Miniaturized RF technology for femtosecond electron microscopy

In this thesis we have taken many of the RF ideas of Ura, Oldfield, and Hawkes from the 1960’s, coupled with modern technology and simulating power to investigate the usage of miniaturized RF technology for femtosecond electron microscopy without the mandatory use of femtosecond lasers. To achieve femtosecond time resolution, a TM110 cavity is used to streak a DC electron beam across a slit, creating ultrashort electron bunches at a regular repetition rate. To create a miniaturized cavity, the TM110 cavity was loaded with ZrTiO4. This reduced the size of the cavity by nearly a factor of 6, and the power was reduced by nearly a full order of magnitude. Characterization of the cavity found that the dielectric loaded cavity operated as it was designed to do, matching both simulations and theory. To test the RF technology developed in our group, a 30 keV SEM beam line was built on top of an optical table. This allows for the beam line to be extended ~2.5 meters beyond the end of the SEM, giving space and room for experiments that would not otherwise be available in a standard electron microscope. This allows for multiple cavity systems to be tested, as well as emittance measurements of a high quality beam, requiring long distances to resolve the small angles. Implementation of the TM110 cavity into the beam line proved quite successful, with the beam behaving as expected. Chapter 5 builds an analytical model which predicts the beam’s behavior through the cavity, and goes a step further to predict the growth in transverse normalized emittance and the energy spread of a beam as it traverses the cavity. The analytical model is then compared to particle tracking simulations, proving to be accurate for high quality beams. The measurements of the emittance demonstrate the behavior predicted by the analytical model, and match with the numerical simulations. An important point realized from the model is that focusing in the cavity makes all the difference, minimizing the emittance and energy spread growth. This is an important detail for using the TM110 cavity, since a high beam quality is always desired to obtain spatial resolution. Chapter 6 detailed a few of the myriad of ideas of which can be done with the RF technology studied within the scope of this project. First, as the project is part of a larger industrial partnership program with the private company FEI, a full simulated study of implementing the TM110 cavity into an existing FEI electron microscope was carried out in cooperation with the company. In the parameter space presented by FEI, the energy spread growth from the cavity in generating 100 fs bunches appears too high, reaching ~10.5 eV FWHM. However, by relaxing a few of the constraints, and changing the parameters of the setup, it was shown that the energy spread growth can be compensated for. Because of the compact size and low power consumption of the dielectric cavity, this makes the cavity an ideal candidate for femtosecond electron bunch generation in a microscope. Following the implementation study, the idea of reducing the repetition rate of the bunches by using a dual mode cavity was presented. This was an idea that stemmed from a "Huh, that is strange…" moment in lab. The moment is seen in Fig. 6.4; it is when the two formerly degenerate modes were being operated simultaneously in one cavity. If the cavity was re-designed to use the two modes, running with two different frequencies, the repetition rate of the electron bunches could then be tailored to match repetition rates in the MHz range, rather than GHz. The original designs of Ura and Oldfield included multiple cavities synchronized to manipulate the beam. However, Oldfield commented in his Ph.D. thesis that the phase control was of the utmost importance. In Sec. 6.3, we demonstrated a high precision of phase control between two cavities, a TM110 streak cavity and a TM010 compression cavity, and supported the results with numerical simulations. With phase control between multiple cavities demonstrated, both synchronized to the same RF signal, the door to time-dependent beam manipulation has flown wide open. Section 6.4 walks right through the multiple cavity door, and presents a two cavity setup to compete with Zewail’s photocathode driven ultrafast electron microscope (UEM). Using two TM110 cavities, femtosecond electron energy loss spectroscopy (FEELS) was simulated using particle tracking software. It was shown that with the two cavities, appropriately phased, an energy resolution o

Kondo effect by controlled cleavage of a single molecule contact

Conductance measurements of a molecular wire, contacted between an epitaxial molecule-metal bond and the tip of a scanning tunneling microscope, are reported. Controlled retraction of the tip gradually de-hybridizes the molecule from the metal substrate. This tunes the wire into the Kondo regime in which the renormalized molecular transport orbital serves as spin impurity at half filling and the Kondo resonance opens up an additional transport channel. Numerical renormalization group simulations suggest this type of behavior to be generic for a common class of metal molecule bonds. The results demonstrate a new approach to single-molecule experiments with atomic-scale contact control and prepare the way for the ab initio simulation of many-body transport through single-molecule junctions.Comment: Main text: 41 pages including references and captions, 9 figures. Supplementary information: 5 pages including 2 figures New experimental and theoretical data supporting initial claims are added. The paper has been reworked from the letter format into a longer versio

Theory and particle tracking simulations of a resonant radiofrequency deflection cavity in TM110_{110} mode for ultrafast electron microscopy

We present a theoretical description of resonant radiofrequency (RF) deflecting cavities in TM$_{110}$ mode as dynamic optical elements for ultrafast electron microscopy. We first derive the optical transfer matrix of an ideal pillbox cavity and use a Courant-Snyder formalism to calculate the 6D phase space propagation of a Gaussian electron distribution through the cavity. We derive closed, analytic expressions for the increase in transverse emittance and energy spread of the electron distribution. We demonstrate that for the special case of a beam focused in the center of the cavity, the low emittance and low energy spread of a high quality beam can be maintained, which allows high-repetition rate, ultrafast electron microscopy with 100 fs temporal resolution combined with the atomic resolution of a high-end TEM. This is confirmed by charged particle tracking simulations using a realistic cavity geometry, including fringe fields at the cavity entrance and exit apertures

Beam pulsing device for use in charged-particle microscopy

A charged-particle microscope comprising: - A charged-particle source, for producing a beam of charged particles that propagates along a particle-optical axis; - A sample holder, for holding and positioning a sample; - A charged-particle lens system, for directing said beam onto a sample held on the sample holder; - A detector, for detecting radiation emanating from the sample as a result of its interaction with the beam; - A beam pulsing device, for causing the beam to repeatedly switch on and off so as to produce a pulsed beam, wherein the beam pulsing device comprises a unitary resonant cavity disposed about said particle-optical axis and having an entrance aperture and an exit aperture for the beam, which resonant cavity is embodied to simultaneously produce a first oscillatory deflection of the beam at a first frequency in a first direction and a second oscillatory deflection of the beam at a second, different frequency in a second, different direction. The resonant cavity may have an elongated (e.g. rectangular or elliptical) cross-section, with a long axis parallel to said first direction and a short axis parallel to said second direction

Cross-calibration of a combined electrostatic and time-of-flight analyzer for energy- and charge-state-resolved spectrometry of tin laser-produced plasma

We present the results of the calibration of a channeltron-based electrostatic analyzer operating in time-of-flight mode (ESA-ToF) using tin ions resulting from laser-produced plasma, over a wide range of charge states and energies. Specifically, the channeltron electron multiplier detection efficiency and the spectrometer resolution are calibrated, and count rate effects are characterized. With the obtained overall response function, the ESA-ToF is shown to accurately reproduce charge-integrated measurements separately and simultaneously obtained from a Faraday cup (FC), up to a constant factor the finding of which enables absolute cross-calibration of the ESA-ToF using the FC as an absolute benchmark. Absolute charge-state-resolved ion energy distributions are obtained from ns-pulse Nd:YAG-laser-produced microdroplet tin plasmas in a setting relevant for state-of-the-art extreme ultraviolet nanolithography

Energy- and charge-state-resolved spectrometry of tin laser-produced plasma using a retarding field energy analyzer

We present a method to obtain the individual charge-state-dependent kinetic-energy distributions of tin ions emanating from a laser-produced plasma from their joint overlapping energy distributions measured by means of a retarding field energy analyzer (RFA). The method of extracting charge state specific parameters from the ion signals is described mathematically, and reinforced with experimental results. The absolute charge-state-resolved ion energy distributions is obtained from ns-pulse Nd:YAG-laser-produced microdroplet tin plasmas in a setting relevant for state-of-the-art extreme ultraviolet nanolithography

Polarization-dependent ponderomotive gradient force in a standing wave

The ponderomotive force is derived for a relativistic charged particle entering an electromagnetic standing wave with a general three-dimensional field distribution and a nonrelativistic intensity, using a perturbation expansion method. It is shown that the well-known ponderomotive gradient force expression does not hold for this situation. The modified expression is still of simple gradient form, but contains additional polarization-dependent terms. These terms arise because the relativistic translational velocity induces a quiver motion in the direction of the magnetic force, which is the direction of large field gradients. Oscillation of the Lorentz factor effectively doubles this magnetic contribution. The derived ponderomotive force generalizes the polarization-dependent electron motion in a standing wave obtained earlier [A.E. Kaplan and A.L. Pokrovsky, Phys. Rev. Lett. $\bm{95}$, 053601 (2005)]. Comparison with simulations in the case of a realistic, non-idealized, three-dimensional field configuration confirms the general validity of the analytical results.Comment: 13 pages, 4 figure

Characterization of angularly resolved EUV emission from 2-μm-wavelength laser-driven Sn plasmas using preformed liquid disk targets

The emission properties of tin plasmas, produced by the irradiation of preformed liquid tin targets by several-ns-long 2 µm-wavelength laser pulses, are studied in the extreme ultraviolet (EUV) regime. In a two-pulse scheme, a pre-pulse laser is first used to deform tin microdroplets into thin, extended disks before the main (2 µm) pulse creates the EUV-emitting plasma. Irradiating 30- to 300 µm-diameter targets with 2 µm laser pulses, we find that the efficiency in creating EUV light around 13.5 nm follows the fraction of laser light that overlaps with the target. Next, the effects of a change in 2 µm drive laser intensity (0.6–1.8 × 1011 W cm−2) and pulse duration (3.7–7.4 ns) are studied. It is found that the angular dependence of the emission of light within a 2% bandwidth around 13.5 nm and within the backward 2π hemisphere around the incoming laser beam is almost independent of intensity and duration of the 2 µm drive laser. With increasing target diameter, the emission in this 2% bandwidth becomes increasingly anisotropic, with a greater fraction of light being emitted into the hemisphere of the incoming laser beam. For direct comparison, a similar set of experiments is performed with a 1 µm-wavelength drive laser. Emission spectra, recorded in a 5.5–25.5 nm wavelength range, show significant self-absorption of light around 13.5 nm in the 1 µm case, while in the 2 µm case only an opacity-related broadening of the spectral feature at 13.5 nm is observed. This work demonstrates the enhanced capabilities and performance of 2 µm-driven plasmas produced from disk targets when compared to 1 µm-driven plasmas, providing strong motivation for the use of 2 µm lasers as drive lasers in future high-power sources of EUV light