Generation of 8–20 μm Mid-Infrared Ultrashort Femtosecond Laser Pulses via Difference Frequency Generation

Modelling of long-wave mid-infrared ultrashort pulse generation via difference frequency generation

In this paper, a model of generating mid-infrared (MIR) ultrashort laser pulse through difference frequency generation (DFG) is established. The pulse evolution relationship among the pump, signal, and idler pulses during the DFG process, as well as the effects of crystal length, pulse energy of the pump and signal lights, pulse width, and other factors on the characteristics of the MIR pulse are explored. Furthermore, through simulations from the time domain to the frequency domain, the spectral characteristics and angular distribution of MIR were analyzed. DFG experimental data are also presented to support the model.

Liquid crystal wave plate operating close to 18 THz

Controlling the properties of mid- and far-infrared radiation can provide a means to transiently alter the properties of materials for novel applications. However, a limited number of optical elements are available to control its polarization state. Here we show that a 15-µm thick liquid crystal cell containing 8CB (4-octyl-4'-cyanobiphenyl) in the ordered, smectic A phase can be used as a phase retarder or wave plate. This was tested using the bright, short-pulsed (∼1 ps) radiation centered at 16.5 µm (18.15 THz) that is emitted by a free electron laser at high repetition rate (13 MHz). These results demonstrate a possible tool for the exploration of the mid- and far-infrared range and could be used to develop novel metamaterials or extend multidimensional spectroscopy to this portion of the electromagnetic spectrum.

High-power, high-repetition-rate tunable longwave mid-IR sources based on DFG in the OPA regime

We demonstrate high-power longwave mid-IR ultrafast sources based on a high-power Er-fiber laser system at 1.55 µm with a 32-MHz repetition rate. Compared with previous 1.03-µm-driven difference frequency generation (DFG), our current configuration allows tighter focusing in the GaSe crystal thanks to an increased damage threshold at 1.55 µm. Consequently, the 1.55-µm-driven DFG can operate in the regime of optical parametric amplification (OPA), in which the mid-IR power grows exponentially with respect to the square root of the pumping power. We experimentally demonstrate this operation regime and achieve broadband mid-IR pulses that are tunable in the 7.7-17.3 µm range with a maximum average power of 58.3 mW, which is also confirmed by our numerical simulation.