Lineshapes and Sensitivity of Spectroscopic Signals of N2 in a Positive Column Collected Using Noise Immune Cavity Enhanced Optical Heterodyne Velocity Modulation Spectroscopy

1
Lineshape and Sensitivity of Spectroscopic
Signals of N2 in a Positive Column Collected
Using NICE-OHVMS
Michael Porambo, Andrew Mills, Brian Siller,
Benjamin J. McCall University of Illinois at
Urbana-Champaign 20 June 2011
2
Outline

• Introduction
• Lineshape Description, Analysis, Ultra-high
  Resolution Spectroscopy
• Sensitivity Comparison
• Summary, Conclusions, Present and Future Work

3
Spectroscopic Techniques
Velocity Modulation Spectroscopy (VMS)1,2
1Gudeman and Saykally, Ann. Rev. Phys. Chem.
1984. 2Stephenson and Saykally, Chem. Rev. 2005.
Optical Heterodyne3 Velocity Modulation
Spectroscopy (OHVMS)4
EOM
3Bjorklund and Levenson, Appl. Phys. B
1983. 4Lindsay, Ph.D. Thesis, University of
Chicago, 2002.
Cavity Enhanced Velocity Modulation
Spectroscopy4,5
4Siller et al. Optics Lett. 2010. 5Mills et al.
Chem. Phys. Lett. 2010.
Noise Immune Cavity Enhanced Optical Heterodyne
Molecular Spectroscopy (NICE-OHMS)6,7 6Ye et al.
J. Opt. Soc. Am. B 1998. 7Foltynowicz et al.
Appl. Phys. B, 2008.
Noise Immune Cavity Enhanced Optical Heterodyne
Velocity Modulation Spectroscopy (NICE-OHVMS)
EOM
4
NICE-OHVMS
5
N2 Signal with NICE-OHVMS
Lamb dips from optical saturation
Sideband-carrier interaction
1 GHz
A. U.
500 MHz
Sideband-sideband interaction
Carrier-carrier interaction
NICE-OHVMS spectrum of Q11(14) of N2 acquired
with 1 GHz heterodyne detection bandwidth.
6
Heterodyne Detection Bandwidth
Relative Frequency (MHz)
As cavity length is scanned, FSR changes.
Laser sidebands do not couple into the cavity as
efficiently, noise immunity suffers. 1.02 GHz (9
FSR) 9 kHz shift in longitudinal mode with
respect to sideband. 113 MHz (1 FSR) 1 kHz
shift in longitudinal mode with respect to
sideband.
7
Absorption and Dispersion
-

8
Absorption and Dispersion

• Absorption and dispersion related by the
  Kramers-Kronig relations.

Example for Gaussian absorption profile
9
Heterodyne Detection Bandwidth
Detector
PZT
TiSapph Laser
EOM
EOM
Absorption
X
9 Cavity FSR 1.02 GHz
1 Cavity FSR 113 MHz
Y
Lock-In Amplifier
Lock-In Amplifier
40 kHz Plasma Frequency
Absorption Signal
Dispersion Signal
Dispersion
X
Y
X
Y
X
Y
10
113 MHz Detection
Dispersion
Absorption
113 MHz Sidebands 1 Cavity FSR
Absorption
Dispersion
Lock-In X Lock-In Y
11
Sub-Doppler Spectra
Absorption
Dispersion
No center Lamb dip in absorption
Spectra calibrated with optical frequency
comb Frequency precision to 1 MHz!
12
Ultra-High Resolution Spectroscopy
Absorption
Dispersion
Red Data Blue - Fit
Red Fit Blue - Data
113 MHz
Sub-Doppler fitting equation modeled as
convolution of Gaussian and Lorentzian absorption
and dispersion profiles (2 absorption/each, 3
dispersion/each) Line center from fit
326,187,572.2 0.1 MHz After correcting for
systematic problems, line center measured to
within uncertainty of 300 kHz!
13
Signal and Noise Calculations
NICE-OHVMS (1 GHz)
OHVMS (1 GHz)
VMS
CEVMS
Signal-to-noise ratio calculated for different
detection techniques under the same conditions.
NICE-OHVMS S/N factor of 2 greater than the
next sensitive technique!
14
Technique Comparison
VMS
OHVMS
CEVMS
NICE-OHVMS
15
Summary and Conclusions

• NICE-OHVMS addresses well challenges in direct
  absorption/dispersion spectroscopy of ions.
• Distinctive, absorption/dispersion lineshape with
  Lamb dips.
• Precise line centers obtained using Lamb dips and
  calibrating to optical frequency comb (1 MHz
  precision).
• S/N greatly improved over VMS, OHVMS, and CEVMS.

16
Present and Future Work

• Vibrational spectroscopy in the mid-IR
• Positive column discharge setup with CW OPO
  (Aculight Argos).
• Study molecular ions of astronomical, fundamental
  chemical interest (e.g., CH5).

Highly sensitive technique for molecular ion beam
detection
McCall group ion beam instrument
Aculight Argos CW OPO http//www.lockheedmartin.co
m/data/assets/ms2/pdf/ArgosSF.pdf

• Direct absorption/dispersion spectroscopy of
  N2 in a fast ion beam.
• Stay tuned for next talk (MI11) on ion beam.

17
Acknowledgments

• McCall Research Group
• Ben McCall
• Andrew Mills
• Brian Siller
• Sources of Funding
• Air Force Research Corp.
• NASA Univ. of Illinois
• Dreyfus
• Packard
• NSF
• Sloan