Wavelength tuning of SOI ring resonator using a liquid crystal

In this example, we will model tuning of an SOI ring resonator filter using liquid crystals (LCs).

Simulation setup

An SOI ring resonator covered with a liquid crystal (LC) cladding is set up in the LC_ring_resonator.fsp simulation file. The evanescent tail of the propagating mode outside the waveguide feels a different refractive index depending on the orientation of of LCs, which result in the change of the effective index of the mode. By modifying the mode effective index, we can tune the resonant wavelength of the ring resonator.

The script file LC_ring resonator_setup.lsf sets up the distribution of LC orientations in the LC_ring_resonator.fsp simulation file. Then you can set up the distribution of LC orientations. The variable "alignment" in the script file controls the orientation. If you set "alignment=0" or "alignment=1", the LCs align along the waveguide or align vertically, respectively. The ordinary index n_o and extraordinary index n_e of the LC is, respectively, n_o =1.53 and n_e =1.71.

For more information about the LC grid attribute which is used to set up the spatially varying orientation of the LC, see LC rotation.

To calculate the change of the effective index for different LC orientations, we also prepare MODE simulation file  LC_waveguide.lms.  In the material database in this simulation file, we define LC materials named LC_x, LC_y and LC_z where the LC is aligned x-, y-, and z- axis, respectively.

Results

Using the LC_waveguide.lms simulation file, the figure below shows the mode profile for x-aligned LC and we can see that there exists electric field distribution outside the waveguide in the region of the LC material. When the orientation of LCs which cover the waveguide is changed, the evanescent field feels different refractive index, which results in the change of the effective index of a waveguide mode.

By modifying the material used for the LC structure, we can also calculate effective indices and evaluate the amount of the change of the effective index for different LC orientations. The figure below shows the effective indices for the fundamental TE mode for different LC orientations together with those for isotropic materials with n_o and n_e. The materials have already been set up in the material database with material names LC_iso1, LC_iso2, LC_x, LC_y, and LC_z.

The figures below show the transmission of the drop and through port for different LC orientations. After setting up the LC orientation using the script file LC_ring resonator_setup.lsf and running the simulation LC_ring_resonator.fsp, you can use Visualizer to plot these figures. The left figure below shows the transmission when LCs are aligned along the waveguide and the right figure shows the transmission when LCs are aligned vertically. As we can see, the resonant wavelength around 1.54 μm shifts down to a lower wavelength when the orientation of the LCs changes from the in-plane orientation to the vertical orientation.

Higher accuracy results

Note that the settings in this example file allow the simulation to be run relatively quickly, and higher accuracy results can be obtained by increasing the mesh accuracy. The ripple artifacts in the above transmission spectrum plots can be removed by increasing the simulation time to allow the fields to fully decay, and this is discussed in more detail at simulation time and frequency domain monitors.

Related publications

1. W. D. Cort, J. Beeckman, T. Claes, K. Neyts, and R. Baset, "Wide tuning of silicon-on-insulator ring resonators with a liquid crystal cladding" Opt. Lett. Vol. 36, No. 19, pp. 3876-3878 (2011)
2. W. D. Cort, J. Beeckman, R. James, F. A. Fernandez, R. Baets, and K. Neyts, "Tuning silicon-on-insulator ring resonators with in-plane switching liquid crystal" J.Opt. Soc. Am. B Vol. 28, No. 1, pp. 79-85 (2011)

See also

LC rotation object, Simple LCD, Twisted nematic LCD, Optical switch, Optical phased array