Roadmap on holography

From its inception holography has proven an extremely productive and attractive area of research. While specific technical applications give rise to 'hot topics', and three-dimensional (3D) visualisation comes in and out of fashion, the core principals involved continue to lead to exciting innovations in a wide range of areas. We humbly submit that it is impossible, in any journal document of this type, to fully reflect current and potential activity; however, our valiant contributors have produced a series of documents that go no small way to neatly capture progress across a wide range of core activities. As editors we have attempted to spread our net wide in order to illustrate the breadth of international activity. In relation to this we believe we have been at least partially successful

Analytical approximations for the period of a nonlinear pendulum

This paper deals with the non-linear oscillation of a simple pendulum and presents an approach for solving the non-linear differential equation that governs its movement by using the harmonic balance method. With this technique it is possible to easily obtain analytical approximate formulas for the period of the pendulum. As we shall see, these formulas show excellent agreement with the exact period calculated with the use of elliptical integrals, and they are valid for both small and large amplitudes of oscillation. The most significant feature of the treatment presented is its simplicity because for the level of approximation considered in this paper the required work can be done “by hand”

Linear response deviations during recording of diffraction gratings in photopolymers

In a holographic recording is advisable that the diffraction efficiency increases linearly with the exposure in a wide zone of the curve of diffraction efficiency versus energetic exposure and the slope of the curve must be approximately constant before saturation in order to improve the energetic sensitivity and to get reproducibility in different recordings with the same kind of photopolymer, although to find examples of deviations to this behavior it is usual. The more important deviation experimentally observed in photopolymers with high thickness happen when the first maximum in the curve is lower than the second one. This effect is opposed to an overmodulation. We present a main hypothesis related to the dye concentration into the layer and with the molecular weight of the polymer chains generated in the polymerization process in order to explain this effect.This work was supported by “Ministerio de Ciencia e Innovación” (FIS2008-05856-C02-01, FIS2008-05856-C02-02) and “Generalitat Valenciana”, Spain (GVPRE/2008/274)

Optimization of a 1 mm thick PVA/acrylamide recording material to obtain holographic memories: method of preparation and holographic properties

Information holographic storage is a very promising technique due to its high theoretical capacity. One of the key factors in developing holographic memories is the need for a suitable recording material which must have certain specific characteristics. In particular, in order to achieve a high storage density it is necessary to work with great thicknesses. One of the essential requirements for holographic memories to be competitive is that the material must have a thickness of 500 μm or more, but it is not easy to find such thicknesses with the photopolymers currently available. In this study, we develop a method of preparing layers of a polyvinyl alcohol/acrylamide based photopolymer approximately 1 mm thick. Optimization of this material makes it possible to obtain good results for the main holographic parameters; diffraction efficiency 70% and energetic sensitivity 50 mJ/cm2.This work was supported by the Ministerio de Ciencia y Tecnología, CICYT, Spain, under project MAT2000-1361-C04-04

Angular responses of the first diffracted order in over-modulated volume diffraction gratings

Kogelnik's coupled-wave (CW) theory has been used for decades to predict the diffraction efficiency of volume diffraction gratings. Although this theory has been applied with success to volume diffraction gratings recorded under a great variety of experimental conditions, its predictions deviate from the actual behaviour whenever the hologram is thin or the refractive index is high. In these cases, it is necessary to use a more general CW theory or the rigorous coupled-wave (RCW) theory. Both of these theories allow for more than two orders to propagate inside the hologram. The difference between them is that in the CW theory the second derivatives that appear in the coupled equations are disregarded. The RCW theory does not incorporate any approximation and thus, since it is rigorous, permits judging the accuracy of the approximations included in Kogelnik's CW theory and the more general CW theory. In this article a comparison between the predictions of the three theories for phase transmission diffraction gratings is carried out. Over-modulated diffraction gratings are also recorded in photographic emulsions in order to study the applicability of Kogelnik's theory in this case. Good agreement between theory and experiment is found for both Kogelnik's theory and the RCW theory formulations in the particular experimental cases studied.This work was supported by the Ministerio de Ciencia y Tecnología, Comisión Interministerial de Ciencia y Tecnología, Spain, under project MAT2000-1361-C04-04 and by the Oficina de Ciencia y Tecnología (Generalitat Valenciana, Spain) under project GV01-130

High-efficiency volume holograms recording on acrylamide and N,N'methylene-bis-acrylamide photopolymer with pulsed laser

In order to achieve a better understanding of the mechanisms of hologram formation and higher diffraction efficiencies in volume gratings stored in acrylamide based photopolymers, a crosslinker (N,N'methylene-bis-acrylamide) has been incorporated in the photopolymer to record holograms by pulsed laser exposure. The presence of this component increases the polymerization rate and refractive index modulation. The recording was performed using a holographic copying process. The original was a grating of 1000 lines/mm processed using silver halide sensitized gelatin. First, the effect of the pulse fluence was investigated. When the pulse fluence was optimized, the results obtained using the new composition of material were compared with those using the composition without a crosslinker. Using a pulsed laser at 532 nm the photopolymer without crosslinker presented diffraction efficiencies slightly less than 60%. On the other hand, when the crosslinker was introduced in the photopolymer composition, the diffraction efficiencies achieved were higher than 85%. The non-linearity of the material's response was also studied comparing the energetic sensitivity, diffraction efficiencies and index modulation of gratings recorded with pulsed and continuous laser exposure. This study was performed fitting the angular scan of each grating using Kogelnik's theory.This work was partially financed by the ‘Conselleria d’Empresa Universitat i Ciència’ (Generalitat Valenciana, Spain) under project GV04A/565 and the ‘Vicerrectorado de Investigación’ (Universidad de Alicante, Spain) under projects GRE04-06 and GRE04-16