Time-resolved Microscopy Research Articles

Time-resolved electrostatic force microscopy using tip-synchronized charge generation with pulsed laser excitation

Nanoscale observation of charge distribution and electric polarization is crucial for understanding and controlling functional materials and devices. In particular, the importance of charge dynamics is well recognized, and direct methods to observe charge generation, transfer, and recombination processes are required. Here, we describe tip-synchronized time-resolved electrostatic force microscopy. Numerical modeling clarifies that the tip-synchronized method provides temporal resolution with the timescale of the cantilever oscillation cycle. This method enables us to resolve sub-microsecond charge migration on the surface. The recombination of photo-excited carriers in a bilayer organic photovoltaic thin film is observed as a movie with a 0.3 µs frame step time resolution. Analysis of the images shows that the carrier lifetime is 2.3 µs near the donor/acceptor interface. The tip-synchronized method increases the range of time-resolved electrostatic force microscopy, paving the way for studies of nanoscale charge dynamics.

Current-induced domain wall oscillations in a nanowire imaged by time-resolved photoemission electron microscopy

Abstract We study reversible domain wall motion in half-ring Ni 80 Fe 20 nanowires on a nanosecond (ns) timescale in a truly current-induced pump-probe experiment using an energy filtered, aberration-corrected photoemission electron microscope. The X-ray magnetic circular dichroism signal is probed at different time delays before, during and after the current pulse in a stroboscopic mode with circularly polarized synchrotron radiation in the energy range of the Fe L 3 -edge (707 eV). We observe lateral domain wall oscillations with a frequency of ∼ 0.4 GHz. Comparing the results to a proposed string model, we find that the domain wall oscillations can be described as string-like asymmetric oscillations.

Photoassisted and multiphoton emission from single-crystal diamond needles.

Practical realization of stable and high brightness sources of ultra-short electron pulses is an important issue in the development of time-resolved electron microscopy for the study of ultra-fast dynamics in materials. Here, we report on the experimental investigation of static (in the dark) and pulsed (under illumination by sub-picosecond laser pulses at 1040 nm) electron emission from single-crystal diamond needles. A significant increase of electron emission current was detected under laser illumination. The nonlinear dependence of the emission current on the laser intensity and on the angle between the needle and the laser beam polarization axis suggests multi-photon emission processes. This interpretation is in agreement with electron spectroscopy measurements performed for electrons emitted at different bias voltages and different laser power levels and repetition rates. The remarkable feature of the diamond emitters is their stability under high average power of laser radiation. This provides a new highly efficient source of photoemitted electrons based on single-crystal diamond.

Simulation of time-dependent ionization processes in acetylene

We have investigated nuclear dynamics in bound and dissociating acetylene molecular ions in a time-resolved reaction microscopy experiment with a pair of few-cycle pulses. Different ionization processes were observed for acetylene. These time-dependent ionization processes are simulated using semi-classical on-the-fly dynamics revealing the underling mechanisms.

Perspective: Towards single shot time-resolved microscopy using short wavelength table-top light sources.

Time-resolved imaging allows revealing the interaction mechanisms in the microcosm of both inorganic and biological objects. While X-ray microscopy has proven its advantages for resolving objects beyond what can be achieved using optical microscopes, dynamic studies using full-field imaging at the nanometer scale are still in their infancy. In this perspective, we present the current state of the art techniques for full-field imaging in the extreme-ultraviolet- and soft X-ray-regime which are suitable for single exposure applications as they are paramount for studying dynamics in nanoscale systems. We evaluate the performance of currently available table-top sources, with special emphasis on applications, photon flux, and coherence. Examples for applications of single shot imaging in physics, biology, and industrial applications are discussed.

Onset of optical emission in femtosecond laser-induced plasmas and its correlation with surface dynamics monitored by pump–probe time-resolved microscopy

Femtosecond time-resolved images acquired by using a two-color pump–probe microscope have been used in combination with optical emission spectroscopy to determine the time scales for the onset of atomic and molecular emission in laser-induced plasmas.

High-Frequency Time-Resolved Scanning Acoustic Microscopy for Biomedical Applications

High-frequency focused ultrasound has emerged as a powerful modality for both biomedical imaging and elastography. It is gaining more attention due to its capability to outperform many other imaging modalities at a submicron resolution. Besides imaging, high-frequency ultrasound or acoustic biomicroscopy has been used in a wide range of applications to assess the elastic and mechanical properties at the tissue and single cell level. The interest in acoustic microscopy stems from the awareness of the relationship between biomechanical and the underlying biochemical processes in cells and the vast impact these interactions have on the onset and progression of disease. Furthermore, ultrasound biomicroscopy is characterized by its non-invasive and non-destructive approach. This, in turn, allows for spatiotemporal studies of dynamic processes without the employment of histochemistry that can compromise the integrity of the samples. Numerous techniques have been developed in the field of acoustic microscopy. This review paper discusses high-frequency ultrasound theory and applications for both imaging and elastography.

Spatial mapping of torques within a spin Hall nano-oscillator

Time-resolved scanning Kerr microscopy (TRSKM) was used to study precessional magnetization dynamics induced by a radio frequency (RF) current within a ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$/Py(5 nm)/Pt(6 nm)/Au(150 nm) spin Hall nano-oscillator structure. The Au layer was formed into two needle-shaped electrical contacts that concentrated the current in the center of a Py/Pt mesa of 4 $\ensuremath{\mu}\mathrm{m}$ diameter. Due to the spin Hall effect, current within the Pt layer drives a spin current into the Py layer, exerting a spin transfer torque (STT). By injecting RF current and exploiting the phase sensitivity of TRSKM and the symmetry of the device structure, the STT and Oersted field torques have been separated and spatially mapped. The STT and torque due to the in-plane Oersted field are observed to exhibit minima at the device center that is ascribed to spreading of RF current that is not observed for DC current. Torques associated with the RF current may destabilize the position of the self-localized bullet mode excited by the DC current and inhibit injection locking. The present study demonstrates the need to characterize both DC and RF current distributions carefully.

Control of plasmon dephasing time using stacked nanogap gold structures for strong near-field enhancement

The construction of metallic nanostructures with strong near-field enhancement is becoming increasingly significant for the practical use of plasmonic devices, such as plasmonic sensors and light-energy conversion systems. Importantly, the near-field enhancement effect depends on the plasmon dephasing time. Here, we propose a method for controlling plasmon dephasing time by utilizing plasmonic coupling for stronger near-field enhancement. Ordered arrays of stacked nanogap gold (Au) structures composed of a metal/insulator/metal nanostructure were fabricated by electron beam lithography and dry etching processes on a niobium-doped titanium dioxide substrate. The dark plasmon mode was excited by the near-field coupling between the upper and lower Au nanostructures separated by an alumina layer with a thickness of 15nm. A strong near-field enhancement effect was induced by the localization of the electromagnetic field between the upper and lower Au nanostructures and the longer plasmon dephasing time based on the excitation of the dark plasmon mode. It is noteworthy that the dephasing time of the dark plasmon mode measured by time-resolved photoemission electron microscopy was extended 3-fold compared with that of the plasmon mode of the Au nanoblock, which can be controlled by the structural design of the stacked nanogap Au structures.

Noncontact Imaging of Ion Dynamics in Polymer Electrolytes with Time-Resolved Electrostatic Force Microscopy.

Ionic-transport processes govern performance in many classic and emerging devices, ranging from battery storage to modern mixed-conduction organic electrochemical transistors (OECT). Here, we study local ion-transport dynamics in polymer films using time-resolved electrostatic force microscopy (trEFM). We establish a correspondence between local and macroscopic measurements using local trEFM and macroscopic electrical impedance spectroscopy (EIS). We use polymer films doped with lithium bis(trifluoromethane)sulfonimide (LiTFSI) as a model system where the polymer backbone has oxanorbornenedicarboximide repeat units with an oligomeric ethylene oxide side chain of length n. Our results show that the local polymer response measured in the time domain with trEFM follows stretched-exponential relaxation kinetics, consistent with the Havriliak-Negami relaxation we measure in the frequency-domain EIS data for macroscopic samples of the same polymers. Furthermore, we show that the trEFM results capture the same trends as the EIS results-changes in ion dynamics with increasing temperature, increasing salt concentration, and increasing volume fraction of ethylene oxide side chains in the polymer matrix evolve with the same trends in both measurement techniques. We conclude from this correlation that trEFM data reflect, at the nanoscale, the same ionic processes probed in conventional EIS at the device level. Finally, as an example application for emerging materials syntheses, we use trEFM and infrared photoinduced force microscopy (PiFM) to image a diblock copolymer electrolyte for next-generation solid-state energy storage applications.

Stochastic Dynamics of a Ferromagnetic Vortex Revealed by Single-Spin Magnetometry

Fast manipulation of nanoscale magnetic textures, such as vortices and skyrmions, underlies devices proposed for next-generation magnetic memory, logic, and sensing. Understanding the switching processes in these structures requires a probe with both nanometer spatial resolution and nanosecond time resolution. This study combines time-resolved magneto-optical microscopy and single-spin magnetometry to unveil the switching of a magnetic vortex between pinning sites, in response to a short magnetic field pulse. The complex dependence of switching probability on pulse duration is explained, revealing a path toward the fast, reliable control required for future nanomagnetic technology.

Time-Resolved Structured Illumination Microscopy for Phase Separation Dynamics of Water and 2-Butoxyethanol Mixtures: Interpretation of "Early Stage" Involving Micelle-Like Structures.

Phase separation dynamics of a water/2-butoxyethanol (2BE) mixture was studied with newly developed time-resolved structured illumination microscopy (SIM). Interestingly, an employed hydrophobic fluorescent probe for SIM showed spectral shifts up to 500 ns after a laser-induced temperature jump, which suggests 2BE micellar-like aggregates become more hydrophobic at the initial stage of phase separation. This hydrophobic environment in 2BE aggregates, probably due to the ejection of water molecules, continued up to at least 10 μs. Time-resolved SIM and previously reported light scattering data clearly showed that the size of a periodic structure remained constant (ca. 300 nm) from 3 to 10 μs, and then the growth of periodic structures having the self-similarity started. We think that the former and the latter processes correspond to "early stage" (concentration growth) and "late stage" (size growth), respectively, in phase separation dynamics. Here we suggest that, in the early stage, the entity to bear 2BE phase be water-poor 2BE aggregates, and the number density of these aggregates would simply increase in time.

Characterization of ultrafast plasmon dynamics in individual gold bowtie by time-resolved photoemission electron microscopy

We report on the investigation of ultrafast dynamics of the plasmonic field in individual gold bowtie nanostructure by combining interferometric time-resolved photoemission electron microscopy with a damped harmonic oscillator model. We experimentally obtain different plasmon dephasing times in the tips of the bowtie nanostructure. In the meantime, we demonstrate that the experimental time-resolved photoemission signal can be used to directly compare resonance frequency and dephasing time of different hot spots. In addition, we find that the plasmon field, which is extracted from the photoemission signal, initially oscillates at the laser field frequency, and finally develops into its eigenfrequency after experiencing a few periods of frequency fluctuation due to the competition between forced and autonomous oscillation of the plasmons.

Structural Fluctuation in Co-Pt Nanoparticles Revealed by Time-resolved High-voltage Electron Microscopy

Structural Fluctuation in Co-Pt Nanoparticles Revealed by Time-resolved High-voltage Electron Microscopy

Time-resolved terahertz time-domain near-field microscopy.

We demonstrate a novel method for measuring terahertz (THz) photoconductivity of semiconductors on length scales smaller than the diffraction limit at THz frequencies. This method is based on a near-field microscope that measures the transmission of a THz pulse through the semiconductor following photoexcitation by an ultrafast laser pulse. Combining back-excitation of the sample using a Dove prism, and a dual lock-in detection scheme, our microscope design offers a flexible platform for near-field time-resolved THz time-domain spectroscopy, using fluences available to typical laser oscillators. Experimental results on a thin film of gallium arsenide grown by metal organic chemical vapor deposition are presented as a proof-of-concept, demonstrating the ability to map the complex conductivity as well as sub-ps dynamics of photoexcited carriers with a resolution of λ/10 at 0.5 THz.

Time resolved imaging of the non-linear bullet mode within an injection-locked nano-contact spin Hall nano-oscillator

Time-resolved scanning Kerr microscopy (TRSKM) has been used to image precessional magnetization dynamics excited by a DC current within a nano-contact (NC) spin Hall nano-oscillator (SHNO). Injection of a radio frequency (RF) current was used to phase lock the SHNO to TRSKM. The out of plane magnetization was detected by means of the polar magneto optical Kerr effect (MOKE). However, longitudinal MOKE images were dominated by an artifact arising from the edges of the Au NCs. Time resolved imaging revealed the simultaneous excitation of a non-linear “bullet” mode at the centre of the device, once the DC current exceeded a threshold value, and ferromagnetic resonance (FMR) induced by the RF current. However, the FMR response observed for sub-critical DC current values exhibits an amplitude minimum at the centre, which is attributed to spreading of the RF spin current due to the reactance of the device structure. This FMR response can be subtracted to yield images of the bullet mode. As the DC current is increased above threshold, the bullet mode appears to increase in size, suggesting increased translational motion. The reduced spatial overlap of the bullet and FMR modes, and this putative translational motion, may impede the injection locking and contribute to the reduced locking range observed within NC-SHNO devices. This illustrates a more general need to control the geometry of an injection-locked oscillator so that the autonomous dynamics of the oscillator exhibit strong spatial overlap with those resulting from the injected signal.

Few cycle pulse laser ablation study of single layer TiO2 thin films using time resolved surface microscopy.

The few cycle optical pulse induced strong field laser solid interaction is a rich area of fundamental and applied research that spans from the study of extreme non-linearities in solids and next generation ultra-broad band high damage threshold optics design and fabrication to peta-Hertz optoelectronics of the future. Our understanding of the extremely non-pertubative phenomena of few cycle pulse (FCP) laser damage and ablation of bulk solids and thin films is still limited. In this work, we present a systematic study of the dynamics of the FCP laser ablation process of single layer TiO2 thin films from 1 ps to 10 ns after a single 9 fs pulse with nominal wavelength of 760 nm interacts with the surface using time-resolved surface microscopy (TRSM) technique. It is observed that FCP ablation craters for certain films exhibit markedly different features when compared to those created by 50 - 150 fs pulses with similar fluences. TRSM measurements also reveal that FCP ablation dynamics strongly depend on the thickness-dependent E-field distribution inside the films (nominally λ/2 vs λ/4), in which the dynamics of free carrier generation due to strong field ionization may play an important role as well. A one-dimensional finite-difference time-domain (FDTD) simulation that takes into account strong field ionization and free carrier absorption is used in conjunction with the TRSM measurements to estimate the excited free carrier density prior to ablation. We also propose a mechanism for the differences in ablation craters between the films based on the FDTD simulation results.

Fabrication and photoluminescent properties of Tb3+ doped carbon nanodots

Carbon nanodots (CNDs) doped with Tb ions were synthesized using different synthetic routes: hydrothermal treatment of a solution containing carbon source (sodium dextran sulfate) and TbCl3; mixing of CNDs and TbCl3 solutions; freezing-induced loading of Tb and carbon-containing source into pores of CaCO3 microparticles followed by hydrothermal treatment. Binding of Tb ions to CNDs (Tb-CND coupling) was confirmed using size-exclusion chromatography and manifested itself through a decrease of the Tb photoluminescence lifetime signal. The shortest Tb photoluminescence lifetime was observed for samples obtained by hydrothermal synthesis of CaCO3 microparticles where Tb and carbon source were loaded into pores via the freezing-induced process. The same system displays an increase of Tb photoluminescence via energy transfer with excitation at 320–340 nm. Based on the obtained results, freezing-induced loading of cations into CNDs using porous CaCO3 microparticles as reactors is proposed to be a versatile route for the introduction of active components into CNDs. The obtained CNDs with long-lived emission may be used for time-resolved imaging and visualization in living biological samples where time-resolved and long-lived luminescence microscopy is required.

Viscoelastic Particle–Laden Interface Inhibits Coffee-Ring Formation

We investigate the evaporation-driven pattern formation in drying drops containing mixtures of polystyrene and soft microgel particles. The well-known coffee-rings that form when drops containing polystyrene particles are dried can be completely undone in the presence of a small quantity of soft colloids. The addition of soft colloids facilitates the adsorption of polystyrene particles to the water-vapor interface leading to a steep increase in their concentration and also imparts viscoelasticity to the interface. Time-resolved video microscopy is used to conclusively show the formation of a gel-like particle-laden interface. The mean square displacement of the polystyrene particles adsorbed to the interface confirms their immobile nature at the interface. This viscoelastic interface almost prevents the bulk flow-assisted migration of polystyrene particles toward the drop edge, leading to the suppression of coffee-ring effect and the formation of uniform particulate deposits.

Adatom dynamics and the surface reconstruction of Si(110) revealed using time-resolved electron microscopy

Surface dynamics lie at the heart of many areas of materials and chemical science, including heterogeneous catalysis, epitaxial growth, and device fabrication. Characterizing the dynamics of surface adsorption, reactions, and diffusion at the atomic scale is crucial to understanding and controlling such processes. Here, we use aberration-corrected scanning transmission electron microscopy to analyze the diffusive behavior of Pt atoms adsorbed on the Si(110) surface and characterize the effects of the electron beam on adatom motion, including a bias introduced by the raster scan of the probe. We further observe the evolution of the Si(110) surface, revealing evidence of developing surface steps attributed to the 16 × 2 surface reconstruction. These results demonstrate a framework for studying complex atomic-scale surface dynamics using aberration-corrected electron microscopy.