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Photo: Niels Rosemann

Synthesis and properties of amorphous molecular materials are by far less well investigated and understood than those of crystalline ones. The members of the Research Unit FOR 2824 study and develop materials that exhibit unique extreme non-linear optical properties, thereby addressing chemical and physical facilitators or inhibitors of crystal formation as well as rotational, vibrational, and electron dynamics in their electronic ground state.
These materials allow, for example, for the generation of directional, low-étendue white-light supercontinua, driven by lower frequency (infrared, IR) irradiation. Such light sources are highly interesting for a variety of purposes, like projection, sensing, endoscopy, optical coherence tomography, and laser cutting. Conventional supercontinuum generation requires complex and expensive pulsed laser sources, which has so far restricted such processes to “academic” applications. New developments for white-light generation (WLG) might revolutionize these technologies.

The research program builds on the recent finding of a class of compounds that convert low-intensity steady-state infrared radiation from commercially available continuous wave (CW) laser diodes into white-light with retention of the spatial emission profile. This is technically less complex and rather inexpensive as compared to the conventional method, as no pulsed lasers are needed. The current challenges lie in the identification and optimization of compounds with appropriate habit that enable this nonlinear optical effect, and to identify the microscopic physical mechanism, i.e., the electronic ground state dynamics.

According to our current, rather phenomenological understanding of this unprecedented effect, two preconditions apparently have to be fulfilled for white-light generation: the compounds need to be amorphous and need to incorporate delocalized π-electron systems. Both conditions are met in powders containing organic adamantanes [(RC)4(CH2)6] or adamantane-shaped clusters of the general formula [(RT)4S6] (R = organic group with an appropriately-sized π-system like Ph = phenyl or Sty = styryl; T = Ge, Sn). The closely related, yet crystalline, compounds [(PhSi)4S4] and [(MeSn)4S6] (Me = methyl), do not show white-light generation, but highly efficient second-harmonic generation (SHG), an alternative non-linear optical response. The physical process behind the observation is not understood yet, hence necessitating a sophisticated study of a library of compounds and the acquisition of much more physical data.

The team of seven principle investigators (PIs) from experimental and theoretical chemistry and physics at Philipps-University Marburg (UMR) and Justus-Liebig University Giessen (JLU) will address the following key goals:

Preparation of a Compound Library

We are preparing a library of two general types of compounds for fine-tuning of their properties: organic adamantane derivatives and heteroadamantane clusters of the general type [(RT)4E6] on the one hand, and higher diamondoids [(RC)xEy], cubane derivatives [(RC)4E4], and cubane-derived clusters [(RT)4(–x)MyE4+z] on the other hand (R = organic substituent; T = C, Si, Ge, Sn; E = O, S, Se, Te, NH, CH2, ON•; M = transition metal), with either homogeneous (one T, one E, one R, one M) or heterogeneous composition (T, T’ and/or E, E’ and/or R/R’ and/or M/M’).

Investigation of Macroscopic Structures

We gain insights into macroscopic structures of pure or blended compounds and their impact on the non-linear optical properties for understanding of the electronic reasons for the non-linear white-light emission and its reliance on either molecular properties (microscopic length scale) or on the (dis)ordered ensemble within an amorphous bulk (mesoscopic or macroscopic length scale).

Technical Applications

The comprehensive knowledge and control of material properties is the key for future technical applications [crystallinity versus non-crystallinity, stability (thermal, air, mechanical), handling (dry, wet), and processing (compacting, sealing, printing)], upon screening of the compounds from purely inorganic through mixed organic-inorganic to purely organic cluster cores. This leads in the pre-design of optical materials for specific applications.

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