# Make a light trap using an object with anomalous dispersion properties and a cavity

## Main Question or Discussion Point

In anomalously dispersive media, the change in phase velocity with wavelength is negative; that is dv/dλ < 0.

I wanted to ask:

(a) whether you could fit a wide range of frequencies into a resonant cavity by taking advantage of anomalous dispersion
(b) whether a material like this exists for the 300nm < λ < 1100nm range and
(c) whether it exhibits the behaviour of n = λο/λd,

where n = index of refraction, λο = wavelength between 300 and 1100nm, and λd = desired wavelength to design the resonant cavity with λd = 300nm.

(d) whether you could design the resonant cavity so that light goes in but doesn't escape easily because of the waveguide modes and because of total internal reflection characteristics.

I am thinking if you could etch tiny rectangular cavities filled with the right medium on the surface of a silicon solar cell and focus the surrounding light into the cavity, then more of the light will be absorbed by the silicon.

What my maths thinking is:
fo=c/λο, c=300Mm/s
v=fo*λd, v = phase velocity
if λd=300nm,
@ λο=300nm, fo=c/λο=1PHz, v=fo*λd=300Mm/s => n=c/v=1
@ λο=1100nm, fo=c/λο=0.272PHz, v=fo*λd=81.82Mm/s =>n=c/v=3.667

## Answers and Replies

Related Classical Physics News on Phys.org

I am thinking if you could etch tiny rectangular cavities filled with the right medium on the surface of a silicon solar cell and focus the surrounding light into the cavity, then more of the light will be absorbed by the silicon.
Interesting application - have you been able to estimate what the theoretical improvement in energy efficiency would be in such a solar cell device?

There are parabolic reflectors for example that do a similar thing - they focus the sun rays whilst tracking the sun as it moves in the sky. I believe that energy enhancements of the order of about 300% are possible in these systems.

The solar cell itself is a very specialised high durability cell that requires a cooling system. (NASA developed one type of these solar cells for some of their satellites and probes)

Interesting application - have you been able to estimate what the theoretical improvement in energy efficiency would be in such a solar cell device?
I'm not sure how much of a difference it would make. There's a lot of issues that I can imagine would affect the outcome. If the cavity is a cube, then the sides of the cube would be 300nm/1.414... = 212nm long for a wavelength at 300nm. I'd need to consider the distance between each cavity so that the walls between each cavity are not so thin that the light goes through the cavity walls. The critical angle of the boundary between the cavity and the next layer of the solar cell must be less than 45 degrees for all frequencies. A micro/nanoscopic lens would have to have the right properties to direct the light within the cavity. I think there would be heating issues. Of course this all depends on whether I understand light dispersion correctly and whether the technology is there at the right price.

I just put the idea out there. I was hoping someone with more knowledge can validate or invalidate the idea.

I'm not sure how much of a difference it would make. There's a lot of issues that I can imagine would affect the outcome. If the cavity is a cube, then the sides of the cube would be 300nm/1.414... = 212nm long for a wavelength at 300nm. I'd need to consider the distance between each cavity so that the walls between each cavity are not so thin that the light goes through the cavity walls. The critical angle of the boundary between the cavity and the next layer of the solar cell must be less than 45 degrees for all frequencies. A micro/nanoscopic lens would have to have the right properties to direct the light within the cavity. I think there would be heating issues. Of course this all depends on whether I understand light dispersion correctly and whether the technology is there at the right price.

I just put the idea out there. I was hoping someone with more knowledge can validate or invalidate the idea.
Can you experimentally validate the application?

(should protect any potentially novel ideas via either secrecy or patent before you publicise them)

Can you experimentally validate the application?
No i can't. I'm not working in sciences/engineering, I just have an interest in solar cells and some knowledge from a university education in electronics engineering. I posted on the forum because I was stuck on the unknowns I outlined earlier and I wasn't sure where to turn to next. I just like to search for the sake of learning.