In this review, the recent development of nano-particle detection using Raman lasers in the whispering gallery mode microcavities is presented. The fabrication of the microcavity, the working principles are given and the recent experimental progress is reviewed. Recent years, the demand for nano-particle sensing techniques was increased, since more and more nano-particles of sizes between 1 nm and 100 nm are employed in areas such as biomedical science and homeland security. In these applications, label-free, rapid and real-time sensing requirements are necessary. Whispering gallery mode (WGM) micro-resonators have high-quality factors and small mode volumes, and have achieved significant progress in the nano-particle sensing field. There are various measurement mechanisms for nano-particle sensing using WGM cavities, including resonance mode broadening, resonance frequency shift, and mode splitting changes. The key point to improve sensing limit is to narrow the resonance mode linewidth, which means reducing the optical cavity losses, or equivalently to enhance quality factor. An important approach to narrowing the mode linewidth is to fabricate active resonators that provide gain and produce laser by doping rare earth irons. According to Schawlow-Townes formula, the linewidth of corresponding laser will be narrower than that of the original optical cavity mode. Active resonators have outstanding performances in particle detection. However, doping process requires complex fabrication steps, and rare earth irons laser demands a certain pumping wavelength band. A new approach is to use low threshold Raman laser in an optical micro-resonator. The binding of nano-particles on WGM micro-resonator induces resonance mode splitting. Raman lasers of the two splitting modes irradiate the same photon detector and generate a beat note signal. By monitoring the jumps of the two split mode differential signals, one can easily recognize the nano-particle binding events, thus achieving real time nanoparticle detection. Using Raman laser in WGM cavities in nano-particle sensing is no longer limited by the stringent requirement of a suitable pump light source, which greatly expands the applicability of this method in different environments. It does not need additional fabrication process as compared with the rare earth doping method. It has also better biological compatibility, which makes it a promising technique in biomedical applications. Recently, two groups, i.e., Li et al. (Proc. Natl. Acad. Sci. 111 14657) from Peking University, and zdemir et al. from University of Washington and Tsinghua University, have successfully completed the demonstration experiments. zdemir et al. (Proc. Natl. Acad. Sci. 111 E3836) have achieved a nano-particle sensing limit down to 10 nm without labelling, and Li et al. (Proc. Natl. Acad. Sci. 111 14657) realized real-time detection of single nano-particles with WGM cavity Raman laser in an aqueous environment. Both experiments have shown the great potential of the new approach. The new technique can also be used in other photonic systems, such as the photonic crystal or metal materials.
