Few-optical-cycle pulses in the near- and mid-infrared by optical parametric amplifiers

Abstract

The generation of light pulses with duration down to a few optical cycles is a major topic in ultrafast optics. The broad gain bandwidths of optical parametric amplifiers (OPAs) have been so far widely exploited for few-cycle pulse generation in the visible, while only limited work has been performed in the near- and mid-infrared spectral region. Near-IR short pulses are interesting for ultrafast spectroscopy as well as for high-harmonic generation and attosecond pulse synthesis, taking advantage of the extended energy cutoff afforded by the longer driving wavelength. The mid-IR spectral range (3–5 µm) overlaps with vibrational transitions of many molecular and solid-state systems, allowing measurement of vibrational dephasing and coherent control of chemical reactions. The OPA phase-matching bandwidth Δω depends on the group-velocity mismatch between signal and idler δ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">si</inf> =1/ν <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gs</inf> −1/ν <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gi</inf> , where ν; <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gs</inf> and ν <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">gi</inf> are the signal and idler group velocities respectively. To the first order one can write Δω∝1/δ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">si</inf> , so that broadband gain is achieved when the group velocities of signal and idler are matched. This condition can occur in the following OPA schemes: (a) in a non-collinear OPA (NOPA), in which the projection of the idler group velocity along the signal direction is equal to the signal group velocity; (b) at degeneracy point ω <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> = ω <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf> ; (c) at some particular frequencies (ω <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</inf> ≠ω <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">i</inf> )out of degeneracy. The characteristics of the non-linear materials are crucial to determine OPA scheme and phase-matching bandwidth [1].