WMD

Science and Technology in the Submillimeter/Terahertz Spectral Region

Frank C. De Lucia

Ohio State UniversityDepartment of PhysicsColumbus, OH, 43210

Overview

What’s a THz?

What’s a ‘Killer Ap’?

Physics of the SMM/THz

Specific Applications Solids Gases

Opportunities - THz + ‘X’

Conclusions and Questions

What’s a THz?

(With a broad definition, what properties are available at a particular choice of frequency?)

There are Established SMM/THz ‘Killer Aps’ Technologies which approach fundamental limits

Fundamental Molecular Studies - Spectroscopy, DynamicsLaboratory AstrophysicsScience in the Field/Remote sensingInterstellar medium, stellar formationUpper atmospheric chemistry

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- atm - atm + 0.8 atm of N2

Two Old, but New, ‘Killer Aps’Identify Need - Competitive SMM/THz Solution? - Do it

-- Clear, but Challenging Paths to Success --

IMAGING ANALYTICAL CHEMISTRY

Widely Promoted ‘Killer Aps’ “are working on a T-ray imaging system that can look through walls, doors, and window curtains to locate people and

weapons within a building” “has also produced a THz method for long-distance sensing of object buried in soil.”

“Cancer cells, especially melanoma tissues, also vibrate in THz and lend themselves to early detection by doctors equipped with THz devices”

“. . . Have already demonstrated . . Passive THz-wave techniques can detect concealed nuclear materials, as well as detect and make images of chemical and radioactive plumes. THz waves interact well with biological molecules, making it possible to remotely detect biological aerosols in less than a minutes with low false-alarm rate.”

“They used envelopes containing various white powders — flour, sugar, talcum powder and spores of a benign species of bacterium, which acted as a surrogate for anthrax — and found that they could detect a characteristic absorption signature for the spores.

“T-rays can detect breast cancer and see underground toxins better than other technologies, such as conventional x-rays.”

“These so-called t-rays can, like x-rays, see through most materials. But t-rays are believed to be less harmful than x-rays. And different compounds respond to terahertz radiation differently, meaning

a terahertz-based imaging system can discern a hidden object’s chemical composition.”

Stage 1: These are powerful ‘public’ Killer Aps. What do we need to do to convince the ‘public’ that we can do them?

Stage 2: Show that there is a competitive SMM/THz Solution.What phenomenology do we need to demonstrate?

Stage 3: What technology do we need to develop to demonstrate?

Physics in the SMM/THz

Degrees of Freedom - What is the Physics?

Energetics and Temperature: h/kT

System and Ambient Noise

Linewidths (Qs), Specificity, Signatures, and Clutter

Illustrative Examples -solids -gases

TemperaturekT (300 K) = 200 cm-1

kT (1.5 K) = 1 cm-1

kT (0.001 K) = 0.0007 cm-1

FieldsqE (electron) >> 100000 cm-1

E (1 D) ~ 1 cm-1

B (electronic) ~ 1 cm-1

B (nuclear) ~ 0.001 cm-1

The THz has defined itself broadly and spans kT

The Energetics

Atoms and MoleculesE (electronic) ~ 50000 cm-1

E (vibrational) ~ 1000 cm-1

E (rotational) ~ 10 cm-1

E (fine structure) ~ 0.01 cm-1

RadiationUV/Vis > 3000 cm-1

IR 300 - 3000 cm-1

FIR 30 - 300 cm-1

THz 3 - 300 cm-1

MW 1 - 10 cm-1

RF < 1 cm-1

Does Thermal Noise ‘Plague’ cw Submillimeter Spectroscopy (Imaging)

Experiments?

SiO vapor at 1700 K

Amplifier noise in 4 K detector

No - You Can’t Even Observe it with a 4 K detector!

Phenomenology:

What is the Physics of Interactions?

Separate into Three Classes According to Linewidth

Low pressure gases: Q ~ 106

Atmospheric pressure gases: Q ~ 102

Solids and Liquids: Q ~ 1 - 100

(are there useful signatures?)

(are these classical or QM?)

VCO10.3 – 10.8 GHz

FrequencyReference10.5 GHz

Mixer

X8 MultiplierW-band

W-band Amplifier75-110 GHz

X3 MultiplierW-band

AmplifierLow Pass Filter10kHz – 1MHz

Harmonic10 MHz Comb

GeneratorAmplifierMixer

Gas Cell Detector

Computer DAQ

FrequencyStandard

x24

FASSST Spectrometer Diagram

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#09 Acrylonitrile Library

Combined Spectrum

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Gas Identification in Mixture of 20 Gases

Blow-ups ofCombined Spectrum

Library Identificationof Acrylonitrile

1 second sweep time over whole spectrum

300 seconds integration on resonance

X 107 sensitivity plus

‘absolute’ specificity

How can this be? Source Brightness!

10-2 photons/pulse/MHz

THE STEALTH ‘KILLER AP’COMMUNICATIONS - WIRELESS TECHNOLOGY*

*The government alone can’t afford to develop the THz, only the market can make us mature

THz + ‘X’ - A search for new approaches to significant problems

Frank C. De Lucia, Department of Physics, Ohio State University, Columbus, OH 43210

Douglas T. Petkie, Department of Physics, Wright State University, Dayton, OH 45435

Robert K. Shelton, Sarah L. Westcott, and Brian N. Strecker, Nomadics, Inc., 1024 Innovation Way, Stillwater, OK 74074

The Importance of ‘X’•THz is unique because of the infancy of its commercial and military applications

•Much of this infancy due to the difficulties of generating and detecting radiation

•However, enormous numbers of important applications in the other spectral regions have resulted from their large investment in systems and applications development – often an additional ‘X’ factor. ‘X’ can be worth Nobel Prize!

RF: MRI (rf +‘X’ = shaped magnetic fields, rf pulse sequences, and signal processing)

Visible: Night Vision (light + ‘X’ = electron multiplication and fluorescence)

•To grow to maturity, the THz needs not only to optimize its technology for native applications (imaging through obscuration, chemical sensing, etc.), but to integrate its attributes with other technologies to address a broader range of challenges competitively.

An Example: ‘X’ for SMM/THz Gas Analysis

1. Gas/Particle Capture and Concentration

2. System Strategy Frequency control and measurement Signal recovery/dynamic range/noise spectra

3. Spectroscopic Theory/Libraries

4. Clutter analysis

5. Information theory

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WHY WE NEED INFORMATION THEORY: THE SPECTRUM OF A 20 GAS MIXTURE

‘X’ = Rydberg Atom Photocathode

What is the photocathode problem in the SMM/THz? 1. There are no materials with a cutoff wavelength this long.

2. If there were, for a room temperature device, the infrared flux would overwhelm the photocathode

input window

Atom source

micro-channel plates

phosphor screen

viewing window

grids

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vacuum can

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518 nm

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0.52 THz

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20f

The Technology:

There is no interconnect problem, either look directly at phosphor screen or use CCD array

Quantitative analysis is favorable*

Laser requirements favorable in comparison to SMM/LO

Discharge plasma excitation may be possible Solid state photocathodes might be possible

*With Professor Douglas Schumacher

The Physics of a Solution:

20d - 18f is strongly allowed: sensitive detector of SMM/THz

Because of selection rules, not energetics, 20d is not sensitive to IR radiation (or for that matter to other SMM/THz)

18f can be selectively field ionized against 20d to produce photoelectron

Most importantly, it has been possible to subject his general idea to a detailed analysis that has led to the solution of the ‘challenges’ and a rather detailed design concept

Conclusions and Questions

What is so favorable about the SMM/THz?

The SMM/THz is very quiet: 1 mW/MHz => 1014 K

Rotational transition strengths peak in the SMM/THz

The SMM/THz combines penatrability with -a reasonable diffraction limit -a spectroscopic capability -low pressure gases have strong, redundant, unique signatures

-solids can have low lying vibrational modes, especially at high THz frequencies

In comparison to the MW, the SMM/THz has a lot of bandwidth

The commercial wireless market will provide us with a cheap technology

It should be possible to engineer small (because of the short wavelength) and low power (because the background is quiet/the quanta is small) devices and systems - e.g. like miniature GC-MS

What is so Challenging about the SMM/THz?Efficient generation of significant tunable, spectrally pure power levels.

The difficulty of the physics which produces signatures in solids.

Need to find a ‘public’ ‘Killer Ap’ that can allow us to rapidly develop ‘X’ like other fields.

Impact of the atmosphere on measurements.

What do We Wish We Knew?What are the signatures of the aforementioned ‘Killer Aps’?

Can we develop a reliable spectroscopic catalog?

What is the science that underlies the spectroscopy?

How do the time and spatial scales of atmospheric fluctuations impact SMM/THz images and spectroscopy?

“The interest of the Navy and other services in this field is so great that the generation, propagation, and detection of such waves are the subject of an expanding research program in the Department of Defense today.”

Rear Admiral R. Bennett, ONRSymposium on Millimeter WavesPolytechnic Institute of Brooklyn31 March 1959

“Now is the time for you workers in the field to come out of hiding and be counted! All is forgiven!”

Leonard R. Weisberg, OUSDREProceedings of the Sixth DARPA/Tri-ServiceMillimeter Wave Conference29 November 1977

Frontispiece: Army’s Near-Millimeter Wave Technology Base Study, November 1979

Now, 25 years later we have gone through a second cycle.

Will there be a third? Or, are we ready to be a mature field?

PEOPLEFrank C. De Lucia - Professor OSU

Eric Herbst - Professor OSUBrenda Winnewisser - Adj. Professor OSUManfred Winnewisser - Adj. Professor OSU

Paul Helminger - Professor USADoug Petkie - Professor WSU

Markus Behnke - Research AssociateAtsuko Maeda - Research AssociateAndrei Meshkov - Graduate StudentIvan Medvedev - Graduate StudentTJ Ronningen - Graduate Student

Laszlo Sarkozy - Graduate StudentDavid Graff - Graduate Student

Bryan Hern - Undergraduate StudentDrew Steigerwald - Undergraduate Student

John Hoftiezer - Electrical Engineer

Optics and Photonics News (August 2003)

“Spectroscopy in the Terahertz Region,” in Sensing with Terahertz Radiation, D. Mittleman, ed. Springer, Berlin (2003).

REFERENCES