Photonics

Photonics

Is this related with Interference? How to avoid this in the simulation

    • DemingMeng
      Subscriber

      Hi,

      I appreciate your help. I met a issue that when I simulate a two layer structure. 90nm TiO2 layer on top of 50um SiC layer. I used E&M simulation and tell the system the loss and refractive index of the two materials. I put two monitor and want to test the reflectance and the transmittance for the structure, which is similar to the spectrum testing. However, it shows a strong vibration in my transmittance spectrum and the reflectance spectrum, as attached. However, during the experiment I did not see this, as attached. My questions are:

      1. Is this related with interference?
      2. How to avoid this because I want the simulation fits the real experiment condition.

      I appreciate your help and look forward your reply.

    • Guilin Sun
      Ansys Employee
      Hi, I believe that the result matches the device, as you are simulating only two layers suspended in air, leading to strong cavity effect. In your experiment, I am sure there is other substrate to support the thin films. Please revise the simulation file based on your experimental device.
      and most importantly, if it is thin film type of device without nano structures, you can simply use stackrt https://support.lumerical.com/hc/en-us/articles/360034406254-stackrt-Script-command to get analytical solution. A similar post is here https://forum.ansys.com/index.php?p=/discussion/28603/simulation-with-broadband-wavelength-range#latest In that post it has only one layer of thin film on the substrate. The simulated result agree well with theory even just mesh accuracy 2 is used.
    • DemingMeng
      Subscriber
      Hi Gsun
      Thank you for your help, but I still have 2 more questions.
      Firstly, When you cite the refractive index from the database, does it only cite the 'n' or both n and k will be cited?

      Secondly, because I will have a 500um thickness of substrate. The substrate itself will have some loss so I want to see the transmittance after the substrate. Where do you recommend me to put the monitor to receive the transmittance?

      I appreciate your help
    • Guilin Sun
      Ansys Employee
      In general, FDTD will consider both n and k, except that the imaginary part k =0, like that case where you can directly assign refractive index GUI:
      For a 500um thick substrate I guess it has much less effect on the total transmittance. I would suggest that you do this:
      1: only contains very little substrate in your simulation; get T;
      2: calculate the actual transmission T(sub_air) between substrate and air from theory, then multiply it to the simulated T, so the modified transmittance will be
      T_final=T*T(sub_air)
      This is a first order approximation to get the actual transmittance in air, which does not consider the cavity effect of the substrate.

      If you really want to get accurate correction for the cavity effect, you can first simulate the field transmission coefficients with small portion of the substrate inside; then you can calculate the Fresnel coefficients at the interface between the substrate and the air; and finally you can form the two interface as a cavity, and calculate the cavity transmittance in theory, provided that you know the transmitted wave is along the axis or you know somehow the phase change of the round-trip cavity between the two interface (in other words, there is no higher order diffraction) . I am sure you can find a text book about the cavity formula. If not, please let me know.

    • DemingMeng
      Subscriber
      Thank you. I understand. May I ask you another question? I do want the cavity effect, which means I want the light bounce inside the strucutre. However, I do not want the light out of the structure interference with each other. Do you know what I can do to achieve my goal?
    • Guilin Sun
      Ansys Employee
      Regarding to cavity effect:
      light interference is natural and in most cases you cannot avoid it. If you want the cavity effect, you can either include the substrate in simulation like your first post. However for 500um or thick usually it is not practical.
      You can develop your own method with the principle I mentioned previously. Since you know the Fresnel's coefficients at the interfaces of the cavity, you will be able to develop a formula or refer some text book.
      Regarding to the interference:
      FDTD simulation solves Maxwell's equations so it is coherent. Single simulation result is the coherent just like from Single Wavelength Laser even the simulation is broadband. You could develop your own method to smooth out the interference by a spectrum window: https://support.lumerical.com/hc/en-us/articles/360034383074-Understanding-temporal-incoherence-in-FDTD
      but you will need to know the actual source spectrum W(f). when the optical path difference exists such as in a cavity, it is the coherent length (CL) that determines the result: the longer the CL and thus the narrower the spectrum of the source, the stronger the interference. You can convert the coherent length into W(f) based on preassumed spectrum function.
      This topic is outside of this Forum support. I would suggest that you discuss this with your professor or supervisor for more details.

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