New Materials for Sustainable Lasers

Illustration: Molecular vibrations lead to high performance laser. (Source: Troan Tran)

Lasers can be energy intensive and many are made using toxic materials like arsenic and gallium. To make lasers more sustainable, new materials and lasing mechanisms must be discovered. Andrea Armani and her team at the USC Viterbi School of Engineering have discovered a new pheno­menon and used it to make a laser with over 40 percent effi­ciency-nearly 10 times higher than other similar lasers. The laser itself is made from a glass ring on a silicon wafer with only a monolayer coating of siloxane molecules anchored to the surface. Thus, it has improved power consumption and is fabricated from more sustainable materials than previous lasers.

The surface Raman laser is based on an ex­tension of the Raman effect, which describes how the interaction of light with a material can induce molecular vibra­tions that result in light emission. One unique feature of this type of laser is that the emitted wavelength is not defined by the electronic transitions of the material, but instead it is determined by the vibra­tional frequency of the material. In other words, the emitted laser light can be easily tuned by changing the incident light. In previous work, researchers have made Raman lasers leveraging the Raman effect in bulk material, like optical fiber and silicon.

Raman lasers have a wide range of appli­cations including military communi­cations, microscopy and imaging, and in medicine for ablation therapy, a minimally invasive procedure to destroy abnormal tissue such as tumors. Armani said she realized that a different strategy might give even higher performing Raman lasers from sus­tainable materials like glass. “The challenge was to create a laser where all of the incident light would be converted into emitted light,” she said. “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.”

Armani said that if conventional Raman lasers were thought of as the old energy-ineffi­cient light bulbs many of us grew up with, this new tech­nology would result in the laser equivalent of energy efficient LED lightbulbs; a brighter result requiring lower energy input. Armani’s interdisciplinary team, comprised of chemists, materials scientists and elec­trical engineers, quickly realized that they could design this type of laser system. Combining surface chemistry and nano­fabrication, they developed a method to precisely form a single monolayer of molecules on a nanodevice.

“Think of the molecule as looking like a tree,” Armani said. “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 attached to the surface of an inte­grated photonic glass ring, which confines an initial light source. The light inside the ring excites the surface-con­strained molecules, which subse­quently emit the laser light. Notably, the efficiency is actually improved nearly 10 times, even though there is less material.

“The surface-constrained molecules enable a new process – Surface Stimulated Raman – to happen,” said Xiaoqin Shen, “This new surface process triggers the boost of the lasing efficiency.” Additionally, just like conven­tional Raman lasing, by simply changing the wavelength of light inside the ring, the emission wavelength from the molecules will change. This flexi­bility is one reason why Raman lasers and now Surface Stimu­lated Raman lasers are so popular across numerous fields including defense, diagnostics, and communications.

Armani said the team managed to bind the molecules to the surface of the glass ring by harnessing the hydroxyl molecule groups on the surface, entities with the formula OH, that contain oxygen bonded to hydrogen, using a process called silani­zation surface chemistry. This reaction forms a single monolayer of precisely oriented indi­vidual molecules.

Armani said the research has the potential to signi­ficantly reduce the input power required to operate Raman lasers as well as impact numerous other appli­cations. “The Raman effect is a funda­mental, Nobel-Prize winning science behavior originally disco­vered in the early 20th century,” Armani said. “The idea of contri­buting something new to this rich field is very rewarding .” (Source: USC)

Reference: X. Shen et al.: Raman laser from an optical resonator with a grafted single-molecule monolayer, Nat. Phot., online 2 December 2019; DOI: 10.1038/s41566-019-0563-7

Link: Dept. of Chemical Engineering and Material Science, University of Southern California, Los Angeles, USA

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