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A New Era of Optical Analysis for Astromaterials

Writer's picture: SamSam

It's been quite a while since my last update, and... wow, a lot has happened! Since I moved to the Lunar and Planetary Laboratory at the University of Arizona, I've had so many amazing opportunities to grow as a scientist and pursue fascinating research across multiple disciplines. Perhaps in the coming months I can write about the fantastic work that our team at NASA JSC accomplished in experimentally investigating sulfide core formation in oxidized planetary bodies or how my LPL collaborators and I were the first to distinguish angstrom-scale (~0.000000001 mm) structural deviations among minerals in the atmosphere of a hot Jupiter-like exoplanet orbiting a star 1,700 light years away. It's been an incredible experience so far, and more opportunities for mind-blowing science come up with each conversation and every cup of coffee.


Today, I'd like to take a moment to highlight some exciting work we're doing at the forefront of developing new tools for geo/astromaterials analysis (i.e., rocks, minerals and meteorites in the lab). I've recently been working with Prof. Khanh Kieu's Nonlinear Optics and Multiphoton Microscopy group, which is just across the street from us in the Wyant School of Optical Sciences at the University of Arizona. We've been using a tool that is otherwise unknown to geologic and planetary sciences - a multiphoton microscope.


Using a femtosecond laser, which emits ultrashort pulses of infrared light, we can stimulate nonlinear optical interactions between photons in a mineral's crystal lattice. The multiphoton microscope earns its moniker because its laser generates such a high flux of photons at its focal point that highly unlikely optical interactions can occur; 2 or 3 photons can simultaneously interact with an electron, imparting the sum of their energies to the electron and exciting it to virtual or fully excited state. As that electron falls back to its ground energy state, it emits a single photon with energy equal to (or in some cases, less than) the sum of those combined photons. The emitted photons are picked up by our detectors, and we can then map out where these signals originate within a sample in three dimensions. Thus, a multi-photon microscope. This tool provides us with a powerful new way to investigate structural and geochemical aspects of rocks and minerals in three dimensions with microscale resolution.


There are two types of signals we can decipher thus far with our microscope: harmonic generations and excitation fluorescence.


Harmonic generations are doubled or tripled frequencies of the incident laser. These highlight structural characteristics like noncentrosymmetic crystal structure in the case of second-order harmonic generation (SHG) and changes in the refractive index for third-order harmonic generation (THG). To demonstrate the utility of these harmonic generations to geomaterials, we collected 3D THG images of a ~45 µm fluid inclusion in a sample of calcite (animated 3D rendering below, left).



These images were collected ~70 µm below the unprepared surface of a calcite crystal (which I bought from a gift shop, but hey that's good enough for a demonstration). In the live-capture THG animation (right), we're looking at a 2D plane through the center of the inclusion. You can see a bright artifact moving around in a kind of random walk, which appears to be a bubble or salt crystal interacting with the laser. The movement of this object confirms that the inclusion is filled with some kind of fluid, rather than a solid mineral or void space. In the near future, we will be able to use Raman spectroscopy to also probe the composition of the fluid, providing an unparalleled ability to image the inclusion's microstructure and composition simultaneously. This will provide researchers with a valuable new approach to studying fluid inclusions that record water-rock interactions from the beginning of the solar system.


We were also able to excite fluorescence in many samples, where an electron is brought to a higher vibrational level within an excited energy state. Some amount of that energy is lost to vibrational decay, and then a photon is emitted as the electron decays to its ground state with energy less than the sum of the incident photons. The effect is similar to what happens when you shine a UV light on a fluorescent mineral, but with many order of magnitude improved resolution. Below is an image from a meteorite that I had in my personal collection, NWA 10421 (it's a family name), a type R6 chondrite. Don't worry about what that means for you non-meteorite folks. Basically, it's a rock from the beginning of the Solar System that never melted.

At the center of the image, showing strong second (red) and third (green) harmonics, is a large grain of the mineral chlorapatite surrounded by fluorescent minerals in the meteorite matrix. The second harmonics indicate that chlorapatite has a noncentrosymmetric structure, which provides some context for the thermal history of the rock. The monoclinic crystal structure variant of chlorapatite is the only variety with noncentrosymmetric structure, and it is only stable below ~200 °C. That suggests this rock cooled very slowly to allow chlorapatite to assume this structure. Overprinted onto the red SHG signals are green THG signals in roughly hexagonal shapes. We think this shows how parts of chlorapatite were altered to their higher symmetry structures, probably during impact-induced heating.


The fluorescence (blue and cyan) in this sample is dominated by the mineral olivine... which is super weird! Olivine is not generally a fluorescent phase, but we think that with the ultrashort pulses of the femtosecond laser, we're able to stimulate enough fluorescence that our detectors are able to resolve fluorescence. However, the most interesting aspect of this is why olivine is fluorescing. The spectrum below was collected for the preceding image and shows intensity of emissions at each wavelength measured.



The broad emission feature produced mostly by olivine peaks at 650 nm, which is coincident with experimental irradiation of olivine in other studies. We suspect that the multiphoton microscope is imaging radiation-induced luminescence, either imparted by cosmic ray exposure in space, or perhaps from radionuclide decay in the meteorite parent body. We're are exploring this possibility through our next set of experiments. It may provide a totally unique way of quantifying the radiation history of meteorites with unprecedented clarity. The manuscript preprint (not yet peer reviewed) for this work can be found here.


We are just scratching the surface of what this incredible tool can do. Stay tuned if you're interested in hearing more about the cool science we're doing here at LPL (maybe next year at the rate I write these).


Image credit: These amazing images were captured by our undergraduate research group member, Colby, who now has nonlinear optical features of minerals burned into his retinae.

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