WMD

1. WMD Science and Technology in the Submillimeter/Terahertz Spectral Region Frank C. De Lucia Ohio State University Department of Physics Columbus, OH, 43210

2. 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

3. What’s a THz? (With a broad definition, what properties are available at a particular choice of frequency?)

4. There are Established SMM/THz ‘Killer Aps’ Technologies which approach fundamental limits Fundamental Molecular Studies - Spectroscopy, Dynamics Laboratory Astrophysics Science in the Field/Remote sensing Interstellar medium, stellar formation Upper atmospheric chemistry

5. Two Old, but New, ‘Killer Aps’ Identify Need - Competitive SMM/THz Solution? - Do it -- Clear, but Challenging Paths to Success -- IMAGING ANALYTICAL CHEMISTRY

6. 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?

7. Physics in the SMM/THz Degrees of Freedom - What is the Physics? Energetics and Temperature: hn/kT System and Ambient Noise Linewidths (Qs), Specificity, Signatures, and Clutter Illustrative Examples -solids -gases

8. The Energetics Atoms and Molecules E (electronic) ~ 50000 cm-1 E (vibrational) ~ 1000 cm-1 E (rotational) ~ 10 cm-1 E (fine structure) ~ 0.01 cm-1 Radiation UV/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 Temperature kT (300 K) = 200 cm-1 kT (1.5 K) = 1 cm-1 kT (0.001 K) = 0.0007 cm-1 Fields qE (electron) >> 100000 cm-1 mE (1 D) ~ 1 cm-1 mB (electronic) ~ 1 cm-1 mB (nuclear) ~ 0.001 cm-1 The THz has defined itself broadly and spans kT

9. 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!

10. 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?)

11. FASSST Spectrometer Diagram VCO 10.3 – 10.8 GHz Frequency Reference 10.5 GHz Frequency Standard Mixer X8 Multiplier W-band Harmonic 10 MHz Comb Generator Amplifier Mixer W-band Amplifier 75-110 GHz Amplifier Low Pass Filter 10kHz – 1MHz x24 X3 Multiplier W-band Computer DAQ Gas Cell Detector

12. Gas Identification in Mixture of 20 Gases #09 Acrylonitrile Library Combined Spectrum Blow-ups of Combined Spectrum Library Identification of Acrylonitrile

13. 1 second sweep time over whole spectrum 300 seconds integration on resonance X 107 sensitivity plus ‘absolute’ specificity

14. How can this be? Source Brightness! 10-2 photons/pulse/MHz

15. THE STEALTH ‘KILLER AP’COMMUNICATIONS - WIRELESS TECHNOLOGY* *The government alone can’t afford to develop the THz, only the market can make us mature

16. 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

17. 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.

18. 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

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

20. grids viewing window T micro-channel plates phosphor screen input window Atom source 0.52 THz vacuum can 22d 20f 18f 20d 18d 16f 518 nm 6p3/2 852 nm 6s1/2 ‘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 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 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

21. Conclusions and Questions

22. 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

23. 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?

24. Frontispiece: Army’s Near-Millimeter Wave Technology Base Study, November 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, ONR Symposium on Millimeter Waves Polytechnic Institute of Brooklyn 31 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, OUSDRE Proceedings of the Sixth DARPA/Tri-Service Millimeter Wave Conference 29 November 1977 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?

25. PEOPLE Frank C. De Lucia - Professor OSU Eric Herbst - Professor OSU Brenda Winnewisser - Adj. Professor OSU Manfred Winnewisser - Adj. Professor OSU Paul Helminger - Professor USA Doug Petkie - Professor WSU Markus Behnke - Research Associate Atsuko Maeda - Research Associate Andrei Meshkov - Graduate Student Ivan Medvedev - Graduate Student TJ Ronningen - Graduate Student Laszlo Sarkozy - Graduate Student David Graff - Graduate Student Bryan Hern - Undergraduate Student Drew Steigerwald - Undergraduate Student John Hoftiezer - Electrical Engineer

26. REFERENCES Optics and Photonics News (August 2003) “Spectroscopy in the Terahertz Region,” in Sensing with Terahertz Radiation, D. Mittleman, ed. Springer, Berlin (2003).