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Photonic Crystals:

Photonic Crystals:

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  1. Photonic Crystals: A Novel Approach to Enhance the Light Output of Scintillation Based Detectors Arno KNAPITSCHa, Etiennette AUFFRAYa, Paul LECOQa a PH-CMX,CERN, Geneva, Switzerland

  2. Outline: • Introduction • Scintillating Crystals • Motivation • Photonic Crystals • Simulations • Monte Carlo • A Frequency- Domain Eigenmode Solver • Results • PhC Fabrication • Sputter Deposition • Electron Beam Lithography • Reactive Ion Etching • Results • Conclusion

  3. Scintillating Crystals ionizing radiation Light emission Scintillating Crystal Cerium-doped Lutetium Yttrium Orthosilicate Bismuth germinate Cerium-doped Lutetium-Yttrium Aluminum Perovskite Cerium-doped Lutetium Aluminum Garnet • What is scintillation? • Emission of light due to an ionizing event • What kind of scintillators are there? • Intrinsic (BWO, BGO) or extrinsic(LYSO:Ce, LuAG:Ce) scintillators • Organic, inorganic, liquid-, plastic, gaseous • Common scintillators in HEP and medical imaging • LYSO:Ce(Lu2-xYxSiO5), BGO(Bi4Ge3O12), LuYAP:Ce(LuxY1-xAlO3), LuAG:Ce(Lu3Al5O7)

  4. Motivation: Application Principle of PET [2] CMS (Compact Muon Solenoid) at the LHC [1] Field of application of heavy inorganic scintillators • High energy physics (HEP), medical imaging (e.g. PET), spectroscopy

  5. Motivation: Increase Nr. of detected Photons Main limiting factor for the light collection efficiency : • Total reflection due to a mismatch of the refractive index of crystal and detector • Snell’s Law: Efficiency: Main factors governing energy- and time resolution: • Detected number of photoelectrons Npe

  6. Motivation: Photonic Crystal incident light reflected light extracted modes How can a photonic crystal help to overcome those limits? • Light extraction due to a periodic grating of the interface:

  7. Photonic Crystals (PhCs) • [3] J. D. Joannopoulos, Photonic crystals – Molding the flow of light, 2008 Photonic crystal basics: • Periodic arrangement of two materials with different index of refraction, in one-, two-, or three dimensions

  8. How does the Photonic Crystal Work? Plain crystal- air interface: (EM – fieldplot [5] ) Crystal- air interface with PhC grating: crystal air crystal air Plane Wave θ>θc Total Reflection at the interface since Extracted Mode (~60% Transmission) θ>θc (0% Transmission) Diffracted modes interfere constructively in the PhC- grating and are therefore able to escape the Crystal

  9. Outline: • Introduction • Scintillating Crystals • Motivation • Photonic Crystals • Simulations • Monte Carlo • A Frequency- Domain Eigenmode Solver • Results • PhC Fabrication • Sputter Deposition • Electron Beam Lithography • Reactive Ion Etching • Results • Conclusion

  10. Simulation: A two Step Approach 1. 2. θc • Look at the angular distribution at the crystal- detector interface with a Monte- Carlo simulation tool (LITRANI [5]) • Take the light distribution from the Monte-Carlo program and simulate the light extraction of a scintillator- PhC- air interface with an eigenmode expansion software (CAMFR [4])

  11. Optimize the PhC Design z a Si3N4 hole depth: d ITO Scintillator y hole diameter: D x • PhC crystal parameters: • Lattice constant: a • Hole diameter: D • Hole depth: d • Optimize the parameters for maximal light transmission over all angles: • Parameters in case of LYSO: • a = 340nm • D = 200nm • d = 300nm

  12. Light Gain Crystal Type: LYSO Crystal measurements: 1.3x2.6x8mm Wrapping: Tyvek Light Gain when comparing to an unstructured Crystal:

  13. Results of different Crystals:

  14. Outline: • Introduction • Scintillating Crystals • Motivation • Photonic Crystals • Simulations • Monte Carlo • A Frequency- Domain Eigenmode Solver • Results • PhC Fabrication • Sputter Deposition • Electron Beam Lithography • Reactive Ion Etching • Results • Conclusion

  15. PhC Fabrication Reactive ion etching reactor Scanning electron Microscope Nano Lithography • PhC is produced in cooperation with the INL (Institut des Nanotechnologies de Lyon) • Three step approach: • Deposition of a pattern transfer material • Patterning of the resist using a scanning electron Microscope • Pattern transfer using reactive ion etching (RIE)

  16. Sputter Deposition z z Si3N4 300nm ITO ITO 70nm 70nm Scintillator Scintillator y y x x Sputtering of two different Materials: • ~70nm of ITO (Indium Tin Oxide) • ~300nm of Si3N4 (Silicon Nitride)

  17. Electron Beam Patterning Electron beam z z z PMMA Resist PMMA Resist PMMA Si3N4 Si3N4 Si3N4 ITO ITO ITO Scintillator Scintillator Scintillator y y y x x x • Deposit of an resist material (PMMA) by spin coating • Writing the PhC pattern into the resist with a scanning electron microscope (SEM) • Removing the exposed areas on the resist with an chemical solvate

  18. Reactive Ion Etching (RIE) Ion Bombardment z z a PMMA Resist Si3N4 Si3N4 Hole depth: 300nm ITO ITO Scintillator Scintillator y y hole diameter: 200nm x x Chemically reactive plasma removes Si3N4 not covered by the resist Change the composition of the reactive plasma to remove the resist (PMMA) without etching the Si3N4

  19. PhC Results D = 200nm a = 340nm Scanning Electron Images:

  20. Outline: • Introduction • Scintillating Crystals • Motivation • Photonic Crystals • Simulations • Monte Carlo • A Frequency domain Eigenmode Solver • Results • PhC Fabrication • Sputter Deposition • Electron Beam Lithography • Reactive Ion Etching • Results • Conclusion • Outlook • Acknowledgement

  21. Conclusion 1. 90% and 110% • Simulations show an light yield enhancement between 80% and 120%

  22. Conclusion 2. The PhC- production process has been adapted to the requirements of the crystal • Due to the ITO layer we have good electrical connectivity from the Si3N4 to the surrounding • The RIE parameters were adapted to the required etching depth without having anisotropic effects on the pattern • Lattice parameters of the PhC could be verified

  23. Outlook • Optical Characterization of the PhC • Light Yield Measurements • Angular Distribution • Compare the measurement- results to the simulations and classify possible deviations • Use the knowledge obtained by the measurements to further optimize the PhC pattern of the next samples

  24. Acknowledgments Many thanks to the staff of the INL – Lyon, especially to J.-L. Leclercq and C. Seassal for their support and advice during my stays in Lyon.

  25. References [1] http://public.web.cern.ch/public/en/LHc/CMS-en.html [2] http://www.nature.com/nrc/journal/v4/n6/box/nrc1368_BX1.html [3] J. D. Joannopoulos, Photonic crystals – Molding the flow of light, 2008 [4] Photonic crystal LEDs - designing light extraction, C. Wiesmann, 2009 [5] CAMFR, (CAvity Modelling FRamework), http://camfr.sourceforge.net [6] LITRANI, http://gentit.home.cern.ch/gentit/litrani/