Pump-probe Microscopy Research Articles

Probing Intrawire, Interwire, and Diameter-Dependent Variations in Silicon Nanowire Surface Trap Density with Pump–Probe Microscopy

Surface trap density in silicon nanowires (NWs) plays a key role in the performance of many semiconductor NW-based devices. We use pump-probe microscopy to characterize the surface recombination dynamics on a point-by-point basis in 301 silicon NWs grown using the vapor-liquid-solid (VLS) method. The surface recombination velocity (S), a metric of the surface quality that is directly proportional to trap density, is determined by the relationship S = d/4τ from measurements of the recombination lifetime (τ) and NW diameter (d) at distinct spatial locations in individual NWs. We find that S varies by as much as 2 orders of magnitude between NWs grown at the same time but varies only by a factor of 2 or three within an individual NW. Although we find that, as expected, smaller-diameter NWs exhibit shorter τ, we also find that smaller wires exhibit higher values of S; this indicates that τ is shorter both because of the geometrical effect of smaller d and because of a poorer quality surface. These results highlight the need to consider interwire heterogeneity as well as diameter-dependent surface effects when fabricating NW-based devices.

The changing landscape of dermatology practice: melanoma and pump-probe laser microscopy.

To present current melanoma diagnosis, staging, prognosis, and treatment algorithms and how recent advances in laser pump-probe microscopy will fill in the gaps in our clinical understanding. Expert opinion and significantly cited articles identified in SCOPUS were used in conjunction with a pubmed database search on Melanoma practice guidelines from the last 10years. Significant advances in melanoma treatment have been made over the last decade. However, proper treatment algorithm and prognostic information per melanoma stage remain controversial. The next step for providers will involve the identification of patient population(s) that can benefit from recent advances. One method of identifying potential patients is through new laser imaging techniques. Pump-probe laser microscopy has been shown to correctly identify nevi from melanoma and furthermore stratify melanoma by aggressiveness. The recent development of effective adjuvant therapies for melanoma is promising and should be utilized on appropriate patient populations that can potentially be identified using pump-probe laser microscopy.

Comparison of pump-probe and hyperspectral imaging in unstained histology sections of pigmented lesions.

Microscopic variations in melanin composition can be mapped through linear and nonlinear optical responses. Though instrumentation to measure linear attenuation is simple and inexpensive, the nonlinear response provides more degrees of freedom with which to spectroscopically resolve pigments. The objective of this study is to assess differences in imaging melanin contrast by comparing hyperspectral (linear) versus pump-probe (nonlinear) microscopy of unstained histology sections of pigmented lesions. The images and analysis we have presented here show that pump-probe uncovers a greater variation in pigment composition, compared with hyperspectral microscopy, and that the two methods yield complimentary biochemical information.

Ultrafast Excited-State Transport and Decay Dynamics in Cesium Lead Mixed Halide Perovskites

While significant research efforts directed toward characterizing the excited-state dynamics of lead halide perovskites have enabled promising advances in photovoltaic, light-emitting diode, and laser technologies, a detailed correlation between composition and functionality in this promising class of materials remains unestablished. We use pump–probe microscopy to characterize both transport and relaxation dynamics in individual crystals of CsPbI2Br, a mixed halide, all-inorganic analogue to the well-studied organic–inorganic hybrid perovskites. In contrast to the methylammonium lead tri-iodide perovskite, we find excited-state dynamics that decay primarily via first-order and Auger mechanisms. By global fitting of power-dependent kinetics collected from individual domains, we find a range of Auger rate constants between 3.3 × 10–30 and 1.5 × 10–28 cm6/s, with negligible contributions from second-order (bimolecular) processes. Direct imaging of the excited-state spatial evolution reveals an average diffu...

Label-Free Saturated Structured Excitation Microscopy

Micro- and nanoscale chemical and structural heterogeneities, whether they are intrinsic material properties like grain boundaries or intentionally encoded via nanoscale fabrication techniques, pose a challenge to current material characterization methods. To precisely interrogate the electronic structure of these complex materials systems, spectroscopic techniques with high spatial resolution are required. However, conventional optical microscopies are limited to probe volumes of ~200 nm due to the diffraction limit of visible light. While a variety of sub-diffraction-limited techniques have been developed, many rely on fluorescent contrast agents. Herein we describe label-free saturated structured excitation microscopy (LF-SSEM) applicable to nonlinear imaging approaches such as stimulated Raman and pump-probe microscopy. By exploiting the nonlinear sample response of saturated excitation, LF-SSEM provides theoretically limitless resolution enhancement without the need for a photoluminescent sample.

Transient absorption imaging of hemes with 2-color, independently tunable visible-wavelength ultrafast source.

Pump probe microscopy is a time-resolved multiphoton imaging technique capable of generating contrast between non-fluorescent pigments based on differences in excited-state lifetimes. Here we describe a fiber-based ultrafast system designed for imaging heme proteins with an independently-tunable pulse pair in the visible-wavelength regime. Starting with a 1060 nm fiber amplifier (1.3 W at 63 MHz, 140 fs pulses), visible pulses were produced in the vicinity of 488 nm and 532 nm by doubling the output of a short photonic crystal fiber with a pair of periodically-poled lithium niobate crystals, providing 5-20 mW power in each beam. This was sufficient for acquiring transient absorption images from unstained cryosectioned tissue.

Perspectives on in situ electron microscopy

In situ transmission electron microscopy (TEM) with the ability to reveal materials dynamic processes with high spatial and temporal resolution has attracted significant interest. The recent advances in in situ methods, including liquid and gas sample environment, pump-probe ultrafast microscopy, nanomechanics and ferroelectric domain switching the aberration corrected electron optics as well as fast electron detector has opened new opportunities to extend the impact of in situ TEM in broad areas of research ranging from materials science to chemistry, physics and biology. In this article, we highlight the development of liquid environment electron microscopy and its applications in the study of colloidal nanoparticle growth, electrochemical processes and others; in situ study of topological vortices in ferroelectric and ferromagnetic materials. At the end, perspectives of future in situ TEM are provided.

Three-dimensional wide-field pump-probe structured illumination microscopy.

We propose a new structured illumination scheme for achieving depth resolved wide-field pump-probe microscopy with sub-diffraction limit resolution. By acquiring coherent pump-probe images using a set of 3D structured light illumination patterns, a 3D super-resolution pump-probe image can be reconstructed. We derive the theoretical framework to describe the coherent image formation and reconstruction scheme for this structured illumination pump-probe imaging system and carry out numerical simulations to investigate its imaging performance. The results demonstrate a lateral resolution improvement by a factor of three and providing 0.5 µm level axial optical sectioning.

Monitoring the dynamics of the surface deformation prior to the onset of plasma emission during femtosecond laser ablation of noble metals by time-resolved reflectivity microscopy

The generation of an expanding plasma during laser ablation is preceded at early times by the formation and evolution of a subsurface melted front. The monitoring of such transient event can't be studied by conventional spectroscopic techniques. Pump-probe femtosecond microscopy allows the following of the surface changes during femtosecond laser ablation taking advantage of the formation of a number of dynamic Newton rings that evolve with time. Measurements at different times allow the quantification of the radial expansion velocity of the molten material. For Au and Ag, expansions in the range of 7000–12,000m/s for Au and 3000–21,000m/s have been calculated depending on the pump energies. Such values correspond to hypersonic velocities with Mach number between 3 and 6.

Screened Charge Carrier Transport in Methylammonium Lead Iodide Perovskite Thin Films.

While organometal halide perovskites are promising for a variety of optoelectronic applications, the morphological and compositional defects introduced by solution processing techniques have hindered efforts at understanding their fundamental properties. To provide a detailed picture of the intrinsic carrier transport properties of methylammonium lead iodide without contributions from defects such as grain boundaries, we utilized pump-probe microscopy to measure diffusion in individual crystalline domains of a thin film. Direct imaging of carrier transport in 25 individual domains yields diffusivities between 0.74 and 1.77 cm2 s-1, demonstrating single-crystal-like, long-range transport characteristics in a thin film architecture. We also examine the effects of excitation density on carrier diffusivity, finding that transport is nearly independent of photoexcited carrier density between 6 × 1017 cm-3 and 4 × 1019 cm-3. Transport modeling of the observed density independence suggests that strong carrier-phonon scattering coupled with a large static relative permittivity is responsible for the unusual transport characteristics of methylammonium perovskite.

Resolution enhancement of pump-probe microscopy with an inverse-annular spatial filter

We have introduced a pupil filter, an inverse-annular pupil filter in a pump-probe photothermal microscope, which provides resolution enhancement in three dimensions. The resolution is probed to be improved in lateral and axial resolution by imaging experiment using 20 nm gold nanoparticles. The improvement in X (perpendicular to the common pump and probe polarization direction), Y (parallel to the polarization direction), and Z (axial direction) are by 15±6, 8±8, and 21±2 % from the resolution without a pupil filter. The resolution enhancement is even better than the calculation using vector field, which predicts the corresponding enhancement of 11, 8, and 6 %. The discussion is made to explain the unexpected results. We also demonstrate the photothermal imaging of thick biological samples (cells from rabbit intestine and kidney) stained with hematoxylin and eosin dye with the inverse-annular filter.

Numerical study of super-resolved optical microscopy with partly staggered beams

The resolving power of optical microscopy involving two or even more beams, such as pump–probe microscopy and nonlinear optical microscopy, can be enhanced both laterally and longitudinally with partly staggered beams. A numerical study of the new super-resolution imaging technology is performed with vector diffraction theory. The influence of polarization is discussed. A resolving power of sub-100 nm and sub-300 nm in the lateral and longitudinal directions, respectively, is achievable.

Dual-wavelength pump-probe microscopy analysis of melanin composition.

Pump-probe microscopy is an emerging technique that provides detailed chemical information of absorbers with sub-micrometer spatial resolution. Recent work has shown that the pump-probe signals from melanin in human skin cancers correlate well with clinical concern, but it has been difficult to infer the molecular origins of these differences. Here we develop a mathematical framework to describe the pump-probe dynamics of melanin in human pigmented tissue samples, which treats the ensemble of individual chromophores that make up melanin as Gaussian absorbers with bandwidth related via Frenkel excitons. Thus, observed signals result from an interplay between the spectral bandwidths of the individual underlying chromophores and spectral proximity of the pump and probe wavelengths. The model is tested using a dual-wavelength pump-probe approach and a novel signal processing method based on gnomonic projections. Results show signals can be described by a single linear transition path with different rates of progress for different individual pump-probe wavelength pairs. Moreover, the combined dual-wavelength data shows a nonlinear transition that supports our mathematical framework and the excitonic model to describe the optical properties of melanin. The novel gnomonic projection analysis can also be an attractive generic tool for analyzing mixing paths in biomolecular and analytical chemistry.

Time-resolved single dopant charge dynamics in silicon.

As the ultimate miniaturization of semiconductor devices approaches, it is imperative that the effects of single dopants be clarified. Beyond providing insight into functions and limitations of conventional devices, such information enables identification of new device concepts. Investigating single dopants requires sub-nanometre spatial resolution, making scanning tunnelling microscopy an ideal tool. However, dopant dynamics involve processes occurring at nanosecond timescales, posing a significant challenge to experiment. Here we use time-resolved scanning tunnelling microscopy and spectroscopy to probe and study transport through a dangling bond on silicon before the system relaxes or adjusts to accommodate an applied electric field. Atomically resolved, electronic pump-probe scanning tunnelling microscopy permits unprecedented, quantitative measurement of time-resolved single dopant ionization dynamics. Tunnelling through the surface dangling bond makes measurement of a signal that would otherwise be too weak to detect feasible. Distinct ionization and neutralization rates of a single dopant are measured and the physical process controlling those are identified.

Transverse pump-probe microscopy of moving breakdown, filamentation and self-organized absorption in alkali aluminosilicate glass using ultrashort pulse laser.

We present time and space resolved transverse pump-probe measurements of the free electron and defect generation induced by nonlinear absorption of ultra short pulsed laser radiation in unhardened Corning Gorilla glass. The applied setup exhibits a 100 fs probe pulse duration and an independent pump pulse duration up to 5 ps. Hence, our work comprises the absorption of ultra short pulsed laser radiation at a wavelength of 800 nm and pulse energies from 10 μJ to 50 μJ up to a delay of 6 ns. Our investigations reveal different absorption regimes like filamentation and moving breakdown as well as the formation of permanent modifications. Finally, the deposition of multiple pulses in the incubation regime is examined, observing a self-organizing absorption effect.

Imaging theory of structured pump-probe microscopy.

With sub-micron spatial resolution and femtosecond temporal resolution, pump probe microscopy provides a powerful spectroscopic probe of complex electronic environments in bulk and nanoscale materials. However, the electronic structure of many materials systems are governed by compositional and morphological heterogeneities on length scales that lie below the diffraction limit. We have recently demonstrated Structured Pump Probe Microscopy (SPPM), which employs a patterned pump excitation field to provide spectroscopic interrogation of sub-diffraction limited sample volumes. Herein, we develop the imaging theory of SPPM in two dimensions to accompany the previously published experimental methodology. We show that regardless of pump and probe wavelengths, a nearly two-fold reduction in spectroscopic probe volume can be achieved. We also examine the limitations of the approach, with a detailed discussion of ringing in the point spread function that can reduce imaging performance.

In-situ and non-destructive focus determination device for high-precision laser applications

A non-destructive, in-line, and low-cost focusing device based on an image sensor has been developed and demonstrated. It allows an in situ focus determination for a broad variety of laser types (e.g. cw and pulsed lasers). It provides stringent focusing conditions with high numerical apertures. This approach does not require sub-picosecond and/or auxiliary lasers, or high fluences above damage thresholds. Applications of this system include, but are not limited to the laser-illumination of micro-electrodes, pump-probe microscopy on thin films, and laser ablation of small samples without sufficient surface area for focus determination by ablation. An uncertainty of the focus position by an order of magnitude less than the respective Rayleigh length could be demonstrated.

Pump-Probe and Time-Resolved Electron Microscopy and Big Data Analysis of Electron-Lattice Dynamics in Strongly Correlated Electron Systems

Pump-Probe and Time-Resolved Electron Microscopy and Big Data Analysis of Electron-Lattice Dynamics in Strongly Correlated Electron Systems

High-harmonic generation at 250 MHz with photon energies exceeding 100 eV

Ultrafast spectroscopy in the extreme ultraviolet demands for ever-higher pulse repetition rates and photon energies. Here, we drive cavity-enhanced high-order harmonic generation (HHG) at a repetition rate of 250 MHz, with 30 fs pulses and an average power of 10 kW. Employing an optimized cavity geometry and a high-pressure gas target, we couple out nanowatt-level harmonics at photon energies around 100 eV. This constitutes an improvement of more than two orders of magnitude over previous megahertz-repetition-rate HHG experiments and paves the way toward high-photon-energy frequency-comb spectroscopy and toward pump-probe photoelectron microscopy and spectroscopy at unprecedented repetition rates.

Super-Resolution Structured Pump–Probe Microscopy

Imaging excited-state dynamics in complex materials provides an important avenue for understanding the role played by micro- and nanoscale structural and compositional heterogeneity in the performance of active electronic and optoelectronic devices. Here, we develop structured pump–probe microscopy (SPPM), a subdiffraction-limited, time-resolved microscopy that incorporates a focused diffraction-limited probe field together with a spatially modulated pump excitation field. We apply the technique to study free carrier dynamics in silicon nanowires, demonstrating far-field ultrafast time-resolved spectroscopy with a 114 nm point-spread function while remaining in the perturbative excitation regime. We predict SPPM will provide an accessible approach for characterizing the ultrafast dynamics of subwavelength regions in complex materials systems.