iGEM TU Delft

# How does the polysilicate covered cell scatter the light?

## Far Field Scattering

### Introduction

Parallel to the light focusing study that investigated if focusing the light with bacteria covered in glass is possible we ran additional models to see how the electromagnetic field scatters further away from our structure. This is called the Far Field Effect and it describes the distribution of the scattered light’s power far away from the structure. Figure 1 shows the main characteristics of a far a far field pattern, the light is moving along the z plane and the particle is in the origin. We cannot measure the lobes in length because this figure shows the form of the light scattered.

From figure 1 two main characteristics of the far field scattering can be seen, the main lobe and the back lobes. The main lobe represents how most of the power is scattered and the minor lobes can be separated to the side lobes and back lobes, they represent the power scattered in undesired directions. Finally, we can see the Half-Power Beam Width (HPBW), that is the angle between the two directions where the intensity of the radiation is half of the beam and the First-Null Beamwidth (FNBW) representing the angular separation between the first nulls of the pattern. (Balanis, 2005)

Even though we cannot extract information about the focusing effect of the of the lenses from the Far Field graphs we get valuable information about how the light is scattering far away from the structure. Because we want to apply our biological microlenses to improve the light captured from optical sensors, having all the light scattered backwards would be really bad. On the other hand, having a big main lobe with very small to none secondary lobes means that most of the light is scattered to the back side of the lens, which is desired.

### COMSOL model

The model described in the focusing study was used as a base to determine the scattering of the light. As shown in figure 2 the 3D model is identical to the focus study because we are investigating exactly the same structure under the same conditions. This time a far field domain was defined in the 3D model so we can see how the light is scattered very far away from the structure.

Figure 3 demonstrates the Far Field effect of the scattered light in a 3D plot. We can see that the main lobe is predominant and some very small side lobes can be seen and even less back lobes.

Figure 4 shows a 2D representation of the Far Field effect. Most of the far field graphs are in 2D because it is easier to read. Here it is easier to see that the main lobe is way bigger than the secondary. The secondary lobes are almost non existent.

## Discussion

In this study we investigated one additional important characteristic of optical devices that is how they scatter light. This adds on our investigation of the light properties of the microlenses. We have now modeled the lenses ability to focus light in the previous question and now we saw how they behave far away from the structure and how much of the incoming light is actually usable and how much lost.

## Conclusion

Parallel to the focusing study we investigated the way the light scatters from our polysilcate covered cell. We argued that the scattering pattern is as important for this application as the focusing because we need more light trapped behind the microlens in the censor. The simulation results showed that the light scatters mostly forward from the lens with little to no back scattering. This means that there is almost no reflection from the microlenses and most of the power is scattered behind the cell. Additionally, we can see that the distribution is very thin forward meaning that there is no dispersion of the energy. Those results in addition with the focusing study indicate that the polysilicate covered cell are a very promising alternative for the conventional micro lenses. Finally those results were confirmed via the spectroscopy measurements which showed that our biological microlences transmit almost perfectly light.

1. Balanis, C. a. (2005). Fundamental Parameters of Antennas. Antenna Theory : Analysis and Design, 27–114. http://doi.org/10.1007/978-