Thermally stable hybrid cavity laser based on silicon nitride gratings.

In this paper, we show the experimental results of a thermally stable Si3N4 external cavity (SiN EC) laser with high power output and the lowest SiN EC laser threshold to our knowledge. The device consists of a 250 µm sized reflective semiconductor optical amplifier butt-coupled to a passive chip based on a series of Si3N4 Bragg gratings acting as narrow reflectors. A threshold of 12 mA has been achieved, with a typical side-mode suppression ratio of 45 dB and measured power output higher than 3 mW. Furthermore, we achieved a mode-hop free-lasing regime in the range of 15-62 mA and wavelength thermal stability up to 80°C. This solves the challenges related to cavity resonances' thermal shift and shows the possibility for this device to be integrated in dense wavelength-division multiplexing (WDM) and heat-intensive optical interconnects technologies.

Breaking Lorentz reciprocity to overcome the time-bandwidth limit in physics and engineering.

A century-old tenet in physics and engineering asserts that any type of system, having bandwidth Δω, can interact with a wave over only a constrained time period Δt inversely proportional to the bandwidth (Δt·Δω ~ 2π). This law severely limits the generic capabilities of all types of resonant and wave-guiding systems in photonics, cavity quantum electrodynamics and optomechanics, acoustics, continuum mechanics, and atomic and optical physics but is thought to be completely fundamental, arising from basic Fourier reciprocity. We propose that this "fundamental" limit can be overcome in systems where Lorentz reciprocity is broken. As a system becomes more asymmetric in its transport properties, the degree to which the limit can be surpassed becomes greater. By way of example, we theoretically demonstrate how, in an astutely designed magnetized semiconductor heterostructure, the above limit can be exceeded by orders of magnitude by using realistic material parameters. Our findings revise prevailing paradigms for linear, time-invariant resonant systems, challenging the doctrine that high-quality resonances must invariably be narrowband and providing the possibility of developing devices with unprecedentedly high time-bandwidth performance.

A comparative study of tuberculosis patients initiated on ART and receiving different models of TB-HIV care.

SETTING: Although health policy in South Africa calls for the integration of services, the effectiveness of different models of integration on patient outcomes has not been well demonstrated. OBJECTIVE: To evaluate the outcomes of coinfected patients starting antiretroviral treatment (ART) in a tuberculosis (TB) hospital who received different models of ongoing care. DESIGN: This cohort study compared outcomes for 271 coinfected patients who started ART in a TB hospital in the Western Cape. After discharge, one group of patients received anti-tuberculosis treatment and ART from different providers, in the same or in different clinics (vertical care). The other group received anti-tuberculosis treatment and ART at the same visit from the same service provider (integrated care). Demographic and clinical data and TB and ART outcomes were compared. RESULTS: The vertical care model had more unfavourable outcomes for anti-tuberculosis treatment (28.7% vs. 5.9%, P < 0.001) and ART (30.1% vs. 7.4%, P < 0.001) than the integrated care model. The vertical care model showed no difference whether services were provided by two service providers in the same or in geographically separate primary health care clinics. CONCLUSION: Patient outcomes were better when TB and HIV care was received from the same service provider at the same visit.

Loss engineered slow light waveguides.

Slow light devices such as photonic crystal waveguides (PhCW) and coupled resonator optical waveguides (CROW) have much promise for optical signal processing applications and a number of successful demonstrations underpinning this promise have already been made. Most of these applications are limited by propagation losses, especially for higher group indices. These losses are caused by technological imperfections ("extrinsic loss") that cause scattering of light from the waveguide mode. The relationship between this loss and the group velocity is complex and until now has not been fully understood. Here, we present a comprehensive explanation of the extrinsic loss mechanisms in PhC waveguides and address some misconceptions surrounding loss and slow light that have arisen in recent years. We develop a theoretical model that accurately describes the loss spectra of PhC waveguides. One of the key insights of the model is that the entire hole contributes coherently to the scattering process, in contrast to previous models that added up the scattering from short sections incoherently. As a result, we have already realised waveguides with significantly lower losses than comparable photonic crystal waveguides as well as achieving propagation losses, in units of loss per unit time (dB/ns) that are even lower than those of state-of-the-art coupled resonator optical waveguides based on silicon photonic wires. The model will enable more advanced designs with further loss reduction within existing technological constraints.