International Research Experience for Undergraduates

Tom Bemben

2002 Participant


Femtosecond Laser Spectroscopy

Femtosecond laser spectroscopy has recently become an area of high interest. It's applications range from exploration of ultrafast electron dynamics to long lived photoexcitations. To progress into a deeper understanding of ultrafast phenomena it is necessary to know the laser pulse length. As there is no conventional electronics that can measure femtosecond pulses my duties this summer included the design and implementation of an autocorrelator, a device that can indirectly estimate the pulse length.

The setup of an autocorrelator involves splitting a laser beam into two beams of equal intensity and focusing them on a nonlinear medium, in this case a Barium Borate Type I crystal. The result is a second harmonic beam (SH), which is then shot onto a combination photodiode and integrating circuit driver with output to an oscilloscope. The photodiode measures the intensities of the SH beams and the driver circuit integrates the products of the two functions to get the autocorrelation function:

Bemben Diagram 01

Other projects included the design of a crystal rotation stage for the second harmonic generation. Considerations had to be made for available space, range of motion, stability, and height. The designs were done on AutoCAD; samples are shown below.

Bemben Diagram 02

Bemben Diagram 03