Fashionable electron microscopy, as a result of picometer wavelength of high-energy electron beam and the current advances in aberration-correction and direct detector applied sciences, permits imaging of matter with atomic decision (1–3). Along with the progress made in electron crystallography, tomography, cryo–single-particle imaging, and different analytical strategies (4–9), it has develop into a central software in lots of fields of analysis from supplies science to biology (10–12). In a typical typical electron microscope, the electron beam is produced by a thermionic or subject emission course of with none management over its temporal habits. With such an electron supply, photos are both static or taken at very long time intervals due to the limitation of the millisecond refresh charge of typical electron detectors. To research the response paths in bodily and chemical transitions past the detector limitations, the superior electron microscope requires a excessive temporal decision.
Managed emission or modulation of a pulsed electron beam is a confirmed technique to provide time-resolved electron microscopy for finding out the elementary dynamical processes of structural and morphological modifications, i.e., ultrafast electron microscopy (UEM) (13–15). A number of strategies have been developed to attain a pulsed electron beam in an electron microscope, such because the electrostatic beam blanker (16–18) and laser-actuated photoemission (19–24), which make nanosecond and sub-picosecond (respectively) dynamics accessible. For the previous, the intrinsic nanosecond length of the electron pulse largely restricts the temporal decision. For the latter, additional optimization of the photoemitted electron pulse utilizing microwave compression (25, 26), terahertz compression (27–31), lively management through radio frequency (RF) optics (32, 33), or photon gating (34, 35) can prolong the temporal decision into the deep femtosecond regime, which finds huge purposes in finding out the transient constructions and morphologies of inorganic and natural supplies (13, 19, 36). Subsequently, the laser-actuated photoemission scheme is at present the first technique for UEM. Nonetheless, a number of boundaries exist for attaining laser-actuated photoemission: Femtosecond lasers may be cumbersome and costly, instrumental modifications are intricate, and beam fluctuation is an intrinsic drawback as a result of inevitable laser pointing instabilities on the cathode. Furthermore, the excitation for samples is usually restricted to the identical femtosecond laser supply, which not solely produces important heating but additionally has been largely incompatible with the examine of system physics, particularly the high-frequency electrodynamics. Electrodynamics of units, significantly within the gigahertz vary, is basically essential as a result of the requirements of world knowledge switch (Wi-Fi, 4G, 5G, processor clock speeds, and so on.) and RF microelectromechanical programs (MEMS) are nearly all within the gigahertz vary (37, 38).
It has been proposed that chopping a steady electron beam by means of the mix of a resonant RF deflection cavity and a small aperture holds promise as a substitute for create quick electron pulses for the implementation of a laser-free UEM (39–42), the place the continual beam is periodically swept throughout the aperture, leading to a pulsed beam conserving the unique peak brightness and power unfold. The benefit is that no femtosecond lasers and intrusive alterations to the electron supply are required. Nonetheless, the resonant RF deflection cavity can solely function at a selected resonance frequency that’s delicate to the ambient thermal fluctuation and requires a really excessive RF energy for actuation. Moreover, the resonant RF deflection cavity will induce a twin pulsed beam as a result of inevitable creation of two pulses with totally different divergence angles in every RF interval. To this point, no ultrafast pump-probe imaging or diffraction by the proposed RF cavity–pushed UEM has been achieved.
Right here, we report the event of a laser-free UEM by integrating a prototype RF-driven electron beam pulser to create quick electron pulses with a tunable repetition charge from 100 MHz as much as 12 GHz, which offers the potential to file ultrafast photos and diffraction patterns of construction transitions. With optimization of the enter RF energy and frequency for the pulser, we achieved a ~10-ps time decision in our instrument. Furthermore, the identical broadband tunable RF sign can be utilized to supply pattern excitation. As a primary demonstration of its functionality for finding out ultrafast dynamics, we carried out a pump-probe examine on electromagnetic (EM) wave propagation dynamics in a microstrip specimen consisting of two interdigitated combs, one of many fundamental constructing blocks for RF MEMS (43). Underneath a 5.25-GHz EM wave excitation, the stroboscopic imaging reveals, in actual time and area, unambiguous temporal oscillating EM fields across the tines of the combs with time-dependent polarization path and energy. Furthermore, a transparent native subject enhancement seems on the corners of every tine. Combining numerical simulations and experimental outcomes, now we have uncovered the electrodynamics of a gigahertz EM wave propagation within the microstrip specimen, which is basically important to the operation of most info processing units and at present inaccessible by different imaging strategies.
RESULTS AND DISCUSSION
We current the conceptual design of the laser-free UEM schematically in Fig. 1A, which outlines the interfacing of the RF-driven pulser system with a transmission electron microscope (TEM). Figure 1B reveals an image of our prototype laser-free UEM system primarily based on a 200-keV TEM (JEOL JEM-2100F, a Lorentz TEM with a Schottky subject emission supply) (44). The pulser, inserted between the electron gun and the microscope’s first condenser lens, consists of two touring wave metallic comb stripline parts with a small chopping aperture between them (inset of Fig. 1). The main points of the design have been described elsewhere (42, 45, 46). Briefly, the highest stripline factor is an electron beam modulator (K1), whereas the underside one is a demodulator (K2), and each function within the touring wave mode. The enter RF indicators to K1 and K2 have the identical frequency and are phase-locked to a grasp oscillator, whereas their amplitude (i.e., energy) and relative part are digitally tunable. When the pulser is pushed by an RF sign with the frequency of f0, a sinusoidal EM subject is generated within the modulator K1, introducing an oscillating transverse momentum kick (within the x–y aircraft, the place z is the optic axis) to the incoming steady electron beam. The beam begins to oscillate within the x–y aircraft and sweep throughout the chopping aperture. The chopping aperture partitions the continual beam into periodic electron pulses with a repetition charge of twof0, as a result of two pulses are created in every RF cycle. Additional downstream, because the pulses enter the demodulator K2, a part and amplitude optimized oscillating EM subject established in K2 totally compensates the transverse momentum induced by K1 to cut back the emittance progress and power unfold of the pulses, preserving the spatial and temporal coherence. Observe that the K2 compensation performs a essential function for resolving the twin pulsed beam problem as a result of modulator-induced transverse momentum on the chopped pulses (39, 40), which is essential to understand the ultrafast pump-probe measurements. As a result of each K1 and K2 function within the touring wave mode, a broadband EM subject with a frequency starting from 50 MHz to six GHz may be established in our present design. Thus, due to the frequency doubling, the repetition charge of the electron pulse is tunable from 100 MHz to 12 GHz. Except in any other case specified, the RF frequency of f0 = 2.625 GHz is used for all of the experimental knowledge introduced on this work.
To carry out true ultrafast pump-probe experiments, the pattern needs to be excited on the similar repetition charge because the probe electron pulses. For the EM wave excitation configuration (Fig. 1), as a result of the repetition charge of the pulsed beam is 2f0, we break up a small a part of the RF sign (~10% of the facility) from the RF supply of K1 by a coupler and doubled its frequency to 2f0 with a frequency doubler. After a downstream amplifier, a part shifter is used to manage the time delay (i.e., part delay) between the excitation EM wave and the probe electron pulse. Final, we use a specifically designed TEM pattern holder with broad bandwidth and low energy loss to effectively ship the EM wave to the pattern (Fig. 1). Furthermore, utilizing superior laser-RF synchronization applied sciences with little pulse jitter (31, 47), the excitation for samples is extensible to laser pulses, particularly, the laser-triggered pump-probe experiments might be carried out as nicely with our instrument and an acceptable laser-compatible pattern holder.
The rationale behind the design of the RF-driven pulser is to understand laser-free UEM whereas preserving the unique modalities of the TEM when the RF activation is off. To check the efficiency of the TEM after integrating the pulser, we recorded a set of imaging and diffraction outcomes underneath the identical situation in each steady beam (typical TEM) mode and pulsed beam mode (Fig. 2). On the most magnification (200,000×) of this Lorentz TEM with a field-free weakly excited goal lens, the bright-field photos of gold nanoparticles in each modes are comparable in each depth profile and distinction (Fig. 2, A and E). For the out-of-focus Fresnel part imaging, each modes present comparable part distinction for a magnetic vortex in a round ferromagnetic permalloy disc (Fig. 2, D and H). For the diffraction assessments, diffraction patterns of gold nanoparticles (Fig. 2, B and F) and a VO2 single crystal (Fig. 2, C and G) had been recorded in each modes, which exhibit no apparent change apart from the anticipated depth decline within the pulsed beam mode. The comparable high quality of imaging and diffraction between the pulsed beam mode and the continual beam mode illustrates the nice efficiency and flexibility of our prototype laser-free UEM.
The temporal decision of the laser-free UEM is especially decided by the length of the chopped electron pulses, which relies on the responsibility cycle of the chopped electron beam and may be altered independently by altering the enter RF energy (Prf), frequency, and/or the chopping aperture measurement. Theoretically, the chopped pulse length is given by τ = γme(d + r)/4qE0l (39), the place γ is the Lorentz issue, me is the electron mass, d is the diameter of the chopping aperture, r is the diameter of the electron beam on the place of the chopping aperture, q is the elementary cost, E0 is the EM subject in K1, and l is the space between K1 and the chopping aperture. Experimentally, when the beam waist on the chopping aperture is smaller than the aperture diameter, the heart beat length τ may be retrieved by measuring the ratio r of the full electron counts per second of the chopped beam and the continual beam with the total beam illuminating on the digicam (40), i.e., τ = r/2f0. As introduced in Fig. 3A, the measured electron pulse length decreases with rising voltage amplitude (U0) of the enter RF supply for the modulator K1 and follows the theoretically anticipated habits
(slot in Fig. 3A) (39). On the most enter RF energy of ~8 W and utilizing the minimal chopping aperture of ~25 μm in our present design, the shortest pulse length of ~10 ps is achieved. In precept, utilizing greater enter RF energy, a better RF frequency, and/or a smaller chopping aperture may obtain shorter and even sub-picosecond or femtosecond electron pulses (40), which holds promise to additional enhance the temporal decision.
To exhibit the ultrafast pump-probe measurement functionality of our laser-free UEM, we carried out an ultrafast imaging examine on the EM wave propagation dynamics in a microstrip consisting of two interdigitated combs (Fig. 3C and fig. S1). Understanding electrodynamics in microstrips is essential, because the oscillating currents and fields are elementary to the operation of just about any info processing system (28). Nonetheless, immediately visualizing the electrodynamics at gigahertz frequencies in microstrips has not been achieved up to now to the most effective of our information due to the dearth of correct transient imaging know-how. The pattern was fabricated on a silicon-on-insulator (SOI) wafer and utilizing a typical SOI microfabrication course of (Supplies and Strategies) and was designed to match the wave impedance at round 5 GHz, which is the frequency regime for the superior 5G wi-fi communication applied sciences. Particularly, the full size of every comb is 1.25 mm, and the tine pitch on each combs is 16 μm. The width and size of every tine are w = 4.5 μm and L = 75 μm, respectively, with a spot between the interleaved tines of g = 3.5 μm (fig. S1). The thickness of the tines alongside the beam-path path is Dz = 25 μm. Within the experiment, the enter terminal of 1 comb was excited by a 5.25-GHz EM wave with an influence of ~1.0 W (Fig. 3A), whereas the output finish of the identical comb was terminated with a 50-ohm load to get rid of sign reflections (Fig. 3C). The opposite comb was held at floor potential. The wavelength of the gigahertz wave for excitation is about 11.5 cm in vacuum however solely about 3 cm in our microstrip due to the big relative permittivity (~12) of the silicon layer. Thus, the 1.25-mm comb pattern spans lower than 5% of a full wave. Underneath the gigahertz wave excitation, the intentional native EM fields across the tines of the interdigitated combs would give a deflection to the imaging electron pulse within the x–y aircraft and end in a change within the picture. As a result of the electron pulse length is sort of 19 instances shorter than the cycle (~190 ps) of the excitation EM wave, it permits us to take stroboscopic photos at a sequence of particular delay instances for time-frozen electrodynamics within the pattern.
First time-resolved photos of EM propagation within the interdigitated comb construction acquired at a magnification of 1200× with an integral time of 1.5 s are proven in Fig. 3D (film S1), the place a set of typical snapshots (two floor tines and one lively tine in between them) at totally different delay instances obtained from the realm indicated by the blue dashed field in Fig. 3C is introduced, revealing a pronounced temporal oscillation or respiratory of the tines within the time-frozen photos. With the delay time rising from 0 ps (time zero was set at a delay level when the beam has no deflection), the width of the center lively tine progressively shrinks first after which broadens, whereas the width of the 2 floor tines progressively broadens first after which shrinks in alternation. Extra particularly, the retrieved width variation (alongside the x path) versus time of the 2 floor tines follows a sinusoidal perform (purple dots in Fig. 3E, solely displaying the information for one of many floor tines), whereas that of the lively tine follows a cosine perform (blue dots in Fig. 3E). By the becoming, it’s discovered that the width variations of each the lively and floor tines present an identical amplitude (~90 nm) and have an an identical frequency of 5.25 ± 0.02 GHz (Fig. 3E), which is in keeping with the frequency of the gigahertz wave for excitation.
For a extra rigorous evaluation of the experimental end result, we denote the spatiotemporal electrical and magnetic fields across the tines as E(x, y, z, t) and B(x, y, z, t), the electron pulse velocity as ve, and the frequency of the excitation wave as f. Contemplating the next circumstances: (i) Dz/ve ≪ 1/f, the electron penetration time by means of the pattern is way shorter than the cycle of the excitation wave, the place Dz is the thickness of the pattern alongside the beam-path path; (ii) the heart beat length is sort of 19 instances shorter than the periodicity of the excitation wave; (iii) the results of magnetic fields are negligible in comparison with these of the electrical fields for the specimen geometry (28); and (iv) the pulsed beam is collimated on the pattern. The approximate change in beam divergence angles αx, y after penetrating the pattern at every place within the beam and at a delay time of t is given by
(28). At a particular delay time t, if the electrical subject vectors (within the x–y aircraft) round a tine level outward from the tine’s floor, then every ray within the pulsed beam is subjected to a field-dependent momentum kick towards the tine’s floor and thus a change of divergence angle αx, y(x, y, t), leading to a beam deflection towards the tine’s floor and a shrinking of the tine within the picture; in distinction, if the electrical subject vectors level towards the tine’s floor, then each the momentum kick and the beam deflection are outward from the tine’s floor, leading to a broadening of the tine within the picture. Subsequently, the noticed inverse temporal respiratory of the lively and floor tines signifies that, upon the EM wave excitation, an oscillating electrical subject perpendicular to
is constructed within the gaps between the lively and floor tines. These photos are a direct reflection of the EM wave propagation course of by means of the interdigitated combs.
Contemplating a collimated beam illumination, the temporal electrical subject Ex, y(x, y, t) across the tine is proportional to the change of the tine’s edge depth profile within the time-frozen photos (28), particularly, a bigger beam deflection corresponds to a bigger native electrical subject. The time-dependent imaging respiratory (tine’s edge variation) at three consultant positions (P1, P2, and P3) round a floor tine is proven in Fig. 3F, as indicated by the coloured arrows within the high panel of Fig. 3D. All of them observe the identical cosine perform however with totally different amplitudes. Level 2 close to the tine’s nook reveals a a lot larger amplitude than the opposite two positions, implying that there’s a substantial native subject enhancement on the corners of the tines within the EM wave propagation course of, which can be mentioned later.
We additional studied the dependence of EM wave propagation dynamics on the excitation energy. Further ultrafast pump-probe imaging measurements had been carried out with totally different excitation powers from 0.5 to 1.0 W, the place an identical temporal respiratory phenomenon of the lively and floor tines occurred in any respect totally different powers. Figure 4A presents the plots of the time-dependent width variation of a floor tine in any respect excitations, through which all of the plots observe a sinusoidal perform with the frequency of 5.25 ± 0.02 GHz (becoming in Fig. 4A), with no part distinction. Whereas their amplitude will increase with rising excitation energy and follows a sq. root dependence of the facility (slot in Fig. 4B), i.e., the amplitude of the temporal oscillating electrical subject erected between the tines, as theoretically anticipated, modifications as a sq. root perform of the excitation energy inside this energy vary.
To additional perceive the experimental observations, we carried out numerical simulations on the EM wave propagation in a microstrip of two interdigitated combs with the identical geometry and supplies (fig. S2). The simulation was carried out by a three-dimensional (3D) EM finite factor evaluation package deal CST Microwave Studio (Supplies and Strategies). Hexahedron and domestically refined meshes had been adopted to get high-resolution EM subject distribution alongside the pattern. A frequency area solver was used to resolve Maxwell’s equations within the cells. The 5.25-GHz RF sign (energy of 1.0 W) excites a touring EM wave, which propagates by means of the 2 interdigitated combs and is totally absorbed by the RF dump (load) on the finish of the pattern (fig. S2).
Figure 5A presents a set of typical snapshots of the simulated electrical subject distribution (projected within the x–y aircraft on the mid-comb thickness) round one lively tine and two adjoining floor tines at totally different delay instances (film S2), the place the arrows point out the path of the fields and the sector energy is encoded within the colour. The pattern is nonmagnetic, and the results of magnetic fields are negligible within the experiment, which aren’t thought-about right here. Clearly, because the EM wave propagates by means of the interdigitated combs underneath investigation, a temporal oscillating electrical subject Ex, y(x, y, t) is immediately established in between the gaps (within the x–y aircraft) of the lively and floor tines, and the electrical subject is perpendicular to the tine’s floor alongside the beam path. Particularly, with time elapses from 0 to 95 ps, the fields level from the lively tine towards the neighboring floor ones and progressively develop from zero to a most amplitude (|Ex| ≈1.7 × 106 V/m) at ~48 ps after which return again to zero at ~95 ps. Additional, from 95 to 190 ps, the electrical fields swap path and progressively improve to a most amplitude (|Ex| ≈1.7 × 106 V/m) at ~143 ps after which decline to zero once more at ~190 ps. This course of is repeated with every EM wave cycle. These temporal oscillating electrical fields would exert a neighborhood subject–dependent momentum kick on the imaging electrons that’s proportional to the native waveform, ensuing within the beam deflection and the respiratory of the tines within the time-frozen photos noticed within the experiment. To point out extra clearly the temporal evolution of the sector distribution, we plot the electrical fields Ex and Ey as a perform of time at three positions close to a floor tine (P1, P2, and P3, in step with that within the experiment proven in Fig. 3D) in Fig. 5 (B and C, respectively). The electrical fields in any respect three positions oscillate in a sinusoidal perform with the frequency of 5.25 GHz however with totally different subject amplitudes. Particularly, the sector energy (Ex) close to the tine’s nook is stronger than the opposite two positions. On the place of P1, Ey is sort of zero, whereas on the place of P3, Ex is nearly zero. These outcomes exhibit that the electrical subject vectors are vertically polarized to the floor of the tines and bear a synchronous oscillation in each path and energy with time, whereas the corners of every tine exhibit a considerable native subject enhancement. To see extra clearly the native subject enhancement, we plotted the electrical subject energy |Ex| (in absolute worth) at t = 20 ps as a perform of place close to the floor of a floor tine in Fig. 5D (2D map of the sector energy is proven in fig. S3), the place the positions are indicated by the purple line with an arrow within the inset. Because the place strikes alongside the pink arrow, the sector energy reveals no obvious change within the parallel hole (from 0 to twenty μm), whereas it reveals a pointy improve close to the nook (place P2 indicated by the pink arrow) after which progressively decreases to zero, indicating the existence of a exceptional nonlinear native subject distribution and an enhancement on the singular factors of the microstrip within the EM wave propagation course of. These nonlinear native subject distribution and enhancement are brought on by the convex floor geometry, the place the smaller radius of the curved floor will end in a better density of the equipotential surfaces and thus a bigger native electrical subject. The outcomes of the simulation are in good settlement with the experimental observations.
In abstract, we developed a laser-free UEM with excessive spatiotemporal decision by integrating an RF-driven pulser with a business TEM, which permits facile operation in each the UEM mode and the traditional TEM mode. It provides a common methodology for EM wave excitation and construction dynamic research in actual time and area through the use of a simple retrofit. We used the laser-free UEM to check the gigahertz EM wave propagation dynamical course of in a microstrip consisting of two interdigitated combs and demonstrated its means for direct visualization of EM subject oscillation with time, revealing subject amplitude, polarization path, and wave propagation on the nanometer-picosecond time scale, which has not been accessible by different imaging strategies. The demonstrated laser-free UEM offers a strong methodology for real-space visualization of electrodynamics in small units working from megahertz to gigahertz frequencies, such because the collective provider dynamics and subject results in miniaturized wi-fi antennas, sensors, and RF MEMS (48). Future optimization of the enter RF waveform and utilizing a smaller chopping aperture may obtain sub-picosecond and even ~100-fs electron wave packets (39, 40), making femtosecond time decision for the laser-free UEM doable. The laser-free UEM can be appropriate with laser, present, and magnetic subject–triggered ultrafast pump-probe measurements utilizing superior laser-RF and electricity-RF synchronization applied sciences (47). With these superior options of the laser-free UEM, we envision the emergence of broad purposes in lots of analysis areas, from supplies physics to biology and cellular communication applied sciences.
Acknowledgments: Funding: This work was supported by the Supplies Science and Engineering Division, Workplace of Fundamental Power Sciences of the U.S. Division of Power (DOE) underneath contract no. DESC0012704. The electron pulser was developed by Euclid Techlabs by means of the DOE’s SBIR grant underneath contract DE-SC0013121. X.F. is grateful to the monetary assist from the Nationwide Nature Science Basis of China (NSFC) (no. 11974191). V.J.G. acquired monetary assist from the Nationwide Institute of Requirements and Expertise (NIST) (70NANB14H253 and 70NANB16H307). The comb pattern was ready on the NIST Heart for Nanoscale Science and Expertise NanoFab. Writer contributions: Y. Zhu, X.F., and J.W.L. conceived the analysis challenge. X.F., C.J., Y. Zhao, and Y. Zhu did the experimental measurements. V.J.G. and J.J.G. ready the pattern. X.F. did the information evaluation and wrote the manuscript with enter from Y. Zhu, E.W., and C.J. E.W. developed the mannequin and carried out the numerical simulations. All of the authors contributed to the dialogue and the revision of the manuscript. Competing pursuits: C.J., J.W.L., and Y. Zhu are inventors on a U.S. patent associated to this work filed by Euclid Techlabs LLC and U.S. Division of Commerce (no. US9,697982, filed on 6 April 2016, revealed on 4 July 2017). C.J., J.W.L., and Y. Zhu are inventors on a U.S. patent associated to this work filed by Euclid Techlabs LLC and U.S. Division of Commerce (no. US10,319556, filed on 2 December 2016, revealed on 11 June 2019). C.J., A.L., E.M., and Y. Zhao are the inventors on a U.S. patent associated to this work filed by Euclid Techlabs LLC (no. US10,515733, filed on 24 April 2019, revealed on 24 December 2019). The authors declare no different competing pursuits. Information and supplies availability: All knowledge wanted to guage the conclusions within the paper are current within the paper and/or the Supplementary Supplies. Further knowledge associated to this paper could also be requested from the authors.