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THEORETICAL AND EXPERIMENTAL INVESTIGATION OF SYNCHRONOUS SILICON NANOWIRE WAVEGUIDE DISPLACEMENT SENSORS

Final Report Summary - NANO-DISP (Theoretical and experimental investigation of synchronous silicon nanowire waveguide displacement sensors)

The project is composed of four phases in total. The first three are towards in-plane, whereas the last one is towards out-of-plane displacement/distance sensing. In the first two phases, phase one and phase two, normally-off and in the third phase normally-on sensors are investigated. While phase one focuses on large-range low-sensitivity sensors, phase two focuses on short-range high-sensitivity ones. This periodic report mentions numerous sensor designs obtained within the concept of the project proposal named 'NANO-DISP: Theoretical and experimental investigation of synchronous silicon nanowire waveguide displacement sensors' towards large-range low-sensitivity and short-range high-sensitivity measurement characteristics as promised in phase one and phase two, respectively, in the first two years. Three dimensional (3D) finite-difference time-domain (FDTD) analysis is utilised in the efforts towards numerical characterisation of the sensors. Phase one involved a single type of sensing mechanism, whereas in phase two, two types of mechanisms are considered. Sensors in phase one are based on waveguide tips with elliptical geometry. Results demonstrated that the angle and length of tip geometry affect optical characteristics of the sensors under consideration. Among all suitable sensors, the highest sensitivity per percent of light intensity is calculated to be 5.74 nm at 80º tip angle for 4.8 µm tip length and 1.5 µm tip width and the lowest sensitivity is calculated to be 9.62 nm at 85º tip angle for 12.0 µm tip length and 1.5 µm tip width. The smallest and largest measurement ranges achievable are foreseen to be 674 nm and 1062 nm, respectively.

First type of the sensors studied in phase two is based on waveguide tips with elliptical geometry similar to those in phase one. The novelty for this type, however, is the use of tip angles below the Brewster's. Among all appropriate sensors in this type are those with 4.8 µm tip length and 1.5 µm tip width. The highest sensitivity is 1.10 nm per percent of light intensity at 10º tip angle and the lowest sensitivity is 5.74 nm per percent of light intensity at 50º tip angle. The smallest and largest measurement ranges achievable based on calculations are predicted to be 135 nm and 674 nm, respectively. Second type of the sensors investigated in phase two is composed of two identical waveguides with tapered tips with abrupt ends. Studies proved that tip width, tip-end size, tip angle and thickness affect optical characteristics. Numerical studies on type two sensors are still on the way for optimal solution, however, so far the highest sensitivity per percent of light intensity is calculated to be 0.97 nm at 9.5º tip angle for 100 nm tip-end size, 300 nm thickness and 600 nm tip width and the lowest sensitivity is calculated to be 3.40 nm at 10.5º tip angle for 200 nm tip-end size, 170 nm thickness and 600 nm tip width. The smallest and largest measurement ranges achievable are calculated to be 97 nm and 330 nm, respectively.

In phase three, short-range high-sensitivity in-plane displacement sensors again, but at normally-on state are being investigated in order to clarify their initial state effects on the sensing characteristics. Towards this goal, three major types of sensor approaches are studied. All sensors utilised in phase three are based on electromagnetic field modulation (EFM). Sensitivities achieved in this phase are from 0.84 nm down to 0.06 nm per percent of light intensity within sub-100 nm distances.

Among major milestones succeeded in the project so far, procurement of two and three dimensional design and file conversion software for use in sensor design process and FDTD analysis software and the workstations for numerical studies can be counted. In addition, micro/nano research centres suitable for successful fabrication of the promised sensors are contacted and agreed. Fabrication of the sensor devices is being carried out at the agreed cleanroom of Bilkent University, Ankara, Turkey's National Nanotechnology Research Centre (UNAM, see http://www.nano.org.tr/). Graduate student researchers are hired, educated and trained and micro/nano-fabrication recipes are clarified after significant number of iterations. Several interesting sensors characterised numerically as mentioned above are fabricated in order to obtain their mechanical and optical characteristics experimentally for verification. Sensor cores where the actual measurement takes place are only 50µm × 80µm in size. The project workpackages are accomplished mostly in the so-called Nanophotonic Systems Research Laboratory (NANOPSYS) at Ozyegin University, Istanbul, Turkey and the very small portion of the rest very recently at Istanbul Technical University (ITU), Istanbul, Turkey. NANOPSYS currently involves three graduate and two undergraduate students as well as the principal investigator (PI). Design and fabrication of home-made vapour HF etching setup for device release is realised with the help of the undergraduate students. Finally, fund search for procurement of both mechanical and optical sensor characterisation setups and their installation are completed. When compared to the project's workplan promised in the proposal, owing to the duration it took to grant the support for the procurement of characterisation setups from a subsequent funding agency, the Scientific and Technological Research Council of Turkey (TUBITAK, see http://www.tubitak.gov.tr/en/ot/10/) in addition to European Community's (EC) Seventh Framework Programme (FP7) Marie Curie International Reintegration Grant (IRG) Programme's Support, the project is delayed approximately by four to six months from the experimental point of view. However, the grant needed for the characterisation setups are ensured properly and as a result, currently, both mechanical and optical characterisation of the sensors promised at phase one are very close to the end, predicted to be completed in about a month. Because of the delay in granting subsequent funding, it hasn't been possible for the research group to fulfill and include the experimental characterisation results into this Periodic Report at hand unfortunately. Nevertheless, in order to make the delay up and to ensure completion of the promised work packages (WPs) fully in the rest of the project's total duration of four years, the research group had already gone ahead to the second phase, phase two and third phase, phase three and completed more than that planned in terms of numerical analysis and design. As a result, we anticipate that all WPs will be fulfilled prior to the end of the project term successfully. Despite difficulties encountered, PI has succeeded to reach the critical mass within the European Union (EU) in financial, infrastructural and researcher aspects in order to ensure microfabrication and characterisation of high-sensitivity embedded optical displacement/distance sensors.