Bragg Reflection Band Research Articles

Broadband wavelength designable achromatic grating based on a cholesteric liquid crystal template

Liquid crystal (LC) gratings have played important roles in light field control due to the advantages of being lightweight, low cost, having no moving parts, and low power consumption. However, the chromatic aberration limits the bandwidth of the LC device and affects the efficiency of the grating. To solve the chromatic aberration issue, a broadband wavelength designable achromatic grating is proposed. Different grating structures are integrated into a single-layer templated cholesteric liquid crystal (CLC) device, and the achromatic diffraction wavelength of the grating can be freely designed from the visible spectral region to the infrared range within the Bragg reflection band of the CLCs. The diffraction intensity of different orders can be changed with the electric field applied to meet the need for dynamic modulation. This grating shows suitable potential applications in optical communication and displays.

Polymer-Stabilized Cholesteric Liquid Crystal Films with Double Reflection Bands Prepared Based on the Competition between Photopolymerization and Photoisomerization.

Composite colors have been widely found in nature. Herein, polymer-stabilized cholesteric liquid crystal (PSCLC) films with composite structural colors were prepared through a one-step photopolymerization approach. The CLC mixtures were prepared using a mixture of acrylates and a mixture of two chiral dopants. One of the chiral dopants is polymerizable, and the other one is photoisomerizable. After photopolymerization, the PSCLC films with double Bragg reflection bands were obtained on the surface of a substrate. The competition between the photopolymerization of the acrylates and the photoisomerization of the chiral dopant was proposed to drive the formation of the double reflection bands. Without oxygen inhibition, the polymerization of the acrylates near the substrate surface was carried out. However, due to oxygen inhibition, the polymerization of the acrylates near the air was retarded. Then, the photoisomerization of the chiral dopant was carried out prior to the polymerization of the acrylates. The wavelengths of the double reflection bands were tunable by changing the concentrations of the acrylates and chiral dopants and the polymerization temperature. Colorful patterns with composite structural colors were prepared, which were suitable for decoration and anti-counterfeiting.

A PSCLC Pattern Prepared Based on Handedness Inversion for Anti-counterfeiting.

Handedness inversion has been widely studied in supramolecular chemistry and material sciences. Herein, a photoisomerizable chiral dopant was synthesized, which could induce the formation of a cholesteric phase with right-handedness. The Bragg reflection band of the cholesteric liquid crystal (CLC) mixture shifted to the long wavelength with extending 365 nm UV light irradiation time. Based on this photochromic property, a colourful polymer-stabilized CLC (PSCLC) film was prepared using a grayscale mask. A handedness reversible CLC mixture was prepared using a mixture of this chiral dopant and S5011. With extending the UV light irradiation time, the handedness of the CLC mixture changed from right- to left-handedness. A patterned PSCLC film was prepared using this CLC mixture. Complementary images were observed under right- and left-handedness circularly polarized lights. The results shown here not only give us a better understanding the competition between photopolymerization and photoisomerization, but also lay the foundations for decoration and anti-counterfeiting.

Colourful patterns prepared using a cholesteric liquid crystal mixture with both thermochromic and photochromic properties

ABSTRACT Colourful polymer-stabilised cholesteric liquid crystal (PSCLC) film attracted much attention for their applications in decoration and anti-counterfeiting. Herein, a chiral dopant was synthesised using trans-p-hexoxycinnamic acid. With the addition of this chiral dopant in a reactive LC mixture, both thermochromic and photochromic phenomena were found. The Bragg reflection band of the CLC mixture shifted to long wavelength with extending 365-nm UV light irradiation time or increasing temperature. The structural colour can cover the visible range. The photochromic property of the CLC mixture was driven by the photoisomerisation of the chiral dopant. Colourful PSCLC patterns were prepared based on the thermochromic and/or photochromic properties and using masks. The results shown here not only give us a better understanding the competition between photopolymerisation and photoisomerisation, but also lay a foundation for the preparation of the decorations.

Colorful patterned polyacrylate films prepared using cholesteric liquid-crystalline mixtures with a smectic order

ABSTRACT Polymer-stabilised cholesteric liquid crystal (PSCLC) films with selective circularly polarised light reflection have attracted much attention for their applications as polarisers, energy-saving windows and for displays. Herein, CLC mixtures were prepared using a nematic LC LC242 and a chiral compound S-6 with enantiotropic SmA and SmC* phases, which exhibited a cholesteric phase with a smectic order. After photopolymerisation, the PSCLC films with fingerprint structure at the surfaces and supramolecular helical structure inside were obtained. Due to the existence of short-range smectic order in the cholesteric structure, the Bragg reflection bands were broadened. For a CLC mixture, with increasing temperature, the short-range smectic order was suppressed, and the selective Bragg reflection band shifted to the short wavelength. Based on this thermochromic behaviour, colourful PSCLC patterns and gratings were prepared, which were able to be applied in decoration and anti-counterfeiting. Since the PSCLC films can be obtained within several seconds under air, large-area films can be prepared on coating lines at low-cost.

Light Intensity-Selective Photopolymerization and Photoisomerization for Creating Colorful Polymer-Stabilized Cholesteric Liquid Crystal Patterns.

Polymer-stabilized cholesteric liquid crystal (PSCLC) films have been widely studied for their application as sensors, polarizers, and reflective windows. However, the preparation of programmable and colorful patterns based on the structural color is still challenging. Herein, the photochromic CLC mixtures were prepared by adding a photoisomerizable chiral additive (CA) and a photoinitiator in the nematic liquid crystal LC242. Under UV irradiation with weak intensity, photoisomerization of the CA was carried out and photopolymerization was suppressed by oxygen inhibition. With extending the irradiation time, the helical pitch of the CLC film increased and the selective Bragg reflection band tended to redshift. Under strong UV irradiation, oxygen inhibition was overcome and photopolymerization dominates the reaction. Therefore, the colorful-patterned PSCLC films were able to be prepared using masks. The results shown here not only give us a better understanding of the effect of oxygen inhibition but also lay the foundations for practical applications such as decoration and optical devices.

Color tuning in thermo-sensitive chiral photonic liquid crystals based on the pseudo-dielectric heating effect

The ability of electro-thermal tuning in center wavelength (λc) of the Bragg reflection band is promoted in cholesteric liquid crystal (CLC) containing a positive-dielectric-anisotropy liquid crystal host (E7) and a binary chiral dopant (R-811 and R-5011). In the CLC mixtures with a fixed R-811 content of 30 wt%, the transition temperature of the smectic-A phase to CLC phase decreases when the R-5011 loading increases. The CLC systems are temperature-sensitive, each exhibiting a temperature range of about 9–12 °C in the CLC phase. Optimization of the content of R-5011 at 1.5 wt% allows for the remarkably effective thermal tuning in λc from 780 nm to 380 nm in response to a temperature increase of merely 3 °C from the room temperature. Under the mechanism of pseudo-dielectric heating, the tunable λc can shift in the entire visible spectrum by an applied voltage as low as 7 Vrms at a frequency of 2 MHz across the cell gap of a 4.8-μm-thick cell. Furthermore, a multifunctional and operation-reversible device is demonstrated by the control of the applied AC voltage, yielding an optical device in which the reflection band can be tuned and the mode can be switched among the reflection, scattering, and transparent states.

2D Bragg Resonators Based on Planar Dielectric Waveguides (from Theory to Model-Based Testing)

Using analytical approaches and CST Microwave Studio 3D simulation, the electrodynamic characteristics of 2D Bragg resonators based on planar dielectric waveguides with doubly periodic corrugation are theoretically analyzed. Such resonators are of great interest to obtain directed narrow-band radiation in heterolasers with large active regions. Model electrodynamic experiments on the “cold” testing of such structures in the millimeter range are performed. Good agreement between the experimental and simulation results is shown, including the existence of the highest-Q mode within the Bragg reflection band in the absence of defects in the periodicity.

Stretchable, flexible, and adherable polarization volume grating film for waveguide‐based augmented reality displays

AbstractWe report a stretchable, flexible, and adherable polarization volume grating (PVG) film and propose a compact optical system for augmented reality displays based on it. The Bragg reflection band shift, deflection angle change, and the mechanical robustness under stretch‐release cycles of the PVG film are also investigated.

Stretchable, flexible, rollable, and adherable polarization volume grating film.

Volume Bragg gratings (VBGs) have many applications, including filters, wavelength multiplexing devices, and see-through displays. As a kind of VBGs, polarization volume gratings (PVGs) based on liquid crystal polymer have the advantages of nearly 100% efficiency, large deflection angle, and high polarization selectivity. However, previous reports regarding PVGs did not address high efficiency, tunable periodicity, and flexibility. Here, we report a stretchable, flexible, and rollable PVG film with high diffraction efficiency. The control of PVG by mechanical stretching is investigated, while the Bragg reflection band shift is evaluated quantitatively. Moreover, we quantified the deflection angle change's behavior, which has promising potential for laser beam steering applications. The mechanical robustness under stretch-release cycles is also scrutinized.

Двумерные брэгговские резонаторы на основе планарных диэлектрических волноводов (от теории к модельному тестированию)

Using analytical approaches and CST Microwave Studio 3D simulations, we have undertaken a theoretical analysis of electrodynamical characteristics of 2D Bragg structures based on planar oversized dielectric waveguides with double-periodical corrugation. Such structures are of significant interest for obtaining directed narrow-band radiation in heterolasers with large dimensions of an active area. Modeling electrodynamical experiments on 'cold' testing of such structures in millimeter wavelength band were conducted. Fine agreement between the experimental results and the simulation results was demonstrated which includes the presence of the highest-quality mode inside the Bragg reflection band in absence of the periodicity defects.

Theoretical and experimental studies of highly selective planar two-dimensional Bragg structures based on dielectric waveguides in the terahertz frequency range

Based on theoretical approach and three-dimensional modeling using the CST Microwave Studio code, planar dielectric two-dimensional Bragg structures in terahertz frequency range were developed and manufactured. Proof-of-principle electrodynamic experiments on the “cold” testing of these structures were carried out. It is shown that the experimental results are in good agreement with the theoretical predicts, including the existence of the highest Q mode inside the Bragg reflection band in the absence of periodicity defects.

A spherical distributed Bragg reflector based on a-Si1–xCx: H and a-SiO2 layers

A spherical distributed Bragg reflector (SDBR) for near-infrared applications is elaborated via plasma-enhanced chemical-vapor deposition. It consists of the alternating quarter-wave layers of amorphous hydrogenized silicon carbide (a-Si1–xC x : H) and amorphous silicon oxide (a-SiO2) deposited onto a glass microsphere. The reflection spectra of the SDBR are measured at different points of its surface. In all cases, the Bragg-reflection bands, as well as the wavelength range of their overlap, are detected, evidencing the presence of a complete photonic band gap in the SDBR obtained.

Morpho‐Butterfly‐Inspired Patterning of Helical Photonic Structures for Circular‐Polarization‐Sensitive, Wide‐Angle Diffuse Reflection

Cholesteric liquid crystals are known to exhibit a circular‐polarization selective Bragg reflection band over a wavelength region determined by their reflective index and pitch. In a planar device where the molecules are aligned unidirectionally at the substrate surface, cholesteric liquid crystals act as flat dielectric mirrors, reflecting light at an angle fulfilling the law of reflection. Here, it is shown that by introducing a random structure inspired by the Morpho didius butterfly into the distribution of the helix phase, the reflected light phase becomes randomized, leading to a diffuse reflection that spans over an angle greater than ±30°. The reflection maintains its circular polarization selectivity, making these materials unique compared to the butterfly itself and other man‐made structures mimicking the butterfly's structure.

A thermally tunable narrowband selector based on a chiral nematic containing a binary thermosensitive chiral dopant mixture

ABSTRACTWe employed a thermosensitive chiral dopant (CD), characterized by its thermoinduced change in helical twisting power (HTP) in terms of the handedness and strength, to shift the Bragg reflection band of a CLC. A thermally tunable photonic bandgap (PBG) device was demonstrated using a one-dimensional asymmetric photonic crystal (PC) infiltrated with the CLC as a defect layer. With a temperature controller to shift the PBG of the CLC, two narrow transmission bands can be tuned in the spectrum dominated by the asymmetric PC structure. This optical device can find applications as a tunable narrowband wavelength selector or an intensity controller.

Bragg reflection band width and optical rotatory dispersion of cubic blue-phase liquid crystals.

The Bragg reflection band width and optical rotatory dispersion of liquid crystalline cholesteric blue phases (BPs) I and II are compared by numerical simulations. Attention is paid to the wavelength regions for which the reflection bands with lowest photon energies appear, i.e., the [110] direction for BP I and the [100] direction for BP II. Finite difference time domain and 4×4 matrix calculations performed on the theoretical director tensor distribution of BPs with the same material parameters show that BP II, which has simple cubic symmetry, has a wider photonic band gap than BP I, which has body centered cubic symmetry, possibly due to the fact that the density of the double-twist cylinders in BP II are twice that in BP I. The theoretical results on the Bragg reflection band width are supported by reflectance measurements performed on BPs I and II for light incident along the [110] and [100] directions, respectively.

Correlated color temperature tunable white LED with a dynamic color filter.

We proposed a new device structure to dynamically tune the correlated color temperature (CCT) of a white light-emitting-diode (WLED). The key component is a dynamic color filter, consisting of a liquid crystal (LC) cell sandwiched between two cholesteric LC films whose Bragg reflection band covers the blue wavelength of the WLED. When a voltage is applied to the LC cell, the transmittance of blue light is changed, while the longer wavelength part remains unaffected, resulting in a tunable CCT. Validated by experiment, our design exhibits several advantages, such as reasonably wide tuning range (6916K to 3253K), low operation voltage (~3.2 V), simple device configuration, and low cost. It is a strong contender for next generation smart lighting.

SHAPE TRANSFORMATION OF WAVE BEAMS FALLING ON BRAGG STRUCTURES

The transformations of the form of wave beams falling on periodic and apodized quasiperiodic Bragg reflectors were considered. Spatial apodization of permittivity contrast of multilayer structures can induce significant decrease of the side lobes. This effect provides much smaller shape deformation of the reflected beam upon incident at angles near the edges of reflection band, as well in Bragg reflection band, in comparison with phenomena in similar periodic structures.

Liquid Crystals: Deformation‐Free, Microsecond Electro‐Optic Tuning of Liquid Crystals (Advanced Optical Materials 3/2013)

The macroscopic alignment of nanosized liquid crystal domains in a polymer matrix with helical order is demonstrated by H. Yoshida and co-workers on page 256. The alignment can be switched to induce a change in the effective refractive index. Nanosizing of liquid crystal domains enables access to fast reorientation dynamics (˜10 μs), while helical order is retained in the polymer matrix, resulting in deformationfree tuning of the Bragg reflection band.

Deformation‐Free, Microsecond Electro‐Optic Tuning of Liquid Crystals

Fast (∼10 μs) and deformation‐free electro‐optic tuning of a liquid crystal is reported, achieved by macroscopic alignment and switching of nanosized, pseudo‐nematic domains. The tuning mode can be achieved by photopolymerizing a mesogenic monomer–liquid crystal mixture in the liquid crystal phase, and forming nanosized pores in the polymer matrix. This concept is particularly effective in liquid crystals with spontaneous structure‐forming capabilities: here this concept is applied to a cholesteric liquid crystal and demonstrate scatter‐free tuning of the Bragg reflection band. It can also lead to new device applications such as thin‐film optical amplitude modulators and linear polarization rotators.