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Trace gas detection has many applications in research and industry; such as breath analysis, environmental monitoring, plasma diagnostics, and fruit quality monitoring [1] . Amongst various methods used for trace gas detection, molecular absorption spectroscopy is a powerful technique, which provides fast, selective, sensitive, and non-destructive detection. The fundamental rotational-vibrational transitions of molecular species are mostly in the mid-infrared (MIR) "fingerprint" (2–20 μm) region, which are significantly stronger than the overtone or combination bands in the near-infrared region. Fourier transform spectroscopy (FTS) is one of the well-known optical techniques, which is capable of broadband detection in the MIR region. However, incoherent thermal light sources, traditionally used in FTS, are omnidirectional and have low spectral brightness. Thus, the maximum achievable interaction length with a gas sample is very limited, resulting in low detection sensitivity. On the other hand, recently developed MIR supercontinuum (SC) light sources can deliver very high spectral brightness compared to the thermal sources or even a synchrotron radiation source [2] . Here, we present a multi-species trace gas sensor based on a MIR SC source, a multi-pass absorption cell, and a compact home-built FTS. For enhancement of the detection sensitivity and reducing the averaging time, we utilized a balanced detection scheme to reduce the relative intensity noise. For this, we measured the two out-of-phase outputs of the interferometer by two detectors. The two interferograms are 180 degrees out-of-phase, but they have an in-phase intensity noise. By subtracting the two outputs, the interferograms are doubled, while the noise is significantly reduced.

Highly responsive ZnO-based photoconductors with a circular electrode scheme are introduced in this paper. Some cost-effective samples were fabricated through simple chemical routes and conventional photolithography methods. Al:ZnO thin films with a high density of hole trap states were prepared after several optimizations during the synthesis process. Circular electrodes with low ohmic contact resistances were formed by optimizing the metallization process. Then, the prepared prototypes were analyzed and their specifications were measured. Performance of the devices was also investigated by using a device simulator. The experimental data showed good agreement with the simulation results. Such that the high responsivity of about 1.9 × 104 AW–1 and signal-to-noise ratio of 9.14 × 103 were recorded after optimization of annealing temperature (at 350 °C), under a weak ultraviolet illumination of 268.27 μW⋅cm–2 at 253 nm wavelength.

A Fourier transform spectrometer (FTS), equipped with a broadband MIR supercontinuum (SC) source, is developed for analyzing CH4/N2 discharge plasma and the kinetics of plasma products (typically C2H6 and HCN) under different discharge currents.

We present a multi-species trace gas sensor based on a fast, compact home-built Fourier transform spectrometer (FTS) combined with a broadband mid-infrared supercontinuum (SC) source. The spectrometer covers the spectral bandwidth of the SC source (2 - 4 µm) and provides a best spectral resolution of 1 GHz in 6 seconds. It has a detection sensitivity of a few hundred of ppbv Hz-1/2 for different gas species. We study the performance of the developed spectrometer in terms of precision, linearity, long-term stability, and multi-species detection. We use the spectrometer for measuring fruit-produced volatiles under different atmospheric conditions and compare the performance with a previously developed scanning grating-based spectrometer.

Traditional Fourier transform spectrometers (FTSs) in an asymmetric configuration can provide both absorption and dispersion spectra of the molecules simultaneously. However, they require a long measurement time, mainly due to the limitations of the signal-to-noise-ration (SNR) as well as maximum speed and travel distance of the retro-reflector mirrors in the FTS. These constraints make FTS inefficient for time-dependent multispecies detection with high SNR in certain specific applications, like plasma and combustion systems. These limitations can be resolved by mid-infrared dual-comb spectroscoy (DCS), which provides unique features of broad spectral bandwidth, high spectral resolution, high detection sensitivity, and simultaneous measurement of absorption and dispersion spectra in the fingerprint region within a short acquisition time. Here, we report time-resolved absorption and dispersion spectroscopy of methane and ethane diluted in helium under static and dynamic electrical discharge plasma, covering a spectral bandwidth of 2850-3150 cm -1 , with a spectral resolution of 0.16 cm -1 [1] . We used an asymmetric configuration for the DCS to measure the absorption and dispersion spectra in a 50-cm-long discharge tube. In this configuration, the gas sample is placed in the beam path of the signal mid-infrared comb, and the transmitted beam interferes with the local mid-infrared comb on a beam splitter. Thus, the molecular absorption and dispersion information is probed by the signal comb, and the local comb is used to retrieve the molecular information through multi-heterodyne down-conversion. The absorption and dispersion spectra retrieved from DCS portray a complete picture of electromagnetic filed interaction with gas phase molecules, which can provide the broadband complex refractive index of the gas samples under discharge.

We present a fast-scanning Fourier transform spectrometer (FTS) in combination with high-repetition-rate mid-infrared supercontinuum sources, covering a wavelength range of 2–10.5 µm. We demonstrate the performance of the spectrometer for trace gas detection and compare various detection methods: baseband detection with a single photodetector, baseband balanced detection, and synchronous demodulation at the repetition rate of the supercontinuum source. The FTS uses off-the-shelf optical components and provides a minimum spectral resolution of 750 MHz. It achieves a noise equivalent absorption sensitivity of ∼10−6 cm−1 Hz−1/2 per spectral element, by using a 31.2 m multipass absorption cell.

An asymmetric time-resolved mid-infrared dual-comb spectrometer with 5 GHz spectral resolution is used to monitor the absorption and dispersion dynamics of different species and their rotational distributions within a non-thermal plasma.

We present a mid-infrared supercontinuum-based trace gas sensor. A lock-in technique is applied to enhance the signal-to-noise ratio by over an order of magnitude, achieving a sensitivity of ~10 ppbv Hz-1/2 for the detection of methane.

We present a multi-species trace gas sensor based on a high-repetition-rate mid-infrared supercontinuum source, in combination with a 30 m multipass absorption cell, and a scanning grating spectrometer. The output of the spectrometer is demodulated by a digital lock-in amplifier, referenced to the repetition rate of the supercontinuum source. This improved the detection sensitivity of the system by a factor 5, as compared to direct baseband operation. The spectrometer provides a spectral coverage of 950 cm−1 (between 2.85-3.90 µm) with a resolution of 2.5 cm−1 in 100 ms. It can achieve noise equivalent detection limits in the order of 100 ppbv Hz−1/2 for various hydrocarbons, alcohols, and aldehydes.

We present a transportable, multi-species, trace gas sensor using a high repetition rate mid-infrared supercontinuum source in combination with a lock-in amplifier, achieving tens of ppbv·Hz-1/2 detection sensitivity for multiple gas species.

We present a multi-species trace gas sensor based on broadband mid-infrared absorption spectroscopy as well as its application to measure fruit-produced volatiles under different atmospheric conditions, demonstrating its potential for monitoring fruit storage rooms.

The detection of gas-phase molecular species at low concentration levels is an attractive research field, supporting a wide variety of industrial, environmental, and biological applications. Among various types of detection methods, mid-infrared (MIR) laser absorption spectroscopy provides a unique opportunity for selective and sensitive detection by targeting the characteristic spectral patterns in the so-called MIR fingerprint region.

We present a fully integrated and transportable multi-species trace gas sensor based on a mid-infrared (MIR) supercontinuum light source. The high brightness (surpassing synchrotron) and ultra-broad spectral bandwidth (2&ndash, 4 &mu, m) of this light source allows simultaneous detection of multiple broadband absorbing gas species. High sensitivity in the sub-ppmv level has been achieved by utilizing an astigmatic multipass cell. A grating-based spectrometer at a scanning rate of 20 Hz is developed employing a balanced detection scheme. A multi-component global fitting algorithm is implemented into a central LabVIEW program to perform real-time data analysis. The obtained concentration values are validated by the standard gas chromatography mass spectrometry (GC-MS) method. Field application of the sensor for quality control of stored fruits at a small scale is demonstrated, involving the detection of ethylene, ethanol, ethyl acetate, acetaldehyde, methanol, acetone, and water simultaneously. The sensor also shows promising potentials for other applications, such as environmental monitoring and biomedical research.

Recent advancement in compact, stable, and broadband mid-infrared supercontinuum (SC) sources has provided attractive scientific and industrial applications in metrology, sensing, and communications [1]. The wide spectral coverage in the so-called molecular fingerprint region is the main advantage of such SC sources, opening up new opportunities for efficient multi-species trace gas detection at short time scale. Presently, there is a lack of sensitive, low-cost, uncooled MIR photodetectors. The alternative approach involves up-converting mid-infrared light into the near-infrared region via non-linear conversion, leveraging the benefits of sensitive and cost-effective CCD detectors [2]. Here, we present the development of a multi-species trace gas sensor utilizing a mid-infrared SC source in combination with a multipass absorption cell and an up-conversion-based spectrometer. The customized SC source (NKT Photonics) has a total power of 450 mW covering the 1.5–4.2 μm spectral window. The astigmatic Herriott multipass cell (AMAC-76, Aerodyne Research) provides an effective optical pathlength of 76 meters improving the detection sensitivity. Details of the up-conversion-based spectrometer (NLIR) have been recently published. [3]. Note that in the present upconversion system the intracavity laser at 1064 nm is generated by pumping the Nd:YVO 4 gain medium at 808 nm instead of 880 nm. In order to enhance the long term stability for single species detection, the amplitude drift of the SC source is minimized by counterbalancing the SC intensity with a non-absorbing reference.

Recent advancements of mid-infrared (MIR) supercontinuum light sources have opened up new possibilities in laser-based trace gas sensing. While the supercontinuum sources inherently support wide spectral coverage, the detection of broadband absorption signals with high speed and low cost is traditionally limited by the MIR detector arrays. In this work, we demonstrate that this limitation can be circumvented by upconverting the MIR signal into the near-infrared (NIR) region, where cost-effective silicon-based detector arrays can be utilized to measure broadband absorption. We also show that, by combining a MIR supercontinuum source with a MIR-to-NIR upconverter and an astigmatic multipass cell, fast detection (~20 ms) of ethane with sub-ppmv sensitivity can be achieved at room temperature. For multi-species detection, a least-square global fitting method is presented, showing a promising potential for applications such as environmental monitoring and biomedical research.

We present mid-infrared trace gas sensing based on a supercontinuum source combined with direct absorption spectroscopy and sum-frequency upconversion, showing broadband detection with a 10−4 Hz−1/2 noise equivalent absorbance sensitivity.

In this letter, we investigated Ti (20 nm)/Al (30 nm)/Au (60 nm) metallic contacts to Al-doped ZnO thin films prepared by the sol–gel method. In order to measure specific contact resistance, circular transfer length method structures were patterned on the samples by a lithography process. The samples were annealed at the different temperatures and electrical characterization ( $I$ – $V$ ) was performed. The annealing process was optimized to produce ohmic contacts with the lowest specific contact resistance. The results showed that the sample annealed at 350 °C had the lowest specific contact resistance of $\sim 3.7\times 10^{-7}~\Omega \cdot \textrm {cm}^{2}$ . Surface and cross section energy dispersive X-ray analysis was used to determine the distribution of different elements during the annealing process. The results indicated effective inter-diffusion of Al and Ti as well as out-diffusion of oxygen, resulting in an increase in the n-type carrier concentration close to the surface of the ZnO. Scanning electron microscopy and atomic force microscopy analysis were used to determine the effect of annealing on the surface morphology.

We present our recent results in the development of mid-infrared spectroscopy systems based on supercontinuum sources for two different field applications: airborne monitoring of atmospheric pollutants and field quality control of agricultural products.

Fourier transform spectroscopy is a well stablished method, with a unique feature of providing broad spectral coverage and high spectral resolution. Incoherent thermal light sources, which are omnidirectional and provide a low spectral brightness, are traditionally used in combination with this method. Therefore, a long averaging time is needed to obtain a high signal-to-noise ratio (SNR) spectrum and the detection sensitivity is usually low for gas phase samples. Newly developed ultra-broadband, low-noise, mid-infrared supercontinuum (SC) light sources show a great potential to replace the thermal sources in Fourier transform spectrometers (FTSs). They are spatially coherent and can deliver orders of magnitude higher spectral brightness compared to thermal sources, while providing equal or even higher spectral bandwidth [1] . The main drawback of SC sources, historically, has been their high relative intensity noise (RIN) due to noise amplification in the nonlinear broadening process [2] . Recent advancements in diode-laser-pumped mid-infrared SC sources provide reduced RIN by utilizing higher repetition rates and shorter pulse durations [3] . The high repetition rate of these sources can also be utilized in combination with a synchronous demodulation technique to reduce the 1/f noise in the detection system. Here we present a home-built FTS, using only out-of-shelf optical components, especially developed to work with ultra-broadband and high repetition rate mid-infrared SC sources. The FTS can potentially work in the 2-11 μm bandwidth with a minimum spectral resolution of 1 GHz suitable for detection of gases at atmospheric pressure. To increase the light-matter interaction length, we employed a multipass cell with 32 m optical path length. Figure 1 demonstrates the performance of the system in combination with the SC source, in three different cases of (a) using a single photodetector operating in the baseband, (b) synchronous demodulation of the photodetector output referenced to the SC repetition rate and (c) utilizing a balanced detection scheme in the baseband. In all of the experiments, the gas sample consists of a mixture of 5 ppm Ethane and 25 ppm Ethyl acetate, diluted in N 2 at atmospheric pressure.

Conventional mechanical Fourier Transform Spectrometers (FTS) can simultaneously measure absorption and dispersion spectra of gas-phase samples. However, they usually need very long measurement times to achieve time-resolved spectra with a good spectral and temporal resolution. Here, we present a mid-infrared dual-comb-based FTS in an asymmetric configuration, providing broadband absorption and dispersion spectra with a spectral resolution of 5 GHz (0.18 nm at a wavelength of 3333 nm), a temporal resolution of 20 &mu, s, a total wavelength coverage over 300 cm&minus, 1 and a total measurement time of ~70 s. We used the dual-comb spectrometer to monitor the reaction dynamics of methane and ethane in an electrical plasma discharge. We observed ethane/methane formation as a recombination reaction of hydrocarbon radicals in the discharge in various static and dynamic conditions. The results demonstrate a new analytical approach for measuring fast molecular absorption and dispersion changes and monitoring the fast dynamics of chemical reactions over a broad wavelength range, which can be interesting for chemical kinetic research, particularly for the combustion and plasma analysis community.