Superchiral Light Emerging from Bound States in the Continuum in Metasurfaces of Si Nanorod Dimers.

Bound states in the continuum (BICs) in all-dielectric metasurfaces enhance light-matter interaction at the nanoscale due to their infinite Q factors and strong field confinement. Among a variety of phenomena already reported, their impact on chiral light has recently attracted great interest. Here we investigate the emergence of intrinsic and extrinsic optical chirality associated with the excitation of BICs in various metasurfaces made of Si nanorod dimers on a quartz substrate, comparing three cases: parallel nanorods (neutral) and shifted and slanted dimers, with/without index-matching superstrate. We analyze both the circular dichroism (CD) of the far-field (FF) interaction and the helicity of the near-field (NF) distribution. We show that the best approach to achieve chiral response in the FF based on extrinsic chirality is to exploit quasi-BICs (q-BICs) appearing in the case of slanted nanorod dimers. By contrast, the helicity density is largely enhanced in the case of shifted dimers, as it presents intrinsic chirality, with values 2 orders of magnitude larger than those of circularly polarized plane waves. These so-called superchiral electromagnetic fields concentrated at the nanoscale within the metasurface hold promise of appealing implications in phenomena such as strong-coupling, photoluminescence emission, or other local light-matter interactions.

Composition, Optical Resonances, and Doping of InP/InGaP Nanowires for Tandem Solar Cells: a Micro-Raman Analysis.

We present a micro-Raman study of InP/InGaP tandem junction photovoltaic nanowires. These nanowires render possible InGaP compositions that cannot be made in thin films due to strain. The micro-Raman spectra acquired along the nanowires reveal the existence of compositional changes in the InGaP alloy associated with the doping sequence. The heavily Zn-doped InxGa1-xP (x is the In molar fraction) side of the tunnel diode is Ga rich, x = 0.25, with respect to the n-type and intrinsic segments of the top cell, which are close to the nominal composition of the NWs (x = 0.35). The p-type end segment is still Ga-rich. Electromagnetic resonances are observed in the tunnel diode. The Raman signal arising from the InGaP side of the tunnel diode is significantly enhanced. This enhancement permits the observation of a Raman mode that can be associated with an LO phonon plasmon coupled mode (LOPCM). This mode has not been previously reported in the literature of InGaP, and it permits the Raman characterization of the tunnel diode. The analysis of this mode and its relation to the LO phonon modes of the alloy, InP-like and GaP-like, allows to establish an apparent one-mode behavior for the phonon plasmon coupling. It indicates that hole plasma couples to the GaP-like LO mode. The LOPCMs are modeled using the Lindhard Mermin formalism for the dielectric function.

Room Temperature Exciton-Polariton Condensation in Silicon Metasurfaces Emerging from Bound States in the Continuum.

We show the first experimental demonstration of room-temperature exciton-polariton (EP) condensation from a bound state in the continuum (BIC). This demonstration is achieved by strongly coupling stable excitons in an organic perylene dye with the extremely long-lived BIC in a dielectric metasurface of silicon nanoparticles. The long lifetime of the BIC, mainly due to the suppression of radiation leakage, allows for EP thermalization to the ground state before decaying. This property results in a condensation threshold of less than 5 µJ cm-2, 1 order of magnitude lower than the lasing threshold reported in similar systems in the weak coupling limit.

Catastrophic optical damage in 808†nm broad area laser diodes: a study of the dark line defect propagation.

We present a study of the propagation of dark line defects (DLDs) in catastrophically damaged 808 nm laser diodes, based on cathodoluminescence (CL) measurements and laser mode propagation simulations. Room temperature CL images show blurred DLDs running parallel to the laser cavity. Remarkably, low temperature images reveal their true morphology: the blurred lines are resolved as parallel narrow discontinuous DLDs. This morphology does not match the usually reported molten front scenario of DLD propagation. Low temperature images show that DLDs consist of a sequence of catastrophic optical damage (COD) events separated a few micrometers from each other. Consequently, a different propagation scheme is proposed. The points where the CODs occur suffer a temperature increase and these hot spots play a capital role in the propagation of the DLDs. Their influence on the beam distribution is modelled using finite element methods. The calculations evidence changes on the intensity distribution of the laser that qualitatively reproduce the DLD shapes. Additionally, the COD events result in the generation of defects in the region that surrounds them. The successive CODs in the discontinuous DLDs are rationalized in terms of the enhanced laser absorption in these sensitized regions where the laser beam is concentrated by thermal lensing.

Tailoring Polarization Conversion in Achiral All-Dielectric Metasurfaces by Using Quasi-Bound States in the Continuum.

Quasi-bound states in the continuum (quasi-BICs) supported in all-dielectric metasurfaces (MTS) are known for their confinement in real space and the notably high values of the quality factor Q. Recently, the properties of quasi-BICs have been employed to achieve polarization conversion with all-dielectric MTS. However, one of the main disadvantages of the current approaches is the dependence on the chirality of either the meta-atoms or their disposition. We present the possibility of achieving polarization conversion by using all-dielectric MTS with square and rectangular lattices of nano-disks. The precise tuning of the lattice and disks parameters allows to transform linearly polarized light into circularly polarized light with near unity polarization rates while maintaining the high Q values of quasi-BICs. Moreover, by using double accidental BICs it is possible to obtain right and left circularly polarized light on demand just by varying the angle of incidence.

Effect of thermal lensing and the micrometric degraded regions on the catastrophic optical damage process of high-power laser diodes.

Catastrophic optical damage (COD) is one of the processes limiting the lifetime of high-power laser diodes. The understanding of this degradation phenomenon is critical to improve the laser power and lifetime for practical applications. In this Letter, we analyze the defect propagation inside the cavity of quantum well (QW) high-power laser diodes presenting COD. For this, we studied the effect of highly localized thermal gradients and degraded regions on the laser field distribution. Finite element method (FEM) simulations are compared to experimental cathodoluminescence (CL) measurements. The presence of micrometric hot spots inside the QW induces the thermal lensing of the laser field. The laser self-focusing inside the cavity eventually generates a new hot spot, and, in a repetitive way, a sequence of hot spots would be created. This would account for the propagation of the dark line defects (DLDs) that are characteristic of this degradation mode.

Local electric field enhancement at the heterojunction of Si/SiGe axially heterostructured nanowires under laser illumination.

We present a phenomenon concerning electromagnetic enhancement at the heterojunction region of axially heterostructured Si/SiGe nanowires when the nanowire is illuminated by a focused laser beam. The local electric field is sensed by micro Raman spectroscopy, which allows the enhancement of the Raman signal arising from the heterojunction region to be revealed; the Raman signal per unit volume increases at least ten times with respect to the homogeneous Si and SiGe nanowire segments. In order to explore the physical meaning of this phenomenon, a three-dimensional solution of the Maxwell equations of the interaction between the focused laser beam and the nanowire was carried out by finite element methods. A local enhancement of the electric field at the heterojunction was deduced. However, the magnitude of the electromagnetic field enhancement only approaches the experimental one when the free carriers are considered, showing enhanced absorption at the carrier depleted heterojunction region. The existence of this effect promises a way of improving photon harvesting using axially heterostructured semiconductor nanowires.