Supercontinuum generation by co-filamentation of two color femtosecond laser pulses

M Vengris,A Varanavičius,A Čepėnas,G Tamošauskas,V Jukna,L Rimkus,N Garejev,A Dubietis

doi:10.1038/s41598-019-45357-y

Abstract

In this paper, we experimentally investigate supercontinuum generation via collinear two-color filamentation in sapphire crystal, by launching two femtosecond pulses at fundamental (1030 nm) and second harmonic (515 nm) wavelengths from an amplified Yb:KGW laser. By changing the time delay between the incident pulses, we observe dramatic changes in the supercontinuum spectrum, transmitted energy, position of the nonlinear focus and intensity distribution along the filamentinduced luminescence traces. In particular, we show that at some delays the two pump wavelengths can assist each other in generating supercontinuum, whilst at other delays large portions of the supercontinuum spectrum are completely extinguished. The transition between supercontinuum generation and its extinction occurs within a very short (20 fs) span of the delay times, despite the fact that the pump pulses are 220 fs long. We propose that the observed non-trivial spectral dynamics can be interpreted by a mechanism, where co-propagating two pump pulses perturb the nonlinear refractive properties of the medium via Kerr effect and generation of free electron plasma thereby affecting pulse splitting and pulse front steepening, which are the key players in the process of supercontinuum generation in a normally dispersive medium.

Highlights

Filamentation of intense femtosecond laser pulses in transparent dielectric media is a universal nonlinear propagation regime, which gives rise to large scale spectral broadening, termed supercontinuum (SC) generation[1]
In this paper we experimentally investigate SC generation via collinear two-color filamentation in sapphire crystal, by launching two femtosecond pulses at fundamental (1030 nm) and second harmonic (515 nm) wavelengths from an amplified Yb:KGW laser system
To summarize the above observations, we demonstrated that co-filamentation of two color femtosecond laser pulses produces a non-trivial nonlinear interaction in the medium

Summary

Introduction

Filamentation of intense femtosecond laser pulses in transparent dielectric media is a universal nonlinear propagation regime, which gives rise to large scale spectral broadening, termed supercontinuum (SC) generation[1]. Owing to its high spatial and temporal coherence, SC is widely used as a seed source in optical parametric amplifiers[3], and as a probe or even pump light in a variety of time-resolved spectroscopic experiments, see e.g.4–6 For such applications, it is important that SC covers as broad spectral range as possible, which becomes especially problematic in the UV, as due to fundamental limitations imposed by the ratio between the bandgap and the energy of incident photon[7], and the chromatic dispersion of nonlinear medium[8]. Even more possibilities are offered by the so-called two-color filamentation, i.e. the nonlinear propagation regime when two ultrashort laser pulses with different carrier wavelengths are launched into the nonlinear medium at the same time This specific filamentation regime was widely studied in gases, such as air, argon, neon and nitrogen. We demonstrate that the SC generation and control of its spectral coverage to a certain degree is possible even in the case when the energies of both interacting pulses are set below the filamentation and SC generation threshold

Methods

Results

Discussion

Conclusion