Usman K. Sapaev

General second-harmonic pulse shaping in grating-engineered quasi-phase-matched nonlinear crystals

We describe a spectrogram-based simulated annealing algorithm for designing quasi-phase-matched crystals capable of producing second harmonic generation pulses of any chosen amplitude and phase profile. The approach applies a new and rapid analytic method for calculating the amplitude and phase of the second harmonic generation pulses generated by a quasi-phase-matched crystal containing an arbitrary grating design. The performance of the algorithm is illustrated by examples of femtosecond second harmonic pulses designed according to various target shapes including single, double and triple Gaussian pulses, positive and negative linear chirp and square, triangular and stepped profiles.

Designer femtosecond pulse shaping using grating-engineered quasi-phase-matching in lithium niobate.

The generation of tailored femtosecond pulses with fully engineered intensity and phase profiles is demonstrated using second-harmonic generation of an Er:fiber laser in an aperiodically poled lithium niobate crystal in the undepleted pump regime. Second-harmonic pulse shapes, including Gaussian, stepped, square, and multiple pulses have been characterized using cross-correlation frequency-resolved optical gating and have been shown to agree well with theory.

Engineered quasi-phase matching for multiple parametric generation.

We develop a rapid and efficient numeric technique for the design of arbitrary quasi-phase matched lattices for parametric generation of single and multiple pulses with any prescribed amplitude and phase profiles from fundamental frequency excitation in the regime of pump depletion. We examine the case of simultaneous of 2nd and 3rd harmonic generation in arbitrary quasi-phase matched gratings taking into account the group velocity mismatch and dispersion.

Femtosecond pulse synthesis by efficient second-harmonic generation in engineered quasi phase matching gratings.

We numerically design quasi-phase matched crystals with domains of arbitrary sizes for second harmonic generation by femtosecond pulses, taking into account both group velocity mismatch and dispersion. An efficient simulated-annealing algorithm is developed to design quasi-phase matching gratings which can yield the desired amplitude and phase of second-harmonic pulses in the presence of pump depletion. The method is illustrated with reference to single, double-hump and chirped fs Gaussian pulses in a lithium niobate crystal.

Efficient high-harmonic generation in engineered quasi-phase matching gratings

We develop an effective numerical algorithm to design engineered quasi-phase-matching gratings in nonlinear crystals for the parametric generation of harmonics of arbitrary order from a fundamental frequency input. We demonstrate the method in the case of lithium niobate excited by a continuous-wave source in the regime of pump depletion.

Pulse shaping via Backward Second Harmonic Generation

We numerically demonstrate pulse shaping by backward frequency doubling of femtosecond laser pulses in engineered quasi-phase-matched waveguides. We employ optimal control theory to access the regime of pump depletion.

Combined action of the bound-electron nonlinearity and the tunnel-ionization current in low-order harmonic generation in noble gases

We study numerically low-order harmonic generation in noble gases pumped by intense femtosecond laser pulses in the tunneling ionization regime. We analyze the influence of the phase-mismatching on this process, caused by the generated plasma, and study in dependence on the pump intensity the origin of harmonic generation arising either from the bound-electron nonlinearity or the tunnel-ionization current. It is shown that in argon the optimum pump intensity of about 100 TW/cm² leads to the maximum efficiency, where the main contribution to low-order harmonics originates from the bound-electron third and fifth order susceptibilities, while for intensities higher than 300 TW/cm² the tunnel-ionization current plays the dominant role. Besides, we predict that VUV pulses at 133 nm can be generated with relatively high efficiency of about 1.5 × 10⁻³ by 400 nm pump pulses.

Quasi-phase-matching for third harmonic generation in noble gases employing ultrasound

We study a novel method of quasi-phase-matching for third harmonic generation in a gas cell using the periodic modulation of the gas pressure and thus of the third order nonlinear coefficient in the axial direction created by an ultrasound wave. Using a comprehensive numerical model we describe the quasi-phase matched third harmonic generation of UV (at 266 nm) and VUV pulses (at 133 nm) by using pump pulses at 800 nm and 400 nm, respectively, with pulse energy in the range from 3 mJ to 1 J. In addition, using chirped pump pulses, the generation of sub-20-fs VUV pulses without the necessity for an external chirp compensation is predicted.

Compensating group-velocity mismatch in parametric frequency generation.

We propose and numerically investigate a simple scheme based on frequency-selective mirrors for compensating group-velocity mismatch during harmonic generation. We address the case of frequency doubling and illustrate the effectiveness of the approach for multipass frequency doubling in LiNbO(3).

Designer femtosecond pulse shaping using grating-engineered quasi-phasematching in lithium niobate

Tailored femtosecond pulses with fully engineered intensity and phase profiles are demonstrated using second-harmonic generation of an Er:fiber laser in an aperiodically-poled lithium niobate crystal. The profiles created include square, stepped, double and triple pulses.