Highly Localized Surface Plasmon Nanolasers via Strong Coupling.

Surface plasmons have robust and strong confinement to the light field which is beneficial for the light-matter interaction. Surface plasmon amplification by stimulated emission of radiation (SPACER) has the potential to be integrated on the semiconductor chip as a compact coherent light source, which can play an important role in further extension of Moore's law. In this study, we demonstrate the localized surface plasmon lasing at room temperature in the communication band using metallic nanoholes as the plasmonic nanocavity and InP nanowires as the gain medium. Optimizing laser performance has been demonstrated by coupling between two metallic nanoholes which adds another degree of freedom for manipulating the lasing properties. Our plasmonic nanolasers exhibit lower power consumption, smaller mode volumes, and higher spontaneous emission coupling factors due to enhanced light-matter interactions, which are very promising in the applications of high-density sensing and photonic integrated circuits.

Electrically programmable solid-state metasurfaces via flash localised heating.

In the last decades, metasurfaces have attracted much attention because of their extraordinary light-scattering properties. However, their inherently static geometry is an obstacle to many applications where dynamic tunability in their optical behaviour is required. Currently, there is a quest to enable dynamic tuning of metasurface properties, particularly with fast tuning rate, large modulation by small electrical signals, solid state and programmable across multiple pixels. Here, we demonstrate electrically tunable metasurfaces driven by thermo-optic effect and flash-heating in silicon. We show a 9-fold change in transmission by <5 V biasing voltage and the modulation rise-time of <625 µs. Our device consists of a silicon hole array metasurface encapsulated by transparent conducting oxide as a localised heater. It allows for video frame rate optical switching over multiple pixels that can be electrically programmed. Some of the advantages of the proposed tuning method compared with other methods are the possibility to apply it for modulation in the visible and near-infrared region, large modulation depth, working at transmission regime, exhibiting low optical loss, low input voltage requirement, and operating with higher than video-rate switching speed. The device is furthermore compatible with modern electronic display technologies and could be ideal for personal electronic devices such as flat displays, virtual reality holography and light detection and ranging, where fast, solid-state and transparent optical switches are required.

CuI as a Hole-Selective Contact for GaAs Solar Cells.

Carrier-selective contacts have emerged as a promising architecture for solar cell fabrication. In this report, the first hole-selective III-V semiconductor solar cell is demonstrated using copper iodide (CuI) on i-GaAs. Surface passivation quality of GaAs is found to be essential for open-circuit voltage (VOC), with good correlation between photoluminescence properties of the GaAs layer and the VOC. Passivation with <10 nm thick In0.49Ga0.51P layers is shown to provide an over 300 mV improvement. Oxygen-rich CuI is formed by natural oxidation in the atmosphere, and the increased oxygen content of ∼10% is validated by energy-dispersive X-ray measurements. The oxygen incorporation is shown to improve hole selectivity and thus solar conversion efficiency. Ultraviolet photoelectron spectroscopy indicates a high work function of ∼6 eV for the oxygen-rich CuI. With optimized GaAs surface passivation and oxygen-rich CuI, a VOC of nearly 1 V and a solar conversion efficiency of 13.4% are achieved. The solar cell structure includes only undoped GaAs, a surface passivation layer, and non-epitaxial CuI contact and is therefore very promising to various low-cost crystal growth methods. The results have a significant impact on III-V solar cell fabrication and costs as it (i) enables fully carrier-selective architectures, (ii) reduces cell fabrication complexity, and (iii) is suitable for layers grown by low-cost crystal growth techniques.

Thin SnxNiyOz Films as p-Type Transparent Conducting Oxide and Their Application in Light-Emitting Diodes.

The development of good-quality p-type transparent conducting oxides (TCOs) is essential to realize the full potential of TCOs for transparent electronics. This study investigates various optical and electrical properties of SnxNiyOz under different deposition conditions to achieve high-performance p-type TCOs. We found that a film with 20% O2/Ar deposited at room temperature exhibits the highest p-type conductivity with a carrier concentration of 2.04 × 1017 cm-3, a resistivity of 14.01 Ωcm, and a Hall mobility of 7.7 cm2 V-1 S-1. We also studied the elemental properties of a SnxNiyOz film and the band alignment at the SnxNiyOz/InP interface and found reasonably large values of the conduction band offset (CBO) and valence band offset (VBO). Finally, we demonstrate stable light-emitting diodes (LEDs) with n-InP nanowires (NWs) conformably coated with a p-SnxNiyOz structure. Several films and devices were fabricated and tested over a span of 6 months, and we observed similar characteristics. This confirms the stability and reliability of the films as well as the reproducibility of the LEDs. We also investigated the temperature-dependent behavior of these LEDs and observed an additional peak due to a zinc blende/wurtzite (ZB/WZ) transition at the InP substrate and NW interface at ∼98 K and below. This study provides promising results of SnxNiyOz as a potential p-type TCO candidate for applications in electronics and optoelectronics.

Flexible InP-ZnO nanowire heterojunction light emitting diodes.

Flexible, substrate-free nanowire (NW) devices are desirable to overcome the extremely challenging task of integrating III-V or III-N semiconductor devices such as LEDs and lasers on a range of optoelectronic circuits or biochips. In this work, we report the demonstration of core-shell p-InP/n-ZnO heterojunction NW array LEDs. The emission from the devices consists of three peaks at room temperature due to conduction band-to-heavy hole band transition, conduction band-to-light hole band transition and recombination at the substrate. At 78 K, an additional peak due to Zn acceptor levels is observed, whereas the peak due to the conduction band-to-light hole band transition quenches. Flexible LEDs are then fabricated by embedding the NW arrays in SU-8 to enable subsequent lift-off from the substrate. Compared with the original on-substrate LED device, broader, red-shifted and multiple peaks are observed from the flexible devices, which may be due to non-uniform strain related effects in the NWs caused by the SU-8 film. A slightly higher series resistance as compared to the on-substrate device and significant Joule heating suggest that good heatsinking is required for these flexible devices. Nevertheless, our study paves a promising way towards flexible and low power LEDs.

Ultralow Threshold, Single-Mode InGaAs/GaAs Multiquantum Disk Nanowire Lasers.

We present single-mode nanowire (NW) lasers with an ultralow threshold in the near-infrared spectral range. To ensure the single-mode operation, the NW diameter and length are reduced specifically to minimize the longitudinal and transverse modes of the NW cavity. Increased optical losses and reduced gain volume by the dimension reduction are compensated by an excellent NW morphology and InGaAs/GaAs multiquantum disks. At 5 K, a threshold low as 1.6 µJ/cm2 per pulse is achieved with a resulting quality factor exceeding 6400. By further passivating the NW with an AlGaAs shell to suppress surface nonradiative recombination, single-mode lasing operation is obtained with a threshold of only 48 µJ/cm2 per pulse at room temperature with a high characteristic temperature of 223 K and power output of ∼0.9 µW. These single-mode, ultralow threshold, high power output NW lasers are promising for the development of near-infrared nanoscale coherent light sources for integrated photonic circuits, sensing, and spectroscopy.

Understanding the role of facets and twin defects in the optical performance of GaAs nanowires for laser applications.

GaAs nanowires are regarded as promising building blocks of future optoelectronic devices. Despite progress, the growth of high optical quality GaAs nanowires is a standing challenge. Understanding the role of twin defects and nanowire facets on the optical emission and minority carrier lifetime of GaAs nanowires is key for the engineering of their optoelectronic properties. Here, we present new insights into the microstructural parameters controlling the optical properties of GaAs nanowires, grown via selective-area metal-organic vapor-phase epitaxy. We observe that these GaAs nanowires have a twinned zinc blende crystal structure with taper-free {110} side facets that result in an ultra-low surface recombination velocity of 3.5 × 104 cm s-1. This is an order of magnitude lower than that reported for defect-free GaAs nanowires grown by the vapor-liquid-solid technique. Using time-resolved photoluminescence and cathodoluminescence measurements, we untangle the local correlation between structural and optical properties demonstrating the superior role of the side facets in determining recombination rates over that played by twin defects. The low surface recombination velocity of these taper-free {110} side facets enable us to demonstrate, for the first time, low-temperature lasing from bare (unpassivated) GaAs nanowires, and also efficient room-temperature lasing after passivation with an AlGaAs shell.

Multiwavelength Single Nanowire InGaAs/InP Quantum Well Light-Emitting Diodes.

We report multiwavelength single InGaAs/InP quantum well nanowire light-emitting diodes grown by metal organic chemical vapor deposition using selective area epitaxy technique and reveal the complex origins of their electroluminescence properties. We observe that the single InGaAs/InP quantum well embedded in the nanowire consists of three components with different chemical compositions, axial quantum well, ring quantum well, and radial quantum well, leading to the electroluminescence emission with multiple wavelengths. The electroluminescence measurements show a strong dependence on current injection levels as well as temperatures and these are explained by interpreting the equivalent circuits in a minimized area of the device. It is also found that the electroluminescence properties are closely related to the distinctive triangular morphology with an inclined facet of the quantum well nanowire. Our study provides important new insights for further design, growth, and fabrication of high-performance quantum well-based nanowire light sources for a wide range of future optoelectronic and photonic applications.