Abstract:
A silicon based terahertz full wave liquid crystal phase shifter is provided. The liquid crystal phase shifter has a first silicon conductive substrate, a second silicon conductive substrate, a plurality of pads, and a liquid crystal. The first and second silicon conductive substrates, instead of the quartz glass and transparent electrode, such as ITO, are used as substrates and to provide electrodes of the liquid crystal phase shifter. Thus, the effect of the liquid crystal phase modulation of the liquid crystal phase shifter in the THz range can be improved.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority benefit of Taiwan Patent Application No. 105132770, filed on Oct. 11, 2016. This invention is partly disclosed in theses entitled “ITO free Silicon Based THz Liquid Crystal Phase Shifter” on July, 2016 completed by Han-Wei Zhang and Dr. Yuan-Yao Lin, and “Silicon Based Ohmic Contact THz Liquid Crystal Phase Shifter” on Jul. 18, 2016 completed by Jun-Ting Guo, Han-Wei Zhang, Chun-Ta Wang, Tsung-Hsien Lin and Chao-Kuei Lee. The contents of which are herein incorporated by reference. 
     FIELD OF THE INVENTION 
     The present invention relates to a liquid crystal phase shifter, and more particularly to a silicon based terahertz full wave liquid crystal phase shifter. 
     BACKGROUND OF THE INVENTION 
     Recently, a variety of terahertz range modulation components have been studied and invented with the development of the terahertz range. In past studies, the phase shifter was made of the nematic liquid crystal, which has large birefringence, and the direction of the nematic liquid crystal was changed by adjusting the electric field to change the refractive index as the phase modulation. 
     In more detail, the phase shifter is made of the nematic liquid crystal, the magnetic field or electric field is used to change the direction of the nematic liquid crystal resulting in a change of the refractive index. The substrates of the liquid crystal phase shifter are mostly made of quartz glass, wherein the quartz has an effect of being highly transparent in the visible light frequency range and the terahertz frequency range. The electrodes need be disposed and have a voltage applied to achieve the birefringence by changing the refractive index of the liquid crystal. The most common method is that a transparent conductive electrode, such as indium tin oxide (ITO) thin film, is plated on the quartz glass to be the electrodes because the quartz glass is not conductive. 
     However, ITO thin film is transparent in the visible light frequency range, but has a high absorption in the terahertz frequency range and cannot be transparent. Thus, ITO thin film is a material with low transparency and high absorption in the terahertz frequency range, and the effect of the liquid crystal phase modulation of the liquid crystal phase shifter is poor. 
     As a result, it is necessary to provide a silicon based terahertz full wave liquid crystal phase shifter to solve the problems existing in the conventional technologies, as described above. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a silicon based terahertz full wave liquid crystal phase shifter which adopts silicon conductive substrates to instead of the quartz glass and transparent electrode, such as ITO, as substrates of the liquid crystal phase shifter to provide electrodes of the liquid crystal phase shifter. Thus, the effect of the liquid crystal phase modulation of the liquid crystal phase shifter in the THz range can be improved. 
     To achieve the above object, the present invention provides a silicon based terahertz full wave liquid crystal phase shifter, which comprises a first silicon conductive substrate, a second silicon conductive substrate, a plurality of pads, and a liquid crystal; wherein the first silicon conductive substrate includes a first inner surface and a first outer surface opposite to the first inner surface, the first inner surface includes a first cell portion and a first conductive portion located at a side of the first cell portion; the second silicon conductive substrate includes a second inner surface facing the first inner surface and a second outer surface opposite to the second inner surface, the second inner surface includes a second cell portion and a second conductive located at a side of the second cell portion; the pads are stacked between the first cell portion and the second cell portion to form a space; the liquid crystal is filled in the space. 
     In one embodiment of the present invention, each of the first silicon conductive substrate and the second silicon conductive substrate is made of a P-type semiconductor. 
     In one embodiment of the present invention, each of the first silicon conductive substrate and the second silicon conductive substrate is made of an N-type semiconductor. 
     In one embodiment of the present invention, the liquid crystal phase shifter includes a first horizontal alignment film coated on the first cell portion of the first inner surface. 
     In one embodiment of the present invention, the liquid crystal phase shifter includes a second horizontal alignment film coated on the second cell portion of the second inner surface. 
     In one embodiment of the present invention, a thickness of the space from the first silicon conductive substrate to the second silicon conductive substrate is 2 mm to 2.2 mm. 
     In one embodiment of the present invention, the silicon based terahertz full wave liquid crystal phase shifter further comprises a first contacting film plated on the first conductive portion of the first inner surface. 
     In one embodiment of the present invention, the first contacting film is made of chromium, aluminum, titanium silicide, titanium nitride, tungsten, molybdenum silicide, platinum silicide, cobalt silicide, or tungsten silicide. 
     In one embodiment of the present invention, the liquid crystal phase shifter includes a second contacting film plated on the second conductive portion of the second inner surface. 
     In one embodiment of the present invention, the second contacting film is made of chromium, aluminum, titanium silicide, titanium nitride, tungsten, molybdenum silicide, platinum silicide, cobalt silicide, or tungsten silicide. 
     As described above, when the phase shift is modulated in the terahertz frequency range by using the nematic liquid crystal, the characteristic of highly transparency and low absorption appear in terahertz frequency range by disposing the first silicon conductive substrate and the second silicon conductive substrate in the liquid crystal phase shifter of the present invention. The phase shift of the liquid crystal can be modulated over 2π in 1 THz frequency range by applying the applied voltage between the first silicon conductive substrate and the second silicon conductive substrate. When the first silicon conductive substrate and the second silicon conductive substrate have low voltage applied after the process of ohmic contact, the phase shift of the liquid crystal shows a trend of near linearity when low voltage is applied. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a silicon based terahertz full wave liquid crystal phase shifter according to a preferred embodiment of the present invention; and 
         FIG. 2  illustrates the comparison of the measurement result of applied voltages and phase shifts at a different terahertz frequency according to a preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The structure and the technical means adopted by the present invention to achieve the above and other objects can be best understood by referring to the following detailed description of the preferred embodiments and the accompanying drawings. Furthermore, directional terms described by the present invention, such as upper, lower, front, back, left, right, inner, outer, side, longitudinal/vertical, transverse/horizontal, etc., are only directions by referring to the accompanying drawings, and thus the used directional terms are used to describe and understand the present invention, but the present invention is not limited thereto. 
     Referring to  FIG. 1 , a silicon based terahertz full wave liquid crystal phase shifter according to a preferred embodiment of the present invention is illustrated. As shown, the liquid crystal phase shifter  100  comprises a first silicon conductive substrate  2 , a second silicon conductive substrate  3 , a plurality of pads  4 , a liquid crystal  5 , a first horizontal alignment film  6 , a second horizontal alignment film, a first contacting film  8 , and a second contacting film  9 , wherein a terahertz wave passes through the liquid crystal phase shifter  100  along a path  101 . The detailed structure of each component, assembly relationships, and principles of operation in the present invention will be described in detail hereinafter. 
     Referring to  FIG. 1 , the first silicon conductive substrate  2  includes a first inner surface  21  and a first outer surface  22 , the first outer surface  22  is opposite to the first inner surface  21 , wherein the first inner surface  21  includes a first cell portion  211  and a first conductive portion  212 , the first conductive portion  212  is located at a side of the first cell portion  211 . 
     Referring to  FIG. 1 , the second silicon conductive substrate  3  includes a second inner surface  31  and a second outer surface  32 , wherein the second inner surface  31  faces the first inner surface  21 , and the second outer surface  32  is opposite to the second inner surface  31 , and the second inner surface  31  includes a second cell portion  311  and a second conductive  312 , the second conductive  312  is located at a side of the second cell portion  311 . 
     In the embodiment of the present invention, each of the first silicon conductive substrate  2  and the second silicon conductive substrate  3  is made of a P-type semiconductor, and a semiconductor resistance of the P-type semiconductor is 1 to 10 (Ohm-cm). In other embodiments, each of the first silicon conductive substrate  2  and the second silicon conductive substrate  3  is also made of an N-type semiconductor, which is not limited from the embodiment of the present invention. In addition, each thickness of the first silicon conductive substrate  2  and the second silicon conductive substrate  3  is 0.53 mm, and an absorption coefficient of the first silicon conductive substrate  2  and the second silicon conductive substrate  3  are less than an absorption coefficient of an indium tin oxide (ITO) thin film in prior art, wherein the a thickness of the ITO thin film is 20 nm. 
     Referring to  FIG. 1 , the pads  4  are stacked between the first cell portion  211  and the second cell portion  311  to form a space, and an insulating colloid coats around the outside of the pads  4 , wherein a thickness of the space from the first silicon conductive substrate  2  to the second silicon conductive substrate  3  is 2 mm to 2.2 mm. In the embodiment of the present invention, the thickness of the space preferably is 2.1 mm, and a sectional area of the space is 5 mm×5 mm, thus the delayed phase shift of the liquid crystal  5  can be achieved to 2π. 
     Referring to  FIG. 1 , the liquid crystal  5  is filled in the space, wherein the liquid crystal  5  is a nematic liquid crystal in the embodiment of the present invention, which changes the arrangement of the liquid crystal  5  by applying an applied voltage between the first silicon conductive substrate  2  and the second silicon conductive substrate  3 . 
     Referring to  FIG. 1 , the first horizontal alignment film  6  is coated on the first cell portion  211  of the first inner surface  21 , and the second horizontal alignment film  9  is coated on the second cell portion  311  of the second inner surface  31 , thus the liquid crystal  5  is horizontal arrangement along a direction between the first silicon conductive substrate  2  and the second silicon conductive substrate  3 . In the embodiment of the present invention, the first horizontal alignment film  6  and the second horizontal alignment film  7  are polyimide (PI) alignment films. 
     Referring to  FIG. 1 , the first contacting film  8  is plated on the first conductive portion  212  of the first inner surface  21 , and the second contacting film  9  is plated on the second conductive portion  312  of the second inner surface  31 . In the embodiment of the present invention, each thickness of the first contacting film  8  and the second contacting film  9  is 100 nm, and the first contacting film  8  and the second contacting film  9  are made of chromium. In other embodiments, each thickness of the first contacting film  8  and the second contacting film  9  is less than 100 nm, and the first contacting film  8  and the second contacting film  9  are also made of aluminum, titanium silicide, titanium nitride, tungsten, molybdenum silicide, platinum silicide, cobalt silicide, or tungsten silicide, which is not limited from the embodiment of the present invention. 
     Referring to  FIG. 2 , an electrode  102  of the first silicon conductive substrate  2  and an electrode  103  of the second silicon conductive substrate  3  have a voltage applied, wherein the applied voltage is 0 V to 15 V. The phase shift of the liquid crystal  5  can be changed with a relative angle by changing the applied voltage. Furthermore, the first contacting film  8  formed on first conductive portion  212  and the second contacting film  9  formed on the second conductive portion  312  generate ohmic contacts respectively due to the first contacting film  8  and the second contacting film  9  being made of chromium. When the first silicon conductive substrate  2  and the second silicon conductive substrate  3  have the voltage applied, the phase shift of the liquid crystal  5  shows a trend of near linearity at low applied voltage after the process of ohmic contact in  FIG. 2 . 
     According to the described structure, the applied voltage is formed between the first silicon conductive substrate  2  and the second silicon conductive substrate  3  by applying voltage to the first conductive portion  212  and the second conductive portion  312  through the electrode  102  and the electrode  103  respectively. In addition, the liquid crystal  5  is arranged along a direction by disposing the first horizontal alignment film  6  and the second horizontal alignment film  7  and generating the terahertz wave passing through the first silicon conductive substrate  2  and the second silicon conductive substrate  3  along the path  101 , so that the phase shift of the liquid crystal  5  can be modulated over 2π in 1 terahertz (THz) frequency range, and the phase shift of the liquid crystal  5  shows a trend of near linearity at low applied voltage. 
     As described above, when the phase shift is modulated in terahertz frequency range by using the nematic liquid crystal, the characteristic of the highly transparent and low absorption appear in the terahertz frequency range by disposing the first silicon conductive substrate  2  and the second silicon conductive substrate  3  in the liquid crystal phase shifter  100  of the present invention. The phase shift of the liquid crystal  5  can be modulated over 2π in 1 THz frequency range by applying the applied voltage between the first silicon conductive substrate  2  and the second silicon conductive substrate  3 . Moreover, when the first silicon conductive substrate  2  and the second silicon conductive substrate  3  are applied the low applied voltage after the process of ohmic contact, the phase shift of the liquid crystal  5  shows a trend of near linearity at low applied voltage. 
     The present invention has been described with a preferred embodiment thereof and it is understood that many changes and modifications to the described embodiment can be carried out without departing from the scope and the spirit of the invention that is intended to be limited only by the appended claims.