAZZETTI, A. LORENZI, A. MONTENERO, Photoinduced effects in hybrid sol-gel materials, J. Sol-Gel Sci. Technol. 37, 201-206 (2006)
13. R. RASCHELLA', I.-G. MARINO, P.P. LOTTICI, D. BERSANI, Polarization gratings in sol-gel thin films investigated through the moving grating technique, SPIE Proceed. 6252, 62522E (2006) ISBN: 0-8194-6311-6
14. R. RASCHELLA', I.-G. MARINO, P.P. LOTTICI, D. BERSANI, A. LORENZI, A. MONTENERO, Photoinduced dichroism in dye-doped hybrid sol-gel films, Optical Materials 28, 909-912 (2006)
15. I.G. MARINO, C. RAZZETTI, P.P. LOTTICI, Wide-field polarimetric analysis of photoinduced birefringence in azo-dye-doped thin films: irradiance and time dependence Applied Physics B (2006) http://dx.doi.org/10.1007/s00340-006-2473-x
16. R. RASCHELLA', I.-G. MARINO, C. RAZZETTI, D. BERSANI, P.P. LOTTICI Modeling and experimental study of photoinduced anisotropy in hybrid sol-gel films, J. Opt. Soc. Am. B (2007) (in press)
17. R. RASCHELLA', I-G. MARINO, D. BERSANI, P.P. LOTTICI Chromophore aggregation and photoinduced dichroism in sol-gel films, J. Non Cristalline Solids (submitted)
18. P.P. LOTTICI, R. RASCHELLA', I-G. MARINO, D. BERSANI, C. RAZZETTI Photoinduced anisotropy in hybrid azo-dye doped sol-gel films, J. Sol Gel Science and Technology (submitted)