Ultrafast pump-probe force microscopy with nanoscale resolution

Azimuthally polarized beams are gaining fundamental importance for near-field force microscopy systems to inspect photoinduced magnetism in special molecules or nanostructures, due to their strong axial magnetic field and vanishing electric field. The magnetic dominant region represents a unique trait of such a beam as a potentially ideal structured light to probe photoinduced magnetism at the nanoscale. Therefore, we present a near-field characterization of an optical, sharply focused azimuthally polarized beam using photoinduced force microscopy, a technique with simultaneous near-field excitation and detection, achieving nanoscale resolution well beyond the diffraction limit. Such a method exploits the photoinduced gradient force on a nanotip, mechanically detected as forced oscillations of the cantilever in an atomic force microscopy system upon external light illumination. The photoinduced force is strongly localized, which that depends only on the near-field signal free from background scattering ph...

Atomic force microscopy (AFM) has developed to become a very versatile nano-scale technique to reveal the three-dimensional (3D) morphology of amyloid aggregates under physiological conditions. However, the imaging principle of AFM is based on measuring the 'force' between a sharp tip and a given nanostructure, which may cause mechanical deformation of relatively soft objects. To avoid the deformation, scanning ion conductance microscopy (SICM) is an alternative scanning probe microscopy technique, operating with alternating current mode. Here we can indeed reveal the 3D morphology of amyloid fibrils and it is capable of exploring proteins with nanoscale resolution. Compared with conventional AFM, we show that SICM can provide precise height measurements of amyloid protein aggregates, a feature that enables us to obtain unique insight into the detailed nucleation and growth mechanisms behind amyloid self-assembly.

When light illuminates the junction formed between a sharp metal tip and a sample, different mechanisms can contribute to the measured photoinduced force simultaneously. Of particular interest are the instantaneous force between the induced dipoles in the tip and in the sample, and the force related to thermal heating of the junction. A key difference between these 2 force mechanisms is their spectral behavior. The magnitude of the thermal response follows a dissipative (absorptive) Lorentzian line shape, which measures the heat exchange between light and matter, while the induced dipole response exhibits a dispersive spectrum and relates to the real part of the material polarizability. Because the 2 interactions are sometimes comparable in magnitude, the origin of the chemical selectivity in nanoscale spectroscopic imaging through force detection is often unclear. Here, we demonstrate theoretically and experimentally how the light illumination gives rise to the 2 kinds of photoinduced forces at the tip-sample junction in the midinfrared. We comprehensively address the origin of the spectroscopic forces by discussing cases where the 2 spectrally dependent forces are entwined. The analysis presented here provides a clear and quantitative interpretation of nanoscale chemical measurements of heterogeneous materials and sheds light on the nature of light-matter coupling in optomechanical force-based spectronanoscopy.

Ultrafast pump-probe microscopy has been utilized as a promising new ultrafast approach. Transient absorption microscopy enables the visualization and localization of the photochemical and photophysical processes in complex systems. Ultrafast diffuse reflectance spectroscopy is of great value for in-situ time-resolved analysis of complex photophysical processes in opaque or highly absorbing materials and devices.We integrated pump-probe transient absorption microscopy (fs-DR-TAM) with ultrafast diffuse-reflectance spectroscopy. We showed that the technique allows the in-situ localization and visualization of charge carrier kinetics within the sample. I will discuss the latest advancements in this area and the integration of different ultrafast microscopy-spectroscopy techniques for sophisticated time-resolved photochemical and photophysical analysis. I will also discuss the advancement in ultrafast image analysis for image segmentation and ultrafast photochemical dynamics deconvolution at a few hundred-nanometer spatial resolution and femtoseconds time resolution.Inorganic perovskite solar cells (PVSCs) have drawn increasing attention because of their stability compared to organic-inorganic devices. We have employed ultrafast microscopy and sophisticated image analysis to understand the photochemical understanding of perovskite solar cells prepared under different experimental conditions.

The prospect of using Tm2+-doped halides for luminescence solar concentrators (LSCs) requires a thorough understanding of the temperature dependent Tm2+ excited states dynamics that determines the internal quantum efficiency (QE) and thereby the efficiency of the LSC. In this study we investigated the dynamics in CaX 2:Tm2+ (X = Cl, Br, I) by temperature- and time-resolved measurements. At 20 K up to four distinct Tm2+ emissions can be observed. Most of these emissions undergo quenching via multi-phonon relaxation below 100 K. At higher temperatures, only the lowest energy 5d–4f emission and the 4f–4f emission remain. Fitting a numerical rate equation model to the data shows that the subsequent quenching of the 5d–4f emission is likely to occur initially via multi-phonon relaxation, whereas at higher temperatures additional quenching via interband crossing becomes thermally activated. At room temperature only the 4f–4f emission remains and the related QE becomes close to 30%. Possible reasons for the quantum efficiency not reaching 100% are provided.

Near-field optical microscopy (NSOM) is a scanning probe technique that allows optical imaging of sample surfaces with nanoscale resolution. Generally, all NSOM schemes rely on illuminating the sample surface and collecting the localized scattered light resulting from the interaction of the microscopes nanoscale probe with the sample surface in the illuminated region. Currently, a new set of nanospectroscopic techniques are being developed using Atomic Force Microscopes to detect optical interactions without detecting any light. One of these approaches is photoinduced force microscopy (PiFM), where local optical forces, originated by the illumination of the tip–sample region, are mechanically detected as forced oscillations of the cantilever of an atomic force microscope operating in a multifrequency mode. In this article we show high resolution nanoimaging via PiFM with a contrast only related to the local refractive index of a sample specifically designed to unambiguously decouple morphology from optica...

The photoinitiated proton-coupled electron transfer (PCET) process in photoacid-based adducts is predominantly governed by the evolution of the electron-proton transfer state. However, such a process is underexplored in the case of photobases as the excited states evolve through multiple competitive channels. Here, we elucidate the excited-state dynamics of a photobase, 4-[4'-(dimethylamino)styryl]pyridine (DMASP), in the presence of hexafluoroisopropanol (HFIP) that enables PCET. Transient absorption measurements show the evolution of a protonated species in the excited state with a time constant of ∼2.5 ps. Fluorescence upconversion measurements reveal the signatures of an emissive intramolecular charge transfer state and a protonated state. The role of such states is further confirmed by time-resolved measurements in the presence of trifluoroacetic acid and computational analysis. Furthermore, the proton-abstraction dynamics of DMASP is analyzed in bulk methanol and butanol solvents. The extent of proton abstraction by DMASP is found to be higher in the presence of HFIP when compared with the normal alcohols over a time period of a few picoseconds.

The tip-sample interaction force measurements in atomic force microscopy (AFM) provide information about materials' properties with nanoscale resolution. The T-shaped cantilevers used in Torsional-Harmonic AFM allow measuring the rapidly changing tip-sample interaction forces using the torsional (twisting) deflections of the cantilever due to the off-axis placement of the sharp tip. However, it has been difficult to calibrate these cantilevers using the commonly used thermal noise-based calibration method as the mechanical coupling between flexural and torsional deflections makes it challenging to determine the deflection sensitivities from force-distance curves. Here, we present thermal noise-based calibration of these T-shaped AFM cantilevers by simultaneously analyzing flexural and torsional thermal noise spectra, along with deflection signals during a force-distance curve measurement. The calibration steps remain identical to the conventional thermal noise method, but a computer performs additional calculations to account for mode coupling. We demonstrate the robustness of the calibration method by determining the sensitivity of calibration results to the laser spot position on the cantilever, to the orientation of the cantilever in the cantilever holder, and by repeated measurements. We validated the quantitative force measurements against the known unfolding force of a protein, the I91 domain of titin, which resulted in consistent unfolding force values among six independently calibrated cantilevers.

A model for the 77 K excited state dynamics in Chlamydomonas reinhardtii in state 1 and state 2

Spin-orbit torque (SOT) is currently the subject of intense research activity due to its potential for switching storage elements in magnetic random access memory[1], generating microwave frequency spin wave auto-oscillations in spin Hall nano-oscillators (SHNOs)[2], and manipulating magnetic textures, e.g. skyrmions[3] in magnetic logic devices. Device operation is underpinned by the spin Hall effect, in which a spin current can be efficiently generated at the interface between a ferromagnetic metal (FM) and a current carrying heavy metal (HM) underlayer due to strong spin-orbit coupling in the HM. The spin current may then exert a SOT on the magnetization of the FM, exciting magnetization dynamics central to device function.A recent study of nano-contacted SHNOs[2] revealed a spatial dependence of the SOTs due to the current distribution within the HM of the device. Moreover, micron and sub-micron scale ferromagnetic elements are known to support non-uniform equilibrium states resulting from strong demagnetizing fields[4]. Since the in-plane (Slonczewski) spin-torque term of the Landau-Lifshitz equation of motion depends upon the relative local orientation of the magnetization and the injected spin polarization, the resulting SOT can be spatially dependent. At the same time the Oersted (Oe) field generated by the DC and RF currents (IDC, IRF) in the HM will also lead to a spatially dependent DC deflection and RF excitation of the magnetization respectively. Therefore, to explore the influence of non-uniform SOTs at the nanoscale, a spatially resolved probe of the resulting magnetization dynamics is required.In this work time-resolved scanning Kerr microscopy (TRSKM) was used to acquire spatiotemporally resolved SOT ferromagnetic resonance (FMR) spectra from an in-plane magnetized Co40Fe40B20(2 nm) ellipse of size 2×0.8 μm2 positioned at the centre of a Pt(6 nm) Hall cross[5]. DC and RF currents were combined using a bias tee and passed through the 2 μm wide Pt lead perpendicular to the long axis of the ellipse, such that the resulting spin polarisation (σ(t)) and Oe-fields (h(t)) were parallel to the long axis of the ellipse, Figure 1(a). Preliminary time-resolved polar Kerr measurements in response to a broadband current pulse allowed the resonance frequency (~2.7 GHz) to be determined for an in-plane bias magnetic field (HB) of 200 Oe, while the time-resolved measurements at HB = 200 Oe in response to an RF current with frequency (fRF) of 2.72 GHz allowed the time delay of node and antinodes of precession at resonance to be determined. Sweeping the magnetic field with the time delay set to a node or an antinode allowed the real or imaginary components of the dynamic susceptibility to be probed respectively. Time-resolved polar Kerr images, Figs. 1(b) and 1(c) revealed that the dynamics at resonance were quasi-uniform at the center of the ellipse, while weaker signal at the edge was consistent with the spatial resolution of ~400 nm. SOT-FMR spectra were therefore acquired from the center of the ellipse for a range of field angles θB and IDC, while fRF was set to 3.2 GHz so that the resonance field was well separated from the field at which switching of the equilibrium magnetization occurred.Optically-detected SOT-FMR spectra acquired with HB applied along the HA (θB = 90°), and IDC = 0 mA showed negligible influence of SOT, i.e. symmetric FMR peaks with respect to field with no broadening or narrowing when magnetized in opposite HA directions. A macrospin calculation reproduced the peaks and a damping parameter α of 0.03 was extracted, similar to the previously reported value for these devices[1]. The coefficient of the in-plane torque term (ST) was assumed to be zero in the HA calculations with IDC = 0 mA. The value of IDC was then increased from 1 mA to 10 mA and spectra measured with HB along the HA, which again showed symmetric FMR peaks. However, when the bias field was applied 30° to either side of the HA, marked asymmetry in both amplitude and linewidth were observed, in addition to a combined one- and two-fold angular dependence of the resonance field due to the DC Oe-field and shape anisotropy respectively. For a particular field history, the linewidth exhibited a marked crossover from broad to narrow as θB was changed from +30° to -30° from the HA. These observations were reproduced by the macrospin calculations, but failed to reproduce the linewidth narrowing/broadening for a single value of ST with α = 0.03, while a smaller value of α = 0.025 reproduced the linewidth of all FMR peaks with ST = (6.75±0.75)×10-7 Oe A-1 cm2 .This work paves the way for spatially resolved characterization of SOT devices at the deep nanoscale, e.g. using near-field magneto-optical techniques[6,7], that is needed to achieve greater understanding of SOT-induced dynamics. **

This thesis presents HELIOS, an in-house laboratory for time-resolved pump-probe spectroscopy with extreme-ultraviolet (XUV) probe radiation. A wide span of pump wavelengths can be generated using commercial laser equipment while XUV probe radiation is generated via a high harmonic generation process in a noble gas delivering probe photons with energies between 20 eV and 72 eV. The XUV beam path features a time-preserving monochromator and was constructed and built in-house. HELIOS features an overall time resolution of about 50 fs when using 800 nm pump and 41 eV probe photons. An energy resolution of 110 meV at 41 eV photon energy can be achieved. HELIOS features two beamlines. One µ-focus beamline with an XUV focal size of about 20 µm can be used with experiments that require such a small XUV focal size as well as with different end stations. The other beamline features a semi-permanently mounted end station for angle-resolved photoelectron spectroscopy under ultra-high vacuum conditions.Experiments demonstrating the usability of HELIOS and the two beamlines are presented. A pump-probe measurement on graphene demonstrates the capability of determining a large part of the k-space in only one measurement due to the use of an ARTOF angle-resolved time-of-flight electron spectrometer. A non-angle-resolved pump-probe measurement on the conducting polymer PCPDTBT demonstrates the high signal-to-noise ratio achievable at this beamline in non-angle-resolved photoelectron-spectroscopy pump-probe measurements. The usability of the µ-focus beamline is demonstrated with time-resolved measurements on magnetic samples employing an in-house-designed spectrometer. These experiments allow the retrieval of element-specific information on the magnetization within a sample employing the transversal magneto-optical Kerr effect (T-MOKE).Additionally, a Fourier transform spectrometer for the XUV is presented, the concept was tested at a synchrotron and it was used to determine the longitudinal coherence of the XUV radiation at HELIOS.

Perspectives on in situ electron microscopy

We introduce a novel instrument combining femtosecond pump-probe spectroscopy and confocal microscopy for spatio-temporal imaging of excited-state dynamics of phase-separated polymer blends. Phenomena occurring at interfaces between different materials are crucial for optimizing the device performances, but are poorly understood due to the variety of possible electronic states and processes involved and to their complicated dynamics. Our instrument (with 200-fs temporal resolution and 300-nm spatial resolution) provides new insights into the properties of polymer blends, revealing spatially variable photo-relaxation paths and dynamics and highlighting a peculiar behaviour at the interface between the phase-separated domains.

A detailed investigation of the spectral and photophysical properties of minocycline (MC) in water at different pHs, solvents of different polarity, and micellar surfactant solutions was carried out in this study. An unusual behavior was highlighted with respect to other tetracyclines due to the presence of an additional dimethylamino group in the MC molecular structure. In particular, four equilibrium constants associated with mono-deprotonation reactions were characterized by steady-state spectroscopy. Femtosecond time-resolved pump-probe and fluorescence up-conversion measurements allowed the dynamics of the lowest excited singlet state of the five different acid-base species of MC to be characterized in terms of lifetimes and transient spectra. Two emissive species associated with keto-enol tautomerism resulting from excited-state intramolecular proton transfer (ESIPT) were revealed with time constants of a few and tens of picoseconds. TD-DFT quantum mechanical calculations were also performed to define the state order and nature of the differently protonated species, together with their absorption spectra. The role of pH proved to be fundamental in modulating the drug charge and therefore the interaction with cationic micelles where the neutral form of MC, that is the biologically active one, resulted efficiently included.

Excimer, a configurational mixing between Frenkel exciton and charge-transfer resonance states, is typically regarded as a trap state that hinders desired energy or charge-transfer processes in artificial molecular assemblies. However, in recent days, the excimer has received much attention as a functional intermediate in the excited-state dynamics such as singlet fission or charge-separation processes. In this work, we show that the relative contribution to charge-transfer resonance of the excimer state in a bay-substituted perylene bisimide dimer cyclophane can be modulated by dielectric properties of the solvents employed. Solvent-dependent time-resolved fluorescence and absorption measurements reveal that an enhancement of charge-transfer resonance in the excimer state is reflected by incomplete symmetry-breaking charge-separation processes from the structurally relaxed excimer state by means of dipolar solvation processes in the high dielectric environment.