We have shown recently that unique optical signatures can be observed with the measurement of ultrashort middle infrared laser pulses that have been transmitted through molecular vapors. Here, we report on an increased signal-to-noise ratio of the pulse measurements by using a cross-correlation technique with a lockin amplifier. Carbon tetrafluoride and dimethyl methylphosphonate (DMMP) cross-correlation signatures are highly discriminated using principal component analysis. A squared exponential Gaussian process regression model is used to quantitatively predict the concentration of DMMP.
Femtosecond laser surface processing (FLSP) is a unique material processing technique that can produce self-organized micro/nanostructures on most materials including metals, semiconductors, and dielectrics. These structures have demonstrated the enhancement of surface properties such as heat transfer and broadband light absorption. The chemical composition and morphology of FLSP structures is highly dependent on processing parameters including background gas composition, pressure, laser fluence, and number of laser pulses. When the laser processing is carried out in open atmosphere, a thick oxide layer forms on the FLSP surface structures due to the high reactivity of the surface with the environmental constituents immediately after laser processing. In this work, N2 and forming gas are used during laser processing in an effort to form a metal nitride on the surface of aluminum. Aluminum nitride is a promising material for enhancing the heat transfer performance of surfaces because of its thermal conductivity, which can be as high as 285 W/m-K, whereas aluminum oxide has a low thermal conductivity (30 W/m-K). Aluminum nitride incorporation into FLSP surfaces has the potential to act as a passivation layer to decrease the oxygen content and increase the thermal conductivity of the surface. Nitrogen incorporation is studied by applying FLSP in air, N2, and a 95% N2/5% H2 mixture. The chemical composition of the FLSP surfaces is determined by X-ray photoelectron spectroscopy (XPS) and energy-dispersive X-ray spectroscopy (EDS). Cross-sectional analysis of the FLSP microstructures is performed using ion beam milling.
The capability to produce femtosecond laser pulses with wavelengths in the atmospheric absorption window requires a new understanding of pulse propagation effects. In this work, we characterize the changes in temporal propagation of middle infrared femtosecond laser pulses by cross-correlation frequency resolved optical gating (XFROG). The temporally distorted infrared pulses are cross-correlated with 800 nm pulses by a four-wave mixing process in air. For the first time, we investigate these propagation effects through gas molecules that are not present in the atmosphere. Each molecule is shown to have a unique effect on the temporal propagation of the pulse that is wavelength dependent. We verify our experimental data with simulations based on a KramersKronig transformation of spectral data from the HITRAN database. The propagation effects are similar to optical free induction decay. Multiple vibrational and rovibrational absorption lines are excited by the middle infrared pulse and constructive interference occurs at various delay times relative to the initial pulse. The constructive interference impresses a unique fingerprint onto the pulse because the spectral lines of each molecule are unique. The fingerprint behaves as a nonlinear function related to the molecular concentration. To account for this, a regression model is developed to predict the concentration of unknown gas species. The middle infrared beam is the only laser beam sensitive to the analytes. Thus, standoff detection is a possibility since the XFROG can be performed locally.
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