Liquid crystals (LCs) are well known as excellent electro-optic materials, and their potential is sufficiently proven when we see the industrial success of the flat-panel display application. An LC device exhibits compactness, lightweight, low power consumption, and low driving voltage for all application fields in the optics and photonics [1
]. Furthermore, some optical control devices, such as variable focusing lenses, beam-steering prisms, and polarization control devices, gained attention in conjunction with conventional LC device applications, such as spatial light modulators and phase shifters [12
]. Recently, it became evident that LC materials have a large birefringence in the wider frequency range of the electromagnetic wave spectra, such as microwave, millimeter wave (MMW), and THz regions [19
The novel frequency regions, which exist between the radio frequency and optics region, were forbidden for a long time because of the technological difficulties, i.e., even the very basic technologies, such as signal generation and detection, were insufficient for consumer applications. However, technological innovation occurred in the THz-sensing technology field upon using the femtosecond laser and photoconductive switch, and the unexplored frequency region including the MMW region gained attention from the high expectation of new application fields.
Furthermore, the next-generation wireless communication standard (i.e., 5G) recently became a popular topic of information and communications technology (ICT). The 5G technology exhibits an extremely short delay time and massive connectivity in conjunction with high-speed communication potential; thus, many people expect that many novel technologies leading to business opportunities will appear beyond smart phones. However, the frequency resources available for wireless communication were successively endangered after mobile phones came into existence, because the number of wireless terminals and the communication traffic increased steeply upon every generation change in the wireless communication standard. It is difficult to achieve the extremely high performance of 5G within the current frequency resources; thus, unexplored higher-frequency regions (i.e., MMW) enter the picture to realize 5G for many novel consumer services.
MMW can propagate with narrow beams using a small antenna, bringing some advantages for energy saving and data security. However, some types of beam control device are mandatory for MMW usage, and low-cost and manageable devices for MMW propagation control are essential for the usage of MMW to become common in our daily life. We believe that LC materials are primary candidates to attain such devices if we reevaluate the technological success history of LC materials in optics. It is evident that LC materials have generally large birefringence in the MMW region as compared to that in the optics region, which is key for attaining variable LC devices working in a specific frequency region. However, molecules of all the commercially available LC materials are designed for display applications, and there is almost no information about their properties in the MMW region. It is essential to develop efficient measurement methods for LC materials to collect more information about excellent LC materials in the MMW region [23
A uniform LC molecular alignment must be maintained in the measurement to evaluate the anisotropy of the LC materials; thus, we investigated a measurement method using a rectangular waveguide combined with a magnetic field [25
]. However, the amount of required LC material is still large, even though the waveguide size becomes smaller in the MMW region. We tried introducing the planar cell structure by using a coplanar waveguide (CPW) to significantly reduce the amount of test sample required [27
]. In this case, another problem occurs; the inserted metal plate, which works as a moving electrical short, destroys the LC molecular alignment layer in one measurement. Firstly, a recent study to overcome this problem using a photo-induced electric short is reviewed. When a suitably strong light is irradiated on the intrinsic semiconductor wafer, the wafer becomes an excellent miller for THz waves due to the densely created photo carriers on the surface [28
]. We fabricated a CPW by using an intrinsic Si wafer as a dielectric material, and the diode laser light was irradiated on a part of the waveguide to create a dense photo-carrier domain that could work as a moving reflection boundary of the MMW without any mechanical contacts.
There are two approaches to develop MMW control devices using LC materials. One involves scaling up the LC optical devices according to the increase in wavelength; these are called quasi-optical devices [30
]. Although the device structure in this approach allows one to simply attain a specific functionality, an extremely thick LC layer is required because the wavelength of the MMW is at least one thousand times that of the optics region. A simple scale up of the LC layer causes fatal degradation of the response speed; thus, we tried introducing a polymer-dispersed LC (PDLC) structure, which consists of a large number of tiny integrated LC cells, by using porous polymethylmethacrylate (PMMA) materials [33
]. We investigated the MMW Fresnel lens, which is a typical quasi-optical device and can be tunable by introducing LC materials. In this case, some kind of machinery process is essential for attaining the Fresnel structure, and direct machining of the bulky porous PMMA material was demonstrated [35
]. If we use a Fresnel structural substrate, the spray deposition method of the porous PMMA is acceptable for attaining a bulky LC layer. In the next section, it is demonstrated that three-dimensional (3D) printers are precise enough to fabricate the Fresnel lens substrate, which becomes the base of the LC Fresnel lens working for MMW control.
Another approach to obtainan MMW LC device is the integration of LC materials into high-frequency electric circuits. The thickness of the LC layer can be significantly reduced if we choose a planar-type waveguide, because we can adopt the conventional sandwich cell structure to attain the LC devices. However, some extra parts must be integrated at the same time as the LC material to complete specific functionality, which leads to a complicated device structure in comparison with the quasi-optical approach. Many types of LC phase shifters, which are one of the important parts for integration circuits, were investigated by integrating the LC material into a suitable waveguide, such as a metal hollow waveguide [20
], microstrip line [39
], or coplanar waveguide [51
In this study, we investigated a planar-type LC phase shifter using a microstrip line. A microstrip line is a very common planar waveguide, which has a large technological background based on its long history. Additionally, we also integrated an LC driving circuit, power divider, frequency elimination filter, and antennas in conjunction with the LC phase shifter to attain specific functional devices, such as a phased array antenna. However, it is normally useless to integrate the LC materials on the microstrip line, because the MMW propagates inside the dielectric material of the substrate. We investigated a novel conversion circuit that can switch the propagation path of the MMW between the dielectric substrate and the upper LC layer to allow the introduction of the LC material onto the surface [47
]. In the last section, we describe the basic performance of a minimal twin array of LC phase shifters connected with a planar antenna.
2. Measurement of Complex Refractive Indices in Millimeter Wave
Most simple and common measurement methods of the refractive index in the MMW region involve introducing an LC material into any type of excellent waveguide and extracting the influence of LC materials by comparing the transmission properties with and without the test sample. The differential measurement of the transmission property caused by the successive small changes of the LC propagation length is a more precise measurement method called the cutback technique, which can eliminate the influence due to aspects other than the LC test material. We developed a measurement method for LC materials using a rectangular waveguide by inserting and moving a metal-tip electrical short, which can precisely change the propagation length of the LC material. A magnetic field was applied during the measurement to align the LC molecules uniformly, which is important for the LC measurement to evaluate its anisotropy [25
]. Although the size of the waveguide becomes small in the MMW region, the mm-size cavity is still extremely large, based on common knowledge of the LC display application. If we utilize aplanar-type waveguide, such as a CPW, the test sample volume can be greatly reduced because a thinner sandwich cell structure can be introduced, such as in an LC display [27
]. However, a metal plate, which works as a reflector of MMW and can change the LC length, must be in contact with the inner surface of the LC cell. The mechanical contact with the internal surface disrupts the LC molecular alignment layer after one measurement. In this section, we try utilizing an optical short, which can work as a moving reflector of the MMW without any mechanical contacts, where the CPW was fabricated using an intrinsic semiconductor, and the electrical short was created by photo-excited carriers. Experiments for the verification of the novel measurement method were performed using the Si substrate itself.
2.1. Principle of Measurement
shows an MMW propagation model on the CPW fabricated using an Si substrate. The incident MMW from the contact probe on the left-hand side propagates to the right, and is reflected at the right-side edge of the CPW. When a pulsed light (905 nm) from the laser diode (LD) is irradiated at a part of the CPW, a temporal electrical short is created, which works as a reflector. Then, a part of the incident MMW is reflected by the optical short. However, the reflectance of the optical short is very small in this stage; thus, a large amount of MMW transmits and is then reflected at the right-side edge. In this case, interference between the reflected MMWs from the optical short and that reflected from the right-side edge is observed. If we move the irradiation position laterally, the length of the propagation path difference between them can be changed. The intensity of the interference changes periodically according to the waveguide’s wavelength, which is influenced by the refractive index of the substrate and upper air. If we introduce an LC material into the CPW, the wavelength in the waveguide is modulated by the refractive index of the LC material. Furthermore, if we focus on the intensity peak, it gradually decreases with the increasing propagation length of the reflected MMW, which is related to the loss of the substrate and the upper introduced material. Here, a bare CPW fabricated using an Si substrate was used for the experiment to check the validity of this method. In this case, the refractive index and loss of the Si substrate could be evaluated, because the loss is negligible for the upper air, while its refractive index is unity.
shows a schematic diagram of our measurement system. A Gunn diode oscillator (40 GHz) was used as the MMW source and it was fed to the CPW by the contact probe through an isolator, attenuator, and circulator in turn. The reflected MMW from the CPW was guided to a diode detector through the isolator, and a temporal change in the interference intensity was observed using an oscilloscope. The synchronized intensity change referring to the LD pulse train was monitored using a lock-in amplifier.
2.2. Evaluation of Refractive Index and Absorption Coefficient
shows a typical measured time waveform appearing on the oscilloscope. The periodic dip waves were observed according to the LD light pulse irradiation and one of them is shown, where the LD pulse repetition is defined at 100 Hz by the external oscillator. The interference between the MMWs reflected at the optical short and at the right-sideend caused a dipping-waveform intensity response. It was observed that the interference disappeared about 50 μs after the LD pulse irradiation, which was determined by the diffusion and recombination phenomena of photo carriers because the laser pulse width was much shorter (100 ns) than the response time. The baseline of the response waveform shows the MMW intensity level reflected from the right-side edge of the CPW without the LD light irradiation, which is much larger than the dip depth of the time waveform. This situation is difficult to understand from Figure 3
, because the wave is observed under the alternating current (AC) coupling mode. The interference wave generally became smaller than the baseline level, and only the dipping wave form was observed. This was probably caused by some absorption phenomenon due to the optical short, which may work not only as a reflector, but also as an absorber of MMW.
The interference effect appeared at the dipping part in the time waveform; thus, we measured the depth change of the dipping wave as a function of the LD light irradiation position, as shown in Figure 4
. It was observed that the depth changed periodically according to the position, which is caused by the MMW propagation path length change between the interference MMWs. The period corresponds to the waveguide wavelength; thus, the refractive index of the waveguide material can be determined from the wavelength. The absorption coefficient can be determined from the decreasing rate of the dipping depth as a function of the propagation length, if we assume the simple Lambert–Beer law for the dip depth dependency.
The interference phenomena could be clearly observed only aroundthe shorter distance position from the contact probe, and it became difficult to observe when the propagation length of the reflected MMW by the optical short became longer. This was probably caused by the significant intensity difference between the interference waves, where the reflected MMW by the optical short was much smaller than that reflected from the right-side end. The reflectance of the optical short must be increased to improve the measurement accuracy by optimizing the CPW, wavelength of the LD, and so on.
presents the evaluation results of the refractive index and absorption coefficient of the Si substrate using the measurement data in Figure 4
. The waveguide wavelength can be directly measured from the periodicity in Figure 4
, and the effective refractive index was calculated in comparison with the vacuum wavelength. The permittivity was calculated from the refractive index, because the refractive index is simply related to relative permittivity in high-frequency regions like MMW. The obtained permittivity was almost the same as that with the mean value of an intrinsic Si semiconductor and air, because the propagating MMW experienced permittivity similarly for both substrate and air. On the other hand, the absorption coefficient was larger than the commonly known value for Si semiconductors. There is almost no absorption by air; thus, the absorption of the CPW comes from the Si substrate. Although the reason for the discrepancy is not clear at this stage, a precise interference model of the reflection MMWs should be introduced to determine the loss parameter instead of the simple calculation model used here. Although some challenges still remain for measuring the LC materials, since a cell structure must be introduced onto the Si substrate surface, the potential of the optical short working as a noncontact movable reflector was verified in the experiment.
In this study, we described some recent investigations intothe application of LC materials in the MMW frequency region. There is insufficient knowledge about specialized LC materials for MMW application; thus, the development of efficient measurement methods for LC materials is essential for exploring excellent materials in this frequency region. We successively investigated the issue using various types of waveguides, and a planar waveguide (CPW) was introduced to significantly decrease the size of the test sample. However, the metal plate inserted into the LC cell, which worked as a moving reflector, destroyed the alignment layer of the LC molecules in one measurement. To overcome this problem, an optical short was introduced using a semiconductor substrate for the microstrip line. An optical short, which can work as a reflection boundary of MMW, was confirmed by irradiating an intense laser pulse to create a high-density photo-carrier domain. The reflection boundary could easily be moved by changing the irradiation position of the laser light. The refractive index and absorption coefficient of the substrate material could be successfully evaluated from the periodicity and attenuation of the MMW peak intensity as a function of the laser light irradiation position.
There are two approaches to develop LC MMW devices. The first involves scaling up an LC optical device according to the increase in the wavelength; this is called aquasi-optical device. A Fresnel lens is a typical quasi-optical device used in the MMW region, and it becomes tunable if we introduce LC materials into the lens. The device structure is generally simple and easy to design following almost the same method used with optical devices. In this study, we demonstrated the basic performance of a Fresnel lens fabricated using a 3D printer. The filament material of the printer had relatively low loss properties in the MMW region, and the fabricated Fresnel lenses exhibited excellent focusing properties, even if some roughness was caused by the laminating process. The LC material was combined with a concave Fresnel lens substrate fabricated by the 3D printer to develop an LC Fresnel lens. A magnetic field was applied to control the LC molecular orientation, and the complementary focusing MMW intensity change was observed upon changing the magnetic field direction.
Another approach to develop LC MMW devices involves integrating LC materials with high-frequency electric circuits. If we adopt a planar wave guide, a thinner sandwich cell structure can be used for MMW LC devices. This is the biggest advantage in comparison with the quasi-optical approach. In this study, we demonstrated an LC phase shifter by combining LC materials with a microstrip line, which is a very common and excellent planar-type waveguide used to construct high-frequency integration circuits. We demonstrated the basic performance of the minimal twin antenna array system, which was attained upon combination with LC phase shifters. It was confirmed that the radiated MMW beam became narrower by the arrays, and the peak intensity tended to increase. A beam-steering angle of about ±12° could be obtained by driving each LC phase shifter with a low voltage; however, the changing property was extremely nonlinear as a function of the applied voltage because of the LC molecular orientation phenomena in the homogeneously aligned LC cell. This approach is advantageous in terms of response speed, because the thinner LC layer is beneficial. However, some other components, such as a power divider, LC driver circuit, and antenna, must be integrated with the LC phase shifter to attain specific functionality.
On the other hand, the device structure in the quasi-optical device is simple for achieving specific functionality. However, a thousand-fold increase of the LC layer is necessary to attain a similar operation to optical devices. A simple increase in the normal cell structure causes a fatal degradation of response speed; thus, we tried adopting the PDLC structure using the porous PMMA material to construct the MMW Fresnel lens. However, the driving method for extremely thick LC still remains a problem that must be solved to develop this type of actual MMW LC device. For any approach, the final device performance is governed by the LC material properties. Exploring novel LC materials for MMW application is still an important issue. Currently, LC materials are not optimized for MMW and/or THz application; thus, we believe that the device performance can be greatly improved when specialized LC materials appear. Further, investigation of the basic performance of any type of LC device is important for the actual application of LC materials to the MMW region.