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Project

Use of chiral conjugated polymers for the optimization of non-reciprocal effects in nanoparticle aggregates

This project aims to optimize the efficiency of optical isolators. Optical isolators are devices that allow light to pass in one direction, but block it in the opposite one. This is very similar to an electrical diode, which allows a current to flow in only one direction. Optical isolators are important, for example, if one wants to protect lasers from back reflections, which can disturb the laser’s operation.  Furthermore, they are a crucial component in optical fibres to ensure a stable data transfer over long distances, like intercontinental data connections. All optical isolators are based on a non-reciprocal optical rotation, which means that a difference in optical rotation between forwards and backwards propagating light is observed. Nowadays several effects are known to induce non-reciprocity, but the one based on Faraday rotation is by far most common. Non-reciprocity in Faraday rotators is introduced by an external magnetic field in combination with a faraday crystal. However, these are difficult and costly to fabricate and have a too large footprint for on-chip integration. Due to a low Verdet constant they are also inefficient and suffer from a narrow spectral asymmetry bandwidth.

Recently it was demonstrated that non-reciprocal optical effects can also be observed in nanocomposites of plasmonic nanoparticles implemented with chiral molecules. This means that such a system can be used directly in optical isolators without the use of additional magnetic fields. Furthermore, the non-reciprocal effects in the nanocomposites are present in an unusually broad wavelength range. Moreover, because no magnetic field is required, and by using nanoparticles the amount of material can be reduced by 4 orders of magnitude compared to Faraday rotators! Combined with an easy layer-by-layer fabrication these systems are very promising candidates for miniaturisation and on-chip integration applications! Nevertheless, the observed non-reciprocal effects are still small, preventing these materials from being used in optical isolators. The small non-reciprocity is the result of several problems. First, the space between the nanoparticles is occupied with quadrupole inactive air and reacted APTMS, which does not contribute to the non-reciprocity. Also the low index of refraction is not in favour of the non-reciprocal effect. Second, the used chiral amino acid has a low chiral activity, leading to a weak non-reciprocal optical.

The goal of this project is to optimise the non-reciprocal properties of the nanoparticle aggregate system using chiral conjugated polymers (CPs). First of all, the nanoparticles will be linked together with a CP linker. Due to their large conjugation lengths CPs are expected to exhibit strong quadrupole effects. Combined with a high refractive index, an increase of the non-reciprocal behaviour will be observed. Second, chirality will be introduced by incorporating a chiral entity in the CP side chain. In that way the linking of the nanoparticles and the introduction of a chiral entity is combined in one process. Next the system will be further optimised. First of all, the optimal distance between the NPs is found by changing the chain length of the CP. Secondly, different types of CPs will be tested to see which ones results in larger AT. Also block copolymers of these different CP will be tested. In the end a durable nanocomposite system with optimised quadrupole contributions and thus optimised non-reciprocal effects will be obtained.

Date:8 Sep 2016 →  7 Sep 2021
Keywords:conjugated polymers, Plasmonic Nanoparticles, Non-reciprocity
Disciplines:Process engineering, Polymeric materials
Project type:PhD project