Simultaneous multi-laser, multi-species trace-level sensing of gas mixtures by rapidly swept continuous-wave cavity-ringdown spectroscopy

The greenhouse-gas molecules CO(2), CH(4), and H(2)O are detected in air within a few ms by a novel cavity-ringdown laser-absorption spectroscopy technique using a rapidly swept optical cavity and multi-wavelength coherent radiation from a set of pre-tuned near-infrared diode lasers. The performance of various types of tunable diode laser, on which this technique depends, is evaluated. Our instrument is both sensitive and compact, as needed for reliable environmental monitoring with high absolute accuracy to detect trace concentrations of greenhouse gases in outdoor air.

Design analysis of doped-silicon surface plasmon resonance immunosensors in mid-infrared range

This paper reports the design analysis of a microfabricatable mid-infrared (mid-IR) surface plasmon resonance (SPR) sensor platform. The proposed platform has periodic heavily doped profiles implanted into intrinsic silicon and a thin gold layer deposited on top, making a physically flat grating SPR coupler. A rigorous coupled-wave analysis was conducted to prove the design feasibility, characterize the sensor's performance, and determine geometric parameters of the heavily doped profiles. Finite element analysis (FEA) was also employed to compute the electromagnetic field distributions at the plasmon resonance. Obtained results reveal that the proposed structure can excite the SPR on the normal incidence of mid-IR light, resulting in a large probing depth that will facilitate the study of larger analytes. Furthermore, the whole structure can be microfabricated with well-established batch protocols, providing tunability in the SPR excitation wavelength for specific biosensing needs with a low manufacturing cost. When the SPR sensor is to be used in a Fourier-transform infrared (FTIR) spectroscopy platform, its detection sensitivity and limit of detection are estimated to be 3022 nm/RIU and ~70 pg/mm(2), respectively, at a sample layer thickness of 100 nm. The design analysis performed in the present study will allow the fabrication of a tunable, disposable mid-IR SPR sensor that combines advantages of conventional prism and metallic grating SPR sensors.

Infrared chemical imaging: spatial resolution evaluation and super-resolution concept

Chemical imaging systems help to solve many challenges in various scientific fields. Able to deliver rapid spatial and chemical information, modern infrared spectrometers using Focal Plane Array detectors (FPA) are of great interest. Considering conventional infrared spectrometers with a single element detector, we can consider that the diffraction-limited spatial resolution is more or less equal to the wavelength of the light (i.e. 2.5-25 microm). Unfortunately, the spatial resolution of FPA spectroscopic setup is even lower due to the detector pixel size. This becomes a real constraint when micron-sized samples are analysed. New chemometrics methods are thus of great interest to overcome such resolution drawback, while keeping our far-field infrared imaging spectrometers. The aim of the present work is to evaluate the super-resolution concept in order to increase the spatial resolution of infrared imaging spectrometers using FPA detectors. The main idea of super-resolution is the fusion of several low-resolution images of the same sample to obtain a higher-resolution image. Applying the super-resolution concept on a relatively low number of FPA acquisitions, it was possible to observe a 30% decrease in spatial resolution.

Compact broadband vibrational sum-frequency generation spectrometer with nonresonant suppression

A compact broadband vibrational sum-frequency spectroscopy (SFG) apparatus is described to study molecules at surfaces and interfaces. Using an étalon as the frequency narrowing device, the visible pulse has a time-asymmetric profile that allows the user to deeply suppress nonresonant background signals that hinder detection of molecular vibrational resonances. Several features of the spectrometer that, in aggregate, improve signal-to-noise ratios by a large factor are described. The spectrometer features a series of interchangeable prealigned sample holders for different applications. Examples of applications are presented where nonresonant suppression greatly improves the ability to study adsorbates on single-crystal surfaces as a function of rotation about the azimuth, and where the rapid data acquisition abilities of the spectrometer are used to study electrochemical transformations on single-crystal electrodes.

Rapid near-infrared fluorescence excitation-emission matrix spectroscopy for multifluorophore characterization using an acousto-optic tunable filter technique

We report on a novel acousto-optic tunable filter (AOTF)-based near-infrared (NIR) fluorescence excitation-emission matrix (EEM) spectroscopy technique for rapid multifluorophore characterization. We implement a unique light filtering module design by using cascaded AOTFs coupled with three orthogonally oriented polarizers to effectively remove the side-ripple artifacts of AOTFs as well as by using a pair of AOTFs coupled with two orthogonally oriented polarizers to improve detection efficiency for high-quality fluorescence EEM acquisitions. NIR fluorescence EEM spectroscopy (41 excitation wavelengths ranging from 550 to 950 nm in 10-nm increments; fluorescence emission from 570 to 1000 nm at 10-nm intervals) can be acquired from fluorescence dyes [e.g., diethylthiatricarbocyanine (DTTC) iodide, oxazine 750, and IR 140] within 10 s or even less, illustrating the potential of the AOTF-based NIR EEM technique developed for rapid multifluorophore analysis and characterization in biochemical and biomedical systems.

Terahertz pulsed spectroscopy and imaging in the pharmaceutical setting - a review

Terahertz pulsed spectroscopy (TPS) and terahertz pulsed imaging (TPI) are two novel techniques for the physical characterization of pharmaceutical drug materials and final solid dosage forms, utilizing spectral information in the far infrared region of the electromagnetic spectrum. This review focuses on the development and performance of pharmaceutical applications of terahertz technology compared with other tools for physical characterization. TPS can be used to characterize crystalline properties of drugs and excipients. Different polymorphic forms of a drug can be readily distinguished and quantified. Recent developments towards a better understanding of the fundamental theory behind spectroscopy in the far infrared have been discussed. Applications for TPI include the measurement of coating thickness and uniformity in coated pharmaceutical tablets, structural imaging and 3D chemical imaging of solid dosage forms.

Infrared differential-absorption Mueller matrix spectroscopy and neural network-based data fusion for biological aerosol standoff detection

An active spectrophotopolarimeter sensor and support system were developed for a military/civilian defense feasibility study concerning the identification and standoff detection of biological aerosols. Plumes of warfare agent surrogates gamma-irradiated Bacillus subtilis and chicken egg white albumen (analytes), Arizona road dust (terrestrial interferent), water mist (atmospheric interferent), and talcum powders (experiment controls) were dispersed inside windowless chambers and interrogated by multiple CO(2) laser beams spanning 9.1-12.0 microm wavelengths (lambda). Molecular vibration and vibration-rotation activities by the subject analyte are fundamentally strong within this "fingerprint" middle infrared spectral region. Distinct polarization-modulations of incident irradiance and backscatter radiance of tuned beams generate the Mueller matrix (M) of subject aerosol. Strings of all 15 normalized elements {M(ij)(lambda)/M(11)(lambda)}, which completely describe physical and geometric attributes of the aerosol particles, are input fields for training hybrid Kohonen self-organizing map feed-forward artificial neural networks (ANNs). The properly trained and validated ANN model performs pattern recognition and type-classification tasks via internal mappings. A typical ANN that mathematically clusters analyte, interferent, and control aerosols with nil overlap of species is illustrated, including sensitivity analysis of performance.

Spherically shaped micro-structured antireflective surfaces

An antireflecting micro-structured interface, working in the resonance domain, and made from a bi-periodic array of semi-spherical hollowing-out in a silicon substrate is presented. Its parameters such as sphere radius and position of sphere centers from the surface are optimized numerically. A simple and robust process is described allowing such kind of antireflective surfaces to be fabricated for the infrared range. Spectral and angular reflectance measurement demonstrates the efficiency of the antireflective micro-structured interface which can easily be adapted for the visible range and for photovoltaic applications by a simple homothetic modification of the micro-structure typical dimensions.

A high spectral sensitivity interferometer based on the dispersive property of the semiconductor GaAs

We develop an interferometer which has high spectral sensitivity based on the dispersive property of the semiconductor GaAs in the near-infrared region. Our experiment demonstrates that the spectral sensitivity could be greatly enhanced by adding a slow light medium into the interferometer and is proportional to the group index of the material. Subsequently the factors which influence the spectral sensitivity of the interferometer are analyzed. Moreover, we provide potential applications of such interferometers using the dispersive property of semiconductor in whole infrared region.

Enhanced 2.0 microm emission and gain coefficient of transparent glass ceramic containing BaF2: Ho3+,Tm3+ nanocrystals

Transparent glass ceramic containing BaF(2):Ho(3+),Tm(3+) nanocrystals has been prepared by melt quenching and subsequent thermal treatment. The precipitation of BaF(2) nanocrystals was confirmed by X-ray diffraction and high-resolution transmission electron microscopy. Intense 2.0 microm fluorescence originating from Ho(3+): (5)I(7) --> (5)I(8) transition was achieved upon excitation with 808 nm laser diode. A large ratio of forward Tm(3+) --> Ho(3+) energy transfer constant to that of backward process indicated high efficient energy transfer from Tm(3+)((3)F(4)) to Ho(3+)((5)I(7)), benefited from the reduced ionic distances of Tm(3+)-Tm(3+) and Tm(3+)-Ho(3+) pairs and low phonon energy environment with the incorporation of rare-earth ions into the precipitated BaF(2) nanocrystals. The results indicate that glass ceramic is a promising candidate material for 2.0 microm laser.

Infrared imaging with a wavefront-coded singlet lens

We describe the use of wavefront coding for the mitigation of optical aberrations in a thermal imaging system. Diffraction-limited imaging is demonstrated with a simple singlet which enables an approximate halving in length and mass of the optical system compared to an equivalent two-element lens.

Suppression of perturbed free-induction decay and noise in experimental ultrafast pump-probe data

We apply a Fourier filtering technique for the global removal of coherent contributions, like perturbed free-induction decay, and noise, to experimental pump-probe spectra. A further filtering scheme gains access to spectra otherwise only recordable by scanning the probe's center frequency with adjustable spectral resolution. These methods cleanse pump-probe data and allow improved visualization and simpler analysis of the contained dynamics. We demonstrate these filters using visible pump/mid-infrared probe spectroscopy of ligand dissociation in carboxyhemoglobin.

Demountable liquid/flow cell for in vivo infrared microspectroscopy of biological specimens

We have developed a liquid/flow cell/chamber allowing infrared measurements of living biological specimens with high spatial resolution under a controlled aqueous environment. This flow chamber features sub-micrometer thick diamond windows exhibiting low spherical and chromatic aberrations. Diamond has excellent transmission properties and minimal dispersion over the entire mid-infrared and visible spectral ranges. In contrast to current commercially available infrared liquid chambers, the flow chamber has a slim profile, which accommodates high resolution/magnification microscope objectives with small working distances, down to 0.6 mm above the chamber and 6 mm below the flow chamber. We have coupled a pump to the flow chamber to provide medium exchange. As an example, we present microspectroscopic infrared maps and spectra of the freshwater green alga Micrasterias sp. in the new flow chamber and compare them to maps obtained with a conventional liquid chamber. Pulse-amplitude-modulated fluorescence measurements on Micrasterias sp. cells inside the new flow chamber have been evaluated to demonstrate the viability of the algal cells.

Investigation of a probe design for facilitating the uses of the standard photon diffusion equation at short source-detector separations: Monte Carlo simulations

We design a special diffusing probe to investigate the optical properties of human skin in vivo. The special geometry of the probe enables a modified two-layer (MTL) diffusion model to precisely describe the photon transport even when the source-detector separation is shorter than 3 mean free paths. We provide a frequency domain comparison between the Monte Carlo model and the diffusion model in both the MTL geometry and conventional semiinfinite geometry. We show that using the Monte Carlo model as a benchmark method, the MTL diffusion theory performs better than the diffusion theory in the semiinfinite geometry. In addition, we carry out Monte Carlo simulations with the goal of investigating the dependence of the interrogation depth of this probe on several parameters including source-detector separation, sample optical properties, and properties of the diffusing high-scattering layer. From the simulations, we find that the optical properties of samples modulate the interrogation volume greatly, and the source-detector separation and the thickness of the diffusing layer are the two dominant probe parameters that impact the interrogation volume. Our simulation results provide design guidelines for a MTL geometry probe.

Design and performance of a flow-through polarization-modulation infrared reflection-absorption spectroscopy cell for time-resolved simultaneous surface and liquid phase detection under concentration and temperature perturbations

Design and performance of a flow-through cell for polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) suitable for simultaneous monitoring of species on surface and in liquid phase on a molecular level at a high time resolution (ca. 1 s) are presented. In particular, the cell was designed to allow periodic concentration and temperature perturbations and thus excite physicochemical phenomena of interest occurring at solid-liquid interfaces. Utilizing the perturbations and spectral responses of both surface and liquid phase species, their dynamic behavior, kinetics, and correlations can be studied. The detection sensitivity is greatly enhanced by the data processing employed in modulation excitation spectroscopy (MES). The cell design is based on a theoretical model. The IR beam path through a multiple-phase system consisting of air, prism, and liquid as well as light reflection at the surface of a sample were considered in order to maximize the detected IR light intensity and absorption by surface molecules. Its high surface sensitivity was demonstrated by CO adsorption on a thin Pt film in a liquid phase. Combination of the PM-IRRAS with concentration MES led to a significant sensitivity enhancement for the detection of surface and liquid phase species. The temperature, tunable in a wide range from 263-343 K, could be controlled within an accuracy of 0.1 K and also modulated periodically in a completely reversible manner, thus allowing accurate temperature MES experiments. With these capabilities, dynamic physicochemical processes at solid-liquid interfaces can be sensitively investigated.

Experimental investigation of evanescence-based infrared biodetection technique for micro-total-analysis systems

The advent of microoptoelectromechanical systems (MOEMS) and its integration with other technologies such as microfluidics, microthermal, immunoproteomics, etc. has led to the concept of an integrated micro-total-analysis systems (microTAS) or Lab-on-a-Chip for chemical and biological applications. Recently, research and development of microTAS have attained a significant growth rate over several biodetection sciences, in situ medical diagnoses, and point-of-care testing applications. However, it is essential to develop suitable biophysical label-free detection methods for the success, reliability, and ease of use of the microTAS. We proposed an infrared (IR)-based evanescence wave detection system on the silicon-on-insulator platform for biodetection with microTAS. The system operates on the principle of bio-optical interaction that occurs due to the evanescence of light from the waveguide device. The feasibility of biodetection has been experimentally investigated by the detection of horse radish peroxidase upon its reaction with hydrogen peroxide.

A perspective on medical infrared imaging

Since the early days of thermography in the 1950s, image processing techniques, sensitivity of thermal sensors and spatial resolution have progressed greatly, holding out fresh promise for infrared (IR) imaging techniques. Applications in civil, industrial and healthcare fields are thus reaching a high level of technical performance. The relationship between body temperature and disease was documented since 400 bc. In many diseases there are variations in blood flow, and these in turn affect the skin temperature. IR imaging offers a useful and non-invasive approach to the diagnosis and treatment (as therapeutic aids) of many disorders, in particular in the areas of rheumatology, dermatology, orthopaedics and circulatory abnormalities. This paper reviews many usages (and hence the limitations) of thermography in biomedical fields.

Experimental study of high sensitivity infrared spectrometer with waveguide-based up-conversion detector(1)

We have developed an up-conversion spectrometer for signals at single photon levels near the infrared region based on a tunable up-conversion detector that uses a periodically poled lithium niobate waveguide as the conversion medium. We also experimentally studied its characteristics including sensitivity, dark count rate, spectral scan speed, signal transfer function of the waveguide, and polarization sensitivity. The overall single photon detection efficiency of the up-conversion spectrometer is about 32%. With its ultra high sensitivity the spectrometer can measure spectra for signals at a level as low as -126 dBm. We have demonstrated the spectrometers high sensitivity by measuring the spectrum of a greatly attenuated multimode emission from a laser diode at the 1310 nm band.

Infrared consequence spectroscopy of gaseous protonated and metal ion cationized complexes

In this article, the new and exciting techniques of infrared consequence spectroscopy (sometimes called action spectroscopy) of gaseous ions are reviewed. These techniques include vibrational predissociation spectroscopy and infrared multiple photon dissociation spectroscopy and they typically complement one another in the systems studied and the information gained. In recent years infrared consequence spectroscopy has provided long-awaited direct evidence into the structures of gaseous ions from organometallic species to strong ionic hydrogen bonded structures to large biomolecules. Much is being learned with respect to the structures of ions without their stabilizing solvent which can be used to better understand the effect of solvent on their structures. This review mainly covers the topics with which the author has been directly involved in research: structures of proton-bound dimers, protonated amino acids and DNA bases, amino acid and DNA bases bound to metal ions and, more recently, solvated ionic complexes. It is hoped that this review reveals the impact that infrared consequence spectroscopy has had on the field of gaseous ion chemistry.

IR absorption and reflectometric interference spectroscopy (RIfS) combined to a new sensing approach for gas analytes absorbed into thin polymer films

Hydrophobic polymer layers (3 microm) were spin-coated on Si or Ge plates and placed in a flow through gas chamber. FTIR reflection spectra of the layers were recorded showing the characteristic IR absorption bands of the polymer and the interference pattern generated by layered structure of the polymer film. Upon exposure of the polymer layer to gaseous analytes enrichment in the polymer film occurred. This was evidenced by the appearance of analyte specific absorption particular in the mid-IR part of the spectrum, as well as by a shift in the interference pattern across the whole spectrum. Qualitative information concerning the analyte was accessible in the mid-IR part of the spectrum, whereas quantitative assessment was obtained from the interference pattern. Polyetherurethane, polydimethylsiloxane, Makrolon and polyisobutylene polymer layers were tested for such IR-RIfS measurements, whereas toluene, o-dichlorobenzene, m-xylene, ethyl acetate and cyclohexane were employed as analytes. There was no influence of water vapour neither on the IR absorptions nor the interference pattern as hydrophobic polymers were used.

High-throughput spectroscopic imaging applied to permeation through the skin

Infrared (IR) spectroscopy has been successfully applied to study the permeation of substances through human skin in a high-throughput manner. The sample of skin was placed on the measuring surface of an attenuated total reflection (ATR) crystal and was divided into several areas. These areas were separated using a specially designed grid created on the surface of the skin and each area was subjected to a different combination of permeant and enhancer. ATR Fourier transform infrared (FT-IR) imaging was applied to measure the permeation of 12 liquid samples through a piece of skin smaller than 5 cm(2). This work demonstrated that, using the ATR-FT-IR imaging method, it is possible to measure and directly compare the transdermal processes of several permeants under identical conditions.

Probing water and biomolecules at the air-water interface with a broad bandwidth vibrational sum frequency generation spectrometer from 3800 to 900 cm(-1)

We have built a broad bandwidth vibrational sum frequency generation (VSFG) spectrometer that can provide high-quality spectra over the range of 3800 to 900 cm(-1). The spectrometer contains a commercial Ti:sapphire based 6 W regenerative amplifier as the master light source, a home-built pulse shaper to produce a narrow bandwidth 800 nm beam, a commercial optical parametric amplifier to generate a broad bandwidth femtosecond infrared (IR) pulse, and a detection system with a monochromator and a charge-coupled device (CCD). We applied this spectrometer to obtain VSFG spectra of a lipid monolayer at the air-water interface in the O-H stretching region (3800-3000 cm(-1)), the C-H stretching region (3100-2700 cm(-1)), the C-D stretching region (2300-2000 cm(-1)), the C=O stretching region (1800-1700 cm(-1)), and the PO(2)(-) symmetric stretching region (1200-1000 cm(-1)). We also obtained the VSFG spectrum of neat water in the O-H stretching region (3800-3000 cm(-1)) and the VSFG spectrum of a protein, alpha-synuclein, in the amide I region (1700-1600 cm(-1)) at the air-water interface. The spectrometer can provide a VSFG spectrum in the O-H stretching region (3800-3000 cm(-1)) without scanning the IR frequency. This feature will be useful in probing water dynamics at interfaces because the free OH and H-bonded OH can be investigated simultaneously. We have also provided instrumental details and discussed further improvements that should be beneficial to other researchers interested in setting up VSFG instrumentation.

A broadband continuous-wave multichannel near-infrared system for measuring regional cerebral blood flow and oxygen consumption in newborn piglets

Near-infrared spectroscopy (NIRS) is a promising technique for assessing brain function in newborns, particularly due to its portability and sensitivity to cerebral hemodynamics and oxygenation. Methods for measuring cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO(2)) have been developed based on broadband continuous-wave NIRS. However, broadband NIRS apparatus typically have only one detection channel, which limits their applicability to measuring regional CBF and CMRO(2). In this study, a relatively simple multiplexing approach based on electronically controlled mechanical shutters is proposed to expand the detection capabilities from one to eight channels. The tradeoff is an increase in the sampling interval; however, this has negligible effects on CBF measurements for intervals less than or equal to 1 s. The ability of the system to detect focal brain injury was demonstrated in piglets by injecting endothelin-1 (ET-1) into the cerebral cortex. For validation, CBF was independently measured by computed tomography (CT) perfusion. The average reduction in CBF from the source-detector pair that interrogated the injured region was 51%+/-9%, which was in good agreement with the CBF reduction measured by CT perfusion (55%+/-5%). No significant changes in regional CMRO(2) were observed. The average regional differential pathlength prior to ET-1 injection was 8.4+/-0.2 cm (range of 7.1-9.6 cm) and did not significantly change after the injury.