Diffractive nonimaging optics

Economic use of energy often depends on efficient management of light. Power generation from solar energy, for example, can involve optical systems that concentrate incident sunlight onto a small area and thereby reduce the required surface area of photovoltaic elements. Similarly, efficient lighting systems require luminaries that direct the emitted light only into the directions where it is needed. For this kind of applications the distribution of light is conventionally controlled using mirrors or lenses. This results in large and bulky systems that are relatively expensive and often impractical to implement. To save both energy and costs, alternative optical systems are required that establish efficient management of light in an inexpensive, compact and lightweight device. A possible design for an inexpensive and flat solar concentrators is based on a plastic or glass light guide. When sunlight is coupled into the light guide it can be guided towards a small solar cell via total internal reflection. Similarly, compact and lightweight luminaries can be produced by extracting light from a side-lit light guide. Both applications require structures that efficiently redirect light from air into the light guide or vice versa. In addition, these structures should result in a well-controlled distribution of light. In this thesis diffraction gratings are studied as possible structures for coupling light into or out of a light guide. Diffraction gratings are optical elements with a periodically varying refractive index. When light is incident onto a grating it gets diffracted into multiple beams (or orders) that all propagate into a different direction. The angle under which a diffracted order propagates after passing through a grating depends on the wavelength of the light, the periodicity (or pitch) of the grating and the angle under which the incident beam encounters the grating. When applied on top of a light guide a grating can couple light from air into that light guide, or vice versa, if the angle of diffraction of a specific order is larger than the critical angle for total internal reflection. This thesis studies what characteristics a grating should have in order to be useful for efficient light management and how the distribution of light can be controlled by adjusting the grating parameters. Two classes of gratings are studied: classical surface-relief gratings and liquidcrystal based polarization gratings. Both can be produced using holographic techniques. Their diffractive properties, in particular their possibility to obtain high diffraction efficiencies and their angular dependence, are compared. It is determined what grating parameters result in high incoupling efficiencies and to what extent both classes of gratings meet the requirements for solar concentrators. The results show that for the pitch sizes needed for solar and lighting applications a high refractive index contrast is required in order to obtain efficient diffraction. Such a refractive index modulation is difficult to establish in polarization gratings, but can be realized in surface-relief gratings. Surface-relief gratings are therefore the best choice for solar concentrators. By studying diffraction by surface-relief gratings at conical angles of incidence, it is shown that efficient in- and outcoupling can be obtained within a specific range of angles of incidence. The conditions for obtaining efficient diffraction are studied. Simulations indicate that efficient diffraction results from a proper choice of the grating thickness and depends on the polarization of the light in- and outside the light guide. These findings are experimentally verified for holographically produced surface-relief gratings. Furthermore, it is shown that efficient diffraction can occur over a wide range of angles when a surface-relief grating is produced using a material with a high refractive index. The angular distribution of light diffracted by gratings shows the same symmetry as the topology of the grating itself. This can be used to control the diffracted radiance. Gratings consisting of square or hexagonal lattices have more topological symmetry than gratings consisting of lines. As a result, their diffraction efficiency is more homogenously spread over a wide range of angles. On the other hand, the topological symmetry can be broken using slanted gratings. This results in high diffraction efficiencies over a narrow range of angles and for a single order of diffraction. For this reason slanted gratings are promising for solar applications. Incorporating diffraction gratings into a concentrator requires a device with a geometrical design that matches the angular dependence of the grating. It is shown how one can use principles from nonimaging optics to design such diffractive concentrators. Two different designs are presented. The first one shows that a grating can be used to reduce the height of a conventional Compound Parabolic Concentrator approximately by a factor 2.9. The second design is based on a tapered light guide, resulting in a compact diffractive concentrator that can be useful in a wide range of applications. The research presented in this thesis shows that diffraction gratings can efficiently diffract light into and out of a light guide. Furthermore, it is shown how the distribution of the diffracted light can be controlled by tuning the grating parameters. This offers possibilities to the design efficient and compact optical systems that can be useful for applications in solar concentrators and luminaries.