Project A:  Chemical Sensing with Microfluidic Delivery of Bio/Chem Materials

We aim to achieve chemical sensing with bio/chem analytes introduced directly into the high optical field of a laser cavity. This is feasible by combining microfluidics with semiconductor lasers, allowing bio/chem analytes to interact with the laser mode. We have made the first demonstration of electrically pumped semiconductor lasers integrated with microfluidics – the lasers operate at room temperature and in single-mode. Distributed Feedback (DFB) Quantum Cascade (QC) lasers were encapsulated by soft lithography, and single-mode lasing was observed with various liquids delivered by microfluidics: isopropanol, methanol, index-matching fluids and water. Moreover, controlled microfluidic tuning of the laser emission wavelength was achieved; the emission wavelength could be continuously tuned by changing the fluids, thus modifying the refractive index environment surrounding the laser.

Figure 1. (a) SEM micrograph of DFB QC laser ridge. (b) Schematic of DFB QC laser chip encapsulated by soft lithography.  (c) Fabricated device with encapsulated laser and  tubes coming out of microfluidic chamber. (d) Emission wavelength of laser can be tuned with different refractive index fluids in microfluidic chamber.

Several novel laser geometries promise to permit even stronger sensitivity to fluids. "Holey" laser structures permit the introduction of fluids directly into the high optical field of the laser cavity. We envision holey quantum cascade lasers with photonic crystal cavities to allow the detection of single molecules. Narrow ridge QC lasers have evanescent optical mode lying outside the ridge – this can be used for strong sensitivity to fluids. By introducing fluids with strong absorption, we can turn off the laser; thus we can detect fluids and even use them to switch on and off a laser.

Project B:  Infrared Optofluidic Spectrometer-on-a-Chip

We design a multifunctional high-resolution Infrared Optofluidic Spectrometer. This combines a high-resolution infrared spectrometer based on many closed spaced DFB QC lasers with microfluidic sample delivery to create chemical analysis systems. Using an array of DFB QC lasers we can cover the "fingerprint region" (500-1500 cm-1) of the infrared, where chemical analytes have signature absorption features. With each individual DFB laser, we can measure absorption at a single wavelength. The emission wavelength of each DFB laser can be tuned a few wavenumbers by changing the temperature. Then with an array of DFB lasers we can map out the absorption spectrum over a large wavelength range.

Figure 2. (a) "Spectrometer on a Chip" schematic. We have a broadly tunable QCL source with DFB laser array integrated with CMOS electronics.  The red dotted line denotes the routing of the pulsed power and DC bias to a specific laser (third from top) and the firing of that laser. (b) Laser array chip placed on top of dime for size comparison. Gold squares on the right portion of the chip are pads for wire-bonding. (c) Laser array chip wire-bonded to the circuit board for testing.

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