We devote this article to the spectral analysis of volume holograms recorded in materials with diffusion-based formation mechanisms. Two media of different natures were examined: polymer material with dye molecules photoattachable to macromolecules (PQ-PMMA) and photochromic additively colored fluorite crystal (CaF2). Differential spectra of holograms refractive index and absorbance were determined by two approaches: numerical approximation of both spectral and angular hologram response (selectivity hypercontour) by coupled-wave theory and processing hologram spectra by Kramers–Kronig dispersion relations. The principles of the two approaches, results of their application, and advantages and disadvantages are discussed. Kramers–Kronig relations can be usefully applied in cases where the nature of holograms formation is well studied, whereas the hypercontour approach is indispensable for obtaining the values of absorbance and refractive index modulation; therefore, a combination of both the techniques is a powerful tool for fundamental investigation of the hologram formation mechanisms.
The article is devoted to the spectral analysis of volume holograms recorded in materials with diffusion-based formation mechanisms. Two media of different nature were examined: polymer material with photosensitive macromolecules (PQ-PMMA) and additively colored solid-state crystal (CaF2). Differential spectra of holograms optical parameters were determined by means of two approaches: numerical approximation of both spectral and angular hologram response (selectivity hypercontour) by Coupled wave theory and processing hologram spectra by Kramers-Kronig dispersion relations. The operation principles, experimental performance, determination results, advantages and disadvantages, as well as validity limits were discussed for both the approaches. Kramers- Kronig relations are operable tool in cases where the nature of holograms formation is well studied, whereas hypercontour approach is indispensable for investigation of hologram formation mechanisms.
A buried depressed-index channel waveguide with a circular cladding and a core diameter of 40 μm is fabricated in a bulk monoclinic 3 at.% Tm:KLu(WO4)2 crystal by femtosecond direct laser writing. In the continuous-wave regime, the Tm waveguide laser generates ∼210 mW at 1849.6 nm with a slope efficiency η of 40.8%. Passively Q-switched operation is achieved by inserting transmission-type 2D saturable absorbers (SAs) based on few-layer graphene and MoS2. Using the graphene-SA, a maximum average output power of ∼25 mW is generated at 1844.8 nm. The pulse characteristics (duration/energy) are 88 ns/18 nJ at a repetition rate of 1.39 MHz.
Volume holographic gratings, both transmission and reflection-type, may be employed as one-dimensional pho- tonic crystals. More complex two- and three-dimensional holographic photonic-crystalline structures can be recorded using several properly organized beams. As compared to colloidal photonic crystals, their holographic counterparts let minimize distortions caused by multiple inner boundaries of the media. Unfortunately, it’s still hard to analyze spectral response of holographic structures. This work presents the results of thick holographic gratings analysis based on spectral-angular selectivity contours approximation. The gratings were recorded in an additively colored fluorite crystal and a glassy polymer doped with phenanthrenequinone (PQ-PMMA). The two materials known as promising candidates for 3D diffraction optics including photonic crystals, employ diffusion-based mechanisms of grating formation. The surfaces of spectral-angular selectivity were obtained in a single scan using a white-light LED, rotable table and a matrix spectrometer. The data expressed as 3D plots make apparent visual estimation of the grating phase/amplitude nature, noninearity of recording, etc., and provide sufficient information for numerical analysis. The grating recorded in the crystal was found to be a mixed phase-amplitude one, with different contributions of refractive index and absorbance modulation at different wavelengths, and demonstrated three diffraction orders corresponding to its three spatial harmonics originating from intrinsically nonlinear diffusion-drift recording mechanism. Contrastingly, the grating in the polymeric medium appeared purely phase and linearly recorded.
Amplitude and phase contributions to mixed volume holographic gratings were extracted from measured contours of angular selectivity. Holograms for the investigation were recorded in the glassy polymer material with phenan-threnequinone (PQ) using the DPSS CW laser (532 nm) and then self-developed due to molecular diffusion of PQ, reaching diffraction efficiency about 40%. Refractive index and absorbance modulation amplitudes of those holograms were obtained as adjustable parameters from theoretical equations by fitting angular dependencies of zeros and 1st orders diffraction efficiency measured at 450, 473, 532, and 633 nm at the different stages of hologram development. Mixed gratings manifest themselves in asymmetrical transmittance selectivity contours with one minimum and one maximum shifted with respect to the Bragg angle, while symmetrical contours with a minimum or a maximum at the Bragg angle are characteristic of pure phase and amplitude gratings, respectively. In the course of a hologram development, it converts from a predominantly amplitude-mixed to almost purely phase one in the case of readout using a light within the absorption band of PQ and maintains the phase nature besides it. The value of refractive index amplitude is ranging from 5×10-6 to 10-4 and the value of absorbance amplitude is up to 140 m-1.
The structurization of holographic planes in holograms recorded in CaF2 crystal with color centers was found. The structurization is apparent in the formation of spiral bundles, which pierce the holographic planes. It testifies to self-organization of color centers in these planes. This process is believed to be linked with colloidal centers, 2D metal islets in the crystal lattice, whose formation and decay during hologram recording at temperatures of 150–190 °C may be considered as a dynamic phase transition that facilitates the generation of stable spatially inhomogeneous (dissipative) structures in the form of bundles. The bundles arise during hologram recording process and remain frozen on cooling of the crystal after the process is finished.
Thick rigid polymer media with diffusive development of gratings are suitable for archive information storage, and especially for 3D holographic optical elements. Diffusion of unreacted molecules of photosensitive dye ensures postexposure growth of diffracted light, which can be followed either by secondary growth, or by some decay caused by displacement of chromophore groups photochemically attached to polymer chains (photoproduct). In a long run, extremely slow, though still finite, diffusion of macromolecules leads to destruction of a holographic grating. Not only the rate, but also the shape of postexposure kinetics noticeably depends on the choice of particular polymer, its degree of polymerization, temperature of processing and thermal history of material: in aged samples, gratings appear more efficient.
The principle of diffusional enhancement has been embodied in the rigid glassy polymer with phenanthrenequinone able to photochemically attach to surrounding macromolecules, thus forming a permanent grating. Owing to material stiffness, it does not suffer from shrinkage and can be made very thick; serving a basis for very stable spectrally selective elements. Replacement of commonly used acrylic glass by polycarbonate ensures further significant improvement of performance and stability of 3D holographic optical elements and memories.
The volume holographic gratings in polymer materials are promising candidates for spectrally selective optical elements, dense data storage, optical communication, and spectral selective imaging. Recently new application of volume reflection-type grating for fast all-optical temporal/spatial light modulators was suggested. Spatial modulation of diffracted red beam by green laser was demonstrated. We also suggest and analyze theoretical model for material with diffusional amplification for different modes of recording: (1) Fast recording with following annealing; (2) Slow recording (real-time grating).
Our experience of work with the photopolymer with diffusive amplification (PDA) as an extremely thick holographic recording medium evidences many attractive properties of this material, especially for such particular applications, as very selective holographic optical elements. However, sometimes the experimentally obtained characteristics of recorded holograms differ from the expected ones. This may be caused by the insufficient knowledge on the material behavior at different regimes and its specific peculiarities. In the present paper we analyze some parameters of PDA, in particular the noise source, and kinetics of the refractive index modulation and diffraction efficiency, as compared with the dynamic photopolymer medium Reoxan. As well as discus the possibilities of application of the PDA material.
In the present paper we consider the possibility to obtain highly selective holographic spectral filters operating in near IR range of spectrum. The properties of photopolymer with diffusive amplification as a medium for recording of such filters are analyzed, namely the mechanism of diffusion enhancement of hologram diffraction efficiency, the dispersion dependence of refractive index variations in a wide spectral range, and its influence on the resulting parameters of the holographic filter. The experimental results on the recording of filters optimized for operation at particular wavelengths of 0.85, 1.06 and 1.5 micrometers are presented.
In the present paper we consider hologram parameters required to obtain highly selective holographic spectral filters operating in near IR range of spectrum. The properties of photopolymer with diffusion amplification as a medium for recording of narrowband spectral filters for near IR, namely the dispersion dependence of refractive index variations in a wide spectral range, and its influence on the resulting parameters of the holographic filter are analyzed. The experimental result on the recording of a filter optimized for operation at 1.06 micrometers and a combined filter for visible and near IR are discussed.
The results of the holographic recording and reconstruction of the time-domain profile of femtosecond laser pulses in the polymer volume medium named Reoxan are presented. The experimental data show the possibility of the effective spectral filtering to control the shape of ultrashort pulses.
The strength of volume holographic grating being uniform throughout material depth, the efficiency of such a grating as a function of angle or wavelength has several side-maxima with magnitude comparable to that of main maximum. The side-maxima can be the origin of noises and lack of selectivity of holographic optical elements and devices such as demultiplexers, filters, etc.
A sensitized photooxidation has been studied in polymer systems based on polymethacrylates and polyimides with anthracene nuclei chemically attached to or include into main chains. Using thermostable polyimide compositions the possibility of reversible photooxidation due to thermal decomposition of bisphenylanthracene was shown.
This report concerns thick phase polymer recording media Reoxan and holographically manufactured optical elements recorded in them both media and elements being developed in our laboratory. Reoxan in traditional realization is realtime medium and dynamic effects of recording take place. That is why reflective holograms (base of narrowband wavelength selective elements) of highefficiency can''t be recorded. We have developed a new recorAding glassy polymer compo sition acting in a " quasidynamic" regime (lowefficiency hologram recording with further selfenhancement). Such a medium gives one an opportunity to obtain both reflective and transinittive hD1ograr of very high diffraction efficiency close to theoretical limit. Parame ters of higheffective holographic spectral selectors recorded as reflective holograms in this medium are also considered in ths report. I . INTRODUCTION. HOLOGRAPHIC SELECTORS The development of coherent optical methods of information transmission storage and transformation is determined by element base level. Specterselective elements play an important role in the field of information treatment. The traditional representatives of such elements are multylayer interference and interference pola rization filters diffraction gratings etc. Higheffective narrowband selectors based or reflective holograms in 3dimension phase recording materials'' can be serios competitors for such devices due to unique proporties of reflective holograms such as high spectral and angular selectivity relatively simple fa5ricticn high efficiency. The earliest wide experimental investigation of thick phase reflective holograms was carried out by dr. Sukhanov arid co-workers
A holographic method was used for investigation of very slow dif fusion of anthracene compounds in glassy polymers. The results show that properties of large organic molecules diffusion differ greatly from those known for small molecules. The diffusion of substituted anthracenes determines mainly the stability of holograms registered in volume phase medium Reoxan. I .
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