0.35%THz pulse conversion efficiency achieved by Ti:sapphire femtosecond laser filamentation in argon at 1 kHz repetition rate

In this study,an optical setup for generating terahertz(THz)pulses through a two-color femtosecond laser filament was carefully designed to achieve a precise overlap of two-color laser pulses in space and time.β-barium borate(BBO),α-BBO,and a dual-wavelength plate were used to compensate the phase delay of the two-color lasers.Tilting ofα-BBO could further realize the precise spatial overlap of the two beams by counteracting the walk-off effect.The maximum out-put THz pulse energy reached 21μJ in argon gas when using a commercial Ti:sapphire laser with a pulse energy of 6 mJ at a 1 kHz repetition rate.The corresponding conversion efficiency exceeded 0.35%.

External focusing dependence of spatial distribution of air lasers during femtosecond laser filamentation in air

The spatial distribution of the forward-propagating amplified spontaneous emission(ASE) of nitrogen molecular ions during femtosecond laser filamentation in air is studied via numerical simulations. The results suggest that the divergence angle and signal intensity are extremely sensitive to the external focal length. Concurrently, we show that the optical Kerr effect plays a significant role in concentrating the directivity of ASE signals, particularly in cases of loose focusing. Furthermore,the simulations demonstrate that ASE signals are enhanced for a tight focus, although the corresponding filament length is shorter. The main physical mechanism underlying this process is the competition between the plasma defocusing and optical Kerr effects. The result is important for filamentation-based light detection and ranging applied to remote sensing.

Femtosecond laser filamentation in simulated atmospheric turbulence[Invited]

The effects of turbulence intensity and turbulence region on the distribution of femtosecond laser filaments are experimentally elaborated.Through the ultrasonic signals emitted by the filaments,it is observed that increasing turbulence intensity and an expanding turbulence active region cause an increase in the start position of the filament and a decrease in filament length,which can be well explained by theoretical calculation.It is also observed that the random perturbation of the air refractive index caused by atmospheric turbulence expands the spot size of the filament.Additionally,when the turbulence refractive index structure constant reaches 8.37×10^(-12)m^(-2/3),multiple filaments are formed.Furthermore,the standard deviation of the transverse displacement of filament is found to be proportional to the square root of the turbulent structure constant under the experimental turbulence parameters in this paper.These results contribute to the study of femtosecond laser propagation mechanisms in complex atmospheric turbulence conditions.

Rapid diagnosis of femtosecond laser filament by single laser shot-induced acoustic pulses[Invited]

Due to the promising applications of femtosecond laser filamentation in remote sensing,great demands exist for diagnosing the spatiotemporal dynamics of filamentation.However,until now,the rapid and accurate diagnosis of a femtosecond laser filament remains a severe challenge.Here,a novel filament diagnosing method is proposed,which can measure the longitudinal spatial distribution of the filament by a single laser shot-induced acoustic pulse.The dependences of the point-like plasma acoustic emission on the detection distance and angle are obtained experimentally.The results indicate that the temporal profile of the acoustic wave is independent of the detection distance and detection angle.Using the measured relation among the acoustic emission and the detection distance and angle,a single measurement of the acoustic emission generated by a single laser pulse can diagnose the spatial distribution of the laser filament through the Wiener filter deconvolution(WFD)algorithm.The results obtained by this method are in good agreement with those of traditional point-by-point acoustic diagnosis methods.These findings provide a new solution and idea for the rapid diagnosis of filament,thereby laying a firm foundation for femtosecond laser filament-based promising applications.

Remote sensing of trace Na element in sea salt aerosol with a sensitivity level of 10 pg/m^(3)[Invited]

We have developed a remote sea salt aerosol fluorescence spectroscopy system integrating a high-power industrial-grade femtosecond laser to enhance detection sensitivity and precision in complex environments.This system successfully detects sea salt aerosol particles,achieving a detection limit of 0.015 ng/m^(3) for neutral Na element(Na I)at 589 nm,with a detection range of 30 m.Our findings demonstrate significant improvements in remote aerosol monitoring,addressing previous challenges in long-range and high-precision sensing with a detection accuracy previously unattainable below 10 ng/m^(3).

Nanotip design for high-resolution terahertz scattering-type scanning near-field optical microscopy

Terahertz(THz)scattering-type scanning near-field optical microscopy(s-SNOM)is an important means of studying and revealing material properties at the nanoscale.The nanotip is one of the core components of THz s-SNOM,which has a decisive impact on the resolution of the system.In this paper,we focus on the theory and design of the nanotip and conduct comprehensive research on it through simulation.The theoretical model is based on full-wave numerical simulation and dipole moment analysis,which can describe the overall nanotip electromagnetic response under the incident field.A comprehensive design model of nanotip geometry,sample materials,and incident field is established to significantly improve the near-field coupling efficiency and spatial resolution to achieve optimal performance.