Tightly focused femtosecond laser pulse in air: from filamentation to breakdown.

The propagation of tightly focused femtosecond laser pulse with numerical aperture of 0.12 in air is investigated experimentally. The formation and evolution of the filament bunch are recorded by time-resolved shadowgraph with laser energy from 2.4 mJ to 47 mJ. The distribution of electron density in breakdown area is retrieved using Nomarski interferometer. It is found that intensity clamping during filamentation effect still play a role even under strong external focusing. The electron density in some interaction zones is higher than 3 × 10(19) cm(-3), which indicates that each air molecule there is ionized.

Widening of Long-range femtosecond laser filaments in turbulent air.

The influence of air turbulence on the long-range filamentation of femtosecond laser pulses has been numerically investigated. Simulations are performed for different parameters of air turbulence and laser pulses. Simulation results indicate that the diameter of filaments formed by free propagated fs laser pulse can be widened to mm level under air turbulence. However, the widening effect can be suppressed if the propagating distance before the on-set position of filamentation becomes shorter. The reduction of non-linear focal length can be realized by pre-focusing of the laser pulse or increasing of the laser intensity. The effect of the inner scale of air turbulence on the filamentation has also been studied.

Spatiotemporal moving focus of long femtosecond-laser filaments in air.

The characteristics of filaments formed by femtosecond-laser pulses freely propagating in air are different from those of filaments generated with a focal lens. A scheme combining (2D+1) modeling of the nonlinear Schrödinger equation and ray-tracing method is proposed to provide a fast estimate of the long-range filamentation process in a single-filament regime. A filament with a length of more than 100m is formed by a 10-mJ , negative chirped 350-fs laser pulse freely propagating in air. A ray-tracing calculation based on the refractive index field obtained from the nonlinear Schrödinger simulation shows that, in the 100-m propagation range, the main mechanism of filamentation is the spatiotemporal moving focus induced by the initial distribution of the laser intensity. The analysis of ray trajectories suggests that the energy exchange between background and filament core due to refocusing of light rays can be induced by Kerr self-focusing without the help of the ionization effect. The plasma defocusing can be observed only at a very short distance on the propagation track, and it prevents the collapse of the laser field.

Aspirin resistance predicts unfavorable functional outcome in acute ischemic stroke patients.

OBJECTIVE: To investigate the prognostic value of aspirin reaction units (ARU) in a 3-month follow-up study in a cohort of Chinese patients with first-ever ischemic stroke. METHODS: Prospective single-center survey of acute ischemic stroke patients receiving aspirin therapy. Two hundred and seventy-five Chinese patients with first-ever ischemic stroke who previously received aspirin therapy were enrolled. ARU was measured using the VerifyNow system. A cutoff of 550 ARU was used to determine the presence of aspirin resistance (AR). RESULTS: Median age at study entry was 67 years (IQR: 59-75) and 142(51.6%) were male. A total of 52 of 275 enrolled patients (18.9%) were AR. Median regression estimated a statistically significant increase in NIHSS score of 0.033 point for every 1-point increase in ARU (95% CI, 0.024 to 0.068; P < 0.001). The unfavorable outcomes distribution across the ARU quartiles ranged between 11.8% (first quartile) to 64.8% (fourth quartile). After adjusting for other established risk factors, in multivariate models comparing the third and fourth quartiles against the first quartile of the ARU, levels of ARU were associated with unfavorable outcome, and the adjusted risk of unfavorable outcome increased by 145% (OR = 2.45 [95% CI 1.46-3.87], P = 0.011) and 317% (4.17[2.76-6.15], P < 0.001), respectively. Similarly, the adjusted risk of mortality increased by 215% (OR = 3.15 [95% CI 1.98-4.73], P = 0.008) and 429% (5.29[4.02-8.17], P < 0.001), respectively. CONCLUSIONS: The results suggest that AR is a meaningful and independent marker to predict short-term functional outcome in patients with ischemic stroke.

Optimization of multiple filamentation of femtosecond laser pulses in air using a pinhole.

The robustness and prolongation of multiple filamentation (MF) for femtosecond laser propagation in air are investigated experimentally and numerically. It is shown that the number, pattern, propagation distance, and spatial stability of MF can be controlled by a variable-aperture on-axis pinhole. The random MF pattern can be optimized to a deterministic pattern. In our numerical simulations, we configured double filaments to principlly simulate the experimental MF interactions. It is experimentally and numerically demonstrated that the pinhole can reduce the modulational instability of MF and is favorable for a more stable MF evolution.

Spatial evolution of multiple filaments in air induced by femtosecond laser pulses.

The spatial evolution of plasma filaments in air induced by femtosecond laser pulses is investigated experimentally. Several major filaments and small scaled additional filaments are detected in the plasma channel. The complicated interaction process of filaments as splitting, fusion and spreading is observed. The major filaments propagate stably, and the small scaled additional filaments can be attracted to the major filaments and merged with them. The major filaments are formed due to the perturbation of initial beam profile and the small scaled filaments are mainly caused by the transverse modulational instability.

Interaction of light filaments generated by femtosecond laser pulses in air.

The interaction of two light filaments propagating in air is simulated. Simulations show that the interaction of the two light filaments displays interesting features such as attraction, fusion, repulsion, and spiral propagation, depending on the relative phase shift and the crossing angle between them. A long and stable channel can be formed by fusing two in-phase light filaments. The channel becomes unstable with the increase of the crossing angle and phase shift. The interaction of two light filaments in different planes is studied and the spiral propagation is observed.