Interaction of energetic hydrogen with surfaces

Issued as Proposal and Progress reports no. 1-2, Project no. G-41-603 (continued by G-41-627

Electrochemical growth of three-dimensionally ordered macroporous metals as photonic crystals

Over the last two decades three dimensionally ordered macroporous (3-DOM) materials have turned out to be very promising in many applications ranging from optics, plasmonics, to catalyst scaffolds. The thesis presents a systematic study on formation and characterisation of 3-DOM metals as photonic crystals. Metals are nearly perfect reflectors with low adsorption at microwave or millimetre wavelengths. Meanwhile they generally absorb visible light because of their negative imaginary part of the dielectric constant that could destroy the band gap in the visible though they. Howevers, for noble metals such as gold, silver and copper, considering the Drude-like behaviour, the adsorption will be small enough to achieve a complete photonic band gap for optical or even shorter wavelengths, with silver performing the best. In order to fabricate the 3-DOM metallic nanostructures, template-directed electrochemical deposition has been employed in which, initially a highly ordered film of submircon sized colloidal spheres is deposited on to electronically conducting substrates, for instance, indium-tin oxide (ITO) coated glass substrate, through evaporation-induced self-assembly; and subsequently it is infiltrated with metallic elements electrochemically reduced from corresponding electrolytes; fiannly removal of the colloidal templating film reveals a metallic film comprised of periodically arranged spherical voids. Field Emission Gun Scanning Electron Microscopy (FEGSEM) was used to examine the surface morphology and periodicity of the 3-DOM metallic films. It revealed that highly ordered structures are homogenous and uniform over a large scale for both the original colloidal templates and metallic inverse structures. However for silver electroplated from either silver thiosulfate or silver chlorate bath, voids in the template are fully infiltrated, including both the interstitial spaces between the colloidal spheres and any cracks between film domains, forming a complete solid network over large length scales; for copper the filling factors are strongly dependent on the bath chemistry and in copper sulfate bath isolated macroporous domains can be formed due to those in the cracks will be dissolved back to the solution while those reduced from copper glycerol bath resulted in fully infiltrated structures. Moreover, angle-resolved reflectance spectroscopy has further confirmed the three-dimensional periodicity and indicated the inverse structures have stop band properties in the visible wavelength region, consistent with variation in the effective refractive index of the films. In addition, surface enhanced Raman scattering (SERS) spectroscopy has been used to evaluate applications of the inverse metals as SERS-active substrates. SERS has nearly exclusively been associated with three noble metals copper, silver (by far the most important) and gold. The 3-DOM metallic thin films possess excellent features for SERS detection arising from their long range periodical void geometry, which gives significant enhancement to Raman intensity. Preliminary measurements have demonstrated the 3-DOM metallic structures are well suited for SERS enhancement. Series spectra from different points of each specimen have given reproducible intensities. Variables associated with Raman intensity such as pore size, dye concentration, and film thickness, have been tuned to achieve maximal enhancement for visible and near-IR wavelengths

An Open Microcavity for Diamond-based Photonics

In recent years, tunable Fabry-Perot microcavities have emerged as a compelling platform for enhancing the flux of coherent photons from single colour centres in solid-state hosts. A prominent example of one such colour centre is the nitrogen-vacancy (NV) centre in diamond. The NV centre has a highly coherent, optically addressable electron spin. Furthermore, the NV centre is a source of single photons. Advances in the creation of entangled spin-photon pairs allow for establishing remote spin-spin entanglement – a key building block in a quantum network. However, the scalability past a few network nodes is limited by modest entanglement rates, in turn limited by the detection efficiency of coherent photons. Limiting factors include the long radiative lifetime and the small branching ratio of “useful” photons into the zero-phonon line (ZPL). However, neither the radiative lifetime nor the branching ratio are rigid features of the NV centre – the flux of ZPL photons can be greatly accelerated in a resonant microcavity. This thesis reports on the realisation of a high-quality tunable Fabry-Perot microcavity embedded with a diamond membrane. However, the diamond alters the cavity performance, rendering the cavity sensitive to surface related losses. Despite operating in a geometry where the standing wave inside the cavity possesses an anti-node at the diamond surface, quality ($\mathcal{Q}$) factors exceeding $100\,000$ were realised. The benefit of this geometry is the strong confinement of the vacuum electric-field to the diamond – the current cavity design allows for the realisation of Purcell factors exceeding 300, thus increasing the fraction of photons emitted into the ZPL from $3\,\%$ to $89\,\%$. The versatile design of the microcavity was demonstrated further by enhancing the Raman transition from the single crystalline diamond. Compared to free-space measurements under likewise identical conditions, a 59-fold intensity enhancement was demonstrated. This enhancement factor encompasses the Purcell effect and the improved detection efficiency provided by the cavity. The Raman transition couples to all cavity modes, allowing for in situ optimising and benchmarking the cavity performance. Additionally, it facilitates coupling to the external single-mode detection optics. Further enhancement of the Raman intensity can be achieved by establishing a double resonant condition, with both the pump laser and the Raman transition being resonant. Resonant recirculation of the pump laser increases the power density inside the cavity, providing a platform with prospects of realising a Raman laser with sub-mW threshold pump power. Exploiting a small thickness gradient in the diamond enabled continuous tuning of the double resonance condition across a spectral window of $\sim1\,\textrm{THz}$. The tuning range is only limited by the travel range of the piezo – with an adequate travel range, continuous tuning is, at least in principle, possible across the entire reflective stopband

International workshop on next generation gamma-ray source

A workshop on The Next Generation Gamma-Ray Source sponsored by the Office of Nuclear Physics at the Department of Energy, was held November 17-19, 2016 in Bethesda, Maryland. The goals of the workshop were to identify basic and applied research opportunities at the frontiers of nuclear physics that would be made possible by the beam capabilities of an advanced laser Compton beam facility. To anchor the scientific vision to realistically achievable beam specifications using proven technologies, the workshop brought together experts in the fields of electron accelerators, lasers, and optics to examine the technical options for achieving the beam specifications required by the most compelling parts of the proposed research programs. An international assembly of participants included current and prospective γ-ray beam users, accelerator and light-source physicists, and federal agency program managers. Sessions were organized to foster interactions between the beam users and facility developers, allowing for information sharing and mutual feedback between the two groups. The workshop findings and recommendations are summarized in this whitepaper

Time-Resolved Photoemission Electron Microscopy: Development and Applications

Time-resolved photoemission electron microscopy (TR-PEEM) belongs to a class of experimental techniquescombining the spatial resolution of electron-based microscopy with the time resolution of ultrafast opticalspectroscopy. This combination provides insight into fundamental processes on the nanometer spatial andfemto/picosecond time scale, such as charge carrier transport in semiconductors or collective excitations ofconduction band electrons at metal surfaces. The high spatiotemporal resolution also offers a detailed view of therelationship between local structure and ultrafast photoexcitation dynamics in nanostructures and nanostructuredmaterials, which is beneficial in exploring new materials and applications in opto-electronics and nano-optics.This thesis describes the investigation of ultrafast photoexcitation dynamics in metal- and III-V semiconductornanostructures using TR-PEEM. We investigate hot carrier cooling in individual InAs nanowires where we findevidence that electron-hole scattering strongly contributes to the intra-band energy relaxation of photoexcitedelectrons on a sub-picosecond time scale and we observe ultrafast hot electron transport towards the nanowiresurface due to an in-built electric field. We demonstrate the combination of TR-PEEM with optical time-domainspectroscopy to enable time- and excitation frequency-resolved PEEM imaging. The technique is applied to GaAssubstrates and nanowires. TR-PEEM is further used to investigate localized and propagating surface plasmonpolaritons. We explore the optical properties of disordered, porous gold nano-particles (nanosponges). Using TRPEEM,we can resolve several plasmonic hotspots with different resonance frequencies and lifetimes within singlenanosponges. We also explore excitation and temporal control of surface plasmon polaritons by means of singlelayeredcrystals of the transition metal dichalcogenide WSe2.In addition, this thesis includes developments in ultrafast optics, aiming to expand the capabilities of the TR-PEEMsetup. We present a setup for generating tunable broadband ultraviolet (UV) laser pulses via achromatic secondharmonic generation. The setup is suitable for operation at high repetition rates and low pulse energies due to its highconversion efficiency. Further, we describe a transmission grating-based interferometer for the generation of stable,phase-locked pulse pairs. Pulse shaping based on liquid crystal technology allows accurate control over the temporalshape of femtosecond laser pulses. We characterize Fabry-Perot interferences affecting the accuracy of such pulseshapers, and we demonstrate a calibration scheme to compensate for these interference effects

First International Conference on Laboratory Research for Planetary Atmospheres

Proceedings of the First International Conference on Laboratory Research for Planetary Atmospheres are presented. The covered areas of research include: photon spectroscopy, chemical kinetics, thermodynamics, and charged particle interactions. This report contains the 12 invited papers, 27 contributed poster papers, and 5 plenary review papers presented at the conference. A list of attendees and a reprint of the Report of the Subgroup on Strategies for Planetary Atmospheres Exploration (SPASE) are provided in two appendices