New Peaks

Scales Nonlinear Optics

G. Ravindra KumarTata Institute of Fundamental ResearchMumbaigrk@tifr.res.in R R Dasari Distinguished Lecture Series, 28 Feb 2005, IIT Kanpur

Light Matter InteractionNormally, Induced Dipole Reradiation (electronic response)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!

50 mJ, 100 fs(0.05J, 10-13s)What is this talk all about?`Peak power 0.5 x1012 WIpeak=1017 W cm-2

E = 1010 V cm-120-100 mkeV-MeVe-Ions Zq+LightX-rays/-raysLight pulse - Spatial Packet (Length approx. 65 micrometers !)less than the breadth of human hair!

Intense Light FieldsExtremely large E fields generated by short pulse, high energy lasersComparison with the intra-matter Coulomb fieldHydrogen atom - 1s electronE ~ 109 V/cm IntensityCurrent Highest Intensity 1021 W/ cm2 !(about 1012 V/cm)

Let us look at the protagonists

Light first..

The LaseRevolution

Small step for MaimanGiant Leap for Laserkind!Bringing the stars down to earth!!

Time evolution of the electric field in a Gaussian-shaped pulseThe laser `projectileA 100 fs pulse has about 80 such cyclesTemporal and spectral width of pulsesI (t)I (l)l = 800 nmPhoton energy = 1.5 eV

FINd:YAGTi:sapphire90 fs200 ps100 fs

A glance at the laser

Next, Matter ..and what happens to it?

Matter under extreme conditionsI = 1016 W cm-2E ~ 109 V/cmRapid ionization of valence electronsTunnelling 1014 - 1015 W cm-2Each atom loses at least one electron. Some can lose as many as 6 ! High intensity Photoeletric Effect

Energy Scales involved

Photon energy - 1.5 ev

Ionization energy (typ.) 10 -100 eV

You see that photon energy does not matter!

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

E(t)cos tLight oscillates electrons !

UP > 106 eV for = 1.06 m & 1019 W/cm2

Each electron interacts with 106 photons !!Acceleration of the ionized electron in the laser field

e - electronic chargeE - electric field in the light wave - wavelength of the laserme - electronic mass

E = 2.75 108 V/cm (1013 W/cm2)UP = 1.1 eV for = 1.06 m > 100 eV for = 10.6 mPonderomotive energy

Energy Scales involved ( again..) Photon energy - 1.5 ev (~ 100 eV)

Ionization energy (typ.) 101 -102 eV >> Photon energy

Energy given to the electron >>>>>>>> both the above!

Intense Laser - Solid InteractionDensity effects: Ionization much more than in the single particle case ( U92+ possible!)Why? additional mechanisms e.g. collisional ionization (particle effect), collective absorption (wave process)High density, high energy plasma formationExtremely complex dynamics

Plasma formation in a solidAcceleration of ionized electrons by light (Oscillation)Collisional absorptionCollisions of these individualelectrons with other particles`Inverse bremsstrahlungResonance AbsorptionExcitation of a plasma wave (Collective effect)Damping of plasma waveHot,dense plasmaInitial ionization of valence electrons by light field

Hot electrons (Fast electrons) Resonance Absorption (> 1015 W cm-2)

P-polarized light at oblique angle of incidence, exciting a plasma wave.POLARIZATION DEPENDENT ABSORPTION IN PLASMAS WHY study Hot electrons?Important for Fast Ignition FusionEmitters of very hard X-ray pulses

Input Laser pulse 300fs1.2 ps after laser pulse3 ps after laser pulseWhere do `hot electrons go?Gremillet et al., PRL 83 (1999) 5015

Coupling of laser light reflectivityTime resolved studiesMagnetic field generationGeneration of X-ray Pulses

Electron and Ion emission

Different perspectives!!

Picosecond Femtosecond duration, Very hard x-ray pulses

T = 40 keVS. Banerjee et al, SPIE, 3886(2000) 596 Hot electrons emit bremsstrahlung

Femtosecond, Hard X-ray Pulses ! P. P. Rajeev et al.Phys. Rev. A , 65, 052903(2002) 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 sRoughness causes ENHANCED emission necessity for an additional mechanism

Physics In ULTRA-INTENSE Light FieldsMatter totally ionizedLarge charge densities Energetic electrons( > 1024 cm-3 )( 103 - 106 eV )Nonequilibrium dynamics - violently driven systemsNon-Maxwellian particle distributionsRelativistic and QED effectsmultiphoton Compton scattering, pair productionNuclear excitation and fusionLaboratory AstrophysicsSunneutron star

Zero to Megagauss in Picoseconds!

Megagauss in picosecondsPhysics News Update #614 dated Nov 20, 2002(American Institute of Physics, NY)Sandhu et al, Phys.Rev.Lett. 89 (2002) 225002

Why study Laser generated magnetic fields? Largest available terrestrially

Magnetic fields mirror electron dynamicsThey 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/DemagnetizationElegant Method Modification of polarization state of laser light (non-contact,highly sensitive)

BHot electron jetsLaser PumpTargetProbe

Setup

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

Generation and damping of B Hot electrons Jhot stream into bulk Return plasma currents compensate The electrical resistivity -1 limits buildup and determines decay of magnetic field. SourceDiffusion

Phenomenological Modeling Evolution equation : dB/dt = S(t) - B/ ,

(Model used by IPR collaborators)Natural decay of the hot e- source produced by the RA.Source due to the fast electron currentsRepresentation of the magnetic diffusion termAssuming exponential source: S(t) = S0 exp(-t/t0)Resistive decay of B from plasma return currentsGOOD FIT with: S0 = 53.7 MG/ps, t0 = 0.7 ps, = 5.6 ps.

GOOD FIT to data : S0 = 53.7 MG/ps, t0 = 0.7 ps, = 5.6 ps. Sandhu et al, Phys.Rev.Lett. 89 (2002) 225002TIFR-IPR

At 1016 W /cm2

IB absorption ~ 10%

Resonance Absorption ~ 30-40%Energy budget for the given laser input: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 StepTowards Efficient Xray emitters.

Metal Nanoparticle coated Targets coated on optically flat Cu disk by high pressure dc sputteringbasic optical characterization by linear reflectivity permittivity changes with size different plasmon resonances different absorption ranges different colored particlesSmall is bountiful !

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 10o incidence

an enhanced intensity ~ 1.4Iin

explains the extra hot e- componentP. P. Rajeev et al., Phys.Rev.Lett. (2003)

Nanotricks yield MegafluxesP. P. Rajeev et al., Phys. Rev. Lett. (2003); Optics Letters (2004) 13-fold enhancement using ellipsoidal particles at 45o 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

13-foldEnhancement!

Concept of fast ignition

Petawatt laser created intense fluxes of MeVElectrons are guided by a carbon fibre plasmaPlasma photonics !Nature (23 Dec 2004)

Nature (23 Dec 2004)

ConclusionsIntense, 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 MeV electrons

Thanks to..

Aditya Dharmadhikari P.K.Kaw (IPR)Pushan AyyubSudip SenguptaP. TanejaAmita Das

Earlier CollaborationS. Banerjee, L.C. Tribedi, R. IssacP.D. Gupta, P.A.Naik and others (CAT)Acknowledgements..

A brief, yet intense ,affair with light !