Jeffrey M. Shainline

Enhanced photoluminescence from nanopatterned carbon-rich silicon grown by solid-phase epitaxy

Photoluminescence from the dicarbon G center in nanopatterned silicon is investigated. Enrichment of silicon with carbon atoms has been achieved by solid-phase epitaxy. When this carbon enrichment is combined with nanopatterning, The authors found a 33-fold increase in the G line emission intensity to a level that is comparable to the band-edge photoluminescence. Significant linewidth broadening is observed and is attributed to lattice strain associated with the nanopatterning.

Observation of giant positive magnetoresistance in a Cooper pair insulator.

Ultrathin amorphous Bi films, patterned with a nanohoneycomb array of holes, can exhibit an insulating phase with transport dominated by the incoherent motion of Cooper pairs (CP) of electrons between localized states. Here, we show that the magnetoresistance (MR) of this Cooper pair insulator (CPI) phase is positive and grows exponentially with decreasing temperature T, for T well below the pair formation temperature. It peaks at a field estimated to be sufficient to break the pairs and then decreases monotonically into a regime in which the film resistance assumes the T dependence appropriate for weakly localized single electron transport. We discuss how these results support proposals that the large MR peaks in other unpatterned, ultrathin film systems disclose a CPI phase and provide new insight into the CP localization.

Subwavelength silicon microcavities.

We present a study of the first silicon microdisk resonators which are smaller than the free-space resonant wavelength in all spatial dimensions. Spectral details of whispering gallery modes with azimuthal mode number m = 4-7 are measured in microdisks with diameters between 1.35 and 1.89microm and are studied at wavelengths from 1.52 to 1.62microm. For the structures considered here, m = 5 is the highest azimuthal mode order in a subwavelength cavity and has measured Q = 1250. These results agree well with theoretical calculations using a finite difference frequency domain method and fit an exponential scaling law relating Q to disk radius via m.

Cooper-pair insulator phase in superconducting amorphous Bi films induced by nanometer-scale thickness variations

Unusual transport properties of superconducting (SC) materials, such as the under doped cuprates, low dimensional superconductors in strong magnetic fields, and insulating films near the Insulator Superconductor Transition (IST), have been attributed to the formation of inhomogeneous phases. Difficulty correlating the behaviors with observations of the inhomogeneities make these connections uncertain. Of primary interest here are proposals that insulating films near the IST, which show an activated resistance and giant positive magnetoresistance, contain islands of Cooper Pairs (CPs). Here we present evidence that these types of inhomogeneities are essential to such an insulating phase in amorphous Bi (a-Bi) films deposited on substrates patterned with nanometer-sized holes. The patterning induces film thickness variations, and corresponding coupling constant variations, that transform the composition of the insulator from localized electrons to CPs. Analyses near the thickness-tuned ISTs of films on nine different substrates show that weak links between SC islands dominate the transport. In particular, the ISTs all occur when the link resistance approaches the resistance quantum for pairs. These observations lead to a detailed picture of CPs localized by spatial variations of the superconducting coupling constant.

Continuous-wave subwavelength microdisk lasers at λ = 1.53 µm

Subwavelength InGaAs/AlInAs microdisk lasers are demonstrated under continuous-wave optical pumping at a heat-sink temperature of 45 K. A 1.49 µm diameter, 209 nm thick microdisk lases in single-mode at a wavelength of 1.53 µm, which is identified as the whispering-gallery mode with the first radial mode number, the fifth azimuthal mode number, and a modal volume of 2.12(λ/n)(3) according to our mode simulation.

Slow light and band gaps in metallodielectric cylinder arrays

We consider two-dimensional three-component photonic crystals wherein one component is modeled as a drude-dispersive metal. It is found that the dispersion relation of light in this environment depends critically on the configuration of the metallic and dielectric components. In particular, for the case of an incident electromagnetic wave with electric field vector parallel to the axis of the cylinders it is shown that the presence of dielectric shells covering the metallic cylinders leads to a closing of the structural band gap with increased filling factor, as would be expected for a purely dielectric photonic crystal. For the same polarization, the photonic band structure of an array of metallic shell cylinders with dielectric cores do not show the closing of the structural band gap with increased filling factor of the metallic component. In this geometry, the photonic band structure contains bands with very small values of group velocity with some bands having a maximum of group velocity as small as .05c.

Driving a Superconductor to Insulator Transition with Random Gauge Fields

Typically the disorder that alters the interference of particle waves to produce Anderson localization is potential scattering from randomly placed impurities. Here we show that disorder in the form of random gauge fields that act directly on particle phases can also drive localization. We present evidence of a superfluid bose glass to insulator transition at a critical level of this gauge field disorder in a nano-patterned array of amorphous Bi islands. This transition shows signs of metallic transport near the critical point characterized by a resistance , indicative of a quantum phase transition. The critical disorder depends on interisland coupling in agreement with recent Quantum Monte Carlo simulations. We discuss how this disorder tuned SIT differs from the common frustration tuned SIT that also occurs in magnetic fields. Its discovery enables new high fidelity comparisons between theoretical and experimental studies of disorder effects on quantum critical systems.

Stimulated emission and emission efficiency enhancement in nanopatterned silicon

1.278μm laser emission has been observed in a SOI structure which has been nanopatterned to contain an array of nanopores. The optical transition is identified to be associated with phononless recombination mediated by the bistable, carbon-related G center. The present work is focused on increasing the luminescence intensity from nanopatterned Si by increasing the number of G centers present in the material. The G center density is increased by increasing the concentration of substitutional atoms in the lattice prior to nanopatterning. To this end, solid-phase epitaxial regrowth of carbon-rich silicon is utilized in order to take advantage of the increased solid solubility of carbon in silicon at the interface between crystalline and amorphous solid silicon.

Directly Pumped Silicon Lasers

Over the past two decades, researchers have tried mightily to enhance silicon’s ability to emit light. Yet electrically pumped silicon lasers remain tantalizingly out of reach. These authors describe how the approaches that have been tried so far might be successfully combined.

Broad Tuning of Whispering-Gallery Modes in Microdisks

Silicon microdisks with dynamically-tunable resonance spectra are achieved with narrow, in-plane silicon electrical contacts in a single lithographic step. Electrical current is passed through the devices to enable thermal tuning via joule heating. A 14nm wavelength shift is demonstrated with 1.6mW power consumption in devices with >20nm free spectral ranges and quality factors exceeding 20,000. Spectral shifts of several modal linewidths can be achieved with tuning powers below 100µW.