Dynamic Bragg Angle Shift in the Course of Volume Hologram Formation

Abstract

Real-time investigation of Bragg angle shift during volume hologram formation is presented in an organic cationic ring-opening polymerization material. Positive as well as negative values of optical shrinkage are found, assumedly related to mechanical deformations of the volume and a change of the average refractive index, respectively. Ruled by the interplay of polymerization and diffusion, the originate grating formation mechanisms prove to represent competing effects regarding the contribution to the optical shrinkage. The influence of sample preparation and holographic exposure procedure on the effects observed is investigated and the usability for minimization of total Bragg resonance detuning is considered.

Spatially Resolved Analysis of Bragg Selectivity

“…Optical shrinkage is accompanied by Bragg angle detuning as a result of altered grating constants or modified refractive index. The Bragg selectivity, however, determines the optical functionality [13,20]. Preliminary results suggest, that a constant functionality beyond high volume stability can be realized by means of an increase of the average refractive index, compensating the impact of mechanical shrinkage associated with photo-induced crosslinking [13].…”

“…Optical shrinkage, comprising of mechanical deformations of the volume on the one hand and a change of the average refractive index on the other hand [14], proceeds inevitably parallel to the grating formation [13]. Low optical shrinkage during holographic recording is desired, since this is linked to a constant functionality of the final optical element.…”

“…Long-term stability depends on OPEN ACCESS intrinsic material features, significantly influencing the course of the grating formation, as well as on temperature and thermal history [18]. The UV curing and baking processes are known to significantly affect the film shrinkage and swelling behaviour [13,19]. The layer thickness represents a crucial factor for the selectivity of optical elements and for storage capacity of memories [18].…”

“…Research efforts are concentrated on material composition, processing and sample fabrication on the one hand [7][8][9][10], and on techniques and methods applied for recording and subsequent analysis on the other hand [7,9,[11][12][13][14][15][16]. In each case, there is a particular need to meet the high demands on precision and repetitive accuracy, accompanying volume holographic operations.…”

“…The second possibility is to apply special material processing, such as pre-imaging, incoherent-light exposure [25,26]. The third approach concentrates on the influence of accessible parameters, namely recording intensity, exposure duration and the material's viscosity to control and adjust the optical shrinkage [13].…”