Düsseldorf 2007 – scientific programme

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Q: Fachverband Quantenoptik und Photonik

Q 59: Quanteneffekte (Lichtstreuung II)

Q 59.4: Talk

Thursday, March 22, 2007, 14:45–15:00, 5E

Amplification of Terahertz pulses in thin gases — •Gerhard Schwaab, Konstanze Schröck, and Martina Havenith — Physikalische Chemie II, Ruhr-Universität Bochum, D-44780 Bochum

We have carried out Terahertz Time-Domain Spectroscopy in a plasma at low pressure and observed a simultaneous absorption and amplification process within each single rotational transition. We were able to model the observation and could show that it is a direct consequence of the short interaction time of the pulsed THz radiation with the plasma. The interaction time is shorter than the average collision time between the molecules. Thus, during measurement time the molecular and photonic states may be considered entangled.

The solution of the time dependent Schrödinger equation yields a linear term that may usually be neglected for long observation times, large frequencies, or non-entangled states. We determine the restrictions for the observation of this effect and calculate the spectrum for a simple diatomic molecule.

We predict an absorption/amplification process that shows a quadratic instead of the typical exponential behaviour. Using this model we are able to explain the spectral features showing a change from emission to absorption as observed previously.

The observed effect should be a general phenomenon for any quantum mechanical system with entangled states when a) the time scale for disturbances exceeds the typical interaction time of the light pulse with the sample and b) the population difference is small.