Security Element Subwavelength Grating Color Saturation
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Solution Overview
Problem
Existing security elements with sub-wavelength gratings fail to provide a significant color effect change when tilted, as they often require a light-absorbing substrate and have limited angle-dependent color filtering properties in transmission.
Innovation Solution
A security element featuring a dielectric substrate with a first periodic line grating structure and a second inverted line grating structure, both formed from a double layer of high-index and metallic materials, creating a phase shift of 90 degrees, which enhances angle-dependent color filtering effects when viewed in transmission.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single line grating structure is used, then the structure is simple, but the color saturation drops significantly when incident light is unpolarized
Solution Approach 1:
The patent combines two line grating structures into a single security element, with each grating having a different orientation (first grating with lines in first direction, second grating with lines in second direction). This merging approach maintains high color saturation for unpolarized incident light while keeping the overall structure relatively simple and manufacturable.
Solution Approach 2:
The patent transitions from a single-plane grating structure to a multi-plane configuration where two gratings are positioned at different heights (first grating at first height, second grating at second height). This dimensional addition enables the structure to handle unpolarized light effectively by providing multiple polarization components.
2Adaptability or versatility
If sub-wavelength gratings are used for color filtering, then angle-dependent color effects are achieved, but the color saturation drops significantly when incident light is unpolarized
Solution Approach 1:
The patent merges two sub-wavelength gratings with orthogonal or angled orientations to create a structure that maintains angle-dependent color filtering while preserving color saturation for unpolarized light. The combined gratings process different polarization components simultaneously.
Solution Approach 2:
The patent changes the structural parameters by introducing a second grating with different orientation and position, transforming the system from handling single polarization to handling unpolarized light while maintaining the angle-dependent color filtering property through the combined effect of both gratings.
3Illumination intensity
If metallic or semi-metallic coatings are used in sub-wavelength gratings, then color saturation in transmission is increased, but the structure requires light-absorbing substrate and shows limited angle-dependent effects
Solution Approach 1:
The patent combines metallic-coated gratings with specific geometric parameters to achieve both high color saturation in transmission and pronounced angle-dependent color filtering. The dual-grating configuration allows the metallic coatings to enhance saturation while the overall structure provides the required angular sensitivity.
Solution Approach 2:
The patent optimizes geometric parameters (grating period, depth, width ratios) and material properties to enable metallic-coated structures to exhibit strong angle-dependent color filtering in transmission, moving beyond the limitations of single-grating configurations.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The security element delivers reproducible and striking color effects when tilted, with improved color saturation and hue changes, suitable for use in documents like banknotes, by utilizing a double line grating structure with a phase shift that ensures enhanced angular dependency and visibility.
Implementation Method 1
They can have color filter properties in the sub-wavelength range if the grating profile is designed in such a way that resonance effects occur in the visible spectral range
Implementation Method 2
Security elements with periodic line grids are known... which have a strong polarizing effect on the reflection or transmission of an incident light beam
Implementation Method 3
Sinusoidal gratings coated with a thin metal film can produce plasmonic resonance effects. These resonances lead to increased transmission in TM polarization
Implementation Method 4
These structures have a strong polarizing effect on the reflection or transmission of an incident light beam
Data Source
Figure 1
Figure 2a~3b
Figure 4a~4b
AI summary
The invention relates to a security element for producing documents of value, such as banknotes, checks or the like, which has: a dielectric substrate (1), a first periodic line grating structure (2), embedded in the substrate (1), made of a plurality of first grating webs (3) running along a longitudinal direction and arranged in a first plane (L1) having first grating gaps (4) located in between, and a second line grating structure (6) of the same period (d) embedded in the substrate (1) and made of second grating webs (7) running along the longitudinal direction and having second grating gaps (8) located in between, wherein the second line grating structure (6), in relation to the first plane (L1), is located above the first line grating structure (2) in a parallel, second plane (L2), and wherein the second line grating structure (6) is formed so as to be inverted with respect to the first line grating structure (2) such that, in plan view of the first plane (L1), the second grating webs (7) lie above the first grating gaps (4) and the second grating gaps (8) lie above the first grating webs (3), wherein the security element (S) produces a color effect when viewed in transmission (T), and the grating webs (3) of the first line grating structure (2) and the grating webs (7) of the second line grating structure (6) are each formed from a double layer, built up from a layer of highly refractive material (3a, 7a) and a layer of metallic material (3b, 7b).