Browsing by Subject "FDTD"

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  • Maconi, Goran; Kassamakov, Ivan; Vainikka, T.; Arstila, Timo; Haeggstrom, Edward (SPIE - the international society for optics and photonics, 2019)
    Proceedings of SPIE
    We simulate the image generated by a microsphere residing in contact on top of an exposed Blu-ray disk surface, when observed by a conventional microscope objective. While microsphere lenses have been used to focus light beyond the diffraction limit and to produce super-resolution images, the nature of the light-sample interaction is still under debate. Simulations in related articles predict the characteristics of the photonic nanojet (PNJ) formed by the microsphere, but so far, no data has been published on the image formation in the far-field. For our simulations, we use the open source package Angora and the commercial software RSoft FullWave. Both packages implement the Finite Difference Time Domain (FDTD) approach. Angora permits us to accurately simulate microscope imaging at the diffraction limit. The RSoft FullWave is able to record the steady-state complex electrical and magnetic fields for multiple wavelengths inside the simulation domain. A microsphere is simulated residing on top of a dielectric substrate featuring sub-wavelength surface features. The scattered light is recorded at the edges of the simulation domain and is then used in the near-field to far-field transformation. The light in the far field is then refocused using an idealized objective model, to give us the simulated microscope image. Comparisons between the simulated image and experimentally acquired microscope images verify the accuracy of our model, whereas the simulation data predicts the interaction between the PNJ and the imaged sample. This allows us to isolate and quantify the near-field patterns of light that enable super-resolution imaging, which is important when developing new micro-optical focusing structures.
  • Papponen, Joni (Helsingin yliopisto, 2022)
    Imaging done with conventional microscopes is diffraction-limited, which sets a lower limit to the resolution. Features smaller than the resolution cannot be distinguished in images. This limit of the diffraction-limit can be overcome with different setups, such as with imaging through a dielectric microcylinder. With this setup it is possible to reach smaller resolution than with a diffraction-limited system, which is called super-resolution. Propagation of light can be modelled with various simulation methods, such as finite-difference time-domain and ray tracing methods. Finitedifference time-domain method simulates the light as waves which is useful for modelling the propagation of light accurately and take into account the interactions between different waves. Ray tracing method simulates the light as rays which requires approximations to the light’s behaviour. This means that some phenomena cannot be taken into account, which can affect the accuracy of the results. In this thesis the model for simulating super-resolution imaging with microcylinder is studied. The model utilizes the finite-difference timedomain method for modelling the near-field effects of the light propagating through the microcylinder and reflecting back from a sample. The reflected light is recorded on the simulation domain boundaries and a near-field-to-far-field transformation is performed to obtain the far-field corresponding to the recorded fields. The far-field is backward propagated to focus a virtual image of the sample, and the virtual image is then used in ray tracing simulation as a light source to focus it to a real image on a detector.