1. Mufit Tecimer1, Karsten Holldack2, Luis R. Elias1
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 ( ),
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: Diamond n~2.37, negligible a )

3. Outcoupling schemes 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)
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. Postprocessing of high rep. rate THz FEL pulses
(Pulse Stacking in an external cavity)
de : coupling ratio do : other cavity losses dc = de + do (total loss)
1.) de = , do = Q ~ 33 (Quality factor) R = (Power reflectivity) trt ~ ns (roundtrip time) trd ~ 1.4 ms (ring down time) Uc ~ 67 Uinc (circul. power), 60% dumping efficiency(?) Ud ~ 40 Uinc
2.) de = , do = Uc ~ 50 Uinc (Q ~ 25) Ud ~ 30 Uinc
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 a broad 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. Q and Si Dielectric Multilayer Mirrors @ 1THz
5 layers
Quarz, Diamond, Si
vacuum
spacer
dielectric cross section:
height:15 mm
width :15, 30, 45 mm
mirror height in most of the waveguide THz FELs :
2 mm mm

8. Variable interlayer spacing
lin. Actuator, fast PZT
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 )
variable spacing
Shifting of high reflectivity bands
Technologies employed:
Thick films and m-mechanical components BESSY)
Metal/dielectric (hybrid) configuration
Au coated metal
High reflectivity mirrors R>99.9%
robust layer structure (single or a few layers necessary to attain/exceed 99.9%

9. FTIR measurements K. Holldack et al, PAC 2005 Proc., p.2239
CSR THz pulse generation
femtoslicing
low-a mode
BESSY II storage ring
low a THz-pulse pattern
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
z-cut Quarz
high resistivity Silicon

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

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. measured using Au+3Q layers
Metal/dielectric (hybrid) configuration
Au+2 x Si wafers : %
Au+2 x Quarz wafers : %
measured using Au+3Q layers
Au+3 x Si wafers : %
Au+4 x Quarz wafers : %
Au+3 x Quarz wafers : %

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 # Subcontr.#R00916

17. Thank you for your attention