The terahertz gap

https://commons.wikimedia.org/wiki/File:Thz_freq_in_EM_spectrum.png?uselang=en-gb

At a thesis proposal talk yesterday, I realized that I hadn't ever written anything specifically about terahertz radiation (THz, or if you're trying to market something, t-rays).   Terahertz (10¹² Hz) is the frequency of electromagnetic radiation higher than microwaves, but lower than what is traditionally labeled the far infrared.  Sometimes called "mm wave" radiation (1 THz would be a free-space wavelength of about 0.3 mm or 300 microns), THz is potentially very useful for communications (pdf, from here), imaging (here, here, here), and range detection (see here for an impressive google project; or here for an article about THz for self-driving cars), among other things.  It's also right around the frequency range of a lot of vibrations in molecules and solids, so it can be used for spectroscopy, though it's also around the energy range where water vapor in the atmosphere can be an efficient absorber.

This frequency region is an awkward middle ground, however.  That's sometimes why it's referred to as the "terahertz gap".

We tend to produce electromagnetic radiation by one of two approaches.  Classically, accelerating charges radiate electromagnetic waves.  In the low frequency limit, there are various ways to generate voltages that oscillate - we can in turn use those to drive oscillating currents and thus generate radio waves, for example.  See here for a very old school discussion.  It is not trivial to shake charges back and forth at THz frequencies, however.  It can be done, but it's very challenging.  One approach to generating a pulse of THz radiation is to use a photoconductive antenna.  Take two electrodes close together on a semiconductor substrate, with a voltage applied between them.  Smack the semiconductor with an ultrafast optical pulse that has a frequency high enough to photoexcite a bunch of charge carriers - those then accelerate from the electric field between the electrodes and emit a pulse of radiation, including THz frequencies.

The other limit we often take in generating light is to work with some quantum system that has a difference in energy levels that is the same energy as the photons we want to generate.  This is the limit of atomic emission (say, having an electron drop from the 2p orbital to the 1s orbital of a hydrogen atom, and emitting an ultraviolet photon of energy around 10 eV) and also the way many solid state devices work (say, having an electron drop from the bottom of the conduction band to the top of the valence band in InGaAsP to produce a red photon of energy around 1.6 eV in a red LED).  The problem with this approach for THz is that the energy scale in question is very small - 1 THz is about 4 milli-electron volts (!).  As far as I know, there aren't naturally occurring solids with energy level splittings that small, so the approach from this direction has been to create artificial systems with such electronic energy gaps - see here.   (Ironically, there are some molecular systems with transitions considerably lower in energy than the THz that can be used to generate microwaves, as in this famous example.)

It looks like THz is starting to take off for technologies, particularly as more devices are being developed for its generation and detection.  SiGe-based transistors, for example, can operate at very high intrinsic speeds, and like in the thesis proposal I heard yesterday, these devices are readily made now and can be integrated into custom chips for exactly the generation and detection of radiation approaching a terahertz.  Exciting times.

Lab question - Newport NPC3SG

Anyone out there using a Newport NPC3SG controller to drive a piezo positioning stage, with computer communication successfully talking to the NPC3SG?  If so, please leave a comment so that we can get in touch, as I have questions.

Faculty position at Rice - theoretical astro-particle/cosmology

Assistant Professor Position at Rice University in

Theoretical Astro-Particle Physics/Cosmology

The Department of Physics and Astronomy at Rice University in Houston, Texas, invites applications for a tenure-track faculty position (Assistant Professor level) in Theoretical Astro-Particle physics and/or Cosmology. The department seeks an outstanding individual whose research will complement and connect existing activities in Nuclear/Particle physics and Astrophysics groups at Rice University (see http://physics.rice.edu). This is the second position in a Cosmic Frontier effort that may eventually grow to three members. The successful applicant will be expected to develop an independent and vigorous research program, and teach graduate and undergraduate courses. A PhD in Physics, Astrophysics or related field is required.

Applicants should send the following: (i) cover letter; (ii) curriculum vitae (including electronic links to 2 relevant publications); (iii) research statement (4 pages or less); (iv) teaching statement (2 pages or less); and (v) the names, professional affiliations, and email addresses of three references.  To apply, please visit: http://jobs.rice.edu/postings/11772.  Applications will be accepted until the position is filled, but only those received by Dec 15, 2017 will be assured full consideration. The appointment is expected to start in July 2018.  Further inquiries should be directed to the chair of the search committee, Prof. Paul Padley (padley@rice.edu).

Rice University is an Equal Opportunity Employer with commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability or protected veteran status.

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Faculty position at Rice - experimental condensed matter

Faculty Position in Experimental Condensed Matter Physics Rice University

The Department of Physics and Astronomy at Rice University in Houston, TX invites applications for a tenure-track faculty position in experimental condensed matter physics.  The department expects to make an appointment at the assistant professor level. This search seeks an outstanding individual whose research interest is in hard condensed matter systems, who will complement and extend existing experimental and theoretical activities in condensed matter physics on semiconductor and nanoscale structures, strongly correlated systems, topological matter, and related quantum materials (see http://physics.rice.edu/). A PhD in physics or related field is required. 

Applicants to this search should submit the following: (1) cover letter; (2) curriculum vitae; (3) research statement; (4) teaching statement; and (5) the names, professional affiliations, and email addresses of three references. For full details and to apply, please visit: http://jobs.rice.edu/postings/11782. Applications will be accepted until the position is filled. The review of applications will begin October 15 2017, but all those received by December 1 2017 will be assured full consideration. The appointment is expected to start in July 2018.  Further inquiries should be directed to the chair of the search committee, Prof. Emilia Morosan (emorosan@rice.edu).  

Rice University is an Equal Opportunity Employer with commitment to diversity at all levels, and considers for employment qualified applicants without regard to race, color, religion, age, sex, sexual orientation, gender identity, national or ethnic origin, genetic information, disability or protected veteran status.

DOE experimental condensed matter physics PI meeting, day 3

And from the last half-day of the meeting:

• Because the mobile electrons in graphene have an energy-momentum relationship similar to that of relativistic particles, the physics of electrons bound to atomic-scale defects in graphene has much in common with the physics that sets the limits on the stability of heavy atoms - when the kinetic energy of the electrons in the innermost orbitals is high enough that relativistic effects become very important.  It is possible to examine single defect sites with a scanning tunneling microscope and look at the energies of bound states, and see this kind of physics in 2d.  
• There is a ton of activity concentrating on realizing Majorana fermions, expected to show up in the solid state when topologically interesting "edge states" are coupled to superconducting leads.  One way to do this would be to use the edge states of the quantum Hall effect, but usually the magnetic fields required to get in the quantum Hall regime don't play well with superconductivity.  Graphene can provide a way around this, with amorphous MoRe acting as very efficient superconducting contact material.  The results are some rather spectacular and complex superconducting devices (here and here).
• With an excellent transmission electron microscope, it's possible to carve out atomically well defined holes in boron nitride monolayers, and then use those to create confined potential wells for carriers in graphene.  Words don't do justice to the fabrication process - it's amazing.  See here and here.
• It's possible to induce and see big collective motions of a whole array of molecules on a surface that each act like little rotors.
• In part due to the peculiar band structure of some topologically interesting materials, they can have truly remarkable nonlinear optical properties.

My apologies for not including everything - side discussions made it tough to take notes on everything, and the selection in these postings is set by that and not any judgment of excitement.  Likewise, the posters at the meeting were very informative, but I did not take notes on those.

DOE experimental condensed matter PI meeting, day 2

More things I learned:

• I've talked about skyrmions before.  It turns out that by coupling a ferromagnet to a strong spin-orbit coupling metal, one can stabilize skyrmions at room temperature.  They can be visualized using magnetic transmission x-ray microscopy - focused, circularly polarized x-ray studies.   The skyrmion motion can show its own form of the Hall effect.  Moreover, it is possible to create structures where skyrmions can be created one at a time on demand, and moved back and forth in a strip of that material - analogous to a racetrack memory.
• Patterned arrays of little magnetic islands continue to be a playground for looking at analogs of complicated magnetic systems.  They're a kind of magnetic metamaterial.  See here.  It's possible to build in frustration, and to look at how topologically protected magnetic excitations (rather like skyrmions) stick around and can't relax.
• Topological insulator materials, with their large spin-orbit effects and surface spin-momentum locking, can be used to pump spin and flip magnets.  However, the electronic structure of both the magnet and the TI are changed when one is deposited on the other, due in part to interfacial charge transfer.
• There continues to be remarkable progress on the growth and understanding of complex oxide heterostructures and interfaces - too many examples and things to describe.
• The use of nonlinear optics to reveal complicated internal symmetries (talked about here) continues to be very cool.
• Antiferromagnetic layers can be surprisingly good at passing spin currents.  Also, I want to start working on yttrium iron garnet, so that I can use this at some point in a talk.
• It's possible to do some impressive manipulation of the valley degree of freedom in 2d transition metal dichalcogenides, creating blobs of complete valley polarization, for example.  It's possible to use an electric field to break inversion symmetry in bilayers and turn some of these effects on and off electrically.
• The halide perovskites actually can make fantastic nanocrystals in terms of optical properties and homogeneity.

DOE experimental condensed matter PI meeting, day 1

I'm pressed for time, so this is brief, but here are some things I learned yesterday:

• An electric field perpendicular to the plane can split and shift the Landau levels of bilayer graphene.  See here.
• The quantum Hall effect in graphene and other 2d systems still has a lot of richness and life in it.
• I have one word for you...."polaritons".
• It's possible to set up a tunneling experiment, from one "probe" 2d electron gas that has a small, tight Fermi surface, into a "sample" 2d electron gas of interest.  By playing with the in-plane magnetic field, the tunneling electrons can pick up momentum in the plane as they tunnel.  The result is, the tunneling current as a function of voltage and transverse fields lets you map out exactly the "sample" electronic states as a function of energy and momentum, like ARPES without the PES part.  See here.
• Squeezing mechanically to apply pressure can actually produce dramatic changes (quantum phase transitions) in unusual fractional quantum Hall states.
• How superconductivity dies in the presence of disorder, magnetic field, and temperature remains very rich and interesting.  The "Bose metal", when magnetic field kills global phase coherence without completely ripping apart Cooper pairs, can be an important part of that transition.  For related work, see here.
• One should be very careful in interpreting ARPES data.  It's entirely possible that not everything identified as some exotic topological material really fits the bill - see here.  On the other hand, sometimes you do see real topologically interesting band structure.
• The DOE still has laptops running Windows XP.

DOE Experimental Condensed Matter PI meeting, 2017

The Basic Energy Sciences program is part of the US Department of Energy's Office of Science, and they are responsible for a lot of excellent science research funding.  The various research areas within BES have investigator meetings every two years, and at the beginning of this coming week is the 2017 PI meeting for the experimental condensed matter physics program.  As I've done in past years,  I will try to write up a bulleted list of things I learn.   (See here, here, and here for the 2013 meeting; see here, here, here, and here for the 2015 meeting).

Good luck and stay safe to those in Florida about to get hit by Hurricane Irma.  It's very different than Harvey (much more of a concern about wind damage and storm surge, much less about total rainfall), but still very dangerous.

Lastly, Amazon seems to have my book available for a surprisingly low price right now ($62, though the list is $85).  I (and my publisher) still have no idea how they can do this without losing money.  

Capillary action - the hidden foe in the physics of floods

There is an enormous amount of physics involved in storms and floods.   The underlying, emergent properties of water are key to much of this.

An individual water molecule can move around, and it can vibrate and rotate in various ways, but it's not inherently wet.  Only when zillions of water molecules get together does something like "wetness" of water even take on meaning.  The zillions of molecules are very egalitarian:  They explore all possible microscopic arrangements (including how they're distributed in space and how they're moving) that are compatible with their circumstances (e.g., sitting at a particular temperature and pressure).  Sometimes the most arrangements correspond to the water molecules being close together as a liquid - the water molecules are weakly attracted to each other if they get close together; at other temperatures and pressures, the most arrangements correspond to the water molecules being spread out as a gas.    Big tropical systems are basically heat engines, powered by the temperature difference between the surface layers of seawater and the upper atmosphere.  That difference in temperatures leads to net evaporation, driving water into the gas phase (by the gigaton, in the case of Hurricane Harvey).  Up in the cold atmosphere, the water condenses again into droplets, and heating the air.  If those droplets are small enough, the forces from adjacent air molecules bouncing off the droplets slow the droplets to the point where they are borne aloft by large-scale breezes - that's why clouds don't fall down even though they're made of water droplets.

There is another feature that comes from the attraction between water molecules and each other, and the attraction between water molecules and their surroundings.   Because of the intramolecular attraction, water molecules would have less energy if they were close together, and therefore having a water-air interface costs energy.  One result is surface tension - the tendency for liquid droplets to pull into small blobs that minimize their (liquid/vapor interface) surface area.

However, sometimes the attractive interaction between a water molecule and some surface can be even stronger than the interaction between the water molecule and other water molecules.  When that happens, a water droplet on such a surface will spread out instead of "beading up".  The surface is said to be hydrophilic.  See here.  This is why some surfaces "like" to get wet, like your dirty car windshield.

Sneaking in here is actually the hidden foe that is known all too well to those who have ever dealt with flooding.  You've seen it daily, even if you've never consciously thought about it.  It's capillary action.  A network of skinny pores or very high surface area hydrophilic material can wick up water like crazy.  Again, the water is just exploring all possible microscopic arrangements, and it so happens that in a high surface area, hydrophilic environment, many many arrangements involve the water being spread out as much as possible on that surface.  This can be to our advantage sometimes - it helps get water to the top of trees, and it makes paper towels work well for drying hands.  However, it can also cause even a couple of cm of floodwater indoors to ruin the bottom meter of sheetrock, or bring water up through several cm of insulation into wood floors, or transport water meters up carpeted stairs.   Perhaps it will one day be economically and environmentally feasible to make superhydrophobic wall and flooring material, but we're not there yet.

(To all my Houston readers, I hope you came through the storm ok!  My garage had 0.8m of water, which killed my cars, but the house is otherwise fine, and the university + lab did very well.)