We investigated negative photoconductivity in graphene using ultrafast terahertz techniques. Infrared transmission was used to determine the Fermi energy, carrier density and mobility of p-type CVD graphene samples. Time-resolved terahertz photoconductivity measurements using a tunable mid-infrared pump probed these samples at photon energies between 0.35eV -1.55eV, approximately one-half to three times the Fermi energy of the samples. Although interband optical transitions in graphene are blocked for pump photon energies less than twice the Fermi energy, we observe negative photoconductivity at all pump photon energies investigated, indicating that interband excitation is not required to observe this effect. Our results are consistent with a thermalized free carrier population that cools by electron-phonon scattering, but inconsistent with models of negative photoconductivity based on population inversion.

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A vacuum feedthrough has been designed at this laboratory to operate at 40 kV and 10−9 Torr. It is small in size, 3.81 cm in diameter and extending 4.45 cm into the vacuum chamber. A selection of commercial connectors is available. The re-entrant design fully shields the conductor for safety, prevention of noise pickup, and protection from environmental effects.

Methods are being examined to fabricate and characteri ze precise multiples tepped foils. Physical vapor deposition of metals onto substrates using precise masking to define each step was evaluated. A process for depositing metal onto preetched substrates to replicate precise steps is being developed. TWL /sr g _ The Bendix Corporation Kansas City Division This report was prepared as an account of work sponsored by the United States Government. Neither the United States, nor the P. 0. Box 1159 United States Department of Energy, nor any of their employees, Kansas City. Missouri 64141 nor any of their contractors, subcontractors, or their employees, makes any warranty, expressed or implied or assumes any legal liability or responsibility for the accuracy, completeness or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights.