Abstract
We propose a mechanism to actively tune optical bistable behavior with the external magnetic field in nonlinear coated nanospheres with a magneto-optical (MO) shell and nonlinear metallic core. We show that such nanostructures can exhibit typical bistable phenomena near surface plasmon resonant wavelengths, which can be modified through the external magnetic fields B. We demonstrate numerically that the optical bistability exists only when the volume fraction of the metallic core is larger than a critical one . Moreover, the bistable behavior is found to be dependent on the incident polarization state as well as the external magnetic field. The application of an external magnetic field does not only increase (or decrease) the upper/lower threshold fields but also changes the critical volume fractions. Such nanostructures with magneto-controllable optical bistability may be designed for us as nonlinear optical nanodevices, such as optical nanoswitches, nanosensors and so on.
© 2016 Optical Society of America
1. Introduction
Optical bistability provides us by controlling light with light, and it just refers to an optical effect where a system exhibits two different values of the local field for a single value of the input field [1–3]. Hence, it can give the optical structures the function to control two distinguishing stable states with the history of the input light. The phenomenon of optical bistability has attracted considerable attention due to its promising applications in nonlinear optical technologies such as all optical switching [4,5], low power lasing [6] and low-threshold optical bistability [7]. In this context, a variational approach [8] and self-consistent mean-field approximation [9,10] were, respectively, proposed to examine the optical bistable behavior of nonlinear plasmonic composites near the surface plasmon resonant wavelengths. Recently, optical bistability and modulational instability in nonlinear plasmonic nanoparticles (NPs) arrays were investigated. And plasmonic kinks, oscillons, solitons, and domain wall due to modulational instability were demonstrated [11]. More recently, a two dimension dielectric/metal composite of coated cylinders has been proposed to demonstrate the nonlocality would lead to the tunable Fano resonances and can significant reduce the switching threshold of the optical bistability in the near field and far field, suggesting a nonlocality enhanced nonlinear optical devices [12].
On the other hand, controlling the surface plasmons with a magnetic field provides further opportunities for designing active plasmonic components. At first, in the presence of an external static magnetic field, the MO materials have different propagation constants for circularly polarized waves with right and left-handedness (RCP and LCP, respectively). This is the reason that MO materials can be utilized to design one-way photonic states [13]. Then, combining MO materials with plasmonic materials such as gold and silver opens new routes in the development of efficient and versatile magneto-plasmonic architectures [14]. For instance, a dielectric/plasmonic cylinders coated with a MO shell can be designed to realize the tunable plasmonic cloaks with an external magnetic field [15,16]. And spherical gain NPs coated with metallic MO shell can operate as a MO spaser amplifying the circularly polarized surface plasmons [17]. More recently, Varytis et al. adopted a rigorous full electrodynamic calculation to investigate strong circular dichroism [18], the plasmon-driven Hall photon currents [19] as well as enhanced Faraday rotation by core-shell magnetoplasmonic NPs [20].
In this paper, we would like to investigate the optical bistable behavior in coated NPs containing a nonlinear plasmonic metallic core and a linear MO shell in the quasistatic limit. We aim at the influence of the MO effect on the bistable hysteresis of the nanostructures. Since the optical features of MO material are purely induced by an external magnetic field, we shall show that can easily control the bistable threshold fields, the bistable region, and the critical volume fraction needed to achieve optical bistability for both CP waves via adjusting the external static magnetic field.
2. Theoretical development
Let us first consider a linear coated magnetoplasmonic nanoparticle consisting of a linear metallic core with the permittivity and radius coated by a MO (Bi:YIG) spherical shell with the permittivity tensor and radius , as shown in Fig. 1. Actually, there are some theoretical discussions on the magnetic-optical enhancement and magnetic spaser based on the present model [17,21], and some works on the experimental synthesis of metallic-MO composite nanostructures in the laboratory [22,23]. For simplicity, we assume that the MO medium magnetization vector (the direction of the external uniform magnetic field B) is parallel to the z-axis. Thus, the permittivity tensor of the shell has the form [24]
where the off-diagonal component is responsible for the “strength” of MO activity of the media. It has a dispersive character and depends upon B, vanishing in its absence. As a consequence, the optical anisotropy and the MO effect of such a material is purely induced by the external magnetic field.In the quasistatic approximation, the size of NP is very small compared with the incident wavelength, the local electric field inside the core can be expressed through the incident field as [25]
the field inside the MO shell may be sought asand the field outside the core-shell NP is written aswhere , , and are some unknown antisymmetric tensors, and is the distance from the center of the system. Employing the Maxwell’s boundary conditions at the inner and outer surfaces, i.e.,we arrive at the following system of equations,where is the nondiagonal part of the permittivity tensor . These equations allow us to determine the tensors , and with the form,with whereOn the other hand,where the element is also related to , , and , and here we don't show the formula of due to its complex form. In the subspace , there are two eigenvalues of the tensor ,Actually, is nothing but the polarizabilities corresponding to the left and right circularly polarized (LCP and RCP, respectively) incident light [25].According to Eqs. (2) and (7), the field in the core can be written as
Then,When the incident light is LCP or RCP (here we omit the harmonic time dependence of the electric field ), the relation between the field in the core and the external applied field can be simplified aswhere is the element of the tensor .Next, we would like to learn nonlinear coated NPs in which spherical metallic core is nonlinear with the field-dependent permittivity . In general, in the weakly nonlinear limit, the contribution from the nonlinear part is much less than the linear one. However, in the strong field case, the nonlinear part may be comparable to or even larger than the linear one. In this connection, we should adopt the self-consistent field theory to evaluate the local filed in the nonlinear core [10,26,27]. Similar as Eq. (16), the relation between the local field in the nonlinear core and the applied filed can be written as
Not that are dependent on , or . As a consequence, Eq. (16) can readily be solved in a self-consistent manner for , and hence the desired optical bistability may be obtained. Here we mention that since both and contain the off-diagonal term in the MO shell, which can be tuned with an external magnetic field. Therefore, in our present nonlinear coated nanodevices, the tunable optical bistability with an external magnetic field can be realized.3. Numerical results
We are now in a position to present the numerical results. The linear permittivity of the metallic core is assumed to be given by the Drude formula , where , and [28]. The parameters for the anisotropic shell with Bi:YIG are , [29].
In Fig. 2, we show the scattering efficiency of the linear coated NP, defined as with the factor given by for LCP, as a function of the incident wavelength . It is evident that the linear scattering efficiency exhibits significant enhancement around the surface plasmon resonant wavelength of the present coated nanosphere. For a given size , as the volume fraction of the core [] increases, the scattering peak is increased, accompanied with the blue-shift of the resonant wavelength, as shown in Fig. 2(a). Actually, for pure metallic NPs (), the surface plasmon resonant wavelength is given by = 343nm. In Fig. 2(b), the resonant wavelengths are almost unchanged as the shell size increases when is fixed. Qualitively, in the quasistatic limit, the equivalent permittivity of the core-shell NPs are only dependent on the volume fraction , not the single size or . Hence, for the fixed , the surface plasmon resonant wavelength keeps invariant with increasing . Here, we would like to mention that the scattering efficiency for RCP (not shown here) is quite similar as those in Fig. 1 for LCP but with a minor difference in the surface resonant wavelength. In this regard, one may adopt , the difference between the scattering efficiency for LCP and RCP incident light to measure the circular dichroism quantitatively, and strong circular dichroism may be observed [18].
The far-field scattering spectra can also be reflected from the near-field properties. In Fig. 3, we show the corresponding field distributions of the coated NPs at the surface plasmon resonant wavelength , and below (or above) the resonant wavelengths. One observes the local-fields in the metallic core coated with the MO shell are almost uniform in both resonant and nonresonant wavelengths due to the dipolar approximation. At the nonresonant incident wavelengths [see Fig. 3 (b) and Fig. 3(c)], the small local fields in the MO shell and the host medium are almost unperturbed, and hence the scattering efficiency is small. On the contrary, at the surface resonant wavelength [see Fig. 3 (a)], the local field is largely enhanced with the resonant state resulting in large scattering efficiency.
The enhancement of local fields within the metallic core near the surface plasmon resonant wavelengths represents an ideal condition to boost the nonlinear response of optical materials. Therefore, we consider the optical switching effects in such nonlinear core-shell metal-MO NPs. For this propose, we consider the nonlinear permittivity of the metallic score to be with the third-order nonlinear susceptibility [28], which is given in the esu-cgs system. Accordingly, the unit of electric field is chosen as statvolt/cm (). In Fig. 4(a), we plot the electric field amplitude in the nonlinear core as a function of the external field for various volume fraction . Bistable responses can be clearly observed. Take as an example, the electric field in the nonlinear core first increases as the incident field increases from zero. When the incident field amplitude reaches the upper threshold field , the electric field amplitude in the core will discontinuously jump to the top stable branch. If the incident field is decreased back from a large value to zero, the electric field in the core will first decrease continuously, and then jump down to the lower stable branch when the incident field amplitude reaches the lower threshold field . We find that the bistable region gets broad, accompanied with the increase in both lower and upper threshold fields as increases. To one’s interest, one predicts there is a critical volume fraction , below which the optical bistable behavior vanishes, as shown in Fig. 4(b).
Since for the MO shell, the off-diagonal term of the permittivity tensor can be adjusted through the magnetic field B, we shows the optical bistability for various values of in both cases of LCP and RCP incident waves in Fig. 5. In the absence of B, we have , and the optical anisotropy and the MO effect of the shell material is absent. In this case, both incident LCP and RCP yield the same optical bistable curve [see the black dashed line in Fig. 5(a)]. For incident RCP, one can see that both the upper and lower threshold fields increases with increasing or the external magnetic fields, as shown in Fig. 5(a) or Fig. 5(b). In addition, broad hysteresis loop for large B is found. On the other hand, for incident LCP, the increase in the external magnetic field (or g) results in the decrease of the upper and lower threshold fields [see Fig. 5(a) or Fig. 5(b)]. Moreover, from Fig. 5(b), the upper threshold fields are dependent on the value of significantly compared with the lower threshold field. And the LCP wave possesses the lower threshold field than the RCP wave. Here, we would like to give some physical explanations about the effect of the external magnetic field on the bistable threshold fields. In essence, the large enhancement of optical nonlinearity results from the enhancement of the spatial local field in the nonlinear metallic core, which will lead to the lower threshold field. The corresponding linear field distributions in such coated nanostructures for LCP incidence for different B are plotted in Fig. 5(c). We note that the magnitude of the electric field in the core for LCP incident wave is indeed enhanced with increasing , and hence the threshold fields become small. Here we conclude that the bistability behavior of the structure depends on the incidence polarization as well as the “strength” of MO effect. This magnetic-controllable optical bistability can directly lead to the design of nonlinear optical nanocircuit components and new-generation tunable sensors [5].
We have mentioned above that the bistability will disappear below the critical value . Here, we shall show the effect of the gyration parameter on the critical volume fraction in Fig. 6. As increases, for incident LCP wave increases while the one for RCP wave decreases. Therefore, the presence of B can also modify the operational volume fraction range of the optical bistability.
4. Conclusion
In this paper, we propose a new nonlinear nanostructure—a plasmonic metallic core coated with MO shell to realize the tunable optical bistable behavior with the external static magnetic field B. In the quasistatic limit, we show that for LCP and RCP incident waves, such nanostructures exhibits different hysteresis loops. Both upper and lower threshold fields decrease (or increase) and the bistable region becomes narrow (or broad) for LCP (or RCP) wave by increasing the strength of external magnetic field B. Moreover, as the strength of MO effect increases, the critical volume fraction for LCP (or RCP) wave increases (or decreases). Since the optical properties of the shell (such as the strength of the MO activity) depend on the external static magnetic field, one can tune the threshold fields and the bistable region for both CP waves by tuning the magnitude of the external magnetic field. In other words, the optical bistability of this kind of nanostructures can be easily controlled by adjusting the static magnetic field. Our study may pave some ways for us to realize tunable nonlinear optical nanodevices such as optical nanoswitches, optical isolators and so on.
Funding
National Natural Science Foundation of China (NSFC) (Grant No. 11374223); National Basic Research Program (Grant No. 2012CB921501); Qing Lan Project; Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions.
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