Canada Day Research Live Blog

Liveblogging from the Ultrafast Spectroscopy Lab on July 1st 2008, check back through the day (or tomorrow) for all my updates.

8:05 am So it’s a national holiday and I volunteered to work today. At least it’s the first time I’ll get to exclusively use the laser in a week, and probably the last time until next week maybe.

I skateboarded to campus today, which at 7:30 am was pretty quick because Whyte Ave was deserted.

Most of campus is abandoned but a few people are around right now.

A speaker on the computer I’m using (nearest to my experiment) is missing so I’m listening to Sonic through a built in tinny speaker.

My goal for today: align the modelocked terahertz spectroscopy setup so I can see a signal. I’ve tried this a couple times before, mostly to no avail. This makes my outlook for the next 6 hours to be pretty dim. At 3:00 pm I’m going with Sonia to the legislature to see the chambers on the one day democracy is open to the public.

8:15 am Laser started and modelocked, output power of 295 mW at somewhere around 30-50 fs pulses (1fs = 10^-15 s). This power is the best it’s been at in a week, and one of the highest I’ve been able to get, maybe the rest of today can go this well.

8:46 am The goal of this experiment is to split the source laser pulse into two, one beam going to the source and generating a terahertz pump beam, the other going through a delay and acting as the probe. Both pulses (the THz pump and optical – 800nm – probe) recombine at the Zinc telluride detector crystal and then go through some splitting optics into a photodetector which I measure through a lock-in amplifier with the computer I’m on that scans the delay (allowing me to measure in real-time the electric field of the terahertz pulse).

As of now I have the probe path almost re-aligned. It seems that every time I go back to this setup it’s completely out of alignment… This doesn’t bode well for the next time I attempt to do this experiment. At the very least, if I’m lucky, today I’ll be able to find the position on the delay stage that corresponds to the overlapping of the two pulses in the detector crystal, the point we call t-zero (t₀).

9:01 am Both beams seem to be aligned now. The probe beam is entering the detector and seems to give a nearly balanced signal. The pump beam is hitting the THz source (a pair of transmission lines with 50 V applied), so the gap reads <5 k ohms, when with no illumination gives a few M ohms. So now I’m scanning the majority of the length of the stage and seeing if any signal jumps.

So while I wait (which is what most of any research in any field is) I can explain a bit of the theory behind this experiment.

The optical pump beam is focussed onto a piece of gallium arsenide, a semiconductor that when illuminated by light below a certain wavelength (816 nm here) it becomes conducting. So with 50 V applied across a small area that is normally insulating, when the optical pulse hits it it becomes conducting. This creates a flow of electrons from one electrode to the other. Accelerating charges create electromagnetic radiation, and in this case it is emitted in the range of terahertz radiation.

This radiation is in between the microwave and infrared regimes, and has several unique properties, such as the ability to penetrate cloth and plastic, while being strongly absorbed by water and reflected by metals.

The set up I’m working on is based around using this radiation for spectroscopic probing of materials. This means we look at the pulse after it travels through free space, and compare it with the pulse after traveling through our material. We then compare the Fourier Transforms (or the frequency spectras) of the pulses and can determine the index of refraction, absorption and conductivity of the sample.

9:21 am Two-thirds through the scan and no signal, I’m going to have to screw around with something in a bit.

One idea I had, but may be hard to implement with such a weak probe beam is to use a different method to source the terahertz radiation. If I put an electro-optic crystal at the source I should be able to generate the same radiation, although it may be too weak to detect.

9:25 am No signal, trying scan for the last bits of the delay.

The delay stage I’m using is 100 mm long, I initially scanned from 15 mm to 85 mm with 20 um steps, which gives a resolution of 133 fs. The pulse I’m looking for is around a picosecond long (or 1000 fs). The delay stage changes the path length of the probe beam, and hopefully (if the optics are positioned right) will cause the beams to pass the same lengths at some point in its scan.

9:40 am Still no signal.

One thing of note, the crystal with the transmission lines has a silicon lens that is pressed against the back of it, unfortunately, on close inspection, it seems the lens has pushed the emitter a bit off of its holder, but luckily not cracking it (GaAs is extremely fragile – and expensive). I also found that my alignment had decided to wander a bit off from where it was on the pump beam, also not a good sign.

I think at this point I’m going to abandon this emitter (for now), as I’ve never seen any good come from it. I should mention that I know this setup can work, since I got a signal with a 2,000 euro emitter that we had on loan to characterize.

I’m going to try a different technique for now, and I will report a bit more on it once I get a scan going.

10:30 am Attempting a new scan now. I’ve changed the source from the transmission lines, which should give a strong signal (8 mV) to a [110] oriented crystal of GaAs, which should produce terahertz by a process called optical rectification.

What happens here is the optical pulse causes the crystal to become polarized, which “looks” like an accelerating current, creating the terahertz radiation.

To set all of this up, I had to first cut the crystal. Unfortunately the crystal supplier gave us square crystals, with the crystal axis not perpendicular to the edges. If you’ve ever cleaved a crystal you’ll realize how much this sucks. Basically, cleaving requires me to scratch one edge with a diamond tip blade, hopefully scoring it along a crystal axis, the crystal then cracks along an axis and splits nicely (think of when you chop wood and it goes right along the grain). Now with this crystal the crack went at about 45o to the edge.

After this I found a washer I could mount my triangular shard to, and drilled a larger aperture. Then I mounted the crystal to the washer with rubber cement (we’re about quality here), and put it in an optics mount.

Finally I realigned the setup, ensuring that the pump and probe beam would overlap on the detector crystal, and then I put the GaAs in place of the source.

I’m now running my scan from 15 mm to 85 mm again. I don’t know if this will really show anything, since the laser I’m using might not have the power to generate the terahertz pulse, but I may be able to detect it using some HgCdTe (mercury cadmium telluride) detectors, but those require liquid helium or liquid nitrogen, which I don’t have access to today (those detectors are also limited to the mid-infrared range, and likely wouldn’t show the main portion of the pulse around 1 THz).

Well back to check my email, Facebook, etc. while I wait for this scan (another major part of research).

10:51 am No signal, no surprise. Mild hunger and frustration begin to set in.

11:09 am I couldn’t find a signal with the prior set up so I’ve moved the generation crystal to the spot where the sample usually goes. This means the THz beam will travel less in air, and therefore be absorbed less by water vapour (which can take 20-30% of the intensity). No guarantees that this will work though. I may have to double check that my delay stage is in the right general position by physically measuring each beam path. I might also eat my sandwiches while I wait too. Work is so exciting sometimes.

11:30 am Still no signals. Sandwiches are a little stale around the edges. And the sudoku I’m working on is taking a while.

11:50 am The pump path length is 67″, the probe path length can be set to 67″ +/- ~5″. It should work.

12:04 pm I returned to using the transmission line, and fine tuned the alignment. It may or may not give anything at all now. Not sure what I plan to do if this fails.

12:49 pm No signals seen at all. Feeling too distracted to get anything meaningful accomplished. I may shut it down now and go read something that will get my blood pressure up.

12:55 pm This ends my liveblogging session.  The lasers off, nothing new is accomplished (aside from mounting a crystal).  Basically this was just another average day in the lab.  I may try this again if I care to.