Wednesday, October 26, 2022

Rice University Academy of Fellows postdoc opportunity, 2023

As I have posted in previous years, Rice has a university-wide endowed honorific postdoctoral program called the Rice Academy of Fellows.   Like all such things, it's very competitive. The new application listing has gone live here with a deadline of January 4, 2023. Applicants have to have a faculty mentor, so in case someone is interested in working with me on this, please contact me via email. We've got some fun, exciting stuff going on!

Sunday, October 16, 2022

Materials labs of the future + cost

The NSF Division of Materials Research has been soliciting input from the community about both the biggest outstanding problems in condensed matter and materials science, and the future of materials labs - what kind of infrastructure, training, etc. will be needed to address those big problems.  In thinking about this, I want to throw out a stretch idea.  

I think it would have transformative impact on materials research and workforce development if there were fabrication and characterization tools that offered great performance at far lower prices than currently possible.  I'd mentioned the idea of developing a super-cheap SEM a while ago. I definitely worry that we are approaching a funding situation where the separation between top universities and everyone else will continue to widen rapidly.  The model of a network of user facilities seems to be how things have been trending (e.g. go to Harvard and use their high-res TEM, if your institution can't afford one).  However, if we really want to move the needle on access and training for a large, highly diverse workforce, it would be incredible to find a way to bring more capabilities to the broadest sweep of universities.   Maybe it's worth thinking hard about what could be possible to radically reduce hardware costs for the suite of materials characterization techniques that would be most important.

Saturday, October 08, 2022

Getting light out of plasmonic tunnel junctions - the sequel

A couple of years ago I wrote about our work on "above threshold" light emission in planar metal tunnel junctions.  In that work, we showed that in a planar tunnel junction, you can apply a bias voltage \(V\) and get lots of photons out at energies quite a bit greater than \(\hbar \omega = eV\).  In the high current regime when there are strong local plasmon resonances, it is possible to drive (steady state) some part of the electronic distribution to very high effective electron temperatures, and then observe radiation from the recombination of those hot carriers.  One neat thing about this is that by analyzing the spectra, it is possible to back out the actual plasmon-modified density of photonic states for emission to the far-field, \(\rho(\hbar \omega)\) of a particular junction.   

In our new paper published this week, we have been able to take this quite a bit further.  In the low current regime with weaker local plasmon resonances, the energy deposited by tunneling electrons is able to diffuse away rapidly compared to the arrival of more carriers, so that kind of carrier heating above isn't important.  Instead, it's been known for a while that the right way to think about light emission in that case is as a process connected to fluctuations (shot noise) in the tunneling current, as demonstrated very prettily here.  Within that mechanism, it should be possible to predict with precision what the actual emission spectrum should look like, given the tunneling conductance, the bias voltage, and \(\rho(\hbar \omega)\).  As shown in the figure, we can now test this, and it works very well.  Take a planar aluminum tunnel junction made by electromigration, and in the high conductance/high current limit, use the hot carrier emission to determine \(\rho(\hbar \omega)\).  Then gently migrate the junction further to lower the conductance and fall out of the hot carrier emission regime.  Using the measured conductance and the previously found \(\rho(\hbar \omega)\), the theory (dashed lines in the right panel) agrees extremely well with the measured spectra (colored data points) with only two adjustable parameters (an overall prefactor, and a slightly elevated electronic temperature that gets the rounding of the emission at the \(eV\) cutoff, indicated by the arrows in the right panel).  

I think this agreement is pretty darn impressive.  It confirms that we have a quantitative understanding of how shot noise (due to the discreteness of charge!) affects light emission processes all the way up to optical frequencies.