Ultrashort signals are integral for conducting high-resolution measurements. In the mid-infrared, the generation of ultrashort pulses is notoriously difficult to achieve and usually requires large optical setups. In our work we use direct sampling to demonstrate the spontaneous generation of stable ultrashort features in the time-domain signal of a mid-infrared quantum cascade laser frequency comb. The full-width at half-maximum of these features is measured to be ~500 fs, right below the Fourier-limit derived from the corresponding optical spectrum and RF-injection can be used for stabilization and manipulation. Using Maxwell-Bloch equation-based simulations, we can reproduce the generation of such features, including the position in relation to the instantaneous frequency and show their width can be lowered even further below the Fourier limit, thus opening new possibilities for high-resolution measurements based on quantum cascade laser frequency combs.
|