- #1
rcc01
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- 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:
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.
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:
- Is the plasma‑dispersion effect in transparent semiconductors/TCOs strong enough at visible wavelengths to produce Δn ≈ 10⁻³–10⁻²?
- Are there known materials (ITO, IGZO, ZnO, etc.) where carrier‑density modulation produces a significant index change while remaining reasonably transparent in the visible?
- 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?
- If this is feasible at near‑IR but not visible, where does the practical cutoff tend to be?
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.