1. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Investigation of Nicro-Optics: Dealing with lenses smaller than sand Matthew Lang Milster Research Group College of Optical Sciences, Tucson AZ

2. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Presentation Outline Nicro-Optics: A new size regime –Fabrication –Simulation –Testing –Handling Applications –Laser diode corrector Ultra small form factor optical pickup –Solid Immersion Lens Cubic crystal birefringence Conclusion

3. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Different categories of sand: Difficult to handle, simulate & test Lenses smaller than what? Very coarse Coarse Medium Fine Very Fine Silt 2000 1000 500 250 125 63 4 Classification Size(  m) Troublesome optics

4. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Fabrication Process: Lithography Photoresist Substrate Mask Exposure Develop Concept art of micro SIL array Etch mask is created from photoresist and transferred to substrate <100  m sag lenses can be made in this fashion Transfer Etch

5. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona The simulation hole: Symptom of a particular size regime? Mini Micro Nicro Nano MoM RCWT FDTD/FDFD Green Function Solvers Boundary Solvers Ray-Based Zemax Code V Oslo SAFE/GBD Light Tools ASAP ?? Few tools exist for modeling arbitrary systems including diffraction and refraction effects Electromagnetic Solvers Fourier Optiscan Diffract GLAD FRED 1m1m 10  m 100  m 1mm 10mm

6. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Non-Sequential Diffraction Calculation (NSDC) A new simulation tool for small scale optics when: –Size regime is too small for ray-based tools, but too large for EM calculators –Rigorous diffraction is required with refraction & reflection through arbitrary surfaces Geometry Facets [Ux 0,Uy 0,Uz 0 ] Reflection/ Transmission [Ux,Uy,Uz] [Ux’,Uy’,Uz’] n’n Example 1 st iteration (source) propagation Example 2 nd iteration propagation Source Facet

7. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona n = 1.0 n = 1.5 n = 1.0 Sequential Diffraction Test Amplitude Phase Used a perfect conjugate lens to test surface decomposition and appropriate refraction upon transmission

8. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona NSDC summary Calculates diffracted field arbitrarily in space Fully vectorized fields Surface interactions Near field (with small enough elements) Complex indices Evanescent effects?

9. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Small Optics Testing Methods Vertical Scanning White Light Profilometry Images entire field of view at once Low NA objectives can’t measure steep surface slopes Stylus Profilometry Can measure somewhat steep surface angles (<60°) Requires surface contact Multiple stitched scans to create surface profile Phase Shifting Interferometry High marginal angles limited by diverger –(NA of 0.95 = marginal angle of 71.8˚) High precision surface mapping (up to ~ /100) Similar systems exist for large mirror metrology… –…but none for micro spherical surface testing

10. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona UA Nicro-Lens Test Setup Custom phase- shifting interferometer with a high NA cone angle –Images pupil of objective lens to give deviation as a function of direction cosine Nicro lenses spherical < /4 out to 42° High-aspect Nicro lens /4 deviation

11. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Application Problem: Mounting & Handling Problem #1: –How is thickness controlled? Solution: –Substrate is lapped off and polished Problem #2: –How do you handle such small lenses? Solution: –Support the lens from above using epoxy and a glass support layer Glass Epoxy GaP Lens trough Glass Epoxy GaP Microlens Array (MEMS Optical)

12. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Mounting: Flat Support Layer Process steps Nicro lens substrate Support Layer Epoxy Lap & Polish Dice/Chamfer Mount Objective

13. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Lapping/Polishing Test Accuracy test goal: lap off material just past the bottom trough around the lens –Very little wedge introduced –Lap distance achieved ±2um (within accuracy of measurement) –Polishing took more material off, but resulted in very smooth surfaces –Mechanical polishing of epoxy needs to be matched to CMP polishing of GaP better to reduce “donut” effect Sites where lens popped out during lapping Glass Epoxy Goal: try to lap just past the trough so the lenses are separate from the substrate Epoxy bump height ~6um

14. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Applications

15. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Laser Diode Corrector A laser diode beam expands more rapidly in greater optically confined direction, less rapidly in the other direction This creates a circular point somewhere in front of the laser facet (aspect ratio ~1) This point is typically 5-10  m away from laser facet (15-30  m if used with a high-index lens) Typical diode beam reaches circular point very close to exit face, (ex. 7  m) Lowering the wavelength reduces the divergence and moves the circular point out from the laser If laser exit medium is GaP, circular point is 3.3x further (ex. 21  m) Same source size 21  m 7m7m

16. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Nicro-Optical System Example Laser Diode Corrector A single anamorphic surface at the circular point can equalize divergence and keep beam aspect ratio close to 1 (circularized) A low power lens further away can collimate the beam Top view Side view With correcting element Epoxy Correcting element Support Layer Collimating microlens x z y z In Air 300um Diverging Light 100um

17. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Laser Diode Corrector Implementation Example Micro Source for Optical Pickup –For data or microscopy applications –A small high-index lens reduces divergence/circularizes laser diode beam –Other optics downstream collimate & focus beam Si Submount Laser High-index circularizing element (Nicro component) Collimating Element Prism Face Focusing Objective Detector 300  m ~1mm

18. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Spot Energy Air gap interface Solid Immersion Lens Wavelength is reduced in medium, =  /n Forms a spot with size: When the medium is close to the bottom surface, this energy is coupled across the gap via evanescent coupling light from laser n Medium

19. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Induced Polarization Air Gap Control Measurement Simulation Small air gap TIR introduces different phase shifts for S & P light at the interface Not reflected for small air gaps due to evanescent coupling Induced polarization signal = precise air gap control Large air gap TIR region

20. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona High-Index SIL Materials: Birefringence Issues High index materials ( n >2) –Diamond ( n = 2.4) –ZnSe ( n = 2.5, >500nm) –GaP ( n = 3.3, >500nm) –Silicon ( n = 4.2, IR) All have cubic flouride crystalline structure Cubic crystals exhibit heptaxial intrinsic birefringence 7 propagation directions with no birefringence!

21. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Gallium Phosphide SIL Birefringence Strange retardance effects achievable spot size Polarization signal due to retardance: Birefringence superimposed on TIR For GaP,  n( 550nm ) = 2.5x10 -5 For OPD  /10, SIL thickness  2mm Another argument to use nicro-SILs! Retardance Orientation Analyzer signal (I/I o ) Crystal orientation [100]

22. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Conclusions A new size regime, “Nicro” optics, is proposed which is classified as a size regime… –That is between conventional “Nano” and “Micro” regimes –Where optical behavior is dominated by diffraction effects –For which simulation tools are not well represented Too big for FDTD, too small for ray-based –Few testing methods which can measure high surface slope Promising applications as: –Beam shaping/Laser diode correction –Ultra-small, very high-index SILs

23. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona Questions?

24. Copyright 2006 Milster Research Group, Optical Sciences Center, University of Arizona