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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.

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.

Using one material system from the near infrared into the ultraviolet is an attractive goal, and may be achieved with (In,Al,Ga)N. This III-N material system, famous for enabling blue and white solid-state lighting, has been pushing towards longer wavelengths in more recent years. With a bandgap of about 0.7 eV, InN can emit light in the near infrared, potentially overlapping with the part of the electromagnetic spectrum currently dominated by III-As and III-P technology. As has been the case in these other III-V material systems, nanostructures such as quantum dots and quantum dashes provide additional benefits towards optoelectronic devices. In the case of InN, these nanostructures have been in the development stage for some time, with more recent developments allowing for InN quantum dots and dashes to be incorporated into larger device structures. This review will detail the current state of metalorganic chemical vapor deposition of InN nanostructures, focusing on how precursor choices, crystallographic orientation, and other growth parameters affect the deposition. The optical properties of InN nanostructures will also be assessed, with an eye towards the fabrication of optoelectronic devices such as light-emitting diodes, laser diodes, and photodetectors.

III-Nitride light emitting diodes (LEDs) are the backbone of ubiquitouslighting and display applications. Imparting directional emission is anessential requirement for many LED implementations. Although optical packaging,nano-patterning and surface roughening techniques can enhance LED extraction,directing the emitted light requires bulky optical components. Opticalmetasurfaces provide precise control over transmitted and reflected waveforms,suggesting a new route for directing light emission. However, it is difficultto adapt metasurface concepts for incoherent light emission, due to the lack ofa phase-locking incident wave. In this Letter, we demonstrate metasurface-baseddesign 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 significantlyfrom transmission, in agreement with an analytical Local Density of OpticalStates (LDOS) model. The results presented in this Letter open a new paradigmfor exploiting metasurface functionality in light emitting devices.

The compliant behavior of densely packed 10 × 10 µm2 square patterned InGaN layers on top of porous GaN is demonstrated. The elastic relaxation of the InGaN layers is enabled by the low stiffness of the porous GaN under layer. High resolution X-ray diffraction measurements show that upon InGaN re-growths on these InGaN-on-porous GaN pseudo-substrates, not only was the regrown layer partially relaxed, but the degree of relaxation of the InGaN pseudo-substrate layer on top of the porous GaN also showed an increase in the a-lattice constant. Furthermore, methods to improve the surface morphology of the InGaN layers grown by metal-organic chemical vapor deposition (MOCVD) were explored in order to fabricate InGaN pseudo-substrates for future optoelectronic and electronic devices. The largest a-lattice constant demonstrated in this study using this improved method was 3.209 Å, corresponding to a fully relaxed InGaN film with an indium composition of 0.056.

The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes(LEDs) is limited due to their poor light extraction efficiency (LEE). Toimprove the LEE of AlGaN LEDs, we developed a fabrication technology to processAlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with highLEE. This process transfers the AlGaN LED epi onto a new substrate bywafer-to-wafer bonding, and by removing the absorbing SiC substrate with ahighly selective SF6 plasma etch that stops at the AlN buffer layer. Weoptimized the inductively coupled plasma (ICP) SF6 etch parameters to develop asubstrate-removal process with high reliability and precise epitaxial control,without creating micromasking defects or degrading the health of the plasmaetching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RFbias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etchselectivity between SiC and AlN. The high SF6 etch selectivity between SiC andAlN was essential for removing the SiC substrate and exposing a pristine,smooth AlN surface. We demonstrated the epi-transfer process by fabricatinghigh light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To furtherenhance the light extraction, the exposed N-face AlN was anisotropically etchedin dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening atroom temperature. This AlGaN TFFC LED process establishes a viable path to highexternal quantum efficiency (EQE) and power conversion efficiency (PCE) UV-CLEDs.

Micro-light-emitting diodes (µLEDs) are gathering significant interest as a technology for emerging micro-displays. However, µLEDs encounter numerous obstacles, including size-dependent efficiency loss, poor efficiency of red µLEDs, and challenges associated with the mass transfer and integration of full-color µLEDs. These issues become more acute in ultra-small µLEDs (<5 µm), which were required by the augmented reality (AR) displays. Here, we discuss the principal challenges faced by µLEDs and explore the possible solutions. We highlight recent advances in InGaN-based RGB µLEDs tailored for AR displays. In particular, we discuss the advancements in ultra-small InGaN µLEDs scaled down to 1 µm, the developments in InGaN red µLEDs, and the implementation of tunnel junction-based cascaded InGaN µLEDs for monolithic integration. [ABSTRACT FROM AUTHOR]

An advanced light-fidelity (Li-Fi) system based on the blue Gallium nitride (GaN) laser diode (LD) with a compact white-light phosphorous diffuser is demonstrated for fusing the indoor white-lighting and visible light communication (VLC). The phosphorous diffuser adhered blue GaN LD broadens luminescent spectrum and diverges beam spot to provide ample functionality including the completeness of Li-Fi feature and the quality of white-lighting. The phosphorous diffuser diverged white-light spot covers a radiant angle up to 120(o) with CIE coordinates of (0.34, 0.37). On the other hand, the degradation on throughput frequency response of the blue LD is mainly attributed to the self-feedback caused by the reflection from the phosphor-air interface. It represents the current state-of-the-art performance on carrying 5.2-Gbit/s orthogonal frequency-division multiplexed 16-quadrature-amplitude modulation (16-QAM OFDM) data with a bit error rate (BER) of 3.1 × 10(-3) over a 60-cm free-space link. This work aims to explore the plausibility of the phosphorous diffuser diverged blue GaN LD for future hybrid white-lighting and VLC systems.

We investigated the properties of indium-doped zinc oxide layers grown by metalorganic chemical vapor deposition on semi-insulating GaN(0001) templates. Specular and transparent films were grown with n-type carrier concentrations up to 1.82 × 1019 cm−3 as determined by Hall measurements, and all In-doped films had carrier concentrations significantly higher than that of a comparable undoped film. For low In flows, the carrier concentration increased accordingly with trimethyl-indium (TMIn) flow until a maximum carrier concentration of 1.82 × 1019 cm−3 was realized. For higher In flows, the carrier concentration decreased with increasing TMIn flow rate. Sheet resistance as low as 185 Ω/sq was achieved for the In-doped films, which is a significant decrease from that of a comparable undoped ZnO film. Our n-type doping studies show that In is an effective dopant for controlling the n-type conductivity of ZnO.

Microscopic photoluminescence was applied to investigate μm-order inhomogeneity of InGaN alloys. Samples had InGaN/GaN multiple-quantum-well structures grown on sapphire substrates at various temperatures, and luminescence was adjusted to be green. Luminescence morphologies of dendritic appearance were observed on as-grown samples. Bright spots luminescing at long wavelengths (green to amber) were formed at high growth temperatures. After annealing at 1000°C, the bright spots disappeared and the dendritic morphology turned into a granular morphology. Because of these μm-order inhomogeneities, it has been suggested that small-scale characterization (sub-μm or smaller) requires special attention in order not to miss effects of μm-order inhomogeneity in InGaN alloys.

We report long-cavity (60.5 λ) GaN-based vertical-cavity surface-emitting lasers with a topside monolithic GaN concave mirror, a buried tunnel junction current aperture, and a bottomside nanoporous GaN distributed Bragg reflector. Under pulsed operation, a VCSEL with a 9 µm aperture had a threshold current density of 6.6 kA/cm2, a differential efficiency of 0.7%, and a maximum output power of 290 µW for a lasing mode at 411 nm and a divergence angle of 8.4°. Under CW operation, the threshold current density increased to 7.3 kA/cm2, the differential efficiency decreased to 0.4%, and a peak output power of 130 µW was reached at a current density of 23 kA/cm2. [ABSTRACT FROM AUTHOR]