Garrek Stemo

I shoot lasers at molecules.

Light-matter strong coupling

When matter interacts with a quantized electromagnetic field under certain conditions, the result is a hybrid quasi-particle called a “polariton”. I study a particular type of polariton called a “vibrational polariton”. These result from strongly coupled molecular vibrations and quantized light. Light is quantized by confining the electromagnetic wave between two highly reflective mirrors. In my research, the mirrors are about 10 to 20 micrometers apart.

Molecule-cavity Experiment
Figure 1. Molecules can couple to a confined electromagnetic field simply by placing them between two highly reflective mirrors. The cavity field and molecular vibrations exchange energy. If the rate of this exchange is faster than molecular dissipative processes or photon leakage out of the cavity, then the system is said to be strongly coupled. The system can then be examined using ultrafast pump and probe laser pulses.

One of the most exciting things about vibrational polaritons is that they can be used to modify chemical reactions. We’re not sure about the details of how this happens, so that’s a major area of focus for the field. This article on quantum control goes into a bit more detail.

Vibrational energy levels and cavity photon energy levels
Figure 2. The energy curve on the left shows the discrete molecular vibrational energy levels. When a transition between two levels has the same energy as a cavity photon (on the right), then the two can couple to form a new system with different energy eigenstates, called the upper and lower polariton. The energy that separates these eigenstates is called the Rabi splitting energy. This represents the rate of energy exchange between the molecules and the quantized electromagnetic field.

Interests

I’m interested in the physics of vibrational polaritons and how they facilitate modified chemical reactivity. Ultrafast laser spectroscopy is the tool for this job. It allows me to study how the molecule-light interaction changes over extremely short time scales (picoseconds). There is still a lot to uncover about these curious quasi-particles. We don’t have a full picture of how vibrational polaritons relax from excited states, for example. The role of polariton coherence, population transfer, and interactions with the reservoir excitations (uncoupled molecules) also need to be studied further to understand how all of these factors might play into modified reactivity.