Dynamic indoor free-space optical communication enabled by beam steering and beam shaping.

Indoor free-space optical communication (FSO) provides orders of magnitude larger usable bandwidth compared to radio-frequency links but suffers from an intrinsic trade-off between areal coverage and received power. In this paper, we report a dynamic indoor FSO system enabled by a line-of-sight optical link featuring advanced beam control capabilities. The optical link herein utilizes a passive target acquisition scheme by combining a beam steering and beam shaping transmitter with a receiver adorned with a ring-shaped retroreflector. When controlled by an efficient beam scanning algorithm, the transmitter is capable of locating the receiver with millimeter-scale accuracy over a distance of 3 m with a full viewing angle of ±11.25∘ in the vertical direction and ±18.75∘ in the horizontal direction within 1.162±0.005s, regardless of the receiver's positions. We also demonstrate 1 Gbit/s data rate with bit error rates below 4×10-7 using an 850 nm laser diode with only 2 mW of output power.

Ultra-compact hybrid silicon:chalcogenide waveguide temperature sensor.

We demonstrate a real-time, reusable, and reversible integrated optical sensor for temperature monitoring within harsh environments. The sensor architecture combines the phase change property of chalcogenide glasses (ChG) with the high-density integration advantages of high index silicon waveguides. To demonstrate sensor feasibility, ChG composition Ge40S60, which is characterized by a sharp phase transition from amorphous to crystalline phase around 415 °C, is deposited over a 50 µm section of a single mode optical waveguide. The phase transition changes the behavior of Ge40S60 from a low loss to high loss material, thus significantly affecting the hybrid waveguide loss around the phase transition temperature. A transmission power drop of over 40dB in the crystalline phase compared to the amorphous phase is experimentally measured. Moreover, we recover the amorphous phase through the application of an electrical pulse, thus showing the reversible nature of our compact temperature sensor. Through integrating multiple compositions of ChG with well-defined phases transition temperatures over a silicon waveguide array, it is possible to determine, in real-time, the temperature evolution within a harsh environment, such as within a nuclear reactor cladding.

Multiplex Sensing of Complex Mixtures by Machine Vision Analysis of TLC-SERS Images.

Thin layer chromatography in tandem with surface-enhanced Raman scattering (TLC-SERS) has demonstrated tremendous potentials as a new analytical chemistry tool to detect a wide range of substances from real-world samples. However, it still faces significant challenges of multiplex sensing from complex mixtures due to the imperfect separation by TLC and the resulting interference of SERS detection. In this article, we propose a multiplex sensing method of complex mixtures by machine vision analysis of the scanning image of the TLC-SERS results. Briefly, various pure substances in solution and the complex mixture solution are separated by TLC followed by one-dimensional SERS scanning of the entire TLC plate, which generates TLC-SERS images of all target substances along the chromatography path. After that, a machine vision method is employed to extract the template images from the TLC-SERS images of pure substance solutions. Finally, we apply a feature point matching strategy based on the Winner-take-all principle, which matches the template image of each pure substance with the mixture image to confirm the existence and derive the position of each target substance in the TLC plate, respectively. Our experimental results based on the mixture solution of five different substances show that the proposed machine vision analysis is highly selective, sensitive and does not require artificial analysis of the SERS spectra. Therefore, we envision that the proposed machine vision analysis of the TLC-SERS imaging is an objective, accurate, and efficient method for multiplex sensing of trace level of target substances from complex mixtures.

Quantitative Sensing of Domoic Acid from Shellfish Using Biological Photonic Crystal Enhanced SERS Substrates.

Frequent monitoring of sea food, especially shellfish samples, for the presence of biotoxins serves not only as a valuable strategy to mitigate adulteration associated health risks, but could also be used to develop predictive models to understand algal explosion and toxin trends. Periodic toxin assessment is often restricted due to poor sensitivity, multifarious cleaning/extraction protocols and high operational costs of conventional detection methods. Through this work, a simplistic approach to quantitatively assess the presence of a representative marine neurotoxin, Domoic acid (DA), from spiked water and crab meat samples is presented. DA sensing was performed based on surface-enhanced Raman scattering (SERS) using silver nanoparticle enriched diatomaceous earth­a biological photonic crystal material in nature. Distinctive optical features of the quasi-ordered pore patterns in diatom skeleton with sporadic yet uniform functionalization of silver nanoparticles act as excellent SERS substrates with improved DA signals. Different concentrations of DA were tested on the substrates with the lowest detectable concentration being 1 ppm that falls well below the regulatory DA levels in seafood (>20 ppm). All the measurements were rapid and were performed within a measurement time of 1 min. Utilizing the measurement results, a standard calibration curve between SERS signal intensity and DA concentration was developed. The calibration curve was later utilized to predict the DA concentration from spiked Dungeness crab meat samples. SERS based quantitative assessment was further complemented with principal component analysis and partial least square regression studies. The tested methodology aims to bring forth a sensitive yet simple, economical and an extraction free routine to assess biotoxin presence in sea food samples onsite.

Quaternion-based Parallel Feature Extraction: Extending the Horizon of Quantitative Analysis using TLC-SERS Sensing.

Quantitative analysis using thin-layer chromatography coupled in tandem with surface-enhanced Raman scattering (TLC-SERS) still remains a grand challenge due to many uncontrollable variations during the TLC developing process and the random nature of the SERS substrates. Traditional chemometric methods solve this problem by sampling multiple SERS spectra in the sensing spot and then conducting statistical analysis of the SERS signals to mitigate the variation of quantitative analysis, while still ignoring the spatial distribution of the target species and the correlation among the multiple sampling points. In this paper, we proposed for the first time a parallel feature extraction and fusion method based on quaternion signal processing techniques, which can enable quantitative analysis using recently established TLC-SERS techniques. By marking three deterministic sampling points, we recorded spatially correlated SERS spectra to constitute an integral representation model of triple-spectra by a pure quaternion matrix. Quaternion principal component analysis (QPCA) was utilized for features extraction and followed by feature crossing among the quaternion principal components to obtain final fusion spectral feature vectors. Support vector regression (SVR) was then used to establish the quantitative model of melamine-contaminated milk samples with seven concentrations (1ppm to 250ppm). Compared with traditional TLC-SERS analysis methods, QPCA method significantly improved the accuracy of quantification by reaching only 7% and 2% quantization errors at 20 and 105 ppm concentration. Validation testing based on reasonable amount of statistic measurement results showed consistently smaller measurement errors and variance, which proved the effectiveness of QPCA method for TLC-SERS based quantitative sensing applications.

Photonic Crystal-Enhanced Fluorescence Imaging Immunoassay for Cardiovascular Disease Biomarker Screening with Machine Learning Analysis.

When myocardial walls experience stress due to cardiovascular diseases, like heart failure, hormone N-terminal pro-B-type natriuretic peptide (NT-proBNP) is secreted into the blood. Early detection of NT-proBNP can assist diagnosis of heart failure and enable early medical intervention. A simple, cost-effective detection technique such as the widely used fluorescence imaging immunoassay is yet to be developed to detect clinically relevant levels of NT-proBNP. In this work, we demonstrate photonic crystal-enhanced fluorescence imaging immunoassay using diatom biosilica, which is capable of detecting low levels of NT-proBNP in solution with the concentration range of 0~100 pg/mL. By analyzing the fluorescence images in the spatial and spatial frequency domain with principle component analysis (PCA) and partial least squares regression (PLSR) algorithms, we create a predictive model that achieves great linearity with a validation R2 value of 0.86 and a predictive root mean square error of 14.47, allowing for good analyte quantification. To demonstrate the potential of the fluorescence immunoassay biosensor for clinical usage, we conducted qualitative screening of high and low concentrations of NT-proBNP in human plasma. A more advanced machine learning algorithm, the support vector machine classification, was paired with the PCA and trained by 160 fluorescence images. In the 40 testing images, we achieved excellent specificity of 93%, as well as decent accuracy and sensitivity of 78% and 65% respectively. Therefore, the photonic crystal-enhanced fluorescence imaging immunoassay reported in this article is feasible to screen clinically relevant levels of NT-proBNP in body fluid and evaluate the risk of heart failure.

Quantitative TLC-SERS detection of histamine in seafood with support vector machine analysis.

Scombroid fish poisoning caused by histamine intoxication is one of the most prevalent allergies associated with seafood consumption in the United States. Typical symptoms range from mild itching up to fatal cardiovascular collapse seen in anaphylaxis. In this paper, we demonstrate rapid, sensitive, and quantitative detection of histamine in both artificially spoiled tuna solution and real spoiled tuna samples using thin layer chromatography in tandem with surface-enhanced Raman scattering (TLC-SERS) sensing methods, enabled by machine learning analysis based on support vector regression (SVR) after feature extraction with principal component analysis (PCA). The TLC plates used herein, which were made from commercial food-grade diatomaceous earth, served simultaneously as the stationary phase to separate histamine from the blended tuna meat and as ultra-sensitive SERS substrates to enhance the detection limit. Using a simple drop cast method to dispense gold colloidal nanoparticles onto the diatomaceous earth plate, we were able to directly detect histamine concentration in artificially spoiled tuna solution down to 10 ppm. Based on the TLC-SERS spectral data of real tuna samples spoiled at room temperature for 0 to 48 hours, we used the PCA-SVR quantitative model to achieve superior predictive performance exceling traditional partial least squares regression (PLSR) method. This work proves that diatomaceous earth based TLC-SERS technique combined with machine-learning analysis is a cost-effective, reliable, and accurate approach for on-site detection and quantification of seafood allergen to enhance food safety.

Ultracompact Silicon-Conductive Oxide Nanocavity Modulator with 0.02 Lambda-Cubic Active Volume.

Silicon photonic modulators rely on the plasma dispersion effect by free-carrier injection or depletion, which can only induce moderate refractive index perturbation. Therefore, the size and energy efficiency of silicon photonic modulators are ultimately limited as they are also subject to the diffraction limit. Here we report an ultracompact electro-optic modulator with total device footprint of 0.6 × 8 µm2 by integrating voltage-switched transparent conductive oxide with one-dimensional silicon photonic crystal nanocavity. The active modulation volume is only 0.06 um3, which is less than 2% of the lambda-cubic volume. The device operates in the dual mode of cavity resonance and optical absorption by exploiting the refractive index modulation from both the conductive oxide and the silicon waveguide induced by the applied gate voltage. Such a metal-free, hybrid silicon-conductive oxide nanocavity modulator also demonstrates only 0.5 dB extra optical loss, moderate Q-factor above 1000, and high energy efficiency of 46 fJ/bit. The combined results achieved through the holistic design opened a new route for the development of next generation electro-optic modulators that can be used for future on-chip optical interconnects.

One-volt silicon photonic crystal nanocavity modulator with indium oxide gate.

The ever-increasing global network traffic requires a high level of seamless integration between optical interconnect systems and complementary metal-oxide-semiconductor (CMOS) circuits. Therefore, it brings stringent requirements for future electro-optic (E-O) modulators, which should be ultracompact, energy efficient, high bandwidth, and in the meanwhile, able to be directly driven by the state-of-the-art CMOS circuits. In this Letter, we report a low-voltage silicon photonic crystal nanocavity modulator using an optimized metal-oxide-semiconductor (MOS) capacitor consisting of an In2O3/HfO2/p-Si stacked nanostructure. The strong light-matter interaction from the accumulated free carriers with the nanocavity resonant mode results in holistic improvement in device performance, including a high tuning efficiency of 250 pm/V and an average modulation strength of 4 dB/V with a moderate Q factor of ∼3700 and insertion loss of ∼6 dB using an ultrashort electrode length of only 350 nm. With 1 V driving voltage over a capacitive loading of only 13 fF, the silicon photonic nanocavity modulator can achieve more than 3 dB extinction ratio with energy consumption of only 3 fJ/bit. Such a low-voltage, low-capacitance silicon nanocavity modulator provides the feasibility to be directly driven by a CMOS logic gate for single-chip integration.

The discovery and optimization of naphthalene-linked P2-P4 Macrocycles as inhibitors of HCV NS3 protease.

Naphthalene-linked P2-P4 macrocycles within a tri-peptide-based acyl sulfonamide chemotype have been synthesized and found to inhibit HCV NS3 proteases representing genotypes 1a and 1b with single digit nanomolar potency. The pharmacokinetic profile of compounds in this series was optimized through structural modifications along the macrocycle tether as well as the P1 subsite. Ultimately a compound with oral bioavailability of 100% in rat, and a long half-life in plasma was obtained. However, compounds in this macrocyclic series exhibited cardiac effects in an isolated rabbit heart model and for this reason further optimization efforts were discontinued.

Microfluidic Diatomite Analytical Devices for Illicit Drug Sensing with ppb-Level Sensitivity.

The escalating research interests in porous media microfluidics, such as microfluidic paper-based analytical devices, have fostered a new spectrum of biomedical devices for point-of-care (POC) diagnosis and biosensing. In this paper, we report microfluidic diatomite analytical devices (µDADs), which consist of highly porous photonic crystal biosilica channels, as an innovative lab-on-a-chip platform to detect illicit drugs. The µDADs in this work are fabricated by spin-coating and tape-stripping diatomaceous earth on regular glass slides with cross section of 400×30µm2. As the most unique feature, our µDADs can simultaneously perform on-chip chromatography to separate small molecules from complex biofluidic samples and acquire the surface-enhanced Raman scattering spectra of the target chemicals with high specificity. Owing to the ultra-small dimension of the diatomite microfluidic channels and the photonic crystal effect from the fossilized diatom frustules, we demonstrate unprecedented sensitivity down to part-per-billion (ppb) level when detecting pyrene (1ppb) from mixed sample with Raman dye and cocaine (10 ppb) from human plasma. This pioneering work proves the exclusive advantage of µDADs as emerging microfluidic devices for chemical and biomedical sensing, especially for POC drug screening.

Plasmonic nanopatch array with integrated metal-organic framework for enhanced infrared absorption gas sensing.

In this letter, we present a nanophotonic device consisting of plasmonic nanopatch array (NPA) with integrated metal-organic framework (MOF) for enhanced infrared absorption gas sensing. By designing a gold NPA on a sapphire substrate, we are able to achieve enhanced optical field that spatially overlaps with the MOF layer, which can adsorb carbon dioxide (CO2) with high capacity. Experimental results show that this hybrid plasmonic-MOF device can effectively increase the infrared absorption path of on-chip gas sensors by more than 1100-fold. The demonstration of infrared absorption spectroscopy of CO2 using the hybrid plasmonic-MOF device proves a promising strategy for future on-chip gas sensing with ultra-compact size.

Ultra-Sensitive Lab-on-a-Chip Detection of Sudan I in Food using Plasmonics-Enhanced Diatomaceous Thin Film.

Sudan I is a carcinogenic compound containing an azo group that has been illegally utilized as an adulterant in food products to impart a bright red color to foods. In this paper, we develop a facile lab-on-a-chip device for instant, ultra-sensitive detection of Sudan I from real food samples using plasmonics-enhanced diatomaceous thin film, which can simultaneously perform on-chip separation using thin layer chromatography (TLC) and highly specific sensing using surface-enhanced Raman scattering (SERS) spectroscopy. Diatomite is a kind of nature-created photonic crystal biosilica with periodic pores and was used both as the stationary phase of the TLC plate and photonic crystals to enhance the SERS sensitivity. The on-chip chromatography capability of the TLC plate was verified by isolating Sudan I in a mixture solution containing Rhodamine 6G, while SERS sensing was achieved by spraying gold colloidal nanoparticles into the sensing spot. Such plasmonics-enhanced diatomaceous film can effectively detect Sudan I with more than 10 times improvement of the Raman signal intensity than commercial silica gel TLC plates. We applied this lab-on-a-chip device for real food samples and successfully detected Sudan I in chili sauce and chili oil down to 1 ppm, or 0.5 ng/spot. This on-chip TLC-SERS biosensor based on diatomite biosilica can function as a cost-effective, ultra-sensitive, and reliable technology for screening Sudan I and many other illicit ingredients to enhance food safety.

Detection of trace organics in Martian soil analogs using fluorescence-free surface enhanced 1064-nm Raman Spectroscopy.

A significant technology challenge in planetary missions is the in situ detection of organics at the sub-part-per-million (ppm) level in soils. This article reports the organic compound detection in Mars-like soils at the sub-ppm level using an ultra-sensitive spectral sensing technique based on fluorescence-free surface-enhanced Raman scattering (SERS), which has a significantly improved sensitivity and reduced fluorescence noise. Raman spectral detection of ppm level organics in Antarctic Dry Valley and Mojave Desert soils have been obtained for the first time, which otherwise are not detected by other Raman spectral techniques.

Broadband on-chip near-infrared spectroscopy based on a plasmonic grating filter array.

We demonstrate an ultra-compact, broadband on-chip near-infrared (NIR) spectroscopy system based on a narrow-band plasmonic filter array. The entire filter array, consisting of 28 individual subwavelength metallic gratings, was monolithically integrated in a thin gold film on a quartz substrate, covering a 270 nm spectra from 1510 nm to 1780 nm. In order to achieve a high spectral resolution, extremely narrow slits are created for the gratings with a polymer waveguide layer on top, generating narrow-band guided-mode resonances through coupling with the surface-plasmon resonances of the metallic gratings. Experimental results show that the transmission bands of the filter array have full width at half-maximum of only 7 nm-13 nm, which is sufficient for NIR spectroscopy. The NIR absorption spectroscopy of xylene using the on-chip plasmonic filter array matches very well with the results from conventional Fourier transform infrared spectroscopy, which proves the great potential for NIR sensing applications.

Effect of finite metallic grating size on Rayleigh anomaly-surface plasmon polariton resonances.

Rayleigh anomalies (RAs) and surface plasmon polaritons (SPPs) on subwavelength metallic gratings play pivotal roles in many interesting phenomena such as extraordinary optical transmission. In this work, we present a theoretical analysis of the effect of finite metallic grating size on RA-SPP resonances based on the combination of rigorous coupled wave analysis and finite aperture diffraction. One-dimensional arrays of gold subwavelength gratings with different device sizes were fabricated and the optical transmission spectra were measured. As the grating size shrinks, the broadening of the RA-SPP resonances is predicted by the theoretical model. For the first order RA-SPP resonances, the results from this model are in good agreement with the spectra measured from the fabricated plasmonic gratings.

Spin angular momentum of surface modes from the perspective of optical power flow.

We show that the spin angular momentum (SAM) carried by a surface mode can be linked to the expectation value, with respect to the distribution of optical power flow, of its decay constant by itself or divided by the product of permittivity and permeability of the medium. Rewriting the formulas for the SAM of a surface mode using the relation between the SAM density and the Poynting vector and then normalizing the light field so that the surface mode carries unit power, we derive novel formulas that show the linear relation between the SAM and those expectation values. The effect of propagation loss is also discussed briefly.

Slow-light effect via Rayleigh anomaly and the effect of finite gratings.

In this Letter, we investigate the slow-light effect of subwavelength diffraction gratings via the Rayleigh anomaly using a fully analytical approach without needing to consider specific grating structures. Our results show that the local group velocity of the transmitted light can be significantly reduced due to the optical vortex, which can inspire a new mechanism to enhance light-matter interactions for optical sensing and photodetection. However, the slow-light effect will diminish as the transmitted light propagates farther from the grating surface, and the slowdown factor decreases as the grating size shrinks.

Microfluidic channels with ultralow-loss waveguide crossings for various chip-integrated photonic sensors.

Traditional silicon waveguides are defined by waveguide trenches on either side of the high-index silicon core that leads to fluid leakage orifices for over-layed microfluidic channels. Closing the orifices needs additional fabrication steps which may include oxide deposition and planarization. We experimentally demonstrated a new type of microfluidic channel design with ultralow-loss waveguide crossings (0.00248 dB per crossings). The waveguide crossings and all other on-chip passive-waveguide components are fabricated in one step with no additional planarization steps which eliminates any orifices and leads to leak-free fluid flow. Such designs are applicable in all optical-waveguide-based sensing applications where the analyte must be flowed over the sensor. The new channel design was demonstrated in a L55 photonic crystal sensor operating between 1540 and 1580 nm.

Enhanced third harmonic generation by organic materials on high-Q plasmonic photonic crystals.

The enhanced optical nonlinearity enabled by localized plasmonic fields has been well studied for all-optical switching processing (AOSP) devices for future optical communication systems. In this work, plasmonic photonic crystals with a nonlinear polycarbonate/polymethine blend cladding layer are designed to enhance the third harmonic generation (THG) at the telecom wavelengths (~1550 nm). Due to the presence of he two-dimensional (2-D) gold nano-patch arrays with improved Q-factor and high local fields, more than 20 × of enhanced THG signals in the hybrid organic-plasmonic nanostructure are experimentally observed. The enhanced THG in the hybrid organic-plasmonic materials suggested that such extraordinary nonlinear effects can be used for AOSP devices and wavelength conversion.