Variable Reflectivity Dielectric Mirrors for THz-FEL Applications

1. Variable Reflectivity Dielectric Mirrors for THz-FEL Applications Mufit Tecimer1, Karsten Holldack2, Luis R. Elias1 1THz FEL Group, University of Hawai’i at Manoa, Honolulu, USA 2Helmholtz Zentrum Berlin, BESSY II, Berlin, Germany IRMMW-THz 2009 Busan, S. Korea September 21, 2009

2. Dielectric Multilayer Mirrors • THz Cavity Ring Down Spectroscopy (2003-2005), • Cavity mirrors for FIR p-Ge Laser, R=99.93% achieved using 3 periods of Si/vacuum gap (2005), • Si multilayer based THz wave filter tested by THz TDS (2008), ……….. • mostly utilizing high resistivity Si multilayers as low loss material (a ~0.02/cm-1) and high index of refraction n~3.41 • Our study of adopting dielectric multilayers as broadband coupler and high reflectivity mirrors for THz Free Electron Lasers (~0.5 THz - 3 THz) as well as for external Pulse Stacker Cavities started in 2007. • As low loss materials: crystalline Quarz (n~2.11) and high resistivity Si.(in between: Diamondn~2.37, negligible a )

3. I. Variable height slot aperture • Slot width : 3.0mm (fixed) - Slot height : 0. to 5.0 mm variable - Optimized Slot height for the FIR region: 1.0mm – 4.0mm • The outcoupling is broad-band and variable, however not efficient. Generated higher order modes end up as Ohmic loss. • Beamsplitter outcoupling using Si wafer (n~3.4). (E. Szarmes, Duke Univ.,1994) Outcoupling schemes II. Beamsplitter Outcoupling (M. Tecimer, FSU-FIR FEL Design Study, July 2007, JLab)

4. Postprocessing of generated THz pulses Pulse Stacking in FELs ( T. Smith, P. Haar Stanford FEL, 1997 ) (M. Tecimer, FSU-FIR FEL Design Study, July 2007, JLab)

5. de : coupling ratio do : other cavity losses dc = de +do (total loss) 1.) de = 0.015 , do = 0.015 Q ~33 (Quality factor) R = 99.8-99.9 (Power reflectivity) trt ~ 42.4 ns (roundtrip time) trd ~ 1.4 ms (ring down time) Uc ~ 67 Uinc (circul. power), 60% dumping efficiency(?) Ud ~ 40 Uinc 2.) de = 0.02 , do = 0.02 Uc ~ 50 Uinc (Q ~ 25) Ud ~ 30Uinc Postprocessing of high rep. rate THz FEL pulses (Pulse Stacking in an external cavity) • Rep. rate of the dumped pulses is determined by the switching IR Laser (~ kHz)

6. Low Loss Dielectric Multilayer Coupler Mirrors for THz FELs Efficient, tunable, broad-band couplers for THz FELs • outcoupling efficiencies with nearly 100%over abroad THz spectral range, (reduced heat load deposited on the mirror/waveguide structure) • well defined adjustment of reflectivity/coupling-ratio (~70 -99%) for a given operational wavelength, • negligible mode distortion at (in/out-) coupling, • (important for coupling into an external pulse stacker cavity) • construction of high reflectivity broad-band THz mirrors. (R>99.9%, exceeding reflectivity of gold coated mirrors)

7. Quarz, Diamond, Si vacuum spacer Q and Si Dielectric Multilayer Mirrors @ 1THz 5 layers dielectric cross section: height:15 mm width :15, 30, 45 mm mirror height in most of the waveguide THz FELs : 2 mm - 10 mm

8. lin. Actuator, fast PZT variable spacing Au coated metal Variable interlayer spacing Fine tunable outcoupling ratio Combination of materials (Quarz, Silicon and Diamond) to match imposed design parameters Intensity modulation, steep change in reflectivity (several tens of percentage per micron) manipulation of THz FEL dynamics ? (mode-locking in long pulse THz FELs, etc ) Shifting of high reflectivity bands Technologies employed: Thick films and m-mechanical components (LIGA @ BESSY) Metal/dielectric (hybrid) configuration High reflectivity mirrors R>99.9% robust layer structure (single or a few layers necessary to attain/exceed 99.9%

9. BESSY II storage ring low a THz-pulse pattern FTIR measurements CSR THz pulse generation • femtoslicing • low-a mode K. Holldack et al, PAC 2005 Proc., p.2239 He-cooled Si Bolometers Bruker IFS 125HR

10. Reflectance tuning using Q and Si Dielectric Multilayer Mirrors high resistivity Silicon z-cut Quarz

11. ~ 0. 1 %/mm ~ 10.0%/mm Q or C 3 x Q Reflectance tuning using Q and Si Dielectric Multilayer Mirrors Intensity modulation (steep change in reflectivity) fine tunable outcoupling ratio

12. FTIR measurements Tilting of the outer layer @ 300mm resolution better than 0.1%

13. FTIR measurements

14. Shifting of the high Reflectivity Bands measured using 105 mm spacer

15. Au+2 x Si wafers : 99.994% Au+2 x Quarz wafers : 99.972% Au+3 x Si wafers : 99.996% Au+4 x Quarz wafers : 99.988% Au+3 x Quarz wafers : 99.985% Metal/dielectric (hybrid) configuration measured using Au+3Q layers

16. Summary • We have experimentally demonstrated that a well defined adjustment of reflectivity and (output) coupling ratio can be accomplished efficiently by using dielectric multilayers in the THz spectral region. • The preliminary experimental results obtained indicate the use of the presented schemes as fast THz modulators. • Metal/dielectric (hybrid) mirrors is an alternative means in constructing high reflectivity mirrors, possibly outperforming the ‘pure’ counterparts. • An intense, stable, broadband CSR source combined with a high resolution FTIR Spectrometer is being used to test the performance of the dielectric mirrors in the THz spectral range. Partially supported by NSF Grant #0520481-Subcontr.#R00916

17. Thank you for your attention