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Integrated microring resonators fabricated on silicon wafers were used as signal transducers to detect alterations in physical traits of attached live mammalian cells. Cell adhesion and growth events could be monitored by the shift in resonance frequency of the microring resonator. Toxic chemical-induced changes in cell motility were rapidly detected based on variations in the fluctuation of resonance frequency. Microring resonators modified with an endothelial cell line (MS1) adhered onto its surface were used to detect the presence of two toxic chemicals, viz. sodium pentachlorophenate and Aldicarb at concentrations above the military exposure guideline levels within a duration of 1 h.

The low-grazing-angle electromagnetic scattering from spilling breaker water waves has been examined using a numerical approach. A moment-method-based electromagnetic technique was used to find the instantaneous scattering from the crests of breakers generated mechanically in a wave tank in the absence of wind. A high-speed imaging system carried on an instrument carriage traveling at the phase velocity of the breakers provided continuous measurements of the temporal evolution of the crest shape, in turn allowing a continuous calculation of the microwave backscatter from the crest. As the wave steepens and a bulge forms on the forward face of the crest, horizontal-to-vertical polarization backscattering ratios (HH/VV) as high as 0 dB are observed. Greatly reduced ratios are observed after breaking when no steep features remain on the wave. A time-dependent Fourier analysis identifies “fast” scattering whose Doppler shifts are nearly equal to or greater than the phase velocity of the breaker, and “slow” scattering whose Doppler shifts are consistent with the orbital motion of the breaker. Comparison of the Doppler shifts with the measured wave profiles shows that the fast scattering identified is associated with very steep surface features that give specular (or nearly specular) reflection points on the surface, but the magnitude and HH/VV ratio of the response to a specific steep feature depends upon its electromagnetic size, thus giving a frequency dependence. The slow scattering identified (in the absence of wind) is correlated with the “turbulent scar” that remains after breaking.

To fight against the explosives-related threats in defense and homeland security applications, a smarter sensing device that not only detects but differentiates multiple true threats from false positives caused by environmental interferents is essential. A new optical detection system is proposed to address these issues by using the temporal and spectroscopic information generated by the surface plasmon coupling emission (SPCE) effect. Innovative SPCE optics have been designed using Zemax software to project the fluorescence signal into clear "rainbow rings" on a CCD with subnanometer wavelength resolution. The spectroscopic change of the fluorescence signal and the time history of such changes due to the presence of a certain explosive analyte are unique and can be used to identify explosives. Thanks to high optical efficiency, reporter depositions as small as 160-μm in diameter can generate a sufficient signal, allowing a dense array of different reporters to be interrogated with wavelength multiplexing and detect a wide range of explosives. We have demonstrated detection and classification of explosives, such as TNT, NT, NM, RDX, PETN, and AN, with two sensing materials in a prototype.

We developed and characterized a Fabry-Perot (FP) sensor module based micro gas chromatography (microGC) detector for multipoint on-column detection. The FP sensor was fabricated by depositing a thin layer of metal and a layer of gas-sensitive polymer consecutively on the endface of an optical fiber, which formed the FP cavity. Light partially reflected from the metal layer and the polymer-air interface generated an interference spectrum, which shifted as the polymer layer absorbed the gas analyte. The FP sensor module was then assembled by inserting the FP sensor into a hole drilled in the wall of a fused-silica capillary, which can be easily connected to the conventional gas chromatography (GC) column through a universal quick seal column connector, thus enabling on-column real-time detection. We characterized the FP sensor module based microGC detector. Sensitive detection of various gas analytes was achieved with subnanogram detection limits. The rapid separation capability of the FP sensor module assembled with both single- and tandem-column systems was demonstrated, in which gas analytes having a wide range of polarities and volatilities were well-resolved. The tandem-column system obtained increased sensitivity and selectivity by employing two FP sensor modules coated with different polymers, showing great system versatility.

We demonstrate the rapid detection of explosive vapors based on a fiber-based optical Fabry-Perot (FP) gas sensor. The sensing probe of the FP sensor is composed of a thin metal layer and a vapor-sensitive polymer layer that are deposited sequentially on a cleaved fiber endface to form an FP cavity. The interference spectrum generated from the reflected light at the metal-polymer and polymer-air interfaces changes upon the absorption of gas analyte. By monitoring the interference shift, we are able to obtain quantitative and knetic information of the interaction between the analyte and the polymer layer. We further assemble the FP sensor with a short fused silica capillary into a sensor module, and employ it in a gas chromotgraphy (GC) system for selevtive rapid on-column detection. In this report, we specifically target 2, 4- dinitrotoluene (DNT) and 2, 4, 6-trinitrotoluene (TNT) for their obvious defense applications. This work could lead to a portable sensor capable of detecting low concentrations of DNT, TNT, and other explosive chemicals.

A versatile, compact, and sensitive fiber-based optical Fabry-Perot (FP) gas sensor is reported in this paper. The sensor probe is composed of a silver layer and a vapor-sensitive polymer layer that are deposited on the cleaved fiber endface to form an FP cavity sequentially. The interference spectrum generated from the reflected light at the silver-polymer and polymer-air interfaces changes upon the absorption of gas analytes. This structure enables using polymer of any refractive index (RI) as the sensing layer, which significantly enhances the sensor versatility. Two polymers of polyethylene glycol (PEG) 400 (RI=1.465-1.469) and Norland Optical Adhesive (NOA) 81 (RI=1.53-1.56) are used as the gas sensing polymer to demonstrate the feasibility of the FP sensor, and show drastically different sensor response to various gas analytes. In addition, we assemble the FP sensor with a short fused silica capillary into a sensor module, and employ it in the gas chromotgraphy (GC) system to investigate its capability as a GC detector for rapid on-column detection.

We demonstrated a novel tandem-column micro-gas chromatography (microGC) based on optofluidic ring resonator (OFRR). The OFRR is a thin-walled fused silica capillary whose interior surface is coated with a polymeric stationary phase. The circular cross section of the OFRR forms the micro-ring resonator and supports whispering gallery modes (WGMs). Via tapered optical fibers in contact with the OFRR, the WGM can be excited externally at any positions along the OFRR capillary, thus enabling multi-point, on-column, real-time detection of vapor molecules flowing through the OFRR. In the present OFRR-based tandem-column-based microGC implementation, a 180 cm long conventional GC column coated with a nonpolar stationary phase was followed by a relatively short OFRR column coated with a polar phase. Two detection positions, one at the inlet of the OFRR and the other a few centimeters downstream, were used to monitor the separation achieved by the first and the second column, respectively. Owing to the multi-point on-column detection that provides complementary retention time information on each chemical compound, co-eluted analytes can be well separated and identified on at least one detection channel and no modulation is needed at the interface of tandem columns. Separation and detection of twelve analytes with various volatilities and polarities within four minutes were demonstrated. In addition, the chromatograms obtained from three different locations along the OFRR column demonstrated the system's capability of on-column monitoring of the separation process for the target analyte in a vapor mixture. Our results will lead to the development of a rapid, simple, and portable microGC system with significantly improved selectivity and chemical identification capabilities.

We demonstrated rapid 2,4-dinitrotoluene (DNT) vapor detection at room temperature based on an optofluidic ring resonator (OFRR) sensor. With the unique on-column separation and detection features of OFRR vapor sensors, DNT can be identified from other interferences coexisting in the analyte sample mixture, which is especially useful in the detection of explosives from practical complicated vapor samples usually containing more volatile analytes. The DNT detection limit is approximately 200 pg, which corresponds to a solid phase microextraction (SPME) sampling time of only 1 second at room temperature from equilibrium headspace. A theoretical analysis was also performed to account for the experimental results. Our study shows that the OFRR vapor sensor is a promising platform for the development of a rapid, low-cost, and portable analytical device for explosive detection and monitoring.

We develop a novel chemical vapor sensing platform based on optofluidic ring resonator (OFRR) for rapid and on-column detection and analysis of a wide range of chemical vapors. The OFRR is a thin-wa lled fused-silica capillary with a diameter of ~100 P m and a few centimeters in length. The circular cr oss-section of the OFRR defines a ring resonator that supports high-Q (>10 6 ) whispering gallery modes or circulating waveguide modes (WGMs). Polymer thin film is coated on the OFRR capillary interior surface as a vapor se nsitive material. The unique structure of the OFRR achieves dual-use of the capillary as the gas delivery channel and as the sensing transducer, avoiding the necessity of building extra gas detection chambers commonly seen in chemical vapor sensors. When vapor molecules pass through the OFRR, the interaction between vapor molecules and the polymer causes polymer refractive index and thickness to change, which leads to a WGM spectral shift. Therefore, by monito ring the WGMs spectrum in time, the quantitative and kinetic information regarding vapor molecule-polymer interaction is acquired. The rapid detection of methanol and hexane vapors representing polar and nonpolar analytes respectively are demonstrated with OFRR vapor sensors. Owing to the unique multipoint on-column detection capability, the OFRR vapo r sensor is studied for the development of the micro-GC gas analyzer. Efficient separation and rapid detection ar e achieved by a few centimeters long OFRR capillary coated with a stationary phase polymer. We further explore the capability of OFRR micro-GC for more challenging explosive detection. The OFRR vapor sensing platform is a promising candidate for the development of rapid, sensitive, simple, portable, and cost-effective micro-gas sensors. Keywords: optofluidic ring resonator; chemical vapor sensor; micro gas chromatography.

We developed a novel on-column micro gas chromatography (microGC) detector using capillary based optical ring resonators (CBORRs). The CBORR is a thin-walled fused silica capillary with an inner diameter ranging from a few tens to a few hundreds of micrometers. The interior surface of the CBORR is coated with a layer of stationary phase for gas separation. The circular cross section of the CBORR forms a ring resonator and supports whispering gallery modes (WGMs) that circulate along the ring resonator circumference hundreds of times. The evanescent field extends into the core and is sensitive to the refractive index change induced by the interaction between the gas sample and the stationary phase. The WGM can be excited and monitored at any location along the CBORR by placing a tapered optical fiber against the CBORR, thus enabling on-column real-time detection. Rapid separation of both polar and nonpolar samples was demonstrated with subsecond detection speed. Theoretical work was also established to explain the CBORR detection mechanism. While low-nanogram detection limits are observed in these preliminary tests, many methods for improvements are under investigation. The CBORR is directly compatible with traditional capillary GC columns without any dead volumes. Therefore, the CBORR-based muGC is a very promising technology platform for rapid, sensitive, and portable analytical devices.

We develop rapid chemical vapor sensors and micro gas chromatography (μGC) analyzers based on the optofluidic ring resonator (OFRR). An OFRR is a micro-sized thin-walled glass capillary; the circular cross-section of the capillary acts as an optical ring resonator while the whispering gallery modes or circulating waveguide modes (WGMs) supported by the ring resonator interact with the vapor samples passing through the capillary. The OFRR interior surface is coated with a vapor-sensitive polymer. The analyte and polymer interaction causes the polymer refractive index (RI) and the thickness to change, which is detected as a WGM spectral shift. Owing to the excellent fluidics, the OFRR vapor sensor exhibits sub-second detection and recovery time with a flow rate of 1 mL/min. On-column separation and detection in the OFRR based μGC system is also demonstrated, showing efficient separation of vapor mixtures and presenting highly reproducible retention time for the individual analyte. Compared to the conventional GC system, the OFRR μGC has the advantage of small size, rapid response, and high selectivity over a short length of column.

Whispering-gallery mode (WGM) resonators such as microspheres, microcylinders, and microrings have been proposed for telecommunication and sensing applications for decades. However, several challenges, such as the robustness of the optical coupling and sample delivery means, were often found in the path of developing them for the real world sensing applications. In this paper, a robust microring platform based on integrated lightwave circuit technology and a tunable laser interrogation system has been demonstrated as high-resolution refractive index and pressure measurement system. By using a "two-point" interrogation scheme and a microring with Q factor of about 24,000, a refractive index sensitivity of about 145 nm/RIU (TM mode) and 205 nm/RIU (TE mode) for two different polarizations and a limit of detection on the order of 10-7 RIU have been demonstrated. For the high-pressure measurement applications, a pressure resolution of 0.05 psia has been achieved.

[1] A two-scale scattering analysis is applied to two series of profiles that represent the measured time evolution of the crests of spilling breaker waves that were mechanically generated and measured in a wave tank. The backscattering at microwave frequencies and low-grazing-angle (LGA) illumination were considered. A deterministic implementation of the perturbation theory that is the foundation of any two-scale scattering model (TSM) is used that allows an instantaneous comparison of TSM cross sections with reference “exact” cross sections found using a numerical moment-method-based electromagnetic approach. TSM proved unable to model the scattering from steep features that appear on the crest both before and after the onset of breaking, even in cases where the horizontal-to-vertical polarization ratios are as low as −10 dB. This shows that the “fast” scattering (with Doppler speeds greater than or equal to the velocity of the breaker) associated with these features is not due to Bragg scattering in the traditional sense in these cases despite the low polarization ratios. TSM is more accurate after the steep features subside and the turbulent “scar” roughness is primarily on the front face of the breaker. However, accuracy is lost as the wave propagates and the scar roughness moves to the crest top and back face of the wave. It appears that a scattering coefficient based on the spectral content of the roughness at the Bragg-resonant condition can provide only a rough estimate of the actual LGA scattering coefficient even after the major steep features have disappeared.