Optical Gain and Multi-Quantum Excitation in Optically Pumped Alkali Atom Rare Gas Mixtures

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1 Physical Sciences Inc. Optical Gain and Multi-Quantum Excitation in Optically Pumped Alkali Atom Rare Gas Mixtures Kristin L. Galbally-Kinney, Wilson T. Rawlins, and Steven J. Davis 20 New England Business Center Andover, MA High Energy/Average Power Lasers and Intense Beam Applications VIII SPIE Photonics West 2014 San Francisco CA 2 February 2014 Paper Acknowledgement of Support and Disclaimer This material is based upon work supported by Air Force Office of Scientific Research under Contract Number FA 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 20 New England Business Center Andover, MA VG14-002

2 Outline VG Alkali atom-rare gas molecules Description of PSI apparatus Alkali atom absorption/gain spectroscopy Multi-quantum excitation Conclusions

3 Transmission Exciplex Effect: Alkali-Rare Gas Collision Pairs Cs-Ar Potential Energy Diagram 1.2 Cs-Ar Absorption Spectra Cs Torr Ar, 448 K VG Cs-Ar Cs-Ar + 75 Torr Ethane Wavelength, nm Van der Waals collision pair provides continuum molecular absorption over several nm spectral range B-state dissociates directly to 2 P 3/2, can lase on either transition Allows efficient coupling of spectrally broad excitation sources to alkali atoms NO LINE NARROWING REQUIRED NIR transitions: How are higher alkali atom states excited?

4 Experimental Approach VG CW excitation of Cs, Rb by 1 W Ti:sapphire laser Ti:S operated as standing wave cavity (~2 GHz linewidth) CW power scaling: DPAL gain, multi-quantum NIR excitation Observe gain by tunable diode laser spectroscopy DPAL mode: pump 2 S 1/2 2 P 3/2, probe 2 P 1/2 2 S 1/2 500 Torr RG + 75 Torr C 2 H 6 XPAL gain not observable for CW system (<10 kw/cm 2 ) Observe NIR side fluorescence with InGaAs array spectrometer XPAL mode: pump various wavelengths in exciplex band 500 Torr Ar, Kr, Xe; no hydrocarbon Observe scaling of NIR emission vs. temperature LIF excitation spectra: structure of exciplex band Short alkali cell lengths: 1 cm, 5 cm Less optically thick to gain-probe laser beam

5 Apparatus for Alkali-Rare Gas Spectroscopy VG Longitudinal pump: 1 W Ti:S laser (2 GHz line width) Co-linear TDL beam for gain measurements Side view for fluorescence spectrometer

6 DPAL/XPAL Gain Measurement Test Bed (Diode laser scanning D 1 line) VG Direct probe of population inversion dynamics Aids in design of optical resonators Portable: take to other facilities Experimental Verification Alkali Cell Probe Beam Ti: S Pump Beam Gain Imaging Extended to spatial imaging of gain Expect significant spatial effects in power scaling Valuable tool for scaling DPAL, XPAL to high powers K-5034

7 Optical Layout for DPAL/XPAL Gain Measurements VG Ti:S laser pumps D 2 transition: F = 3 or 4 Ethane collisions produce emission on D 1 transition TDL laser probes absorption/gain on D 1 : F = 3 and 4

8 Relative Absorbance Relative Absorbance Collisional Broadening Effect Computed D 1 Absorption Spectra: Cs Low Pressure, Doppler broadening F" = 4 F" = 3 F' = 3 4 F' = 3 4 Cs 2 S 1/2 2 P 1/2, 894 nm VG High Pressure, collisional broadening Computed Spectrum Cs D 1 Multiplet 500 Torr Kr + 75 Torr C 2 H K Relative Frequency, MHz Relative Frequency, MHz Collisional broadening greatly expands required scan range High optical thickness at elevated temperatures

9 5 cm Cell, 70 C Ti:S pump: 2 S 1/2 (F =4) Absorption/Gain Spectra Cs( 2 S 1/2,F 2 P 1/2,F ), 894 nm 500 Torr Kr + 75 Torr C 2 H 6 VG cm Cell, 100 C Ti:S pump: 2 S 1/2 (F =3) -8 Spectra retain memory of which state is pumped => Incomplete collisional redistribution

10 Scaling of DPAL Gain with Excitation Power Density VG Possible loss process at higher power, higher [Cs]

11 Lumped 3-Level Model for Small Signal Gain 3 L e v e l S y s t e m VG k c [ M ] 2 P 3 / 2 2 P 1 / 2 I B 1 3 A 2 1 Observed gain plateau is due to bleaching Predicted maximum gain is consistent with data Distributed over 4 states Observed roll-off >10 kw/cm 2 may be due to energy pooling loss of 2 P 3/2 1 n n n IB 2 S 1 / n IB31 A31 kc k M M 1 3 M n A n c n 2 A21 kq 1 n TOT IB 31 n A31 IB n2 n3 21 J

12 Multi-quantum Excitation of Cs(I) Fluorescence Excitation Near 852 nm VG Cs Torr Kr, 473 K

13 Blue Cs(7 2 P) Emission at excitation = 852 nm Cs Torr Kr, 473 K VG (a) (b) Two-photon resonances leading to 7 2 P occur at other wavelengths: 911, 919 nm (7 2 P) 884, 885 nm (6 2 D) 822 nm (8 2 S)

14 Signal Intensity, counts/s 3/2 3/2 3/2 5/2 1/2 5/2 3/2 1/2 1/2 1/2 Signal Intensity 3/2 1/2 5/2 3/2 3/2 3/2 1/2 1/2 1/2 3/2 Infrared Cs(I) Fluorescence: InGaAs Array Spectrometer Spectral Resolution = 0.3 nm Excitation of CsXe near 852 nm VG FTIR Spectrometer Spectral Resolution = 2 cm -1 (0.002 nm) E S J' 6 2 P J" 1.6E-03 Quartz Transmission P J' D J" 1.4E E E E E E P J' 7 2 S J" 5 2 D J' 6 2 P J" E E Wavelength, mm -2.0E Wavelength, mm Initial observations at very low pump power (~100 mw) Collisional energy pooling or 2-photon pumping via exciplex? Expect significant process at high pump power, high T

15 NIR Transitions Observed in Cs-Xe, Rb-Kr = mm, Ti:S Power Density kw/cm 2 VG Cs Rb

16 Cs, Rb Near-IR Excitation Spectra: 4 2 F States Pump on D 2 : ~8 kw/cm 2 VG F states are >1000 cm -1 above 2-photon energy for 852 nm Likely collisional energy pooling Examine scaling with pump power, temperature (ground state concentration)

17 Fluorescence Signal, counts/s Cs(7 2 S 1/2 ) Fluorescence vs. Pump Power Cs-Xe: mm (7 2 S 1/2 6 2 P 3/2 ) 200 C 180 C 160 C 140 C 120 C 100 C 90 C VG Ti:S Average Power Density, kw/cm 2 Similar results for RbKr, Rb(6 2 S 1/2, 4 2 F) transitions ½-order scaling with pump intensity indicates complex excitation mechanism

18 NIR Excitation: Variation with Pump Wavelength Cs-Xe, 1469 nm (7 2 S 1/2 6 2 P 3/2 ) Pump Power Density ~8 kw/cm 2 VG Significant excitation in red wing of D 2 Blue wing is weaker Broadening and spectral structure near D 2 line Blue (7 2 P) fluorescence visible down to 840 nm (band head)

19 NIR Cs(I) Fluorescence Excitation: Cs-Ar 1012 nm (4 2 F 5/2,7/2 5 2 D 5/2 ) 1469 nm (7 2 S 1/2 6 2 P 3/2 ) VG

20 NIR Cs(I) Fluorescence Excitation: Cs-Xe 1012 nm (4 2 F 5/2,7/2 5 2 D 5/2 ) 1469 nm (7 2 S 1/2 6 2 P 3/2 ) VG

21 NIR Cs(I) Fluorescence Excitation: Cs-Kr 1012 nm (4 2 F 5/2,7/2 5 2 D 5/2 ) 1469 nm (7 2 S 1/2 6 2 P 3/2 ) VG

22 NIR Cs(I) Fluorescence Excitation: Ar, Kr, Xe Cs(7 2 S 1/2 6 2 P 3/2 ), 1469 nm VG

23 Summary and Conclusions CW gain scaling for Cs/Kr/C 2 H 6 mixtures suggests loss process at higher pump power, larger Cs concentrations Multiphoton excitation or collisional energy pooling? Pump D 2 lines in Cs/Xe, Rb/Kr mixtures: strong NIR fluorescence in several lines 1-4 mm Upper state energies 2-3 ev Excitation of 2 F states above 2-photon energy: not multiphoton Scaling with pump power and T are sub-linear, indicates collisional up-pumping Observe excitation of Cs(7 2 S 1/2, 4 2 F) in Ar, Kr, Xe via exciplex band Complicated band structures Evidence points to complex optical and collisional excitation mechanism Possible loss process for optically pumped 2 P states Potential for NIR laser transitions VG

24 Acknowledgements VG Daniel Maser, University of Colorado William Kessler Michael Heaven Emory University High Energy Laser Joint Technology Office Air Force Office of Scientific Research