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1, 2 , 2 , 2 , * 1
1  School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798
2  Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, #08-02 Innovis Tower, Singapore 138634


Refractive index sensing method based on silicon ring resonator is used to build the sensing platform, for its potential to be CMOS compatible and can be easily integrated with other sensors and control elements [1]. An integrated volatile organic compounds (VOC) sensor is presented in this paper. Current device is tested with acetone vapor, and the detection limit reached 200 ppm, which is lower than the human safety level [2]. By making a ring resonator array, the proposed sensor has a potential to detect and distinguish different gas species. 

Figure 1(a) shows the sensing platform with silicon ring resonator as the key component. A straight waveguide is used to couple laser into the ring structure. The width and height of the waveguide and the ring is 450 and 220 nm, respectively. The ring structure is coated by a thin absorption layer and covered in a microfluidic channel, acting as the main sensing unit. Figure 1(b) shows the SEM image of one of the sensing ring.

After light passing through the resonator, resonance peaks can be seen on the output spectrum. As shown in Figure 2(a), the linewidth of each peak is less than 30 pm, which offers a Q-factor of more than 5×104. This resonance peak, however, is very sensitive to the refractive index and thickness of the absorption layer. Gas absorb inside the layer will affect the resonance peak position. Figure 2(b) shows the mode distribution in the waveguide after the absorption layer coating. The strong light field in the absorption layer will make the device sensitive to refractive index and thickness change.

To demonstrate this sensing performance, a 20-nm polydimethylsiloxane is spin coated on top of the device to absorb VOC vapor. Acetone is chosen as the target gas. Gas is injected to the chip surface via a microfluidic channel. Figure 3(a) shows the monitored peak position as a function of time. The periods with white background represents the condition when pure N2 gas is injected to the channel. On the other hand, the periods with gray background represents the condition when different concentrations of acetone vapor are injected in the channel. Stable performance and fast response is observed. The response time constant is about 20 s. Figure 3(b) indicates a linear relationship between acetone concentration and resonance peak position. Based on the experimental results, the sensitivity is 1.7 pm/1000 ppm and the detection limit is 200 ppm.

In conclusion, a gas sensor for VOC based on silicon photonics ring resonator system is presented in this paper. Sensitivity of 1.7 pm/1000 ppm and detection limit of 200 ppm are achieved, which is lower than the human safety level. This makes it a potential application for indoor air quality detection and monitoring.

Keywords: VOC, Sensor, Ring resonator