We present an investigation on the electrical and optical properties of tapered quantum cascade lasers emitting at 14-15 μm, based on the InAs/AlSb system. In tapered lasers the active zone volume is increased to obtain higher optical power outputs without degrading the beam quality. Devices with three different taper angles of 1°, 2° and 3° were examined in terms of electrical, optical, and spectral properties and were compared with conventional ridge waveguide lasers.
Photothermal Spectroscopy (PTS) is an indirect analytical technique in which the optical signal is directly proportional to the laser emission intensity. This direct dependence on the laser power means that - in contrast to more conventional transmission-absorption techniques - PTS fully benefits from the high power of novel tunable mid-infrared laser sources such as Quantum Cascade Lasers (QCLs). In particular, QCLs equipped with an external cavity (EC) allow broad tunability which can be exploited in the detection of liquids identified by broad absorption bands. To achieve high sensitivity in PTS it is also important to choose a sensitive mode of transducing photothermal signal. Among the PTS transduction techniques photothermal interferometry (i.e. the detection of the phase change resulting from sample heating) stands out due to its high sensitivity. In this work, we use an EC-QCL in a photothermal interferometry PTS setup for trace water detection. We employ a HeNe laser-based Mach-Zehnder Interferometer (MZI) with liquid flow-cells inserted in the two arms. An EC-QCL emitting in the range of 1570-1730 cm-1 is arranged co-linear to the analyte arm of the interferometer and used to target the bending mode (𝜈2 ~ 1645 cm-1) of water molecules in different matrices. Highest linearity and sensitivity are ensured by locking the MZI at its quadrature point via an active-feedback loop. Fluctuations and drifts are further minimized by means of temperature stabilization. When benchmarking the system against commercial FTIR spectrometers it is shown to be in excellent agreement with regards to band shapes, band positions and relative intensities and to compare favorably in terms of sensitivity. Achieved limits of detection (LODs) for water in chloroform and jet-fuel are in the low ppm range. Higher LODs orders of magnitude were obtained indeed for the case of water in ethanol. An analysis of the matrix influence on the PTS signal’s strength has been carried out. Results show how the choice of the matrix dramatically influences limits of detection and limits of quantification (LOQs).
We report on the detection of nitric oxide using an Interferometric Cavity-Assisted Photothermal Spectroscopy (ICAPS) gas sensor in combination with a DFB-QCL emitting at 1900 cm-1 as excitation source. In ICAPS, a probe laser is coupled to a Fabry-Perot interferometer acting as an optical transducer of thermal effects. A wavelength modulation approach of the probe diode laser was employed, to actively lock its wavelength to the point of highest sensitivity and linearity of the interferometric fringe for a stable readout. A normalized noise equivalent absorption of 5·10-6 Wcm-1Hz-1/2 was achieved corresponding to 1.4 ppm of NO.
We report on the detection of the isotopologues 12CH4 and 13CH4 by employing a quartz-enhanced photoacoustic spectroscopy (QEPAS)-based sensor. By properly selecting the exciting light source and the working conditions, two absorption lines, having a negligible cross-section ratio temperature coefficient of -6.7‰/°C and a cross section ratio of ~ 0.06 for a natural abundance of each isotope, can be targeted. The QEPAS signal of the two isotopologues was acquired for mixtures in nitrogen of methane in natural abundance in a wide range of concentrations (0.02%-20%) showing a non-linear trend with high concentrations and a constant ratio comparable with the cross-section.
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