image: The combination of high-Q WGMs and nanoplasmonics enables ultrahigh-sensitivity acquisition of molecular fingerprints.
Credit: by Wenbo Mao, Yihang Li, Xuefeng Jiang, Zhiwen Liu, Lan Yang
Light-matter interaction is one of the most fundamental ways to explore the physical world. While the reflection and refraction of light reveal the structural characteristics of matter, inelastic scattering of light, like Raman scattering, encodes the molecular fingerprint of chemical bonds into the shifts of photon energy. The molecular vibration is reflected by optical spectrum information, thereby providing an important feature for identifying materials. Raman spectroscopy has been widely used in environmental detection, food safety, biomedical monitoring, and material science. However, Raman scattering is so weak that usually only one Raman photon is produced for every billions incident photons. Researchers have been exploring and developing various mechanisms and structures to enhance Raman signals over the past decades. Surface-enhanced Raman spectroscopy, or SERS, represents one of the most promising platforms, where metallic nanostructures are introduced to significantly enhance the electromagnetic field near the sample surface and tailor the targets’ density of energy states in favor of Raman scattering.
On the other hand, whispering-gallery-mode (WGM) microresonators have emerged as frontrunners to enhance light-matter interactions. These structures act as a "reservoir" of light, allowing light to travel millions of roundtrips in a sub-millimeter cavity, accumulating energy density and enhancing light-matter interactions. WGM microresonators have found numerous applications ranging from single-molecule detection, chip-scale laser sources, spectroscopy, biomedical imaging, etc. The combination of these two platforms, i.e., SERS and WGM microresonators, can significantly boost light-matter interaction both spatially and temporally.
In a recent paper published in Light: Science & Application, a team of researchers led by Professor Lan Yang from Washington University in St. Louis collaborated with Prof. Zhiwen Liu from Penn State University demonstrated a novel WGM microprobe for 2D mapping of enhanced Raman spectroscopy across a large surface area of various samples. They also showcased the WGM microprobe integrated with nano-plasmonic SERS substrate for enhanced Raman signals revealing molecular fingerprints of different materials. Nano-plasmonic hotspots are coupled with WGMs via a phase-matched cavity-antenna coupling mechanism to maximize the spontaneous Raman scattering from various chemical and biochemical samples. Two-orders-of-magnitude enhancement was reported on top of the stand-alone nano-plasmonic hotspots conventionally excited by a focused free-space light beam. The team also showed the compatibility of the microprobe with different types of SERS substrate, including lithographically defined nano-plasmonic hotspot array and commercial SERS substrate paper, demonstrating the versatility of the novel platform. More interestingly, by mounting the probe onto a translation stage, a two-dimensional hyperspectral Raman imaging with signal enhancement was realized with only sub-milliwatt continuous-wave pump power. The reported results opened the door to explore inelastic light-matter interaction enhancement with WGM-plasmonic hybrid resonance, offering a versatile tool for Raman-based material analysis and chemical imaging.
"This work is an example of our group’s interest in developing innovative technologies enabled by WGM microresonators," said Dr. Lan Yang, Edwin H. & Florence G. Skinner Professor of Electrical & Systems Engineering in the McKelvey School of Engineering. “We are curious about exploring new features and capabilities enabled by WGM as a platform and its integration with other structures. In this work, we found that phase-matched coupling between WGM and plasmonic nanoantennas could enhance the intensity of hybrid optoplasmonic modes. Such an effect can be used to further enhance the Raman signals through a WGM microprobe that can scan across a large surface area.”
“In some ways, the WGM microprobe platform works similarly to an atomic force microscopy (AFM) tip. We also tried to demonstrate and evaluate the potential of this platform from an apparatus development point of view. This motivates our study of enhancing conventional SERS and 2D scanned Raman imaging using the microprobe configuration,” said Wenbo Mao, the first author of this work. He is excited about the potential applications the platform presents. “We are confident to say that this new technique could unlock many opportunities for a better and more compact SERS test platform.”
Yihang Li, the co-first author of this work, finally concluded, "probably the over-simplified way to understand our work is: we developed a versatile and scannable contact lens to SERS samples."
Journal
Light Science & Applications