how to export high resolution graph from FDTD solutions directly?
I am not aware of an easy way to change the dpi, if this is what you mean, although you should have control on the total number of pixels, ie, by adjusting the size when you are saving the image. That said, you could may be try plotting in matlab, where you might be able to have control on the resolution. Or export the data to other plotting tools.
If you familiar with MATLAB and interested in making a high quality figure using it, you can refer to the this blog. I could create high quality 1D or 2D figures suitable for scientific journals after I had learned how to make good quality scientific graphs via MATLAB from this blog.