The success of laser-based trace gas sensing techniques crucially depends on the availability and performance of tunable laser sources combined with appropriate detection schemes. Besides near-infrared diode lasers, continuously tunable midinfrared quantum cascade lasers and nonlinear optical laser sources are preferentially employed today. Detection schemes are based on sensitive absorption measurements and comprise direct absorption in multi-pass cells as well as photoacoustic and cavity ringdown techniques in various configurations. We illustrate the performance of several systems implemented in our laboratory. These include time-resolved multicomponent traffic emission measurements with a mobile CO 2 -laser photoacoustic system, a diode-laser based cavity ringdown device for measurements of impurities in industrial process control, isotope ratio measurements with a difference frequency (DFG) laser source combined with balanced path length detection, detection of methylamines for breath analysis with both a near-IR diode laser and a DFG source, and finally, acetone measurements with a heatable multipass cell intended for vapor phase studies on doping agents in urine samples.PACS 33.20.Ea; 42.62.Fi; 42.72.Ai; 87.64.km; 92.60 Trace gases play a key role in various areas such as air pollution, climate research, industrial process control, agriculture, food industry, volcanology, workplace safety and medical diagnostics. There exist numerous gas sensing devices based on different detection principles like gas chromatography (GC), mass spectrometry (MS) or combinations of the two (GC-MS), chemiluminescence, Fourier transform infrared spectroscopy (FTIR), electrochemical sensors, colorimetry, etc. In recent years, laser-spectroscopic sensing devices have attracted a lot of interest (see e.g., [1]) as they enable high detection sensitivity (down to sub-ppb concentrations) and selectivity (including differentiation between isotopomers and isomers), multicomponent capabil- Trace gas sensor requirements with corresponding approach for laser-based detection system ity and large dynamic range (several orders of magnitude in concentration), and generally neither sample-preparation nor -preconcentration are required. However, the laser source characteristics in terms of available wavelengths, tunability, linewidth, power, operation temperature, etc., as well as the combination with appropriate sensitive detection schemes are crucial for the success of laser-based sensing. Table 1 lists the main sensing requirements and the corresponding approaches using laser-based methods. Ideally, one system can fulfill all requirements. The combination of broad continuous tuning with a narrow linewidth and a certain power level favors laserbased systems. Laser sourcesTunable near-infrared (near-IR) lasers are readily available, notably diode lasers equipped with an external cavity. These external cavity III-V diode lasers (ECDLs) represent excellent sources in terms of availability, compactness, robustness and tuning characteri...
In silicon heterojunction solar cells, thin amorphous silicon layers passivate the crystalline silicon wafer surfaces. By using in situ diagnostics during plasma-enhanced chemical vapor deposition ͑PECVD͒, the authors report how the passivation quality of such layers directly relate to the plasma conditions. Good interface passivation is obtained from highly depleted silane plasmas. Based upon this finding, layers deposited in a large-area very high frequency ͑40.68 MHz͒ PECVD reactor were optimized for heterojunction solar cells, yielding aperture efficiencies up to 20.3% on 4 cm 2 cells.
Low-temperature (≤200 °C) epitaxial growth yields precise thickness, doping, and thermal-budget control, which enables advanced-design semiconductor devices. In this paper, we use plasma-enhanced chemical vapor deposition to grow homo-epitaxial layers and study the different growth modes on crystalline silicon substrates. In particular, we determine the conditions leading to epitaxial growth in light of a model that depends only on the silane concentration in the plasma and the mean free path length of surface adatoms. For such growth, we show that the presence of a persistent defective interface layer between the crystalline silicon substrate and the epitaxial layer stems not only from the growth conditions but also from unintentional contamination of the reactor. Based on our findings, we determine the plasma conditions to grow high-quality bulk epitaxial films and propose a two-step growth process to obtain device-grade material.
The silane dissociation efficiency, or depletion fraction, is an important plasma parameter by means of which the film growth rate and the amorphous-to-microcrystalline silicon transition regime can be monitored in situ. In this letter we implement a homebuilt quantum cascade laser-based absorption spectrometer to measure the silane dissociation efficiency in an industrial plasma-enhanced chemical vapor deposition system. This infrared laser-based diagnostic technique is compact, sensitive, and nonintrusive. Its resolution is good enough to resolve Doppler-broadened rotovibrational absorption lines of silane. The latter feature various absorption strengths, thereby enabling depletion measurements over a wide range of process conditions.
Following the ingestion of only 5.1 mL of D2O, a mid-infrared laser spectrometer determines the D/H isotope ratio increase in exhaled water vapor for the first time, to the best of our knowledge. This increase is still detectable several weeks after the heavy water intake. Collected breath samples are directly transferred into a high-temperature multipass cell operated at 373 K. No breath sample preparation is required. Aside from the capability to hinder unwanted condensation, measurements at elevated temperatures offer other advantages such as a lower temperature dependence of the delta value or the possibility to vary the intensity of absorption lines. We lay the foundation for many laser-based clinical applications. As an example, we measure a total body water weight of 55.2%+/-1.8% with respect to the total body weight, in agreement with the normal value of the male population.
The emission of fundamental and harmonic radiation from the rear side of thin foils in the thickness range 50-460 nm irradiated by intense frequency doubled Ti:sapphire laser pulses of the duration of 150 fs and intensities up to a few 10(18) W/cm(2) was investigated. Following up a previous study of the rear side harmonic emission [Teubner, Phys. Rev. Lett. 92, 185001 (2004)], we measured the emission efficiencies, polarization properties, and the spectral shapes of the fundamental frequency and the second harmonic. Rear side emission is only observed when the obliquely incident laser light is p -polarized. Particle-in-cell (PIC) simulations indicate that the foils remain strongly overdense during the interaction with the laser pulse and that the rear side emission is caused by energetic electron bunches which are generated at the front side by resonance absorption. They are accelerated into the foil and drive strong plasma oscillations at the fundamental and higher harmonic frequencies.
In absorption spectroscopy, infrared spectra of heated gases or condensed samples in the vapor phase are usually recorded with a single pass heated gas cell. This device exhibits two orders of magnitude lower sensitivity than the high-temperature multipass cell presented in this article. Our device is a novel type of compact long path absorption cell that can withstand aggressive chemicals in addition to temperatures up to 723 K. The construction of the cell and its technical features are described in detail, paying special attention to the mechanisms that compensate for thermal expansion and that allow the user to vary the optical path length under any thermal or vacuum condition. The cell may be used with a laser source or implemented within a Fourier transform infrared spectrometer. Its design is compatible with optical arrangements using astigmatic mirrors or spherical mirrors in a Herriott configuration. Here we implement a homebuilt Herriott-type cell with a total optical path length of up to 35 m. In order to demonstrate the feasibility of the cell, methane and water vapor absorption lines showing dissimilar temperature effects on line intensity were recorded with the help of a mid-infrared laser source tunable between 3 and 4 microm. Emphasis is put on lines that are too weak to be recorded with a single pass cell.
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