Optical and spin properties of nitrogen vacancy centers in bulk and nanocrystalline diamond

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Chemistry, 2013.Cataloged from PDF version of thesis.Includes bibliographical references.The NV center is becoming a very hot topic in many areas of science, including, Physics, Chemistry, Biology, and Quantum Information. The Degen group has focused on a specific application of the NV center, namely scanning magnetometry. In my time in the group, I focused on building a microscope and studying NV centers in bulk and nanocrystalline diamond. I began by building a confocal microscope which was capable of observing and determine single NV centers. I made measurements on the photon statistics of different defects to determine if they were single emitters or multiple emitters. I also made microwave frequency magnetic measurements to determine the spin properties of single NV centers by measuring their couplings to neighboring paramagnetic nuclei as well as to a spin bath. Through these efforts, I was able to successfully confirm that the microscope was capable of identifying and measuring single NV centers and their properties. Lastly, I worked on the first steps of improving our understanding of NV centers in bulk diamond crystals. The goal of magnetometry involves putting the NV center as close to the diamond surface as possible. I made measurements that were aimed at studying the spin and coherence properties of the NV when it was within 10 nm of the diamond surface. These studies provided insight into the interactions of the NV center with the diamond surface.by Benjamin Kwasi Ofori-Okai.S.M

Terahertz generation by beamlet superposition

We analytically show how a superposition of beamlets produces terahertz radiation with greater spatial homogeneity and efficiency compared to tilted-pulse-fronts generated by diffraction gratings. The advantages are particularly notable for large pump bandwiths and beam sizes, alluding to better performance in the presence of cascading effects and higher energy pumping. A theory of terahertz generation using a superposition of beamlets is developed. It is shown how such an arrangement produces a distortion free tilted-pulse-front. Closed form expressions for terahertz spectra and transients in three spatial dimensions are derived. Conditions for obtaining performance parity and bounds for optimal parameters are furnished

THz generation using a reflective stair-step echelon

We present a novel method for THz generation in lithium niobate using a reflective stair-step echelon structure. The echelon produces a discretely tilted pulse front with less angular dispersion compared to a high groove-density grating. The THz output was characterized using both a 1-lens and 3-lens imaging system to set the tilt angle at room and cryogenic temperatures. Using broadband 800 nm pulses with a pulse energy of 0.95 mJ and a pulse duration of 70 fs (24 nm FWHM bandwidth, 39 fs transform limited width), we produced THz pulses with field strengths as high as 500 kV/cm and pulse energies as high as 3.1 $\mu$J. The highest conversion efficiency we obtained was 0.33%. In addition, we find that the echelon is easily implemented into an experimental setup for quick alignment and optimization.Comment: 19 pages, 4 figure

Nonlinear two-dimensional terahertz photon echo and rotational spectroscopy in the gas phase

Ultrafast two-dimensional spectroscopy utilizes correlated multiple light-matter interactions for retrieving dynamic features that may otherwise be hidden under the linear spectrum. Its extension to the terahertz regime of the electromagnetic spectrum, where a rich variety of material degrees of freedom reside, remains an experimental challenge. Here we report ultrafast two-dimensional terahertz spectroscopy of gas-phase molecular rotors at room temperature. Using time-delayed terahertz pulse pairs, we observe photon echoes and other nonlinear signals resulting from molecular dipole orientation induced by three terahertz field-dipole interactions. The nonlinear time-domain orientation signals are mapped into the frequency domain in two-dimensional rotational spectra which reveal J-state-resolved nonlinear rotational dynamics. The approach enables direct observation of correlated rotational transitions and may reveal rotational coupling and relaxation pathways in the ground electronic and vibrational state.Comment: 31 pages, 14 figure

Direct experimental visualization of waves and band structure in 2D photonic crystal slabs

We demonstrate for the first time the ability to perform time resolved imaging of terahertz (THz) waves propagating within a photonic crystal (PhC) slab. For photonic lattices with different orientations and symmetries, we used the electro-optic effect to record the full spatiotemporal evolution of THz fields across a broad spectral range spanning the photonic band gap. In addition to revealing real-space behavior, the data let us directly map the band diagrams of the PhCs. The data, which are in good agreement with theoretical calculations, display a rich set of effects including photonic band gaps, eigenmodes and leaky modes.National Science Foundation (U.S.) (Grant no. 1128632)National Science Foundation (U.S.) (NSF GRFP Fellowship)Canadian Institutes of Health Research (Fellowship

The homogenization limit and waveguide gradient index devices demonstrated through direct visualization of THz fields

Electromagnetic homogenization approximation calculates an effective refractive index of a composite material as a weighted average of its components, and has found uses in gradient refractive index and transformation optics devices. However, the utility of the homogenization approximation is hindered by uncertainty in its range of applicability. Harnessing the capability of time-resolved imaging provided by the terahertz polaritonics platform, we determined the dispersion curves of slab waveguides with periodic arrays of holes, and we quantified the breakdown of the homogenization approximation as the period approached the terahertz wavelength and the structure approached the photonic bandgap regime. We found that if the propagation wavelength in the dielectric waveguide was at least two times as large as the Bragg condition wavelength, the homogenization approximation held independent of the detailed geometry, propagation direction, or fill fraction. This value is much less demanding than the estimate of 10:1 often assumed for homogenization. We further used the experimental capabilities to extract the effective refractive index of the photonic crystals in the homogenization approximation limit, and we used this to analyze the predictive strength of analytical formulas. These formulas enabled rapid design of a Luneburg lens and a bi-directional cloak in a waveguide platform without the need for numerical simulations. Movies of terahertz waves interacting with these structures, which were fabricated using femtosecond laser machining, reveal excellent performance. The combination of an analytical formula and confidence in the homogenization approximation will aid in fast design and prototyping of gradient index devices.National Science Foundation (U.S.) (Grant 1128632)HDTRA Grant (1-12-1-0008)National Science Foundation (U.S.). Graduate Research Fellowship Progra

Rapid and Precise Determination of Zero-Field Splittings by Terahertz Time-Domain Electron Paramagnetic Resonance Spectroscopy

Zero-field splitting (ZFS) parameters are fundamentally tied to the geometries of metal ion complexes. Despite their critical importance for understanding the magnetism and spectroscopy of metal complexes, they are not routinely available through general laboratory-based techniques, and are often inferred from magnetism data. Here we demonstrate a simple tabletop experimental approach that enables direct and reliable determination of ZFS parameters in the terahertz (THz) regime. We report time-domain measurements of electron paramagnetic resonance (EPR) signals associated with THz-frequency ZFSs in molecular complexes containing high-spin transition-metal ions. We measure the temporal profiles of the free-induction decays of spin resonances in the complexes at zero and nonzero external magnetic fields, and we derive the EPR spectra via numerical Fourier transformation of the time-domain signals. In most cases, absolute values of the ZFS parameters are extracted from the measured zero-field EPR frequencies, and the signs can be determined by zero-field measurements at two different temperatures. Field-dependent EPR measurements further allow refined determination of the ZFS parameters and access to the g-factor. The results show good agreement with those obtained by other methods. The simplicity of the method portends wide applicability in chemistry, biology and material science.Comment: 36 pages, 30 figures, 1 tabl

The impact of sodium contamination in tin sulfide thin-film solar cells

Through empirical observations, sodium (Na) has been identified as a benign contaminant in some thin-film solar cells. Here, we intentionally contaminate thermally evaporated tin sulfide (SnS) thin-films with sodium and measure the SnS absorber properties and solar cell characteristics. The carrier concentration increases from 2 × 10[superscript 16] cm[superscript −3] to 4.3 × 10[superscript17] cm[superscript−3] in Na-doped SnS thin-films, when using a 13 nm NaCl seed layer, which is detrimental for SnS photovoltaic applications but could make Na-doped SnS an attractive candidate in thermoelectrics. The observed trend in carrier concentration is in good agreement with density functional theory calculations, which predict an acceptor-type Na[subscriptSn] defect with low formation energy.United States. Department of Energy (SunShot Initiative, Contract No. DE-EE0005329)National Science Foundation (U.S.) (Grant No. CHE-11115577)Alexander von Humboldt FoundationNational Science Foundation (U.S.). Graduate Research Fellowship ProgramMIT Energy Initiative (Fellowship)United States. Department of Energy. Office of Energy Efficiency and Renewable Energy (Postdoctoral Research Award)National Science Foundation (U.S.) (Award No. DMR-08-19762)National Science Foundation (U.S.). Center for Nanoscale Systems (Award No. ECS-0335765

Transient terahertz photoconductivity measurements of minority-carrier lifetime in tin sulfide thin films: Advanced metrology for an early stage photovoltaic material

Materials research with a focus on enhancing the minority-carrier lifetime of the light-absorbing semiconductor is key to advancing solar energy technology for both early stage and mature material platforms alike. Tin sulfide (SnS) is an absorber material with several clear advantages for manufacturing and deployment, but the record power conversion efficiency remains below 5%. We report measurements of bulk and interface minority-carrier recombination rates in SnSthin films using optical-pump, terahertz-probe transient photoconductivity (TPC) measurements. Post-growth thermal annealing in H2S gas increases the minority-carrier lifetime, and oxidation of the surface reduces the surface recombination velocity. However, the minority-carrier lifetime remains below 100 ps for all tested combinations of growth technique and post-growth processing. Significant improvement in SnSsolar cell performance will hinge on finding and mitigating as-yet-unknown recombination-active defects. We describe in detail our methodology for TPC experiments, and we share our data analysis routines in the form freely available software.Chemistry and Chemical Biolog