Controlling the flux of photons is crucial in many areas of science and technology. Artificial materials with nano-scale modulation of the refractive index, such as photonic crystals, are able to exercise such control and have opened exciting new possibilities for light manipulation. An interesting alternative to such periodic structures is the class of materials known as quasi-crystals, which offer unique advantages such as richer Fourier spectra. Here we introduce a novel approach for designing such richer Fourier spectra, by using a periodic structure that allows us to control its Fourier components almost at will. Our approach is based on binary gratings, which makes the structures easy to replicate and to tailor towards specific applications. As an example, we show how these structures can be employed to achieve highly efficient broad-band light trapping in thin films that approach the theoretical (Lambertian) limit, a problem of crucial importance for photovoltaics.

Subwavelength imaging requires the use of high numerical aperture (NA) lenses together with immersion liquids in order to achieve the highest possible resolution. Following exciting recent developments in metasurfaces that have achieved efficient focusing and novel beam-shaping, the race is on to demonstrate ultra-high NA metalenses. The highest NA that has been demonstrated so far is NA=1.1, achieved with a TiO2 metalens and back-immersion. Here, we introduce and demonstrate a metalens with high NA and high transmission in the visible range, based on crystalline silicon (c-Si). The higher refractive index of silicon compared to TiO2 allows 2 us to push the NA further. The design uses the geometric phase approach also known as the Pancharatnam-Berry phase and we determine the arrangement of nano-bricks using a hybrid optimization algorithm (HOA). We demonstrate a metalens with NA = 0.98 in air, a bandwidth (FWHM) of 274 nm and a focusing efficiency of 67% at 532 nm wavelength, which is close to the transmission performance of a TiO2 metalens. Moreover, and uniquely so, our metalens can be front-immersed into immersion oil and achieve an ultra-high NA of 1.48 experimentally and 1.73 theoretically, thereby demonstrating the highest NA of any metalens in the visible regime reported to the best of our knowledge. The fabricating process is fully compatible with CMOS technology and therefore scalable. We envision the front-immersion design to be beneficial for achieving ultra-high NA metalenses as well as immersion metalens doublets, thereby pushing metasurfaces into practical applications such as high resolution, low-cost confocal microscopy and achromatic lenses.Metasurfaces are artificial sheet materials of sub-wavelength thickness that modulate electromagnetic waves mainly through photonic resonances [1][2][3]. Their properties are based on the ability to control the phase and/or polarisation of light with subwavelength-scale dielectric or metallic nano-resonators [4,5]. Correspondingly, metasurfaces are able to alter every aspect of transmitting or reflecting beams, achieving various extraordinary optical phenomena including deflection [6 -8], retro-reflection [9, 10], polarization conversion [4, 11 -14], focusing [15 -17] and beam-shaping [18], with a nanostructured thin film alone. Focusing metasurfaces -namely metalenses -are amongst the most promising optical elements for practical applications [19,20], e.g. for cell phone camera lenses [21,22] or ultrathin microscope objectives [23,24], since their subwavelength nanostructures are able to provide more precise and efficient phase control compared to binary amplitude and phase Fresnel zone plates .

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Stilbazolium dimers were designed and synthesized in which methylene groups of different lengths link the two chromophores. The second-order nonlinear optical property has been detected from their Langmuir-Blodgett monolayers. Photocurrent generation as well as other photophysical properties such as absorption and fluorescence were investigated. Comparing the dimers with the stilbazolium monomer, we found strong second harmonic generation (SHG) signals and remarkable enhanced photoelectric conversion (PEC) quantum yields from 1,3-Bis [(E)-4-(2-(4-(N-methyl, N-octadecylamino) phenyl) ethenyl) pyridinyl]-alkane dibromide (B3) and 1,5-Bis [(E)-4-(2-(4-(N-methyl, N-octadecylamino) phenyl) ethenyl) pyridinyl]-alkane dibromide (B5). The structures of the Langmuir-Blodgett films of these dyes are assumed. Although these dimers are center-symmetric (or quasi-center-symmetric) molecules, they can form the ordered non-center-symmetric (in normal direction) monolayer by the compress force and the molecular interaction between the amphiphilic dye molecules and the water molecules at the air/water interface. The influences of bias, donor/acceptor, soluble oxygen, and light intensity to photocurrent generation were studied; our results provide a new perspective to improve the performance of functional molecular material without any change of the chemical structure of the chromophore itself.