Faculty Mentor: Shanti Deemyad (Physics & Astronomy, University of Utah)

Introduction

The Diamond Anvil Cell (DAC) is a powerful tool that allows scientists to put materials under high static pressure. These pressures can go beyond those found in the center of the Earth. At high pressures, materials behave strangely, resulting in bizarre phenomena like colossal magnetoresistance and superconduction. If properly understood, these phenomena promise to improve technologies such as magnetic memory storage devices, magnetic-field sensors, and particle accelerators.

But, to study why these materials behave unexpectedly we must collect light for microscopy and spectroscopy measurements. Presently, when performing these measurements through the DAC much of the light is lost due to the small angle of total internal reflection (TIR) causing all light attempting to exit at an angle greater than 24.4° to be reflected back into the diamond. Additionally, because of the high index of refraction of diamonds (2.4) the light rays that do escape are bent far away from the optical axis. This makes it unrealistic to collect even just the 24.4° range of light rays that escape the diamond. Finally, due to partial reflection we lose 17% of all the light we put into our measurement. To improve this reality, we designed an optical system that substantially reduces the deviation of light rays from the optical axis, and greatly reduces the magnitude of partial reflection through use of motheye inspired subwavelength nanocone structures, resulting in increased light collection and higher resolution measurements.

Methods

Using established geometric optics theory, we modeled the optical system to derive an equation for the exit angle of the light ray as a function of its emission angle. We then used multivariable calculus techniques to find the set of variables that results in the most collimated light. Leveraging a geometric optics software, Geogebra, and a wave optics software, RSoft, we simulated the theoretical models to check our results. Finally, experimentally we created an optical setup to measure the change in size of the light cone when light travels through air versus diamond. We utilized a Raman spectroscopy system to see how adding the lens affects the magnitude of the Raman signal and the sensitivity of our measurement.

Results

We found that our simulations agreed strongly with our theory. We found that the light collimation changes depending on the index of refraction of the components of our optical system with lenses made from lower index materials having worse average collimation. An optimal system made from high index material would result in ray deviations of less than 2° for the diamond dimensions accessible to us. However, both of our experiments were inconclusive. We did find that for spectroscopy measurements, the placement of our system was very important as significant deviations of the system from the optical axis resulted in no Raman signal. We were unable to conclude whether or not the optical system significantly increased our Raman Signal, and by how much our optical system changes the ray deviation.

Conclusion

We found that our simulations agreed strongly with our theory. We found that the light collimation changes depending on the index of refraction of the components of our optical system with lenses made from lower index materials having worse average collimation. An optimal system made from high index material would result in ray deviations of less than 2° for the diamond dimensions accessible to us. However, both of our experiments were inconclusive. We did find that for spectroscopy measurements, the placement of our system was very important as significant deviations of the system from the optical axis resulted in no Raman signal. We were unable to conclude whether or not the optical system significantly increased our Raman Signal, and by how much our optical system changes the ray deviation.

Further work should be done on proving these results experimentally through precise placement of the lens for Raman Spectroscopy, and on computationally modeling how AR coatings and moth-eye inspired optical gradient mediums affect the amount of unwanted reflection.

Acknowledgement

Thank You Dr. Saveez Saffarian, Dr. Rajesh Menon, Willis Holle, Anukriti Ghimire, Gavin Farley, and Tyler Walker for your support during this project! Thank you to the University of Utah Office of Undergraduate Research for supporting this project.

Bibliography

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