III-Nitride light emitting diodes (LEDs) are the backbone of ubiquitous lighting and display applications. Imparting directional emission is an essential requirement for many LED implementations. Although optical packaging 1 , nano-patterning 2,3 and surface roughening 4 techniques can enhance LED extraction, directing the emitted light requires bulky optical components. Optical metasurfaces provide precise control over transmitted and reflected waveforms, suggesting a new route for directing light emission. However, it is difficult to adapt metasurface concepts for incoherent light emission, due to the lack of a phase-locking incident wave. In this Letter, we demonstrate metasurface-based design of InGaN/GaN quantum-well structures that generate narrow, unidirectional transmission and emission lobes at arbitrary engineered angles. We show that the directions and polarization of emission differ significantly from transmission, in agreement with an analytical Local Density of Optical States (LDOS) model. The results presented in this Letter open a new paradigm for exploiting metasurface functionality in light emitting devices. Light emitting diodes (LED) are rapidly enabling solid-state solutions to commercial lighting applications. Imparting unidirectionality to LEDs is a challenging, problem with a host of applications 5-7 awaiting a scalable solution. Directional emission is naturally observed in lasing systems, 13,14 where all of the resonators are emitting coherently. By modifying the local density
Two-dimensional
hybrid metal halide perovskites (2D perovskites)
are attractive for light-emitting devices and other applications because
their emission is tunable across the visible spectrum. The emission
profile of 2D perovskites can be broadened via a variety of mechanisms
and is further complicated by the presence of impurities. Here, the
challenge of making phase-pure films in Ruddlesden–Popper phases
[(A′)2(A)
n−1B
n
X3n+1 structure]
is overcome by using a single A/A′-site cation, ethylammonium
(EA), whose optimal size also prohibits the formation of off-target
phases. In the (EA)2(EA)
n−1Pb
n
Br3n+1 family, the low-energy, broad emission observed in bulk crystals
is reduced in spin-cast, polycrystalline films. This decrease in broad
emission, attributed to phonon-mediated processes, is correlated with
the strain in polycrystalline films that is observed by X-ray scattering.
Photothermal deflection spectroscopy shows that strain also increases
the electronic disorder near the free exciton absorbance. Broad emission
in films can be recovered by slowing growth kinetics, which removes
the strain acquired from spin-casting and increases the domain size.
These results help extend the utility of 2D perovskites by suggesting
design rules for the growth of thin films with the targeted phase
and emission.
Phased-array metasurfaces have been extensively used for wavefront shaping of coherent incident light. Due to the incoherent nature of spontaneous emission, the ability to similarly tailor photoluminescence remains largely unexplored. Recently, unidirectional photoluminescence from InGaN/GaN quantum-well metasurfaces incorporating one-dimensional phase profiles has been shown. However, the possibility of generating arbitrary two-dimensional waveforms—such as focused beams—is not yet realized. Here, we demonstrate two-dimensional metasurface axicons and lenses that emit collimated and focused beams, respectively. First, we develop off-axis meta-axicon/metalens equations designed to redirect surface-guided waves that dominate the natural emission pattern of quantum wells. Next, we show that photoluminescence properties are well predicted by passive transmission results using suitably engineered incident light sources. Finally, we compare collimating and focusing performances across a variety of different light-emitting metasurface axicons and lenses. These generated two-dimensional phased-array photoluminescence waveforms facilitate future development of light sources with arbitrary functionalities.
Phased-array metasurfaces
grant the ability to arbitrarily shape
the wavefront of light. As such, they have been used as various optical
elements including waveplates, lenses, and beam deflectors. Luminescent
metasurfaces, on the other hand, have largely comprised uniform arrays
and are therefore unable to provide the same control over the wavefront
of emitted light. Recently, phased-array control of the wavefront
of spontaneous emission has been experimentally demonstrated in luminescent
phased-array metalenses and beam deflectors. However, current luminescent
metasurface beam deflectors exhibit unidirectional emission for only
p-polarized light. In this paper, we use a reciprocal simulation strategy
to explain the polarization disparity and improve the directionality
of incoherent emission from current quantum-well emitting phased-array
metasurfaces. We also design complementary metasurfaces to direct
emission from systems where emission originates from alternate quantum
mechanical processes.
Topotactic transformations involve structural changes between related crystal structures due to a loss or gain of material while retaining a crystallographic relationship.
2The perovskite oxide La0.7Sr0.3CoO3 (LSCO) is an ideal system for investigating phase transformations due to its high oxygen vacancy conductivity, relatively low oxygen vacancy formation energy, and strong coupling of the magnetic and electronic properties to the oxygen stoichiometry. While the transition between cobaltite perovskite and brownmillerite (BM) phases has been widely reported, further reduction beyond the BM phase lacks systematic studies. In this work, we study the evolution of the physical properties of LSCO thin films upon exposure to highly reducing environments. We observe the rarely-reported crystalline Ruddlesden-Popper (RP) phase, which involves the loss of both oxygen anions and cobalt cations upon annealing where the cobalt is found as isolated Co ions or Co nanoparticles. First principles calculations confirm that the concurrent loss of oxygen and cobalt ions is thermodynamically possible through an intermediary BM phase. The strong correlation of the magnetic and electronic properties to the crystal structure highlights the potential of utilizing ion migration as a basis for emerging applications such as neuromorphic computing.
Higher-frequency microwave ablation is appealing because it allows for more flexibility in antenna design. A critical issue concerning the feasibility of higher-frequency microwave ablation, considering its strong dependence on heat diffusion to grow thermal lesions, is its performance in strongly perfused environments. This work shows that higher-frequency microwave ablation achieves thermal lesions comparable to those from microwave ablation performed at conventional frequencies in both non- and strongly perfused environments.
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