1. Atmospheric Remote Sensing Via Infrared-Submillimeter Double Resonance
Sree H. Srikantaiah, Jennifer Holt, Christopher F. Neese
Frank C. De Lucia
Department of Physics, Ohio State University
Dane J. Phillips
IERUS Technologies
Henry O. Everitt
AMRDEC, Redstone Arsenal

2. Double resonance spectroscopy to transcend pressure broadening effects at Atmospheric pressure
GHz 2T 1T
500mT
100mT
Traditional smm /THz spectroscopy has absolute specificity at low pressures due to highly resolved rotational lines - great technique for sensing at low pressures
At atmospheric pressure this specificity and sensitivity is lost due to pressure broadening (5GHz linewidths)
Difficulty due to added problems in detection of molecular resonances due to atmospheric fluctuations
But the linewidths at atmospheric pressure make this much harder
At bottom, briefly note, but do not elaborate on the Specificity and sensitivity points
(this is the introduction to the next chart)

3. Double resonance (DR) sensor : Methodology
Example 12CH3F
Pulsed/CW
CO2 Laser
THz Detector
THz Source
Gas Cell
Choose suitable IR pump laser – absorption line coincidence
Monitor modulation of the linked rotational transitions (at THz frequencies) initiated by the IR pump laser
Establish a 3D specificity matrix (IR pump, THz probe, Time constant of the DR trace) with as many points as possible
- to increase specificity for a particular molecule
Spectral and temporal features

4. Optimum regime for atmospheric pressure DR sensor
The collisional relaxation rate at atmospheric pressure is ~ 100ps
Saturate (or come as close as you can) in 100 ps
Probe the directly pumped rotational transitions before collisions equilibrate states.
Linewidths of 10 GHz → large gain bandwidth for DR signal due to coalescing of closely spaced pressure broadened rotational lines getting pumped simultaneously

5. Experimental Setup 100 ps pump laser system TEA laser : 100ns
Plasma switch : 25ns (10ps falling edge)
Hot cell : < 100ps
CO2 Hot Cell
OFID
~ 100ps
TEA CO2 laser
50ns 30 Hz
0.15 J
Spectrometer
Plasma shutter
High speed IR detector
THz Heterodyne Receiver
THz source
Sample cell
Fast Data acquisition system
Scope: 6GHz, 40 GS/s
Mixer: 40 GHz Bandwidth
Detector diode: < 1ns rise time
DR signal
6 GHz , 40 GS/s Digitizing Oscilloscope
Trigger

6. 25 ns HWHM , ~100 ps falling edge
100 ps IR Laser system
Transverely Excited Atmospheric pressure Laser (Source)
OFID filter
(Hot Cell at 400o C)
Plasma shutter (Fast Switch)
TEA Laser output
50 ns HWHM
Plasma Switch output
25 ns HWHM , ~100 ps falling edge
CO2 Hot cell output
~100 ps HWHM

7. P-type Ge Hot Hole detector: True pulse-width measurement
relaxation time : ~ 2ps
Expected rise time (1cm crystal) : 100ps
Detector performance comparison
100 ps pulse !

8. Experimental Results Sample : 12CH3F (1 Torr of Gas)
Pressure range : T (air)
Pump : 9P20 CO2 Laser excitation (Rabi Frequency = ~ 10 GHz)
Probe: GHz (V3=1 , J12,K2, Q branch)
Cell : 2mm x 4mm cross section , 30 cm long rectangular waveguide cell
Experimental Results
few collisions
106 collisions
1 collision
At 760 T, 1 collision = 100 ps
Goal : pumping rate comparable to Collision rate
(Rabi Frequency dependent)
Phase 1: 100 ns pulse
Phase 2: Plasma switch - 25ns pulse
Phase 3: 100 ps pulse
K relaxation
Modify the energy level diagram depicting movement of population
Rabi frequency is large enough to pump all the Ks but not adjacent Js
J relaxation

9. Pump Laser pulse temporal shape: Nature of Double Resonance signature
IR Pump Laser pulse
(Plasma switch o/p)
-25 ns pulse width
-10ps cutoff transient
500ns pulse structure leak – remnant of the TEA laser pulse after plasma switch shuts off
Corresponding Double resonance signal
Pump Laser pulse temporal shape: Nature of Double Resonance signature
25 ns
500 ns
Fast component
Slow component

10. Phase 2: Double resonance with IR plasma switch pump
DR signal at
700 T
DR signal evolution with pressure
3.5 % modulation at 700 T
S/N of 35:1

11. Pressure dependent Collision time vs DR Signal
81 %
Pressure dependent Collision time vs DR Signal
IR pump pulse
10 T
90 %
3.5 %
100 T
66 %
700 T

12. Illustration of Physics
At low pressure Larger J non-equilibrium
Pump on
Pump off
Pump on
Pump off
Vibrational State equilibrium
Vibrational State equilibrium
Pump off (vibrational level Equilibrium)
Pressure broadening on probe
Pump access to more J states
Reduction of J non- equilibrium
Pump on (J – non equilibrium)
Approaches Vibrational state equilibrium
Add Energy level diagram
Equilibration with other excited vibrational states

13. Effects of Rabi Frequency and Pressure broadening
Only Δνrabbi gain ∼ 10 GHz
Δνrabbi gain ∼ 10 GHz
Pressure broadening ∼ 5 GHz
Pump on
(J non Equilibrium)
Pump off
(Vibrational Equilibrium)
Δν gain ~ 100 MHz
(pressure broadening + K structure)
Δν gain ~ 5 GHz
Rabi Frequency: 13.5 GHz
Doppler width = 36 MHz
Pressure broadened linewidth = 4.6 GHz (FWHM) – Atmosphere

14. Phase 3: DR signal with 100 ps IR pump
OFID Hot Cell output
Double resonance 760 T
No DR with 100 ps pulse !!
100 ps IR pump
Power Comparison:
Avg Power ( Energy) [Peak power]
TEA Laser : mW (50 mJ) [0.5 MW]
Plasma Switch : mW (7 mJ) [0.35 MW]
Hot Cell (100 ps pulse) : µW (2µJ) [0.02 MW]
Very low peak power !!

15. 100 ps (one collision) Physics
Pump population concentrated in ∼ 1 J
Many collisions distribute population into many Js
1 Collision
260 collisions ~ equilibrates the vibrational pool
1 Collision doesn’t equilibrate the vibrational pool
260 Collisions
Rabi Frequency does not scale with Pressure
But must be big enough to pump on a 100 ps time scale
25 ns (1 collision) experiment is a surrogate for the 100ps (1 collision) experiment

16. Summary
IR – submillimeter Double resonance studied with 25 ns IR pump.
Rabi Frequency Effects
Pressure Broadening Effects
With a 25 ns pump , Data at 700 T indicates quick equilibration of pumped population into the vibrational manifold.
Δα when pump is on (local J non-equilibrium) is only twice the value of Δα when pump is off (vibrational equilibrium)
25 ns (1 collision) experiment is a good surrogate for the 100ps (1 collision) experiment – very clearly depicts the signal enhancement achieved by staying in the 1 Collision limit

17. Defense Threat Reduction Agency
This work was supported by grants from:
Defense Threat Reduction Agency
(DTRA)