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E206 Terahertz Radiation from the FACET Beam

E206 Terahertz Radiation from the FACET Beam. Alan Fisher and Ziran Wu SLAC National Accelerator Laboratory. SAREC Review SLAC 2014 September 15–17. Topics. Tuning FACET for peak THz: a new record Collaborations with THz users (E218 and new proposal) EO spectral decoding

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E206 Terahertz Radiation from the FACET Beam

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  1. E206Terahertz Radiationfrom the FACET Beam • Alan Fisher and Ziran Wu • SLAC National Accelerator Laboratory SAREC Review SLAC 2014 September 15–17

  2. Topics Fisher: E206 THz • Tuning FACET for peak THz: a new record • Collaborations with THz users (E218 and new proposal) • EO spectral decoding • Near-field enhancement • Patterned foils • Grating structure • THz transport calculations

  3. FACET THz Table • Table top is enclosed and continuously purged with dry air to reduce THz attenuation by water vapor. Fisher: E206 THz

  4. Peak THz: Michelson Interferometer Scans Tuning Compression for Peak THz Before After Fisher: E206 THz

  5. Peak THz: Spectra Tuning Compression for Peak THz Before After • Tuning extended spectrum to higher frequencies • Modulation due to: • Water-vapor absorption (12% humidity, later reduced to 5%) • Etalon effects in the detector Fisher: E206 THz

  6. Peak THz: Reconstructing the Electron Bunch Tuning Compression for Peak THz Before After • Requires compensation for DC component, which is not radiated. • Kramers-Kronig procedure provides missing phase for inverse Fourier transform of spectrum. Fisher: E206 THz

  7. Peak THz: Knife-Edge Scans for Transverse Size Horizontal Vertical Fisher: E206 THz

  8. Peak THz: Energy and Electric Field Fisher: E206 THz • Joulemeter reading and adjustments 3.8 V Joulemeter 2 6-dB attenuator 1/50 Amplifier gain 2 Beamsplitter 1/(700 V/J) Detector calibration 4 THz correction= 1.7 mJ • Kramers-Kronig without DC compen-sation gives longitudinal profile of field. • Pulse energy and knife-edge scans give peak field: 0.6 GV/m. • Focused with a 6-inch off-axis parabolic mirror. Focusing with a 4-inch OAP should give 0.9 GV/m.

  9. Modeling Emission from a Conducting Foil • Calculates emission on a plane 200 mm from the foil • Model includes finite foil size, but not effect of 25-mm-diameter diamond window: • ~30% reflection losses • Long-wave cutoff • Calculated energy consistent with measured 1.7 mJ Fisher: E206 THz

  10. FACET Laser brought to THz Table Fisher: E206 THz • Ti:Sapphire was transported to the THz table last spring • The laser enables several new experiments on the THz table: • Materials studies • E218 (Hoffmann, Dürr) • New proposal from Aaron Lindenberg • Electron-laser timing • Strong electro-optic signal used to find overlap timing for E218 • Scanned EO measurement outside the vacuum • Plan to make this a single-shot measurement • Switched mirror on a silicon wafer

  11. Layout of the THz Table for User Experiments 800nm, ~150fs, 9Hz, 1mJ W. Polarizer /4 /2 Polarizer Pyro P. Diode EO Crystal ND Filter  PEMDet. BS VO2 Sample Translation Stage  Pyrocam PD PD Laser Path from IP Table CCD E218 Setup Fisher: E206 THz

  12. Scanned Electro-Optic Sampling Fisher: E206 THz • Mercury-cadmium-telluride detector and fast scope used to time THz and laser within 150 ps • Precise timing overlap from EO effect in GaPand ZnTe • Direct view of THz waveform • Scan affected by shot-to-shot fluctuations in electron beam and laser • Consider electro-optic spectral decoding for shot-by-shot timing…

  13. Single-Shot Timing: Electro-Optic Spectral Decoding • From a collaboration with M. Gensch, Helmholtz Center in Dresden (HZDR) • Demonstrated timing resolution >2 fs • Simulate 150-fs (RMS) electron beam • With and without 60-fs notch • Add ±10-fs beam jitter relative to laser • Code benchmarked in Dresden • Adjust laser chirp to ~1 ps FWHM • Calculation: spectrometer resolves jitter • Ocean Optics HR2000+ spectrometer • Fiber-coupled to gallery Model of electron bunch Calculated spectrometer display Fisher: E206 THz

  14. Single-Shot Timing: Switched Mirror Test with Laser-Generated THz Pulse Fisher: E206 THz • THz incident on silicon at Brewster’s angle: full transmission • Fast laser pulse creates electron-hole pairs • Rapid transition to full reflection • Time of transition slewed across surface by different incident angles • Pyroelectric camera collects both transmitted and incident THz pulses • Goal: ~20 fs resolution • Depends on laser absorption depth and carrier dynamics on fs timescale

  15. Sommerfeld Mode: THz Transport along a Wire Fisher: E206 THz • THz diffracts quickly in free space • Large mirrors, frequent refocusing • Waveguides are far too lossy • Sommerfeld’s mode transports a radially polarized wave outside a cylindrical conductor • Low loss and low dispersion • Mirror can reflect fields at corners • Calculated attenuation length: a few meters • Far better than waveguide, but too short to guide THz out of tunnel • But near field should be enhanced at the tip

  16. Enhanced Near Field at a Conical Tip LCu = 1 mm (Wire section) RCu = 1 mm (Copper wire radius) Lcone= 6 mm (Conical tip length) Frequency = 1 THz Sommerfeld Mode Input Mode Focuses along the Tip Ziran Wu Copper Wire: Straight and Conical Sections • Assuming high coupling efficiency for CTR into the Sommerfeld mode on the wire • Subwavelength (~/3) focusing at the tip:More than factor of 10 field enhancement Tip modal area ~ 100um dia. Fisher: E206 THz

  17. CTR from Patterned Foils: Polarization Vertical Horizontal Total THz intensityon a plane200 mm from foil Uniform foil: Radially polarized Quadrant Mask Pattern Quadrant pattern: Linear polarization Fisher: E206 THz • Instead of a uniform circular foil, consider a metal pattern • Deposit metal on silicon, then etch

  18. CTR from Patterned Foils: Spectrum 1.6 3.2 1.5 3.0THz 1.4 2.8 Fisher: E206 THz • Grating disperses spectrum. Period selects 1.5 THz. • 30° incidence with a 15° blaze (equivalent to 45° incidence on flat foil): 1st order exits at 90° • Small central hole might be needed for the electron beam

  19. Longitudinal Grating in Fused Silica • Silica dual-grating structure (εr= 4.0) • 55 periods of 30 µm: 15-µm teeth and 15-µm gaps • Simulated for q = 3 nCand σz = 30 µm Field Monitor From TR k E0 e- 4.4 THz Fromgrating 3.41 mJ/pulse at 4.4 THz (162 GHz FWHM) Multi-cycle radiation ~ 0.6 GV/m TR at grating entrance Fisher: E206 THz

  20. Copper-Coated Fused Silica Grating Field Monitor • Silica grating with copper coating • 11 periods of 30 µm: 15-µm teeth and 15-µm gaps • Simulated for q = 3 nCand σz = 30 µm Metal Coating Electron bunch e- Metal Coating 2.91 mJ/pulseof narrow-bandemission at3.275 THz ~ 10 GV/m Multi-cycle radiation Fisher: E206 THz

  21. THz Transport Line Elliptical mirror pair 1-THz Component 100 mm Matlab model, 200 mm from foil Zemax propagation to image plane 10 m y (mm) Fisher: E206 THz • 8-inch evacuated tubing with refocusing every ~10 m • Zemax models with paraboloidal, ellipsoidal, or toroidal focusing mirrors • Insert fields from CTR source model into Zemax model of transport optics. • Use Zemax diffraction propagator for each frequency in emission band. x (mm)

  22. Summary Fisher: E206 THz • Record THz measured in the spring 2014 run: 1.7 mJ • Improved transverse optics • Tuned compression to peak the THz • Began first THz user experiments • Electro-optic signal was timed and measured outside vacuum • Plans • User experiments • A variety of THz sources with different polarization, spectrum, energy • Calculation tools for diffraction in THz transport line

  23. Q&A Fisher: E206 THz • What are the remaining scientific questions about THz generation? • Modeling coherent transition or diffraction radiation • Debate about the transition from near field to “pre-wave zone” to far field • Theoretical effective source size is very large (meters): a ≈ γλ • Effect of smaller foil and beampipe? • Near field (Fresnel zone): Distance L ≤ a • Where does near field really end? • Far field (Fraunhoferzone) distance is kilometers: L > a2/λ = γ2λ • Pre-wave zone in the middle • Multiple stages and formation length • Alternative structures • Modeling THz transport • Diffraction codes were written for lasers and do not model THz sources • Unusual spatial, temporal, spectral properties • Approximations not intended for such long wavelengths • Fresnel, Fraunhofer, transition from plane wave to spherical wave

  24. Q&A Fisher: E206 THz • Compare the FACET source to THz generated by a laser on a foil. • The foil experiments generate ~ 1 µJ of THz. • In these experiments, the THz is used as a diagnostic, not as an intense source.

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