Coupling a Bessel Beam and Damaging Nanowires
by Daniel M. Todd
Supervisor: Dr. Carlos Trallero
This summer, I had the fortune of participating in two projects: one concerning coupling a Bessel beam through a fiber while the other was concerned with producing single-crystalline gold nanowires to use in studying damage thresholds of those nanowires.
This project focuses on answering the question of whether or not we can improve the transmission efficiency of light through a hollow-core fiber by using a Bessel beam. When a standard (Gaussian) laser beam passes through one of these fibers, it leaves the fiber as a Bessel beam but with significantly reduced power. We hope that by using a special optical lens called an Axicon, can we turn the Gaussian beam into a Bessel beam prior to the fiber and then pass it through the fiber with less loss of light.
· Generate Bessel beam with Axicon
· Propagate Bessel beam through hollow core fiber
· Characterize the Bessel beam by fitting a spatial mode
This image demonstrates the overarching purpose of the project: we want to make the transmission of light through the fiber as efficient as possible so that we can produce high-energy, few-cycle pulses rather than lower-energy, many-cycle pulses.
Here is a good representation of what the Axicon lens does: it takes a Gaussian beam (left) and refracts the light in such a way to make the trademark design of the Bessel beam in the near-field (middle). This design is composed of an intense central mode surrounded by concentric rings of increasing radius and decreasing intensity. In the far-field (right), a Bessel beam takes on the shape of a single ring as all of the central modes quickly drop in intensity.
This Axicon setup is used to generate an approximation of a Bessel beam by passing a Gaussian beam through the Axicon lens. This setup uses a 15mW Helium-Neon laser with wavelength 632.8nm. The neutral density filter is used to control the intensity of the initial beam and the telescope is used to widen the beam into the Axicon. The 500mm lens is used to focus the beam so that it can fit into the 250 micron opening of the hollow core fiber.
This is an example of our experimental data: this false-color image of the mode of our produced Bessel beam was taken at a distance of 190mm away from the Axicon.
*Note: This image was taken during a slightly different setup than seen above. Rather than a 500mm lens, there is a 100mm lens and the position of the lens and Axicon are switched. The Axicon receives the laser beam first, then at 100mm away, the 100mm lens focuses the beam down to what we see here.
I was able to work with graduate student Adam Summers and help him with his current study focused on finding the damage thresholds for single-crystalline gold nanowires. We want to expand his initial experiment to study the effects on damage threshold of:
· Pulse duration
· Other metals
But to do this, it requires the fabrication of many single-crystalline gold nanowires.
Grow gold (Au) nanowires on glass slides
Study damage threshold of Au nanowires
Long wavelength and few-cycle pulses
Control polarization of electric field relative to crystal axis
Overall, I was able to produce multiple gold (Au) electrodes on glass. The image on the lower left is a prototype slide I used for tracing slices I had to make in the material, and was made with the Solidworks CAD software. The final design as seen in the lower right provided up to 20 electrode gaps (100 μm average)
How I prepared the Electrode slides:
· Placed tape on methanol-cleaned slides
· Sliced tape along lines, removed pieces
· Evaporated Chromium, then Gold onto slides
o Used a Varian VE-10 Evaporator
· Removed remaining tape to reveal electrode gaps
The above image demonstrates how the Varian VE-10 evaporator (or metal deposition machine works). After placing the desired metals in wires baskets, a current is run across the wire under vacuum to heat the metals very quickly. The metals evaporate upward onto downward-facing slides and the metal sticks and solidifies on the cool surface of the slide. We do this first with chromium then again with gold in order ensure the gold will stick to the slide.
The Nanowire Setup is used to focus a high-intensity beam onto an Au nanowire in order to observe damaging intensities. The lamp structure provides an adjustable backlight to view the nanowire. The laser setup can adjust intensity using the half-wave plate and polarizing beam cube combination and focuses the 32 or 108 fs pulse onto the nanowire. In my proposed experiment, there would be another half-wave plate just prior to L3 in order to change the polarization of the electric field with respect to the nanowire.