Optical bistability in defective photonic multilayers doped by graphene

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1 Opt Quant Electron (2017)49:163 DOI /s Optical bistability in defective photonic multilayers doped by graphene Dong Zhao 1,2 Zhou-qing Wang 1 Hua Long 1 Kai Wang 1 Bing Wang 1 Pei-xiang Lu 1,3 Received: 14 February 2017 / Accepted: 18 March 2017 Springer Science+Business Media New York 2017 Abstract We study the optical bistability (OB) in photonic multilayers doped by graphene sheets, stacking two Bragg reflectors with a defect layer between the reflectors. OB stems from the nonlinear effect of graphene, so the local field of defect mode (DM) could enhance the nonlinearity and reduce the thresholds of bistability. The structure achieves the tunability of bistability due to that the DM frequency and transmittance could be modulated by the chemical potential. Bistability thresholds and interval of the two stable states could be remarkably reduced by decreasing the chemical potential. A lager Bragg periodic number could increase the localizing of field, but the graphene loss may decrease the intensity of transmission light. We have concluded an appropriate periodic number to achieve OB. The study suggests that the tunable bistability of the structure could be used for all-optical switches in optical communication systems. Keywords Optical bistability Nonlinear optics Graphene 1 Introduction Optical bistability (OB) is a nonlinear phenomenon in which the transmission properties of light vary with the incident light power. The behavior can be used for all-optical switches, limiters, transistors and memory elements (Soljacić et al. 2002; Svensson et al. 1990; & Bing Wang wangbing@hust.edu.cn School of Physics and Wuhan National Laboratory for Optoelectronics, Huazhong University of Science and Technology, Wuhan , China School of Electronics Information and Engineering, Hubei University of Science and Technology, Xianning , China Laboratory for Optical Information Technology, Wuhan Institute of Technology, Wuhan , China

2 163 Page 2 of 9 D. Zhao et al. Yanik et al. 2003; Brandonisio et al. 2012). A typical bistable device usually consists of distributed feedback (DFB) structure and nonlinear material (He and Liu 1999). The refractive index of the nonlinear material is a nonlinear function of the incident intensity, which is the nonlinear effect OB originating from. The key point to achieve OB is that the light-induced refractive-index variation should be large enough (Noskov and Zharov 2006). Combining distributed feedback and Fabry Perot (DFB/FP) could make the intensity thresholds of bistability lower compared to a single DFB configuration (He and Cada 1992). Photonic crystal (PC) has also been implemented for bistability both theoretically and experimentally (Wang et al. 2008; Lepeshkin et al. 2004; Husakou and Herrmann 2007). PC has a photonic band gap structure, and the bistability in PC stems from the shifting of the band edge. Due to the excitation of highly localized bulk plasmon polariton, subwavelength metal-nonlinear dielectric multilayers proceeded to reduce the thresholds of OB (Chen et al. 2009). Low threshold OB for the resonant modes was experimentally demonstrated in one-dimensional (1D) PC at visible frequencies (Sreekanth et al. 2015). A Kerr-type nonlinear defect layer was introduced in the center of 1D PC to obtain OB curve of the transmitted light intensity (Wang et al. 1997). The physical mechanism of this doped structure for bistability is dynamic shifting of the defect mode frequency. Moreover, thresholdtunable OB was achieved in doped photonic multilayers (Kong et al. 2013; Mehdian et al. 2013; Centeno and Felbacq 2000). Optical bistability at terahertz frequencies has been realized in sandwich structures embedded with graphene (Dai et al. 2015a, b; Wang et al. 2016a, b; Solookinejad 2016), which utilized the surface plasmon polaritions (SPPs) of graphene. SPPs coupled quantum cascade detector could enhance optical absorption for mid-long-infrared pixels (Li et al. 2016). Fabry Preot cavity with graphene (Jiang et al. 2015) and graphemecovered nonlinear interface (Xiang et al. 2014) have also been used to achieve tunable optical bistability. Graphene possesses various unique optical, electronic, and mechanical properties (Huang et al. 2017; Qin et al. 2016; Wang et al. 2016c;Wang et al. 2017a;Ke et al. 2016) and its large third-order nonlinear coefficient has been theoretically predicted (Mikhailov 2007) and experimentally verified (Hendry et al. 2010; Hong et al. 2013). SPPs on graphene have stronger confinement and relatively less propagation loss in comparison with that on metals (Ke et al. 2015; Liu et al. 2016), so graphene provides new ways to envision light propagation at the nanoscale and ultra-fast switching (Fan et al. 2016; Wang et al. 2017b; Tong et al. 2017; Qin and Zhu 2017; Li et al. 2017; Zhang et al. 2017). The surface conductivity of graphene depends on chemical potential, which can be tuned by chemical doping or gate voltage (Huang et al. 2014). In this work, we design a distributed-feedback Bragg structure doped by graphene sheets. The graphene sheets locate at the interfaces of each dielectric layer and the middle of the defect layer. The structure combines DFB and FP structure, which has a local enhanced effect of defect mode (DM) (also called resonant mode) in near-infrared light range. The energy of the DM is mainly located at the defect layer, which could increase the nonlinear effect of graphene. OB results from the intensity-induced change of the effective dieletric constant of graphene, so the localizing of field could decrease the threshold of OB. The frequency and the transmittance of DM can be fine-tuned by the chemical potential of graphene, which could be used for modulating the threshold of OB. We numerically study light propagation through such special photonic multilayer structure and achieve tunable OB. The tunability of OB results from the variation in surface conductivity of graphene. The difference in output intensity levels of the two stable states is greatly affected by the chemical potential. Bragg periodic number and the detuning of incidence wave to DM can also change the bistable curve profile.

3 Optical bistability in defective photonic multilayers doped Page 3 of Theory The 1D multilayer stack used in this work is shown in Fig. 1, where A and B are lower and higher refractive index materials respectively, C is defect layer, and G are graphene sheets. A and B are alternatively placed in the multilayer stack of dielectric materials, Graphene sheets are located at the interfaces of dielectric layer and the middle of the defect layer. The optical thickness of layers A, B and C are quarter-wavelength with d a = n a k 0 /4, d b = n b k 0 /4 and d c = n c k 0 /2 respectively, where k 0 is the midgap wavelength of PC in normal incidence. We consider a plane wave normally incident onto the 1D PC. Graphene is treated as a thin film with an equivalent thickness of D, then, the relative equivalent permittivity of graphene could be given by e g = 1? ir g g 0 /(k 0 D) (Wang et al. 2016a, b), where k 0 = 2p/k is the incidence wave number in vacuum, g 0 is the vacuum resistivity, r g is the total surface conductivity, and k is the free space wavelength of the incidence wave. When considering nonlinear effect, r g can be written as r g = r 1? r 3 E == 2, where r 1 is the linear surface conductivity, r 3 is the nonlinear surface conductivity coefficient, and E // is the in-pane component of the electric field. The linear surface conductivity of graphene may be modeled using Kubo formula (Chen and Alù 2011; Thongrattanasiri et al. 2012), which depends on incidence wavelength k, chemical potential l c, thickness D, and relax time s. We choose the excitation wavelength in the range of near-infrared light, and set the parameters as D & 1 nm (Wang et al. 2012), and s = 0.5 ps in the following study. The nonlinear conductivity coefficient is (Smirnova et al. 2014) r 3 ¼ i 3 e 2 ev 2 F l c ð1þ 8 p h l c x x where V F & c/300 is the Fermi velocity. The dielectric constant can be rewritten as e g = e 1? v (3) g E == 2, where linear dielectric constant is e 1 = 1? ir 1 g 0 /(k 0 D). Consequently, the cubic volume susceptibility can be expressed as v (3) g = ir 3 g 0 /(k 0 D). Here we use the TMM and assume the samples are nonmagnetic dielectric media. If the wave is obliquely incident from the air background upon the PC, the electric and magnetic fields in two interfaces of one layer can be correlated via a transfer matrix (Fang et al. 1987) 0 cosu M l ¼ l i sinu g l A l ð2þ ig l sinu l cosu l Fig. 1 Schematic of the 1D photonic multilayers. A and B are lower and higher refractive index dielectrics respectively, C is defect layer, and G are graphene sheets. The blue line represents the intensity distribution of defect mode. It shows that the main energy localizes at the defect layer. (Color figure online)

4 163 Page 4 of 9 D. Zhao et al. where u l = 2pd l (e l - sin 2 h) 1/2 /k, d l is the thickness of transmission medium, and h is the incident angle. For TE wave, g l = (e 0 /l 0 ) 1/2 (e l - sin 2 h) 1/2, and for TM wave, g l = e l (e 0 / l 0 ) 1/2 /(e l - sin 2 h) 1/2. It should be noted that for Kerr-type nonlinear material, since its refractive index is dependent on the localized field intensity, the transfer matrix should be solved by nonlinear method. So the nonlinear layer are usually discretized into many sublayers (Kong et al. 2013), then, the field intensity and refractive index can be considered constant in each sublayer if the number of sublayers is large enough. Here, graphene is taken as nonlinear Kerr-like homogeneous ultra-thin film material, and it is sufficiently thin corresponding to the incidence wavelength. Hence, there is no need to divide it into sublayers. The electric field intensity and refractive index of the lth layer graphene can be derived by E l ¼ M H l l E lþ1 H lþ1 where E l, H l, E l?1 and H l?1 are the electromagnetic field intensity at two interfaces of graphene respectively, and M l is the transfer matrix of the lth graphene. The matrix element in the M l contains parameter e l, which can be approximately given by e l = e 1? v (3) g je lþ1 j 2. The process in essence is a nonlinear correction of linear permittivity. So, taking into account the nonlinear surface conductivity, the equivalent permittivity of graphene is intensity-dependent. On the whole, the electric and magnetic fields of the first and last layers can be associated by E 1 ¼ M H 1 E nþ1 H nþ1 ¼ m 11 m 12 Enþ1 m 21 m 22 H nþ1 where m 11, m 12, m 21, and m 22 are the matrix elements of the total transfer matrix M connecting EM fields at the incident and exit ends. Therefore the transmission coefficient can be obtained by transfer matrix 2g t ¼ 0 ð5þ m 11 g 0 þ m 12 g 0 g nþ1 þ m 21 þ m 22 g 0 where g 0 = g n?1 = (e 0 /l 0 ) 1/2 (1 - sin 2 h) 1/2. The transmittance parameter T is identified as T = tt* = I t /I i, where I t and I i are incident and transmitted intensities, respectively. ð3þ ð4þ 3 Results and discussions The basic photonic bandgap of the defective PC as shown in Fig. 1 is Dx gap = 4x 0 arcsinjðn b n a Þ=ðn b þ n a Þj 2. p (Yariv et al. 2007), where x 0 = 2pc/k 0 is the center of band gap and k 0 = 1.55 lm is the free space wavelength, A is MgF 2 and B is ZnS. The linear refractive index of the layer materials are n a = 1.38, n b = 2.35, and n c = 1.5. Dielectrics A and B alternate in Z-axis to form a symmetrical structure about the defect layers. The Bragg periodic number is N = 4 in each side. A series of graphene sheets are embedded in the structure. The DM appears in the middle of the forbidden band as shown in Fig. 2a. When an incident wave has the same frequency as the DM frequency, it could pass through the structure without reflection. The intensity distribution of the DM presents that the main energy localizes at the defect layer. Light-induced refractive-index variance is proportional

5 Optical bistability in defective photonic multilayers doped Page 5 of 9163 Fig. 2 a Transmittance spectrum for several chemical potentials of graphene. b Transmittance versus chemical potential for incidence wavelength k = 1.55 lm. c DM versus chemical potential. The blue line represents the peak value of transmittance, and the red line represents the frequency shift of DM. d Real part (blue line) and imaginary part (red line) of surface conductivity (r g ) of graphene as a function of chemical potential respectively. (Color figure online) to the local light intensity, so the graphene embedded in the defect layer is the main source for dispersive optical bistability. The function of the graphene sheets at the interfaces of dielectrics A and B is tuning the system characters. That is controlling on the frequency shift of defect mode and switching thresholds of bistability. Figure 2a presents the transmittance for l c = 0.40, 0.45 and 0.50 ev. For a special incidence wavelength k = 1.55 lm, the transmittance and detuning could be modulated by chemical potential as shown in Fig. 2b. The graphene thickness has little impact on the wavelength shift of defect mode. The transmittance reaches the peak value 0.96 and the detuning is zero at l c = 0.58 ev. So this characteristic can be used for manipulating the thresholds of OB. It could be found that, when chemical potential is less than 0.39 ev, the transmittance of DM equals to zero, and while it exceeds 0.39 ev, the value rapidly steps up to 1 as shown (the blue line) in Fig. 2c. To explain the phenomena, we plot the curves of surface conductivity of graphene versus the chemical potential as shown in Fig. 1d. It could be seen that the blue curves in Fig. 1c and d are symmetry about the horizontal axis. As l c \ 0.39 ev, the real part of the surface conductivity of graphene is close to 0.6 S, which corresponds to the loss of material in physics, so the transmittance of DM is almost zero. While l c [ 0.39 ev, the value sharply jumps down to zero, and the transmittance of DM almost steps up to 1. The frequency shift curve of DM in Fig. 2c corresponds with the imaginary part of the surface conductivity of graphene in Fig. 2d. The red curve shows that, as l c \ 0.41 ev, the wavelength of DM is red-shift companied with l c increasing, while the wavelength of DM is blue-shift with an increasing in chemical potential as l c [ 0.41 ev. In order to demonstrate how graphene sheets couple with defect layer, we sampled three chemical potentials l c = 0.50, 0.60 and 0.70 ev. The three resonant wavelengths are k r = , and lm, respectively. If the incident wavelength is exactly

6 163 Page 6 of 9 D. Zhao et al. Fig. 3 Bistability phenomenon for Bragg periodic number N = 4. a Dependence of transmittance on incident intensity for several chemical potentials. b Bistability profile of transmitted intensity as a function of incident intensity. c Upper (blue line) and lower (red line) threshold of bistability versus chemical potential. a c with incidence wavelength k = 1.57 lm. d Critical chemical potential achieving bistability for different incidence wavelengths. (Color figure online) equal to the resonant wavelength, the OB behavior is not obtained (Wang et al. 2008). The incidence wavelength is set as k = 1.57 lm. The incidence frequency is close to the three special resonant modes and could guarantee a certain amount of detuning. As introduce the nonlinearity, the strong nonlinear effect makes the defect modes to move toward the incident wave frequency (Soljacić et al. 2002). For the nonlinear graphene embedded in our geometry, the transmittance varies with the local intensity, i.e. the incident intensity, as shown in Fig. 3a. Therefore bistable phenomena occur in the 1D PC when DM frequencies almost equal to incident light frequencies as shown in Fig. 3b. It also demonstrates that the bistability curve can be modulated by chemical potential. In the previous achievements, as the incidence wavelength is invariant, the detuning of the resonant wavelength to the incident wavelength is proportional to the chemical potential. This rightly explains the interval between the up-threshold and the down-threshold gradually expands as chemical potential increases in Fig. 3c. For a given incidence wave, the bistability occurs only when the incidence wavelength equals to the resonant mode under considering nonlinearity influence. The resonant mode is a function of chemical potential, so there is a critical chemical potential where the detuning of the two waves is zero. It means that there is no OB below the critical chemical potential. In Fig. 3c, as the given incidence wavelength is 1.57 lm, the critical chemical potential for OB is about 0.5 ev. Figure 3d depicts the curve of the critical chemical potential versus incidence wavelength. There is no OB in the area under the critical curve. It shows that the critical chemical potential approaches the saturation value 3.9 ev as the incidence wave increases, because the real part of the surface conductivity of graphene sharply drops to zero near potential 3.9 ev as shown in Fig. 2d. In other words, a little variation in the chemical potential results in a large conductivity difference near 3.9 ev. The linear transmittance spectra with different Bragg periodic number N are given in Fig. 4. For different N, the results show that the transmittances of defect mode are different

7 Optical bistability in defective photonic multilayers doped Page 7 of 9163 Fig. 4 a Transmittance spectrum for Bragg periodic number N = 2, 4, 6, with incidence wavelength k = 1.57 lm and chemical potential l c = 0.50 ev. b Defect mode versus Bragg periodic number. The blue line represents the peak value of transmittance, and the red line represents the frequency shift. (Color figure online) Fig. 5 Upper (solid line) and lower (dotted line) threshold of bistability versus Bragg periodic number N, the blue line for incidence wavelength k 1 = 1.57 lm and the red line for k 2 = 1.60 lm respectively, with chemical potential l c = 0.70 ev. (Color figure online) in Fig. 4a. It demonstrates that N is bigger, the resonant peak is sharper, and the transmittance of the defect mode peak drops as N increases. Distinctly, the loss is more serious as N is bigger, so the transmittance of resonant mode reduces step by step with N increasing as shown in Fig. 4b. Almost no light could go through the structure for N = 10. The transmittance of defect mode is close to 1 and the loss is very small when N is less than 2. As N [ 2, the resonant frequency is invariant. It shows that N = 4 or 5 is the appropriate number, because the defect mode simultaneously has a narrow line width and a large transmittance. The difference values in output intensity levels of the two stable states are proportional to layer pair number and detuning as shown Fig. 5. For example, the output intensity interval is smaller for the incidence wavelength k = 1.57 lm than 1.60 lm with a fixed N. It is remarkable that, for N \ 4, there is no OB existing in the structure with an incidence wavelength 1.57 lm. Increasing the incidence wavelength to 1.60 lm, the critical layer pair number is N = 3. 4 Conclusion In summary, we have investigated the bistable phenomenon in a defective PC doped by graphene in near-infrared frequencies. The frequency of DM and threshold of bistability could be separately modulated by changing the chemical potential of graphene, without reconfigurating the multilayer structure. The transmittance curve of defect mode and the

8 163 Page 8 of 9 D. Zhao et al. frequency curve are similar to the figure of the real part and the imaginary part of the surface conductivity of graphene respectively. The threshold values can also be modulated by the Bragg periodic number. The detuning and interval of threshold are proportional to chemical potential and Bragg periodic number. The threshold can be remarkably reduced by decreasing chemical potential or Bragg periodic number. The results show that N = 4 or 5 is the appropriate number for achieving OB. As N and incidence wavelength are fixed, there exists critical chemical potential for OB. The calculations further demonstrate that, for a certain chemical potential and incidence wavelength, the threshold interval increases as N increases. The bistability behavior of the proposed object can be applied in communication systems, such as optical switching. Acknowledgements This work is supported by the 973 Program (Grant No. 2014CB921301), the National Natural Science Foundation of China (Grant Nos , ), Natural Science Foundation of Hubei Province (Grant No. 2015CFA040). References Brandonisio, N., Heinricht, P., Osborne, S., Amann, A., O Brien, S.: Bistability and all-optical memory in dual-mode diode lasers with time-delayed optical feedback. IEEE Photonics J. 4(1), (2012) Centeno, E., Felbacq, D.: Optical bistability in finite-size nonlinear bidimensional photonic crystals doped by a microcavity. Phys. Rev. B 62(12), (2000) Chen, P.Y., Alù, A.: Atomically thin surface cloak using graphene monolayers. ACS Nano 5(7), (2011) Chen, J., Wang, P., Wang, X., Lu, Y., Zheng, R.S., Ming, H., Zhan, Q.: Optical bistability enhanced by highly localized bulk plasmon polariton modes in subwavelength metal-nonlinear dielectric multilayer structure. Appl. Phys. Lett. 94(8), (2009) Dai, X., Jiang, L., Xiang, Y.: Low threshold optical bistability at terahertz frequencies with graphene surface plasmons. Sci. Rep. 5, (2015a). doi: /srep12271 Dai, X., Jiang, L., Xiang, Y.: Tunable optical bistability of dielectric/nonlinear graphene/dielectric heterostructures. Opt. Express 23(5), (2015b) Fan, Y., Wang, B., Wang, K., Long, H., Lu, P.: Plasmonic zener tunneling in binary graphene sheet arrays. Opt. Lett. 41(13), (2016) He, J., Cada, M.: Combined distributed feedback and Fabry Perot structures with a phase matching layer for optical bistable devices. Appl. Phys. Lett. 61(18), (1992) He, G.S., Liu, S.H.: Physics of Nonlinear Optics. World Scientific, New York (1999) Hendry, E., Hale, P.J., Moger, J., Savchenko, A.K.: Coherent nonlinear optical response of graphene. Phys. Rev. Lett. 105, (2010) Hong, S.Y., Dadap, J.I., Petrone, N., Yeh, P.C., Hone, J., Jr, R.M.O.: Optical third-harmonic generation in graphene. Phys. Rev. X 3, (2013) Huang, C., Ye, F., Sun, Z., Chen, X.: Tunable subwavelength photonic lattices and solitons in periodically patterned graphene monolayer. Opt. Express 22(24), (2014) Huang, H., Ke, S., Wang, B., Long, H., Wang, K., Lu, P.: Numerical study on plasmonic absorption enhancement by a rippled graphene sheet. J. Lightwave Technol. 35(2), (2017) Husakou, A., Herrmann, J.: Steplike transmission of light through a metal-dielectric multilayer structure due to an intensity-dependent sign of the effective dielectric constant. Phys. Rev. Lett. 99(12), (2007) Jiang, L., Guo, J., Wu, L., Dai, X., Xiang, Y.: Manipulating the optical bistability at terahertz frequency in the fabry-perot cavity with graphene. Opt. Express 23(24), (2015) Ke, S., Wang, B., Huang, H., Long, H., Wang, K., Lu, P.: Plasmonic absorption enhancement in periodic cross-shaped graphene arrays. Opt. Express 23(7), (2015) Ke, S., Wang, B., Qin, C., Long, H., Wang, K., Lu, P.: Exceptional points and asymmetric mode switching in plasmonic waveguides. J. Lightwave Technol. 34(22), (2016) Kong, X.K., Liu, S.B., Zhang, H.F., Wang, S.Y., Bian, B.R., Dai, Y.: Tunable bistability in photonic multilayers doped by unmagnetized plasma and coupled nonlinear defects. IEEE J. Sel. Top. Quantum Electron. 19, (2013)

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