Richard L. Sandberg

High numerical aperture tabletop soft x-ray diffraction microscopy with 70-nm resolution.

Light microscopy has greatly advanced our understanding of nature. The achievable resolution, however, is limited by optical wavelengths to ≈200 nm. By using imaging and labeling technologies, resolutions beyond the diffraction limit can be achieved for specialized specimens with techniques such as near-field scanning optical microscopy, stimulated emission depletion microscopy, and photoactivated localization microscopy. Here, we report a versatile soft x-ray diffraction microscope with 70- to 90-nm resolution by using two different tabletop coherent soft x-ray sources—a soft x-ray laser and a high-harmonic source. We also use field curvature correction that allows high numerical aperture imaging and near-diffraction-limited resolution of 1.5λ. A tabletop soft x-ray diffraction microscope should find broad applications in biology, nanoscience, and materials science because of its simple optical design, high resolution, large depth of field, 3D imaging capability, scalability to shorter wavelengths, and ultrafast temporal resolution.

Three-dimensional structure determination from a single view

The ability to determine the structure of matter in three dimensions has profoundly advanced our understanding of nature. Traditionally, the most widely used schemes for three-dimensional (3D) structure determination of an object are implemented by acquiring multiple measurements over various sample orientations, as in the case of crystallography and tomography, or by scanning a series of thin sections through the sample, as in confocal microscopy. Here we present a 3D imaging modality, termed ankylography (derived from the Greek words ankylos meaning ‘curved’ and graphein meaning ‘writing’), which under certain circumstances enables complete 3D structure determination from a single exposure using a monochromatic incident beam. We demonstrate that when the diffraction pattern of a finite object is sampled at a sufficiently fine scale on the Ewald sphere, the 3D structure of the object is in principle determined by the 2D spherical pattern. We confirm the theoretical analysis by performing 3D numerical reconstructions of a sodium silicate glass structure at 2 Å resolution, and a single poliovirus at 2–3 nm resolution, from 2D spherical diffraction patterns alone. Using diffraction data from a soft X-ray laser, we also provide a preliminary demonstration that ankylography is experimentally feasible by obtaining a 3D image of a test object from a single 2D diffraction pattern. With further development, this approach of obtaining complete 3D structure information from a single view could find broad applications in the physical and life sciences.

Aspect ratio dependence of auger recombination and carrier multiplication in PbSe nanorods.

Nanomaterials with efficient carrier multiplication (CM), that is, generation of multiple electron-hole pairs by single photons, have been the object of intense scientific interest as potential enablers of high efficiency generation-III photovoltaics. In this work, we explore nanocrystal shape control as a means for enhancing CM. Specifically, we investigate the influence of aspect ratio (ρ) of PbSe nanorods (NRs) on both CM and the inverse of this process, Auger recombination. We observe that Auger lifetimes in NRs increase with increasing particle volume and for a fixed cross-sectional size follow a linear dependence on the NR length. For a given band gap energy, the CM efficiency in NRs shows a significant dependence on aspect ratio and exhibits a maximum at ρ ∼ 6-7 for which the multiexciton yields are a factor of ca. 2 higher than those in quantum dots with a similar bandgap energy. To rationalize our experimental observations, we analyze the influence of dimensionality on both CM and non-CM energy-loss mechanisms and offer possible explanations for the seemingly divergent effects the transition from zero- to one-dimensional confinement has on the closely related processes of Auger recombination and CM.

Coherent X-Ray Diffraction Imaging

For centuries, lens-based microscopy, such as optical, phase-contrast, fluorescence, confocal, and electron microscopy, has played an important role in the evolution of modern science and technology. In 1999, a novel form of microscopy, i.e., coherent diffraction imaging (also termed coherent diffraction microscopy or lensless imaging), was developed and transformed our conventional view of microscopy, in which the diffraction pattern of a noncrystalline specimen or a nanocrystal was first measured and then directly phased to obtain a high-resolution image. The well-known phase problem was solved by combining the oversampling method with iterative algorithms. In this paper, we will briefly discuss the principle of coherent diffraction imaging, present various implementation schemes of this imaging modality, and illustrate its broad applications in materials science, nanoscience, and biology. As coherent X-ray sources such as high harmonic generation and X-ray free-electron lasers are presently under rapid development worldwide, coherent diffraction imaging can potentially be applied to perform high-resolution imaging of materials/nanoscience and biological specimens at the femtosecond time scale.

Tabletop soft-x-ray Fourier transform holography with 50 nm resolution

We present what we believe to be the first implementation of Fourier transform (FT) holography using a tabletop coherent x-ray source. By applying curvature correction to compensate for the large angles inherent in high-NA coherent imaging, we achieve image resolution of 89 nm using high-harmonic beams at a wavelength of 29 nm. Moreover, by combining holography with iterative phase retrieval, we improve the image resolution to <53 nm. We also demonstrate that FT holography can be used effectively with short exposure times of 30 s. This technique will enable biological and materials microscopy with simultaneously high spatial and temporal resolution on a tabletop soft-x-ray source.

Multiexciton Dynamics in Infrared-Emitting Colloidal Nanostructures Probed by a Superconducting Nanowire Single-Photon Detector

Carrier multiplication (CM) is the process in which absorption of a single photon produces multiple electron-hole pairs. Here, we evaluate the effect of particle shape on CM efficiency by conducting a comparative study of spherical nanocrystal quantum dots (NQDs) and elongated nanorods (NRs) of PbSe using a time-resolved technique that is based on photon counting in the infrared using a superconducting nanowire single-photon photodetector (SNSPD). Due to its high sensitivity and low noise levels, this technique allows for accurate determination of CM yields, even with the small excitation intensities required for quantitative measurements, and the fairly low emission quantum yields of elongated NR samples. Our measurements indicate an up to ∼60% increase in multiexciton yields in NRs versus NQDs, which is attributed primarily to a decrease in the electron-hole pair creation energy. These findings suggest that shape control is a promising approach for enhancing the CM process. Further, our work demonstrates the effectiveness of the SNSPD technique for the rapid screening of CM performance in infrared nanomaterials.

Ultrafast visualization of crystallization and grain growth in shock-compressed SiO2

Pressure- and temperature-induced phase transitions have been studied for more than a century but very little is known about the non-equilibrium processes by which the atoms rearrange. Shock compression generates a nearly instantaneous propagating high-pressure/temperature condition while in situ X-ray diffraction (XRD) probes the time-dependent atomic arrangement. Here we present in situ pump–probe XRD measurements on shock-compressed fused silica, revealing an amorphous to crystalline high-pressure stishovite phase transition. Using the size broadening of the diffraction peaks, the growth of nanocrystalline stishovite grains is resolved on the nanosecond timescale just after shock compression. At applied pressures above 18 GPa the nuclueation of stishovite appears to be kinetically limited to 1.4±0.4 ns. The functional form of this grain growth suggests homogeneous nucleation and attachment as the growth mechanism. These are the first observations of crystalline grain growth in the shock front between low- and high-pressure states via XRD.

Quasi-phase matching and characterization of high-order harmonic generation in hollow waveguides using counterpropagating light

We review recent experimental and theoretical work on the use of counterpropagating light to enhance high-order harmonic generation through all-optical quasi-phase matching. Also presented is a new technique for measuring the coherence of high harmonics in the nonlinear medium. This information is crucial for understanding the process of harmonic generation over extended distances, as well as for effective enhancement using quasi-phase matching techniques.

Extreme ultraviolet lithography: From research to manufacturing

The authors explore the critical issues remaining for the introduction of extreme ultraviolet lithography (EUVL) in semiconductor manufacturing. Among all technical issues, source power appears to be the most significant challenge that the technology is facing at this time. The lack of sufficiently high-power sources integrated in the first generation of full-field commercial scanners has profound implications on the remaining issues, and therefore on the risk associated with the insertion of EUVL technology. At the core of the problem is the cost of ownership of EUVL, which depends most heavily on source power. Moreover, the lifetime of the scanner mirrors and of the masks can only be tested properly with high-power sources on these first scanners. Without the ability to perform these tests, the technology might suffer unrecoverable delays. Fortunately, there has been good progress in the development of laser-produced plasma sources, which appear to be the most capable candidates for high-power scalabili...

A comparison of uranium oxide and nickle as single-layer reflectors from 2.7 to 11.6 nm

We present the measured reflectances (Beamline 6.3.2, ALS at LBNL) of naturally oxidized uranium and naturally oxidized nickel thin films from 2.7 to 11.6 nm at 5°, 10°, and 15° grazing incidence. These show that uranium, as UO2, can fulfill its promise as the highest known single surface reflector for this portion of the soft x-ray region, being nearly twice as reflective as nickel in the 5-10 nm region. This is due to its large index of refraction coupled with low absorption. Nickel is commonly used in soft x-ray applications in astronomy and synchrotrons. (Its reflectance at 10° exceeds that of Au and Ir for most of this range.) We prepared uranium and nickel thin films via DC-magnetron sputtering of a depleted U target and resistive heating evaporation respectively. Ambient oxidation quickly brought the U sample to UO2 (total thickness about 30 nm). The nickel sample (50 nm) also acquired a thin native oxide coating (<2nm). Though the density of U in UO2 is only half of the metal, its reflectance is high and it is relatively stable against further changes. There are important discrepancies between UO2’s actual reflectance with those predicted by the atomic scattering factor model indicative of the need to determine the actual constants of UO2.