Time Domain THz Spectroscopy and Related Sensing Applications

Transcription

1 VG Time Domain THz Spectroscopy and Related Sensing Applications David J. Cook, Brian K. Decker, Gami Dadusc, and Mark G. Allen Physical Sciences Inc. 20 New England Business Center Andover, MA June 2, 2003 Physical Sciences Inc. 20 New England Business Center Andover, MA 01810

2 Target Sensor Specifications Structure Penetrating Detect through clothing, within sealed packaging, container, shipping materials VG Personnel Screening Rapid screening of personnel and handheld materials (backpacks, briefcases, coolers,...) Chemical Specificity Able to detect and classify illicit compounds (e.g., explosives) Non-Invasive/Non-Destructive/Non-Detectable Harmless undetectable to living beings, material under inspection Automated Threat Detection Straightforward chemometric-type spectral identification for automated alarm and classification

3 Why THz? VG THz spectral range of µm Structure Penetrating Lack of Mie and Rayleigh scattering when compared to IR/VIS Chemical Specificity Need THz Spectral signatures: rotational lines (gas); phonon bands (crystal); large amplitude collective modes (macromolecule) Non-Invasive/Non-Destructive/Non-Detectable THz is nonionizing; need low power levels (<< 1 mw/cm 2 ) for sensing Automated Threat Detection and Personnel Screening Presence of spectral signatures leads to non-subjective algorithms

4 Implementation Concepts Portal Screening VG Transmission mode for automated material receiving Backscatter (DIAL) mode for personnel screening Schematic concept of container-penetrating sensor for hidden explosives. Transmission mode (for conveyed containers) is shown at the top; backscatter mode (for personnel) is shown at the bottom.

5 Challenges VG Atmospheric Transmission Standoff range ~ m (or more) is necessary for sensing threats Container Penetration Need sufficient throughcontainer power for measurement. Containers must not generate false positives Container Spectral Signatures Need specific spectral signatures for target materials

6 HITRAN Simulation of Atmospheric Absorption VG Base e extinction; 25 C, 30% relative humidity; 7.13 Torr H 2 O; 760 Torr total pressure; all other gases standard 1976 atmosphere Long range (100 m to 1 km) sensors limited to sub 1 THz frequencies Multiple windows exist above 1 THz for short range applications

7 The PSI Time Domain THz Spectrometer VG MHz 1 Watt 70 fs Ti:Sapphire laser Biased emitter source (Planken, et al., Rev. Sci. Inst., 73, 1715) ZnTe E-O Detection

8 Spectrometer Characterization VG Five decades of dynamic range THz

9 TD-THz Noise Performance VG Measure 15 consecutive scans ~1 min ea Calculate individual spectra and mean () I Calculate log n ν 10 I() ν

10 Container Penetration: clothing VG Due to reduced scattering, THz radiation penetrates inhomogenous materials Example spectra of clothing materials Transmission through many common clothing materials at relevant wavelengths

11 Container Penetration: mailing materials VG Example spectra mailing materials Good penetration of common mail materials at relevant wavelengths

12 Container Penetration: packaging materials VG Example spectra packaging materials Good penetration of common mail materials at relevant wavelengths

13 Explosive Samples for Spectroscopy Identify THz spectral signatures associated with commonly used high explosives VG Focus on pure high explosive substances, rather than on formulations. (Measure pure RDX rather than C-4.) Would ideally use free-standing pellets of pure polycrystalline material; sandwich pellets between polyethylene windows. Extinction previously unknown: Obtained 1mm thick and 250 micron thick samples.

14 High Explosive Samples for Time Domain THz Spectroscopy VG High Explosive Samples Studied RDX TNT HMX PETN NH 4 NO 3

15 Target Spectral Signatures: PETN & TNT VG PETN (250 µm thick) TNT (250 µm thick)

16 Target Spectral Signatures: HMX & RDX VG RDX (250 µm thick) HMX (250 µm thick)

17 Target Spectral Signatures: NH 4 NO 3 VG NH 4 NO 3 (250 µm thick) Restrahlen bands lead to strong absorption for > 1.5 THz

18 Remaining Challenges VG Tunable, Quasi-CW THz Laser Present technology limited to low power (< 1 mw) with little or no tunability. PSI is investigating current pumped QCL lasers (Tredicucci) and OPO/DFG based sources (in conjunction with PSI subsidiary Q-Peak) Sensitive Detectors Present technology favors cryogenic bolometers or photoconductors DIAL Apparatus Diffuse reflection from containers, clothing, and actual high explosive formulations not measured