E206 Terahertz Radiation from the FACET Beam Alan Fisher and Ziran - PowerPoint PPT Presentation

E206 Terahertz Radiation from the FACET Beam Alan Fisher and Ziran Wu SLAC National Accelerator Laboratory SAREC Review SLAC 2014 September 15–17 1

Topics § 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 Fisher: E206 THz 2

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

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

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 5

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 6

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

Peak THz: Energy and Electric Field § 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. Fisher: E206 THz 8

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 9

FACET Laser brought to THz Table § 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 Fisher: E206 THz 10

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

Scanned Electro-Optic Sampling § Mercury-cadmium-telluride detector and fast scope used to time THz and laser within 150 ps § Precise timing overlap from EO effect in GaP and 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… Fisher: E206 THz 12 12

Single-Shot Timing: Electro-Optic Spectral Decoding Model of electron bunch Calculated spectrometer display § Simulate 150-fs (RMS) electron beam § With and without 60-fs notch § Add ±10-fs beam jitter relative to laser § Adjust laser chirp to ~1 ps FWHM § Calculation: spectrometer resolves jitter § Ocean Optics HR2000+ spectrometer § Fiber-coupled to gallery Fisher: E206 THz 13 13

Single-Shot Timing: Switched Mirror § 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 Test with Laser-Generated THz Pulse Fisher: E206 THz 14

Sommerfeld Mode: THz Transport along a Wire § 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 Fisher: E206 THz 15

Enhanced Near Field at a Conical Tip L Cu = 1 mm (Wire section) Sommerfeld Mode Input R Cu = 1 mm (Copper wire radius) L cone = 6 mm (Conical tip length) Frequency = 1 THz 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 (~ l /3) focusing at the tip: More than factor of 10 field enhancement Tip modal area ~ 100um dia. Fisher: E206 THz 16

CTR from Patterned Foils: Polarization § Instead of a uniform circular foil, consider a metal pattern § Deposit metal on silicon, then etch Horizontal Vertical Total THz intensity on a plane 200 mm from foil Uniform foil: Radially polarized Quadrant Mask Pattern Quadrant pattern: Linear polarization Fisher: E206 THz 17

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

Longitudinal Grating in Fused Silica § Silica dual-grating structure ( ε r = 4.0) § 55 periods of 30 µm: 15-µm teeth and 15-µm gaps Field Monitor § Simulated for q = 3 nC and σ z = 30 µm k 4 E 0 From 3.5 TR 3 e- Intensity (a.u.) 2.5 2 4.4 THz 10 x 10 1 1.5 From 1 3.41 mJ/pulse grating at 4.4 THz 0.5 0.5 (162 GHz FWHM) Multi-cycle radiation 0 0 1 2 3 4 5 6 7 0 E z (V/m) Frequency (THz) 0.8 0.6 ~ 0.6 GV/m -0.5 0.4 0.2 E z (GV/m) TR at grating -1 0 entrance -0.2 -0.4 -1.5 0 2 4 6 8 10 12 14 16 -0.6 Time (ps) -0.8 Fisher: E206 THz 19 6 7 8 9 10 11 12 13 14 15 Time (ps)

Copper-Coated Fused Silica Grating § Silica grating with copper coating Metal Coating § 11 periods of 30 µm: 15-µm teeth and 15-µm gaps e- Field Monitor § Simulated for q = 3 nC and σ z = 30 µm 9 x 10 Metal Coating 8 11 6 x 10 2.5 4 Electron bunch 2 E z (V/m) 2 0 -2 6 -4 1.5 -6 E z (V/m) 5 2.91 mJ/pulse ~ 10 GV/m -8 of narrow-band 1 -10 4 Intensity (a.u.) 2 2.5 3 3.5 4 4.5 5 5.5 6 emission at Time (ps) 3.275 THz 3 0.5 Multi-cycle radiation 2 0 1 -0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6 0 0 1 2 3 4 5 6 7 8 9 10 Time (ps) Frequency (THz) Fisher: E206 THz 20

THz Transport Line § 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. Elliptical mirror pair 1-THz Component 100 mm Matlab model, 200 mm from foil Zemax propagation to image plane y (mm) 10 m Fisher: E206 THz 21 x (mm)

Summary 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 Fisher: E206 THz 22