Hi,
I have a question about reflection, transmission, and absorption of light.
Is there a material that can absorb (or reflect) a light in one-way?
For example, there is a substrate that has a cavity structure on the surface.
When light irradiation from the top of it, it absorbs (or reflects) all(100%) the light from the surface with the aid of the cavity structure.
Then, how about the light irradiated from the bottom of it (flat surface)?
Can the light be emissive into the air through the substrate? Or will it be absorbed(or reflected) like the light irradiated from the top?
Thanks.
Dear Won,
the picture for the backside irradiation is wrong. The light beam coming from the backside doesn't penetrate the structure in a straight line. The surface boundary reflects the light beam like the upper incoming rays. The reflectivity depends on the angle with respect to the surface. Furthermore, for the intensity of reflected and transmitted light the Fresnel law must be taken into account.The intensity depends on the incident angle and the ratio of the refractive indices. You can have total reflection. In the sum the behaviour of light beams is complementary to the opposite side.
For a high absorption the ratio of wavelength and roughness must be taken into account. If the wavelength is higher than the roughness, the boundary behaves like a smooth surface. Therefore you must adapt the lengthscales to get a high absorption.
With regards
R. Mitdank
From my experience (optical thin films):
- transmittance will be the same either the light is incident from the front, or from the back
- reflectance will be the same also, in case of transparent materials (no absorption, no scattering)
- however, reflectance and absorption can be different for light coming from the front and from the back if there is some absorption in one of the material, or some source of scattering inside the structure.
Dear Won Bae,
There are materials those are available for absorbing or reflecting at high percentage. When you refer to a light, you had better to specify what kind of optical bandwith or wavelength range is that material can have high absorption or reflection.
I agree with Rüdiger that your diagram on backside was wrong.
Do you look for coating materials for high reflection or absorption at a certain wavelength range?
Best wishes
Xuhua
Dear Won,
Super black materials, whose reflectivity is less than 0.5%. For example, the reflectivity of carbon nanotube was only 0.045%. You might want to read: ACS Nano 6(2012) 10475; Nano Letters 8(2008) 446 and Carbon N.Y.. 2017; 119:10-20.
Best regards,
Gedvidas
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