Two-photon grayscale lithography for free-form micro-optical arrays.

Compared to standard rotationally symmetric macroscopic optical components, free-form micro-optical arrays (FMOAs), sometimes termed microstructured optical surfaces, offer greater design freedom and a smaller footprint. Hence, they are used in optical devices to deliver new functionalities, enhanced device performance, and/or a greater degree of miniaturization. But their more complex surface shape is a challenge for traditional manufacturing technologies, and this has triggered a substantial effort by research institutes and industry to develop alternative fabrication solutions. Two-photon polymerization (2PP) is a promising additive manufacturing technology to manufacture 3D optical (micro)structures. The manufacturing times involved are, however, often impractically long, especially for the excellent surface quality required for optical applications. Recently, Nanoscribe GmbH has reduced manufacturing times substantially with the introduction of so-called two-photon grayscale lithography (2GL). However, its acceleration potential and consequent impact on surface quality have, to the best of our knowledge, yet to be reported. A direct comparison between 2PP and 2GL indicates that, for the investigated FMOA, 2GL is around five times faster than 2PP and also delivers better surface quality. This study therefore confirms the potential of 2GL to manufacture complexly shaped FMOAs.

Surface topology and functionality of freeform microlens arrays.

Nonsymmetric (also known as freeform) optical components have attracted a great deal of academic and industrial attention due to the substantial benefits they have demonstrated in imaging and nonimaging optical systems. Additionally, freeform microlens arrays (FMLAs) are very promising with regard to the growing demand for device miniaturization and cost reduction. As a flip side, FMLAs entail specific challenges in design, manufacturing, and characterization. Here we report on the latter and present an innovative characterization strategy that makes it possible to assess the quality of FMLAs quickly and accurately. The precisely measured surface topology of FMLAs was accurately represented using nonuniform rational basis-spline (NURBS) and its optical response was predicted by means of ray-tracing simulations. We show that for reliably measured surface topology, the results are in excellent agreement with the experimental measurements. We also show that, compared to previous studies, illuminance levels displayed in a logarithmic scale are more adequate for low light levels and represent a closer match to nonlinear human visual perception. We believe that the method presented here will contribute to speeding up the FMLA manufacturing process, one of the current downsides of this promising technology.

Non-contact printing of optical waveguides using capillary bridges.

Non-contact printing methods such as inkjet, electro hydrodynamic, and aerosol printing have attracted attention for their precise deposition of functional materials that are needed in printed electronics, optoelectronics, photonics, biotechnology, and microfluidics. In this article, we demonstrate printing of tapered optical waveguides with losses of 0.61 ± 0.26 dB/cm, with the best performing structure achieving 0.19 dB/cm. Such continuous features are indispensable for successfully printing functional patterns, but they are often corrupted by capillary forces. The proposed inkjet printing method uses these forces to align liquid bridges into continuous features, enabling the printing of smooth lines on substrates with arbitrary contact angles.

Optical enhancement of a printed organic tandem solar cell using diffractive nanostructures.

Solution processable organic tandem solar cells offer a promising approach to achieve cost-effective, lightweight and flexible photovoltaics. In order to further enhance the efficiency of optimized organic tandem cells, diffractive light-management nanostructures were designed for an optimal redistribution of the light as function of both wavelength and propagation angles in both sub-cells. As the fabrication of these optical structures is compatible with roll-to-roll production techniques such as hot-embossing or UV NIL imprinting, they present an optimal cost-effective solution for printed photovoltaics. Tandem cells with power conversion efficiencies of 8-10% were fabricated in the ambient atmosphere by doctor blade coating, selected to approximate the conditions during roll-to-roll manufacturing. Application of the light management structure onto an 8.7% efficient encapsulated tandem cell boosted the conversion efficiency of the cell to 9.5%.

Diffractive nanostructures for enhanced light-harvesting in organic photovoltaic devices.

We present in-coupling gratings for improving the performance of thin film organic solar cells. The impact of the grating on the absorption in the active layer is modeled and explained using a standard cell architecture. An increase in absorption of 14.8% is predicted and is shown to be independent from the active material. The structure is then applied on blade-coated devices and yields an efficiency improvement of 12%. The angular behavior of the structures is measured showing superior performance for two dimensional gratings. By simulating the current generation for different angles and illumination conditions, we predict a total yearly increase of the generated current of 12% using an optimized grating. The fabrication of these structures, moreover, is compatible with roll-to-roll production techniques, thus making them an optimal solution for printed photovoltaics.

Design, fabrication and optical characterization of quantum cascade lasers at terahertz frequencies using photonic crystal reflectors.

We designed, fabricated and characterised electrically injected quantum cascade lasers with photonic crystal reflectors emitting at terahertz frequencies (3.75 THz). These in-plane emitting structures display typical threshold current densities of 420 A/cm2 and output powers of up to 2 mW under pulsed excitation. The emission characteristics are shown to be robust, as with increasing current the emission remains singlemode with no drift in wavelength, this results from the narrow reflectivity band of the photonic crystal reflectors.