Scott D. Carpenter

Effect of Pulse Shape on the Efficiency of Multiphoton Processes: Implications for Biological Microscopy

The effects of spectral shape on two photon fluorescence excitation are investigated experimentally using an acousto-optic pulse shaper to modify femtosecond pulses from a Ti:sapphire laser. By using different spectral window shapes, we find that the measured two photon efficiency can vary by a factor of 2 for differently shaped spectra with the same full width at half maximum. We find that these effects are described well by a simple model assuming transform-limited pulses. The fact that even small changes in the spectral wings can significantly affect the efficiency of nonlinear processes has implications for biological multiphoton imaging, where it may be desirable to minimize sample exposure to radiation and maximize fluorescence or harmonic efficiency.

Reflectron design for femtosecond electron guns

We apply the reflectron principle to design an electron gun capable of delivering femtosecond duration electron pulses. The gun is based on photoemission and uses a high repetition rate femtosecond laser system. We show that even at low laser power electrons are generated in a multiphoton process, generating an electron beam with a significant energy spread. The temporal broadening resulting from this distribution can be partially compensated by a reflective electron mirror. Our reflectron features a gridless mirror that focuses the electron pulses in space and time. Computer simulations highlight the important design parameters and demonstrate that pulse durations of hundreds of femtoseconds are achievable, even in cases of broad kinetic energy distributions. The reflectron is very accommodating toward space charge effects, making it an attractive choice in applications requiring high electron densities.

Experimental adaptive optimization of mass spectrometer ion optic voltages using a genetic algorithm

We use a genetic algorithm (GA) to optimize the mass resolution and detection efficiency of a multi-photon ionization, time-of-flight mass spectrometer. The algorithm uses experimental fitness functions to optimize eight voltages supplied to a set of ion optics. The GA optimized the ion detection efficiency by a factor of 10 and the mass resolution by a factor of 11 over previous settings obtained from computer simulations of the instrument. This experiment highlights the usefulness of adaptive algorithms to the experimental optimization of multidimensional search problems commonly found in modern laboratories.

Feedback Quantum Control of Population Transfer Using Shaped Femtosecond Pulses

Quantum control is extended to complex molecular systems by using experimental feedback to control the acousto-optic tailoring of ultrashort pulses.

Self-learning optical system based on a genetic-algorithm driven spatial light modulator

We demonstrate the applicability of a genetic algorithm (GA) to control the focus of an adaptive optical system using a liquid crystal spatial light modulator. The optical setup and the algorithm applied are set to fitness type reinforcement for learning. The particular GA developed optimizes the phase shifts in 32 independent pixels, and is biased towards approximating continuous functions that suit the focusing problem. The learning process is demonstrated to work reliably even in the presence of experimental noise.