ANSYS Lumerical: Multilevel Diffractive Lenses Design Part 1

Hello, in this demo series we'll be discussing how ANSYS workflow could be used to design a full-fledged optical system.

In this particular demo, I'll be discussing how we can design micron-scale optical devices which can be used for imaging applications like for virtual reality or for biomedical applications. More specifically, we'll be using multi-layer diffraction gratings to obtain desired optical responses.

On this slide, I'm trying to illustrate the advantages of multi-layer diffraction gratings compared to metal lenses. Both of them can be used to shape the beam to give a particular optical response.

However, the practicality of multi-layer diffraction gratings and their performance make them much more desirable compared to metal lenses. Metal lenses typically need to be very small, much smaller than the wavelength of light, and are thus very difficult to fabricate.

Diffractive multi-layer diffraction gratings, on the other hand, can achieve similar or even superior optical responses while ensuring that small structures don't need to be fabricated. They are therefore much more practical to fabricate.

Additionally, the thickness of your final device is limited by the support substrate. Increasing the thickness from lambda/2 to 2 times lambda will not significantly affect the size of your devices. This is how diffraction, refraction, or bending of light typically happens.

In the case of a standard refractive lens, increasing the numerical aperture requires increasing the thickness. However, with diffraction, you can increase the diffraction principle and obtain different optical responses and numerical apertures without changing the thickness.

This is a typical example of a standard lens, a standard refractive lens. To increase the numerical aperture, you would have to increase the thickness.

On the other hand, with diffraction, you would increase the diffraction principle and obtain different optical responses and numerical apertures without changing the thickness. This is how a typical multi-layer diffraction grating would be.

A standard diffractive grating has a particular height, and there are regions where the material has been completely removed.

In this demo, we will be showing a multi-level diffractive lens, where the modulation of heights is not only between zero and one but for all the different values between zero and one.

A metal lens is much harder to fabricate, as the structures have to be prepared very carefully and are much smaller than the features in the multi-layer multi diffraction multi layer diffraction gratings. These structures have been studied in great detail in the reference provided.

The table gives a summary of the performance for both narrow band and broadband, for desired numerical aperture, focal length, and wavelength of operation.

We will be designing this based on the algorithm mentioned in the paper, a DBS algorithm, which uses an iterative procedure to modulate the height of your structure and obtain the MDL device.

You start with some initial values of heights and perform iterations, optimizing the figure of merit (FOM) to obtain the required height profile. Here, we'll be talking about an example for a particular range of wavelengths. We're choosing 460, 540, and 620 nanometers. The focal length is 10 mm.

The number of grooves is 280. The width of each groove is 3 microns. The material we choose is a particular photoresist polymer, which is a dispersive material. The height modulation will be dependent on the desired optical response. This is essentially the process for designing.

You specify your desired optical response, obtain the height profile using the DBS algorithm, import it into numerical FDTD, design the optical response, perform FDTD simulations to obtain near field data, and then use the numerical far field function to obtain far field profiles at long distances.

Once you have obtained a structure, you run your simulations and then analyze the near fields. These can be stored in a particular file for far field calculations. We then load the near field profile, specify some parameters, and use the far field exact 2D function to obtain the far field profiles.

This is what we have obtained for the MDL designed for 460, 540, and 620 nanometers. We can now look at how the actual structure looks like.

You can specify your design parameters, source, FDTD region, add a plane wave source, add near field monitors, and then read the data from the near field monitor. This data can be used to save in a separate file for far field analysis. The far field analysis is something we are doing here.

You can read in data from the near field for a desired frequency and then perform far field calculations for a particular distance. This allows us to see how the fields will look like for distances of the order of a few mm.

Using numerical methods, we are able to perform far field analysis for several mm and test the performance of MLD gratings. This is the first step towards building a full-fledged optical system for a particular application based on certain specifications.

The far fields and the field profile obtained in this FDTD can be exported and used in ZMAX for further calculations and for further beam propagation, which we will see in the upcoming videos. Thank you and see you soon.