E206 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 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 § 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- cutoff § Calculated energy consistent with measured 1.7 mJ

Fisher: E206 THz 9 FACET 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 Polarizer l/4 W. Polarizer Pyro PD P. Diode EO Crystal PD ND Filter PEM BS ß Det. VO2 Sample E218 ß Translation Pyrocam Setup 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 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 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

Sommerfeld Mode Input LCu = 1 mm (Wire section) RCu = 1 mm (Copper wire radius) Lcone= 6 mm (Conical tip length) = 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

Fisher: E206 THz Tip modal area ~ 100um 16dia. 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): 1st order exits at 90° § Small central hole might be needed for the electron beam

1.4 1.5 1.6 2.8 3.0 3.2 THz

Fisher: E206 THz 18 Longitudinal Grating in Fused Silica

§ Silica dual-grating structure (εr= 4.0) § 55 periods of 30 µm: 15-µm teeth and 15-µm gaps Field Monitor § Simulated for q = 3 nC and σz = 30 µm k 4 E0 3.5 From TR 3 ) e- .

u 2.5 . a ( y

t 2 i s 10 n 4.4 THz x 10 e 1 t 1.5 n I 1 3.41 mJ/pulse From 0.5 0.5 at 4.4 THz grating Multi-cycle radiation (162 GHz FWHM) 0 0 1 2 3 4 5 6 7

) 0 0.8 Frequency (THz) m / V

( 0.6 z ~ 0.6 GV/m E -0.5 0.4

0.2 ) m TR at grating / -1 V 0 G (

entrance z

E -0.2

-0.4 -1.5 0 2 4 6 8 10 12 14 16 Time (ps) -0.6 -0.8 Fisher: E206 THz 6 7 8 9 10 11 12 13 14 15 19 Time (ps) Copper-Coated Fused Silica Grating

Metal Coating § Silica grating with copper coating e- § 11 periods of 30 µm: 15-µm teeth and 15-µm gaps Field Monitor § Simulated for q = 3 nC and σz = 30 µm

9 x 10 Metal Coating 8 11 x 10 6 2.5 4 Electron bunch 2 )

2 m 0 / V (

z -2 E 6 -4 1.5 -6 ) 5 2.91 mJ/pulse

m -8 ~ 10 GV/m /

V 1 of narrow-band

( -10 ) 4 2 2.5 3 3.5 4 4.5 5 5.5 6 . z u emission at Time (ps) . E a ( 3.275 THz y

t 3 0.5 i s n e Multi-cycle radiation t n

I 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 )

10 m m m ( y

Fisher: E206 THz x (mm) 21 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