Carlton Farley

Interferometric measurement of laser heating in praseodymium-doped YAG crystal

Temperature measurement is required for many applications but can be difficult in some cases. Laser heating or cooling studies demand accurate measurements of temperature changes. A Michelson interferometer configuration has been used to investigate laser heating in solids. An analytical formula was derived to estimate the temperature change from the fringe count by taking into account the temperature dependence of the sample length and refractive index. When 115 mW of a focused Ar+ laser beam (488 nm) passes through a Pr(3+)-doped YAG sample, its temperature increased by 11.7±1.0 K along the beam path due to nonradiative relaxation. The power dependence of the fringe count/movement was recorded. The temperature change was estimated by the interferometric method and is in agreement with that measured by a thermocouple.

Effect of Pore Size and Film Thickness on Gold-Coated Nanoporous Anodic Aluminum Oxide Substrates for Surface-Enhanced Raman Scattering Sensor.

A sensitive surface enhanced Raman scattering chemical sensor is demonstrated by using inexpensive gold-coated nanoporous anodic aluminum oxide substrates. To optimize the performance of the substrates for sensing by the Surface-enhanced Raman scattering (SERS) technique, the size of the nanopores is varied from 18 nm to 150 nm and the gold film thickness is varied from 30 nm to 120 nm. The sensitivity of gold-coated nanoporous surface enhanced Raman scattering sensor is characterized by detecting low concentrations of Rhodamine 6G laser dye molecules. The morphology of the SERS substrates is characterized by atomic force microscopy. Optical properties of the nanoporous SERS substrates including transmittance, reflectance, and absorbance are also investigated. Relative signal enhancement is plotted for a range of substrate parameters and a detection limit of 10−6 M is established.

Surface-enhanced Raman spectroscopy scattering from gold-coated ceramic nanopore substrates: effect of nanopore size

Abstract. The dependence of magnitude of the electric near-field on the separation between metal nanoparticles for surface-enhanced Raman spectroscopy (SERS) substrates was experimentally verified. Diameters of gold-coated nanopores in a ceramic alumina substrate were varied to study the charge buildup near interparticle junctions and its effect on the enhancement factor due to SERS. The substrates were characterized by sensing a Rhodamine dye and calculating the associated Raman enhancement factors. Decreasing Au interparticle distance increases the electric near-field and shifts the plasmon resonance peak accordingly.

Short Distance Standoff Raman Detection of Extra Virgin Olive Oil Adulterated with Canola and Grapeseed Oils

A short distance standoff Raman technique is demonstrated for detecting economically motivated adulteration (EMA) in extra virgin olive oil (EVOO). Using a portable Raman spectrometer operating with a 785 nm laser and a 2-in. refracting telescope, adulteration of olive oil with grapeseed oil and canola oil is detected between 1% and 100% at a minimum concentration of 2.5% from a distance of 15 cm and at a minimum concentration of 5% from a distance of 1 m. The technique involves correlating the intensity ratios of prominent Raman bands of edible oils at 1254, 1657, and 1441 cm–1 to the degree of adulteration. As a novel variation in the data analysis technique, integrated intensities over a spectral range of 100 cm–1 around the Raman line were used, making it possible to increase the sensitivity of the technique. The technique is demonstrated by detecting adulteration of EVOO with grapeseed and canola oils at 0–100%. Due to the potential of this technique for making measurements from a convenient distance, the short distance standoff Raman technique has the promise to be used for routine applications in food industry such as identifying food items and monitoring EMA at various checkpoints in the food supply chain and storage facilities.

Standoff Raman measurement of nitrates in water

The identification and real time detection of explosives and hazardous materials are of great interest to the Army and environmental monitoring/protection agencies. The application and efficiency of the remote Raman spectroscopy system for real time detection and identification of explosives and other hazardous chemicals of interest, air pollution monitoring, planetary and geological mineral analysis at various standoff distances have been demonstrated. In this paper, we report the adequacy of stand-off Raman system for remote detection and identification of chemicals in water using dissolved sodium nitrate and ammonium nitrate for concentrations between 200ppm and 5000ppm. Nitrates are used in explosives and are also necessary nutrients required for effective fertilizers. The nitrates in fertilizers are considered as potential sources of atmospheric and water pollution. The standoff Raman system used in this work consists of a 2-inch refracting telescope for collecting the scattered Raman light and a 785nm laser operating at 400mW coupled with a small portable spectrometer.

Mach-Zehnder interferometric measurement of laser heating/cooling in Yb3+: YAG

A 915 nm T:Sapphire laser was used to excite luminescence from Yb3+ doped YAG crystal. The excited ions undergo nonradiative relaxation followed by strong emission at 1030 nm. The heating produced by the nonradiative relaxation increases the sample temperature. A Mach-Zehnder interferometer was setup with 514.5 nm Ar+ laser beam. Laser heating causes the interferometer fringes to move. A mathematical formula was developed to estimate the change in sample temperature from the fringe count. When 300mW Ti:Sapphire laser beam was focused through the YAG:Yb3+crystal its temperature increased by 6.9°C. This technique works equally well to measure the solid sample temperature changes in laser cooling studies.

Photodegradation of melanin by an interferometric technique

Photodegradation of melanin thin films is investigated for a UVA wavelength of 355 nm and a UVC wavelength of 244 nm. The technique involves interferometric exposure of melanin with two coherent beams from a low-power UV laser. The periodic photodegradation-grating pattern is monitored by diffraction of a second low-power He-Ne laser. Dependence of the photodegradation rate on UV intensity as well as the effect of ambient humidity is investigated and explained with a simple model. The technique has promise for investigating photo-induced effects in other biomolecular substrates as well.

Stand-off and up-close Raman detection of nitrates buried in sand and soils

Raman measurements, using a 785nm laser, are taken of Ammonium Nitrate and Sodium Nitrate buried in sand. Nitrate is kept in clear plastic containers and buried underneath sand at various depths. Raman measurements are then taken at distances of 5m and 20m, with the sand being completely dry as well as completely wet. A different set of experiments was conducted with Nitrate buried in sand in a glass container, where no Raman signal was seen in dry sand. Water was then added at the edge of the container and allowed to migrate to the bottom. Raman measurements are then taken at a distance of 7mm over time to detect Nitrates brought to the surface by water as it wicks to the surface.

Effect of film thickness on localized surface plasmon enhanced chemical sensor

A highly-sensitive, reliable, simple and inexpensive chemical detection and identification platform is demonstrated. The sensing technique is based on localized surface plasmon enhanced Raman scattering measurements from gold-coated highly-ordered symmetric nanoporous ceramic membranes fabricated from anodic aluminum oxide. To investigate the effects of the thickness of the sputter-coated gold films on the sensitivity of sensor, and optimize the performance of the substrates, the geometry of the nanopores and the film thicknesses are varied in the range of 30 nm to 120 nm. To characterize the sensing technique and the detection limits, surface enhanced Raman scatterings of low concentrations of a standard chemical adsorbed on the gold coated substrates are collected and analyzed. The morphology of the proposed substrates is characterized by atomic force microscopy and the optical properties including transmittance, reflectance and absorbance of each substrate are also investigated.

Raman detection of MNA in solid rocket fuels

A sensitive Raman spectroscopy technique is used for detection and possible quantification of the propellant stabilizer, nmethyl nitroaniline (MNA), in solid rocket propellants used in multiple domestic missile systems. Over time, the energetic ingredients of the propellant will degrade and react with the stabilizer, causing issues with the propellant useful safe life. Currently, there are no non-destructive analytical techniques for which MNA can be detected in solid rocket fuel inside a missile. Therefore, after a certain amount of time, missiles in inventory must be disassembled and tested for reliability and safety. This methodology is labor intensive, costly, and time consuming so a less intrusive approach is warranted to determine a missile useful safe life. Raman spectroscopy provides a possible solution to this problem, where a small fiber optic probe line may be inserted into the rocket motor of the missiles, which can be tested within seconds without the need for dismantling the missiles. A 785 nm portable Raman analyzer is used for all measurements reported in this paper with integration times ranging from 10 to 60 s. It is found that Raman sensing is a viable option for detection of MNA in solid rocket fuels.