Nanocomposite Photoanisotropic Materials for Applications in Polarization Holography and Photonics

Abstract

Photoanisotropic materials, in particular azodyes and azopolymers, have attracted significant research interest in the last decades. This is due to their applications in polarization holography and 4G optics, enabling polarization-selective diffractive optical elements with unique properties, including circular polarization beam-splitters, polarization-selective bifocal lenses, and many others. Numerous methods have been applied to increase the photoinduced birefringence of these materials, and as a result, to obtain polarization holographic elements with a high diffraction efficiency. Recently, a new approach has emerged that has been extensively studied by many research groups, namely doping azobenzene-containing materials with nanoparticles with various compositions, sizes, and morphologies. The resulting nanocomposites have shown significant enhancement in their photoanisotropic response, including increased photoinduced birefringence, leading to a higher diffraction efficiency and a larger surface relief modulation in the case of polarization holographic recordings. This review aims to cover the most important achievements in this new but fast-growing field of research and to present an extensive comparative analysis of the result, reported by many research groups during the last two decades. Different hypotheses to explain the mechanism of photoanisotropy enhancement in these nanocomposites are also discussed. Finally, we present our vision for the future development of this scientific field and outline its potential applications in advanced photonics technologies.

Keywords: azopolymers; nanocomposite materials; nanoparticles; photoanisotropic materials; polarization holographic gratings; surface relief gratings.

Figure 1

TEM/SEM images of NPs with different composition and shape: (a) TEM image of the nanocomposite film made from the copolymer P_1–2 doped with ZnO nanoparticles. In the inset is shown the selected area electron diffraction image (reprinted with permission from [139]); (b) TEM image of TiO₂ nanoparticles in PAZO thin film (reprinted with permission from [148]); (c) TEM image of a MFI zeolite rectangular nanoparticle (reprinted with permission from [151]); (d) TEM image of the chalcogenide NPs (GeTe₄)_100−xCu_x (reprinted with permission from [164]); (e) SEM image of carbon nanotubes (reprinted with permission from [137]); (f) SEM image of carbon nanofibers (reprinted with permission from [137]); (g) TEM image of upconverting hexagonal nanoparticles (scale bar is 200 nm) (reprinted with permission from [152]); (h) TEM image of Au colloidal solution (reprinted with permission from [160]); (i) TEM image of Au NPs (reprinted with permission from [156]); (j) TEM image of Ag (reprinted with permission from [155]); (k) SEM image of Ag nanocubes. The inset shows a TEM image of silver nanocubes (scale bar is 100 nm) (reprinted with permission from [165]); (l) TEM image of goethite nanorods (reprinted with permission from [150]).

Figure 2

Chemical structures of some azopolymers used as NPM components in the studies of Berberova et al. [128,141,158], Nedelchev et al. [129,139,140,150], Falcione et al. [132], Fernandez et al. [146], Mateev et al. [147,162], Nazarova et al. [148,156,166], Stoilova et al. [149,163], Hautala et al. [161], Kang et al. [145], Achimova et al. [135], and Vijayakumar et al. [167].

Figure 3

Chemical structures of some azodyes used as components of the nanocomposite photoanisotropic materials investigated by Schneider et al. [168], Basuki et al. [169], Shah et al. [142], and Hu et al. [170].

Figure 4

Dependence of the birefringence enhancement ratio on the concentration for different non-metallic NPs (ZnO, SiO₂, MFI) in one and the same azopolymer matrix, namely the azopolymer P1 [139,144,151].

Figure 5

(a) Birefringence kinetics of PAZO matrix doped with different non-metallic NPs: ZnO [128], TiO₂ [147], (GeTe₄)₈₅Cu₁₅ [149], and goethite (α-FeOOH) [150] for their optimal concentrations; (b) optical scheme for birefringence kinetics measurement.

Figure 6

Birefringence kinetics of the azopolymer PAZO doped with different metallic NPs: Au [156,158], Ni, and Cu complexes [162,163] for their optimal concentrations.

Figure 7

Optical scheme for polarization holographic and surface relief grating recording.

Figure 8

Diffraction efficiencies kinetics of thin films of PAZO doped with different NPs: ZnO [141], TiO₂ [148], α-FeOOH [129], Au [158], Ni and Cu complexes [162,163] for their optimal concentrations.

Figure 9

Three-dimensional AFM images of the surface relief gratings, formed in nanocomposite materials during polarization holographic recording. In all cases the azopolymer used was PAZO. The dopant NPs concentrations and types are: (a) 2% Cu complexes; (b) 1% Ni complexes; (c) 10% goethite nanorods; (d) 16 a.u. Au NPs with size 40 nm; (e) 1% TiO₂ NPs with size 21 nm; (f) 3% TiO₂ NPs with size 21 nm.