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UNIVERSITY OF MARYLAND AT COLLEGE PARK Trapping and destruction of long range high intensity optical/plasma filaments by molecular quantum wakes S. Varma, Y.-H. Chen, and H. M. Milchberg Institute for Research in Electronics and Applied Physics Dept. of Electrical and Computer Engineering Dept. of Physics HEDLP - 2008 Support: DoE, NSF, JHU-APL Some applications of filaments • directed energy • triggering and guiding of lightening • remote detection: LIDAR, LIBS • directed, remote THz generation Introduction to Filamentation • High power, femtosecond laser beams propagating through air form extremely long filaments due to nonlinear self-focusing ((3)) dynamically balanced by ionization and defocusing. 0 neff = n0 + ngas + nplasma Pcr ~ 2/8n0n2 What does a filament look like? 5 mm 0.8Pcr 1.3Pcr 1.8Pcr 2.3Pcr 2.8Pcr 3.5 mJ Filament images at increasing power (Pcr occurs at 1.25 mJ for a 130fs pulse) “prompt” and “delayed” optical response of air constituents + - + + + - - - + - Atoms: 1% argon Laser polarization Prompt electronic response Delayed inertial response + + + + + + - - - - + + + + - - - - - Molecules: 78% nitrogen, 21% oxygen Laser field alignment of linear gas molecules Classical picture E p induced dipole moment molecular axis p/ / -laser field applies a net torque to the molecule -molecular axis aligns along the E field -delayed response (ps) due to inertia intense laser field (~1013 W/cm2) random orientation E “some” alignment time-dependent refractive index shift n(t ) 2 N 1 cos 2 t n0 3 degree of alignment cos 2 1/ 3 cos 2 1/ 3 < >t : time-dependent ensemble average n0=n(random orientation) Field alignment and “revivals” of rotational wavepacket Quantum description of rigid rotor j, m exp(i j t ) eigenstate even where j E j / 2πcBj ( j 1) (j: ≥0 integer) B h(8 2cI )1 (“rotational constant”) I : moment of inertia Rotational wavepacket j ,m a j ,m j , m exp(i j t ) An intense fs laser pulse “locks” the relative phases of the rotational states in the wavepacket Quantum revival of rotational response The time-delayed nonlinear response is composed of many quantized rotational excitations which coherently beat. t = Tbeat t=0 We can expect the index of refraction to be maximally disturbed at each beat. Single-shot Supercontinuum Spectral Interferometry (SSSI) – Imagine a streak camera with 10fs resolution! A pump pulse generates transient refractive index n (r, t) x Pump pulse Imaging lens Probe Ref. z medium y Probe Ref. CCD Imaging spectrometer Probe and Ref. • Temporally stretched (chirp) for long temporal field of view (~ 2 ps). • ~100 nm bandwidth supercontinuum gives ~10 fs resolution. Extract probe (x, t) to obtain n(x, t). Experimental setup and sample interferogram 0 ps ~ 2 ps Sample interferogram 250 mm N2O gas 652nm 723nm Chen, Varma, York and Milchberg, Opt. Express 15, 11341 (2007) Rotational wavepacket of D2 and H2 molecules P=7.8 atm I=4.4x1013 W/cm2 room temperature Rotational quantum “wakes” in air TN2 , ¾TO2 Vg pump vg pump SSSI measurement showing alignment and anti-alignment “wake” traveling at the group velocity of the pump pulse. Pump-probe filament experiment 2m filament f/300 focusing Object plane Polarizing beamsplitter CCD Filaments are trapped/enhanced or destroyed TN2 , ¾TO2 A 5 mm B C 8.08.0 D 8.4 8.4 8.8 8.8(ps) (ps) Trapped filaments are ENHANCED White light generation, filament length and spectral broadening are enhanced. Aligning filament (left) and probing filament (right), misaligned Both beams collinear, probe filament coincident with alignment wake of N2 and O2 in air CCD camera saturation Conclusions • SSSI enables us to probe refractive index transients with ~10fs resolution over 2ps in a single shot, allowing us to observe room-temperature molecular alignment. • A high intensity laser filament propagating in the quantum wake of molecular alignment can be controllably and stably trapped and enhanced, or destroyed. • Applications: directed energy, remote sensing, etc... Pump power Response near t=0 A 0.68Pcr 1.12Pcr 1.72Pcr A 2.20Pcr 2.60Pcr (ps) 3.72Pcr (ps) Increasing aligning pulse energy laser scan (probe=3.4Pcr) Spectral broadening The spatio-temporally varying refractive index of the wake of molecular alignment causes predictable spectral modulation and broadening of the probe filament. Filament spectrum v. delay A B C D Alignment v. delay E C E A B D Molecular rotational wavepacket revivals mode-locking analogy: coherent sum of longitudinal modes typ. spectrum modes pulse width ≈ (round trip time) / (# of modes) T=8.2ps T/2 Example: N2 3T/4 T/4 nitrogen ps peak width ≈T / jmax(jmax+1) ~ 40 fs for N2 1D spatially resolved temporal evolution of O2 alignment 0T 0.5T 0.25T • pump peak intensity: 2.7x1013 W/cm2 • 5.1 atm O2 at room temperature • T=11.6 ps x (mm) (fs) 0.75T 1T 1.25T x (mm) (ps) Introduction to Filamentation • High power, femtosecond laser beams that propagate through air form extremely long filaments due to nonlinear self-focusing ((3)) dynamically balanced by ionization and defocusing. • Filaments can propagate through air up to 100s of meters, and are useful for remote excitation, ionization and sensing. Rotational wavepacket of H2 molecules at room temperature Experiment: Fourier transform BH 2=61.8 cm1 T=270 fs Lineout at x=0 Calculation: The pump intensity bandwidth (~2.5x1013 s-1) is even less adequate than in D2 to populate j=2 and j=0 states. Weaker rotational wavepacket amplitude. P=7.8 atm I=4.4x1013 W/cm2 H2 0.3010-24 cm3 T Charge density wave in N2 at 1 atm vg • Filament ionization fraction ~10-3 2x1016 cm3 • ~0.5% ponderomotive charge separation at enhanced intensity ~5x1014 W/cm2 over 50100 fs alignment transient Ne~ 1014 cm-3 Quantum beat index bucket E~ 0.75 MV/cm • Many meters of propagation Experimental setup and sample interferogram 110 fs high pressure exp gas cell (up to ~8 atm) 1 kHz Ti:Sapphire regenerative amplifier ~300 mJ P: pinhole BS: beamsplitter HWP: /2 plate SF4: dispersive material xenon gas cell (1-2 atm) supercontinuum (SC) Michelson interferometer 0 ps ~ 2 ps Sample interferogram 250 mm N2O gas 652nm Optical Kerr effect ((3)) and the molecular rotational response in the gas induce spectral phase shift and amplitude modulation on the interferogram. Both spectral phase and amplitude information are required to extract the temporal phase (refractive index). 723nm Experimental setup and sample interferogram 110 fs high pressure exp gas cell (up to ~8 atm) 1 kHz Ti:Sapphire regenerative amplifier ~300 mJ P: pinhole BS: beamsplitter HWP: /2 plate SF4: dispersive material xenon gas cell (1-2 atm) supercontinuum (SC) Michelson interferometer 0 ps ~ 2 ps Sample interferogram 250 mm N2O gas 652nm Optical Kerr effect ((3)) and the molecular rotational response in the gas induce spectral phase shift and amplitude modulation on the interferogram. Both spectral phase and amplitude information are required to extract the temporal phase (refractive index). 723nm
