Terahertz imaging using the Jefferson Lab - FEL high power broadband terahertz source J. Michael Klopf a), Matthew Coppinger b), Nathan Sustersic b), James Kolodzey b), and Gwyn P. Williams a) a) Jefferson Lab Free Electron Laser Facility, Newport News, VA 23606 USA b) University of Delaware, Newark, DE 19716 USA
Outline Generating high power THz at the Jefferson Lab - FEL Benefits and applications of THz imaging Challenges of THz imaging THz imaging results Conclusions
Conventional and Accelerator Based Sources 2 2 2ea 4 Larmor's Formula: Power = (cgs units) 3 γ 3c ~100 V THz e - -> 115 MeV a GaAs 100V E = = 10 4 10 m sub-psec laser pulse 6 V m F 10 V 10 ( 3 10 ) = = = 2 6 m.5mev /c 0.5 10 17 10 m 2 sec 6 6 8 2 GaAs sub-sec laser pulse 2 8 2 c (3 10 ) 17 m a = = 10 ρ 1 if ρ = 1 m ρ sec γ = 225 so γ 4 ~ 10 9!!!! THz 2
Coherent Synchrotron Radiation Considering Short e - Bunches Electric field electron(s) E/N Intensity E 2 N 2 2 2 ddidi I = = N = ddω d dω ω dω N 2 2 2 e ω 2 4 π c super-radiant enhancement r n ˆ ( n ˆ β ) e i ω r [ t n ˆ r ( t ) / c ] dt 2 incoherent synchrotron radiation from N e - s time freq. (1/time)
Coherent Synchrotron Radiation Effect of Pulse Length Watts/cm -1 100000 10000 1000 100 10 1 0.1 0.01 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 Frequency (THz) 0.1 1 10 100 MHz 100 pc 150 x 150 mr 0.1 ps 0.3 ps 1.0 ps 54 W 1E-10 1 10 100 1000 Frequency (cm -1 ) 540 W 840 W
Jefferson Lab THz Beamline Optics Vertical Position 1.0 0.5 0.0 M4 F3 Vertical Position 3 2 1 0-1 -2 THz To User Facility Funding US Army NVL 1.0-0.5-1.0x10-1 m -100mm -50 0 50 100 200x200mm Horizontal Position F2 Vertical Position 3 2 1 0-3x10-2 m -30mm 60x60mm -20-10 0 10 20 30 Horizontal Position 0.5-1 -2 Vertical Position 0.0-0.5 M2-3x10-2 m 60x60mm -30mm -20-10 0 10 20 30 Horizontal Position -1.0x10-1 m -100mm -50 0 50 100 Horizontal Position 200x200mm M1 4 2 200x200mm Vertical Position 0-2 -4x1 0-2 m Optical calculations by Oleg Chubar, Paul Dumas -40mm -20 0 20 40 Horizontal Position
Measured JLab FEL THz Spectrum Wavenumber (cm -1 ) 0.30 0.25 0 20 40 60 80 100 120 140 JLab - FEL THz spectrum τ p ~ 350 fs 0.30 0.25 Intensity (arb. units) 0.20 0.15 0.10 0.20 0.15 0.10 0.05 0.05 0.00 0 1 2 3 4 THz 0.00
THz Beam Profile Near a Focus Measured Calculated
Benefits and Applications of THz Imaging THz light is non-ionizing and is believed to be safe for human exposure at sufficient power levels for imaging THz light can penetrate many fabrics and packaging materials and thus holds great potential for security fields in detecting concealed weapons Spectral signatures of certain biological and chemical agents have been measured at THz frequencies Detection of basal cell carcinomas and tooth cavities has been demonstrated in the THz region of the spectrum Other applications are only now being discovered due to the relatively unexplored THz gap (protein dynamics, superconductivity, magneto-optics, electro-optics, nonlinear optics)
Challenges of THz Imaging Providing sufficient THz power to illuminate a large field of view and to image in real time Properly collecting the reflected THz radiation from the target region (transmission mode generally not useful) Filtering of the THz induced thermal IR Properly imaging onto a detector array Creating imaging arrays designed specifically for THz imaging
THz Induced Thermal IR Raw Beam Data ON Beam Processed OFF Data paper target imaging target paper target imaging target Images taken using the stock Ge lens THz passes through paper target and is reflected off of the imaging target Heating due to absorption of THz heats the paper and the imaging target, producing the thermal IR seen above
THz Induced Thermal IR 7500 Raw Beam Data ON 7400 THz Thermal Effects & Prompt THz 6885 6880 Beam Processed OFF Data THz Thermal Effects Magnified (Same Time Interval) 7300 Intensity 6875 6870 6865 6860 Intensity 7200 7100 6855 6850 6845 0 5 10 15 20 25 30 35 40 45 50 Time (sec) 7000 6900 Thermal Prompt 6800 0 5 10 15 20 25 30 35 40 45 50 Time (sec)
THz Imaging Schematic beamline THz filter/lens visible camera mirror 1 mirror 2 THz camera mirror 3 object moves/rotates 2 Watts of broadband light onto 75mm x 75mm field. ~10 4 camera elements, so 200 microwatts per pixel. Scattering ~ 0.1%, so 0.2 microwatts per pixel. Noise level, 1 nanowatt, so S/N is ~200.
THz Imaging Layout
Test Pattern Imaging Target
THz Imaging Covered Target Raw Data Processed Data CD mailer covering cloth covering
Test of Imaging Resolution Raw Data Processed Data Raw THz images are processed to reduce the background and improve contrast Current configuration resolved down to the 1mm wide contact pads Polyethylene lens filtered the thermal IR, but does not image well
THz Transmission of Lens/Filter Materials 0.8 0.00 1.00 THz 2.00 3.00 1 A-10 window (Ge) Ge window (~3 mm) Si window (~3.3 mm) poly window (~4.6 mm) Normalized THz spectrum 1 0.8 transmission 0.6 0.4 0.6 0.4 0.2 0.2 0 10 30 50 70 90 110 wavenumber (cm -1 ) 0
Conclusions We have a high power THz source capable of illuminating a large field of view which can be imaged at full video rates Initial results have resolved features down to 1mm Filtering of the thermal IR is necessary to utilize the important properties of THz radiation Better imaging optics are required to improve resolution and contrast HRFZ-Si (NovaPhase/ThorLabs) and picarin (Microtech Instruments) lenses will be tested Development of sensor arrays designed specifically for THz imaging must be pursued (NIST - Boulder) Development of compact high power THz source will enable deployed systems (Advanced Energy Systems)
Jefferson Lab & U. of Delaware Team