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Bringing ultrafast (nanosecond and below) temporal resolution to transmission electron microscopy (TEM) has historically been challenging. Despite significant recent progress in this direction, it remains difficult to achieve sub-nanosecond temporal resolution with a single electron pulse, in real-time (i.e., duration in which the event occurs) imaging. To address this limitation, here, we propose a methodology that combines laser-assisted TEM with computational imaging methodologies based on compressed sensing (CS). In this technique, a two-dimensional (2D) transient event [i.e. (x,y) frames that vary in time] is recorded through a CS paradigm, which consists of spatial encoding, temporal shearing via streaking, and spatiotemporal integration of an electron pulse. The 2D image generated on a camera is used to reconstruct the datacube of the ultrafast event, with two spatial and one temporal dimensions, via a CS-based image reconstruction algorithm. Using numerical simulation, we find that the reconstructed results are in good agreement with the ground truth, which demonstrates the applicability of CS-based computational imaging methodologies to laser-assisted TEM. Our proposed method, complementing the existing ultrafast stroboscopic and nanosecond single-shot techniques, opens up the possibility for single-shot, real-time, spatiotemporal imaging of irreversible structural phenomena with sub-nanosecond temporal resolution.

We present an innovative implementation of the solid-state-biased coherent detection (SSBCD) technique, which we have recently introduced for the reconstruction of both amplitude and phase of ultra-broadband terahertz pulses. In our previous works, the SSBCD method has been operated via a heterodyne scheme, which involves demanding square-wave voltage amplifiers, phase-locked to the THz pulse train, as well as an electronic circuit for the demodulation of the readout signal. Here, we demonstrate that the SSBCD technique can be operated via a very simple homodyne scheme, exploiting plain static bias voltages. We show that the homodyne SSBCD signal turns into a bipolar transient when the static field overcomes the THz field strength, without the requirement of an additional demodulating circuit. Moreover, we introduce a differential configuration, which extends the applicability of the homodyne scheme to higher THz field strengths, also leading a two-fold improvement of the dynamic range compared to the heterodyne counterpart. Finally, we demonstrate that, by reversing the sign of the static voltage, it is possible to directly retrieve the absolute THz pulse polarity. The homodyne configuration makes the SSBCD technique of much easier access, leading to a vast range of field-resolved applications.

In this work, the time-domain integration of broadband terahertz (THz) pulses via a tapered two-wire waveguide (TTWWG) is reported. Such a guiding structure consists of two metallic wires separated by a variable air gap that shrinks down to a subwavelength size as the movement takes from the waveguide input to its output. It is shown that while an input THz pulse propagates toward the subwavelength output gap, it is reshaped into its first-order time integral waveform. In order to prove the TTWWG time integration functionality, the THz pulse is detected directly within the output gap of the waveguide, so as to prevent the outcoupling diffraction from altering the shape of the time-integrated THz transient. Since the time-domain integration is due to the tight geometrical confinement of the THz radiation in a subwavelength gap volume, the TTWWG operational spectral range can easily be tuned by judiciously changing both the output gap size and the tapering angle. The results lead to the physical realization of a broadband, analog THz time integrator device, which is envisioned to serve as a key building block for the implementation of complex and ultrahigh-speed analog signal processing operations in THz communication systems.

Bringing ultrafast temporal resolution to transmission electron microscopy (TEM) has historically been challenging. Despite significant recent progress in this direction, it remains difficult to achieve sub-nanosecond temporal resolution with a single electron pulse imaging. To address this limitation, here, we propose a methodology that combines laserassisted TEM with computational imaging methodologies based on compressed sensing (CS). In this technique, a twodimensional (2D) transient event [i.e. (x, y) frames that vary in time] is recorded through a CS paradigm. The 2D streak image generated on a camera is used to reconstruct the datacube of the ultrafast event, with two spatial and one temporal dimensions, via a CS-based image reconstruction algorithm. Using numerical simulation, we find that the reconstructed results are in good agreement with the ground truth, which demonstrates the applicability of CS-based computational imaging methodologies to laser-assisted TEM. Our proposed method, complementing the existing ultrafast stroboscopic and nanosecond single-shot techniques, opens up the possibility for single-shot, spatiotemporal imaging of irreversible structural phenomena with sub-nanosecond temporal resolution.

Enhanced image contrast has been seen at graphene-layered steps a few nanometers in height by means of photon-induced near-field electron microscopy (PINEM) using synchronous femtosecond pulses of light and electrons. The observed steps are formed by the edges of graphene strips lying on the surface of a graphene substrate, where the strips are hundreds of nanometers in width and many micrometers in length. PINEM measurements reflect the interaction of imaging electrons and induced (near) electric fields at the steps, and this leads to a much higher contrast than that achieved in bright-field transmission electron microscopy imaging of the same strips. Theory and numerical simulations support the experimental PINEM findings and elucidate the nature of the electric field at the steps formed by the graphene layers. These results extend the range of applications of the experimental PINEM methodology, which has previously been demonstrated for spherical, cylindrical, and triangular nanostructures, to shapes of high aspect ratio (rectangular strips), as well as into the regime of atomic layer thicknesses.

Plasmon Probe When light of certain wavelengths strikes a metal surface, it sets the metal's electrons in motion along trajectories termed “plasmon modes.” Yurtsever et al. (p. 59 ; see the Perspective by Batson ) constructed a type of electron microscope that can probe the electric fields associated with this process by measuring the energy of a separate pulse of electrons that is bounced off the surface immediately (less than trillionths of a second) after the light strikes. The spatial resolution was sufficient to map intensities in distinct regions of a single silver nanoparticle.

Converging on Dynamics Electron diffraction is a versatile technique for discerning atomic-level structure, but the data emerge averaged over the micron–scale area sampled by the electrons, and so blur local distinctions in systems that aren't strictly periodic. A recent approach to minimizing this problem has been to focus the electron beam impinging on the sample. Yurtsever and Zewail (p. 708 ) have now applied convergent focusing to an ultrafast electron diffraction apparatus and were thus able to resolve picosecond structural dynamics in local regions tens of nanometers across. The technique was used to probe heterogeneous temperature changes in laser-heated silicon.

Complex structural dynamics at the nanoscale requires sufficiently small probes to be visualized. In conventional imaging using electron microscopy, the dimension of the probe is large enough to cause averaging over the structures present. However, by converging ultrafast electron bunches, it is possible to select a single nanoscale structure and study the dynamics, either in the image or using electron diffraction. Moreover, the span of incident wave vectors in a convergent beam enables sensitivity levels and information contents beyond those of parallel-beam illumination with a single wave vector Bragg diffraction. Here, we report the observation of propagating strain waves using ultrafast Kikuchi diffraction from nanoscale volumes within a wedge-shaped silicon single crystal. It is found that the heterogeneity of the strain in the lateral direction is only 100 nm. The transient elastic wave gives rise to a coherent oscillation with a period of 30 ps and with an envelope that has a width of 140 ps. The origin of this elastic deformation is theoretically examined using finite element analysis; it is identified as propagating shear waves. The wedge-shaped structure, unlike parallel-plate structure, is the key behind the traveling nature of the waves as its angle permits "transverse" propagation; the parallel-plate structure only exhibits the "longitudinal" motion. The studies reported suggest extension to a range of applications for nanostructures of different shapes and for exploring their ultrafast eigen-modes of stress-strain profiles.

Localized electric fields that are induced optically exhibit unique phenomena of fundamental importance to nanoplasmonics. In recent years, they have been considered for efficient photovoltaic and light harvesting devices, single molecule detection, biomolecular labeling and manipulation, and surface enhanced Raman scattering [1]. Several methodologies have been utilized for probing the near-fields, and these include optical methods, cathodoluminesce and transmission electron microscopy [2]. The latter probes the fields with sub-nanometer precision but the measured quantity is the dielectric response of the system to an impinging swift electron, and not to a photon excitation.

The function of many nano- and microscale systems is revealed when they are visualized in both space and time. Here, we report our first observation, using four-dimensional (4D) electron microscopy, of the nanomechanical motions of cantilevers. From the observed oscillations of nanometer displacements as a function of time, for free-standing beams, we are able to measure the frequency of modes of motion and determine Young's elastic modulus and the force and energy stored during the optomechanical expansions. The motion of the cantilever is triggered by molecular charge redistribution as the material, single-crystal organic semiconductor, switches from the equilibrium to the expanded structure. For these material structures, the expansion is colossal, typically reaching the micrometer scale, the modulus is 2 GPa, the force is 600 microN, and the energy is 200 pJ. These values translate to a large optomechanical efficiency (minimum of 1% and up to 10% or more) and a pressure of nearly 1,500 atm. We note that the observables here are real material changes in time, in contrast to those based on changes of optical/contrast intensity or diffraction.

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2006