Noureddine Melikechi

Soil Diversity and Hydration as Observed by ChemCam at Gale Crater, Mars

The ChemCam instrument, which provides insight into martian soil chemistry at the submillimeter scale, identified two principal soil types along the Curiosity rover traverse: a fine-grained mafic type and a locally derived, coarse-grained felsic type. The mafic soil component is representative of widespread martian soils and is similar in composition to the martian dust. It possesses a ubiquitous hydrogen signature in ChemCam spectra, corresponding to the hydration of the amorphous phases found in the soil by the CheMin instrument. This hydration likely accounts for an important fraction of the global hydration of the surface seen by previous orbital measurements. ChemCam analyses did not reveal any significant exchange of water vapor between the regolith and the atmosphere. These observations provide constraints on the nature of the amorphous phases and their hydration.

High-sensitivity absorption measurement in water and glass samples using a mode-mismatched pump-probe thermal lens method

We report on a mode-mismatched pump-probe thermal lens experiment performed to measure absorption coefficients as small as 10−8 cm−1 in liquid and solid samples. We take advantage of the large axial dimension of the thermal lens by focusing the pump beam in the presence of a collimated probe beam. We measure an absorption coefficient at 533 nm of (6.8±0.1)×10−4 cm−1 and of (3±0.3)×10−6 cm−1 for distilled water and BK7 optical glass, respectively.

Trace element geochemistry (Li, Ba, Sr, and Rb) using Curiosity's ChemCam: Early results for Gale crater from Bradbury Landing Site to Rocknest

The ChemCam instrument package on the Mars rover, Curiosity, provides new capabilities to probe the abundances of certain trace elements in the rocks and soils on Mars using the laser-induced breakdown spectroscopy technique. We focus on detecting and quantifying Li, Ba, Rb, and Sr in targets analyzed during the first 100 sols, from Bradbury Landing Site to Rocknest. Univariate peak area models and multivariate partial least squares models are presented. Li, detected for the first time directly on Mars, is generally low ( 100 ppm and >1000 ppm, respectively. These analysis locations tend to have high Si and alkali abundances, consistent with a feldspar composition. Together, these trace element observations provide possible evidence of magma differentiation and aqueous alteration.

Pump–probe mode-mismatched thermal-lens Z scan

We report on a pump–probe mode-mismatched thermal-lens Z-scan method for the measurement of small absorption coefficients. In this method the pump light beam is focused into the sample to induce a thermal lens, which is tested by a collimated probe-light beam. Comparison between mode-matched and mode-mismatched Z-scan schemes is performed by use of a Fresnel-diffraction approximation model. This method is used to measure the absorption of distilled water and optical glass in the near-infrared and visible regions of the spectrum.

Pump-probe thermal lens near-infrared spectroscopy and Z-scan study of zinc (tris) thiourea sulfate

We demonstrate a novel use of the pump-probe thermal lens spectroscopy and Z-scan techniques for the measurement of the small absorption coefficient in a semiorganic, nonlinear optical crystal zinc (tris) thiourea sulphate. We measure the absorption spectrum and the absolute values of the absorption of the crystal in the near-infrared region (700–980 nm). A maximum absorption coefficient of ∼0.04 cm-1 is reported.

Laser damage studies in zinc (tris) thiourea sulfate: Nonlinear optical crystal

We report on a detailed study of laser damage in a semiorganic nonlinear optical crystal, zinc (tris) thiourea sulfate using eight nanosecond laser pulses with a repetition rate of 10 Hz at wavelengths of 532 and 1064 nm. The damage morphology shows a correlation with the mechanical hardness anisotropy. The crystal shows a damage threshold measured to be 0.6±0.1 GW/cm2 for a single 532 nm laser pulse which reduces to 0.12±0.01 GW/cm2 as the number of shots is increased to 10 000. Single shot laser damage at 1064 nm wavelength shows an onset estimated to be about 3 GW/cm2. Multiple shots at 1064 nm result in laser ablation hindering the determination of a damage threshold. We suggest a possible role of negative thermal effects resulting in the high damage threshold observed in this crystal.

The effect of irradiation wavelength bandwidth and spot size on the scraping depth and temperature rise in composite exposed to an argon laser or a conventional quartz–tungsten–halogen source☆

OBJECTIVES The purpose of this study was to investigate the effect of the spectral distribution of the curing irradiation near the maximum excitation wavelength of the photo-initiator and the effect of the irradiation spot size on the scraping depth-of-cure and temperature rise in a resin composite for both an argon laser and a quartz-tungsten-halogen lamp. METHODS Using bandpass filters, the spectral outputs of an argon laser and a quartz-tungsten-halogen lamp were restricted to pass selected wavelengths on to a commercial camphorquinone-based resin composite and the depth-of-cure, using scraping methods, was measured. The temperature rise in composite was measured for some of the above-mentioned sources. The spot sizes for both sources were varied and the scraping depth was measured. Lateral curing or the extent of curing away from the focused spot was also measured. RESULTS For constant power density and exposure time, an irradiation spectral distribution closer to the photo-initiator excitation peak yielded a higher scraping depth than a broadband spectral distribution for both sources. Under similar conditions, the argon laser resulted in a lower temperature rise in the composite than the lamp. For the same total energy imparted to the resin composite, the scraping depth increased with reducing spot size of the curing irradiation. Furthermore lateral curing of the composite well beyond the irradiation spot size was observed. SIGNIFICANCE The spectral and spatial characteristics of the curing irradiation need to be carefully considered as these affect the scraping depth-of-cure and temperature rise in a resin composite.

High manganese concentrations in rocks at Gale crater, Mars

The surface of Mars has long been considered a relatively oxidizing environment, an idea supported by the abundance of ferric iron phases observed there. However, compared to iron, manganese is sensitive only to high redox potential oxidants, and when concentrated in rocks, it provides a more specific redox indicator of aqueous environments. Observations from the ChemCam instrument on the Curiosity rover indicate abundances of manganese in and on some rock targets that are 1–2 orders of magnitude higher than previously observed on Mars, suggesting the presence of an as-yet unidentified manganese-rich phase. These results show that the Martian surface has at some point in time hosted much more highly oxidizing conditions than has previously been recognized.

Chemical variations in Yellowknife Bay formation sedimentary rocks analyzed by ChemCam on board the Curiosity rover on Mars

The Yellowknife Bay formation represents a similar to 5m thick stratigraphic section of lithified fluvial and lacustrine sediments analyzed by the Curiosity rover in Gale crater, Mars. Previous works have mainly focused on the mudstones that were drilled by the rover at two locations. The present study focuses on the sedimentary rocks stratigraphically above the mudstones by studying their chemical variations in parallel with rock textures. Results show that differences in composition correlate with textures and both manifest subtle but significant variations through the stratigraphic column. Though the chemistry of the sediments does not vary much in the lower part of the stratigraphy, the variations in alkali elements indicate variations in the source material and/or physical sorting, as shown by the identification of alkali feldspars. The sandstones contain similar relative proportions of hydrogen to the mudstones below, suggesting the presence of hydrous minerals that may have contributed to their cementation. Slight variations in magnesium correlate with changes in textures suggesting that diagenesis through cementation and dissolution modified the initial rock composition and texture simultaneously. The upper part of the stratigraphy (similar to 1m thick) displays rocks with different compositions suggesting a strong change in the depositional system. The presence of float rocks with similar compositions found along the rover traverse suggests that some of these outcrops extend further away in the nearby hummocky plains.

Experimental and theoretical investigation of thermal lensing effects in mode-locked femtosecond Z-scan experiments

We show that the closed aperture refractive Z-scan signatures using high-repetition rate ultrafast laser systems are strongly influenced by thermal lensing. We demonstrate that a stationary thermal lens develops even in the case of very low linear absorption. In addition, we have developed a stationary thermal lens model using the Gaussian decomposition method and applied it for the first time to Z-scan experiments and find good agreement between theory and experiment.