The Attenuated Total Reflection Method

The advancement of thin-film electro-optic (EO) materials and devices depends on reliable characterization and a thorough understanding of structure–property relationships that govern their EO performance. Here, we reinvent the attenuated total reflection (ATR) technique using the prism coupler method, which was proposed over three decades ago but remained largely underdeveloped, for standard characterization of thin-film EO coefficients (r₃₃ and r₁₃). The ATR technique detects the optical modes of a sharp drop in reflectivity (R) at specific critical angles in the spectrum, caused by the evanescent wave at the prism/air interface coupling into the slab waveguide through optical tunneling. For EO waveguides, substantial reflectivity change (ΔR) of ATR modes can be induced by small refractive index changes (Δn of 10^-4 to 10^-3 ) of the Pockels effect at the extrema of first or second derivatives, enabling efficient lock-in-free detection and determination of r-coefficients. Further multi-mode and frequency-selective ATR measurements on benchmark EO materials, including thin-film lithium niobate (TFLN) and EO polymer SEO100, led to the accurate determination of r-coefficients. The study validates the Gaussian function as a better model for representing the ATR spectra of waveguide films, providing a global approximation of optical modes than the traditional Taylor expansions. Compared to the Teng-Man reflection technique, the new ATR technique stands out for its improved accuracy, high traceability, effective self-calibration, simple instrumentation, rapid data processing, quantitative determination of both surface plasmon resonance mode and dielectric slab waveguide modes, and for organic electro-optic materials, obtaining primary molecular properties of push-pull chromophores for materials development.

The infrared spectra of thin solid films of carotenoids on supports were investigated using multiple attenuated total reflection (ATR) spectroscopy technique. The specimens were obtained by the evaporation of 1 or 2 drops of appropriate solution of carotenoid on the both sides of quickly rotating ATR element. The carotenoids investigated were β-carotene, canthaxanthin, and lycopene. Retinal and retinyl acetate as related compounds were also studied. Multiple ATR infrared spectra of amorphous and crystalline films were recorded and compared with the corresponding spectra of the compounds in solutions and in bulk crystalline state (in KBr disks). The spectra of amorphous films were shown to be almost identical to the spectra of investigated compounds in solutions whereas in the spectra of crystalline (or partly crystalline) films the same band splittings as in the spectra of crystalline specimens appear. The values of the effective film thickness were calculated for all the compounds investigated and it was shown that the films studied were thin surface ones. The ATR technique was applied to the investigation of reactivity of the polyenes, patrticularly with the atmospheric oxygen in the dark and under illumination by visible and ultraviolet light. The oxidation is accompained by the graduate disappearence of the δ=C-H absorption band at 966 cm−1. The consumption of polyenic double bonds is accompained by the formation of carbonyl and hydroxyl absorption bands. Mechanisms of the initial stages of the process are proposed for different polyenes.

An optical fiber sensor based on attenuated total reflectance (ATR) for extraction chemical information from highly scattering turbid materials has been evaluated. The influence of particles on bulk absorption and ATR transmitted spectra of micron-sized graphite flakes and spherical glassy carbon suspensions were investigated. The ATR transmitted spectra of coiled fiber-optic sensor in those suspensions with various concentrations are insensitive to scattering of suspended particles, especially for graphite flake suspensions. The reason for different influence of graphite flakes and spherical glassy carbon particles suspensions on e ATR spectra analyzed. This study demonstrates that fiber-optic sensor based on ATR technique is a feasible technique in application for monitoring turbid suspensions.

Numerical evaluation of the Kramers-Kronig analysis used in reflection spectroscopy has been carried out in order to find out optimal experimental conditions that minimize errors on complex refractive indices. By the use of the spectral simulation and the complex refractive index based on dispersion theory, we compared six kinds of experimental setups: external (EXT), internal (INT), and attenuated total reflection (ATR) geometries for both perpendicular (s)-and parallel (p)-polarized incident beams. Applying the so-called Dignam method to the simulated spectra, we evaluated the errors on the complex refractive indices. As far as physical and chemical properties of a sample are not taken into account, following conclusions are obtained: (1) ATR technique with p-polarization gives the smallest error at an optimal incident angle near by the critical angle.(2) If two kinds of measurements are permitted, the combined use of INT with s-polarization and ATR with p-polarization minimizes the error.

In the course of developing EO polymers, a convenient and fast method to obtain the electrooptic (EO) coefficients, r33 and r13, is highly desirable. Some of the existing EO measurement techniques such as Fabry-Perot interferometry(1) and ellipsometry(2) require metal deposition and electrode processing for each test sample. Therefore, they are not suited for daily sample testing. Second harmonic generation (SHG) is an indirect method to measure the EO coefficients(3). It is usually performed with 1.064 Nd:YAG laser and encounters difficulties with polymers that contain high µβ chromophores because these chromophores usually have red-shifted absorption peaks that causes the Kleiman symmetry, a fundamental assumption of this technique, to break down. Attenuated total reflection (ATR) can directly measure the EO coefficients with no restriction on the wavelength of the absorption peak. One measurement scan provides the refractive index, the thickness, and an EO coefficient. Typically, the thin film electrode in contact with the test sample is made of Au or Ag in conventional ATR techniques(4). These metal thin films are soft and not durable for repeated measurements. The curve fitting algorithm for data processing(4, 5) is also inconvenient for fast sample evaluation. In this paper, a modified ATR technique for routine EO measurement is presented which does not require electrode preparation for each test sample and uses a simple algorithm for data processing.

By combining attenuated total reflection (ATR) Fourier transform infrared (FTIR) difference spectroscopy with perfusion of chemical compounds on membrane proteins deposited on the surface of an internal reflection prism, we demonstrate that redox transitions in various bioenergetic systems can be probed. After partial drying of reaction centers (RCs) from Rb. sphaeroides on the ATR prism, the film was rehydrated with buffer. The IR beam penetrates the ATR prism and the evanescent wave propagates through the sample film which is enclosed in a flow cell. The rRedox state changess in the RCs film were manipulated by changing the chemical composition (ferricyanide, ferrocyanide or dithionite) of the flowing buffer. Illumination of the same film was also performed to generate the P+QA- charge-separated state. Chemically-induced ATR/FTIR difference spectra were recorded ofcorresponding to (a) the oxidation of the primary donor (P+/P); and to (b) the reduction of the primary quinone acceptor (QA-/QA); were obtained, as well as ; as well as (c) the light-induced P+QA-/PQA difference spectrum. The calculated QA-/QA spectrum (c-a) agrees with the chemically-induced spectrum (b). ATR/FTIR spectroscopic studies on RCs from Rps. viridis wand PS I will be also presented. More generally, the perfusion-induced ATR/FTIR technique can be widely applied to investigate molecular vibrational changes triggered by redox- and pH-changes, or ligand binding.

Surface melting and crystallization behavior of polyhydroxyalkanoates studied by attenuated total reflection infrared spectroscopy

We show the existence of an inherent property of evanescent electromagnetic waves: spin-momentum locking, where the direction of momentum fundamentally locks the polarization of the wave. We trace the ultimate origin of this phenomenon to complex dispersion and causality requirements on evanescent waves. We demonstrate that every case of evanescent waves in total internal reflection, surface states and optical fibers/waveguides possesses this intrinsic spin-momentum locking. We also introduce a universal right-handed triplet consisting of momentum, decay and spin for evanescent waves. We derive the Stokes parameters for evanescent waves which reveal an intriguing result - every fast decaying evanescent wave is inherently circularly polarized with its handedness tied to the direction of propagation. We also show the existence of a fundamental angle associated with total internal reflection (TIR) such that propagating waves locally inherit perfect circular polarized characteristics from the evanescent wave. This circular TIR condition occurs if and only if the ratio of permittivities of the two dielectric media exceeds the golden ratio. Our work leads to a unified understanding of this spin-momentum locking in various nanophotonic experiments and sheds light on the electromagnetic analogy with the quantum spin hall state for electrons.

Phospholipid-protein balance in affective disorders: Analysis of human blood serum using Raman and FTIR spectroscopy. A pilot study

Fourier transform infrared (FTIR) spectroscopy is an experimental technique used initially for qualitative and quantitative analysis of organic compounds, providing specific information on molecular structure, chemical bonding and molecular environment. For many years, FTIR has been successfully employed for studying a wide variety of proteins, enzymes, nucleic acids, lipids and glycolipids and photobiological systems. Time‐resolved (tr)‐FTIR spectroscopy can monitor reactions of the amino acids, the ligands and specific water molecules in the active centre of a protein in the time range from nanoseconds to seconds, thereby providing a detailed understanding of the molecular reaction mechanism. Key Concepts: FTIR provides several advantages compared to other spectroscopic techniques. FTIR spectroscopy is widely used for the investigation of secondary protein structure. To obtain information on individual groups, we have to look at difference spectra. In order to derive information on the reaction mechanism, the infrared bands have to be assigned to molecular groups of the protein. The QM/MM (quantum mechanics/molecular mechanics) approach can be used to calculate the infrared spectra of proteins. With the ATR (attenuated total reflection) technique it is possible to immobilise a protein on a surface and study reactions with ligands or other proteins. Biochemical changes of tissues and body fluids can be monitored using FTIR. By means of infrared microscopes spatial resolved information with a resolution of a few micrometer can be obtained.

We report on a theoretical and experimental study of the energy transfer between an optical evanescent wave, propagating in vacuum along the planar boundary of a dielectric material, and a beam of sub-relativistic electrons. The evanescent wave is excited via total internal reflection in the dielectric by an infrared (λ = 2 μm) femtosecond laser pulse. By matching the electron propagation velocity to the phase velocity of the evanescent wave, energy modulation of the electron beam is achieved. A maximum energy gain of 800 eV is observed, corresponding to the absorption of more than 1000 photons by one electron. The maximum observed acceleration gradient is 19 ± 2 MeV/m. The striking advantage of this scheme is that a structuring of the acceleration element's surface is not required, enabling the use of materials with high laser damage thresholds that are difficult to nano-structure, such as SiC, Al2O3 or CaF2.

We presented a micrometer size tapered fiber tip for real-time refractive index measurement of live cell. The tip is cut with a specific tilted angle. Only the cutting surface of tip apex is coated with gold nanoparticles (GNPs) by selective bonding method. The evanescent wave at the apex will excite the localized surface plasmon resonance (LSPR) of GNPs. With enhanced scattering at the resonance wavelength, the tip reflection will experience a loss, which is analogous to attenuated total reflection (ATR). By measure the reflection beam, a wavelength dependent loss dip in the reflection spectrum or an intensity loss of a specific wavelength could be used to detect the refractive index of live cell.

Surface enhanced infrared absorption spectroscopy based on gold nanostars and spherical nanoparticles

Strong anion-exchange materials carry a positive charge that allows them to trap and concentrate anions while releasing other anions. Here, we introduced a quaternary ammonium anion exchange group into mesoporous silica films coated on attenuated total reflection (ATR) crystals to enrich nitrate from aqueous phase in the volume probed by the evanescent field. The ion-exchange and enrichment capabilities of the films were characterized using FTIR spectroscopy. Thereby fast analyte enrichment and full sensor recovery were observed. In addition, high enrichment factors of up to 1600 were achieved. After characterization, these coated ATR crystals were used in a dedicated ATR-IR filtometer comprising a Fabry-Pérot filter detector unit and a miniaturized thermal emitter with a overall dimensions of only 80 mm × 120 mm × 70 mm. The filter covered the spectral region between 1250–1800 cm−1 allowing for recording IR spectra of nitrate enriched into the mesoporous silica film. The sensor was calibrated using the Langmuir adsorption model as calibration function. From this a limit of detection of 1.2 mg L−1 was derived for the ATR-IR filtometer. This emphasizes the high potential of functionalized mesoporous silica films combined with low-cost filtometers for portable water sensors.

We have demonstrated the application of broadband absorption spectroscopy in a liquid-core optical ring resonator. An initial proof of concept of the broadband liquid-core optical ring resonator (BLCORR) was constructed using a thinned-wall, 250-μm-inner-diameter fused silica capillary, tapered multimode optical fibers for input and output coupling, and a light-emitting diode (LED) source. When compared with standard cuvette measurements, an apparent path length as high as 5 cm was observed for methylene blue (MB). MB is a cationic dye that exhibits strong surface interaction with bare silica. Bromothymol blue (BTB), on the other hand, has a similar absorbance spectrum but does not share this same surface activity. On comparing these two dyes, the apparent path length for MB was found to reach more than 50 times that of BTB, confirming the expectation that the sensing region being probed is largely within the evanescent field at the inner surface of the capillary. The BLCORR may also inherit, from attenuated total reflection (ATR) spectroscopy, the ability to analyze highly concentrated chromophores. Concentrations of BTB as high as 10(-2) and 10(-3) M were easily distinguished from each other at the λ(max) in the BLCORR, whereas this was not the case in a 4-mm cuvette cell. Our presented device employs commercially available materials and could incorporate well into microfluidic systems. These benefits, along with the demonstrated ability to take enhanced surface absorbance measurements in a capillary, give the BLCORR potential in a variety of applications.