Muhammad Nurhuda

Generation of 5 fs, 0.5 TW pulses focusable to relativistic intensities at 1 kHz

We have demonstrated the generation of 5 fs, 0.5 TW pulses at 1 kHz repetition rate using a pulse compression technique in a hollow fiber with a pressure gradient. Owing to the excellent beam quality by passing through the hollow fiber, the beam after pulse compression could be focused to a nearly diffraction-limited spot size. We obtained for the first time a peak intensity as high as 5x10(18) W/cm(2) in the 2-cycle regime.

Propagation dynamics of femtosecond laser pulses in a hollow fiber filled with argon: constant gas pressure versus differential gas pressure

We investigate the dynamics of femtosecond laser pulses propagating in a hollow fiber filled with argon, through a full numerical solution of the nonlinear Schrodinger equation. The simulation results show that, if the intensity is low and no ionization takes place, the spatial profile of the beam does not change very much so that its propagation model may be simplified to a one-dimensional model. If the intensity is high and ionization takes place, the spatial dynamics as well as temporal dynamics become very complicated because of self-focusing and defocusing. It is found that, for the same value of the B integral, self-focusing inside a hollow fiber can be substantially suspended by a differential gas pressure technique, where the gas pressure is set to be a minimum at the entrance and then increased with the propagation distance. Numerical simulations show that using such a technique, the energy transmitted during propagation inside hollow fiber is significantly enhanced, and the spatial phase is also improved.

Generation of high-energy high-order harmonics by use of a long interaction medium

We investigate the energy scaling of high-order harmonics in Ar, using a loosely focused beam in a phase-matched condition. Adjusting the argon gas density and the pump laser focusing condition produces a total output harmonic energy as high as 0.7 μJ in the spectral region 34.8–25.8 nm (the corresponding orders of the 23rd–31st harmonics); 27th-order harmonic (29.6-nm) energy attained is as high as 0.3 μJ with an almost perfect spatial profile. High efficiency and good spatial quality are achieved simultaneously.

Generalization of the Kerr effect for high intensity, ultrashort laser pulses

We have investigated the nonlinear susceptibility of atoms induced by high intensity, ultrashort laser pulses using a numerical solution of the time-dependent Schrodinger equation. We found that the instantaneous nonlinear susceptibility becomes saturated at high intensity. We also found that the saturation is closely linked to depletion of the ground state. Based on the numerical results, a simple model that generalizes the nonlinear susceptibility of atoms for high intensity, ultrashort laser pulses is proposed. We also investigated the ionization-induced dipole moment and found that the amplitude of the dipole moment induced by an ionized electron is, in general, smaller than that induced by a free-electron, and is attributable to a residual interaction between the ionized-electron and its parent ion.

Compression of intense ultrashort laser pulses in a gas-filled planar waveguide

We report on the compression of intense ultrashort laser pulses for the purpose of producing few-cycle optical pulses at the multimillijoule level by using a planar waveguide (PWG). The PWG is composed of two parallel glass slabs separated by a gap of 100 microm and mounted in a gas chamber filled with a noble gas. In comparison with the conventional hollow-fiber-based pulse compression technique, the use of a PWG enables the injection of high-energy ultrashort pulses, because the input laser beam is confined only in the lateral direction perpendicular to the waveguide plane. Using this technique, we demonstrate the generation of 12 fs, 2 mJ laser pulses in an argon-filled PWG.

Control of self-phase modulation and plasma-induced blueshifting of high-energy, ultrashort laser pulses in an argon-filled hollow fiber using conjugate pressure-gradient method

A proposal for spectral broadening of an intense laser pulse with energy 15 mJ and a pulse duration of 40 fs in an argon-filled hollow fiber, using conjugate pressure-gradient method, is presented. The design, to be referred as conjugate pressure-gradient method, shall consist of a pair of hollow-fiber segments, each with gas pressure that increasingly and decreasingly varies, respectively. It is expected that the occurrence of early self-focusing can be avoided, as well as the elimination of the additional nonlinear processes at hollow fiber exit. Simulation using 15 mJ, 40 fs laser pulses shows that, in the intensity regime where ionization is important, excellent spectral broadening and spectral phase of the pulses can be obtained using the proposed conjugate pressure gradient method. The method therefore shall be useful for the spectral broadening of high-energy, ultrashort laser pulses.

Spatiotemporal dynamics of high-intensity femtosecond laser pulses propagating in argon

The spatiotemporal dynamics of high-intensity femtosecond laser pulses propagating in argon have been investigated experimentally and the results compared with numerical calculations. After the beam expands and refocuses, the pulse splits in two. While the subcomponent expands immediately, the main component propagates as a filament. The pulse duration of the main component is less than half the initial pulse duration and the spectrum is broadened by a factor of two.

SATURATION OF NONLINEAR SUSCEPTIBILITY

The nonlinear susceptibility of atoms interacting with an intense laser field has been investigated using numerical solution of the time-dependent Schrodinger equation. It was found that the behavior of nonlinear susceptibility strongly depends on the intensity; it increases linearly in low intensity regime (Kerr nonlinearity), then saturates at higher intensity. Furthermore, it has also been found that the nonlinear susceptibility is substantially influenced by the onset of ionization.

Plasma-induced spectral broadening of high-energy ultrashort laser pulses in a helium-filled multiple-pass cell

We investigated the possibility of plasma-induced spectral broadening of high-energy ultrashort laser pulses in a helium-filled multipass cell (MPC) through a series of full numerical simulations of the extended nonlinear Schrodinger equation. It was found that the gas pressure must be set low so that the propagation dynamics can be controlled only by plasma defocusing. Simulations using 100 mJ, 40 fs laser pulses in the MPC, which is 6 m long and has a mirror of 3.1 m radius at each end, showed that if the gas pressure is set within the range of 40-130 Pa, then the relevant spectral broadening can be obtained after five passes, yielding compressed pulses of a 4.7-6.4 fs width. The ratio of the energy of the compressed pulse to the output pulse is found to be within 58-88%.

ENERGY TRANSMITTANCE AND SPATIAL PHASE IMPROVEMENT OF INTENSE ULTRASHORT LASER PULSES IN GAS-FILLED HOLLOW FIBER USING PRESSURE GRADIENT METHOD

A pressure gradient method for spectral broadening of intense-femtosecond laser pulses in gas-filled hollow fiber is proposed. The simulations using input energy of 6 mJ and pulse duration of 40 fs have shown that using the same value of ∫ p(x)dx, the energy transmittance can be enhanced by a factor of 25% compared to that of using constant gas pressure while the global spatial phase is also improved.