Holographic Optical Elements

Optical elements such as lenses, beam splitters, diffraction gratings and filters can be produced by holographic imaging. These holographic optical elements (HOE) have the advantage of being cheap (due to their simple design, small size, low weight) and are easily reproducible by embossing polymer materials. They are wavelength selective and have a high diffraction efficiency, corresponding to large apertures of conventional lenses.

Figure 9: Holographic optical elements (HOE) (a) transmission axial; (b)transmission non-axial; (c)production steps of surface relief HOE [2]

HOEs are most important in optoelectronics, in particular, for optical memory discs and diode lasers. Diode lasers have a very large angle of divergence, which can be compensated by the installment of HOE focusing elements. The simplest HOE is a Fresnel zone plate which can act as a focusing lens (see the Fresnel hologram recording in Fig. 6 ). If the HOE is illuminated by a reference beam of $I_{R}$ at the same angle of incidence as the recording, it can act as a classical optical element. In general, off-axis HOEs (Fig. 9) are more useful than the on-axis variety. Reconstructing off-axis, not only reduces the reflection losses but also avoids interference from higher diffraction orders. The theoretical diffraction efficiency of a 3-D transmission phase HOE is 100% (Table 1). The actual $\eta$ is less than this, however, and can be improved only when the medium is coated with an antireflection layer. Reflection losses can be decreased by using coatings with lower refractive indices (for glasses $n=1.5$ and for normal incidence the reflection coefficient $R$ is 4%, for materials with $n=2.5$ like As$_2$S$_3$, the reflection coefficient is $R=18\%$)[2]. The materials used in HOE are subject to the same imperfections as conventional optical elements (lenses, objectives). Minimum aberrations⁵ can be achieved only when the image is reconstructed at the same wavelength and the same angle of incidence as that at which it was recorded. This is not usually possible in real time recording materials. In order to obtain high diffraction efficiencies, the readout must be done at another wavelength ( $\lambda_1 \neq \lambda_2$) which leads to a drastic increase in aberrations. Conventional silver halide materials also have some limitations for HOE production, caused by a change in the interference pattern structure during chemical processing (development). Therefore, special recording materials are necessary for HOE production. One of the best materials for HOE production are photosensitive resists. In these materials, the primary holographic recording is followed by subsequent etching to produce a surface profile which is additionally covered with a metal layer (Fig. 9c). Significant progress has been obtained in computer generated holograms using laser beams, X-rays or electron beams [11,12,13].