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

Abstract

We report the generation of extremely broadband and inherently phase-locked mid-infrared pulses covering the 5 to 11 µm region. The concept is based on two stages of optical parametric amplification starting from a 270-fs Yb:KGW laser source. A continuum seeded, second harmonic pumped pre-amplifier in β-BaB 2 O 4 (BBO) produces tailored broadband near-infrared pulses that are subsequently mixed with the fundamental pump pulses in LiGaS 2 (LGS) for mid-infrared generation and amplification. The pulse bandwidth and chirp is managed entirely by selected optical filters and bulk material. We find an overall quantum efficiency of 1% and a mid-infrared spectrum smoothly covering 5-11 µm with a pulse energy of 220 nJ at 50 kHz repetition rate. Electro-optic sampling with 12-fs long white-light pulses directly from self-compression in a YAG crystal reveals near-single-cycle mid-infrared pulses (32 fs) with passively stable carrier-envelope phase. Such pulses will be ideal for producing attosecond electron pulses or for advancing molecular fingerprint spectroscopy.

A New Generation of Ultrafast Oscillators for Mid-Infrared Applications

“…EOS probes the birefringence of the crystal that has been modulated by the electric field of the mid-infrared pulse, with the measurements being performed as a function of the delay between both pulses. However, the recorded EOS signal on the short-wavelength side is limited by the duration of the probe pulse [318], and broader frequencies spans are only detectable for decreasing sampling pulse durations. Typically, it is said that the probe pulse has to be shorter than a half-cycle of the oscillating field to be observed [49], and a common way to achieve this is to split off part of the oscillator output for further nonlinear spectral broadening and compression [315,318].…”

“…However, the recorded EOS signal on the short-wavelength side is limited by the duration of the probe pulse [318], and broader frequencies spans are only detectable for decreasing sampling pulse durations. Typically, it is said that the probe pulse has to be shorter than a half-cycle of the oscillating field to be observed [49], and a common way to achieve this is to split off part of the oscillator output for further nonlinear spectral broadening and compression [315,318]. In nonlinear media, the propagation of few-cycle pulses is influenced by the χ (3) -effects of self-focusing and self-phase modulation, and a proper control of the material parameters can result in a significant reduction in pulse duration.…”

Second-harmonic generation and self-phase modulation of few-cycle mid-infrared pulses

“…The mid-infrared pulses used for our experiments are obtained from a non-collinear optical parametric amplifier composing white-light seed generation and broadband parametric amplification of the near-infrared part around 1200 nm followed by type-I difference frequency mixing with the 1030 nm pulses from the driving laser (Pharos PH1-20, Light Conversion) [20]. The output mid-infrared pulses have a pulse energy of 400 nJ at a repetition rate of 50 kHz and a central wavelength of 7 μm.…”

“…The spectrum covers 5.4-11 μm (27.3-55.5 THz) at −20 dB intensity, and the ultrashort electric field has a pulse duration of ∼34 fs or ∼1.4 optical cycles (intensity full width at half-maximum [FWHM]). Figure 1(a) shows an electro-optic sampling measurement, made with a 20 μm thick GaSe crystal and 12 fs visible probe pulses from a supercontinuum source [20] of the electric field, as it enters our optical setup. Figure 1(b) depicts the experiment.…”

“…The fundamental mid-infrared pulses driving our experiments have a passively stable carrier-envelope phase (CEP) [20] and the second-harmonic pulses should therefore also be CEP-stable. The fundamental and second-harmonic pulses are therefore suitable for coherent short-pulse synthesis [27,28].…”