C. Pina-Hernandez, A. Koshelev, G. Calafiore, M. Sainato, S. Dhuey, S. Cabrini, K. Munechika
Keywords: nanoimprint lithography, high-refractive index polymer, printable photonics
Summary:The development of advanced photonic circuits working in the visible light promises a revolution in a broad range of areas from bio-chemical sensing to quantum computing. Over the last years, novel nanophotonic devices were demonstrated but are mainly still limited to research laboratories due to their complex fabrication and challenging scalability for large areas. Here we present a novel approach to drastically simplify the fabrication process via nanoimprint lithography (NIL) and demonstrate the first printed active photonic crystals with embedded quantum dots (QDs), fabricated by a powerful route involving the combination of top-down and bottom-up approaches1-3. One- and two-dimensional photonic crystal slabs were patterned by direct printing of functional TiO2 based high refractive index materials. Thermally stable CdSe/CdS core/shell quantum dot nanocrystals (active medium) were synthesized and uniformly incorporated to form an active hybrid organic-inorganic matrix with high refractive index (n=2.1). The active hybrid material was successfully patterned by direct nanoimprint techniques on hard and flexible substrates with high resolution and fidelity. The printed devices exhibited excellent optical properties, which led to enhanced light absorption by the QDs (Figure 1). Fluorescence intensities of QDs were compared between the regions with and without printed photonic crystals and a fluorescence enhancement factor of ~ 10 was measured from the patterned regions. Furthermore, we show that the degree of photoluminescence enhancement is highly frequency dependent. The largest fluorescence enhancements comes from photonic crystals with the guided mode resonance frequency that spectrally overlaps with the excitation and emission frequency of the QDs used for this study. In contrast, when there is no spectral overlap, we do not observe any fluorescence enhancement (Figure 1). Arrays of photonic crystals nanocavities were imprinted in a single step with quality factors (Q) of 1000 (Figure 2). Advanced applications are under current development including optically pumped nanolasers and high efficiency light emitting diodes. This work represents a powerful and cost-effective route for the development of numerous nanophotonic structures and devices that will lead to the emergence of new applications.