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. 2014 Sep 2;111(37):E3836–E3844. doi: 10.1073/pnas.1408283111

Fig. 1.

Fig. 1.

Experimental setup and measurement method. (A) Schematic illustration of the experimental setup used in the study of Raman gain-enhanced detection of single nanoparticles using mode splitting. A differential mobility analyzer with a nozzle is used to deposit nanoparticles into the mode volume of the resonator one by one. Light from a pump laser is coupled to the WGM of a microtoroid resonator by a fiber-taper coupler. The residual pump, Stokes photons, and the Raman laser were out-coupled from the resonator with the same fiber taper. The silicon chip with the silica microtoroids was placed on a 3D nanopositioning stage to precisely tune the distance between the fiber taper and the microtoroid. A fiber polarization controller (PC) was used to change the polarization state of the pump laser to maximize the coupling efficiency. The pump light and the Raman laser light (probe light in case of below lasing threshold operation) are separated from each other using a wavelength division multiplexer (WDM) and detected with photodetectors (PD) connected to an oscilloscope (OSC). (Inset) The top view of a fiber-taper-coupled silica microtoroid resonator taken by an optical microscope. FC, fiber connector. (B) Transmission spectra of the silica microtoroid obtained in the 1,450-nm and 1,550-nm bands at low optical power. The resonances in these bands are separated from each other by 11.67 THz, which lies within the Raman gain spectra given in C, Insets. (C) Typical transmission spectra obtained in the experiments for the pump in the 1,450-nm band and Raman gain and the laser in the 1,550-nm band when the wavelength of the pump laser (blue) is scanned in time. The sawtooth-like waveform is due to the thermal broadening of the resonance line. As we move along this sawtooth-like form in time, more pump light is coupled into the resonator, and cavity power build-up sufficient to produce Raman gain and lasing is achieved as seen in the increased signal in the transmission obtained in the 1,550-nm band (red). (Insets) The Raman process and the Raman gain spectra for silica, which implies that Raman gain is provided within a band of 5–30 THz away from the pump light.