Main Page/PHYS 4210/SonoluminescenceDev

Sonoluminescence

Sonoluminescence is the process by which a gas bubble trapped at the antinode of an ultrasonic standing wave emits visible radiation. This strange phenomenon will be the platform on which 3-dimensional standing waves and black-body radiation will be investigated.

Introduction

Single bubble sonoluminescence, hereafter abbreviated as SL, was discovered in the late 1980's and has received a great deal of attention. This remarkable process involves the trapping of gas bubble at the antinode of a ultrasonic standing wave in a liquid.

3D Standing Waves

In order for the trapping of gas bubbles, a 3-dimensional ultrasonic standing wave need to be attained. The resonant frequency of a the standing wave will occur if each of the 3 dimensions has a length which is equal to an integer number of half wavelengths of the acoustic wave. One will notice that the shape of the liquid cell is rectangular, having the length and width being equal. The height of the cell, and hence a value of great importance for this experiment, is set by the volume of water added to the cell.

The Ultrasonic Horn

The ultrasonic horn is used to deliver acoustic power to the volume of water. Internally, the horn contains a series of annular shaped disc transducers which are bolted into its base. The basic structure and shape of the horn is designed to efficiently couple the pressure waves generated from the transducers to the narrow stem of the horn. All of the transducers are the same. They consist of a ceramic material which has been prepared in such a manner as to have a permanent polarization. In other words,there are specialized capacitors. As a charged is placed across this capacitor there is a force generated across the ends, and the capacitor wants to separate. Since the transducers are compressed, this repulsive force does not physically expand the disc but does produce dynamic pressure. As the charge across the transducer is reversed, there is now an attractive force which results in a negative pressure amplitude. The capacitance of the particular tranducers used is 11 nF. In order to efficiently couple electrical power into the transducers, it is advantageous to connect an inductor in series to achieve electrical resonance as given by the formula:

The Cell

The cell is a plastic container onto which is epoxied a small ceramic transducer that serves as a microphone. Since this transducer is not compressed small fluctuations in its diameter produce a measurable signal. BY attaching this transducer to the bottom of the cell, one can easily detect when the pressure is the cell in in resonance by looking for large amplitude sine waves on a oscilloscope at the ultrasonic frequency. The cell is provided with a black shroud which serves which has a circular opening on one side to allow for light from the SL to enter the photomultiplier tube.

The Photomultiplier Tube

A photomultiplier tube (PMT) is a device which coverts one incident photon into a large pulse of electrons which can then be read out on an oscilloscope. The photon passes through the glass enclosure of the PMT an liberates an electron from a coated surface, this electron is then accelerated by static electric fields into dynode 1, and this initial electron free many more electrons. This bunch of electron then accelerates into dynode 2 freeing a larger bunch of electrons. This process continues many times such that even one incident photon can produce a measurable number of electrons at the output. A PMT is a very sensitive optical detector. Due to the high sensitivity, having too many photons entering the PMT can damage the device, and this should be avoided. Even with the black shroud covering the cell, you should not apply power to the PMT without also shutting of the room lights.

The Fluke 412B power supply will provide the high voltage required to bias the dynodes of the PMT. Typically, the PMT should be run at XXXX V. The PMT will output a negative going pulse with a sharp rise time and a relatively slow fall time. The amplitude of this pulse is related to the number of photons hitting the PMT during a particular pulse. In this experiment, the effect of Sonoluminescence only occurs once during an acoustic wave period, hence, the SL bubbles is flashing at around 27 kHz. The PMT will output pulses synchronized with the acoustic frequency whose amplitude is related to the brightness of the sonoluminescence.