New Method Could Lead to Higher Performance Lasers for Various Applications

Lasers are used for many different things, right from encrypting communications to entertaining animals, like cats.

Above: In current Raman lasers, molecules suspended in a solution vibrate and emit light. Below: By attaching the molecules to a surface, the research team is able to constrain the molecules’ motion as they vibrate and emit light, creating a more efficient laser. Image Credit: Nishant Tripathi.

Regrettably, the same lasers can also be energy-intensive, and a majority of them are developed using harmful materials such as gallium and arsenic. Therefore, innovative lasing mechanisms and materials should be identified so that lasers can be made more sustainable.

At the University of Southern California (USC) Viterbi School of Engineering, Professor Andrea Armani and her colleagues have identified a novel phenomenon that they utilized to make a laser with more than 40% efficiency. This efficiency is virtually 10 times higher than that of other analogous lasers.

In essence, the laser itself is fabricated from a glass ring on a silicon wafer, using just a monolayer coating of siloxane molecules that are adhered to the surface. This laser is made from more sustainable materials than previous lasers and has better power consumption properties.

The research was performed by Armani along with her co-authors Xiaoqin Shen and Hyungwoo Choi from Mork Family Department of Chemical Engineering and Material Science of USC; Dongyu Chen from Ming Hsieh Department of Electrical and Computer Engineering of USC; as well as Wei Zhao from the Department of Chemistry at the University of Arkansas at Little Rock. The study has been reported in the Nature Photonics journal.

Based on an extension of the Raman effect, the surface Raman laser demonstrates how molecular vibrations are induced by the interaction between a material and light, and how these vibrations result in the emission of light.

This kind of laser has one special trait—that is, the emitted wavelength is determined by the material’s vibrational frequency and not defined by the material’s electronic transitions. To put this in simpler terms, the laser light that is emitted can be effortlessly adjusted by altering the incident light.

In earlier studies, scientists have fabricated Raman lasers by exploiting the Raman effect in “bulk” material, for example, silicon and optical fiber.

Raman lasers are used in many different applications such as imaging, microscopy, military communications, and even medicine for ablation therapy. This therapy is a minimally invasive procedure that kills tumors and other similar abnormal tissues.

According to Armani, the Ray Irani Chair in Chemical Engineering and Materials Science at USC, she realized that a unique approach might result in higher-performing Raman lasers made from sustainable materials such as glass.

The challenge was to create a laser where all of the incident light would be converted into emitted light. In a normal solid-state Raman laser, the molecules are all interacting with each other, reducing the performance. To overcome this, we needed to develop a system where these interactions were reduced.

Andrea Armani, Professor, Viterbi School of Engineering, University of Southern California

Armani further added that if traditional Raman lasers were considered as the obsolete energy-inefficient light bulbs that were known before, then this novel technology would lead to the laser equivalent of LED light bulbs that are also energy-efficient; that is, brighter result demanding lower energy input.

The interdisciplinary team of Armani included electrical engineers, materials scientists, and chemists who readily realized that this kind of laser system could possibly be developed. Using a combination of nanofabrication and surface chemistry, the researchers devised a technique to accurately create only a single monolayer of molecules on a nanodevice.

Think of the molecule as looking like a tree,” Armani added. “If you anchor the base of the molecule to the device, like a root to a surface, the molecule’s motion is limited. Now, it can’t just vibrate in any direction. We discovered that by constraining the motion, you actually increase the efficiency of its movement, and as a result, its ability to act as a laser.”

The molecules are fixed to the surface of an inbuilt photonic glass ring. This ring limits an initial light source. The surface-constrained molecules are excited by the light within the glass ring. These molecules subsequently produce the laser light. Specifically, the efficiency is essentially enhanced virtually 10 times in spite of less material.

The surface-constrained molecules enable a new process, called Surface Stimulated Raman, to happen. This new surface process triggers the boost of the lasing efficiency.

Xiaoqin Shen, Study Co-Lead Author, University of Southern California

The other co-lead author of the study is Hyungwoo Choi.

Moreover, just like traditional Raman lasing, the molecules’ emission wavelength is changed by simply altering the wavelength of light within the glass ring. This flexibility is one of the reasons why Raman lasers—currently known as surface stimulated Raman lasers—are very popular across various fields such as communications, diagnostics, and defense.

According to Armani, the researchers harnessed the hydroxyl molecule groups on the surface of the glass ring to bind the molecules to the same surface. Hydroxyl molecule groups are entities with the formula OH containing oxygen attached to hydrogen, through a process known as silanization surface chemistry. This reaction creates a single monolayer of individual molecules that are accurately oriented.

For Armani, this finding represents an interesting project—a project she has been exploring since her days as a PhD student.

This is a question I’ve been wanting to look into for a while, but it just wasn’t the right time and the right place and right team to be able to answer it,” she added.

According to Armani, the study could considerably decrease the input power needed to use Raman lasers and, at the same time, impact many other applications.

The Raman effect is a fundamental, Nobel-Prize winning science behavior originally discovered in the early 20th century. The idea of contributing something new to this rich field is very rewarding.

Andrea Armani, Professor, Viterbi School of Engineering, University of Southern California

The study was financially supported by the Office of Naval Research and the Army Research Office of U.S. Army Research Laboratory.


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