Physics PhD Thesis Defense by Mustafa Eryürek

KOÇ UNIVERSITY

GRADUATE SCHOOL OF SCIENCES & ENGINEERING

PHYSICS

PhD THESIS DEFENSE BY MUSTAFA ERYÜREK

Title: Optofluidic Sensing with Optical Microresonators

Speaker: Mustafa Eryürek

Time: February 27, 2018, 13.00

Place: ENG 208

Koç University

Rumeli Feneri Yolu

Sariyer, Istanbul

Thesis Committee Members:

Prof. Alper Kiraz (Advisor, Koç University)

Prof. Ali Serpengüzel (Koç University)

Prof. İskender Yılgör (Koç University)

Prof. Zafer Ziya Öztürk (Gebze Technical University)

Asist. Prof. Kenan Çiçek (Iğdır University)

Abstract:

In this thesis, detection of various fluids using optical microresonators is described.

As optical microresonators, different geometries are used such as microdisks, microrings, microspheres, and microcylinders. The sensing mechanism relies on the change of optical properties when there is a change in the amount of a quantity of interest. Modes of the optical microresonators, also known as the whispering gallery modes (WGMs), are analyzed for the changes, and their spectral positions or quality factors are tracked and employed as sensitive indicators of the changes. Sensing of air humidity and hydrogen gas, liquid refractometry and contamination detection in liquid microspheres are demonstrated in the context of this thesis.

In humidity sensing work, air humidity is sensitively detected using SU-8 polymer microdisk resonators. These resonators are produced with a single-step fabrication and they provide a repeatable and long-term-stable sensing platform. When these SU-8 microresonators are coated with Pd, they can also operate as hydrogen gas sensors. Using this idea, hydrogen gas sensing using Pd-coated SU-8 microdisk resonators is presented in the hydrogen concentrations well below its flammable limit of 4%.

Apart from gas sensing, detection in liquid environment is also studied in two cases.

In the first case, the refractive index of the liquid environment is detected using optical fiber resonators (OFRs). These OFRs are easily fabricated from standard optical fibers and they provide refractive index sensitivity in the order of 10^^-5 RIU. Refractive index detection results as well as the analytical calculations of the expected response of OFR sensors are discussed. In the second case, contamination of the oil droplets is investigated. The experiments performed in microfluidic chips and the droplets are held in position using optical trapping.

Based on the Q factors of the droplets, the material content inside the droplets can be predicted.

Finally, hydrogen gas sensing using polymer and Pd-coated OFRs is presented. These OFRs present even higher sensitivity than the Pd-coated SU-8 microdisk resonators. OFR sensors are easier to prepare and more flexible than the SU-8 microdisk sensors. Hydrogen concentrations down to 1000 ppm are successfully detected, with a potential detection limit less than 100~ppm. For all the sensing applications, detection results and underlying sensing mechanisms are presented, and they are compared with the state-of-the-art WGM-based sensors.