The unit of force calculated by volumetric and MST monitors in FDTD


I have a question regarding the Lumerical FDTD. I tried different analyzers to calculate the optical force applied on a ZnO particle, which was placed in the focus of 1064 nm laser. The particle was ~1 um size and placed 50 nm away from the center of the focus. Given my experience, the optical force should be around several pN. However, the three analyzers give completely different results. And it’s difficult to figure out the units of the force. I wonder if someone knows the units.

I tried three analyzers:

  1. One is from my former colleague. And he told me that the unit of the result should be pN/(W/um^2)
  2. “optical_force_volumetric” from the LDTD software.
  3. “optical_force_MST” from the LDTD software.

The result from the first one was tens of thousands of pN. The resultant optical trapping stiffness was ~200 pN/nm, which was too high to be true.

The second one gave a total force ~3.65e-024 (“F_total vs lambda/f”), and the third one gave ~3.4e-024 (“F_total vs lambda/f”). Both seemed too low.

The second one (volumetric) also gave the F in different directions (“F vs x,y,z, lambda/f”). But the F could be as high as 0.01 or as low as 1e-009.

Does anyone have any idea about the units?



My guess is that the output has a unit of Newton per watt and you can get the exact force by multiplying the power of the optical power of you tweezer.

Best Wishes,

Vivek (KX Champion)

Hi Vivek,

Thank you for your advice.

The problem is that the magnitudes of those numbers are so different. As I mentioned, the volumetric analyzer gave a 10^-24 total force. However, the force in Y direction, where the particle was displaced 50 nm from the equilibrium position, is 10^-2.

My guess is that their units include some length scale.