Second ELI Nuclear Physics Workshop Bucharest – Magurele, 1-2 February 2010

1. Second ELI Nuclear Physics Workshop Bucharest – Magurele, 1-2 February 2010 ULTRASHORT PULSE, HIGH INTENSITY LASERS Dan C. Dumitras, Razvan Dabu Department of Lasers, National Institute for Laser, Plasma and Radiation Physics, Bucharest, Romania http://www.inflpr.ro

2. Intense Laser Fields The relativistic regime IL > 1018 W/cm2 results in a plethora of novel effects: X-ray generation, -ray generation, relativistic self-focusing, high-harmonic generation, electron and proton acceleration, neutron and positron production, as well as the manifestation of nonlinear QED effects

3. Relativistic/Ultra-relativistic Regimes Relativistic regime:1 < a0 < 100, a02 = ILλL2/(1.37 x 1018 Wμm2/cm2) where a0 is the normalized electric field amplitude, IL and λL are the laser intensity and wavelength At a0 = 1 the electron mass increases by 21/2; the limit a0 ~ 100 corresponds to the 100 TW class lasers Ultra-relativistic regime:IL > 1023 W/cm2 (a0 ~ 102 – 104) • in this novel regime, positrons, pions, muons and neutrinos could be produced as well as high-energy photons • this largely unexplored intensity territory will provide access to physical effects with much higher characteristic energies and will regroup many subfields of contemporary physics: atomic physics, plasma physics, particle physics, nuclear physics, gravitational physics, nonlinear field theory, ultrahigh-pressure physics, astrophysics and cosmology • the ultra-relativistic regime opens possibilities of: • extreme acceleration of matter so that generation of very energetic particle beams of leptons and hadrons becomes efficient • efficient production (~ 10%) of attosecond or even zeptosecond pulses by relativistic compression occurring at rate of 600/a0 [as] • study of the field – vacuum interaction effects

4. Interaction Regimes and Targets

5. Peak Power -Pulse Duration Conjecture (Mourou,Brasov 2009) 1) To get high peak laser power wemust decrease the pulse duration 2) To get short laser pulses wemust increase the intensity • Picosecond science (10 ps – to a few hundredth fs): 25 years • Femtosecond science (from a few hundredths fs to a few fs): 18 years • Attosecond science (from a few hundredths as to a few as): it will take at least next 15 years the most important achievements are yet to come(Svelto, Brasov 2009)

6. Ultra-Short Pulses by Laser Mode-Locking Optical-Fiber Compression: 6 fs (1987) nJ Hollow-Fiber Compression: 4,5 fs (1997) mJ

7. From Femtosecond to Attosecond 4 fs 80 as

8. Ultrashort Pulse Lasers Basic elements essential to a fs laser: - a broadband gain medium ( >> 1 THz); p  1/, theultra-short pulse duration is inversely proportional to the phase-locked spectral bandwidth - a laser cavity - an output coupler - a dispersive element - a phase modulator - a gain-loss process controlled by the pulse intensity or energy

9. Ti-Sapphire Lasers The gain rod in a Ti:sapphire laser can cumulate the functions: - gain (source of energy) - phase modulator (through the Kerr effect) - loss modulation (through self-lensing) - gain modulation

10. High Power Amplifiers In a laser amplifier the energy extraction efficiency is a function of the ratio of the energy density and the saturation fluence of the laser material For ultrashort pulses, the energy density of light at the surface and in the volume of the optical elements is limited by the onset of nonlinear effects and laser damage due to the high peak power Hence, an ultrashort pulse cannot be amplified efficiently

11. Principle of CPA – Chirped Pulse Amplification (Mourou 1985) Idea: to stretch (and chirp) a fs pulse from an oscillator (up to 10,000 times), increase the energy by linear amplification, and thereafter recompress the pulse to the original pulse duration and shape During amplification, the laser intensity is significantly decreased in order - to avoid the damage of the optical components of the amplifiers; - to reduce the temporal and spatial profile distortion by non-linear optical effects during the pulse propagation For the amplification to be truly linear, two essential conditions have to be met by the amplifier: - the amplifier bandwidth exceeds that of the pulse to be amplified; - the amplifier is not saturated

12. Pulse Chirping • A chirped pulse is a signal in which the carrier frequency has a small time dependence • In particular, it has a lineartime-varying instantaneous frequency: • The chirping results in a spectral broadening of the pulse, i.e., it extends the range of frequency components contained in the pulse • In general, a pulse can be chirped by passing it through a medium with a nonlinear refractive index, i.e., a medium in which the refractive index depends upon the electric field • In a CPA scheme, a large bandwidth ultrashort pulse is chirped in a stretcher based on diffraction gratings i(t) = 0 + βt A chirped gaussian signal pulse, where the instantaneous frequency grows with time

13. Pulse Stretching • The diffraction angle of the first order is • sinθ = /d – sinθin • where d is the grating period • A greater wavelength (red) is diffracted at a larger angle • A pair of plane ruled gratings with their faces and rulings parallel has the property of producing a time delay that is increasing function on wavelengths • The grating provides a large negative group-velocity dispersion (GVD); if a telescope is added between the gratings, the sign of the dispersion can be inverted (positive GVD) • Stretching is obtained with a combination of diffraction gratings and a telescope (such a combination of linear elements does not modify the original pulse spectrum) • During this process the blue portion of the pulse travels a longer path length than the red portion of the beam

14. Pulse Compression Pulse compression of a chirped pulse using a grating pair which provides negative GVD • The red-shifted wavelengths of the pulse that arrive at the first grating are diffracted more than the blue-shifted wavelengths, and arrive at different portion of the second grating than the blue wavelengths • During this process the red portion of the pulse travels a longer path length than the blue portion of the beam • After diffracting from the second grating and recombining with the blue wavelengths, the total pulse has been compressed in time sincethe blue components have caught up with the red components

15. Amplified Spontaneous Emission - ASE • ASE is a severe problem in fs pulse amplification • It is produced because the pump pulse is much longer than the fs pulse to be amplified • ASE reduces the available gain and decreases the ratio of signal (amplified fs pulse) to background (contrast), or even can cause lasing of the amplifier, preventing amplification of the seed pulse • Solutions to reduce ASE: using of saturable absorbers for a favorable steepening of the leading pulse edge; cross polarized wave (XPW) generation; segmentation of the amplifier in multiple stages

16. Prelasing (ns and ps Laser Pre-pulses) • Prelasing: laser action, occurring during the pump phase in an amplifier, resulting from the residual feedback of the various interfaces in the optical path • Pre-pulses are produced by: - bad orientation of the reflective optics (reflection on the back side)  gives a ~ 10 ps pre-pulse - strong nonlinear effects  give ps pre-pulses - leakage in the regenerative amplifier  gives a ns pre-pulse • Solutions to reduce pre-pulse intensity: the use of Pockels cells and/or Faraday rotators, ps-pumped OPCPA

17. Spectral shaping using acousto-optical programmable gain control filter (AOPGCF) - Mazzler (a) (b) TEWALAS laser spectra: (a) without active Mazzler; (b) optimized by Mazzler. Mauve line – FEMTOLASERS oscillator; yellow line – after first multi-pass amplifier; white line – after second multi-pass amplifier

18. Correction of spectral phase dispersionusing acousto-optical programmable dispersion filter (AOPDF) - Dazzler Temporal distortion of the amplified re-compressed pulse is produced by: - dispersion and phase distortions introduced by the laser amplifier system - spectral gain narrowing in Ti:sapphire amplifiers (a) (b) TEWALAS:Pulse duration measurements using SPIDER (a) with Dazzler phase correction; (b) without phase correction. All cases: with spectrum correction by Mazzler

19. Optical Parametric Chirped Pulse Amplification – OPCPA (Piskarskas 1992) • Idea: to replace the laser gain media of a CPA system by a nonlinear crystal • Key principle of OPCPA: A broad bandwidth linearly chirped signal pulse is amplified with an energetic and relatively narrow-band pump pulse of approximately same duration • Amplification by stimulated emission is substituted by optical parametric amplification of the signal pulse in the presence of a pump pulse • Requirements: precise time/space synchronization of signal and pump pulses; high intensity and high quality pump beams; short pump pulse duration

20. Advantages and disadvantages of OPCPA Advantages: • High gain in a single pass (up to ten orders of magnitude per cm) • Broad bandwidth (ultrashort re-compressed pulses) • Parametric amplification is possible in a wide range of wavelengths • Negligible thermal loading • High signal – noise contrast ratio • High energy and peak power levels in available large nonlinear crystals, no transversal lasing • One avoids the problems of power losses by ASE in high-gain laser amplifiers Disadvantages: • The requirement to match the pump and signal pulse duration • The requirement for a high intensity and high beam quality for pump pulse • The limited aperture of most available nonlinear crystals • The complicated details of phase-matching issues

21. High-Intensity Laser System • Front-End: - large bandwidth Ti:sapphire oscillator, optical stretcher and low energy Ti:sapphire amplifiers - large bandwidth Ti:sapphire oscillator, stretcher and ultra-broad-band non-collinear optical parametric chirped pulse amplification (NOPCPA) in BBO, LBO, DKDP crystals • Power amplifiers: - Ti:Sapphire power amplifier chain pumped by high-energy nanosecond SHG Nd:YAG, Nd:glass lasers - large aperture DKDP-NOPCPA amplifiers pumped by high energy nanosecond SHG Nd:glass lasers • Pulse compression and beam focusing: - large diffraction gratings temporal compressor - adaptive optics (deformable mirrors)

22. pulse energy European PW lasers and projects And more to come … 10 J 1 KJ 1 J 100 J 100 mJ 10 PW Petal APOLLON RAL RAL Jena 1 PW MPQ LULI 100 TW GSI CLPUSalamanca 10 TW INFLPR power 1 TW 100 GW 10 fs 10 GW 100 fs 1 ns 10 ps 1 ps 100 ps pulse length

23. and European visions 10 PW 10 PW 1 PW 1 PW 100 TW 100 TW PFS 10 TW 10 TW power power 1 TW 1 TW 100 GW 100 GW 10 GW 10 GW Peak power chart: State-of-the-art pulse energy 10 J 1 KJ 1 J 100 J 100 mJ CPA fusion RAL LULI JENA OSAKA MBI Osaka RAL LULI Livermore LOA JAERI CUOS Osaka CPA table top Jena CUOS Celia LUND ATLAS MBI CUOS INFLPR Shanghai Brookhaven 10 fs 100 fs 10 ps 1 ns 1 ps 100 ps Data from OECD - Global Science Forum pulse length

24. PW Laser Systems: reported, projects, concepts λ = central wavelength, τ =pulse duration, R = repetition rate, P = peak power

25. INFLPR - TEWALAS

26. Possible solutions for 10-PW ELI-RO laser A) OPCPA based laser system (910-nm central wavelength): Front-End → very broad-band signal radiation at 910-nm central wavelength generated by chirp-compensated collinear OPA. High power OPCPA in large aperture DKDP crystals • B1) Hybrid laser system at 800 nm central wavelength: • Front-End based on OPCPA in nonlinear crystals (BBO, LBO) • High power amplification in Ti:sapphire crystals Proposed solution or • B2) Ti:sapphire amplifiers at 800 nm central wavelength : • Front-End based on Ti:sapphire amplification • High power amplification in Ti:sapphire crystals

27. 2xFRONT END DPSL-pumped OPCPA AMPLIFIERS Ti:Sapphire pumped by ns Nd:YAG & Nd:Glass lasers DIAGNOSTICS A3 +A4+ A5 POWER AMPLIFIERS >300 J A1 + A2 BOOSTERS > 4 J, 10Hz FE1: 10-20 mJ BW > 120 nm TCP = 50 ps 0.1-1 kHz C > 10^12 BEAM TRANSPORT IN VACUUM COMPRESSOR >200 J COMPRESSOR 200 J A3 +A4+ A5 POWER AMPLIFIERS >300 J A1 + A2 BOOSTERS > 4 J, 10Hz BEAM TRANSPORT IN VACUUM INFLPR COMPRESSOR 200 J COMPRESSOR >200 J TARGETS FE2: > 100 mJ BW > 80 nm TCP= 1-2 ns 10-100 Hz C > 10^12 A3 +A4+ A5 POWER AMPLIFIERS >300 J A1 + A2 BOOSTERS > 4 J, 10Hz BEAM TRANSPORT IN VACUUM COMPRESSOR 200 J COMPRESSOR >200 J Φ = 1-20 μm IΣ = 3 x 1023 -24W/cm2 TEST COMPRESSOR TARGETS DIAGNOSTICS BW – Spectral bandwidth, C – intensity contrast, TCP- chirped pulse duration, TC – re-compressed pulse duration, Φ – focused laser beam diameter, IΣ – intensity on target ELI-RO Nuclear Laser Facility Layout Concept of 3 x 10 PW amplifier chains 2 x FRONT END DPSSL-pumped OPCPA / ns SHG Nd:YAG pumped Ti:S 3-chains AMPLIFIERS Ti:Sapphire pumped by ns SHG Nd:YAG & Nd:Glass lasers

28. Thank You ! ELI-NP