Nonlinear opportunities

Topological insulators — fascinating materials whose bulk is insulating but whose surface can be conductive with well-defined spin textures — may turn out to be useful for creating novel nonlinear optical devices. That's the conclusion of a study into their properties, which has revealed that they can exhibit a broadband, ultrafast nonlinear effect called spatial self-phase modulation (SSPM). Bingxin Shi and co-workers from Hunan University and Shenzhen University in China observed and studied SSPM in a topological insulator made from nanosheets of bismuth telluride (Bi₂Te₃) dispersed in alcohol (Appl. Phys. Lett. 107, 151101; 2015). They report that the sample exhibits broadband SSPM from the ultraviolet (400 nm) to the near-infrared (1,070 nm) due to the presence of a large, ultrafast, broadband third-order optical nonlinearity.

The scientists synthesized ultrathin Bi₂Te₃ nanosheets by a solvothermal method that involved dissolving bismuth chloride (BiCl₃) and sodium selenide (Na₂TeO₃) in ethylene glycol and then heating to 200 °C. The result was the creation of hexagonal-based plates of uniform size (400–600 nm across), as determined by field-emission scanning electron microscopy (pictured; left). The nanosheets were dispersed in an alcohol solution, which was then poured into a quartz cuvette for optical characterization.

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Femtosecond laser light with a central wavelength of 800 nm, pulse duration of 100 fs and repetition rate of 1 kHz was focused onto the sample and the transmitted light was collected by a CCD (charge-coupled device) camera placed 5 cm behind the sample. The authors observed the generation of diffraction rings due to SSPM (pictured; right).

They report that the formation of the diffraction rings showed complex temporal behaviour; the rings rapidly appear, and then the upper half of the rings begin to collapse, finally the structure becomes stable after about half a second. The distortion in the upper part of the rings is believed to be due to thermal effects and gravity. Tiny gas bubbles moving upward were generated as a result of the absorbed laser energy, causing non-uniform density distribution of the Bi₂Te₃ nanosheets in the solvent and consequently distorting the intensity distribution of the diffraction rings. The SSPM diffraction rings were also observed for laser illumination at wavelengths of 400 nm and 1,070 nm. The study indicates that the nonlinear refractive index n_2,single of the Bi₂Te₃ nanosheets is ∼10⁻¹⁵ m² W⁻¹, corresponding to the third-order nonlinear susceptibility of ∼10⁻⁹ esu.