Learning Forum

[homero.zapata]

Hello,

I'm interested in simulating a gaussian beam highly focused by an immersion microscope ( NA = 1.48 ). The considered wavelength is 785nm. We would like to consider this in the simulations using a gaussian source with a numerical aperture higher than the theoretical limit of NA = 1. Please, in order to carry this out would it be possible to consider the refractive index of the immersion oil and allow it to go beyond the theoretical limit ?

Thank you

[Kirill]

Dear Subscriber,

You should be able to set up the medium, define the optical components if needed, and configure the beam source. Please refer to the article Plane wave and beam source - Simulation object for guidance. Once set up, you can perform beam propagation and analysis according to Maxwell’s equations.

The nominal NA of the beam is limited by the refractive index of the medium. However, the actual field distribution of a tightly focused vectorial beam can significantly differ from the expectations based on a paraxial Gaussian beam approximation.

In other words, while you cannot violate fundamental physical laws, you should consider the full vectorial nature of high-NA beams, as they exhibit effects such as polarization-dependent focusing, longitudinal field components, and deviations from paraxial theory.

Best regards,
Kirill

 

[homero.zapata]

Hello Kirill,

 

Thank you for your answer.

Indeed when the blog develops the numerical aperture as n*sin(a) the medium refractive index parameter is not shown in the source configuration window.

How do we access to this parameter? 

Is it deduced from the material where the source is positioned ? Or from the refractive index of the FDTD simulation box ?

I forgot to specify in the precedent post that we're running simulations at the nanometer scale and that the medium of the structure and the one of the source injection is not the same.

So, in order to avoid simulating the interface oil/ glass/ air we're performing an an injection of the Gaussian source inside the glass. medium.

So, we're wondering if it would be possible to consider a different medium before the injection ?

Typically accessing the n from the n*sin(a) equation.

Thank you.

 

Best regards.

[Kirill]

Dear Subscriber,

There is no refractive index setting within the source settings, as the refractive index is automatically determined by the object in which the source is embedded.
The background index in the FDTD settings defines the refractive index of the surrounding medium in the simulation region when no other objects are present.
As mentioned in the article Plane wave and beam source - Simulation object, the NA setting accounts for "the medium in which the source is found." You simply need to set the correct effective NA for the beam in the glass.

Best regards,
Kirill

[homero.zapata]

Hello Kirill,

Perfect thank you very much for the precission. It helps a lot.

I have one last question. So if I want to simulate the oil immersion objective I can create layer of polymer with the right refractive index.
I am limited by the computational ressources. Do you know any adviced minimal thickness to consider in the glass layer after the oil glass interface ?
The real distance is in the micron range but we don't have enough ressources to compute this.

Thank you

Best Regards,

Romeo.

[Kirill]

Dear Romeo,

Could you please provide a screenshot or sketch of your structure? What is the sequence of polymer, oil, and glass, including their thicknesses? Also, where are you positioning the source and detectors?
What are the limitations of your simulation? Is it constrained by RAM (simulation volume size) or CPU/GPU (simulation time)?

Best regards,
Kirill

[homero.zapata]

Hello Kirill,
Thank you for your answer.
Here I tried to represent typical sizes of the fabricated sample:

The main limitation factor beying the meshing resolution as to simulate the effect we need to resolve our structured layer wuth a mesh size of 5nm.
The typical FDTD window we used until now has a size span of 2600nm X 5600nm X 1100nm ( Where we injected the source inside the glass layer with a thickness that was of only 500nm). I would like to think that the thickness of the materials is not that relevant (because we don't consider them dispersive).

In terms of simulation capabilities I think that is the CPU/GPU performance ( computation time ) because we need one week to obtain one simulation set.
Ourserver has 768GB RAM and two AMD EPYC 9374F (32 cores) processors.

Do you think that I should inject through the oil layer by considering a thin layer of glass ?

Thank you

Best regards
Romeo
  

[Kirill]

 

Dear Romeo,

Thanks for sharing! This simulation certainly involves multiple complexities. Direct propagation through ~150 µm of glass at a 785 nm wavelength, combined with high NA, might be quite challenging.

Have you considered placing the source inside the glass, closer to the glass-gold interface?

If that’s not an option, but you only need to vary the top structure and don’t require full beam propagation through the entire glass thickness each time, you could split the simulation into parts. You can place a monitor at a reasonable propagation distance from the source, record the data, save it to a file, and then use it as a source for subsequent simulations. You may need to repeat this several times. See the example: Using monitor data to define the spatial field profile of a source in FDTD. Once you’ve completed the full propagation to the glass-gold interface, you can reuse the imported data for further simulations.

Using a thinner layer of glass may not produce the exact same result due to the vectorial nature of the beam, but it would still be interesting to compare.

Best regards,
Kirill