# Presentation: 358198

## High Q Micro cavity Based Smart Bio Chemical Sensors

**Sudhaprasanna Padigi**** 1****, Nirupama Bulusu ****2**** , Shalini Prasad ****1**
**1**** Department of ****Electrical and Computer Engineering, ****2**** Department of Computer Science**
**Portland State University, OR**

**ABSTRACT**
- With the focus on bio terrorism and environmental sensing, there is need for development of  smart integrated bio-chemical sensors. We investigate Whispering Gallery Mode based High Q micro cavity as a bio-chemical sensor. We integrate the lab-on-a-chip approach with High Q technology to develop highly selective and smart sensors for in-situ detection of air-borne  bio-chemical agents. We have investigated the ring based micro-cavity structure as a sensor. We identify the detection of agents through the changes observed in the frequency response as a result of enzyme-substrate interactions.

- With the focus on fabrication of low cost sensors, we have adopted the silica-on-silicon material system for the fabrication of micro cavity sensors. The silica is deposited using TEOS (Tetra-ethoxysilane) as a precursor. The oxide is deposited using the Plasma enhanced chemical vapor deposition (PECVD) technique. Silica is transparent at infra-red wavelength and is more suitable for sensing applications as many chemical agents have a specific detection signatures at the infra-red wavelengths.

**MATERIALS AND METHODS**

**Figure 2**. The optical micro cavities are integrated with dielectric waveguides on a chip. The input waveguide is butt-coupled to a laser diode for excitation of the optical micro cavity through evanescent coupling. The evanescently coupled light excites the Whispering Gallery Modes of the optical micro cavity leading to very high field enhancements inside the micro cavity. On resonance, the light from the micro resonator is coupled out evanescently into an output waveguide. The output light is detected by a photo detector butt-coupled to the output wave guide. The performance of the micro cavity is characterized by the Quality (Q) factor and the Finesse (F) value..

**ACKNOWLEDGEMENTS**

**DISCUSSION AND FUTURE WORK**

- We have theoretically demonstrated the advantages of integration of microfabrication techniques with high Q technology at the device level and it’s applications at the system level to develop specific applications for bio-chemical sensing in the Sensor networks domain. We would like to further explore different geometries of the micro cavities at the device level and the possibility of integrating them with the current system.

- We thank Kolar Sunder for her help in preparing the poster. We also would like to thank Portland State University for Summer faculty enhancement grant and ONAMI. This work is also partly supported by the NSF Grant# 0423728 and the Provost's PSU Foundation Faculty Development Award.

**REFERENCES**

- [1 ] T.J. Kippenberg, S.M.Spillane, D.K. Armani and K.J. Vahala,
-       Applied Physics Letters Volume 83, Number 4, July 2003.

**Figure 4.** Snapshots of the simulation of micro-ring cavity with dimensions on the order of 10μm at different time steps. The simulations were performed in CFD-RC® simulation tool from ESI.

**Figure 5.** The Optical micro cavity based bio-chemical sensor is integrated with *BIOSENS, *a bio chemical sensor board being developed at Department of  Computer Science, Portland State University. The* BIOSENS * will be integrated onto an existing wireless transmission platform like Mica Z ® from Xbow Inc.

- The ring micro cavity resonator was simulated using the CFD-RC simulation tool. The CFD-RC is based on the Effective Index Method (EIM). The EIM is approximated from a 3-Dimensional multi-physics problem to a 2-Dimensional multi-physics problem. This approximation though is in a 2-D form, it attempts to account for some of the 3-D physical effects like waveguide dispersion, edge diffraction and bending loss. The underlying equation for EIM is  based on a 2-D scalar wave equation:

**Figure 6. **The *BIOSENS *sensor boards are integrated with the pre-existing wireless transmission platforms like Mica Z ® from XBOW and are meshed together to form a wireless mesh network, enabling distributed and heterogeneous sensor networks to perform distributed and heterogeneous sensing in a given area.

**Figure 1****.** The optical micro cavities are fabricated on the silica-on-silicon material system using a combination of wet etch and dry etch techniques. The substrate is spin coated with photo resist and patterned using standard contact lithography. The patterned substrates are etched using a 5:1BHF(Buffered Hydrofluoric Acid) solution to create openings for further etching of silicon. This is then followed by etching of silicon using a combination of KOH (Potassium hydroxide) etching and RIE (Reactive Ion etching) etching. This followed by the final step of photo resist striping using Acetone.

**RESULTS**

**Figure 3.  **The surface of the micro cavity is chemically modified to enable enzyme-substrate interactions. The bio-chemical agents are immobilized on the surface and this leads to the change in the Q-factor and the Finesse value leading the detection of the bio-chemical agents. The agents can be characterized by the change in the frequency spectrum of the micro cavity.

**PROCESS FLOW FOR THE FABRICATION OF MICROCAVITIES**

**SILICA ON SILICON**

**APPLYING PHOTO RESIST**

**PATTERNING USING CONTACT **
**              ****LITHOGRAPHY **

**5:1 BHF ETCHING OF SILICA**

**KOH/RIE ETCHING OF SILICON**

**PRINCIPLE OF OPERATION OF MICRO CAVITY**

**PRINCIPLE OF OPERATION**
** ****OF MICRO CAVITY AS A **
**BIO-CHEMICAL SENSOR.**

**SIMULATION RESULTS**

**DEVELOPMENT OF SYSTEM-ON-A-CHIP**

**CHALLENGES**

**DEVICE LEVEL**
- Integration of the dielectric wave guides with the optical micro   cavities.
- Coupling of laser diode with the input waveguide
- Coupling of photo detector with the output waveguide
- Agent specific surface functionalization
- Minimization of losses due to surface scattering and bending loss.

**SYSTEM LEVEL**
- Interfacing of the micro photonic sensor with the micro electronic signal processing capability.
- Interfacing of the* BIOSENS  *bio-chemical sensor board with the pre-existing Mica z motes for distributed and heterogeneous sensing applications.

**CONCLUSION**

**DISTRIBUTED SENSOR NETWORKING APPLICATION **