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Asymmetric nanostructures can mimic a chiral response when circular polarized light interacts with the structures under particular angle of incidence [1]. This phenomenon is called ‘extrinsic chirality’ and usually is present under linear optical investigation with low visibility. Due to the fact that optical second harmonic generation is possible only in samples with some degree of asymmetry, this can be used in order to investigate the extrinsic chirality with a background free technique, thus inducing a high visibility of the artificial circular dichroism [2,3].
Here we present the second harmonic generation (SHG) measurements obtained on samples composed by GaAs nanowires grown on silicon.
The wires present resonant leaky modes around 800nm and at 400nm due to the high refractive index contrast ratio between wires and air, even if these wavelengths lie on the absorption band of GaAs [4]. The measurements performed on this sample present good SHG signal due to the second order nonlinear term of GaAs, but did not present any circular dichroism (SHG-CD).
By coating the sample with a 20nm thin layer of gold deposited asymmetrically, by evaporating the metal only from one side of the nanowires, the symmetry of the structure is broken, thus induced high SHG-CD.
The SHG-CD is measured by shining the sample with circular polarized pump light at the fundamental wavelength of 800nm and by revealing the second harmonic signal at 400nm in s or p polarization, as a function of sample rotation.
Four samples were measured with GaAs wires of about 5 micron in length with different diameters ranging from 140nm to 200nm. In this case it is possible to explore different resonance conditions and different SHG-CD is revealed. For each sample, the measured were carried out before and after the asymmetric gold layer deposition, thus allowing direct comparison of the results.
References
[1] A. Belardini, M. C. Larciprete, M. Centini, E. Fazio, C. Sibilia, D. Chiappe, C. Martella, A. Toma, M. Giordano, and F. Buatier de Mongeot, Phys. Rev. Lett. 107, 257401 (2011).
[2] A. Belardini, M. Centini, G. Leahu, D. C. Hooper, R. Li Voti, E. Fazio, J. W. Haus, A. Sarangan, V. K. Valev, C. Sibilia, Sci. Rep. 2016, 6, 31796.
[3] G. Leahu, E. Petronijevic, A. Belardini et al., Adv. Optical Mater. 2017, 1601063 (2017).
[4] G. Leahu, E. Petronijevic, A. Belardini, M. Centini, R. Li Voti, T. Hakkarainen, E. Koivusalo, M. Guina, C. Sibilia. Sci. Rep. 7, 2833 (2017).
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