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Invited presentation by Prof. B. Witzingmann
The development of supercontinuum sources is advancing fast in the last decades. As do all nonlinear effects, the supercontinuum generation strongly relies on the nonlinearity of the active material. This nonlinearity may be greatly enhanced in specially designed photonic-crystal fibers, making supercontinuum sources widely available. Nevertheless, pulsed lasers are required to supply high enough field strengths to overcome the threshold for supercontinuum generation.
We study a new cluster based class of nonlinear media that exhibits ultra-low thresholds for supercontinuum generations, thus enabling the use of a low coast steady-state laser diode as the driving laser [1].
The clusters are composed of a tin sulfide based core that is surrounded by four organic ligands. The core adopts an adamantane-like architecture, [Sn4S6]. It has a tetrahedral shape and thus lacks inversion symmetry, enabling nonlinear processes. The four ligands (R = 4-(CH2=CH)-C6H4) are consolidating the structure of the core. Yet, as they are randomly oriented around the Sn-C bonds, they are also preventing any long-range order in the solid phase of the compound. As a result, the compound is obtained as a white powder with totally frustrated order.
This powder has been studied in respect to its optical properties. When irradiated with a continuous-wave infrared laser of sufficient intensity it emits a warm white spectrum that is virtually independent from the pump-wavelength in a range of 725-1050 nm. Lowering the pump intensity, however, changes the spectral weight to the red, similar to dimming of thermal emitters. The input-output characteristics, however, exclude a thermal process as the source of the observed white-light. Additionally semi-classical calculations of the white-light generation process are performed, underlining this statement.
[1] Rosemann N.W., et al.; Science, 2016, 352, 1301-1304
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