Few-cycle mid-infrared pulse generation, characterization, and coherent propagation in optically dense media

Abstract

The generation of four- to five-cycle mid-infrared pulses using a single-stage potassium niobate optical parametric amplifier (OPA) is demonstrated. The OPA is pumped by a gain-switched Ti:Sapphire regenerative amplifier and is seeded with the near-infrared portion of a white light continuum. The OPA is continuously tunable from 2700 to 4700 nm, and maintains sub-65 fs pulses that are fully characterized using cross-correlation frequency-resolved optical gating (XFROG). These are the shortest near-transform-limited pulses reported over this large infrared spectral range. This apparatus is used to measure ultrafast vibrational responses, specifically, pulse profiles modified by the free-induction decay (FID) of O–H oscillators. Both the intensity and instantaneous phase of these pulses are determined after traversing samples of isotopically diluted water (HDO in D2O) using XFROG, representing a new application of the XFROG technique to the study of (dipolar) molecular responses. Pronounced beating of the trailing edges of FID-modified pulses is observed even below optical densities of one, and the details of these features are found to depend on the chirp of the field used to generate the FID. These results indicate that discretion should be used in the selection of sample conditions for nonlinear infrared spectroscopic measurements.

Optical Parametric Generators and Amplifiers

“…PPKTP, on the other hand, should have large broadband gain at around 2.8 µm (signal of 1.7 µm) for a 1.064 µm pump. A similar analysis for a collinear OPA, using birefringence phase-matched crystals in the MIR region, have been reported in the literature: for pumping at the Ti:Sapphire amplifier wavelengths of KTP isomorphs [12,64,65], BBO [64], KNbO 3 [1,5,66], MgO:LiNbO 3 [67]; for type-I and type-II parametric interaction in AgGaS 2 pumped by Cr:Forsterite (1.25 µm) [68,69] and by 1.1 µm-1.6 µm OPA [70]; and for ZnGeP 2 OPG pumped at near 2 µm and operating in the 2.5 µm -10 µm wavelength region [71].…”

“…Femtosecond pulses in this region are of interest primarily for ultrafast spectroscopy. For instance, sub-100 fs pulses in the region between 3 µm and 4 µm are required for investigation of hydrogen-bounded liquids by measuring relaxations of X-H (X = N, O) stretching vibrations [1]. Another example can be stretching vibrations of carbonyl bonds in proteins whose absorption lines fall between 5 µm and 6 µm [1].…”

Pulse shaping and its characterization of mid-infrared femtosecond pulses: Toward coherent control of molecules in the ground electronic states

“…They simplified the method by using 800 nm reference light derived from the zero-order diffraction of the compressor in a chirped-pulse amplifier and mixed with the mid-infrared pulse in a LiNbO 3 crystal. In previous SFG-XFROG measurements, mid-infrared pulses (3-20 lm) were mixed with the near-infrared fundamental pulse in GaSe [31], KNbO 3 [32,33], and LiNbO 3 [34] to obtain the visible SFG output. Recently, we have introduced a AgGaGeS 4 crystal for more efficient SFG-XFROG system in the wavelengths between 3 lm and 11 lm [35].…”

Characterization of mid-infrared femtosecond pulses [Invited]

“…Other interesting variants of FROG have also been demonstrated, in the mid-IR, such as frequency-resolved pump-probe [49], based on third-order rather than second-order optical nonlinearities, and cross-correlation FROG methods [50][51][52].…”