Holographic properties of azobenzene oligomers with differently bonded chromophore groups

Abstract

PACS 42.40. Lx, 42.70.Jk, 42.70.Ln Holographic properties of two types (I and II, specified in the text) of azobenzene oligomers (ABO) have been experimentally studied in the spectral region of low absorption at 633 nm. In the case of type I ABO the chromophore groups were covalently bonded to either tolyl-polyurethane or hexamethylenepolyurethane matrix. In the case of type II ABO azobenzene chromophores were dispersed in polystyrene matrix, and their concentration was varied from 0.005 to 0.200 mol/l. Samples were in the form of 10 µm thick films. In both cases the diffraction efficiency exposure time dependences were measured for the holographic grating period of 2 µm at 633 nm. The diffraction efficiency of more than 2% and the specific recording energy down to 30 -60 J/(cm 2 %) were achieved in both cases. In the case of type I ABO the best results were achieved with 4-nitronaphtylazobenzene chromophore groups in hexamethylenepolyurethane matrix. The chromophore concentration threshold within 0.01 -0.04 mol/l range, the holographic efficiency growth with chromophore concentration and the photosensitization effect were found in the case of type II ABO. 1 Introduction Organic materials have been extensively studied as recording materials for scalar and vector holography [1, 2] because they allow for wider tailoring of properties than inorganic materials. Azocompounds are among the most efficient holographic recording materials, which we have studied, also in previous papers [3]. In this paper we report on further studies of azobenzene oligomers (ABO), for holography in the red spectral region, which is of special theoretical and practical interest. First, the unclear physical mechanism [4] that enables recording in the spectral region with very low absorption is of large interest. Second, the hologram recording in this region allows one to obtain higher diffraction efficiency (DE) and to use cheaper lasers. Two groups of materials have been studied: ABO with covalently bonded chromophore groups (type I ABO) and ABO with dispersed (without the covalent bondings, type II ABO) ones. The matrices were also varied. The polyurethane structure was modified by tolyl (Tl) or hexamethylene (HM) in the case of type I ABO (Fig. 1). (These matrices we further designate as Tl and HM matrices.) Polystyrene matrix was used in the case of type II ABO. Each type of samples has its advantages and disadvantages. Oligomers with chromophore groups covalently bonded to the matrix exhibit higher DE and photosensivity yet the synthesis standardization is difficult. On the contrary, ABO with dispersed chromophore groups until now have exhibited lower holographic efficiency but their synthesis is much easier and more predictable. In this work we show that their efficiency can be comparable to that of the first group.

Efficient surface relief grating generated in azobenzene-containing material using an He–Ne laser

“…A small surface corrugation of 30 nm was also observed in a two-step process where the films were irradiated in a first step with homogeneous UV light followed by holographic exposure to red light [6]. Red light has also been used for holographic inscription of a polarization grating in amorphous chalcogenides [7] and recently for SRG inscription in azobenzene-containing materials [8][9][10]. However, the efficiency of SRG formation was not high, with a maximum of diffraction efficiency reaching 8% for the best material [8].…”

“…Such gas lasers are expensive and require efficient water cooling. As reported so far the results of SRG inscription with an He-Ne laser [6,[8][9][10] were not very effective, but it is still of interest to use such a laser source. Thus in this paper, we demonstrated the possibility to inscribe very effective SRG using the red light of an He-Ne laser.…”