**4. Volume holographic applications**

planes of the structure. As an example, the middle and right hand side of **Figure 8** show two planes of a three‐dimensional holographic grating with hexagonal close packing crystal structure. It was produced by four mutually coherent exposure beams. The lateral distance of neighboring crystal units is 2 μm, to be read from **Figure 8**. The grating constant perpen‐

Optical microscopy provides local information on the grating in the context of geometry and dimensionality. No information on the optical functionality, such as on the Bragg selectivity, is provided. However, this information is accessible according to the relation between structure and function and can be derived from spatial‐resolved analysis. In case of transmission type gratings, this can be accomplished by means of scanning the lateral plane to obtain local val‐ ues of the grating parameters. Local values of thickness d, refractive index contrast Δn, as well as grating period Λ and the grating slant Φ can be obtained. The lateral scan method is keeping track of the hologram shape, which is determined by the material response to the

**Figure 8.** Optical microscopic imaging of volume phase gratings in photosensitive polymer: one‐dimensional grating (left hand side) and 3D photonic grating under variation of the microscopic focal plane (middle and right hand side).

Probing only a fraction of the exposed area is primarily for the purpose of measuring precision [29, 40]. However, it also enables scanning of the grating by moving the sample perpendicular to the optical axis. A sequence of rotation scans through the grating diam‐ eter constitutes a lateral scan. This analytical method allows the determination of the holo‐ gram characteristics along the sample surface. Thereupon, it is possible to compare and track respective properties from the center of the grating to the edges, corresponding to the areas of highest and lowest recording intensity. As a consequence, spatial sequences of the grating

**Figure 9** illustrates the principle of lateral scanning. The ratio of probe beam to exposure beam diameter was 1:6. The local diffraction efficiency is displayed along the lateral posi‐ tion of five different volume holograms. The gratings were recorded with different exposure dose. The respective energy density of exposure (E) is displayed in **Figure 9**. The lateral scan

The results from spatially resolved investigations of the grating constant reveal the influence of the recording intensity and exposure duration on the Bragg selectivity [30]. The advantage

*π*

<sup>2</sup> ) in case of E > 330 mJ/cm2

.

parameters are derived, providing insight into the local material characteristics.

reveals the material response, resulting in overmodulation (*ν* > \_\_

dicular to the image plane amounts to 22 μm (not shown) [39].

**3.5. Spatially resolved diffraction analysis**

16 Holographic Materials and Optical Systems

Gaussian intensity distribution of the recording beams [30].

The range of applications for volume holographic materials spreads wide across several disciplines. Holographic optical elements can perform the functions of mirrors, lenses, gratings or combinations of them to be applied for scanning, splitting, focusing and con‐ trolling of laser light in optical devices. The corresponding scope of applications reaches from light‐guiding in general to more complex systems and operational areas, specific technologies, metrology such as holographic interferometry, through to potential use for consumer electronics such as for display technologies, as well as the many applications associated with the exploding bandwidth in meeting the demands of internet traffic and related data storage. Furthermore, holography and photolithography may arise in a pow‐ erful combination to create complex structures for micromechanical and photonic devices with potential applications not only in optics and electronics, but also in tissue engineer‐ ing, cell biology and medical science as well.

Some selected application areas are described in more detail below.

#### **4.1. Integrated optics**

Photosensitive polymers are highly interesting materials for applications in integrated optics [6, 29]. This particularly applies with respect to the multi‐functional applicability as well as the ability to optimize and miniaturize respective components.

Information and communication technologies are subject to a continuing shift toward optical solutions. All optical devices are needed to evade the current drawback of electro‐optical chips, slowing down the whole process by forcing to work at the speed of electronics. Another objective is to reduce feature sizes in the course of miniaturization of components.

Future photonic devices such as electro‐optical chips may incorporate micro‐lasers and holo‐ graphic optical elements (HOEs) for optical computations, interconnects and memory sys‐ tems, possibly forming smaller and cheaper computer parts with higher performance [41]. Furthermore, HOEs may pave the way for the future of optical information technologies with all optical switchers, optical interconnects, (De‐) multiplexers and narrow‐spectral bandwidth filters, as well as photonic crystals with the potential to create integrated optical devices, capa‐ ble of all optical signal processing.

The field of applications for integrated optics can further be enlarged. HOEs are also capa‐ ble to efficiently redirecting light with the aim to improve light collection in solar cells [42]. Volume holographic optical elements (vHOEs) with tuneable angular and spectral Bragg selectivity, produced in instant developing photopolymer film, have recently been reported for use as lightweight, thin and flat optical elements for photovoltaic applications [43, 44].

#### **4.2. Security technologies**

Hologram encoding refers to the representation of the complex wavefield at the hologram plane, capable to encrypt information. The microscopic structure of a hologram is hard to replicate, constituting the particular applicability of holographic parts for security features. Embossed holograms are usually applied for mass production of cheap holograms for secu‐ rity applications, for instance on credit cards. But photopolymer holography and volume holographic parts in particular could provide considerable benefits for advanced security solutions. A volume hologram may not be copied by embossing.

Photopolymer holograms are expected to play an increasingly important role in security and authentication markets due to tight color control and strikingly realistic dimensions resulting in the unique looks of recorded images compared to embossed holograms. Photopolymer holograms can be individualized or serialized [41]. Furthermore, advanced holographic security labels could be used to fully exploit the capabilities of volume holographic systems. To further enhance the level of security, additional functionalities can be achieved by incorpo‐ ration of nanoparticles. The nanoparticle‐rich grating planes result in an additional security level of volume holographic labels, attained by means of specific characteristics, such as local photoluminescence in the patterned microscopic structure [37].

While two‐dimensional surface holograms, currently in use, may in principle be copied by means of a point‐by‐point survey and imitation, the only possibility to reproduce three‐

dimensional volume holograms is optical reconstruction of the original hologram and inter‐ ference with the reference wave. As a consequence, the exploitation of the photosensitive volume in all three dimensions could result in a considerable increase of the security level. Recently, 2.5D nanostructures based on holographic surface‐relief Bragg gratings have been demonstrated, which show tuneable diffraction in the visible spectrum and can further be combined with additional functionalities for enhanced multilevel security [45].

#### **4.3. Biomedicine**

**4.1. Integrated optics**

18 Holographic Materials and Optical Systems

ble of all optical signal processing.

**4.2. Security technologies**

Photosensitive polymers are highly interesting materials for applications in integrated optics [6, 29]. This particularly applies with respect to the multi‐functional applicability as well as the

Information and communication technologies are subject to a continuing shift toward optical solutions. All optical devices are needed to evade the current drawback of electro‐optical chips, slowing down the whole process by forcing to work at the speed of electronics. Another

Future photonic devices such as electro‐optical chips may incorporate micro‐lasers and holo‐ graphic optical elements (HOEs) for optical computations, interconnects and memory sys‐ tems, possibly forming smaller and cheaper computer parts with higher performance [41]. Furthermore, HOEs may pave the way for the future of optical information technologies with all optical switchers, optical interconnects, (De‐) multiplexers and narrow‐spectral bandwidth filters, as well as photonic crystals with the potential to create integrated optical devices, capa‐

The field of applications for integrated optics can further be enlarged. HOEs are also capa‐ ble to efficiently redirecting light with the aim to improve light collection in solar cells [42]. Volume holographic optical elements (vHOEs) with tuneable angular and spectral Bragg selectivity, produced in instant developing photopolymer film, have recently been reported for use as lightweight, thin and flat optical elements for photovoltaic applications [43, 44].

Hologram encoding refers to the representation of the complex wavefield at the hologram plane, capable to encrypt information. The microscopic structure of a hologram is hard to replicate, constituting the particular applicability of holographic parts for security features. Embossed holograms are usually applied for mass production of cheap holograms for secu‐ rity applications, for instance on credit cards. But photopolymer holography and volume holographic parts in particular could provide considerable benefits for advanced security

Photopolymer holograms are expected to play an increasingly important role in security and authentication markets due to tight color control and strikingly realistic dimensions resulting in the unique looks of recorded images compared to embossed holograms. Photopolymer holograms can be individualized or serialized [41]. Furthermore, advanced holographic security labels could be used to fully exploit the capabilities of volume holographic systems. To further enhance the level of security, additional functionalities can be achieved by incorpo‐ ration of nanoparticles. The nanoparticle‐rich grating planes result in an additional security level of volume holographic labels, attained by means of specific characteristics, such as local

While two‐dimensional surface holograms, currently in use, may in principle be copied by means of a point‐by‐point survey and imitation, the only possibility to reproduce three‐

solutions. A volume hologram may not be copied by embossing.

photoluminescence in the patterned microscopic structure [37].

objective is to reduce feature sizes in the course of miniaturization of components.

ability to optimize and miniaturize respective components.

In tissue engineering, cell biology and medical science, many applications become accessible through explicit control over molecular structure and mechanical properties, such as elasticity, cross‐linking degree or surface morphology of certain biomaterials [46, 47]. In view of such applications, a combination of holographic and photolithographic processing may be used to create complex structures for micromechanical and photonic devices [2, 48]. Here again, vol‐ ume holography in photosensitive polymers is of high interest for practical applications, with respect to the high flexibility and optimization ability of subsequent devices [41]. The opti‐ cal functionality can be applied for the use as holographic sensors [49]. Photonic crystals are promising candidates for biosensors and bioassays. With view to the interrelation of function and structure, functional structures of different PC materials are linked with respective sens‐ ing mechanisms [50]. Optical structures have already been incorporated in hydrogels for diag‐ nostics. Bragg grating‐based hydrogel sensors as well as hydrogel microlenses are utilized as optical sensors. In both cases, a significant change in the refractive index or rather high diffrac‐ tion efficiency is required to ensure good functionality [51]. Substantial benefits of holographic patterning concern the fabrication flexibility. Furthermore, hydrogels can also be structured photolithographically, taking advantage of diffusion processes. Three‐dimensional structuring at the microscale results in the ability to spatially tailor biomechanical and biochemical material properties [52].

Beyond the specific material characteristics, discussed in Section 2.1.1, there are additional requirements for the composition of novel biomedical material systems, such as biocom‐ patibility and non‐toxicity [53]. Investigations on cellular behavior on one‐ and two‐dimen‐ sional surface topography may be used to evaluate cytotoxicity and cytocompatibility as a function of mechanical properties, such as the cross‐linking degree. Selective cell adhesion and spreading can be observed, depending on the (bio)chemical, physical and mechanical properties [54].

For applications in tissue engineering and medical science, the desired functionality could be achieved with a combination of optical structuring of the volume and specific modification of the surface. With view to the possibilities of miniaturization, this could result in the design of advanced biomedical implants, sensing systems and diagnostic tools for in vivo studies. In particular, intra‐ocular lenses (IOLs), implanted in place of the natural eye lens in ophthalmic surgery, open up the prospect of substantial improvement, such as to overcome the system‐ atic induction of higher order aberrations or to enhance the approximation to the original function of the natural eye lens [55, 56]. This affects not only the visual ability but also some more complex abilities of the eye such as accommodation (the capability of the eye to focus sharply on close‐up and distant objects) or brightness‐darkness adaptation. With regard to the design, diffractive approaches are feasible and highly favorable in many respects com‐ pared with conventional refractive designs [57, 58]. Systems currently in use are limited to surface patterns, providing combined diffractive‐refractive structures. Volume holographic systems, with potential special features involved, have not yet been applied. However, the use of refractive‐diffractive optical properties by means of an integration of volume structuring, in combination with surface modification could provide highest possible functionality and applicability. Besides design, performance and functionality, biocompatibility, especially in case of foldable hydrogel lenses, must be ensured [59, 60]. Prospective IOLs could fulfill their function with an optically structured volume, leaving the surface free for other functionalities. Thus, late postoperative opacification of implanted lenses, resulting in glare and misty vision, might be addressed based on surface modifications for specific bio‐interaction [60]. In any case, the integration of optical functionality into the volume of intra‐ocular lenses might suc‐ ceed according to the leading idea *from structure to function* and would be accompanied with considerable benefits.
