Laser heterodyning detector for frequency-domain ultrafast spectroscopy

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Abstract
The attainable frequency range of ordinary heterodyning and super-heterodyning frequency- domain fluorescence instrumentation is limited by the response of the optical detector rather than by the harmonic content of the light source. The replacement of the photomultiplier detector by a 6 (mu) microchannel plate detector has improved the frequency response from 500 MHz to 10 GHz. A new method is developed to detect fast, excited state processes by extending to the frequency-domain the well known pump/probe (absorption) technique used in the time-domain. The upper frequency limit attainable with this method is limited only by the pulse width of the light sources. For picosecond pulse lasers this limit extends to hundreds of gigahertz. A theoretical determination of the basic equations is given, and data are shown for the excited state decay of a rhodamine 6G sample in ethylene glycol.

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