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Ultrafast Meets Ultrasensitive  

To study the pure and un-interrupted quantum dynamics of a molecule, you need to isolate the molecule from its neighbors by getting it in the gas phase, preferably cold and free from collisions. You might want to have a few neighbor molecules to see how that affects the dynamics, a few partners joined together in a so-called “cluster”, but you want control over those neighbors so you can make a systematic study.

Scientists can produce cold, isolated molecules and clusters using the techniques of molecular beams, in which gas jets are shot into a vacuum. The trouble with molecular beams is that you don’t get very many molecules to work with. The density of molecules (molecules/liter) in a molecular beam is typically about 10 million times lower than in air and 10 billion times lower than a liquid, so very sensitive techniques are needed to record signals from these extremely dilute samples.

At Stony Brook, Prof. Tom Allison's group, using a special type of laser called a frequency comb, and optical resonators to passively amplify minute signals, has recently demonstrated a nearly 4 orders of magnitude improvement in the sensitivity of ultrafast spectroscopy, such that signals can easily be recorded from the “designer” molecules and clusters that can only be produced in molecular beam. The instrument can record changes in the absorption of the probe pulses to a few parts in 1010. The sensitivity enhancement comes from resonating the laser pulses in optical cavities, one for the pump pulses and a second for the probe pulses, which requires precise control of the electric-field of the laser pulses so they can be coherently added and stored. The techniques they have demonstrated in the visible region of the electromagnetic spectrum and can also be used in the UV and infrared, and thus applied to a wide range of fundamental problems in molecular physics.

The results are published in Optica, the OSA’s premier high-impact journal, see more here.


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