SINGLE-QUANTUM-WELL LIGHT-EMITTING-DIODES DEMONSTRATED AS EXCITATION SOURCES FOR NANOSECOND PHASE-MODULATION FLUORESCENCE LIFETIME MEASUREMENTS

We have characterized the output of inexpensive, commercially availabl e single quantum well (SQW) blue and green light emitting diodes (LEDs ). The SQW LEDs were amplitude modulated with the output from a freque ncy generator while biased through a bias tee with 5 mA of current. Th e blue SQW LED produced 800 mu W of light centered at 466 nm with a -3 dB bandwidth of 58 MHz. The green SQW LED produced 543 mu W of light centered at 522 nm, with a -3 dB bandwidth of 26 MHz. Modulated light was available to approximately 100 MHz, allowing the measurement of ns fluorescence lifetimes. The fluorescence lifetime of a standard fluor ophore (fluorescein) was measured in the frequency domain using the ph ase-modulation technique, and gave results similar to those obtained w ith a 488 nm argon ion laser modulated with a Pockels cell. To demonst rate the usefulness of the SQW LED source, we also performed measureme nts with the fluorescent pH indicator SNAFL-2. Again, these results co mpared favorably with those obtained with the laser. When compared to a laser modulated with a Pockels cell, the SQW LEDs were smaller, less expensive, required less power, generated less heat, and required les s alignment. The ability to modulate the SQW LEDs at high frequencies, along with the blue or green outputs, allow their use as inexpensive light sources in fluorescence lifetime optical sensors and even fluoro meters. (C) 1996 American Institute of Physics.