On the generation of polarization-dependent supercontinuum and third harmonic in air

Abstract

Filamentation and supercontinuum (SC) generation occur in transparent media during the propagation of intense femtosecond laser pulses. We report experimental results of polarization-dependent SC generation and third harmonic generation (THG) in air using intense 40 fs, 800 nm pulses under varying focusing conditions. We observe that tight focusing enhances the extent of the SC compared to when there is weak external focusing. Moreover, we observe that when the incident beam is linearly polarized the SC yield is more than that obtained using circularly polarized light of the same energy, but this difference reduces as focusing becomes tighter and depolarization begins to take effect. We have also carried out measurement for THG in air under the same conditions as for SC generation. A THG efficiency of 0.5% is measured for linearly polarized light in air. Although conservation of spin angular momentum precludes THG with circularly polarized light, we do observe THG with circularly polarized light in our experiments because of depolarization effects. We show that THG measurements allow in situ measurements of the extent to which incident light is depolarized.

Pulse repetition rate effect on the plasma inside femtosecond laser filament in air

“…The propagation of powerful femtosecond laser pulses in air is a very complicated phenomenon that involves a number of nonlinear processes, including four-and six-wave mixing [1,2] , odd harmonics [3,4] , self-focusing and filamentation [5] , self-phase modulation [6] , supercontinuum generation [7,8] , and so on. The self-focusing phenomenon during the nonlinear propagation leads to the ionization of air molecules.…”

Pulse repetition-rate effect on the intensity inside a femtosecond laser filament in air

“…The dynamic equilibrium between the Kerr self-focusing and plasma-defocusing effects leads to filamentation [1][2][3] . Due to its potential applications in supercontinuum emission [4][5][6] , extreme ultraviolet (EUV) emission [7,8] , air lasing [9,10] , THz radiation [11,12] , remote sensing [13] , guiding of corona discharges [14] , machining [15] and weather control [16] , filamentation of femtosecond laser pulses in optically transparent media has attracted a great deal of attention in recent years. One of the most profound effects during femtosecond filamentation is intensity clamping phenomena.…”

Loss of polarization of elliptically polarized collapsing beams

“…However, the change in the beams polarization itself as a result of its collapse remains largely unexplored. Since several applications of laser filamentation including HHG, THz generation and supercontinuum generation are polarization sensitive [38][39][40], investigating the polarization state of collapsing beams is crucial [41][42][43]. In some of the studies, molecular alignment and delayed birefringence acting on the probe were investigated [44][45][46].…”