Super-octave longwave mid-infrared coherent transients produced by optical rectification of few-cycle 25-μm pulses

Sergey Vasilyev,Peter G Schunemann,Igor S Moskalev,Viktor O Smolski,Andrey V Muraviev,Jeremy M Peppers,Sergey B Mirov,Kevin Zawilski,Konstantin L Vodopyanov,Mike Mirov,Valentin P Gapontsev

doi:10.1364/optica.6.000111

Abstract

Femtosecond laser sources and optical frequency combs in the molecular fingerprint region of the electromagnetic spectrum are crucial for a plethora of applications in natural and life sciences. Here we introduce Cr2+-based lasers as a convenient means for producing super-octave mid-IR electromagnetic transients via optical rectification (or intra-pulse difference frequency generation, IDFG). We demonstrate that a relatively long, 2.5 μm, central wavelength of a few-cycle Cr2+:ZnS driving source (20 fs pulse duration, 6 W average power, 78 MHz repetition rate) enabled the use of highly nonlinear ZnGeP2 crystal for IDFG with exceptionally high conversion efficiency (>3%) and output power of 0.15 W, with the spectral span of 5.8–12.5 μm. Even broader spectrum was achieved in GaSe crystal: 4.3–16.6 μm for type I and 5.8–17.6 μm for type II phase matching. The results highlight the potential of this architecture for ultrafast spectroscopy and generation of broadband frequency combs in the longwave infrared.

Highlights

Few-cycle laser sources with a spectral span exceeding an octave in the longwave portion of the mid-IR spectrum (5–20 μm, 15–60 THz) are vital for a number of applications, ranging from molecular fingerprinting with dual optical frequency combs [1], IR nano-imaging [2], Fourier transform infrared nanospectroscopy absorption spectroscopy [3], time-resolved optical spectroscopy, and studies of ultrafast dynamics [4,5] to high-field science [6]
Remarkable examples include supercontinuum generation (SCG) in chalcogenide fibers driven by an optical parametric amplifier (OPA) [7], and augmentation of the OPA spectrum by SCG via cascaded quadratic nonlinearities in crystals [8]
Another large family of super-octave mid-IR sources is based on frequency divideby-2 optical parametric oscillators (OPOs) that feature broad bandwidth, low pumping thresholds [9], high conversion efficiency [10], and the capability to scale the average power of an octave-spanning output to watt level [11]

Summary

Introduction

Few-cycle laser sources with a spectral span exceeding an octave in the longwave portion of the mid-IR spectrum (5–20 μm, 15–60 THz) are vital for a number of applications, ranging from molecular fingerprinting with dual optical frequency combs [1], IR nano-imaging [2], Fourier transform infrared nanospectroscopy (nano-FTIR) absorption spectroscopy [3], time-resolved optical spectroscopy, and studies of ultrafast dynamics [4,5] to high-field science [6]. Another large family of super-octave mid-IR sources is based on frequency divideby-2 (subharmonic) optical parametric oscillators (OPOs) that feature broad bandwidth, low pumping thresholds [9], high conversion efficiency [10], and the capability to scale the average power of an octave-spanning output to watt level [11].

Results

Conclusion