Measured Transmission Spectra Research Articles

Transmission Spectroscopy of the Earth–Sun System to Inform the Search for Extrasolar Life

Abstract Upcoming NASA astrophysics missions such as the James Webb Space Telescope will search for signs of life on planets transiting nearby stars. Doing so will require coadding dozens of transmission spectra to build up sufficient signal to noise while simultaneously accounting for challenging systematic effects such as surface/weather variability, atmospheric refraction, and stellar activity. To determine the magnitude and impacts of both stellar and planet variability on measured transmission spectra, we must assess the feasibility of stacking multiple transmission spectra of exo-Earths around their host stars. Using our own solar system, we can determine if current methodologies are sufficient to detect signs of life in Earth’s atmosphere and measure the abundance of habitability indicators, such as H2O and CO2, and biosignature pairs, such as O2 and CH4. We assess the impact on transmission spectra of Earth transiting across the Sun from solar and planetary variability and identify remaining unknowns for understanding exoplanet transmission spectra. We conclude that a satellite observing Earth transits across the Sun from beyond L2 is necessary to address these long-standing concerns about the reliability of coadding planet spectra at UV, optical, and infrared wavelengths from multiple transits in the face of relatively large astrophysical systematics.

Recording of carbon clusters and (–СН2–)n radicals in a vacuum at the site of irradiation (in situ) of polyethylene

For the first time, absorption bands of C atoms and carbon clusters C2, C3, C4, C5, and C6 were found in irradiated polyethylene. The relationship between the type and mass of –CH2–, –C2H4–, –C3H6–, –C4H8–, – C5H10–, – C6H12 – free radicals and clusters of C, C2, C3, C4, C5, and C6 with their ionization energy in electron-irradiated polyethylene has been established. The results are based on the measured transmission spectra of the films and calculated values of the absorption bands after each dose of electron exposure in a vacuum at the site of irradiation (in situ). A linear dependence of the energy position of absorption bands on the type and mass of free radicals and clusters has been obtained. By the areas of the absorption bands of free radicals and carbon clusters the contribution of each defect to their total concentration has been determined depending on the absorbed dose in the range of 104–107Gy. The established dependences correlate with the known regularities in the formation of colloidal and color centers during irradiation of alkali-halide crystals, photographic materials, and oxide compounds.

Transfer matrix based reverse engineering approach to analyse annealing induced changes in optical transmission of TiO2/Ag/TiO2/Ag multilayer

Measured optical transmission spectrum of photonic structure comprising TiO2/Ag/TiO2/Ag multilayer is reverse engineered by employing transfer matrix method with three different multilayer models to analyse the factors responsible for the annealing induced changes in the transmission. The multilayer is made to have a single peak or a passband in the visible region, so that the spectral changes due to annealing can be observed more vividly. Prior to the multilayer deposition, single layer thin film of Ag and TiO2 are prepared using asymmetric bipolar pulsed dc sputtering and their optical constants such as refractive index and extinction co-efficient are determined. The multilayer is prepared by four-layer sequential deposition of Ag and TiO2 on BK7 glass substrate. The deposited TiO2/Ag/TiO2/Ag multilayer is annealed in the temperature range 100–300 °C. The optical transmission of the annealed multilayers is measured using spectrophotometer. The measured peak transmission and spectral bandwidth of the multilayer filter decreases with annealing, which is ascribed to inter-diffusion at the interfaces, and increasing absorption at the interfaces and in the silver layers of the multilayer. The ascription is resolved from the layers properties and interface parameters obtained by reverse engineering the measured transmission spectra. The rough interfaces in the multilayer are modeled as thin homogeneous layers following Bruggeman effective medium approximation, and the interface parameters are found to be varying with annealing.

In-plane excitation of a topological nanophotonic corner state at telecom wavelengths in a cross-coupled cavity

In silicon photonics, the cavity mode is a fundamental mechanism to design integrated passive devices for on-chip optical information processing. Recently, the corner state in a second-order topological photonic crystal (PC) rendered a global method to achieve an intrinsic cavity mode. It is crucial to explore such a topological corner state in silicon photonic integrated circuits (PICs) under in-plane excitation. Here, we study both theoretically and experimentally the topological nanophotonic corner state in a silicon-on-insulator PC cavity at a telecommunications wavelength. In theory, the expectation values of a mirror-flip operation for the Bloch modes of a PC slab are used to characterize the topological phase. Derived from topologically distinct bulk polarizations of two types of dielectric-vein PCs, the corner state is induced in a 90-deg-bend interface, localizing at the corner point of real space and the Brillouin zone boundary of reciprocal space. To implement in-plane excitation in an experiment, we fabricate a cross-coupled PC cavity based on the bend interface and directly image the corner state near 1383 nm using a far-field microscope. Finally, by means of the temporal coupled-mode theory, the intrinsic Q factor of a cross-coupled cavity (about 8000) is retrieved from the measured transmission spectra. This work gives deterministic guidance and potential applications for cavity-mode-based passive devices in silicon PICs, such as optical filters, routers, and multiplexers.

Low frequency ship vibration isolation using the band gap concept of sandwich plate-type elastic metastructures

Elastic metamaterial is a type of artificial composite material/structure with unique subwavelength physics property, which shows attractive potential application in low-frequency sound and vibration control. In this paper, a ship vibration isolation method using the band gap concept of sandwich plate-type elastic metastructures is proposed to solve the low frequency vibration and noise control of marine power system. The flexural wave propagation and low-frequency vibration isolation characteristics of sandwich plate-type elastic metamaterials are investigated by using the finite element method combination with Solid-Shell multi-physics coupling model and Bloch theory. Numerical results demonstrate that the proposed sandwich plate-type elastic metastructures possess significant low-frequency subwavelength flexural wave band-gap and vibration isolation characteristics under the premise of structural strength and stiffness. The formation mechanism of flexural band gap is mainly attributed to the mode coupling of the local resonant mode and the Lamb wave mode of sandwich plate. The experimental measurements of flexural vibration transmission spectra were conducted to validate the accuracy and reliability of the numerical calculation method. In addition, the influences of geometrical parameters on the flexural wave band gap are studied. On this basis, the potential application of the proposed ship vibration isolation method in the marine power system is explored.

Detection of DNA Bases via Field Effect Transistor of Graphene Nanoribbon With a Nanopore: Semi-Empirical Modeling.

DNA sequencing techniques are critical in order to investigate genes' functions. Obtaining fast, accurate, and affordable DNA bases detection makes it possible to acquire personalized medicine. In this article, a semi-empirical technique is used to calculate the electron transport characteristics of the developed z-shaped graphene device to detect the DNA bases. The z-shaped transistor consists of a pair of zigzag graphene nanoribbon (ZGNR) connected through an armchair graphene nanoribbon (AGNR) channel with a nanopore where the DNA nucleobases are positioned. Non-equilibrium Green's function (NEGF) integrated with semi-empirical methodologies are employed to analyze the different electronic transport characteristics. The semi-empirical approach applied is an extension of the extended Hückel (EH) method integrated with self-consistent (SC) Hartree potential. By employing the NEGF+SC-EH, it is proved that each one of the four DNA nucleobases positioned within the nanopore, with the hydrogen passivated edge carbon atoms, results in a unique electrical signature. Both electrical current signal and transmission spectrum measurements of DNA nucleobases inside the device's pore are studied for the different bases with modification of their orientation and lateral translation. Moreover, the electronic noise effect of various factors is studied. The sensor sensitivity is improved by using nitrogen instead of hydrogen to passivate the nanopore and by adding a dual gate to surround the central semiconducting channel of the z-shaped graphene nanoribbon.

Artificial bee colony algorithm: a novel strategy for optical constants and thin film thickness extraction using only optical transmittance spectra for photovoltaic applications

An effective approach to determine thin film thickness (d) and optical constants (n,k,α) from transmittance spectrum with interference fringes is proposed. The developed strategy is based on applying the artificial bee colony (ABC) algorithm and Cauchy dispersion model. The accuracy test of this method has been assessed by using simulated and real tests. Simulated test is used to check the ability of ABC algorithm to determine the parameters of simulated transmittance spectra. Real test deals with the investigation of the determination approach on experimental measured transmittance spectra. Those spectra were measured from six elaborated samples of amorphous hydrogenated silicon (a-Si:H) thin films with different thicknesses, which will be used as an eco-friendly layer for solar cell applications. The obtained results noticeably show the high effectiveness of the developed strategy to accurately determine the thin film thickness and optical constants.

Fabrication and characterization of CuAlO2/ 4H–SiC heterostructure on 4H–SiC (0001)

In this work, CuAlO2 films were prepared on 4H–SiC (0001) using the sol-gel method. The surface morphology, structure and optical characteristics of the prepared films were studied. A homogeneous distribution of fine rhombohedral crystal structural grains is observed on the 4H–SiC substrate using scanning electron microscope (SEM). The film was confirmed to consist predominantly of delafossite CuAlO2 phase and impurity spinel CuAl2O4 phase via X-ray diffraction (XRD). The direct optical bandgap estimated from the measured transmittance spectra is approximately 3.85eV, while the indirect bandgap is 1.78 eV. Four emission peaks are observed in the photoluminescence (PL) spectra measured at room temperature. The emission peaks around 360 nm and 380 nm are the near band edge emission (NBE) of the prepared CuAlO2 films and the 4H–SiC substrate, respectively. The other two emission peaks around 410 nm and 470 nm arises from defects of Cu-vacancy. The fabricated p-CuAlO2/n-4H–SiC heterojunction diode exhibits typical rectification. Additionally, the turn-on voltage and rectification ratio at ± 3 V are determined to be 1.6 V and 1.26 × 103, respectively. A further 7.15 × 109 times increase of the hole injection capacity could be acquired potentially through an analysis of the energy band structure, which suggests that high injection capacity SiC bipolar devices could be feasibly achieved by utilizing CuAlO2 as the p + emitter.

The free carriers loss, free carriers susceptibility, and optical VEL and SEL functions in ZnO films doped by Cu, Al, and Cu-Al composite nanoparticles

The purpose of this work is to investigate the optical constants of ZnO films doped by Cu, Al, and Cu-Al nanoparticles. The reflectance and the transmittance spectra measurements were applied for calculating these optical constants in the range of 200–2500 nm. It can be seen that ZnO films doped with Al nanoparticles have a maximum value nanoparticles size of about 23.73 nm. With increasing annealing temperature for all wavelength range, due to the improvement of crystalline structure and hence decreasing defects, the extinction coefficient of films decreased. The theoretical extrapolation of e1 versus λ2 at low wavelengths values provides the dielectric constant e∞ and for the ZnO films doped with Cu nanoparticles, it has a maximum value of 4.61. The ZnO films doped by Al nanoparticles have a minimum value of $${\chi }_{c}$$ in all energy range. The dissipation factor tan δ of ZnO films doped by Cu nanoparticles especially in high energy has a maximum value. SEL functions in ZnO films doped by Cu nanoparticles have the maximum value.

Observation of frequency band-switchable topological edge modes using a 2 bit coding acoustic topological insulator

Here, the frequency band-switchable topologically protected edge state transport is realized in simulation and experiment based on a 2 bit coding acoustic topological insulator that consists of two layers of sonic crystals arrayed by the Helmholtz resonant triangle-lattice scatters with two distinct rotation angles. The acoustic topological phase transition is revealed and the gapless frequency bands are predicted. Experimentally measured transmission spectra and simulated pressure fields show good agreement with the predicted results. The error between the measured and the predicted results is illustrated by introducing a slit into the nested scatters, which is comparable to the manufacturing accuracy of the commercial 3D printer. Our work provides a simple method of coding to achieve the frequency-switchable acoustic topological edge modes, and paves a promising way to design the intelligent, programmable, and flexible acoustic devices.

Optical Constants of a Solar System Organic Analog and the Allende Meteorite in the Near- and Mid-infrared (1.5–13 μm)

Abstract Measurements of visible and near-infrared reflection (0.38–5 μm) and mid-to-far-infrared emission (5–200 μm) from telescope and satellite remote-sensing instruments make it possible to investigate the composition of planetary surfaces via electronic transitions and vibrational modes of chemical bonds. Red spectral slopes at visible and near-infrared wavelengths and absorption features at 3.3 and 3.4 μm observed in circumstellar disks, in the interstellar medium (ISM), and on the surfaces of solar system bodies are interpreted to be due to the presence of organic material and other carbon compounds. Identifying the origin of these features requires measurements of the optical properties of a variety of relevant analog and planetary materials. Spectroscopic models of dust within circumstellar disks and the ISM, as well as planetary regoliths, often incorporate just one such laboratory measurement despite the wide variation in absorption and extinction properties of organic and other carbon-bearing materials. Here we present laboratory measurements of transmission spectra in the 1.5–13 μm region and use these to derive real and imaginary indices of refraction for two samples: (1) an analog to meteoritic insoluble organic matter and (2) a powdered Allende meteorite sample. We also test our refractive index retrieval method on a previously published transmission spectrum of an Mg-rich olivine. We compare optical measurements of the insoluble organic matter analog to those of other solar system and extrasolar organic analogs, such as amorphous carbon and tholins, and find that the indices of refraction of the newly characterized material differ significantly from other carbonaceous samples.

Surface activation of poly(methyl methacrylate) for microfluidic device bonding through a H2O plasma treatment linked with a low-temperature annealing

This paper reports on a rapid, lossless process for the bonding of poly(methyl methacrylate) (PMMA) substrates for use in microfluidic devices for blood tests, utilizing H2O plasma, and H2O linked rapid thermal annealing (RTA) treatments. The bonding of PMMA produced with H2O plasma linked with RTA was analyzed, and its effect on the bonding of free radicals was investigated. The PMMA surface treatment was performed at constant RF power and H2O vapor flow but changing plasma treatment times in both processes. The surface modification of the PMMA subjected to the H2O plasma treatment was studied by optical emission spectroscopy, which confirmed that the relative intensity of hydroxyl radicals and oxygen emission peaks. The surface of the PMMA after being subjected to the plasma linked with RTA treatment was also characterized. The free energy increased with the plasma treatment time. The tensile test results revealed that the strength of OH bonds on the PMMA surface increased with a high plasma treatment time of 120 s. After bonding, UV–visible transmission spectrum measurements revealed that the transparency of the microfluidic device increased. We demonstrated the bonding of a microfluidic device with plasma treatment without deformation. This method provides an ideal bonding technique for the manufacture of microfluidic devices with PMMA. This technique has more advantages than other bonding techniques.

Effect of nitrogen flow on the properties of carbon nitride films deposited by electron cyclotron resonance plasma-enhanced chemical vapor deposition

Amorphous hydrogenated carbon nitride (a-CNx:H) films were fabricated via electron cyclotron resonance microwave plasma-enhanced chemical vapor deposition technology using N2 and C2H2 as nitrogen and carbon sources, respectively. The effects of the N2/C2H2 gas flow ratio on the composition, bond structure, scratch resistance, and optical properties of the a-CNx:H films were investigated. X-ray photoluminescence spectroscopy and Raman spectroscopy results indicated that the N/C atomic ratio, carbon 1s sp3/sp2 ratio, and nitrogen 1s sp3/sp2 ratio increased with increasing N2/C2H2 gas ratio. Pin-on-disk wear tests conducted with a pin made of ultrahigh-molecular weight polyethylene indicated that all the a-CNx:H films exhibited good scratch resistance; however, rubbing with a stainless-steel ball weakened the wear resistance of the a-CNx:H films as the N2/C2H2 gas ratio changed from 5/30 to 30/30. The measured transmittance spectra showed that nitrogen incorporation into the carbon films increased their transmittance in the visible to near-infrared regions. The optical bandgap of the a-CNx:H films was obtained by the Tauc method, which demonstrated that when the N2/C2H2 gas flow ratio changed from 0/30 to 30/30, the optical bandgap increased from 1.18 to 1.86 eV. Our results suggest that a-CNx:H films can be used as optical and protective coatings.

Microstructured optical fibers sensor modified by deep eutectic solvent: Liquid-phase microextraction and detection in one analytical device

A sensitive optical sensor based on hollow core microstructure optical fibers modified with deep eutectic solvent was produced for the first time. An easy procedure for the modification of hollow-core microstructure optical fibers with deep eutectic solvent was developed. Deep eutectic solvents based on natural monoterpenoids and fatty acids were investigated for glass surface modification. The sensor was used for the determination of non-steroidal anti-inflammatory drugs (mefenamic acid, diclofenac, flurbiprofen and ketoprofen) in human urine samples. The mechanism of the sensor response was investigated and discussed. Liquid-phase microextraction of non-steroidal anti-inflammatory drugs was implemented in deep eutectic solvent phase supported in the inner surface of hollow-core microstructure optical fibers followed by transmission spectra measurement in one analytical device. The preconcentration step performed directly in the analytical device allowed to obtain high sensitivity and selectivity. The limits of detection calculated from the calibration plots based on 3σ were 3 μg L−1 for all target analytes.

Gemini/GMOS optical transmission spectroscopy of WASP-121b: signs of variability in an ultra-hot Jupiter?

ABSTRACT We present ground-based, spectroscopic observations of two transits of the ultra-hot Jupiter WASP-121b covering the wavelength range ≈500–950 nm using Gemini/GMOS. We use a Gaussian process framework to model instrumental systematics in the light curves, and also demonstrate the use of the more generalized Student’s-T process to verify our results. We find that our measured transmission spectrum, whilst showing overall agreement, is slightly discrepant with results obtained using HST/STIS, particularly for wavelengths shortwards of ≈650 nm. In contrast to the STIS results, we find evidence for an increasing bluewards slope and little evidence for absorption from either TiO or VO in our retrieval, in agreement with a number of recent studies performed at high-resolution. We suggest that this might point to some other absorbers, particularly some combination of recently detected atomic metals, in addition to scattering by hazes, being responsible for the excess optical absorption and observed vertical thermal inversion. Our results are also broadly consistent with previous ground-based photometry and 3D GCM predictions, however, these assumed different chemistry to our retrievals. In addition, we show that the GMOS observations are repeatable over short periods (days), similar to the HST/STIS observations. Their difference over longer periods (months) could well be the result of temporal variability in the atmospheric properties (i.e. weather) as predicted by theoretical models of ultra-hot Jupiters; however, more mundane explanations such as instrumental systematics and stellar activity cannot be fully ruled out, and we encourage future observations to explore this possibility.

Optical Polymer Waveguides Fabricated by Roll-to-Plate Nanoimprinting Technique

The paper reports on the properties of UV-curable inorganic-organic hybrid polymer multimode optical channel waveguides fabricated by roll-to-plate (R2P) nanoimprinting. We measured transmission spectra, refractive indices of the applied polymer materials, and optimized the R2P fabrication process. Optical losses of the waveguides were measured by the cut-back method at wavelengths of 532, 650, 850, 1310, and 1550 nm. The lowest optical losses were measured at 850 nm and the lowest average value was 0.19 dB/cm, and optical losses at 1310 nm were 0.42 dB/cm and 0.25 dB/cm at 650 nm respectively. The study has demonstrated that nanoimprinting has great potential for the implementation of optical polymer waveguides not only for optical interconnection applications.

PRISA: a user-friendly software for determining refractive index, extinction co-efficient, dispersion energy, band gap, and thickness of semiconductor and dielectric thin films

A user-friendly software PRISA has been developed to determine optical constants (refractive index and extinction co-efficient), dispersion parameters (oscillator energy and dispersion energy), absorption co-efficient, band gap and thickness of semiconductor and dielectric thin films from measured transmission spectrum, only. The thickness, refractive index, and extinction co-efficient of the films have been derived using Envelope method proposed by Swanepoel. The absorption co-efficient in the strong absorption region is calculated using the method proposed by Connel and Lewis. Subsequently, both direct and indirect bandgap of the films is estimated from the absorption co-efficient spectrum using Tauc plot. The software codes are written in Python and the graphical user interface is programmed with tkinter package of Python. It provides convenient input and output of the measured and derived data. The software has a feature to cross check the results by retrieving transmission spectrum using the values of refractive index, extinction co-efficient, and thickness obtained from Envelope method. The performance of the software is verified by analyzing numerically generated transmission spectra of a-Si:H amorphous semiconductor thin films, and experimentally measured transmission spectra of electron beam evaporated HfO2 dielectric thin films as examples. PRISA is found to be much simpler and accurate as compared to the other freely available softwares. To help researchers working on thin films, the software is made freely available at https://www.shuvendujena.tk/download.

3D integrated wavelength demultiplexer based on a square-core fiber and dual-layer arrayed waveguide gratings.

We present a 3D integrated wavelength demultiplexer using a square-core fiber (SCF) and matched dual-layer arrayed waveguide gratings (AWGs). The SCF works as a 3D fiber multimode interference device, which splits the input light into symmetric four spots. The spots are then coupled to a pitch-matched 4-waveguide network, each connecting an AWG. Interface waveguides are designed to improve the coupling efficiency between the SCF and the dual-layer chip. The four AWGs are designed with different central wavelengths and a large free spectral range (FSR) of 120 nm. To reach a small and uniform insertion loss among different channels, only the central channels of each AWG are used for demultiplexing. The device is fabricated on a polymer platform. The upper and lower layers of the chip are fabricated using the same photolithography mask but rotated 180° so that 4 different AWG designs can be mapped to a single chip. The measured transmission spectra of the four AWGs cover a bandwidth of 112 nm. The insertion loss variation is smaller than 1.4 dB. The designed device can find applications in fiber optic sensing, communication, and astronomy.

Online detection of soluble solids content and maturity of tomatoes using Vis/NIR full transmittance spectra

To meet the demand for high-quality tomatoes of different commercial purposes, the optimal models for predicting soluble solids content (SSC)and classifying maturities of tomatoes were developed simultaneously based on the Vis-NIR spectra collected by an online full transmittance spectrum measurement system. The intensity of full transmittance spectra at maturities of green, turning, pink, light red and red increased with their mature stages, indicating that the optical properties of tomatoes correlated significantly with their growth stages. Hence, the full transmittance has the potential to distinguish the maturities of tomatoes. The prediction models of SSC and classification models of maturity were built by partial least squares (PLS) regression and extreme learning machine (ELM) classifier based on the spectral data obtained by different detection orientations and different preprocessing methods, respectively. The results showed that detection orientations significantly affected the performance of SSC prediction models and maturity classification models, and the best model for SSC prediction was developed using the full wavelength spectra preprocessed by multiplicative scatter correction (MSC) with T1 orientation (the stem-top axis is vertical to conveyor belt). For samples in the prediction set, the correlation coefficient (Rp) and root mean square error (RMSEP) and residual predictive deviation (RPD) of the best SSC prediction model were 0.75, 0.27 °Brix and 1.49, respectively. The best classification model for five classes was built based on the first ten principle components (PC) of spectra preprocessed by MSC with T1 orientation. The success rate of model for classifying the total, green, turning, pink, light red and red stages were 80.08%, 94.62%, 77.31%, 62.69%, 78.85%, 89.62%, respectively. To further investigate the classification ability of ELM classifier and simplify the classification complexity of maturities, the initial five maturities were converted into three maturities, namely immature, intermediate (turning ​+ ​pink) and mature (light red ​+ ​red). Compared with the classification model of five classes, model that was established for classifying three types of maturities obtained better results with accuracies of 91.31%, 95.38%, 89.23% and 91.35% for total, immature, intermediate and mature stages, respectively, based on the full wavelength spectra pretreated by MSC with T1 orientation. Overall, it was optimal for detection models of SSC and maturity by combining T1 orientation with MSC preprocessing, and the full transmittance mode used in this study has further potential applications for quality evaluation of agriculture products.

Sensitive Detection of Goat αs1-Casein Using Tapered Optical Fiber Sensor

We present a new approach for the detection of α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s1</sub> -casein, a milk allergen, using tapered optical fiber sensor. The single-mode optical fiber of 125 μm diameter was tapered to produce waist diameter and length of 12 μm and 15 mm, respectively. The fiber sensor was functionalized using sodium hydroxide, (3-aminopropyl) triethoxysilane, and glutaraldehyde to the tapered region. Anti-peptide polyclonal antibody from N-terminal sequence of α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s1</sub> -casein was immobilized onto the tapered region, which subsequently bound to the introduced targeted α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s1</sub> -casein. Measurement of transmission spectrum was performed within the wavelength range of 1500-1600 nm. Results showed that the shift of the wavelength in the fabricated sensor was linearly proportional to the increment of α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s1</sub> -casein from 0.1 to 1.0 nM. The sensitivity of the sensor towards α <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s1</sub> -casein was measured at 1.139 nm/nM. Specificity was validated using bovine α-casein as a negative control with no significant wavelength shift. The sensor was tested using goat milk sample in three different dilutions to illustrate the applicability of the sensor in food matrices.