Terahertz vacuum

No, not a super-cool vacuum that picks up 10¹² particles per second, but my normal terahertz spectroscopy setup under 10-5 Torr of pressure (equivalent to the high reaches of the Earth’s themosphere).

In today’s experiment I am probing wire grid polarizers.  Essentially wires of 200nm thick gold on a glass (fused SiO₂) substrate that are 10 microns wide. The wires are separated by 10 microns gaps and vary in length from 5 millimetres to 10 microns long.

Since the wavelengths of the pulses I’m sending through are centred around 300 microns, this means at the extreme end of my devices I have subwavelength devices.

By doing terahertz time-domain spectroscopy on these samples, we should be able to figure out the limits for convential polarizers and determine whether (and where) the Drude model of conductivity breaks down.

The unfortunate part of today is that since the final sample mount isn’t finished for the vacuum chamber, which will attach to a cryostat, I have to break vacuum every time I want to move my sample and probe a different length of wire. The lucky part is vacuum is achieved in a mere 5 minutes.

So why use a vacuum for this experiment? Typically terahertz experiments are done in a nitrogen purged environment. The reason for purging (or vacuum) is that water is the sworn enemy of terahertz radiation. More specifically water molecules have several rotational modes that correspond to frequencies in the terahertz range. These modes absorb much of the incident radiation. Essentially terahertz radiation gets damped by damp air. By filling a chamber completely with nitrogen you eliminate most of the water vapour. Using a vacuum chamber accomplishes the same thing but is much cooler (and allows for a cryostat to be used inside as well).

This material is the work that I’ll be presenting at this year’s EngPhys/IEEE/Physics Undergraduate Research Symposium, which I worked to launch last year. After that I’ll hopefully take the work to CUPC.