Joe Petrus

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A square lattice of shallow, noncylindrical holes in GaAs is shown to act as a phononic crystal (PnC) reflector. The holes are produced by wet-etching a GaAs substrate using a citric acid:H2O2 etching procedure and a photolithographed array pattern. Although nonuniform and asymmetric etch rates limit the depth and shape of the phononic crystal holes, the matrix acts as a PnC, as demonstrated by insertion loss measurements together with interferometric imaging of surface acoustic waves propagating on the GaAs surface. The measured vertical displacement induced by surface phonons compares favorably with finite-difference time-domain simulations of a PnC with rounded-square holes.

Precise and accurate U–Pb LA–ICPMS dating of many U-bearing accessory minerals (e.g. apatite, allanite, titanite and rutile) is often compromised by common Pb. LA–ICPMS dating of these U-bearing accessory phases typically requires a matrix-matched standard, and data reduction is often complicated by variable incorporation of common Pb not only into the unknowns but also particularly into the reference material. We present here a general approach to common Pb correction in U–Pb LA–ICP–MS dating using a modified version of the VizualAge U–Pb data reduction package for Iolite (VizualAge_UcomPbine). The key feature of the method is that it can correct for variable amounts of common Pb in any U–Pb accessory mineral standard as long as the standard is concordant in the U/Pb (and Th/Pb) systems following common Pb correction. Common Pb correction of the age standard can be undertaken using either the 204Pb, 207Pb or 208Pb(no Th) methods, and the approach can be applied to raw data files from all widely used modern multi-collector and single-collector ICPMS instruments.

VizualAge_UcomPbine first applies a common Pb correction to the user-selected age standard integrations and then fits session-wide “model” U–Pb fractionation curves to the time-resolved U–Pb standard data. This downhole fractionation model is applied to the unknowns and sample-standard bracketing (using a user-specified interpolation method) is used to calculate final isotopic ratios and ages. 204Pb- and 208Pb(no Th)-corrected concordia diagrams and 204Pb-, 207Pb- and 208Pb(no Th)-corrected age channels can be calculated for user-specified initial Pb ratio(s). All other conventional common Pb correction methods (e.g. intercept or isochron methods on co-genetic analyses) can be performed offline.

The approach was tested on apatite and titanite age standards (for which there are independent constraints on the U–Pb crystallization age) using a Thermo Scientific iCAP-Qc (Q–ICP–MS) coupled to a Photon Machines Analyte Excite 193 nm ArF Excimer laser. Madagascar apatite, OLT1 titanite and R10 rutile were used as primary standards and were corrected for variable common Pb using the new VizualAge_UcomPbine DRS. The secondary Durango (31.44 ± 0.18 Ma) apatite standard yielded a U–Pb TW concordia intercept age of 31.97 ± 0.59 Ma (MSWD = 1.09; primary standard corrected by the 207Pb-method) and a U–Pb concordia age of 31.82 ± 0.40 Ma (MSWD = 1.4; primary standard corrected by the 204Pb-method). McClure Mountain (523.51 ± 1.47 Ma) yielded a U–Pb TW concordia intercept age of 524.5 ± 3.7 Ma (MSWD = 0.72) while the Fish Canyon Tuff (28.201 ± 0.046 Ma) and Khan (522.2 ± 2.2 Ma) titanite standards yielded U–Pb TW concordia intercept ages of 28.78 ± 0.41 Ma (MSWD = 1.4) and 520.9 ± 3.9 Ma (MSWD = 4.2) respectively. The suitability of the 208Pb(no Th)-correction is demonstrated by the agreement between a U–Pb TW concordia intercept age of 452.6 ± 4.7 Ma (MSWD = 0.89) and a 208Pb(no Th)-corrected TW concordia age of 448.6 ± 4.5 Ma (MSWD = 1.4) on a c. 450 Ma rutile which exhibits variable incorporation of common Pb.

A range of LA–ICPMS U–Pb dating applications are presented and include U–Pb dating of apatite from > 3.8 Ga gneisses from Akilia, SW Greenland. These apatites host 13C-depleted graphite inclusions that are interpreted as biogenic in origin and representing the oldest indications of life on Earth. The U–Pb age profiles on single apatite grains presented here are characteristic of Pb loss by volume diffusion with core–rim age differences of up to 300 Ma. These data explain the scatter and poor precision of earlier U–Pb apatite age determinations on Akilia apatite. Other LA–ICPMS dating applications include U–Pb apatite dating as a rapid method for determining the age of mafic intrusions, U–Pb titanite and apatite dating of ash fall tuffs, determining temperature–time histories using multiple U–Pb thermochronometers and improving concordance in LA–ICPMS primary zircon standard datasets by analysing young, common Pb-bearing primary zircon standards that have not accumulated significant radiation damage.

The occurrence of ca. 360 Ma greisen-related molybdenite mineralization in the Clayton Hill biotite–muscovite leucomonzongranite (<1 km2) is unusual for the southwestern Meguma terrane of the Maritime Appalachians, where 380 Ma granite-related Sn mineralization dominates. The presence of extensive hornfelsing and occurrence of mineralized (Sn–W–Cu–Zn) skarns in the area suggests a larger intrusion occurs at depth. The intrusion and mineralization are constrained to 361 Ma based on concordant Re–Os (molybdenite) and 40Ar/39Ar (muscovite) ages, which equates to other ca. 360 Ma granites in the area, hence magmatism of this age is more widespread than previously considered. The intrusion is highly evolved (74 wt.% SiO2), metaluminous (A/CNK = 1–1.05; A/CNK = molar ratio [Al2O3/(CaO + Na2O + K2O)]) and depleted in cafemic and transition elements. Rare-earth elements (∑REE = 106–23 ppm) are strongly fractionated with light rare-earth elements (LREE) enriched (La/LuN = 10), and negative Eu anomalies. Biotite is Fe-rich (Fe/(Fe + Mg) = 0.75), and both biotite and muscovite are F deficient. These geochemical features indicate affinity to the other 360 Ma granites and suggest a similar petrogenesis with some A-type affinity. Mineralization occurs as molybdenite-bearing greisens and rusty miaroles lined with pyrite and enrichment in Cu–As–Bi–Au. Stable isotopes (O, S) indicate greisen formation at 450 °C from fluids with δ18OH2O = 7‰ and δ34SH2S = 2‰–3‰, which suggests an A- or I-type magmatic reservoir rather than a peraluminous magma. These data indicate the presence of a widespread magmatic event that is 20 Ma younger than the 380 Ma Sn-base metal event in southern Nova Scotia (e.g., East Kemptville). That this event is the same age as the world class Mount Pleasant Mo–Sn–W–Zn–In deposit of southern New Brunswick indicates additional potential for significant granite-related mineralization in this part of Atlantic Canada.

VizualAge, a new computer software tool for analysing U-Pb data obtained by laser ablation-inductively coupled plasma-mass spectrometry, was developed. It consists of a data reduction scheme (DRS) for Iolite (a general mass spectrometry data analysis tool) as well as visualisation routines. In addition to the U/Pb and Th/Pb ages calculated by Iolite’s U-Pb geochronology DRS, VizualAge also calculates 207Pb/206Pb ages and common Pb corrections for each time-slice of raw data. Importantly, VizualAge allows one to display a live concordia diagram for visualising data on such a diagram as an integration interval is being adjusted. This provides instantaneous feedback regarding discordance, uncertainty, error correlation and common Pb. Several zircon data sets were used to illustrate how the live concordia could be used as a powerful inspection tool, revealing a single analysis to consist of zones of concordance, metamict areas, as well as inherited cores or younger overgrowths. VizualAge also constructs histograms, conventional and Tera-Wasserburg type concordia diagrams, as well as 3D U-Th-Pb and total U-Pb concordia diagrams. The precision and accuracy of data reduced with VizualAge are demonstrated with examples of the Plešovice, Temora-2 and Penglai zircon reference materials. Data for zircon from the Long Lake Batholith (Wyoming craton) were used to illustrate how VizualAge calculated common Pb corrections and helped to expose as yet unexplained difficulties with accurately determining 204Pb.

A new program capable of rapidly and accurately reducing a two-dimensional X-ray diffraction pattern obtained from a Debye–Scherrer or Gandolfi camera to a quantitative digital diffractogram is described. By implementing geometric optimization routines based on the symmetry and sharpness of diffraction lines, the optimal configuration of the camera, X-ray path, and sample can be determined, thus providing the best possible set of integration parameters. Results for NIST standard reference materials 640c (Si) and 676 (Al2O3) are included to illustrate program functionality.

The unique frequency range and robustness of surface acoustic wave (SAW) devices has been a catalyst for their adoption as integral components in a range of consumer and military electronics. Furthermore, the strain and piezoelectric fields associated with SAWs are finding novel applications in nanostructured devices. In this thesis, the interaction of SAWs with periodic elastic structures, such as photonic or phononic crystals (PnCs), is studied both computationally and experimentally.

To predict the behaviour of elastic waves in PnCs, a finite-difference time-domain simulator (PnCSim) was developed using C++. PnCSim was designed to calculate band structures and transmission spectra of elastic waves through two-dimensional PnCs. By developing appropriate boundary conditions, bulk waves, surface acoustic waves, and plate waves can be simulated. Results obtained using PnCSim demonstrate good agreement with theoretical data reported in the literature.

To experimentally investigate the behaviour of SAWs in PnCs, fabrication procedures were developed to create interdigitated transducers (IDTs) and PnCs. Using lift-off photolithography, IDTs with finger widths as low as 1.8 µm were fabricated on gallium arsenide (GaAs), corresponding to a SAW frequency of 397 MHz. A citric acid and hydrogen peroxide wet-etching solution was used to create shallow air hole PnCs in square and triangular lattice configurations, with lattice constants of 8 µm and 12 µm, respectively. The relative transmission of SAWs through these PnCs as a function of frequency was determined by comparing the insertion losses before and after etching the PnCs. In addition, using a scanning Sagnac interferometer, displacement maps were measured for SAWs incident on square lattice PnCs by Mathew (Creating and Imaging Surface Acoustic Waves on GaAs, Master’s Thesis). Reasonable agreement was found between simulations and measurements. Additional simulations indicate that SAW waveguiding should be possible with a PnC consisting of air holes in GaAs.

The phononic properties of a commonly used photonic plate were also determined. Band structure simulations of the plate displayed no complete elastic band gaps. However, transmission simulations indicated that a pseudo-gap may form for elastic waves polarized in the sagittal plane.

An oscillating electric field at 1.356 GHz was used to promote the resonant energy transfer process: 43d(5/2) + 43d(5/2) -> 45p(3/2) + 41f between translationally cold Rb-85 Rydberg atoms. The ac Stark shifts due to this dressing field created degeneracies between the initial and final two-atom states of this process. The ac field strength was scanned to collect spectra which are analogous to dc electric-field-induced resonant energy transfer spectra. Different resonances were observed for different magnetic sublevels involved in the process. Compared to earlier work performed at higher frequencies, the choice of dressing frequency and structure of the spectra may be intuitively understood, by analogy with the dc field case.

The ac Stark effect was used to induce resonant energy transfer between translationally cold Rb-85 Rydberg atoms. When a 28.5 GHz dressing field was set at specific field strengths, the two-atom dipole-dipole process 43d(5/2)+43d(5/2)-> 45p(3/2)+41f was dramatically enhanced, due to induced degeneracy of the initial and final states. This method for enhancing interactions is complementary to dc electric-field-induced resonant energy transfer, but has more flexibility due to the possibility of varying the applied frequency.

Dome-shaped gold nanoparticles (with an average diameter of 10.5 nm) are grown on H-terminated Si(100) substrates by simple techniques involving electro- and electroless deposition from a 0.05 mM AuCl3 and 0.1 M NaClO4 solution. XPS depth profiling data (involving Au 4f core-level and valence band spectra) reveal for the first time the formation of gold silicide at the interface between the Au nanoparticles and Si substrate. UV-visible diffuse reflectance spectra indicate that both samples have surface plasmon resonance maxima at 558 nm, characteristic of an uniform distribution of Au nanoscale particles of sufficiently small size. Glancing-incidence XRD patterns clearly show that the deposited Au nanoparticles belong to the fcc phase, with the relative intensity of the (220) plane for Au nanoparticles obtained by electroless deposition found to be notably larger than that by electrodeposition.

A microwave dressing field was used to induce resonant energy transfer in translationally cold Rydberg atoms. The 85Rb Rydberg atoms were obtained by laser excitation of cold atoms in a magneto-optical trap. When the amplitude of a 1.356 GHz dressing field was scanned, the two-atom dipole-dipole process 43d5/2 + 43d5/2 → 45p3/2 + 41f5/2,7/2 was enhanced due to the induced degeneracy of the initial and final states. The resulting spectrum had a series of resonant field amplitudes corresponding to different magnetic sublevels possible for the states involved. The scanned field amplitude was calibrated using microwave spectroscopy of the 43d5/2 − 41f transition under the influence of non-resonant dressing fields. The calibrated resonant field amplitudes agree well with ac Stark shift calculations performed using the Floquet approach. This method for enhancing interactions is complementary to dc electric field induced resonant energy transfer, but benefits from the ability to shift energy levels in either direction by choice in frequency.

Resonant energy transfer between cold Rydberg atoms was used to determine Rydberg atom energy levels, at precisions approaching those obtainable in microwave spectroscopy. Laser cooled Rb-85 atoms from a magneto-optical trap were optically excited to 32d(5/2) Rydberg states. The two-atom process 32d(5/2)+32d(5/2)-> 34p(3/2)+30g is resonant at an electric field of approximately 0.3 V/cm. This process is driven by the electric dipole-dipole interaction, which is allowed due to the partial f character that the g state acquires in an electric field. The experimentally observed resonant field, together with the Stark map calculation is used to make a determination of the Rb-85 ng-series quantum defect delta(g)(n=30)=0.00405(6).