Moth-Eye Anti-Reflective Coating

1. Influence of reflected light

  • When using a mobile phone under the big sun, or the TV screen reflects fluorescent light or sunlight, the screen cannot be seen clearly.
  • The principle of solar cells is to convert light energy into electrical energy. When the reflectivity of the solar panel is too high, the sunlight entering the panel will decrease and the conversion rate of light energy will be reduced.
  • When taking pictures with backlighting, there will always be light spots (ghosting) or glare in the photos taken.

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2. The principle of reflection

When light enters another medium from one medium, if the refractive indexes of the two mediums are different, the light will reflect and refract at the interface of the two mediums. As the refractive index difference between the two media is greater, the interface reflectivity R is greater. Reflectance R=[(n1-n2)/(n1+n2)]2

※When light enters glass from air, the refractive index of air isnair=1,the refractive index of glass inglass=1.5,and its interface reflectivity R=(0.5/2.5)2=0.04=4%, so when light enters glass from air, there will be 4% of the light is reflected.


3. Method to reduce reflection

1. Utilize the characteristics of reflectance change and wavelength-dependent characteristics caused by surface structure and shape of the material surface fluctuations.

2. Use the transition absorption between energy bands in the semiconductor layer.

3. Use the film interference phenomenon in the multilayer optical film with different refractive index to achieve the anti-reflection effect.

※Plating a layer of MgF2(n=1.38)anti-reflective film on the glass surface, the reflectance when light enters MgF2from the air is (0.38/2.38)2=0.025, and when light enters the glass from MgF2 , it is reflected.

The reflectivity is(0.12/2.88)2=0.0017, the total reflectivity is 0.025+0.0017=2.67%

Although the above methods can reduce reflection, they also have many disadvantages, ex: image fogging, can only be used in a specific wavelength range, complicated and expensive manufacturing process, low yield...etc.


4. Moth Eye Surface Structure

In 1967, Bernhard discovered the nocturnal moth whose eyes are composed of a collection of fine hexagons of about tens of μm. The surface of these hexagons also has fine irregularities with a diameter of about 100n to 200nm, and its evolution characteristics It can prevent the reflection of the eyes so that natural enemies or prey can not find themselves, and can efficiently gather the weaker light at night to see the surroundings clearly.




5. Sub-wavelength structure

        Scientists have produced a similar structure based on the moth’s eye-a subwavelength structure. According to the effective medium theory (EMT), the structural refractive index is equivalent to a multilayer film with a graded refractive index, which can not only effectively reduce Reflectivity also solves the process problem between plating multilayer anti-reflective films and the selection restriction of anti-reflective film materials.


Artificial sub-wavelength structure



6. The principle of sub-wavelength structure

  • When the period length of the shutter is much smaller than the wavelength of the incident light, only zero-order reflected light will be produced optically, and there will be no high-order diffracted light, and its optical properties can be regarded as a homogeneous film due to the equivalent medium theory.



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The shutter structure can be equivalent to a homogeneous film with a fixed refractive index

  


  • The design of the sub-wavelength structure utilizes the above-mentioned characteristics, and the tiny protrusions can be regarded as overlapping of many different periodic shutters far smaller than the wavelength of the incident light to form a structure with a graded refractive index.


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The continuous shutter structure can be equivalent to a film with a graded refractive index




7. Application of sub-wavelength structure

CANON, SHARP and many manufacturers have applied this technology to camera lenses, LCD TV panels, solar panels and optical components to reduce reflectivity.




references

[1]  SHARP patent number US20120234794 A1

[2]  Philippe Lalanne, Mike Hutley, The optical properties of artificial media structured at a subwavelength scale,2015

[3]  Michael Berger, Moth eyes inspire self-cleaning antireflection nanotechnology coatings,2008

[4]  Quartz nanometer array as a secondary wavelength structure with anti-reflection properties, News from the Center for Instrument Research, Issue 109