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A novel design of decorating microsphere surface with concentric rings to modulate the photonic nanojet (PNJ) is investigated. By introducing the concentric ring structures into the illumination side of the microspheres, a reduction of the full width at half maximum (FWHM) intensity of the PNJ by 29.1%, compared to that without the decoration, can be achieved numerically. Key design parameters, such as ring number and depth, are analyzed. Engineered microsphere with four uniformly distributed rings etched at a depth of 1.2 μm and width of 0.25 μm can generate PNJ at a FWHM of 0.485 λ (λ = 400nm). Experiments were carried out by direct observation of the PNJ with an optical microscope under 405 nm laser illumination. As a result, shrinking of PNJ beam size of 28.0% compared to the case without the rings has been achieved experimentally. Sharp FWHM of this design can be beneficial to micro/nanoscale fabrication, optical super-resolution imaging, and sensing.
© 2015 Optical Society of America
1. Introduction
In traditional optics, the spatial resolution of lithography and microscopy is ultimately limited by the diffraction of light wave. One way to achieve resolution down to several nanometers is the so-called super-resolved fluorescence microscopy [1], which is, however, limited to fluorescence samples and does not break the diffraction limit of the lens systems [2]. Recent years have also witnessed the development of all-optical superresolution methods. For example, the use of ultra-thin metallic surface structure, known as plasmonic metasurface, has offered an important approach for breaking the diffraction limit of lens systems [3–6]. In the previous work, the principles of metasurface wave were systematically proposed [5], forming the foundation to overcome the diffraction limit via metasurfaces. Based on the short-wavelength property and directional transmission of surface plasmon, 22 nm linewidth has been experimentally realized at 365 nm wavelength, greatly surpassing the optical diffraction limit [6].
On the other hand, photonic nanojet (PNJ) has been emerged as a high intensity beam with sub-wavelength (sub-λ) width that can propagate over a much longer distance than incident wavelength (λ). The phenomenon is usually associated with incident light irradiation on lossless dielectric microcylinder and microsphere of diameter larger than λ, where the PNJ emerges from the shadow-side surface of the structures. These specific properties are not possessed by classical Gaussian beams generated by high numerical aperture objectives. Chen et al. first reported it in 2004 through finite-difference time-domain (FDTD) modeling of cylindrical structures under plane wave illumination [7]. Since then, many research groups have reported a broad range of microsphere diameters from 2λ to more than 50λ theoretically and experimentally [8–23]. Due to its unique characteristics, PNJ of the conventional microspheres finds a variety of applications. Combining micro-silica beads with the femtosecond laser illumination, optical nano-lithography with a feature size of 200 – 300 nm can be achieved [24–32]. Microspheres can also enhance the backscattering intensity of the emitters which are located in the PNJ region [7, 10, 20, 33]. When combining microspheres with a solution of Rhodamine B dyes, two-photon fluorescence up to 30% has been demonstrated [19]. Other applications include super-resolution imaging [34–36], enhanced optical detection [33], broadband low loss waveguide [17,18] and high density optical data storage [37].
To improve the optical properties, such as propagation distance and beam width of PNJ, several works have been carried out to modulate the PNJ via different structure designs. Two-layer dielectric microsphere/microcylinder [38] and gradient index microsphere [39] were proposed by combining different refractive index materials to adjust the propagation distance and focusing of the PNJ theoretically. Elongation of the working distance has been achieved with a large beam width (0.89 λ) [40]. However, the large FWHM value of this work limits the focusing resolution. Meanwhile, single material structure designed as a hemisphere shape shell has been proposed theoretically, and it demonstrated excellent focusing capability with a long working distance [41]. However, the intensity of the PNJ is weak. So far, to modulate PNJ generated by decorating the microsphere surface with micron-scale structures has not been reported. The purpose of the surface engineering is to combine functional micro-scale structures with conventional microspheres to improve FWHM and working distance of PNJ. Micro-scale structures have demonstrated excellent abilities in tuning light propagation direction. If it can be structured in a three dimensional configuration and fabricated on the microsphere surface, the modulation of the PNJ can be achieved in this novel approach.
In this paper, we theoretically and experimentally demonstrate the improvement of PNJ properties for microspheres with the concentric rings being etched into the illumination side of the surface. Finite-difference time-domain (FDTD) technique was adopted for numerical simulations of the engineered microspheres. Various parameters which contribute to the PNJ, such as ring number and depth, are analyzed. Experiments are carried out using an optical microscope with a high sensitivity CCD camera to verify the modulation effect of the concentric-ring microsphere (CRMS) with a single ring and four rings being fabricated on the illumination side.
2 Concentric ring microsphere (CRMS) design and fabrication
The silica microspheres employed in this paper are brought commercially (Bangs Laboratories, Inc). As the size of the microsphere is small, which is around 10 μm, a thin gold membrane was designed to carry the microspheres during experiments. These home-made gold membranes were fabricated by conventional UV lithography and gold electroplating. The detailed fabrication process are listed as the follows: first, a soda lime blank (Nanofilm, Wetlake Village, Califormia) with 100 nm thick chromium and 530 nm thick layer of AZ1518 photoresist was patterned by a direct-write laser system (Heidelberg Instruments uPG 101). Then, a 500 µm thick silicon wafer was cleaned and covered with thin layers of Cr/Au (100 nm/50 nm) as an adhesion and plating base. It was then deposited with 5 µm thick AZ9260 resist by spin coating and then exposed by UV light in a Mask & Bond Aligner (Karl Suss, MA8/BA6). After resist developing, the remaining resist mold was used for gold electroplating to build the gold layer. The AZ 9260 resist and Au plating base were then removed by acetone and gold etchant. Finally, the whole gold membranes were released from the substrate by Cr etching.
The microspheres were dispersed onto the gold membrane by immersing the whole structure into diluted water. Then, the membrane was dried in ambient air. The concentric ring structures were fabricated on the surface of the microspheres using FEI DA 300 Focus Ion Beam (FIB) system. Applying 30 KV and 50 nA of liquid metal Gallium ion sources, the rings at an average ring width (outer ring radius minus inner ring radius) of 0.25 µm were milled. The inner radius of the first ring is 0.5 µm and the distance between the adjacent rings is 0.25 µm. The dependence of the PNJ of the concentric ring microspheres (CRMS) on ring number and depth were studied with Lumerical 3D FDTD software [42]. The simulations were carried out by setting the incident wave as x axis polarized plane wave with 400 nm wavelength, and the material of the microspheres was selected as silica. 2D intensity figures were plotted in the yz plane and the FWHM intensity of the PNJ was evaluated at the strongest intensity points. The working distance of each configuration is defined as the distance between the bottom spherical surface to the strongest intensity point of PNJ.
Figure 1 illustrates the configuration of the CRMS with Fig. 1(a) as the experimental setup used for capturing the images of the PNJ in the xy plane. The PNJ was observed by a Nikon Eclipse LV100ND optical microscope under 405 nm laser illumination (linear polarized at a power of 40 mW, from ONDAX Company). Light passing through the engineered microspheres is focused at the shadow side, propagates through a 150x objective lens (NA = 0.9), and then captured by a Nikon DS-Qi2 CCD camera. Figures 1(b) and 1(c) show SEM images of the top and cross-section views of a 4-ring CRMS located in a gold membrane. It can be observed that uniformly distributed concentric rings with smooth edge and uniform depth were fabricated. The concentric rings have an average width of 0.25 μm with a machining error of 10 nm. The depth of the rings was around 1.2 μm with a machining error of 0.2 μm. During the experiment, the engineered microspheres, which were held on a gold membrane, were manipulated by a nano-stage at a movement step of 50 nm in the z direction.
Fig. 1 Configuration of the CRMS. (a) Schematic of observing PNJ by an optical microscope. The lens located between the objective lens and CCD represents for the focusing lenses in the optical microscope. (b) top and (c) side views of a 4 ring CRMS.
3. Results and discussion
3.1 Photonic nanojet (PNJ) dependence on ring number
In numerical simulations, the ring number is changed from 0 to 6 to study its influence on the beam width of the PNJ. The depth of all the rings is 1.2 μm. The gold membrane is ignored in simulation as no obvious change in FWHM and working distance has been observed. As an example, the cross section view of a 4-ring CRMS is shown in Fig. 2(a). The incident light illuminates from the top, passes through the concentric rings, and was focused at the shadow side of the microspheres. Light intensity distributions in the yz plane for 0, 2 and 4 rings CRMS are shown in Figs. 2(b) to 2(d), respectively. Color bar with arbitrary units was applied for each configuration under an unitary incident wave. Compared to the microsphere without the decoration of Fig. 2(b), the 4-ring CRMS of Fig. 2(d) demonstrates a more converged beam at the focal plane. More specifically, Fig. 2(e) shows the light intensity distribution along the y axis at the highest intensity point of the PNJ for the rings number from 0 to 6. An obvious decrease in FWHM can be observed and there are no significant side lobes.
Fig. 2 PNJ generated by CRMS with 0 to 6 etched rings on the illumination side of CRMS. (a) Cross-section view of the CRMS; (b)-(d) light intensity distribution of CRMS with 0, 2 and 4 rings in the yz plane; (e) light intensity distribution along y axis at the highest intensity points of the PNJ. (f) Dependence of FWHM and working distance of the PNJ on ring number.
The simulation results show that the etched rings on microspheres can efficiently reduce the FWHM of the PNJ from 274.2 nm (no ring, 0.686 λ) to 182.8 nm (6 rings, 0.457 λ), which corresponds to a reduction of 33.3%. As shown in Fig. 2(f), a rapid decrease of FWHM values is observed when ring number changes from 0 to 3. When 4 concentric rings are decorated on the microsphere, the FWHM of the PNJ is 194.3 nm (0.486 λ), corresponding to a reduction of 29.1%. It can be observed in Fig. 2(f) that the working distance and light intensity are also reduced with ring number. When concentric ring structures are introduced, scattering of incident light occurs at the top surface of the microspheres. Light irradiates on the rings and is scattered at the air-glass boundaries. Light intensity within the microspheres is modified and the optical properties of the PNJ generated are tuned. The physics of the light interaction with materials at nano-scale is complicated, especially considering a spherical surface. Therefore, in this paper, we analyze the optical properties of the PNJ with FDTD numerical solutions. To balance the FWHM and working distance, the structures with 4 rings are chosen as an optimal design for further simulation and experimental demonstration.
3.2 PNJ dependence on ring depth
Ring depth, which is calculated as the distance from the surface of microsphere to the bottom of the rings, also exerts a significant impact on the PNJ. The ring number is fixed at 4 to study the influence of ring depth on the FWHM and working distance of PNJ. Figure 3 shows the FDTD simulation of PNJs generated by CRMS as ring depth is varied from 0 to 1.6 μm on the illumination side of CRMS. Figures 3(a) to 3(d) show the light intensity distribution in the yz plane. Without the rings, microspheres generate a FWHM of 274.2 nm (0.686 λ). In comparison, when the ring depth is 1.2 µm, FWHM can be modulated to 194.3 nm (0.486 λ), corresponding to a reduction of 29.1%. When comparing Figs. 3(a) and 3(d), we observe the beam width of the PNJ is smaller, indicating a highly converged intensity distribution. Figure 3(e) shows the FWHM along y axis of each configuration at the strongest intensity point. It can be observed that for each configuration, no obvious side lobe is generated. Figure 3(f) shows the dependence of FWHM and working distance on the ring depth. It was observed that when the etching depth is shallow (0.2 to 0.6 µm), FWHM of the PNJ is large (~260 nm) with a long working distance. Increasing the etching depth from 1.0 to 1.6 µm results in the smaller FWHM (~200 nm) and a shorter working distance.
Fig. 3 FDTD simulation of PNJs generated by CRMS with ring depth changed from 0 to 1.6 μm. (a) - (d): Intensity distribution of CRMS with ring depth of 0, 0.8, 1.2 and 1.6 μm in the yz plane. (e) Comparisons of the intensity along the y axis for different configurations at the highest intensity points of the PNJ. (f) FWHM and working distance versus ring depth.
It can be concluded that the smallest FWHM of the PNJ can be achieved at a depth of 0.8 μm. When the depth is shallower than 0.8 μm, the changes in PNJ’s FWHM and working distance are not obvious. When the depth is larger than 0.8 μm, the working distance demonstrates no significant change but the FWHM increases slightly. Modulation done at a shallow depth (1.0 μm) is sufficient to achieve a sharp PNJ. It can be summarized from the results that etching depth between 0.8 to 1.4 μm can generate a sharp focus and the average working distance is around 0.8 μm. Therefore, ring depth of 1.2 μm is chosen to evaluate the focusing capability experimentally.
3.3 Experimental evaluation of the PNJ by CRMS
To verify the modulation of the CRMS, experiments of direct observation of PNJ via an optical microscope were carried out to compare the PNJ generated by microspheres with and without the decorations. Figure 4 shows the experimental results obtained by the CRMS decorated with: (a) 4 rings, (b) single ring and (c) microsphere only. Inner and outer radii for the single ring in Fig. 4(b) are 2.0 and 2.25 μm, respectively. The ring depth is 1.2 μm and uniform for all the ring structures. The xy plane normal to the longitudinal direction of the PNJ were captured with 50 nm per step by a nano-stage. Ten raw images of each configuration are shown in Figs. 4(d) to 4(f), demonstrating the gradual change in light distribution along z axis. The normalized intensity plots of the PNJ for each configuration is shown in Figs. 4(g) to 4(i). It can be observed from the experiment that no obvious side lobe is present, and the working distance decreases with the ring number, which agrees well with the simulation.
Fig. 4 Experimental results of the PNJ generated under 405 nm laser illumination by (a) 4 rings (b) single ring CRMS and (c) microsphere only. 10 raw images of light intensity distribution along z axis for (d) 4 ring microsphere; (e) 1 ring microsphere and (f) microsphere only are listed. The images are taken with a separation of 50 nm in z axis, 10 images are chosen for each configuration to show the change at the focal plane. The intensity distributions along horizontal direction are plotted in (g), (h) and (i), respectively.
It can be observed that the FWHM of the PNJ generated by CRMS with 4 rings is sharper on average than that of single ring CRMS and microsphere only. For the 4-ring CRMS configuration, PNJ at a FWHM of 247.1 nm was observed. Compared to the results obtained by microsphere only (343.1 nm), significant reduction of FWHM (28.0%) was achieved. This modulation strength agrees well with the simulation results. Meanwhile, for a single ring CRMS, FWHM of 272.1 nm was obtained, corresponding to a modulation strength lower than the 4-ring CRMS. However, due to the scattered energy by the engineered structures, PNJ by CRMS possesses a lower light intensity.
4. Conclusions
In summary, the modulation of PNJ generated by the engineered microspheres was presented. By decorating the silica microspheres with the concentric rings, the dependence of PNJ and working distance on ring number and depth has been studied. When the ring number decorated on CRMS is changed from 0 to 6, both the FWHM of the PNJ and working distance decrease gradually. Next, the ring depth of a 4-ring CRMS is varied from 0 to 1.6 μm. When the depth is between 0 to 0.6 μm, weak modulation strength was observed. At a depth of 1.2 μm, FWHM of the PNJ is reduced by 29.1%. However, the working distance and light intensity were also reduced. This is due to the scattering effect by the rings which modulates the propagation direction. Experiments of direct observation of the PNJ were carried out to verify this modulation strength. It was observed that microspheres without the ring structure decoration demonstrated a PNJ at a FWHM of 343.1 nm (0.85 λ). After the four concentric rings were engineered on the microsphere surface, the PNJ with a FWHM of 247.1 nm (0.61 λ) was observed. This corresponds to a significant reduction of 28.0% and shows good agreement with the simulation results.
Acknowledgments
This research is supported by the National Research Foundation, Prime Minister’s Office, Singapore under its Competitive Research Program (CRP Award No. NRF-CRP10-2012-04).
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