The ideal portable microfluidic chip-based fluorescence detection system would include an integrated optical microsensor capable of detecting and flexibly discriminating among a wide range of simultaneous fluorescence emission signals. We present a compact optical microspectrometer based on a standing-wave architecture (1), and we will discuss results of spectral discrimination of optical test sources in the visible (488 nm - 665 nm) and near infrared (633 nm - 866 nm). Spectral resolution of 4 nm (λ = 633 nm) was recently achieved with an integrated near infrared prototype (Figs. 1a-1b). With a surface-normal , linear optical design, the 17 × 13 × 1 mm device is well-suited to integration with a microfluidic chip. Whereas most microspectrometers (commonly grating-based devices with a detector array) have poor spectral multiplexing characteristics, our standing-wave microspectrometer has the multiplexing advantage and simple single detector readout of a Fourier transform (FT) spectrometer. Microspectrometers also usually have fixed spectral resolution, however our device can be easily configured for a particular sensing task by tuning its spectral resolution, allowing real-time optimization of sensitivity. With a visible device, continuous resolution tuning from 72 nm to 6 nm (λ = 633 nm) was demonstrated (Fig. 2). This device is based on the standing-wave transform spectrometer architecture with a moving mirror and a partially transmitting photodetector. Incident light reflecting off the mirror creates an optical standing wave. While the mirror scans along the optical beam axis, the detector samples the moving standing wave; the FT of the resulting time-varying photocurrent yields the optical spectrum. This architecture offers the same advantages as other FT spectrometers, but in an optically 1-D system; this eliminates the need for a beamsplitter and reference mirror. Hybrid integration allows implementation of any photodetector material, and thus detection in any wavelength range. Our near infrared device contains a GaAs/AlGaAs photodiode (2), and our visible device contains an a- SiC:H/a-Si:H photodiode (3). In both cases the mirror component is a parallel-plate electrostatically driven Si MEMS actuator. High-amplitude harmonic oscillation, up to 52 μm at 800 Hz, enables fast, continuous spectral analysis. With an actuator capacitance of only 4.2 pF, it should draw < 0.1% of the power drawn by a typical piezoelectric transducer. We will introduce two unique physical properties of this device that can be used to suppress scattered pump light in fluorescence experiments. The minimum mirror-detector distance can be set by adjusting the actuator drive amplitude; this reduces the sensitivity to sources with low spectral coherence, which allows for preferential suppression of any broadband light source such as a broadband lamp pump. Also, the photodetector thickness can be designed so that there is a minimum of generated AC current for one wavelength, and a relatively strong AC signal for nearby wavelengths; this enables suppression of laser pump light with preferential detection of fluorescence emission. In conclusion, we have investigated and will discuss a number of intriguing capabilities of the standing- wave microspectrometer, with particular attention to those that could make this device useful in microsystems dependent on analysis of fluorescence emission.
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