Energy deposition from focused terawatt laser pulses in air undergoing multifilamentation

Laser filamentation is responsible for the deposition of a significant part of the laser pulse energy in the propagation medium. We found that using terawatt laser pulses and relatively tight focusing conditions in air, resulting in a bundle of co-propagating multifilaments, more than 60 % of the pulses energy is transferred to the medium, eventually degrading into heat. This results in a strong hydrodynamic reaction of air with the generation of shock waves and associated underdense channels for each short-scale filament. In the focal zone, where filaments are close to each other, these discrete channels eventually merge to form a single cylindrical low-density tube over a $\sim 1 \mu\mathrm{s}$ timescale. We measured the maximum lineic deposited energy to be more than 1 J/m.Comment: 7 pages, 7 figure

Study of ortho-to-paraexciton conversion in Cu2_2O by excitonic Lyman spectroscopy

Using time-resolved $1s$-$2p$ excitonic Lyman spectroscopy, we study the orthoexciton-to-paraexcitons transfer, following the creation of a high density population of ultracold $1s$ orthoexcitons by resonant two-photon excitation with femtosecond pulses. An observed fast exciton-density dependent conversion rate is attributed to spin exchange between pairs of orthoexcitons. Implication of these results on the feasibility of BEC of paraexcitons in Cu$_2$O is discussed

Self-compression of optical laser pulses by filamentation

International audienceDuring the propagation of intense femtosecond laser pulses in a transparent medium, pulse shortening can occur without external guiding. Experimental evidence for this effect and a description of its physical origin are presented. Nearly single cycle pulses at 800 nm with an energy of 0.120 mJ can be obtained with excellent beam quality. Carrier envelope offset phase (CEP) stability is conserved or even improved after the nonlinear propagation stage. Prospects for further improvement are discussed

Unusual stability of a one-parameter family of dissipative solitons due to spectral filtering and nonlinearity saturation

International audienceThe stability of a one-parameter family of dissipative solitons seen in the cubic-quintic complex Ginzburg-Landau equation is studied. It is found that an unusually strong stability occurs for solitons controlled by the spectral filtering and nonlinearity saturation simultaneously, consistently with the linear stability analysis and confirmed by large-perturbation numerical simulations. Two universal types of bifurcations in the spectrum structure are demonstrated

Fine control of terahertz radiation from filamentation by molecular lensing in air

International audienceWe demonstrate a method to control remotely the terahertz (THz) source in air based on the bifilamentation of femtosecond laser pulses. By fine tuning the time delay between the two pulses, a significant modulation of the THz intensity from bifilamentation is observed. The phenomenon is attributed to the molecule quantum lensing effect around the air molecule revival time, which changes the separation between the two neighboring plasma producing filaments

Polarization analysis of terahertz radiation generated by four-wave mixing in air

International audienceWe examine the generation of terahertz by optical rectification of fundamental infrared beam with its first harmonic in ionized air. From polarization measurements we identify an important, yet so far unreported cross term (X(3)xyxy + X(3)xyyx) of the nonlinear susceptibility tensor. Omission of this term leads to an error by a factor 10^6 of the THz intensity obtained with certain polarization configurations

Calorimetric detection of the conical THz radiation from femtosecond laser filaments in air

International audienceThe spectral distribution of the conical terahertz (THz) emission from a femtosecond laser filament in air is measured with a bolometric detector and a set of filters, confirming that the main part of the emission lies between 0.5 and 3 THz. The efficiency of this THz emission is compared with that obtained in air via Four Waves Mixing of femtosecond laser pulses at ω and 2 ω in the presence of a plasma

Auger decay, Spin-exchange, and their connection to Bose-Einstein condensation of excitons in Cu_2O

In view of the recent experiments of O'Hara, et al. on excitons in Cu_2O, we examine the interconversion between the angular-momentum triplet-state excitons and the angular-momentum singlet-state excitons by a spin-exchange process which has been overlooked in the past. We estimate the rate of this particle-conserving mechanism and find a substantially higher value than the Auger process considered so far. Based on this idea, we give a possible explanation of the recent experimental observations, and make certain predictions, with the most important being that the singlet-state excitons in Cu_2O is a very serious candidate for exhibiting the phenomenon of Bose-Einstein condensation.Comment: 4 pages, RevTex, 1 ps figur