Liquid Crystal-Tuned Planar Optics in Terahertz Range: History
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Terahertz waves of higher frequencies compared to microwave and radio frequency have shown great potential in radar detection and high-speed wireless communication. To spatially control the wavefront of terahertz beams, various novel components, such as terahertz filters, polarization converters and lenses, have been investigated. Metamaterials and metasurfaces have become the most promising technique for the free manipulation of terahertz waves. Metadevices integrated with liquid crystals have been widely used in active terahertz devices.

  • liquid crystal
  • metamaterial
  • tunable
  • terahertz

1. Introduction

The explosive expansion of data traffic in wireless communication systems prompts the use of spectra with higher carrier frequencies to increase the bandwidth and provide larger capacity. The terahertz (THz) spectrum, between microwave and infrared waves (frequency range 0.1 to 10 THz), theoretically supports a communication capacity of terabits per second (Tbps) [1]. THz is considered the most promising technology for next-generation wireless communication, and is known as “the last piece of the RF spectrum puzzle for communication systems” [2]. THz waves can penetrate most nonmetallic and nonpolar materials, such as cloth, paper, and wood, making them very suitable for nondestructive detection and security checks [3][4]. Currently, one of the biggest obstacles to THz communication is the absorption of water vapor in the atmosphere which makes it difficult to transmit high fidelity over long distances [5]. Therefore, it is necessary to develop highly efficient and compact passive and active devices, such as focusing [6], deflecting [7] and filtering [8] devices, to control the propagation of THz beams in free space.
Compared with electrical or optical counterparts, THz devices are far from mature. THz exceeds the cutoff frequencies of many semiconductor amplifiers and mixers, making them incompatible with existing RF components [9]. High-speed amplitude and phase coding based on photonics provide an opportunity for high-efficiency THz modulations [10][11]. Early refracted or diffracted THz devices are made of polymers or crystals, and their bulky sizes and fixed functions limit the miniaturization, integration, and versatility of THz apparatuses. Recently, metamaterials and metasurfaces based on subwavelength artificial electromagnetic microstructures have become excellent means for THz wavefront shaping [12][13][14]. Research on metamaterials began in 1968, when Veselago first proposed a medium with negative permeability [15]. In 1996, negative effective permeability was experimentally demonstrated by significantly enhanced magnetic resonance using artificial structures with periodic split ring resonators (SRRs) [16]. Early metamaterials were used as frequency selective filters [17]. With the development of micro/nano processing and electromagnetic simulations, research on artificial electromagnetic microstructures has been extended to the THz band [18]. By properly designing the unit structures, various filters [19], polarization converters [20], lenses [21] and other components have been achieved. The modulation of electromagnetic waves by metamaterials mainly depends on the surface plasmon resonance (SPR) of the metallic microstructure, which is usually narrowband and has high backscattering loss and ohmic loss, especially in the THz band [22]. To improve efficiency, all-dielectric metasurfaces [23] and multilayer metal metamaterials [24] have been adopted.

2. Properties of LCs and LC-Based THz Devices

Known for their dominant role in information displays, LCs, featuring both fluidity and anisotropy, can also be adopted for dynamic THz wave manipulations. The birefringence, Δn, is a key parameter of the LC which determines the function of wavefront regulation. For instance, LC THz modulators work on accumulated phase modulations, which are related to the birefringence and thickness of LCs. Thick cells suffer from slow responses and poor alignments; thus, large birefringence is in high demand. The properties of 5CB at 0.3–1.4 THz were measured 20 years ago, with a Δn of 0.21 [25]. E7 exhibits a birefringence range of 0.130–0.148 at 0.2–2.0 THz [26]. In 2012, the birefringence of 18-series LCs was tested, and that of 1825 reached 0.36 at 0.7–3 THz. Moreover, the absorption of these LCs was negligible in the experiments [27][28]. In 2012, NJU-LDn-4 was synthesized, exhibited an average birefringence of 0.306 in 0.4–1.6 THz and kept the nematic phase in a large temperature range covering room temperature [29]. The nanoparticles doped E7 LC leads to a 10% increase in birefringence in 0.3–3 THz [30].
As a typical element, the filter plays an important role in the terahertz band. The birefringence of LC is adopted for terahertz filters. Pan’s group produced Lyot filters based on the combination of a magnetic field-controlled parallel-oriented LC cell exhibiting a fixed phase delay, and vertically oriented LC cells with an adjustable phase delay. Through changing the direction of the magnetic field, a modulation range of 40% at 0.388–0.564 THz was demonstrated [31]. Later, magnetically tuned Solc filters were made using two cells with vertically oriented LCs. The modulation bandwidths were 0.176–0.293 THz and 0.474–0.794 THz, respectively. The corresponding modulation ranges were 66.5% and 67.5%, separately [32]. A 60 GHz band stop filter at approximately 0.3 THz, was realized by electrically controlling multiple LC layers [33]. A notch filter with a bandwidth of 0.35–0.7 THz was realized through electrically switching a single-layer LC cell [34]. THz filters based on LC-filled photonic crystals were theoretically investigated as well [35]. In these early works, almost all of them suffered from thick LC layers slowing down the response and increasing the absorption loss. The stacking of multiple LC cells reduces the thickness of a single cell but induces a more complicated configuration and extra interface loss.
The birefringence of the LC is also adopted for phase shifters. Through electrically driven nematic LC [36][37], polymer-stabilized LC [38] and hydrogen-bonded LC [39], different THz phase shifters were realized. THz LC wave plates were demonstrated in a similar way. A broadband tunable THz wave plate made of NJU-LDn-4 was reported [40]. In this element, metal wire grids and a few layers of graphene were used as conductive electrodes. Due to the large birefringence of NJU-LDn-4 in the THz band, a broadband quarter wave plate and a half wave plate were produced. By further introducing a double-layer structure, the bandwidth can be further expanded to 0.5–2.5 THz for a quarter wave plate. Compared to the transmitted THz LC wave plates, the reflective ones require only 50% LC to achieve the same modulation, which accelerates the modulation and reduces the insertion loss [41]. Chang’s group used a 600 μm dual-frequency LC (DFLC) to form a cell whose birefringence is frequency dependent. Via tuning the frequency from 1 kHz to 100 kHz under a constant electric field, a tunable quarter wave plate above 0.68 THz and a half wave plate above 1.33 THz were realized [42].
Besides the above devices, LCs have been utilized in abundant THz applications. The THz beam was deflected by an electrically controlled LC wedge cell [43]. By injecting E7 into the groove of a silicon grating, the intensity ratio between the 0th order and 1st order was magnetically tuned from 4:1 to 1:2 at 0.3 THz [44]. The twisted nematic LC cell accomplished the linear polarization conversion of the incident THz beam, and applying an electric field further adjusted the polarization [45]. Polarization-dependent imaging was achieved by inserting a uniformly oriented LC polymer film with different orientations compared to the orthogonal polarizers. The output intensity was varied by adjusting the orientations, which can be used as attenuators or switches [46]. In previous works, the LC alignments were either controlled by a strong magnetic/electric field or by mechanical rubbing. Both of them were restricted to homogeneous alignment. The appearance of photoalignment [47] makes the micropatterning of LC achievable.

3. LC-Integrated Plasmonic Metadevices

Plasmonic metamaterials consist of artificial metallic micro/nanostructures. By tailoring the structures and shapes of metamaterial units, functions not available with natural materials, such as [48], invisible cloaks [49], and perfect absorption [50], can be achieved. To realize dynamic or switchable functions, metamaterials are usually combined with semiconductors [51], graphene [52], vanadium dioxide [53], and other functional materials. LCs have attracted special attention due to their mature and cost-efficient fabrication [54]. However, they still suffer from slow operating speed as well as limited modulation depth, and there is still a long way toward practical active metadevices.
In recent years, a lot of THz absorbers, phase shifters and polarization converters based on LC-integrated metamaterials have been presented. Because of their applications in sensing and imaging fields, THz absorbers have been widely concerned [55][56][57]. In 2011, a tunable bandpass filter was theoretically designed, which uses a woodpile metallic photonic crystal as a resonator and electrode and LCs as a defect layer to fill the photonic crystal [58]. In 2013, Shrekenhamer et al. demonstrated LCs embedded into metamaterials to form metal–LC–metal absorbers, and 30% amplitude modulation of absorption at 2.62 THz and over 4% bandwidth adjustment were presented [59]. By combining different geometries of plasmonic resonators, a multiband tunable metamaterial absorber can be realized [60]. A few layers of porous graphene are integrated into the metamaterial to apply a uniform electric field to LCs and achieved an amplitude modulation of ~80% at a voltage of 10 V [61]. In addition to metamaterial absorbers, modulators are also used to modulate THz wave amplitudes. Different THz modulators based on LC-integrated metamaterials have been reported. However, most of these devices operate in reflection mode, limiting their applications in THz systems. Yang et al. reported a metadevice that worked in transmission, with high modulation depth and low insertion loss, by integrating two layers of metamaterials with LCs [62]. In 2018, an LC-integrated metadevice operating in transflective mode was reported. The comb electrodes exhibited polarization selectivity and realized a dynamic electromagnetically induced transparency analog (EIT) in transmission mode, and a dynamic absorber in reflection mode [63].
For LC-only THz devices, the required thickness of the LC layer is on the submillimeter scale, inducing a high driving voltage and slow response. The integration of metamaterials with LCs can significantly improve the response while maintaining a large phase shift. Buchnev et al. achieved an amplitude modulation of 20% and a phase modulation of 40° by hybridizing 12 μm thick LCs with metasurfaces [64]. In 2019, Sasaki et al. designed a polarization converter using an LC/metal grid configuration, reducing the LC thickness by two orders of magnitude compared to pure LC-based devices [65]. Benefiting from the wide-band characteristics of the metal gratings, broadband linear polarization generation can be achieved by mixing LCs as a refractive index variable environmental medium to actively tune the Fabry-Perot-like resonance in two orthogonally arranged metal gratings [66].
Dynamic THz beam steering has great potential in high-speed wireless communication, high-resolution imaging, and radar. Traditional radar relies on mechanical scanning for continuous beam steering to capture the target [67]. Phased arrays are considered good candidates for beam steering in the microwave band [68]. However, in the THz band, the high loss of the semiconductor switch restricts the phase shift. In 2014, Cui et al. proposed a programmable metasurface with the phase of each pixel unit switchable between 0 and π. Therefore, digitized dynamic manipulation of electromagnetic waves was realized by programming the coding sequences [69]. In 2020, Wu et al. designed a programmable LC-integrated metasurface by coding the “0/1” phase to dynamically control beam steering, and the maximum deflection angle reached 32° [70]. Buchnev et al. adopted the LC sandwiched by S-type metamaterials to demonstrate an efficient and ultrathin spatial phase modulator [71]. By applying a gradient voltage, continuous phase retardation and high-resolution spatial phase control were achieved. To extend the range of phase modulation, Cui et al. used two layers of asymmetric metamaterials to spatially manipulate the transmitted THz beam, and demonstrated double-layer beam steering with a maximum deflection angle of 30° [72]. The above designed programmable metasurfaces are all based on binary phase coding. To suppress unnecessary high-order diffraction, Xu et al. proposed a multibit coding scheme based on resonant switching and achieved maximum single-beam scanning of ±21° with significantly improved efficiency [73]. STMs can spatially address THz amplitudes by binary coding each pixel as well. In 2014, Savo et al. designed a STM with 8 × 8 pixels using metamaterial absorbers. Each pixel was independently controlled by external FPGAs, and the average modulation depth of the reflectivity reached 75% [74]. Recently, Hu et al. designed a transflective STM by integrating LCs with a Fano-resonant metasurface and pixelated interdigital electrodes. The modulation depth of each pixel reached 38.8% and 61.1% for the transmissive and reflective modes, respectively [75]. STMs also play important roles in THz imaging.

4. Tunable Dielectric Metasurfaces Based on LCs

For metallic metamaterials, a phase change from 0 to 2π cannot be achieved by changing the resonant frequency. Moreover, the low forward-scattering discounts the efficiency of wavefront modulation. Therefore, researchers introduced the concept of a metasurface into the THz band and designed a THz dielectric metasurface. Similar to glass in visible light, high-impedance silicon is transparent in the THz band. Additionally, silicon is perfectly compatible with existing nanofabrication technology, making it an ideal candidate for THz planar optics. By coding the phase of the THz wavefront, functions such as focusing, deflection and vortex beam generation can be realized. By appropriately designing the unit structure of dielectric metasurfaces, a subwavelength phase gradient can be realized in the two-dimensional plane. The phase introduced by dielectric metasurfaces is generally based on two different principles. One is the resonance phase related to the structural parameters of the unit. Dielectric meta-atoms interact with incident THz waves to excite Mie resonance and add a certain phase to the incident wave [76][77].
In the THz band, silicon-based metasurfaces are often fabricated by photolithography and deep reactive ion etching (DRIE). The manufacture is matured and available for mass production. As the function of the dielectric metasurface is strictly bound to the structure, it is totally fixed once the structure is fabricated. Methods for the realization of a tunable THz dielectric metasurface have become an important research direction. To introduce tunability to the THz dielectric metasurface, LCs are integrated. The tunability is divided into two species. One uses LCs as a birefringent medium to provide a tunable anisotropic refractive index. It allows LCs to be used as a tunable wave plate or a special environmental medium. In 2018, Hu et al. combined large birefringent NJU-LDn-4 NLCs in the THz band, with a circular silicon column array metasurface to fabricate a THz absorber, with an adjustable power absorption rate and resonant peak position and which has a 47% modulation depth at 0.79 THz. In 2020, Chang et al. made use of the controllable anisotropy of DFLCs. They combined DFLCs with a silicon column array metasurface to fabricate a wide-band THz modulator that can manually control its achromaticity and birefringence. This device can be turned from OFF state (no polarization conversion effect) to a controllable waveplate, exhibiting over 97% polarization conversion ratio (PCR) in 0.97–1.3 THz as a half-wave plate, or over 96% PCR in 0.67–1.3 THz as a quarter-wave plate [78]. In addition, they used polymer-dispersed liquid crystals (PDLCs) as the birefringent medium in another work. Driven by an electric field, the orientations of LCs rotate in the droplets of PDLCs, which in turn causes anisotropy variation and a reduction in dispersion in a wider bandwidth compared to the original silicon gradient grating metasurface [79]. In the same year, Hu et al. designed a linear polarization-multiplexing dielectric bifocal metalens, in which two orthogonal linear polarization incident lights have two different foci. They introduced LCs to select the outgoing light of different polarizations to achieve the bifocal function [80]. In 2021, Hu et al. used LCs as the medium of silicon metalens to control the environmental refractive index, thus changing the focal length of the metalens between 8.3 mm and 10.5 mm [81]
The other way is to adopt LCs to supply an extra PB phase modulation. Moreover, the phase modulation can be temporarily erased by an external field. This gives the dielectric THz metasurface another method for tunability. In 2020, Hu et al. combined the PB phase of LC with the resonant phase of the silicon metalens, enabling a switch between an achromatic lens and a large chromatic lens. This introduced a new approach for the combination of LCs and metasurfaces in the THz band, which has great potential in THz imaging and communications [82]. By orienting the LCs into a PB phase grating pattern, an electrically adjustable focus separation between incident LCP waves and RCP waves for an off-axis bifocal THz metalens was achieved [80]. In 2022, Chang et al. fabricated a silicon-based PB phase metasurface and deflected incident light with different chirality in different directions, which realizes a dynamic modulation depth of >94% and maximum efficiency of over 50% for the different spin states. At the same time, the PB phase of the LCs is used to change the deflection direction of light under the same chirality incident lightd, and a magnetic field is introduced to control the orientation of the LCs, making subsequent communication applications possible.

This entry is adapted from the peer-reviewed paper 10.3390/app13031428


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