Oana Malis

Improvement of second-harmonic generation in quantum-cascade lasers with true phase matching

About a 100-fold improvement of the second-harmonic generation in a quantum-cascade laser with integrated optical nonlinearity was obtained by including phase-matching considerations in the design of the deep-etched ridge waveguide. The waveguide layer structure was optimized to minimize the phase mismatch of the zero-order mode of the fundamental light with the second-order transverse mode of the second-harmonic light. Exact phase matching is made possible by the faster decrease of the modal refractive index of the fundamental light with decreasing ridge width relative to the refractive index of the second-harmonic light. Up to 240 μW of the second-harmonic power and a nonlinear power conversion efficiency of up to 36 mW/W2 were achieved.

Si(100) surface morphology evolution during normal-incidence sputtering with 100–500 eV Ar+ ions

Grazing incidence small-angle x-ray scattering and atomic force microscopy have been used to systematically investigate the evolution of Si(100) surface morphology during normal-incidence Ar+ sputtering as a function of ion energy in the range of 100–500 eV. For ion energy ranges of 100–300 eV, two structures with distinct individual length scales and behaviors form on the surface. There is a smaller scale (lateral size of 20–50 nm) morphology that grows in scattering intensity and coarsens with time. There is also a larger scale (lateral size of approximately 100 nm) morphology that grows in scattering intensity but does not coarsen significantly in the time scales studied. At higher energies (400–500 eV), sputtering causes the Si(100) surface to become smoother on length scales smaller than 200 nm.

Repeatable low-temperature negative-differential resistance from Al0.18Ga0.82N/GaN resonant tunneling diodes grown by molecular-beam epitaxy on free-standing GaN substrates

Low-aluminum composition AlGaN/GaN double-barrier resonant tunneling structures were grown by plasma-assisted molecular-beam-epitaxy on free-standing c-plane GaN substrates grown by hydride-vapor phase epitaxy. Clear, exactly reproducible, negative-differential resistance signatures were observed from 4 × 4 μm2 devices at 1.5 V and 1.7 V at 77 K. The relatively small value of the maximum peak-to-valley ratio (1.03) and the area dependence of the electrical characteristics suggest that charge transport is affected by leakage paths through dislocations. However, the reproducibility of the data indicates that electrical traps play no significant role in the charge transport in resonant tunneling diodes grown by molecular-beam-epitaxy under Ga-rich conditions on free-standing GaN substrates.

Terahertz intersubband absorption in non-polar m-plane AlGaN/GaN quantum wells

We demonstrate THz intersubband absorption (15.6–26.1 meV) in m-plane AlGaN/GaN quantum wells. We find a trend of decreasing peak energy with increasing quantum well width, in agreement with theoretical expectations. However, a blue-shift of the transition energy of up to 14 meV was observed relative to the calculated values. This blue-shift is shown to decrease with decreasing charge density and is, therefore, attributed to many-body effects. Furthermore, a ∼40% reduction in the linewidth (from roughly 8 to 5 meV) was obtained by reducing the total sheet density and inserting undoped AlGaN layers that separate the wavefunctions from the ionized impurities in the barriers.

Near-infrared intersubband absorption in molecular-beam epitaxy-grown lattice-matched InAlN/GaN superlattices

Strong near-infrared intersubband absorption is observed directly at room temperature in silicon-doped lattice-matched InAlN/GaN superlattices grown by molecular-beam epitaxy on GaN templates grown by hydride vapor-phase epitaxy. X-ray diffraction characterization of the heterostructures indicates excellent layer thickness uniformity and low interface roughness. For 2–4.5 nm quantum wells, the intersubband transition energies span the technologically relevant range between 2.3 and 2.9 μm. The experimental results are in good agreement with calculations of the transition energies using a conduction band offset of 1 eV and spontaneous polarization of 3 MV/cm.

Quasi-coherent thermal emitter based on refractory plasmonic materials

The thermal emission of refractory plasmonic metamaterial - a titanium nitride 1D grating - is studied at high operating temperature (540 {\deg}C). By choosing a refractory material, we fabricate thermal gratings with high brightness that are emitting mid-infrared radiation centered around 3

An in situ real-time x-ray diffraction study of phase segregation in Au–Pt nanoparticles

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Improvement of near-infrared absorption linewidth in AlGaN/GaN superlattices by optimization of delta-doping location

m. We demonstrate experimentally that the thermal excitation of plasmon-polariton on the surface of the grating produces a well-collimated beam with a spatial coherence length of 32{\lambda} (angular divergence of 1.8{\deg}) which is quasi-monochromatic with a full width at half maximum of 70 nm. These experimental results show good agreement with a numerical model based on a two-dimensional full-wave analysis in frequency domain.

Surface morphology evolution of m-plane (11¯00) GaN during molecular beam epitaxy growth: Impact of Ga/N ratio, miscut direction, and growth temperature

In situ real-time x-ray diffraction was used to study phase segregation and coarsening of Au-Pt nanoparticles supported on silica powder, and porous alumina membranes. Contrary to the expectations from the bulk phase diagram, silica supported Au-Pt nanoparticles have an alloyed structure that is preserved even after extensive annealing at temperatures as high at 700 degrees C. In stark contrast, alumina supported Au-Pt nanoparticles exhibit a rich phase behaviour that is sensitive to alloy composition and the details of the synthesis process. In particular, low-density as-prepared Au(41)Pt(59) nanoparticles exhibit the signature of incipient phase segregation that develops into full phase separation during annealing at high temperature.

Homogeneous AlGaN/GaN superlattices grown on free-standing (11¯00) GaN substrates by plasma-assisted molecular beam epitaxy

We report a systematic study of the near-infrared intersubband absorption in AlGaN/GaN superlattices grown by plasma-assisted molecular-beam epitaxy as a function of Si-doping profile with and without δ-doping. The transition energies are in agreement with theoretical calculations including many-body effects. A dramatic reduction of the intersubband absorption linewidth is observed when the δ-doping is placed at the end of the quantum well. This reduction is attributed to the improvement of interface roughness. The linewidth dependence on interface roughness is well reproduced by a model that considers the distribution of well widths measured with transmission electron microscopy.