1. Introduction

As one type of non-diffraction waves, Airy beams have simulated considerable research interest owing to their intriguing diffraction-free, self-accelerating, self-healing, and abruptly autofocusing properties [1–4]. It is widely believed that Airy beams can greatly promote a number of applications including biomedical treatment [5], non-linear effects [6,7], optical micromanipulation [8], and chip-scaled signal processing [9,10]. Conventional methods to generate Airy beams require a complex and bulky optical system including a Fourier transform lens and a spatial light modulator [4], which hinders their applications in micro/nano photonic applications.

Metasurfaces, a two-dimensional version of metamaterials, have provided an unprecedented approach to control electromagnetic waves by locally modulating the phase, amplitude and/or polarization of the scattered field [11–13]. Thus, metasurfaces have shown great potentials to reshape the wave-front of light, leading to a variety of fascinating wave-manipulation effects including light bending [11,14,15], undirectional surface plasmon coupling [12,16], invisibility cloaks [17,18], flat lenses [13,19], holography [20,21], and generation of vortex beam [11,22]. A number of ultra-compact Airy beam generators for either free-space light [2,23–28] or surface plasmons (SPs) [1,29–35] have been successfully realized for micro/nano photonic applications based on metasurfaces. Typically, the free-space Airy beams are produced by using nanorods [2,23] or C-shaped aperatures [24,27,28] to simultaneously modulate the phase and amplitude. In parallel with the above advances, the plasmon Airy beam on a metal surface has attracted special attentions due to its potential applications in the fields of plasmonic circuitry and surface optical manipulation. The past several years have witnessed a rapid development with a great deal of theoretical and experimental studies on the plasmon Airy beam. Initially the plasmon Airy beams were experimentally demonstrated using diffraction grating [1,30–32,34] and non-periodical nanocave array on a metal surface [29,35]. Such kinds of configurations enable the excitation of the SPs as well as phase modulation of the excited SPs. However, the excitation of SPs highly relies on linearly-polarized incidence whose polarization should be along the grating period. The orthogonally linearly polarized incidence fails to excite the SPs propagating along the desired direction so that Airy plasmons could not be formed in principle. As one of the typical building blocks of metasurfaces, subwavelength metallic slit resonators are widely used for SPs manipulation on a metal surface [36–38]. Through the careful design of suitable subwavelength slit resonators and the prescribed arrangement of their spatial distributions, it has been proven capable of flexibly controlling the excitation of SPs as well as modulating their amplitude and phase with different polarization incidences. Many practical applications with metallic slit resonators including polarization-controlled asymmetrical excitation [36], wave-front manipulation [37], and generation of Airy beam [38] for SPs have been proposed and demonstrated. However, we emphasize here that the Airy plasmon generation strategies examined thus far (plasmonic gratings or slit resonators), rely on a certain linearly polarized incidence for plasmonic gratings [1,30–32,34], or circularly polarized incidence for slit resonators [38]. It can be inferred that, based on these configurations mentioned above no Airy plasmon could be generated if the polarization state is switched to the orthogonal linear polarization for plasmonic gratings in [1,30–32,34], or a linearly polarized incidence is used for slit resonators in [38]. Consequently, exploitation of new metasurface design applicable to both of linearly and circurly polarizations would afford more freedom in controlling the generation of Airy plasmons, and thus promote new applications in various micro/nano photonic applications.

In this paper, we have proposed a metasurface design that is capable of generating Airy plasmons under different polarization states with different arrangements of the paired nanoslits. With the proper design of the meta-atoms, i.e., paired nanoslit resonators, the phase distributions of the excited SPs along the nanoslit column direction can be adjusted to follow the Airy phase distributions for either linearly- or circularly-polarized incidence. Airy plasmons can thus be formed on a metal surface with different polarization incidences. Distinct from the previously reported configurations, our proposal can be applicable to x-, y-, left-handed circular polarized (LCP), and right-handed circular polarized (RCP) incidences.

2. Results

2.1 Design principle

For a sub-wavelength nanoslit resonator in a metal film, SPs can be excited only if the incident light contains electric-field component perpendicular to the long axis of the nanoslit resonator [$E_1$, and $E_2$ in Fig. 1(a)]. The electric fields $E_1$, and $E_2$ can be expressed as $E_1\text{=}{\overrightarrow{e}}_1\cdot \overrightarrow{E}$, and $E_2\text{=}{\overrightarrow{e}}_2\cdot \overrightarrow{E}$, where ${\overrightarrow{e}}_1$, and ${\overrightarrow{e}}_2$ represent the unit vectors oriented prependicularly to the the long axis of left and right nanoslit resonators, respectively, and $\overrightarrow{E}$ denotes the incident electric field. It has been previously demonstrated that SP field excited by a single nanoslit resonator can be approximately seen as an in-plane magnetic dipole [37], where the theory of SP generation by a dipole has been studied analytically in [39]. Furthermore, when such nanoslits are arranged in a column with a spacing that is smaller than the SP wavelength, the generated SPs are plane waves that propagate perpendicularly to the nanoslit column [36]. By arranging the nanoslit resonators in double columns spaced by $a$ [Fig. 1(a)], the launched SPs from each nanoslit resonator will interfere constructively and thus form a new wave-front that is perpendicular to the column. To generate plasmon Airy beams [Fig. 1(b)], the amplitude and phase distributions of the excited SPs should abide by the truncated Airy function as shown in Figs. 1(c) and 1(d). The phase shift of SPs by the meta-atom, i.e., paired nanoslit resonators, can be well controlled by the orientation angles (${\theta }_1$, ${\theta }_2$) as well as the spacing, $a$. Assuming a beam of light illuminates the paired nanoslit resonators with normal incidence, its electric field, $\overrightarrow{E}$, can be deemed as a combination of x-component and y-component, $A_x$ ($A_y$), and ${\varphi }_x$ (${\varphi }_y$) represent the amplitude and phase of x-component (y-component), respectively. Considering two points M and Q that have a distance of $b$ from the left and right columns, respectively, the x- and z-componets of the electric field of the excited SPs at point M (Q) contributed by the left andright nanoslit resonators are proportional to $E_L^M$ ($E_L^Q$) and $E_R^M$ ($E_R^Q$) [36,37]

 

Fig. 1 (a) Schematic of double columns of nanoslit resonators (left side), where one pair of nanoslit resonators (a meta-atom) are spaced with $a$ (right side). The left and right slits are illuminated by electric fields of $E_1$, and $E_2$ with the orientation angles being represented by ${\theta }_1$, and ${\theta }_2$, respectively. The orientation angles ${\theta }_1>0$ (${\theta }_2<0$) are defined as the anticlockwise (clockwise) angles with respect to x-axis. (b) Schematic of the plasmon Airy beam generator on a 200 nm thick gold film. (c) The amplitude and (d) phase profiles for the truncated Airy function with $n$ = 0.001, and $Y_0$ = 1000 nm, where $n$ is a positive value to ensure truncated Airy beams, and $Y_0$ is a transverse scale.

Download Full Size | PDF

To get the electric field components of the launched SPs at point M (Q) as shown in Fig. 1(a), the near-field coupling or scattering effects from the adjacent nanoslit column on the SPs generation by a nanoslit column should be taken into account. To evaluate the influence of the near-field coupling or scattering effects on the generated SP fields, we have simulated the SPs field generated by the individual nanoslit column and the SPs field in the presence of the two nanoslit columns. The simulation work is conducted by three-dimensional (3D) finite difference time domain (FDTD) method with a commercial software FDTD Solutions [40]. It is apparent from Fig. 2(a) that, the linearly superposed SPs fields excited by individual nanoslit columns match closely with the SPs fields generated when both of the two columns are present [Fig. 2(a)], indicating the near-field coupling or scattering effects can be ignored [36]. As a result, the electric field components of the launched SPs at point M (Q) are proportional to the superposition of $E_L^M$ ($E_L^Q$) and $E_R^M$ ($E_R^Q$) [36,37]

 

Fig. 2 (a) The electric field distributions of $E_z$ along the x direction for the linearly superposed SPs field excited by individual nanoslit columns (blue solid line) and for the generated SPs field in the presence of the two nanosilt columns (red dashed line) with ${\theta }_1$ = 45°, ${\theta }_2$ = −45° for LCP incidence. (b, c) Simulated phase maps of $E_z$, for the excited SPs with (b) ${\theta }_1^P$ = 45°, ${\theta }_2^P$ = −45°, and with (c) ${\theta }_1^N$ = −45°, ${\theta }_2^N$ = 45°, respectively, for LCP incidence. (d) Schematic of the Airy beam generator for circularly-polarized incidences. (e, f) Simulated electric field distributions of $|E_z{|}^2$ 10 nm above the metal surface with $Y_0$ = 1000 nm for (e) LCP, and (f) RCP incidences, respectively, under the illumination of a Gaussian beam with the beam waist of 12 μm along both the x and y directions. In the simulation, the gold nanoslit is 50 nm in width and 200 nm in length, and the permittivity of gold can be retrieved to [41], and other geometrical parameters are set as $a$ = 298 nm and $c$ = 200 nm.

Download Full Size | PDF

For a RCP or LCP incidence, $A_x=A_y=A$, and ${\varphi }_y-{\varphi }_x=\pm \pi /2$, the electric fields $E_1$, and $E_2$can be expressed as $E_1\text{=}A{e}^{i{\varphi }_x}(\cos {\theta }_1\pm i\sin {\theta }_1)$, and $E_2\text{=}A{e}^{i{\varphi }_x}(\sin {\theta }_1\mp i\cos {\theta }_1)$, respectively. Then, Eq. (7) can be written as

To generate the phase distributions of Airy function for SPs along the y direction, two meta-atoms with different orientation angles (marked as ${\theta }_1^P$ and ${\theta }_1^N$) are used to yield the alternative appearing 0 and π phase profile. According to Eq. (8), a phase shift of π can be introduced as the orientation angles satisfy ${\theta }_1^P\text{-}{\theta }_1^N\text{=}\pm \pi /2$. It is worth noting here, this condition is applicable for both the LCP and RCP incidences.

For a linearly polarized (LP) incidence, the electric fields $E_1$, and $E_2$ can be denoted as $E_1\text{=}{A}_x{e}^{i{\varphi }_x}\cos {\theta }_1$, and $E_2\text{=}{A}_x{e}^{i{\varphi }_x}\sin {\theta }_1$ for x polarization, and $E_1\text{=}{A}_y{e}^{i{\varphi }_y}\sin {\theta }_1$, and $E_2\text{=-}{A}_y{e}^{i{\varphi }_y}\cos {\theta }_1$ for y polarization. Then, Eq. (7) can be expressed as

The truncated one-dimensional Airy function is described as

It is worthy of note here, the design strategy of generating Airy plasmons with double-column nanoslits is such general that one can use it to realize Airy plasmons in any spectral regime. First of all, the required SPs can be excited on the metal-air interface by use of nanoslits so long as the width, $W$, and the length,$L$, of the nanosilt meet the condition as $W<<L<{\lambda }_{spp}$, where${\lambda }_{spp}$ represents the effective wavelength of the SPs, and the metal is sufficiently thick to avoid the coupling of SPs on both sides of the metal layer. Secondly, the orientation angles used to produce field intensity and phase distributions of SPs under different polarizations are independent of the working wavelength and plasmonic materials. Once the geometrical parameters of the nanoslit is selected, the spacing, $a$, between the double slits are determined by ${\beta }_{r}a=\pi$. To implement Airy plasmons, once the suitable plasmonic materials (Al, Au and Ag for visible and infrared frequencies, and graphene and semiconductor materials for terahertz frequencies) are chosen for the working wavelengths, one can design the structural parameters of the nanoslits for generating Airy plasmons by using the design principle mentioned above.

2.2 Generation of Airy plasmons with circularly-polarized incidences

To validate the phase control method, we have simulated the excitation of the SPs with two columns of the nanoslit resonators [left side of Fig. 1(a)] under LCP incidence of 633 nm wavelength. The simulated phase maps of the electric field component, $E_z$, for the SPs with one meta-atom (${\theta }_1^P$ = 45°, ${\theta }_2^P$ = −45°) and the other meta-atom (${\theta }_1^N$ = −45°, ${\theta }_2^N$ = 45°) are presented in Figs. 2(b) and 2(c), respectively. We can see that the excited SPs propagate away

from both columns of the nanoslit resonators along x-axis, and a π phase difference between the two meta-atoms is formed, which is well consistent with that predicted by Eq. (8). The two meta-atoms can also introduce a phase difference of π for RCP incidence (not shown here). We further periodically arrange the two meta-atoms along the y direction [Fig. 2(d)], where the phase shift offered by the individual meta-atom abides by truncated Airy function. Namely, a pair of nanoslits with the rotation angles (${\theta }_1^P$ = 45°, ${\theta }_2^P$ = −45°) is used to offer zero phase shift, and another pair of nanoslits with the rotation angles (${\theta }_1^N$ = −45°, ${\theta }_2^N$ = 45°) is employed to provide π phase shift. As a result, the phase profile along the y direction follows the phase distributions required by the truncated Airy function presented in Fig. 1(d). Once upon the illumination of LCP or RCP, Airy plasmon beams can be generated on both sides of the nanoslit resonators [Figs. 2(e) and 2(f)].

2.3 Generantion of Airy plasmons with linearly-polarized incidences

It can be easily inferred from Eq. (11) that, to produce Airy phase distributions with either x- or y-polarized incidence, the orientation angles for both meta-atoms can be varied within a wide range. With a proper combination of the orientation angles of ${\theta }_1^P$ and ${\theta }_1^N$, the proposed scheme can be designed to operate with x- or y- polarization dependence. If ${\theta }_1^P$ and ${\theta }_1^N$ satisfy ${\theta }_1^P\in \text{(-}\pi /4,\pi /4)\text{,}{\theta }_1^N\in [-\pi /2,\text{-}\pi /4),(\pi /4,\pi /2]$, and ${\theta }_1^P+{\theta }_1^N=\pi /2$, the proposal is applicable to x-polarized incidence. If ${\theta }_1^P$ and ${\theta }_1^N$ satisfy ${\theta }_1^P\in (0,\pi /4),{\theta }_1^N\in (\text{-}\pi /2,-\pi /4)$, and ${\theta }_1^P+{\theta }_1^N=0$, the proposal can work for y-polarized incidence. It can be easily inferred that, it is impossible to find a set of (${\theta }_1^P$, ${\theta }_1^N$) that enables the generation of Airy plasmons for both x- and y-polarized incidences. In other words, the proposal cannot be designed to generate Airy plasmons regardless of polarization. As an example, we have chosen$({\theta }_1^P,{\theta }_2^P)=\left(90°,0°\right)$, $({\theta }_1^N,{\theta }_2^N)=\left(90°,0°\right)$ as the orientation angles for the two meta-atoms to justify the polarization selectivity for the generation of Airy plasmons [Fig. 3(a)]. The simulated field distributions on the metal surface in Figs. 3(b) and 3(c) clearly confirm our prediction. It is apparent that x-polarized incidence produces Airy plasmons on both sides of the slit columns [Fig. 3(b)], while y-polarized incidence does not work [Fig. 3(c)]. The generation of Airy plasmon beam can be switched from the x-polarization dependence to y-polarization dependence by taking $({\theta }_1^P,{\theta }_2^P)=\left(45°,-45°\right)$, $({\theta }_1^P,{\theta }_2^P)=\left(-45°,45°\right)$ [Fig. 3(d)]. In contrast to Figs. 3(b) and 3(c), the generation of Airy plasmons can merely occur under y-polarized incidence [Figs. 3(e) and 3(f)].

 

Fig. 3 (a) Schematic of the Airy plasmon generator under the incidence of x-polarization dependence. Simulated electric field distributions of $|E_z{|}^2$ 10 nm above the metal surface for (b) x- and (c) y-polarized incidences as the orientation angles (${\theta }_1$, ${\theta }_2$) of the two meta-atoms are (90°, 0°), (0°, 90°), respectively. (d) Schematic of the Airy plasmon generator under the incidence of y-polarization dependence. Simulated electric field distributions of $|E_z{|}^2$ 10 nm above the metal surface for (e) x- and (f) y-polarized incidences as the orientation angles (${\theta }_1$, ${\theta }_2$) of the two meta-atoms are (45°, −45°), (−45°, 45°), respectively. In the simulations, all the other structural parameters are the same as those in Fig. 2.

Download Full Size | PDF

Figures 2(e), 2(f), 3(e) and 3(f) show that, y-polarized, LCP, and RCP incidences share the same orientation angles of (${\theta }_1^P$ = 45°, ${\theta }_2^P$ = −45°) and (${\theta }_1^N$ = −45°, ${\theta }_2^N$ = 45°) to generate the Airy plasmons by coincidence. Actually, a π/2 difference between the orientation angles, ${\theta }_1^P-{\theta }_1^N$, for the two meta-atoms is not a prerequisite to yield Airy plasmons for linearly-polarized incidences. Nevertheless, a π/2 difference between the orientation angles should be pre-designed under LCP and RCP incidences. This means that, with a proper combination of (${\theta }_1^P$, ${\theta }_2^P$), and (${\theta }_1^N$, ${\theta }_2^N$) for the two meta-atoms, it is highly expected to design an Airy plasmon generator that can merely operate under a particular linearly-polarized incidence. To justify our prediction, we have examined the electromagnetic response as the difference between the orientation angles, ${\theta }_1^P-{\theta }_1^N$, for the two meta-atoms is far from π/2. When the orientation angles (${\theta }_1$, ${\theta }_2$) for the two meta-atoms are set to be (30°, −60°), (60°, −30°), respectively, a π phase shift for the two meta-atoms is introduced for x-polarized incidence [Fig. 4(a)]. We can see that the dominant lobe of Airy plasmons appears though the scattering field significantly blocks the formation of Airy plasmon beam [Fig. 4(b)]. We have extracted the field intensity profile of ($|E_z{|}^2$) at x = 13.5 μm with $Y_0$ = 1000 nm in Fig. 4(b), which is basically consistent with the field intensity profile of Airy function [Fig. 4(f)]. No Airy plasmon beam is generated for y-polarized incidence since the excited SPs for the two meta-atoms propagate in phase [Fig. 4(c)]. According to Eq. (8), the two meta-atoms yield alternative appearing 0 and π/3 phase profile for CP incidences, which deviates significantly from the phase distributions of Airy function. It is therefore not surprising that the resultant field intensity distributions do not resemble Airy function distributions under LCP and RCP incidences [Figs. 4(d) and 4(e)]. For the y-polarized and CP incidences, the extracted field intensity profiles are significantly deviated from Airy function [Figs. 4(g)-4(i)]. It thus suggests that only if the field intensity profile is consistent with Airy function that Airy plasmons could be possibly generated for the presented scheme.

 

Fig. 4 (a) Schematic of Airy plasmon generator for x-polarization incidence. Simulated electric field distributions of $|E_z{|}^2$ 10 nm above the metal surface for (b) x-, (c) y-polarized incidences, and (d) LCP, (e) RCP incidences with the orientation angles (${\theta }_1$, ${\theta }_2$) for the two meta-atoms as (30°, −60°), (60°, −30°), respectively. In the simulations, all the other structural parameters are the same as those in Fig. 2. (f-i) The field intensity profiles of ($|E_z{|}^2$) extracted from the datum at x = 13.5 μm in (b-e).

Download Full Size | PDF

2.4 Properties of the Airy plasmons

We next investigate the optical properties of Airy beams in terms of nondiffracting, and self-healing, and autofocusing effects. The number of slit pairs is an important parameter when optimizing the performance of the Airy SP generator. If the range of $y/Y_0$ is kept fixed at −10 to 4, a larger number of slit pairs will lead to a larger value of $Y_0$. The corresponding numbers of slit pairs are 29, 36, and 47, respectively, with $Y_0$ = 600, 750, and 1000 nm, respectively. Figures 5(a)-5(c) present the field intensity distributions on the metal surface for three different values of the transverse scale ($Y_0$ = 600, 750, and 1000 nm, respectively) under LCP incidence. The transverse acceleration and propagation distance without diffraction are related to the value of $Y_0$ [2,4]. A smaller $Y_0$ leads to significantly increased transverse acceleration. Besides, a longer non-diffraction distance can be obtained with a larger $Y_0$. This can be easily understood by the concepts of propagating and evanescent waves as discussed in [42]. During the propagation, the evanescent components gradually increase and finally dominate the Airy plasmons, resulting in the diffraction effect. The spectrum of propagating waves with a larger $Y_0$ is wider than that with a smaller $Y_0$, resulting in a longer propagation distance of the non-diffraction. In our work, we have taken $Y_0$ = 1000 nm (47 pairs of nanoslits) as an example to demonstrate the performance of the Airy plasmon beam in terms of non-diffraction length. We have extracted the intensity profiles ($|E_z{|}^2$) at x = 13.5 μm with $Y_0$ = 1000 nm in Fig. 5(c), which is basically consistent with the field intensity distributions of an ideal Airy beam [see Fig. 5(d)]. The fact that the main lobe maintains its intensity profile and tends to accelerate during propagation further demonstrates that the Airy plasmons have strong capability of resisting the influence of scattering field and diffraction.

 

Fig. 5 (a-c) Simulated electric field distributions of $|E_z{|}^2$ 10 nm above the metal surface for $Y_0$ = 600 nm (a), 750 nm (b), and 1000 nm (c), respectively. (d) The field intensity profiles ($|E_z{|}^2$) extracted from the datum at x = 13.5 μm in (c). In the simulation, all the other parameters are the same as those in Fig. 2.

Download Full Size | PDF

The non-diffraction property for the Airy plasmons is evaluated by the full width at half maximum (FWHM) of the main lobe along the x direction. To avoid the influence of the noise field near the nanoslits on the field distributions of the main lobe, the extraction of the FWHM is made at x = 13 μm, a little bit far from the nanoslits. The non-diffraction property is strictly defined as the FWHM is within the range of 1.3-1.8 μm. The propagation distance without diffraction can thus reach approximately 37 μm, nearly 3.7-fold of the propagation length of SPs (~10.1 μm). It should be noted that the non-diffraction length is independent on the propagation loss of the SPs but is limited by the finite size of the initial Airy SPs [2], which is associated with the number of slit pairs for the current design. A larger propagation loss will decrease the field intensity of the Airy plasmon beam but not influence the non-diffraction length. The FWHM quickly increases in the diffracting zone [gray shaded region in Fig. 6(a)] and the non-diffraction feature of the Airy plamons gradually fades out. For the self-healing property, it is studied by etching a squared hole of 1 μm × 1 μm along the propagating path of the main lob on the right side of nanoslit column [see the black squares in Figs. 6(b)-6(e)]. For all the cases under different polarization incidences shown in Figs. 2(d), 3(a), 3(d), and 4(a), the main lob of the Airy plasmons is significantly disturbed near the squared hole, but recovers quickly after leaving. The generated Airy plasmons can recover as well after leaving a bigger obstacle of a rectangle-shaped hole of 1 μm × 2 μm [Fig. 6(f)]. These simulation results indicate that the presented configuration is rather robust to resist the disturbance.

 

Fig. 6 (a) The FWHM of the main lobe versus the propagation distance. The black dashed lines represent 1.3, and 1.8 μm, respectively. (b-e) Simulated electric filed distributions of $|E_z{|}^2$ for $Y_0$ = 1000 nm as a squared hole of 1 μm × 1 μm (denoted as a black square) is placed at (x, y) = (12.5 μm, 4.25 μm) for the cases in Figs. 2(d), 3(a),3(d), and 4(a), respectively. (f) Simulated electric field distributions of $|E_z{|}^2$ for $Y_0$ = 1000 nm when the squared hole in (b) is a rectangle-shaped hole of 1 μm × 2 μm. In the simulations, all the other parameters are the same as those in Figs. 2-4.

Download Full Size | PDF

Last but not the least, the Airy plasmon beam can be used for the demonstration of abruptly autofocusing effect on a metal surface. Such an effect here can be realized by symmetrically arranging two Airy plasmon generators with respect to y = 0, as schematically shown in Figs. 7(a) and 7(b). The geometrical parameters for both of the upper (y>0) and lower (y<0) Airy plasmon generators are the same as those in Fig. 2. We can see the focusing effect (appearing around 36 μm from the nanoslit column) on both sides of the nanoslit resonators under a CP incidence [Figs. 7(c) and 7(d)]. To further demonstrate the focusing effect, we have extracted the normalized field intensity profiles along the dashed lines denoted in Figs. 7(c) and 7(d), which show an obvious focusing effect with a FWHM of $1.7{\lambda }_0$ [Fig. 7(e)]. Since the Airy plasmons have strong self-healing property as mentioned above, such a configuration shows significant superiority over the conventional plasmonic focusing lens that rely on the phase accumulation during light propagation [43]. We hope that the autofocusing property of the generated Airy plasmons should find various potential applications, such as medical examination and nano-particle manipulation [44].

 

Fig. 7 (a) Autofocusing effect by double Airy plasmon generators (symmetrical with respect to y = 0) with a distance, $d$, of 200 nm. The truncated Airy function provided by positive- and negative-going nanoslit resonators ranges from −2.7 to 9.9 along the y direction. (b) Schematic of the metasurface used in (c, d). (c, d) Simulated electric filed distributions of $|E_z{|}^2$ for the designed autofocusing Airy plasmons with $Y_0$ = 1000 nm under (c) LCP and (d) RCP incidences with a Gaussian beam waist of 22 μm. (e) The normalized field intensity profiles of ($|E_z{|}^2$) along the white dashed lines denoted in (c, d). In the simulations, all the other parameters are the same as those in Fig. 2.

Download Full Size | PDF

3. Conclusion

In conclusion, we have proposed a strategy of generating Airy plasmons with paired nanoslit resonators. The phase distributions of the excited SPs along the nanoslit column direction can be tuned to follow the Airy phase distributions for either linearly- or circularly-polarized incidence. Distinct from the conventional method that operates under a certain polarization state for the incident light, our proposal can generate Airy plasmons under different polarization states with a proper adjustment of the orientation angles of the paired nanoslit resonators. The unique properties of Airy plasmons including the non-diffraction, self-healing, and autofocusing behaviours have been demonstrated with FDTD simulations. Finally, it is worth emphazising, our approach provides the extra dimension to control the generation of Airy beams with light polarization, which can thus stimulate new micro/nano photonic designs for polarization-controlled plasmonic circuits and surface optical manipulation.