FAQ

General

Q: How should I determine the numerical aperture (NA) of a square lens?

If you need the NA for the 'maximum' light collection angle of the lens, then there is no universally agreed definition of it (at least it seems). It shall be up to you whether you use the sine angle for the mid point of a side or a corner of the square lens.

If you need a NA for your incident beam, then you need to use the beam diameter when calculating it (assuming the beam diameter is smaller than the lens aperture). Thus, in this case, the lens shape doesn't matter.

SMD Tool

Q: Are business organizations allowed to use SMD Tool?

Yes, SMD Tool is free for all, including personal and commercial users.

Q: What range of phase should I use to arrange unitcells in Unitcell Arranger?

Different FDTD software may export phase values in different ranges – i.e. -180° to 180° or 0° to 360°. While Target Phase Generator in SMD Tool generates phases in the range of -180° to 180°, Unitcell Arranger takes unitcell phase data given in any range between -360° to 360°. This means that you can use either -360° to 0°, -180° to 180°, or 0° to 360°. Here phases with a 360° difference are all identical: -360° = 0° = 360°, for example.

Q: How can I incorporate the substrate effect in Lens Simulator module?

Imaging mode allows users to input the substrate thickness (t_sub). With Focusing mode, the software doesn’t know any information about the substrate. However, its effects can be incorporated into the unitcell data (.csv) by simulating unitcells with a thin layer of substrate in FDTD software.

If the metalens operates at normal incidence angle, placing the lens so the substrate faces the incident beam is another trick to work around the substrate effect. In such cases, the incident wave is still a plane wave inside the substrate, and the phase delay along the substrate thickness is the same for all unitcells.