The strain in GaN epitaxial layers grown on silicon (111) substrates by metalorganic vapor phase epitaxy has been investigated. The insertion of AlN/GaN superlattices was found to decrease the stress sufficiently for avoiding crack formation in an overgrown thick (2.5 μm) GaN layer. X-ray diffraction and photoluminescence measurements are used to determine the effect of these AlN/GaN superlattices on the strain in the subsequent GaN layers. A reduction of threading dislocation density is also observed by transmission electron microscopy and atomic force microscopy when such superlattices are used. Strong band edge photoluminescence of GaN on Si(111) was observed with a full width at half maximum of the bound exciton line as low as 6 meV at 10 K. The 500 arcsec linewidth on the (002) x-ray rocking curve also attests the high crystalline quality of GaN on Si (111), when using these AlN/GaN superlattices.
Controlling light properties with diffractive planar elements requires full-polarization channels and accurate reconstruction of optical signal for real applications. Here, we present a general method that enables wavefront shaping with arbitrary output polarization by encoding both phase and polarization information into pixelated metasurfaces. We apply this concept to convert an input plane wave with linear polarization to a holographic image with arbitrary spatial output polarization. A vectorial ptychography technique is introduced for mapping the Jones matrix to monitor the reconstructed metasurface output field and to compute the full polarization properties of the vectorial far field patterns, confirming that pixelated interfaces can deflect vectorial images to desired directions for accurate targeting and wavefront shaping. Multiplexing pixelated deflectors that address different polarizations have been integrated into a shared aperture to display several arbitrary polarized images, leading to promising new applications in vector beam generation, full color display and augmented/ virtual reality imaging.
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