1. Optomechanical cantilever device for displacement sensing and variable attenuator 1 Peter A Cooper, Christopher Holmes Lewis G. Carpenter, Paolo L. Mennea, James C. Gates, Peter G.R. Smith Planar Optical Materials group Photonics West 2014

2. Outline 2 Blue: Silica layers Red: Silicon substrate Describe the motivation between fabrication of silica glass micro cantilever array on silicon substrate Describe in detail the fabrication procedure Present characterization for mechanical actuation 500 microns

3. Context 3 What is the motivation for combining optical elements with microstructures? Enhancement of tuning effects New sensor/actuator geometries Cantilever 1 MicrobeamMembrane 2 [1] Lewis G Carpenter et al “Integrated optic glass microcantilevers with Bragg gratings interrogation” Optics Express 18 (2010) [2] C Holmes et al “Miniaturization of Bragg-multiplexed membrane transducers” J. Micromech 22 (2012)

4. Project overview and motivations 4 The integration of optical components into a glass cantilever for high resolution force sensing An variable attenuator compatible with piezoelectric actuation A platform for manipulating particles or cells with optical forces Demonstration of novel physical dicing methods in integrated optics

5. Fabrication - FHD 5 Layers of silica are deposited on the silicon using Flame Hydrolysis Deposition (FHD) Central layer doped with germanium to produce photosensitivity to UV light. Silica soot deposited from gas precursor SiCl 4 Dopants can added with other halide gases

6. 6 UV writing process used to define channel waveguide in core layer Interference pattern from overlapping beams can be used to simultaneously define Bragg gratings Mode dimensions compatible with low loss coupling to optical fibers Typical spectrum showing Gaussian apodized gratings Fabrication – UV Writing

7. 7 Dicing for optical surfaces Commercial dicing saw used for dicing wafers Air-bearing spindle runs at 20,000 with better than 1 micron run-out Suitable for structures with micron precision Diamond impregnated blade widths ranging from 250um to 10 um available

8. 8 Dicing for optical surfaces Commercial dicing saw used for dicing wafers Air-bearing spindle runs at 20,000 with better than 1 micron run-out Suitable for structures with micron precision Diamond impregnated blade widths ranging from 250um to 10 um available

9. 9 Fabrication of the device A Loadpoint Microace dicing saw was used to define 7 grooves through the silica and into the silicon in plunge cut mode An additional groove is diced with a 10 micron width blade at an angle of 8 degrees from perpendicular to the previous grooves. 1mm

10. 10 Fabrication of the device The cantilevers are undercut using a potassium hydroxide wet etch which selectively removes the silicon. A 25% KOH solution at 75C was used etched for approximately 5 hours. 1mm

11. 11 Characterization-mechanical Scanning Electron Microscope (SEM) reveals cantilevers bend upwards out of plane

12. 12 Characterization-mechanical Zescope White Light Interferometer used to further measure the deflection after etch release. 32 µm 15 µm

13. 13 Actuation Optical Fiber Cantilevers Cantilevers are actuated by cleaved optical fiber (diameter 125 micrometers) Two types of actuation are possible. Pushing or one or two cantilevers simultaneously

14. 14 Cantilevers are actuated by cleaved optical fiber (diameter 125 microns) Two types of actuation are possible. Pushing or one or two cantilevers simultaneously Actuation

15. Coupling Angular Alignment Theory 15 This is derived from the overlap integral of the mode exiting one fibre to the mode of the second fibre. Fiber optic angular misalignment (from Ghatak, Introduction to Fiber Optics)

16. Characterization When the device is actuated the Bragg peaks from the side of the device opposite to the coupling point become visible Bragg gratings at different wavelengths are placed either side of the cavity provide way of measuring coupling 1520nm 1540nm 1560nm 1580nm 1590nm 1580nm 1530nm 1550nm 1570nm

17. Characterization The optical coupling goes through a maximum which occurs when the angle between the two cantilevers is at a minimum Bragg grating reflectivites used to measure coupling across central cavity ~20 dB of attenuation for TE and TM modes The sensitivity of reflectivity to translation over the central 10µm of the reflectivity various by 0.8 dB

18. 18 Summary A new type of dual-cantilever microstructure has been demonstrated which can act as either a displacement sensor or a variable attenuator The use of Bragg gratings allows quantitative measurement of the loss and suppression ratio of the device which was found to be ~20 dB for both the TE mode and for the TM mode Next step maybe piezoelectric actuation through deposited layers or external piezoelectric device

19. Acknowledgements 19

20. Thank you for listening 20 Website http://planarphotonics.com Peter Cooper p.cooper@soton.ac.uk