Fig. 1. Experimental setup. (a) Coherent emission of multiple Raman laser lines from a liquid droplet resonator. The green arrow represents the pump; red arrows represent forward and backward stimulated Raman emission. (b) Micrograph of our 78 μm diameter silicone oil droplet coupled to a tapered fiber (shown below). (c) Energy-level diagram illustrating three states involved in the Raman spectra. (d) Experimental setup for characterizing the resonator’s optical quality factor by slowly scanning the laser frequency through resonance and measuring the bandwidth of the depth in the transmission. Also, fast scan through resonance similarly allows measuring of the optical quality, but at the temporal domain, relying on the optical ringdown effect. (e) Experimental setup for measuring Raman laser, where the backward Raman laser is directed to an optical spectrum analyzer (OSA) and to a photodetector (PD).

Fig. 2. Measuring the optical quality factor. (a) Scanning the pump laser through one of the resonances charges the resonator with light that decays later on. We fit an exponential decay (red) to provide the photon lifetime and measure a quality factor of 250 million. (b) Repeating this measurement in the frequency domain, while scanning relatively slowly, reveals a Q of 160 million. We find measurement in (a) more reliable since it is proof against broadening mechanisms.

Fig. 3. Experimentally measured threshold, power, and efficiency for the microdroplet Raman laser. Raman laser power outcoupled via the fiber, as a function of the pump input power. We fit the experimental data (circles) to the sum of two linear functions; one represents the spontaneous emission, and the other represents the stimulated emission. A knee shape at 160 μW indicates the transition from spontaneous emission to stimulated emission at input power generally referred to as the lasing threshold. The slope efficiency here is 18%. R squared is 0.98 and 0.9995 for the spontaneous and stimulated fits, respectively. The size of the circles corresponds to the resolution of our measurement.

Fig. 4. Experimentally measured Raman laser lines and their corresponding calculated molecular vibrations. (a) We calculate the vibrational modes of polydimethylsiloxane (which include three repeating monomer units) using the ADF module of SCM software. A movie describing the dynamics involved in these S1-2 vibrations appears in the Supplementary Material. Our pump-mode wavelength is 778.2 nm, and its quality factor is 250 million. Our Raman line wavelengths are 792.2, 991.1, and 1230.8 nm.

Fig. 5. Experimental results. (blue) Raman spectrum of 56 μm diameter droplet resonator made from silicone oil with a viscosity of 1000 mPa·s. (black) Control group: Raman spectrum obtained using a commercial Raman spectrometer (Horiba Jobin Yvon LabRAM HR Evolution). The two spectra are provided together (a) for showing the higher power of the stimulated emission, then (b) for zooming in to the control group experiment to provide its finer details. The Raman laser lines near 1000, 3000, and 6000  cm−1 [blue line in (a)] occur in multiple modes.