Research – Page 2

Autumn School on Correlated Electrons

For those interested in The Physics of Correlated Insulators, Metals and Superconductors, a workshop is being held 25-29 September 2017 at Forschungszentrum Jülich. The lecture notes of one of the papers can be found at arXiv:1708.07183. A list of previous years’ workshop topics and links to the lecture notes can be found here.

Singularities and FEM stability

Engineers often think of finite element method (FEM) calculations as routine, as they indeed are for well-posed mathematical situations. But under certain interesting conditions, this is no longer true – one has to work hard to deal with the singularities. In her thesis work, Camille Carvalho developed a finite element method for computing the scattering of electromagnetic waves from sharp corners, as in nanophotonic waveguides. The method given applies to 2.5D (the third dimension being translationally trivial), like one would find in a waveguide with corners defined by crystal planes, for example.

It seems to me that this approach may find use not only in the case of plasmonic waveguides, but also for certain situations in EUV metrology wherein plasmons contribute to the observed scattering.

Furthermore, it would be interesting if a fully 3D method could be developed. It may enhance our insight for photon-assisted field emission, which we discussed in a previous post.

References:

Dr. Carvalho’s website, where one finds a convincing visual, and links, among which are:
Dr. Carvalho’s thesis presentation (français)
Bonnet-Ben Dhia, Carvalho, Chesnel, Ciarlet, “On the use of Perfectly Matched Layers at corners for scattering problems with sign-changing coefficients”, arXiv:1511.01932

3D NIL from two-photon masters

While nano-imprint lithography (NIL) nears commercial viability for semiconductor applications, perhaps this is less true for nano-photonics. A preprint posted to the arXiv today outlines an apparently successful, albeit slow, demonstration of “three dimensional” NIL, starting from masters written by two-photon lithography. (Nanoscribe, for example, makes suitable equipment for this step.) The master has positive tone, while an intermediate replica of PDMS has negative tone. The latter is used to cast positive tone replicas on substrates.

The micrographs are quite impressive, revealing reproduction of even the sidewall texturing resulting from the two-photon lithography of the master.

Although the authors claim the technique could be used to print structures with overhangs, the examples shown are all strictly convex. For convex geometries, I see no advantage over electron-beam masters, except possibly capital outlay.

Reference: R Saive, C R Bukowsky, H A Atwater, “Three-dimensional nanoimprint lithography using two-photon lithography master samples”, arXiv:1702.04012

UTEM with laser-driven field emitter

If one turns down the temperature of a standard, zirconiated tungsten, Schottky cathode, it will still emit electrons when photo-activated. By arranging the polarization of the photon beam, one maintains the usual tight, high-brightness emission characteristic of Schottky TFE, albeit at considerably reduced current.

A group at Göttingen has adapted a JEOL JEM-2100F for this purpose. In an informative preprint, they give a quite complete description of the setup and its performance. In a project like this one expects a few difficulties, and these they find: limited beam current and stability issues, to name two. Given the reduced tip temperature, the zirconium does not migrate adequately to maintain the steady state population at the apex. Apparently the tip can be regenerated.

While column setup can be performed with the tip at normal TFE temperature, my experience has always been that the beam position changes depending on tip temperature. I look forward to seeing how this works out.

Reference: A Feist, et al., “Ultrafast transmission electron microscopy using a laser-driven field emitter: femtosecond resolution with a high coherence electron beam”, arXiv:1611.05022.

2016 NNI Strategic Planning Stakeholder Workshop

The U.S. National Nanotechnology Initiative is inviting participation in their planning workshop to be held 19-20 May 2016 in Washington, D.C.

Click here for the announcement.