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Photonic crystal-based waveguide terahertz wave Set-reset latch
Accepted Manuscript Title: Photonic crystal-based waveguide terahertz wave Set-Reset latch Authors: Jian-Zhong Sun, Jiu-Sheng Li PII: DOI: Reference:
S0030-4026(17)30849-5 http://dx.doi.org/doi:10.1016/j.ijleo.2017.07.031 IJLEO 59427
To appear in: Received date: Accepted date:
16-4-2017 10-7-2017
Please cite this article as: Jian-Zhong Sun, Jiu-Sheng Li, Photonic crystal-based waveguide terahertz wave Set-Reset latch, Optik - International Journal for Light and Electron Opticshttp://dx.doi.org/10.1016/j.ijleo.2017.07.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photonic crystal-based waveguide terahertz wave Set-Reset latch Sun Jian-Zhong, Li Jiu-Sheng* Centre for THz Research, China Jiliang University, Hangzhou 310018, China
*E-mail: [email protected] Abstract: In terahertz system, controlling terahertz wave propagation is a very important issue. In this letter, we have proposed and demonstrated a terahertz wave Set-Reset latch based on two-dimensional photonic crystal. The present device consists of two photonic crystal NOR gates and two photonic crystal multimode-interfere structures. Finite-difference time-domain method is used to analyze and confirm the propagation properties of the device. The total size of our proposed Set-Reset latch is equal to 85a×40a. The response time of the Set-Reset latch is 3.25ps. Our study suggests that the Set-Reset latch has great potential in future terahertz wave information processing integrated circuit. Keywords: terahertz wave, Set-Reset latch, photonic crystal 1. Introduction In the last decade, terahertz science and technology has gotten many researchers’ attention because of its unique properties and potential applications. Nowadays, the terahertz technologies are diversified applications including communication, imaging, security detection, and so forth [1-4]. Although, a great of progresses has been made in terahertz devices such as sources [5-6], detectors [7-8], switches [9], quarter-wave plates [10], absorbers [11], and modulators [12-13], as terahertz wave signal processing device such as terahertz logic circuit has attached less attention. To our best knowledge, all logic gate circuits reported in literatures are used to manipulate light [14-18]. As key components in signal processing networks, terahertz logic circuit is being greatly in demand and is expected to be the main supporting techniques in future terahertz information networks. Therefore, it is urgent to design a terahertz logic circuit to meet the development of terahertz technology. Furthermore, to design a compact logic circuit which can be used in the scaled integration implementation in the terahertz regime is presently very challenging. As we all know, photonic crystals offer the potential to generate exceedingly small microcircuits capable of handling and manipulating terahertz wave at dimensions on the order of operating wavelength. Therefore, photonic crystals have great advantages in designing compact logic circuit. Moreover, two-dimensional photonic crystals are widely employed to design optical and terahertz devices because of the ease of mathematical analysis and fabrication. In this letter, we have proposed a novel terahertz wave Set-Reset (S-R) latch configuration of two NOR gates and two multimode-interfere (MMI) structures based on two-dimensional photonic crystals which have been created by a square lattice of cylindrical silicon rods (with refractive index n=3.45) embedded in SiO2. The terahertz wave propagation in the time domain of the Set-Reset latch has been simulated and analyzed by using finite-difference time-domain method. The unique logic circuit properties of the device exhibit tremendous promise for applications in terahertz photonic integrated circuits networks. 2. Device design and analysis Fig. 1(a) shows the proposed terahertz wave Set-Reset latch based on two dimensional photonic crystal, which is composed of two NOR gates (marked NOR gate 1 and 2) and two multimode-interfere structures (denoted MMI Region 1 and 2). As shown in this figure, a two dimension planar photonic crystal structure consists of a square lattice array of cylindrical silicon rods with the refractive index of n=3.45. The background material is SiO2 with refractive index of 1.45 and the radius of the silicon rod is equal to r=0.2a. The lattice constant and the radius of rods are chosen to be a=100μm and r=20μm,
respectively. The Set-Reset latch has two control singal input ports marked S and R, and two output ports marked Q and Q’, which represents two stable output states (i.e. one is named the logic ‘0’, the other is named the logic ‘1’.). The logic circuit diagram of the presented Set-Reset latch is shown in Fig. 1(b). During the working progress, the four input ports of A, B, C and D are always set at the state ‘1’. While the two control singal input ports (Set and Reset) have been introduced in the Set-Reset latch, the outputs of the device are generated at a rapid response time later. The present output value (Qn+1) often depends on the previous output value (Qn), which depends on the past values of the input ports. Figure 2 shows that schematic view of the photonic crystal NOR gates in our proposed terahertz wave Set-Reset latch. The NOR gate consists of two ring resonators embedded in two parallel input line defect photonic crystal waveguides (The waveguide length is of L1=13a) and two section parallel coupling line defect photonic crystal waveguides (The waveguide length is of L2=8a), and a line defect waveguide locates in the middle of two dimensional photonic crystal. The NOR gate includes two input signal ports (S or R, and G1 or G2), a probe signal port (A or B) and an output port (F1 or F2). The NOR logic gate is excited from (A or B) port with terahertz wave signal and exits from (F1 or F2) port, which is controlled by (S or R) port and (G1 or G2) port. Figure 3(a)~3(d) show the steady state electric field distribution of the NOR gate when the input ports (S or R)
and (G1 or G2) are set to be
different stages (i.e. ‘1’ or ‘0’). Here, we use the truth table to analysis the working mechanism of the NOR gate. The overall operation of NOR gate is depicted in Table 1. Obviously, the designed construction can realize the function of NOR gate well. Figure 4 shows the configuration of the multimode-interfere structure in our proposed terahertz wave Set-Reset latch. The MMI structure has two input ports (C or D, and F1 or F2) and two output ports (G1 or G2, and Q or Q’). The length of the MMI region is set to be d=14a. The steady state electric field distribution of the MMI region are shown in Figs.5(a)~5(b). From the figures, one can see that the MMI structure is excited from both input ports (C or D) and (F1 or F2) with terahertz wave signal and exits from output (G1 or G2), while the output port (Q or Q’) almost can not collected any signal. However, in Fig.5(b), when the MMI structure is only excited from input port (C or D) with terahertz wave signal and exits from output port (Q or Q’), there are no terahertz signal at output port (G1 or G2).
3. Results analysis and discussion In this study, the optimized values of the terahertz wave Set-Reset latch design parameters are as follows: a=100μm, r=0.2a, d=14a, L1=13a, and L2=8a. According to analysis as discussed before, the novel terahertz wave Set-Reset latch is proposed using two photonic crystal NOR gates combined with two photonic crystal MMI structures, see Fig.1. The finite-difference time-domain method has been used to numerically demonstrate the terahertz wave propagation behaviors in the proposed device. The steady state electric field distributions of the Set-Reset latch are shown in Figs.6(a)~6(e). Firstly, supposing that the control signal input port S=1 is entered NOR gate 1 to prevent the terahertz wave signal of input port A from passing and reaching to MMI region 1, and then we can get the output Q=1. For the case of the control signal input port R=0, the terahertz wave signal of input port B is passing through NOR gate 2 and reaching to MMI region 2, and then the output Q’=0. That is to say, when S=1 and R=0, the output ports of the Set-Reset latch shows Q=1 and Q’=0, as illustrated in Fig. 6(a). Secondly, the control signal input port S is switched from ‘1’ to ‘0’ and R remains equal to ‘0’ (i.e. R=0), the two output ports Q and Q’ remain Q=1 and Q’=0, as shown in Fig. 6(b). From the figure, it can be noted that the terahertz wave from MMI region 2 prevents the terahertz wave signal of input
port A from passing NOR gate 1 and reaching to MMI region 1, therefore the output of the Set-Reset latch Q remains equal to ‘1’. Similarly, since the control signal input port R remains equal to ‘0’, the terahertz wave signal of input port B is passing through NOR gate 2 and can not transmiting through the MMI region 2, finally the output of the Set-Reset latch Q’ is not changed (i.e. Q’ remains equal to ‘0’). Accordingly, suppose that S=0 and R changes from ‘0’ to ‘1’ as depicted in Fig. 6(c), the control signal input port R=1 is entered NOR gate 2 to prevent the terahertz wave signal of port B from passing and reaching to the MMI region 2, the output of the Set-Reset latch Q’ becomes ‘1’ (i.e. Q’=1). As for the control signal input port S=0, the terahertz wave signal of port A is passing through NOR gate 1 and reaching to MMI region 1, consequently the output of the Set-Reset latch Q changes from ‘1’ to ‘0’ (i.e. Q=0). Fig. 6(d) shows the output of the Set-Reset latch Q’=1 and Q=0 for the case of the control signal input port S remains equal to ‘0’ and the R switches from ‘1’ to ‘0’. When the control signal input port R varies from ‘1’ to ‘0’, the feedback terahertz wave signal from NOR gate 2 prevents the terahertz wave signal of port B from passing and reaching to MMI region 2, and then the output of the Set-Reset latch Q’ remains equal to ‘1’ (i.e. Q’=1). In addition, when S=0, the terahertz wave signal of port A is passing through NOR gate 1 and reaching to MMI region 1, the output Q remain equal to ‘0’ (i.e. Q=0). That is to say, when the input port R changes from ‘1’ to ‘0’ and the input port S keeps its state, the output ports Q’ and Q will not change their states. Correspondingly, providing that S=R=1, the control signal input ports S and R are entered NOR gates 1 and 2 to prevent the terahertz wave signals of input ports A and B from reaching to MMI region 1 and MMI region 2, respectively, at last the outputs of the Set-Reset latch Q’=1 and Q=1, as depicted in Fig.6 (e). But this state is not stable, and will be varied with the change of the state of S and R rapidly. Based on the calculated electric field distribution in the Set-Reset latch shown in Fig.6, the truth table of the Set-Reset latch is depicted in Table.2. Analyzing the truth table, one can see that the S and R can set or reset the state of the output signal Q, and when the S or R switches to be ‘0’, the state of Q can be kept. Likewise, the Q’ becomes the next state as the input signal S and R switches. Moreover, Fig.7 shows the response time chart of the different states of the Set-Reset latch. According to the Fig.7(a), provided that at time t=0ps, the control signal input port S=0 and R=1, and the outputs of the Set-Reset latch Q=Q’=0. When t=3.25ps, the output ports of the Set-Reset latch reaches stable states Q’=1 and Q=0. After this time, when the S is still equal to ‘0’ and R varies from ‘1’ to ‘0’, the outputs Q’ and Q remains equal to Q’=1 and Q=0. Furthermore, in the Fig.7(b), providing that at t=0ps, when the input port S=1 and R=0, and the outputs of the Set-Reset latch Q=Q’=0. When t=3.25ps, the output ports of the Set-Reset latch attains stable states Q’=0 and Q=1. After this time, when R is still equal to ‘0’ and the S changes from ‘1’ to ‘0’, the outputs Q’ and Q remains equal to Q’=0 and Q=1. 4. Conclusion We propose a scheme for a Set-Reset latch using two photonic crystal NOR-gates and MMI-structures in terahertz frequency regime. By utilizing finite different time domain method, the propagation performance of the device is demonstrated. The total dimensions of the proposed Set-Reset latch are not more than 85a×40a. The response time of this Set-Reset latch is less than 3.25ps. It is expected that the proposed Set-Reset latch has potential applications in future for compact terahertz wave information processing integrated circuit to increase the performance of the terahertz wave network. Acknowledgments The author gratefully acknowledges the financial support from the National Natural Science
Foundation of China under Grant No.61379024. References 1. I.Ho, X.Guo, X.Zhang, “Design and performance of reflective terahertz air biased coherent detection for time-domain spectroscopy,” Opt. Express, 18, 2872-2883 (2010) 2. X.Wu, X.Pan, B. Quan, X.Xu,C.Gu, L.Wang, “Self-referenced sensing based on terahertz metamaterial for aqueous solutions,”Appl. Phys. Lett., 102, 151109 (2013) 3. J. Federici, L. Moeller, “Review of terahertz and subterahertz wireless communications,” J.Applied Physics, 107, 111101 (2010) 4. S.Koenig, D.Diaz, J.Antes, F.Boes, R.Henneberger, A.Leuther, A.Tessmann, R.Schmogrow, D.Hillerkuss, R.Palmer, T.Zwick, C.Koos, W.Freude, O.Ambacher, J.Leuthold, I.Kallfass, “Wireless sub-THz communication system with high data rate,” Nat. Photonics, 7, 977-981 (2013) 5. N.Yu, Q.Wang, M.Kats, J.Fan, S.Khanna, L.Li, A. Davies , E. Linfield, F.Capasso, “Designer spoof surface plasmon structures collimate terahertz laser beams,” Nat. Mater, 9, 730-735 (2010) 6. L.Luo, I.Chatzakis, J.Wang, F.Niesler, M.Wegener, T.Koschny, C.Soukoulis, “Broadband terahertz generation from metamaterials,” Nat. Commun., 5, 3055 (2014) 7. M.Vitiello, D.Coquillat, L.Viti, D.Ercolani, F.eppe, A.Pitanti, F.Beltram, L.Sorba, W.Knap, A.Tredicucci, “Room-temperature terahertz detectors based on semiconductor nanowire field-effect transistors,”Nano Lett., 12, 96-101 (2012) 8. L.Vicarelli, M.Vitiello, D.Coquillat, A.Lombardo, A.Ferrari, W.Knap, M. Polini, V. Pellegrini, A. Tredicucci, “Graphene field-effect transistors as room-temperature terahertz detectors,” Nat. Mater., 11, 865-871 (2012) 9. T.Lv, Z.Zhu, J.Shi, C.Guan, Z.Wang, T.Cui, “Optically controlled background-free terahertz switching in chiral metamaterial,” Opt. Lett., 39, 3066-3069 (2014) 10. D.Wang, Y.Gu, Y.Gong, C.Qiu, M.Hong, “An ultrathin terahertz quarter-wave plate using planar babinet-inverted metasurface,” Opt. Express, 23, 11115-11122 (2015) 11. B.Wang, L.Wang, G.Wang, W.Huang, X. Li, X.Zhai, “Frequency continuous tunable terahertz metamaterial absorber,” J. Lightwave Technol., 32,1183-1189 (2014) 12. D.Shrekenhamer, C.Watts, W.Padilla, “Terahertz single pixel imaging with an optically controlled dynamic spatial light modulator,” Opt. Express, 21, 12507-12518 (2013) 13. Y.Bai, K.Chen, H.Liu, T.Bu, B.Cai, J.Xu, Y.Zhu, “Optically controllable terahertz modulator based on electromagnetically induced transparency like effect,” Optics Commun., 353, 83-89 (2015) 14. Z.Mohebbi, N.Nozhat, F.Emami, “High contrast all-optical logic gates based on 2D nonlinear photonic crystal,” Opt. Commun., 355, 130-136 (2015) 15. H.Taleb, K.Abedi, “Design of a low-power all-optical NOR gate using photonic crystal quantum-dot semiconductor optical amplifiers,” Opt. Lett., 39, 6237-6240 (2014) 16. C.Tang, X.Dou, Y.Lin, H.Yin, B.Wu, Q.Zhao, “Design of all-optical logic gates avoiding external phase shifters in a two-dimensional photonic crystal based on multi-mode interference for BPSK signals,” Opt. Commun., 316, 49-55 (2015) 17. S.Zeng, Y.Zhang, B.Li, “Ultrasmall optical logic gates based on silicon periodic dielectric waveguides,” Photon. Nanostruct., 8, 32-37 (2010) 18. J.Bai, J.Wang, J.Jiang, X. Chen, Y.Qiu, Z.Qiang, “Photonic NOT and NOR gates based on a single compact photonic crystal ring resonator,” Appl. Opt., 48, 6923-6927 (2009)
Fig.1 Configuration (a) and logic circuit (b) of our proposed terahertz wave Set-Reset latch Fig.2 Schematic view of the NOR gate structures in our proposed terahertz wave Set-Reset latch Fig.3 Steady state electric field distribution of the NOR gate (a) Both input ports (S or R) and (G1 or G2) are set ‘0’; (b) Input ports (S or R)and (G1 or G2) is set to be ‘1’ and ‘0’, respectively; (c) Input ports (S or R) and (G1 or G2) is set to be ‘0’ and ‘1’, respectively; (d) Both input ports (S or R) and (G1 or G2) are set to be ‘1’ Fig.4 Configuration of the MMI region in our proposed terahertz wave Set-Reset latch Fig.5 Steady state electric field distribution of the MMI region (a) Both input port (C or D) and (F1 or F2) are set ‘1’; (b) Input port (C or D) and (F1 or F2) are set to be ‘1’ and ‘0’, respectively. Fig.6 Steady state electric field distribution of Set-Reset latch (a) input port S is ‘1’ and R is ‘0’; (b)input port S changes from ‘1’ to ‘0’, and R remains ‘0’; (c) input port S remains ‘0’ and R changes from ‘0’ to ‘1’; (d) input port S remains ‘0’, and R changes from ‘1’ to ‘0’; (e) Both input ports S and R are set to be ‘1’ Fig.7 Response time chart of the different states of the Set-Reset latch
(a)
(b) Fig.1 Configuration (a) and logic circuit (b) of our proposed terahertz wave Set-Reset latch
Fig.2 Schematic view of the NOR gate structures in our proposed terahertz wave Set-Reset latch
(a)
(b)
(c)
(d)
Fig.3 Steady state electric field distribution of the NOR gate (a) Both input ports (S or R) and (G1 or G2) are set ‘0’; (b) Input ports (S or R)and (G1 or G2) is set to be ‘1’ and ‘0’, respectively; (c) Input ports (S or R) and (G1 or G2) is set to be ‘0’ and ‘1’, respectively; (d) Both input ports (S or R) and (G1 or G2) are set to be ‘1’
Fig.4 Configuration of the MMI region in our proposed terahertz wave Set-Reset latch
(a)
(b)
Fig.5 Steady state electric field distribution of the MMI region (a) Both input port (C or D) and (F1 or F2) are set ‘1’; (b) Input port (C or D) and (F1 or F2) are set to be ‘1’ and ‘0’, respectively.
(a)
(b)
(c)
(d)
(e) Fig.6 Steady state electric field distribution of Set-Reset latch (a) input port S is ‘1’ and R is ‘0’; (b)input port S changes from ‘1’ to ‘0’, and R remains ‘0’; (c) input port S remains ‘0’ and R changes from ‘0’ to ‘1’; (d) input port S remains ‘0’, and R changes from ‘1’ to ‘0’; (e) Both input ports S and R are set to be ‘1’
(a)
(b) Fig.7 Response time chart of the different states of the Set-Reset latch
Tab.1 Truth table of NOR gate Input Port S
Input Port G1
Output Port F1
or Port R
or Port G2
or Port F2
0
0
1
1
0
0
0
1
0
1
1
0
Tab.2 Truth table of our proposed Set-Reset latch S
R
Qn+1
Q'n+1
State
0
0
Qn
Q'n
Hold
1
0
1
0
Set
0
1
0
1
Reset
1
1
1
1
Invalid
S0030-4026(17)30849-5 http://dx.doi.org/doi:10.1016/j.ijleo.2017.07.031 IJLEO 59427
To appear in: Received date: Accepted date:
16-4-2017 10-7-2017
Please cite this article as: Jian-Zhong Sun, Jiu-Sheng Li, Photonic crystal-based waveguide terahertz wave Set-Reset latch, Optik - International Journal for Light and Electron Opticshttp://dx.doi.org/10.1016/j.ijleo.2017.07.031 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photonic crystal-based waveguide terahertz wave Set-Reset latch Sun Jian-Zhong, Li Jiu-Sheng* Centre for THz Research, China Jiliang University, Hangzhou 310018, China
*E-mail: [email protected] Abstract: In terahertz system, controlling terahertz wave propagation is a very important issue. In this letter, we have proposed and demonstrated a terahertz wave Set-Reset latch based on two-dimensional photonic crystal. The present device consists of two photonic crystal NOR gates and two photonic crystal multimode-interfere structures. Finite-difference time-domain method is used to analyze and confirm the propagation properties of the device. The total size of our proposed Set-Reset latch is equal to 85a×40a. The response time of the Set-Reset latch is 3.25ps. Our study suggests that the Set-Reset latch has great potential in future terahertz wave information processing integrated circuit. Keywords: terahertz wave, Set-Reset latch, photonic crystal 1. Introduction In the last decade, terahertz science and technology has gotten many researchers’ attention because of its unique properties and potential applications. Nowadays, the terahertz technologies are diversified applications including communication, imaging, security detection, and so forth [1-4]. Although, a great of progresses has been made in terahertz devices such as sources [5-6], detectors [7-8], switches [9], quarter-wave plates [10], absorbers [11], and modulators [12-13], as terahertz wave signal processing device such as terahertz logic circuit has attached less attention. To our best knowledge, all logic gate circuits reported in literatures are used to manipulate light [14-18]. As key components in signal processing networks, terahertz logic circuit is being greatly in demand and is expected to be the main supporting techniques in future terahertz information networks. Therefore, it is urgent to design a terahertz logic circuit to meet the development of terahertz technology. Furthermore, to design a compact logic circuit which can be used in the scaled integration implementation in the terahertz regime is presently very challenging. As we all know, photonic crystals offer the potential to generate exceedingly small microcircuits capable of handling and manipulating terahertz wave at dimensions on the order of operating wavelength. Therefore, photonic crystals have great advantages in designing compact logic circuit. Moreover, two-dimensional photonic crystals are widely employed to design optical and terahertz devices because of the ease of mathematical analysis and fabrication. In this letter, we have proposed a novel terahertz wave Set-Reset (S-R) latch configuration of two NOR gates and two multimode-interfere (MMI) structures based on two-dimensional photonic crystals which have been created by a square lattice of cylindrical silicon rods (with refractive index n=3.45) embedded in SiO2. The terahertz wave propagation in the time domain of the Set-Reset latch has been simulated and analyzed by using finite-difference time-domain method. The unique logic circuit properties of the device exhibit tremendous promise for applications in terahertz photonic integrated circuits networks. 2. Device design and analysis Fig. 1(a) shows the proposed terahertz wave Set-Reset latch based on two dimensional photonic crystal, which is composed of two NOR gates (marked NOR gate 1 and 2) and two multimode-interfere structures (denoted MMI Region 1 and 2). As shown in this figure, a two dimension planar photonic crystal structure consists of a square lattice array of cylindrical silicon rods with the refractive index of n=3.45. The background material is SiO2 with refractive index of 1.45 and the radius of the silicon rod is equal to r=0.2a. The lattice constant and the radius of rods are chosen to be a=100μm and r=20μm,
respectively. The Set-Reset latch has two control singal input ports marked S and R, and two output ports marked Q and Q’, which represents two stable output states (i.e. one is named the logic ‘0’, the other is named the logic ‘1’.). The logic circuit diagram of the presented Set-Reset latch is shown in Fig. 1(b). During the working progress, the four input ports of A, B, C and D are always set at the state ‘1’. While the two control singal input ports (Set and Reset) have been introduced in the Set-Reset latch, the outputs of the device are generated at a rapid response time later. The present output value (Qn+1) often depends on the previous output value (Qn), which depends on the past values of the input ports. Figure 2 shows that schematic view of the photonic crystal NOR gates in our proposed terahertz wave Set-Reset latch. The NOR gate consists of two ring resonators embedded in two parallel input line defect photonic crystal waveguides (The waveguide length is of L1=13a) and two section parallel coupling line defect photonic crystal waveguides (The waveguide length is of L2=8a), and a line defect waveguide locates in the middle of two dimensional photonic crystal. The NOR gate includes two input signal ports (S or R, and G1 or G2), a probe signal port (A or B) and an output port (F1 or F2). The NOR logic gate is excited from (A or B) port with terahertz wave signal and exits from (F1 or F2) port, which is controlled by (S or R) port and (G1 or G2) port. Figure 3(a)~3(d) show the steady state electric field distribution of the NOR gate when the input ports (S or R)
and (G1 or G2) are set to be
different stages (i.e. ‘1’ or ‘0’). Here, we use the truth table to analysis the working mechanism of the NOR gate. The overall operation of NOR gate is depicted in Table 1. Obviously, the designed construction can realize the function of NOR gate well. Figure 4 shows the configuration of the multimode-interfere structure in our proposed terahertz wave Set-Reset latch. The MMI structure has two input ports (C or D, and F1 or F2) and two output ports (G1 or G2, and Q or Q’). The length of the MMI region is set to be d=14a. The steady state electric field distribution of the MMI region are shown in Figs.5(a)~5(b). From the figures, one can see that the MMI structure is excited from both input ports (C or D) and (F1 or F2) with terahertz wave signal and exits from output (G1 or G2), while the output port (Q or Q’) almost can not collected any signal. However, in Fig.5(b), when the MMI structure is only excited from input port (C or D) with terahertz wave signal and exits from output port (Q or Q’), there are no terahertz signal at output port (G1 or G2).
3. Results analysis and discussion In this study, the optimized values of the terahertz wave Set-Reset latch design parameters are as follows: a=100μm, r=0.2a, d=14a, L1=13a, and L2=8a. According to analysis as discussed before, the novel terahertz wave Set-Reset latch is proposed using two photonic crystal NOR gates combined with two photonic crystal MMI structures, see Fig.1. The finite-difference time-domain method has been used to numerically demonstrate the terahertz wave propagation behaviors in the proposed device. The steady state electric field distributions of the Set-Reset latch are shown in Figs.6(a)~6(e). Firstly, supposing that the control signal input port S=1 is entered NOR gate 1 to prevent the terahertz wave signal of input port A from passing and reaching to MMI region 1, and then we can get the output Q=1. For the case of the control signal input port R=0, the terahertz wave signal of input port B is passing through NOR gate 2 and reaching to MMI region 2, and then the output Q’=0. That is to say, when S=1 and R=0, the output ports of the Set-Reset latch shows Q=1 and Q’=0, as illustrated in Fig. 6(a). Secondly, the control signal input port S is switched from ‘1’ to ‘0’ and R remains equal to ‘0’ (i.e. R=0), the two output ports Q and Q’ remain Q=1 and Q’=0, as shown in Fig. 6(b). From the figure, it can be noted that the terahertz wave from MMI region 2 prevents the terahertz wave signal of input
port A from passing NOR gate 1 and reaching to MMI region 1, therefore the output of the Set-Reset latch Q remains equal to ‘1’. Similarly, since the control signal input port R remains equal to ‘0’, the terahertz wave signal of input port B is passing through NOR gate 2 and can not transmiting through the MMI region 2, finally the output of the Set-Reset latch Q’ is not changed (i.e. Q’ remains equal to ‘0’). Accordingly, suppose that S=0 and R changes from ‘0’ to ‘1’ as depicted in Fig. 6(c), the control signal input port R=1 is entered NOR gate 2 to prevent the terahertz wave signal of port B from passing and reaching to the MMI region 2, the output of the Set-Reset latch Q’ becomes ‘1’ (i.e. Q’=1). As for the control signal input port S=0, the terahertz wave signal of port A is passing through NOR gate 1 and reaching to MMI region 1, consequently the output of the Set-Reset latch Q changes from ‘1’ to ‘0’ (i.e. Q=0). Fig. 6(d) shows the output of the Set-Reset latch Q’=1 and Q=0 for the case of the control signal input port S remains equal to ‘0’ and the R switches from ‘1’ to ‘0’. When the control signal input port R varies from ‘1’ to ‘0’, the feedback terahertz wave signal from NOR gate 2 prevents the terahertz wave signal of port B from passing and reaching to MMI region 2, and then the output of the Set-Reset latch Q’ remains equal to ‘1’ (i.e. Q’=1). In addition, when S=0, the terahertz wave signal of port A is passing through NOR gate 1 and reaching to MMI region 1, the output Q remain equal to ‘0’ (i.e. Q=0). That is to say, when the input port R changes from ‘1’ to ‘0’ and the input port S keeps its state, the output ports Q’ and Q will not change their states. Correspondingly, providing that S=R=1, the control signal input ports S and R are entered NOR gates 1 and 2 to prevent the terahertz wave signals of input ports A and B from reaching to MMI region 1 and MMI region 2, respectively, at last the outputs of the Set-Reset latch Q’=1 and Q=1, as depicted in Fig.6 (e). But this state is not stable, and will be varied with the change of the state of S and R rapidly. Based on the calculated electric field distribution in the Set-Reset latch shown in Fig.6, the truth table of the Set-Reset latch is depicted in Table.2. Analyzing the truth table, one can see that the S and R can set or reset the state of the output signal Q, and when the S or R switches to be ‘0’, the state of Q can be kept. Likewise, the Q’ becomes the next state as the input signal S and R switches. Moreover, Fig.7 shows the response time chart of the different states of the Set-Reset latch. According to the Fig.7(a), provided that at time t=0ps, the control signal input port S=0 and R=1, and the outputs of the Set-Reset latch Q=Q’=0. When t=3.25ps, the output ports of the Set-Reset latch reaches stable states Q’=1 and Q=0. After this time, when the S is still equal to ‘0’ and R varies from ‘1’ to ‘0’, the outputs Q’ and Q remains equal to Q’=1 and Q=0. Furthermore, in the Fig.7(b), providing that at t=0ps, when the input port S=1 and R=0, and the outputs of the Set-Reset latch Q=Q’=0. When t=3.25ps, the output ports of the Set-Reset latch attains stable states Q’=0 and Q=1. After this time, when R is still equal to ‘0’ and the S changes from ‘1’ to ‘0’, the outputs Q’ and Q remains equal to Q’=0 and Q=1. 4. Conclusion We propose a scheme for a Set-Reset latch using two photonic crystal NOR-gates and MMI-structures in terahertz frequency regime. By utilizing finite different time domain method, the propagation performance of the device is demonstrated. The total dimensions of the proposed Set-Reset latch are not more than 85a×40a. The response time of this Set-Reset latch is less than 3.25ps. It is expected that the proposed Set-Reset latch has potential applications in future for compact terahertz wave information processing integrated circuit to increase the performance of the terahertz wave network. Acknowledgments The author gratefully acknowledges the financial support from the National Natural Science
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Fig.1 Configuration (a) and logic circuit (b) of our proposed terahertz wave Set-Reset latch Fig.2 Schematic view of the NOR gate structures in our proposed terahertz wave Set-Reset latch Fig.3 Steady state electric field distribution of the NOR gate (a) Both input ports (S or R) and (G1 or G2) are set ‘0’; (b) Input ports (S or R)and (G1 or G2) is set to be ‘1’ and ‘0’, respectively; (c) Input ports (S or R) and (G1 or G2) is set to be ‘0’ and ‘1’, respectively; (d) Both input ports (S or R) and (G1 or G2) are set to be ‘1’ Fig.4 Configuration of the MMI region in our proposed terahertz wave Set-Reset latch Fig.5 Steady state electric field distribution of the MMI region (a) Both input port (C or D) and (F1 or F2) are set ‘1’; (b) Input port (C or D) and (F1 or F2) are set to be ‘1’ and ‘0’, respectively. Fig.6 Steady state electric field distribution of Set-Reset latch (a) input port S is ‘1’ and R is ‘0’; (b)input port S changes from ‘1’ to ‘0’, and R remains ‘0’; (c) input port S remains ‘0’ and R changes from ‘0’ to ‘1’; (d) input port S remains ‘0’, and R changes from ‘1’ to ‘0’; (e) Both input ports S and R are set to be ‘1’ Fig.7 Response time chart of the different states of the Set-Reset latch
(a)
(b) Fig.1 Configuration (a) and logic circuit (b) of our proposed terahertz wave Set-Reset latch
Fig.2 Schematic view of the NOR gate structures in our proposed terahertz wave Set-Reset latch
(a)
(b)
(c)
(d)
Fig.3 Steady state electric field distribution of the NOR gate (a) Both input ports (S or R) and (G1 or G2) are set ‘0’; (b) Input ports (S or R)and (G1 or G2) is set to be ‘1’ and ‘0’, respectively; (c) Input ports (S or R) and (G1 or G2) is set to be ‘0’ and ‘1’, respectively; (d) Both input ports (S or R) and (G1 or G2) are set to be ‘1’
Fig.4 Configuration of the MMI region in our proposed terahertz wave Set-Reset latch
(a)
(b)
Fig.5 Steady state electric field distribution of the MMI region (a) Both input port (C or D) and (F1 or F2) are set ‘1’; (b) Input port (C or D) and (F1 or F2) are set to be ‘1’ and ‘0’, respectively.
(a)
(b)
(c)
(d)
(e) Fig.6 Steady state electric field distribution of Set-Reset latch (a) input port S is ‘1’ and R is ‘0’; (b)input port S changes from ‘1’ to ‘0’, and R remains ‘0’; (c) input port S remains ‘0’ and R changes from ‘0’ to ‘1’; (d) input port S remains ‘0’, and R changes from ‘1’ to ‘0’; (e) Both input ports S and R are set to be ‘1’
(a)
(b) Fig.7 Response time chart of the different states of the Set-Reset latch
Tab.1 Truth table of NOR gate Input Port S
Input Port G1
Output Port F1
or Port R
or Port G2
or Port F2
0
0
1
1
0
0
0
1
0
1
1
0
Tab.2 Truth table of our proposed Set-Reset latch S
R
Qn+1
Q'n+1
State
0
0
Qn
Q'n
Hold
1
0
1
0
Set
0
1
0
1
Reset
1
1
1
1
Invalid