Nonlinear optical susceptibilities χ(2) of nitridosilicate powders
Introduction
The frequency conversion of coherent radiation via parametric devices like optical parametric oscillators and frequency doublers has found increasing attention in recent years. New materials with high nonlinear optical susceptibilities and high damage thresholds are of special interest for such devices. Materials with noncentrosymmetric crystal structures like nitridosilicates with their superior mechanical properties and their high thermal stability are attractive candidates. No investigations of the nonlinear optical properties of nitridosilicates have been reported in the literature as yet.
Most novel materials are only available in powder form and standard techniques for the determination of the susceptibility tensor cannot be used. Initial information is available from powder methods which measure an average of the tensor components. A frequently used powder method is the Kurtz–Perry-Method [1] in which light at the fundamental frequency penetrates the sample and generates a signal at the second harmonic (SH) frequency. This technique suffers from the fact that phase matching in the powder crystallites may occur and the results strongly depend on the relation between the—in most of the cases—unknown coherence length and the size of the crystallites.
To avoid this problem, a method using a total reflection geometry has attracted increasing attention in recent years which is called SHEW-method (Second Harmonic Wave generated by an Evanescent Wave) [2], [3]. Laser light is totally reflected at an interface between a material with high refractive index and the sample under investigation. The second harmonic signal generated in this geometry is proportional to a weighted average of all tensor components of the material [4].
In this paper the Kurtz–Perry and the SHEW-method are used to measure the optical nonlinearity χeff(2) of nitridosilicates. Simultaneously, we obtain information on the refractive indices from the SHEW investigations. After a short review of the SHEW technique we present experimental results. The averaged effective Figure of Merit Meff=deff2/n2ωnω2 with deff=0.5χeff(2), which is a measure for the efficiency of the nonlinear interaction process, is found to be of the same order of magnitude as that of LiIO3 in the most efficient samples and very high refractive indices between 2 and 3 result from our findings.
Section snippets
Preparation of the powder samples
M2[Si5N8] with M=Ca, Sr, Ba. A mixture of the respective alkaline earth metal (Sr: 70.0 mg, 0.799 mmol, ABCR, 99.95%, Ba: 120.1 mg, ABCR, 99.9%, 0.874 mmol, Ca: 32.0 mg, 0.798 mmol, ABCR, 99.997%) and Si(NH)2 (116.0 mg, 2.00 mmol) [5] was thoroughly mixed under argon atmosphere in a glove box and transferred into a tungsten crucible positioned in a water-cooled silica glass reactor of a radiofrequency furnace (type IG 10/200 Hy, frequency: 200 kHz, electrical output: 0–12 kW, Hüttinger, Freiburg)
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