Surface visualisation of bacterial biofilms using neutral atom microscopy.

The scanning helium microscope (SHeM) is an emerging technology that uses a beam of neutral helium atoms to form images with extreme surface sensitivity and non-destructive qualities. Here, we present the first application of SHeM to image bacterial biofilms. We demonstrate that SHeM uniquely and natively visualizes the outermost surface of extracellular polymeric substance in the absence of contrast agents and dyes and without inducing radiative damage.

The field of taxonomy is critically important for the identification, conservation, and ecology of biological species. Modern taxonomists increasingly need to employ advanced imaging techniques to classify organisms according to their observed morphological features. Moreover, the generation of three-dimensional datasets is of growing interest; moving beyond qualitative analysis to true quantitative classification. Unfortunately, biological samples are highly vulnerable to degradation under the energetic probes often used to generate these datasets. Neutral atom beam microscopes avoid such damage due to the gentle nature of their low energy probe, but to date have not been capable of producing three-dimensional data. Here we demonstrate a means to recover the height information for samples imaged in the scanning helium microscope (SHeM) via the process of stereophotogrammetry. The extended capabilities, namely sparse three-dimensional reconstructions of features, were showcased via taxonomic studies of both flora (Arabidopsis thaliana) and fauna (Heterodontus portusjacksoni). In concert with the delicate nature of neutral helium atom beam microscopy, the stereophotogrammetry technique provides the means to derive comprehensive taxonomical data without the risk of sample degradation due to the imaging process.

Optimization of a constrained linear monochromator design for neutral atom beams

Neutral helium atom microscopy, also referred to as scanning helium microscopy and commonly abbreviated SHeM or NAM (neutral atom microscopy), is a novel imaging technique that uses a beam of neutral helium atoms as an imaging probe. The technique offers a number of advantages such as the very low energy of the incident probing atoms (less than 0.1 eV), unsurpassed surface sensitivity (no penetration into the sample bulk), a charge neutral, inert probe and a high depth of field. This opens up for a range of interesting applications such as: imaging of fragile and/or non-conducting samples without damage, inspection of 2D materials and nano-coatings, with the possibility to test properties such as grain boundaries or roughness on the Å ngström scale (the wavelength of the incident helium atoms) and imaging of samples with high aspect ratios, with the potential to obtain true to scale height information of 3D surface topography with nanometer resolution: nano stereo microscopy. However, for a full exploitation of the technique, a range of experimental and theoretical issues still needs to be resolved. In this paper we review the research in the field. We do this by following the trajectory of the helium atoms step by step through the microscope: from the initial acceleration in the supersonic expansion used to generate the probing beam over the atom optical elements used to shape the beam (resolution limits), followed by interaction of the helium atoms with the sample (contrast properties) to the final detection and post-processing. We also review recent advances in scanning helium microscope design including a discussion of imaging with other atoms and molecules than helium.

A ray tracing method for predicting contrast in atom beam imaging is presented. Bespoke computational tools have been developed to simulate the classical trajectories of atoms through the key elements of an atom beam microscope, as described using a triangulated surface mesh, using a combination of MATLAB and C code. These tools enable simulated images to be constructed that are directly analogous to the experimental images formed in a real microscope. It is then possible to understand which mechanisms contribute to contrast in images, with only a small number of base assumptions about the physics of the instrument. In particular, a key benefit of ray tracing is that multiple scattering effects can be included, which cannot be incorporated easily in analytic integral models. The approach has been applied to model the sample environment of the Cambridge scanning helium microscope (SHeM), a recently developed neutral atom pinhole microscope. We describe two applications; (i) understanding contrast and shadowing in images; and (ii) investigation of changes in image formation with pinhole-to-sample working distance. More generally the method has a broad range of potential applications with similar instruments, including understanding imaging from different sample topographies, refinement of a particular microscope geometry to enhance specific forms of contrast, and relating scattered intensity distributions to experimental measurements.

A method is described for investigating the ionization produced in helium by collisions between quasi-stationary atoms—that is, atoms possessing only therm al velocities—and a beam of neutral helium atoms with kinetic energy less than 100 eV. Ionization is shown to begin when the kinetic energy of the bombarding atoms is twice as great as the minimum kinetic energy which electrons m ust possess in order to ionize helium.

We herein report on the microfabrication of a Si(111) surface with a negative/positive contrast by atom lithography using a neutral metastable helium atom beam (He-MAB) and a self-assembled monolayer (SAM) of octadecyltrichlorosilane (OTS). The OTS SAM bonded directly to the silicon surface as a resist and was exposed to He-MAB through a stencil mask to yield a latent image in it. Using chemical etching to develop and transfer the latent image directly onto the underlying silicon substrate, a square silicon micromesa and a microwell matrix with a nanoscale edge resolutions of approximately 100 nm on the Si(111) surface were fabricated. The negative/positive patterning mechanism was discussed in terms of the damage of the SAM resist under the irradiation of He-MAB and the possible effects of contamination.

The scenario of “electron capture and loss” has been recently proposed for the formation of negative ion and neutral atom beams with up to MeV kinetic energy [S. Ter-Avetisyan, et al., Appl. Phys. Lett. 99, 051501 (2011)]. Validation of these processes and of their generic nature is here provided in experiments where the ion source and the interaction medium have been spatially separated. Fast positive ions accelerated from a laser plasma sourceare sent through a cold spray where their charge is changed. Such formed neutral atom or negative ion has nearly the same momentum as the original positive ion. Experiments are released for protons,carbon, and oxygen ions and corresponding beams of negative ions and neutral atoms have been obtained. The electron capture and loss phenomenon is confirmed to be the origin of the negative ion and neutral atom beams. The equilibrium ratios of different chargecomponents and cross sections have been measured. Our method is general and allows the creation of beamsof neutral atoms and negative ions for different species which inherit the characteristics of the positive ion source.

Image formation in the scanning helium microscope

The paper is aimed at describing measurement methodology for characterizing quality of a silicon wafer applied as a focusing element in the Scanning Helium Microscope and continues the struggle against phenomena decreasing accuracy. The focusing mirror being the heart of the system and the decisive factor, which defines the resolution of the microscope, indicates the importance of testing methods. The systems made specially for this purpose provide the ability to create maps of the surface shape, thickness and surface roughness of the wafers. The paper shows many multidisciplinary issues associated with the measurements procedures and concludes with discussion on accuracy limits.

A design for a pinhole scanning helium microscope

Helium atoms are an established, non-invasive probe of surfaces. The interaction between the atom and the surface, in the thermal energy regime, is predominantly one of elastic scattering that gives surface sensitivity without perturbation to the electronic or physical properties of the surface itself. Recent work has been directed at beam focusing, with the aim of creating a microprobe. All the key atom-optical components necessary for the construction of a scanning helium microscope (SHeM) are now in place and such a microscope is currently under development. We present an overview of the technique, with particular reference to the potential contrast mechanisms.

We describe recent developments in the fabrication of an atom-optical mirror for focusing thermal helium atoms. A bent silicon crystal is, in principle, capable of high intensity, low aberration helium reflection; manipulation of the mirror's macrostructure minimizes optical aberrations in the device whilst chemical control over the mirror's microstructure ensures high intensity reflection. Incorporation of the atom mirror into a novel Scanning Helium Microscope (SHeM) is outlined, in the context of surface-structural studies. In particular, we refer to the expected operation, contrast mechanisms and resolution of such an instrument.

Manipulation of atomic and molecular beams is essential to atom optics applications including atom lasers, atom lithography, atom interferometry and neutral atom microscopy. The manipulation of charge-neutral beams of limited polarizability, spin or excitation states remains problematic, but may be overcome by the development of novel diffractive or reflective optical elements. In this paper, we present the first experimental demonstration of atom focusing using an ellipsoidal mirror. The ellipsoidal mirror enables stigmatic off-axis focusing for the first time and we demonstrate focusing of a beam of neutral, ground-state helium atoms down to an approximately circular spot, (26.8±0.5) μm×(31.4±0.8) μm in size. The spot area is two orders of magnitude smaller than previous reflective focusing of atomic beams and is a critical milestone towards the construction of a high-intensity scanning helium microscope.

Non-statistical excitation of the magnetic substates of the 1P 1 level of group II metal atoms in collision with 800 eV helium atoms