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

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E206 Terahertz Radiation from the FACET Beam

SAREC Review SLAC 2014 September 15–17

Alan Fisher and Ziran Wu

SLAC National Accelerator Laboratory

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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

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FACET THz Table

Fisher: E206 THz

Table top is enclosed and continuously purged with dry air to reduce THz attenuation by water vapor.

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Peak THz: Michelson Interferometer Scans

Fisher: E206 THz

Tuning Compression for Peak THz Before After

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Peak THz: Spectra

Fisher: E206 THz

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

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Peak THz: Reconstructing the Electron Bunch

Fisher: E206 THz

§ Requires compensation for DC component, which is not radiated. § Kramers-Kronig procedure provides missing phase for inverse Fourier transform of spectrum.

Tuning Compression for Peak THz Before After

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Peak THz: Knife-Edge Scans for Transverse Size

Fisher: E206 THz

Horizontal Vertical

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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.

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Modeling Emission from a Conducting Foil

Fisher: E206 THz

§ 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

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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

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Layout of the THz Table for User Experiments

800nm, ~150fs, 9Hz, 1mJ

CCD

• P. Diode

BS ND Filter ß l/2 Polarizer

ß

Pyro EO Crystal VO2 Sample PEM Det. Pyrocam

Translation Stage l/4 PD PD

• W. Polarizer

Fisher: E206 THz

E218 Setup Laser Path from IP Table

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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 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…

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Single-Shot Timing: Electro-Optic Spectral Decoding

Fisher: E206 THz

§ 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

Model of electron bunch Calculated spectrometer display

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Single-Shot Timing: Switched Mirror

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

Test with Laser-Generated THz Pulse

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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

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LCu = 1 mm (Wire section) RCu = 1 mm (Copper wire radius) Lcone= 6 mm (Conical tip length) Frequency = 1 THz

Enhanced Near Field at a Conical Tip

Fisher: E206 THz

§ 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 Sommerfeld Mode Input Copper Wire: Straight and Conical Sections Mode Focuses along the Tip

Tip modal area ~ 100um dia.

Ziran Wu

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CTR from Patterned Foils: Polarization

Fisher: E206 THz

§ Instead of a uniform circular foil, consider a metal pattern

§ Deposit metal on silicon, then etch

Uniform foil: Radially polarized Quadrant pattern: Linear polarization

Horizontal Vertical Total

THz intensity

• n a plane

200 mm from foil

Quadrant Mask Pattern

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CTR from Patterned Foils: Spectrum

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

1.5

3.0

THz 1.6

3.2

1.4

2.8

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Longitudinal Grating in Fused Silica

Fisher: E206 THz

2 4 6 8 10 12 14 16

• 1.5
• 1
• 0.5

0.5 1 x 10

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Time (ps) Ez (V/m)

TR at grating entrance Multi-cycle radiation

6 7 8 9 10 11 12 13 14 15

• 0.8
• 0.6
• 0.4
• 0.2

0.2 0.4 0.6 0.8

Time (ps) Ez (GV/m)

~ 0.6 GV/m

1 2 3 4 5 6 7 0.5 1 1.5 2 2.5 3 3.5 4

Frequency (THz) Intensity (a.u.)

From grating 4.4 THz

3.41 mJ/pulse at 4.4 THz (162 GHz FWHM)

§ Silica dual-grating structure (εr= 4.0)

§ 55 periods of 30 µm: 15-µm teeth and 15-µm gaps

§ Simulated for q = 3 nC and σz = 30 µm e- k E0

Field Monitor

From TR

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Copper-Coated Fused Silica Grating

Fisher: E206 THz

§ Silica grating with copper coating

§ 11 periods of 30 µm: 15-µm teeth and 15-µm gaps

§ Simulated for q = 3 nC and σz = 30 µm e-

Metal Coating Metal Coating

Field Monitor 1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

• 0.5

0.5 1 1.5 2 2.5 x 10

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Time (ps) Ez (V/m) Electron bunch Multi-cycle radiation

2 2.5 3 3.5 4 4.5 5 5.5 6

• 10
• 8
• 6
• 4
• 2

2 4 6 8 x 10

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Time (ps) Ez (V/m)

~ 10 GV/m

1 2 3 4 5 6 7 8 9 10 1 2 3 4 5 6

Frequency (THz) Intensity (a.u.)

2.91 mJ/pulse

• f narrow-band

emission at 3.275 THz

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THz Transport Line

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.

1-THz Component Matlab model, 200 mm from foil Zemax propagation to image plane Elliptical mirror pair

100 mm 10 m

x (mm) y (mm)

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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