WO2017007431A1

WO2017007431A1 - Microsphere for generating a photonic nanojet

Info

Publication number: WO2017007431A1
Application number: PCT/SG2016/050319
Authority: WO
Inventors: Minghui Hong; Xudong Chen; Mengxue WU
Original assignee: National University Of Singapore
Priority date: 2015-07-09
Filing date: 2016-07-08
Publication date: 2017-01-12

Abstract

The present invention relates to a microsphere for generating a photonic nanojet. The present invention relates to the microsphere, its method of manufacture and a device for using the microsphere. In an aspect of the present invention, there is provided a microsphere for generating a photonic nanojet at a shadow-side of the microsphere, the microsphere comprising an illumination-side opposite the shadow-side, wherein the surface of the illumination-side is modified.

Description

MICROSPHERE FOR GENERATING A PHOTONIC NANOJET

Field of the invention The present invention relates to a microsphere for generating a photonic nanojet. The present invention relates to the microsphere, its method of manufacture and a device for using the microsphere.

Background of the invention

Transmission of light through micro-scale particles exhibits interesting optical properties and attracts research attention for decades.

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, which is, however, limited to fluorescence samples and does not break the diffraction limit of the lens systems. 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. In the previous work, the principles of metasurface wave were systematically proposed, 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.

In 2004, it was reported the existence of a highly converged non-evanescent propagating beam generated at the shadow side of a plane wave illuminated microcylinder (through finite- difference time-domain (FDTD) modelling of cylindrical structures) under plane wave illumination. It was named as a photonic nanojet (PNJ) to differ its properties from conventional Gaussian beam generated by optical lenses, 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. Many research groups have reported a broad range of microsphere diameters from 2λ to more than 50λ theoretically and experimentally. 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. Microspheres can also enhance the backscattering intensity of the emitters which are located in the PNJ region. When combining microspheres with a solution of Rhodamine B dyes, two-photon fluorescence up to 30% has been demonstrated. Other applications include super-resolution imaging, enhanced optical detection, broadband low loss waveguide and high density optical data storage.

The characteristics of the PNJ includes: (a) the minimum value of the full-width at half- maximum (FWHM) can break the classical diffraction limit for microspheres; (b) the optical path length along propagation direction can extend longer than a few wavelength (λ); and (c) it is a non-evanescent phenomenon that exists for a large range of diameters of microsphere and microcylinders.

These advantages find the PNJ a wide range of applications, such as optical super-resolution, nano-scale optical lithography, enhanced optical detection, broadband low loss waveguide, and high density optical data storage.

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 and gradient index microsphere 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). 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. However, the intensity of the PNJ is weak.

Hence, there exists a need for an improved microsphere design.

The listing or discussion of an apparently prior-published document in this specification should not necessarily be taken as an acknowledgement that the document is part of the state of the art or is common general knowledge. Any document referred to herein is hereby incorporated by reference in its entirety.

Summary of the invention

It is therefore an object of the present invention to provide for an engineered microsphere that is able to modulate the photonic nanoject generated under plane wave illumination. An advantage of the present invention is to shrink the beam width of the photonic nanojets generated by such microsphere and facilitates various new applications.

In a first aspect of the present invention, there is provided a microsphere for generating a photonic nanojet at a shadow-side of the microsphere, the microsphere comprising an illumination-side opposite the shadow-side, wherein the surface of the illumination-side is modified.

There is nothing in the state of the art that suggests the modulation of a photonic nanojet (PNJ) generated by decorating the microsphere surface with micron-scale structures. The purpose of the surface engineering is to combine functional micro-scale structures with conventional microspheres to improve the FWHM of the 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. By "microsphere", it is meant to include any small spherical particles with diameters in the micrometer range. In an embodiment of the present invention, the diameter of the microsphere may be about between 4 μιη to 10 μιη. The term may also be used to include any microsphere lens array that may be used over a substrate of a microfluidic device, and may be used as part of an optical detection method based on the photonic nanojet phenomenon which transports nano-objects in the microfluidic channel with a depth comparable to the longitudinal dimension of the photonic nanojet. In the present invention, the microsphere may be a dielectric microsphere. Preferably, the surface modification comprises a pattern etched on the illumination-side surface of the microsphere. In an embodiment, the pattern comprises a ring structure that is etched on the surface of the microsphere. Preferably, a plurality of concentric rings are etched. By "concentric rings", it is meant to include any rings, circles or other circular structures that share the same center. In an embodiment, the rings may have different radii but share the same center in a way that is similar to an archery target which features evenly spaced concentric circles that surround a "bullseye" (center). There may be any number of concentric rings, for example from 1 to 6. Preferably, 4 equally spaced apart concentric rings are etched onto the illumination-side surface of the microsphere. In such a configuration, and as described above in which the concentric rings share the same center, the radius of the ring closest the center is about 0.5 μιη, and the radius of each of the subsequent rings is between 2.0 μιη to 2.25 μιη. Preferably, each ring has an inner and outer radii, the distance between the inner and outer radii of each ring is between 2.0 μιη and 2.25 μιη. Preferably, each ring has a width of about 0.25 μιη. Preferably, the depth of each of the ring etched on the surface is the same.

In an alternative embodiment, the pattern etched on the illumination-side surface of the microsphere is a spiral structure. By "spiral", it is meant to include any curve which emanates from a point and moving farther away as it revolves around the point. The point may be similar to the center of the concentric rings described above. Preferably, the depth of the pattern etched on the surface is between 0.2 μιη to 1.6 μιη.

Preferably, the microsphere is made from any material that is optically transparent or semi- transparent.

Preferably, the photonic nanojet generated from the microsphere has a full-width at half- maximum (FWHM) value between 194.3 nm to 272.1 nm. More preferably, the photonic nanojet generated from the microsphere has a FWHM value of 247.1 nm.

In a second aspect of the present invention, there is provided a device for generating a photonic nanojet wherein the device comprises a microsphere according to the first aspect of the present invention. Preferably, the microsphere is supported by a holder. In an embodiment, the holder is a thin gold membrane.

Preferably, the device further comprises an optical or objective lens for focusing the photonic nanojet generated at the shadow-side of the microsphere.

Preferably, the microsphere is immersed in oil or water.

Preferably, the device further comprises an illumination source for illuminating the illumination-side of the microsphere. In an embodiment of the present invention, the illumination source is a laser beam.

In a third aspect of the present invention, there is provided a method for fabricating a microsphere, the method comprising: (a) providing a microsphere, the microsphere having a shadow-side for generating a photonic nanojet and an illumination-side opposite the shadow- side for exposure to an illumination source; and (b) modifying the surface the of the illumination-side. Preferably, the illumination-side surface is modified by etching a pattern on the surface. The patterns that may be etched on the surface are described above. Preferably, the pattern may be a ring structure or a spiral structure etched on the illumination-side surface by a focused ion beam, UV lithography or electrobeam lithography, or by any other etching process known to the skilled addressee.

In a fourth aspect of the present invention, there is provided a microsphere method for generating a photonic nanojet, the method comprising: (a) providing a microsphere according to the first aspect of the invention; (b) illuminating the illumination-side of the microsphere.

Preferably, the illumination-side is illuminated with a 405 nm laser.

Preferably, the method further comprises focusing the photonic nanojet exiting the shadow- side of the microsphere.

Preferably, the method further comprises manipulating the microsphere by a nano-stage at a movement step of about 50 nm in a z axis direction.

Preferably, the method generates a photonic nanojet having a full-width at half-maximum (FWH M) value that is reduced by about 28% to 34% compared to photonic nanojets generated by conventional microspheres.

Since 2004, when photonic nanojet was first introduced, many researches have been carried out to study and improve its optical properties, such as beam width, working distance, and light intensity. However, there is no prior work relating to carving concentric rings into the surface of the microspheres to achieve a reduction of full width at half maximum intensity of the photonic nanojet by about 30%.

Shrinking down the beam size of the photonic nanojets of engineered microspheres have many advantages in various fields. Multiple methods have been carried out and yet they suffer from drawbacks of high loss and complexity in fabrication. Advantageously, the present invention demonstrates a feasible design by decorating microsphere surfaces with functional micro-structures to modulate the photonic nanojets and achieve beam spot size beyond the diffraction limit.

In order that the present invention may be fully understood and readily put into practical effect, there shall now be described by way of non-limitative examples only preferred embodiments of the present invention, the description being with reference to the accompanying illustrative figures.

Brief description of the Figures

In the Figures:

Figure 1 Configuration of the CRMS, (a) Schematic of observing PNJ by an optical microscope; (b) top and (c) side views of a 4 ring CRMS.

Figure 2 Photonic nanojet 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.

Figure 3 FDTD simulation of PNJs generated by CRMS with ring depth changed from 0 to 1.6 μιη on the illumination side of CRMS, (a) - (d): I ntensity distribution of CRMS with ring depth of 0, 0.8, 1.2 and 1.6 μιη 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.

Figure 4 Experimental results of the photonic nanojet produced under 405 nm laser illumination by (a) 4 ring, (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 intensity distributions along horizontal direction are plotted in (g), (h) and (i), respectively. Figure 5 Poynting vector plotted in FDTD software to show energy flow in the microspheres having with and without patterns etched on the surface. Figure 6 E field distribution pattern of microsphere.

Figure 7 E field distribution pattern and data plotted in a graph for simulation results showing relationship of etching rings on microspheres and FWHM of PNJ generated. Figure 8 The SEM image and E field distribution patterns showing relationship of position of ring structures on the microspheres and FWHM of PNJ generated.

Figure 9 E field distribution patterns. Figure 10 shows the E field distribution patterns and comparison with the different FWHM values.

Figure 11 Simulation results for a high refractive index (n=2) microsphere with 4 concentric rings immersed in water (n=1.515).

Figure 12 is a cross-section view of the experimental setup using the microsphere according to an embodiment of the present invention.

Figures 13 (a) shows the structure fabricated on the microsphere, Fig. 13(b) is the cross- section intensity distribution pattern and FWHM value at the highest intensity according to another embodiment of the present invention.

Detailed description of the preferred embodiments In the present invention, a novel engineered microsphere by etching concentric rings on the illumination side to improve the PNJ properties was devised. We 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) 5 with a single ring and four rings being fabricated on the illumination side.

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 μιη and width of 0.25 μιη 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.

With reference to the Figures, the present invention relates to a microsphere 5 having a shadow-side 15 and an illumination-side 10. The microsphere 5 may have a diameter between 4 μιη to 10 μιη, and may be made from any optically transparent or semi-transparent material. A beam of any light source may be used to illuminate the illumination-side 10 of the microsphere 5 to generate a photonic nanojet (PNJ) on the shadow-side 15. The surface of the illumination-side 10 of the microsphere 5 is modified. As described above, any form of modification may be made to the surface including any physical modification that affects the surface structure of the microsphere 5.

In an embodiment of the present invention, the surface of the illumination-side 10 is a pattern etched on it. As shown in the Figures, circular rings in the form of concentric circles or rings 20 are etched on the surface. Figure 1(c) shows a cut-away portion of the microsphere 5 to reveal a partial cross-sectional view. The microsphere 5 may be 1 ring etched on the surface, or a series of plurality of rings 20 etched on the surface. Advantageously, 4 rings may be etched as is shown as an exemplary embodiment of the present invention. Each ring may have a radius, a width (spanning a portion of the surface of the microsphere) and a depth (spanning an area etched into the microsphere) as will be set out in detail below. Figure 2(a) is a cross sectional view of the microsphere 5 and shows how each ring is etched on the surface and into a portion of the microsphere 5.

In an alternative embodiment, the pattern that is etched on the surface of the illumination- side 10 of the microsphere 5 is a spiral structure (shown in Figure 13(a)). Such a structure may be similar to a concentric ring structure but the curve that is etched on the surface of the microsphere is continuous. The spiral structure may have a width and depth that is similar to those used for the concentric ring structure. The spiral ring structure may have a ring width of about 250 nm, a ring depth of about 1.4 μιη from the top of the microsphere 5, and the illumination carried out in an x-axis linear polarisation with a 405 nm wavelength laser beam. Figure 13(b) shows the intensity of the yz cross-section and the FWHM of the PNJ generated by the microsphere 5 along the y axis. The FWHM value is 218 nm, which is corresponding to 0.54λ.

The etching may be carried out by any suitable lithography process. The pattern etched onto the surface of the microsphere is a 3D pattern.

The microsphere 5 may be used as part of a device for generating a PNJ. For example, as one shown in Figure 1. In an embodiment, the microsphere may be held by a holder 25, for example a thin gold membrane or any other suitable structure or material. An objective lens 30 may be placed on the shadow-side 15 of the microsphere 5 to focus the PNJ that is generated at the shadow-side 15.

In operation, the microsphere 5 may be immersed in oil or water. The immersion may be carried out by adding either oil or water to the microsphere 5, i.e. exposing both the illumination-side 10 and the shadow-side 15 of the microsphere 5 to either oil or water to create a homogeneous ambient environment for the microsphere 5. An advantage of such an immersion includes having the wavelength of the incident beam being reduced in the immersion medium which, in turn, results in a sharper focus of the PNJ that is generated by the microsphere 5. This is because the effective wavelength of the incident beam is shorter in an immersion medium, thus resulting in a sharper focus of the PNJ.

An illumination source such as a laser beam may be positioned on the side of the illumination- side 10 of the microsphere 5 to emit on the microsphere 5 a laser beam. A high sensitivity CCD camera 35 is used to verify the modulation effect of the microsphere 5. A further lens 32, which may be part of an optical system, may be placed intermediate the objective lens 30 and CCD camera 35 for further focusing. The illumination may be carried out in an x-axis linear polarization with a 405 nm wavelength laser beam.

Figure 12 shows a cross-sectional view of the experimental setup with microsphere 5 when used for imaging. A sample substrate 40 for imaging may be placed on the illumination-side 10 of the microsphere 5 either near or directly on the microsphere 5. In an embodiment, the sample substrate 40 may be a Blu-Ray disk. Other samples that may be used include any substrates having a flat surface. A quartz/Cr substrate 45 may be placed on the shadow-side 15 of the microsphere 5 for holding the microsphere 5.

The etching of the pattern 20 on the surface of the illumination-side 10 of the microsphere may be carried out by any suitable process known to the skilled addressee. Examples include etching using a focussed ion beam, UV lithography or electrobeam lithography.

Examples and data extracted from simulation and experiments are described in detail below.

Materials and methods

1. 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, the diameter of which may be between 4 μιη to 10 μιη, a thin gold membrane (about 5 μιη thick) was designed to carry or hold the microsphere 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 μιη 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 μιη 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 μιη were milled. The inner radius of the first ring is 0.5 μιη and the distance between the adjacent rings is 0.25 μιη. The dependence of the PNJ of the concentric ring microspheres (CRMS) on ring number and depth were studied with Lumerical 3D FDTD software. 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 ON DAX 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 μιη with a machining error of 10 nm. The depth of the rings was around 1.2 μιη with a machining error of 0.2 μιη. 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.

2. Results and discussion

(a) Photonic nanojet (PNJ) dependence on ring number To show the strong modulation effect on PNJ of the engineered microspheres, FDTD simulations were carried out. 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 μιη. 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. Colour bar with arbitrary units was applied for each configuration under an unitary incident wave. Compared to the microsphere without the etching (or 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. 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, the optical properties of the PNJ were analysed 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.

Figure 5 shows the Poynting vector that is plotted in the FDTD software which indicates the energy flow in the microspheres without [Fig. 5(a)] and with [Fig. 5(b)] ring structures. The lines were added by joining the Poynting vectors together to show the trend. The incident beams in near the edge transmitted vertically as they were guided by the etched rings before they converged. Here, the rings functioned as small waveguides. After adding the rings, these beams focused near the same focal point in the center. Even when the incident wavelength is changed from 405 nm to 550 nm, the super-focusing effect of the concentric ring microsphere still existed. In an experiment, the diameter of the microsphere was 10 μιη. The inner radius of the first ring was 0.5 μιη. The difference between outer and inner diameters for each ring was 0.25 μιη, and the distance between adjacent rings was also 0.25 μιη. In this simulation, it was shown that structure with 6 rings had the smallest FWHM. In general, structures with 4-6 rings could focus better. For a microsphere with no engineered rings on the surface, the FWHM of the nanojet was simulated as 353.3 nm (0.64λ) at the maximum intensity, located 0.93 μιη away from the microsphere surface. This can be seen in the E field distribution pattern shown in Figure 6. Simulation results showed that etching rings on microspheres could indeed reduce the FWHM, as shown by the E field distribution pattern and data plotted in a graph in Figures 7(a) and (b). As the number of rings increased from 0 to 4, the FWHM of the PNJ reduced from 353.3 nm to 292.0 nm for 17.2%. Microsphere with 4 - 6 rings had similar FWHM. In addition to the studying the relationship between the number of rings etched on the microsphere and the FWHM of the PNJ generated by the microsphere, simulations of one ring etched at different positions and their combined effect were also done to investigate the influence of ring position. The wavelength of the incident light is 550 nm. 10 μιη diameter silica microspheres were used for focusing. For the microspheres with 1 ring, the inner radius is 0.5 μιη, 1 μιη, 1.5 μιη, and 2 μιη, respectively. The difference between the outer and inner diameters for each ring was 0.25 μιη. The etching depth is 0.8 μιη for all rings. The SEM image and E field distribution patterns are shown in Figure 8. The results showed that the focusing property of microsphere with 1 ring was similar and was only slightly better than a pure microsphere with no ring structure on the surface.

Table 1 below summarises the data obtained in the experiments and simulations showing the relationship of the FWHM generated by the microsphere of the present invention and the number of rings etched on the microsphere.

ure ii»c3fo¾) ¾¾ t oiit risgs.

Figure 9 shows the E field distribution patterns when the microsphere diameter is 4 μιη; depth of each ring is 0.8 μιη; incident wavelength is 400 nm; and the ring numbers 0 to 4 to investigate the E field distribution and the different FWHM values.

Figure 10 shows the E field distribution patterns and comparison with the different FWHM values when the microsphere diameter is 4 μιη; number of rings = 4; ring depth = 0.8 μιη; incident wavelength = 400 nm. Refractive index of the microsphere was changed from 1.2 to 1.6. (In air ambient). Simulation for high refractive index concentric ring microsphere in immersion condition was also carried out. Simulation result for a high refractive index (n=2) microsphere with 4 concentric rings immersed in water (n=1.515) are shown in Figure 11. As the relative index was set to 1.50, the nanojet was formed outside of the microsphere. The FWHM of the nanojet is 221 nm (0.41λ).

(b) 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 μιη 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). I n comparison, when the ring depth is 1.2 μιη, 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 μιη), FWHM of the PNJ is large (about 260 nm) with a long working distance. I ncreasing the etching depth from 1.0 to 1.6 μιη results in the smaller FWHM (about 200 nm) and a shorter working distance. The working distance of the microsphere 5 is defined as the distance between the shadow-side 15 surface of the microsphere 5 and the highest intensity point of the PNJ along the optical axis of the PNJ.

It can be concluded that the smallest FWHM of the PNJ can be achieved at a depth of 0.8 μιη. When the depth is shallower than 0.8 μιη, the changes in PNJ's FWHM and working distance are not obvious. When the depth is larger than 0.8 μιη, the working distance demonstrates no significant change but the FWHM increases slightly. Modulation done at a shallow depth (1.0 μιη) is sufficient to achieve a sharp PNJ. It can be summarized from the results that etching depth between 0.8 to 1.4 μιη can generate a sharp focus and the average working distance is around 0.8 μιη. Therefore, ring depth of 1.2 μιη is chosen to evaluate the focusing capability experimentally.

(c) 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 μιη, respectively. The ring depth is 1.2 μιη 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.

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.

3. Conclusions

In summary, the present invention is a novel way to tune the PNJ by decorating concentric ring structures on the microsphere surfaces. Significant reduction of about 30% in FWHM of the PNJ were achieved numerically and good agreements were found with experiment. This design facilitates applications which require nano-scale beams with high intensity. 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 μιη. When the depth is between 0 to 0.6 μιη, weak modulation strength was observed. At a depth of 1.2 μιη, 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.

Whilst there has been described in the foregoing description preferred embodiments of the present invention, it will be understood by those skilled in the technology concerned that many variations or modifications in details of design or construction may be made without departing from the present invention.

Claims

1. A microsphere for generating a photonic nanojet at a shadow-side of the microsphere, the microsphere comprising:

(a) an illumination-side opposite the shadow-side,

wherein the surface of the illumination-side is modified.

2. The microsphere according to claim 1, wherein the surface modification comprises a pattern etched on the illumination-side surface of the microsphere.

3. The microsphere according to claim 2, wherein the pattern comprises a ring structure.

4. The microsphere according to claim 3, wherein the pattern comprises a plurality of concentric rings.

5. The microsphere according to claim 4, comprising four equally spaced apart concentric rings.

6. The microsphere according to claim 5, wherein the concentric rings share the same center, the radius of the ring closest the center is about 0.5 μιη, and the radius of each of the subsequent rings is between 2.0 μιη to 2.25 μιη.

7. The microsphere according to claim 5, each ring having an inner and outer radii, the distance between inner and outer radii of each ring is between 2.0 μιη and 2.25 μιη.

8. The microsphere according to claim 5, wherein each ring has a width of about 0.25 μιη.

9. The microsphere according to claim 5, wherein the depth of each of the ring etched on the surface is the same.

10. The microsphere according to claim 2, wherein the pattern comprises a spiral structure.

11. The microsphere according to any one of the preceding claims, wherein the depth of the pattern etched on the surface is between 0.2 μιτι to 1.6 μιτι.

12. The microsphere according to any one of the preceding claims, wherein the microsphere is made of an optically transparent or semi-transparent material.

13. The microsphere according to any one of the preceding claims, wherein the diameter of the microsphere is between 4 μιη to 10 μιη.

14. The microsphere according to any one of the preceding claims, wherein the photonic nanojet generated from the microsphere has a full-width at half-maximum (FWHM) value between 194.3 nm to 272.1 nm.

15. The microsphere according to claim 14, wherein the photonic nanojet generated from the microsphere has a FWHM value of 247.1 nm.

16. A device for generating a photonic nanojet, the device comprising a microsphere according to any one of claims 1 to 15.

17. The device according to claim 16, further comprising a holder for supporting the microsphere.

18. The device according to claim 17, wherein the holder is a thin gold membrane.

19. The device according to any one of claims 16 to 18, further comprising an optical lens for focusing the photonic nanojet generated at the shadow-side of the microsphere.

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SUBSTITUTE SHEETS (RULE 26)

20. The device according to any one of claims 16 to 19, wherein the microsphere is immersed in oil or water.

21. The device according to any one of claims 16 to 20, further comprising an illumination source for illuminating the illumination-side of the microsphere.

22. The device according to any one of claims 16 to 21, wherein the illumination source is a laser beam.

23. A method for fabricating a microsphere, the method comprising:

(a) providing a microsphere, the microsphere having a shadow-side for generating a photonic nanojet and an illumiriation-side opposite the shadow- side for exposure to an illumination source; and

(b) modifying the surface the of the illumination-side.

24. The method according to claim 23, wherein the illumination-side surface is modified by etching a pattern on the surface.

25. The method according to claim 24, wherein the pattern is a ring structure or a spiral structure etched on the illumination-side surface by a focused ion beam, UV lithography or electrobeam lithography.

26. A method for generating a photonic nanojet, the method comprising:

(a) providing a microsphere according to any one of claims 1 to 15;

(b) illuminating the illumination-side of the microsphere.

27. The method according to claim 26, wherein the illumination-side is illuminated with a 405 nm laser.

28. The method according to any one of claims 26 or 27, further comprising focusing the photonic nanojet exiting the shadow-side of the microsphere.

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SUBSTITUTE SHEETS (RULE 26)

29. The method according to any one of claims 26 to 28, further comprising manipulating the microsphere by a nano-stage at a movement step of about 50 nm in a z axis direction.

30. The method according to any one of claims 26 to 29, wherein the full-width at half- maximum (FWHM) value of the photonic nanojet is reduced by about 28% to 34%.

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SUBSTITUTE SHEETS (RULE 26)