Three-Dimensional Resolvable Plasmonic Concentric Compound

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Three-dimensional Resolvable Plasmonic Concentric Compound Lens: Approaching the Axial Resolution from Microscale to Nanoscale Kai-Hao Chang, Yen-Chun Chen, Wen-Hao Chang, and Po-Tsung Lee ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b01003 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 8, 2017

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ACS Photonics

Three-dimensional

Resolvable

Plasmonic

Concentric Compound Lens: Approaching the Axial Resolution from Microscale to Nanoscale Kai-Hao Chang1, Yen-Chun Chen2, Wen-Hao Chang2, and Po-Tsung Lee1* 1

Department of Photonics, College of Electrical and Computer Engineering, National Chiao Tung University, Hsinchu 300, Taiwan 2

Department of Electrophysics, College of Science, National Chiao Tung University, Hsinchu 300, Taiwan * Corresponding Author: Po-Tsung Lee, E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: We propose the design and working principle of a plasmonic concentric compound lens (CCL) comprising inner circular nanoslits and outer circular nanogrooves. Dual-wavelength operations have been achieved for 650 and 750 nm at nanoscale and microscale focal lengths along with their depth of focus (DOF). By tuning the arrangement of nanogrooves, the axial resolution can be modulated and the narrowest DOF is achieved by a design of gradually decreasing groove width. For the ultra-high tunability of axial resolution, DOF over 400 nm for both working wavelengths is also achieved. We not only developed an approximate-perturbedfocus model for explaining the performance of DOF but also found an extraordinary way to improve the resolution. The enhanced resonance of central disk as nannoantenna in CCL also has great influence on nanofocusing with different deigns of outer nanogrooves. This work provides new sight of focusing ability governed by the general optical nanogrooves. The optimized CCL shows excellent focusing performance with a lateral resolution down to 0.32 λ (λ = 650 nm), which is the best resolving ability achieved thus far in the near field region with a long focal length up to 500 nm.

KEYWORDS: plasmonic lens, subwavelength focusing, axial resolution, depth of focus, dualwavelength.


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To explore science and structures at nanoscale, subwavelength imaging techniques surmounting the diffraction limit have been widely studied by many methods, such as, mass spectral imaging1, photoactivated localization microscopy (PALM)2, near-field scanning nanoscopy3, and techniques utilizing metamaterial lens systems4. Among these methods, the PALM has been well developed and applied to many biological applications for detecting ultrasmall cells (< 25 nm). Recently, many types of plasmonic lens (PL) systems have been utilized for the application of PALM. To obtain subwavelength-resolution imaging, the circular zone plates and slits are ideal candidates for PL owing to the strongly-enhanced evanescent optical “needle” field for approaching narrow focusing feature5. On the other hand, the grating can transfer high-order diffracted modes containing the evanescent waves into far field6. By using this simple structure, the PL may achieve powerful super-resolution at distance from the near field to far field. The rich optical focusing properties of plasmonic zone plate have been widely investigated. Various designs for different applications such as bioimaging7, data storage8, nanolithography9, and optical trapping10 have been realized. However, seldom research results are related with the depth of focus (DOF). For the development of the PL, incorporating the beaming effect is important associated with large DOF. Recently, With the development of nanoscale techniques, three-dimensionally resolved imaging is crucial not only for optical data storage but also for biological engineering. Recently, the multilayered optical near-field recording disc has been proposed for achieving high-density image storages11. The improvement in the axial resolution is thus essential to obtain well resolved target images, such that the optical signal can be extracted from the layer-by-layer recording pattern. In the context of biological imaging, the images of blood and cancer cells have been resolved with an ordinary lens, whose axial resolutions are


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limited within several micrometers12. To develop novel biological engineering techniques, the subwavelength DOF of focal spot in axial systems should be small enough to resolve the Cajal body (0.3–1 µm), paraspeckle (0.2–1 µm), and the mitochondria (0.5–1.0 µm) inside a cell. The PALM techniques for three-dimensional subwavelength imaging have been developed with a resolution below 100 nm13. Furthermore, a super powerful method of stimulated emission depletion can narrow down the fluorescence image by using two laser beams14. However, the supporting objective lens suffers the diffraction limit ( λ / 2n sin(α ) of lateral resolution, and

λ /( n sin(α ))2 of axial resolution, where α and n denote the aperture angle of lens and refractive index), since the mixed signal collected from dye molecules is indistinguishable for different depths at nanometer scales. Hence, the plasmonic lens may be the solution for replacing the traditional objective lens. However, achieving a long focal length with a subwavelength focal spot remains difficult. Although the recently developed super-oscillation lens can provide a small focal spot along with a long focal length, complex Fourier-transform based pattern is hard to fabricate and the tuning method of axial resolution has not been discussed previously. In fact, the techniques for improving the axial-resolution need novel kinds of plasmonic devices, and the corresponding fundamental physical mechanism and theory should be developed immediately. The dual-wavelength or multi-wavelength operations of micro lens have been developed for optical storage and fluorescence imaging. For optical storage, optical pickup uses two laser diodes with wavelengths at 785 nm for CD and 650 nm for denser DVD15. For fluorescence imaging, the multicolor imaging techniques are utilized to reconstruct and visualize threedimensional objects. Multiple proteins in a pathway can be investigated using multiple fluorophore tags16.

However, these applications have not been demonstrated utilizing PL.

Besides, PL has the potential to achieve not only subwavelength resolution but also advanced


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depth-resolving ability, which will boost the functionality of optical storage and fluorescence imaging. In this paper, we propose the plasmonic concentric compound lens (CCL) with dualwavelength operations and discuss their corresponding axial resolutions for microscale and nanoscale. Based on this design, the coarse scanning for large region and the subsequent fine scanning can be performed at two different wavelengths. The operating wavelengths are selected to meet the biological window for penetrating the tissue. An ultra-small focal spot can be created and utilized under the short wavelength operation mode. Furthermore, the axial resolution can be modeled theoretically and confirmed by experimental results of tuning the outer nanogroove arrangements of the concentric compound lens. This work provides the details of this unique method of controlling the DOF which could be applied to PALM, for other depth-resolved imaging techniques as well. RESULTS AND DISCUSSION

The plasmonic CCL has been designed combining the circular nanoslits and nanogrooves, to act as the inner and the outer lens for different operating wavelengths. Figure 1a illustrates the designed PL as the probe for resolving images at different penetration depths. Different focusing mechanisms produced by different regions of the lens can realize the designed focal length and DOF at microscale and nanoscale. The SEM image of the plasmonic CCL is shown in Figure 1b. To avoid oxidation of metal and have stable plasmonic properties, we choose gold for our CCL. Silver and aluminum exposed in the air suffer oxidation17, and the change of the dielectric permittivity from oxidation will perturb the optimized focusing condition. Furthermore, aluminum with strong absorption at visible wavelength is not suitable for our device.


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The design principles of the inner and outer lens of the plasmonic CCL are based on the total constructive interference, but with different methods. The inner circular nanoslits having different slit widths are gradually moved radially outward to produce a focusing wave-front with a phase delay for compensating the optical path difference up to the focal point18. The distribution of phase delay along the r-axis for the inner lens can be expressed in terms of cylindrical coordinate as18, 19 2 2 2πf 2π f + r φ(r ) = . λ λ

(1)

Here, f is the focal length and λ is the working wavelength, fixed as 650 nm for the inner lens. This relationship between phase delay and position along the y-axis is shown in the central plot of Figure 1c. Next, the phase delays caused by different slit widths were calculated, and they could be extracted from the wavevectors of the metal-insulator-metal configuration. The analytical formula for the propagating wavevector β, which is crucial for the phase delay βd, can be written as18, 19

2

2 o air

tanh( β - k ε w/2) =

-ε air β 2 - k o2 ε m ε m β 2 - k o2 ε air

.

(2)

Here, ko represents the wavevector in free space, w is the slit width, and εair and εm are the permittivity in air and metal, respectively. To determine the geometrical parameters of the inner lens, slits with specific widths can be chosen at specific positions by matching the phase delay condition (green circles), as shown in Figure 1c. To design the outer lens, we use the outer nanogrooves to generate the scattering field constructively at the focal point, as shown in Figure 1d. Each of the gratings are selected at proper positions to satisfy the condition for constructive interference. Previously, the focusing


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effect of PL with circular grooves has been demonstrated by following a simple descriptive formula20. However, since this formula is derived from theory based on the surface plasmon polariton (SPP) waves propagating from the central slit instead of the circular slit, it should be modified. After modification, the formula based on the principle of constructive interference can be expressed as

φ(r) =

2π λ

f 2 + r2 -

2π λ

f 2 + ro2 +

2π ( r - ro ) - φo = 2mπ , λspp

(3)

where λspp and φ o are the effective wavelength and the initial phase constant, respectively, and

ro = yo is the position of the slit as indicated in Figure 1d. In addition, the portion 2π ( r - ro ) - φo is the phase contributed from the SPPs on the surface, similar to that reported λspp previously21, as shown in the center of Figure 1d. Finally, as shown in the right side of Figure 1d, the positions of these nanogrooves can be obtained by eq 3 ( φ(r) - 2mπ = 0 ). The positions indicated by the blue circles and the order numbers (with different “m” numbers) are calculated. The important feature of this design, i.e., the dual-wavelength operations, are valid because the SPPs propagate though the outer nanogrooves only at λ = 750 nm, which is important for a single focusing spot with a short wavelength at λ = 650 nm. Otherwise, there is no significant difference in the focusing between the long and short working wavelengths. To combine the inner and outer lenses and demonstrate the dual-wavelength operations of plasmonic CCL, simulations using COMSOL Multiphysics software based on the finite element method (FEM) has been performed to verify the design, as shown in Figure S1. The short and long wavelengths are taken as 650 and 750 nm, respectively. In the design, target focal lengths are initially taken as 300 and 1500 nm, and the simulated results reveal 250 and 1610 nm,


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respectively (shown in Figures S1a and c, respectively). The distinctive focusing effects are observed with the different focal lengths and their DOF at the microscale and nanoscale regimes. To achieve a smaller focal point with a small side lobe, the geometrical parameters for tuning the nanogrooves have been carefully investigated and discussed in the Supporting Information (in Figures S1 and S2). After our calculation for optimization, the modified CCL (m = 1–4) shows the distinctive focusing performance of less side lobe effect, compared with the original CCL (m = 0–3). In addition, our design principles can be further utilized for multi-wavelength operations. We demonstrate the triple-wavelength operations using eq. S1 in Figure S3 with circular nanogrooves at calculated positions instead of complex meta-nanostructures. Effect of nanogrooves on the DOF. To investigate the effect of geometry on the DOF, we

analyze and discuss the interference of diffracted electromagnetic fields in free space. According to the design principles, each nanogroove contributes constructively to the phase with the multiple of 2π to generate the ideal focusing spot22. However, the large DOF is observed due to the additional phase contributed from each nanogroove, as illustrated in Figure 2a. The phase change of the propagating wave from coupling out of the nanoslit has been found in the interference pattern23, but the focusing effect has not been taken into consideration in the previous work. Here, a modified model is used to analyze the groove’s effect on the DOF using the following redefined formula:

2π 2π z( m )2 + r 2 z( m )2 + ro2 λ λ , 2π + ( r - ro ) - φo + φm = 2mπ λspp

φ(z( m)) =

(4)

where ro = yo is the position of the outermost slit as indicated in Figure 2a. The groovescontributed phase φm , from each nanogroove is extracted from the simulation of the distribution


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of the field Hx along the propagating axis. The term z(m) represents the slightly perturbed position of the focus from the m-th nanogroove, when the additional phase shift is taken into consideration. Here we present four different types of PLs as PL-A (fixed groove width), PL-B (decreased groove width), PL-C (increased groove width), and Ref (without nanogrooves) shown in Figure 2b. With these designs of different arrangements of the positions and the groove widths of the outer lens, the DOF can be tuned and optimized under different conditions of interference. The detailed design parameters for each PL are listed in Table S1. Among these designs, for the case of PL-A with the optimized focusing power intensity from Figure S1b, we select the width, w = 240 nm. Then, we reset the width from 240 to 120 nm at steps of 40 nm in the outward direction for PL-B, and follow the inverse setting for PL-C. The number of nanogrooves strongly affects the focal spot size. The minimum number of nanogrooves is 4, for ensuring sufficient lateral resolution. It has been investigated in the Supporting Information (Figure S2). The magnetic field distributions, Hx for the four cases in the y-z plane, at λ = 750 nm are presented with different focusing effects, as shown in Figure 2c. We also extract the field distribution Hx, along the y-axis for calculating the additional phase shift caused by the nanogrooves. Figure 2d shows the cases of PL-A and Ref which follow a sinusoidal distribution from the center of the outermost slit to the outmost nanogroove. The shifted phase φm of the field distribution Hx of PL-A can be understood as a combination of outward and back reflected SPP waves from the nearby nanogrooves providing additional accumulated phase. Furthermore, the difference of the mismatched phase φm should be considered and has been incorporated in eq 4 for solving the perturbed z(m) with a variation in the number m. Using this calculation, numerically solved z(m) for m = 1 to 4 for the four PLs are shown in Figure 2e. The diffracted electromagnetic waves from each nanogroove with different focal positions z(m) form a broad


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DOF, which can be tuned with the design of the outer nanogrooves. To investigate the width effect on the focusing properties in the design of PL-A, the total range of z(m) from m = 1 to 4 for 160 and 240 nm groove widths are obtained as 470 and 381 nm, respectively. We find that the perturbed z(m) are different for different values of m; for the 1st and 2nd nanogrooves they are at high z position, and for the 3rd and 4th, at low z position. The z(m) positions of all the values of m for the case of 160 nm groove width are higher than those of the 240 nm groove width. We can predict that the PL-B will have the smaller DOF because it can be viewed as the combination of low z(m) position of the 1st and 2nd nanogrooves by wider width and the high z(m) position of the 3rd and 4th nanogrooves by narrower width. Based on these principles, we

can design various kinds of PLs by arranging nanogrooves with different widths at calculated positions for desired DOF. Furthermore, we find the smallest spanning region as 362 nm for PLB and the largest as 512 nm for PL-C. To confirm this result, we simulate the distribution of power flow along the z-axis, as shown in Figure 2f. The widest power line shape is found for the case of PL-C and the smallest corresponds to PL-B. The results obtained from simulation are consistent with the estimated DOF by varying z(m) from the lowest to the highest. This analytical method can be treated as an approximate-perturbed-focus model for describing the DOF. This model is useful because the phase information of focusing is presented in terms of the spatial distribution for each nanogroove in a meaningful way. In Figure 2g, the DOF along the zaxis and the focusing spot size along the y-axis for the four PLs are compared. PL-B shows the best focusing performance with a subwavelength focusing spot of 339 nm and a minimum axial resolution of 1141 nm. In order to provide the required resolution for different targets and also considering the time consumed for scanning, two additional designs are presented for the wide range of DOF, as shown in Figures S4, S5, S6 and S7. The focusing performances and their


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associated physical insights are all discussed in the Supporting Information. For these kinds of CCLs, the normalized transmittance (pure transmitted power ratio from small aperture excluding other physical factors24) is ranging from 0.25 to 0.7 and the maximum is observed for PL-B at λ = 650 nm. The maximum diffraction efficiency for λ = 750 nm is around 0.15 for PL-C. To summarize the performances of all designs (details are provided in the Supporting Information and Table S2), for the working wavelength of λ = 650 nm, we find the smallest DOF as 287 nm with the m numbers corresponding to 0, 2, 3, 4 in the case of PL-B and the largest DOF as 789 nm with the m numbers being 1, 2, 3, 4 in the case of PL-A. For the working wavelength of λ = 750 nm, the smallest DOF is 1001 nm for PL-B with the m numbers being 2, 3, 4, 5 and the largest DOF is 1428 nm for PL-C with the m numbers 0, 2, 3, 4. We achieve the ultra-high tunability of DOF beyond 400 nm for both working wavelengths (maximum 502 nm DOF at λ = 750 nm). These focusing performances are much better than previous studies22, 25. For investigating the tuning ranges of focal length and DOF, we simulate the extreme cases in the Supporting Information with results presented in Figures S8 and S9. The range of tunable DOF depends on the size of lens and targeted image quality. The larger plasmonic lens results in the better focusing performance and narrower DOF22, 26, which is verified and shown in Figure S9. Analysis of the measured DOF and FWHM. The fabrication and measurement of various

CCLs have been conducted to confirm our simulation results. The originally designed CCL (m = 0–3) and the modified CCL (m = 1–4) of PL-B and PL-C have been fabricated by deposition and patterning using electron-gun evaporation and focus ion beam (FIB). The near-field measurements have been performed for both working wavelengths at different heights to obtain the DOF.


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For the original CCL (m = 0–3), the axial resolutions shown in Figure 3a for λ = 650 nm are 383 and 326 nm (fitted by the Gaussian function) for the simulated and measured DOF, respectively. For λ = 750 nm, the axial resolutions are 1246 and 1092 nm at microscale for large axial resolution application. To explain the discrepancy between the simulation and experiment, the curvature effect due to imperfect fabrication has been investigated and is represented in Figure S10, where the two-dimensional FEM has been performed owing to the restriction to create such structures in 3D by the COMSOL software. The focal position and DOF have a strong relation with the curvature r. Figure 3b reveals the focusing spot presented in the power intensity distribution under the y-polarized incident wave for each case. We can see that the presence of strong and asymmetry side lobes around the central focusing spot at λ = 650 nm. The asymmetric intensity distribution of focal spot is caused by the imperfect shape of plasmonic CCL during fabrication. The grain generated in the deposited metal thin film and the ion charging during focus ion beam (FIB) milling process are responsible for this imperfection, which distorts the focal spot, resulting the asymmetric focal spot observed in our measured data. (detailed discussion is provided in the Supporting Information, in Figure S11). In addition, as shown in Figure 3c, the FWHM of the experimental results along the polarized direction are 570 and 758 nm for the cases of λ = 650 and λ = 750 nm, respectively. To verify the design principles of tuning DOF, the PL-B and PL-C have been fabricated and measured at both wavelengths, at λ = 650 and 750 nm. Figure 4a shows the SEM images of PLB and PL-C with the gradually decreasing and increasing widths, respectively. Figure 4b shows the measured power intensity distributions along the z-axis for the original (m = 0, 1, 2, 3), PL-B (m = 1, 2, 3, 4), and PL-C (m = 1, 2, 3, 4) at both working wavelengths. According to the experimental results, PL-C, original, and PL-B show the highest, medium and lowest DOF at λ =


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750 nm, demonstrating the tunability of the design. This is consistent with the predicted axial resolving power obtained from numerical simulations and the analytical model mentioned above. For the case at λ = 650 nm, the smallest DOF among these PLs is observed for original (DOF of 326 nm, subwavelength scale). Although there are no definite design principles for optimizing the focusing performance at the short wavelength operation, one of them could be the increase of focal length and DOF of PL-B caused by the beaming effect of the outer nanogrooves. Figure 4c shows the simulated and measured focal spots at each focal plane corresponding to each case. These profiles of measured focusing spot are in good agreement with the simulated results. Tiny focal spots at λ = 650 nm and the wider distribution at λ = 750 nm are distinctively observed. To investigate the focusing performance of lateral resolution, the cross section of focal spot is shown in Figure 4d. The simulated and measured power intensity profiles for PL-B and PL-C at each focal plane at λ = 650 nm have been compared in Figure 4d. From the fitting analysis, the measured focal spot size for PL-B is 213 nm (around 0.32λ), which is smaller than the value of 308 nm (0.47λ) for PL-C. Comparing with the original CCL, there is a significant reduction in the side lobe on the focal plane. The outstanding focusing ability, an ultra-small lateral focusing spot and a longer focal length than previous works27, 28, 29, is experimentally demonstrated. This improved nanofocusing is owing to the enhanced resonating effect caused by the surrounding nanogrooves with decreasing groove width (detailed analysis in Figure 5), which distorts the quasi-spherical shaped 3D focusing spot into an elongated elliptical sphere from the case of the original CCL changing to the PL-B. In other words, we sacrifice the axial resolution to reduce the size of the lateral focusing spot and increase the focal length, maintaining its resolving ability at around 0.97λ, which, in comparison with the other PL30,

31

systems, is sufficient for the

demanded application. For the focusing performance at λ = 750 nm, the measured focal lengths


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of PL-B and PL-C are 1200 and 1300 nm, respectively. The measured value of the reduced focal spot of PL-B is 668 nm, which is better than that of PL-C with lateral resolution of 800 nm, as shown on the right side of Figure 4d. To further understand the mechanism of excellent nanofocusing in PL-B at λ = 650 nm, the fundamental light and matter interaction via plasmonic resonance has been analyzed employing several models. By our verification, the enhanced evanescent waves can be supported by the composite diffracted evanescent wave (CDEW)32 and enhanced resonance of the optical antennas33. For CDEW, the constructive interference of evanescent waves boosts the transmission by either nanogratings or nanogrooves. The enhanced desorption power carried by CDEW leads the maximum transmitted power even in the non-plasmonic dielectric material32. For the latter one, the resonance of nanoantenna can be enhanced by the outer grating providing the accumulated phase at constructive condition. The evanescent wave associated with the enhancement factor of electric field is also increased, which should narrow down the resolution of nanofocusing. Although lots of models can describe the plasmonic behaviors of nanogrooves in different aspects, more than one phenomenon exist and it is hard to be distinguished in real situation. From our calculation in Figure 5a, sharp transmittance peak of PL-A (fixed groove width for studying the most simple case) revealed by the CDEW is consistent with the simulated peak of intensity of Ez-field 20 nm above the center of PL-A. This result gives the evidence that the resonating nanoantenna is accompanied by enhanced evanescent waves. Figure 5b shows the simulated spectra of intensity of Ez-field for PL-A, PL-B, PL-C and Ref. PL-B exhibits the strongest intensity at λ = 650 nm, which is confirmed by the measured near-field distributions of PL-B and PL-C at z = 20 nm shown in Figure 5c. Around 3 times of amplification at the center


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of the CCL is observed. In addition, more evidence that PL-B can support the evanescent-wave mediated transmission even in the dielectric CCL is shown in Figure S12. According to these analyses, the evanescent waves can be amplified through the constructive condition of CDEW and resonating effect of nanoantenna, which provides explanation for the nanofocusing performance of the lateral resolution between PL-B and PL-C. From the experimental results, the proposed method of tuning DOF by the nanogroove arrangement is verified. Excellent lateral and axial resolutions are achieved for PL-B at λ = 650 nm. Furthermore, the variation of DOF can be described by our proposed approximateperturbed-focus model, which is also confirmed by the experiment. Finally, the phenomenon of resonating nanoantenna strongly enhances the amplitude of evanescent waves. This effect associated with narrow focal spot in experiment has not been investigated before.

CONCLUSION.

In summary, the plasmonic CCL has been designed, fabricated, and demonstrated with dualwavelength operations. The theoretical formula based on the interference mechanism is employed to decide the focusing performances. The microscale and nanoscale focal spots are observed at λ = 750 and 650 nm, respectively. These design principles have been further utilized successfully for multi-wavelength operations in our simulation. In addition, the focusing properties can be improved by optimizing the outer nanogrooves. We effectively reduce the size of the side lobe of the focusing beam by removing the nearest nanogroove. Furthermore, the proposed approximate-perturbed-focus model can intuitively describe the effect of nanogrooves on the DOF by considering the additional contributed phase. We establish the designs of ultrahigh freedom to tune the range of DOF more than 400 nm by arranging the nanogrooves with


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different m numbers. The optimized cases of PL-B show the smallest resolutions at microscale and nanoscale for working wavelengths at λ = 750 and 650 nm. We obtain the ultra-small subwavelength focusing spot (0.32 λ) at λ = 650 nm, which is the narrowest focal spot realized in the experiment with the extended focal length of around 500 nm. Finally, we not only demonstrate a potential plasmonic device but also provide the physical insight for nanofocusing. The resonating nanoantenna associated with evanescent waves can be enhanced by the design of outer nanogrooves, which is further verified by experiment. This novel PL with powerful resolvable resolution can be used to obtain different images and information at different depths, such as in PALM and near-field optical data storage systems. This plasmonic device can also provide a convenient way to implement the nanoscopy, especially for improving axial resolution without any signal processing34.

METHODS

Numerical Method. FEM calculations based on the commercial software (COMSOL 3.5) were used to simulate the focusing properties of CCL. The dielectric constant of Au is taken from a previous work35. In addition, transverse magnetic (TM)-polarized planer wave was setup (electric-field E parallel to the y-axis), for the source of excitation. Then, the power intensity distributions of axial and x-y plane are obtained and the different directional resolutions can be analyzed. Fabrication of CCL. The fabrication process of CCL is simple, and can be achieved in two steps. The glass was selected as a substrate and the gold film was deposited by the electron-gun evaporation (EBX-8C, ULVAC). Then, the pattern of CCL was sketched by the focus ion beam (FIB) milling technique (Ga+ ion, Helios NanoLab 600i dual beam, FEI). The fine structure was implemented by the operation at 30 kV and 10 pA beam current.


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Measurement of Near-Fields. The near-field scanning optical microscope (NSOM, NT-MDT NTEGRA Solaris) was used to image the near-field distribution at different heights from the surface to the height of the focal plane. The self-made fiber was mounted on a tuning fork, whose aperture is around 150 nm and is coated with 130-nm-thick silver film on the outer surface of the taper of the fiber for ensuring high collection efficiency36. Besides, the shear-force feedback was employed with electrically biased voltage for scanning the images at different heights. In addition, the white light laser (SuperK EXTREME supercontinuum lasers, NKT) used as a source, whose wavelength can be selected by the filter, was incident on the back-side of the sample and transmitted through the CCL. Finally, the near-field is sampled by the probe and transmitted through the fiber into the photomultiplier tube for signal detection.

ASSOCIATED CONTENT Supporting Information Supporting information is available free of charge via the internet http://pubs.acs.org. Figures S1-S2 show the optimization of and geometry effect on focusing. Figure S3 demonstrates the multi-wavelength operations of CCL. Figures S4–S7 depict the designs of CCL for ultra-wide tuning range of DOF. Figures S8-S9 present the tuning ranges of focal length and DOF. Figures S10-S11 show the influence of shape on focusing properties. Figure S12 shows the CDEW mediated radiative power from the ZnO-based CCL. AUTHOR INFORMATION Corresponding Author * Corresponding Author: Po-Tsung Lee, E-mail: [email protected]


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Author Contributions This design, fabrication, and measurement had been performed by Kai-Hao Chang. Yen-Chun Chen assisted during measurement and crucial perspectives of analytical model and Prof. WenHao Chang supervised the process of measurement. Prof. Po-Tsung Lee supervised and integrated all the works. This manuscript was written through contributions of all authors, and all authors have given approval to the final version of the manuscript.

ACKNOWLEDGMENT The authors acknowledge the financial support from Ministry of Science and Technology (MOST), Taiwan, under 103-2221-E-009-096-MY3. We sincerely thank the Center of Nanotechnology, Materials Science, and Microsystems for the supporting facility of FIB at National Tsing Hua University.


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Figures


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\ Figure 1. (a) Illustration of plasmonic concentric compound lens (CCL) for dual-wavelength operations for the application of PALM with the dye doped tissue. (b) SEM image of CCL designed with m number 0, 1, 2, 3. (c) The circular nanoslits are designed as the inner lens at λ = 650 nm. The central and rightmost plots are used to determine the slit width at proper position by matching the same phase delay condition (green circle denoting). (d) The circular grooves are designed as the outer lens at λ = 750 nm. The central plot shows the phase accumulation of SPPs propagating. The right plot presents the positions of groove by solving eq 2 for different m numbers.


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Figure 2. (a) Illustration of the model describing the broadening of DOF by considering the additional phase. (b) Illustration of the proposed different plasmonic CCLs for DOF modulation, which are PL-A (fixed groove width), PL-B (decreased groove width), PL-C (increased groove width), and Ref (no grooves). (c) The Hx-field distributions in the y-z plane of the three types of groove arrangements as the outer lens, PL-A (w = 240 nm), PL-B, PL-C, and Ref at λ = 750 nm. (d)

The phase shift of PL-A compared with the Ref case in terms of Hz distribution. (e) The numerically solved z position of the eq 4 for mth nanogroove. (f) The power intensity distributions along the z-axis and (g) the focusing performances of DOF and FWHM for these four cases.


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Figure 3. (a) The simulated and measured power intensity distributions of the original CCL (m = 03) present nanoscale and microscale focal lengths and DOF at λ = 650 and 750 nm, respectively. (b) The simulated and measured power intensity distributions of focal spots are shown at each focal position. The incident wave is polarized parallel to the y-axis. (c) The power intensity distributions of the original CCL along the y-axis are plotted both in simulation and experiment at λ = 650 nm and 750 nm, respectively.


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Figure 4. (a) SEM images of PL-B and PL-C. (b) The measured power intensity distributions along the z-axis of PL-B and PL-C are shown for λ = 650 nm and λ = 750 nm. The original CCL is presented for comparison. (c) The simulated and measured focal spots of PL-B and PL-C at each focal plane. (d) The measured and simulated power intensities along the y-axis show the narrower focal spot for the PL-B at both working wavelengths.


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Figure 5 (a) The simulated spectra of transmittance and intensity of Ez-field 20 nm above the center of PL-A. (b) The spectra of intensity of Ez-field for PL-A, PL-B, PL-C and Ref are calculated. Different PLs provide varied field enhancement for the central nanodisk as resonating nanoantenna. (c) The measured power intensity distributions of PL-B and PL-C at z = 20 nm. The rightmost plot shows the cross sections along the y-axis.


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Three-Dimensional Resolvable Plasmonic Concentric Compound

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