Femtosecond Laser Filamentation

Peach Lu, Hung-Wei Sun

Selected Topics in Ultrafast Optics

Outline

Filamentation Physics Self-focusing and Plasma defocusing

Slice-by-Slice model

Intensity Clamping

Filamentation Evolution

Advanced Topics and Applications Multiple Filamentation

Nonlinear Interaction in Filaments

Self-Pulse Compression and Self-Spatial Mode Filtering

Other Applications

2

Motivation

Laboratoire d'Optique Applique, Palaiseau, France Photo taken by Prof. Chen on 2014/03/14

White light (Supercontinuum) was observed when a high intensity Ti-Sapphire laser was focused into a nonlinear crystal.

Filamentation has a strong potential to produce single isolated attosecond burst.

3

Self-focusing and Tunnel Ionization

Deutsches Elektronen-Synchrotron Chin, See Leang, Femtosecond Laser Filamentation

Self-focusing effect occurs when the peak power of the pulse is large enough to overcome the linear diffraction.

Compared to multiphoton ionization, tunnel ionization can be seen as quasi-static.

4

Slice-by-slice model

Filamentation is a series of plasma.

Can the self-focusing effect reach "infinity and beyond" ?

5

Plasma defocusing

A gas where a large fraction of the atoms is ionized.

The change of refractive index is 42 202 The phase velocity could be larger than speed of light.

6

Intensity Clamping Couairon and Mycyrowicz, Phys. Rep. 441, 47 (2007)

A larger peak intensity would increase the volume of the filament : the intensity is still clamped.

The clamped intensity in air is independent of pressure.

7

Filamentation Evolution

Chin, See Leang, Femtosecond Laser Filamentation Fraunhofer-Institut fr Optronik, Systemtechnik und Bildauswertung

The central/strongest slice self-focuses to the nearest point, while the front part keeps on self-focusing.

The back part encounters the plasma left behind by the front part, which giving rise to a very complex intensity distribution.

The whole process just repeats again and again and again......

8

Questions

1. Suppose we have two pulses with the same peak intensity, which one

is easier to self-focus and form a filament?

The ultrashort one (100 fs) ?

2. When does the filamentation come to an end?

3. A unlucky dust accidentally get in the path of the filament s core.

Will the subsequent filamentation be forced to terminate?

9

Outline

Filamentation Physics Self-focusing and Plasma defocusing

Slice-by-Slice model

Intensity Clamping

Filamentation Evolution

Advanced Topics and Applications Multiple Filamentation

Nonlinear Interaction in Filaments

Self-Pulse Compression and Self-Spatial Mode Filtering

Other Applications

10

Mulitiple Filamentation

Chin, See Leang, Femtosecond Laser Filamentation L. Berge,et. al, Rep. Prog. Phys. 70 (2007)

Spatial irregularity induces local self-focusing. Cooperation or competition between filaments? Multiple filamentation is the basic limitation for the long distance control.

Hard to generate strong filaments at distances larger than 100 meters.

11

Paper Review: Externally refuelled optical filaments

The background reservoir can provide energy to the filament core. The low intensity dress acts like an artificial photon bath. Filaments extended to 220 cm was demonstrated in their experiment.

Maik Scheller, et.al., Nature Photonics (2014)

12

SPM, Self-Steepening and Supercontinuum

Rothenberg, et. al, Opt. Lett (1992)

SPM induced by both the neutral gas and the generated plasma. Self-steepening due to the interaction between trailing edge and plasma. Focusing the 800 nm pump pulse at 1.4 TW (60 mJ/ 42 fs) in air

generating 200 nm 14 m supercontinuum was observed by Thberge,

et. al in 2008.

13

Self-Pulse Compression

The front part becomes narrower as the pulse propagates. The back part spreads out into the background reservoir. A single-cycle pulse can be achieved.

http://www.steingrube-home.de/science-daniel.html

14

Self-Spatial Mode Filtering

The lowest spatial mode would dominate. The higher order mode would self-focus later, encountering the plasma

generated by lowest one.

It also occurs during the nonlinear interaction in the filaments.

Chin, See Leang, Femtosecond Laser Filamentation Thberge, et. al, PRL 97 (2006)

15

Paper Review: Self-Compression by filamentation

A Mysyrowicz, et.al., New Journal of Physics 10 (2008)

16

Paper Review: Tunable Ultrashort Laser Generation

The duration of the 4WM pulse can be altered by lengthening the filament. The conversion efficiency can be as high as 60% in argon gas (1.5 atm). The mode quality and energy stability are all excellent because the 4WM

pulse is locked with the intensity-clamped and spatial self-filtered filament.

Thberge, et. al, PRL 97 (2006)

17

Comparison with the hollow fiber

Self-steepening + Third-order Harmonics : Shorter wavelengths. Higher throughput and less sensitive to experimental conditions. The ability of self-compression to single-cycle pulse.

L. Galmann, et. al, Appl. Phys. B 86 (2007)

18

Paper Review: Filamentation for Weather Control

Filamentation can induce condensation nuclei in air. The temperature gradient in the chamber was maintained using a cold

plate.

Jingjing Ju, et.al., PRE (2013)

19

Paper Review: Filaments for Atmospheric Analysis

J. Kasparian,et. al, Science 301 (2003)

20

Summary

21

Filamentation is the region where the intensity of the laser is

clamped owing to the equilibrium between self-focusing and plasma

defocusing effect.

The intensity clamping enables a stable and high conversion

efficiency nonlinear interaction.

The self-action of the filamentation, such as self-spatial mode filtering

and self-compression, makes itself a widely-used tool in many

applications.

How to optimize your system to generate a really long and stable

filamentation still remains a popular issue in this field.

Femtosecond Laser FilamentationOutlineMotivationSelf-focusing and Tunnel Ionization Slice-by-slice modelPlasma defocusing Intensity Clamping Filamentation EvolutionQuestionsOutlineMulitiple FilamentationPaper Review: Externally refuelled optical filamentsSPM, Self-Steepening and SupercontinuumSelf-Pulse CompressionSelf-Spatial Mode FilteringPaper Review: Self-Compression by filamentationPaper Review: Tunable Ultrashort Laser GenerationComparison with the hollow fiber Paper Review: Filamentation for Weather ControlPaper Review: Filaments for Atmospheric AnalysisSummary