Filters Based on TSLCs and Templated-TSLCs: History
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Contributor:
Changli Sun
,
Jiangang Lu

An optical filter is one of the indispensable devices in massive and high-speed communication, optical signal processing, and display. Twist-structure liquid crystals, cholesteric liquid crystals, blue-phase liquid crystals, and sphere-phase liquid crystals show potential application in optical filters originating from the periodic nanostructures. Wavelength and bandwidth tuning can be controlled via temperature, electric fields, light, angle, spatial control, and templating technology. 

  • twist-structure liquid crystals
  • filter
  • photonic properties
  • templating technology

1. Introduction

Wavelength-selective and band-pass filters are fundamental and essential devices in optical communication for satisfying the acute need for massive and high-speed communication. They are the essential components of optical wavelength-division multiplexing and orthogonal frequency-division multiplexing systems in optical fiber communication [1][2][3], visible light communication [4][5], and microwave communication [6]. Liquid crystals (LCs) are promising materials in the optical communication field due to the advantages of low driving power, low power consumption, high birefringence, and large electro–optic effect [7][8][9]. LC is a state of matter between isotropic liquid and solid phases, possessing both the fluidic characteristics of liquid and the order properties of crystals [10][11]. The introduction of chirality to the LC system has an important impact on the properties [11]. Twist-structure liquid crystals (TSLCs) are a class of variant LCs with twisted LC molecules, consisting of blue-phase LCs (BPLCs), cholesteric LCs (CLCs), and sphere-phase LCs (SPLCs) [12]. TSLCs show a good application in optical communication devices, such as wavelength-selective filters, optical attenuators, optical switches, and beam steerers [13][14][15]. The wavelength range of TSLCs can be tuned by doping different concentrations of chiral dopant, forming spatial gradients, designing device structure, applying temperature, or irradiating the material with ultraviolet light [16][17][18][19], which makes them attractive in tunable optical filters [20][21][22][23]. Gao et al. reported the development of templated TSLCs and summarized the potential photonic applications, including lasing, optical filters, grating, etc. The following discussion focuses on optical filters based on TSLCs, promising stimuli-responsive materials for wavelength and bandwidth tuning. 

2. Filters Based on TSLCs and Templated-TSLCs

TSLC filters can be triggered by several stimuli to generate structural change, resulting in a shift of the Bragg reflection wavelength and variation in the bandwidth. The effects of templating technology, temperature, electricity, light irradiation, incident angle, and spatial control on TSLC filters are presented below.

2.1. Templating Technology

Templating is one approach that transfers the features of a host medium into a guest matrix through a set of chemical and physical processes. It is a replication of fundamental features under structural inversion [24]. A polymer template can be prepared by photopolymerizing LC pre-polymers and then washing out the remaining molecule mixtures [25][26]. A variety of LCs, including nematic LCs, chiral nematic LCs, and pre-polymers, are candidates for materials refilled into polymer templates [27]. Filters based on templating technology have the advantages of high reflectivity, multiple reflection peaks, and a flexibly changeable photonic band gap (PBG) [28][29].
A multi-layer templated BPLC filter reflecting multi-wavelength without intermediate dielectric layers was fabricated (Figure 1a) [30]. To obtain the template, the glass substrates of the cell were separated, and the polymer-stabilized BPLC film was placed into ethanol to wash out the residual LC, chiral dopant, nonreactive monomers, and the photo-initiator. After laminating the templates of different reflection wavelengths and refilling nematic LCs into the multi-layer template, the BPLCs were reconstructed. The templated-BPLC filter showed a narrow reflection bandwidth (<15 nm), good angular stability, and stable reflection with a temperature shift.
Figure 1. (a) The fabrication process of the multi-layer templated BPLC filter. Reproduced with permission from Ref. [30]. MDPI, 2019. (bi) Flow chart of multiple refilling process. (bii) Transmittance spectra of templated-BPLC, CLC, and dual-wavelength LC filter. Reproduced with permission from Ref. [31]. MDPI, 2021. (ci) Materials used in experiments. (cii) Transmission spectra of multi-chiral CLC filters with single layer. Reproduced with permission from Ref. [32]. MDPI, 2021.
Compared to the LC filter with a multi-layer structure, a single-layer structure LC filter could significantly simplify the device structure and streamline the fabrication process. Hence, a multi-wavelength TSLC filter and a bandwidth-tunable CLC filter of a single-layer structure were implemented with a multiple wash-out–refilling process [31]. By refilling a CLC with a different pitch from that of the target template into a BPLC template, a single-layer LC filter with multi-reflection peaks was obtained (Figure 1b). By refilling a CLC template with CLCs of adjacent pitch sequentially, a bandwidth-tunable single-layer filter could be realized. The FWHM of the bandwidth-scalable CLC filter could be continuously broadened by 96% when compared with that of the original filter.
To improve the maximum reflectance of a single-layer CLC filter, a high-reflectivity CLC filter reflecting both right- and left-circularly polarized light was proposed [32]. A filter with hyper-reflectivity was obtained by refilling a left-handed (LH) CLC into a right-handed (RH) CLC template (Figure 1c). The RH polymer-stabilized CLC precursors consisted of BPH006, R5011, C3M, TMPTA, and IRG184. The refilling LH CLC mixtures comprised BPH006 and S811. The hyper-reflectivity was related to the wavelength consistency. Different from the single-handed LC filter, the multi-chiral LC filter showed hyper-reflectivity due to the coexistence of right- and left-handedness.
A single-layer LC filter, multi-wavelength LC filter, multi-phase LC filter, and multi-chiral LC filter could be realized using the templating technology. The TSLC filter with templating technology featured high flexibility, high reflectivity, a wide tunable range, and good stability. The handedness of the template, the phase of the refilling LCs, and the wash-out–refill process were important factors for achieving a TSLC filter based on the templating technique.

2.2. Temperature Variation

The reflection bands of the filters associated with the helical pitch, order parameter, and refractive indices are related to temperature due to the thermodynamic behavior of the LC molecules [33][34][35]. The temperature-dependent characteristics of the LC filters cover the central wavelength and Bragg reflection bandwidth. For BPLCs, the temperature dependence of the Kerr constant, which is related to the induced birefringence and pitch length, is of great relevance and of fundamental importance [36][37].
In order to improve the reflectivity, a polarization-independent tunable optical filter combining LH and RH CLCs as a unit was demonstrated (Figure 2a) [38]. The bandwidth of the reflection band decreased as the reflection band of CLC-1 red-shifted with decreasing temperature and that of CLC-2 blue-shifted with increasing temperature. The bandwidth of the high reflectivity CLC filter could be adjusted from 10 to 70 nm, and the central wavelength could vary from 573 to 500 nm with the temperature ranging from 23 to 50 °C.
Figure 2. (ai) Combination of RH-CLC and LH-CLC. (aii) Schematic diagram of the experiment setup. (aiii) Reflection spectra of the filter. Reproduced with permission from Ref. [38]. Copyright 2014 The Japan Society of Applied Physics. (b) Transmission spectra of the filter versus (bi) temperature and (bii) electric field. Reproduced with permission from Ref. [39]. MDPI, 2019. (c) The mechanism illustration of the holding treatment (ci,cii). Reproduced with permission from Ref. [40]. MDPI, 2019.
In addition to the CLC filter, a near-infrared SPLC filter with a low operating electric field and large temperature gradient was proposed (Figure 2b) [39]. During the cooling process from the sphere phase to N*, the structure varied from a 3-DTS to a helical twist structure. Due to the sensitivity of the 3-DTSs to external stimuli, a central wavelength tuning range from 1580 nm to 1324 nm with a temperature gradient of 42.7 nm/K was obtained. In addition, an electrical central wavelength adjustment of over 76 nm with an operating electric field of 0.3 V/µm was realized.
Considering the effect of temperature on the LC filter, the performance of the sectional polymerization process on the tunable TSLC filters was demonstrated [40]. As the temperature decreased rapidly, the pitch of TSLCs at the bottom close to the temperature controller was shortened owing to the helical-twisting power variation, while that at the top remained due to the long distance from the temperature controller (Figure 2c). The reflection bandwidth of the CLC filter and the BPLC filter could be widened by the holding treatment from 120 nm to 220 nm and from 45 nm to 140 nm, respectively.
The tuning of the central wavelength and the bandwidth of the TSLCs was based on the temperature-dependent pitch variation, refractive indices change, and the reorientation of the LC molecules. Several factors influencing the reflection band had a strong relationship with temperature, including the helical-twisting power of the chiral dopants, elastic constants, Kerr constant, viscosity, and the order parameters of the LCs. The temperature responses of the TSLCs were critical for their application in filters.

2.3. Electric Field Modulation

Among various stimuli, the electric field shows good feasibility and high efficiency in inducing the reorientation of the LC molecules [41]. For BPLCs, three typical and progressive effects of the electric field are known, including a local reorientation of the LC director, a distortion of the cubic lattice, and a phase transition to lower ordered phases [42]. The reflection bandwidth of the polymer-stabilized CLCs with negative dielectric anisotropy can be changed by direct current (DC) electric fields due to the absorption of cations by the polymer network [43].
The electrical tuning of the central wavelength and the bandwidth of the CLC bandpass filters in the infrared (3-5 μm) was reported [44]. The substrates coated with silver nanowires and graphene mid-wave infrared (MWIR) transparent electrodes were fabricated. Under a DC field of 110 V, the central wavelength of the filter eventually reached 4.90 μm in the MWIR band. With a voltage ranging from 0 to 20 V, the reflection band was broadened and extended to cover a wavelength range of 2500–4200 nm, obtaining a bandwidth of nearly 2000 nm.
A wavelength shift of 141 nm was realized by electric control on a partly polymerized chiral LC [45]. The LC filter consisted of a mixture of photopolymerizable LC, non-reactive nematic LC MDA, and a chiral dopant. Upon applying an electric field, the ordinary refractive index of the LC contributed, and the refractive indices of the mixture decreased. Since the pitch was maintained by the polymer template, the photonic band edges both programmed a blue shift. The filter featured high reflectivity over the reflection band and possessed a switching-on time of 50 µs and a switching-off time of 20 µs.
Polymer-stabilized CLCs with negative dielectric anisotropy featuring a large magnitude (exceeding 1500 nm) and invertible tuning under a DC field were reported [46]. The optical response and relaxation behavior upon the application of a DC electric field were strongly related to the viscoelastic properties of the polymer network. The nonlinear distortion of the pitch and electromechanical response of the polymer network contributed to the reflection variation.
Dynamic control of the reflection band in the monodomain polymer-stabilized BPLCs upon an electric stimulus was demonstrated [47]. Compared with achiral monomers, chiral monomers contributed to the stability of the double-twist cylinders due to the intrinsic chiral structure. The internal structure of the mixture could be well maintained even under a strong electric field since the polymer network was formed in both disclination lines and bulk LCs. The spectral range reached ~216 nm by adjusting the chiral monomer and LC concentrations in the mixture. Via the application of an electric field, the total reflection tuning range could be 241 nm, and the switching time was <350 ms.
A new electro–optic phenomenon that a single selective reflection splits into two distinct reflections under an electric field was observed in the CLC phase [48]. The distinctive phenomena originated from the introduction of side-chain liquid crystalline mesogens and were dependent on the compositions of the mixtures. The distinguished control on two CLC elements was demonstrated. The CLCs showed functional opportunities in several devices, such as spectrum-variant polarization filters.
The electric field could change the refractive indices of the LCs with dielectric anisotropy by tilting the LC molecules, resulting in the shift of the reflection band. The variation in the reflection band induced by the electric field included reflection-band splitting, red shift, and left shift. The value, direction, frequency, and mode of the electric field might have an influence on filter performance.

2.4. Light Control

Light control is a preferred external stimulus for LCs due to the advantages of remote, temporal, and spatial manipulation [49][50]. In such LC systems, photoresponsive chiral switches, mainly composed of chiral centers and photoswitches, are generally doped into nematic LC materials [50]. Photoswitches undergo configurational changes upon photoisomerization, such as the reversible trans-cis isomerization of azobenzenes and menthone derivatives [51][52]. The isomerization leads to the variation in the helical twisting power, the pitch length, and, consequently, the selective reflection of the LCs [53].
The properties of photoresponsive self-assembled chiral–azobenzene-doped BPLCs were investigated [54]. During the cooling process, surface alignment could contribute to the induction of more uniform and diverse BP structures, containing BP II, BP I and BPS-like phases. The photoisomerizations of trans→cis and cis→trans occurred when chiral–azobenzene-doped dopants were irradiated by light with wavelengths of 405 and 450 nm, respectively. The photoisomerization process contributed to isothermal phase transition and reflection-band shifting, improving the possibility of all-optical control on BPLCs.
Electric and photo control on an oblique helicoidal cholesteric (ChOH) LC doped with an azoxybenzene derivative were investigated [55]. Light irradiation caused trans-cis photoisomerization of azoxybenzene dopants, leading to variations in the pitch and diffractive properties of ChOH. The maximum central wavelength shift of the Bragg reflection reached about 235 nm by the photoisomerization of 5 wt% achiral molecules, indicating the ultra-sensitivity of ChOH to light irradiation.
Azobenzenes and menthone derivatives doped LCs underwent isomerization with light control, resulting in changes in the twist structure, pitch, and birefringence. Photo-induced manipulation had the advantages of remote control, fast response time, and ease of addressability. The factors affecting filter performance contained the light irradiation time, the wavelength of the light stimuli, and the intensity of the light irradiation.

This entry is adapted from the peer-reviewed paper 10.3390/polym14224898

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