Chemical routes to two-dimensional photonic crystal sensors equipped with nano/microcavities

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The development of simple, small, and highly sensitive sensors for detecting hazardous substances is one of the hot topics in both academic research and commercial applications. In this context, optical sensors based on nano/microstructured silicon are highly interesting and different configurations such as Mach-Zehnder interferometers, gratings, waveguides and photonic crystal devices have been investigated. These sensors can show extraordinary sensing performance if their geometrical lay-out is well designed and specific optical read-out methods are applied. Most promising for achieving extremely high sensitivities are optical sensors based on two-dimensional photonic crystal slabs. The sensors are often realized by creating a periodic array of holes in a thin silicon slab which can exhibit a photonic band gap (stop band) characterized by the prevention of light propagation at a range of frequencies. Thereby, narrow resonance linewidths in their optical spectra are achieved making them ideal candidates for sensor applications. Furthermore, by introducing point or line defects into the periodic lattice of holes optical nano/microcavities are formed facilitating a strong confinement of light. Despite the high demand for these sensors, two-dimensional photonic crystal slabs are almost exclusively fabricated by top-down methods, e.g. photo- and e-beam lithography. These methods allow for a precise definition of the geometric dimensions of nanostructures, including their size and morphology, but require sophisticated equipment. In contrast, bottom-up approaches provide simple and cost-efficient methods for structuring large areas using self-assembly processes. Especially in colloidal lithography tremendous progress was made towards the realization of highly ordered and hierarchical nanostructures. However, until now the fabrication of nanomaterials with site-selective variations in their dimensions on the micro/nanoscale by colloidal lithography was barely investigated even though simple, fast and cost-efficient production methods for these kinds of materials are highly desirable. This project aims at the fabrication of 2D photonic crystal sensors equipped with nano/microcavities using a combination of colloidal lithography and metal-assisted etching of silicon-based substrates. For this purpose, the exceptional properties of stimuli-responsive hydrogel microgels will be exploited in order to provide independent control over lattice constants and colloid diameters in two-dimensional arrays of hydrogel microgels deposited on the substrate surface. Resulting colloidal masks will be used for preparing gold nanodisc arrays which will subsequently be employed for metal-assisted etching of the silicon-based substrate. Thereby, 2D photonic crystal sensors equipped with nano/microcavities will be realized in a simple and cost-efficient way on large areas.