Achievement

IGERT trainee Nick Karpowicz studied the underlying physics of the generation and detection of terahertz waves in gases or THz gas photonics. Through ab initio calculations based on the time-dependent Schrodinger equation, he was able to precisely solve the interaction between high-intensity two-color laser pulses and atoms that result in the asymmetric ionization process that releases THz radiation. This was the 1st step in the two-step model introduced in his paper, "Coherent terahertz echo of tunnel ionization in gases", Phys. Rev. Lett. 102, 093001 (2009). The 2nd step is the "echo", where the ionized electron wave packets interact with the surrounding gas and plasma, releasing a 2nd coherent burst of THz waves. Experiments in a pulsed gas jet confirmed the theory. The same theoretical framework, when applied to detecting terahertz waves in gases, revealed that the underlying physical mechanism was different, and could be explained through four-wave mixing.