Light-emitting diodes and photodiodes in the deep ultra-violet range for absorption photometry in liquid chromatography, capillary electrophoresis and gas sensing

Absorbance measurement in the deep ultra-violet range (below 300 nm) has been one of the most widely used detection methods for analytical techniques as a large number of organic compounds have strong absorption bands in the deep UV region. The use of incandescent or discharge lamps coupled to a monochromator for the wavelength selection in a conventional UV detector makes it complex and costly. Light-emitting diodes (LEDs) for the deep UV range commercially available in recent years have become potential alternatives to thermal light sources. LEDs with their relatively narrow emission bandwidths (typically 20 nm) are well suited for absorption photometry in which a monochromator is not required. This dissertation, therefore, concerns the utilization LEDs and photodiodes (PDs) in the deep UV range as radiation sources and light detectors, respectively for absorption photometry in high-performance liquid chromatography (HPLC), capillary electrophoresis (CE) and gas sensing. LEDs were known to perform as light detectors. In measuring systems based on LEDs as light sources, PDs have been normally employed for detection devices. The practical reasons for the use of LEDs as alternatives to PDs, however, have not been demonstrated. Only an advantage of cost-saving was pointed out. In the first project, the performance of LEDs in the light intensity measurement was investigated and compared to that of standard silicon PDs in three different measuring configurations: current follower mode to measure to photocurrents, photovoltaic mode to determine the voltage developed across the diode on irradiation without load and discharge time mode to measure the rate to discharge the junction capacitance of diodes. LEDs as detectors were generally found to be adequate for the analytical work but PDs offered higher sensitivity and linearity as well as provided stable readings with faster settling times. Absorbance detectors for narrow-column HPLC (250 μm inner diameter) and CE (50 μm inner diameter) based on deep UV-LEDs and PDs selective for emission wavelengths were developed and evaluated in the quantification of model compounds at 255 and 280 nm. Absorbance measurements were directly obtained by the use of a beam splitter and PDs for reference signals and a logarithmic ratio amplifier-based circuitry to emulate the Lambert-Beer’s law. Narrow-column HPLC is useful for the applications in which the reduction in eluent consumption is desired or only limited amount of samples is available when utmost sensitivity is not required. In CE, the use of a capillary as the separation channel to minimize the peak broadening downscales the detection window to micrometer range which is even much narrower than that of a narrow-bore HPLC. This makes the design and construction of these LED-based detectors for narrow detection channels more challenging than for a standard HPLC as the higher efficiency for light coupling and stray light avoidance is essentially required. Additionally, high mechanical stability is needed to minimize the noise resulted from mechanical fluctuations. The performance of these optical devices at two measured wavelengths was excellent in terms of the baseline noise (low μAU range), linearity between absorbance values and concentrations (correlation coefficients > 0.999) and reproducibility of peak areas (about 1%). Not only was the potential of a deep UV-LED as a radiation source for absorption spectroscopy investigated for separation techniques but also for the detection of benzene, toluene, ethylbenzene and the xylenes compounds in the gas phase at 260 nm. In the first part of this work, its performance in the acoustic waves excitation was preliminarily investigated with some different measuring systems for the detection of the toluene vapor. It was found that the intensity of a deep UV-LED was insufficient to produce detectable acoustic signals. This was followed by the construction of an absorbance detector for the determination of these target compounds based on the combination of a deep UV-LED and PDs. This optical device was designed to use optical fibers for the light coupling from the LED to a measuring cell and a reference PD, that allows removing a beam splitter previously required for detectors of a narrow column HPLC and CE. Its performance with regard to linearity and reproducibility was satisfactory. Detection limits of about 1 ppm were determined. It could be concluded that viable absorbance detectors for narrow-column HPLC, CE and gas sensing based on deep UV-LEDs and PDs as light sources and light detectors, respectively can be constructed. The performance of these inexpensive LED-based optical devices with regard to linearity, reproducibility and baseline noise was satisfactory and found to be comparable to that of more complex and expensive commercial detectors. These detectors with features of low power consumption and small size are useful for portable battery-powered devices

Spectroscopic measurement of volatile organic compounds as biomarkers for human breath analysis

I composti BTEX - benzene, toluene, etilbenzene e xileni - sono tra i composti organici volatili (COV) più pericolosi, pertanto il loro rilevamento altamente sensibile e selettivo può fornire informazioni chiave in molte applicazioni, dal monitoraggio ambientale all'analisi dell'espirato. I sensori ottici basati su tecniche di spettroscopia di assorbimento laser (LAS) possono rappresentare una soluzione per il rilevamento di BTEX poiché questi composti mostrano caratteristiche di assorbimento forti e distinte nella regione spettrale 13 - 15 μm. Tra queste tecniche, la spettroscopia fotoacustica a diapason di quarzo (QEPAS) e la spettroscopia termoelastica indotta dalla luce (LITES) rappresentano due tecniche efficaci. L'attività di ricerca svolta durante il mio dottorato di ricerca in Industria 4.0 presso il Politecnico di Bari si è concentrato sullo sviluppo di sensori ottici basati su tecniche QEPAS e LITES per il rilevamento di benzene, mirando la banda di assorbimento target centrata a 14,85 μm utilizzando un Laser a cascata quantica (QCL) non commerciale. I sensori QEPAS e LITES sviluppati hanno mostrato un'eccellente risposta lineare e sono stati ottenuti limiti di rilevamento minimi stimati (MDL), rispettivamente di 13 ppb e 105 ppb, con una costante di tempo del lock-in amplifier di 100 ms.BTEX compounds - benzene, toluene, ethylbenzene, and xylenes - are among the most hazardous Volatile Organic Compounds (VOCs), thus their highly sensitive and selective detection can provide key information in many applications, ranging from environmental monitoring to breath analysis. Optical sensors based on laser absorption spectroscopy (LAS) techniques can represent a viable solution for BTEX detection since these compounds show strong and distinct absorption features in the spectral region 13 - 15 μm. Among these techniques, Quartz-Enhanced Photoacoustic Spectroscopy (QEPAS) and Light-Induced Thermoelastic Spectroscopy (LITES) represent two effective techniques. The research activity carried out during my Ph.D. program in Industry 4.0 at the Polytechnique University of Bari was focused on the development of optical sensors based on QEPAS and LITES techniques for benzene detection, targeting the absorption band centred at 14.85 μm using a non-commercial Quantum Cascade Laser (QCL) source. The developed QEPAS and LITES sensors showed an excellent linear response and estimated minimum detection limits (MDL) of 13 ppb and 105 ppb, respectively, were obtained with a lock-in time constant of 100 ms

An Induced Environment Contamination Monitor for the Space Shuttle

The Induced Environment Contamination Monitor (IECM), a set of ten instruments integrated into a self-contained unit and scheduled to fly on shuttle Orbital Flight Tests 1 through 6 and on Spacelabs 1 and 2, is described. The IECM is designed to measure the actual environment to determine whether the strict controls placed on the shuttle system have solved the contamination problem. Measurements are taken during prelaunch, ascent, on-orbit, descent, and postlanding. The on-orbit measurements are molecular return flux, background spectral intensity, molecular deposition, and optical surface effects. During the other mission phases dew point, humidity, aerosol content, and trace gas are measured as well as optical surface effects and molecular deposition. The IECM systems and thermal design are discussed. Preflight and ground operations are presented together with associated ground support equipment. Flight operations and data reduction plans are given

Development of compact and innovative optical sensors for trace gas detection

Trace gas sensing is crucial in several application fields. In this thesis, Quartz Enhanced Photoacoustic Spectroscopy (QEPAS)-based sensors were studied, designed and employed to detect gas traces. Quartz Tuning Forks (QTFs) are employed as transducers in this sensing technique. Studies of electrical, mechanical and resonance characteristics of QTFs allowed improvements of QEPAS sensors performances. These innovative sensors were employed to both monitor the presence of pollutants in air and control the quality of mechanical components. Different detection strategies to detect multiple gas traces in mixtures were investigated. Finally, the design of a Semi-Integrated QEPAS sensor is described. The research activity related to the Industrial Ph.D. Program and presented in this thesis was carried out within the framework of PON RI 2014-2020 “Dottorato di Ricerca a Caratterizzazione Industriale” (DOT130W083), and in collaboration with two industries, i.e., MASMEC S.p.a. (Italy) and THORLABS GmbH (Germany)

Evaluation and study of advanced optical contamination, deposition, measurement, and removal techniques

A program is described to design, fabricate and install an experimental work chamber assembly (WCA) to provide a wide range of experimental capability. The WCA incorporates several techniques for studying the kinetics of contaminant films and their effect on optical surfaces. It incorporates the capability for depositing both optical and contaminant films on temperature-controlled samples, and for in-situ measurements of the vacuum ultraviolet reflectance. Ellipsometer optics are mounted on the chamber for film thickness determinations, and other features include access ports for radiation sources and instrumentation. Several supporting studies were conducted to define specific chamber requirements, to determine the sensitivity of the measurement techniques to be incorporated in the chamber, and to establish procedures for handling samples prior to their installation in the chamber. A bibliography and literature survey of contamination-related articles is included

Femtosecond laser techniques for fabrication of QTF and tailoring of its optical properties for gas sensing applications

This Ph.D. thesis reports on the development of femtosecond (fs) laser machining processes fabrication of QTF and tailoring of its optical properties for gas sensing applications . Pursuing such a strategy would allow for the exploitation of many advantages of fs-laser processing such high precision and quality of processing, low environmental impact and flexible production processes. The issue related to the transparency of quartz in the 1 to 5 μm range has been addressed by developing a blackening approach for quartz crystals. This strategy consists in the laser surface texturing of the target material, in order to enhance its optical absorption in a particular wavelength range. In particular, a femtosecond pulsed laser was used to create matrices-like patterns on the surfaces of quartz crystal wafers. These matrix patterns were designed to reduce and flatten the quartz transmittance across the wavelength range of interest. Finally, a proof of concept was demonstrated by implementing two laser-textured QTF as photodetector in a LITES setup for detection of two water vapor absorption features at 1.39 μm and 7.38 μm. An environmental-sustainable method for QTF prototype production was developed through direct laser cut of quartz wafers exploiting femtosecond laser ablation. This activity was performed in collaboration with the FOLAS Lab research group in their facilities at the Faculty of Mechanical Engineering at University of Ljubljana. This method exploited a milling channel approach to cut through the complete depth of the target wafers. This process was characterized as function of the femtosecond laser working parameters and was then employed for the cut of actual QTFs. The resonant properties of the laser-cut devices were evaluated experimentally by means of photoacoustic excitation and they were found to be comparable both to their simulated behavior and to the performances of standard QTFs, thus confirming the validity of this manufacturing technique

Effects of magnetic field, doping and local strain on excitons in two-dimensional semiconductors

Two-dimensional materials are nanostructures that consist of atomically thin sheets of layered materials. An important subgroup are semiconducting transition metal dichalcogenides, which in the limit of a single layer exhibit a transition from an indirect to a direct band gap. These materials show multiple unique optical properties such as a strong light-matter interaction, strongly bound excitons and circular dichroism. In this thesis, mono- and bilayers of tungsten diselenide and tungsten disulfide were studied using optical spectroscopy. First, the spectrum of a charge tunable mono- and bilayer of tungsten diselenide was studied in magnetic field to characterize the exciton spectral lines in the neutral and negatively charged regime. These measurements were complemented by a theoretical model based on density functional theory, which is able to calculate the magnetic shift throughout the Brillouin zone. These numerical calculations showed excellent quantitative agreement with previous reports and extend those models by providing predictions for momentum-indirect excitons. Next, these findings were used to study the spectrum of monolayer tungsten disulfide. A sample was embedded in a field effect structure made from graphene, which achieved negative, neutral as well as positive charge carrier concentrations. It was shown, that apart from a shift in the band gap energy, the exciton spectrum can be described analogously to tungsten diselenide. Finally, the behavior of a suspended monolayer of tungsten diselenide was studied under local strain. Using a low temperature atomic force microscope based on a quartz tuning fork with a transparent diamond tip, real-time measurement of the photoluminescence spectrum at simultaneous application of local force was achieved. It was shown that under local action of the tip the spectrum exhibits a redshift, characteristic for tensile strain. At high force, the brightening of dark excitons through the hybridization of dark excitons with localized states was observed. At even higher force, the tip caused a spontaneous and permanent deformation of the sample structure while the spectrum changed to a set of narrow, bright spectral lines, some of which remained present after release of the tip.Zweidimensionale Materialien sind Nanostrukturen, die aus wenigen Lagen geschichteter Materialien hergestellt werden. Eine wichtige Untergruppe stellen halbleitende Übergangsmetalldichalkogenide dar, die im Extremfall einer Einzellage einen Übergang zu einem direkten Halbleiter erfahren. Diese Materialien zeigen mehrere einzigartige optische Eigenschaften wie starke Licht-Materie-Wechselwirkung, stark gebundene Exzitonen und zirkularen Dichroismus. In dieser Arbeit wurden Einzel- und Doppellagen aus Wolframdiselenid und Wolframdisulfid mithilfe optischer Spektroskopie untersucht. Zunächst wurde das Spektrum einer ladungsdurchstimmbaren Einzel- und Doppellage aus Wolframdiselenid im Magnetfeld untersucht um deren Spektrallinien im neutralen und negativ geladenen Regime durch ihre charakteristische Aufsspaltung zu charakterisieren. Diese wurde mit einem theoretischen Modell auf Basis von Dichtefunktionaltheorie verglichen, welches es erreicht, die magnetische Verschiebung innerhalb der gesamten Bandstruktur zu berechnen. Die numerischen Berechnungen zeigen exzellente quantitative Übereinstimmung mit vorangegangenen Studien und erweitern diese um Vorhersagen für impulsindirekte Exzitonen. Anschließend wurden diese Erkenntnisse angewandt um das Spektrum von einzellagigem Wolframdisulfid zu untersuchen welches in eine Feldeffektstruktur aus Graphen eingebettet wurde. Dadurch wurde es erreicht, sowohl negative als auch positive Ladungsträgerkonzentrationen zu induzieren. Es wurde gezeigt, dass das Spektrum sich unter Berücksichtigung der größeren Bandlücke analog zu dem von Wolframdiselenid verhält. Zuletzt wurde das Verhalten einer freitragenden Einzellage aus Wolframdiselenid unter lokalen Verspannungen untersucht. Mithilfe eines kryogenen Rasterkraftmikroskops, bestehend aus einer Quarzstimmgabel und einer daran befestigten transparenten Diamandspitze, wurde es erreicht, in Echtzeit unter punktueller Krafteinwirkung das Photolumineszenzspektrum zu vermessen. Es zeigte sich, dass unter lokaler Einwirkung der Spitze das Spektrum eine Rotverschiebung erfährt, die für Zugverspannung charakteristisch ist. Bei hoher Verschiebung wurde eine Aufhellung dunkler Exzitonen durch Hybridisierung von dunklen Exzitonen mit lokalisierten Zuständen beobachtet. Bei noch höherer Krafteinwirkung verursachte die Spitze eine spontane und permanente Verformung der Probenstruktur, während das Spektrum in eine Gruppe schmaler, hell leuchtender Spektrallinien zerfiel, die auch nach Beendigung der Krafteinwirkung sichtbar blieben

A Cryogenic Silicon Interferometer for Gravitational-wave Detection

The detection of gravitational waves from compact binary mergers by LIGO has opened the era of gravitational wave astronomy, revealing a previously hidden side of the cosmos. To maximize the reach of the existing LIGO observatory facilities, we have designed a new instrument that will have 5 times the range of Advanced LIGO, or greater than 100 times the event rate. Observations with this new instrument will make possible dramatic steps toward understanding the physics of the nearby universe, as well as observing the universe out to cosmological distances by the detection of binary black hole coalescences. This article presents the instrument design and a quantitative analysis of the anticipated noise floor