Rihong Li

Advances in intense femtosecond laser filamentation in air

This is a review of some recent development in femtosecond filamentation science with emphasis on our collective work. Previously reviewed work in the field will not be discussed. We thus start with a very brief description of the fundamental physics of single filamentation of powerful femtosecond laser pulses in air. Intensity clamping is emphasized. One consequence is that the peak intensity inside one or more filaments would not increase significantly even if one focuses the pulse at very high peak power even up to the peta-watt level. Another is that the clamped intensity is independent of pressure. One interesting outcome of the high intensity inside a filament is filament fusion which comes from the nonlinear change of index of refraction inside the filament leading to cross beam focusing. Because of the high intensity inside the filament, one can envisage nonlinear phenomena taking place inside a filament such as a new type of Raman red shift and the generation of very broad band supercontinuum into the infrared through four-wave-mixing. This is what we call by filamentation nonlinear optics. It includes also terahertz generation from inside the filament. The latter is discussed separately because of its special importance to those working in the field of safety and security in recent years. When the filamenting pulse is linearly polarized, the isotropic nature of air becomes birefringent both electronically (instantaneous) and through molecular wave packet rotation and revival (delayed). Such birefringence is discussed in detailed. Because, in principle, a filament can be projected to a long distance in air, applications to pollution measurement as well as other atmospheric science could be earned out. We call this filamentation atmospheric science. Thus, the following subjects are discussed briefly, namely, lightning control, rain making, remote measurement of electric field, microwave guidance and remote sensing of pollutants. A discussion on the higher order Kerr effect on the physics of filamentation is also given. This is a new hot subject of current debate. This review ends on giving our view of the prospect of progress of this field of filamentation in the future. We believe it hinges upon the development of the laser technology based upon the physical understanding of filamentation and on the reduction in price of the laser system.

Can we reach very high intensity in air with femtosecond PW laser pulses

In the course of femtosecond pulse filamentation in atmospheric density gases, the peak intensity is always limited by optical-field-induced ionization. This intensity clamping phenomenon is universal in all the cases we studied, namely, single and multiple filament regimes with and without external focusing using pulses of up to subpetawatt level. Even in the tight focusing cases, the clamped intensity along the propagation direction does not exceed 30% of the global intensity maximum. The remarkable shot-to-shot stability of the clamped intensity (better than 1% of the maximum value) is revealed both experimentally and numerically in a single filament regime in air.

Intense photoluminescence at 2.7 μm in transparent Er 3+ :CaF 2 -fluorophosphate glass microcomposite

Er3+ doped CaF2-fluorophosphate (FP) glass microcomposites were produced by heat-treating the mixture of Er3+:CaF2 precipitate and FP glass powder above the melting temperature of the FP glass. The appearance of CaF2 crystallites in the resulting composites was confirmed by x ray diffraction. Despite the fact that the average diameter of the crystallites was around 10 μm as revealed by the micromorphology study, a transparent composite was obtained by matching the refractive index of FP glass to that of CaF2. Intense IR fluorescence at around 2.7 μm was observed in the composite, implying the composite would be a promising candidate for IR lasers and amplifiers.

Nonlinear Optical Properties of Graphene and Carbon Nanotube Composites

Jun Wang1a, Yu Chen1b, Rihong Li1a, Hongxing Dong1a, Long Zhang1a, Mustafa Lotya2, Jonathan N. Coleman2 and Werner J. Blau2 1aKey Laboratory of Materials for High-Power Laser, Shanghai Institute of Optics and Fine Mechanics, Chinese Academy of Sciences 1bKey Laboratory for Advanced Materials, Department of Chemistry, East China University of Science and Technology 2School of Physics and the Centre for Research on Adaptive Nanostructures and Nanodevices (CRANN), Trinity College Dublin 1China 2Ireland

Femtosecond laser filamentation with a 4 J/60 fs Ti:Sapphire laser beam: Multiple filaments and intensity clamping

We used both the backscattered nitrogen fluorescence signal (BSF) and ICCD fluorescence side imaging methods to study the filament light intensity with a 4 J/60 fs Ti:sapphire laser beam. It has been concluded that even the laser power is increased by 100 times in our experiment the peak laser intensity in the filament only has a little change. We attribute this phenomenon to the result of intensity clamping and the competition of multiple filaments.

Transverse evolution of a plasma channel in air induced by a femtosecond laser.

We investigate the evolution of filamentation in air by using a longitudinal diffraction method and a plasma fluorescence imaging technique. The diameter of a single filament in which the intensity is clamped increases as the energy of the pump light pulse increases, until multiple filaments appear.

Transient-grating self-referenced spectral interferometry for infrared femtosecond pulse characterization

We propose a technique for measuring infrared femtosecond pulses: transient-grating self-referenced spectral interferometry. Based on this technique, we built an extremely simple, alignment-free device and successfully characterized both 38 fs pulses at 800 nm and sub-two-cycle 10 fs pulses at 1.75 μm.

Control of electron-seeding phase in a cascaded laser wakefield accelerator

Two segments of plasmas with different densities, which are operated as the electron injector and accelerator, respectively, are designed to realize a cascaded laser wakefield accelerator. Particle-in-cell simulations indicate that the further acceleration of the electrons in the second uniform-density plasma relies on the injection and acceleration in the first stage. It is found that electrons trapped in the second wakefield period in the first stage can be seeded into the next stage with an optimized phase for efficient acceleration and reducing in the relative energy spread. And finally a 700 MeV electron beam with a relative rms energy spread about 0.6% and the normalized transverse emittance of 1.4π mm mrad was obtained after a 5.5-mm-long acceleration in a dark-current free cascaded laser wakefield accelerator. Our results demonstrate that, for a given laser energy, choices in laser and plasma parameters strongly affect the output electron beam energy and quality, and that all of these parameters c...

Tunable and white light emitting AlPO4 mesoporous glass by design of inorganic/organic luminescent species

The realization of tunable and white light emitting sources employed by UV-LED with single-host phosphors has been an exciting development in the search for high luminous efficiency and excellent color rendering index white-light source. A tunable and white light emitting mesoporous glass was prepared by utilizing both inorganic/organic (Europium/coumarin) luminescent species in the meso-structure. The tunable and white light emission was deliberately designed by CIE calculation based on the individual emission spectra, which was realized by tailoring the emission of Eu2+/Eu3+ ions and coumarin 535 in sol-gel AlPO4 mesoporous glass. This simple and versatile procedure is not limited in the combination of rare earth and organic dye and is therefore extendable to other luminescent species in meso-structure for color-tunable efficient solid-state lighting sources.

Laser-filament-induced condensation in an inverted cloud chamber

We have demonstrated experimentally that firing a laser filament in a subsaturated zone of a cloud chamber of inversed temperature profile could induce water condensation. Initially, through ion chromatography performed on samples collected from a receptacle placed under the laser, the presence of HNO3 was observed. The existence of this HNO3, as well as digital camera images of very tiny droplets close to the laser axis viewed at an angle perpendicular to the chamber, suggest that the condensation could be due to the laser. A second experiment measuring the growth of the water droplets using a cooled receptacle confirmed the existence of laser-induced condensation in these conditions.