Our fluidic microcavity is made of a liquid jet formed by ejecting ethanol vertically through a deformed orifice of a near-elliptical shape. As the liquid column advances, modulation in surface profile spontaneously occurs because the surface tension of liquid acts as a restoring force for an initially noncircular cross section as illustrated in Fig. 5a. A small segment of a few micron thickness of the liquid column at one of extreme positions of surface modulation then acts as a two-dimensional microcavity for the optical wave. The cavity boundary shape can be determined by forward shadow diffraction of a laser beam incident on the jet column49, and it is approximately a quadru-octapole given by , where and . For spectroscopic observations, the liquid contains dye molecules which emit fluorescence when optically excited. When the small segment of the jet column comprising a deformed cavity is excited by a pump laser as seen in Fig. 5b, the fluorescence from dye molecules is enhanced at cavity resonances as shown in Fig. 3a. This enhancement comes from the cavity quantum electrodynamics effect44. In this cavity-modified fluorescence spectrum from the microjet cavity, we typically observe 4 ~ 5 groups of cavity resonances or modes. Each mode group is a sequence of resonances with a well-defined free spectral range.