1. Spectroscopic Engineering in the Submillimeter
Frank C. De Lucia
Department of Physics
Ohio State University
June 19, 2013
Columbus, Ohio
I can hear some of you saying, “that’s an odd title for a meeting like this- what’s he going to talk about? We spectroscopists are scientists – not engineers. We do QM Hamiltonians and fit spectral constants to many decimal places.

2. Submillimeter Spectrum of Nitric Acid
Before you conclude that I’m not one of you, in the spirit of this meeting let me establish my credentials by showing a slide with so many numbers that you can’t read it.

3. Even Better - Perturbations

4. Outline The Underlying Physics
Two Examples: Microwave Limb Sounder and ALMA
Other Examples
Opportunities/The Submillimeter Engineer’s Tool Kit
Two legacy applications: Sensors and Imaging
Engineering non-ambient environments
Cold molecules
Molecular ions
Plasmas
Mass market technology to enable powerful ~ ‘free’ systems

5. From Where Did We Come? Giants of Spectroscopic Science:
Hertzberg Wilson Dennison Nielsen
Townes
The First Submillimeter Engineer
Motivation for development of the maser:
Molecular generator to address the submillimeter source problem
Molecules as engineering medium to accomplish this
Last 150 pages of his book devoted to engineering
Frequency standards, analytical chemistry, spectrometers, . .
Townes went on to be a astronomical engineer, but not as a servant, but as a leader in the field of astronomy: first polyatomic molecule in space
Notice that the spectroscopic engineers are posed in front of blackboards – Townes is pointing at the state-selector in his maser

6. With Whom Are We Going There? The Three Cultures*
Where Are We Going?
With Whom Are We Going There?
The Three Cultures*
THz/Optical
Optical Society of America,
“THz Spectroscopy and Imaging Applications”
Toronto, June 14, 2011
Millimeter/Electronic (Engineering)
IEEE International Microwave Show 2011
“Workshop on MM-Wave and Terahertz Systems”
Baltimore, MD, June 6, 2011
Submillimeter/Electronic (Scientific)
International Astronomical Union,
“The Molecular Universe”
Toledo Spain, June 2, 2011
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With apologies to C. P. Snow, “The Two Cultures”
We are not in a spectroscopic vacuum, but there is far too little communications.
Two summers ago I was giving three talks in the two weeks ahead of this spectroscopy conference. In preparing for the talks I became concerned that they had too much overlap. Then I realized that it was unlikely that anyone would be at more than one of these talks. Then I realized that it was unlikely that anyone at one of these talks would even know anyone at either of the other two talks.
This has many negative consequences, not the least of which is that much time and money is devoted to finding bad solutions that were solved decades ago by one of the other communities. This seems undiminished to this day.

7. Do We Know Each Other?
As a recent example, I call your attention to this web site. Many of us think that we do ‘high frequency’ work – often for a number of decades. A quick look seems to show that none of us are in this list.
In fairness it’s a two way street, I hardly recognize many in this list.

8. Radiation and Interactions: Orders of Magnitude
1018 K
1017 K
1016 K
1015 K
1014 K
1013 K
1012 K
1011 K In
1 MHz
1010 K
109 K
108 K
107 K
106 K
100 GHz kT
I said at the start that I wanted to use physics as a tool for tying much of this together and especially for looking towards opportunities in the future.
kT(300 K) = 6000 GHz => thermal emission from both atmospheric and astronomical sources
kT (3 K) = 60 GHz => thermal emission from space/cryogenic sources
For samples in thermal equilibrium, Doppler broadening is proportional to frequency
Optimum sample quantity is then proportional to frequency

9. The THz is VERY Quiet even for CW Systems in Harsh Environments
Experiment: SiO vapor at ~1700 K
But what about noise?
For a long time the THz-TDS community in many papers (which are referenced in the paper listed on this chart) said that cw SMM systems were, “plagued by noise” and that pulsed THz-TDS systems were required to overcome this limitation. Since I had always used cw system and never seen ANY background noise, I was puzzled and assume that this nonsense would go away. It didn’t, so (after great struggles with referees – I am grateful to a knowledgeable editor who immediately saw what was going on) I published the paper above. Briefly state they missed the difference between power and noise (about 7 orders of magnitude) and the fact that there is nothing special about 1 cm^2 of area – what counts in the area of a mode (another four orders of magnitude). This seems to have worked since the “plagued by noise” discussions seems to have stopped – but this still is the paper doomed to be cited rarely.
All noise from 1.6 K detector system
1 mW/MHz -> 1014 K

10. ABSORPTION COEFFICIENTS
Number Boltzmann Einstein Photon
Density Factor Coefficient Size
(in long wavelength limit)

11. Frequency and Temperature Factors
(Partition function and degeneracy)
(Pressure broadening = Doppler broadening)
The strong temperature and frequency function has a major impact on spectroscopic engineering.
10 GHz GHz:
300 K - 3 K:
1000 K - 1 K: x 107

12. Low Atmospheric Clutter Background [The miracle of the Microwave]
Nitric acid at ~ 1 ppb is first ‘clutter molecule’ in low pressure sample
Whereas atmospheric clutter is a major limitation in the IR (largely due to water and carbon dioxide), it has little impact in the SMM/THz. A careful simulation shows that even in the polluted atmosphere the clutter limit to the detection of molecules on the Army’s list of toxic industrial chemicals in below 1 ppt.

13. The Physics is very Favorable:
Simple, but powerful systems to study small, fundamental molecules are possible
Today
Commercial availability of submillimeter components makes possible much more sophisticated and flexible systems
This talk is about the spectroscopic engineering that involves these systems
Here is a system we built more than 40 years ago. It was very simple, but the physics, especially of the desirable small, fundamental molecules is so favorable that it was possible to observe and model their spectroscopy.

14. Epitome of Spectroscopic Engineering: JPL’s Microwave Limb Sounder
A Priori Predicted Spectral Signature of the Atmosphere
Needed, Sought, and Achieved ‘Complete’ Spectroscopic Model via
Quantum Mechanical Models:
The ‘Pickett’ Program
___________________
Required careful knowledge of atmospheric concentrations and temperatures
An engineered spectroscopic data base: (1) selection of molecules and states,
(2) table of results for use by non-experts
Employed a generation of spectroscopist -> accomplished atmospheric scientists
Note the use of QM model
A priori predicted spectral response for the satellite – like building a bridge; you get one design for something you are going to launch
I believe that this was the motivation for the development of HP’s SPFIT. SPCAL,
Contrast this with Astro, why different

15. Enabled a Complete Spectroscopic Model of the Atmosphere in the Millimeter/Submillimeter

16. The ALMA Spectroscopy Problem is Much More Challenging:
A Spectroscopic Engineering Work in Progress
people linage
technical linage

17. Completeness and Intensity
Calibration in Orion
No a priori catalog of Orion
Many more detectable species
Narrower lines
Larger molecules with complex perturbations
Four full sessions at this meeting
Requires a different kind of engineering than MLS
This figure of our results is taken from the NSF website
_____________
Figure courtesy of NSF

18. A Contribution to the Engineering: Complete Experimental Models
Challenges for Quantum Mechanical Models
Completeness: Excited Vibrational States (hard to analyze perturbed states)
Frequency calculations: Extrapolations in J and K
Intensities: Especially in flexible molecules

19. Completeness in Ethyl Cyanide
Experimental
QM Catalog
ALMA
CES Simulation at 190 K

20. Frequency Calculation [perturbed states are hard to calculate]QM
Vinyl Cyanide

21. Intensities in Methanol [and other flexible molecules?]
Usually in the SMM/THz you measure frequencies and calculate intensities. There are two reasons: (1) measurement of absolute intensity is hard, (2) we believe that the measurement of the dipole moment via the Stark Effect, combined with the angular momentum operators from the frequency fit will lead to accurate calculation of intensities.
These results from our intensity calibrated experimental spectra show that this is not true for molecules like methanol (which have internal rotation).

22. Other Examples of Spectroscopic Engineering
Gordy: Brought spectroscopic technology to astronomy/engineering problem
Flygare: Electronic time domain techniques for spectroscopy
Claude Woods: Brought spectroscopic insight, to engineering problem, and launched ion spectroscopy in the mm/submm
Krupnov, Burenin: Backward Wave Oscillator techniques for submillimeter spectroscopy
Belov et al.: BWO lamb dip spectroscopy
RAD-3 Spectrometer
Liebe: Propagation models
Pate: Modern digital implementation of electronic time domain techniques
Crowe, Hesler (VDI): A commercial, broadband mm/submm technology
Herschel and SOFIA:
mm/submm has long history of spectroscopic engineering (service to ourselves, service to others (NSF: broader impact!!!!) subtle distinction – you want both – but it depends on what you do
Herschel and SOFIA: Too many people to name – technical engineering and spectroscopic engineering – of the same people doing both

23. A piece of Spectroscopic Engineering History: The First mm/submm Astronomy
Accomplished new science
Used heterodyne third harmonic mixer for receiver (technology from spectroscopy)
Humidity in Durham ended astronomy at Duke, but graduate student (Burrus) at time went on to build the receivers for the Bell Labs Penzias/Wilson millimeter wave astronomy group
From Townes (How the LaserHhappened), “Charlie Burrus, who was just down the hall from us, had developed the diodes and mixer assembly for a millimeter-wave (pre-optical fiber days) broadband communications system We returned to the NRAO 36 ft with a higher frequency Burrus receiver.”
Bell labs gave Burrus diodes to many other labs to start millimeter wave radio astronomy.
Burrus was hired to do millimeter wave communications (pre optical fiber).

24. What is in the Submillimeter Spectroscopic Engineer’s Tool Kit?
What is the Physics?
Strong molecular interactions
Small Doppler widths
Highly specific fingerprints (Erot << kT)
Very quiet background
Low diffraction relative to microwave
Penetration of materials and hostile environments
What are the enablers?
Very bright electronic sources
Flexible and agile control
Potential for very low cost

25. Some Submillimeter Opportunities
Well known and well represented at this meeting
Astronomy and Astrophysics
Gas sensors and process control
Remote sensing of the upper atmosphere
Well known in other communities
Imaging
Non-ambient environments
Cold molecules (hv/kT ~ 1)
Non-thermal (e.g. plasmas) (quiet and transparent in SMM)
Laser diagnostics
Ions and free radicals
Impact of mass market technologies
A black art commercial (expensive) almost FREE
Several of these are well represented at this meeting, so I will only acknowledge them at this point (but I might use to illustrate underlying physics in the context of applications familiar to those at this meetin)
Can we engineer or exploit useful environments – optimized for SMM
It depends on who is asking. This community and this conference have always pitched a ‘big tent’ IAC and TAM are to be commended.
BM and TAM will carry on this tradition at Illinois

26. Two SMM/THz Legacy ‘Public’ Applications
-- Clear, but Challenging Paths to Success --
IMAGING ANALYTICAL CHEMISTRY
Only want to acknowledge here. These will happen. A major issue is cost, which I will address in a few minutes when I talk about technology.

27. Non-ambient Environments
Low temperature environments
Traps and beams
Plasmas
Molecular ions
Here I would like to pick out a couple that I find interesting

28. Cold Molecules: Quantum Collisions
300 K K
_________________________________
The cold atom community has become interested in recent years, but I would like to talk about molecules and spectroscopy that most of us in this room would recognize – not Di-atoms
Correspondence Principle
The predictions of the quantum theory for the behavior of any physical system must correspond to the prediction of classical physics in the limit in which the quantum numbers specifying the state of the system become very large.

29. An Experimentalist’s History and Perspective
Pioneering Theory of Green and Thaddeus
Explore New Experimental Regimes
What is the physics in the regime where kT ~ hvr ~Vwell?
Erot ~ Ewell ~ kT

30. Pressure broadening by Helium
Typical Spectra – HCN
Pressure broadening by Helium

31. Engineering of Plasmas for Spectroscopy
J. Chem. Phys. 78, 2312 (1983).

32. Molecular Ions at Low Temperature
Minimal Electron Beam Heating
Molecular Ions at Low Temperature
11.2 K
28 K

33. MA01 Low Temperature Trapping: From Reactions to Spectroscopy
S. Schlemmer, O. Asvany, and S. Brunken Universitat zu Koln
Traps can be a powerful and flexible tool in the submillimeter
23 K
I hope that you had the opportunity to hear Stephan Schlemmer’s Plenary talk Monday Morning. Instrument of the type shown here are exceptionally flexible and interesting.

34. Plasma Diagnostics in a Discharge Laser*
In the submillimeter plasmas are transparent and quiet
Experimental arrangement for the measurement of number density and temperatures in the plasma of an HCN discharge laser.
Relaxation of excited vibrational state population that leads to the HCN laser
Vibrational temperatures of HCN (100) and CO (v=1). Gas mixture was N2:CH4:CO = 1:2:2 for a total pressure of 200 mTorr.
__________________________________
*D. D. Skatrud and F. C. De Lucia, "Dynamics of the HCN Discharge Laser," Appl. Phys. Lett., Vol. 46, pp , 1985.

35. Semiconductor Plasma Diagnostics Applied Materials Semiconductor
CF2 Concentration
Applied Materials Semiconductor
Plasma Reactor
Temperature
Y. Helal, et al. WH09

36. The Technology Future How close are we?
High resolution, easily calibrated, and flexible submillimeter technology from the wireless community will become essentially free.
These will not be ‘toy’ systems.
This technology can also require little space and little power.
How close are we?
Wireless HDTV communications link at 60/240 GHz
Custom integrated CMOS Rx/Tx in 200 – 300 GHz region
Off the shelf family of chips/modules to 100 GHz

37. A SiGe BiCMOS 16-Element Phased-Array Tx/Rx for 60GHz Communications*
Combined Tx/Rx 16 Channel Evaluation Board
We have used this chip set to do spectroscopy
Integration includes synthesizer, modulator, and steered phased array
Applications include wireless HDTV
Single ‘engine’ flexible enough for communications, imaging, spectroscopy
Extension to 240 GHz under discussion
*Courtesy of Alberto Valdes-Garcia and Arun Natarajan, Watson Laboratory, IBM

38. CMOS Integrated Engine for 200-300 GHz
Antennas: Rashaunda Henderson (UT-D)
Receiver: Bhaskar Banerjee (UT-D)
Let’s not denigrate the EE
This engine is widely applicable: spectroscopy, communications, imaging
Transmitter: Kenneth O (UT-D)
With integrated synthesizer
Currently less microwave power (~0.1 mW) than III-V
(Prototype: Summer 2013)

40. Off the Shelf System Hardware
Wireless Components: To 100 GHz - Chip costs <$100

41. Symbiosis among Spectroscopy and Spectroscopic Engineering
Type 1: Submillimeter spectroscopic analysis is a key component of system designed to address broader problems
Astronomy, Atmospheric Science, Chemistry, Sensors, . . .
Type 2: Submillimeter spectroscopists develop technology of importance to other fields
Astronomy, Imaging, Communications, . . .
Type 3: Molecules/spectroscopy provide engineering building blocks
Lasers, Masers, . . .
Type 4: Engineer molecular environments for spectroscopy
Molecular ions, traps, cold molecules, . . .
I’ve used ‘submillimeter spectroscopic engineering’ in several contexts
I will use this in an attempt to tie everything together

42. Summary We love the science of spectroscopy
A mature submillimeter spectroscopy makes spectroscopic engineering possible. Well defined (but sometimes complex) theory.
Favorable physics in submillimeter
Rotational fingerprint is strong, specific, and ubiquitous
Available technology – go from hardest to easiest
Wireless technology promises to make the submillimeter particularly interesting because the inexpensive technology can also be very powerful
Absolute frequency calibration and spectral agility
‘Zero’ instrument width
High brightness temperatures
Quiet, low clutter backgrounds
Systems can be very small and low power – photons are small
Spectroscopic engineering in the submillimeter is many faceted and provides an accelerating symbiotic family of opportunities

43. Students, Coworkers, and Colleagues The Spectroscopic Community
Acknowledgements
Students, Coworkers, and Colleagues
The Spectroscopic Community
My inability to list acknowledgments here and my selective use of examples in the talk in no way diminishes my appreciation of all of the contributions
I would also like to acknowledge to contributions of the wider spectroscopic community - while the final responsibility of the opinions expressed here is mine they owe there origins to many
Finally, I would like to thank the many agencies that have seen a relation between what we love to do and their mission. If you look closely you will see that the Army Research Office logo is a little larger. They have played a seminal role in the development of much of this ‘Spectroscopic Engineering’. You will also note that they are the sponsor of this conference shown on the front cover.