Pump-probe Microscopy Research Articles

A pre-time-zero spatiotemporal microscopy technique for the ultrasensitive determination of the thermal diffusivity of thin films.

Diffusion is one of the most ubiquitous transport phenomena in nature. Experimentally, it can be tracked by following point spreading in space and time. Here, we introduce a spatiotemporal pump-probe microscopy technique that exploits the residual spatial temperature profile obtained through the transient reflectivity when probe pulses arrive before pump pulses. This corresponds to an effective pump-probe time delay of 13ns, determined by the repetition rate of our laser system (76MHz). This pre-time-zero technique enables probing the diffusion of long-lived excitations created by previous pump pulses with nanometer accuracy and is particularly powerful for following in-plane heat diffusion in thin films. The particular advantage of this technique is that it enables quantifying thermal transport without requiring any material input parameters or strong heating. We demonstrate the direct determination of the thermal diffusivities of films with a thickness of around 15nm, consisting of the layered materials MoSe2 (0.18 cm2/s), WSe2 (0.20 cm2/s), MoS2 (0.35 cm2/s), and WS2 (0.59 cm2/s). This technique paves the way for observing nanoscale thermal transport phenomena and tracking diffusion of a broad range of species.

Time-resolved transmission electron microscopy for nanoscale chemical dynamics.

The ability of transmission electron microscopy (TEM) to image a structure ranging from millimetres to Ångströms has made it an indispensable component of the toolkit of modern chemists. TEM has enabled unprecedented understanding of the atomic structures of materials and how structure relates to properties and functions. Recent developments in TEM have advanced the technique beyond static material characterization to probing structural evolution on the nanoscale in real time. Accompanying advances in data collection have pushed the temporal resolution into the microsecond regime with the use of direct-electron detectors and down to the femtosecond regime with pump-probe microscopy. Consequently, studies have deftly applied TEM for understanding nanoscale dynamics, often in operando. In this Review, time-resolved in situ TEM techniques and their applications for probing chemical and physical processes are discussed, along with emerging directions in the TEM field.

Optical bulk-boundary dichotomy in a quantum spin Hall insulator

The bulk-boundary correspondence is a critical concept in topological quantum materials. For instance, a quantum spin Hall insulator features a bulk insulating gap with gapless helical boundary states protected by the underlying Z2 topology. However, the bulk-boundary dichotomy and distinction are rarely explored in optical experiments, which can provide unique information about topological charge carriers beyond transport and electronic spectroscopy techniques. Here, we utilize mid-infrared absorption micro-spectroscopy and pump–probe micro-spectroscopy to elucidate the bulk-boundary optical responses of Bi4Br4, a recently discovered room-temperature quantum spin Hall insulator. Benefiting from the low energy of infrared photons and the high spatial resolution, we unambiguously resolve a strong absorption from the boundary states while the bulk absorption is suppressed by its insulating gap. Moreover, the boundary absorption exhibits strong polarization anisotropy, consistent with the one-dimensional nature of the topological boundary states. Our infrared pump–probe microscopy further measures a substantially increased carrier lifetime for the boundary states, which reaches one nanosecond scale. The nanosecond lifetime is about one to two orders longer than that of most topological materials and can be attributed to the linear dispersion nature of the helical boundary states. Our findings demonstrate the optical bulk-boundary dichotomy in a topological material and provide a proof-of-principal methodology for studying topological optoelectronics.

Externally-triggerable optical pump-probe scanning tunneling microscopy with a time resolution of tens-picosecond

Photoinduced carrier dynamics of nanostructures play a crucial role in developing novel functionalities in advanced materials. Optical pump-probe scanning tunneling microscopy (OPP-STM) represents distinctive capabilities of real-space imaging of such carrier dynamics with nanoscale spatial resolution. However, combining the advanced technology of ultrafast pulsed lasers with STM for stable time-resolved measurements has remained challenging. The recent OPP-STM system, whose laser-pulse timing is electrically controlled by external triggers, has significantly simplified this combination but limited its application due to nanosecond temporal resolution. Here we report an externally-triggerable OPP-STM system with a temporal resolution in the tens-picosecond range. We also realize the stable laser illumination of the tip-sample junction by placing a position-movable aspheric lens driven by piezo actuators directly on the STM stage and by employing an optical beam stabilization system. We demonstrate the OPP-STM measurements on GaAs(110) surfaces, observing carrier dynamics with a decay time of sim 170 ps and revealing local carrier dynamics at features including a step edge and a nanoscale defect. The stable OPP-STM measurements with the tens-picosecond resolution by the electrical control of laser pulses highlight the potential capabilities of this system for investigating nanoscale carrier dynamics of a wide range of functional materials.

Nondestructive Nonlinear Optical Microscopy Revealed the Blackening Mechanism of Ancient Chinese Jades.

Jade is most valued in Chinese culture since ancient times. For unearthed jade artifacts, the alteration color resulting from weathering effects and human activities provides information for cultural heritage conservation, archaeology, and history. Currently, the noninvasive 3-dimensional characterization of jade artifacts with high chemical and spatial resolution remains challenging. In this work, we applied femtosecond pump-probe microscopy and second harmonic generation microscopy techniques to study the black alteration of an ancient jade artifact of the late Spring and Autumn period (546 to 476 BC). The direct cause of the "mercury alteration" phenomena was discovered to be the conversion of metacinnabar from buried cinnabar in the tomb. Furthermore, a 3-dimensional optical reconstruction of the black alteration was achieved, providing a high-resolution method for analyzing the blackening mechanism without the need of sample damage. Our approach opens up new opportunities to extract microscopic spatiochemical information for a broad range of alteration colors in jade artifacts.

Label-Free Imaging of Humic Substance Bioaccumulation by Pump–Probe Microscopy

Humic substances (HS) are the most abundant forms of natural organic matter on the earth surface. Comprised of decomposed plant and animal materials rich in carbon, oxygen, hydrogen, nitrogen, and sulfur complexes, HS facilitate global carbon and nitrogen cycling and the transport of anthropogenic contaminants. While it is known that HS also interact with organisms at different trophic levels to produce beneficial and harmful effects whether HS exert these biological effects through accumulation remains unknown. Current radiolabeling techniques, which only detect the amount of accumulated radiolabels, cannot visualize the transport and accumulation behavior of HS. Here, using a label-free method based on pump-probe microscopy, we show HS entered the protozoan Tetrahymena thermophila, zebrafish embryos, and human cells and exerted direct effects on these organisms. HS accumulated in the nucleus of T. thermophila, chorion pore canals of zebrafish embryos, and nucleus of intestinal and lung cells in a concentration- and time-dependent way. Epigenetic and transcriptomics assays show HS altered chromatin accessibility and gene transcription in T. thermophila. In zebrafish larvae, HS induced neurotoxicity, altering spontaneous muscle contraction and locomotor activity. Detailed images showing HS accumulation in our study reveal new insights on the ecological and environmental behavior of HS.

Thermo-mechanical coupling induced Near-threshold double pulses femtosecond laser ablation enhancement in titanium

Near-threshold femtosecond laser surface treatment of titanium produces functional nanostructures, which has received much attention in applications and scientific researches. However, researches on the mechanism of near-threshold temporally-shaped femtosecond laser ablation of titanium are scarce, because the difficulty of investigating the modification mechanism of surface properties by sub-pulses below the ablation threshold. In this study, the double-pulse femtosecond laser ablation with various pulse separation was investigated. We found that the maximum ablation area appeared around 1 ps pulse separation at different fluences, and the ablation enhancement weakened with the increase of the fluence. The ultrafast pump–probe microscopy technique and ultrafast thermoelasticity model were used to investigate the mechanism. The result of pump–probe microscopy technique indicated that the enhancement of the area was caused by the decrease of transient reflectivity in the early stages which facilitated the subsequent sub-pulse energy absorption. The theoretical results indicate that the maximum ablation enhancement occurred at 1 ps, which is due to the combination of thermal and mechanical effects. The maximum of long axis diameter of periodic structure also appeared at 1 ps in multi-point processing. These findings have significant implications for the modulation of titanium surface nanostructures and providing theoretical support for processing.

Efficient pump-probe sampling with a single-cavity dual-comb laser: Application in ultrafast photoacoustics

Ultrafast pump-probe measurements are used to characterize various samples, such as biological cells, bulk, and thin-film structures. However, typical implementations of the pump-probe apparatus are either slow or complex and costly hindering wide deployment. Here we combine a single-cavity dual-comb laser with a simple experimental setup to obtain pump-probe measurements with ultra-high sensitivity, fast acquisition, and high timing precision over long optical delay scan ranges of 12.5 ns that would correspond to a mechanical delay of about 3.75 m. We employ digital signal balancing to obtain shot-noise-limited detection compatible with pump-probe microscopy deployment. Here we demonstrate ultrafast photoacoustics for thin-film sample characterization. We measured a tungsten layer thickness of (700 ± 4) Å with shot-noise-limited detection. Such single-cavity dual-comb lasers can be used for any pump-probe measurements and are especially well-suited for ultrafast photoacoustic studies such as involving ultrasonic echoes, Brillouin oscillations, surface acoustic waves and thermal dynamics.

Direct observation of ultrafast singlet exciton fission in three dimensions

We present quantitative ultrafast interferometric pump-probe microscopy capable of tracking of photoexcitations with sub-10 nm spatial precision in three dimensions with 15 fs temporal resolution, through retrieval of the full transient photoinduced complex refractive index. We use this methodology to study the spatiotemporal dynamics of the quantum coherent photophysical process of ultrafast singlet exciton fission. Measurements on microcrystalline pentacene films grown on glass (SiO2) and boron nitride (hBN) reveal a 25 nm, 70 fs expansion of the joint-density-of-states along the crystal a,c-axes accompanied by a 6 nm, 115 fs change in the exciton density along the crystal b-axis. We propose that photogenerated singlet excitons expand along the direction of maximal orbital π-overlap in the crystal a,c-plane to form correlated triplet pairs, which subsequently electronically decouples into free triplets along the crystal b-axis due to molecular sliding motion of neighbouring pentacene molecules. Our methodology lays the foundation for the study of three dimensional transport on ultrafast timescales.

Contrast mechanisms in pump-probe microscopy of melanin.

Pump-probe microscopy of melanin in tumors has been proposed to improve diagnosis of malignant melanoma, based on the hypothesis that aggressive cancers disaggregate melanin structure. However, measured signals of melanin are complex superpositions of multiple nonlinear processes, which makes interpretation challenging. Polarization control during measurement and data fitting are used to decompose signals of melanin into their underlying molecular mechanisms. We then identify the molecular mechanisms that are most susceptible to melanin disaggregation and derive false-coloring schemes to highlight these processes in biological tissue. We demonstrate that false-colored images of a small set of melanoma tumors correlate with clinical concern. More generally, our systematic approach of decomposing pump-probe signals can be applied to a multitude of different samples.

Imaging ultrafast evolution of subwavelength-sized topography using single-probe structured light microscopy

Imaging ultrafast processes in femtosecond (fs) laser–material interactions such as fs laser ablation is very important to understand the physical mechanisms involved. To achieve this goal with high resolutions in both spatial and temporal domains, a combination of optical pump–probe microscopy and structured illumination microscopy can be a promising approach, but suffers from the multiple-frame method with a phase shift that is inapplicable to irreversible ultrafast processes such as ablation. Here, we propose and build a wide-field single-probe structured light microscopy (SPSLM) to image the ultrafast three-dimensional topography evolution induced by fs lasers, where only a single imaging frame with a single structured probe pulse is required for topography reconstruction, benefiting from Fourier transform profilometry. The second harmonic of the fs laser is used as the structured probe light to improve spatial lateral resolution into the subwavelength region of ∼ 478 nm , and the spatial axial and temporal resolutions are estimated to be ∼ 22 nm and ∼ 256 fs , respectively. With SPSLM, we successfully image the ultrafast topography evolution of a silicon wafer surface impacted by single and multiple fs pulses. The variable formation and evolution of the laser induced periodic surface structures during an ultrashort time are visualized and analyzed. We believe that SPSLM will be a significant approach for revealing and understanding various ultrafast dynamics, especially in fs laser ablation and material science.

High ambipolar mobility in cubic boron arsenide revealed by transient reflectivity microscopy.

Semiconducting cubic boron arsenide (c-BAs) has been predicted to have carrier mobility of 1400 square centimeters per volt-second for electrons and 2100 square centimeters per volt-second for holes at room temperature. Using pump-probe transient reflectivity microscopy, we monitored the diffusion of photoexcited carriers in single-crystal c-BAs to obtain their mobility. With near-bandgap 600-nanometer pump pulses, we found a high ambipolar mobility of 1550 ± 120 square centimeters per volt-second, in good agreement with theoretical prediction. Additional experiments with 400-nanometer pumps on the same spot revealed a mobility of >3000 square centimeters per volt-second, which we attribute to hot electrons. The observation of high carrier mobility, in conjunction with high thermal conductivity, enables an enormous number of device applications for c-BAs in high-performance electronics and optoelectronics.

Label-Free Delineation of Human Uveal Melanoma Infiltration With Pump-Probe Microscopy.

Uveal melanoma (UM) is the most frequent primary intraocular malignancy in adults, characterized by melanin depositions in melanocytes located in the uveal tract in the eyes. Differentiation of melanin species (eumelanin and pheomelanin) is crucial in the diagnosis and management of UM, yet it remains inaccessible for conventional histology. Here, we report that femtosecond time-resolved pump-probe microscopy could provide label-free and chemical-specific detection of melanin species in human UM based on their distinct transient relaxation dynamics at the subpicosecond timescale. The method is capable of delineating the interface between melanoma and paracancerous regions on various tissue conditions, including frozen sections, paraffin sections, and fresh tissues. Moreover, transcriptome sequencing was conducted to confirm the active eumelanin synthesis in UM. Our results may hold potential for sensitive detection of tumor boundaries and biomedical research on melanin metabolism in UM.

Ultrafast Pump–Probe Microscopy on 2D Transition Metal Dichalcogenides

Although microscopic techniques have been used to characterize transition metal dichalcogenides (TMDs), direct observation of charge carrier dynamics distribution in TMDs with diverse shapes remains unexplored. Herein, ultrafast pump–probe microscopy (UPPM) is employed to reveal the carrier dynamics distribution in molybdenum disulfide (MoS2) and tungsten disulfide (WS2) monolayer of four shapes: triangular (t‐MoS2), curved triangular (c‐MoS2), triangular (t‐WS2), and hexagonal (h‐WS2). Monitoring the photon transmission T at 1.55 eV after pumping with a photon energy of 3.1 eV, a negative ΔT/T occurs in t‐MoS2 and c‐MoS2, while a positive ΔT/T is detected in t‐WS2 and h‐WS2 after 3–7 ps time evolution. This distinctive behavior is attributed to deep/shallow defects below the conduction band minimum (CBM) in MoS2 and WS2. Spatial‐independent ΔT/T is observed in t‐MoS2 and t‐WS2, while the ΔT/T in c‐MoS2 has a rapid decay of photoexcited carriers at the vertices and curved edges. Additionally, a threefold symmetry of ΔT/T is revealed in h‐WS2, attributed to the dissimilar occupation of defect states near the h‐WS2 CBM. This work paves the way for examining charge carrier dynamics of various shapes of TMDs and provides a unique microscopic method for studying the charge carrier dynamics in emerging TMDs heterostructures.

Comparison of ultrashort pulse ablation of gold in air and water by time-resolved experiments

Laser ablation in liquids is a highly interdisciplinary method at the intersection of physics and chemistry that offers the unique opportunity to generate surfactant-free and stable nanoparticles from virtually any material. Over the last decades, numerous experimental and computational studies aimed to reveal the transient processes governing laser ablation in liquids. Most experimental studies investigated the involved processes on timescales ranging from nanoseconds to microseconds. However, the ablation dynamics occurring on a sub-nanosecond timescale are of fundamental importance, as the conditions under which nanoparticles are generated are established within this timeframe. Furthermore, experimental investigations of the early timescales are required to test computational predictions. We visualize the complete spatiotemporal picosecond laser-induced ablation dynamics of gold immersed in air and water using ultrafast pump-probe microscopy. Transient reflectivity measurements reveal that the water confinement layer significantly influences the ablation dynamics on the entire investigated timescale from picoseconds to microseconds. The influence of the water confinement layer includes the electron injection and subsequent formation of a dense plasma on a picosecond timescale, the confinement of ablation products within hundreds of picoseconds, and the generation of a cavitation bubble on a nanosecond timescale. Moreover, we are able to locate the temporal appearance of secondary nanoparticles at about 600 ps after pulse impact. The results support computational predictions and provide valuable insight into the early-stage ablation dynamics governing laser ablation in liquids.

Hidden surface photovoltages revealed by pump probe KPFM

In this work, we use pump-probe Kelvin probe force microscopy (pp-KPFM) in combination with non-contact atomic force microscopy (nc-AFM) under ultrahigh vacuum, to investigate the nature of the light-induced surface potential dynamics in alumina-passivated crystalline silicon, and in an organic bulk heterojunction thin film based on the PTB7-PC71BM tandem. In both cases, we demonstrate that it is possible to identify and separate the contributions of two different kinds of photo-induced charge distributions that give rise to potential shifts with opposite polarities, each characterized by different dynamics. The data acquired on the passivated crystalline silicon are shown to be fully consistent with the band-bending at the silicon-oxide interface, and with electron trapping processes in acceptors states and in the passivation layer. The full sequence of events that follow the electron–hole generation can be observed on the pp-KPFM curves, i.e. the carriers spatial separation and hole accumulation in the space charge area, the electron trapping, the electron–hole recombination, and finally the electron trap-release. Two dimensional dynamical maps of the organic blend photo-response are obtained by recording the pump-probe KPFM curves in data cube mode, and by implementing a specific batch processing protocol. Sample areas displaying an extra positive SPV component characterized by decay time-constants of a few tens of microseconds are thus revealed, and are tentatively attributed to specific interfaces formed between a polymer-enriched skin layer and recessed acceptor aggregates. Decay time constant images of the negative SPV component confirm that the acceptor clusters act as electron-trapping centres. Whatever the photovoltaic technology, our results exemplify how some of the SPV components may remain completely hidden to conventional SPV imaging by KPFM, with possible consequences in terms of photo-response misinterpretation. This work furthermore highlights the need of implementing time-resolved techniques that can provide a quantitative measurement of the time-resolved potential.

Time resolved studies reveal the origin of the unparalleled high efficiency of one nanosecond laser ablation in liquids

Laser ablation in liquid is a scalable nanoparticle production method with applications in areas like catalysis and biomedicine. Due to laser-liquid interactions, different energy dissipation channels such as absorption by the liquid and scattering at the ablation plume and cavitation bubble lead to reduced laser energy available for nanoparticle production. Ultrashort pulse durations cause unwanted nonlinear effects in the liquid, and for ns pulses, intra-pulse energy deposition attenuation effects are to be expected. However, intermediate pulse durations ranging from hundreds of picoseconds up to one nanosecond have rarely been studied in particular in single-pulse settings. In this study, we explore the pico- to nanosecond pulse duration regimes to find the pulse duration with the highest ablation efficiency. We find that pulse durations around 1–2 ns enable the most efficient laser ablation in liquid since the laser beam shielding by the ablation plume and cavitation bubble sets in only at longer pulse durations. Furthermore, pump-probe microscopy imaging reveals that the plume dynamics in liquids start to differ from plume dynamics in air at about 2 ns after pulse impact.

Sequential Activation of Molecular and Macroscopic Spin-State Switching within the Hysteretic Region Following Pulsed Light Excitation.

Molecular spin-crossover (SCO) compounds constitute a promising class of photoactive materials exhibiting efficient photoinduced phase transitions (PIPTs). Taking advantage of the unique, picture-perfect reproducibility of the spin-transition properties in the compound [Fe(HB(1,2,4-triazol-1-yl)3 )2 ], the spatiotemporal dynamics of the PIPT within the thermodynamic metastability (hysteretic) region of a single crystal is dissected, using pump-probe optical microscopy. Beyond a threshold laser-excitation density, complete PIPTs are evidenced, with conversion rates up to 200 switched molecules per absorbed photon. It is shown that the PIPT takes place through the sequential activation of two (molecular and macroscopic) switching mechanisms, occurring on sub-microsecond and millisecond timescales, governed by the intramolecular and free energy barriers of the system, respectively. The main finding here is that the thermodynamic metastability has strictly no influence on the sub-millisecond switching dynamics. Indeed, before this millisecond timescale, the response of the crystal to the laser excitation involves a gradual, molecular conversion process, as if there wereno hysteresis loop. Consequently, in this regime, even a 100% photoinduced conversion may not give rise to a PIPT. These results provide new insight on the intrinsic dynamical limits of the PIPT, which is an important issue from a technological perspective.

Super-resolution photoacoustic imaging based on saturation difference of transient absorption

As a label-free, non-invasive, and high-contrast imaging technology, photoacoustic microscopy (PAM) is widely used in biomedical science. However, due to the restriction of diffraction limit, the optical resolution of PAM has not been effectively improved. In this paper, in order to break the diffraction limit, a patterned competition based on saturable transient absorption effect is introduced to realize the super-resolution imaging into the conventional photoacoustic microscopy. We have provided a physical model of photoacoustic microscopy based on transient absorption saturation difference (TASD-PAM) and verified its effectiveness by analyzing the photophysical properties of our sample and imaging it with a sub-diffraction lateral resolution. Compared to reported pump-probe microscopy, which is based on stimulated emission depletion or ground-state depletion process, our method can realize the label-free super-resolution imaging of dark materials that cannot emit fluorescence. This technique enables observing the microstructure of dark materials in a way that defies the diffraction limit, which will expand the application range of photoacoustic microscopy and open up a new way for super-resolution imaging.

Time-resolved ultrafast laser ablation dynamics of thin film indium tin oxide.

The interaction of ultrashort laser pulses above the ablation threshold of thin-film indium tin oxide (ITO) is examined with pump-probe microscopy. We are able to observe photomechanical spallation at delay times of hundreds of picoseconds, which plays a stronger role near the ablation threshold of 0.17 J/cm2. A phase explosion may also be observed at tens of picoseconds, playing a stronger role for increasing peak fluences. As one exceeds the material removal efficiency maximum near 0.6 J/cm2, a second spallation is observable in the center of the irradiated spot at a delay time of one nanosecond and corresponds to a crater depth of 50 nanometers. No discernable ridge formation has been observed. We recommend an industrial processing window of at least two pulses per position with a peak fluence between 0.6-1.0 J/cm2.