Optical autocorrelator that uses a surface-emitting second-harmonic generator on (211)B GaAs

An optical autocorrelator grown on a (211)B GaAs substrate that uses visible surface-emitted second-harmonic generation is demonstrated. The (211)B orientation needs TE mode excitation only, thus eliminating the problem of the beating between the TE and TM modes that is required for (100)-grown devices; it also has the advantage of giving higher upconversion efficiency than (111) growth. Values of waveguide loss and the difference in the effective refractive index between the TE(0) and TE(1) modes were also obtained from the autocorrelation experiment

Sidelobes in the response of arrayed waveguide gratings caused by polarization rotation

Earlier it was observed that polarization rotation in an AWG built from birefringent waveguides can result in sidelobes in its response. This effect was measured in a polarization sensitive AWG with an orthogonal layout. Now we investigate through detailed simulation whether this effect also exists in polarization desensitised AWGs. It is shown that a dispersion compensated AWG does not suffer from a polarization sidelobe. Alternatively, the AWG can be designed to minimize polarization rotation to suppress the sidelobe

Whole-genome sequencing reveals host factors underlying critical COVID-19

Critical COVID-19 is caused by immune-mediated inflammatory lung injury. Host genetic variation influences the development of illness requiring critical care1 or hospitalization2,3,4 after infection with SARS-CoV-2. The GenOMICC (Genetics of Mortality in Critical Care) study enables the comparison of genomes from individuals who are critically ill with those of population controls to find underlying disease mechanisms. Here we use whole-genome sequencing in 7,491 critically ill individuals compared with 48,400 controls to discover and replicate 23 independent variants that significantly predispose to critical COVID-19. We identify 16 new independent associations, including variants within genes that are involved in interferon signalling (IL10RB and PLSCR1), leucocyte differentiation (BCL11A) and blood-type antigen secretor status (FUT2). Using transcriptome-wide association and colocalization to infer the effect of gene expression on disease severity, we find evidence that implicates multiple genes—including reduced expression of a membrane flippase (ATP11A), and increased expression of a mucin (MUC1)—in critical disease. Mendelian randomization provides evidence in support of causal roles for myeloid cell adhesion molecules (SELE, ICAM5 and CD209) and the coagulation factor F8, all of which are potentially druggable targets. Our results are broadly consistent with a multi-component model of COVID-19 pathophysiology, in which at least two distinct mechanisms can predispose to life-threatening disease: failure to control viral replication; or an enhanced tendency towards pulmonary inflammation and intravascular coagulation. We show that comparison between cases of critical illness and population controls is highly efficient for the detection of therapeutically relevant mechanisms of disease

Sidelobes in the response of arrayed waveguide gratings caused by polarization rotation

Earlier it was observed that polarization rotation in an AWG built from birefringent waveguides can result in sidelobes in its response. This effect was measured in a polarization sensitive AWG with an orthogonal layout. Now we investigate through detailed simulation whether this effect also exists in polarization desensitised AWGs. It is shown that a dispersion compensated AWG does not suffer from a polarization sidelobe. Alternatively, the AWG can be designed to minimize polarization rotation to suppress the sidelobe

Modulation of the second-order nonlinear tensor components in multiple-quantum-well structures

In this paper, we present experimental results which demonstrate that quantum-well intermixing techniques can be used to modulate the magnitude of the second-order nonlinear coefficient /spl chi//sup (2)/. Impurity-free vacancy disordering with SiO/sub 2/ and Ga/sub 2/O/sub 3/ caps was used to modulate the position of the band edge and hence, the magnitude of /spl chi//sub eff//sup (2)/. Using a coupled quantum-well structure we were able to demonstrate modulation of the d/sub 33/ tensor components associated with the asymmetric structure and of the d/sub 14/ component associated with the bulk crystal structure

Control of the second- and third-order nonlinearities in GaAs-AlGaAs multiple quantum wells

We report the use of impurity-free vacancy disordering techniques to control the nonlinear optical properties of GaAs–AlGaAs multiple quantum wells. These processes result in a shift in the position of the absorption edge to higher energy and have been used to modify the second- and third-order nonlinear coefficients. Working at photon energies just below the half bandgap we have observed a reduction of ∼ 50% in the value of nonlinear refractive coefficient n2 for a bandgap shift of around 40 nm. This change arises due to the combined effects of increasing the bandgap and increasing the detuning. The process can also result in a modulation in the magnitude of the second-order susceptibility χ(2) coefficient and provides a potential mechanism for realizing quasi-phase-matched structures

Quantum-well intermixing for the control of second-order nonlinear effects in AlGaAs multiple-quantum-well waveguides

We present experimental evidence to demonstrate the feasibility of a promising new quasi-phase-matching technique in AlGaAs multiple-quantum-well waveguides. Non-phase-matched second-harmonic-generation measurements indicate that, for sub-half-bandgap excitation near 1.5  µm, quantum-well intermixing by impurity-free vacancy disordering results in a reduction of the nonlinear susceptibility

Realization of phase grating comb reflectors and their application to widely tunable DBR lasers

Widely tunable InP lasers often require comb reflectors as an integral part of their design. It is desirable to achieve comb reflectors with as "top-hat" like a response as possible. There are several ways to do this, but one simple and practical way is by utilizing a phase grating. Previously these have been discussed theoretically, but here we will describe the fabrication of such gratings and how they can find application as the reflectors in widely tunable InP lasers. The example devices which we will discuss show that phase-grating-based distributed Bragg reflector lasers can have tuning ranges of around 35 nm combined with excellent sidemode suppression ratio of better than 45 dB at the mode centers

Modification of the second-order optical nonlinearities in AlGaAs asymmetric multiple quantum well waveguides by quantum well intermixing

We demonstrate that a quantum well intermixing technique can be used to control the second-order nonlinearity χ(2)zzz in an AlGaAs asymmetric coupled quantum well waveguide structure at 1.52 μm. Photoluminescence measurements also indicate that the spatial resolution of the impurity-free vacancy disordering process used for quantum well intermixing is better than 1.5 μm which should be sufficient for first-order quasiphase-matched second harmonic generation