Angular Reflectance Spectrum Research Articles

Real-time hybrid angular-interrogation surface plasmon resonance sensor in the near-infrared region for wide dynamic range refractive index sensing

Herein, we integrated angle-scanning surface plasmon resonance (SPR) and angle-fixed SPR as a hybrid angular-interrogation SPR to enhance the sensing performance. Galvanometer-mirror-based beam angle scanning achieves a 100-Hz acquisition rate of both the angular SPR reflectance spectrum and the angle-fixed SPR reflectance, whereas the use of near-infrared light enhances the refractive index (RI) sensitivity, range, and precision compared with visible light. Simultaneous measurement of the angular SPR reflectance spectrum and angle-fixed SPR reflectance boosts the RI change range, RI resolution, and RI accuracy to 10–1–10–6 RIU, 2.24 × 10−6 RIU, and 5.22 × 10−6 RIU, respectively. The proposed hybrid SPR is a powerful tool for wide-dynamic-range RI sensing with various applications.

Common-mode plasmon sensing scheme as a high-sensitivity compact SPR sensor.

A deep metal grating enables quasi-phase-matched simultaneous excitation of two counterpropagating surface plasmon modes by means of its +1st and -2nd diffraction orders. The resulting angular reflection spectra of the scattered -1st and zeroth orders exhibit three interleaved zeros and maxima in a range centered around the Littrow angle. The spectra differ thoroughly from the usual reflection dip resulting from single-order plasmon coupling that produces strong absorption. The zeroth and -1st orders exhibit two crossing angles enabling high-sensitivity common-mode detection schemes designed to reject variations in source power and environmental noise. The proof of concept and experimental assessment of this new surface plasmon resonance (SPR) sensing scheme are demonstrated by monitoring gases in a pressure-controlled chamber. A limit of detection (LOD) of 2 × 10-7 refractive index unit (RIU) was achieved.

Estimation of Leaf Water Content of Different Leaves from Different Species Using Hyperspectral Reflectance Data

Water content of individual leaves or vegetation canopies is a significant variable in plant physiological processes. The water content of the vegetation leaves plays a very significant role. The ability of hyperspectral advanced technology to accurately evaluate leaf and canopy water content has improved large-scale measures. Due to the presence of water absorption band in near and SWIR wavelength range, electromagnetic spectrum will allow us to correctly measure the leaf water content. Three different parameters were used to describe the water status: Equivalent Water Thickness (EWT), Gravimetric Water Content (GWC), and Plant Water Concentration (PWC), with leaf multi angular reflectance spectrum, to find sensitive spectral indices, to correctly assess water content of leaves in a wide range of plant species. Using spectral indices derived from multi angular reflectance spectra, we looked into the possibility for predicting leaf water content of six species in the study area. To analyze the status of leaf water, three different forms of hyperspectral indices were evaluated, including the Simple Ratio (SR), Normalized ratio wavelength (ND) and Double Difference ratio (DDn). To look over the possibility of predicting the leaf water status of the species in the study field, we proposed four new indices. The results showed that EWT is comparatively more sensitive to trace leaf water status than GWC and PWC. The best-established EWT indices were (R905-R1795)/(R1905-R1935), R1350/R1390, (R840-R1565)/(R840+R1565) and (R925-R1625)/ (R925+R1625) and the performance of the proposed hyper-spectral indices surpassed the performance of other indices in this study. The mentioned indices were then further analyzed on LOPEX and ANGERS databases for validation of our suggested indices and we come up with better results. This study indicates that spectral indices can be used and could be more reliable to predict leaf water content, but future studies will need to include more plant species and field data. The newly developed indices can be used to estimate EWT using simple laboratory measurements, making them helpful for agricultural environmental sciences and ecology related studies.

Estimation of Leaf Water Content of Different Leaves from Different Species Using Hyperspectral Reflectance Data

Water content of individual leaves or vegetation canopies is a significant variable in plant physiological processes. The water content of the vegetation leaves plays a very significant role. The ability of hyperspectral advanced technology to accurately evaluate leaf and canopy water content has improved large-scale measures. Due to the presence of water absorption band in near and SWIR wavelength range, electromagnetic spectrum will allow us to correctly measure the leaf water content. Three different parameters were used to describe the water status: Equivalent Water Thickness (EWT), Gravimetric Water Content (GWC), and Plant Water Concentration (PWC), with leaf multi angular reflectance spectrum, to find sensitive spectral indices, to correctly assess water content of leaves in a wide range of plant species. Using spectral indices derived from multi angular reflectance spectra, we looked into the possibility for predicting leaf water content of six species in the study area. To analyze the status of leaf water, three different forms of hyperspectral indices were evaluated, including the Simple Ratio (SR), Normalized ratio wavelength (ND) and Double Difference ratio (DDn). To look over the possibility of predicting the leaf water status of the species in the study field, we proposed four new indices. The results showed that EWT is comparatively more sensitive to trace leaf water status than GWC and PWC. The best-established EWT indices were (R905-R1795)/(R1905-R1935), R1350/R1390, (R840-R1565)/(R840+R1565) and (R925-R1625)/ (R925+R1625) and the performance of the proposed hyper-spectral indices surpassed the performance of other indices in this study. The mentioned indices were then further analyzed on LOPEX and ANGERS databases for validation of our suggested indices and we come up with better results. This study indicates that spectral indices can be used and could be more reliable to predict leaf water content, but future studies will need to include more plant species and field data. The newly developed indices can be used to estimate EWT using simple laboratory measurements, making them helpful for agricultural environmental sciences and ecology related studies.

Nanoplasmonic methods in angular spectroscopy of nanoscale biological objects

The paper presents the results of calculating the angular spectra of light reflection under the condition of excitation of surface plasmons in the Kretschman scheme. The silver layer in this scheme plays the role of a reference material, the minimum in the angular spectrum of which serves as a reference point for the shift of the minimum of the angular spectrum when a layer of the studied biological material is added to the considered layered system, which were melanin and biological tissue. As a result of the work, specific pronounced minima in the angular spectra were obtained, which make it possible to identify these materials with a high degree of accuracy due to the narrow resonance peaks in the angular reflection spectra, which are spaced from the peak in the reflection from the silver film by certain angles. The method contains all the characteristic features of the resonance spectroscopy method; the conditions for the excitation of a surface plasmon at the metal boundary act as resonance conditions.

Quantitative Analysis of Thickness and pH Actuation of Weak Polyelectrolyte Brushes

Polymer brushes are widely used as surface coatings for various inert, functional, or responsive interfaces. If the polymer can alter its protonation state (a polyelectrolyte (PE)), the brush can switch between a collapsed and swollen state with pH, which enables applications such as nanoscale actuators. However, changes in brush height as the polymer alters its charge state are not straightforward to measure accurately. Here, we show how surface plasmon resonance can be used to determine the thickness of PE brushes both in their charged and neutral states. We use different methods to measure the heights of brushes consisting of poly(acrylic acid) and the polybasic poly(2-(diethylamino)ethyl methacrylate), both prepared by atom transfer radical polymerization. We find polymers in solution that can act as refractive index probes, which do not interact with the grafted polyelectrolytes, thus providing an “exclusion height” of the brush. Importantly, the angular reflection spectrum can be used to directly id...

Multiple resonant excitations of surface plasmons in a graphene stratified slab by Otto configuration and their independent tuning

Multiple resonant excitations of surface plasmons in a graphene stratified slab are realized by Otto configuration at terahertz frequencies. The proposed graphene stratified slab consists of alternating dielectric layers and graphene sheets, and is sandwiched between a prism and another semi-infinite medium. Optical response and field distribution are determined by the transfer matrix method with the surface current density boundary condition. Multiple resonant excitations appear on the angular reflection spectrum, and are analyzed theoretically via the phase-matching condition. Furthermore, the effects of the system parameters are investigated. Among them, the Fermi levels can tune the corresponding resonances independently. The proposed concept can be engineered for promising applications, including angular selective or multiplex filters, multiple channel sensors, and directional delivery of energy.

Couple molecular excitons to surface plasmon polaritons in an organic-dye-doped nanostructured cavity

In this work, we demonstrate experimentally the hybrid coupling among molecular excitons, surface plasmon polaritons (SPPs), and Fabry-Perot (FP) mode in a nanostructured cavity, where a J-aggregates doped PVA (polyvinyl alcohol) layer is inserted between a silver grating and a thick silver film. By tuning the thickness of the doped PVA layer, the FP cavity mode efficiently couples with the molecular excitons, forming two nearly dispersion-free modes. The dispersive SPPs interact with these two modes while increasing the incident angle, leading to the formation of three hybrid polariton bands. By retrieving the mixing fractions of the polariton band components from the measured angular reflection spectra, we find all these three bands result from the strong coupling among SPPs, FP mode, and excitons. This work may inspire related studies on hybrid light-matter interactions, and achieve potential applications on multimode polariton lasers and optical spectroscopy.

Label-Free Detection of Tumor Angiogenesis Biomarker Angiopoietin 2 Using Bloch Surface Waves on One Dimensional Photonic Crystals

We describe the design and fabrication of biochips based on 1-D photonic crystals supporting Bloch surface waves for label-free optical biosensing. The optical properties of Bloch surface waves are studied in relation to the geometry of the photonic crystals and on the properties of the dielectric materials used for the fabrication. The planar stacks of the biochips are composed of silica, tantala, and titania that were deposited using plasma-ion-assisted evaporation under high-vacuum conditions. The biochip surfaces were functionalized by silanization, and appropriate fluidic cells were designed to operate in an automated platform. An angularly resolved ellipsometric optical sensing apparatus was assembled to carry out the sensing studies. The angular operation is obtained by a focused laser beam at a fixed wavelength and detection of the angular reflectance spectrum by means of an array detector. The results of the experimental characterization of the physical properties of the fabricated biochips show that their characteristics, in terms of sensitivity and figure of merit, match those expected from the numerical simulations. Practical application of the sensor was demonstrated by detecting a specific glycoprotein, Angio-poietin 2, that is involved in angiogenesis and inflammation processes. The protocol used for the label-free detection of Angiopoietin 2 is described, and the results of an exemplary assay, carried out at a relatively high concentration of 1 μg/ml, are given and confirm that an efficient detection can be achieved. The limit of detection of the biochips for Angiopoietin 2, based on the protocol used, is 1.5 pg/mm2 in buffer solution. The efficiency of the label-free assay is confirmed by independent measurements carried out by means of confocal fluorescence microscopy.

Angularly resolved ellipsometric optical biosensing by means of Bloch surface waves.

In label-free biosensing, a continuous improvement of the limit of detection is necessary to resolve the small change of the surface refractive index produced by interacting biomolecules at a very small concentration. In the present work, optical sensors based on one-dimensional photonic crystals supporting Bloch surface waves are proposed and adopted for label-free optical biosensing. We describe the implementation of an angularly resolved ellipsometric optical sensing scheme based on Bloch surface waves sustained by tantala/silica multilayers. The angular operation is obtained using a focused beam at fixed wavelength and detection of the angular reflectance spectrum by means of an array detector. The results show that the experimental limit of detection for a particular photonic crystal design is 6.5 × 10(-7) refractive index units (RIU)/Hz(1/2) and further decrease could be obtained. For the first time, we report on the practical application of this technique to a cancer biomarker protocol that aims at the detection of a specific glycoprotein (angiopoietin 2) involved in angiogenesis and inflammation processes.

Omnidirectional reflectance and vanishing Brewster angle of disordered semiconductor nanopillar surface with gradually changed refractive index

The geometrical-factor dependent Brewster angle shift in angular reflection spectra of the subwavelength nanopillar roughened semiconductor antireflective surface is investigated. Modeling of the nanopillar roughened surface evidently accounts for the shift and vanishing of the high-contrast transverse magnetic (TM) TM-mode angular reflectance spectrum with enlarging nanopillar height. The Brewster angle down-shifts by 20° with corresponding reflectance minimum increasing from <0.01% to 5.3% as the height of nanopillar increases from 0 to 240 nm, and the Brewster angle vanishing phenomenon is persistently observed until its disappearance with the nanopillar height enlarging up to 240 nm, leaving a slightly increased reflectance at larger incident angles.

Experimental observation of the propagation-dependent beam profile distortion and Goos–Hänchen shift under the surface plasmon resonance condition

We accurately measure the profiles of a reflected divergent beam under a very common surface plasmon resonance configuration in both the near- and far-field regions. Through analysis of the normalized beam shapes, significant propagation-dependent distortion is experimentally revealed. The results are in very good agreement with the theoretical estimations, considering the nonlinear phase distribution within the angular range covered by the focused beam. The corresponding minimal positions of the angular reflectance spectrum measured at different locations could shift as much as a few tens of millidegrees. The propagation-dependent Goos–Hänchen shifts are also accurately obtained, and they significantly deviate from the estimates based on the traditional linear phase response but match very well with the simulation results from our model. This verifies our previous theoretical predictions of the propagation-dependent beam distortion, and it could have further implications for various sensing and device applications.

T-shaped plasmonic array as a narrow-band thermal emitter or biosensor

A T-shaped plasmonic array is proposed for application as an effective thermal emitter or biosensor. The reflection and thermal radiation properties of a T-shaped array are investigated theoretically. The angular dependent reflectance spectrum shows a clear resonant dip at 0.36 eV for full polar angles. No other significant localized resonant mode is found in the investigated spectral range from 0.12 eV to 0.64 eV. According to the Kirchhoff's law, the thermal radiation of the proposed structure can lead to a sharp peak at 3.5 microm with low sideband emission. We have also found that the T-shaped structure filled with organic material such as PMMA with different thicknesses (10 nm -50 nm) can lead to significant shift of the resonance wavelength. Thus, the T-shaped structure can also be used as a good sensor for organic materials.

Determination of dielectric constants of substrates and thin films using angular dependent IR reflection spectra

Three different methods for the treatment of angular dependent reflection spectra - Kramers-Kronig-Transformation, Direct Search of dielectric constants and Dispersion Analysis - were applied and the results compared. The Dispersion Analysis was used to determine the dielectric function of glass, a gel layer and the glass surface. The dielectric functions were improved by folding with a Gauss function.

IR transmission and reflection study of lamellar silicon grating-wafer structures

IR normal transmission and angular reflection spectra are studied for lamellar silicon grating-wafer structures. The grating dimensions are comparable with the radiation wavelengths 2.2–22 μm. The spectra were measured using the FTIR system. Numerical simulations of the spectra were carried out using an operator-vector version of rigorous-coupled wave analysis. The theoretical results are in agreement with the measured spectra. The effect of grating structure on the manifestation of two inherent light absorption mechanisms of bulk silicon-interstitial oxygen vibrations and lattice phonons—is discussed. The results indicate the possibility of obtaining a narrow spectral absorber.

Analysis of the morphology and the optical properties of sputtered molybdenum particles

The optical constants of thin sputtered molybdenum layers, embedded in a ceramic-metal composite produced by a batch sputtering deposition system were analyzed. This was accomplished by assuming a multilayer system for a tin oxide-molybdenum cermet and calculating the optical constants from angular and polarization dependent reflection and transmission spectra. These optical constants differ strongly from those for sputtered bulk material obtained ellipsometrically. A good agreement between measured effective refractive indices for cermets and effective medium calculation was found, if these optical constants for molybdenum were used in the effective medium calculations. Differences to the optical constants of the cermet determined ellipsometrically were explained by the birefringence of the cermet. The size and the shape of the embedded particles were investigated with an atomic force microscope.

Surface polariton in the ATR angular spectra of a thin iron film bounded by dielectric layers

The complex dielectric constant of Fe has a large imaginary part and small real part throughout most of the visible region (∊i ≫ |∊r|). Under this condition a thin film of Fe(≤ 20 nm), bounded by a dielectric, can support a surface polariton. For p-polarized fields the antisymmetric solution to the dispersion relation has a nearly real propagation vector parallel to the surface. Hence this antisymmetric polariton can be excited optically. We use the attenuated total reflection (ATR) configuration with three successive layers, cryolite-Fe-cryolite, evaporated onto the base of a glass prism. The surface polariton is detected as a narrow minimum in the ATR angular spectrum. We choose the frequency at which ∊r = 0 and then an incident p-polarized plane wave yields zero energy propagation in the Fe film in the direction parallel to the plane on the film. There are also other broader minima in the p-polarized spectrum corresponding to large field oscillations localized in the second dielectric. Minima in the s-polarized spectrum correspond to oscillations localized in the first dielectric. In addition the angular reflectance spectrum of both polarizations contains sharp maxima caused by resonance standing wave patterns which persist through the incident medium as well as the film system.