stackpurcell -- why is there a very large radiated power loss when comparing emission into air?

abackerabacker Member Posts: 1

Hi, Is there a resource explaining the differences in calculating the "result.density.downward" and "result.density.downward_into_air" fields of the output of the stackpurcell solver in Lumerical FDTD?

I am comparing the results from the example application: "optimizing farfield emission of multilayer stack". When I compare radiated power density (result.density.downward versus result.density.downward_into_air) I find a HUGE power drop, even at normal incidence! According to the fresnel equations, only ~4% of power should be directly reflected for normally incident light at an air-glass interface, while the simulations predict ~60% power loss.


  • gsungsun Posts: 954Ansys Employee

    For dipole radiation in stack devices, description is only at this page: stackpurcell - Script command

    Please note that, Fresnel coefficients describe the r/t for plane wave, with given propagation direction of the plane wave, whereas dipole radiation is anisotropic.

    "only ~4% of power should be directly reflected for normally incident light at an air-glass interface" is for single interface such as air/SiO2, not for stacks. Stacks have cavity effect which can enhance the transmission or reflection depending on the structure. If the simulation agrees roughly with the analytical results, the simulation settings should have no significant defects.

  • abackerabacker Posts: 3Member

    I am not entirely sure if the cavity resonance is an accurate cause for the power-loss into air in this case... In the example "optimizing Fairfield emission into multilayer stack" The thickness of the SiO2 layer is set to 0. Will changing this thickness have any effect on the "into_air" transmission? Since the glass SiO2 layer is normally much larger than the wavelength, should this be set as a "incoherent" layer in the stack solver to get accurate results?

    Is there any more documentation on downward vs. downward_into_air outputs for the stackpurcell solver? The page on the stackpurcell solver page does not go into much detail describing the specific differences between these two outputs.

    Thanks again for your help!!!

  • gsungsun Posts: 954Ansys Employee

    The cavity effect definitely changes the transmission and reflection, since dipole radiated power changes with environment materials.

    the SiO2's 0 thickness is just for convenience. you can change it to whatever the value. STACK functions only need to know the 1st material's property, and its thickness does not affect the result, since now we are in mathematical world, not in physics world. Although ultimately all devices are inside air/vacuum, the 1st material in STACK functions is considered as semi-infinite. You can add its limited thickness if you want to counter the bottom SiO2/air interface (add one more material air as the first material), or simply begins z from the very bottom of SiO2 (the output is to SiO2).

    There is no other document other than the function page. Consider the stacks are infinitely large in xy, and limited thickness in z, it has two outputs: one if to z<0 (in your case it is SiO2), and the other side is z>total thickness in the last material (in your case it seems it is air).

    In this stack:

    The bottom material is the 1st material, air, and the top is the last material, n=1.5. So it has TWO outputs, and only two outputs.

    I hope this clarifies the issue.

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