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Photonic design and optimization of thin-film polycrystalline diamond waveguides are shown, serving as advanced evanescent field transducers in the mid-infrared fingerprint regime (2000–909 cm–1; 5–11 μm). Design constraints inherent to optical/system considerations and the material were implemented in a finite element method (FEM)-based simulation method that allowed three-dimensional modeling of the overall structure. Thus, lateral mode confinement, attenuation in the direction of radiation propagation, and physical resilience were evaluated. In a final step, the designed structures were fabricated, and their utility in combination with a broadly tunable external cavity quantum cascade laser for chemical sensing of a liquid phase analyte was demonstrated.

In den letzten Jahren ist insbesondere in der Sicherheitstechnik eine steigende Nachfrage nach neuen Technologien zur Erkennung von sicherheitskritischen Materialien zu beobachten. Der Terahertz (THz)-Frequenzbereich, der sich im elektromagnetischen Spektrum zwischen Mikrowellen und Infrarot befindet, ist aufgrund seiner Eigenschaften prädestiniert für Anwendungen in der Sicherheitstechnik. Viele Materialien, wie beispielsweise Sprengstoffe besitzen resonante Absorptionen, sogenannte spektrale Fingerabdrücke im THz-Frequenzbereich, welche die Erkennung dieser Materialien erlauben. In Kombination mit der Transparenz vieler Kleidungs- und Verpackungsmaterialien liegt die Anwendung von THz-Technologie zum Erkennen versteckter Sprengstoffe nahe. Die Voraussetzungen für eine erfolgreiche Erkennung von energetischen Materialien mit THz-Technologie müssen in zwei Bereichen geschaffen werden. Diese zwei Bereiche sind geeignete THz-Technologien zur Materialerkennung einerseits und detaillierte spektroskopische THz-Analysen andererseits. Diese Arbeit untersucht beide Aspekte durch die Evaluation einiger geeigneter Detektortechnologien zur THz-Materialerkennung sowie spektroskopische Untersuchungen einer Auswahl von interessanten energetischen Materialien. Im Rahmen dieser Arbeit entworfene THz-Spektroskopie-Aufbauten in Transmission und Reflexion werden zur THz-Analyse energetischer Materialien verwendet. Die Transmissionsspektroskopie mit einer hohen Bandbreite von 0,2 - 6,6 THz dient der Ermittlung des Absorptionskoeffizienten als quantitative Größe und ermöglicht erstmalig die zweifelsfreie Messung einer Resonanz des Sprengstoffs RDX bei 4,2 THz. Die Variabilität der spektralen Fingerabdrücke aufgrund von Additiven, Inhomogenitäten der Proben und Präparation werden spektroskopisch untersucht. Die dabei festgestellte Präparationsabhängigkeit kann zu einer inhomogenen Verbreiterung einer Resonanz führen und stellt somit eine Herausforderung für die THz-Erkennung von Sprengstoffen dar. In dieser Arbeit werden einige interessante Detektortechnologien bezüglich des Potentials der THz-Sprengstofferkennung untersucht. Ein in Kooperation mit Projektpartnern entwickeltes echtzeitfähiges THz-Bildgebungssystem bei 800 GHz basierend auf einer Zeile von 32 subharmonischen Mischern sowie ungekühlte antennengekoppelte Thermoelemente, ebenfalls bei 800 GHz, werden charakterisiert. THz-Bildgebung an sicherheitskritischen Objekten wird mit verschiedenen THz-Bildgebungstechnologien demonstriert. Die THz-Bildgebung von Sprengstoffproben resultiert in einer Erkennung eines Sprengstoffs anhand einer resonanten Absorption mit verschiedenen THz-Technologien. Dabei wird die THz-Erkennung der Resonanz eines Sprengstoffs mit einem Thermoelement als THz-Leistungsdetektor weltweit erstmalig demonstriert. Abschließend wird das Potential der THz-Erkennung energetischer Materialen kritisch diskutiert und eine Einschätzung zur zukünftigen Entwicklung gegeben. In recent years, there is a growing demand for technology to identify and recognize materials relevant to security. The terahertz (THz) frequency range, situated between microwaves and infrared in the electromagnetic spectrum, has certain properties making it predestined for security applications. Many materials, e.g. energetic materials (explosives), exhibit resonant absorptions, so called spectral fingerprints, in the THz-frequency range which facilitate their recognition. In combination with the transparent nature of many clothing- and packaging materials to THz-radiation, the application of THz-technology for the recognition of hidden explosives is obvious. In order to successfully recognize energetic materials with THz-technology, requirements in two areas have to be fulfilled, namely suitable THz-technologies for material detection and detailed spectroscopic THz-analyses of energetic materials. This thesis investigates both areas by evaluation of suitable detector technologies for THz material recognition as well as an analysis of spectroscopic experiments on a selection of interesting energetic materials. THz time-domain spectroscopy setups in transmission and reflection, which where developed in the context of this thesis, are used to analyze the energetic materials. Spectroscopy in transmission mode, facilitating a very high bandwidth from 0,2 - 6,6 THz, is used to determine the absorption-coefficient as a quantitative measure and enabled the author to measure a clear resonance of the explosive RDX at 4,2 THz for the first time. The variability of spectral fingerprints due to additives, inhomogeneities and preparation of explosives are investigated via THz-spectroscopy. Results show that an inhomogeneous broadening of a resonance can occur due to the preparation of the energetic material. This is a challenge for THz-based recognition of explosives. Furthermore, several THz-imaging technologies are investigated regarding their respective potential for THz explosives recognition. A realtime THz-imaging system based on a row of 32 sub-harmonic mixers, developed in cooperation with project partners, as well as a system based on uncooled antenna-coupled thermocouples, both operating at 800 GHz, are characterized. THz-imaging on objects relevant to security is demonstrated using different THz-imaging technologies. The outcome of THz-imaging of samples of explosives is their recognition based on resonant absorptions using different THz-imaging technologies. In this thesis, the THz-recognition of resonances of explosives using thermocouples as the THz power detector is demonstrated globally for the first time. To conclude, the potential of the THz-recognition of energetic materials is discussed and an assessment of future directions for research is given.

Der pyroelektrische Sensor LIM-262-ZH ist für die nicht-dispersive Infrarot-Gasanalyse zur Bestimmung der CO2-Konzentration geeignet. Dieser Artikel be­schreibt ein für den Schaltungsentwickler nützliches SPICE Makromodell. Frequenzgang und transientes Verhalten werden hinreichend genau modelliert. An­passung der Modellparameter ermöglicht die Simula­tion von anderen pyroelektrischen Sensoren. Die Infrar­ot-Lichtintensität wird als elektrische Stromstär­ke abgebildet. The pyro-electric sensor LIM-262-ZH supports non-dispersive infrared analysis of CO2 concentration in gases. This paper describes a SPICE macro model useful for electronic circuit designers. Frequency and transient characteristics are modeled with appropriate accuracy. By adaptation of the model parameters other pyro-electric sensors may be used in simulations too. The incident and absorbed infrared light intensity is modeled as electrical current.

We have investigated the influence of the final states of bound-to-continuum transitions within the conduction band of asymmetric quantum well structures on the photocurrent. This influence manifests itself by an energy-dependent oscillation of the current direction. We observe pronounced oscillations at zero bias voltage in a double quantum well structure, induced by an asymmetric excitation into continuum states with positive and negative momentum, i.e. by a photogalvanic effect (PGE). If this effect is superimposed on an asymmetric backrelaxation, similar oscillations are observed in the spectrum when the latter asymmetry is compensated by an external electric field. Theoretically, we find a strong relation between the PGE and a quantum interference effect occurring in the a strong re continuum.

Ill., graph. Darst.

Ill., graph. Darst.

We report on the use of SIMOX substrates for fabrication of IR radiation thermopiles by CMOS technology. SIMOX technology enables the fabrication of single-crystal leads on thin silicon or silicon oxide membranes. Two p-type silicon/aluminium thermopiles on different membranes are presented. A thermopile with a 4 μm thick, epitaxially grown silicon membrane has a responsivity of 18 V/W and a time constant of 3 ms. Using a 2.5 μm thick silicon oxide membrane, a responsivity of 150 V/W and a time constant of 25 ms are achieved. In addition, the fabrication and properties of an IR absorber made in CMOS technology are presented.

Ill., graph. Darst.

Ill., graph. Darst.

Magnetotransport and photoluminescence (PL) measurements on InAs/(Galn)Sb superlattices (SLs) .grown by molecular-beam epitaxy on GaSb substrates at different substrate temperatures are reported. With increasing growth temperature, a transition of the SLs from residual n-type to residual p-type doping was observed. For n-type samples, a decrease in the electron concentration leads to a strong increase in the PL intensity. In contrast, the PL intensity of p-type samples is only weakly dependent on the hole concentration. This correlation can be used to control the residual doping of the SLs.

Ill., graph. Darst.

Ill., graph. Darst.

Ill., graph. Darst.

Using the free-electron laser facility FELBE, the authors have studied the influence of the intensity on the quadratic autocorrelation measured with two-photon quantum well infrared photodetectors (QWIPs). At high illumination powers, the shape of the autocorrelation trace is affected by photocurrent saturation of the two-photon QWIP. They describe the saturation mechanism by different analytical models taking account of the photocurrent nonlinearity in analogy to linear QWIPs and give conditions where true quadratic behavior can be observed. While these studies were carried out at 77 K, properties of two-photon QWIPs at room temperature will also be addressed.

Ill., graph. Darst

(de)Infrared gas measurement, CO2, OEM system, Ill., graph. Darst.

We report on the time-resolved photocurrent response of an InGaAs/GaAs quantum-well infrared photodetector stimulated by infrared fs-laser pulses (∼9 μm wavelength). We observe two dynamic photocurrent components, which are driven by the optical excitation and by the photoinduced space charge, respectively. The ratio of the time-averaged values of these two components allows us to determine the photoconductive gain. These gain values exhibit a good quantitative agreement with the gain obtained from continuous waves measurements of the peak responsivity. Furthermore, we present a detailed study of the decay time constant of the slow component as a function of the temperature and the bias voltage and compare our results with a recent theoretical model.

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A recently demonstrated transport mechanism giving rise to novel photovoltaic multiquantum well intersubband infrared detectors is analyzed theoretically. It is shown that optimized photovoltaic devices have detectivities that can be even larger than those of photoconductive ones. In addition, they have a high dynamic range and are particularly suited for the large photon fluxes occurring in the 8-12 mym regime.

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Within this dissertation two new measurement-techniques which reduce the power-consumption of an infrared-gas-sensor (IR) are introduced. By using the maximum of the sensor-signals first derivative, the author was able to reduce the power-consumption of an carbon-dioxide-sensor (CO2) for ventilation control by a factor greater than 200, in comparison to the normal operation mode. Furthermore a new method for an automated internal compensation of the radiation power drift of the IR-source is introduced. In contrast to the known dual-wavelength method which requires an additional IR-detector, working on an second not absorbed (from the target gas) wavelength, the new technique just uses a second rarely driven IR-source equal to the first one. By the comparison of the two signals, the one from the aged first source with the one from the almost not ageing second source, the radiation-power drift of the first source can be compensated. In addition to these new techniques and methods for driving an IR-gas-sensor, research results are presented, to introduce the reader in the different design-factors to be considered, to develop an application-optimised IR-CO2-sensor. Because of the contradictory influences of the length of the gas-chamber on the sensor-signal without gas and on the dependency of the sensor-signal on different gas-concentrations, an optimal length of the gas chamber can be found for most applications. This optimum depends on the target gas and the desired concentration-range.

We report on a monolithic integrated infrared sensor system consisting of a thermopile, a sensor measuring the chip temperature and an amplifier stage fabricated in a CMOS process on SIMOX (separation by implanted oxygen) wafers. A responsivity of 209 V W−1 and a normalized detectivity D ∗ of 1.3 × 108 cm Hz 1 2 W−1 are found for thermopiles with single-crystalline p-Si/n-polysilicon thermocouples on silicon oxide/silicon nitride membranes. A first analysis of the thermal influence of the power consumed by the circuitry on the thermopile voltage indicates that the sensor performance is not deteriorated by the integrated electronic circuitry.

We report on a novel GaAs:Si/(AlGa)As multiquantum well photodetector structure which exhibits photoresponse maxima in both the 8 - 12 and 3 - 5 Mym spectral regions. The relative intensity of these maxima strongly depends on the bias voltage, demonstrating the potential of such a structure as a tunable two-color intersubband detector.

We present results concerning the properties of anodic fluoride films grown on Hg1-xCdxTe with values of x between 0.2 and 0.3. Analysis of the growth rate indicated that the film was only partially fluoridized, with a relatively high amount of HgTe and/or elemental tellurium present. X-ray photoelectron spectroscopy confirmed the presence of two different tellurium peaks, one corresponding to tellurium bonded fluorine, and the other to tellurium bonded mercury or the elemental tellurium. The films were characterized by optical properties such as refractive index and band gap, electrical properties such as dielectric constant and resistivity, and chemical properties such as their reactivity in various solvents. It is shown that the anodic fluoride layers behave as an insulator, with a strong tendency to become oxidized. The electrical characteristics of the anodic fluoride-HgCdTe interface were determined from capacitance-voltage measurements of metal/insulator/semiconductor devices.

Continuous tuning over the entire 8-12 mym wavelength range is demonstrated for the intersubband absorption resonance in n-doped GaAs/AlxGa1-xAs multiple quantum-well structures following partial interdiffusion of the well and barrier layers via rapid thermal annealing. The data indicate that redshifting of the intersubband absorption resonance arises both from interdiffusion-induced modification of the confining potential and from a decrease in the depolarization shift. The later effect is due in part to a decrease in the freecarrier concentration within the Si-doped quantum wells following rapid thermal annealing. Significant diffusion of the localized Si dopant is also observed over the range of annealing temperatures investigated here. Calculated values of the Al-Ga interdiffusion coefficient, as a function of anneal temperature, indicate that Si diffusion through the heterointerfaces contributes substantially to layer intermixing.

Intersubband transitions in GaAs/Al sub x Ga sub 1-xAs quantum wells have been studied as a function of the doping level by Raman scattering and infrared absorption. The n-type dopant concentration placed in the well was varied between 1 x 10 high 18 and 8 x 10 high 18 cm high -3. With increasing doping level Raman scattering reveals a frequency down-shift of the single-particle intersubband transition and a shift to higher frequencies of the collective intersubband plasmon-phonon mode. The resonance in infrared absorption follows closely the collective mode observed in Raman scattering, demonstrating clearly that the doping-dependent depolarization shift of the absorption peak is an important parameter that must be taken into account in device design. Possible models for the frequency down-shift of the single-particle transition are also discussed.

We present a quantum-well infrared photodetector that allows us to switch between a quadratic and a linear detection mode. We employ an asymmetric detector design where intersubband transitions in a three-level system give rise to a quadratic power dependence down to very low excitation densities. Upon reversing the bias voltage, however, the intermediate subband acts as the ejection channel due to highly efficient tunneling, thus leading to a two-level system with linear photoresponse.

An array of photovoltaic infrared sensors with 12 mym cutoff wavelength has been fabricated for the first time in a narrow-gap semiconductor layer grown heteroepitaxially on Si. Heteroepitaxy is achieved using intermediate stacked epitaxial CaF2-SrF2-BaF2 buffer layers to overcome the large lattice as well as thermal expansion mismatch between narrow-gap PbSnSe and Si. The IR sensors exhibit resistance-area products up to 0.3 (ohm cm square) at 77 K. This corresponds to sensitivities which are above the 300 K background noise limit and only 2 - 5 times lower than those of state of the art HgCdTe sensors on CdZnTe substrates with the same cutoff wavelengths.

We show that the dark current noise spectrum of an In(0.30)Ga(0.70)As/GaAs quantum-well infrared photodetector (QWIP) is characterized by two plateau-like frequency regions. At high frequencies, the observed noise current is due to the generation-recombination noise of carriers, which have been emitted thermionically from the quantum wells into the continuum. In the low-frequency regime, an additional contribution to the noise current is caused by the redistribution of space charges, that occurs on a time scale similar to the dielectric relaxation time.

Diamond has excellent optical properties including broadband transmissivity, low self-absorption, and a high refractive index, which have prompted its use for optical sensing applications. Thin- film diamond strip waveguides (DSWGs) combined with tunable quantum cascade lasers (tQCLs) providing an emission wave- length range of 5.78–6.35 mm (1735–1570 cm"1) have been used to obtain mid-infrared (MIR) spectra of proteins, thereby enabling the analysis of their secondary structure via the amide I band. Three different proteins were analyzed, namely bovine serum albumin (BSA), myoglobin, and g-globulin. The secondary structure of BSA and myoglobin has a major contribution of a-helices, whereas g-globulins are rich in b-sheet structures, which is reflected in the amide I band. A comparison of the spectra obtained via the combination of the tQCL and DSWG with spectra obtained using conventional Fourier transform infrared (FTIR) spectroscopy and a commercial diamond attenuated total reflection (ATR) element has been performed. It is shown that the main features evident in FTIR-ATR spectra are also obtained using tQCL-DSWG sensors., acceptedVersion

Photonic design and optimization of thin-film polycrystalline diamond waveguides are shown, serving as advanced evanescent field transducers in the mid-infrared fingerprint regime (2000���909 cm���1; 5���11 ��m). Design constraints inherent to optical/system considerations and the material were implemented in a finite element method (FEM)-based simulation method that allowed three-dimensional modeling of the overall structure. Thus, lateral mode confinement, attenuation in the direction of radiation propagation, and physical resilience were evaluated. In a final step, the designed structures were fabricated, and their utility in combination with a broadly tunable external cavity quantum cascade laser for chemical sensing of a liquid phase analyte was demonstrated., publishedVersion

We report on the experimental determination of the different contributions to the noise current in quantum-well infrared photodetectors (QWIPs). A detailed description of the measurement set-up and experimental methods is given. The noise properties of conventional bound-to-continuum QWIPs and four-zone QWIPs will be discussed. The noise current of these devices is dominated by g-r noise and by shot noise, respectively.

Diamond has excellent optical properties including broadband transmissivity, low self- absorption and a high refractive index, which have prompted its use in for optical sensing applications. Thin-film diamond strip waveguides (DSWGs) combined with tunable quantum cascade lasers (tQCLs) providing an emission wavelength range of 5.78-6.35 μm (1735-1570 cm-1) have been used to obtain mid-infrared (MIR) spectra of proteins, thereby enabling the analysis of their secondary structure via the amide I band. Three different proteins were analyzed, namely bovine serum albumin (BSA), myoglobin, and γ-globulin. The secondary structure of BSA and myoglobin has a major contribution of -helices, whereas γ-globulins are rich in β-sheet structures, which is reflected in the amide I band. A comparison of the spectra obtained via the combination of the tQCL and DSWG with spectra obtained using conventional Fourier transform infrared (FTIR) spectroscopy and a commercial diamond attenuated total reflection (ATR) element has been performed. It is shown that the main features evident in FTIR-ATR spectra are also obtained using tQCL-DSWG sensors., submittedVersion

Mid-infrared sensors (MIR) based on attenuated total reflection (ATR) spectroscopy provide a robust and sensitive platform for the detection of low levels of organic molecules dissolved in water. The chemical and partitioning properties of the polymer membrane play an important role and are crucial to MIR-ATR sensor performance. Although the MIR-ATR sensor has been used to quantify dissolved hydrocarbons in a wide range of aquatic environments, very little is reported on the impact of aqueous solution composition and polymer membrane aging on the hydrocarbon – polymer partitioning process. In the present study we investigated a number of factors that may affect the sensor sensitivity and analytical performance during quantification of hydrocarbons in water. Calibration curves were obtained as a function of salinity and it was revealed that the response factors are generally higher in the presence of salts. We show that reliable hydrocarbon analysis can be achieved in water provided that the sensor is calibrated using appropriate standard solutions and corrected for salinity/ionic strength variations. Sensor measurements were also performed over a range of crude oil – water mixtures, and these studies revealed that the total petroleum hydrocarbon concentration in solution (between the 0 and 20 ppm range) has a negligible effect on the analyte-polymer partitioning process. In terms of sensor durability, the polymerfilm displayed remarkable repeatability and long-term stability in water over a two-month testing period ., acceptedVersion