Two-week Update

Haven't really been keeping up with the posts over the past few weeks.

For the most part, I spent a large portion of time working on a laser locking circuit which would combine peak-locking and DAVLL schemes.

In short, peak-locking is a very accurate (can lock laser to a specific hyperfine transition), but has a very narrow capture range and can be easily disturbed so that it unlocks. DAVLL is more robust, having a larger capture range and is able to maintain the laser's frequency with a range. The drawback is that the DAVLL scheme is not the most accurate, having a tendency to drift over time.

The purpose of attempting to combine these techniques is to have the accuracy of the peak-locking scheme and the robustness of the DAVLL scheme. So far, our attempt at combining these two methods is through direct addition (with adjustable gain) by feeding the signals through a summing amplifier. I completed the circuit earlier this week but only had limited testing opportunities since John wanted me to work on the PCB layout for AOM controllers.

Front view of the locking circuit, showing the various inputs.

 Top view of the locking circuit, showing the rats-nest that is the wiring.

The results of our limited testing showed that the circuit is somewhat quirky, not quite locking the laser as well as we had hoped. Possible issues may be the selection of the components used in the circuit, especially with integrator values. As an example, if the integrator provided too much feedback to the peak-locking scheme, it could possibly produce increasingly strong oscillations due to it over-correcting and losing lock that way. Another issue may be different locking points for the peak-locking and DAVLL schemes. One signal would tell the laser to correct to one particular frequency while the other signal may push the laser towards another frequency.


See Diode Laser Frequency Stabalization for a better explanation.

We also presented our mid-term talks this past Friday. Didn't quite go as well (for me) as I had expected. The "prompt" was to convey what we've been doing (not necessarily the overall objective), so I skimmed over a lot of details to focus on what I was doing with the laser locking circuit. Perhaps, I skimmed over too much when I compare mine to what others have done. I posted my slides below.

As a side note, I am using KiCad to layout the PCBs. It's a pretty neat open-source software package which allows you to design and layout circuits. I've messed a bit with the TinyCad-FreePCB combination, but I decided to work with KiCad instead because I feel that it more smoothly incorporates schematic designs (well, at least makes it easier) with one complete package compared to crossing over different programs.

KiCad Wiki
KiCad Step-by-Step Tutorial
KiCad Libraries

The finished saturated absorption laser spectroscopy setup.

Red = pump beam.
Orange/Yellow = Probe beams.

Second Week

Nothing much changed throughout the second week, did more of the same. On Tuesday, I called Isomet and Crystal Technology to obtain more information and price quotes for several models of AOMs for John Lorenz.

Most of my Wednesday was spent in laser safety training to become an unrestricted user for class 3B and 4 lasers (Laser Safety). Most of the lasers I'd be using in the lab are up to a few hundred milliwatts of power, but it was probably a good idea to obtain the training anyways. Most of the information was commonsense, but there was some interesting information, such as being able to injure the eye through expanding gases.

The rest of my week was spent on learning more about saturated absorption. John spend a fair amount of time explaining what we would be expecting to see in the saturated absorption, such as the orientation of the peaks for both hyperfine transitions and crossover resonances. In short, the laser frequency the atoms see are due to Doppler shifts caused by the atoms moving either towards or away from the laser, leading to Doppler broadening. If I remember correctly, if the Doppler broadened transitions are wider than the hyperfine transitions, we should expect to see crossover resonances because the atoms in the velocity of the atoms causes them to see different light frequencies and to become excited to a state not necessarily tuned to by the frequency of the laser. This causes the atoms to be absorb (reduced transmitted laser intensity) light of certain frequencies and not others depending on the state population of the atoms. Pumping atoms out of one state causes an increased transmission of light corresponding to that transition. If the width of the Doppler broadened transition is smaller than the frequency between hyperfine transitions, no crossover resonances should occur. This is because the atoms are not moving fast enough to produce enough of a Doppler shift to become excited into a different state than the one the laser is tuned to.

On Friday, I began setting up a layout for a saturated absorption laser spectroscopy. It should be a relatively simple setup on the optics table, but the space restrictions made it difficult to separate the probe beam from the pump beam. After a bit of fiddling, the setup is mostly complete with only a few adjustments to be made on Monday.

Driving to and My First Week at Purdue

The Drive

**I would have posted this sooner, but I was (and still am) having trouble accessing the internet in my room**

I arrived at Purdue around 9:30pm Saturday evening (May 22, 2010), after about 12 hours driving the 630 miles or so from Gaithersburg, MD to West Lafayette, IN (see Google Map below). And it only cost me about $65.00 too.

View Larger Map

The drive wasn't as bad, nor as tiring, as I expected it to be. I rather enjoyed going through western Maryland and West Virginia, especially going up, down, and through the mountains, but after that, it was pretty boring. Driving through Ohio and Indiana, I found both those states to be pretty flat with lots of wide open spaces and 65/70 mph speed limits.

Driving through western Maryland.

Arriving at Hawkins Hall (room 252), where I'll be staying for the 10 weeks of the REU, I found the dorm and the room to be quite nice. The room is larger, cleaner, and comes with more furniture/storage space. Apparently, it's a dorm for graduate students, complete with (from what I've heard) weekly laundered sheets.

My room in Hawkins 252.

Having had the option of flying, I'm glad I decided to drive. It's VERY helpful when the nearest department store is a 10 min drive away and you need to buy food to supplement the $300 meal plan (which works out to be a little over $4.00/day if you buy food everyday).

First Week of Research

For the most part, my first week in the lab was about what I expected it to be with John, the graduate student I'm working with, being at the DAMOP (Division of Atomic, Molecular and Optical Physics) conference in Texas. Most of my time involved me searching for various parts on the internet, such as Acousto-Optic Modulators and obtaining price quotes and shipping times. The rest of my time was spent teaching myself quantum mechanics (or at least attempting to). During this time, two other graduate students working in the same lab, Adeel Altaf and Dionysios Antypas, have been instrumental in getting me acquainted with the lab, the physical concepts involved in the research being done, and any other questions I had.

My space in SB45.

Outside of computer work, I spent some time yesterday on a laser setup to practice locking a laser to hyperfine transition frequencies. The purpose of locking the laser to a particular hyperfine transition is to keep atoms in the MOT (magneto-optical trap) that will be used for photoassociation out of a state which another laser will be unable to access. The laser I worked with will provide the frequency which pumps atoms out of the undesired ground state by exciting atoms which de-excite to that undesired ground state back to the same excited energy level. The spectroscopy technique used to find the hyperfine transition frequencies of the Rb is quite ingenious, (mostly) eliminating Doppler broadening (an excellent explanation can be found at DOPPLER-FREE SATURATED ABSORPTION SPECTROSCOPY: LASER SPECTROSCOPY).

After my first week, I am both excited and intimidated about the coming weeks. I am intimidated from my interactions with the graduate students, I feel that I would be unable to reach their level of expertise and knowledge of physics, even after many years of study. This has me questioning my own ability to get into, let alone succeed, in graduate school. I am also excited to be able to have the opportunity of working in the lab with such talented people. During the 10 weeks, I hope that I will be able to both fill in gaps in my knowledge (such as optics training which St. Mary's does not offer) as well as contribute something meaningful to the project(s).

Background Information

Here are some information and images from John Lorenz about what they've been doing:

As I described before, I currently have a dual species MOT and am about ready to take atom number measurements - the density of out atoms in our trap are an important parameter in photoassociation.  I have attached a diagram of my experimental setup and a picture (experimental_setup.jpg and experimental_pic.jpg respectively).  The don't reflect the most current state of things, but they give a rough idea.  Right now I have 3 diode lasers (DL) and a tapered amplifier (TA) (which amplifies light from one of the diode lasers) to create the trap.  They are all locked to a given frequency with homemade locking circuits and AOM's or EOM's - which are devices which can shift the frequency of light in a crystal by up to a few hundred megahertz.  One of the DL's is homemade.  I will soon put a new DL on the table (also homemade) for the purposes doing the Photoassociation.  The kind of data I will be taking this summer are images of the MOT (as seen in mot.jpg) and molecular spectra.  I've included a fluorescence spectra of a lithium dimer taken from a heat pipe (fluorescence.jpg) - fluorescence spectra collect light from the fluorescing molecules in a resonant laser beam and record energy level as peaks.  My spectra will look for decreases in MOT fluorescence when molecules are formed, so energy levels will be dips instead of peaks - this is called trap loss spectroscopy. 

experimental_pic.jpg

experimental_setup.jpg

fluorescence.jpg

MOT.jpg

Photoassociation Project

Communicating with Dr. Dan Elliott and his Ph.D. student John Lorenz, I found out that I would be working on the photoassociation (photoassociation - the association of two atoms, under the influence of laser light, to form a diatomic molecule) project.

We are working on an experimental project in which our goal is to produce ultracold LiRb molecules in an atomic Magneto-Optical Trap (MOT).  We are currently able to cool and trap rubidium atoms and lithium atoms concurrently.  Researchers in other laboratories have already demonstrated the ability to assemble other diatomic molecules in a MOT through a process known as Photoassociation, and we will use similar schemes to demonstrate this in LiRb.  LiRb is attractive in that it has a relatively large permanent electric dipole moment.  We also have in mind a scheme to drive this molecule to its lowest electronic, vibrational and rotational state that looks promising, although we have a lot of work to do before we achieve this goal.  Ultimately, we look to creating superposition states that might be of interest as quantum logic gates.

Accepted to Two REUs

I found out I got accepted to the Purdue Physics REU on March 24, 2010:

Dear Roger,

In response to your application we're delighted to make you an offer to join our summer 2010 REU program at Purdue. The REU fellowship carries a stipend of $450 per week. Room and board and on-campus Residence Hall are also covered by the program. A travel allowance is available. More details about the program may be found on our website:

http://www.physics.purdue.edu/reu/

Please confirm that you are still considering Purdue as one of the options for summer research by March 26th. Once confirmed, we will contact you with the details of your summer research project. Once you have a chance to discuss the research project with your future adviser we will ask you to formally accept (or reject) this offer.

Sincerely,
Prof. Sergei Savikhin
Department of Physics
Purdue University

I applied for REUs last year, but didn't get accepted to any, so I was pretty stoked about being accepted, especially to a large university like Purdue.

Then, on March 25, 2010, I got a call from Dr. John Noe at the Stony Brook Laser Teaching Center, asking about what I did the previous summer (which I spent at St. Mary's studying quantum walks). After a few days, and phone calls from from Dr. Erlend Graf and Dr. Harold Metcalf,  I found out I also got accepted to the Stony Brook Physics REU.

After that, I had an arduous week of debating between where I wanted to go (with both places constantly switching positions). My primary goal for this summer was to experience as many different aspects of physics as possible so that I have a better idea of what I want to focus on, and this just nudged Purdue over Stony Brook, which I felt offered me more of an opportunity to experience working in an active physics research laboratory.

I would have been excited to have been accepted to an REU at all, let alone to both Purdue and Stony Brook.