Subwavelength Grating Security Element for Non-Reciprocal Polarization
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Solution Overview
Problem
Existing security elements for valuable documents that utilize polarization effects to differentiate between front and back views are complex and not suitable for cost-effective mass production.
Innovation Solution
A security element featuring two periodic line grid structures with subwavelength periods, rotated by an azimuth angle of 40° to 50° relative to each other, creating dispersive polarization filtering effects that differ when viewed from the front and back, allowing for non-reciprocal transmission and color motif recognition.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If liquid crystal layers with opposite linear polarization are used to achieve non-reciprocity in transmission, then polarization effects are achieved, but the structure becomes complex and unsuitable for cost-effective mass production
Solution Approach 1:
The patent extracts the polarization function from complex liquid crystal systems and implements it using simple dielectric subwavelength gratings. By removing the liquid crystal layers and replacing them with periodic dielectric structures that inherently provide polarization-dependent transmission, the design achieves the same optical effect with dramatically reduced structural complexity and manufacturing difficulty.
Solution Approach 2:
The patent replaces the mechanical/liquid crystal-based polarization system with a photonic crystal structure. The subwavelength periodic dielectric gratings create effective medium properties that produce polarization effects through geometric configuration rather than through liquid crystal orientation mechanisms, enabling simpler fabrication and mass production.
2Reliability
If subwavelength gratings with period less than 450 nm are used, then semi-transparency and dispersive polarization effects are achieved, but manufacturing precision requirements increase
Solution Approach 1:
The patent optimizes the grating period parameter to be less than 450 nm, which is below the diffraction limit for visible light. This parameter choice enables the grating to function as an effective medium with polarization-dependent transmission while remaining manufacturable with standard semiconductor fabrication techniques. The specific range balances optical performance with manufacturing capability.
Solution Approach 2:
The patent divides the grating structure into periodic units with specific geometric parameters. By segmenting the structure into repeating subwavelength elements, the design achieves complex optical functionality through simple geometric repetition, reducing the need for high precision in individual feature dimensions while maintaining overall performance.
3Reliability
If two grid structures are rotated by 40° to 50° relative to each other, then non-reciprocal transmission with different color motifs is achieved, but structural complexity increases
Solution Approach 1:
The patent introduces asymmetry by rotating the second subwavelength grating by 40° to 50° relative to the first grating. This asymmetric configuration creates non-reciprocal transmission properties where light traveling in opposite directions experiences different polarization filtering, enabling different color motifs to be observed from the front and back sides of the security element.
Solution Approach 2:
The patent adds rotational orientation as an additional degree of freedom to the grating configuration. By introducing the angular dimension (40°-50° rotation) in addition to the planar grating structure, the design achieves non-reciprocal optical effects without significantly increasing manufacturing complexity, as the rotation can be implemented through standard photolithography alignment procedures.
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 solution enables the production of a mass-producible security element with distinct dispersive polarization effects in transmission, providing effective color differentiation and enhanced security features without the complexity of previous designs.
Implementation Method 1
a structure that generates different polarization effects in the visible spectral range when viewed from the front and back in transmission
Implementation Method 2
Each grid structure generates a polarization effect, which manifests as polarization filtering that is dispersive, i.e., wavelength-dependent
Implementation Method 3
The line grid structures are made of metallic, high-refractive-index, dielectric, or semi-metallic material
Implementation Method 4
The horizontal surfaces are coated with a high-refractive-index dielectric
Data Source
Figure 1~11b
Figure 2~3
Figure 4a~4b
AI summary
The invention relates to a security element for producing value documents, such as banknotes, checks, or the like, having the following: a dielectric substrate (7, 8) and a first grating structure (2), which is embedded in the substrate (7, 8) and lies on a first plane and which is periodic with a period of not more than 450 nm in a first spatial direction (5) and generates a polarization effect in transmission observation. A second grating structure (4) which lies on a second plane that is spaced from the first plane is embedded in the substrate, said second grating structure generating a second polarization effect which differs from the first polarization effect in transmission observation and being periodic with a period of not more than 450 nm in a second spatial direction (6). The first and the second spatial directions (5, 6) lie at an angle of 40° to 50° relative to each other.