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We report the existence of a second type of recycling mode that occurs for air-seeded bubbles. Observation of period doubling in both the stable, the first type, and the second type of recycling mode, together with simultaneous measurement of the relative phase of light emission compared to the drive, shows that the instability boundaries of period doubling and bubble extinction are mainly determined by the bubble size irregardless of the gas composition. The second type mode seems to represent a link between single-bubble and multibubble sonoluminescence.

The size of the light-emitting region is a key parameter toward understanding the light-emitting processes in a sonoluminescing bubble. Here we present measurements of interference effects from particles with a diameter of approximately 2 microm situated 6-10 microm from a sonoluminescing bubble. From the angular size of the pattern and from an estimated distance to the particles we conclude that the light-emitting region of a sonoluminescing bubble is smaller than commonly believed [see, e.g., Nature (London) 398, 402 (1999)]. We argue that an upper limit of the size of the light-emitting region is approximately 200 nm.

We present direct observations of period doubling in the flash to flash pulse heights in single-bubble sonoluminescence. States involved are stable, spherically symmetry broken. Observations are made using seven detectors distributed in the equatorial plane of the bubble. Contrary to earlier experiments by Holt et al. [Phys. Rev. Lett. 72, 1376 (1994)], where period doubling was observed in the time intervals between flashes but not in the pulse heights, we observe period doubling in pulse heights, but no corresponding period doubling is seen in the time intervals. In parameter space the period doubling is observed below the n=2 shape instability boundary line where extinction is shown to take place.

We present observations of stable spherical symmetry broken states in single bubble sonoluminescence including observations of period doubled states. States observed involve both spatially oriented states and states with a tumbling symmetry axis. The observations are made using a fiber based four-channel correlation scheme. The measurements are made both with and without narrow band optical filters. The symmetry broken states are seen in all cases even using a 650+/-40-nm filter. This fact may be used to distinguish between different theories for the light emission. Prior to the measurements reported here, theoretical attempts to explain observations of period doubling bifurcation phenomena in single bubble sonoluminescence were centered on radially bifurcated collapses. The present experiments show unequivocally that the observations are primarily a result of breaking the spherical symmetry in the bubble collapse. Period doubling will at most show up as secondary effects in the total light output, if at all.

Using a fiber-based four-channel correlation scheme to investigate spatial and temporal correlations, we show that observations of period-doubling phenomena in single bubble sonoluminescence are primarily a result of spontaneously breaking the spherical symmetry in the bubble collapse and, at most, may show up as secondary effects in the flash-to-flash spatially integrated light output.