Published: Dec. 5, 2022

Thariq Shanavas is a fourth-year physics PhD student who works in the Gopinath Lab in electrical, computer and energy engineering. He was recently first author on a paper in the journal APL Photonics that reported the first observation of cascaded forward stimulated Brillouin scattering in a microresonator platform.

We asked Shanavas to share some more on this new laser technique, as well as his experiences as a graduate student at CU Boulder.

Can you tell us a little about this paper and its impact?

Our work focuses on making inexpensive, high-quality lasers using newly discovered materials. Lasers are used in medical imaging, optical disc drives, laser printers, barcode scanners, DNA sequencing instruments, fiber-optic communication, semiconducting chip manufacturing, laser surgery, and skin treatments among a plethora of other applications. Our research group aims to explore novel applications of lasers by building laser light sources in regimes where few options are currently available.

The color of light is characterized by a parameter called its wavelength. For example, red light has a wavelength of about 700 nanometers (nm), green light about 500 nanometers, and blue light about 450 nanometers. Colors invisible to the human eye have a wavelength either less than 400 nanometers or greater than 800 nanometers.

Among other cool stuff, our group is interested in building lasers in the infrared region, which is light with wavelength between 800 nanometers and 20,000 nanometers. The infrared region is often called the “chemical fingerprint” region because many chemicals absorb light at very specific wavelengths in this range.

For example, water absorbs light at 2662 nm, 2734 nm, and 6269 nm. So if the light from a distant star bounces off a planet and the reflected light is missing these wavelengths, it is likely that there is water in the planet’s atmosphere! Similarly, we could shine a laser through an unknown chemical sample and deduce its composition by looking at the wavelengths of the light that is absorbed. This technique is called spectroscopy and is widely used to identify and characterize chemicals.

For many applications of lasers, including spectroscopy, we require that the laser emit light at a very specific wavelength, with minimal spread around the target wavelength. This “spread” is called the linewidth of a laser and is one of the parameters that characterize the quality of a laser.

One way to reduce this spread is to bounce light off of phonons in a material. Without going into too much detail, phonons are the quanta of mechanical vibrations the same way photons are the quanta of light. This process is called Brillouin scattering. Our work demonstrates a new way to enhance this process in an extremely compact volume that is less than a hundredth of an inch across. We have achieved this by using an optical “resonator” in the shape of a microscopic sphere, which allows the light to circulate along its edge and build up in a closed path.

While we demonstrated a laser that emits light at wavelength 1550 nm, our technique can be used to make a laser at higher wavelengths too! Besides the uses in spectroscopy we talked about earlier, we expect our work to advance the field of integrated photonics, such as sensors-on-a-chip for measuring the concentration of ammonia in the atmosphere. In our paper, we also highlight some ways our research could be used to make/improve electronic frequency synthesizers, which are used in modern devices like radio receivers and GPS systems.

What is the next step for this research project?

Our demonstration was the first of its kind. Therefore, we used the simplest design of a “resonator” that allowed us to demonstrate the type of Brillouin scattering we were interested in – a sphere resonator. For future work, we plan on using our in-house designed microscopic wedge resonators with configurable angles, diameters, and thicknesses. We believe it would improve the brightness of our laser by at least 100 times, unlocking more new and exciting applications!

What’s your favorite part about the research you do and/or the lab you work in?

I like the collaborative nature of our work! Over the course of this project, I got to work with material scientists, physicists, and electrical engineers and drew inspiration from their unique expertise.

I also found the open-ended nature of our research very exciting. When I started this work, I had a very different vision for what the end product would look like. But along the way, I ran into several limitations of what is allowed by the laws of nature. My expectations and vision evolved with my understanding of laser physics, until almost we were done with the final product.