AsianScientist (Oct. 8, 2014) – Researchers led by Professors Xiao Yun-Feng and Gong Qihuang from Peking University have developed a novel microcavity Raman-laser sensor capable of single nanoparticle detection in both air and in an aqueous environment. Their results have been published in the Proceedings of the National Academy of Sciences.
The microcavities they use are called whispering gallery mode (WGM) optical microcavities, which are analogous to the acoustic whispering gallery in St. Paul Cathedral in London and the echo wall in the Temple of Heaven in Beijing. Due to the low propagation loss and small size of such microcavities, photons can circulate inside for up to hundreds of thousands of times, thus dramatically increasing light-matter interactions.
In this work, researchers detected nanoparticles using stimulated Raman scattering (also called Raman lasing) in the microcavities. Raman scattering is an inelastic light scattering process, whereby the energy of an incident photon is modified by an inelastic interaction with the cavity material.
The cavity material changes its energy state and the photon loses an equivalent amount of energy. The scattered light has a lower frequency and is called the Stokes emission. The energy difference between the incoming photon and the scattered Stokes photon matches a molecular vibration in the cavity material.
Stimulated Raman scattering occurs in both counter-propagating traveling cavity modes (i.e., clockwise and counterclockwise modes) which are initially degenerate. When the Raman laser encounters a nanoscale object, it will split into two Raman lines, because the nano-object induces a coupling between the two counter-propagating modes through backscattering.
When both split modes lase simultaneously, a beat note is produced, with the beat frequency corresponding to the Raman mode splitting. Thus, by monitoring the beat frequency of the split-mode Raman lasers, ultrasensitive nanoparticle detection can be realized.
“This mode splitting sensing mechanism is robust to many noise sources, such as thermal noise and laser frequency noise, since the two split modes share exactly the same noise,” said Prof. Xiao. “Moreover, Raman laser has much narrower line width compared to a passive microcavity, and thus to significantly lower the detection limit.”
“In order to demonstrate the feasibility of this sensing mechanism, we first did a control experiment in air. We transferred single nanoparticles to and from the microcavity using a nanofiber, using a technology previously developed by our group, and measured the beat frequency changes after each transfer process,” said Li Bei-Bei, a Ph.D. student in Prof. Xiao’s group and first author of the study.
“As expected, every time a nanoparticle is transferred to the cavity surface, the beat frequency of the Raman laser experienced an abrupt change.”
Using this method, real-time single nanoparticle detection in an aqueous environment was then performed, where particles as small as 20 nanometers were successfully detected.
“Another advantage of this sensing mechanism is that Raman gain is an inherent property of almost any material, so we do not need to dope the cavity with an additional gain medium. Furthermore, since Raman scattering occurs under practically any pump wavelength, this sensing method also loosens the requirement of specific wavelength bands for the pump laser,” said Prof. Xiao.
“Due to these advantages, this Raman-laser sensor represents a significant step towards practical microlaser sensors.”
The article can be found at: Li et al. (2014) Single Nanoparticle Detection Using Split-mode Microcavity Raman Lasers.
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Source: Peking University.
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