Recent results of our studies into optical effects where sub-micron length scales play a pivotal role are presented. We start with a discussion of fine optical features produced by relatively large objects, and then move on to consider the big effects that can be produced by sub-micron structures. Topics covered include fine structure in the optical field of microlenses and gratings, and then further down in length scale from microstructured surfaces to resonant filters, photonic crystal waveguides and metallic nanoparticles. For each step we demonstrate potential applications in which such a length scale can present important advantages, as well as discussing some of the disadvantages and challenges in the design and fabrication of such elements. We particularly highlight the sensitivity of many of the structures to small variations in optical situation (e.g. geometry, orientation, material, polarization) leading significant optical effects for small-scale changes. Methods for the characterization of optical fields produced by objects at these smaller dimensions are also presented.
We present recent applications of one-dimensional (1D) and two-dimensional (2D) periodic structures. The structures were designed using rigorous diffraction theory and produced by modern micromachining techniques (electron beam writing, optical lithography). In addition, interferometric recording of periodic structures was investigated in order to fabricate periodic structures with arbitrary profile shapes.
Multiple beam interference lithography is an efficient method to produce very small periodic surface-relief structures over large surfaces. For a recording wavelength of 413nm, the period of these structures ranges from a few hundreds nanometer to a few micrometers. Applications of these structures range from antireflective surfaces, when the gratings have sub-wavelength periods, to various types of diffractive optical elements. Using a single exposure with two-beam interference, the profile of the obtained structures is sinusoidal. Superimposing, however, several such exposures with increasing spatial frequencies, also non-sinusoidal structures can be obtained. By decomposing the desired profile in its Fourier components, it is possible to obtain virtually any profile of structure, depending only on the number of Fourier components that the optical setup can produce. Various kinds of periodic structures are shown, starting with simple one-dimensional gratings up to more complex two-dimensional structures, such as an array of micro pyramids. Therefore, we gave a demonstration of the great potential of this method for realizing optical components. The advantages of the method are the simultaneous fabrication of relatively large arrays in a quick and simple manner and the accurate control of the periodicity of the structure.
We present the results of the application of zero-order diffraction gratings for optical variable devices (OVD's) for document security. Zero-order gratings have periods which are smaller than the wavelength of light; to describe accurately the optical properties of the zero-order gratings, we have applied rigorous electromagnetic theory, which we have compared to experimental measurements. We studied the diffractive behavior of zero-order grating varies locally in a predetermined manner. In the latter case, the resulting surface profile can exhibit variations in the diffraction properties, for example, a moire pattern. Furthermore, we have developed diffractive surface-reliefs which are a combination of a high-frequency, zero-order grating with large-period gratings, the addition of the zero-order grating to a large-period grating results in a surface relief with novel diffractive properties.