Field-Induced Heliconical Cholesteric Liquid Crystal Structure
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Solution Overview
Problem
Direct application of an electric field to cholesteric liquid crystals for color control leads to non-uniform distortion of the periodic structure, resulting in non-uniform coloration and reflectivity, and existing methods for tuning the reflection color have limitations such as broadening of the reflection band and decrease in reflectance.
Innovation Solution
The introduction of an oblique helicoidal director state (Noh*) in chiral nematic liquid crystals, achieved by applying an electric field, which allows for electrical control of the pitch and cone angle of the heliconical structure, enabling tunable color reflections within the visible spectrum with a narrow reflection bandwidth and millisecond switching times.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If an electric field is applied to cholesteric liquid crystals to control reflected color, then the reflection color can be tuned, but the periodic structure becomes distorted in a non-uniform manner resulting in non-uniform coloration and reflectivity
Solution Approach 1:
The patent applies an electric field to change the pitch parameter of the helical structure, transforming it from a right-angle helix to an oblique helicoid with adjustable pitch and cone angle. This parameter change enables continuous color tuning across the visible spectrum while maintaining structural homogeneity through the field-induced oblique helicoidal state.
2Adaptability or versatility
If the pitch of the helix is changed to tune reflection color, then the reflection wavelength can be adjusted, but the reflection bandwidth broadens and reflectance decreases
Solution Approach 1:
The patent dynamically adjusts both the pitch and cone angle of the helical structure through electric field control. By simultaneously optimizing these two parameters in the oblique helicoidal state, the system achieves wavelength tuning while maintaining narrow reflection bandwidth and high reflectance, overcoming the limitations of static pitch-only control.
3Adaptability or versatility
If an electric field perpendicular to the helix axis is applied to change reflection wavelength, then the wavelength can be tuned, but fringe-fields locally distort the homogeneity of the structure
Solution Approach 1:
The patent transitions from controlling only the pitch (one-dimensional control) to simultaneously controlling pitch and cone angle (two-dimensional control) through electric field application. This dimensional expansion in parameter space allows wavelength tuning while maintaining structural homogeneity by optimizing both parameters together in the oblique helicoidal state.
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
This approach allows for broad range color switching with a narrow reflection bandwidth, suitable for applications like reflection displays and tunable lasers, by transforming the chiral nematic state into an oblique helicoidal state with an electric field, maintaining structural homogeneity and enhancing reflectance.
Implementation Method 1
direct application of an electric field to cholesteric LCs to control the reflected color
Implementation Method 2
The field-induced modification of the helix of the N* material is typically performed by one of two ways: (1) changing the pitch of the helix
Implementation Method 3
The selective reflection is a manifestation of the periodic helical organization of the cholesteric phase. When macroscopically organized in the Grandjean texture (uniform standing helix), the chiral nematic satisfies the condition for a reflection of light as defined by the Bragg Equation.
Implementation Method 4
the central wavelength of the reflection bandgap is defined as: λp=nP, where P is the pitch length of the helical twist of the director and n is the average refractive index of the liquid crystal
Implementation Method 5
the reflection bandwidth of N* is defined by Δλ=ΔnP, where Δn is the birefringence of the LC
Data Source
AI summary
A diffraction grating comprises a liquid crystal (LC) cell configured to apply an electric field through a cholesteric LC material that induces the cholesteric LC material into a heliconical state with an oblique helicoid director. The applied electric field produces diffracted light from the cholesteric LC material within the visible, infrared or ultraviolet. The axis of the heliconical state is in the plane of the liquid crystal cell or perpendicular to the plane, depending on the application. A color tuning device operates with a similar heliconical state liquid crystal material but with the heliconical director axis oriented perpendicular to the plane of the cell. A power generator varies the strength of the applied electric field to adjust the wavelength of light reflected from the cholesteric liquid crystal material within the visible, infrared or ultraviolet.