Stable,intense supercontinuum light generation at 1 kHz by electric field assisted femtosecond laser filamentation in air

Supercontinuum(SC)light source has advanced ultrafast laser spectroscopy in condensed matter science,biology,physics,and chemistry.Compared to the frequently used photonic crystal fibers and bulk materials,femtosecond laser filamentation in gases is damage-immune for supercontinuum generation.A bottleneck problem is the strong jitters from filament induced self-heating at kHz repetition rate level.We demonstrated stable kHz supercontinuum generation directly in air with multiple mJ level pulse energy.This was achieved by applying an external DC electric field to the air plasma filament.Beam pointing jitters of the 1 kHz air filament induced SC light were reduced by more than 2 fold.The stabilized high repetition rate laser filament offers the opportunity for stable intense SC generation and its applications in air.

The 1 PW/0.1 Hz laser beamline in SULF facility

In this paper,we report the recent progress on the 1 PW/0.1 Hz laser beamline of Shanghai Superintense Ultrafast Laser Facility(SULF).The SULF-1 PW laser beamline is based on the double chirped pulse amplification(CPA)scheme,which can generate laser pulses of 50.8 J at 0.1 Hz after the final amplifier;the shot-to-shot energy fluctuation of the amplified pulse is as low as 1.2%(std).After compression,the pulse duration of 29.6 fs is achieved,which can support a maximal peak power of 1 PW.The contrast ratio at-80 ps before main pulse is measured to be 2.5×10^-11.The focused peak intensity is improved by optimizing the angular dispersion in the grating compressor.The maximal focused peak intensity can reach 2.7×10^19W/cm2 even with an f/26.5 off-axis parabolic mirror.The horizontal and vertical angular pointing fluctuations in 1 h are measured to be 1.89 and 2.45μrad,respectively.The moderate repetition rate and the good stability are desirable characteristics for lasermatter interactions.The SULF-1 PW laser beamline is now in the phase of commissioning,and preliminary experiments of particle acceleration and secondary radiation under 300–400 TW/0.1 Hz laser condition have been implemented.The progress on the experiments and the daily stable operation of the laser demonstrate the availability of the SULF-1 PW beamline.

High-Sensitivity Gas Detection with Air-Lasing-Assisted Coherent Raman Spectroscopy

Remote or standoff detection of greenhouse gases,air pollutants,and biological agents with innovative ultrafast laser technology attracts growing interests in recent years.Hybrid femtosecond/picosecond coherent Raman spectroscopy is considered as one of the most versatile techniques due to its great advantages in terms of detection sensitivity and chemical specificity.However,the simultaneous requirement for the femtosecond pump and the picosecond probe increases the complexity of optical system.Herein,we demonstrate that air lasing naturally created inside a filament can serve as an ideal light source to probe Raman coherence excited by the femtosecond pump,producing coherent Raman signal with molecular vibrational signatures.The combination of pulse self-compression effect and air lasing action during filamentation improves Raman excitation efficiency and greatly simplifies the experimental setup.The air-lasing-assisted Raman spectroscopy was applied to quantitatively detect greenhouse gases mixed in air,and it was found that the minimum detectable concentrations of CO_(2) and SF_(6) can reach 0.1%and 0.03%,respectively.The ingenious designs,especially the optimization of pump-seed delay and the choice of perpendicular polarization,ensure a high detection sensitivity and signal stability.Moreover,it is demonstrated that this method can be used for simultaneously measuring CO_(2) and SF_(6) gases and distinguishing ^(12)CO_(2) and ^(13)CO_(2).The developed scheme provides a new route for high-sensitivity standoff detection and combustion diagnosis.

Multiple-fringe structure of attosecond transient absorption spectrum driven by mid-infrared laser

We theoretically investigate the delay-dependent attosecond transient absorption spectra in the helium atom dressed by an infrared laser pulse in the wavelength range of 800–2400 nm. By numerically solving the three-dimensional time-dependent Schrdinger equation, we find that the absorption spectrogram exhibits a multiple-fringe structure for using the mid-infrared dressing pulse. The quantitative calculation of the transition matrix between different Floquet states provides direct evidence on the origin of the multiple-fringe structure.Our result shows that the wavelength of the dressing pulse is an important parameter and the unique feature of attosecond transient absorption spectroscopy can be induced in the mid-infrared regime.

Editorial

It is accidental, but interesting to celebrate the i0th anniversary of Chinese Optics Letters （COL） after revival from the ＂most important＂ day in 2012, December 21, which is closely related to Mayan calendar. The last decade sees the development of optics in China and around the world, and Nobel Prize awarded to scientists for their excellent work in fiber optics, quantum optics, etc. Up to now, most research results are still published in journals, and COL has been trying to publish the most excellent papers for authors and readers.

Monoenergetic proton beam accelerated by single reflection mechanism only during hole-boring stage

Multidimensional instabilities always develop with time during the process of radiation pressure acceleration,and are detrimental to the generation of monoenergetic proton beams.In this paper,a sharp-front laser is proposed to irradiate a triple-layer target(the proton layer is set between two carbon ion layers)and studied in theory and simulations.It is found that the thin proton layer can be accelerated once to hundreds of MeV with monoenergetic spectra only during the hole-boring(HB)stage.The carbon ions move behind the proton layer in the light-sail(LS)stage,which can shield any further interaction between the rear part of the laser and the proton layer.In this way,proton beam instabilities can be reduced to a certain extent during the entire acceleration process.It is hoped such a mechanism can provide a feasible way to improve the beam quality for proton therapy and other applications.

Inertial confinement fusion driven by long wavelength electromagnetic pulses

A method for inertial confinement fusion driven by powerful long wavelength electromagnetic pulses(EMPs), such as CO2 laser pulses or high power microwave pulses, is proposed. Due to the high efficiency of generating such long wavelength electromagnetic pulses, this method is especially important for the future fusion electricity power. Special fuel targets are designed to overcome the shortcomings of the long wavelength electromagnetic pulses.