Innovative Ultrafast Laser System for Scanning Photoionization Imaging Microscopy
Abstract
Scanning photoelectron imaging microscopy (SPIM) is a unique technique that combines two powerful and complementary approaches- scanning microscopy and velocity map imaging. The latter allows for the measurement of full 3D vector momenta when electrons are photoemitted from single plasmonic nanostructures by ultrashort (around 100 fs) laser pulses. The resulting electrons are projected by a set of electrostatic lenses onto a position sensitive detector, from which initial velocity vectors and thus electron momenta can be reconstructed. These photoelectron momentum distributions tell us important information about the isolated nano sample, such as the electronic structure (e.g., work function or Fermi level) or even chirality of the sample from additional information on the distribution of photo-ejected electron spin angular momenta. Most importantly, the SPIM concept combines photoemission from a single nanostructure with spatial scanning microscopy. As a result, laser beam can be tightly focused into a diffraction limited spot with electrons generated from a single, spatially well-defined nanostructure. Thus, the laser spot can be scanned to record spatially resolved information on the sample to permit correlated (SEM, TEM) measurements of these nanostructures with spatial resolution on the less than 10 nm scale. As a result, such SPIM experimental capabilities provide a wealth of multidimensional data on well characterized single nanostructures with spatial, energy, and vector linear momentum and spin angular momentum resolution on the hot electron dynamics. Additionally, electron relaxation dynamics can be obtained from dual color multiphoton SPIM pump-probe studies. Such data encode dynamical information about hot electron processes induced by the pump laser in single plasmonic nano samples on timescales down to a few tens to hundreds of femtoseconds. For this project, we plan on purchasing a state-of-the-art ultrafast laser system designed to generate ultrashort laser pulses (t less than 120 fs) with broadly tunable wavelengths from the near UV to the near IR (340 nm - 1300 nm). To achieve this, the laser system is a combination of two components, an oscillator generating laser pulses at fixed wavelengths and an optical parametric oscillator (OPO) enabling smooth continuous tuning of wavelengths. These two oscillator-OPO components are commonly achieved as two separate systems; however, such designs have significant drawbacks in terms of overall system stability. In order to maximize simplicity of operation and data collection, we aim for a laser system in which these two components are built into a single unit to increase long-term stability and power, as well as exploit the most recent development in ultrafast optics away from a traditional Ti-Sapphire architecture towards newer and more reliable Yb systems.Such an advanced laser system provides a fundamentally new experimental tool to exploit several scientific directions with important technological applications. For example, we will study photoemission dynamics from engineered plasmonic nanostructures (e.g., Au-Ag nanorods, nanoshells, nanostars), based on polarization-frequency dependent multiphoton photoionization. These results are important for future photovoltaics where nanomaterials can enhance the efficiency of solar cells, or in photocatalysts where plasmonic materials provide hot electrons to enhance chemical reactions. Another direction involves the use of ultrafast polarization dependent illumination of chiral materials on thin metallic surfaces to control electron spin-based transport through chiral molecules, which is at the core of an emerging field of chiral induced spin selectivity (CISS). Though still incompletely understood, such chiral effects look to be important for future spintronic devices or even quantum computers working at room temperature where calculations may be performed by manipulating electron spins.
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Mar 07, 2024
- Source ID
- FA95502310663
Entities
People
- David J. Nesbitt
Organizations
- Air Force Office of Scientific Research
- Regents of the University of Colorado
- United States Air Force