Physical Optical Gain and Multi-Quantum Excitation in

Physical Sciences Inc. 20 New England Business Center Andover, MA 01810

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 Physical Sciences Inc.

20 New England Business Center

Andover, MA 01810

High Energy/Average Power Lasers and Intense Beam Applications VIII

SPIE Photonics West 2014

San Francisco CA

2 February 2014

Paper 8962-05

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

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Physical Sciences Inc.

Distribution Statement A: Approved for Public Release, Distribution Unlimited.

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Outline

Alkali atom-rare gas “molecules”

Description of PSI apparatus

Alkali atom absorption/gain spectroscopy

Multi-quantum excitation

Conclusions

-1



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Exciplex Effect: Alkali-Rare Gas Collision Pairs

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

NIR transitions: How are higher alkali atom states excited?

Cs-Ar Potential Energy Diagram

0

0.2

0.4

0.6

0.8

1

1.2

800 820 840 860 880 900 920

Wavelength, nmT

ran

sm

iss

ion

Cs-Ar

Cs-Ar + 75 Torr Ethane

Cs + 500 Torr Ar, 448 K

Cs-Ar Absorption Spectra -2



Experimental Approach

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 2S1/2 → 2P3/2, probe 2P1/2 ↔ 2S1/2

– 500 Torr RG + 75 Torr C2H6

– XPAL gain not observable for CW system (<10 kW/cm2)

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

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Apparatus for Alkali-Rare Gas Spectroscopy

• Longitudinal pump: 1 W Ti:S laser (2 GHz line width)

• Co-linear TDL beam for gain measurements

• Side view for fluorescence spectrometer

-4



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DPAL/XPAL Gain Measurement Test Bed (Diode laser scanning D1 line)

Direct probe of population

inversion dynamics

Aids in design of optical

resonators

Portable: take to other

facilities

Extended to spatial

imaging of gain

– Expect significant spatial

effects in power scaling

– Valuable tool for scaling

DPAL, XPAL to high powers

Ti: SPump Beam

Alkali Cell

ProbeBeam

Experimental Verification

K-5034

← Gain Imaging

↓

-5



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Optical Layout for DPAL/XPAL

Gain Measurements

• Ti:S laser pumps D2 transition: F” = 3 or 4

• Ethane collisions produce emission on D1 transition

• TDL laser probes absorption/gain on D1: F” = 3 and 4

-6



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Collisional Broadening Effect Computed D1 Absorption Spectra: Cs

• Collisional broadening greatly expands required scan range

• High optical thickness at elevated temperatures

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

-30000 -20000 -10000 0 10000 20000 30000

Relative Frequency, MHz

Rela

tive A

bso

rba

nce

Computed Spectrum

Cs D1 Multiplet

500 Torr Kr + 75 Torr C2H6

295 K

Low Pressure, Doppler broadening

Cs 2S1/2 – 2P1/2, 894 nm

0

0.1

0.2

0.3

0.4

0.5

0.6

-8000 -6000 -4000 -2000 0 2000 4000 6000 8000

Relative Frequency, MHz

Rela

tive A

bso

rba

nce

F" = 4 F" = 3

F' = 3 4 F' = 3 4

High Pressure, collisional broadening

-7



Absorption/Gain Spectra Cs(2S1/2,F” ↔ 2P1/2,F’), 894 nm

5 cm Cell, 70 °C

Ti:S pump: 2S1/2(F”=4)

1 cm Cell, 100 °C

Ti:S pump: 2S1/2(F”=3)

500 Torr Kr + 75 Torr C2H6

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↓ ↓

• Spectra retain “memory” of which state is pumped

=> Incomplete collisional redistribution

-8



Scaling of DPAL Gain with

Excitation Power Density VG14-002

• Possible loss process at higher power, higher [Cs]

-9



“Lumped” 3-Level Model for Small Signal Gain

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A 2 1 I B 1 3

2

1

3

3 L e v e l S y s t e m

2

2

P 1 / 2

S 1 / 2

2 P 3 / 2 k c [ M ]

J - 9 6 9 3

1

3131

133 n

MkAIB

IBn

c

3

21

2 nMkA

Mkn

Q

c

13

221331311

IB

nAnAIBn

321 nnnnTOT

• Observed gain plateau is due to bleaching

• Predicted maximum gain is consistent with

data

– Distributed over 4 states

• Observed roll-off >10 kW/cm2 may be due to

energy pooling loss of 2P3/2

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Multi-quantum Excitation of Cs(I) Fluorescence

Excitation Near 852 nm

Cs + 500 Torr Kr, 473 K

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“Blue” Cs(72P) Emission at excitation = 852 nm

(a) (b)

Cs + 500 Torr Kr, 473 K

Two-photon resonances leading to 72P occur at other wavelengths:

• 911, 919 nm (72P)

• 884, 885 nm (62D)

• 822 nm (82S)

-12



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Infrared Cs(I) Fluorescence: Excitation of CsXe near 852 nm

-500

0

500

1000

1500

2000

2500

3000

3500

4000

1320 1340 1360 1380 1400 1420 1440 1460 1480 1500

Wavelength, mm

Sig

na

l In

ten

sit

y, c

ou

nts

/s

7 2SJ' → 6

2PJ"

1/2

→1

/2

1/2

→3

/2

7 2PJ' - 5

2DJ"

3/2

→3

/2

3/2

→5

/2

1/2

→5

/2

InGaAs Array Spectrometer

Spectral Resolution = 0.3 nm

-2.0E-04

0.0E+00

2.0E-04

4.0E-04

6.0E-04

8.0E-04

1.0E-03

1.2E-03

1.4E-03

1.6E-03

1.8E-03

2.9 3 3.1 3.2 3.3 3.4 3.5 3.6 3.7

Wavelength, mmS

ign

al In

ten

sit

y

5 2DJ' → 6

2PJ"

3/2

→1

/2

5/2

→3

/2

3/2

→3

/2

7 2PJ' → 7

2SJ"

3/2

→1

/2

1/2

→1

/2

Quartz Transmission

FTIR Spectrometer

Spectral Resolution = 2 cm-1 (0.002 nm)

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

-13



NIR Transitions Observed in Cs-Xe, Rb-Kr

= 1.0 – 1.5 mm, Ti:S Power Density 0.5 – 8 kW/cm2

Cs Rb

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Cs, Rb Near-IR Excitation Spectra: 42F States Pump on D2: ~8 kW/cm2

42F 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)

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Cs(72S1/2) Fluorescence vs. Pump Power

10

100

1000

10000

0.1 1 10

Flu

ore

scen

ce S

ign

al,

co

un

ts/s

Ti:S Average Power Density, kW/cm2

Cs-Xe: 1.469 mm (72S1/2 62P3/2)

200 C

180 C

160 C

140 C

120 C

100 C

90 C

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• Similar results for RbKr, Rb(62S1/2, 42F) transitions

• ½-order scaling with pump intensity indicates

complex excitation mechanism

-16



NIR Excitation: Variation with Pump Wavelength Cs-Xe, 1469 nm (72S1/2 → 62P3/2)

Significant excitation in red wing of D2

– Blue wing is weaker

– Broadening and spectral structure near D2 line

Blue (72P) fluorescence visible down to 840 nm (band head)

Pump Power Density ~8 kW/cm2 VG14-002 -17



NIR Cs(I) Fluorescence Excitation: Cs-Ar

1012 nm (42F5/2,7/2 → 52D5/2)

1469 nm (72S1/2 → 62P3/2)

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NIR Cs(I) Fluorescence Excitation: Cs-Xe

1012 nm (42F5/2,7/2 → 52D5/2)

1469 nm (72S1/2 → 62P3/2)

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NIR Cs(I) Fluorescence Excitation: Cs-Kr

1012 nm (42F5/2,7/2 → 52D5/2)

1469 nm (72S1/2 → 62P3/2)

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NIR Cs(I) Fluorescence Excitation: Ar, Kr, Xe Cs(72S1/2 → 62P3/2), 1469 nm

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Summary and Conclusions

CW gain scaling for Cs/Kr/C2H6 mixtures suggests loss process at higher pump power, larger Cs concentrations – Multiphoton excitation or collisional energy pooling?

Pump D2 lines in Cs/Xe, Rb/Kr mixtures: strong NIR fluorescence in several lines 1-4 mm – Upper state energies 2-3 eV – Excitation of 2F states above 2-photon energy: not multiphoton – Scaling with pump power and T are sub-linear, indicates collisional

up-pumping

Observe excitation of Cs(72S1/2, 42F) 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 2P states – Potential for NIR laser transitions

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Acknowledgements

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Daniel Maser

Physical Sciences Inc., University of Colorado

William Kessler


Michael Heaven

Emory University

High Energy Laser Joint Technology Office

Air Force Office of Scientific Research

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Physical Optical Gain and Multi-Quantum Excitation in