G. Krampert

An apertureless near-field scanning optical microscope and its application to surface-enhanced Raman spectroscopy and multiphoton fluorescence imaging

We describe a home-built apertureless near-field scanning optical microscope and present preliminary results of its operation. Raman scattering from samples of polydiacetylene para-toluene sulphonate, and two-photon-induced fluorescence from crystallites of coumarin I dye are strongly enhanced in the presence of a sharp gold-coated atomic force microscope tip. We verify the dependence of the scattered intensity on the polarization of the incident beam relative to the tip axis. Finally, we show near-field fluorescence images taken in the presence of a strong far-field background whose spatial resolution is limited by the size of the tip.

Adaptive shaping of femtosecond polarization profiles

We report the experimental implementation of femtosecond polarization pulse shaping within an adaptive learning loop. This technique makes it possible to optimally and automatically generate light fields in which intensity, momentary frequency, and light polarization (i.e., ellipticity and orientation) change as a function of time within a single femtosecond laser pulse. By use of second-harmonic generation as a feedback signal in an evolutionary algorithm, specific phase- and polarization-modulated laser pulses are generated. Material dispersion and time-dependent modulations of the polarization state can be compensated. These experiments demonstrate the feasibility of adaptive quantum control experiments with polarization-shaped femtosecond laser pulses.

Femtosecond learning control of quantum dynamics in gases and liquids: technology and applications

Abstract Obtaining active control over quantum-mechanical systems is a fascinating prospect in modern physics. In recent years, quantum control has developed as a means of actively controlling the outcome of light-matter interactions. The basic principles of adaptive femtosecond quantum control are discussed. Specifically shaped femtosecond laser pulses are used to direct the dynamical evolution of quantum wavefunctions into desired target channels. This is achieved in a ‘closed-loop’ implementation wherein experimental feedback signals are evaluated by a learning algorithm. In this iterative optimization scheme, knowledge of the Hamiltonian is not required. Specific experimental examples are automated compression of amplified femtosecond laser pulses and control of gas-phase molecular photodissociation in Fe(CO)5, CpFe(CO)2Cl and CH3CHOHCOOH. Additionally, selective control of molecular excitation is demonstrated under liquid-phase conditions. As a further technological development, the technique of femtosecond polarization pulse shaping is discussed briefly, which makes it possible to vary the polarization state of light, intensity and oscillation frequency in a flexible manner on an ultrashort timescale.

Rotation-translation device for condensed-phase spectroscopy with small sample volumes

We present and characterize an experimental device for optical spectroscopy with small sample volumes contained in a thin film. Employing rotational and translational motion, the sample transport speeds are high enough to offer a new sample volume for each interaction in time-resolved spectroscopy experiments working with a 1kHz repetition rate. This is especially suited for ultrafast femtosecond spectroscopy such as transient absorption spectroscopy or fluorescence upconversion. To reduce photodegradation and effects from local thermal heating, a large sample area is scanned in contrast to conventional devices with either only rotation or translation movements. For characterization of the setup, transient absorption experiments are carried out using the rotation-translation device and a conventional flow-cell setup, which exhibit similar signal-to-noise ratio in the two cases. The effects of photodegradation and diffusion are also investigated, demonstrating the suitability of the device for time-resolve...

Adaptive Femtosecond Quantum Control

Since the advent of quantum mechanics there has been a desire to understand in detail the behavior of quantum systems. This quest has been accompanied by the implicit dream to not only be able to observe in a passive way, but in fact also to actively control quantum-mechanical processes. The key question in quantum control is: Can one find external control parameters which guide the temporal evolution of quantum-mechanical systems in a desired way, even if this evolution is very complex?

Teaching lasers to twist molecules

We report on optimal control of the photoisomerization reaction of 3,3-diethyl-2,2-thiacyanine iodide (NK88) dissolved in methanol. Enhancement as well as reduction of the relative yield of cis- to trans-isomers are achieved.

Adaptive polarization control of molecular dynamics

We demonstrate that the use of time-dependent light polarization opens a new level of control over quantum systems. With K2 molecules we show that polarization-shaped laser pulses increase the multiphoton-ionization yield compared to linearly-polarized laser pulses.