Speaker: Dr. Maria Richter

Title: Rotational Quantum Beat Laser

Abstract: 

As known since Einstein, a medium in a lower state will absorb resonant light
propagating through it, while emission of new photons will be stimulated when
the medium is excited. In all standard lasers, light amplification generally
requires population inversion between the upper and the lower states to break
the reciprocity between absorption and stimulated emission. However, if a
medium is prepared in a carefully crafted superposition state, quantum
interference may fully suppress absorption while leaving stimulated emission
intact, opening the possibility of lasing without inversion. Preparing and
maintaining the required quantum superposition might appear as an additional
complication, requiring auxiliary laser light. Here we show that lasing without
inversion arises naturally, with no extra effort, during  propagation of
intense femtosecond laser pulses in air. All that is required is the coherent
excitation of rotational wave packets in two dipole-coupled electronic states,
which happens automatically due to laser-induced molecular alignment in intense
light fields. The effect is general and should enable inversionless
amplification of broadband radiation in many molecular gases, opening new
opportunities for remote sensing.