Emerging gradient metasurfaces represent a new class of diffraction optical components. Through elaborate engineering of planar arrangements of subwavelength optical antennas, metasurfaces are capable of imparting arbitrary phase profiles on to the incident light, thereby enabling devices such as holograms, complex lenses, and beam splitters. However, the traditional approach for designing reflective gradient metasurfaces fails for simple beam deflection if the angle included by the incident and the anomalously reflected beam is large. Recently, it has been shown that this shortcoming, which results from the impedance mismatch at the interface and parasitic reflections, can be eliminated by proper metasurface engineering. Here, we report the design, fabrication, and characterization of an optical metasurface, which reflects normally impinging light at around 1550 nm wavelength at an angle of 80°with respect to the surface normal. Under these conditions, the calculated and measured results show a reflection efficiency that exceeds previous results by a factor of 2. We believe that our findings are an important step toward high-efficiency devices for general wavefront manipulation.

Existing photoresists for 3D laser lithography that can be removed after development in a subtractive manner typically suffer from harsh cleavage conditions. Here, we report chemoselectively cleavable photoresists for 3D laser lithography based on silane crosslinkers, allowing the targeted degradation of 3D printed microstructures under mild conditions. Three bifunctional silane crosslinkers carrying various substitutions on the silicon atom are synthesized. The photoresists are prepared by mixing these silane crosslinkers with pentaerythritol triacrylate and a two-photon photoinitiator. The presence of pentaerythritol triacrylate significantly enhances the direct laser written structures with regard to resolution, while the microstructures remain cleavable. For the targeted cleavage of the fabricated 3D microstructures, simply a methanol solution including inorganic salts is required, highlighting the mild cleavage conditions. Critically, the photoresists can be cleaved selectively, which enables the sequential degradation of direct laser written structures and allows for subtractive manufacturing at the micro- and nanoscale.

CommuniCationeven larger than for bulk ZnO crystals. [4][5][6][7][8][9][10][11][12][13][14] These publications have also highlighted some of the important links between the structural properties of the thin films and their nonlinear optical susceptibility. In the work by Cao et al., thin films of ZnO were fabricated on sapphire substrates by pulsed laser ablation. [5] The second-order susceptibility was determined to be 13.40 pm V (2) 1. Furthermore, it was shown that the second-order susceptibility decreases with increasing crystalline quality and with increasing film thickness. This finding was attributed to a strong contribution of grain boundaries and interfaces to the second-order susceptibility. Another group highlighted the strong dependence of the structural and second-order nonlinear properties on the deposition conditions and the choice of the substrate. [4] Clearly, different deposition techniques lead to different growth behavior, hence different structural properties and thus different nonlinear optical properties. Atomic-layer deposition (ALD) is a fabrication technique that is particularly well-suited for the needs of optical integration. ALD is CMOS-compatible and allows for an inexpensive and conformal deposition on a large variety of substrates with a precision down to a monolayer. [15] Unfortunately, ZnO crystallites grown by ALD seem to have properties unfavorable for a strong second-order nonlinear response as there is not a single publication covering this topic. As we will show in this publication, the weak second-order response can be ascribed to the random orientation of the ZnO crystallites in pure ALD-grown ZnO thin films. In general, there are only few publications showing the realization of ALD-grown second-order nonlinear thin films and these are solely based on the idea of a strong interface-connected response. [16,17] In this Communication, we use the concept of nanolaminates to tune the second-order nonlinear properties as it has previously been demonstrated for the mechanical, linear optical, and also the third-order nonlinear optical properties of ALD-grown thin films. [18][19][20] Using this approach, we are able to control the ZnO crystallite growth and thus, for the first time, achieve a strong bulk second-order nonlinear susceptibility in ALD-grown thin films.The literature on ZnO thin films suggests that two key factors have to be controlled to maximize the second-order susceptibility in ALD-grown ZnO films. The first key factor is a well-defined ZnO crystallite orientation to avoid mutual cancellation of the contributions of neighboring misaligned crystallites. Without a preferential direction within a plane parallel to the substrate, the substrate normal is the only extraordinary direction. Indeed, ZnO films with a strong nonlinear

The determination of the second-order susceptibility (χ(2)) of thin film samples can be a delicate matter since well-established χ(2) measurement methodologies such as the Maker fringe technique are best suited for nonlinear materials with large thicknesses typically ranging from tens of microns to several millimeters. Here we compare two different second-harmonic generation setups and the corresponding measurement methodologies that are especially advantageous for thin film χ(2) characterization. This exercise allows for cross-checking the χ(2) obtained for identical samples and identifying the main sources of error for the respective techniques. The development of photonic integrated circuits makes nonlinear thin films of particular interest, since they can be processed into long waveguides to create efficient nonlinear devices. The investigated samples are ABC-type nanolaminates, which were reported recently by two different research groups. However, the subsequent analysis can be useful for all researchers active in the field of thin film χ(2) characterization.

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In-chip direct laser writing of a centimeter-scale acoustic micromixer van 't Oever J., Spannenburg N., Offerhaus H., van den Ende D., Herek J., Mugele F. An elasto-mechanical unfeelability cloak made of pentamode metamaterials T. Dreidimensionale optische Tarnkappe realisiert N. Stenger, T. Ergin and M. Wegener Physik in unserer Zeit 41, 218 (2010) Rhombicuboctahedral three-dimensional photonic quasicrystals A. Ledermann, M. Wegener and G. von Freymann Adv.

Nanolaminate metamaterials recently attracted a lot of attention as a novel second-order nonlinear material that can be used in integrated photonic circuits. Here, we explore theoretically and numerically the opportunity to enhance the nonlinear response from such nanolaminates by exploiting Fano resonances supported in grating-coupled waveguides. The enhancement factor of the radiated second harmonic signal compared to a flat nanolaminate can reach values as large as 35 for gold gratings and even 7000 for MgF 2 gratings. For the MgF 2 grating, extremely high-Q Fano resonances are excited in such all-dielectric system that result in strong local fields in the nonlinear waveguide layer to boost the nonlinear conversion. A significant portion of the nonlinear signal is also strongly coupled to a dark waveguide mode, which remains guided in the nanolaminate. The strong excitation of a dark mode at the second harmonic frequency provides a viable method for utilizing second-order nonlinearities for light generation and manipulation in integrated photonic circuits. Osellame, B. Hecht, L. Duò, F. Ciccacci, and M. Finazzi, "Mode matching in multiresonant plasmonic nanoantennas for enhanced second harmonic generation," Nat. Nanotechnol. 10(5), 412-417 (2015). 9. J. Butet, P.-F. Brevet, and O. J. F. Martin, "Optical second harmonic generation in plasmonic nanostructures:From fundamental principles to advanced applications," ACS Nano 9(11), 10545-10562 (2015). 10. J. Lee, M. Tymchenko, C. Argyropoulos, P.-Y. Chen, F. Lu, F. Demmerle, G. Boehm, M.-C. Amann, A. Alù, and M. A. Belkin, "Giant nonlinear response from plasmonic metasurfaces coupled to intersubband transitions," Nature 511(7507), 65-69 (2014