Cavity-Enhanced Velocity Modulation Spectroscopy

Brian Siller, Andrew Mills & Benjamin McCallUniversity of Illinois at Urbana-Champaign

Positive column discharge cell◦ High ion density, rich chemistry◦ Cations move toward the cathode

Velocity Modulation Spectroscopy

Plasma Discharge Cell

+1kV -1kV

Positive column discharge cell◦ High ion density, rich chemistry◦ Cations move toward the cathode◦ Ions absorption profile is Doppler-shifted

Velocity Modulation Spectroscopy

Plasma Discharge Cell

+1kV -1kV

Laser

Detector

Positive column discharge cell◦ High ion density, rich chemistry◦ Cations move toward the cathode◦ Ions absorption profile is Doppler-shifted

Velocity Modulation Spectroscopy

Plasma Discharge Cell

-1kV +1kV

Laser

Detector

Positive column discharge cell◦ High ion density, rich chemistry◦ Cations move toward the cathode◦ Ions absorption profile is Doppler-shifted

Drive with AC voltage◦ Ion Doppler profile alternates red/blue shift◦ Laser at fixed wavelength◦ Demodulate detector signal at modulation frequency

Velocity Modulation Spectroscopy

Plasma Discharge CellDetector

Laser

Want strongest absorption possible Signal enhanced by modified White cell

◦ Laser passes through cell unidirectionally◦ Can get up to ~8 passes through cell

Velocity Modulation Spectroscopy

Plasma Discharge Cell

Laser

Detector

Optical cavity acts as a multipass cell◦ Number of passes =◦ For finesse of 150, get ~100 passes

Must actively lock laser wavelength/cavity length to be in resonance with one another

DC signal on detector is extremely noisy◦ Velocity modulation with lock-in amplifier

minimizes effect of noise on signal detection

Cavity Enhanced Absorption Spectroscopy (CEAS)

Laser

CavityDetector

CavityTransmission

Error Signal

Pound-Drever-Hall LockingTi:Sapph Laser

EOM

PZT

Lock Box

14MHz

Detector

Detector

CEVMS Setup

Lock-In Amplifier

Transformer

Cavity Mirror Mounts

Audio Amplifier

Laser

40 kHz

CEVMS Setup

Doppler profile shifts back and forth Red-shift with respect to one direction of the

laser corresponds to blue shift with respect to the other direction

Net absorption is the sum of the absorption in each direction

Extracting N2+ Absorption Signal

Abso

rpti

on S

trength

(A

rb.

Unit

s)

Relative Frequency (GHz)

V (kV)

t (μs)

Extracting N2+ Absorption Signal

Abso

rpti

on

Relative Frequency

Demodulate detected signal at twice the modulation frequency (2f)

Can observe and distinguish ions and neutrals◦ Ions are velocity modulated◦ Excited neutrals are concentration modulated◦ Ground state neutrals are not modulated at all

Extracting N2+ Absorption Signal

Typical Scan of Nitrogen Plasma

Cavity Finesse 150 30mW laser power

N2+ Meinel Band

N2* first positive band

Second time a Lamb dip of a molecular ion has been observed (first was DBr+ in laser magnetic resonance technique)1

Used 2 lock-in amplifiers for N2

+/N2*

1M. Havenith, M. Schneider, W. Bohle, and W. Urban; Mol. Phys. 72, 1149 (1991).

N2+

◦ Velocity directly dependent on voltage◦ No significant phase shift with respect to voltage

N2*

◦ 78° phase shift with respect to N2+ signal

◦ Peak N2* density occurs when rate of formation

equals rate of destruction

Phase Analysis

V (kV)

t (μs)

Peak N2*

Density

N2+

◦ Velocity directly dependent on voltage◦ No significant phase shift with respect to voltage

N2*

◦ 78° phase shift with respect to N2+ signal

◦ Peak N2* density occurs when rate of formation

equals rate of destruction◦ Analogous to Earth’s heating/cooling cycle with

the sun Sun is brightest at noon (peak voltage) Hottest time of day is 5pm (peak N2

* density) 5 hour time delay in 24 hour day = 75° phase shift

Phase Analysis

Potentially orders of magnitude more sensitivity

Allows for saturation spectroscopy◦ Resolve Doppler-blended lines◦ Better line center determination◦ Width of Lamb dip allows more information to be

extracted from spectra

(See talk FD02 for much more detail)

Advantages Over Traditional Velocity Modulation

Better isolation of plasma noise◦ Faraday cage◦ Dedicated grounding

Improved cavity locking◦ Less noise induced by lock to cavity◦ Allow for locking to a higher finesse cavity

Heterodyne Detection

Potential Improvements

Improves upon absorption strength of velocity modulation experiments

Allows for Doppler-free spectroscopic studies of molecular ions

Very general technique

To unlock full potential, must first minimize plasma noise with careful experimental setup

CEVMS Summary

McCall Group Funding

◦ Air Force◦ NASA◦ Dreyfus◦ Packard◦ NSF◦ Sloan

Acknowledgements