Why Use Terahertz? Prof Douglas J Paul Department of Electronics and Electrical Engineering Terahertz Spectrum kBT at 300 K Energy (meV) 2 3 4 5 7 10 20 30 40 50 40 30 6 7 10 Wavelength (µm) 1000 700 500 300 200 100 70 Frequency (THz) 0.3 10 0.4 0.5 0.7 20 1 30 2 40 50 70 3 100 Wavenumber (cm–1) D.J. Paul Electronic + Electrical Engineering 4 5 200 300 Terahertz Issues Advantages: Low energy (meV), non-ionising radiation (Water is strong absorber) – application dependent: contrast Many molecular rotational and vibrational absorption modes Many materials are transparent to THz Disadvantages: Metals are opaque at THz Water is strong absorber Most 300 K objects have blackbody spectrum in THz D.J. Paul Electronic + Electrical Engineering Terahertz Technology Time domain systems: pulsed lasers with antenna or non-linear crystals Systems: TeraView, Picometrics, Nikon Both frequency and time (depth) information Able to form 3D images with time information Coherent detection to improve signal to noise System cost presently dominated by pulsed fs laser Frequency domain systems: many examples Normally only 2D frequency information D.J. Paul Electronic + Electrical Engineering Why Use THz? THz will only be used if: it provides different / new information and / or it is cheaper than any competing technology it is safer than existing techniques THz key advantages: spectral fingerprinting – far-infrared spectroscopy non-ionising imaging many materials are transparent The biggest advantages for THz are for applications than require ALL of above D.J. Paul Electronic + Electrical Engineering Perceived Problems for Terahertz Far-infrared spectroscopy is an old field (1950s !) Perception that water absorbs everything at THz All you’ll see is the blackbody spectrum Terahertz technology and systems are expensive To sell numbers of THz systems, these perceptions need to be overcome D.J. Paul Electronic + Electrical Engineering Non-ionising Radiation Terahertz gap energies: 1 to 40 meV c.f. ionising medical imaging: > 10 keV CT, X-ray, γ-ray or PET Chest CT – 5.8 mSv * Chest x-ray – 0.1 mSv * c.f. security imaging systems > 10 keV X-ray e.g. Rapiscan Rapiscan 2000 – 0.1 µSv Why use THz? : safer imaging modality than CT, X-ray, γ-ray or PET *P.C. Shrimpton et al., U.K. HPA, “Doses from CT: Examinations in the U.K.” (2003) D.J. Paul Electronic + Electrical Engineering Terahertz Exposure Limits Safety analysis based on 2 standards for 2.6 µm to 20 mm (115 THz to 15 GHz): American National Standard for the Safe Use of Lasers (ANSI Z136.1) IEEE Standard for Safety Levels with Respect to Human Exposure to Radio Frequency Electromagnetic Fields (C95.1) Maximum average beam power of 3 mW Pulsed MPEs derived for near-infrared with longer duration pulses > 10 ps E. Berry et al., J. Laser Apps. 15, 192 (2003) D.J. Paul Electronic + Electrical Engineering Spectral Windows: Linear Scale Wavelength (m) 10-1 10-2 10-3 10-4 Atmospheric Absorption 1.0 10-5 10-6 10-7 thermal visible imaging 10-8 10-9 0.8 0.6 “Terahertz gap” 0.4 0.2 micro wave 0.0 109 1010 1011 1012 1013 1014 1015 1016 1017 1018 Frequency (Hz) Useful transparent regions: microwave and sound, 8-12 µm and visible D.J. Paul Electronic + Electrical Engineering Atmospheric Absorption: Log Scale Wavelength Systems: QinetiQ ThruVision Rapiscan 3mm 1000 Attenuation (dB/km) mm-wave good transmission for security imaging 30mm 10000 300µm 30µm 3µm H2O Excessive rain (150mm/h) H2O O2 100 10 Fog (0.1gm–3) visibility 50m CO2 Heavy rain (25mm/h) H2O O2 1 0.1 H2O CO2 CO2 Drizzle (0.25mm/h) 20˚C : 1 atm H2O : 7.5gm–3 H2O 0.01 0.01 H2O O3 mm 0.1 sub-mm terahertz 1 infrared 10 visible 100 Frequency (THz) Useful windows exists between H2O absorption lines D.J. Paul Electronic + Electrical Engineering 300nm 1000 Water Mean Absorption Coefficient Energy (eV) -6 6 Water absorption (cm–1) 10 4 10 2 10 10 -5 -4 10 -3 10 -2 10 radio µwave mm rotational + vibration mode frequencies are effectively opaque -1 10 0 10 THz 1 10 IR 10 Vis UV 0 10 -2 10 -4 10 8 10 9 10 10 10 11 10 12 10 13 10 14 10 15 10 16 10 Frequency (Hz) M.R. Querry et al., Handbook of Optical Constants II, p1067 (1991) D.J. Paul Electronic + Electrical Engineering Refractive Index Water Energy (meV) 600 0 4 6 8 10 12 5 α n 4 400 3 300 2 200 1 100 0 Refractive index Absorption (cm–1) 500 2 0 0.5 1 1.5 2 2.5 3 0 Frequency (THz) J. Bertie, Appl. Spectroscopy 50, 1047 (1996) D.J. Paul Electronic + Electrical Engineering What Applications is THz Useful For? Terahertz astronomy Medical imaging low volume, high cost issue: clinical trial times Production monitoring Security imaging (weapon and illicit material identification) Drug discovery and formulation No killer application so far Material characterisation etc.......... D.J. Paul Electronic + Electrical Engineering Terahertz Security Applications Absorption (AU and offset) Explosives and narcotics identification Postal screening Semtex PE4 Reading letters in an envelope RDX PETN HMX TNT Security screening ceramic disc metal blade y-axis x-axis 0 1 2 3 4 Frequency (THz) Images from TeraView Ltd. D.J. Paul Electronic + Electrical Engineering inside fleece jacket x-axis Terahertz Security 10m Stand-off Imaging 1.56 THz 350 GHz Visible CO2 laser LO heterodyne single point system 2 minute acquisition time per frame What frequencies gives good contrast and useful transmission in real environments? J.C. Dickson et al., Proc. SPIE 6212, 62120Q (2006) D.J. Paul Electronic + Electrical Engineering Imaging Depth into Leather -2 1.6 10 I = I0 e−α·depth -2 Depth (m) 1.4 10 –19 NEP 10 α = 30.8 cm–1 at 1 THz W/√Hz hot electron bolometers APL 85, 519 (2004) -2 1.2 10 QCLs -2 1.0 10 1.2 THz α// = 31.8 cm–1 at 1.042 THz 4.4 THz -3 8.0 10 α = 27.2 cm–1 –13 NEP 10 -3 6.0 10 W/√Hz at 1.042 THz time domain systems He cooled Si bolometer Proc. SPIE 6212, 62120E-1 (2006) -3 4.0 10 -7 10 -6 10 -5 10 -4 10 -3 10 -2 10 Source power (W) -1 10 0 10 Component and systems improvements required for 1 cm depth imaging D.J. Paul Electronic + Electrical Engineering Sources of Radiation Across the Spectrum Wavelength (µm) Output power (Watts) 1000 100 10 1 100 free electron InP laser 10 IMPATT MMICs III-V QCLs 1 Gunn gas 10K klystrons 100m III-V 5K p-Ge pin 4K 10m QCL BWOs 77K 1m Lead salt lasers 10K 100µ InGaAs Si impurity lasers 4K 10µ RTDs Schottky diodes 1µ photomixer 100n Photonics Electronics 10n photoconductive antenna 1n 100p 0.1 1 10 100 Frequency (Terahertz) D.J. Paul Electronic + Electrical Engineering 1000 FTIR Spectra of TriNitro Toluene (TNT) DFT calculation FTIR transmission diffuse reflection Most explosives have strong spectral features >2 THz H.-B Liu, X.-C. Zhang, Proc. NATO ARW “THz Freq. Detection and Identification of Materials and Objects” (2006) D.J. Paul Electronic + Electrical Engineering Differentiation to Masking Agents Some work to differentiate masking agents in time-domain systems H.-B Liu, X.-C. Zhang, Proc. NATO ARW “THz Freq. Detection and Identification of Materials and Objects” (2006) D.J. Paul Electronic + Electrical Engineering H3C Raman Spectroscopy of TATP 9 18 O H3C 45 CH3 A sy m . O -O Sym. O -O C R 2O S tr e tc h in g CH 3 B e n din g Sy m . D.J. Paul Electronic + Electrical Engineering CH3 O O O O Frequency (THz) 27 36 T ATP O H3C A.J. Pena et al., Proc SPIE 5778, 347 (2005) 3 CH3 A sy m . 54 Security Uses of THz Fast people imaging systems easiest at mm-wave (< 1 THz) System trade-off: resolution versus transmission versus contrast Competition from low dose, soft x-ray systems (public acceptance?) “Spectral fingerprinting” of illicit materials easier above 2 THz Present ion mobility spectrometers not perfect: high false positives Identification of biological materials would have enormous market (PCR is far too slow) D.J. Paul Electronic + Electrical Engineering (a) Visible Nodular BCC from Forehead In Vitro (c) Histology section (d) Terahertz THz image intensity section 0.8 (b) Intensity (a.u.) 0.7 0.6 0.5 30 80 130 180 230 280 330 380 430 Distance (pixels) V.P. Wallace et al., British J. Dermatology 151, 424 (2004) D.J. Paul Electronic + Electrical Engineering Absorption Coefficient for Skin and Water Energy (meV) Absorption coefficient (cm–1) 300 250 2.5 3 3.5 4 4.5 5 5.5 6 deionised water skin adipose tissue (fatty) stirated muscle 200 150 100 50 0 0.5 TPI @ 300 K 0.75 1 1.25 1.5 Frequency (THz) A.J. Fitzgerald et al., J. Biol. Phys. 29, 123 (2003) D.J. Paul Electronic + Electrical Engineering Issues with Terahertz Medical Imaging Clinical trials and acceptance takes time and costs money Epidermis has α ~ 130 cm–1 at 1 THz –> depth resolution of < 5 mm Good for surface or shallow cancers near a surface which can be reached (skin, oral, prostate?, breast?, etc..) Needs cheaper THz systems for high market penetration D.J. Paul Electronic + Electrical Engineering B-Scan THz Images of Ibruprofen Coatings Sample A time domain systems with depth info Sample B A.J. Fitzgerald et al., J. Pharma. Sci. 94, 177 (2005) D.J. Paul Electronic + Electrical Engineering Measuring Coatings on Ibuprofen Tablets Sample B Impulse function (au) 2.0 Sample A 1.5 1.0 0.5 0.0 –0.5 0 5 10 15 time (ps) A.J. Fitzgerald et al., J. Pharma. Sci. 94, 177 (2005) D.J. Paul Electronic + Electrical Engineering Conclusions Terahertz does have useful applications with potential markets THz advantages: spectral fingerprinting – far-infrared spectroscopy non-ionising imaging many materials are transparent The biggest advantages for THz are for applications than require ALL of above THz will only be used if: it provides different / new information and / or it is cheaper than any competing technology it is safer than present technology D.J. Paul Electronic + Electrical Engineering