Six-cycle mid-infrared source with 3.8 μJ at 100 kHz

Design and simulation of a single-cycle source tunable from 2 to 10 micrometers

We present the design of a novel single-cycle infrared source tunable from 2 to 10 μm. We simulate the optical parametric amplification (OPA) in BBO and the difference frequency generation (DFG) in AGS based on coupled second-order three-wave nonlinear propagation equations. We combine this with the unidirectional pulse propagation equation, which models the generation of the initial supercontinuum seed in sapphire and the final self-compression in YAG, ZnS, and GaAs, respectively. The obtained results indicate that single-cycle pulses can be produced in a tunable range of 2 to 10 μm.

Intracavity gain shaping in millijoule-level, high gain Ho:YLF regenerative amplifiers

We demonstrate intracavity gain shaping inside a 2 μm Ho:YLF regenerative amplifier with a spectral bandwidth of 2.9 nm broadened to 5.4 nm, corresponding to Fourier-limited pulses of 1 ps duration. The intracavity gain shaping is achieved by using a simple etalon, which acts as a frequency-selective filter. The output of the regenerative amplifier is amplified by a single-pass amplifier, and we achieve total energy of 2.2 mJ and pulse duration of 2.4 ps at 1 kHz with pulse fluctuations <1%. The amplifier chain is seeded by a home-built mode-locked holmium-doped fiber oscillator.

Impact of atmospheric molecular absorption on the temporal and spatial evolution of ultra-short optical pulses

We present a rigorous study on the impact of atmospheric molecular absorption on the linear propagation of ultrashort pulses in the mid-infrared wavelength region. An ultrafast thulium-based fiber laser was employed to experimentally investigate ultrashort-pulse propagation through the atmosphere in a spectral region containing several strong molecular absorption lines. The atmospheric absorption profile causes a significant degradation of the pulse quality in the time domain as well as a distortion of the transverse beam profile in the spatial domain. Numerical simulations carried out in the small signal limit accurately reproduce the experimental observations in the time domain and reveal that the relative loss in peak power after propagation can be more than twice as high as the relative amount of absorbed average power. Although their nature is purely linear (i.e. the intensities considered are sufficiently low) the discussed effects represent significant challenges to performance-scaling of mid-infrared ultrafast lasers operating in spectral regions with molecular absorption bands. Guidelines for an efficient mitigation of the pulse quality degradation and the beam profile distortion are discussed.

Strong-field plasmonic photoemission in the mid-IR at <1 GW/cm² intensity

We investigated nonlinear photoemission from plasmonic films with femtosecond, mid-infrared pulses at 3.1 μm wavelength. Transition between regimes of multi-photon-induced and tunneling emission is demonstrated at an unprecedentedly low intensity of <1 GW/cm(2). Thereby, strong-field nanophysics can be accessed at extremely low intensities by exploiting nanoscale plasmonic field confinement, enhancement and ponderomotive wavelength scaling at the same time. Results agree well with quantum mechanical modelling. Our scheme demonstrates an alternative paradigm and regime in strong-field physics.

Nondegenerate parametric generation of 2.2-mJ, few-cycle 2.05-μm pulses using a mixed phase matching scheme

We describe the production of 2.2-mJ, ∼6 optical-cycle-long mid-infrared laser pulses with a carrier wavelength of 2.05 μm in a two-stage β-BaB2O4 nondegenerate optical parametric amplifier design with a mixed phase matching scheme, which is pumped by a standard Ti:sapphire chirped-pulse amplification system. It is demonstrated that relatively high pulse energies, short pulse durations, high stability, and excellent beam profiles can be obtained using this simple approach, even without the use of optical parametric chirped-pulse amplification.

Self-compression to sub-3-cycle duration of mid-infrared optical pulses in dielectrics

We present the first demonstration of sub-3 cycle optical pulses at 3.1 μm central wavelength generated through self-compression in the anomalous dispersion regime in a dielectric. The pulses emerging from this compact and efficient self-compression setup could be focused to intensities exceeding 10(14) W/cm(2), a suitable range for high field physics experiments. Numerical simulations performed with a 3D nonlinear propagation code, provide theoretical insight on the processes involved and support our experimental findings.

Ionization with low-frequency fields in the tunneling regime

Strong-field ionisation surprises with richness beyond current understanding despite decade long investigations. Ionisation with mid-IR light has promptly revealed unexpected kinetic energy structures that seem related to unanticipated quantum trajectories of the electrons. We measure first 3D momentum distributions in the deep tunneling regime (γ = 0.3) and observe surprising new electron dynamics of near-zero momentum electrons and extremely low momentum structures, below the eV, despite very high quiver energies of 95 eV. Such level of high-precision measurements at only 1 meV above the threshold, despite 5 orders higher ponderomotive energies, has now become possible with a specifically developed ultrafast mid-IR light source in combination with a reaction microscope, thereby permitting a new level of investigations into mid-IR recollision physics.

Generation of high-fidelity few-cycle pulses at 2.1 μm via cross-polarized wave generation

We demonstrate the generation of temporally clean few-cycle pulses at 2.1 μm by shortening of 6-optical-cycle pulses via cross-polarized wave (XPW) generation in BaF(2), CaF(2) and CVD-Diamond crystals. By combining spectra and single-shot third-order intensity cross-correlation traces in a novel Bayesian pulse retrieval technique, we measured pulse durations of 20 fs, corresponding to 2.8 optical cycles. Our results show that XPW generation in the infrared could provide a high-fidelity source of few-cycle pulses for strong-field physics applications. It could also serve as an injector for high-peak power ultrafast mid-IR wavelength parametric amplifiers.

Mid-infrared optical parametric amplifier based on a LGSe crystal and pumped at 1.6 μm

We demonstrate the generation of 22.6 μJ of combined energy at 3 μm for sub-300fs pulses at a repetition rate of 1 kHz using a LGSe optical parametric amplifier (OPA). The LGSe OPA is pumped by the 140-fs 1.6 μm pulses from a 300-mW KTA optical parametric chirped pulse amplifier (OPCPA) based on an all-optical synchronization scheme. By using a highly-nonlinear fiber, the output of an erbium-doped fiber laser operating at 1560 nm is shifted to 1050 nm in order to coherently seed a Nd:YLF regenerative amplifier. The LGSe OPA is seeded using the MIR coming from the amplification of the 1.6 μm in the OPCPA.

Generation of carrier-envelope phase-stable two optical-cycle pulses at 2 μm from a noncollinear beta-barium borate optical parametric amplifier

We report on the generation of two optical-cycle, carrier-envelope phase-stable pulses with energy of 15 μJ at central wavelength of 2 μm. Pulses of 15 fs (2.3 optical cycles) duration are obtained by difference-frequency generation, which at the same time provides passive carrier-envelope phase stabilization, and noncollinear optical parametric amplification in beta-barium borate crystal, which is shown to provide broad phase-matching bandwidth if seeded by pulses in the 1.6-2.6 μm wavelength range. Pulse compression is achieved by means of a simple propagation through the optical setup and by precisely controlling the initial chirp of the pulses to be frequency downconverted.

Broadband noncollinear optical parametric amplification without angularly dispersed idler

We report a new scheme for direct generation of broadband angular-dispersion-free mid-IR idler pulses via noncollinear optical parametric amplification when group-velocity matched wavelengths cannot be found and the traditional noncollinear geometry fails to increase the phase-matching bandwidth. The scheme does not require any post-amplification idler angular dispersion compensation. We derive and interpret the condition for broadband amplification and absence of idler angular dispersion. A broadband angular-dispersion-free 2.15 μm idler pulse is generated as an experimental demonstration. We identify the potential of the scheme to generate a broadband 3.5 μm idler, with a bandwidth supporting a sub-two-cycle pulse.

Direct observation of two-color pulse dynamics in passively synchronized Er and Yb mode-locked fiber lasers

We report direct experimental observation of interesting pulse synchronization dynamics in a cavity-combined Er and Yb mode-locked fiber lasers by measuring the relative position between the two-color pulses in the shared fiber section. The influence of the 1.03 μm pulse on the 1.56 μm single pulse as well as bound soliton pairs can be clearly identified as an effective phase modulation through the XPM effect with the walk-off effect taken into account. For the 1.56 μm single pulse under synchronization, the dependence of the relative position variation and the center wavelength shift on the cavity mismatch detuning is found analogous to the typical characteristics of FM mode-locked lasers with modulation frequency detuning. Moreover, depending on the cavity mismatch, the passively synchronized 1.56 μm bound soliton pairs are found to exhibit two different dynamical behaviors, i.e., phase-locked (in-phase) as well as non-phase-locked. The physical origins for these two kinds of bound soliton pairs are investigated experimentally by disclosing their locations with respective to the copropagating 1.03 μm pulse.

Sub-250-mrad, passively carrier-envelope-phase-stable mid-infrared OPCPA source at high repetition rate

An all-optical and passively carrier-to-envelope-phase-stabilized (CEP-stabilized) optical parametric chirped pulse amplification (OPCPA) system is demonstrated with sub-250-mrad CEP stability over 11 min and better than 100 mrad over 11 s. This is achieved without any electronic CEP stabilization loop for 160 kHz pulse repetition rate in the few cycle regime.