GRK-1203 Workshop Oelde Watching a laser pulse at work - Laser-plasma interaction seen on ultra-short timescales - Ralph Jung - Institut für Laser und Plasmaphysik -

What kind of work can a laser pulse do? Heat Zum Vergleich: 2x10^21 W/cm^2 entspricht… Ionize Push Illuminate

The potentially most popular examples for high-intensity laser-plasma interaction Zum Vergleich: 2x10^21 W/cm^2 entspricht…

Summary Watching a laser pulse at work - Laser-plasma interaction seen on ultra-short timescales - Task: Observe optically the breakdown of the the material caused by an ultra-short laser pulse. Zum Vergleich: 2x10^21 W/cm^2 entspricht… What happens there (ionization) ? How can you observe the ionization front (in principle) ? How looks a typical pump-probe experiment (practically) ? Data: High-resolution photos of the interaction and comparison with 3D-PIC simulations.

What happens there? - A typical laser-plasma experiment - Laser as a „black box“ Focusing optics Target volume Zum Vergleich: 2x10^21 W/cm^2 entspricht…, Limit im Verstärker bis Selbstfokussierung einsetzt: einige GW/cm^2, proportional zur Intensität, nicht zur Fluenz (Energie), typische Streckung 10.000x, sweep = linearer Frequenzverlauf, vershciedene Frequenzen erscheinen zu unterschiedlichen Zeiten, das Gitter dispergiert das Spektrum und streckt den Puls dadurch zeitlich Wavelength: l Pulse energy: E Pulse duration: Dt Focal spot diameter: typically 5-10 mm (FWHM)

Modern ultra-short pulse lasers are based on the CPA-technique ~ nJ ~ nJ Zum Vergleich: 2x10^21 W/cm^2 entspricht…, Limit im Verstärker bis Selbstfokussierung einsetzt: einige GW/cm^2, proportional zur Intensität, nicht zur Fluenz (Energie), typische Streckung 10.000x, sweep = linearer Frequenzverlauf, vershciedene Frequenzen erscheinen zu unterschiedlichen Zeiten, das Gitter dispergiert das Spektrum und streckt den Puls dadurch zeitlich Amplification:106-1012 Verstärkung: 106-1012 Few-Cycle Pulse 5 fs, 100 Gigawatt Sub-10-fs-Pulse 5 fs, 100 Gigawatt ~ mJ ~ mJ „Chirped Pulse Amplification“ [Strickland and Mourou, 1985]

Modern ultra-short pulse lasers are based on the CPA-technique ~ nJ ~ nJ Zum Vergleich: 2x10^21 W/cm^2 entspricht… Amplification:106-1012 Verstärkung: 106-1012 Ultra-high power laser 500 fs, 1 Petawatt Sub-10-fs-Pulse 5 fs, 100 Gigawatt ~ kJ ~ kJ „Chirped Pulse Amplification“ [Strickland and Mourou, 1985]

What happens there? - A typical laser-plasma experiment - Laser as a „black box“ Focusing optics Target volume Zum Vergleich: 2x10^21 W/cm^2 entspricht…, Limit im Verstärker bis Selbstfokussierung einsetzt: einige GW/cm^2, proportional zur Intensität, nicht zur Fluenz (Energie), typische Streckung 10.000x, sweep = linearer Frequenzverlauf, vershciedene Frequenzen erscheinen zu unterschiedlichen Zeiten, das Gitter dispergiert das Spektrum und streckt den Puls dadurch zeitlich

Ionization mechanisms I = 0 W/cm2 E = 0 V/m Example: H-Atom Bohr radius: The phase of the electric field of the pulse after the interaction with the gas was measured with a SPIDER. Electric field: Light intensity: Electron undisturbed 1965 Rubi Laser: Ionization !

Ionization mechanisms I = 1.1012 W/cm2 E = 2.7.109 V/m Example: H-Atom Bohr radius: Perturbation theory Electric field: Light intensity: Multi-Photon Ionization 1965 Rubi Laser: Ionization !

Ionization mechanisms I > 1.1012 W/cm2 E > 2.7.109 V/m Example: H-Atom Bohr radius: Also: I < 10^13: Perturbation theory, Hier: Löse zeitabhängige Schrödinger Gleichung in Single-Active-Electron Approximation (SAE), problematisch, da Wellenpaket wegläuft (continuum-continuum-transitions) bei höher werdenden Intensitäten. The multiphoton ionization (MPI) of noble gases by strong laser fields can be well described by time dependent quantum mechanical calculations within the single active electron approximation (SAE). -> Sequential ionization Electric field: Light intensity: Above Threshold Ionization 1965 Rubi Laser: Ionization !

Ionization mechanisms I = 1.1014 W/cm2 E = 2.7.1010 V/m Example: H-Atom Bohr radius: Electric field: Light intensity: Tunneling Ionization

Ionization mechanisms I = 3.45.1014 W/cm2 E = 5.1.1011 V/m Example: H-Atom Bohr radius: The phase of the electric field of the pulse after the interaction with the gas was measured with a SPIDER. Electric field: Light intensity: Barrier Suppression Ionization

Ionization mechanisms The phase of the electric field of the pulse after the interaction with the gas was measured with a SPIDER.

Ionization mechanisms Barrier Suppression Ionization The phase of the electric field of the pulse after the interaction with the gas was measured with a SPIDER. Augst et. al. PRL 63 (20), 2212 (1989)

Ionization mechanisms Keldysh ionization rates (1965), extended form by Ammosov, Delone an Krainov (1986) ADK-Theory For hydrogen like atoms, Influence of pulse duration: Pulse duration may be relevant ! Take also level depletion into account Nagaya et. al. J. Phys. Soc. of Japan, 75 (2006) 044302

What happens there? - A typical laser-plasma experiment - Laser as a „black box“ Focusing optics Target volume Zum Vergleich: 2x10^21 W/cm^2 entspricht…, Limit im Verstärker bis Selbstfokussierung einsetzt: einige GW/cm^2, proportional zur Intensität, nicht zur Fluenz (Energie), typische Streckung 10.000x, sweep = linearer Frequenzverlauf, vershciedene Frequenzen erscheinen zu unterschiedlichen Zeiten, das Gitter dispergiert das Spektrum und streckt den Puls dadurch zeitlich e- + + e- e- +

Optical properties of the plasma channel created Way of a beam path through the plasma: Zum Vergleich: 2x10^21 W/cm^2 entspricht… s

Hence: Diagnose the plasma with a second, well timed laser pulse Typical setup of a „pump-probe“ experiment Zum Vergleich: 2x10^21 W/cm^2 entspricht… Image the plasma channel onto a camera Optical probing : shadowgraphy, interferometry, polarimetry etc. Temporal resolution given by probe pulse duration

The sub-10-fs laser system used here Central wavelength: l0 = 800 nm Pulse duration (fwhm): t = 8 fs Pulse energy: E = 150 mJ Intensity in focus: I = 1-2 . 1016 W/cm2 + Hollow-fibre + compressor Pulse consists only of few optical cycles 3.5.1011 V/m Electric field Pre-pulse level so low that no measureable pre-plasma produced 3 mm High ADK-ionization rates expected Time or space

Gastarget The pulse ionizes the gas and an optically transparent plasma is produced x x Pulsed gas jet: Supersonic expansion, Mach 3.3 with up to 3.1020 particles/cm3 (~10 bar) Laser pulse z z 10 fs are a slap of light of just 3 microns in length Focus diamater about 5 microns Gases: Helium, Neon, Argon, Nitrogen

Geometrie des „pump-probe“ Experiments Geometry of „pump-probe“ experiment x (Kalorimetrie/ Spektroskopie) Pulsed gas jet: Supersonic expansion, Mach 3.3 with up to 3.1020 particles/cm3 (~10 bar) „Pump“- pulse z y Gases: Helium, Neon, Argon, Nitrogen „Probe“- pulse (opt. imaging / shadowgrams / interferograms)

Focused shadowgraphy Explain focused shadowgraphy

Focused shadowgraphy

Focused shadowgraphy

Focused shadowgraphy

Focused shadowgraphy

Focused shadowgraphy

A „Pump-Probe“- experiment using an ultra-short optical probe pulse BBO 40 microns in thickness, 10 x microscope objective, High magnification 44 x, High resolution of better than 1 micron elliptical illuminating spot of about 50 µm x 150 µm (both FWHM). group velocity dispersion effects in BBO (D2 = 37,0 fs2/mm) is of the order of less than 0.1%. View inside Target Chamber

Typische Aufnahmen des Plasmakanals High-resolution images of the ionization front using „focused shadowgraphy“ Example: Helium at a neutral density of n0 = 2 – 3 . 1019 cm-3  = 0 fs  = +30 fs Zeitauflösung ~ 10 fs (Ortsauflösung 1 mm)

From the shadowgrams the electron density within the channel can be obtaind Plasma Plasma Neutral gas Neutralgas Feld ionization Feldionisation

3D-Particle-In-Cell simulation of the ionization front

3D-Particle-In-Cell simulation of the ionization front Simulated electron density produced by pump-pulse

3D-Particle-In-Cell simulation of the ionization front

3D-Particle-In-Cell simulation of the ionization front Simulation of deflection

3D-Particle-In-Cell simulation of field-ionization within the gas Plasma-Simulation-Code (PSC) (Hartmut Ruhl) Electron density at t = 50,0 fs Elektronendichte t = 66,7 fs Electron density at t = 66,7 fs He2+ He+ . 1019 cm-3 Example: Helium at a neutral density of n0 = 1 . 1019 cm-3

3D-Particle-In-Cell simulation of field-ionization within the gas Plasma-Simulation-Code (PSC) (Hartmut Ruhl) <Z> = 1,5 <Z> = 1,5 Electron density at t = 50,0 fs Electron density at t = 66,7 fs . 1019 cm-3 Example: Helium at a neutral density of n0 = 1 . 1019 cm-3

Plasma-Simulation-Code (PSC) (Hartmut Ruhl) 3D-PIC simulation of the interaction between plasma channel and probe pulse Plasma-Simulation-Code (PSC) (Hartmut Ruhl) Electron density at t = 50,0 fs Shadowgram at t = 74,7 fs . 1019 cm-3 Example: Helium at a neutral density of n0 = 1 . 1019 cm-3

Summary What happens there (ionization) ? Task: Observe optically the breakdown of the the material caused by an ultra-short laser pulse. What happens there (ionization) ? Ionization mechanism (Multiphoton, Tunneling, BSI, ADK-Rates) Zum Vergleich: 2x10^21 W/cm^2 entspricht… How can you observe the ionization front (in principle) ? Optical properties of a laser-produced plasma, „Pump-Probe“-technique, trajectories of probe-puls rays How looks a typical pump-probe experiment (practically) ? Experimental setup, some pictures High-resolution photos of the interaction and comparison with 3D-PIC simulations. Time-resolved shadowgrams, PIC results