Download - E206 Terahertz Radiation from the FACET Beam
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E206Terahertz Radiation
from the FACET Beam
SAREC ReviewSLAC
2014 September 15–17
Alan Fisher and Ziran WuSLAC National Accelerator Laboratory
2Fisher: E206 THz
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
3Fisher: E206 THz
FACET THz Table
Table top is enclosed and continuously purged with dry air to reduce THz attenuation by water vapor.
4Fisher: E206 THz 4
Peak THz: Michelson Interferometer Scans
Tuning Compression for Peak THz
Before After
5Fisher: E206 THz 5
Peak THz: Spectra
Tuning Compression for Peak THz
Before After
Tuning extended spectrum to higher frequenciesModulation due to:
Water-vapor absorption (12% humidity, later reduced to 5%) Etalon effects in the detector
6Fisher: E206 THz 6
Peak THz: Reconstructing the Electron Bunch
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
7Fisher: E206 THz 7
Peak THz: Knife-Edge Scans for Transverse Size
Horizontal Vertical
8Fisher: E206 THz 8
Peak THz: Energy and Electric Field
Joulemeter reading and adjustments3.8 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.
9Fisher: E206 THz
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
10Fisher: E206 THz
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
11Fisher: E206 THz
Layout of the THz Table for User Experiments
800nm, ~150fs, 9Hz, 1mJCCD
P. Diode
BSND Filter
/2 Polarizer Pyro
EO Crystal
VO2 Sample
PEMDet.
PyrocamTranslation
Stage
/4PD
PD
W. Polarizer
E218 Setup
Laser Path from IP Table
12Fisher: E206 THz 12
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…
13Fisher: E206 THz 13
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 bunchCalculated spectrometer display
14Fisher: E206 THz
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 timescaleTest 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 (~/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
17Fisher: E206 THz
CTR from Patterned Foils: Polarization
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 intensityon a plane
200 mm from foil
Quadrant Mask Pattern
18Fisher: E206 THz
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
1.53.0
THz
1.63.2
1.42.8
19Fisher: E206 THz
Longitudinal Grating in Fused Silica
0 2 4 6 8 10 12 14 16-1.5
-1
-0.5
0
0.5
1x 10
10
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
0.2
0.4
0.6
0.8
Time (ps)
Ez (
GV
/m)
~ 0.6 GV/m
0 1 2 3 4 5 6 70
0.5
1
1.5
2
2.5
3
3.5
4
Frequency (THz)
Inte
nsi
ty (
a.u
.)
Fromgrating
4.4 THz
3.41 mJ/pulseat 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
FromTR
20Fisher: E206 THz
Copper-Coated Fused Silica Grating
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 µme-
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
0.5
1
1.5
2
2.5x 10
11
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
0
2
4
6
8x 10
9
Time (ps)
Ez (
V/m
)
~ 10 GV/m
0 1 2 3 4 5 6 7 8 9 100
1
2
3
4
5
6
Frequency (THz)
Inte
nsi
ty (
a.u
.)
2.91 mJ/pulseof narrow-bandemission at3.275 THz
21Fisher: E206 THz
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.
1-THz Component
Matlab model, 200 mm from foil Zemax propagation to image plane
Elliptical mirror pair
100 mm
10 m
x (mm)
y (m
m)
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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
23Fisher: E206 THz
Q&A
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 (Fraunhofer zone) 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
24Fisher: E206 THz
Q&A
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.