Pulsed electron beam sources are increasingly in demand, especially as probes for ultrafast science and as patterning tools in lithography applications. Plasmonic nanostructures have been shown to enhance multiphoton photocurrent from metal cathodes by orders of magnitude, but for many applications it is also important to characterize the transverse electron beam properties. Here, we present preliminary work demonstrating a DC photogun setup for characterizing plasmonic photoemitters at Lawrence Berkeley National Laboratory, including measurement of photocurrent and the asymmetric transverse phase space of photoemitted beams from plasmonic nanogroove resonator arrays. This lays the groundwork for future studies of emerging plasmon-enhanced photoemitter designs, such as plasmonic lens nanoemitters.
We discuss our experiments that apply ultrafast electron diffraction (UED) to study structural dynamics of the phase transition in single crystal tantalum ditelluride, TaTe2, a quasi-2D quantum material which exhibits a trimer superstructure at cryogenic temperatures. Intense near-infrared (NIR) pulses at 1030 nm are employed to quench the low temperature, atomically ordered state and the process is captured by ultrashort bunches of electrons as a function of pump-probe time delay. The diffraction signatures of the trimer superstructure recover on picosecond time scales. These measurements of TaTe2 underscore moreover the applicability of the HiRES UED beamline at Lawrence Berkeley National Laboratory (LBNL) to probe ultrafast structural dynamics of complex materials.
Ultrafast electron diffraction (UED) has become a leading technique for investigation of structural dynamics in solids providing high spatial and temporal resolutions. Radio frequency (RF) based photoinjectors providing Mega-electron-volt (MeV) scale electron beams are improving the source brightness and instrument versatility and are largely responsible for advancement of the field of structural dynamics. At Lawrence Berkeley National Laboratory (LBNL), an RF photoinjector gun for ultrafast structural studies using UED has been in development and is now producing high-quality scientific results. Here we describe some factors that enable UED of materials at LBNL and present some exemplary results.
Nanoscale electron pulses are increasingly in demand, including as probes of nanoscale ultrafast dynamics and for emerging light source and lithography applications. Using electromagnetic simulations, we show that gold plasmonic lenses as multiphoton photoemitters provide unique advantages, including emission from an atomically at surface, nanoscale pulse diameter regardless of laser spot size, and femtosecond-scale response time. We then present fabrication of prototypes with sub-nm roughness via e-beam lithography, as well as electro-optical characterization using cathodoluminescence spectromicroscopy. Finally, we introduce a DC photogun at LBNL built for testing ultrafast photoemitters. We discuss measurement considerations for ultrafast nanoemitters and predict that we can extract tens of pA photocurrent from a single plasmonic lens using a Ti:Sa oscillator. Altogether, this lays the groundwork to develop and test a broad class of plasmon-enhanced ultrafast nanoemitters.
Using time-resolved cathodoluminescence imaging, we measure the pixel-by-pixel cathodoluminescence decay of Mn2+ dopants in cesium lead chloride perovskite microplates. This measurement generates a spatially resolved map of the excited state decay dynamics of the Mn2+ dopants, which suggest an explanation for enhanced Mn2+ emission near the surface of the microplate. Near the surface, the contribution from the longer lifetime component increases, which implies that the population of excited Mn2+ is higher near the surface. This may arise due to the increased probability of carrier recombination at a Mn2+ dopant near the surface, possibly enabled by an increased concentration of traps.
We report on experimental activities on HiRES, a novel ultrafast electron diffraction beamline under development at LBNL. The instrument provides high-flux of relativistic electron pulses with sub-picosecond duration, which are then shaped in transverse and longitudinal phase space producing small spot sizes with femtosecond resolution. Alternatively beam shaping can be used to achieve large lateral coherence lengths for chemical and biological applications.
Recent work on laser-induced crystallization of thin films and nanostructures is presented. Characterization of the morphology of the crystallized area reveals the optimum conditions for sequential lateral growth in a-Si thin films under
high-pulsed laser irradiation. Silicon crystal grains of several micrometers in lateral dimensions can be obtained
reproducibly.
Laser-induced grain morphology change is observed in silicon nanopillars under a transmission electron microscopy (TEM) environment. The TEM is coupled with a near-field scanning optical microscopy (NSOM) pulsed laser processing system. This combination enables immediate scrutiny on the grain morphologies that the pulsed laser
irradiation produces. The tip of the amorphous or polycrystalline silicon pillar is transformed into a single crystalline
domain via melt-mediated crystallization. The microscopic observation provides a fundamental basis for laser-induced
conversion of amorphous nanostructures into coarse-grained crystals.
A laser beam shaping strategy is introduced to control the stochastic dewetting of ultrathin silicon film on a foreign
substrate under thermal stimulation. Upon a single pulse irradiation of the shaped laser beam, the thermodynamically
unstable ultrathin silicon film is dewetted from the glass substrate and transformed to a nanodome. The results suggest that the laser beam shaping strategy for the thermocapillary-induced de-wetting combined with the isotropic etching is a
simple alternative for scalable manufacturing of array of nanostructures.
KEYWORDS: Crystals, Silicon, Near field scanning optical microscopy, Near field optics, Semiconductor lasers, Laser processing, Semiconductors, Laser crystals, Scanning electron microscopy, Nanowires
Recent research results are presented where lasers of different pulse durations and wavelengths have been coupled to
near-field-scanning optical microscopes (NSOMs) through apertured bent cantilever fiber probes and atomic force
microscope (AFM) tips in apertureless configurations. Experiments have been conducted on the surface modification of
metals and semiconductor materials. By combining nanoscale ablative material removal with subsequent chemical
etching steps, ablation nanolithography and patterning of fused silica and crystalline silicon wafers has been
demonstrated. Confinement of laser-induced crystallization to nanometric scales has also been shown. In-situ observation of the nanoscale materials modification was conducted by coupling the NSOM tips with a scanning electron
microscope (SEM). Nucleation and growth of semiconductor materials have been achieved by laser chemical vapor
deposition (LCVD) at the nanoscale level. Locally selective growth of crystalline silicon nanowires with controlled size,
heterogeneity and nanometric placement accuracy has been accomplished.
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