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 .

eprints@whiterose.ac.uk https://eprints.whiterose.ac.uk/ Reuse Unless indicated otherwise, fulltext items are protected by copyright with all rights reserved. The copyright exception in section 29 of the Copyright, Designs and Patents Act 1988 allows the making of a single copy solely for the purpose of non-commercial research or private study within the limits of fair dealing. The publisher or other rights-holder may allow further reproduction and re-use of this version -refer to the White Rose Research Online record for this item. Where records identify the publisher as the copyright holder, users can verify any specific terms of use on the publisher's website. TakedownIf you consider content in White Rose Research Online to be in breach of UK law, please notify us by emailing eprints@whiterose.ac.uk including the URL of the record and the reason for the withdrawal request.Photonic band-structure effects in the reflectivity of periodically patterned waveguides We report sharp resonant features in the reflectivity spectra of semiconductor waveguides patterned with periodic lattices of deep holes. The resonances arise from coupling of incident light to the photonic bands of the lattice. By varying the reflection geometry, large parts of the photonic band structure are determined. A scattering matrix treatment is used to obtain theoretical spectra which agree well with experiment. The waveguide is shown to have an important influence on the band structure, including marked polarization mixing and significant energy up-shifts. ͓S0163-1829͑99͒50548-1͔Ever since the discovery of diffraction anomalies in gratings by Wood, 1 the reflectivity properties of patterned surfaces have been widely studied. Similar resonant anomalies occur in many circumstances when a periodic patterning is applied to a surface supporting excitations which momentum conservation forbids from coupling to external photons by momentum conservation. Resonant anomalies appear when these forbidden modes become allowed as a result of inplane band-structure effects arising from the patterning. Such effects constitute the basic physics underlying recent observations of surface plasmon polariton resonances 2 and photonic band gaps ͑PBG's͒, 3 optical transmission through subwavelength hole arrays, 4 and sharp spectral features in shallow grating waveguide structures. 5In this paper we investigate the reflectivity properties of two-dimensional photonic crystal dielectric waveguides. In contrast to previously studied shallow gratings, these structures are patterned with deep air holes etched through the waveguide and into the cladding layer.6 As a result, significant band gaps open up in the photonic Brillouin zone, leading to parabolic dispersions ͑so-called ''heavy photons''͒ at high symmetry points. Many new phenomena with potential optoelectronics applications have been predicted for PBG structures, including control of spontaneous emission, 7 gap solitons, 8 optical limiting and nonlinearities. 9 The presence of the waveguide is crucial to these appli...

Charged (X*) and neutral ͑X͒ exciton recombination is reported in the photoluminescence spectra of single In͑Ga͒As quantum dots. Photoluminescence excitation ͑PLE͒ spectra show that the charged excitons are created only for excitation in the barrier or cladding layers of the structure, consistent with their charged character, whereas the neutral excitons in addition show well-defined excitation features for resonant excitation of the dots. The PLE spectra for X and X* exhibit a clear anticorrelation in the region of the wetting layer transition, showing that they compete for photocreated carriers.

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Dielectric metasurfaces support resonances that are widely explored both for far-field wavefront shaping and for near-field sensing and imaging. Their design explores the interplay between localised and extended resonances, with a typical trade-off between Q-factor and light localisation; high Q-factors are desirable for refractive index sensing while localisation is desirable for imaging resolution. Here, we show that a dielectric metasurface consisting of a nanohole array in amorphous silicon provides a favourable trade-off between these requirements. We have designed and realised the metasurface to support two optical modes both with sharp Fano resonances that exhibit relatively high Q-factors and strong spatial confinement, thereby concurrently optimizing the device for both imaging and biochemical sensing. For the sensing application, we demonstrate a limit of detection (LOD) as low as 1 pg/ml for Immunoglobulin G (IgG); for resonant imaging, we demonstrate a spatial resolution below 1 µm and clearly resolve individual E. coli bacteria. The combined low LOD and high spatial resolution opens new opportunities for extending cellular studies into the realm of microbiology, e.g. for studying antimicrobial susceptibility.