Radiation Sources for Spectroscopy and Imaging in the Submillimeter/Terahertz Frank C. De Lucia

1. Radiation Sources for Spectroscopy and Imaging in the Submillimeter/Terahertz Frank C. De Lucia Ohio State University Advisory Group on Electron Devices February 28, 2007 Arlington, VA

2. Terms of Reference 1. Assess state of R&D for compact SUBMM sources 2. Review DoD/Government needs and applications 3. Identify technical and operational limits for SUBMM source technology 4. Review foreign activities and programs 5. Determine commercial involvement in source technology 6. Identify opportunities in device design, fabrication and supporting technologies with potential for breakthroughs 7. Assess novel and hybrid approaches for THz generation/amplification 8. Create a THz source technology development roadmap 9. Understand the Signature Science of the targets of interest - The 2nd Gap in the Electromagnetic Spectrum =>This 2nd gap negatively impacts our ability to develop APPROPRIATE technology

3. Attributes of the THz Technology: The region is very quiet and very sensitive detectors are possible (Sources are very bright: 1 mW in 100 Hz corresponds to a temperature of 1018 K Phenomenology/Signatures: Penetration of dielectric materials (decreases rapidly with frequency - scatter and absorption) Low pressure gases have strong and unique rotational signatures Complex solids have low lying vibrational states in the THz, but these are much less studied and characterized Active and Passive Images are complex and different from those in other spectral regions Applications Established Scientific ApplicationsClear Paths to Public ApplicationsWidely Discussed Public Applications _________________________________________________________________________________________________ Astronomy Imaging Through Obstructions Remote Explosive Detection Atmospheric Science Dust, Clothing Remote Detection of Gases Laboratory Spectroscopy Point Gas Sensors T-Ray Medical Imaging Plasma Diagnostics Spectroscopic Imaging of Cancer Physical Chemistry Imaging Through Obstructions Walls Remote Detection of Bio

4. Signature Science and Appropriate Figures of Merit => Quantitative end-to-end designs

5. Spectral Width/Frequency Reference As a Basis for a Discussion of Matching and Developing Appropriate Source Technology with Applications of Interest << 1 MHz Fundamental Oscillators/Amplifiers (BWOs,TWTs, GUNNs, Klystrons) Harmonic Generation OPFIR Femtosecond Demodulation 1 - 100 MHz Quantum Cascade Lasers Cw/Mode Locked Laser Driven Photomixers a few GHz Pulsed Laser Driven Mixing Broadband (resolution via FT detection) FTFIR THz-TDS > x 106 SMM/THz phenomena have a larger range of spectral widths (> x109) We need appropriate Figures of Merit for sources, detectors, and systems

6. Source/Target Bandwidth Limited Brightness (K) 1022 1021 1020 1019 1018 1017 1016 1015 100 Hz - Active Imager 1 mW 1. Broadband and pulsed sources share much of the same physics and follow similar curves. Because of the rapid roll-off, it is important to ask how much brightness they have at a particular frequency. 2. Consider the ‘dynamic range’ associated with a 100 W light bulb in the context of its usefulness as a spectroscopic source at 1 THz. 3. A typical heterodyne receiver will have a noise temperature of 3000 K

7. Source/Target Bandwidth Limited Brightness (K) 1018 1017 1016 1015 1014 1013 1012 1011 1 MHz - Spectroscopic Line 1 mW 1. Broadband and pulsed sources share much of the same physics and follow similar curves. Because of the rapid roll-off, it is important to ask how much brightness they have at a particular frequency. 2. Consider the ‘dynamic range’ associated with a 100 W light bulb in the context of its usefulness as a spectroscopic source at 1 THz. 3. A typical heterodyne receiver will have a noise temperature of 3000 K

8. Source/Target Bandwidth Limited Brightness (K) 1014 1013 1012 1011 1010 109 108 107 10 GHz - Atmospheric Line 1 mW 1. Broadband and pulsed sources share much of the same physics and follow similar curves. Because of the rapid roll-off, it is important to ask how much brightness they have at a particular frequency. 2. Consider the ‘dynamic range’ associated with a 100 W light bulb in the context of its usefulness as a spectroscopic source at 1 THz. 3. A typical heterodyne receiver will have a noise temperature of 3000 K

9. Source/Target Bandwidth Limited Brightness (K) 1013 1012 1011 1010 109 108 107 106 100 GHz - Solid Resonance 1 mW 1. Broadband and pulsed sources share much of the same physics and follow similar curves. Because of the rapid roll-off, it is important to ask how much brightness they have at a particular frequency. 2. Consider the ‘dynamic range’ associated with a 100 W light bulb in the context of its usefulness as a spectroscopic source at 1 THz. 3. A typical heterodyne receiver will have a noise temperature of 3000 K

10. Two SMM/THz Legacy ‘Public’ Applications -- Clear, but Challenging Paths to Success -- IMAGING ANALYTICAL CHEMISTRY Engineering Progress and Signature Science R & D will Impact the Breadth of Applicability

11. Why is there a ‘Clear Path’ to Public Analytical Chemistry? Signatures: A well understood spectroscopic signature science foundation is in place False Alarms: False alarm rates in complex environments have been studied and can be shown to be low because of the number of resolution elements and ‘complex redundancy’ of molecular fingerprints Clutter: Background clutter/interference at trace levels have been studied and can be shown to be low Appropriate Technology Developed: Compact, high resolution solid state sources based on diode harmonic generation technology have been developed Potential for Low Cost: Rapid expansion of wireless communication technology to higher frequency is rapidly reducing the cost of the power amplifiers to drive this diode harmonic generation technology

13. Sensor System Figures of Merit Sensitivity - ‘Dynamic Range’ is widely abused 1. Only source power in the signature bandwidth (Brightness - W/Hz) is useful - the rest often causes additional noise (a fundamental limit for FTFIR) 2. Noise and dynamic range example: - 1 mW in a 100 Hz bandwidth, 3000K noise temperature =>dynamic range of >140 db - in ideal noise limited spectrometer, the minimum detectable absorption with 1 second of integration time is only - 90 db This is good for the imager because the bandwidth of the receiver can be matched to the source and frame rate of the imager But people who build spectrometersshould never discuss dynamic range because the detection of a small amount of power in a narrow bandwidth is fundamentally different than the detection of a small change in a large amount of power.

14. Source Brightness vs System Noise (K) Source Brightness (K)System Noise (K) 1018 1017 1016 1015 1014 1013 1012 1011 1.7 x 1010 1.7 x 109 1.7 x 108 1.7 x 107 1 MHz - Spectroscopic Line 1 mW 1. To keep graph simple an integration time (1 microsecond) that corresponds to the spectral linewidth is used at 1 mW this provides a S/N of ~106. In a more optimized system, an integration time of ~ 1 second might be used, and a S/N of ~109 results. 2. Unless the noise temperature of the receiver is higher than the system noise (which results from the addition of the thermal noise voltage to the carrier signal), it is not important.

19. Why is there a ‘Clear Path’ to Public Imaging? Heritage: Many special purpose, single pixel, imagers have been built over the last 40+ years Detectors: - scientifically we understand - in single element receivers we can approach well understood fundamental limits Transmit power: - acceptable solid state sources for some applications exist Propagation: -overall absorption generally known -impact of fluctuations noise less clear Signatures/targets/clutter: -nature of active images complex, but large contrast in images provides opportunities -strategies to minimize impact of obscuration needed Practicality: Where can we get to on sensitivity-speed-size-cost tradeoff in a FPA? These are not show stoppers, but the answers will determine the Breath of Application

20. 640 GHz

25. But no extra time/power required

26. Angular Diversity To Average Away Speckle: Move Imaging Mirror by its Diameter (Independent of distance) D Some TIFT illumination scheme are multimode and do this automatically with a very large number (10000?) of modes Specular Reflection: Number of Illumination angles to insure that one is normal:

27. Modes and Angles: Active and Passive Imaging in the THz For a single mode, 100 Hz bandwidth, 300 K, the thermal power/noise is ~4 x 10-19 W 1 mW in 100 Hz corresponds to a noise temperature of ~1018 K A reasonable receiver noise temperature is 3000 K For diffuse target, the number of return modes is NAD = (spot size/wavelength)2 ~ 100 (our system in portrait mode) For a specular target, the number of return modes is 1 Floodlight limit: If an illuminator of power PI is used to flood light (i.e. fill all modes) of an object whose scale is l, in a 100 Hz bandwidth the temperature/mode is Withl = 1 m, l = 0.5 mm TI ~2 x 1011 K Random illumination limit: A practical way to get spotlight illumination would be to illuminate the whole room or ‘urban canyon' assume a 10% reflection, and let the target come into equilibrium with the room. If we let l= 100 m, then TI ~2.5 x 106K. This is a very bright light bulb for a focal plane array like TIFT and the angular diversity will largely eliminate coherent effects and the need for ‘strategic angles’

28. What is so favorable about the SMM/THz? What are the Opportunities? The SMM/THz combines penetrability 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 Rotational transition strengths peak in the SMM/THz The SMM/THz is very quiet: 1 mW/MHz => 1014 K The commercial wireless market will provide us with a cheap technology Favorable Underlying Physics: It should be possible to engineer small (because of the short wavelength), high spectral purity (because we can derive via multiplication from rf reference) and low power (because the background is quiet/the quanta is small) devices and systems

29. What is so Challenging about the SMM/THz? Efficient generation of significant tunable, spectrally pure power levels Practical broadband frequency control and measurement The need to develop systems without knowledge of the phenomenology Impact of the atmosphere

30. What Needs to be Done to Enable the SMM/THz Spectral Region? 1. Source and detector figures of merit appropriate for different classes of applications. A better match between technologies and applications. 2. What are the signatures of solids? Distribution in frequency relative to penetration? 3. Classical penetrability, scatter, and specular reflection as a function of frequency and material. 4. What are the signatures of clutter for scenarios of interest? With this knowledge can we develop strategies to overcome related limits? Meaningful decisions about source development directions require quantitative and comprehensive understanding not only of the sources, but also of their interactions with detectors (noise), target signatures, and clutter and their respective figures of merit.