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The precise detection of volatile organic compounds plays a pivotal role in addressing environmental concerns, industrial safety, and medical diagnostics. The accurate identification and quantification of these compounds because of their ubiquity and potential health hazards has fueled the development of advanced sensing technologies. This work presents a sensing system in the realm of long-wavelength infrared spectroscopy for achieving enhanced selectivity and sensitivity of benzene, toluene, and propane detection through quartz-enhanced photoacoustic spectroscopy. High-resolution gas spectroscopy is made possible by the use of specially designed InAs/AlSb-based quantum cascade lasers, emitting in the wavelength range 13–15 μm, and quartz tuning forks. The sensor system, characterized by its robustness and precision, demonstrates exceptional capabilities in benzene, toluene, and propane detection. The system's capacity for practical applications in environmental monitoring and medical diagnostics is demonstrated by its ability to distinguish these volatile organic compounds with a minimum detection limit of 113 ppb, 3 ppb, and 3 ppm for toluene, benzene, and propane at an integration time of 10 s, even in complex gas matrices. This work advances gas sensing technology while also offering insightful information on spectral interferences, a persistent problem in the field. The results usher in a new era of sophisticated and reliable gas sensing techniques meeting the growing demand for precise volatile organic compounds detectors for environmental monitoring purposes. [ABSTRACT FROM AUTHOR]

Infrared laser technology over the last decades has led to an increasing demand for optical detectors with high sensitivity and a wide operative spectral range suitable for spectroscopic applications. In this work, we report on the performance of a custom quartz tuning fork used as a sensitive and broadband infrared photodetector for absorption spectroscopy. The photodetection process is based on light impacting on the tuning fork and creating a local temperature increase that generates a strain field. This light-induced, thermoelastic conversion produces an electrical signal proportional to the absorbed light intensity due to quartz piezoelectricity. A finite-element-method analysis was used to relate the energy release with the induced thermal distribution. To efficiently exploit the photo-induced thermoelastic effects in the low-absorbance spectral region of quartz also, chromium/gold layers, acting as opaque surface, have been deposited on the quartz surface. To demonstrate the flat response as photodetectors, a custom tuning fork, having a fundamental resonance frequency of 9.78 kHz and quality factor of 11 500 at atmospheric pressure, was employed as photodetector in a tunable diode laser absorption spectroscopy setup and tested with five different lasers with emission wavelength in the 1.65–10.34 μm range. A spectrally flat responsivity of ∼2.2 kV/W was demonstrated, corresponding to a noise-equivalent power of 1.5 nW/√Hz, without employing any thermoelectrical cooling systems. Finally, a heterodyne detection scheme was implemented in the tunable diode laser absorption spectroscopy setup to retrieve the resonance properties of the quartz tuning fork together with the gas concentration in a single, fast measurement. [ABSTRACT FROM AUTHOR]

An optical gas sensing technique based on in-plane quartz-enhanced photoacoustic spectroscopy (IP-QEPAS) is reported. In IP-QEPAS, the laser beam is aligned in the plane of the quartz tuning fork (QTF) to increase the interaction area between the acoustic wavefront and the QTF. A custom T-shaped QTF with a prong length of 9.4 mm and a resonance frequency of 9.38 kHz was designed and employed in the IP-QEPAS sensor. For comparison, the traditional QEPAS sensor in which the laser beam is perpendicular to the QTF plane (PP-QEPAS) is investigated with the same operating conditions. Theoretical calculations of strain and displacement of the QTF prong were performed to support the advantage of using the IP-QEPAS technique. By selecting water vapor as the gas target, the IP-QEPAS sensor results in a signal more than 40 times higher than that measured with the PP-QEPAS configuration, confirming the potential of this approach. [ABSTRACT FROM AUTHOR]

A highly sensitive trace gas sensor based on light-induced thermoelastic spectroscopy (LITES) and a custom quartz tuning fork (QTF) is reported. The QTF has a T-shaped prong geometry and grooves carved on the prongs' surface, allowing a reduction of both the resonance frequency and the electrical resistance but retaining a high resonance quality factor. The base of the QTF prongs is the area maximizing the light-induced thermoelastic effect. The front surface of this area was left uncoated to allow laser transmission through the quartz, while on the back side of the QTF, a gold film was coated to back-reflect the laser beam and further enhance the light absorption inside the crystal. Acetylene (C2H2) was chosen as the target gas to test and validate the LITES sensor. We demonstrated that the sensor response scales linearly with the laser power incident on the prong base, and the optimum signal to noise ratio was obtained at an optical power of 4 mW. A minimum detection limit of ∼325 ppb was achieved at an integration time of 1 s, corresponding to a normalized noise equivalent absorption coefficient of 9.16 × 10−10 cm−1W/√Hz, nearly one order of magnitude better with respect to the value obtained with a standard 32.768 kHz QTF-based LITES sensor under the same experimental conditions. [ABSTRACT FROM AUTHOR]

We compared the thermal performance of mid-IR GaInAs/AlInAs quantum cascade lasers (QCLs) having identical gain medium, but different heat sinking configurations. By using a two-dimensional anisotropic thermal model, we have calculated the temperature profiles and the heat flow patterns of ridge waveguide QCLs, either buried or planarized, and mounted substrate-side or epilayer-side down. Device planarization with Y2O3:Si3N4 dielectric layers gives an ∼7% reduction of the device thermal resistance with respect to InP buried heterostructures. If this planarization is combined with thick gold electroplating and epilayer-side mounting of the device, the thermal resistance is reduced by ∼34% and ∼50%, respectively, with respect to conventional ridge waveguide structures. [ABSTRACT FROM AUTHOR]

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 80–250 K, 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 400 K. 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 4 μm have better thermal properties than the buried structures. [ABSTRACT FROM AUTHOR]

We employ a microprobe photoluminescence (PL) technique to determine the thermal resistance and wall-plug efficiency of narrow-ridge interband cascade lasers emitting at 3.8 μm. Using two different semiconductor epilayers as integrated thermometers, the local lattice temperature is extracted from the PL spectra and the wall-plug efficiency (ηw) derived from the slope of the temperature increase versus electrical power. The maximum ηw at 78 K is found to be 8.1±0.5%, and a fit to the lattice temperature gradient implies cross-plane thermal conductivities of 4.5–6.5 W/m K for the short-period InAs/AlSb superlattice cladding and of ≈1.5–4 W/m K for the active region. [ABSTRACT FROM AUTHOR]

We present a gas sensing system based on quartz-enhanced photoacoustic spectroscopy (QEPAS) employing a monolithic distributed-feedback quantum cascade laser (QCL) array operated in a pulsed mode as a light source. The array consists of 32 quantum cascade lasers emitting in a spectral range from 1190 cm−1 to 1340 cm−1. The optoacoustic detection module was composed of a custom quartz tuning fork with a prong spacing of 1 mm, coupled with two micro-resonator tubes to enhance the signal-to-noise ratio. The QEPAS sensor was validated by detecting the absorption of the P- and R-branches of nitrous oxide. The measurements were performed by switching the array QCLs in sequence while tuning their operating temperature to retrieve the fine structure of the two N2O branches. A sensor calibration was performed, demonstrating a linear responsivity for N2O:N2 concentrations from 1000 down to 200 parts-per-million. With a 10 s lock-in integration time, a detection sensitivity of less than 60 parts-per-billion was achieved permitting the monitoring of nitrous oxide at global atmospheric levels. [ABSTRACT FROM AUTHOR]

We have investigated the band-to-band photoluminescence of In0.53Ga0.47As/AlAs0.56Sb0.44 quantum-cascade devices operating at λ∼4.3 μm, based either on normally grown or nominally As-terminated interfaces. This technique allowed us to probe the spatial distribution of conduction electrons as a function of the applied voltage and to correlate the quantum design of devices with their thermal performance. [ABSTRACT FROM AUTHOR]

A double antinode excited quartz-enhanced photoacoustic spectroscopy (DAE-QEPAS) spectrophone, employing a custom-made quartz tuning fork (QTF) and operating at the 1stt overtone resonance mode is reported. The signal phase variation along the QTF prong was investigated, and a piezoelectric transducer was introduced to compensate the phase shift between two QTF separated 1st overtone antinode points. Two sets of acoustic micro-resonators were optimized and assembled at two antinode points to improve the spectrophone performance. With the two antinodes excited by one laser source, the DAE-QEPAS spectrophone attained a sensitivity gain factor of ∼100 times and ∼3 times with respect to the 1st overtone resonances of a bare custom QTF and a standard onbeam QEPAS spectrophone, respectively. H2O was selected as the target analyte and a detection limit of ∼230 ppb was obtained by the DAE-QEPAS spectrophone for a 1 s integration time, corresponding to a normalized noise equivalent absorption coefficient of 1.73×10-9cm-1·W·Hz-1/2. [ABSTRACT FROM AUTHOR]

A quartz enhanced photoacoustic spectroscopy (QEPAS) sensor, employing an erbium-doped fiber amplified laser source and a custom quartz tuning fork (QTF) with its two prongs spaced 800 lm apart, is reported. The sensor employs an acoustic micro-resonator (AmR) which is assembled in an "on-beam" QEPAS configuration. Both length and vertical position of the AmR are optimized in terms of signal-to-noise ratio, significantly improving the QEPAS detection sensitivity by a factor of ~40, compared to the case of a sensor using a bare custom QTF. The fiber-amplifier-enhanced QEPAS sensor is applied to H2S trace gas detection, reaching a sensitivity of ~890 ppb at 1 s integration time, similar to those obtained with a power-enhanced QEPAS sensor equipped with a standard QTF, but with the advantages of easy optical alignment, simple installation, and long-term stability. [ABSTRACT FROM AUTHOR]

A quartz-enhanced photoacoustic spectroscopy sensor system was developed for the sensitive detection of hydrogen peroxide (H2O2) using its absorption transitions in the v6 fundamental band at 7∼.73 lm. The recent availability of distributed-feedback quantum cascade lasers provides convenient access to a strong H2O2 absorption line located at 1295.55 cm-1. Sensor calibration was performed by means of a water bubbler that generated titrated average H2O2 vapor concentrations. A minimum detection limit of 12 parts per billion (ppb) corresponding to a normalized noise equivalent absorption coefficient of 4.6×10-9 cm-1W/Hz1/2 was achieved with an averaging time of 100 s [ABSTRACT FROM AUTHOR]

We measured the time dependence of the local lattice temperature in terahertz quantum cascade lasers with surface plasmon waveguides. The time constants for heat extraction from the active region (0.15–0.29 μs) are approximately ten times shorter than those associated with heat extraction from the substrate, thereby showing the dominant role of the substrate-heat sink coupling. Thermal diffusivities D=0.052 cm2/s (Ds=3.9 cm2/s) and thermal diffusion lengths μ=1.2 μm (μs=31 μm) in the active region (substrate) have been extracted by fitting the solution of the heat transport equation to the experimental data. [ABSTRACT FROM AUTHOR]

We extracted the electronic temperatures, the thermal resistance (RL=11.5 K/W), the cross-plane thermal conductivity [k⊥=2.0±0.1 W/(K m)], and the thermal boundary resistance [TBR=(4.1–9.3)×10-10 K/W m2] in strain-compensated Ga0.609In0.391As/AlIn0.546As0.454 quantum-cascade lasers operating at 4.78 μm in continuous wave up to 15 °C and in pulsed mode up to 40 °C. Submonolayer thick InAs and AlAs δ layers are included in the active region to increase the conduction band discontinuity. We found that potential interface broadening caused by the insertion of these δ layers allows for a 43% improvement of the thermal conductivity with respect to conventional lattice-matched GaInAs/AlInAs heterostructures. [ABSTRACT FROM AUTHOR]

Improved optical power performance of bound-to-continuum quantum-cascade lasers operating at 2.83 THz is reported. Peak optical powers of 100 mW at 4 K and power conversion-efficiencies as high as ηw=(5.5±0.4)% in continuous wave at 40 K 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±0.11 W/A. [ABSTRACT FROM AUTHOR]

The authors extracted the thermal resistance (RL=9.6 K/W) and the electrical power dependence of the electronic temperature (Re=12.5 K/W) of Ga0.47In0.53As/Al0.62Ga0.38As1-xSbx quantum-cascade lasers (QCLs) operating at 4.9 μm, in the lattice temperature range of 60–90 K. The low electron-lattice coupling constant α=10.4 K cm2/kA can be related to the beneficial effect of the high conduction band offset, peculiar to the GaInAs/AlGaAsSb material system, on the electron leakage. The authors found an active region cross-plane thermal conductivity value k⊥=1.8±0.1 W/(K m), which is approximately three times larger than that measured in QCLs with GaInAs/AlInAs heterostructures. [ABSTRACT FROM AUTHOR]

The electronic temperatures of the conduction subbands, the local lattice temperature, and the electron-lattice energy relaxation times (τE) in bound-to-continuum GaAs/AlxGa1-xAs quantum cascade lasers, operating at 2.9 and 2 THz, are reported. This information has been gathered from the analysis of microprobe photoluminescence spectra collected during device continuous wave operation. We find that the electronic distributions in both the active region and the injector are thermalized and that all subbands share the same electronic temperature. The efficiency of electron cooling increases with the conduction band offset. [ABSTRACT FROM AUTHOR]

We compare the thermal properties of THz quantum cascade lasers (QCLs) fabricated with metal-metal optical waveguides based on Au/Au or In/Au wafer bonding. This information was obtained from the analysis of microprobe band-to-band photoluminescence spectra measured on devices operating in continuous wave (cw). The experimental normalized thermal resistances (RL*), show that the use of Au/Au wafer bonding optimizes the heat dissipation. Comparison with surface-plasmon based THz QCLs, demonstrates that the use of metal-metal wafer bonding can allow cw operation at progressively higher temperatures. [ABSTRACT FROM AUTHOR]

We measured the thermal resistance (R=20.1 K/W) and the electrical power dependence of the electronic temperature (Re=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) ∼50 times lower than THz QCLs with resonant-phonon active region scheme. [ABSTRACT FROM AUTHOR]

We compare the electronic temperatures and the population inversion both below and above the lasing threshold in three quantum-cascade lasers (QCLs) operating at 2.8 THz, 3.2 THz, and 3.8 THz using microprobe band-to-band photoluminescence. In the lasing range, while the ground-state temperature remains close to the lattice one (90 K–100 K), the upper radiative state heats up to ∼200 K. 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. [ABSTRACT FROM AUTHOR]

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. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

We have determined the shear stress associated with the temperature gradient in quantum cascade lasers operated in continuous-wave mode. This information was obtained as a function of the electrical power using a combination of microprobe photoluminescence and anti-Stokes/Stokes Raman measurements in ridge-waveguide GaAs structures mounted epilayer down to the heat sink. At electrical power densities in the order of ∼5 kW/cm(2), the strain in the cladding layers at the edges of the laser ridges reaches the critical value for the creation of misfit dislocations. Above 10-12 kW/cm(2), extended defect formation and eventual device failure are observed. [ABSTRACT FROM AUTHOR]

Observation of the nonequilibrium optical phonons population associated with electron transport in quantum-cascade lasers is reported. The phonon occupation number was measured in the range 75-280 K by using a combination of microprobe photoluminescence and Stokes/anti-Stokes Raman spectroscopy. The excess phonon population is observed to decrease as the lattice temperature increases. From the nonequilibrium phonon population, we extracted interface phonon lifetimes of 5 ps at 75 K and 2 ps at 280 K. [ABSTRACT FROM AUTHOR]

The local temperature of quantum-cascade lasers operating in continuous wave mode is reported. This information is extracted from the thermal shift of the band-to-band photoluminescence peaks in the AlInAs and InP cladding layers of quantum-cascade laser facets using a high-resolution microprobe setup. Interpolation by means of a two-dimensional heat diffusion model allows to obtain the temperature profile and the thermal conductivity in the waveguide core. Comparison between substrate and epilayer-side mounted lasers shows the superior thermal dissipation capability of the latter, and explains their better performance with respect to threshold current and maximum operating temperature. [ABSTRACT FROM AUTHOR]

We report on the determination of thermal resistance, facet temperature profile, and heat flux of GaAs/Al[sub 0.33]Ga[sub 0.67]As quantum-cascade lasers operating in pulsed mode, using a microprobe band-to-band photoluminescence technique. The thermal resistance of epilayer-side mounted lasers is ∼30% smaller than that of substrate-side mounted ones. The dependence of the thermal resistance on the injection conditions and its correlation with the output power is also reported. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A single-tube on-beam quartz enhanced photoacoustic spectroscopy (SO-QEPAS) spectrophone, which employs a custom-made quartz tuning fork (QTF) having a prong spacing of 700 lm and operating at the 1st overtone flexural mode, is reported. The design of QTF prong geometry allows the bare QTF to possess twice higher Q-factor values for the 1st overtone resonance mode falling at ∼17.7 kHz than in the fundamental resonance mode at ∼2.8 kHz, resulting in an 8 times higher QEPAS signal amplitude when operating in the 1st overtone resonance mode. Both the vertical position and length of the single-tube acoustic micro-resonator (AmR) were optimized to attain optimal spectrophone performance. Benefiting from the high overtone resonance frequency and the quasi 1st harmonic acoustic standing waves generated in the SO-QEPAS configuration, the AmR length is reduced to 14.5 mm. This allows the realization of compact spectrophone and facilitates the laser beam alignment through the QTF þ AmR system. The signal enhancement in the overtone resonance mode and the high acoustic coupling efficiency between the AmR and QTF in the SOQEPAS configuration yields an overall sensitivity enhancement factor of ∼with respect to the bare custom QTF operating in the fundamental resonance mode. [ABSTRACT FROM AUTHOR]

This review aims to discuss the latest advancements in quartz-enhanced photoacoustic spectroscopy (QEPAS) based trace-gas sensing. Starting from the QEPAS basic physical principles, the most used QEPAS configurations will be described. This is followed by a detailed theoretical analysis and experimental study regarding the influence of quartz tuning forks (QTFs) geometry on their optoacoustic transducer performance. Furthermore, an overview of the latest developments in QEPAS trace-gas sensor technology employing custom QTFs will be reported. Results obtained by exploiting novel micro-resonator configurations, capable of increasing the QEPAS signal-to-noise ratio by more than two orders of magnitude and the utilization of QTF overtone flexural modes for QEPAS based sensing will be presented. A comparison of the QEPAS performance of different spectrophone configurations is reported based upon signal-to-noise ratio. Finally, a novel QEPAS approach allowing simultaneous dual-gas detection will be described. [ABSTRACT FROM AUTHOR]

The electron population in the excited miniband of quantum cascade structures with intrinsic superlattice active regions is extracted from the fine structure analysis of spontaneous interminiband electroluminescence spectra. At current densities typical of laser thresholds, the electrons injected into the excited miniband of a (GaInAs)[sub 6 nm]/(AlInAs)[sub 1.8 nm] superlattice are described by a nonequilibrium thermal distribution characterized by temperatures T[sub e]>200 K, much higher than the lattice temperature T[sub L]=15 K. © 2000 American Institute of Physics. [ABSTRACT FROM AUTHOR]