Low-cost frequency-domain methods of ruby fluorescence lifetime measurements

Three different light sources whose intensities varied sinusoidally with time were developed and used to measure the fluorescence lifetime of ruby with the frequency-domain method. The first light source consisted of a 5-mW Nd:YAG green laser pointer and a rotating plastic polarizer. The intensity of laser light at 532 nm was observed to vary sinusoidally at a frequency twice the frequency of the rotation due to the law of Malus. The rotation frequency was controlled by a variable-speed motor. Light from the second source came from a modulated blue light-emitting diode. It was driven by a function generator. The third light was also a green laser pointer and a rotating chopper. The shape of the laser beam exiting the chopper was scrambled by aligning the beam to pass through a long plastic rod. The intensity of light scattered from the end of the rod varied sinusoidally with time. Its frequency of variation was controlled by the variable-speed chopper. Each light source was used to illuminate a ruby crystal to induce red fluorescence from the crystal. Its intensity and the fluorescence were detected by identical photo detectors whose signals were displayed by an oscilloscope. The phase shift (&phis;) between the light excitation and fluorescence signals was measured. It provided the fluorescence lifetime (tau) according to the frequency-domain relationship, tau = tanfw . The fluorescence lifetime measured with the three different light sources was consistent with one another. The average value was 3.9 ms and the standard deviation was 0.3 ms. It agreed with reported values in the literature.;In addition, a novel derivation of the frequency-domain relationship, tau = tanfw , was presented by solving the rate equation relating the populations of the relevant energy levels. It describes the fluorescence as a quantum process rather than a convolution of the excitation function. It provides a better understanding of the fluorescence process.