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Ultrafast electron microscopy (UEM) has made it possible to visualize laser-induced time-dependent processes in materials with a high spatial and temporal resolution. This analytical method is based on sample excitation (pumping) by a pulsed laser beam and its probing by time-synchronized photoelectron bunches in a transmission electron microscope column. Using a digital 2D detector in an experimental setup, one can record a “molecular movie” as a sequence of images representing the behavior of material in the space–time continuum. High temporal resolution is of particular interest in the problems related to detailed analysis of various laser-induced transient processes occurring in thin films, observation of the response of adsorbed molecules to nonequilibrium structural changes of the surface, surface and volume dynamics in semiconductors, generation of coherent acoustic and optical phonons, surface melting of nanoparticles, and nonequilibrium structural dynamics of phase transformations. This review presents the state of the art of the UEM method, where picosecond, femtosecond, and subfemtosecond probe pulses are used to reveal the structural dynamics of condensed matter.

The generation of coherent optical phonons in a polycrystalline antimony film sample has been investigated using femtosecond electron diffraction method. Phonon vibrations have been induced in the Sb sample by the main harmonic of a femtosecond Ti:Sa laser (λ = 800 nm) and probed by a pulsed ultrashort photoelectron beam synchronized with the pump laser. The diffraction patterns recorded at different times relative to the pump laser pulse display oscillations of electron diffraction intensity corresponding to the frequencies of vibrations of optical phonons: totally symmetric (A 1g ) and twofold degenerate (E g ) phonon modes. The frequencies that correspond to combinations of these phonon modes in the Sb sample have also been experimentally observed.

We have developed an ultrafast transmission electron microscope for studying dynamic processes in samples excited by femtosecond laser pulses and for probing transient processes occurring under irradiation by a pulsed (~7 ps) photoelectron beam with an adjustable delay with respect to the excitation pulse. A 75-keV photoelectron beam is formed using a silver photocathode irradiated by a femtosecond laser beam. This microscope is shown to have a high spatial resolution in the photoelectron regime: nanoscale in the imaging regime and atomic in the electron diffraction regime. It is used for experimental observation of ultrafast interaction of a laser-induced electron cloud with a pulsed photoelectron beam. Using this effect, a method of spatial and temporal alignment of excitation laser and probe electron pulsed beams on a sample is experimentally implemented.

The generation of coherent optical phonons in an antimony film has been directly observed by the femtosecond electron diffraction method. The sample has been excited by a femtosecond laser pulse (λ = 800 nm) and probed with a pulsed photoelectron beam. Oscillations of the intensity corresponding to vibration frequencies of optical phonons excited by the laser have been observed in the obtained diffraction patterns: totally symmetric (A 1g) and twofold degenerate (E 2g) phonon modes of antimony and their combinations.

Time-resolved electron microscopy is based on the excitation of a sample by pulsed laser radiation and its probing by synchronized photoelectron bunches in the electron microscope column. With femtosecond lasers, if probing pulses with a small number of electrons—in the limit, single-electron wave packets—are used, the stroboscopic regime enables ultrahigh spatiotemporal resolution to be obtained, which is not restricted by the Coulomb repulsion of electrons. This review article presents the current state of the ultrafast electron microscopy (UEM) method for detecting the structural dynamics of matter in the time range from picoseconds to attoseconds. Moreover, in the imaging mode, the spatial resolution lies, at best, in the subnanometer range, which limits the range of observation of structural changes in the sample. The ultrafast electron diffraction (UED), which created the methodological basis for the development of UEM, has opened the possibility of creating molecular movies that show the behavior of the investigated quantum system in the space-time continuum with details of sub-Å spatial resolution. Therefore, this review on the development of UEM begins with a description of the main achievements of UED, which formed the basis for the creation and further development of the UEM method. A number of recent experiments are presented to illustrate the potential of the UEM method.

Aberrations of off-axis parabolic mirrors are investigated and quantitatively described by means of computer simulation. The results of the simulation are confirmed experimentally with the use of a Shack–Hartmann sensor. An algorithm for compensation of aberrations and digital restoration of the corrected image of a microscopic object is developed and applied in a holographic circuit that contains an off-axis parabolic mirror.

We demonstrate the use of a conical capillary in a scanning probe microscopy for surface analysis. The probe can measure photoemission from a substrate by transmitting photoelectrons along the capillary as a function of probe position. The technique is demonstrated on a model substrate consisting of a gold reflecting layer on a compact disc which has been illuminated by an unfocused laser beam with a wavelength 400 nm, from a femtosecond laser with a beam size of 4 mm. A quartz capillary with a 2-µm aperture has been used in the experiments. The period of gold microstructure, shown to be 1.6 µ, was measured by the conical probe operating in shear force mode. In shear force regime, the dielectric capillary has been used as a “classical” SPM tip, which provided images reflecting the surface topology. In a photoelectron regime photoelectrons passed through hollow tip and entered a detector. The spatial distribution of the recorded photoelectrons consisted of periodic mountain-valley strips, resembling the surface profile of the sample. Submicron spatial resolution has been achieved. This approach paves the way to study pulsed photodesorption of large organic molecular ions with high spatial and element resolution using the combination of a hollow-tip scanner with time-of-flight technique.

One of the most fundamental problems of modern natural science is the direct observation of atomic motions in the course of various processes. For this purpose, in the experiment it is necessary to provide high spatial-temporal resolution. The solution to this problem is achieved by using a pulsed electron beam of ultrashort duration to create a stroboscopic diffraction pattern in the method of time-resolved electron diffraction (TRED). Three types of experimental schemes have been developed at our lab. The experimental complex includes (i) 20-keV table-top apparatus for TRED, (ii) 75-keV ultrafast transmission electron microscope and (iii) lensless table-top device for femtosecond electron diffraction. The obtained experimental results are presented.

A new type of microscopy based on scanning in vacuum by a beam of charged particles transmitted through a hollow probe has been implemented. This approach provides controllable motion of spatially localized ion, electron, molecular (atomic), and soft X-ray beams and investigation of the surface in the shear force mode. In the photoelectron mode, in which electrons are transmitted through a 2-μm quartz capillary, a surface profile of gadolinium irradiated by 400-nm femtosecond laser pulses has been visualized with a subwave spatial resolution. The new method of microscopy opens an opportunity of investigations in the field of nanometer local photodesorption of molecular ions (one of the last ideas of V.S. Letokhov).

A method is demonstrated which combines the ionisation of free molecules by a sharply focused femtosecond laser beam and projection microscopy in a divergent electric field. The electric field is produced in vacuum between a metallic tip and a flat positionsensitive charged particle detector. The method enables investigation of photoionisation processes in low-density gases with a subdiffraction spatial resolution and can be used as well in profile measurements for sharply focused, intense laser beams. In a demonstration experiment, a femtosecond laser beam with a peak intensity of ~1014 W cm-2 was focused to a 40-μm-diameter waist in vacuum near a millimetre-size tip and ~2-μm spatial resolution was achieved. According to our estimates, the use of a sharper tip will ensure a submicron spatial resolution, which is a crucial condition for the spatial diagnostics of sharply focused short-wavelength VUV radiation and X-rays.

A method for measuring the Gaponov-Miller force (GMF) is demonstrated based on the deflection of a picosecond photoelectron beam exposed to tightly focused intense femtosecond laser radiation. It is shown experimentally that the action of this force produced by femtosecond laser pulses linearly depends on their intensity. The method can be used to verify the correctness of measuring the duration of an ultrashort electron bunch based on the GMF.

A conducting organic polymer is visualized on a laser photoelectron projection microscope, which is based on Letokhov’s concept and has a nanometer spatial resolution. Photoelectron images of polyaniline (which is the most promising representative of conducting polymers) with a magnification of ∼105 have been obtained when a 100-nm quartz capillary coated with a film of this material was irradiated by femtosecond laser pulses. The projection photoelectron method using 400- and 800-nm laser radiation has made it possible to directly reveal the existence of the redox heterogeneity of the organic polymer, which is due to the contact of the sections of polyaniline with different oxidation degrees and strongly affects the electric conductivity of the sample.

The space-velocity distribution of electrons propagating in vacuum can be deformed by the ponderomotive potential produced by high-intensity femtosecond laser pulses, which makes it possible to subsequently separate such electrons from the initial beam. It is shown that optical modification of electron beams with kinetic energies on the order of 100 eV by femtosecond laser radiation with an intensity from 1014 to 1018 W/cm2 makes it possible to form electron beams with a duration on the order of 50–100 fs. Examples of optical control over the shape of electron beams, based on deflection, reflection, focusing, and splitting of electron beams, are considered.

A method based on an original electron microscope created for investigating photoelectron beams is presented. It ensures a nanometer spatial resolution and picosecond time resolution. Electrons appearing when a metal needle is irradiated by femtosecond laser pulses are transmitted through a dielectric microcapillary and are subjected to a ponderomotive potential created by femtosecond laser radiation focused near the capillary tip. The position-sensitive detection scheme allows for the detection of the spatial profile of a photo-electron beam with a magnification of K ≅ 4 × 104. The time structure of the photoelectron beam is visualized by scanning the delay time between laser pulses irradiating the needle and a laser pulse focused near the capillary tip.

A nanolocalized electron beam emitted from silicon nanotubes has been used to image the aperture of a quartz capillary with a channel diameter of 100 nm. An analogous nickel-coated capillary was used as a tip for imaging deposited organic nanostructures by means of femtosecond laser photoelectron projection microscopy (LPPM). Organic nanocomplexes deposited onto the tip surface change the dependence of the photoelectron response signal on the energy density of probing femtosecond laser pulses. An analysis of the LPPM images of capillary nanotips shows that the spatial resolution achieved in these experiments is on a level of 5 nm.

The possibility of the desorption of complicated molecular complexes by soft X rays resulting from a solid target irradiated by a single sharply focused femtosecond laser pulse with an energy of several millijoules has been experimentally demonstrated for polyaniline, which is an organic conducting polymer. X-ray desorption and photodesorption of polyaniline by femtosecond laser pulses have been compared using a time-of-flight mass spectrometer. The results provide the possibility of studying surfaces with spatial nanoresolution and high elemental (chemical) selectivity, as well as observing the photodesorption with a high temporal resolution.

Organic nanocomplexes obtained by applying an organic dye (Coumarin 153) solution onto the surface of a metal nanotip were studied by femtosecond laser photoelectron projection microscopy. The tip was a 100-nm-diameter capillary covered with a thin nickel layer. The image of a through hole of the capillary was a convenient reference for interpretation of the results. The spatial resolution achieved in these experiments was equal to about 5 nm.

The method of ultrahigh-spatial-resolution field-emission projection imaging of nonconducting tips is implemented experimentally. An image of a glass microcapillary tip was obtained for the first time by the nonscanning method with spatial resolution no worse than 20 nm.

A combination of femtosecond laser techniques and nanomicroscopy creates promising new possibilities for studying both matter and a whole number of ultrafast physical processes. The results obtained at the Institute for Spectroscopy as a result of developing a new area of femtosecond nanomicroscopy are presented.

Extended periodic microstructures are formed on a thin gold film by femtosecond laser radiation. The structure length is an order of magnitude larger than the typical size of the irradiated region. The detection of gold photoions generated in the process of the microstructuring ensures a high-sensitivity on-line diagnostics of this phenomenon.

Nanolocalized ion packets have been produced due to the transmission of photoions, which are emitted from a refractory metal emitter irradiated by femtosecond laser radiation with a high repetition rate, through a quartz nanocapillary. With the use of such a pulsed nanobeam, a miniature ion microscope has been created and the image of a dielectric-nanoaperture fragment has been obtained.

A directed photoelectron beam is obtained when photoelectrons from a sharp metal tip irradiated by femtosecond laser pulses are passed through a quartz nanocapillary. Such a nanolocalized photoelectron train makes it possible to conduct experiments with simultaneous femtosecond time and nanometer spatial resolutions.

The spatial distribution of light intensity in the near field is studied by observing the photoelectron projection images of a subwavelength nanoaperture. The imaging electrons are obtained as a result of a two-photon external photoelectric effect induced in the aperture formed at the end of an optical fiber by femtosecond pulses of the second-harmonic radiation (410 nm) of a Ti:sapphire laser. The light-field distribution in the aperture is not distorted by any near-by object (such as a medium containing fluorescent molecules), which allows nonperturbing measurement of such a distribution.

The analysis of the possibilities of microstructuring (micromachining) of various solids by sharply focused femtosecond laser (fsl) capable of providing for a high intensity for moderate pulse energies remains quite topical. Usually the process takes place immediately at the focal spot and an extended structure can only be formed by scanning the sample. In our experiments focusing fsl pulses into a vacuum setup we produced extended microstructures (microdamages) on a few tens nm thickness gold foil deposited onto a fused-quartz substrate. The structure length has been an order of magnitude larger than the typical size of the irradiated region. The detection of Au + photoions ensured a highly-sensitivite on-line diagnostics of this phenomenon and in the limit can be used for the probing of the picostructuring of solids irradiated by a laser. In the next step we are "cloning" the microstructure (microdamage) from one surface to another when there is a few tens microns vacuum-spacing between them and only one surface with a metal film is "directly" irradiated by fsl pulses. For the expertiment we prepared two similar fused-quartz prisms and pressed one sample to another. The clearance is determined by the thickness of a spacer (Cu-foil). The possible explanations are given.

The ultrafast e - -bunches produced by femtosecond laser (fsl) radiation are powerful tool in modern physics to observe different ultrashort processes induced by fsl pulses and to reach high spatio-temporal resolution. In Letokhov's projection microscope such a beam transfers an information from a tip with a sample illluminated by fsl pulses to a detector. Here the fsl radiation provides for an effective photoelectron multiphoton emission without significant heating of the sample. Two organic samples, formed from dye molecules and the organic conducting polymers has been vizualized using the nanocapillary as a tip. The advantages of the nanocapillary tip are described in the report. The temporal resolution depends upon the pulse duration of the e - -beam, τ e . One of the most powerful way to measure τ e is to use the Gaponov-Miller force (GMf), or the ponderomotive force, which the electrons experience in the inhomogeneous field of a focused laser pulse. Such a force helped us to demonstrate the «instantaneousness» of multiphoton emission process from solid targets. As the beam propagated, it spread in time. Using GMf we temporally characterized the e - -pulse transmitted through microcapillary (which can be the basis of the promising scanning microscope) and combined spatial nanoresolution and picosecond temporal resolution. Also the ultrafast microlocalized e - -beam is an ideal tool to measure the GMf created by focused fsl pulses and to characterize very intense laser beam profile in-situ. In principal, such approach may allow for direct subwave spatio-temporal probing of superintense laser beam profiles.

Our experimental activity in the field of femtosecond electron optics is described. The final purpose is to generate and characterize a sub-100-fs femtosecond electron beam.

It is shown that the high-resolution laser photoelectron microscope with subwavelength-spatial resolution can be used for an absolute values of the two-photon external photoelectric effect measurements with high (a few nm—scale) localization. The spatial distribution of light intensity in the near field is studied by observing the photoelectron projection images of a subwavelength nanoaperture. The imaging electrons are obtained as a result oftwo-photon external photoelectric effect induced in the aperture formed at the end of an optical fiber by femtosecondpulses of the second-harmonic radiation (410 mu) of a Ti:sapphire laser. The light-field distribution in the aperture isnot distorted by any near-by object, which allows the first nonperturbing measurement of such a distribution. Key words: femtosecondpulses, two-photon ionization, laser photoelectron microscope. 1. INTRODUCTION. The near-field optical microscopy makes it possible to obtain optical images of various objects with a spatialresolution much better than the optical wavelength ? and is at present a widespread investigation method in biophysics,nanotechnology, surface physics, etc. (see, for example, Refs.1 for review). The most important element in the near-fieldoptical microscopy is the light source (detector) with an aperture smaller than 2. It is precisely the aperture size that

It is shown that field emission microscopy and related methods can be used to analyze the metal coated fiber tips, which nowadays are the most frequently used sensor for the scanning near-field optical microscopy (SNOM). Metal free and thus non field-emitting aperture for the light transmission on the tip apex can be directly seen and its parameters can be measured, which is very important for the interpretation of SNOM data.

Pulsed desorption of organic conducting polymer by XUV-photons, formed by a thin capillary collimator, has been investigated. Short-wavelength radiation has been resulted from a metal target irradiated by a sharply focused Ti:Sa laser beam (0.8 μm, 40 fs, 3 mJ∕pulse) and has been filtered by Mylar-gold substrate. Single shot and 1-kHz pulses regimes of driving femtosecond laser have been compared using a time-of-flight mass spectrometer. A stepwise function of photoion signal vs laser pulse energy has been observed.

A nanolocalized electron beam has been formed by electrons passing through a quartz microcapillary. The authors have demonstrated that such electrons are capable of producing the image of a dielectric nanoaperture. Using the capillary and a metal tip illuminated by femtosecond laser pulses, the authors have developed a photoelectron source based on nanolocalized photoelectron bunches with low angular divergence. It allows the observation of femtosecond-laser-induced processes on a surface with high temporal and spatial resolutions.

This paper discusses the passage of monoenergetic radiation through rectangular, cylindrical, and ring-shaped test holes. It is assumed that the sources are located at the far end and on the lateral surface of the test hole. Formulas are derived for calculating the radiation flux in test holes passing through both the shield and the core of the reactor. The results of the theory are compared with experiment.