Characterizing 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
1
Topics
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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
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FACET THz Table
Table top is enclosed and continuously purged with dry air to reduce THz attenuation by water vapor.
Fisher: E206 THz
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Peak THz: Michelson Interferometer Scans
Tuning Compression for Peak THz
Before
Fisher: E206 THz
After
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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
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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
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Peak THz: Knife-Edge Scans for Transverse Size
Horizontal
Fisher: E206 THz
Vertical
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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 compensation 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
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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-mmdiameter diamond window:
§ ~30% reflection losses
§ Long-wave cutoff
§ Calculated energy consistent
with measured 1.7 mJ
Fisher: E206 THz
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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
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Layout of the THz Table for User Experiments
Laser Path
from IP Table
800nm, ~150fs, 9Hz, 1mJ
CCD
l/2
Polarizer
l/4
Pyro
W. Polarizer
PD
P. Diode
EO
Crystal
BS
ND Filter
ß
VO2
Sample
Fisher: E206 THz
ß
Pyrocam
PD
PEM
Det.
Translation
Stage
E218
Setup
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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
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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
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Single-Shot Timing: Switched Mirror
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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
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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
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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 (~l/3) focusing at the tip:
More than factor of 10 field enhancement
Fisher: E206 THz
Tip modal area ~ 100um 16
dia.
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
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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
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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 nC and σz = 30 µm
Field Monitor
k
4
E0
From
TR
3.5
3
Intensity (a.u.)
e10
1
x 10
2.5
2
4.4 THz
1.5
1
0.5
0.5
0
0
0
1
2
0.8
0.6
-0.5
4
5
6
~ 0.6 GV/m
Ez (GV/m)
0.4
TR at grating
entrance
-1
-1.5
3
Frequency (THz)
z
E (V/m)
Multi-cycle radiation
From
grating
3.41 mJ/pulse
at 4.4 THz
(162 GHz FWHM)
0.2
0
-0.2
-0.4
0
2
4
6
8
Time (ps)
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12
14
16
-0.6
-0.8
6
7
8
9
10
11
Time (ps)
12
13
14
15
19
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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-
§ Simulated for q = 3 nC and σz = 30 µm
Field Monitor
9
8
Metal Coating
11
2.5
x 10
6
x 10
4
Electron bunch
E (V/m)
2
z
2
0
-2
6
-4
1.5
-10
2
2.5
3
3.5
4
4.5
5
5.5
Time (ps)
0.5
Multi-cycle radiation
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Intensity (a.u.)
1
5
~ 10 GV/m
-8
z
E (V/m)
-6
2.91 mJ/pulse
of narrow-band
emission at
3.275 THz
4
3
2
0
1
-0.5
1
1.5
2
2.5
3
3.5
Time (ps)
Fisher: E206 THz
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4.5
5
5.5
6
0
0
1
2
3
4
5
6
7
8
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Frequency (THz)
20
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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
10 m
Fisher: E206 THz
Zemax propagation to image plane
y (mm)
Matlab model, 200 mm from foil
x (mm)
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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
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