Distinctive signature of indium gallium nitride quantum dot lasing in microdisk cavities.

Low-threshold lasers realized within compact, high-quality optical cavities enable a variety of nanophotonics applications. Gallium nitride materials containing indium gallium nitride (InGaN) quantum dots and quantum wells offer an outstanding platform to study light-matter interactions and realize practical devices such as efficient light-emitting diodes and nanolasers. Despite progress in the growth and characterization of InGaN quantum dots, their advantages as the gain medium in low-threshold lasers have not been clearly demonstrated. This work seeks to better understand the reasons for these limitations by focusing on the simpler, limited-mode microdisk cavities, and by carrying out comparisons of lasing dynamics in those cavities using varying gain media including InGaN quantum wells, fragmented quantum wells, and a combination of fragmented quantum wells with quantum dots. For each gain medium, we use the distinctive, high-quality (Q ∼ 5,500) modes of the cavities, and the change in the highest-intensity mode as a function of pump power to better understand the dominant radiative processes. The variations of threshold power and lasing wavelength as a function of gain medium help us identify the possible limitations to lower-threshold lasing with quantum dot active medium. In addition, we have identified a distinctive lasing signature for quantum dot materials, which consistently lase at wavelengths shorter than the peak of the room temperature gain emission. These findings not only provide better understanding of lasing in nitride-based quantum dot cavity systems but also shed insight into the more fundamental issues of light-matter coupling in such systems.This is the author's accepted manuscript. The final version is available from PNAS at http://www.pnas.org/content/111/39/14042.abstract

Long stokes shifts and vibronic couplings in perfluorinated polyanilines

We report the effect of surfactant addition on the optical properties of perfluorinated polyanilines synthesized through liquid–liquid interfaces. We obtained very long Stokes shifts, 205 nm, for oligomers derived from a hydrofluoroether–water system in the presence of Triton X-100 as a surfactant, and vibronic fine features from a toluene-water system

Deterministic optical polarisation in nitride quantum dots at thermoelectrically cooled temperatures.

We report the successful realisation of intrinsic optical polarisation control by growth, in solid-state quantum dots in the thermoelectrically cooled temperature regime (≥200 K), using a non-polar InGaN system. With statistically significant experimental data from cryogenic to high temperatures, we show that the average polarisation degree of such a system remains constant at around 0.90, below 100 K, and decreases very slowly at higher temperatures until reaching 0.77 at 200 K, with an unchanged polarisation axis determined by the material crystallography. A combination of Fermi-Dirac statistics and k·p theory with consideration of quantum dot anisotropy allows us to elucidate the origin of the robust, almost temperature-insensitive polarisation properties of this system from a fundamental perspective, producing results in very good agreement with the experimental findings. This work demonstrates that optical polarisation control can be achieved in solid-state quantum dots at thermoelectrically cooled temperatures, thereby opening the possibility of polarisation-based quantum dot applications in on-chip conditions

Direct generation of linearly polarized single photons with a deterministic axis in quantum dots

We report the direct generation of linearly polarized single photons with a deterministic polarization axis in self-assembled quantum dots (QDs), achieved by the use of non-polar InGaN without complex device geometry engineering. Here, we present a comprehensive investigation of the polarization properties of these QDs and their origin with statistically significant experimental data and rigorous k·p modeling. The experimental study of 180 individual QDs allows us to compute an average polarization degree of 0.90, with a standard deviation of only 0.08. When coupled with theoretical insights, we show that these QDs are highly insensitive to size differences, shape anisotropies, and material content variations. Furthermore, 91% of the studied QDs exhibit a polarization axis along the crystal [1–100] axis, with the other 9% polarized orthogonal to this direction. These features give non-polar InGaN QDs unique advantages in polarization control over other materials, such as conventional polar nitride, InAs, or CdSe QDs. Hence, the ability to generate single photons with polarization control makes non-polar InGaN QDs highly attractive for quantum cryptography protocols

Ultrafast, Polarized, Single-Photon Emission from m-Plane InGaN Quantum Dots on GaN Nanowires.

We demonstrate single-photon emission from self-assembled m-plane InGaN quantum dots (QDs) embedded on the side-walls of GaN nanowires. A combination of electron microscopy, cathodoluminescence, time-resolved microphotoluminescence (μPL), and photon autocorrelation experiments give a thorough evaluation of the QD structural and optical properties. The QD exhibits antibunched emission up to 100 K, with a measured autocorrelation function of g(2)(0) = 0.28(0.03) at 5 K. Studies on a statistically significant number of QDs show that these m-plane QDs exhibit very fast radiative lifetimes (260 ± 55 ps) suggesting smaller internal fields than any of the previously reported c-plane and a-plane QDs. Moreover, the observed single photons are almost completely linearly polarized aligned perpendicular to the crystallographic c-axis with a degree of linear polarization of 0.84 ± 0.12. Such InGaN QDs incorporated in a nanowire system meet many of the requirements for implementation into quantum information systems and could potentially open the door to wholly new device concepts

Structure-activity correlations for Brønsted acid, Lewis Acid, and photocatalyzed reactions of exfoliated crystalline niobium oxides

Exfoliated crystalline niobium oxides that contain exposed but interconnected NbO6 octahedra with different degrees of structural distortion and defects are known to catalyze Brønsted acid (BA), Lewis acid (LA), and photocatalytic (PC) reactions efficiently but their structure–activity relationships are far from clear. Here, three exfoliated niobium oxides, namely, HSr2Nb3O10, HCa2Nb3O10, and HNb3O8, are synthesized, characterized extensively, and tested for selected BA, LA, and PC reactions. The structural origin for BA is associated mainly with acidic hydroxyl groups of edge-shared NbO6 octahedra as proton donors; that of LA is associated with the vacant band position of Nb5+ to receive electron pairs from substrate; and that of PC is associated with the terminal Nb=O of NbO6 octahedra for photon capture and charge transfer to long-lived surface adsorbed substrate complex through associated oxygen vacancies in close proximity. It is believed that an understanding of the structure–activity relationships could lead to the tailored design of NbOx catalysts for industrially important reactions

P3_2_The Space Elevator

This paper examines the feasibility for construction of a space elevator through simple physical analysis of a basic model. Beyond this consideration is made for further development of the model with the hope of ultimately creating a successful simulation which can be applied to a real future setting

P3_ 1_Saving The Earth

Each year hundreds of asteroids will impact the Earth, the majority simply burn up in the atmosphere while those reaching the surface cause only localised damage at the point of impact. However, were an asteroid of significant size (>1km in diameter) to impact the Earth there would be disastrous climatic changes to the whole globe which could conceivably result in the deaths of a large population of the Human race, or possibly even extinction. With this in mind H. Teller proposes a feasibility study for the deflection of large asteroids that are on a collision course with Earth using nuclear weapons (JST Vol. 7 No. 1, 2008)