Special Issue "Liquid Crystal THz Photonics: Materials, Devices and Applications"

A special issue of Applied Sciences (ISSN 2076-3417). This special issue belongs to the section "Optics and Lasers".

Deadline for manuscript submissions: closed (31 October 2018)

Special Issue Editor

Guest Editor
Prof. Dr. Ci-Ling Pan

Department of Physics, National Tsing Hua University, Hsinchu 30013, Taiwan
Website | E-Mail
Interests: ultrafast, liquid crystal and THz photonics; laser science and applications

Special Issue Information

Dear Colleagues,

Terahertz (THz) science and technology have advanced significantly over the last three decades. Applications are abundant in topics such as material characterization, data communication, biomedicine, 3D imaging, and environmental surveillance. These developments were hampered somewhat as crucial quasi-optic components, such as phase shifters, modulators, attenuators, polarizers, waveguides, beam splitters and deflectors in the THz range, are still relatively underdeveloped.

Recently, a number of tunable THz devices employing liquid crystals (LCs) have attracted considerable attention. Partly, the unexpected large birefringence of liquid crystals and low attenuation in the THz frequency range is responsible for the realization of such advances. Specially designed liquid crystals with the above-mentioned desirable characteristics have been reported lately. The response time of liquid crystal THz photonic devices typically much slower than similar devices for the visible. Continuing research development is essential.

In this Special Issue, recent advances in the field of liquid crystal THz photonics are presented. Topics include the optical properties of liquid crystals in the THz frequency range, design and synthesis of liquid crystals for THz applications, THz liquid crystal devices with liquid crystal enabled functionalities and their applications. Novel approaches with combination of liquid crystals and engineered materials such as metamaterials and photonic crystals are also covered.

Prof. Dr. Ci-Ling Pan
Guest Editor

Manuscript Submission Information

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Keywords

  • liquid crystal
  • nematic
  • isotropic
  • lyotropic
  • ferroelectric
  • birefringence
  • absorption
  • terahertz
  • far-infrared
  • millimeter wave
  • optics
  • phase shifter
  • photonics
  • optical and photonic devices (polarizer, grating, beam deflector, filter, spatial light modulator)

Published Papers (9 papers)

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Research

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Open AccessFeature PaperArticle Twisted Nematic Liquid-Crystal-Based Terahertz Phase Shifter using Pristine PEDOT: PSS Transparent Conducting Electrodes
Appl. Sci. 2019, 9(4), 761; https://doi.org/10.3390/app9040761
Received: 16 January 2019 / Revised: 10 February 2019 / Accepted: 18 February 2019 / Published: 21 February 2019
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Abstract
For this study, we demonstrated three different types of twisted nematic (TN) liquid crystal (LC) terahertz (THz) phase shifters using pristine poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) thin films as transparent conducting electrodes (TCEs). The transmittance of spin-coated pristine PEDOT: PSS thin film was as high [...] Read more.
For this study, we demonstrated three different types of twisted nematic (TN) liquid crystal (LC) terahertz (THz) phase shifters using pristine poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT: PSS) thin films as transparent conducting electrodes (TCEs). The transmittance of spin-coated pristine PEDOT: PSS thin film was as high as 92% in the frequency range of 0.2–1.2 THz. This is among the highest reported. Several TN-LC cells were constructed in a comparative study, which confirmed the reliability of pristine PEDOT: PSS as a TCE layer for THz phase shifter applications. The highest phase shift, required root-mean-square (RMS) driving voltage, and threshold voltage achieved by devices tested were 95.2° at 1 THz, 7.2 VRMS, and 0.5 VRMS, respectively. The thickness of the LC layer for the phase shifter was 250 µm, approximately half as thick as previous designs. In addition, the pristine PEDOT: PSS-based TN-LC phase shifter exhibited a figure-of-merit (FOM) value of approximately 6.65 degree·dB−1·V−1. This compared favorably with previously reported homogeneously aligned phase shifters with an FOM of 2.19 degree·dB−1·V−1. Our results indicated that a twisted nematic LC cell with pristine PEDOT: PSS thin films as electrodes is a good combination for a THz phase shifter and wave plates as well as other LC-based THz devices. Full article
(This article belongs to the Special Issue Liquid Crystal THz Photonics: Materials, Devices and Applications)
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Open AccessArticle High-Transmittance 2π Electrically Tunable Terahertz Phase Shifter with CMOS-Compatible Driving Voltage Enabled by Liquid Crystals
Appl. Sci. 2019, 9(2), 271; https://doi.org/10.3390/app9020271
Received: 13 November 2018 / Revised: 23 December 2018 / Accepted: 8 January 2019 / Published: 14 January 2019
Cited by 1 | PDF Full-text (2326 KB) | HTML Full-text | XML Full-text
Abstract
We have investigated tunable terahertz (THz) phase shifters that are based on a sandwiched liquid crystal (LC) cell with indium–tin–oxide (ITO) nanowhiskers (NWhs) as transparent electrodes. More than 360° of phase shift at 1.0 THz was achieved at a driving voltage as low [...] Read more.
We have investigated tunable terahertz (THz) phase shifters that are based on a sandwiched liquid crystal (LC) cell with indium–tin–oxide (ITO) nanowhiskers (NWhs) as transparent electrodes. More than 360° of phase shift at 1.0 THz was achieved at a driving voltage as low as ~2.6 V (rms). This is approximately 40 times smaller than that reported in previous works using an electrically tuned LC device. Significance of the NWhs in reducing the required voltage is demonstrated. Overall transmittance of the device is as high as 30%, which is accountable by absorption losses of ITO NWhs, quartz substrate and LC. Experimental results are in good agreement with a theoretical formulism while taking into account super-thick LC cells (~1 mm) and pretilt angles. We also propose and demonstrate a novel THz technique for measuring pretilt angles of liquid crystals. Full article
(This article belongs to the Special Issue Liquid Crystal THz Photonics: Materials, Devices and Applications)
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Open AccessArticle Visible Measurement of Terahertz Power Based on Capsulized Cholesteric Liquid Crystal Film
Appl. Sci. 2018, 8(12), 2580; https://doi.org/10.3390/app8122580
Received: 27 October 2018 / Revised: 4 December 2018 / Accepted: 10 December 2018 / Published: 12 December 2018
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Abstract
We demonstrate a new method to detect terahertz (THz) power using a temperature-supersensitive capsulized cholesteric liquid crystal film based on the thermochromic and thermodiffusion effect, which is clearly observed. A quantitative visualization of the THz intensity up to 4.0 × 103 mW/cm [...] Read more.
We demonstrate a new method to detect terahertz (THz) power using a temperature-supersensitive capsulized cholesteric liquid crystal film based on the thermochromic and thermodiffusion effect, which is clearly observed. A quantitative visualization of the THz intensity up to 4.0 × 103 mW/cm2 is presented. The diameter of the color change area is linearly dependent on the THz radiation power above 0.07 mW in the steady state. Moreover, the THz power can be detected for 1 sec of radiation with a parabolic relation to the color change area. The THz power meter is robust, cost-effective, portable, and even flexible, and can be used in applications such as THz imaging, biological sensing, and inspection. Full article
(This article belongs to the Special Issue Liquid Crystal THz Photonics: Materials, Devices and Applications)
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Open AccessArticle Tunable Liquid Crystal Based Phase Shifter with a Slot Unit Cell for Reconfigurable Reflectarrays in F-Band
Appl. Sci. 2018, 8(12), 2528; https://doi.org/10.3390/app8122528
Received: 23 October 2018 / Revised: 5 December 2018 / Accepted: 5 December 2018 / Published: 6 December 2018
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Abstract
An electrically tuned phase shifter based on the single slot unit cell and liquid crystal for a reconfigurable reflectarray antenna was presented. The simulation and measured results obtained at the F-band were used to demonstrate that the dielectric properties of a nematic state [...] Read more.
An electrically tuned phase shifter based on the single slot unit cell and liquid crystal for a reconfigurable reflectarray antenna was presented. The simulation and measured results obtained at the F-band were used to demonstrate that the dielectric properties of a nematic state liquid crystal could be exploited to realize a slot unit cell phase shifter for the reconfigurable reflectarrays antennas. By reducing the inhomogeneous of the external electric field in the liquid crystal layer, a lower control voltage and improved model accuracy was obtained. In the experiments test, the achieved saturation bias voltage (10 V) was smaller than the previously reported structure, and the phase shift of the reflectarray greater than 300° was achieved in the frequency range 121.5–126 GHz. The maximum phase shift of 306° occurred at 124.5 GHz. Full article
(This article belongs to the Special Issue Liquid Crystal THz Photonics: Materials, Devices and Applications)
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Open AccessArticle Electrically Tunable Propagation Properties of the Liquid Crystal-Filled Terahertz Fiber
Appl. Sci. 2018, 8(12), 2487; https://doi.org/10.3390/app8122487
Received: 29 October 2018 / Revised: 20 November 2018 / Accepted: 20 November 2018 / Published: 4 December 2018
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Abstract
A bandgap-guiding microstructured fiber for terahertz (THz) radiation was designed by infiltrating the cladding air holes with nematic liquid crystal. Structural parameter dependence of the photonic bandgaps, polarization-dependent bandgap splitting, and electrically tunable propagation properties of the designed fiber were investigated theoretically by [...] Read more.
A bandgap-guiding microstructured fiber for terahertz (THz) radiation was designed by infiltrating the cladding air holes with nematic liquid crystal. Structural parameter dependence of the photonic bandgaps, polarization-dependent bandgap splitting, and electrically tunable propagation properties of the designed fiber were investigated theoretically by using the finite-element method. An external electric field applied across the designed fiber can broaden the effective transmission bandwidth and achieve single-mode single-polarization guidance. Flattened near-zero group-velocity dispersion of 0 ± 1 ps/THz/cm was obtained for the y-polarized fundamental mode within a broad frequency range. Our results provide theoretical references for applications of liquid crystal-filled microstructured fiber for dynamic polarization control and tunable fiber devices in THz frequency. Full article
(This article belongs to the Special Issue Liquid Crystal THz Photonics: Materials, Devices and Applications)
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Open AccessArticle Electrically Tunable Hydrogen-Bonded Liquid Crystal Phase Control Device
Appl. Sci. 2018, 8(12), 2478; https://doi.org/10.3390/app8122478
Received: 30 October 2018 / Revised: 23 November 2018 / Accepted: 25 November 2018 / Published: 3 December 2018
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Abstract
Terahertz waves have attracted much attention mainly because of their potential in imaging, security checking, nondestructive testing, and information and communication technologies. In the past few years, there has been an extensive effort to investigate terahertz wave control devices. Liquid crystal (LC) devices [...] Read more.
Terahertz waves have attracted much attention mainly because of their potential in imaging, security checking, nondestructive testing, and information and communication technologies. In the past few years, there has been an extensive effort to investigate terahertz wave control devices. Liquid crystal (LC) devices are strong candidates for high-performance terahertz wave control devices because of their controllability at low drive voltages and their low power consumption. In this study, we fabricated an electrically tunable phase control device by using a hydrogen-bonded LC material. We investigated the performance of the LC phase shifter by using a far infrared continuous wave laser. We also estimated the birefringence and absorption properties of the hydrogen-bonded LC at 2.5 THz by using Jones matrix calculations. The measurements and calculation results indicated that the hydrogen-bonded LC showed no dichroism at 2.5 THz. Based on the absorption properties, we believe that it could be a strong candidate for use in future terahertz devices. Full article
(This article belongs to the Special Issue Liquid Crystal THz Photonics: Materials, Devices and Applications)
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Open AccessArticle Fast-Tunable Terahertz Metamaterial Absorber Based on Polymer Network Liquid Crystal
Appl. Sci. 2018, 8(12), 2454; https://doi.org/10.3390/app8122454
Received: 18 October 2018 / Revised: 21 November 2018 / Accepted: 21 November 2018 / Published: 1 December 2018
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Abstract
This paper introduces a tunable metamaterial absorber (MA) based on polymer network liquid crystal (PNLC) in the terahertz (THz) frequency band. Under the optimal polymerization condition, through electrical control of the orientation of the PNLC embedded in the frequency selective surface, the resonant [...] Read more.
This paper introduces a tunable metamaterial absorber (MA) based on polymer network liquid crystal (PNLC) in the terahertz (THz) frequency band. Under the optimal polymerization condition, through electrical control of the orientation of the PNLC embedded in the frequency selective surface, the resonant frequency of the absorber can be tuned from 416.5 to 405.0 GHz, corresponding to fractional frequency bandwidth of 2.8%. The experimental results show that the proposed MA based on the PNLC offers an adjustment time of 10 ms and recovery time of 85 ms, which is significantly faster than the tunable metamaterial devices based on conventional nematic liquid crystal (LC). Full article
(This article belongs to the Special Issue Liquid Crystal THz Photonics: Materials, Devices and Applications)
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Open AccessCommunication Liquid Crystal Tunable Dielectric Metamaterial Absorber in the Terahertz Range
Appl. Sci. 2018, 8(11), 2211; https://doi.org/10.3390/app8112211
Received: 30 October 2018 / Revised: 7 November 2018 / Accepted: 8 November 2018 / Published: 10 November 2018
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Abstract
In this paper, we propose a tunable dielectric metamaterial absorber in the terahertz (THz) range. The absorber is composed of a silicon pillar array embedded in a liquid crystal (LC) layer, which is sandwiched by two graphene electrodes. By way of varying the [...] Read more.
In this paper, we propose a tunable dielectric metamaterial absorber in the terahertz (THz) range. The absorber is composed of a silicon pillar array embedded in a liquid crystal (LC) layer, which is sandwiched by two graphene electrodes. By way of varying the applied bias, the LC orientation can be continuously tuned. At a saturated bias, all LCs are vertically driven, and an absorption peak of 0.86 is achieved at 0.79 THz. When the bias is turned off, the same LCs are horizontally aligned, and the absorption peak degenerates into two smaller ones. A 47% modulation depth at 0.79 THz is obtained via numerical simulation with experimental feasibility considered. Such an active THz dielectric absorber may be utilized as part of various active THz apparatuses in THz imaging, sensing, switching, and filtering. Full article
(This article belongs to the Special Issue Liquid Crystal THz Photonics: Materials, Devices and Applications)
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Review

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Open AccessReview Potential of Liquid-Crystal Materials for Millimeter-Wave Application
Appl. Sci. 2018, 8(12), 2544; https://doi.org/10.3390/app8122544
Received: 31 October 2018 / Revised: 28 November 2018 / Accepted: 3 December 2018 / Published: 8 December 2018
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Abstract
In this study, we reviewed three topics regarding the application of liquid-crystal (LC) materials to millimeter-wave (MMW) devices. It is essential to develop useful measurement methods for refractive indices of LC materials in the MMW region. Herein, a novel measurement method using optical [...] Read more.
In this study, we reviewed three topics regarding the application of liquid-crystal (LC) materials to millimeter-wave (MMW) devices. It is essential to develop useful measurement methods for refractive indices of LC materials in the MMW region. Herein, a novel measurement method using optical short is demonstrated using a Si semiconductor substrate. There are two approaches to develop MMW LC devices. One is the quasi-optical approach, which involves scaling up the optical components, and the other approach involves integrating the LC materials into high-frequency electric circuits. A three-dimensional (3D) printer is used to fabricate the Fresnel lens, which is a typical quasi-optical device useful in the MMW region, where we can develop the tunable lens by introducing LC materials. A planar-type MMW waveguide is advantageous for integrating the LC materials to develop LC MMW devices using the second approach. We investigated a useful microstrip-line-type LC phase shifter by developing a novel conversion circuit to introduce the LC material onto the dielectric substrate surface. A phase shifter is an important MMW component that is used to attain a phased array antenna system, and a minimal twin antenna array is demonstrated using the microstrip-line-type LC phase shifters. Full article
(This article belongs to the Special Issue Liquid Crystal THz Photonics: Materials, Devices and Applications)
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