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

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.

Popular Abstract

Objects with dimensions in the order of nanometer (1 millionth fraction of 1mm), have special characteristics that enable them with the potential to be applied in a variety of technologies, including solar cells and LEDs. Light behaves differently when it interacts with special materials with such small dimensions. In the current work, we try to explain this interaction between light and these objects called nano-structures, specifically an infinitely long single indium phosphide (InP) nanowire and an infinite periodic array of such nanowires.
Essentially, a nanowire is a one dimensional nano-structure with the length from a few tens of nanometers to several micrometers and the radius around 50-100 nanometers (1000 times thinner than a human hair). InP is a semiconductor with electrical conductivity between that of insulators (e.g. glass) and metals (e.g. copper). Furthermore, semiconductors can absorb the energy of the light and convert it to electricity in opto-electronics devices such as photovoltaic cells and photo-detectors. This study has been done theoretically via semi analytical methods for simple systems like a single nanowire and via numerical methods for more complicated systems like the array of nanowires.
The calculations show that the absorption mechanism of light in the single nanowire depends on the radius of the nanowire. Due to the small size of the nanowire, part of the light is captured inside the nanowire where it is absorbed and part of it leaks out from the nanowire. Then, in the periodic array, this leaked light can interact with the neighboring nanowires and change the absorption pattern. Therefore, the absorption in the periodic array not only depends on the radius of the nanowires but also on the period of the structure, that is, on the distance between the nanowires.
We show that for specific combinations of the period of the array and the radius of the constituent nanowires, the leaked light from a single nanowire interferes constructively inside the neighboring nanowires. This constructive interference can boost the absorption up to 18 times in contrast to that in a single nanowire. Through such resonant absorption, the nanowires can absorb 200 times stronger than the same amount of InP material in bulk form. The direct consequence of this is achieving higher efficiency of photo-devices exploiting the interaction of light and semiconductor nanowires. A higher efficiency facilitates lower utility of the material and a potentially lower cost.

• author: Hosseinnia, Ali
• supervisor: Nicklas Anttu; Mats-Erik Pistol
• organization: Solid State Physics; Department of Physics
• course: FYSM60 20142
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: Coupling, Optical resonances, Horizontal nanowires, Periodic Array, Indium Phosphide, Finite Element Method, Scattering Matrix Method, Eigen function method, Mie Theory, Scattering, Absorption, Nano-Optics, Nanowire
• language: English
• id: 4933718
• date added to LUP: 2015-01-20 22:27:40
• date last changed: 2015-06-02 09:34:28