1 E206 Terahertz Radiation from the FACET Beam SAREC Review SLAC 2014 September 15–17 Alan Fisher and Ziran Wu SLAC National Accelerator Laboratory

2 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

3 FACET THz Table Fisher: E206 THz Table top is enclosed and continuously purged with dry air to reduce THz attenuation by water vapor.

44 Peak THz: Michelson Interferometer Scans Fisher: E206 THz Tuning Compression for Peak THz BeforeAfter

55 Peak THz: Spectra Fisher: E206 THz Tuning Compression for Peak THz BeforeAfter  Tuning extended spectrum to higher frequencies  Modulation due to:  Water-vapor absorption (12% humidity, later reduced to 5%)  Etalon effects in the detector

66 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 BeforeAfter

77 Peak THz: Knife-Edge Scans for Transverse Size Fisher: E206 THz Horizontal Vertical

88 Peak THz: Energy and Electric Field Fisher: E206 THz  Joulemeter reading and adjustments 3.8 VJoulemeter  26-dB attenuator  1/50Amplifier gain  2Beamsplitter  1/(700 V/J)Detector calibration  4THz 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.

9 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

10 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

11 Layout of the THz Table for User Experiments 800nm, ~150fs, 9Hz, 1mJ CCD P. Diode BS ND Filter  /2 Polarizer  Pyro EO Crystal VO 2 Sample PEM Det. Pyrocam Translation Stage /4 PD W. Polarizer Fisher: E206 THz E218 Setup Laser Path from IP Table

12 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…

13 Single-Shot Timing: Electro-Optic Spectral Decoding Fisher: E206 THz  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 bunch Calculated spectrometer display

14 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

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 L Cu = 1 mm (Wire section) R Cu = 1 mm (Copper wire radius) L cone = 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

17 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 on a plane 200 mm from foil Quadrant Mask Pattern

18 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): 1 st order exits at 90°  Small central hole might be needed for the electron beam THz

19 Longitudinal Grating in Fused Silica Fisher: E206 THz TR at grating entrance Multi-cycle radiation ~ 0.6 GV/m From grating 4.4 THz 3.41 mJ/pulse at 4.4 THz (162 GHz FWHM)  Silica dual-grating structure (ε r = 4.0)  55 periods of 30 µm: 15-µm teeth and 15-µm gaps  Simulated for q = 3 nC and σ z = 30 µm e- k E0E0 Field Monitor From TR

20 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 σ z = 30 µm e- Metal Coating Field Monitor Electron bunch Multi-cycle radiation ~ 10 GV/m 2.91 mJ/pulse of narrow-band emission at THz

21 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 foilZemax propagation to image plane Elliptical mirror pair 100 mm 10 m x (mm) y (mm)

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

23 Q&A Fisher: E206 THz 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 > a 2 /λ = γ 2 λ  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

24 Q&A Fisher: E206 THz 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.