I’ve spent the past two years studying the Nitrogen-Vacancy (NV) center, a point-defect in diamonds. You can think about this physically in the following way: First, imagine a perfect diamond lattice. Now, take out two carbon atoms that are right next to each other, and finally, replace one with a nitrogen atom and leave the other as an empty space. The result is a system that can be thought of like an “atom” within the diamond lattice, meaning that a single defect has quantum mechanical degrees of freedom that can be manipulated. Not only that, it turns out that this defect has a wide set of properties that make it a good candidate for applications in fields ranging from biology to quantum computing.
When I started to work with NV centers, there had been a lot of research done on understanding the defect itself when it is deep within a macroscopic piece of diamond or when it is in a nanoscale diamond crystal. A lot had been learned about the electronic and magnetic properties of the defect. It had been shown in the past that the defect can be used to measure static magnetic fields and that it can report on the amount of different isotopes of carbon that are present within the diamond itself.
An exciting new development is that people are figuring out how to use NVs to report on the material surrounding the diamond. This is because many of the properties of the NV center have very predictable responses to environmental changes. My research focused on using the defect to obtain structural information on (a.k.a. take a picture of) micometer to nanometer scale objects. This technique would be used to look inside objects in a manner similar to how an MRI is used to look inside people. Not only would it be able to provide full three-dimensional structural information, but it could be used to do material characterization as well. In order to do that, we needed to understand the properties of the defect, not just in the case where it was very deep in the diamond and the defect effectively saw an infinite amount of diamond in all directions, but also when it was very close to the diamond surface and where the defect responded to the external environment.
What we’ve learned so far is that, indeed, the distance from the surface does seem to change the properties of the NV center. Not surprisingly, a shallow defect (one that is only 1 nanometer away from the surface of a macroscopic piece of diamond) behaves differently from a defect deep in the diamond. What is surprising is that the shallow defect also behaves differently from defects in tiny nanometer-sized diamond crystals. This means there’s still more to learn before we can use NV centers for nanoscale magnetic imaging.
At the end of my first two years at MIT, I was confronted with the choice of moving with my lab to ETH Zurich and continuing this exciting work or switching projects and staying at MIT. I chose to remain at MIT and currently the work that I’m doing involves time-resolved spectroscopy in the Nelson Group.
The idea behind time-resolved spectroscopy is to measure how a system of interest, such as an atom, molecule, or material, evolves after a controlled stimulus. You do something to the system, wait some known amount of time, and then probe the system. This means that time-resolved spectroscopy is inherently focused on looking at changes and dynamics. This was really what drew me to time-resolved spectroscopy, because I could think of it like taking a movie. For instance, we launch a wave, and sometime after the wave is launched, we take a snapshot of it. By changing the amount of time between the moment the wave is launched and the moment the photograph is captured, we record the full evolution of the wave. By stitching all the snapshots together, we build up a movie of the wave. Looking at the data is like watching a video in super-super-duper slow motion.
The first of the projects that I’m working on involves studying the transfer of mechanical energy between different kinds of molecular motion. The question that we’re asking is: How is energy transferred from collective motions of groups of molecules into the higher frequency oscillations that eventually lead to bond breaking and chemical reactions? Time-resolved measurements are required for understanding such phenomena because energy is transferred so quickly (typically in a picosecond, or one millionth of a millionth of a second), and other techniques are too slow to watch what happens. We start a collective oscillation in a molecular crystal and see where that energy goes and how long it takes for it to dissipate into internal motions. In particular, we watch how energy makes its way into vibrations that will lead to the breaking of chemical bonds and the initiation of chemical reactions.
Another project is focused on studying how light propagates in periodically structured materials called photonic crystals. Photonic crystals have been studied and discussed theoretically for almost a century, and a host of experimental work has been done in different parts of the electromagnetic spectrum. I’m working on studying them at terahertz frequencies. This frequency range is particularly exciting because our technique makes it possible to make a movie of the electromagnetic waves as they propagate through the photonic crystal. This will give us direct feedback on the behavior of light in these crystals, which will help us design materials with different responses and functions. Ultimately, we could use these results to design devices that can manipulate the flow of light in powerful new ways and could be instrumental in building new computers that use light instead of electricity to carry information.