Generation of few-cycle infrared pulses from a degenerate dual-pump OPCPA

Performance improvement of a nonlinear temporal filter by using cascaded femtosecond optical parametric amplification

In this paper, we report that the conversion efficiency and spectrum of femtosecond optical parametric amplification (fs-OPA) can be significantly enhanced by employing a compact cascaded femtosecond OPA (CF-OPA) scheme with the self-compensation of the temporal walk-off between two nonlinear gain media. Correspondingly, the gain related temporal contrast can also be improved. The feasibility of the CF-OPA method using three cascaded BBO crystals is numerically and experimentally analyzed. Moreover, by replacing the conventional fs-OPA with the CF-OPA and optimizing the design, the performance of a nonlinear temporal filter combining cross-polarized wave generation and fs-OPA is comprehensively improved. The experimental results demonstrate the superiority of the CF-OPA scheme, which can generate high-performance cleaned pulses at 1 kHz repetition rate with energy of 340μJ, energy fluctuation below 0.9% (RMS), spectral width of 97 nm (FWHM), Fourier-transform-limited pulse width of 12 fs and temporal contrast better than 10^-12. To the best of our knowledge, this is the first reported temporal walk-off self-compensated quasi-collinear CF-OPA geometry adopting three cascaded BBO crystals, which can be easily generalized to other wavelengths or nonlinear crystals. The above nonlinear temporal filter with a CF-OPA scheme has the rarest comprehensive parameters, which can provide excellent seed pulses for PW and 10 PW class femtosecond laser systems.

Identification of tunneling and multiphoton ionization in intermediate Keldysh parameter regime

Quantitative identification of tunneling ionization (TI) and multiphoton ionization (MPI) with Keldysh parameter γ in intermediate regime is of great importance to better understand various ionization-triggered strong-field phenomena. We theoretically demonstrate that the numerical observable ionization delay time is a more reliable indicator for characterizing the transition from TI to MPI under different laser parameters. Using non-linear iterative curve fitting algorithm (NICFA), the detected time-dependent probability current of ionized electrons can be decoupled into weighted TI and MPI portions. This enables us to confirm that the observed plateau-like structure in ionization delay time picture at the intermediate γ originates from the competition between TI and MPI processes. A hybrid quantum and classical approach (HQCA) is developed to evaluate the weights of TI and MPI electrons in good agreement with NICFA result. Moreover, the well separated TI and MPI electrons using HQCA are further propagated classically for mapping their final momentum, which well reproduces the experimental or ab-initio numerical calculated signatures of ionized electron momentum distribution in a rather broad γ regime.

Noncollinear and nonlinear pulse propagation

A novel method for numerical modelling of noncollinear and nonlinear interaction of femtosecond laser pulses is presented. The method relies on a separate treatment of each of the interacting pulses by it's own rotated unidirectional pulse propagation equation (UPPE). We show that our method enables accurate simulations of the interaction of pulses travelling at a mutual angle of up to 140°. The limit is imposed by the unidirectionality principal. Additionally, a novel tool facilitating the preparation of noncollinear propagation initial conditions - a 3D Fourier transform based rotation technique - is presented. The method is tested with several linear and nonlinear cases and, finally, four original results are presented: (i) interference of highly chirped pulses colliding at mutual angle of 120°, (ii) optical switching through cross-focusing of perpendicular beams (iii) a comparison between two fluorescence up-conversion processes in BBO with large angles between the input beams and (iv) a degenerate four-wave mixing experiment in a boxcar configuration.

Ultrabroadband microjoule 1.8 μm laser pulse from a single-stage broadband pumped OPA

We propose a broadband pumped optical parametric amplification scheme for the generation of a micojoule few-cycle pulse centered at 1.8 μm. Owing to the opposite chirp of the broadband pump and seed pulses, an idler pulse with an FWHM bandwidth of 434 nm is obtained, which can be effectively compressed to a near-transform-limited duration of 13.1 fs by simply compensating for the linear chirp. The picojoule-level seed is amplified to microjoule level in a single stage with a pump pulse of 100 μJ, corresponding to an overall conversion efficiency of 9%. The presented scheme is potentially applicable in various nonlinear crystals with different kinds of femtosecond laser systems, which provides not only an efficient approach of down-converting a near-infrared laser with moderate energy to the mid-infrared region, but also a suitable seeding source in high-energy OPCPA systems.

Ultrabroadband tunable OPA design using a spectrally broadened pump source

A robust optical parametric amplifier (OPA) scheme is proposed in which we have experimentally achieved broadband bandwidth for a collinear OPA design that exhibits good beam quality and spatial distribution using a type-I β barium borate crystal. The applied pump pulses are simultaneously spectrally broadened and temporally stretched in a multi-plate system before being used to amplify the temporally stretched white light. In this case, the phase matching can be obtained for a broad range of wavelengths, and we have managed to achieve bandwidths three times broader than a conventional narrowband pumped collinear OPA. With a bandwidth as broad as 400 nm centered at 1400 nm, as well as a broadband angular dispersion-free idler, the signal bandwidth supports transform-limited pulses as short as 7.5 fs which correspond to sub-two optical cycles for this center wavelength. Furthermore, the system is easily tunable over a 400 nm range of bandwidths starting from 1100 to 1500 nm. The proposed scheme provides a versatile near-infrared/middle-infrared source with a broad bandwidth and fine tuning capability which paves the way for ultrafast spectroscopy and strong field applications.

Extending plasma channel of filamentation with a multi-focal-length beam

We propose a novel scheme that lengthens the plasma channel in filamentation with a multi-focal-length beam. Instead of one focal length introduced by a conventional convex lens, the multi-focal-length beam modulated by a spatial light modulator (SLM) produces a filament in an extended range with limited but strictly manipulated laser energy. The results show that the scheme is capable of doubling the filament length compared to a single-lens scheme with a 2-mJ input pulse. The filament location and length can be simply tuned by altering the spatial amplitude and phase or employing higher energies. Furthermore, the extended filament length leads to the generation of a broadened continuum ranging from visible (VIS) to infrared (IR) domain. This versatile scheme offers an efficient tool for the development of a variety of applications involving ultrafast nonlinear optics.