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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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E206Terahertz Radiationfrom the FACET Beam • Alan Fisher and Ziran Wu • SLAC National Accelerator Laboratory SAREC Review SLAC 2014 September 15–17
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
FACET THz Table • Table top is enclosed and continuously purged with dry air to reduce THz attenuation by water vapor. Fisher: E206 THz
Peak THz: Michelson Interferometer Scans Tuning Compression for Peak THz Before After Fisher: E206 THz
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
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
Peak THz: Knife-Edge Scans for Transverse Size Horizontal Vertical Fisher: E206 THz
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.
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
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
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
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…
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
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
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
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
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
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
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
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
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)
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
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
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.