Investigating the potential for entangled two-photon excited fluorescence imaging

May 21, 2021|

Setting bounds on the absorption cross-sections of molecular systems. There has been a long-running controversy regarding the “quantum advantage” for multiphoton excitation of molecules with entangled photons and if quantum multiphoton imaging can be realized. Although theoretical proposals have been advanced for decades, no experimental work (with the exception of a publication by Jeff Kimble’s group in the 1990s) appeared in the literature until 2006 when reports from a small number of groups began to emerge of a large quantum enhancement (e.g. up to 10 orders of magnitude) of the two photon excitation rate using entangled pairs compared to classical light. Last year, a paper describing a microscope based on the “entangled two-photon absorption” (E2PA) effect was published in Journal of the American Chemical Society. On the other hand, it has emerged from discussions at scientific meetings that many researchers have failed to replicate the results in these numerous publications, or to find any other evidence for this enhancement. As a result, there is considerable skepticism of the publications making these remarkable claims. Unfortunately, these negative results haven’t been published and therefore a rigorous basis for resolving the controversy hasn’t yet been established. Finally, new experiments at JILA have finally set upper-bounds for the E2PA cross-sections in molecular fluorophores, including those investigated in previous reports. We performed both classical and quantum light excitation in the same optical transmission and fluorescence-based systems with rigorously characterized states of light and measurement sensitivities. We find that E2PA cross-sections are at least four to five orders of magnitude smaller than previously reported. Our results imply that the signals and images reported in previous publications are artifacts. Although we don’t expect this contribution to be the last word on the subject, this work introduces a new level of experimental rigor that will lead towards new designs for quantum microscopes and sensors.

M. Parzuchowski, A. Mikhaylov, M. D. Mazurek, R. N. Wilson, D. J. Lum, T. Gerrits, C. H. Camp, M. J. Stevens, R. Jimenez, “Setting Bounds on Entangled Two-Photon Absorption Cross Sections in Common Fluorophores,” Physical Review Applied, 15, 044012 , (2021).

Nondestructive, high-resolution, chemically specific 3D nanostructure characterization using phase-sensitive EUV imaging reflectometry

May 20, 2021|

Next-generation nano and quantum devices have increasingly complex 3D structure. As the dimensions of these devices shrink to the nanoscale, their performance is often governed by interface quality or precise chemical or dopant composition. A STROBE team from CU Boulder, UCLA, UC Berkeley, as well as laser and nanoelectronics industry partners, worked together for 4 years to design, construct and commission the first phase-sensitive extreme ultraviolet imaging reflectometer. It combines the excellent phase stability of tabletop coherent extreme UV (EUV) light sources, the unique chemical- and phase-sensitivity of coherent EUV imaging, and state-of-the-art algorithms. This tabletop microscope can non-destructively probe surface topography, layer thicknesses, and interface quality, as well as dopant concentrations and profiles. High-fidelity imaging was achieved by implementing phase sensitive imaging at different angles, by using advanced methods to mitigate noise and artifacts in the reconstructed image, and by using a high-brightness, EUV source with excellent intensity and wavefront stability. These measurements were validated through multiscale electron and atomic force microscopy imaging to show that this approach has unique advantages compared with others. Critical to this project were new photon and electron-based imaging methods, advanced algorithms, unique samples, as well STROBE advances in tabletop coherent imaging in transmission and reflection mode. Several STROBE trainees received awards for this effort.

Tanksalvala, C. L. Porter, Y. Esashi, B. Wang, N. W. Jenkins, Z. Zhang, G. P. Miley, J. L. Knobloch, B. McBennett, N. Horiguchi, S. Yazdi, J. Zhou, M. N. Jacobs, C. S. Bevis, R. M. Karl, P. Johnsen, D. Ren, L. Waller, D. E. Adams, S. L. Cousin, C. Liao, J. Miao, M. Gerrity, H. C. Kapteyn, M. M. Murnane, “Nondestructive, high-resolution, chemically specific 3D nanostructure characterization using phase-sensitive EUV imaging reflectometry,” Science Advances, 7, eabd9667, (2021).

New phase retrieval methods enabled by the world’s fastest electron detector

May 19, 2021|

The need for rapid and accurate image analysis is increasing in electron microscopy studies of nanomaterials. With newly developed fast, high-efficiency electron detectors and automated imaging protocols, incorporating electron microscopy into high throughput materials design efforts is becoming possible. These new capabilities strongly motivate automated methods to extract relevant structural features, such as nanoparticle size, shape, and defect content, from high resolution transmission electron microscopy (HRTEM) data to link these features to bulk properties and study the influence of heterogeneity on bulk behavior. In general, protocols that surpass the accuracy of traditional image analysis and do not require time-consuming manual analysis are needed. Recent advances in image interpretation using deep learning using machine learning make it a promising route toward automatic interpretation of HRTEM micrographs.

In this STROBE collaboration, we demonstrate a pipeline to detect and classify regions of interest in HRTEM micrographs. Our pipeline uses a convolutional neural net (CNN) to identify crystalline regions (nanoparticles) from an amorphous background in the images, and then feeds individual regions of interest into a random forest classifier to detect whether or not they contain a crystallographic defect. Our CNN has a lightweight U-Net architecture and accurately segments a diverse population of nanoparticles with only a small number of training images. After segmentation, individual nanoparticle regions can be isolated and fed directly into existing python tools to extract size and shape statistics. To detect the presence of defects in nanoparticle regions, we implement a random forest classifier. We demonstrated the random forest classifier’s ability to detect stacking faults in the CdSe subset of identified nanoparticles. Both the CNN and classifier demonstrate state of the art performance at their respective tasks. While this work focuses on HRTEM images of nanoparticles supported on a carbon substrate, in principle the tool can be used to detect any regions of crystallinity in HRTEM data.

K. Groschner, C. Choi, M. C. Scott, “Machine Learning Pipeline for Segmentation and Defect Identification from High-Resolution Transmission Electron Microscopy Data,” Microscopy and Microanalysis, 1-8, (2021).

Compressive and adaptive nano imaging for enhanced speed and content

May 18, 2021|

Scattering scanning near-field optical microscopy (s-SNOM) provides for spectroscopic imaging from molecular to quantum materials with few nanometer deep sub-diffraction limited spatial resolution. However, conventional acquisition methods are often too slow to fully capture a large field of view spatio-spectral dataset. Through this collaboration, STROBE researchers, at CU Boulder and the ALS –Berkeley, demonstrated how the data acquisition time and sampling rate can be significantly reduced while maintaining or even enhancing the physical or chemical image information content. The novel data acquisition and mathematical concepts implemented are based on advanced data compressed sampling, matrix completion, and adaptive random sampling. This research is of particular interest in synchrotron based nano-imaging facilities. This work paves the way to true spatio-spectral chemical and materials nano-spectroscopy with a reduction of sampling rate by up to 30 times.

Labouesse, S. C. Johnson, H. A. Bechtel, M. B. Raschke, R. Piestun, “Smart Scattering Scanning Near-Field Optical Microscopy,” ACS Photonics, 7, 3346-3352, (2020).

Atomic structures determined from digitally defined nanocrystalline regions

May 17, 2021|

Three-dimensional (3D) structures of molecules determined from nanoscale regions of crystalline arrays could potentially illuminate the subtle differences that engender crystal defects or the multiple states accessible to subpopulations of molecules within an ensemble. A step toward this goal involves the extraction of meaningful diffraction data from 3D regions on the nanoscale. This is achieved using a near-parallel electron beam designed to illuminate sub-10nm regions of a sample. Scanning such a beam across a sample allows for digital logic to be applied to the measured data, facilitating the expostfacto assortment of information and reduction from desired 3D subvolumes.

A STROBE team from UCLA, UC Berkeley and LBNL collaborated to determine the first molecular structures determined by 4DSTEM. The structures were determined from a digitally defined subregion of a nanocrystal. After collecting TB of data, the team obtained reconstructions that revealed the atomic structure of a peptide, and showed that radiation damage imparted on the sample during data collection was not prohibitive for structure determination. Compared to other approaches, the approach allows for a much greater degree of control and obviates the need for spatial separation of samples. New methods, algorithms, enhanced microscopes and advanced sample preparation techniques developed by the STROBE collaboration were key to enabling the success of this project.

Gallagher-Jones, K. C. Bustillo, C. Ophus, L. S. Richards, J. Ciston, S. Lee, A. M. Minor, J. A. Rodriguez, “Atomic structures determined from digitally defined nanocrystalline regions,” IUCrJ, 7, (2020).

STROBE solved a century-old scientific problem: Determining the 3D atomic structure of amorphous solids

April 13, 2021|

Amorphous solids such as glass, plastics and rubber are ubiquitous in our daily life and have broad applications ranging from telecommunications to electronics and solar cells. However, due to the lack of any crystal-like long-range order, the traditional X-ray crystallographic methods for extracting the three-dimensional (3D) atomic arrangement of amorphous solids simply do not work. STROBE advanced atomic electron tomography to determine the 3D atomic positions and chemical species of an amorphous solid for the first time – with a stunning precision of 21 picometer. We found that instead of long-range order characteristic of crystals such as diamond, this amorphous metallic glass had regions of short- and medium-range order. Moreover, although the 3D atomic packing is disordered, some regions connect with each other to form crystal-like networks, which exhibit translational but no orientational order. Looking forward, we anticipate this approach will open the door to determining the 3D atomic coordinates of a wide range of amorphous solids, whose impact on non-crystalline solids may be comparable to the first 3D crystal structure solved by x-ray crystallography over a century ago.

Y. Yang, J. Zhou, F. Zhu, Y. Yuan, D. Chang, D. S. Kim, M. Pham, A. Rana, X. Tian, Y. Yao, S. Osher, A. K. Schmid, L. Hu, P. Ercius and J. Miao, “Determining the three-dimensional atomic structure of an amorphous solid”, Nature 592, 60–64 (2021).

Charging-driven coarsening and melting of a colloidal nanoparticle monolayer at an ionic-liquid vacuum interface

November 20, 2020|

Colloidal materials are a platform for studying self-assembly as well as the bottom-up creation of next generation hierarchical materials, and controllably perturbing their collective dynamics is an important step towards directing their assembly. In a liquid droplet, silica nanoparticles collect on the surface and organize to form an ordered 2D lattice. A STROBE research team led by Naomi Ginsberg (UC Berkeley) investigated these monolayers on a low vapor pressure ionic liquid, allowing experiments to be performed under the vacuum environment of a scanning electron microscope. Alongside imaging the particles, the electron beam serves as a perturbative tool for controllably charging the colloidal lattice. As particles charge, they sink into the droplet reducing the monolayer’s density and driving a melting transition. These findings will provide new insights for understanding phase transitions in soft materials and analogous atomic crystals.

Bischak, C.G., Raybin, J.G., Kruppe, J.W., Ginsberg, N.S., "Charging-driven coarsening and melting of a colloidal nanoparticle monolayer at an ionic-liquid vacuum interface." Soft Matter, 16, 9578-9589 (2020).

World’s Smallest ‘Refrigerator’

August 4, 2020|

Thermoelectric devices represent a potentially transformative technology, one that could revolutionize power generation and temperature control.  While they are robust, compact, noiseless, and have no moving parts, thermoelectric devices are implemented only in a few niche applications because of their low efficiency compared to conventional, compression-based heat engines. According to well-grounded theoretical considerations, thermoelectric materials might be made more efficient than their bulk counterparts via tailored nanostructuring.  Given the large upside, even small improvements in thermoelectric materials might bring us to a tipping point where thermoelectric devices are routinely employed for recovering waste heat and refrigerating food.

A STROBE team led by Chris Regan (UCLA) has developed new imaging techniques for characterizing thermoelectric devices at the nanoscale, and has demonstrated these techniques on the smallest refrigerator ever constructed.  Their thermoelectric refrigerator has an active volume of about 1 cubic micrometer, which is too small to be seen with the naked eye.   Viewed in a microscope, it demonstrates its cooling abilities by forming a single dewdrop instantaneously when electrical power is applied. This work is continuing in collaboration with researchers at the STROBE/PEAQS partner institutions Fort Lewis College and Norfolk State University.

Electron-Transparent Thermoelectric Coolers Demonstrated with Nanoparticle and Condensation Thermometry, Hubbard, et al., ACS Nano, 11510-11517, (2020).

Full characterization of ultrathin 5nm low-k dielectric films: Influence of thickness and dopants on the mechanical properties

June 26, 2020|

The demand for faster, more efficient, and more compact nanoelectronic devices, like smartphone chips, requires engineers to develop increasingly complex designs. To achieve this, engineers use layer upon layer of very thin films – as thin as only a couple strands of DNA – with impurities added, to tailor the function. However, the presence of these necessary impurities and extreme thinness degrades the material strength, reducing its performance and making it more likely to fail. To date, it was simply not possible to test the stiffness or compressibility of the thinnest of these ultra-thin films. Now, by using laser-like beams at very short wavelengths – beyond the ultraviolet region of the spectrum – scientists were finally able to measure the mechanical properties of these films. What they learned was surprising: as the layers thinned, the mechanical properties dramatically deteriorated, becoming nearly 10 times flimsier than expected. Additionally, the presence of impurities can be more detrimental to the film’s strength than the effect of its thinning. These findings will influence the design of next generation electronic and other nanoscale devices.

T. D. FrazerJ. L. KnoblochJ. N. Hernández-CharpakK. M. Hoogeboom-PotD. NardiS. YazdiW. ChaoE. H. AndersonM. K. TrippS. W. KingH. C. KapteynM. M. MurnaneB. AbadPhysical Review Materials4073603(2020).

Molecular Syringe

April 29, 2020|

Bacteriocins are contractile molecular syringes — nanomachines produced by one bacterium that can puncture the cell membrane of another bacterium to deliver a lethal punch. In this week’s issue of Nature and featured on the cover, STROBE UCLA scientist Hong Zhou and his colleagues present high-resolution structures of the bacteriocin pyocin R2 from P. aeruginosa – in both its preand post-contraction states. The results allow the researchers to suggest in detail how the molecular syringe works, offering insight into how R-type bacteriocins might be developed into a new class of antimicrobials. This work was featured in the April 2020 cover of Nature.

Ge et al., Action of a minimal contractile bactericidal nanomachine, Nature 580, pages 658–662 (2020).
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