Single Infrared Spectrum Enables Simultaneous Identification of (Bio)Chemical Nature and Particle Size of Microorganisms and Synthetic Microplastic Beads.

Populations of nearly identical chemical and biological microparticles include the synthetic microbeads used in cosmetic, biomedical, agri-food, and pharmaceutical industries as well as the class of living microorganisms such as yeast, pollen, and biological cells. Herein, we identify simultaneously the size and chemical nature of spherical microparticle populations with diameters larger than 1 µm. Our analysis relies on the extraction of both physical and chemical signatures from the same optical spectrum recorded using attenuated total reflection (ATR)-Fourier transform infrared (FTIR) spectroscopy. These signatures are the spectral resonances caused by the microparticles, which depend on their size and the absorption peaks revealing their chemical nature. We validate the method first on separated and mixed groups of spherical microplastic particles of two different diameters, where the method is used to calculate the diameter of the microspherical particles. Then, we apply the method to correctly identify and measure the diameter of Saccharomyces cerevisiae yeast cells. Theoretical simulations to help in understanding the effect of size distribution and dispersion support our results.

Polarized light diffuse reflectance FT-NIR MEMS spectrometer enabling the detection of powder samples through a thin plastic layer.

Polarized scattered light Fourier transform infrared (FTIR) spectroscopy is used for measuring the absorbance of highly scattering materials overcoming the multiple scattering effect. It has been reported for in vivo for biomedical applications and in-field for agricultural and for environmental monitoring. In this paper, we report a polarized light microelectromechanical system (MEMS)-based FTIR in the extended near infrared (NIR) that utilizes a bistate polarizer in a diffuse reflectance measurement setup. The spectrometer is capable of distinguishing between single backscattering from the uppermost layer and multiple scattering from the deep layers. The spectrometer has a spectral resolution of 64c m -1 (about 16 nm at a wavelength of 1550 nm) and operates in the spectral range of 4347c m -1 to 7692c m -1 (1300 nm to 2300 nm). The technique implies de-embedding of the MEMS spectrometer polarization response by normalizing its effect; this is applied on three different samples: milk powder, sugar, and flour in plastic bags. The technique is examined on different scattering size particles. The scattering particles diameter's range is expected to vary from 10 µm to 400 µm. The absorbance spectra of the samples are extracted and compared to the direct diffuse reflectance measurements of the samples, showing good agreement. By using the proposed technique, the calculated error for the flour was decreased from 43.2% to 2.9% at 1935 nm wavelength. The wavelength error dependence is also reduced.

Potential of a Miniature Spectral Analyzer for District-Scale Monitoring of Multiple Gaseous Air Pollutants.

Gas sensors that can measure multiple pollutants simultaneously are highly desirable for on-site air pollution monitoring at various scales, both indoor and outdoor. Herein, we introduce a low-cost multi-parameter gas analyzer capable of monitoring multiple gaseous pollutants simultaneously, thus allowing for true analytical measurement. It is a spectral sensor consisting of a Fourier-transform infrared (FTIR) gas analyzer based on a mid-infrared (MIR) spectrometer. The sensor is as small as 7 × 5 × 2.5 cm3. It was deployed in an open-path configuration within a district-scale climatic chamber (Sense City, Marne-la-Vallée, France) with a volume of 20 × 20 × 8 m3. The setup included a transmitter and a receiver separated by 38 m to enable representative measurements of the entire district domain. We used a car inside the climatic chamber, turning the engine on and off to create time sequences of a pollution source. The results showed that carbon dioxide (CO2) and water vapor (H2O) were accurately monitored using the spectral sensor, with agreement with the reference analyzers used to record the pollution levels near the car exhaust. Furthermore, the lower detection limits of CO, NO2 and NO were assessed, demonstrating the capability of the sensor to detect these pollutants. Additionally, a preliminary evaluation of the potential of the spectral sensor to screen multiple volatile organic compounds (VOCs) was conducted at the laboratory scale. Overall, the results demonstrated the potential of the proposed multi-parameter spectral gas sensor in on-site gaseous pollution monitoring.

Complex Kernel-based spectrum reconstruction algorithm for cascaded Fabry-Perot interferometric spectrometer.

A method to calculate the spectrum of the light incident on a cascaded Fabry-Perot interferometric spectrometer from the detector signal versus the scanning mirror position is presented. The method is based on modifying the Fabry-Perot integral equation to reduce possible spectrum reconstruction errors that arise due to inaccurate determination of the optical path difference reference position and the dependence on the dispersion of the cavity material. A transformation algorithm that employs the suggested kernel modification is derived and tested. The presented algorithm is then compared to the conventional kernel, showing spectral error reductions by larger than 20 dB.

Continuous Monitoring of Air Purification: A Study on Volatile Organic Compounds in a Gas Cell.

Air pollution is one of the major environmental issues that humanity is facing. Considering Indoor Air Quality (IAQ), Volatile Organic Compounds (VOCs) are among the most harmful gases that need to be detected, but also need to be eliminated using air purification technologies. In this work, we tackle both problems simultaneously by introducing an experimental setup enabling continuous measurement of the VOCs by online absorption spectroscopy using a MEMS-based Fourier Transform infrared (FTIR) spectrometer, while those VOCs are continuously eliminated by continuous adsorption and photocatalysis, using zinc oxide nanowires (ZnO-NWs). The proposed setup enabled a preliminary study of the mechanisms involved in the purification process of acetone and toluene, taken as two different VOCs, also typical of those that can be found in tobacco smoke. Our experiments revealed very different behaviors for those two gases. An elimination ratio of 63% in 3 h was achieved for toluene, while it was only 14% for acetone under same conditions. Adsorption to the nanowires appears as the dominant mechanism for the acetone, while photocatalysis is dominant in case of the toluene.

Odd excitation of symmetric multimode interference structures.

In this work, we study the odd excitation of symmetric multimode interference (MMI) integrated optical structures. We develop a simple formula for the imaging length in the guide under odd excitation in the MMI structures using the symmetry of the structure. The obtained analytical results are verified by numerical calculations using the beam propagation method. The developed model is applied for both 2D and 3D MMI structures successfully. It also allows building functional devices such as power splitters or mode converters for the odd/asymmetric modes of the waveguide.

Autoregressive superresolution microelectromechanical systems Fourier transform spectrometer.

In this work, the application of the superresolution autoregressive (AR) model to enhance the resolution of the microelectromechanical systems (MEMS) Fourier transform infrared spectrometer (FTIR) spectrometer is studied theoretically and experimentally. The effect of the number of spectral lines, the spacing between the lines, the resolution of the MEMS FTIR spectrometer and the signal-to-noise ratio (SNR) on the prediction accuracy is addressed for different targeted prediction resolutions. The effect of the SNR on applying the AR model is studied. Then, the AR model is applied to experimental data obtained using the MEMS FTIR for the different cases of single spectral line, xenon lamp lines and gas cells containing different gas mixtures. It is found that enhancement up to 4× can obtained in the case of the single line, while an enhancement of about 2-2.5× can be obtained in the case of multilines without having false spectral lines.

Optical modeling of black silicon using an effective medium/multi-layer approach.

In this work, black silicon (BSi) structures including nanocones and nanowires are modeled using effective medium theory (EMT), where each structure is assumed to be a multilayer structure of varying effective index, and its optical scattering in the infrared range is studied in terms of its total reflectance, transmittance and absorptance using the transfer matrix method (TMM). The different mechanisms of the intrinsic absorption of silicon are taken into account, which translates into proper modeling of its complex refraction index, depending on several parameters including the doping level. The model validity is studied by comparing the results with the rigorous coupled wave analysis and is found to be in good agreement. The effect of the aspect ratio, the spacing between the structure features and the structure disordered nature are all considered. Moreover, the results of the proposed model are compared with reflectance measurements of a fabricated BSi sample, in addition to other measurements reported in the literature for Silicon Nanowires (SiNWs). The TMM along with EMT proves to be a convenient method for modeling BSi due to its simple implementation and computational speed.

Transmission-enabled fiber Fabry-Perot cavity based on a deeply etched slotted micromirror.

In this work, we report the analysis, fabrication, and characterization of an optical cavity built using a Bragg-coated fiber (BCF) mirror and a metal-coated microelectromechanical systems (MEMS) slotted micromirror, where the latter allows transmission output from the cavity. Theoretical modeling, using Fourier optics analysis for the cavity response based on tracing the propagation of light back and forth between the mirrors, is presented. Detailed simulation analysis is carried out for the spectral response of the cavity under different design conditions. MEMS chips of the slotted micromirror are fabricated using deep reactive ion etching of a silicon-on-insulator substrate with different device-etching depths of 150 µm and 80 µm with aluminum and gold metal coating, respectively. The cavity is characterized as an optical filter using a BCF with reflectivity that is larger than 95% in a 300 nm range across the E-band and the L-band. Versatile filter characteristics were obtained for different values of the MEMS micromirror slit width and cavity length. A free spectral range (FSR) of about 33 nm and a quality factor of about 196 were obtained for a 5.5 µm width aluminum slit, while an FSR of about 148 nm and a quality factor of about 148 were obtained for a 1.5 µm width gold slit. The presented structure opens the door for wide spectral response transmission-type MEMS filters.

Transformation algorithm and analysis of the Fourier transform spectrometer based on cascaded Fabry-Perot interferometers.

The Fourier transform spectrometer based on cascaded Fabry-Perot interferometers is analyzed, where one of the interferometers has a fixed length, while the other is scanning. We propose a method to reconstruct the spectrum correctly based on solving the integral equation of the overall response of the cascaded interferometers. The method is tested for different design parameters and noise conditions. Low reconstruction error below -80 dB is found to be achievable.

In-plane coupled Fabry-Perot micro-cavities based on Si-air Bragg mirrors: a theoretical and practical study.

In-plane Fabry-Perot cavities based on deeply etched Bragg mirrors are used in many microphotonic applications including sensing, telecom, and swept laser devices. A main limitation to their performance is the small free spectral range (FSR) and low finesse. The FSR limits the dynamic range or the wavelength tuning range, while the linewidth limits the resolution. In this work, we propose coupled Fabry-Perot micro-cavities that greatly enhance the FSR, besides reducing the linewidth, which lead to higher finesse and better performance. The proposed structure is modeled and etched on Si substrate to a depth of 150 µm using the deep reactive ion etching technology. Optical measurements indicate an enhanced FSR of more than 140 nm and a quality factor of 3152 using coupled cavities as compared to only 9 nm FSR for a single cavity of the same length. The over-etching and surface roughness, being the main effective fabrication tolerances, are modeled and extracted from the measurements.

Single-longitudinal-mode broadband tunable random laser.

In this work, we demonstrate a broadband tunable single-longitudinal-mode (SLM) random laser based on Rayleigh backscattering in a standard single-mode fiber. The wide tuning range of this SLM fiber laser over 1500-1570 nm is demonstrated with a linewidth of 4.5-30 kHz. The tuning is achieved using a tunable bandpass Fabry-Perot filter, and a semiconductor optical amplifier is used as the wide-bandwidth gain medium. The laser is able to operate in the S + C + L band.

Planar broad-band and wide-angle hybrid plasmonic IMI filters with induced transmission for visible light applications.

This work presents a technique for the design of visible optical filters using a hybrid plasmonic insulator-metal-insulator (IMI) structure. The proposed IMI visible light filter exhibits high transmission (∼91%) and an insertion loss of ∼0.4 dB with almost an omnidirectional field-of-view (0°-70°), a feature that is important for light collection in miniaturized devices. The proposed design also has a minimal polarization dependent loss of 0.2 dB at an angle of incidence of 60°. The effects of design parameters on the filter's performance are studied. Design rules of the filter are deduced along with physical justifications of the obtained results.

Analysis of dual coupler nested coupled cavities.

Coupled ring resonators are now forming the basic building blocks in several optical systems serving different applications. In many of these applications, a small full width at half maximum is required, along with a large free spectral range. In this work, a configuration of passive coupled cavities constituting dual coupler nested cavities is proposed. A theoretical study of the configuration is presented allowing us to obtain analytical expressions of its different spectral characteristics. The transfer function of the configuration is also used to generate design curves while comparing these results with analytical expressions. Finally, the configuration is compared with other coupled cavity configurations.

Angle-tolerant hybrid plasmonic filters for visible light communications.

This work presents what we believe is a novel design of a hybrid plasmonic-transmission blue filter for visible light communication systems that employ yellow phosphor-coated blue light-emitting diodes. The proposed filter balances the trade-off between transmission performance and tolerance to variation in angles of incidence (AOIs) while maintaining a low cost with limited complexity design. The designed filter operation is based upon quasi-plasmon mode excitation in a hybrid structure of alternating layers of silver and titanium dioxide over a silica substrate. A primary design approach for a hybrid plasmonic filter of five alternating layers is illustrated in detail. Needle optimization technique is further applied to achieve the required filter performance. The designed filter has an insertion loss of ∼1 dB over a spectral range of 400-485 nm and a minimal close to zero polarization-dependent loss for a wide range of AOI (slightly above 50°). The tolerance of the proposed design against fabrication errors is also tested. The performances of the proposed filters are tested for individual and simultaneous variations from the designed thicknesses, with a ±10% standard deviation from each layer's thickness.

Fourier transform spectrometer based on Fabry-Perot interferometer.

We analyze the Fourier transform spectrometer based on a symmetric/asymmetric Fabry-Perot interferometer. In this spectrometer, the interferogram is obtained by recording the intensity as a function of the interferometer length. Then, we recover the spectrum by applying the discrete Fourier transform (DFT) directly on the interferogram. This technique results in spectral harmonic overlap and fictitious wavenumber components outside the original spectral range. For this purpose, in this work, we propose a second method to recover the spectrum. This method is based on expanding the DFT of the interferogram and the spectrum by a Haar or box function. By this second method, we recovered the spectrum and got rid of the fictitious spectral components and spectral harmonic overlap.

Box-like filter response using multimode single-ring microresonators.

In this work we suggest a new technique to form a box-like filter response using a single microring resonator. The new design is based on exploiting the multimode propagation in the ring. The technique is implemented in the case of a two-mode ring resonator for different designs. The obtained results show that the new designs allow getting 1 to 10 dB spectral width shape factor better than 0.4, as verified by the FDTD simulation at different wavelengths and for different spectral widths. This new design methodology opens the door for more compact optical filters with controlled box-like response.

Design of an InGaAsP/InP compact integrated optical depolarizer.

In this paper, we propose and demonstrate two integrated optics InGaAsP/InP depolarizer circuits. The first one is based on the Lyot depolarizer configuration, while the second one is based on the Mach-Zehnder configuration. Detailed simulation, using three-dimensional full vectorial beam propagation method, shows that a high-index contrast material allows the design of a compact polarization insensitive depolarizer. For the first design, an output degree of polarization (DOP) less than 0.1 is obtained for light sources with spectral widths larger than 25 nm, while for the second one, output DOP less than 0.06 is obtained for light sources with spectral widths larger than 40 nm.

Performance evaluation of a metal-insulator-metal surface plasmon resonance optical gas sensor under the effect of Gaussian beams.

In this work, the performance of a nonconventional IR surface plasmon resonance (SPR) gas sensor structure based on the use of a metal-insulator-metal (MIM) structure is studied. This MIM-based sensor structure gives enhanced performance five times better than the conventional MI SPR optical gas sensors. The performance of the SPR gas sensors is studied under the effect of oblique incident Gaussian beams with different spot sizes, and the performance enhancement of the MIM structure is confirmed for different spot sizes. The simulation technique used to generate the results is also verified by comparing them to actual experimental results available in the literature.

Dispersion compensation in Fourier domain optical coherence tomography.

In this work, we propose a numerical technique to compensate for errors due to dispersion effects in Fourier domain optical coherence tomography. The proposed technique corrects for errors in depth measurements and resolution loss due to dispersion. The results show that, by using this technique, errors in thickness measurement are reduced from about 5% to less than 0.1% depending on the sample length and the amount of dispersion. Also, an improvement in the resolution from about 50 µm to less than 10 µm is demonstrated.