_Appl. Sci._**2014**, \(4\), 19-27; doi:10.3390/app4010019  
_Applied sciences_  
**ISSN 2076-3417**  
www.mdpi.com/journal/applsci  
**Imaging of Volume Phase Gratings in a Photosensitive Polymer, Recorded in Transmission and Reflection Geometry**  
**[PERSON] 1,* and [PERSON] 2**  
to a descriptive representation of spatial characteristics, this particularly applies to the visual depiction of recorded sub-micrometer structures. However, imaging of volume phase gratings poses a considerable challenge with respect to image contrast and resolution [7]. On the one hand this can be traced back to the small refractive index contrast to be depicted, which is typically in the order of \(10^{-3}\)[10, 14]. On the other hand it is of course attributed to the small size of the micro- and nanostructures. In general, the choice of methods for imaging is determined by the specific property which is contributing to the contrast of the image. This is, in the case of volume phase gratings, assigned to the characteristic physico-chemical feature, constituting the grating by means of a periodic modulation. In addition, the dimension, orientation and geometry of the structure are connected with geometry and resolution of the imaging technique. In view of the significance of the imaging task, the pros and cons of possible techniques, such as optical microscopy, electron microscopy and atomic force microscopy, must be considered. Special forms of light microscopy, such as confocal laser scanning, have been applied, in which the signal contrast is assumedly attributed to the remaining fluorescence of the photoinitiation system [15]. In this context, it must be stressed that it is usually inappropriate to apply fluorescent or dyes as contrast agents to achieve higher image contrast in low-contrast volume hologram scenes. The reason for this is that agglomeration of the contrast media along the grating planes, e.g., by means of diffusion, is prerequisite to achieve the desired effect, which cannot be ensured. But to achieve contrast in electron microscopy, conductive species are necessarily required, such as in the case of incorporated nanoparticles [14, 16]. As a consequence, transmission electron microscopy (TEM) is used to evaluate the degree of nanoparticle assembly [11, 14, 17]. Scanning force microscopy (SFM) may be applied additionally, to map surface modulations [13, 14]. Nanocomometric materials are also the subject of investigations by luminescence microscopy [11]. However, mapping of nanoparticles yields only a limited description of lattice structures, not necessarily identical with the phase grating of interest, which is linked to the diffractive properties. In fact, it has been demonstrated that photoinsensitive nanoparticles experience counterdiffusion during grating buildup [18]. Finally, the imaging task is becoming increasingly complex without conductive species and a fortiori taking account of the three-dimensionality of volume phase gratings. However, it is the third dimension in particular to which the specific applicability of volume holograms can be attributed [19, 20]. Obviously, imaging of phase gratings becomes more difficult, the smaller the amplitude and the period of the associated refractive index profile, which is generally the case for reflection gratings, especially in the presence of so-called parasitic gratings. The designation parasitic grating refers to structures recorded unintentionally, which result in a reduction of the dynamic range and are therefore undesirable. However, in view of the etymological origin of the term _parasitic_ (from Greek \(\pi\alpha\rho\dot{\alpha}\)-(para) \"beside, by\" and \(\vec{\alpha}\)\(\vec{\mathrm{t}}\)\(\vec{\mathrm{  
term _uni-directional_ secondary gratings, providing an additional description in view of the diffractive properties, as opposed to the multi-directional propagation of waves in case of noise gratings. In this paper we show microscopic images of transmission and reflection gratings as well as of uni-directional secondary gratings.