Wenzel Salzmann

Coherent control with shaped femtosecond laser pulses applied to ultracold molecules

We report on coherent control of excitation processes of translationally ultracold rubidium dimers in a magneto-optical trap by using shaped femtosecond laser pulses. Evolution strategies are applied in a feedback loop in order to optimize the photoexcitation of the Rb{sub 2} molecules, which subsequently undergo ionization or fragmentation. A superior performance of the resulting pulses compared to unshaped pulses of the same pulse energy is obtained by distributing the energy among specific spectral components. The demonstration of coherent control to ultracold ensembles opens a path to actively influence fundamental photoinduced processes in molecular quantum gases.

Theoretical model for ultracold molecule formation via adaptive feedback control

We theoretically investigate pump-dump photoassociation of ultracold molecules with amplitude- and phase-modulated femtosecond laser pulses. For this purpose, a perturbative model for light-matter interaction is developed and combined with a genetic algorithm for adaptive feedback control of the laser pulse shapes. The model is applied to the formation of 85Rb2 molecules in a magneto-optical trap. We find that optimized pulse shapes may maximize the formation of ground state molecules in a specific vibrational state at a pump-dump delay time for which unshaped pulses lead to a minimum of the formation rate. Compared to the maximum formation rate obtained for unshaped pulses at the optimum pump-dump delay, the optimized pulses lead to a significant improvement of about 40% for the target level population. Since our model yields the spectral amplitudes and phases of the optimized pulses, the results are directly applicable in pulse shaping experiments.

Characteristic oscillations in the coherent transients of ultracold rubidium molecules using red and blue detuned pulses for photoassociation

We investigate the interaction of femtosecond laser pulses with an ensemble of ultracold rubidium atoms by applying shaped excitation pulses with two different types of spectral filtering. Although the pulses, which are frequency filtered with a high pass, have no spectral overlap with molecular states, we observe coherent molecular transients. Similar transients obtained with nearly transform-limited pulses, where only the atomic resonance is removed, reveal two differing oscillatory components. The resulting transients are compared among themselves and supported with quantum dynamical simulations which indicate a photoassociation process. The effect is due to the strong field interaction of the pulse with the colliding atom pair.

The interaction of a spectrally cut laser-pulse with a two-level atom

In this article we present a theoretical model for the light–matter interaction of a two-level atom and a femtosecond laser pulse, spectrally cut in the vicinity of the atomic resonance. The model utilizes a transformation into an interaction picture from a bare atomic state basis and is applicable to two-level systems. This transformation simplifies some of the dynamics of the interaction. A complementary intuitive picture is also presented and describes the interaction of the induced dipole with the pulses transients. The calculations are in excellent agreement with experimental measurements performed on an ultracold rubidium gas.

Oxygen measurements at high pressures using a low cost, polarisation stabilized, widely tunable vertical-cavity surface-emitting laser

Vertical-cavity surface-emitting lasers (VCSELs) are used for oxygen monitoring via tunable diode laser spectroscopy at 760nm wavelength. For the desired application, novel polarization-stable laser diodes based on AlGaAs were developed. We present measurements of the pressure-broadening coefficients of the electric dipole forbidden oxygen A-Band b1Σg + -> X3Σg + transition at 760nm. For the first time the pressure-broadening coefficients were determined with a temperature tuned vertical-cavity surface-emitting laser. Because of special techniques of polarization stabilization with a combination of a dielectric surface grating and a surface relief the VCSELs have a mode hop-free tuning range of more than 7nm and a sidemode suppression of more than 30dB. This provides a low cost laser diode system with a wide tuning range, which enables the possibility of simultaneous measurement of temperature, pressure and oxygen concentration in air, high pressure measurements and also a higher accuracy of oxygen concentration measurements due to averaging over 18 absorption lines.