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

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 Cr 2-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 Cr 2 :ZnS driving source (20 fs pulse duration, 6 W average power, 78 MHz repetition rate) enabled the use of highly nonlinear ZnGeP 2 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.

Octave-spanning single-cycle middle-infrared generation through optical parametric amplification in LiGaS2

“…Direct broadband laser sources are rare in the MIR due to lack of suitable materials. Researchers are therefore invoking nonlinear optical conversions, for example difference frequency generation or optical rectification in nonlinear crystals [10][11][12][13] such as GaSe or AgGaS 2 [14,15], filaments [16,17] or plasmas [18]. A quite new material is LiGaS 2 (LGS) [19][20][21][22][23][24] which offers a broadband transparency range (0.32-11.6 µm) and a large bandgap of ∼4 eV that leads to very weak two-photon absorption for Ti:sapphire-based (∼800 nm) or Yb-based (∼1030 nm) laser pulses.…”

Optical frequency metrology in the bending modes region

“…A powerful solution to both issues is direct comb spectroscopy 19,20 , which exploits millions of laser modes that compose the comb spectrum to directly probe molecular absorption over extremely large bands and with absolute frequency calibration 21 . Thanks to a time-domain counterpart formed by a coherent train of femtosecond laser pulse 20 , frequency combs are ideal sources for efficient nonlinear frequency conversion deep into the mid-infrared (MIR) 22 , also fostered by innovative solutions for laser gain media 23 , nonlinear materials 24 and frequency conversion schemes [25][26][27] . However, it has only been recently that snapshots of entire bands have been acquired by a dual-comb approach 28 at high temporal and spectral resolution in the LWIR, from 6.7 to 16.7 µm (1500-600 cm −1 ) 29,30 .…”

Intra-pulse difference-frequency generation of mid-infrared (27–20 μm) by random quasi-phase-matching

“…This wavelength range can be covered by numerous different schemes, each with their own strengths and weaknesses in terms of complexity, simultaneous bandwidth, power, efficiency, and pulse durations [3][4][5][6][7][8]. Yet, the most popular method of coherent broadband MIR generation remains nonlinear downconversion from the nearinfrared-a spectral region where many high-power driving lasers are available [4,7,[9][10][11][12][13][14][15][16][17][18]. Obviously, this process requires a suitable nonlinear medium that is transparent for the pump, signal, and idler wavelengths.…”