4. Experimental Results of Hydrogen Sensing Experiments Optical Sensor for Hydrogen Based on a Palladium-Coated Polymer Microresonator Mustafa Eryürek1, Yasin Karadağ2, Nevin Taşaltın3, Necmettin Kılınç4 and Alper Kiraz1,5 1. Department of Physics, Koç University, Rumelifeneri Yolu, 34450 Sarıyer İstanbul, Turkey; 2. Department of Physics, Marmara University, 34722, Göztepe İstanbul, Turkey; 3. TUBİTAK MRC Materials Institute, Sensor Materials-Photonic Technologies Lab. 41470, Gebze Kocaeli, Turkey; 4. Department of Mechatronics Engineering, Niğde University, 51245 Niğde, Turkey; 5. Koç University TÜPRA޸ Energy Center (KÜTEM), Koç University, Rumelifeneri Yolu Sarıyer, 34450 İstanbul, Turkey 1. Motivation 4. Experimental Results of Hydrogen Sensing Experiments Developing hydrogen gas sensor based on palladium-coated polymer microresonator for Reversible operation at room temperature. Detection of hydrogen gas at low concentrations (down to 0.3%). 2. Sample Preparation µ SU-8 polymer microresonators are fabricated using UV photolithography. Parameters: 4ml SU-8 2002 is placed on 4-inch thick-oxide Si wafer. Spin-coating at 3000 rpm for 60s. The thickness of the SU-8 layer is 1200 nm. Soft-baking at 95oC for 2 minutes. UV Exposure for 20-60 s. Post-exposure baking at 95oC for 1 minute. Development for 1 minute. Then, Pd is coated on the microresonators concentrically using lift-off technique. The thickness of the Pd layer is 220 nm. Radius of the Pd is kept smaller than the SU-8 to avoid absorption loss in the whispering gallery modes due to the Pd metal. 3. Experimental Setup Resonance shifts are measured using Pd-coated and uncoated devices. The presence of Pd layer improves the sensitivity by 20-fold. The lower detection limit is 0.3% which is well below the flammable limit (4%) [2]. Response and recovery times are 167, 81, 50s and 55, 73, 163s for 0.3%, 0.5% and 1%. Increase in the response time is attributed to the faster adsorption of hydrogen into the Pd as the hydrogen concentration increases. Decrease in the recovery time is attributed to the delayed desorption of hydrogen as the hydrogen concentration increases. There is a linear relation between the hydrogen concentration and the resonance shift upto 1% hydrogen. Then upto 1.5%, saturation is observed. The sensor degredates irreversibly after 1.5% because of the phase transition in Pd [1]. 5. Conclusion An optical hydrogen gas sensor is presented based on the elastic nature of the polymer microdisk resonator. Sensor operates at room temperature for hydrogen concentrations well below the flammable limit. Device shows linear response between 0.3% to 1% hydrogen concentrations. Sensor has a dynamic range from 0.3% to 1.5%. The dynamic range can be increased using Pd alloys instead of Pd [1]. The lower detection limit of the device (0.3%) can be decreased using more sophisticated fabrication techniques and fabricating higher-Q optical microresonators. References: [1] J.-S. Noh, J. M. Lee, and W. Lee, "Low-dimensional palladium nanostructures for fast and reliable hydrogen gas detection," Sensors 11, 825-851 (2011). [2] İ. Dinçer, "Technical, environmental and exergetic aspects of hydrogen energy systems" International Journal of Hydrogen Energy 27, 265-285 (2002). A tunable laser is coupled to the SU-8 waveguide using butt-coupling method. The transmission spectrum is measured under nitrogen environment for reference. Hydrogen gas is introduced to the environment and tranmission is measured. Hydrogen interacts with Pd which results volumetric expansion in Pd [1]. This expansion is transferred to the SU-8 microresonator. Whispering gallery resonances shift because of radius-dependent resonance condition. The resonance shifts are calculated with respect to the hydrogen concentration. Acknowledgements: This work is supported by TÜBİTAK (Grant No. 111T803) Koç University Nano-Optics Research Laboratory, Rumeli Feneri Yolu, Sarıyer, İstanbul 34450 Turkey, Email: meryurek@ku.edu.tr