1. Introduction

Similar to the nondiffracting beam in free space [1], nondiffracting surface plasmon polaritons (SPP) beam, which maintains self-healing and non-divergence properties in two-dimensional space can effectively avoid the inherent diffraction and transmission loss of SPP and has attracted considerable attention [2]. Among them [2–5], the Bessel-like SPP beam, which has a straight track and a restricted lateral profile within a certain distance, has potential applications in near-field optical trapping and on-chip interconnect circuits [2,6–14]. Meanwhile, the SPP Bottle beam, which features a single bottle or an array of bottles, i.e., alternating regions of bright (or dark) foci surrounded by low (or intense) intensities [8,15–19], can be used to optically sort micro-particles by trapping those with a specific size, and have potential applications in near field optical trapping, micromanipulation, and fixed-point Raman enhancement [11,20], etc. To this end, high-efficiency excitation and dynamic manipulation of the nondiffracting SPP beam are important prerequisites for practical application in these above-mentioned technologies [21,22].

In recent years, in order to obtain higher excitation efficiency for the nondiffracting SPP beams, the emitter structure of nondiffracting SPP beams has been developed from traditional groove or grating structure to complex polarization-coupled slit array [2,6] or special nanoantenna array [19]. Meanwhile, recent advances in near-field SPP wave modulation have led researchers to develop more flexible means to dynamically manipulate the propagation and distribution of the nondiffracting SPP beams. For instance, by changing the polarization, phase, and incidence angle of the incident light or utilizing the tunability of the chemical potential of graphene to change the dielectric environment, the spatial intensity distribution, launching direction, and waveform type of the nondiffracting SPP beam can be dynamically manipulated [2,6,8,14,18]. Despite the above-mentioned progress, the dynamically manipulating types of the nondiffracting SPP beam still need to be enriched to satisfy various applications. Considering that in the application of on-chip interconnection circuits, SPPs with different excitation wavelengths can be encoded and carry different information by wavelength division multiplexing technology to enhance the freedom of information transmission [23,24]. Meanwhile, in the applications of near-field optical trapping and surface-enhanced Raman spectroscopy (SERS), different wavelengths can affect the optical force of the manipulating particles and the speed of aggregation, respectively [25,26]. Thus, the realizations of multi-wavelength high-efficiency excitation and wavelength-controlled dynamic manipulation of the Bessel-like SPP beam and the SPP Bottle beam are of great significance for realizing the above-mentioned applications. However, to the best of our knowledge, the Bessel-like SPP beam or SPP Bottle beam devices with high coupling efficiency and wavelength-dependent directional launching have yet been reported.

In this paper, the plasmonic devices that allow for wavelength-dependent dynamic manipulation of Bessel-like SPP beams and SPP Bottle beams are designed. The proposed devices are composed of several compact elements etched in a gold film which is a single compact element consisting of one groove and another auxiliary resonant groove. Different from the conventional groove or ridge structure that equally split SPP power to propagate from the boundary to both sides, the proposed compact coupling element directs all of the SPP power of the matched wavelength to one side, resulting in higher collecting efficiency. Moreover, compared with previous studies in which Bessel-like SPP beams or SPP Bottle beams launching in one direction [6,7,10,15,17], here the desired nondiffracting SPPs beam launching in a specific direction can be readily controlled by tuning the incident light wavelength, demonstrating that the dynamic Bessel-like SPP beams and SPP Bottle beams in this work can provide more degrees of freedom in practical applications and inspire the design of other related novel dynamic SPPs devices.

2. Methods

The simulation of near-field distribution control was performed with commercial software FDTD solutions (Lumerical, Ltd) based on the finite-difference time-domain method (FDTD). The gold film with a thickness of 450nm is placed on the SiO₂ substrate. Meanwhile, the thickness of the gold film is much larger than the skin depth of Au [27], which ensures sufficient coupling of the SPP field only at the Au/vacuum interface. The dielectric constant of gold was fitted to experimental data from Johnson and Christy [28]. The calculations employ a total field scattered field plane wave source that allows monitoring of either the total field (including light and SPP) or pure SPP interference field only (without incident light). The mesh size nearby the sample was 5nm × 5nm × 5nm, and the total simulation area was blocked by perfectly matched layers (PMLs) to reduce artificial reflections from the boundaries. A convergence study had performed, and the error was within the acceptable limit.

3. Results and discussion

Bessel-like SPP beam manipulation device. The schematic diagram of the Bessel-like SPP beam manipulation device is shown in Fig. 1. In analogy with the previous symmetric tilt grating or groove array that can excite Bessel-like SPP [6,7,10], the directional structure consists of four compact coupling elements (named 1 to 4, respectively) which are both arranged obliquely along the Y-axis with an angle θ=10° and etched on the gold film with a thickness of 450nm. As shown in Fig. 1 illustration, the length of the compact coupling element is L=5μm, and spacing between the compact coupling elements on the left and right sides is d=500nm. The compact elements 1 and 2 are symmetrically distributed about the X-axis, so with compact elements 3 and 4. The schematic diagram of the single compact coupling element is shown in the illustration. Each compact coupling element is composed of one main groove and an auxiliary resonant groove. The design principles of the compact coupling element are detailed in Ref. [29]. Briefly, under normal incidence light illumination, different from the traditional single groove structure that only excites the symmetric first mode of SPP, the auxiliary groove added in the compact coupling element will effectively excite two SPP modes (i.e., the symmetric first mode and the asymmetric second mode) in the coupling element. Meanwhile, the relative phase between these two SPP modes strongly depends on the wavelength of the excitation light and the size (width and depth) of the groove. Thus, by properly designing the dimensions of the groove, two SPP modes can constructively interfere on one side of the compact coupling element and destructively interfere on the opposite side. As a result, by properly adjusting the wavelength of the incident light, the directional excitation of SPP can be switched between the two sides of the compact coupling element. Follow this principle, the width of the auxiliary groove is selected to be 200nm(∼0.25λ) to ensure that only the first mode of SPP is excited in the auxiliary groove [29]. The width of the main groove is selected as 450nm to maintain high extinction ratio and directionality of the SPP at different wavelengths (for more detailed information, please refer to the Supplement 1 Figure S1). Meanwhile, the gradual change of the height of the main and auxiliary groove can lead to periodic modulation of interference mode in a certain direction [29], and we select the heights of the main groove and the auxiliary groove as h₁=87nm and h₂=33nm. By this presented parameter assemble, directional excitation of Bessel-like SPP beams accompany with relatively high contrast ratio can be achieved. Thus, the compact coupling element (one of the four elements in the structure) has the ability to wavelength-selecting the emission direction of SPP and can maintain this characteristic within the range of 0° to 40° with the polarization angle of the light relative to the element normal, so this can meet the prerequisites for generating and dynamically manipulating nondiffracting SPP (for more detailed information, please refer to the Supplement 1 Figure S2). As to the precision required in the fabrication of the several elements in the structure, we noted that the formation of the Bessel-like SPP beam requires the SPP excited by the upper and lower elements to maintain continuous and stable constructive interference [7]. This requires that the elements on the upper and lower sides (compact coupling elements 1 and 3 or 2 and 4) need to be as identical as possible. However, minor fabricating errors of the geometrical parameters such as width and height of the grooves, the length and tilt angle of the elements between the elements on the left and those on the right side (compact coupling elements 1 (3) and 2 (4)) are allowed, which can still maintain the ability of directional excitation of SPP.

 

Fig. 1. Schematic diagram of the combination structure for efficient and wavelength-manipulated directional launching of non-diffraction Bessel-like SPP device, which is composed of the four compact coupling elements. Illustration on the left: details of the combination structure. Illustration on the right: cross-section of the single compact coupling element that is composed of one main groove and another auxiliary resonant groove.

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The designed results of the Bessel-like SPP beam manipulation device are summarized in Fig. 2. For the 700nm and 800nm normal incident light, the electric field direction is parallel with X-axis. The near field intensity profiles of the generated SPPs beams at the plane 5nm above the device are shown in Figs. 2(a) and 2(b), respectively. Due to the coupling efficiency of the compact coupling element is different under different wavelengths excitation, which leads to different excitation intensities of nondiffracting SPP at different wavelengths. Thus, in order to present nondiffracting SPP clearly, we have chosen different color bars in Fig. 2. It is clear that the generated SPP beams can only propagate towards the positive or negative X-axis for the incidence with 800nm or 700nm. Meanwhile, the SPPs beam generated by the directional structure exhibits the main lobe with maxima in the center for both cases of 700nm and 800nm, which clearly show that the profiles of the generated SPP beams are both corresponding to the zeroth-order Bessel function. In fact, when the excitation wavelength is selected between 700nm and 800 nm, the control effect of the directional excitation of nondiffracting SPP degrades that will result in propagation of the SPP in both directions (see Supplement 1 Figure S2). This is due to the relative phase of the two SPP modes (i.e., the symmetric first mode and the asymmetric second mode) strongly depends on the wavelengths of the excitation light.

 

Fig. 2. (a, b) The FDTD simulated near field intensity distribution maps (Ez²) of Bessel-like SPP in the directional structure under laser excitation at different wavelengths (700nm, 800nm), respectively. (c, d) The transverse intensity distribution of the Bessel-like SPP beam at a specific propagation distance from the excitation structure at different excitation wavelengths (700nm, 800nm) in (a, b), respectively. (e, f) The near field intensity distribution maps (Ez²) of Bessel-like SPP in the groove structure under laser excitation in FDTD simulation at different wavelengths (700 nm, 800 nm), respectively. The element in groove structure has only one ordinary groove. The area of the groove structure and the color bar in e (f) is select as the same as in a (b). The red boxes in (a-b, and e-f) represent the range covered by the light field. The white boxes in (a-b, and e-f) represent the position of the structure in the simulation.

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The normalized SPP field intensity distributions with 700nm excitation along the Y-axis at X= -15µm, -20µm, -25µm in Fig. 2(a) are extracted and plotted in Fig. 2(c). The full width at half-maximum (FWHM) of main lobe waist calculated through the curves shown in Fig. 2(c) is about 1.05µm, 1.13µm, 1.49µm (corresponding to X= -15µm, -20µm, -25µm). Meanwhile, the normalized SPP field intensity distributions with 800nm excitation along the Y-axis at X=15µm, 20µm, 25µm in Fig. 2(b) are extracted and plotted in Fig. 2(d). The full width at half-maximum (FWHM) of main lobe waist calculated through the curves shown in Fig. 2(d) is about 1.1 µm, 1.39 µm, 1.64 µm (corresponding to X = 15 µm, 20 µm, 25 µm). It is noted that the normalized transversal intensity distributions along X=-15μm in 700 nm and X=15μm in 800 nm can more clearly demonstrate that the profiles of the generated SPP beams correspond to the zeroth-order Bessel function. Moreover, the excited SPP beams propagating within the region from 15μm to 25μm keep a tiny divergence angle (43.97mrad in 700 nm and 53.94mrad in 800 nm, respectively), which indicate the diffraction invariance of the generated Bessel-like SPP beam within a certain distance. Similar to that shown in previous works [2,7], the maximum nondiffracting length of the Bessel-like SPP beams in our case can be estimated geometrically from ${X_{max}} = D\textrm{sin}\theta $ (where D is the length of a single compact coupling element, and θ is the inclination angle of a single compact coupling element in the structure). Substituting the parameters D=5μm and θ=10° in the simulation, ${X_{max}} \approx \,28.79\mathrm{\mu}\textrm{m}$ in this scheme can be obtained, which is consistent with the results in the simulation. And the maximum nondiffracting length ${X_{max}}$ can be further longed for by increasing the length of the single compact element or decrease the tilt angle of the compact coupling elements in the structure. Thus, there is a tradeoff between the compactness of the device and the propagation length of the Bessel-like SPP beam, which should be properly balanced according to the specific applications [2]. Here the SPP beam profiles are derived from the SPPs generated by the 1(2) element and the 3(4) elements, from which the generated SPPs are in phase and can interfere constructively along with the negative or positive X-axis. It clearly shows that directional launching of the Bessel-like SPPs beam to the left or right side can be achieved only by tuning the wavelength of the normal incidence light.

Meanwhile, to demonstrate the proposed directional structure has a higher efficiency coupling excitation capability compared to the nondiffracting device that is composed of the ordinary groove (only removing auxiliary resonant groove from the structure of Figure1, the cross-section is displayed in inset of Fig. 2(e)). The near field intensity profiles of the ordinary groove as shown in Figs. 2(e) and 2(f), in which the width, length, and tilt angle of the groove structure are consistent with the directional structure in Fig. 2(a). The height and width of the groove element are 120nm and 650nm, which are consistent with the whole structure size of the compact coupling elements shown in Fig. 2(a). As Figs. 2(e) and 2(f) display, the groove structure will also emit Bessel-like SPP beams with equal intensity on both sides of the groove under the normal incidence. Comparing Figs. 2(a) and 2(e), it can be clearly observed that, under the same incident light intensity and irradiation area, the intensity of the Bessel-like beam excited on the left side of the directional structure is much larger than that excited on the traditional groove structure. The same phenomenon can also be observed in the comparison of Figs. 2(b) and 2(f). Meanwhile, in order to quantitatively compare the enhancement of the directional coupling efficiency, we further calculated the coupling efficiency of the single compact coupling element (one of the four elements in the structure, which is composed of the main and auxiliary groove) and the single groove on the gold film through the formula [30] $C = \frac{{{P_{SPP}}}}{{{P_{Light}}}}$, where $\,{P_{SPP}}$ is the power of the SPP propagating at the air-gold interface and ${P_{Light}}$ is the power of the light. We extracted the powers of the SPP propagating to the left and the right from the single compact coupling element and the single groove, and calculated the coupling efficiency. The results show that the coupling efficiencies of the single compact coupling element are C_left=25.83%, C_right=11.13% at 700nm and C_left=2.38%, C_right=16.06% at 800nm, respectively. Correspondingly the coupling efficiency of the single groove are C_left=15.77%, C_right=15.77% at 700nm and C_left=9.86%, C_right=9.86% at 800nm, respectively. Therefore, we can see that both the directional coupling efficiency of the single compact coupling element at 700 nm and at 800 nm, respectively, are around 1.63-fold of those of the single groove. The above simulation results show that, compared with the groove structure that approximately equally split SPP power to propagate from the boundary to both sides [8], the proposed directional structure has a higher excitation efficiency, which can greatly enhance its application ability in on-chip interconnection circuits and particle manipulation.

The self-healing property is an important characteristic of Bessel-like SPP beams, which is useful to greatly reduce the loss resulting from any disturbance, such as the roughness of the metal surface, during the transmission [2,7]. To confirm this feature, we introduce a defect in the forward path of the main lobe of the Bessel beam in the form of a cylindrical gold obstacle with a diameter of 200nm and a height of 70nm on the gold film 10μm away from the structure. As shown in Fig. 3(a), after passing through a cylindrical obstacle, the unidirectional excited Bessel-like SPP beam can still propagate in a straight line to the left and is almost unaffected by the obstacle. Moreover, the intensity distribution is essentially the same as the one without the obstacle in Fig. 2(a). To quantitatively evaluate the intensity distribution decreases of the Bessel-like SPP beam after facing the Au cylindrical obstacle, we introduced a relative attenuation rate parameter $\,\mathrm{\eta }\,$ ($\mathrm{\eta } = 1 - \textrm{E}_{{\textrm{z}_1}}^2\textrm{ / }\textrm{E}_{{\textrm{z}_0}}^2$, where $\textrm{E}_{{\textrm{z}_1}}^2$ and $\textrm{E}_{{\textrm{z}_0}}^2$ corresponds to the intensity of the SPP beam with and without obstacle, respectively). We extracted the intensity of the SPP beam at 5μm behind the obstacle (X=${\pm} $ 15μm). And the attenuation rates of the Bessel-like SPP beams in Fig. 3 are ${\mathrm{\eta }_{700}}$=10.08% and ${\mathrm{\eta }_{800}}$=3%, respectively. Although the intensity of the main lobe decreases, it is still the strongest of all the lobes. The same phenomenon can also be observed in the 800nm case in Fig. 3(b). The simulation confirms the unique self-healing properties of the Bessel-like SPP beam excited from the directional structure in both cases. On the other hand, when the traditional planar SPP beam facing an Au cylindrical obstacle, unlike the Bessel-like SPP beam that will transmit along the original path, it will exhibit strong scattering, and its transmission path will be strongly affected by the obstacle. Moreover, the calculated attenuation rate of the planar SPP beam at 5μm after the obstacle (X=15μm) is 6.27% at 800nm, which is about two-fold larger energy loss than that of the Bessel-like SPP (for more detailed information, please refer to the Supplement 1 Figure S3). Thus, this result furtherly demonstrates the self-healing property of the Bessel-like SPP beam. Besides, the self-healing property exhibited in the compact coupling element (composed of main and auxiliary grooves) is found to be applicable to the case of the single-groove structure used for the generation of nondiffracting SPP in Fig. 2(e). Physically, the extraordinary self-healing property of the Bessel-like SPP beam is essentially derived from the large lateral size of the total field including the side lobes (compared to the narrow main lobe) [7]. Thus, when the size of the obstacle (d) is much smaller than the lateral size of the total field (F) (d<1/25F [31]), the number, size, and position of the obstacle will not affect the self-healing phenomenon obviously (for more detailed information, please refer to the Supplement 1 Figure S4). However, the obstacle can cause the energy loss of the Bessel-like SPP beam due to the scattering by the obstacle as well as the interaction of the SPP with the formed localized surface plasmons at the obstacle. Meanwhile, due to sufficiently overlapping of the SPPs launched by the upper and lower elements is an indispensable precondition for full formation of the Bessel-like SPP beam [7], the formation position of the Bessel SPP beam in this work is about 5-10μm away from the structure. However, it should be pointed out that the Bessel-like SPP beam from the compact coupling element still maintains the unique properties of the Bessel-like SPP beam, such as self-healing, non-divergent properties, and the profile with Bessel function.

 

Fig. 3. (a, b) The FDTD near-field intensity distribution maps $({E_z^2} )$ of the Bessel-like SPP beam generated by the directional structure after facing a cylindrical obstacle at different excitation wavelengths (700nm, 800nm). The white circle in the figure is the marked gold cylindrical obstacle with a diameter of 200nm, a height of 70nm.

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SPP Bottle beam manipulation device. The SPP Bottle beam has good applicability in particle manipulation, near-field optical trapping, and surface-enhanced Raman spectroscopy (SERS). Therefore, based on the mechanism of forming the SPP Bottle beam through the superposition and interference of Bessel-like SPP beam and plane SPP wave [17], we can design the wavelength-dependent directional launching SPP Bottle beam device with similar means presented in the previous section. As shown in Fig. 4(a), the SPP Bottle beams manipulation device consists of the five compact coupling elements. The 1-4 elements are arranged obliquely along the Y-axis with an angle θ that is similar to the Bessel-like SPP device. And the single compact element 5 is adding along the Y-axis in the structure center region. Similar to the directional launching SPP structure, the 1-4 elements are symmetrically distributed relative to the X-axis. In the single compact element, the width and height of the main groove are ${h_1}$=87nm, ${w_1}$=450nm, and corresponding parameters of the lower groove are $\,{h_2}$=33nm, ${w_2}$=200nm, respectively. In the SPP Bottle beams manipulation device, as illustrated in Fig. 4, the length and the tilt angle of the 1-4 elements are ${L_2}$=10μm and θ=20° and the length of the center element 5 is ${L_1}$=6μm, respectively. The spacing between the compact elements on the left and right sides is d=650nm. It should be noted that, although the parameters (length and angle) of the compact coupling element in Fig. 4(a) are different from those in Fig. 1, it still maintains the ability to manipulate SPP directional excitation (for more detailed information, please refer to the Supplement 1 Figure S2). The selected lengths and the tilt angles in our work are enough for the presentation of the phenomenon meanwhile without facing the challenge of the huge amount of simulation. The parameters of the incident light and gold layer are the same as those used in the Bessel-like SPP device. Similar to the situation in Fig. 1, the directionally excited SPPs generated by 1-4 elements will propagate and interfere to form the Bessel-like SPP beam. Meanwhile, the formed Bessel-like SPP beam will interfere with the plane SPP wave excited from the center element 5 at the x-y plane, and SPP beams with the Bottle profile can be constructed in the central area along the positive or negative x-direction.

 

Fig. 4. (a) Schematic diagram of the combination structure consisted of five compact elements to realize wavelength-controlled manipulation of the SPP Bottle beams. Illustration on the left: the structure is formed by the specific spatial distribution and combination of five elements. Illustration on the right: the single compact coupling element is containing one main groove and another auxiliary groove. (b, c) The FDTD simulation results of wavelength-dependent directional launching of SPP Bottle beam in the directional structure under laser excitation at different wavelengths (700nm, 800nm), respectively. The monitors are located at the plane 5 nm above the device. (d) The transverse intensity distribution of the SPP Bottle beam at a specific propagation distance from the excitation structure at b. (e) The curves of the normalized light intensity along the X-axis at Y = 0 μm, as illustrated by the white dotted line in b, c. The red boxes in (b-c) represent the region covered by the light.

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Figures 4(b) to 4(e) show the simulated results on the demonstration of wavelength-dependent dynamic manipulation of the SPP Bottle beam. Figures 4(b) and 4(c) display the generated SPP field intensity distribution for 700nm and 800nm light excitation, respectively. It can be seen that under the normally incident light, the three SPP plane waves excited on the device side will propagate and interfere, and the resulting interference pattern constitutes a lattice of bottles with constant size and spacing. Meanwhile, for 700nm light, the generated SPP Bottle beam is launching on the left side of the device. In contrast, for 800nm light, the generated bottle beam is launching on the right side of the device. Therefore, by controlling the incident light’s wavelength, the SPP Bottle beam directional launching from the device can be selectively obtained. Thus, the wavelength-dependent directional launching of SPP Bottle beams has been achieved.

The normalized SPP field intensity distributions along the Y-axis at X=-20µm, -30µm, -35µm in Fig. 4(b) are extracted and plotted in Fig. 4(d). The simulation results reveal the evolution of the transverse field distributions of the SPP Bottle beams in propagating. It can more clearly be seen that the excited SPP Bottle beams have profiles which alternating regions of bright (or dark) foci surrounded by low (or intense) intensities. Meanwhile, the SPP Bottle beams feature a one dimension line of intensity peaks and valleys that repeats itself during propagation, remaining homogeneous due to its nondiffracting character, which corresponds to the unique Talbot effect [15,17].

Meanwhile, Fig. 4(e) displays the curves of the normalized light intensity of SPP Bottle beams along the X-axis at Y = 0μm, as illustrated by the white dotted line in Figs. 4(b) and 4(c), respectively. Considering the limitation of the nondiffracting length [15], we can only get the correct periodicity of the formed SPP Bottle beam at the second envelope (within the position with an absolute value of nearly 10μm-30μm) in the curve. It shows that the periodicity of the SPP Bottle beam is 12.21μm in 700nm and 13.58μm in 800nm, respectively. Meanwhile, the average spatial periodicity of the SPP Bottle beam by the interference of Bessel-like SPP beam and plane SPP wave along the X-axis can be expressed as $T = {\lambda _{SPP}}/({1 - cos \theta } )$, where ${\lambda _{SPP}}$ is the wavelength of the SPP and $\theta $ is the tilt angle of four compact elements. The numerical periodicities of the SPP Bottle beams are 11.25μm in 700nm and 12.98μm in 800nm, which are close to the simulation results. Similar to the case in Bessel-like SPP beams, the maximum nondiffracting distance of SPP Bottle beams can also be longer by increasing the length of the compact element or decrease the tilt angle of the compact element in the structure. Therefore, compared to the passive manipulation of the SPP Bottle beams by changing the angle $\theta $ or the shape of the structure in previous studies [15,17], tuning the wavelength of the incident light to dynamically manipulate the size of the SPP Bottle in our device can prove more flexibility in practical applications.

4. Conclusions

In conclusion, with the Bessel-like SPP beam manipulation device, i.e., the elaborately designed combination structure of the four compact elements, we have demonstrated that the Bessel-like SPP beam can be selectively unidirectionally launching on both sides of the device by modulating the wavelength of the incident light. And the Bessel-like SPP beams with the self-healing and non-divergence properties can be highly efficiently excited in this device. Similar to the ability of a wavelength division multiplexing plasmonic device to encode and transmit SPP beams of different wavelengths to different paths [23,24], our device can transmit and encode Bessel SPP beams of different wavelengths along a specific path, which can increase the freedom of information transmission in the application of on-chip interconnect circuits.

Meanwhile, in the SPP Bottle beam manipulation device, by changing the wavelength of the incident light, SPP Bottle beams of different wavelengths can be efficiently excited, and the direction of emission can be dynamically selected, which can be applied to particle manipulation and screening in near-field optical trapping and surface-enhanced Raman spectroscopy (SERS) [25,26]. Besides, the size of the single bottle can be further dynamically controlled by changing the wavelength of the incident light, which can be applied to near-field optical trapping to optically screen particles by capturing particles of a specific size [8].

Moreover, the nondiffracting SPP beams’ dynamic manipulation device can be realized in other wavebands by scaling the parameters of the compact element and structure. Besides, the sample structures can be fabricated using focused ion beam (FIB) milling on a 450-nm-thick gold film that was evaporated on a glass substrate experimentally [29]. Thus, the design scheme of the proposed device provides a new means for constructing plasmonic devices with wavelength-dependent dynamic manipulation of nondiffracting SPP beams.