Search

The biotechnological transformation process of hydrocortisone to 16α-hydroxy hydrocortisone, a precursor of the widely used anti-inflammatory drug desfluorotriamcinolone, performed by Streptomyces roseochromogenes , requires high conversion ratios, low by-product formation, and a good product recovery after microbial conversion. To reach these targets, in this research, new whole cell transformation and chromatographic purification approaches were exploited. The optimal pH and temperature settings for the microbial conversion were first studied in shake flasks and then in 2.5-L batch experiments. Pulsed feedings during fermentations, coupled with a two step substrate addition approach, pushed to a transformation ratio of 63.0 ± 3.0%, starting from 1 g L −1 of substrate. By employing a DO-stat feeding strategy in 22-L fed-batches the transformation ratio increased up to 80.0 ± 2.0% with a ratio of 16α-hydroxy hydrocortisone on the total products of 94.5 ± 0.4%. In addition a new protocol for the 16α-hydroxy hydrocortisone purification, directly from the broth supernatant, was set up by using reverse phase chromatography reaching a 85.0 ± 2% product recovery with a product purity grade of 93.0 ± 2.0%.

We investigate the optical anisotropy of single nanofibers realized by electrospinning a conjugated polymer. Polarized infrared and micro-Raman measurements evidence a higher degree of molecular orientation in fibers processed from tetrahydrofuran solutions, with respect to samples spun from mixture with dimethyl sulfoxide. The fraction of ordered molecules is correlated to the fibers morphology. Polarized photoluminescence highlights a larger red shift (60 meV) for spectra from fibers processed from tetrahydrofuran, confirming the higher achieved molecular order resulting in reduced interchain separation and hence excitonic emission with lower transition energies compared to that from randomly aligned molecules. Conjugated polymer fibers are obtained, emitting light with polarization ratios up to 5, usable as polarized photonic nanosources.

Chondroitin sulfate is a glycosaminoglycan widely used as active principle of anti-osteoarthritis drugs and nutraceuticals, manufactured by extraction from animal cartilaginous tissues. During the manufacturing procedures, another glycosaminoglycan, the keratan sulfate, might be contemporarily withdrawn, thus eventually constituting a contaminant difficult to be determined because of itsstructural similarity. Considering the strict regulatory rules on the pureness of pharmaceutical grade chondrotin sulfate there is an urgent need and interest to determine the residual keratan sulfate with specific, sensitive and reliable methods. To pursue this aim, in this paper, for the first time, we set up a multi-analytical and preparative approach based on: i) a newly developed method by high performance anion-exchange chromatography with pulsed amperometric detection, ii) gas chromatography-mass spectrometry analyses, iii) size exclusion chromatography analyses coupled with triple detector array module and on iv) strong anion exchange chromatography separation. Varied KS percentages, in the range from 0.1 to 19.0% (w/w), were determined in seven pharmacopeia and commercial standards and nine commercial samples of different animal origin and manufacturers. Strong anion exchange chromatography profiles of the samples showed three or four different peaks. These peaks analyzed by high performance anion-exchange with pulsed amperometric detection and size exclusion chromatography with triple detector array, ion chromatography and by mono- or two-dimensional nuclear magnetic resonance revealed a heterogeneous composition of both glycosaminoglycans in terms of sulfation grade and molecular weight. High molecular weight species (>100 KDa) were also present in the samples that counted for chains still partially linked to a proteoglycan core.

We report on single mode optical transmission of hollow core glass waveguides (HWG) coupled with an external cavity mid-IR quantum cascade lasers (QCLs). The QCL mode results perfectly matched to the hybrid HE11 waveguide mode and the higher losses TE-like modes have efficiently suppressed by the deposited inner dielectric coating. Optical losses down to 0.44 dB/m and output beam divergence of similar to 5 mrad were measured. Using a HGW fiber with internal core size of 300 mu m we obtained single mode laser transmission at 10.54 mu m and successful employed it in a quartz enhanced photoacoustic gas sensor setup

One of the top priorities in the development of quantum cascade laser sources is the optimization ofthe heat transport dynamic. We review here our experimental studies on the thermal properties of state ofart quantum cascade lasers operating both in the mid-IR and THz ranges. The experimental approach isbased on the investigation of the band-to-band photoluminescence signals, collected during device continu-ous wave operation. We measured the lattice temperature profile on the device front facet and using thesedata as inputs, we extract the heat dissipation patterns, the in-plane and the cross-plane active region thermalconductivitiesandthethermalboundaryresistanceforquantumcascadelasersbasedondifferentmaterialsystems.PACSnumbers:61.46.–w,61.82.Fk,68.35.Ct

"A multi-step plasma procedure for the fabrication of high-quality SiO2/Si interfaces and the functionalization of silicon dioxide surfaces has been developed for microelectronic manufacturing industry. The substrate activation based on a plasma-enhanced chemical vapour deposition (PECVD) has been developed in order to obtain high quality SiO2/Si heterostructures. The SiO2 films have been then exposed to additional PECVD processes in order to modify the dielectric surface by polar groups. The advantage of this method consists in the possibility of activating the substrate, depositing and functionalizing high-quality SiO2 films in a single run process, at low temperature, and by means of the same equipment. The reported method has also been exploited to produce device-quality silicon nitride/silicon heterostructures; preliminary results are shown. (C) 2007 Elsevier B.V. All rights reserved."

We have measured the local lattice temperature distribution and modeled the heat transport in all classes of quantum cascade lasers operating both in the mid-infrared and terahertz ranges. All relevant active regions based on GaAs/AlGaAs, GaInAs/AlGaAsSb, GaInAs/AlInAs/InP material systems have been investigated. A common feature of such complex multiple heterostructures is the strong anisotropy of thermal conductivity, its cross-plane component being much smaller than the in-plane one. Bulk contributions to this phenomenon are negligible, whereas a dominant role is played by the presence of abrupt sub-nanometer sized interfaces. The presence of a high density of interfaces causes phonon interference effects, which inherently limit the heat extraction. The values of the thermal boundary resistance have been extracted from our experimental data and compared among several devices. The possibility of generating stimulated emission of phonons in terahertz quantum cascade lasers will be also discussed. (C) 2007 Elsevier B.V. All rights reserved.

We measured the lattice temperature (T-L) distribution, the cross-plane thermal conductivity (k(perpendicular to)), and the thermal boundary resistance (TBR) of the GaAs/Al0.15Ga0.85As quantum cascade lasers (QCLs) operating at 2.83 THz in the heat sink temperature range 45-300 K. This information was extracted from the analysis of microprobe hand-to-hand photoluminescence in QCLs operating in continuous wave. Both k(perpendicular to) and TBR decrease monotonically at increasing temperature, the main influence on k(perpendicular to) arising from the high density of interfaces.

We have investigated plasmonic excitations at the surface of Bi_{2}Se_{3}(0001) via high-resolution electron energy loss spectroscopy. For low parallel momentum transfer q_{∥}, the loss spectrum shows a distinctive feature peaked at 104 meV. This mode varies weakly with q_{∥}. The behavior of its intensity as a function of primary energy and scattering angle indicates that it is a surface plasmon. At larger momenta (q_{∥}~0.04 Å^{-1}), an additional peak, attributed to the Dirac plasmon, becomes clearly defined in the loss spectrum. Momentum-resolved loss spectra provide evidence of the mutual interaction between the surface plasmon and the Dirac plasmon of Bi_{2}Se_{3}. The proposed theoretical model accounting for the coexistence of three-dimensional doping electrons and two-dimensional Dirac fermions accurately represents the experimental observations. The results reveal novel routes for engineering plasmonic devices based on topological insulators.

We report about fabrication and characterization of semiconductor nanowire-based field effect transistor devices which can act as detectors for electromagnetic radiation in the THz frequency range. The detection mechanism is based on the nonlinear transfer characteristic of the transistor, which is used to realize signal rectification; the small capacitance related to the nanowire small cross section is beneficial in allowing a good device sensitivity up to 1.5 THz at room temperature. Due to the extreme flexibility with which semiconductor nanowires can be grown, we discuss how the basic, homogeneous InAs or InSb nanowire FETs can be improved to realize smarter devices and functionalities.

To monitor the density of photo-generated charge carriers on a semiconductor surface, we demonstrate a detectorless imaging system based on the analysis of the optical feedback in terahertz quantum cascade lasers. Photo-excited free electron carriers are created in high resistivity n-type silicon wafers via low power (≅40 mW/cm2) continuous wave pump laser in the near infrared spectral range. A spatial light modulator allows to directly reconfigure and control the photo-patterned intensity and the associated free-carrier density distribution. The experimental results are in good agreement with the numerical simulations.

We report on a set of high-sensitivity terahertz spectroscopy experiments making use of QCLs to detect rotational molecular transitions in the far-infrared. We demonstrate that using a compact and transportable cryogen-free setup, based on a quantum cascade laser in a closed-cycle Stirling cryostat, and pyroelectric detectors, a considerable improvement in sensitivity can be obtained by implementing a wavelength modulation spectroscopy technique. Indeed, we show that the sensitivity of methanol vapour detection can be improved by a factor approximate to 4 with respect to standard direct absorption approaches, offering perspectives for high sensitivity detection of a number of chemical compounds across the far-infrared spectral range.

Nanometer size field effect transistors can operate as efficient resonant or broadband terahertz detectors, mixers, phase shifters and frequency multipliers at frequencies far beyond their fundamental cut-of frequency. This work is an overview of some recent results concerning the low temperatures operation, linearity, and circular polarization studies of nanometer scale field effect transistors for the detection of terahertz radiation. Also first results on graphene transistors are discussed. © (2013) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.

We demonstrate the phase-locking of a 2.5 THz Quantum Cascade Laser (QCL) to an air-propagating, 6 octaves-spanning, THz Optical Frequency Comb Synthesizer (OFCS). We directly observe the beating signal using a commercial detector, therefore proving that this approach provides a mature metrological tool in the THz region. We show that only a small fraction (

International audience; Heterostructured InAs/InSb nanowire (Nw) based field effect transistors (FET) have been fabricated and tested as Terahetz radiation detectors. While responsivity and noise equivalent power compare with the ones of InAs nanowire detectors, the presence of small-gap InSb semiconductor gives rise to interesting physical effects such an increase of the detected signal with charge injection through the wire, at odds with standard FET-detectors. Additionally, the photodetected signal voltage changes its sign after a threshold gate bias, which we explain considering surface-related transport and field asymmetries imposed by the use of a lateral gate electrode.

We report on the optical coupling between hollow core waveguides and external cavity mid-IR quantum cascade lasers (QCLs). Waveguides with 1000 mu m bore size and lengths ranging from 2 to 14 cm, with metallic (Ag)/dielectric (AgI or polystyrene) circular cross-section internal coatings, have been employed. Our results show that the QCL mode is perfectly matched to the hybrid HE11 waveguide mode, demonstrating that the internal dielectric coating thickness is effective to suppress the higher losses TE-like modes. Optical losses down to 0.44 dB/m at 5.27 mu m were measured in Ag/polystyrene-coated waveguide with an almost unitary coupling efficiency.

We report experimental evidence of intrinsic linewidth values approaching the quantum limit in far-infrared quantum cascade lasers through the measurement of their frequency-noise power spectral density and a thorough investigation of the physics behind it.

We demonstrate that azimuthally polarized micro-ring THz quantum-cascade lasers can be coupled to hollow metallic waveguides with efficiencies ≥96% and perfectly matched with the TE 01 waveguide mode, giving attenuation losses < 3dB/m. The use of Ag coated-flexible pipe waveguides allows propagation of either vertically polarized or azimuthally polarized QCL beams with propagation losses as low as 2.1–4.4 dB/m and bending losses lower than 1.2 dB.

We review the basic microscopic phenomena controlling the extraction of energy from the active regions of terahertz quantum cascade lasers. Using time-resolved microprobe photoluminescence, we measure the in-plane (v//) and cross-plane (v) heat transfer velocities and find a strong anisotropy, with v// slightly reduced by 30% and v reduced by a factor of 6.5 with respect to the corresponding bulk values. Combining this information with measurements of the cross-plane thermal conductivity enables us to estimate a phonon mean free path value L ~ 70 nm at 80 K, reduced by about one order of magnitude with respect to the bulk value in GaAs, but much larger than the average spacing between the interfaces in the active region. The latter finding is consistent with a heat transport model considering both thermal boundary resistance and phonon dispersion modification.

We demonstrate that azimuthally polarized micro-ring THz quantum-cascade lasers can be coupled to hollow metallic waveguides with efficiencies ≥96% and perfectly matched with the TE 01 waveguide mode, giving attenuation losses < 3dB/m. The use of Ag coated-flexible pipe waveguides allows propagation of either vertically polarized or azimuthally polarized QCL beams with propagation losses as low as 2.1-4.4 dB/m and bending losses lower than 1.2 dB.

We report on a promising experimental approach to efficiently couple quantum cascade laser (QCL) sources fabricated in either surface emitting micro-ring resonator or standard edge emitting geometry with terahertz (THz) hollow waveguides. We show that the THz beam of a QCL can be guided into flexible silver-coated polycarbonate waveguides having length in the range 4-12 cm with coupling efficiencies > 80%. The dominant optical modes propagating through the waveguides can be selected by varying the polarization of the incoming QCL beam, the coupling geometries, and/or bending the hollow waveguide. Either the lowest loss TE(11) or the TE(01) mode can be selected and guided through the waveguide with propagation losses in the range 2.1-4.4 dB/m and bending losses lower than 1.2 dB.

Experimental evidence of a physical phenomenon characteristic of semiconductor lasers, i.e. the cooling of the electrons above the threshold for stimulated emission, is reported. By using a terahertz quantum-cascade laser as a prototype structure we show that this effect is directly related with the internal quantum efficiency (η int ).

A detailed time resolved experimental investigation of the thermal parameters causing the temperature performance degradation of THz QCLs will be presented. Possible technological solutions aimed to the extension of the operating temperatures of THz QCLs will be discussed.

We report on the realization of a optoacoustic sensor based on mid-infrared quantum-cascade lasers (QCLs) for the detection of nitric oxide (NO) and formaldehyde (CH(2)O). A resonant photoacoustic cell equipped with 4 electret microphones was excited in its first longitudinal mode by the modulated laser light. A detection limit for of 300 parts in 10(9) (ppbv) for NO and 150 ppbv for CH(2)O is found, using distributed feedback QCLs operating in pulsed mode at 5.34 mu m and 5.6 mu m, respectively.

The nature of the electron distribution and the electron-lattice energy relaxation phenomena in all classes of quantum cascade lasers operating in the THz range, namely, resonant-phonon, bound-to-continuum, and interlaced photon-phonon designs are reviewed. Thermalized hot-electron distributions are found in all cases. However, electronic temperatures of individual conduction subband are strongly influenced by the specific quantum design and the actual electron-lattice energy relaxation channels. A wealth of information was obtained both below and above laser threshold from the analysis of micro-probe band-to-band photoluminescence spectra recorded with a spatial resolution of approximate to 2 mu m. The influence of the detailed knowledge of the hot electron distributions on the design of improved THz quantum cascade lasers aiming at high temperature operation will be discussed.

We have measured the electronic temperature of the upper laser level (TE u) during continuous-wave operation in THz quantum-cascade lasers and found an experimental correlation with the occurrence of stimulated emission and the internal quantum efficiency.

We compare a set of experimental lattice temperature profiles measured in a surface-emitting terahertz (THz) quantum-cascade laser (QCL) with the results of a 2-D anisotropic heat diffusion model. We evaluate the temperature dependence of the active of the cross-plane thermal conductivity (kappa(perpendicular to)) region which is known. to be strongly anisotropic due to its superlattice-like nature. Knowledge of (kappa(perpendicular to)) and its temperature dependence is crucial in order to improve the temperature performance of THz QCLs and this has been used to investigate the longitudinal lattice temperature distribution of the active region and to compare the thermal properties of metal-metal and semi-insulating surface-plasmon THz optical waveguides using a 3-D anisotropic heat diffusion model.

The use of a high-electron mobility semiconductor nanowire as transistor channel has recently allowed the extension of the spectral coverage of THz field-effect transistor detectors up to 1.5 THz. In this report, we demonstrate efficient operation of a field-effect transistor detector based on a semiconductor nanowire at a much higher frequency, 2.8 THz, with a responsivity ?5 V/W in a bandwidth ?100 kHz, thus proving the full potential of such approach for the detection of THz quantum cascade lasers. Finally, such a THz sensing system is exploited to perform raster scan transmission imaging, with high spatial resolution, signal-to-noise ratio, and acquisition rate. © 2014 AIP Publishing LLC.

The wall-plug efficiency and the thermal resistance of bound-to-continuum THz quantum-cascade lasers are extracted from the analysis of micro-probe photoluminescence spectra, via the direct measurement of the lattice temperature as a function of the electrical-power.

We present a comparative Monte-Carlo analysis of a set of GaAs-based THz quantum cascade lasers, allowing us to identify the effects limiting the device performance in the threshold region. Reasonable agreement with experiment is found.

Observation of non-equilibrium optical phonons population associated with electron transport in THz quantum cascade lasers is reported. The phonon occupation number was measured by using a combination of micro-probe photoluminescence and Stokes/Anti-Stokes Raman spectroscopy. Energy balance analysis allows us to estimate the phonon relaxation rate, that superlinearly increases with the electrical power in the range 1.5 W - 1.95 W, above laser threshold. This observation suggests the occurrence of stimulated emission of optical phonons.

Plasma-enhanced chemical vapour deposition has been applied to the fabrication of high-quality SiO2/Si interfaces and to the functionalization of the silicon dioxide surfaces for organic thin film transistor applications. The advantage of the method herein reported resides in the possibility of activating the substrate, depositing and functionalizing high-quality SiO2 films in a single-run process, at low temperature. The structural properties of silicon dioxide samples have been studied by infrared spectroscopy, angle resolved and depth profiling X-ray photoelectron spectroscopy. The electronic properties have been retrieved from the leakage current values and the Fowler-Nordheim current plots. (c) 2007 Elsevier B.V. All rights reserved.

We report the first measurements of the conduction subband electronic temperatures and local lattice temperature in THz quantum cascade lasers based on different active region and optical waveguide configurations both below and well above the laser threshold. These results have been achieved by using a non-invasive microprobe photoluminesce technique during device continuous wave operation.

We measured the thermal resistance (R=20.1 K/W) and the electrical power dependence of the electronic temperature (R-e=27.0 K/W) of THz quantum-cascade lasers (QCLs) with bound-to-continuum active region scheme in the lattice temperature range of 30-100 K. This information, obtained from the analysis of microprobe photoluminescence spectra for QCLs operating at 2.5 THz, gives an electron-lattice energy relaxation rate (0.1 ps(-1)) similar to 50 times lower than THz QCLs with resonant-phonon active region scheme.

We compare the electronic temperatures and the electron lattice coupling in mid-infrared and terahertz GaAs-based quantum cascade lasers (QCLs). Thermalized hot-electron distributions are found in both classes of QCLs. The results illustrate the influence of the quantum design on the electron-lattice energy relaxation rates, that ultimately determine the device thermal performance.

We have investigated the transport properties of In0.53Ga0.47As/AlAs0.56Sb0.44 quantum cascade lasers operating at λ ∼ 4.3 µm, by means of band-to-band photoluminescence (PL). The evolution of the PL bands with the applied electric field allows a detailed analysis of the injection and emission efficiency. A clear correlation between the device thermal performance and the electron distribution has been found.

The electronic temperatures, the lattice temperature and the electron-lattice energy relaxation rate in bound-to-continuum THz quantum-cascade lasers are extracted from the analysis of micro-probe photoluminescence spectra and compared with results for resonant-phonon structures.

A recent study initiated by the European Space Agency aimed at identifying the most promising technologies to significantly improve on the generation of coherent electromagnetic radiation in the THz regime. The desired improvements include, amongst others, higher output powers and efficiencies at increasingly higher frequencies, wider tunability and miniaturization. The baseline technologies considered revolve around Photomixing and novel laser based technologies compared to all electronic techniques. Some of the most significant findings will be presented together with technological developments and experimental results selected for medium to short term development. These technologies include advanced p-i-n photomixer with superlattice structures and, THz quantum cascade lasers. Recent results achieved in these fields will be put into the potential perspective for the respective technology in the future.

We report on the evidence of saturation effects in a rotational transition of CH3OH around 2.5 THz, induced by a free-running continuous-wave quantum cascade laser (QCL). The QCL emission is used for direct-absorption spectroscopy experiments, allowing to study the dependence of the absorption coefficient on gas pressure and laser intensity. A saturation intensity of 25 mu W/mm(2), for a gas pressure of 17 mu bar, is measured. This result represents the initial step towards the implementation of a QCL-based high-resolution sub-Doppler THz spectroscopy, which is expected to improve by orders of magnitude the precision of THz spectrometers. (C) 2015 AIP Publishing LLC.

Tuning of the emission wavelength in GaAs/AlxGa1-xAs bound-to-continuum quantum cascade lasers with different Al mole fractions (x) is reported. By varying x in the range of 0.37-0.52, a shift of the emission wavelengths of over 4 mu m has been observed. Using this method, laser action in the range of 11.2-15.3 mu m at temperatures T >= 260 K has been demonstrated with a record value of similar to 340 K for GaAs based QCLs operating at 13.5 mu m. (C) 2008 American Institute of Physics.

We report on the investigation of the electronic and thermal properties of high performance terahertz (THz) quantum cascade lasers (QCLs) based on a bound-to-continuum scheme and grown by molecular beam epitaxy (MBE) by a commercial provider. The local lattice temperatures, the subband electronic temperatures and the electron-lattice coupling constant have been extracted from the analysis of microprobe band-to-band photoluminescence spectra measured on devices operating in continuous wave (cw). We found that the electronic distributions in both the active region and the injector are thermalized and that all the subbands share the same electronic temperatures. The measured device thermal conductance C * = 4.6 W/(K×m) reflects the good thermal management of the investigated devices. The electron-lattice coupling constant α = 78.5 Kcm 2 /kA is a factor 1.65 higher than in mid-IR QCLs thus indicating the inefficient cooling of the electronic ensemble caused by the strong reduction of electron-LO phonons scattering channels.

New frontiers for high precision THz spectroscopy are presented. Current limits can be overcome by exploiting saturation effects on molecular transitions and coupling THz QCL radiation to an optical cavity. © 2014 OSA.

The electronic temperatures of individual subbands, the lattice temperature and the population inversion both below and above the laser threshold of THz quantum cascade lasers are extracted by the analysis of interband /spl mu/-probe photoluminescence spectra.

Thanks to the recent progresses in photonics and nanotechnology, Terahertz (THz) radiation is finding increasing use in a wide range of applications. Since their first demonstration, THz Quantum Cascade Lasers (QCLs) have been the most important sources for a large range of applications and also for fundamental research. An important characteristic of THz QCLs is the very low intrinsic linewidth of their emission [1], at a level of tens of Hz, and this further qualifies them as ideal metrological sources. Moreover thanks to recent improvements in THz technology, robust and versatile metrological frequency references are available [2-4], enabling absolute measurements of THz frequencies.

Progress in the development of tunable, highly coherent sources directly linked to the primary frequency standard, in the mid/far-IR range, with a number of applications including molecular sensing and imaging, will be discussed. © OSA 2013.

We report on the experimental observation of non-equilibrium longitudinal and transverse optical phonons populations associated with electron transport in resonant-phonon THz quantum-cascade lasers and compare the results with the outcome of Monte Carlo simulations.