Luminescence from Spherically and Aspherically Collapsing Laser Induced Bubbles

Light emission from bubbles in liquids has received increased attention with the advent of single bubble levitation in sound fields [1]. In this Letter we present investigations of the light emission from single collapsing cavitation bubbles: single cavitation bubble luminescence (SCBL). Our method makes use of focused laser light to produce largely empty bubbles in a liquid, a method used earlier for cavitation bubble dynamics studies [2,3]. In contrast to acoustically driven bubbles in SBSL (single bubble sonoluminescence) [1] and MBSL (multibubble sonoluminescence) [4] experiments, laser induced bubbles do not need a sound field for a strong collapse and for light emission [5]. Laser induced bubbles can be made to collapse in adjustable aspherical geometry and thus may settle some questions about the origin of the luminescence. SCBL may also serve as a link between SBSL and MBSL in that the aspherical environment present in MBSL can be simulated in SCBL. Figure 1 shows a sketch of the experimental arrangement used to investigate the bubble dynamics and the light emission during bubble collapse. A Q-switched Nd:YAG laser delivers single laser pulses of 8 ns width and up to 20 mJ energy at a wavelength of 1064 nm. The laser light is focused with an aberration minimized lens system into a cuvette filled with bidistilled water (85 mm 3 85 mm 3 75 mm, water at room temperature and air saturated). A focus angle of 25± places the focus 27 mm from the cuvette wall. This allows a nearly spherical geometry in the surrounding liquid for the investigated bubble sizes ranging from 0.8 mm to 1.5 mm in radius. Single bubbles are nucleated at the focus of the laser pulse when the dielectric breakdown threshold is reached, and a rapidly expanding plasma is formed. The recombining plasma gives rise to a gasand vapor-filled bubble that grows to a maximum radius depending on the laser pulse parameters, in particular, pulse length and pulse energy [6]. The bubble collapses down to a minimum size until the compressed gas in the bubble leads to a rebound. During the collapse process, the bubble dissipates energy, mainly through emission of acoustic transients, and therefore reexpands to a smaller, second maximum bubble radius. The bubble dynamics is resolved with an image converter camera at 227.000 framesys (Imacon 700, Hadland Photonics) with