Owing to their superior optical properties, semiconductor nanopillars/nanowires in one-dimensional (1D) geometry are building blocks for nano-photonics. They also hold potential for efficient polarized spin-light conversion in future spin nano-photonics. Unfortunately, spin generation in 1D systems so far remains inefficient at room temperature. Here we propose an approach that can significantly enhance the radiative efficiency of the electrons with the desired spin while suppressing that with the unwanted spin, which simultaneously ensures strong spin and light polarization. We demonstrate high optical polarization of 20%, inferring high electron spin polarization up to 60% at room temperature in a 1D system based on a GaNAs nanodisk-in-GaAs nanopillar structure, facilitated by spin-dependent recombination via merely 2–3 defects in each nanodisk. Our approach points to a promising direction for realization of an interface for efficient spin-photon quantum information transfer at room temperature—a key element for future spin-photonic applications.

Mie resonator arrays formed by embossing titanium dioxide (tio 2) nanoparticles (nps) from solution are investigated as optical coatings for anti-reflection applications. Compacted nanoparticle assemblies offer unique possibilities to tailor the effective refractive index (RI). Here, we demonstrate a simple table-top, low pressure, and low temperature method to fabricate structured optical coatings. TiO 2 nanostructures in the form of nanodisks support Mie resonances in the visible wavelength spectrum and exhibit strong forward scattering into the high index substrates, making them suitable as broadband anti-reflection coatings for solar cells. TiO 2 np-based nanodisk arrays are designed, fabricated, and characterized regarding their anti-reflection properties on Si, GaAs, and InP substrates and solar cells. Detailed finite-difference time-domain simulations are performed to optimize the tio 2 NP-based Mie resonator arrays for the broadband anti-reflection as well as to explain the measured reflectance spectra. The solar-weighted reflectance is used as a figure of merit (FoM). TiO 2 nanodisk arrays on Si show a FoM of ~ 7% in the 400-1,100 nm wavelength spectrum; similar values are obtained for GaAs and InP substrates. TiO 2 nanodisk arrays embossed directly on prefabricated planar single-junction Si, GaAs, and InP solar cells result in an appreciable increase (~ 1.3 times) in the short-circuit current densities. Recently, sub-wavelength dielectric Mie resonator arrays have been reported for applications such as omnidirectional broadband anti-reflection. 1,2 Si nanodisk arrays on Si substrates show low average surface reflectance over the visible-NIR wavelength region. 3 Surface reflection reduction 4-7 plays a major part in increasing the performance of solar cells. For inorganic semiconductor solar cell materials, e.g., Si and III-Vs, due to their high refractive indices (~ 3-4) the reflectance loss (~ 30-40%) is significant. To reduce this, the solar cell surface can either be structured directly or an additional (structured) optical coating can be used. Direct structuring of solar cells can degrade its performance due to process induced defects and surface recombination and invariably requires additional passivation procedures/coatings. Such issues become more significant for thin film solar cells (thickness of ~ 2 µm or less). These limitations may be overcome by depositing a structured optical layer instead. Anti-reflection coatings (ARCs) include commonly used traditional thin-film dielectrics (e.g., silicon dioxide (SiO 2) and silicon nitride (Si x N y)), metal nanoparticles, 8,9 and dielectric nanostructures 3,10,11. While thin film dielectrics are easier to fabricate, multilayers are often required to achieve broadband anti-reflection. Metallic nanoparticles suffer from parasitic absorption in the metal and the resonances are highly sensitive to the RI of the matrix below or around the nanoparticles. 12 High-index dielectric (e.g., Si) Mie resonator arrays placed on Si or substrates with similar refract...

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Nanostructures composed of dielectric, metallic or metalo-dielectric structures are receiving significant attention due to their unique capabilities to manipulate light for a wide range of functions such as spectral colors, anti-reflection and enhanced light-matter interaction. The optical properties of such nanostructures are determined not only by the shape and dimensions of the structures but also by their spatial arrangement. Here, we demonstrate the generation of vivid colors from nanostructures composed of spatially disordered metalo-dielectric (In/InP) nanopillar arrays. The nanopillars are formed by a single-step, ion-sputtering-assisted, self-assembly process that is inherently scalable and avoids complex patterning and deposition procedures. The In/InP nanopillar dimensions can be changed in a controlled manner by varying the sputter duration, resulting in reflective colors from pale blue to dark red. The fast Fourier transform (FFT) analysis of the distribution of the formed nanopillars shows that they are spatially disordered. The electromagnetic simulations combined with the optical measurements show that the reflectance spectra are strongly influenced by the pillar dimensions. While the specular and diffuse reflectance components are appreciable in all the nanopillar samples, the specular part dominates for the shorter nanopillars, thereby leading to a glossy effect. The simulation results show that the characteristic features in the observed specular and diffused reflectance spectra are determined by the modal and light-scattering properties of single pillars. While the work focuses on the In/InP system, the findings are relevant in a wider context of structural color generation from other types of metalo-dielectric nanopillar arrays.

A novel ZnO dry etching approach is introduced using reactive ion beam etching of thick sol-gel ZnO layers for controlled nanodisk/nanocone array fabrication. In this approach the same system can be used for the colloidal lithography mask (silica particles) size reduction by a fluorine-based chemistry and etching of the ZnO nanostructures by a CH 4 /H 2 /Ar chemistry. This resulted in a ZnO:SiO 2 etch selectivity of ∼3.4 and etch rate of ∼56 nm/min. Thick sol-gel ZnO layers, nanodisk arrays and (truncated) nanocone arrays were fabricated and their optical properties analyzed by finite-difference time-domain simulations and spectrally-resolved total/specular reflectivity measurements. The demonstrated broadband omnidirectional anti-reflection, controlled nanostructure period/geometry and low absorption in the visible-NIR spectrum makes these sol-gel ZnO nanostructures very interesting for many optoelectronic applications, including photovoltaics. ZnO is a wide bandgap (∼3.37 eV at room temperature) II-VI semiconductor. Predominantly it crystalizes in the Wurtzite crystal structure and bulk ZnO has a refractive index of n ≈ 1.9-2.2 in the NIR-visible wavelength range. Some of the features of ZnO interesting for photovoltaic applications are its high transparency in the visible-NIR spectrum, a wide bandgap, a large exciton binding energy (60 meV at room temperature), non-centrosymmetric structure, strong room temperature luminescence and biocompatibility. 1,2ZnO has been studied extensively as a candidate for a number of applications, e.g., transparent conductive electrodes, optical waveguides, piezo-electric transducers, acoustic wave devices, conductive gas sensors, solar cell windows and biosensing. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] Here, a sol-gel method has been developed and used for the fabrication of (relatively) thick porous polycrystalline ZnO layers. A similar method has already been reported for thin films by, e.g., Gohdsi et al. 34 and Choudhury et al. 44 Here the method has been extended for obtaining well-defined thick ZnO films (up to ∼2 μm) by using a sequential drop-cast approach in a 'vacuum' environment. The 'building' of these sequential ZnO layers to one thick layer provides a method to tune specific layer thicknesses on the micro/nanoscale. Other approaches for obtaining thick porous ZnO layers have been reported. [45][46][47] The sol-gel method is attractive since it is simple, cheap and straight forward. In addition, this method offers the flexibility to provide ZnO layers on a variety of surfaces. Sol-gel layers, consisting of crystals with c-axis orientation vertical to the surface, can be * Electrochemical Society Member.z E-mail: dvisser@kth.se obtained when the substrate material has a similar Wurtzite crystal structure to ZnO, e.g., GaN, SiC and (0001)-sapphire. For other types of substrates dense sol-gel layers, composed of non-specific direction, can still be obtained. In this work, thick ZnO layers have been fabricated and nanostructured on a Si substrate. These laye...