Search

An all-optical photoinduced lensing method is used to excite and monitor acoustic waves in liquids. Following optical absorption, the laser pulse induces a localized temperature gradient that launches pressure waves in the excited region at the nanosecond time scale. This generates a lens-like optical element in the sample. A probe laser beam senses the refractive index change due to the acoustic and thermal effects. Piezo-optic and thermo-optic coefficients govern how the refractive index of a material changes in response to mechanical stress and temperature variations, respectively. These effects are connected to the physical properties of the liquids and can be accessed by theoretically describing the intensity signal. A complete set of physical properties of ten liquids are quantitatively described in this work. These effects find applications in a wide range of fields, from optical communication, ultrasonic imaging, and sensing to adaptive optics and fundamental research. [ABSTRACT FROM AUTHOR]

The investigation of molecules in the mid-IR spectral range has revolutionized our understanding in many fields such as atmospheric chemistry and environmental sensing for climate research or disease monitoring in medical diagnosis. While the mid-IR analysis of gas-samples is already a mature discipline, the spectroscopy of liquids is still in its infancy. However, it is a rapidly developing field of research, set to fundamentally change our knowledge of dynamical processes of molecules in liquid-phase. In this field, mid-IR plasmonics has emerged as breakthrough concept for miniaturization, enabling highly-sensitive and -selective liquid measurement tools. In this review, we give an overview over current trends and recent developments in the field of mid-IR spectroscopy of molecules in liquid phase. Special attention is given to plasmon-enhanced concepts that allow measurements in highly compact sensor schemes. Nowadays, they reach full monolithic integration, including laser, interaction section and detector on the same chip, demonstrating unprecedented operation in situ and real-time analysis of chemical processes. [ABSTRACT FROM AUTHOR]

Laser-based mid-infrared (mid-IR) photothermal spectroscopy (PTS) represents a selective, fast, and sensitive analytical technique. Recent developments in laser design permits the coverage of wider spectral regions in combination with higher power, enabling for qualitative reconstruction of broadband absorption features, typical of liquid or solid samples. In this work, we use an external cavity quantum cascade laser (EC-QCL) that emits in pulsed mode in the region between 5.7 and 6.4 µm (1770–1560 cm−1), to measure the absorption spectrum of a thin film of polymethyl methacrylate (PMMA) spin-coated on top of a silicon nitride (Si3N4) micro-ring resonator (MRR). Being the PTS signal inversely proportional to the volume of interaction, in the classical probe–pump dual beam detection scheme, we exploit a Si3N4 transducer coated with PMMA, as a proof-of-principle for an on-chip photothermal sensor. By tuning the probe laser at the inflection point of one resonance, aiming for highest sensitivity, we align the mid-IR beam on top of the ring’s area, in a transversal configuration. To maximize the amplitude of the photoinduced thermal change, we focus the mid-IR light on top of the ring using a Cassegrain reflector enabling for an optimal match between ring size and beam waist of the excitation source. We briefly describe the transducer design and fabrication process, present the experimental setup, and perform an analysis for optimal operational parameters. We comment on the obtained results showing that PTS allows for miniaturized robust sensors opening the path for on-line/in-line monitoring in several industrial processes.

This paper presents a simple, highly efficient method for analyzing single-walled carbon nanotube (SWNT) bundles based on (1) ultrasound-assisted solubilization/dispersion of SWNTs in the ionic liquid 1-butyl-3-methylimidazolium tetrafluoroborate, (2) encapsulation of the nanotubes in sodium dodecyl sulfate micelles, and (3) analysis by capillary electrophoresis. The process by which SWNTs disperse in the ionic liquid was studied by Raman spectroscopy. No degradation of SWNTs was observed under mild sonication conditions. The shape and position changes observed in the Raman spectral bands for the nanotubes are ascribed to debundling and interaction with the ionic liquid. Separation of solubilized SWNTs was accomplished by using a 50 mM formic acid solution at pH 2.0 as background electrolyte and a potential of -10 kV. Under these conditions, separation was completed within only 4 min. Eighteen peaks for SWNTs were identified in the analysis of commercial SWNT bundles. The two types of bundles studied exhibited distinct, highly characteristic electrophoretic profiles which could be used to control SWNTs purity.

A Fabry-P6rot quantum cascade laser (QCL) was used as a powerful light source for mid infrared (MIR) detection in flow injection analysis. The QCL lased at several wavelengths close to each other within a few wavenumbers (990-1010 cm(super -1)), hence fitting well to the broad absorption bands of molecules in liquid phase. As compared with that of a state-of-the-art Fourier transform spectrometer, the signal-to-noise ratio could be improved by a factor of 50. Additionally, by using a QCL as the light source, optical path lengths of more than 100 (mu)m could be used even in aqueous matrixes which reduced the danger of cell clogging. In the example shown here phosphate was determined in Diet Coke samples. The flow injection system used allowed the measurement of the sample at two pH values (5 and 13) at which the analyte was present as H(sub 2)PO(sub 4)(super -) and PO(sub 4)(super 3-), respectively. As the analytical readout the difference in IR absorption of H(sub 2)PO(sub 4)(super -) and PO(sub 4)(super 3-) at the laser wavelengths was taken. The FIA-QCL measurements were corroborated by ion chromatography which was used as a reference method.

Two approaches for automated preparation of a multi-component calibration solution in sequential injection analysis (SIA) using one single standard for each component are proposed. The key to both approaches is the generation of highly reproducible injection sequences in the flow system. In the first approach, single standards of three different components are stacked in the holding coil in well-defined sequences. Upon flow reversal and transport to the detector, the stacked zones disperse into each other producing exactly defined mixtures of the three standards which can be recorded as a function of time. The second approach is based on complete mixing of the three single standards in different ratios prior to detection. For this purpose, repeated short sequences of small volumes of standard and distilled water are aspirated into the holding coil, which results in a homogeneous calibration solution upon flow reversal and transport to the detector. The proposed approaches have been applied to simultaneous determination of glucose, fructose, and sucrose in aqueous standards as well as in soft drinks using Fourier transform infrared spectroscopic detection and multivariate partial least-squares data analysis. The obtained results were compared with external reference methods to confirm the results obtained by the multivariate SIA-FTIR technique. Typical deviations from the results obtained by the reference method were in the order of 2.5, 4.4, and 2.8 g/L (2.5, 4.1, and 2.1% of total sugar) for glucose, fructose, and sucrose, respectively.

In this work, a flow system containing a micromachined lamella-type porous silicon reactor and a novel mid-IR fiber-optic flow cell were used for the enzymatic determination of sucrose in aqueous solution. The method relies on the enzymatic hydrolysis of sucrose to fructose and glucose catalyzed by [Beta]-fructosidase and on the acquisition of FT-IR spectra before and after complete reaction. [Beta]-Fructosidase was covalently bound to the porous silicon surface of the channels in the microreactor. The porous silicon was achieved by anodization of the silicon reactor in a HF/ethanol mixture. For the measurement of small amounts of aqueous solution, a miniaturized flow cell was developed which consisted of two Ag[Cl.sub.x][Br.sub.1-x] fiber tips (diameter, 0.75 mm) coaxially mounted in a PTFE block at a distance of 23 [[micro]meter]. The flowing stream was directed through the gap of the two fiber tips which served to define the optical path length and to bring the focused mid-IR radiation to the place of measurement. Using this construction, a probed volume of [approximately]10 nL was obtained. The calibration curve was linear between 10 and 100 mmol/L sucrose. Furthermore, the potential of this method was demonstrated by the analysis of binary sucrose/glucose mixtures showing no interference from glucose and by the successful determination of sucrose in real samples.

In this paper we show a novel design of high Q-factor silicon nitride (SiN) 1D photonic crystal (PhC) cavities side-coupled to curved waveguides, operating with both silica and air cladding. The engineering of the etched 1D PhC cavity sidewalls angle allows for high Q-factors over a wide range of upper cladding compositions, and the achievement of the highest calculated Q-factor for non-suspended asymmetric SiN PhC structures. We show the employment of these type of SiN PhC cavities in hybrid external cavity laser (HECL) configuration, with mode-hop free single mode laser operation over a broad range of injected currents (from 25 mA to 65 mA), milliwatts of power output (up to 9 mW) and side-mode suppression ratios in the range of 40 dB. We demonstrate the operation of these devices as compact and energy efficient optical sensors that respond to refractive index changes in the surrounding medium the measurement of sodium chloride (from 0% to 25%) and sucrose (from 0% to 25%) in aqueous solution. In HECL configuration, the RI sensor exhibits a 2 orders of magnitude improvement in detection limit compared to the passive microcavity. We also discuss the possibility for applying these devices as novel transducers for refractive index changes that are induced by analyte specific absorption of infrared radiation by the target analytes present in gas or liquid phase.