Learning Forum

[SAURAV GAUTAM]

Hi,

While measuring transmittance of silicon film , i am getting oscillation in spectra when there's a gap between PML and material.

. may i know what could be the explaination of this and way to reduce it?

 

thanksz

 

 

[Guilin Sun]

This is physical, since there is a gap between the thin film and the PML. This means you are simulating a cavity formed by the thin film. Please make sure the thin film has substrate or not. Even with substrate, under proper conditions, such "oscillation" can happen when there is resonance. 

When there are air gaps the cavity ends are in air/background material. When there is a substrate, one end of the cavity is in the substrate whereas the other one is still in air. Anything that forms two materail interfaces potentially can have resonance in broader spectrum, when it fulfils optical-path-difference is integer number of wavelength. 

[SAURAV GAUTAM]

thank you,

i have slight confusion about cavity formation because PML supposed to absorb all input field so what's causeing the interference? 

there's no substrate, i just want to measure transmitted field, so is possible to modify measure geometry and get those value for different wavelength.

[Guilin Sun]

not the PML, but the material interfaces that forms the cavity. Think how a typical cavity is formed. The thin film without substrate is the same. If you follow the cavity R/T derivation processs, you will know how the formula acts and where the phase difference is created.