Vincenzo Spagnolo

Quartz-Enhanced Photoacoustic Spectroscopy: A Review

A detailed review on the development of quartz-enhanced photoacoustic sensors (QEPAS) for the sensitive and selective quantification of molecular trace gas species with resolved spectroscopic features is reported. The basis of the QEPAS technique, the technology available to support this field in terms of key components, such as light sources and quartz-tuning forks and the recent developments in detection methods and performance limitations will be discussed. Furthermore, different experimental QEPAS methods such as: on-beam and off-beam QEPAS, quartz-enhanced evanescent wave photoacoustic detection, modulation-cancellation approach and mid-IR single mode fiber-coupled sensor systems will be reviewed and analysed. A QEPAS sensor operating in the THz range, employing a custom-made quartz-tuning fork and a THz quantum cascade laser will be also described. Finally, we evaluated data reported during the past decade and draw relevant and useful conclusions from this analysis.

Measurement of subband electronic temperatures and population inversion in THz quantum-cascade lasers

We compare the electronic temperatures and the population inversion both below and above the lasing threshold in three quantum-cascade lasers (QCLs) operating at 2.8THz, 3.2THz, and 3.8THz using microprobe band-to-band photoluminescence. In the lasing range, while the ground-state temperature remains close to the lattice one (90K–100K), the upper radiative state heats up to ∼200K. From the measured thermal resistance and the power dependence of the ground-state electronic temperature, we get a value of the electron-lattice energy relaxation rate comparable with that typical of midinfrared QCLs.

Photoacoustic Techniques for Trace Gas Sensing Based on Semiconductor Laser Sources

The paper provides an overview on the use of photoacoustic sensors based on semiconductor laser sources for the detection of trace gases. We review the results obtained using standard, differential and quartz enhanced photoacoustic techniques.

Ppb-level detection of nitric oxide using an external cavity quantum cascade laser based QEPAS sensor

Geometrical parameters of micro-resonator for a quartz enhanced photoacoustic spectroscopy sensor are optimized to perform sensitive and background-free spectroscopic measurements using mid-IR quantum cascade laser (QCL) excitation sources. Such an optimized configuration is applied to nitric oxide (NO) detection at 1900.08 cm(-1) (5.26 µm) with a widely tunable, mode-hop-free external cavity QCL. For a selected NO absorption line that is free from H(2)O and CO(2) interference, a NO detection sensitivity of 4.9 parts per billion by volume is achieved with a 1-s averaging time and 66 mW optical excitation power. This NO detection limit is determined at an optimal gas pressure of 210 Torr and 2.5% of water vapor concentration. Water is added to the analyzed mixture in order to improve the NO vibrational-translational relaxation process.

Part-per-trillion level SF6 detection using a quartz enhanced photoacoustic spectroscopy-based sensor with single-mode fiber-coupled quantum cascade laser excitation.

A sensitive spectroscopic sensor based on a hollow-core fiber-coupled quantum cascade laser (QCL) emitting at 10.54 μm and quartz enhanced photoacoustic spectroscopy (QEPAS) technique is reported. The design and realization of mid-IR fiber and coupler optics has ensured single-mode QCL beam delivery to the QEPAS sensor. The collimation optics was designed to produce a laser beam of significantly reduced beam size and waist so as to prevent illumination of the quartz tuning fork and microresonator tubes. SF(6) was selected as the target gas. A minimum detection sensitivity of 50 parts per trillion in 1 s was achieved with a QCL power of 18 mW, corresponding to a normalized noise-equivalent absorption of 2.7×10(-10) W·cm(-1)/Hz(1/2).

Optical sensor for real-time monitoring of CO 2 laser welding process

An optical sensor for real-time monitoring of laser welding based on a spectroscopic study of the optical emission of plasma plumes has been developed. The welding plasmas electron temperature was contemporarily monitored for three of the chemical species that constitute the plasma plume by use of related emission lines. The evolution of electron temperature was recorded and analyzed during several welding procedures carried out under various operating conditions. A clear correlation between the mean value and the standard deviation of the plasmas electron temperature and the quality of the welded joint has been found. We used this information to find optimal welding parameters and for real-time detection of weld defects such as crater formation, lack of penetration, weld disruptions, and seam oxidation.

Terahertz quartz enhanced photo-acoustic sensor

A quartz enhanced photo-acoustic sensor employing a single-mode quantum cascade laser emitting at 3.93 Terahertz (THz) is reported. A custom tuning fork with a 1 mm spatial separation between the prongs allows the focusing of the THz laser beam between them, while preventing the prongs illumination. A methanol transition with line-strength of 4.28 × 10−21 cm has been selected as target spectroscopic line. At a laser optical power of ∼ 40 μW, we reach a sensitivity of 7 parts per million in 4s integration time, corresponding to a 1σ normalized noise-equivalent absorption of 2 × 10−10 cm−1W/Hz½.

Thermal modeling of GaInAs∕AlInAs quantum cascade lasers

We measured the facet temperature profiles of GaInAs∕AlInAs quantum cascade lasers (QCLs) operating in continuous wave mode by means of microprobe photoluminescence. These results were used to evaluate the in-plane (k‖) and the cross-plane (k⊥) thermal conductivities of the active region and to validate a two-dimensional model for the anisotropic heat diffusion in QCLs. In the temperature range of 80–250K, k⊥ monotonically increases with temperature and remains one order of magnitude smaller than the thermal conductivities of bulk constituent materials. We found an excellent agreement between the calculated and experimental values of the thermal resistance of GaInAs∕AlInAs QCLs operating in continuous wave up to 400K. Comparison between the calculated thermal performances of QCLs sharing the same active region structure, but having either a buried or a ridge waveguide, shows that devices with Au contact layers thicker than 4μm have better thermal properties than the buried structures.

Simultaneous measurement of the electronic and lattice temperatures in GaAs/Al0.45Ga0.55As quantum-cascade lasers: Influence on the optical performance

We measured the electronic and lattice temperatures in steady-state operating GaAs/AlGaAs quantum-cascade lasers, by means of microprobe band-to-band photoluminescence. Thermalized hot-electron distributions with temperatures up to 800 K are established. The comparison of our data with the analysis of the temperature dependence of device optical performances shows that the threshold current is determined by the lattice temperature.

Terahertz quantum cascade lasers with large wall-plug efficiency

Improved optical power performance of bound-to-continuum quantum-cascade lasers operating at 2.83THz is reported. Peak optical powers of 100mW at 4K and power conversion-efficiencies as high as ηw=(5.5±0.4)% in continuous wave at 40K were measured. The ηw values were assessed via an experimental method based on the analysis of the local lattice temperature as extracted by microprobe photoluminescence versus electrical power. From the measured ηw values they extracted a slope efficiency value 0.41±0.11W∕A.