Alex Bryant

Ultra-low-loss on-chip resonators with sub-milliwatt parametric oscillation threshold

On-chip optical resonators have the promise of revolutionizing numerous fields including metrology and sensing; however, their optical losses have always lagged behind their larger discrete resonator counterparts based on crystalline materials and flowable glass. Silicon nitride (Si3N4) ring resonators open up capabilities for optical routing, frequency comb generation, optical clocks and high precision sensing on an integrated platform. However, simultaneously achieving high quality factor and high confinement in Si3N4 (critical for nonlinear processes for example) remains a challenge. Here, we show that addressing surface roughness enables us to overcome the loss limitations and achieve high-confinement, on-chip ring resonators with a quality factor (Q) of 37 million for a ring with 2.5 {\mu}m width and 67 million for a ring with 10 {\mu}m width. We show a clear systematic path for achieving these high quality factors. Furthermore, we extract the loss limited by the material absorption in our films to be 0.13 dB/m, which corresponds to an absorption limited Q of at least 170 million by comparing two resonators with different degrees of confinement. Our work provides a chip-scale platform for applications such as ultra-low power frequency comb generation, high precision sensing, laser stabilization and sideband resolved optomechanics.

Synthesis and characterization of a BaGdF5:Tb glass ceramic as a nanocomposite scintillator for x-ray imaging.

Transparent glass ceramics with embedded light-emitting nanocrystals show great potential as low-cost nanocomposite scintillators in comparison to single crystal and transparent ceramic scintillators. In this study, cubic structure BaGdF5:Tb nanocrystals embedded in an aluminosilicate glass matrix are reported for potential high performance MeV imaging applications. Scintillator samples with systematically varied compositions were prepared by a simple conventional melt-quenching method followed by annealing. Optical, structural and scintillation properties were characterized to guide the design and optimization of selected material systems, aiming at the development of a system with higher crystal volume and larger crystal size for improved luminosity. It is observed that enhanced scintillation performance was achieved by tuning the glass matrix composition and using GdF3 in the raw materials, which served as a nucleation agent. A 26% improvement in light output was observed from a BaGdF5:Tb glass ceramic with addition of GdF3.

Laser shock compression induced crystallization of Ce3Al metallic glass

Laser shock compression studies on Ce3Al metallic glass performed using a 3 J Nd:YAG laser indicate shock-induced crystallization, evidenced by the presence of a two-wave/stepped particle velocity profile and structural changes observed via X-ray Diffraction (XRD) analysis of recovered material. A direct shock-compression setup was designed with 25 μm thick Ni driver foil, 40 μm thick Ce3Al metallic glass ribbon, and 3 mm thick poly(methyl methacrylate) (PMMA) backer window for use with input laser energies varying from 100 to 2000 mJ and corresponding estimated peak pressures of 1.4 to 4.1 GPa in Ce3Al. At shock pressures below ∼1.8 GPa (300 mJ laser input energy), samples were recovered showing no obvious deformation or structural changes evidenced via XRD analysis. At higher laser energies and shock pressures above the elastic limit, samples were recovered showing visible deformation and crystallization evidenced by Rietveld analysis of diffraction patterns. The corresponding velocity profiles also showed a stepped wave structure, increasing in magnitude with energy. The overall results reveal possible densification of the glass due to delocalization of 4f electrons in Ce at lower laser shock pressures and increased crystallization with preferred orientation and distortion of the nanocrystals at higher shock compression conditions.

Shock compression induced devitrification of amorphous Ce3Al melt-spun ribbons

Shock compression experiments were performed on amorphous Ce3Al melt-spun ribbons using the 50 J laser shock loading system at the Omega laser facility. A multi-layered sample of 2mm total thickness with 1 mm × 1.5 mm width and 40 µm thick ribbons sandwiched with 6 µm epoxy was used as the target in order to study the effects of varying pressures (due to attenuation) on the transformation behavior. The shock-induced changes were characterized post-mortem via synchrotron XRD analysis. Comparisons of the initial amorphous, thermally devitrified (at 500°C), and shock compressed samples indicate devitrification under high pressure shock compression into the thermodynamically stable hexagonal α-Ce3Al intermetallic phase, similar to the thermally devitrified samples.

High quality factor Si 3 N 4 ring resonators achieved by surface roughness reduction

We demonstrate high-confinement Si3N4 ring resonators with a quality factor of 15.6 million. We show that ultra-high quality factors are achievable by using a process that addresses surface roughness on all interfaces of the waveguides.