Absorption of light through isolated and coupled resonances in horizontal InP nanowire arrays

University essay from Lunds universitet/Fasta tillståndets fysik; Lunds universitet/Fysiska institutionen

Abstract: We have studied the interaction of light with two types of closely related nanostructures: a single horizontal InP nanowire and an infinite periodic array of such nanowires. The study has been done theoretically by calculating the absorption cross section of the nanowires via a semi analytical method, the Mie theory, and two numerical methods, the scattering matrix method (SMM) and the finite element method (FEM) to perform electromagnetic modeling. The absorption spectra obtained by the Mie theory show strong polarization dependency. Also, we have noticed that the peaks in the spectra red shift by increasing the radius of the nanowires. To study this redshift, we assumed that the nanowires can be seen as optical waveguides that can capture the light in certain eigenmodes much like a whispering gallery mode. Due to this, a semi analytical eigenfunction method has been employed to investigate the eigenmodes in the nanowire. The results show that the eigenmodes are the origin of the Mie resonances and redshift by increasing the radius of the nanowire. By moving to study the optical response of the periodic array consisting of infinitely many nanowires, an additional set of optical resonances is introduced to the absorption in the nanowires. These resonances are due to Bragg grating condition for constructive interference of scattered light between neighboring nanowires, and these resonances depend on the period of the structure. These new resonances are called lattice resonance throughout this thesis. We show that for specific combination of the period of the array and the radius of the constituent nanowires, the lattice resonances couple with the single nanowire resonances. This coupling can boost the absorption in the array by a factor of 18 compared to that in single nanowires. Through such resonant absorption, the nanowires can absorb 200 times stronger than the same amount of InP material in bulk form.

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