Intense few-cycle pulses at mid-infrared (MIR) wavelengths provide a powerful tool for the phase control of condensed matter. One important parameter for the description of the electric field of such few-cycle pulses is the carrier envelope phase (CEP), which defines the temporal offset of the peak of the intensity envelope from the nearest peak of the carrier wave. The generation of pulses with reproducible electric field, necessary for some of our experiments, calls for the control of the CEP.

In our labs the mid-infrared light pulses are generated via difference-frequency mixing of the output of two different near-infrared optical parametric amplifiers (OPA), that are seeded by the same white light continuum. Given the properties of the amplification process, those near-IR pulses are locked in phase, thus generating intrinsically CEP stable pulses in the mid-infrared.

Frequency-tunable, few-cycle mid-infrared light pulses generated via difference-frequency mixing of the outputs of two OPAs seeded by the same white light continuum. Left: Electric fields measured via electro-optic sampling; right: corresponding frequency spectra obtained by Fourier transformation.

Nonetheless, small drifts of the CEP over time occur, due to thermal and mechanical drifts in the setup. For the detection and compensation of such CEP drifts, we designed a scheme which relies on the combination of nonlinear optics and interferometry [1].

The availability of precisely tailored pulses in the MIR spectral region might open new pathways for the control of correlated electronic systems, especially when the excitations strongly depend on the driving electric filed rather than the intensity. In order to manipulate light in the time-frequency domain, it is necessary to control the spectral phase of the pulses.

In our lab the pulse shaping in the MIR is achieved by an all-reflective pulse shaper, designed in a 4f geometry, with a deformable mirror in the Fourier Plane.

The choice of an all-reflective setup was dictated by the fact that material absorption at mid-IR wavelengths prevents the applicability of well established pulse shaping techniques based on Liquid Crystal Spatial Light Modulators (LC-SLMs) or acousto-optic modulators (AOMs).

Our pulse shaper allows us to compress mid-IR pulses, to impart them spectral phases on demand (second and third order phases have been demonstrated), and to split the mid-IR light into two time-delayed pulses [2]. 

These tailored MIR pulses will be employed in experiments aimed to the coherent control of phase transitions in condensed matter.

Related Experiment

Optical Parametric Amplifiers (on the breadboard) and Difference Frequency Generation setup. The 800mn light is converted to Mid-Infrared radiation, ranging from 4 to 20 µm.

Close up on the second stage of one OPA.

Close up on the DFG setup.

Andrea tunes the OPAs to get the desired Mid-Infrared wavelength.