Last night I was awake until 4.5 am, first to work on a new crystal for nonlinear interactions and second to listen to Democrats 2008 US presidency debate on CNN. For the first one, I concluded (then in the morning checked with other colleagues) that results of a newly publishes paper (2007) that claims to observing SHG in a new crystal are completely wrong! It was reported by an Ukrainian group of Crystal researchers that I think are not professional in Nonlinear Optics.
In Dem debate, Hillary's attack on other candidates especially Obama and Edwards was strange for me because we can find that how deep is their beliefs for future plans as US president and as a Democrate. But their open discussion in every topic was really warming because this type of talks is not allowed or even forbidden with accusation of weakening government and even prisoning in asian countries especially middle east.
The most interesting and in my opinion very knowlegeable person among them is Joe Biden, who is very relaxed when is not talking but very serious and straight when explaining his views.
The promising point in all of their talks was a united view in bringing peace into ME and open talks with al nations there, especially Iran.
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Second harmonic generation in liquid crystals
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F.Kentischera, R. Macdonalda
aOptical Institute of the Technical University of Berlin
Straße des 17. Juni 135, 10623 Berlin, Germany
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SECOND HARMONIC GENERATION
Second Harmonic Generation (SHG) is a three wave mixing nonlinear optical process.Under certain conditions, two incident beams with the same frequency f1 are converted more or less efficiently into light with the double frequency 2f1, as shown in the figure below. In a corpuscular (quantum theoretical) picture, two photons of the energy hf1 are annihi- lated and simultaneously a photon of the energy 2hf1 is created. Only half of the photons are generated as annihilated.
Keywords:
SHG
linear electro-optical effect
sum and difference frequencies
parametric processes
polarization response
molecular hyperpolarizability ß
symmetry of the medium
macroscopic coefficients Xijk
bulk SHG
noncentrosymmetric media
phasemachting
Maker-Fringes
Figure 1: Illustration of SHG
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EXPERIMENTAL SETUP FOR SHG MEASUREMENTS
'Klick' on the link in the header, to have a look at our experimental setup for SHG measurements. Currently our experi- mental setup is modified to make spatially resoluted SHG measurements (SHG-Microscopy).
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INVESTIGATED BANANA-SHAPED LIQUID CRYSTALLINE MOLECULES
In the figures below, the investegated banana shaped molecules are shown. They belong to the alkyloxy- and alkyl- homologues series which were synthezised by Heppke and coworkers at the I.-N.-Stranski Institute at the Technical Institute of the TU Berlin. In 1996 the synthesis of bent-shaped molecules and their liquid crystalline properties were reported by Takezoe et al. (J. Mater. Chem. 6(7), 1231 (1996)) for the first time. From that date on many international research groups started to investegate the properties of these interesting new marterials.
Figure 2: Skech of a banana shaped liquid crystalline molecule by Andy Warhol
Figure 3: Chemical Structure of banana shaped molecules ( (R=OCnH2n+1 alcyloxy- R=CnH2n+1 alcyl-homologues)
The molecules exhibit sereveral thermodynamic stable phases, with different symmetry dependent, nonlinear optical properties. For the first time in the world, we investigated these phases by SHG measurements.We determined the symmetry properties and the nonlinear coefficients of some of the phases by angle-, polarization-, temperature- and electric field-dependent measurements. In the electric field induced, ferroelecric state of the so called B2 phase the largest value is the d31 coefficient with 16,5 pm/V. This value is the largest nonlinear coefficient reported for organic liquid crystals so far.
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ACKNOWLEDGEMENTS
This work was supported by the Sonderforschungsbereich 335 ‘Anisotrope Fluide’ (SFB 335) of the Deutsche Forschungsgesellschaft (DFG).
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Publications
[TU Berlin] [Physics Department] [Institute of Optics] [Liquid Crystal Group Home]
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This page is maintained by F. Kentischer Last update: 12/98
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