In the applications for Holographic Optical Elements (HOE) our photomaterial has shown interesting performances. Because the recording process of the hologram modifies the spatial resolution of the previously calculated adaptive filter, the characterization of materials in terms of diffraction efficiency, image fidelity, refractive index modulation, dark diffusion mechanism, scattering and noise gratings and linear and non-linearity behavior becomes compulsory.
Diffraction orders, generated in a phase transmission volume hologram,
monitored in a computer screen. It is appreciable, by naked eye,
the quality of the diffraction efficiency. The hologram was recorded in
a photopolymerizable glass.
High-resolution scanning electron
microscopy (SEM) image of the holographic grating.
The characterization of the recording process requires as well the modeling of the photo mechanism involving photopolymerization and light modulation. The Colburn-Haines mechanism explains, on an overall basis, the formation of a permanent refractive index modulation within the volume of the photopolymerizable glass. Migration of monomer molecules to the illuminated zones, and their subsequent polymerization, induces the formation of a permanent modulated structure (a holographic grating). The presence of HRIS enhances the values of the refractive index modulation Dn.
Formation of a permanent spatial modulation of refractive index within the volume of a photopolymerizable glass by single monomer (left) and monomer-and-HRIS (right) diffusion following the diffraction pattern generated by interference of two mutually coherent laser beams. Light color (yellow) represents low refractive index fringes. Dark color (orange) the high refractive index fringes.
F. del Monte, Ó. Martínez-Matos, J. A. Rodrigo, M. L. Calvo, and P. Cheben, "A Volume Holographic Sol-Gel Material with Large Enhancement of Dynamic Range by Incorporation of High Refractive Index Species," Adv. Mater. 18, 2014-2017 (2006).
- Applications with Photopolymerizable glasses incorporating High Refractive Index Species (HRIS).
- Experimental detection of the optical Pendellösung effect
The dynamical theory of X-ray diffraction by crystals, first formulated by Ewald in 1916, predicts the Pendellösung effect. The theory and experimental verifications were discussed in detail by Batterman and Cole in 1964. There is coherent splitting of the incident beam resulting in a radiation periodic beating at different depths of the grating. Pendellösung fringes were first observed in 1968 by C.G. Shull for the interference formed by the diffraction of slow neutron beams by thick crystals of SiO2 with thickness ranging from ~0.3 cm to 1 cm. In 1996 Cheben and Calvo predicted theoretically the observation of the Pendellösung effect for certain types of volume holographic gratings
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The experimental evidence was achieved with a holographic grating recorded in a photopolymerizable glass incorporating HRIS.
M. L. Calvo, P. Cheben, Ó. Martínez-Matos, F. del Monte, and J. A. Rodrigo, "Experimental Detection of the Optical Pendellösung Effect," Phys. Rev. Lett. 97, 084801 (2006).
|Reproduction of part of a sequence of images for observing the structure of the first-diffraction-order, in presence of Pendellösung effect.|
- Polarization and phase-shift properties of high spatial frequency holographic gratings
The polarization properties of transmission volume holographic phase gratings recorded in a photopolymerizable glass modified with high refractive index species have been observed. The gratings were recorded by the interference of two parallel s-polarized writing beams with orthogonal propagation directions. High optical quality, scattering, and diffraction efficiency of 99.4% are achieved. Degrees of polarization of 0.987 and 0.999 are obtained for transmitted and diffracted light, respectively. Furthermore, phase analysis of the transmitted light reveals a phase discontinuity of pi at the Bragg angle.
Transmitted intensity for p-polarized (a) and s-polarized (b) grating illumination with a divergent Gaussian beam.
(c) Interference pattern of (b) with an s-polarized collimated beam. Interferogram detail (zoom 3X), showing the fringe interleaving. (d) Intensity profile along axis for image (a) (curve I) and image (b) (curve II). III Angular dependence of diffraction efficiency of an s-polarized collimated beam obtained by subtraction of profiles II and I.
, , , and , "Polarization and phase-shift properties of high spatial frequency holographic gratings in a photopolymerizable glass," Opt. Lett. 34, 485-487 (2009).
- Holographic gratings recorded in photopolymerizable glass for femtosecond pulse laser beams tailoring
The photopolymerizable glass exhibits remarkable properties for applications in the femtosecond regime of a laser beam. The diffraction efficiency of the Volume Phase Holographic Gratings (VPHG) keeps constant until a critical exposition time (also known as Laser Induced Damage Threshold), for which the efficiency declines, and the samples exhibited damage (see graphic.
Dependence of the diffraction efficiency of the volume holographic grating recorded in photopolymerizable glass, incorporating HRIS, for various pulse energy values in mJ, with the exposition time. Samples properties remained unaltered after 107 pulses of 0.6 mJ, (say the order of 0.75 TW/cm2 per pulse).
M. P. Hernández-Garay, O. Martínez-Matos, J. G. Izquierdo, M. L. Calvo, P. Vaveliuk, P. Cheben, and L. Bañares, "Femtosecond spectral pulse shaping with holographic gratings recorded in photopolymerizable glasses," Opt. Express 19, 1516-1527 (2011).
The remarkably high damage threshold of the photopolymerizable glass provides the feasibility for generating particular laser beams in the femtosecond regime. The developed technique is based upon an arrangement of two VPHG’s operating in parallel configuration. It exploits the spatial coherence properties of the incoming laser beam in a compact and robust setup that mitigates angular and spatial chirp.
Free-space propagation of femtosecond pulsed beams generated with the designed VPHG setup. (a) Near-field (b) and far-field propagation of LG1,2 beam is displayed. The LG0,6+LG0,-6 beam propagation in (c) near field and (d) far-field regimes. The labels in the figure indicate propagation distances.
Ó. Martínez-Matos, José A. Rodrigo, M. P. Hernández-Garay, J. G. Izquierdo, R. Weigand, M. L. Calvo, P. Cheben, P. Vaveliuk, and L. Bañares, "Generation of femtosecond paraxial beams with arbitrary spatial distribution," Opt. Lett. 35, 652-654 (2010).
- Quality control of beams recorded onto a holographic storage medium
The combination of both, computer generated holograms (CGH) and analog holography provides excellent results for controlling the quality, beam purity, of arbitrary paraxial optical beams. For that purpose, the holographic photomaterial used for holographic storage medium was the photopolymerizable glass synthesized at GICO-UCM. Additionally, an iterative algorithm that allows recovering the phase of the generated beam from the measurement of its Wigner distribution projections was applied. Its performance has been analyzed for laser beams of different symmetry: Laguerre-Gaussian, Hermite-Gaussian and spiral ones, which are obtained experimentally by a computer generated hologram (CGH) implemented on a programmable spatial light modulator (SLM). The figure shows the experimental results.
Holographic storage of particular laser beams: (a) Holographically recorded LG+2,4 beam onto a photopolymerizable glass and its power spectrum (b). (c) Retrieved phase of the reconstructed beam and its wavefront distortion (d). (e) Reconstruction of LG+3,4 +LG-3,4 beam recorded on a photopolymerizable glass incorporating HRIS. Diffraction efficiency near 90%.
J. A. Rodrigo, T. Alieva, A. Cámara, Ó. Martínez-Matos, P. Cheben, and M. L. Calvo, "Characterization of holographically generated beams via phase retrieval based on Wigner distribution projections," Opt. Express 19, 6064-6077 (2011).
- New sustainable holographic processes: toward green holography
In another aspect of the investigation in photomaterials, and given the hazardous nature of some of the involved components, such as the monomer and sol-gel precursors, the introduction of nonvolatile substances in the design, synthesis and applications, is having a primary interest for developing sustainable chemical processes. In particular, we have introduced Ionic liquids (ILs) as an alternative to solvents in chemical reactions.
ILs are organic salts having an ionic structure (hence the name) and which are liquid at ambient temperature. Their extreme low non-volatility prevents the evaporation of toxic substances which is typical of other solvents, and also affects the diffusion dynamics of particles inside the glass matrix.
The photograph shows the image taken by Scanning Electron Microscope (SEM) of a VPHG implemented in a photopolymerizable glass containing IL. IL concentration was in the actual case 7.6%. Sample thickness was 30 mm and grating period 20 mm. The presence of a 50 lines/mm grating is apparent, as well as the generation of certain inhomogeneities with diameters the order of 3 mm.
, , , , and "Photopolymerizable glasses incorporating high refractive index species and ionic liquid: A comparative study", J. Appl. Phys. 109, 053106 (2011).