Optical sensing of nanoscale objects with ultrahigh sensitivity is highly desirable for applications in various fields, such as in early-stage diagnosis of human diseases and in environmental monitoring, as well as in homeland security. The ultimate sensitivity of a nanoparticle sensor is expected to be at the level of asingle nanoparticle response.Very recently, a group led by Prof. Yun-Feng Xiao and Prof. Qihuang Gongfrom Peking University, has developed a novel microcavity Raman-lasersensor, and realized single nanoparticle detection in both air and in an aqueous environment. The smallest nanoparticle they detected in water was 20 nanometers in radius. Their new experimental results were publishedin Proceedings of National Academy of SciencesonlineEarly Edition (DOI: 10.1073/pnas.1408453111).

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 lowpropagation 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. Therefore, WGM microcavities areideal platforms for label-free, ultrasensitive detection of single nano-objects.

In this work, researchersdetected nanoparticles using stimulated Raman scattering (also called Raman lasing) in the microcavities.Raman scattering is an inelastic light scattering process, whereby the energy of anincident photonis 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 molecularvibration in the cavity material, as shown in Figure 2(a).A typical Raman spectrum of the silica microcavity is plotted in Figure 2(b), with the pump in the 680 nm wavelength band.

Stimulated Ramanscattering 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 (as shown in Figure 2(c)), 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, as shown in Figure2(d) of the above figure.

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. Moreover, Raman laser has much narrower linewidth compared to a passive microcavity, and thus to significantlylower the detection limit.In order to demonstrate the feasibility of this sensing mechanism, the researchers first did a control experiment in air. They transferred single nanoparticles to and from the microcavity using a nanofiber, using a technology previously developed by the same group, and measured the beat frequency changes after each transfer process. As expected, every time a nanoparticle is transferredto 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, with the smallest particle of 20 nanometers in radius 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. Due to these advantages, this Raman-laser sensor represents a significant step towards practical microlaser sensors.