New Peaks Scales Nonlinear Optics G Ravindra Kumar

New Peaks Scales Nonlinear Optics G. Ravindra Kumar Tata Institute of Fundamental Research Mumbai grk@tifr. res. in R R Dasari Distinguished Lecture Series, 28 Feb 2005, IIT Kanpur

Light – Matter Interaction Normally, Induced Dipole Reradiation (electronic response) , k, E ´, k´, E´ 1. Optical interactions depend on the Electric field in the light wave. 2. Valence/outer `bound’ electrons that respond to this field. But, 3. Does this idea work when you go to high light Intensities? NO!

What is this talk all about? Ipeak=1017 W cm-2 50 m. J, 100 fs (0. 05 J, 10 -13 s) E = 1010 V cm-1 20 -100 m `Peak’ power 0. 5 x 1012 W e- ke. V-Me. V Ions Zq+ Light X-rays/ -rays Light pulse - Spatial Packet (Length approx. 65 micrometers !) less than the breadth of human hair!

Intense Light Fields Extremely large E fields generated by short pulse, high energy lasers Comparison with the intra-matter Coulomb field Hydrogen atom - 1 s electron E ~ 109 V/cm Intensity Current Highest Intensity – 1021 W/ cm 2 ! (about 1012 V/cm)

Let us look at the protagonists……… Light first……. .

The Lase. Revolution Small step for Maiman Giant Leap for Laserkind! Bringing the stars down to earth!!

Nd: YAG FI 200 ps 100 fs Ti: sapphire Ar+ Pump Ti: S Osc. 90 fs

A glance at the laser …

Next, Matter ……. . and what happens to it?

Matter under extreme conditions single atom I = 1016 W cm-2 E ~ 109 V/cm High intensity Photoeletric Effect + Rapid ionization of valence electrons Tunnelling Over the barrier 1014 - 1015 W cm-2 > 1015 W cm-2 Each atom loses at least one electron. Some can lose as many as 6 !

Energy Scales involved Photon energy - 1. 5 ev Ionization energy (typ. ) – 10 -100 e. V You see that photon energy does not matter!

Intense, Femtosecond Light - Matter Interaction broad features Matter intrinsically unstable, ionization (multiple) inevitable `Intensity’ of light matters, not the wavelength (photon energy) Highly transient interaction, `impulse’ excitation (d - function like? ) Structure and dynamics completely coupled `dynamic’ structure? ! Creation of new states of matter The coulomb binding field becomes perturbative, not the light field!

Light oscillates electrons ! E(t)cos t 10 -100 nm Vosc = 100 -1000 lattice spacings in a solid

Ponderomotive energy Acceleration of the ionized electron in the laser field e - electronic charge E - electric field in the light wave - wavelength of the laser me - electronic mass Acceleration 1017 g !!! E = 2. 75 108 V/cm (1013 W/cm 2) UP = 1. 1 e. V for = 1. 06 m > 100 e. V for = 10. 6 m l e R iv t a UP > 106 e. V for = 1. 06 m & 1019 W/cm 2 Each electron interacts with 106 photons !! ic t is

Energy Scales involved ( again…. . ) Photon energy - 1. 5 ev (~ 100 e. V) Ionization energy (typ. ) – 101 -102 e. V >> Photon energy Energy given to the electron >>>> both the above!

Intense Laser - Solid Interaction Ionization much more than in the single particle case ( U 92+ possible!) Why? Density effects: additional mechanisms e. g. collisional ionization (particle effect), collective absorption (wave process) High density, high energy plasma formation Extremely complex dynamics

Plasma formation in a solid Initial ionization of valence electrons by light field Acceleration of ionized electrons by light (Oscillation) Repetitive processes Collisional absorption Collisions of these individual electrons with other particles `Inverse bremsstrahlung’ Resonance Absorption Excitation of a plasma wave (Collective effect) Damping of plasma wave Hot, dense plasma

POLARIZATION DEPENDENT ABSORPTION IN PLASMAS Resonance Absorption (> 1015 W cm-2) P-polarized light at oblique angle of incidence, exciting a plasma wave. ‘Hot’ electrons (‘Fast’ electrons) WHY study Hot electrons? Important for Fast Ignition Fusion Emitters of very hard X-ray pulses

Where do `hot’ electrons go? Input Laser pulse 300 fs 1. 2 ps after laser pulse Gremillet et al. , PRL 83 (1999) 5015 3 ps after laser pulse

Different perspectives!! • Coupling of laser light – reflectivity • Time resolved studies • Magnetic field generation • Generation of X-ray Pulses • Electron and Ion emission

Hot electrons emit bremsstrahlung o Picosecond Femtosecond duration, Very hard x-ray pulses T = 40 ke. V S. Banerjee et al, SPIE, 3886(2000) 596

Femtosecond, Hard X-ray Pulses ! • bremsstrahlung emission from polished and unpolished targets at 1 x 1016 Wcm-2 • p-polarized light is used throughout • surface topography should have had detrimental effects as “some ‘p’ becomes ‘s’ • Roughness causes ENHANCED emission • necessity for an additional mechanism P. P. Rajeev et al. Phys. Rev. A , 65, 052903(2002)

Physics In ULTRA-INTENSE Light Fields Matter totally ionized Large charge densities ( > 1024 cm-3 ) Energetic electrons ( 103 - 106 e. V ) Sun Nonequilibrium dynamics - violently driven systems Non-Maxwellian particle distributions Gigantic magnitudes Magnetic fields 109 G Electric field 1010 V cm-1 Pressure 109 bars. Temperature 108 K ( for e- ) n ro t u ne star Relativistic and QED effects multiphoton Compton scattering, pair production Nuclear excitation and fusion Laboratory Astrophysics

Zero to Megagauss in Picoseconds! Sandhu et al, Phys. Rev. Lett. 89 (2002) 225002 “Megagauss in picoseconds” Physics News Update #614 dated Nov 20, 2002 (American Institute of Physics, NY)

Why study Laser generated magnetic fields? • Largest available terrestrially • Magnetic fields mirror electron dynamics They also control them! (specially fast/relativistic electrons) • Understanding them important for Laser Fusion • Potential applications in futuristic information storage, isotope separation, MCD etc…

How to measure B ? Direct Methods: • Induction Probes • Magnetization/Demagnetization Elegant Method • Modification of polarization state of laser light (non-contact, highly sensitive)

Laser Pump Hot electron jets B Target Probe

Setup

TIFR + IPR Giant Magnetic Pulse ! Sandhu et al, Phys. Rev. Lett. 89 (2002) 225002 Magnetic field pulse profile for p- polarized pump at 1016 W cm-2

Generation and damping of B • Hot electrons Jhot stream into bulk • Return plasma currents compensate Source • The electrical resistivity -1 limits buildup and • determines decay of magnetic field. Current loops Cold e- Laser Hot e. Solid Diffusion Plasma layer

Phenomenological Modeling Evolution equation : d. B/dt = S(t) - B/ , Source due to the fast electron currents Assuming exponential source: Representation of the magnetic diffusion term S(t) = S 0 exp(-t/t 0) Resistive decay of B from plasma return currents GOOD FIT with: S 0 = 53. 7 MG/ps, t 0 = 0. 7 ps, = 5. 6 ps. Natural decay of the hot esource produced by the RA. (Model used by IPR collaborators)

TIFR-IPR Sandhu et al, Phys. Rev. Lett. 89 (2002) 225002 GOOD FIT to data : S 0 = 53. 7 MG/ps, t 0 = 0. 7 ps, = 5. 6 ps.

Energy budget for the given laser input: At 1016 W /cm 2 IB absorption ~ 10% Resonance Absorption ~ 30 -40% The rest is not coupled !

Plasmas reflect light very well… (40 -50%) The reflected light carries information about the plasma (density, scale length, temperature…. ) However, there lies the problem… how do you couple more light in? It is indeed possible to couple upto 90% of incident light!! HOW? We address this now!

A `Small’ Step. Towards Efficient Xray emitters….

Small is bountiful ! Metal Nanoparticle coated Targets • coated on optically flat Cu disk by high pressure dc sputtering • basic optical characterization by linear reflectivity • permittivity changes with size – different plasmon resonances – different absorption ranges – different colored particles Drude fits

Enhanced Hard Xray emission from metal nanoplasmas • using spherical and ellipsoidal nanoparticles (b ~ 15 nm) • 3 -4 fold enhancement in the x-ray yield at 10 o incidence • an enhanced intensity ~ 1. 4 Iin explains the extra hot e • component P. P. Rajeev et al. , Phys. Rev. Lett. (2003)

Surface Plasmons Def: Electromagnetic surface waves (‘p’) which exist at the interface between 2 media whose e have opposite signs. E kz Hy kx dielectric (e>0) +++ --- metal (e <0) Surface plasma oscillations: . fluctuations of the charge on a metal boundary

Nanotricks yield Megafluxes • 13 -fold enhancement using ellipsoidal particles at 45 o at 6 x 1014 W cm-2 • spherical particles continue to give 3 -4 fold enhancements • Very good agreement with the model • Almost an order of magnitude increase in the effective intensity using ellipsoidal particles • explains the observed temperature and yield P. P. Rajeev et al. , Phys. Rev. Lett. (2003); Optics Letters (2004) 13 -fold Enhancement!

Concept of fast ignition

Petawatt laser created intense fluxes of Me. V Electrons are guided by a carbon fibre plasma Plasma photonics ! Nature (23 Dec 2004)

Nature (23 Dec 2004)

Conclusions Intense, Ultrashort light interaction with matter – Exciting scientific frontier! Picosecond, Megagauss (5 ps, 27 MG) magnetic pulses demonstrated in femtosecond laser produced plasmas. • Enhanced, femtosecond x-ray emission • Guiding of intense fluxes of Me. V electrons

Thanks to………. .

Acknowledgements…. . Aditya Dharmadhikari Pushan Ayyub P. Taneja P. K. Kaw Sudip Sengupta Amita Das (IPR) Earlier Collaboration S. Banerjee, L. C. Tribedi, R. Issac P. D. Gupta, P. A. Naik and others (CAT)

A brief, yet intense , affair with light !