Abstract:
The present invention discloses a high-contrast photonic crystal AND logic gate, comprising a five-port two-dimensional photonic crystal, a nonlinear cavity unit and a Y-shape AND logic gate unit; and it includes a reference-light input port, two signal-input ports, a system signal-output port and an idle port; the nonlinear cavity unit is coupled with the Y-shape AND logic gate unit. The structure of the present invention, which is compact in structure and ease of integration with other optical logic elements, not only can realize the functions of the high-contrast photonic and logic gate, but also has high contrast of high and low logic output; and is widely applicable to optical communication bands.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation application of PCT Application No. PCT/CN2015/097842 filed on Dec. 18, 2015 which claims priority to Chinese Patent Application No. 201410799867.8 filed on Dec. 19, 2014, the entire contents of which are hereby incorporated by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to two-dimensional (2D) photonic crystal (PhC) optical AND logic gate. 
       BACKGROUND OF THE INVENTION 
       [0003]    In 1987, the concept of PhC was proposed separately by E. Yablonovitch from United States Bell Labs who discussed how to suppress spontaneous radiation and by S. John from Princeton University who made discussions about photonic localization. A PhC is a material structure in which dielectric materials are arranged periodically in space, and is usually an artificial crystal comprising of two or more materials having different dielectric constants. 
         [0004]    With the emergence of and in-depth research on PhC, people can control the motion of photons in a PhC material more flexibly and effectively. In combination with traditional semiconductor processes and integrated circuit technologies, design and manufacture of PhC and devices thereof have continually and rapidly marched towards all-optical processing, and PhC has become a breakthrough for photonic integration. In December 1999, PhCs was recognized by the American influential magazine  Science  as one of the top-ten scientific advances in 1999, and therefore has become a hot topic in today&#39;s scientific research field. 
         [0005]    An all-optical logic device mainly includes an optical amplifier-based logic device, a non-linear loop mirror logic device, a Sagnac interference type logic device, a ring cavity logic device, a multi-mode interference logic device, an optical waveguide coupled logic device, a photoisomerized logic device, a polarization switch optical logic device, a transmission grating optical logic device, etc. These optical logic devices have the common shortcoming of large size in developing large-scale integrated optical circuits. With the improvement of science and technology in recent years, people have also done research and developed quantum optical logic devices, nano material optical logic devices and PhC optical logic devices, which all conform to the dimensional requirement of large-scale photonic integrated optical circuit. For modern manufacturing processes, however, the quantum optical logic devices and the nanomaterial optical logic devices are very difficult to be manufactured, whereas the PhC optical logic devices have competitive advantages in terms of manufacturing process. 
         [0006]    In recent years, PhC logic devices have become a hot area of research drawing widespread attentions, and it is highly likely for them to replace the current widely-applied electronic logic devices in the near future. 
       SUMMARY OF THE INVENTION 
       [0007]    The present invention is aimed at overcoming the defects of the prior art and providing a high-contrast PhC AND logic gate which is compact in structure, high in contrast of the high and low logic outputs, and easy to integrate with other optical logic elements. 
         [0008]    The technical solution proposal adopted by the invention to solve the technical problem is as follows: 
         [0009]    An high-contrast PhC AND logic gate, wherein the high-contrast PhC AND logic gate is a structure of five-port 2D PhC, comprising a nonlinear cavity unit and a Y-shape AND logic gate unit including a first reference-light input port, two system signal-input ports, a first system signal-output port and a first idle port; the nonlinear cavity unit is coupled with the Y-shape AND logic gate unit. 
         [0010]    The nonlinear unit is a 2D PhC cross-waveguide nonlinear cavity; and the nonlinear cavity unit includes a second reference-signal input port, an intermediate signal-input port, a second system signal-output port and a second idle port. 
         [0011]    The Y-shape AND logic gate unit includes two signal-input ports and an intermediate signal-output port. 
         [0012]    The intermediate signal-input port of the nonlinear cavity unit is connected with the intermediate signal-output port of the Y-shape AND logic gate unit. 
         [0013]    The 2D PhC cross-waveguide nonlinear cavity includes a high-refractive-index dielectric pillar and the 2D PhC cross intersected waveguide is a four-port network; a left port of the four-port network is the second reference-light input port, a lower port of the four-port network is the intermediate signal-input port, an upper port of the four-port network is the second system signal-output port, and a right port of the four-port network is the second idle port; two mutually-orthogonal quasi- one-dimensional (1D) PhC structures are placed in two waveguide directions crossed at a center of the cross waveguide; a dielectric pillar is arranged in a middle of a cross waveguide, the dielectric pillar is made of a nonlinear material, a cross section of the dielectric pillar is square, polygonal circular, or oval, and a refractive index of the dielectric pillar is 3.4 or another value more than 2; a dielectric constant of a rectangular linear-dielectric pillars clinging to the central dielectric pillar and close to the signal-output port is equal to that of the central dielectric pillar under low-light-power conditions; the quasi-1D PhC structures and the dielectric pillars constitute a waveguide defect cavity. 
         [0014]    The twelve rectangular high-refractive-index linear-dielectric pillars and one square dielectric pillar are arranged in the center of the 2D PhC cross-waveguide nonlinear cavity in a form of the quasi-1D PhC along longitudinal and transverse waveguide directions, the central dielectric pillar clings to four adjacent rectangular linear-dielectric pillars and a distance there between is 0, and every two adjacent rectangular linear-dielectric pillars are spaced 0.2668 d from each other. 
         [0015]    The Y-shape AND logic gate unit is of a three-port waveguide network PhC structure, the lower ports of the three-port network are respectively the two signal-input ports, and the upper port of the three-port waveguide is the immediate signal-output port; a dielectric pillar is made of a nonlinear material arranged at the intersection of the three-port waveguide, and the dielectric pillar is a circular nonlinear-dielectric pillar; and a radius of the nonlinear-dielectric pillar is the same as that of the linear-dielectric pillar. 
         [0016]    The cross section of the high-refractive-index linear-dielectric pillar of the 2D PhC is circular, elliptic, polygonal or triangular. 
         [0017]    The background filling material for the 2D PhC is air or a low-refractive-index dielectric having a refractive index less than 1.4. 
         [0018]    The 2D PhC structure is a (2m+1)×(2n+1) array structure, where m is an integer more than or equal to 4, and where n is an integer more than or equal to 7. 
         [0019]    The cross section of the high-refractive-index linear-dielectric pillar of the 2D PhC is circular, elliptic, polygonal or triangular. 
         [0020]    The background filling material for the 2D PhC is air or a low-refractive-index dielectric having a refractive index less than 1.4. 
         [0021]    The 2D PhC structure is a (2m+1)×(2n+1) array structure, where m is an integer more than or equal to 4, and where n is an integer more than or equal to 7. 
         [0022]    The PhC logic device of the present invention is widely applied to optical communication bands. Compared with the prior art, it has the following advantages: 
         [0023]    1. Compact in structure, and ease of integration with other optical logic elements; 
         [0024]    2. The PhC logic device can directly carry out all-optical logic functions of “AND”, “OR”, “NOT” and the like, is a core device for realizing all-optical computing. 
         [0025]    3. Through the amplitude transform characteristic of the nonlinear cavity, not only can the functions of the high-contrast PhC logic gate be realized, but also the contrast of high and low logic outputs is high; and 
         [0026]    4. Strong anti-interference capability and high in computing speed. 
         [0027]    These and other objects and advantages of the present invention will become readily apparent to those skilled in the art upon reading the following detailed description and claims and by referring to the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]    The present invention is more specifically described in the following paragraphs by reference to the drawings attached only by way of example. 
           [0029]      FIG. 1  is a structural diagram of a high-contrast PhC AND logic gate of the present invention. 
           [0030]    In  FIG. 1 , indications are: nonlinear cavity unit  01 , Y-shape AND logic gate unit  02 , reference-light input port  11 , first signal-input port  12 , second signal-input port  13 , idle port  14 , system signal-output port  15 , first rectangular high-refractive-index linear-dielectric pillar  16 , second rectangular high-refractive-index linear-dielectric pillar  17 , square nonlinear-dielectric pillar  18 , circular high-refractive-index linear-dielectric pillar  19 , circular nonlinear-dielectric pillar  20   
           [0031]      FIG. 2( a )  is a structural diagram of the Y-shape AND logic gate unit  02  shown in  FIG. 1 . In  FIG. 2( a ) , wherein indicated are: first signal-input port  12 , second signal-input port  13 , and immediate signal-output port  32   
           [0032]      FIG. 2( b )  is a structural diagram of the nonlinear cavity unit  01  shown in  FIG. 1 . In  FIG. 2( b ) , wherein indicated are: reference-light input port  11 , immediate signal-input port  31 , idle port  14  and system second signal-output port  15 . 
           [0033]      FIG. 3  is a waveform diagram of the basic logic functions for  FIG. 2( b ) . 
           [0034]      FIG. 4  is the output signal waveform of “Output  1 ” in  FIG. 2( a ) ; 
           [0035]      FIG. 4  is the logic signal-output waveform of “Output  2 ” in  FIG. 1 ; 
           [0036]      FIG. 4  is the logic signal-output waveform of “Output  3 ” in  FIG. 1 ; 
           [0037]      FIG. 5  is a logic operation truth table for  FIG. 2( b ) . 
       
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
       [0038]    The terms a or an, as used herein, are defined as one or more than one, The term plurality, as used herein, is defined as two or more than two. The term another, as used herein, is defined as at least a second or more. 
         [0039]    As shown in  FIG. 1 , the high-contrast PhC AND logic gate of the present invention is a structure of five-port PhC, and includes a nonlinear cavity unit  01  and a Y-shape AND logic gate unit  02 ; the high-contrast PhC AND logic gate includes a reference-light input port, two signal-input ports, a system signal-output port and an idle port; the nonlinear cavity unit  01  is a 2D PhC cross-waveguide nonlinear cavity, and realizes given logic functions by using the preceding-stage logic output as a logic input according to the logic operation characteristics itself; the Y-shape AND logic gate unit  02  is of a three-port Y-shape PhC structure, performs AND logic operation on input signals and includes two signal-input ports and an immediate signal-output port; the PhC AND logic gate  02  is of a PhC structure of a three-port waveguide network, wherein the lower ports of the three-port network are respectively the two signal-input ports, and the upper port of the three-port network is the immediate signal-output port; a dielectric pillar  20  made of a nonlinear material is arranged at the junction of a three-port waveguide, wherein the dielectric pillar  20  is a circular nonlinear-dielectric pillar, which is made of a Kerr shape nonlinear material, and has a dielectric constant of 5 under low-light-power conditions; and the radius of the nonlinear-dielectric pillar  20  is the same as that of the circular high-refractive-index linear-dielectric pillar  19 . 
         [0040]    The nonlinear cavity unit  01  is a 2D PhC cross-waveguide nonlinear cavity and is a 2D PhC cross-waveguide four-port network formed by high-refractive-index dielectric pillars, wherein the left port of the four-port network is a reference signal-input port, the lower port of the four-port network is an intermediate signal-input port, the upper port of the four-port network is a signal-output port, and the right port of the four-port network is an idle port; two mutually-orthogonal quasi-1D PhC structures are placed in two waveguide directions crossed at the center of a cross-waveguide, wherein a dielectric pillar is arranged in the middle of the cross waveguide, the dielectric pillar is a square nonlinear-dielectric pillar; the dielectric pillar is made of a nonlinear material, the cross section of the dielectric pillar is square, polygonal circular, or oval, and the refractive index of the dielectric pillar is 3.4 or another value more than 2; and the dielectric constant of a rectangular linear pillar clinging to the central dielectric pillar and close to the signal-output port is equal to that of the central dielectric pillar under low-light-power conditions; the quasi-1D PhC structures and the dielectric pillar constitute a waveguide defect cavity. Twelve rectangular high-refractive-index linear-dielectric pillars and one square nonlinear-dielectric pillar are arranged in the center of the 2D PhC cross-waveguide nonlinear cavity in the form of a quasi-1D PhC along longitudinal and transverse waveguide directions, the first rectangular high-refractive-index linear-dielectric pillar  16  has a refractive index of 3.4; the second rectangular high-refractive-index linear-dielectric pillar has a dielectric constant being the same as that of a nonlinear-dielectric pillar under low-light-power conditions, every two adjacent rectangular linear-dielectric pillars are spaced 0.2668 d from each other, and the central square nonlinear-dielectric pillar in the cross-waveguide nonlinear cavity is made of a Kerr type nonlinear material, and a dielectric pillar constant of 7.9 under low-light-power conditions; the central square nonlinear-dielectric pillar clings to the four adjacent rectangular linear-dielectric pillars and the distance there between is 0; circular high-refractive-index linear-dielectric pillar  19  in the cross-waveguide nonlinear cavity is made of a Si nonlinear material, and has a refractive index of 3.4. 
         [0041]    The present invention is based on the Photonic Bandgap (PBG) characteristic, quasi-1D PhC defect state, tunneling effect and optical Kerr nonlinear effect of the 2D PhC cross-waveguide nonlinear cavity shown in  FIG. 1 , wherein the nonlinear cavity unit  01  realizes high-contrast PhC AND logic gate functions. Introduced first is the basic principle of the PhC nonlinear cavity in the present invention: a 2D PhC provides a PBG with certain bandwidth, a light wave with its wavelength falling into this bandgap can be propagated in an optical circuit designed inside the PhC, and the operating wavelength of the device is thus set to certain wavelength in the PBG; the quasi-1D PhC structure arranged in the center of the cross waveguide and the nonlinear effect of the central nonlinear-dielectric pillar  18  together provide a defect state mode, which, as the input light wave reaches a certain light intensity, shifts to the operating frequency of the system, so that the structure produces the tunneling effect and signals are output from the output port  5 . 
         [0042]    For the lattice constant d of 1 μm and the operating wavelength of 2.976 μm, referring to the 2D PhC cross-waveguide nonlinear cavity  01  shown in  FIG. 2( b ) , and for a signal A input from the port  11 , and a signal B input from the port  31  shown by the upper two diagrams in  FIG. 3 , the logic output waveforms are obtained and indicated at the lower part in  FIG. 3 . A logic operation truth table for the structure shown in  FIG. 2( b )  can be obtained according to the logic operation characteristic shown in  FIG. 3 , as illustrated in  FIG. 5 . In  FIG. 5 , C indicates a current state Q n , and Y indicates a signal output of the output port  15  of the nonlinear cavity unit, i.e., the next state Q n+1 . A logic expression of the nonlinear cavity unit can be obtained according to the truth table. 
         [0000]        Y=AB+BC    (1)
 
       That is 
       [0043]        Q   n+1   =AB+BQ   n    (2)
 
         [0044]    Referring to the PhC Y-shape AND logic gate structure shown in  FIG. 2( a ) , a signal C is input to the port  12 , a signal D is input to the port  13 , and the output signal waveform of the port  32  is as shown by “Output  1 ” in  FIG. 4 . The nonlinear cavity unit  01  is coupled with the Y-type AND logic gate unit  02 , i.e., the intermediate signal-input port  31  of the nonlinear cavity unit  01  is connected with the intermediate signal-output port  32  of the Y-shape AND logic gate unit  02 , and it is supposed that the AND logic output signal of the Y-shape AND logic gate unit  02  is G, i.e., the AND logic output signal G of the Y-shape AND logic gate unit  02  is the input signal of the intermediate signal-input port  31  of the nonlinear cavity unit  01 . At the moment, reference light E=1 is input to the reference-light input port  11  of the high-contrast PhC AND logic gate, and it can be obtained from formula 2: 
         [0000]        Q   n+1   =G    (3)
 
         [0045]    Finally, the system output port  15  will output the high-contrast AND logic signal G. 
         [0046]    The 2D PhC structure of the device of the present invention is a (2m+1)×(2n+1) array structure, where m is an integer more than or equal to 4, and where n is an integer more than or equal to 7, Design and simulation results will be provided below in an embodiment given in combination with the accompanying drawings, wherein the embodiment is exemplified by a 17×27 array structure, and design and simulation results are given, taking the lattice constant d of the 2D PhC array being 1 μm and 0.5208 μm respectively as an example. 
       Embodiment 1 
       [0047]    Referring to that shown in  FIG. 1 , the lattice constant d is 1 μm; the operating wavelength is 2.976 μm; the radius of the circular high-refractive-index linear-dielectric pillar  19  is 0.18 μm; the long sides of the first rectangular high-refractive-index linear-dielectric pillar  16  are 0.613 μm, and the short sides are 0.162 μm; the size of the second rectangular high-refractive-index linear-dielectric pillar  17  is the same as that of the first rectangular high-refractive-index linear-dielectric pillar  16 ; the side length of square nonlinear-dielectric pillar  18  is 1.5 μm, and the third-order nonlinear coefficient is 1.33×10 −2  μm 2 /V 2 ; and the distance between every two adjacent rectangular linear-dielectric pillars is 0.2668 μm; the radius of the circular nonlinear-dielectric pillar  20  is 0.18 μm, and the third-order nonlinear coefficient is 1×10 −4  μm 2 /V 2 ; 
         [0048]    Referring to the structure shown in  FIG. 2( a ) , a signal C and a signal D are respectively input to the signal-input port  12  and the signal-input port  13 . The waveforms of the signal C and the signal D are shown in  FIG. 4 , and the output signal of the intermediate signal-output port  32  of the Y-shape AND logic gate unit  02  is shown by “Output  1 ” in  FIG. 4 . The logic output having logic amplitude output lower than 0.5 P 0  is set as logic 0, and the logic output having logic amplitude output higher than 0.5 P 0  is set as logic 1. It can be obtained that the output amplitude of the logic 1 of the intermediate signal-output port  32  of the Y-shape AND logic gate unit  02  is about 1.88 P 0 , the output amplitude of the logic 0 is about 0.47 P 0  (except the condition that the two inputs are 0), and the contrast of the high and low logics is about 6 dB. The intermediate signal-output port  32  of the Y-shape AND logic gate unit  02  shown in  FIG. 2( a )  is coupled with the intermediate signal-input port  31  of the nonlinear cavity unit  01  shown in  FIG. 2( b ) , and the structure shown in  FIG. 1  can thus be obtained. Similarly, signal-input Port  12  and signal-input Port  13  are respectively input by the signal C and the signal D shown in  FIG. 4 . The logic output waveform of the high-contrast PhC AND gate of the present invention as shown by “Output  2 ” can be obtained for the lattice constant d=1 μm and the operating wavelength is 2.976 μm. For the waveform of “Output  2 ” in  FIG. 4  shows, the logic 1 of the system output  15  oscillates at a high amplitude interval, and is continually converged to the amplitude of 2.125 P 0 ; the amplitude of the logic 0 of the system output  15  is 0.006 P 0 , and the low logic amplitude is well suppressed. The high and low logic contrast of the system output  15  is more than 25 dB. 
       Embodiments 2 
       [0049]    The lattice constant d is 0.5208 μm; the operating wavelength is 1.55 μm; the radius of the circular high-refractive-index linear-dielectric pillar  19  is 0.0937 μm; the long sides of the first rectangular high-refractive-index linear-dielectric pillar  16  are 0.3193 μm, and the short sides are 0.0844 μm; the size of the second rectangular high-refractive-index linear-dielectric pillar  17  is the same as that of the first rectangular high-refractive-index linear-dielectric pillar  16 ; the side length of square nonlinear-dielectric pillar  18  is 0.7812 μm, and the third-order nonlinear coefficient is 1.33×10 −2  μm 2 /V 2 ; and the distance between every two adjacent rectangular linear-dielectric pillars is 0.1389 μm; the radius of the circular nonlinear-dielectric pillar  20  is 0.0937 μm, and the third-order nonlinear coefficient is 1×10 −4  μm 2 /V 2 ; 
         [0050]    As shown in  FIG. 1 , signal-input Port  12  and signal-input Port  13  are respectively input by the signal C and the signal D shown in  FIG. 4 . The logic output waveform of the high-contrast PhC AND gate of the present invention as shown by “Output  3 ” can be obtained for the lattice constant d=0.5208 μm and the operating wavelength is 1.55 μm. 
         [0051]    As the waveform of “Output  3 ” in  FIG. 4  shows, the amplitude of the logic 1 of the system output  15  oscillates along 2.05 P 0 , and gradually becomes stable. The amplitude of the logic 0 of the system output  15  is 0.008 P 0 , and the low logic amplitude is well suppressed. The high and low logic contrast of the system output  15  is more than 24 dB. 
         [0052]    While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.