Nanosecond Broadband Spectroscopy For Laser-Driven Compression Experiments Dylan K. Spaulding, R. Jeanloz Department of Earth and Planetary Science, University of California, Berkeley307 McCone Hall, Berkeley, CA, 94720-4767 B.A. Remington, D.G. Hicks, G.W. Collins Lawrence Livermore National Laboratory, 7000 East Avenue, Livermore, CA, 94550
Outline Motivation Shock Compression U P Ramp Compression Sample Shock Front U S? How do we generate high-pressure and temperature in the laboratory? Experiments and Diagnostics Proposal and proof of concept for measuring broadband reflectivity
Extreme Chemistry at Ultra-High Pressure Stixrude & Jeanloz Insulator CEA, LLNL Semiconductor Metal Hicks et al., PRL, 2006
Single Wavelength Reflectivity Reveals That the Deep Earth May Contain Metallic-like Silicate Liquids Reflectivity Fused Silica (SiO 2 ) Melt Quartz (SiO 2 ) Hicks et al., PRL, 2006 Reflectivity Enstatite (MgSiO 3 ) Spaulding et al., in prep Pressure (GPa) Pressure (GPa) For SiO 2, a 2-fold increase in temperature (5,000K 10,000K) gives 3 orders of magnitude increase in conductivity (1 1000/(ohm-cm)). MgSiO 3 appears to show similar behavior
Nanosecond Broadband Reflectivity Measurements Broadband reflectometry is a potentially powerful tool for probing electronic structure in condensed matter. However, for HED measurements, a sufficiently bright source is required. Existing broadband light sources for HED measurements: Gas guns, s: Incandescent flashlamps Long pulse, ns: No broadband light sources exist in the ns regime Short-pulse, ps, fs: Supercontinuum generation in crystals/liquids (self-phase modulation) Goal: 1) To develop broadband reflectometry in the ns regime using non-linear pumping of an optical fiber 2) To perform the first time-resolved probing of laser-shock states
Experiments Generating High Pressures and Temperatures in the Laboratory Janus Laser Nd:glass 2 Beams: 1kJ/beam 1053nm, 527nm, 351nm The OMEGA Laser Nd:glass Wavelength: UV 60 beams, 30kJ The NIF 192 beams, 2Megajoules Wavelength: 351nm Images: Jlf.llnl.gov, lle.rochester.edu, NIF
Laser-Driven Compression Different Kinetics, Different Processes Decaying Shock Multi-Shock Spaulding Lee et al, 2006 5 10 15 20 5 10 15 20 Steady Shock Ramp Compression McWilliams J.P. Davis (2005) Smith et al. 2007 5 10 15 20
Measuring High Pressure and Temperature States DIAGNOSTICS TARGET Streaked Optical Pyrometer Records thermal emission from sample for calculation of absolute temperature Reflectivity Monitor Observes changes in sample optical properties 532nm Probe Beam DRIVE 2 Janus Beams Variable Pulse Shape 527nm Up to ~500J/Beam Amplitude 1mm Decaying Shock Interferometer Delay Element VISAR (Velocity Interferometry System for Any Reflector) Measures Doppler shift from moving surfaces to determine shock and particle velocities Position
Temperature Measurements With Streaked Optical Pyrometry (SOP) Optical Train γ Hamamatsu C7700 Streak Camera e - V(t 1 ) Time V(t 0 ) Calibrate against a source of known spectral radiance (tungsten ribbon filament) to determine the relationship between measured intensity and temperature Intensity ( T ) P L (, T ) T ( ) S ( ) d system S x VISAR (532nm)
Complications in Optical Pyrometry Is the Shock Front Transparent to Thermal Emission? Shock Front U P U S t=t 0 t=t 0 +Δt Broadband reflectivity allows Determination of wavelengthdependent emissivity as a function of time for better graybody temperature correction www.ecse.rpi.edu
Supercontinuum Generation Via Non-Linear Pumping Of An Optical Fiber Q-Switched ND:YAG 532nm 1-10ns Pulse 10-35mJ Spectrometer Streak Camera 532nm 23 meters, 50µm core fiber Supercontinuum generation Broadband Output Target Assembly Self Phase Modulation Induced change in refractive index Stimulated Raman Scattering Inelastic photon scattering Four-Photon Mixing Interference of three fields results in creation of a fourth
Possible Layout for White Light Supercontinuum Generation Using Existing Jupiter GCR Laser (For Calibration and Alignment Purposes) 632nm 532nm Flipper Mirror Dielectric Dielectric Pulsed GCR (Nd:YAG) Laser From Janus Bay (10ns FWHM, 150µJ, 532nm) Fast Lens Dielectric Cube Beamsplitter Output to Camera, Streaked Spectrometer 23m, 50µm Core Fiber Fast Lens Flipper Mirror Photodiode (Calibration Only) Flipper Mirror Polarizer Waveplate Broadband Output to Target Chamber Oscilloscope (Calibration Only)
Time-Resolved Imaging Diagnostics VISAR PELLICLE BEAM SPLITTER DICHROIC BEAM SPLITTER M1 TO SPECTROMETER ND FILTER WHEEL 532nm NOTCH FILTER LIGHT-TIGHT ENCLOSURE IRIS M5 M3 L2 IRIS M4 L1 M2 HAMAMATSU C7700 STREAK STREAK CAMERA CAMERA 300g/mm GRATING L3 FIBER OPTIC TIME FIDUCIAL M1-4: 3 diameter mirrors M5: 4 diameter mirror L1,2: 1m achromat lenses L3: 146mm fast achromat Dichroic beam splitter may be replaced with a 50/50 beamsplitter depending on VISAR performance for better spectral response Spectrometer (Acton SpectraPro 2300i or fiber-coupled Ocean Optics HR4000) is along the east wall of the Janus target bay. The latter could be placed in the enclosure, replacing M2 with the pellicle beamsplitter and eliminating M1.
Calibrations System must be spectrally and temporally calibrated for reliable data Spectral calibration achieved using the two lasers (532nm and 632 nm) and Ne and/or He/Ar lamps Temporal calibrations are provided for the streak camera and can be checked using a comb generator Ambient reflectivity measurements can be taken from a static target by pulsing the laser without driving the sample. A mirror or polished Al target would allow characterization of the extent of spectral broadening
Proof of Concept Broadband output Green Laser 23m, 50 m core fiber
Proof of Concept: Spectrally Resolved Reflectivity on Shocked SiO 2 Spectral broadening up to >800nm was observed using a standard fiber pumped at 532nm. Shift to longer wavelengths appears to deplete energy in shorter wavelengths. Time 0 Wavelength 532nm 850nm SiO 2 Al Time Direction of increasing pump energy: Black Blue Green Red 20ns Shock Breakout Space 1100 Quartz ~ 0.8 Mbar SiO 2 ~0.8 Mbar 1000 Before break-out 900 800 After break-out Intensity 700 600 500 400 300 200 500 600 700 800 900 Wavelength (nm)
Conclusions Time-resolved spectroscopy of shock and ramp-compressed states has direct implications for high-pressure chemistry and will also serve to improve uncertainty in measurements from other optical diagnostics A bright broadband nanosecond source was successfully demonstrated > 30 J/pulse (more than enough for streak camera) Reflectivity measurements on Terrestrial mantle compositions (SiO 2, MgO and MgSiO 3 ) suggest the possibility of dense, metallic-like fluid states Acknowledgements: Dylan Spaulding thanks the Krell Institute for their generous funding. The authors wish to thank Kjell Tengsdall for his assistance in acquiring materials for these experiments and Dwight Price, Joe McDonald and Jim Hunter (LLNL) for their assistance at Janus.