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The vast majority of conventional photovoltaic (PV) module technologies suffer from inactive areas, for example, due to the front contact finger grids or interconnection lines, which are essential for efficient extraction of the photocurrent. Invisibility cloaking by freeform surfaces is a new concept to guide incident light of the entire solar spectrum and all angles of incidence away from these inactive areas to the active areas of the PV modules. In this work, a freeform surface design is incorporated into a standard encapsulation layer of conventionally front glass covered PV modules. For a copper indium gallium diselenide (CIGS) thin-film PV module, a relative improvement in power conversion efficiency of 6.5% is presented, which demonstrates that the entire optical losses of the inactive area can be recovered. The design of the freeform surface is optimized based on rigorous ray-tracing simulations, which further allow discriminating and quantifying the underlying optical effects. Moreover, in order to demonstrate the scalability of the freeform surface cloak, a prototype CIGS thin-film PV module (comprised of nine monolithically interconnected solar cells) exhibiting fully-cloaked interconnection lines is presented.

The ability to create superoleophobic surfaces repellent toward low-surface-tension liquids is important for various applications, and has been recently demonstrated using re-entrant or doubly re-entrant microtopography. Liquid droplets on such surfaces feature composite liquid-solid-air interfaces, whereas composite liquid-lubricant-air interfaces would have potential for additional repellency. Here, the development of a novel slippery superoleophobic surface with low adhesion is demonstrated via combining doubly re-entrant microtopography with slippery lubricant-infused porous surfaces. This is realized by using 3D direct laser writing to fabricate doubly re-entrant micropillars with dedicated nanostructures on top of each pillar. The top nanostructures stabilize the impregnated slippery lubricant, while the re-entrant geometry of the micropillars prevents lubricant from spreading. The slippery layer reduces the adhesion of liquid to the pillars, as proved using scanning droplet adhesion microscopy (SDAM), while the doubly re-entrant micropillars make the surface superoleophobic. This novel interface combining two extremes, superoleophobicity and slippery lubricant-infused surface, is of importance for designing superoleophobic and superhydrophobic surfaces with advanced liquid repellent, anti-icing, or anti-fouling properties.

In transformation optics, coordinate transformations are usually mapped onto equivalent (meta-)material parameter distributions. In 2015, we introduced an approach mapping coordinate transformations onto dielectric free-form surfaces. We presented model experiments on cloaking of reflective contact fingers on solar cells. We now report on the fabrication of masters by 3D laser lithography used for soft imprinting. For prototype silicon heterojunction solar cells investigated under 1-sun illumination, we demonstrate the predicted 9% relative efficiency increase. We additionally show that our approach is adaptable to Lambertian sources, thereby cloaking light-emitting diode contacts to achieve spatially homogeneous emission.

In this paper an on-chip device capable of wavelength-selective generation of vortex beams is demonstrated. The device is realized by integrating a spiral phase-plate onto a MEMS tunable Fabry-Perot filter. This vortex-MEMS filter, being capable of functioning simultaneously in wavelength and orbital angular momentum (OAM) domains at around 1550 nm, is considered as a compact, robust and cost-effective solution for simultaneous OAM- and WDM optical communications. Experimental spectra for azimuthal orders 1, 2 and 3 show OAM state purity >92% across 30 nm wavelength range. A demonstration of multi-channel transmission is carried out as a proof of concept.

In this paper, we experimentally demonstrate simultaneous wavelength and orbital angular momentum (OAM) multiplexing/demultiplexing of 10 Gbit/s data streams using a new on-chip micro-component-tunable MEMS-based Fabry-Perot filter integrated with a spiral phase plate. In the experiment, two wavelengths, each of them carrying two channels with zero and nonzero OAMs, form four independent information channels. In case of spacing between wavelength channels of 0.8 nm and intensity modulation, power penalties relative to the transmission of one channel do not exceed 1.45, 0.79 and 0.46 dB at the hard-decision forward-error correction (HD-FEC) bit-error-rate (BER) limit 3.8 × 10sup-3/supwhen multiplexing a Gaussian beam and OAM beams of azimuthal orders 1, 2 and 3 respectively. In case of phase modulation, power penalties do not exceed 1.77, 0.54 and 0.79 dB respectively. At the 0.4 nm wavelength grid, maximum power penalties at the HD-FEC BER threshold relative to the 0.8 nm wavelength spacing read 0.83, 0.84 and 1.15 dB when multiplexing a Gaussian beam and OAM beams of 1st, 2nd and 3rd orders respectively. The novelty and impact of the proposed filter design is in providing practical, integrable, cheap, and reliable transformation of OAM states simultaneously with the selection of a particular wavelength in wavelength division multiplexing (WDM). The proposed on-chip device can be useful in future high-capacity optical communications with spatial- and wavelength-division multiplexing, especially for short-range communication links and optical interconnects.

In this work, detailed energy yield modelling based on experimental data of all-angle invisibility cloaks for solar cell contact fingers is used to underline their high improvement potential under realistic conditions.

We demonstrate an on-chip device capable of wavelength-selective generation of vortex beams, which is realized by a spiral phase plate integrated onto a microelectromechanical system (MEMS) tunable filter. This vortex MEMS filter, being capable of functioning simultaneously in both wavelength and orbital-angular-momentum (OAM) domains at the 1550 nm wavelength regime, is considered as a compact, robust, and cost-effective solution for simultaneous OAM- and wavelength-division multiplexed optical communications. The experimental OAM spectra for azimuthal orders 1, 2, and 3 show an OAM state purity >92% across a wavelength range of more than 30 nm.

We discuss invisibility cloaking of metal grid electrodes on Lambertian light emitters by using dielectric free-form surfaces. We show that cloaking can be ideal in geometrical optics for all viewing directions if reflections at the dielectric-air interface are negligible. We also present corresponding white-light proof-of-principle experiments that demonstrate close-to-ideal cloaking for a wide range of viewing angles. Remaining imperfections are analyzed by ray-tracing calculations. The concept can potentially be used to enhance the luminance homogeneity of large-area organic light-emitting diodes.

We present a light-extraction approach allowing for single-pass and omniangle outcoupling of light from light-emitting diodes (LED). By using transformation optics, we perceive a feasible graded-index structure that is a transition from the LED exit facet to a low refractive index region with expanded space that represents air. Apart from the material dispersion of the constituents, our approach is wavelength independent. The suggested extractor is geometrically compact with size parameters comparable to the width of an LED and therefore well adapted for pixelated LEDs. A beam-expanding functionality is possible while fully preserving the outcoupling efficiency by applying index and geometry truncation.

Nontransparent contact fingers on the sun-facing side of solar cells represent optically dead regions which reduce the energy conversion per area. We consider two approaches for guiding the incident light around the contacts onto the active area. The first approach uses graded-index metamaterials designed by two-dimensional Schwarz–Christoffel conformal maps, and the second uses freeform surfaces designed by one-dimensional coordinate transformations of a point to an interval. We provide proof-of-principle demonstrators using direct laser writing of polymer structures on silicon wafers with opaque contacts. Freeform surfaces are amenable to mass fabrication and allow for complete recovery of the shadowing effect for all relevant incidence angles.

We employ coordinate transformations to design two different optical structures for cloaking of reflective front contacts on solar cells. The cloaking performance is verified in a proof-of-principle experiment and using ray tracing simulations.

This work demonstrates a performance improvement of state-of-the-art silicon solar cells by cloaking their metal contact fingers. The cloaking free-form surfaces are fabricated on silicon heterojunction solar cells using direct laser writing of polymers and subsequent soft imprinting. Cloaking performance is determined experimentally by measuring spatially resolved and angle-resolved current generation and the spectral response of the cell. The short-circuit current density of the cell increases by 7.3%; its power-conversion efficiency is enhanced by 9.3%. Overcompensation of the shadowing loss is found to be caused by improved light-gathering and light-trapping in the polymer layer. The experimental findings are in good agreement to ray-tracing simulations.

We demonstrate a new tunable MEMS-based WDM&OAM Fabry-Perot filter for simultaneous wavelength (WDM) and Orbital Angular Momentum (OAM) (de)multiplexing. The WDM&OAM filter is suitable for dense on-chip integration and dedicated for the next generation of optical interconnects utilizing all three degrees of freedom of the electromagnetic waves: wavelength, polarization, and OAM. The WDM&OAM filter consists of two Distributed Bragg Reflectors (DBRs), (see Fig. 1a, b): a bottom one fixed to the substrate and a movable top MEMS DBR. An applied tuning current, changing the resonator length, extends the top DBR and hence selects the central filter wavelength. A spiral phase mask on the top switches the OAM order by ±1, ±2, etc. For a detailed description of the structure and fabrication of the device, please refer to [1, 2]. The MEMS filter shows a full-width at half-maximum (FWHM) bandwidth of about 0.2 nm and a free spectral range (FSR) of about 126 nm. The phase mask provides sufficient OAM state purity in a 35 nm window around 1550 nm, covering well the whole C-band.