* The development of spectrometers and their application to the detection of gas phase pollutants. In particular generation continuous wave Terahertz radiation by photomixing.
* Photoinscription of glass materials by femotsecond radiation including the structural analysis of irradiated glasses.
* Fabrication and characterisation of chalcogenide glasses.
Abstract: The glass-forming region in the pseudo-ternary CdSeâAgIâAs2Se3 system was determined. Measurements including differential scanning calorimetry (DSC), density, and X-ray diffraction were performed. The effect resulting from the addition of CdSe or AgI has been highlighted by examining three series of different base glasses. The characteristic temperatures of the glass samples, including glass transition (Tg), crystallisation (Tx), and melting (Tm) temperatures are reported and used to calculate their ÎT = Tx â Tg and their Hruby, Hr = (Tx â Tg)/(Tm â Tx), criteria. Evolution of the total electrical conductivity Ï and the room temperature conductivity Ï298 was also studied. The terahertz transparency domain in the 50â600 cmâ1 region was pointed for different chalcogenide glasses (ChGs) and the potential of the THz spectroscopy was suggested to obtain structural information on ChGs.
Abstract: The case of symmetric tops CH3X (X = Br, Cl, F, â¦) perturbed by non-polar diatoms Y2 (Y = N2, O2, â¦)
is analyzed from the viewpoint of theoretical collisional broadening of their rotational lines observed in
atmospheric spectra. A semi-classical approach involving an exponential representation of the scattering
operator and exact trajectories governed by the isotropic potential is presented. For the first time the
active molecule is strictly treated as a symmetric top and the atom-atom interactions are included in the
intermolecular potential model. It is shown for the CH3Cl-O2 system that these interactions contribute
significantly to the line width for all values of the rotational quantum numbers J and K. Additional testing
of modifications required in the semi-classical formalism for a correct application of the cumulant
expansion is performed and it is shown that the use of the cumulant average on the rotational states of the
perturbing molecule leads to entirely negligible effects for the not very strongly interacting CH3Cl-O2
system. In order to check the theoretical predictions and to extend the scarce experimental data available
in the literature to higher values of the rotational quantum numbers, new measurements of room
temperature O2-broadened CH3Cl rotational lines are carried out by a photomixing continuous-wave
terahertz spectrometer. The experimental line widths extracted with a Voigt profile model demonstrate an
excellent agreement with theoretical results up to very high J-values (J = 31, 37, 40, 45, 50).
Abstract: The generation of cw-THz radiation by photomixing is particularly suited to the high resolution spectroscopy of gases; nevertheless, until recently, it has suffered from a lack of frequency metrology. Frequency combs are a powerful tool that can transfer microwave frequency standards to optical frequencies and a single comb has permitted accurate (10â8) THz frequency synthesis with a limited tuning range. A THz synthesizer composed of three extended cavity laser diodes phase locked to a frequency comb has been constructed and its utility for high resolution gas phase spectroscopy demonstrated. The third laser diode allows a larger tuning range of up to 300 MHz to be achieved without the need for large frequency excursions, while the frequency comb provides a versatile link to be established from any traceable microwave frequency standard. The use of a single frequency comb as a reference for all of the cw-lasers eliminates the dependency of synthesized frequency on the carrier envelope offset frequency. This greatly simplifies the frequency comb stabilization requirements and leads to a reduced instrument complexity.
Abstract: The alkali metal halide doping of gallium-sulfide glasses yields improvements in the optical, thermal and glass forming properties. To understand these improvements, the short-range order of xCsCl(1 - x)Ga2S3 glasses was probed by Raman spectroscopy. Raman spectra have been interpreted using density functional theory (DFT) harmonic frequency calculations on specific clusters of GaS4H4 and/or GaS3H3Cl tetrahedral subunits. The assignment of the observed vibrational bands confirms the main structural conclusions obtained with X-ray and neutron diffraction experiments and gives some new insights into the gallium-network present in the xCsCl(1 - x)Ga2S3 glasses. At the lowest concentration, the observed spectrum may be interpreted with small clusters such as dimers and trimers connected by corner-sharing (CS) GaS4H4 tetrahedral subunits. The vibrational fingerprints of tri-clusters with three-fold coordinated sulfur atoms have also been identified; however, no Raman signature of chlorine-doped subunits has been found to be caused by their insufficient intensity. For higher CsCl concentrations, distinct spectral features corresponding to chlorine-doped clusters appear and are increasing in intensity with x. In other words, undoped and Cl-doped tetrahedra coexist in the xCsCl(1 - x)Ga2S3 glasses. The added chlorine atoms induce a fragmentation of the glass network and replace the sulfur atoms in the CS tetrahedral environment. The comparison of the observed spectra with theoretical predictions and diffraction data favoured one-fold coordinated chlorine atoms in the glass network.
Abstract: The high brilliance of the AILES beamline at the SOLEIL synchrotron facility has been exploited for the study of the gas-phase vibrational spectra of weakly volatile organophosphorous compounds. The propagation of the synchrotron radiation in long path length gas cells allowed improvements in the sensitivity limits and spectral coverage compared with a previous study, performed by our group with conventional thermal sources. A ppm level detection in the entire IR domain up to terahertz (THz) frequencies has been realized for dimethyl methylphosphonate (DMMP), trimethyl phosphate (TMP), triethyl phosphate (TEP), and diethyl (2-methylallyl)phosphonate (DEMaP). In the present study, the assignment of the gas-phase vibrational and the conformational analysis of the two most stable conformers of DMMP and TMP have been extended to the torsional THz spectra in the 20-120 cm-1 range. The improvement of the S/N ratio below 600 cm-1 has permitted for the first time a gas-phase conformational analysis of the two weakly volatile and highly flexible TEP and DEMaP compounds. The experimental far-infrared (FIR)/THz spectra have been studied taking into account four low-energy conformers determined by means of high level of theory quantum chemistry calculations. Finally, due to its particularly low vapor pressure, the detection of gas-phase tributyl phosphate (TBP) in the FIR domain was unsuccessful. Nevertheless, the mid-IR/near-IR spectra of TBP recorded in a multipass cell heated to 355 K have been assigned with the harmonic vibrational predictions of the most stable conformer.
Abstract: We report the first successful high-resolution gas phase study of the âparallelâ band of DMSO at 380 cm1 associated with the ν11 bending vibrational mode. The spectrum was recorded with a resolution of 0.0015 cm1 using the AILES beamline of the SOLEIL synchrotron source, the IFS 125 FTIR spectrometer and a multipass cell providing an optical path of 150 m. The rotational constants and centrifugal corrections obtained from the analysis of the resolved rotational transitions reproduce the spectrum to the experimental accuracy.
Abstract: A standard DFB diode laser operating at 1.55 mm has been used with an EDFA to construct a hybrid cw-THz spectrometer. Frequency steps of as small as 4 MHz are demonstrated by current control whereas larger spectra in excess of 400 GHz have been realized using temperature control.
Abstract: A transverse electromagnetic horn antenna is monolithically integrated with a low temperature grown GaAs vertical photodetector on a silicon substrate forming a vertically integrated photomixer. Continuous-wave terahertz radiation is generated at frequencies up to 3.5 THz with a power level reaching 20 nW around 3 THz. Microwave and material concepts allow both qualitative and quantitative explanations of the experimental results. The thin film microstrip line topology has been adapted for active devices by an AuâAu thermocompression layer transfer technique and seems to be a promising generic tool for a new generation of efficient terahertz devices.
Abstract: A frequency doubled erbium doped modelocked fiber frequency comb is used to implement a THz photomixing synthesizer. The useful THz linewidth is in order of 150 kHz and has been assessed along with the frequency accuracy by spectroscopic measurements demonstrating a relative accuracy of 10â8 at frequencies around 1 THz. The THz synthesizer is used to implement a THz spectrometer to study the rotational absorption spectrum of carbonyl sulfide (OCS). Measurement of the principal transitions between 813 GHz and 1283 GHz allowed the properties of the THz spectrometer to be compared with competing techniques, and demonstrates the potential of the THz photomixing synthesizer as an alternative means to explore the THz domain.
Abstract: A review is provided of sources and detectors that can be employed in the THz range before the description of an opto-electronic source of monochromatic THz radiation. The realized spectrometer has been applied to gas phase spectroscopy. Air-broadening coefficients of HCN are determined and the insensitivity of this technique to aerosols is demonstrated by the analysis of cigarette smoke. A multiple pass sample cell has been used to obtain a sensitivity improvement allowing transitions of the volatile organic compounds to be observed. A solution to the frequency metrology is presented and promises to yield accurate molecular line center measurements.
Notes: This article belongs to the special issue Laser Spectroscopy and Sensing
Abstract: The room-temperature nitrogen- and oxygen-broadening coefficients of hydrogen cyanide spectral lines have been measured in the 0.5â3 THz (17â100 cmâ1) frequency range (purely rotational transitions with 5less-than-or-equals, slantJless-than-or-equals, slant36) by a continuous-wave terahertz spectrometer based on a photomixing source. An improved version of the Robert and Bonamy semiclassical formalism has been used to calculate the oxygen-broadening coefficients resulted in a good agreement with these measurements. The nitrogen and oxygen data are combined to provide the air-broadening coefficients as used by the HITRAN database. A significant difference is observed between the measured and tabulated values for transitions with high values of the rotational quantum number. A new polynomial representation is suggested for inclusion in HITRAN. A similar polynomial expression has been derived for the nitrogen broadening to aid the studies of Titan's atmosphere.
Abstract: Recent advances in the development of monochromatic continuous-wave terahertz sources suitable for high resolution gas phase spectroscopy and pollution monitoring are reviewed. Details of a source using an ultra fast opto-electronic photomixing element are presented. The construction of a terahertz spectrometer using this source has allowed spectroscopic characterisation and application studies to be completed. Analysis of H2S and OCS under laboratory conditions are used to demonstrate the spectrometer performance, and the determination of the transition line strengths and pressure self broadening coefficients for pure rotational transitions of OCS. The spectral purity 5 MHz, tunability 0.3 to 3 THz, and long wavelength [approximate]200 [mu]m of this source have been exploited to identify and quantify numerous chemical species in cigarette smoke. The key advantages of this frequency domain are its high species selectivity and the possibility to make reliable measurements of gas phase samples heavily contaminated by aerosols and particles.
Abstract: A terahertz time domain spectroscopy experiment is used to study the coherent re-emission after exciting more than 60 energy rotational states of OCS molecules in gas phase. Due to the regular structure of the absorption spectrum of such linear molecules, a set of subsequent pulses separated by 82.6 ps is re-radiated from the vapour and recorded up to 450 ps. A model based on a linear response of the gas and by use of âMaxwellâBlochâ equations has permitted the re-emitted free induced decay to be investigated. Spectroscopic parameters, such as rotational constant, centrifugal distortion coefficient and relaxation times are responsible for the temporal shape and so can be evaluated. The influence of the optical thickness to access the relaxation times is discussed.
Abstract: A transverse electromagnetic horn antenna is monolithically integrated with a standard ultrafast interdigitated electrode photodetector on low-temperature-grown GaAs. Continuous-wave terahertz radiation is generated at frequencies up to 2 THz with a maximum power of approximately 1 µW at 780 GHz. Experimental variations in the terahertz power as function of the frequency are explained by means of electromagnetic simulations of the antenna and the photomixer vicinity.
Abstract: Sulphide glasses doped with rare-earth ions have been demonstrated to be suitable for photonic applications such as optical amplifiers, up-converters and fiber lasers. The substitution of metal halides into the glass network has been shown to result glasses with desirable properties in terms of quantum efficiency and fiber manufacture [J.R. Hector, J. Wang, D. Brady, M. Kluth, D.W. Hewak, W.S. Brocklesby, D.N. Payne, Journal of Non-Crystalline Solids 239 (1998) 176]. To assist in the understanding of this improvement a structural analysis of glasses with a composition xCsCl(1 â x)Ga2S3 has been undertaken in order to examine the nature of the gallium environment. Information collected by high energy X-ray diffraction and neutron diffraction have been analyzed to permit the identification of the structural units as Ga centered tetrahedra. The interconnection between the tetrahedra was found to be predominantly corner sharing.
Notes: Keywords: Neutron diffraction/scattering; X-ray diffraction; Glasses; Chalcogenides; Chalcohalides; Rare-earths in glasses; Short-range order
Abstract: The fabrication of InGaAs/InP uni-travelling-carrier photodiodes integrated with broadband horn antennas and demonstration of photomixing up to 1.8 THz are reported. A radiated power of 1.1 µW at 940 GHz was measured for a photocurrent of only 2.75 mA (50 mW optical power), demonstrating the high efficiency of the device. The conversion efficiency obtained has been increased by more than a decade compared to the best reported values in the literature. The photomixer is used to identify the 1411 GHz water vapour absorption line at atmospheric pressure.
Abstract: This paper describes the performance of a coherent homodyne spectrometer to preserve phase information of the THz electric field. Thereby, optical constants of a high resistive silicon sample are presented between 200 GHz and 1.2 THz. The phase shift introduced by such a thick sample is much greater than 2 pi, and the manner of solving the ambiguity of the modulo 2 pi is discussed and can be used to surround the refractive index mainly at high frequency where the signal to noise ratio decreases. The analysis of the coherent homodyne signal as a function as the frequency at a fixed delay line permits deduction of sample thickness or measurement of distance without any mechanical displacement, and so facilitates potential ranging and tomography studies.
Abstract: Terahertz time domain spectroscopy and photomixing have been used alongside one another for the detection and the quantification of small polar species in mainstream cigarette smoke. The broadband submillimeter source used in time domain spectroscopy allowed a rapid and simultaneous detection of several pure rotational transitions of hydrogen cyanide (HCN) and carbon monoxide (CO) in realistic conditions of pressure and temperature. The spectral purity of the continuous wave terahertz source produced by photomixing, permitted the concentrations of these molecules to be measured at pressures of tens of hPa. Moreover, at lower pressure, traces of formaldehyde (H2CO) have been unambiguously identified at frequencies above 1 THz. A comparison with chemical analytical methods has been completed for each molecule highlighting the advantages of the direct measurement by THz spectroscopy.
Abstract: Photomixing and coherent homodyne detection have been used to measure the absorption and dispersion of dielectric media. The signal to noise ratio was sufficient to allow the dispersion and absorption characteristics of a polytetrafluoroethylen (Teflon) sample to be extracted up to a frequency of 1.6 THz. Due to the relatively narrow spectral linewidth of terahertz source, estimated to be in order of 5 MHz, the anomalous dispersion in the vicinity of a rotational transition of carbonyl sulfide is clearly observed, with a refractive index variation of about 10(-4) being measured at frequencies within 40 MHz of the transition. (c) 2006 American Institute of Physics.
Abstract: Continuous-wave terahertz spectroscopy by photomixing is applied to the analysis of mainstream cigarette smoke. Using the wide tunability of the source, spectral signatures of hydrogen cyanide (HCN), carbon monoxide (CO), formaldehyde (H2CO), and water (H2O) have been observed from 500 to 2400 GHz. The fine spectral purity allows direct concentration measurement from the pure rotational transitions of HCN and CO. The quantification of the measurement was validated by the means of a calibration gas containing CO. The potential of this technique for trace gas detection is demonstrated with an estimated detection limit of HCN equal to 9 parts in 10(6). (c) 2006 Optical Society of America.
Abstract: A spectrometer operating in the 100-2000 GHz range and allowing for absolute line strength measurements has been developed. The continuous wave terahertz radiation is generated by mixing two Ti:Sapphire laser beams in a vertically integrated low temperature grown GaAs (LTG-GaAs) photomixer. Pure rotational lines of (OCS)-O-16-C-12-S-32 in the ground vibrational state have been considered for J values up to 90. Observed self-broadening parameters are in agreement with those deduced from infrared experiments. For the first time in the submillimeter range, absolute line strengths have been determined, allowing for a determination of the electric dipole moment in good agreement with the value previously obtained from Stark effect measurements. (c) 2006 Elsevier Inc. All rights reserved.
Abstract: The photoinscription of standard telecommunications glass materials by femtosecond radiation is shown to induce significant mesoscopic structure. The scattering intensity for irradiated glasses is close to two orders of magnitude greater than that of unexposed material. Anomalous small-angle X-ray scattering (ASAXS) around the germanium K-edge for the silica and germanium doped silica regions of a fibre preform is used to demonstrate that identical structures are induced in both glass materials, with germanium displaying a capacity to isomorphically replace silicon in the case of the germanium doped silica. Analysis of measured scattering indicates that photo-inscribed features are produced at two. distinct scales with typical radii of R approximate to 20 angstrom and R-min approximate to 200 angstrom. (c) 2005 Elsevier B.V. All rights reserved.
Abstract: Design requirements for an 8000 frame/s dual-wavelength ratiometric chemical species tomography system, intended for hydrocarbon vapor imaging in one cylinder of a standard automobile engine, are examined. The design process is guided by spectroscopic measurements on iso-octane and by comprehensive results from laboratory phantoms and research engines, including results on temporal resolution performance. Novel image reconstruction techniques, necessary for this application, are presented. Recent progress toward implementation, including details of the optical access arrangement employed and signal-to-noise issues, is described. We present first cross-cylinder IR absorption measurements from a reduced channel-count (nontomographic) system and discuss the prospects for imaging. (c) 2005 Optical Society of America.
Abstract: The tomographic measurement of the residual stress profile in femtosecond-laser irradiated standard SMF-28 germanium-doped telecommunication fiber is demonstrated. The fiber is irradiated with weakly focused pulses to realize long-period fiber gratings. In the irradiated grating regions, an asymmetrical increase in axial core stress up to 6.2 kg/mm(2) is found. The increase in stress is attributed to a densification of the irradiated glass matrix. The stress-induced anisotropic index distribution is calculated and related to the absolute index change in the irradiated regions. (C) 2004 American Institute of Physics.
Abstract: Long-period gratings (LPGs) have been inscribed by femtosecond laser radiation into fiber optics with Ge-doped and pure silica cores. Grating parameters of 16-dB rejection and 0.3-dB insertion loss are obtained for a Ge-doped fiber core. Numerical calculation of the transmission characteristics yielded a good correlation between the measured and synthetic data showing that an induced refractive index change up to 3 X 10(-4) can be achieved. Inscription of an LPG in pure silica fiber core is demonstrated and thought to be first achieved in this fiber type. Greater laser intensity is required in pure silica compared with Ge-doped cores to produce a refractive index change of similar magnitude.
Abstract: The generation of continuous coherent THz radiation by mixing two cw Ti:Sa laser beams with a well-controlled frequency separation for a new scheme of vertically integrated low temperature grown GaAs (LTG-GaAs) spiral photomixer is reported. For this new photomixer device used in THz emission, the LTG-GaAs active layer is sandwiched between the two parallel metal plates of a high-speed photodetector loaded by a broadband spiral antenna. We have exploited the advantage of a higher delivered power in the low part of the spectrum (<2000 GHz), while a low RC time constant planar interdigitated detector was used at the upper frequency. The performances of the spectroscopic setup in terms of spectral resolution (5 MHz), tunability and frequency capability are assessed by measurements of the pure rotational spectra of hydrogen sulfide (H2S) up to 3000 GHz.
Abstract: We report on an optical fiber probe, coupled to a 404-nm laser diode, as a fluorescence sensor for monitoring of commercial gasolines. The principle of operation of the sensor is based on quantifying the intensity of the Stokes-shifted fluorescence from some of the heavier polycyclic aromatic hydrocarbons CxHy, (x,y) greater than or equal to (14,10) present in gasolines as minor constituents. The normalized efficiency of the optical fiber probe, as a function of its geometry is calculated in the cases of single-fiber and parallel dual-fiber designs. The spatial and temporal resolutions achievable by the sensor are discussed as a function of design parameters The performance of the sensor is investigated experimentally for commercial gasolines in the liquid and gas phase. The optimal excitation wavelength for such sensors is investigated in the range of 350-400 nm. The linear sensitivity to vapor concentrations of retail gasoline fuel is demonstrated in the range of 4%-125% of combustion stoichiometry at 10 bar and 180 degreesC. Statistical processing of the data from the sensor allows distinction to be made between different forecourt gasoline suppliers, as well as fuel varieties (unleaded, low sulfur, etc.).
Abstract: A fluorescence-based fiber-optic probe is implemented for use with a blue/UV laser diode to form a rugged, nonintrusive fuel sensor. Measurements are obtained from a test vessel controlled to replicate the temperature and pressure environment in a typical combustion engine. A fuel containing a specific polycyclic aromatic hydrocarbon (perylene, C20H12) is used to obtain wavelength resolved information and concentration variation characteristics. Emission intensity is examined in atmospheres with and without oxygen at pressures from 1 to 10 bar. The signals from a retail gasoline fuel present a greater challenge as they result from a complex mixture of hydrocarbon components. Detection of gasoline vapor concentrations in the range of 4% to 125% of stoichiometry is demonstrated.
Abstract: The recent availability of reliable and relatively low-cost GaN based semi-conductor laser diode sources emitting at 404 nm has opened many new areas for fluorescence based measurements. This article characterizes the fluorescence behavior of commercial liquid-phase gasoline samples using such excitation sources. Comparison is drawn with the emission when excited using broad-band sources at shorter wavelengths (340 nm). Here, 404 nm is shown to selectively excite the larger CxHy polycyclic aromatic hydrocarbons (PAH) commonly found as minor constituents of gasoline, mainly for (x, y) greater than or equal to (14, 10). Both Stokes and anti-Stokes shifted emission was observed in all the gasoline tested and in some PAH samples. The fluorescence is usually superimposed on Raman scattered laser light, arising from vibrations within the basic benzene structures. The fluorescence features of the gasoline samples tested were found to be broadly similar, but, because of its distinctive spectroscopic features, the fluorescence arising from benzo(a)pyrene was found to be one of the main variants. More generally, principal component analysis of the spectra was able to highlight differences between both the sample provenance and the fuel variety.
Abstract: We report an All-Opto-Electronic tomography system that is sensitive to hydrocarbon vapour distribution, or liquid spray distribution, with temporal resolution of over 3000 frames per second. A tomography system comprising 32 channels has been built and tested. For chemical sensitivity to saturated hydrocarbons, we exploit the principle of Near Infra-Red (NIR) absorption at 1700 nm relative to a reference wavelength, using laser diode sources whose technology is based on that of the communications industry. Images are obtained from a laboratory set-up incorporating both gaseous injection and a liquid Gasoline Direct Injection (GDI) system. The performance of a prototype system on a running GDI engine is reported. The difficulty in performing concentration measurements of the gaseous fuel within the liquid spray region is shown, and means to improve this performance are discussed. However, it has been found possible to image the liquid spray cone using attenuation of the reference beam. These images correlate well with other techniques [1].
Abstract: The spatial distribution of chemical species can be a critical determinant of chemical reactor performance. The spatial variation of air-fuel ratio in a combustion chamber of an internal combustion engine has significant influence on fuel efficiency and emissions. We report the development of a fiber-based near-Infrared absorption tomography system, to measure the distribution of hydrocarbons in cylinder. It has been successfully applied to transient gas injections. The technique exploits the specific (but weak) hydrocarbon absorption of 1.7 mum radiation, which wavelength has only recently become accessible by availability of solid-state alloptoelectronic components. A standard telecommunications laser was also deployed to measure reference information. The measurement space is sampled by 32 dual- wavelength fiber-coupled measurement paths. The logarithm of the ratio of the two measurements yields the path Integral of the hydrocarbon absorption, and hence, of concentration. The path integral is measured with typically 28 dB signal-to-noise ratio. Single-channel characterization shows that the technique is readily calibrated for temperature and pressure effects, over the region 70-150 degreesC and 1-10 bar. Tomographic reconstruction of different gaseous hydrocarbon flows has been achieved with spatial resolution of the order D/5, where D is the vessel diameter. Temporal resolution of about 1000 frames/s is demonstrated. (C) 2001 SPIE and IS&T.
Abstract: The spatial distribution of chemical species can be a critical determinant of the performance of chemical reactors. One such reactor is the combustion chamber of the Internal Combustion engine. This paper presents a design for the measurement of hydrocarbon concentration distribution within a running engine using near infra-red absorption tomography. The fundamentals of the technique, and design parameters for the equipment are discussed. By utilising micro-optic components, a minimally invasive system is feasible and by utilising advanced laser/photodetector combinations, good temporal performance is anticipated. (C)2000 Elsevier Science S.A. All rights reserved.
Abstract: A novel concept is introduced for application to chemical species tomography. The technique is based on fluorescence, using collimated sources and detectors. It does not require a complex inversion step to calculate the distribution of the desired parameter. Suitable gaseous fuels, dopants, and sources are assessed for potential use in a fuel imaging system within an internal combustion engine. The technique has difficulty in meeting the temporal requirements in such a gas-phase application. However, first data are presented from liquid-phase experiments that demonstrate the technique in operation. (C)2000 Elsevier Science S.A. All rights reserved.
