Chemical Profiles of the Oxides on Tantalum in State of the Art Superconducting Circuits.

Over the past decades, superconducting qubits have emerged as one of the leading hardware platforms for realizing a quantum processor. Consequently, researchers have made significant effort to understand the loss channels that limit the coherence times of superconducting qubits. A major source of loss has been attributed to two level systems that are present at the material interfaces. It is recently shown that replacing the metal in the capacitor of a transmon with tantalum yields record relaxation and coherence times for superconducting qubits, motivating a detailed study of the tantalum surface. In this work, the chemical profile of the surface of tantalum films grown on c-plane sapphire using variable energy X-ray photoelectron spectroscopy (VEXPS) is studied. The different oxidation states of tantalum that are present in the native oxide resulting from exposure to air are identified, and their distribution through the depth of the film is measured. Furthermore, it is shown how the volume and depth distribution of these tantalum oxidation states can be altered by various chemical treatments. Correlating these measurements with detailed measurements of quantum devices may elucidate the underlying microscopic sources of loss.

Gap-plasmon enhanced water splitting with ultrathin hematite films: the role of plasmonic-based light trapping and hot electrons.

Hydrogen is a promising alternative renewable fuel for meeting the growing energy demands of the world. Over the past few decades, photoelectrochemical water splitting has been widely studied as a viable technology for the production of hydrogen utilizing solar energy. A solar-to-hydrogen (STH) efficiency of 10% is considered to be sufficient for practical applications. Amongst the wide class of semiconductors that have been studied for their application in solar water splitting, iron oxide (α-Fe2O3), or hematite, is one of the more promising candidate materials, with a theoretical STH efficiency of 15%. In this work, we show experimentally that by utilizing gold nanostructures that support gap-plasmon resonances together with a hematite layer, we can increase the water oxidation photocurrent by two times over that demonstrated by a bare hematite film at wavelengths above the hematite bandgap. Moreover, we achieve a six-fold increase in the oxidation photocurrent at near-infrared wavelengths, which is attributed to hot electron generation and decay in the gap-plasmon nanostructures. Theoretical simulations confirmed that the metamaterial geometry with gap plasmons that was used allows us to confine electromagnetic fields inside the hematite semiconductor and to enhance the surface photochemistry.

Plasmon-Enhanced Photoelectrochemical Water Splitting for Efficient Renewable Energy Storage.

Photoelectrochemical (PEC) water splitting is a promising approach for producing hydrogen without greenhouse gas emissions. Despite decades of unceasing efforts, the efficiency of PEC devices based on earth-abundant semiconductors is still limited by their low light absorption, low charge mobility, high charge-carrier recombination, and reduced diffusion length. Plasmonics has recently emerged as an effective approach for overcoming these limitations, although a full understanding of the involved physical mechanisms remains elusive. Here, the reported plasmonic effects are outlined, such as resonant energy transfer, scattering, hot electron injection, guided modes, and photonic effects, as well as the less investigated catalytic and thermal effects used in PEC water splitting. In each section, the fundamentals are reviewed and the most representative examples are discussed, illustrating possible future developments for achieving improved efficiency of plasmonic photoelectrodes.

Probing the ultrafast solution dynamics of a cyanine dye in an organic solvent interfaced with water.

Dependence of ultrafast dynamics on the excited state evolution and ground state recovery of a cyanine dye (IR125) in dichloromethane (DCM) solvent interfaced with neat water is presented. We use degenerate pump-probe transient absorption spectroscopy to show that the excited-state dynamics of the dye molecule is strongly dependent on the position of the measurements from bulk DCM solution to the solution near the water layer. The decay component of the transient corresponding to the excited state lifetime increases from bulk DCM solution to its interface with water. Such results show that the effect of the presence of water layer over the dye solution in DCM extends several micrometers, indicating the surfactant nature of the IR125 molecules, and provides us a measure of the penetration of water into the DCM layer. The initial ultrafast decay component (coherent spike) directly correlates to the pulse-width of our near-transform limited pulses used in these experiments. This approach of measuring the excited state decay of a dye across an immiscible liquid interface can provide important characteristics of microtransport across such interfaces.

Stable optical trapping of latex nanoparticles with ultrashort pulsed illumination.

Here we report how ultrafast pulsed illumination at low average power results in a stable three-dimensional (3D) optical trap holding latex nanoparticles which is otherwise not possible with continuous wave lasers at the same power level. The gigantic peak power of a femtosecond pulse exerts a huge instantaneous gradient force that has been predicted theoretically earlier and implemented for microsecond pulses in a different context by others. In addition, the resulting two-photon fluorescence allows direct observation of trapping events by providing intrinsic 3D resolution.

Control of laser induced molecular fragmentation of n-propyl benzene using chirped femtosecond laser pulses.

We present the effect of chirping a femtosecond laser pulse on the fragmentation of n-propyl benzene. An enhancement of an order of magnitude for the relative yields of C3H3+ and C5H5+ in the case of negatively chirped pulses and C6H5+ in the case of positively chirped pulses with respect to the transform-limited pulse indicates that in some fragmentation channel, coherence of the laser field plays an important role. For the relative yield of all other heavier fragment ions, resulting from the interaction of the intense laser field with the molecule, there is no such enhancement effect with the sign of chirp, within experimental errors. The importance of the laser phase is further reinforced through a direct comparison of the fragmentation results with the second harmonic of the chirped laser pulse with identical bandwidth.