Researchers at JILA, a joint institute of the National Institute of Standards and Technology (NIST; Boulder, CO) and the University of Colorado (Boulder), are developing a method of imaging nanostructures that is intended to enhance and complement existing microscopy techniques; it does this by combining diffraction-limited optical excitation with detection of photons and low-kinetic-energy photoelectrons.1
The researchers ultimately hope to combine confocal-fluorescence or Raman spectroscopy with time-resolved photoelectron imaging spectroscopy for applications such as the investigation of local plasmonic effects in nanostructures. So far, they have demonstrated a scanning-photoionization microscopy (SPIM) technique that could, in principle, yield spectroscopic information in thin nano- and mesostructured polycrystalline metal patterns, spatially resolved down to the single-molecule level.
Their experimental apparatus enables simultaneous measurement of optical-penetration depth and two-photon photoemission cross section from a diffraction-limited spot size in a photolithographically patterned polycrystalline gold film placed on a glass cover slip. Comparison of these measurements with atomic-force-microscopy scans indicates that photoionization contrast varies as a function of electron escape depths and thickness variations across the sample. The researchers also implemented a simple form of spatially resolved photoemission spectroscopy, which they expect to enhance using time-of-flight electron-energy analysis.
Ti:sapphire light source
The light source in their scanning-photoionization microscopy setup was a Kerr-lens passively modelocked Ti:sapphire oscillator driven by a frequency-doubled diode-pumped solid-state Nd:YVO4 (vanadate) pump laser. Frequency doubling and dispersion compensation yielded 415 nm, 100 fs pulses with 0.18 nJ pulse energies, which were focused to a near-diffraction-limited spot at the sample by a Schwarzschild-type reflective microscope objective. The sample was scanned over the stationary laser beam using a combination of independently operated coarse and fine translation stages (see Fig. 1).
The researchers performed fluorescence measurements by collecting red-shifted photons with the microscope objective and focusing them through a confocal pinhole into a single-photon-counting photomultiplier module, enabling subsequent generation of confocal-microscopy images. In addition, multiphoton absorption enabled photoionization imaging using a Faraday cup that collected photoelectrons to produce a photocurrent, which the researchers measured using a picoammeter as a transimpedance amplifier. Electron kinetic-energy distributions were also obtained by slowly varying the Faraday-cup bias.
The researchers observed photoemission at three distinct intensity levels: zero detectable photocurrent from the glass cover slip (a wide-band-gap insulator), a small but finite photocurrent from the solid parts in the polycrystalline thin metal film, and strong photoemission at the metal film edges (see Fig. 2).
