In the last decades, the organic molecular crystals have been considered as potential substitutes for inorganic crystals in photonics for optical signal processing and integrated optics showing large second order non-resonant nonlinearities and fast response times. Their most significant advantages compared to inorganics are the high values for nonlinear coefficients, large birefringence values, high damage thresholds in laser beam and large transparency domain. These properties are associated with the presence of the delocalized π electrons cloud and, donor and acceptor substituent groups, which are generating important inductive and mezomeric effects at the molecular level and significant optical non-linear effect at the macroscopic scale.

Crystalline aromatic derivatives compounds such as meta-dinitrobenzene (m-DNB) and benzil are interesting for the above mention applications because they show large transparency domain, and theoretically study have anticipated high non-linear coefficients on one hand and the π-electrons systems that favour the emission property by fluorescence and/or intermolecular energy transfer on the other hand.

Additionally, substituted organics could show specific properties like important two photon absorption fluorescence emission (TPF) and are interesting for potential applications in frequency up conversion lasing, optical power limiting, three-dimensional (3D) fluorescence imaging, 3D optical data storage, 3D lithographic micro fabrication and photodynamic therapy.

In particular, the interest in studying crystalline benzil and m-DNB is explained mainly by their behaviour similar to that of the inorganic wide-gap semiconductors. This interest is also justify by the particularities of their molecular and crystalline structure that favors the appearance of the free space having diameter (~3 Å for both m-DNB and benzil evaluated from the molecular chemical structure taking into account the geometry of the molecule [1]) adequate for the inclusion of foreign atoms or molecules (doping), or for the development of specific internal microstructures of a different phase.

The type of dopant incorporation in the host matrix, substitutionally or interstitially, depends on the shape, volume and intermolecular bonding of the dopant molecule and, on the geometrical similarity between dopant and matrix. The investigation of the effect of the dopant incorporation and organization inside an organic matrix is important for obtaining a new class of materials combining the properties of the both components.

This paper presents some studies concerning the incorporation of inorganic (silver, iodine, sodium) and organic (naphthalene, m-DNB) single dopant in benzil or m-DNB matrix and of m-DNB+iodine; naphthalene+iodine) double dopant in benzil or m-DNB matrix.

Because both m-DNB and benzil are characterized by good thermal stability at the melting point (89^oC and 95^oC respectively), these organic molecular crystals have been grown from the melt in a Bridgman-Stockbarger vertical configuration (thermal gradient at the growth interface: 5-35^oC/cm; moving speed of the growth interface: 0.5-3 mm/h), which has been adapted to counteract the supercooling and low thermal conductivity characterizing the big molecule organic compounds. The starting material was purified by chemical methods/vacuum distillation/multiple passages (~20) zone refining process. The dopant concentration in wt% was well controlled.

Thick crystalline wafer (1.5-3 mm) with a diameter of ~8-12 mm, have been obtained by cutting these pure and doped ingots with a wire saw machine and mechanically polishing the slices with a mixture of alumina powder in ethyleneglycol.

We have analysed the dopant/matrix systems from the point of view of the growth interface stability emphasising the stability limits and the experimental conditions (temperature gradient, moving speed, concentration gradient) for initiating the instabilities associated with defects in the final crystal.

To obtain high quality crystals, the temperature distribution both in melt and crystal must be accurately controlled. Controlling the local thermal distribution in the heater during the solidification process determined by the position of the growth ampoule in the heater we can control the position and shape of the solid-liquid interface. For this purpose we propose the numerical modelling of the heat transfer during the process of aromatic derivatives crystallization by firstly solving the steady-state heat transport model equation and subsequently considering the pseudo-transient heat transfer in the central zone of the furnace, simulated in a time-dependent model, assuming that the moving speed of the ampoule was equal to the solidification rate [2].

We have investigated the effect of the experimental conditions (T_{hot zone} and ampoule position for charge melting; T_{cold zone} and ampoule position for initiating the crystallization) on the growth interface shape and evaluated the interface deflection (curvature). This value was correlated with the growth interface shape, which is an important parameter associated with the structural quality of the crystal.

We have also realized a comparative study between the structural/morphological/optical (transmission, emission) properties of the pure and doped molecular crystals. We have evidenced an exponential sub band gap absorption process and estimated the corresponding Urbach energy. This parameter which is correlated with topological and/or structural defects is determined by two types of factors correlated: 1. to the growth regime (temperature gradient, moving speed); 2. to selected dopant.

We have analysed the effect of the doping on the second harmonic generation (SHG) and TPF signals (Fig. 1a and 1b), emphasising the dependence of the maximum intensity on the beam position, moving the beam focus across the wafer surface and depth inside the sample (Fig. 2). The effect of the dopant on the SHG will be estimated evaluating the non-linear coefficient of pure and doped organic crystal from the dependence of the SH intensity on the power of the incident laser beam.

References

[1] A. Stanculescu, J. Optoelectron. Adv. Mater. 9 (2007) 1329.

[2] F. Barvinschi, A. Stanculescu, F. Stanculescu, J. Cryst. Growth 317 (2010) 23.

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