We have designed and realized a three-dimensional invisibility-cloaking structure operating at optical wavelengths based on transformation optics. Our blueprint uses a woodpile photonic crystal with a tailored polymer filling fraction to hide a bump in a gold reflector. We fabricated structures and controls by direct laser writing and characterized them by simultaneous high-numerical-aperture, far-field optical microscopy and spectroscopy. A cloaking operation with a large bandwidth of unpolarized light from 1.4 to 2.7 micrometers in wavelength is demonstrated for viewing angles up to 60 degrees.

In this work, the authors realize stable and highly efficient wide‐bandgap perovskite solar cells that promise high power conversion efficiencies (PCE) and are likely to play a key role in next generation multi‐junction photovoltaics (PV). This work reports on wide‐bandgap (≈1.72 eV) perovskite solar cells exhibiting stable PCEs of up to 19.4% and a remarkably high open‐circuit voltage (VOC) of 1.31 V. The VOC‐to‐bandgap ratio is the highest reported for wide‐bandgap organic−inorganic hybrid perovskite solar cells and the VOC also exceeds 90% of the theoretical maximum, defined by the Shockley–Queisser limit. This advance is based on creating a hybrid 2D/3D perovskite heterostructure. By spin coating n‐butylammonium bromide on the double‐cation perovskite absorber layer, a thin 2D Ruddlesden–Popper perovskite layer of intermediate phases is formed, which mitigates nonradiative recombination in the perovskite absorber layer. As a result, VOC is enhanced by 80 mV.

Sustained stimulated emission under continuous-wave (CW) excitation is a prerequisite for new semiconductor materials being developed for laser gain media. Although hybrid organic-inorganic lead-halide perovskites have attracted much attention as optical gain media, the demonstration of room-temperature CW lasing has still not been realized. Here, we present a critical step towards this goal by demonstrating CW amplified spontaneous emission (ASE) in a phase-stable perovskite at temperatures up to 120 K. The phase-stable perovskite maintains its room-temperature phase while undergoing cryogenic cooling and can potentially support CW lasing also at higher temperatures. We find the threshold level for CW ASE to be 387 W cm -2 at 80 K. These results indicate that easily-fabricated single-phase perovskite thin films can sustain CW stimulated emission, potential at higher temperatures as well, by further optimization of the material quality in order to extend the carrier lifetimes.

Supercritical fluid reactive deposition was used for the deposition of highly dispersed platinum nanoparticles with controllable metal content and particle size distribution on beta-cyclodextrin. The average particle size and size distribution were steered by the precursor reduction conditions, resulting in particle preparations <20, <100, and >100 nm as characterized by transmission electron microscopy and scanning electron microscopy (SEM). These particle preparations of different size distributions were used to address the question as to whether metallic platinum particles are able to invade cells of the gastrointestinal tract as exemplified for the human colon carcinoma cell line HT29 and thus affect the cellular redox status and DNA integrity. Combined focused ion beam and SEM demonstrated that platinum nanoparticles were taken up into HT29 cells in their particulate form. The chemical composition of the particles within the cells was confirmed by energy-dispersive X-ray spectroscopy. The potential influence of platinum nanoparticles on cellular redoxsystems was determined in the DCF assay, on the translocation of Nrf-2 and by monitoring the intracellular glutathione (GSH) levels. The impact on DNA integrity was investigated by single cell gel electrophoresis (comet assay) including the formation of sites sensitive to formamidopyrimidine-DNA-glycosylase. Platinum nanoparticles were found to decrease the cellular GSH level and to impair DNA integrity with a maximal effect at 1 ng/cm(2). These effects were correlated with the particle size in an inverse manner and were enhanced with increasing incubation time but appeared not to be based on the formation of reactive oxygen species.

We report on the performance and stability of distributed feedback lasers based on the solution-processed methylammonium lead iodide perovskite (CH3NH3PbI3). The CH3NH3PbI3 layers are processed via solution-casting in ambient atmosphere onto nanoimprinted second order Bragg gratings. This way, we achieve highly polarized surface-emitted lasing at room temperature with a linewidth of less than 0.2 nm and a laser threshold of 120 kW/cm2. The lasing is stable; no change in the laser emission within 15 h of pulsed excitation with a repetition rate of 1 kHz (corresponding to &gt;5 × 107 pulses) is observed, exceeding the stability achieved for solution processed organic semiconductor lasers. Furthermore, adjustment of the grating period allowed the lasing wavelength to be varied over the entire bandwidth of the amplified spontaneous emission (between 781 and 794 nm). The fabrication process of nanoimprinting followed by solution-casting of the gain material demonstrates that stable CH3NH3PbI3 lasers are compatible with scalable production technologies and offers a route towards electrically pumped diode architectures.

We report on digitally printed distributed feedback lasers on flexible polyethylene terephthalate substrates based on methylammonium lead iodide perovskite gain material. The perovskite lasers are printed with a digital drop-on-demand inkjet printer, providing full freedom in the shape and design of the gain layer. We show that adjusting the perovskite ink increases the potential processing window and decreases the surface roughness of the active layer to less than 7 nm, which is essential for low lasing thresholds. Prototype inkjet-printed perovskite lasers processed on top of nanopatterned rigid as well as flexible substrates are demonstrated. Optimized perovskite gain layers printed on PET substrates demonstrated lasing and showed a linewidth of 0.4 nm and a lasing threshold of 270 kW/cm. In addition, printing of a distinct shape shows a high level of uniformity, demonstrated by a low spatial resolved full width half maximum variation over the whole printing area. These results reveal the possibilities of digital printed perovskite layers towards large-scale and low-cost laser applications of arbitrary shape.

description of the efforts on developing perovskite lasers is given in recent review articles. [19][20][21][22] As the field advances, new approaches for patterning perovskite films are becoming essential. More specifically, a lithographic approach that can define desired perovskite patterns is highly needed in order to utilize these materials for the realization of integrated perovskite photonic devices. Generally speaking, effective lithographic patterning requires three main capabilities. First, the ability to cover the material (in our case perovskites) with a layer of resist and pattern it with sub-micrometer resolution using a scalable process. Second, employment of the patterned resist as an etch mask for complete removal of the unprotected perovskite film. Third, removal of the residual resist mask from the perovskite film. Since metal halide perovskites are unstable soluble materials that are sensitive to a variety of solvents and gases, [23] accomplishing all of the above steps without damaging the film is highly challenging. During the past few years, several attempts have been made in order to address this challenge. Lyashenko et al. [24] demonstrated the ability to pattern perovskites using photolithography followed by SF 6 etching. This approach was utilized to define patterns in photoresists on top of methyl-ammonium lead iodide (MAPbI 3 ) films but was limited to micrometer scale features due to diffraction effects in photolithography. In addition, the chemical reaction that was used in order to etch the material only modified the exposed areas chemically (converting the material to PbF 2 ) and did not remove the perovskite layer completely as desired from a proper etching technique. Zhang et al. [25] demonstrated a different approach where PMMA layers were deposited on top of a MAPbBr 3 film and patterned using electron beam lithography (EBL) followed by dry Cl etching. Although this method can potentially generate sub-micrometer features, it is based on a serial patterning technique (EBL) and is thus limited in terms of speed and cost. In addition, the scanning electron microscopy (SEM) images of the patterned surface suggest that the etching process left a residual layer on the surface (possibly PbCl 2 ) and was unable to remove the perovskite completely. A slightly different approach to pattern perovskite films, involving lift-off instead of etching, was recently successfully demonstrated. [26] However, this technique necessitates the evaporation of perovskite films and is not compatible with the standard spin-coating film preparation method which is simple, inexpensive, and renders these materials A complete lithographic scheme for thin metal halide perovskite films is demonstrated and utilized for the realization of perovskite micro lasers. The process consists of nanoimprint lithography followed by ion beam milling. It is simple, fast, scalable, and exhibits sub-micrometer resolution. The optical properties of the perovskite films are obtained by employing analytical tools as well as by cha...

Metal halide perovskites are recently attracting strong attention due to their potential in solar cells, LEDs, and lasers. Here, we demonstrate the broad spectral tuning of the optical gain characteristics of triple cation (containing methylammonium, formamidinium and cesium) mixed-halide perovskite thin films. We explicitly study the interrelation between amplified spontaneous emission (ASE) thresholds, the operational stability of the gain, and the material composition. The incorporation of cesium and a deficiency of lead in the precursor solutions is found to be crucial for low ASE thresholds and the improved stability of mixed-halide perovskites. We tune the photoluminescence in mixed-halide perovskites between peak wavelengths of 510 and 790 nm by exchanging the halide from iodide to bromide and chloride in small steps of 10%, while preserving the narrowband emission below a linewidth of 130 meV for all mixtures. The optical gain under ns-excitation can be tuned over a significant portion of this spectral window; we observe ASE emission in regions between 545 to 555 nm and 680 to 810 nm. This is a significant step towards perovskite lasers operating through a broad portion of the visible to near infrared spectrum. References and links1. S. Kazim, M. K. Nazeeruddin, M. Grätzel, and S. Ahmad, "Perovskite as Light Harvester: A Game Changer in Photovoltaics," Angew. Chem. Int. Ed. Engl. 53(11), 2812-2824 (2014). 2. S. D. Stranks and H. J. Snaith, "Metal-halide perovskites for photovoltaic and light-emitting devices," Nat.Nanotechnol. 10(5), 391-402 (2015). Tress, A. Abate, A. Hagfeldt, and M. Grätzel, "Cesium-containing triple cation perovskite solar cells: improved stability, reproducibility and high efficiency," Energy Environ. Sci. 9(6), 1989-1997 (2016