Spectroscopic Engineering in the Submillimeter

Frank C. De LuciaDepartment of PhysicsOhio State University

June 19, 2013Columbus, Ohio

Submillimeter Spectrum of Nitric Acid

Even Better - Perturbations

OutlineThe 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

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, . .

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__________________With apologies to C. P. Snow, “The Two Cultures”

Do We Know Each Other?

kT

Radiation and Interactions: Orders of Magnitude

1018 K1017 K1016 K1015 K1014 K1013 K1012 K1011 K

In 1 MHz

In 1 MHz

1013 K1012 K1011 K1010 K109 K108 K107 K106 K

In 100 GHz

In 100 GHz

kT(300 K) = 6000 GHz => thermal emission from both atmospheric and astronomical sourceskT (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

The THz is VERY Quiet even for CW Systems in Harsh Environments

Experiment: SiO vapor at ~1700 K

All noise from 1.6 K detector system

1 mW/MHz -> 1014 K

mn NFmc

1 e hmn / kT Bmn hmn

ABSORPTION COEFFICIENTSNumber Boltzmann Einstein PhotonDensity Factor Coefficient Size

8 3

3h2 m g n2

gx, y, z

1

hmn / kT (in long wavelength limit)

Frequency and Temperature Factors

mn 8 2

3ck

N

FmT

mn2 m g n

2

gx, y, z

mnT 5 / 2 (Partition function and degeneracy)

1 (Pressure broadening = Doppler broadening)

mn 3

T 5 / 2

10 GHz - 1000 GHz: 106

300 K - 3 K: 105

1000 K - 1 K: 3 x 107

Low Atmospheric Clutter Background[The miracle of the Microwave]

Nitric acid at ~ 1 ppb is first ‘clutter molecule’ in low pressure sample

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

Epitome of Spectroscopic Engineering:JPL’s Microwave Limb Sounder

Needed, Sought, and Achieved ‘Complete’ Spectroscopic Model via

Quantum Mechanical Models:

The ‘Pickett’ Program http://spec.jpl.nasa.gov/___________________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

A Priori Predicted Spectral Signature of the Atmosphere

Enabled a Complete Spectroscopic Model of the Atmosphere in the

Millimeter/Submillimeter

The ALMA Spectroscopy Problem is Much More Challenging:

A Spectroscopic Engineering Work in Progress

Completeness and Intensity Calibration in Orion

_____________ Figure courtesy of NSF

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

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

Completeness in Ethyl Cyanide

Experimental

QM Catalog

ALMA

CES Simulation at 190 K

Vinyl Cyanide

Frequency Calculation[perturbed states are hard to calculate]

QM

Intensities in Methanol [and other flexible molecules?]

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:

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

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

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

Two SMM/THz Legacy ‘Public’ Applications -- Clear, but Challenging Paths to Success --

IMAGING ANALYTICAL CHEMISTRY

Non-ambient Environments

Low temperature environments

Traps and beams

Plasmas

Molecular ions

LN2Reservoir

LHe Reservoir

Buffer Gas Line Pot Pumping Line

Cell/Pot

Continuous LHe Fill Line

Vacuum Jacket

4K and 77K Heat Shields

40 cm

50 cm

Sample Gas Injector

COLLISIONAL COOLING APPARATUS

Sample Gas Injector

Expeimental Cell

Liquid Helium Pot

Buffer Gas Line

Pot PumpingLine

Millimeter WaveProbe Path

Cold Molecules: Quantum Collisions

Lb

2Em

300 K 1 K_________________________________

L ~ 30J ~ 10

L ~ 2J 1

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.

An Experimentalist’s History and PerspectivePioneering Theory of Green and Thaddeus

Explore New Experimental Regimes What is the physics in the regime where kT ~ hr ~Vwell?

LN2Reservoir

LHe Reservoir

Buffer Gas Line Pot Pumping Line

Cell/Pot

Continuous LHe Fill Line

Vacuum Jacket

4K and 77K Heat Shields

40 cm

50 cm

Sample Gas Injector

COLLISIONAL COOLING APPARATUS

Sample Gas Injector

Expeimental Cell

Liquid Helium Pot

Buffer Gas Line

Pot PumpingLine

Millimeter WaveProbe Path

Erot ~ Ewell ~ kT

Typical Spectra – HCNPressure broadening by Helium

J. Chem. Phys. 78, 2312 (1983).

Engineering of Plasmas for Spectroscopy

Molecular Ions at Low Temperature

Minimal Electron Beam Heating

11.2 K

28 K

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

Experimental arrangement for the measurement of number density and temperatures in the plasma of an HCN discharge laser.

Plasma Diagnostics in a Discharge Laser*In the submillimeter plasmas are transparent and quiet

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. 631-633, 1985.

Relaxation of excited vibrational state population that leads to the HCN laser

Semiconductor Plasma Diagnostics

Temperature

CF2 Concentration

Y. Helal, et al. WH09

Applied Materials Semiconductor Plasma Reactor

The Technology Future

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

A SiGe BiCMOS 16-Element Phased-Array Tx/Rx for 60GHz Communications*

*Courtesy of Alberto Valdes-Garcia and Arun Natarajan, Watson Laboratory, IBM

Combined Tx/Rx 16 Channel Evaluation Board

•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

Transmitter: Kenneth O (UT-D)

Receiver: Bhaskar Banerjee (UT-D)

CMOS Integrated Engine for 200-300 GHz

Antennas: Rashaunda Henderson (UT-D)

(Prototype: Summer 2013)•With integrated synthesizer•Currently less microwave power (~0.1

mW) than III-V

Off the Shelf System Hardware

Wireless Components: To 100 GHz - Chip costs <$100

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, . . .

SummaryWe love the science of spectroscopyA mature submillimeter spectroscopy makes spectroscopic engineering possible. Well defined (but sometimes complex) theory.

Favorable physics in submillimeterRotational fingerprint is strong, specific, and ubiquitous

Available technology – go from hardest to easiestWireless 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

Acknowledgements

Students, Coworkers, and Colleagues

The Spectroscopic Community