Silicon Platform for Mid-infrared Optofluidic Sensors
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Silicon Platform for Mid-infrared Optofluidic Sensors Pao Lin1, Hao-Yu Greg Lin2, Vivek Singh1, Neil Sunil Patel1, Lionel Kimerling1, Anuradha Murthy Agarwal1 1 2
Microphotonics Center, MIT, Cambridge, Massachusetts, USA. Harvard University, Cambridge, Massachusetts, USA.
ABSTRACT Mid-Infrared optofluidics based silicon sensor platforms are demonstrated. Silicon is a great candidate for mid-infrared optofluidics for the following reasons: (1) Silicon has a broad transmission window up to 7 um (2) Silicon offers CMOS compatible and monolithic fabrication (3) Silicon has high chemical resistance that can withstand high temperature, acid/base solution and organic solvents. (4) Silicon is a non-toxic environmentally friendly material. The fabricated mid-infrared optofluidic sensor can replace bulky instruments, such as FTIR, with a lab-on-achip system, while achieving much higher sensitivity. INTRODUCTION Chemical sensors using integrated photonics have attracted significant attention because of their potential for large area environmental monitoring and high throughput screening for biomedical discovery.[1] Advanced technologies using absorbance, Surface Plasmon Resonance (SPR), and fluorescence detection have been developed to realize chip-scale optical sensors. [2-6] For instance, chemical sensors using micro-ring resonators have been fabricated, and ppm level detectivity was demonstrated.[7] However, since these sensors are based mainly on the measurement of refractive index changes, specific identification of chemical compounds is difficult.[8] Similarly, Surface Plasmon Resonance (SPR) sensors require labeling techniques to enable specific analyte detection. Our robust air-clad pedestal silicon sensors offer a trace chemical analyte detection scheme using characteristic Mid-IR absorption spectra to simultaneously perform quantitative (target concentration) and qualitative (compound recognition) analysis.[9] Specifically, Mid-IR spectra can “fingerprint” molecular structures within functional groups present in the chemical analytes, enabling label free detection. Undoubtedly, lab-on-a-chip broadband Mid-IR sensors are highly desirable in many applications such as remote real-time sensing of trace toxins and detection of contaminants. EXPERIMENTAL SET-UP TO TEST MID-INFRARED OPTOFLUIDIC SENSORS Figure 1 (a) illustrates the experimental set-up used to evaluate the performance of our chemical sensors. The light source is a pulsed laser with a wavelength tunable from λ=2.4 μm to λ=3.8 μm, a pulse repetition rate of 150 kHz, a pulse duration of 10 nano seconds, an average power of 150 mW and a line width of 3 cm-1. Using a reflective lens the probe light is collimated into a 9 µm core and 125 µm cladding single mode fluoride fiber. Light emitted from the fiber is then butt –coupled into the waveguide of the on-chip sensor as shown in Figure 1 (b), where the core of the Mid-IR fiber is lined up with the smooth cleaved front facet of the waveguide. The alignment between the optical fiber and the waveguide is performed u
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