Free‑Carrier Index Modulation in Thin Transparent Layers at Visible Wa

[rcc01]

TL;DR

Checking whether a thin transparent solid‑state layer (TCO/semiconductor) with tunable free‑carrier density can produce a useful refractive‑index change (Δn ~10⁻³–10⁻²) at visible wavelengths for phase modulation.

I’m an electronics engineer trying to understand the physical limits of using free‑electron density to modulate the refractive index of a thin, transparent layer at visible wavelengths. My original thought involved a low‑density gas plasma, but based on recent feedback here, I now understand that the plasma frequency of lab‑scale gas plasmas is far too low to produce a meaningful index change at optical frequencies.

A previous thread here helped me understand that a low‑density gas plasma would require centimeter‑scale thicknesses to achieve a π phase shift at visible wavelengths, which is not practical for my application.

Before I move forward with any fabrication work, I want to sanity‑check a reframed version of the idea.

Reframed concept: Instead of a low‑density gas plasma, consider a solid‑state layer (e.g., a transparent semiconductor or TCO) where the free‑carrier density can be modulated electrically. These materials can reach much higher electron densities (10¹⁸–10²¹ cm⁻³), so the effective plasma frequency is much closer to the visible/near‑IR range.

The question is whether a thin, transparent solid‑state layer with tunable carrier density could produce a useful refractive‑index change (Δn on the order of 10⁻³ to 10⁻²) at visible wavelengths, sufficient for phase modulation in a layer thickness on the order of a few microns or less.

What I’m trying to understand:

1. Is the plasma‑dispersion effect in transparent semiconductors/TCOs strong enough at visible wavelengths to produce Δn ≈ 10⁻³–10⁻²?
2. Are there known materials (ITO, IGZO, ZnO, etc.) where carrier‑density modulation produces a significant index change while remaining reasonably transparent in the visible?
3. Are there fundamental limits (e.g., absorption, Kramers–Kronig constraints, scattering) that would prevent achieving a π phase shift in a thin layer using free‑carrier modulation?
4. If this is feasible at near‑IR but not visible, where does the practical cutoff tend to be?

I’m not trying to build a display device at this stage — just trying to determine whether the underlying physics supports the idea on paper before I ask a fabrication lab to make a small test coupon.

Any guidance, references, or sanity checks would be greatly appreciated. I’m still learning the optics side, so please forgive any naive assumptions.

P.S. In a previous thread, members here helped me understand that a low‑density gas plasma cannot produce a significant refractive‑index change at visible frequencies without requiring centimeter‑scale thicknesses. That feedback prompted me to explore solid‑state free‑carrier approaches instead.

 

[renormalize]

TL;DR: Checking whether a thin transparent solid‑state layer (TCO/semiconductor) with tunable free‑carrier density can produce a useful refractive‑index change (Δn ~10⁻³–10⁻²) at visible wavelengths for phase modulation.

I'm by no means expert on this topic, but doped-silicon optical modulators can apparently accomplish this (although not necessarily in the specific single-layer configuration you're proposing):

This diagram is taken from the freely-available 2010 Nature review article: https://www.researchgate.net/publication/232752299_Silicon_optical_modulators_Review.
I suggest you read through this paper and use it as a starting point to drill-down further into the literature, in order to judge if your ideas can really improve on existing optical-modulation technology.