Resonant gratings with an etch-stop layer and a fabrication-error tolerant design.

Sub-wavelength gratings (SWG) have shown much promise for applications such as lightweight high bandwidth reflectors, polarising filters and focusing lenses. Unfortunately, grating performance may be rapidly degraded through variability in grating dimensions. We demonstrate, in particular, how an error in depth of etch can be detrimental to the performance of zero contrast grating reflectors. We mitigate the impact of this fabrication error through the introduction of an etch stop layer and in so doing we experimentally realise a high bandwidth reflector based on this modified structure. Another common fabrication error is variation in the duty-cycle of fabricated gratings. This duty-cycle variation can weaken grating performance, however we demonstrate that grating designs that exhibit tolerance to duty-cycle fluctuation can be identified through simulation. Finally, we discuss the impact of lateral etching and the resulting sidewall concavity. We present our approach for numerically predicting the spectral response from such a grating and also for convenience we outline an approach for quickly approximating grating performance. Good agreement is observed between these numerical predictions and measurements made on a HCG with concave sidewalls.

Monolithic InGaAs Nanowire Array Lasers on Silicon-on-Insulator Operating at Room Temperature.

Chip-scale integrated light sources are a crucial component in a broad range of photonics applications. III-V semiconductor nanowire emitters have gained attention as a fascinating approach due to their superior material properties, extremely compact size, and capability to grow directly on lattice-mismatched silicon substrates. Although there have been remarkable advances in nanowire-based emitters, their practical applications are still in the early stages due to the difficulties in integrating nanowire emitters with photonic integrated circuits. Here, we demonstrate for the first time optically pumped III-V nanowire array lasers monolithically integrated on silicon-on-insulator (SOI) platform. Selective-area growth of InGaAs/InGaP core/shell nanowires on an SOI substrate enables the nanowire array to form a photonic crystal nanobeam cavity with superior optical and structural properties, resulting in the laser to operate at room temperature. We also show that the nanowire array lasers are effectively coupled with SOI waveguides by employing nanoepitaxy on a prepatterned SOI platform. These results represent a new platform for ultracompact and energy-efficient optical links and unambiguously point the way toward practical and functional nanowire lasers.

Realization of high-contrast gratings operating at 10 µm.

We present a new material pairing that can be used to realize high-contrast gratings at wavelengths of 10 µm and greater. Using only optical lithography, the material pair solves the absorption issue limiting the popular Si/SiO2 pairing from operation above 6 µm. We describe the obstacles that exist with the currently used grating materials for this wavelength range and outline why our chosen materials overcome this obstacle. We numerically demonstrate that gratings utilizing these materials are capable of wideband high reflectivity. We experimentally show that the spectral response of gratings that are fabricated using such a process show good agreement with theoretically predicted performance.

Heterogeneously-Grown Tunable Tensile Strained Germanium on Silicon for Photonic Devices.

The growth, structural and optical properties, and energy band alignments of tensile-strained germanium (ε-Ge) epilayers heterogeneously integrated on silicon (Si) were demonstrated for the first time. The tunable ε-Ge thin films were achieved using a composite linearly graded InxGa1-xAs/GaAs buffer architecture grown via solid source molecular beam epitaxy. High-resolution X-ray diffraction and micro-Raman spectroscopic analysis confirmed a pseudomorphic ε-Ge epitaxy whereby the degree of strain varied as a function of the In(x)Ga(1-x)As buffer indium alloy composition. Sharp heterointerfaces between each ε-Ge epilayer and the respective In(x)Ga(1-x)As strain template were confirmed by detailed strain analysis using cross-sectional transmission electron microscopy. Low-temperature microphotoluminescence measurements confirmed both direct and indirect bandgap radiative recombination between the Γ and L valleys of Ge to the light-hole valence band, with L-lh bandgaps of 0.68 and 0.65 eV demonstrated for the 0.82 ± 0.06% and 1.11 ± 0.03% strained Ge on Si, respectively. Type-I band alignments and valence band offsets of 0.27 and 0.29 eV for the ε-Ge/In(0.11)Ga(0.89)As (0.82%) and ε-Ge/In(0.17)Ga(0.83)As (1.11%) heterointerfaces, respectively, show promise for ε-Ge carrier confinement in future nanoscale optoelectronic devices. Therefore, the successful heterogeneous integration of tunable tensile-strained Ge on Si paves the way for the design and implementation of novel Ge-based photonic devices on the Si technology platform.

GaSb/GaAs type-II quantum dots grown by droplet epitaxy.

We demonstrate the formation of GaSb quantum dots (QDs) on a GaAs(001) substrate by droplet epitaxy using molecular beam epitaxy. The high crystal quality and bimodal size distribution of the QDs are confirmed using atomic force and transmission electron microscope images. A staggered type-II QD band structure is suggested by a photoluminescence peak that is blue shifted with increasing excitation intensity, a large emission polarization of 60%, and a long carrier decay time of 11.5 ns. Our research provides a different approach to fabricating high quality GaSb type-II QDs.