Articles you may be interested inHigh-order optical harmonic generation as a process of single atom interaction with sub-relativistic single-and multicolor laser fields AIP Conf.A bright metastable helium ͑He ‫ء‬ ͒ beam is collimated sequentially with the bichromatic force and three optical molasses velocity compression stages. Each He ‫ء‬ atom in the beam has 20 eV of internal energy that can destroy a molecular resist assembled on a gold coated silicon wafer. Patterns in the resist are imprinted onto the gold layer with a standard selective etch. Patterning of the wafer with the He ‫ء‬ was demonstrated with two methods. First, a mesh was used to protect parts of the wafer making an array of grid lines. Second, a standing wave of = 1083 nm light was used to channel and focus the He ‫ء‬ atoms into lines separated by / 2. The patterns were measured with an atomic force microscope establishing an edge resolution of 80 nm. Our results are reliable and repeatable.

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The increase in molecular orientational fluctuations as a phase transition is approached is an interesting and dramatic phenomenon, but associated undergraduate experiments are rare. We present an advanced undergraduate experiment on light scattering from the molecular orientational fluctuations in liquid crystals. In the high-temperature, disordered phase of a liquid crystal, small clusters of parallel-oriented molecules form spontaneously and scatter light. The size of the fluctuations and the consequent light scattering increase dramatically as the temperature is reduced approaching the weakly first-order phase transition to the ordered liquid crystal phase. We discuss a simplified theory that captures the essential physics.

Pretransitional fluctuations in the isotropic phase of liquid crystalline and non-liquid crystalline alkyl cyanobiphenyls have been investigated using light scattering and magnetic birefringence measurements. We find evidence for a virtual isotropic-nematic phase transition in short-chain alkyl cyanobiphenyls with no observable nematic phase. The measured temperature dependence of fluctuations is well-described by mean-field theory. Virtual phase transition temperatures extrapolated from separate light scattering and magnetic birefringence experiments are in good agreement. Landau-de Gennes model parameters for the compounds investigated are calculated from the experimental results.

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