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Steven J. Davis, Kristin L. Galbally-Kinney, William J. Kessler, and Wilson T. Rawlins

Dynamics of Alkali Atom Excitation and Population Inversion in Optically Pumped Rare-Gas Exciplex Systems. Steven J. Davis, Kristin L. Galbally-Kinney, William J. Kessler, and Wilson T. Rawlins Physical Sciences Inc. Andover, MA 01810 2010 Annual Directed Energy Symposium Bethesda, MD

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Steven J. Davis, Kristin L. Galbally-Kinney, William J. Kessler, and Wilson T. Rawlins

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  1. VG10-209 Dynamics of Alkali Atom Excitation and Population Inversion in Optically Pumped Rare-Gas Exciplex Systems Steven J. Davis, Kristin L. Galbally-Kinney, William J. Kessler, and Wilson T. Rawlins Physical Sciences Inc. Andover, MA 01810 2010 Annual Directed Energy Symposium Bethesda, MD November 2010 Acknowledgement of Support and Disclaimer This material is based upon work supported by Air Force Office of Scientific Research under Contract Number FA9550 07 1 0575. Any opinions, findings and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of Air Force Office of Scientific Research. Distribution Statement A: Approved for Public Release; Distribution is Unlimited

  2. Outline VG10-209 • Alkali atom-rare gas “molecules” • Description of PSI apparatus • Absorption spectroscopy of exciplex species • Multi-photon excitation • Alkali atom absorption/gain spectroscopy • Summary

  3. Exciplex Effect: Alkali-Rare Gas Collision Pairs VG10-209 Cs-Ar Potential Energy Diagram Cs-Ar Absorption Spectra • Van der Waals collision pair provides continuum molecular absorption over several nm spectral range • B-state dissociates directly to 2P3/2, can lase on either transition • Allows efficient coupling of spectrally broad excitation sources to alkali atoms – NO LINE NARROWING REQUIRED

  4. Apparatus for Alkali-Rare Gas Spectroscopy VG10-209 • Longitudinal pump: 0.5 W Ti:S laser • Co-linear TDL beam for gain measurements • Side view for fluorescence spectrometer

  5. Exciplex Absorption and Fluorescence Spectroscopy Cs, Rb + He, Ar, Kr, Xe Distribution Statement A: Approved for Public Release, Distribution Unlimited.

  6. Laser Excitation of Cs(62P) Fluorescence via CsKr Exciplex Absorption VG10-209 Direct pumping of D1, D2 lines throughout exciplex band CsKr(BX) Fluorescence Intensity (within Exciplex Band)

  7. Absorption Spectroscopy: General Observations VG10-209 • Rare gas effects for Ar, Kr, Xe are similar EXCEPT: • Bandhead moves to shorter wavelengths for smaller rare gas atoms • Broadening effect Ar > Kr > Xe • UIUC: combine rare gases to fill in exciplex absorption spectrum between bandhead and D2 line • C2H6 effect enhances exciplex absorption • Mechanism is unknown • Are there other collision partners that do this? • Emory Univ.: potential energy calculations for Rb + CH4 in progress • Exciplex effect is small for He: not significant in conventional DPAL

  8. Emission Spectra Produced by Multiphoton Absorption Distribution Statement A: Approved for Public Release, Distribution Unlimited.

  9. Multiphoton Excitation of Cs(I) FluorescenceExcitation Near 852 nm VG10-209 Cs + 500 Torr Kr, 473 K

  10. “Blue” Cs Doublet Emission at excitation = 852 nm VG10-209 Cs + 500 Torr Kr, 473 K (b) (a)

  11. Multiphoton Excitation of Infrared Alkali Transitions by Atom and Exciplex Absorption VG10-209 From: Sharma et al., APL38, 209 (1981) • Infrared lines can be lased via multiphoton excitation of higher states

  12. Infrared Cs(I) Fluorescence:Excitation of CsXe at 852 nm VG10-209 FTIR Spectrometer Spectral Resolution = 2 cm-1 (0.002 nm) InGaAs Array Spectrometer Spectral Resolution = 0.3 nm • Excitation observed at very low pump power (~100 mW) • implies 2-photon pumping via intermediate exciplex state • will be prominent process at high pump power

  13. Cs-Ar Energy Level Diagrams:Pathways for Resonant 2-Photon Excitation VG10-209 • Potential energy curves for Cs-Ar suggest possibility of efficient two-photon pumping path

  14. Gain Spectroscopy and Imaging Distribution Statement A: Approved for Public Release, Distribution Unlimited.

  15. DPAL/XPAL Gain Measurement Test Bed(Diode laser scanning D1 line) VG10-209 • Direct probe of population inversion dynamics • Aids in design of optical resonators • Portable: take to other facilities • Can extend to spatial imaging of gain • Expect significant spatial effects in power scaling • Valuable tool for scaling DPAL to high powers

  16. Optical Layout for DPAL/XPAL Gain Measurements VG10-209

  17. Computed D1 Absorption Spectra: CsCollisional Broadening Effect VG10-209 Cs 2S1/2 – 2P1/2, 894 nm Low Pressure, Doppler broadening High Pressure, collisional broadening • Collisional broadening greatly expands required scan range • High optical thickness at elevated temperatures

  18. Absorption/Gain Spectra:Cs(2S1/2,F”=42P1/2,F’), 894 nm500 Torr Kr + 75 Torr C2H6, 338 K VG10-209 Pump Laser: 2S1/22P3/2, 852 nm W/cm2 GAIN ABSORPTION • Continuing work: investigate absorption and gain dynamics • for DPAL, XPAL configurations: Cs, Rb, K

  19. Dependence of Gain on Pump Power VG10-209

  20. State-Selected Absorption and Saturation VG10-209

  21. Imaging Gain Diagnostic:A Critical Tool for Power Scaling of Alkali Lasers VG10-209 • Single pass unsaturated gain • Provides estimate for available output power • Spatially resolved gain • Can measure both longitudinal and transverse gain • Predicts output beam quality • Applicable to static and flowing systems • Effects of heat deposition, fluid dynamics, radiation trapping, particles • Key data to optimize resonator design, output power, and beam quality Concept:

  22. 3-D Image of D1 Gain, AbsorptionCs + 500 Torr Kr + 75 Torr C2H6 VG10-209 • Probe beam diameter > pump beam diameter • Gain profile follows Gaussian profile of pump beam • Elevated base absorbance: effects of radiation trapping

  23. Summary Exciplex effect: Ar, Kr, Xe Broadens absorption to several nm, direct inversion of D2 line Ethane enhances exciplex absorption: mechanism? other collision partners? Laser-induced fluorescence: tracks molecular potentials Evidence for direct excitation of D1, longer B-state lifetime Multiphoton excitation of infrared atomic transitions (1 to 4 mm): possible laser candidates Observe exciplex-assisted pumping of higher Cs* states Excitation mechanism may involve “real” molecular states rather than “virtual” intermediates – more efficient process Likely significant phenomenon at moderate to high pump power Optical diagnostic tools required for gain, key species concentrations Fiber-coupled gain diagnostics in progress for Rb, Cs Emphasize high-resolution spectral, spatial information – imaging VG10-209

  24. Acknowledgements M. C. Heaven, J. Merritt, J. Han Emory University J. G. Eden, J. D. Readle, C. J. Wagner, J. J. Coleman, N. L. Dias, V. B. Verma University of Illinois at Urbana-Champaign J. T. Verdeyen, D. L. Carroll CU Aerospace T. Madden, D. Hostutler AFRL HEL-JTO and AFOSR

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