THz Holy Grails: Opportunities, Challenges, and Critical Paths

Frank C. De LuciaOhio State University

AMRDECJanuary 19, 2006

PEOPLE

Frank C. De Lucia - Professor OSUDoug Petkie - Professor WSUEric Herbst - Professor OSU

Brenda Winnewisser - Adj. Professor OSUManfred Winnewisser - Adj. Professor OSU

Paul Helminger - Professor USAAtsuko Maeda - Research AssociateIvan Medvedev - Research AssociateAndrei Meshkov - Graduate StudentTJ Ronningen - Graduate Student

Laszlo Sarkozy - Graduate StudentDavid Graff - Graduate StudentCory Casto - Graduate Student

Kerra Fletcher - Graduate StudentBryan Hern - Undergraduate Student

Drew Steigerwald - Undergraduate StudentJohn Hoftiezer - Electrical Engineer

“Whispered Excitement

about the THz”Graham Jordan

Opening Plenary Presentation SPIE Symposium: Optics/Photonics in Security and Defense Bruges, Belgium, 26 September, 2005

The New York Times - July 11, 2005High-Tech Antiterror Tools: A Costly,

Long-Range Goal

Millimeter wave machines . . .use trace amounts of heat released by objects . . .to create images that can identify hidden bombs . . . from about 30 feet away.

Terahertz radiation devices can create images of concealed objects as well as identify the elemental components of a hidden item.

The terahertz devices may be more promising since they could sound an alarm if someone entering a subway or train station had traces of elements used in bombs on them.

Frontispieces from Army NMMW study of 1979

“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, OUSDREProc. of the 6th DARPA/Tri-ServiceMillimeter Wave Conference 29 November 1977

Now and Then in the MM,NMM,SMM,THz, FIR

OverviewWhat’s a THz? What’s the Phenomenology?

Opportunities/Applications

Well established, but not ‘Public’: Science, Astrophysics, and Atmospheric

Clear paths to ‘Public’ Legacy applications: Imaging and Gas Sensors

Holy Grails (THz . . .)

The THz: A technology in search of signatures

Critical paths to the Holy Grails Science

Technology

Concluding remarks

There is both a technical and scientific ‘gap in the electromagnetic spectrum’

What’s a THz?0.1 THz ~1/60 kT - clearly classical = 3 mm

10 THz > kT - quantum regime = 0.03 mm

30 THz ~ frequency of CO2 laser = 0.01 mm

What’s a Name Mean?Millimeter, NearMillimeter, Submillimeter,

Terahertz, Far Infrared

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

Jumping the ‘gap in the electromagnetic spectrum’ is not the same as closing it

Phenomenology:

What is the Physics of the 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?)

Parallel and On Going MM/SMM Science[Field Technology and Systems Grew out of Lab Science]

NASA JPL catalog

HITRAN, KOLN, GEISA data bases

Data Base Meetings HITRAN

NASA

The GEISA/IASI spectroscopic database

Two MW/MM Legacy ‘Public’ Applications -- Clear, but Challenging Paths to Success --

IMAGING ANALYTICAL CHEMISTRY

A CLEAR PATH TO

NON-SPECTROSCOPIC IMAGING

Challenges and/or Unknowns:

Sensitivity of non-heterodyne arrays

Local oscillator power for heterodyne arrays

Illuminator power for active arrays

Penetration of obscurants

Target signatures

Spatial and temporal fluctuations of the atmosphere and obscurants (clutter)

How do you look at THz images?

The World at 650 GHz

A Sampler

Corey Casto

Douglas T. Petkie

Frank C. De Lucia

Another illustration of the noticeable difference in the image caused by the presence of facial hair. Optically the left and right sides of the faces only subtly differ.

Conceptsvs.

The Laws of Physics[A Specific Illustration of a Larger Point: What’s a THz?]

Widely Promoted ‘Holy Grails’ “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 objects 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 rates.”

“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.”

1: These are powerful ‘public’ Holy Grails.

2: Which of these can be achieved?

3: What are the waystations on their critical paths?

Signatures: Explosives Spectra

MORE INTERCOMPARISIONS

This problem of repeated presentation of clearly spurious results is not restricted to THz solid state spectra - It exists in the gas phase as well.

The solid line shows the reflectivity of the meat part normalized of the reflectivity of the fat part of Black Forrest ham averaged on three points each.

From THz-Bridge

Are any of us willing to say that we are sure that the sharp lines are spurious?

A HOLY GRAIL:

REMOTE SPECTROSCOPIC IMAGING

A significant fraction of those ‘interested in the THz’ are drawn by this application in one form or another

Signature, with perfect atmo model

Systems: Remote Spectroscopic SensingGas Phase Example: 100 m, 1 ppm plume => 10-2 absorption fraction, with 10 GHz linewidth

sharp lines: 10-7 detectable (noise limits), 105 resolution elements

broad lines: 10-1 detectable (clutter limits), <102 resolution elements

Solids:

What is the concentration and absorption fraction (in reflection)?

What is the signature, the linewidths, the clutter?

Are their equivalent double resonance schemes for solids?

3-D Specificity Matrix

5-6

4-5

3-4

5 GHz

200 250 300 GHz

CH3F - KNOWN ENERGY TRANSFER PARAMETERS

J

5 4 3

5 4

3 K=3

InfraredPump

modulation

= 0 modulation

Net modulation

Pump and Probes THz Modulation

6

EXPERIMENTAL ARRANGEMENT

Mini CO2

TEA Laser

THz cwsource

THz Detector

1 m

1 atmosphere air~ 10-3 – 10 -6 dilution

Computer

50 ns

FastDigitizer

Questions:

How does energy transfer in overlapping spectra of large molecules work?

Does the high pump intensity modulate the clean atmosphere? How specific are the relaxation times? At atmospheric pressure, do most gases of interest have overlap with laser?

Is Their a Similar Scheme for Solids?

It’s hard to say!

All of the ‘THz’ signatures are empirical

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

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

ABSTRACT

The THz is unique among spectral regions because of the relative infancy of its commercial applications. Much of this infancy has been due to the well known difficulties of generating and detecting radiation. However, the enormous number of important applications in each of the other spectral regions has resulted at least as much from their large investment in systems and applications development – an ‘X’ factor - as from the technological maturity of the spectral region. Examples in the radio region include magnetic resonance imaging (rf +‘X’ = shaped magnetic fields, rf pulse sequences, and signal processing) and cruise missiles (rf + ‘X’ = rocket and guidance system). In the visible, Night Vision (light + ‘X’ = electron multiplication and fluorescence) serves as an example. 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. In this paper we will discuss the underlying physics of interactions in the THz to see how they lead to both the attractive and limiting features of the spectral region, while at the same time providing hints about how to overcome these limitations by considering ‘X’. Specific examples of ‘X’ will be provided and the authors will welcome comments, suggestions, and ideas from the audience.

SPIE Meeting - Orlando - March 28, 2005

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?The need to develop systems without knowledge of the phenomenology

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

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?

Science Along the Critical Path(Signatures and Clutter)

What are the spectral signatures of solids? a. What is the physics? Is it more difficult than that of gas phase phenomena? b. How do transmission and reflection/scattering signatures differ? c. How narrow are the spectral features? d. Are there schemes for resolution/specificity improvement? e. Is it possible to develop reproducible and well founded spectral catalogues?

What is the nature of clutter for solids? More specifically, most of the published fingerprints of explosives and some biological chemicals consist of a few very broad lines. These might still be useful fingerprints IF most common materials (dirt, clothing, ordinary powders (soap, sugar, starch. etc.)) do not have similar features. a. What are the clutter spectra due to these common materials? b. How complex are the spectra due to mixtures of these materials? c. How do systems separate the broad spectral features of solids from background effects and clutter?

Science Along the Critical Path (continued)

What are the gas phase spectral signatures a. Large molecules b. Non-ambient conditions c. Are their double resonance signatures? d. What is the equilibrium and typical vapor pressures of these large molecules?

What do we know about the penetration of materials (clothing, building materials, particulates, etc.)? a. How does it vary with frequency? b. How do regions of good transmission compare with the frequencies of fingerprint spectra? c. How do we define and characterize the materials (e. g. water content and density of weave)?

What is the impact of the atmosphere? a. What are the time and spatial scales of atmospheric fluctuations? b. How do they compare with the corresponding scales of imaging systems?

c. How many information points (windows) are available as a function of range?

Technology Along the Critical Path

Because a very large number of THz technology solutions have been detailed in the literature, we will not attempt to be exhaustive.

Technology based on commercial ‘wireless’ developments: This approach appears to be the only technology in the foreseeable future that will provide sources at a price that will allow SMM/ THz applications to move to the ‘public’ - Only the market can afford THz, not the government.

Low Temperatures: a. Many of the source power limitations are dramatically reduced with cooled detectors. b. Many signatures in solids become stronger and more specific. c. Cooling is also a strong enabler for focal plane arrays. What is the time line for fieldable cooling approaches, for quantum cooling on a chip? How might these coolers compare in power and size with the size and power requirements for THz local oscillators?

A rediscovery of FTFIR technology: a. FTFIR is a very competitive and cost effective in many applications

Non-traditional Approaches: a. Double resonance approaches for remote sensing

Concluding Remarks

There are Legacy Applications which can become the first ‘public’ applications of the SMM/THz. There are clear but challenging paths to success.

There are also ‘Holy Grail’ Applications which have attracted wide attention to the THz for which there are significant phenomenological uncertainties.

There are 2 ‘Gaps in the Electromagnetic Spectrum’. The long discussed technical one A less discussed phenomenological/scientific one