Methylammonium lead iodide perovskites are considered direct bandgap semiconductors. Here we show that in fact they present a weakly indirect bandgap 60 meV below the direct bandgap transition. This is a consequence of spin-orbit coupling resulting in Rashba-splitting of the conduction band. The indirect nature of the bandgap explains the apparent contradiction of strong absorption and long charge carrier lifetime. Under hydrostatic pressure from ambient to 325 MPa, Rashba splitting is reduced due to a pressure induced ordering of the crystal structure. The nature of the bandgap becomes increasingly more direct, resulting in five times faster charge carrier recombination, and a doubling of the radiative efficiency. At hydrostatic pressures above 325 MPa, MAPI undergoes a reversible phase transition resulting in a purely direct bandgap semiconductor. The pressure-induced changes suggest epitaxial and synthetic routes to higher efficiency optoelectronic devices

Metallic and dielectric nanostructures have highly tunable resonances that have been used to increase light absorption in a variety of photovoltaic materials and device structures. Metal nanowires have also emerged as a promising candidate for high-performance transparent electrodes for local contacts. In this Letter we propose combining these electrical and optical functions. As a first step, we use rigorous solutions to Maxwell's equations to demonstrate theoretically extreme absorption in semiconductor thin films wrapped around metal nanowires. We show that there are two key principles underlying this extraordinary light trapping effect: (1) maximizing the absorption of each individual resonance by ensuring it is critically coupled and (2) increasing the total number of degenerate resonances. Inserting a metal core into a semiconductor nanowire creates such a degeneracy: polarization-dependent Mie resonances are transformed into polarization-independent Fabry-Pérot-like resonances. We demonstrate that, by reducing the polarization sensitivity and increasing the number of critically coupled modes, this hybrid coaxial nanowire geometry substantially outperforms solid semiconducting nanowires, even though the semiconductor volume is significantly reduced. These results suggest that metal nanowires with semiconductor shells might be ideal building blocks for photovoltaic and solar fuel applications.

Quantum emitters such as the diamond nitrogen-vacancy (NV) center are the basis for a wide range of quantum technologies. However, refraction and reflections at material interfaces impede photon collection, and the emitters’ atomic scale necessitates the use of free space optical measurement setups that prevent packaging of quantum devices. To overcome these limitations, we design and fabricate a metasurface composed of nanoscale diamond pillars that acts as an immersion lens to collect and collimate the emission of an individual NV center. The metalens exhibits a numerical aperture greater than 1.0, enabling efficient fiber-coupling of quantum emitters. This flexible design will lead to the miniaturization of quantum devices in a wide range of host materials and the development of metasurfaces that shape single-photon emission for coupling to optical cavities or route photons based on their quantum state.