Person:

Hu, Evelyn

Thin-film InGaN photonic crystal (PhC) light-emitting diodes (LEDs) with a total semiconductor thickness of either 800 nm or 3.45 μm were fabricated and characterized. Increased directional radiance relative to Lambertian emission was observed for both cases. The 800-nm-thick PhC LEDs yielded only a slight improvement in total light output over the 3.45-μm-thick PhC LEDs. Simulations indicate that, except for ultrathin devices well below 800 nm, the balance between PhC extraction and metal absorption at the backside mirror results in modal extraction efficiencies that are almost independent of device thickness, but highly dependent on mirror reflectivity.

This letter reports on high extraction efficiency light-emitting diodes (LEDs) based on embedded two-dimensional air-gap photonic crystals (PhCs). High refractive index contrast provided by the air gaps along with high interaction of the embedded PhCs with the guided light resulted in an efficient extraction of all guided modes in the LED, in contrast to the common surface PhC configuration. Embedded PhC LEDs presented an enhanced directional light emission compared to non-PhC LEDs. High extraction efficiency, close to unity, provided by the encapsulated embedded PhC LEDs demonstrates the capability of this approach to achieve high efficiency devices with directional light emission.

Here we describe the fabrication and characterization of a plasmonic nanocavity formed in the narrow gap between a Ag nanowire and a flat Ag substrate. The fluorescence spectrum of nanocrystals within the gap was strongly modified by the cavity modes, showing peaks of position and width (Q ∼ 30–60) in quantitative agreement with numerical calculations. At gap spacings of ∼ 15 nm, the noncavity background fluorescence is largely quenched by the Ag substrate, while the modal fluorescence remains strong, indicating that gap-type structures are more robust to fluorescence quenching.

Highly soluble, non-aggregated colloidal wurtzite InN nanocrystals were obtained through an ambient pressure, low-temperature method followed by post-synthesis treatment with nitric acid.

The light extraction efficiency of photonic-crystal (PhC) light-emitting diodes (LEDs) relies on the competition between the PhC extraction and dissipation mechanisms of the guided light within the LED. This work presents the experimental determination of the PhC extraction length of each guided mode and the absorption coefficient of the active region (AR) and quantum wells (QWs) from the observation of the LED far-field emission using a high-resolution angle-spectrum-resolved measurement. The angular and spectral linewidths of the extracted guided modes reveal, depending on the spectral range, the modal extraction length of the PhCs, the AR absorption length, or a combination of both. Modes with a high confinement with the QWs presented a shorter absorption length compared with their extraction length by a shallow surface PhC (95-nm-deep), meaning that the AR absorption was a more efficient mechanism than the PhC extraction. The measured modal extraction length of the shallow surface PhC varied in the range of 55–120 μm, which determines the minimum dimensions of the device and the maximum acceptable dissipation length for an efficient extraction of the guided light by the PhCs. This paper presents also a discussion on the PhC designs that yield PhC extraction lengths shorter than other dissipation lengths, a fundamental requirement for high-efficiency PhC LEDs. The same technique was also applied to estimate the absorption coefficient of the InGaN-based QWs, and can be extended to experimentally determine losses by metallic layers from electrical contacts or other dissipation mechanisms, which are parameters of interest to a broader class of optoelectronic devices, not only PhC LEDs.

Metallic optical systems can confine light to deep subwavelength dimensions, but verifying the level of confinement at these length scales typically requires specialized techniques and equipment for probing the near field of the structure. We experimentally measured the confinement of a metal-based optical cavity by using the cavity modes themselves as a sensitive probe of the cavity characteristics. By perturbing the cavity modes with conformal dielectric layers of subnanometer thickness using atomic layer deposition, we find the exponential decay length of the modes to be less than 5% of the free-space wavelength ((\lambda)) and the mode volume to be of order (\lambda^3/1000). These results provide experimental confirmation of the deep subwavelength confinement capabilities of metal-based optical cavities.

We present a design of plasmonic cavities that consists of two sets of 1-D plasmonic crystal reflectors on a plasmonic trench waveguide. A 'reverse image mold' (RIM) technique was developed to pattern high-resolution silver trenches and to embed emitters at the cavity field maximum, and FDTD simulations were performed to analyze the frequency response of the fabricated devices. Distinct cavity modes were observed from the photoluminescence spectra of the organic dye embedded within these cavities. The cavity geometry facilitates tuning of the modes through a change in cavity dimensions. Both the design and the fabrication technique presented could be extended to making trench waveguide-based plasmonic devices and circuits.

Fabrication of devices designed to fully harness the unique properties of quantum mechanics through their coupling to quantum bits (qubits) is a prominent goal in the field of quantum information processing (QIP). Among various qubit candidates, nitrogen vacancy (NV) centers in diamond have recently emerged as an outstanding platform for room temperature QIP. However, formidable challenges still remain in processing diamond and in the fabrication of thin diamond membranes, which are necessary for planar photonic device engineering. Here we demonstrate epitaxial growth of single crystal diamond membranes using a conventional microwave chemical vapor deposition (CVD) technique. The grown membranes, only a few hundred nanometers thick, show bright luminescence, excellent Raman signature and good NV center electronic spin coherence times. Microdisk cavities fabricated from these membranes exhibit quality factors of up to 3000, overlapping with NV center emission. Our methodology offers a scalable approach for diamond device fabrication for photonics, spintronics, optomechanics and sensing applications.

We investigated the effect of fluorine-terminated diamond surface on the charged state of shallow nitrogen vacancy defect centers (NVs). Fluorination is achieved with (CF_4) plasma, and the surface chemistry is confirmed with x-ray photoemission spectroscopy. Photoluminescence of these ensemble NVs reveals that fluorine-treated surfaces lead to a higher and more stable negatively charged nitrogen vacancy ((NV^−)) population than oxygen-terminated surfaces. (NV^−) population is estimated by the ratio of negative to neutral charged NV zero-phonon lines. Surface chemistry control of (NV^−) density is an important step towards improving the optical and spin properties of NVs for quantum information processing and magnetic sensing.

InGaN-based active layers within microcavity resonators offer the potential of low threshold lasers in the blue spectral range. Here, we demonstrate optically pumped, room temperature lasing in high quality factor GaN microdisk cavities, containing InGaN quantum dots (QDs) with thresholds as low as (0.28 mJ/cm^2). The demonstration of lasing action from GaN microdisk cavities with QDs in the active layer, provides a critical step for the nitrides in realizing low threshold photonic devices with efficient coupling between QDs and an optical cavity.