1

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

15

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

16

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)

22

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.