Patent Publication Number: US-10317696-B2

Title: Electromagnetic wave focusing device and optical apparatus including the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority from Korean Patent Application No. 10-2016-0069386, filed on Jun. 3, 2016, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference. 
     BACKGROUND 
     1. Field 
     Apparatuses consistent with exemplary embodiments relate to optical structures and apparatuses including the same, and more particularly, to electromagnetic wave focusing devices and optical apparatuses including the same. 
     2. Description of the Related Art 
     As nano device technology has developed, various options for manufacturing thin film type/ultra-compact type optical elements and optical devices have been developed. Also, interest has increased in technologies that promise a high resolving power beyond the resolution limit of existing optical systems. High resolving power technology may have a ripple effect on many fields requiring high-resolution imaging, such as biotechnology and analysis technology. In particular, a non-fluorescent type “true” super-resolution technology is desired, as compared to a “functional” super-resolution technology using fluorescent materials. 
     In the case of a Faraon type metasurface, lens performance is implemented by arranging numerous amorphous silicon (a-Si) nanoposts on a substrate. However, since absorption coefficients of nanoposts are large in the visible light region, it is difficult to use nanoposts in the visible light region. Also, since nanoposts having smaller sizes are necessary for use in wavelength regions, the processing difficulty is increased. 
     In the case of a Pendry superlens, an image is formed in a point area by using a metamaterial having a negative (−) refractive index regardless of a diffraction limit. However, a complicated and difficult process is needed for manufacturing a Pendry superlens. Also, it is almost impossible to manufacture a metamaterial having a negative refractive index in various visible light wavelength regions, and there is a fundamental limitation such as light loss by metal. 
     In the case of near-field scanning optical microscopy (NSOM), although resolution may be increased by using a near field of a metal tip, it is required that a distance between the metal tip and a sample is shorter than a wavelength. Accordingly, there is the difficulty that a user is required to have high-level proficient technique, expensive equipment is necessary, and coupling efficiency is low. 
     In order for common users/customers to use products with high resolving power/super-resolution imaging technology, various requirements, such as, mass producibility, price competitiveness, miniaturization, user-friendly user interface (UI), etc., may be desired. 
     SUMMARY 
     One or more exemplary embodiments provide electromagnetic wave focusing devices having superior performance. 
     One or more exemplary embodiments provide electromagnetic wave focusing devices which may implement a high resolving power/super-resolution. 
     One or more exemplary embodiments provide electromagnetic wave focusing devices having a small thickness and which are easy to manufacture. 
     One or more exemplary embodiments provide electromagnetic wave focusing devices having simple structures. 
     One or more exemplary embodiments provide optical apparatuses including the electromagnetic wave focusing device. For example, a microscope (super-resolution microscope) employing the electromagnetic wave focusing device may be provided by one or more exemplary embodiments. 
     Additional exemplary aspects and advantages will be set forth in part in the description which follows and, in part, will be apparent from the description, or may be learned by practice of the presented exemplary embodiments. 
     According to an aspect of an exemplary embodiment, an electromagnetic wave focusing device includes a plurality of material members, each at least partially surrounding a reference point and located at a distance from the reference point different from distances at which all other of the plurality of material members are located. In other words, the plurality of material members may be arranged substantially concentrically around the reference point. The plurality of members are arranged at non-uniform (non-periodic) intervals with respect to each other and are configured to focus electromagnetic waves at a point in space, the electromagnetic waves being focused after passing through the plurality of material members. 
     The plurality of material members may define a first interval between a first pair of two adjacent material members, a second interval between a second pair of two adjacent material members, such that the second interval is different from the first interval. Optionally, a third interval is defined between a third pair of two adjacent material members, where the third interval is different from the first and second intervals. 
     Intervals between the plurality of material members may increase with distance from the reference point. 
     Intervals between the plurality of material members may decrease with distance from the reference point. 
     When a width of each of the material members is defined as a distance between an inner circumference and an outer circumference thereof, at least two of the plurality of material members may have different widths from each other. 
     Widths of the plurality of material members may increase or decrease with distance from the reference point. 
     Widths and intervals of the plurality of material members sequentially may increase or decrease with distance from the reference point. 
     Each of the plurality of material members may have a ring structure. 
     Each of the plurality of material members may include any one of dielectric and semiconductor. 
     Each of the plurality of material members may include any one of Si, Ge, GaP, SiOx, SiNx, and an oxide semiconductor, and the oxide semiconductor may include at least one of Zn, In, Ga, and Sn. 
     At least two of the plurality of material members may have different thicknesses from each other. 
     The plurality of material members may include first and second material members, at least one of the first and second material members may have a multilayer structure, and the number of material layers constituting the first material member and the number of material layers constituting the second material member may be different from each other. 
     Thicknesses of the plurality of material members may range from several tens of nanometers (nm) to several micrometers (μm). 
     An entire width of the plurality of material members may range from about 0.5 μm to about 50 μm. 
     A numerical aperture (NA) of the electromagnetic wave focusing device may be equal to or greater than 0.3. 
     The electromagnetic wave focusing device may be configured to output an output light having a full width at half maximum (FWHM) that is less than ½ of a wavelength of an incident light. 
     The electromagnetic wave focusing device may further include a transparent substrate, in which the plurality of material members are provided on a surface of the transparent substrate. 
     According to an aspect of another exemplary embodiment, a optical apparatus includes the electromagnetic wave focusing device including a plurality of material members, each at least partially surrounding a reference point and located at a distance from the reference point different from distances at which all other of the plurality of material members are located. In other words, the plurality of material members may be arranged substantially concentrically around the reference point. The plurality of material members may be arranged at irregular intervals with respect to each other and are configured to focus electromagnetic waves at a point in space, the electromagnetic waves being focused after passing through the plurality of material members. 
     According to an aspect of another exemplary embodiment, a electromagnetic wave focusing device for focusing electromagnetic waves at a point in space, the electromagnetic wave focusing device including a plurality of material members, each at least partially surrounding a reference point and located at a distance from the reference point different from distances at which all other of the plurality of material members are located. In other words, the plurality of material members may be arranged substantially concentrically around the reference point. The plurality of members are arranged at intervals which are determined to satisfy spatial coherence with the electromagnetic waves with distance from the reference point, and each of the widths is defined as a distance between an outer side and an inner side of each of the plurality of material component. 
     The intervals and widths of the plurality of material members may increase with distance from the reference point. 
     The intervals and widths of the plurality of material members may decrease with distance from the reference point. 
     An optical apparatus including the electromagnetic wave focusing device that includes a plurality of material members, each at least partially surrounding a reference point and located at a distance from the reference point different from distances at which all other of the plurality of material members are located. In other words, the plurality of material members may be arranged substantially concentrically around the reference point. The plurality of members are arranged at intervals and with widths which are defined to satisfy spatial coherence with the electromagnetic waves with distance from the reference point, and each of the widths is defined as a distance between an outer circumference and an inner circumference of each of the plurality of material component. 
     According to an aspect of another exemplary embodiment, a microscope includes an objective lens unit arranged facing an object to be observed, the objective lens unit including the electromagnetic wave focusing device that includes a plurality of material members, each at least partially surrounding a reference point and located at a distance from the reference point different from distances at which all other of the plurality of material members are located. The plurality of material members may be arranged at irregular intervals with respect to each other and may be configured to focus electromagnetic waves at a point in space, the electromagnetic waves being focused after passing through the plurality of material members, an electromagnetic wave source unit irradiating electromagnetic waves toward the objective lens unit, and an image providing unit showing an image of the object obtained through the objective lens unit. 
     The microscope may have a resolving power that is less than ½ of a wavelength of the electromagnetic waves incident upon the electromagnetic wave focusing device. 
     The microscope may be configured to obtain an image of the object by scanning the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee. 
       These and/or other exemplary aspects and advantages will become apparent and more readily appreciated from the following description of the exemplary embodiments, taken in conjunction with the accompanying drawings in which: 
         FIG. 1  is a plan view of an electromagnetic wave focusing device according to an exemplary embodiment; 
         FIG. 2  is a cross-sectional view taken along a line A-A′ of a plurality of material members of  FIG. 1 ; 
         FIG. 3  is a cross-sectional view conceptually illustrating focusing of electromagnetic waves by an electromagnetic wave focusing device according to an exemplary embodiment; 
         FIG. 4  is a graph showing intensities of electromagnetic waves by positions in a direction in which electromagnetic waves are input, when the electromagnetic waves are incident upon the electromagnetic wave focusing device according to an exemplary embodiment; 
         FIG. 5  is a graph showing intensities of electromagnetic waves output through a medium when a series of electromagnetic wave pulses are irradiated onto the medium to satisfy a coherence condition, thereby explaining a principle to be applied to the electromagnetic wave focusing device according to an exemplary embodiment; 
         FIGS. 6 to 8  are graphs showing results of evaluating a beam focusing effect for each wavelength of electromagnetic waves (incident light) perpendicularly incident on an electromagnetic wave focusing device according to an exemplary embodiment, by using a finite-difference time-domain (FDTD) simulation method; 
         FIG. 9  is a graph showing a result of evaluating a beam focusing effect of electromagnetic waves (incident light) perpendicularly incident on an electromagnetic wave focusing device according to another exemplary embodiment, by using the FDTD simulation method; 
         FIG. 10  is a graph explaining a full width at half maximum (FWHM) of the focused beam (electromagnetic wave) of  FIG. 9 ; 
         FIGS. 11A, 11B, and 11C  are graphs showing results of evaluating a beam focusing effect of electromagnetic waves (incident light) perpendicularly incident on an electromagnetic wave focusing device according to another exemplary embodiment, by using the FDTD simulation method; 
         FIG. 12  is a plan view for explaining a design condition of an electromagnetic wave focusing device according to an exemplary embodiment; 
         FIG. 13  is a cross-sectional view for explaining an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 14  is a cross-sectional view for explaining an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 15  is a cross-sectional view for explaining an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 16  is a plan view of an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 17  is a plan view of an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 18  is a plan view of an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 19  is a plan view of an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 20  is a plan view for explaining an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 21  is a plan view for explaining an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 22  is a cross-sectional view of a structure of an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 23  is a cross-sectional view of a structure of an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 24  is a plan view of an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 25  is a plan view of an electromagnetic wave focusing device according to another exemplary embodiment; 
         FIG. 26  is a plan view of an electromagnetic wave focusing device according to another exemplary embodiment; and 
         FIG. 27  illustrates an optical apparatus (microscope) employing an electromagnetic wave focusing device according to an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Various exemplary embodiments will now be described more fully with reference to the accompanying drawings in which exemplary embodiments are shown. 
     It will be understood that when an element is referred to as being “connected” or “coupled” to another element, it can be directly connected or coupled to the other element or intervening elements may be present. In contrast, when an element is referred to as being “directly connected” or “directly coupled” to another element, there are no intervening elements present. As used herein the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It will be understood that, although the terms “first”, “second”, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of exemplary embodiments. 
     Spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary term “below” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing particular exemplary embodiments only and is not intended to be limiting of exemplary embodiments. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
     Exemplary embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of exemplary embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, exemplary embodiments should not be construed as limited to the particular shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of exemplary embodiments. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which exemplary embodiments belong. It will be further understood that terms, such as those defined in commonly-used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     Reference will now be made in detail to electromagnetic wave focusing devices and optical apparatuses including the electromagnetic wave focusing devices according to exemplary embodiments which are illustrated in the accompanying drawings. In the drawings, the width and thicknesses of layers and regions are exaggerated for clarity of the specification and for convenience of explanation. Like reference numerals refer to like elements throughout. 
       FIG. 1  is a plan view of an electromagnetic wave focusing device D 10  according to an exemplary embodiment. 
     Referring to  FIG. 1 , the electromagnetic wave focusing device D 10  may include a plurality of material members (i.e., elements or components) E 10 , E 20 , and E 30 , together forming a structure encompassing a reference point C 1  on a plane. The material members E 10 , E 20 , and E 30  may be located at difference distances from the reference point C 1 . The material members E 10 , E 20 , and E 30  may include, for example, a first material member E 10 , a second material member E 20 , and a third material member E 30 . The first, second, and third material members E 10 , E 20 , and E 30  may be sequentially arranged in increasing distance from the reference point C 1 . The material members E 10 , E 20 , and E 30  may have a coaxial ring structure in which each of the material members has a different size. Although three material members E 10 , E 20 , and E 30  are used in the present exemplary embodiment, it is merely exemplary and four or more material members may be used. 
     The material members E 10 , E 20 , and E 30  may be arranged at non-uniform intervals (i.e., non-periodic intervals or irregular intervals). In other words, the interval between the first material member E 10  and the second material member E 20  and the interval between the second material member E 20  and the third material member E 30  may be different from each other. Also, at least two of the material members E 10 , E 20 , and E 30  may have different widths. The width of each of the material members E 10 , E 20 , and E 30  may be defined to be a distance between an outermost side or circumference and an innermost side or circumference of the material members E 10 , E 20 , and E 30 , respectively. All of the material members E 10 , E 20 , and E 30  may have different widths. Accordingly, the intervals and widths of the material members E 10 , E 20 , and E 30  may vary in their distance from the reference point C 1 . The material members E 10 , E 20 , and E 30  may have intervals and widths determined to satisfy spatial coherence with electromagnetic waves (not shown) to be incident thereon. 
     The intervals between the material members E 10 , E 20 , and E 30  may increase with distance of the material member from the reference point C 1 . The widths of the material members E 10 , E 20 , and E 30  may increase with distance from the reference point C 1 . When the intervals of the material members E 10 , E 20 , and E 30  increase with distance from the reference point C 1 , the widths of the material members E 10 , E 20 , and E 30  may also increase with distance from the reference point C 1 . In this case, the intervals and widths of the material members E 10 , E 20 , and E 30  may sequentially (continuously) increase with distance from the reference point C 1 . According to another exemplary embodiment, the intervals of the material members E 10 , E 20 , and E 30  may decrease with distance from the reference point C 1 . The widths of the material members E 10 , E 20 , and E 30  may decrease with distance from the reference point C 1 . When the intervals of the material members E 10 , E 20 , and E 30  decrease with distance from the reference point C 1 , the widths of the material members E 10 , E 20 , and E 30  may also decrease with distance from the reference point C 1 . In this case, the intervals and widths of the material members E 10 , E 20 , and E 30  may sequentially (continuously) decrease with distance from the reference point C 1 . 
     The material members E 10 , E 20 , and E 30  may be arranged on one single surface of a substrate SUB 10 . The substrate SUB 10  may be a transparent substrate formed of a transparent material, for example, quartz or glass. The material of the substrate SUB 10  is not limited to quartz or glass and any of variety of materials may be employed therefor. 
       FIG. 2  is a cross-sectional view taken along a line A-A′ of the material members E 10 , E 20 , and E 30  of  FIG. 1 . 
     Referring to  FIG. 2 , a width w 1  of the first material member E 10 , a width w 2  of the second material member E 20 , and a width w 3  of the third material member E 30  may be different from one another. For example, it may be that w 1 &lt;w 2 &lt;w 3 . An interval s 1  between the first material member E 10  and the second material member E 20  and an interval s 2  between the second material member E 20  and the third material member E 30  may be different from each other. For example, it may be that s 1 &lt;s 2 . When the w 1 , s 1 , w 2 , s 2 , and w 3  are respectively represented by d 1 , d 2 , d 3 , d 4 , and d 5 , it may be that d 1 &lt;d 2 &lt;d 3 &lt;d 4 &lt;d 5 . This shows that the widths and intervals of the material members E 10 , E 20 , and E 30  gradually increase with distance from the reference point C 1 . In this case, while maintaining a ratio of d 1 , d 2 , d 3 , d 4 , and d 5 , the size of each of d 1 , d 2 , d 3 , d 4 , d 5  may be modified. For example, the size of each of d 1 , d 2 , d 3 , d 4 , and d 5  may be changed to ½ times, 2 times, 3 times or the like. In  FIG. 2 , although d 1 , d 2 , d 3 , d 4 , and d 5  sequentially increase with distance from the reference point C 1 , this is merely exemplary and a size relation between d 1 , d 2 , d 3 , d 4 , and d 5  may be variously changed. 
     In the exemplary embodiment of  FIGS. 1 and 2 , the material members E 10 , E 20 , and E 30  may be composed of a dielectric material or a semiconductor material. For example, the material members E 10 , E 20 , and E 30  may include any one of Si, Ge, GaP, SiOx, SiNx, and semiconductor oxide. The semiconductor oxide may include at least one of Zn, In, Ga, and Sn. The semiconductor oxide may include, for example, ZnO, InSnO, GaInZnO, and HfInZnO. The Si may be amorphous silicon (a-Si) or polycrystalline silicon (poly-Si). The SiOx may be, for example, SiO 2 , and the SiNx may be, for example, Si 3 N 4 . When the material members E 10 , E 20 , and E 30  are formed of Si, e.g., a-Si, the material members E 10 , E 20 , and E 30  may be formed by using an existing Si-based semiconductor process. Accordingly, the material members E 10 , E 20 , and E 30  may be formed at relatively low costs. However, the material of the material members E 10 , E 20 , and E 30  is not limited thereto and various materials may be employed therefor. For example, the material members E 10 , E 20 , and E 30  may include any one selected from a group consisting of AlGaAs, GaAs, AlAs, InGaAlAs, AlGaInAs, and InP. 
     The thicknesses of the material members E 10 , E 20 , and E 30  may be about several tens of nanometers (nm) to about several micrometers (μm). For example, the thicknesses of the material members E 10 , E 20 , and E 30  may be about 50 nm to about 2 μm, or about 100 nm to about 1 μm. The thicknesses of the material members E 10 , E 20 , and E 30  may be less than the wavelength of electromagnetic waves (incident light) determined to be incident upon the electromagnetic wave focusing device D 10 . 
     The entire width (outermost diameter) of the material members E 10 , E 20 , and E 30  may be about several hundred nanometers to about several tens of micrometers. For example, the entire width (outermost diameter) of the material members E 10 , E 20 , and E 30  may be about 0.5 μm to about 50 μm, or about 1 μm to about 10 μm. As such, the electromagnetic wave focusing device D 10  having superior properties may be implemented by using a combination of simple patterns in a small size within about 10 μm. 
     Also, the distance (unit width) between the outermost side or circumference and the innermost side or circumference of each of the material members E 10 , E 20 , and E 30  may be equal to or greater than about 50 nm or equal to or greater than about 100 nm. The intervals between the material members E 10 , E 20 , and E 30  may be equal to or greater than about 50 nm or equal to or greater than about 100 nm. As such, the unit width of each of the material members E 10 , E 20 , and E 30  and the intervals between the material members E 10 , E 20 , and E 30  are relatively large, the material members E 10 , E 20 , and E 30  may be easily formed (patterned) without processing difficulties. 
       FIG. 3  is a cross-sectional view conceptually illustrating focusing of electromagnetic waves by the electromagnetic wave focusing device D 10  according to an exemplary embodiment. 
     Referring to  FIG. 3 , electromagnetic waves input from one side of the electromagnetic wave focusing device D 10  may be focused by the electromagnetic wave focusing device D 10  and output to the other side of the electromagnetic wave focusing device D 10 . In this state, the electromagnetic wave focusing device D 10  may be provided on a lower surface of the substrate SUB 10 , and the electromagnetic waves incident on an upper surface of the substrate SUB 10  may be output toward the lower surface of the substrate SUB 10 . 
     The electromagnetic wave focusing device D 10  according to an exemplary embodiment may be a “flat plate-type optical device (flat optics)”. Since the electromagnetic wave focusing device D 10  focuses light, the electromagnetic wave focusing device D 10  may be function as a “lens”, and may be referred to as a “thin flat lens” when considering the shape and size of the electromagnetic wave focusing device D 10 . When the electromagnetic wave focusing device D 10  is formed of silicon (Si), the electromagnetic wave focusing device D 10  may be a “silicon-based thin lens”. Also, the electromagnetic wave focusing device D 10  may have a small thickness of a subwavelength and implement electromagnetic properties by using a plurality of artificial material patterns. Accordingly, from this point of view, the electromagnetic wave focusing device D 10  may be referred to as a “metasurface structure”. Also, when the material patterns, that is, the material members of  FIG. 1 , are formed of dielectric or semiconductor materials, the electromagnetic wave focusing device D 10  may be referred to as a “dielectric metasurface structure” or “semiconductor metasurface structure,” respectively. The electromagnetic wave focusing device D 10  may be a “non-metallic metasurface structure” in which no metal is included. Also, the electromagnetic wave focusing device D 10  may be referred to as a “passive meta-lens” having a simple structure. 
       FIG. 4  is a graph showing intensities of electromagnetic waves (E field) by positions in a direction in which electromagnetic waves are input, when the electromagnetic waves are incident upon the electromagnetic wave focusing device according to an exemplary embodiment. A wavelength of the incident electromagnetic waves E 1  is about 868 nm. 
     Referring to  FIG. 4 , it may be seen that, when the electromagnetic waves E 1  having a intensity of “1” are incident upon the electromagnetic wave focusing device D 10 , the electromagnetic waves E 1  may have a intensity equivalent to about “24” at a position corresponding to the reference point C 1  of the electromagnetic wave focusing device D 10 . In other words, the electromagnetic waves E 1  having an intensity of “1” are reinforced to be the electromagnetic waves have an intensity corresponding to about “24”. Accordingly, it may be seen that the electromagnetic waves E 1  are focused/reinforced by the electromagnetic wave focusing device D 10  according to an exemplary embodiment. 
       FIG. 5  is a graph showing intensities of electromagnetic waves output through a medium when a series of electromagnetic wave pulses, which is an on-off series of electromagnetic waved according to time, are irradiated onto the medium to satisfy a coherence condition, thereby explaining a principle applicable to the electromagnetic wave focusing device according to an exemplary embodiment. 
     Referring to  FIG. 5 , it may be seen that electromagnetic waves having a relatively large intensity are output in a region in which input electromagnetic wave pulses are modulated, that is, in a transient region. Also, it may be seen that, when a coherence condition of an on-off series is satisfied, electromagnetic waves having a very large intensities are output at a particular time point (dotted area). This shows that when the time pulse of the input electromagnetic waves satisfies the coherence condition with respect to the medium, electromagnetic waves of very large intensities are output at a particular time point. 
     The electromagnetic wave focusing device D 10  described with reference to  FIG. 1  to  FIG. 3  may be implemented by appropriately spatially mapping the coherence condition of the time pulse of  FIG. 5 . The electromagnetic waves having very large intensities may be output at the reference point C 1  or in an area adjacent thereto, as illustrated in  FIG. 4 , by using the electromagnetic wave focusing device D 10 . 
       FIGS. 6 to 8  are graphs showing results of evaluating a beam focusing effect for each wavelength of electromagnetic waves (incident light) perpendicularly incident on an electromagnetic wave focusing device according to an exemplary embodiment, by using a finite-difference time-domain (FDTD) simulation method.  FIG. 6  illustrates a case in which the wavelength of the incident electromagnetic waves is about 400 nm (blue);  FIG. 7  illustrates a case in which the wavelength of the incident electromagnetic waves is about 550 nm (green); and  FIG. 8  illustrates a case in which the wavelength of the incident electromagnetic waves is about 700 nm (red). The electromagnetic wave focusing device used in  FIGS. 6 to 8  has the structure described with respect to  FIGS. 1 and 2 . In this case, d 1 , d 2 , d 3 , d 4 , and d 5  were respectively about 0.24 μm, about 0.37 μm, about 0.5 μm, about 0.64 μm, and about 0.77 μm. Also, a distance from the reference point (center point) C 1  to a first material member (ring pattern) E 10  was about 0.1 μm. Additionally, a dummy pattern is further provided outside the third material member E 30  and separated therefrom. In this case, the interval between the third material member E 30  and the dummy pattern was about 0.91 μm. The material members (ring pattern) E 10 , E 20 , and E 30  are all formed of a-Si, and the thicknesses of the material members E 10 , E 20 , and E 30  were about 100 nm. 
     Referring to  FIGS. 6 to 8 , it may be seen that, even when the wavelength of the incident electromagnetic waves is changed within a region of visible light (400-700 nm), a focusing effect of a beam at one point in each wavelength region is obtained. It seems that, even when an absorption coefficient with respect to visible light is high by using a-Si, an effect of focusing the electromagnetic waves at one point is obtained because not only the high refractive index properties of a-Si are used, but also the spatial coherence effect, that is, a light-matter coherent interaction effect, is used. Also, when the electromagnetic wave focusing device is applied to a lens unit, the numerical aperture (NA) of the electromagnetic wave focusing device may be equal to or greater than about 0.3. For example, the NA of the electromagnetic wave focusing device may be about 0.3-0.8. Also, it is expected that, even when the electromagnetic waves are incident upon the focusing apparatus in a non-perpendicular direction, an effect of focusing electromagnetic waves at various wavelengths may be obtained through design change and design optimization. 
       FIG. 9  is a graph showing a result of evaluating a beam focusing effect of electromagnetic waves (incident light) perpendicularly incident on an electromagnetic wave focusing device according to another exemplary embodiment, by using the FDTD simulation method. The electromagnetic wave focusing device used in  FIG. 9  has the structure described with respect to  FIGS. 1 and 2 , and the widths and intervals of the material members E 10 , E 20 , and E 30 , that is, d 1 , d 2 , d 3 , d 4 , and d 5 , may be half (½) of the values described with reference to  FIGS. 6 to 8 . The thicknesses of the material members E 10 , E 20 , and E 30  are about 100 nm, and the wavelength of the incident electromagnetic waves is about 400 nm. 
     Referring to  FIG. 9 , it may be seen that a beam focusing effect, that is, beams are focused at one point is obtained. 
       FIG. 10  is a graph for explaining a full width at half maximum (FWHM) of the focused beam (electromagnetic wave) of  FIG. 9 . In  FIG. 10 , the X-axis denotes a position in an X-axis direction and the Y-axis denotes an absolute value, that is, |E/E 0 |, of the intensity ratio of output electromagnetic waves E to the incident electromagnetic waves E 0 . 
     Referring to  FIG. 10 , it may be seen that the FWHM of a focused beam (electromagnetic waves) is about 188 nm. Since the wavelength of the incident electromagnetic waves is about 400 nm, the FWHM (188 nm) of the focused beam (electromagnetic waves) is less than half of the wavelength (400 nm) of the incident electromagnetic waves. It may be seen from the above result that a high resolving power equal to or less than a subwavelength is obtained by using the electromagnetic wave focusing device according to an exemplary embodiment. It may be seen that, considering that the resolving power limit of a conventional optical system is about 250 nm or about 200 nm, a high resolving power/super-resolution over the resolving power limit of a general optical system is obtained by using the electromagnetic wave focusing device according to an exemplary embodiment. When the electromagnetic wave focusing device is applied to an optical apparatus, for example, a lens unit of a microscope, a high resolving power/super-resolution microscope may be obtained. 
       FIGS. 11A, 11B, and 11C  are graphs showing results of evaluating a beam focusing effect of electromagnetic waves (incident light) perpendicularly incident on an electromagnetic wave focusing device according to another exemplary embodiment, by using the FDTD simulation method.  FIG. 11A  illustrates a case in which the wavelength of the incident electromagnetic waves is about 450 nm.  FIG. 11B  illustrates a case in which the wavelength of the incident electromagnetic waves is about 550 nm.  FIG. 11C  illustrates a case in which the wavelength of the incident electromagnetic waves is about 650 nm. The structure of the electromagnetic wave focusing device used in  FIG. 11  is the same as that of the electromagnetic wave focusing device described with reference to  FIGS. 6 to 8 , except that the thickness of material members is about 940 nm. In this case, it is evaluated how the beam focusing properties are changed according to a change in the thickness of material members forming the electromagnetic wave focusing device. 
     Referring to  FIGS. 11A, 11B, and 11C , it may be seen that, even when the wavelength of the incident electromagnetic waves is changed within a visible light region, superior beam focusing effect is obtained. Also, it may be seen that a difference in the beams focusing position (height) between  FIG. 11A ,  FIG. 11B , and  FIG. 11C  is as small as about 2 to 3 μm. This means that, even if the wavelength (color) of the incident electromagnetic waves is changed, the position of a focused beam is not changed much. In other words, it means that chromatic aberration decreases. The chromatic aberration of the electromagnetic wave focusing device according to an exemplary embodiment may be within about 5 μm or 3 μm. Accordingly, it may be seen that the properties and efficiency of the electromagnetic wave focusing device may be improved through the change/control of designs and dimensions of material members forming the electromagnetic wave focusing device, by utilizing the present inventive concept. 
     In designing the electromagnetic wave focusing device according to an exemplary embodiment, the widths of and intervals between the material members forming a focusing apparatus may be determined by the following mathematical equations. The following mathematical equations are described with reference to  FIG. 12 .  FIG. 12  illustrates that a plurality of material members E 11 , E 21 , E 31 , and E 41 , each having a ring shape, are arranged at different distances with respect to the reference point (center point) C 1 . 
     
       
         
           
             
               
                 
                   
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     In Mathematical Equation 1, “z i ” denotes a distance (width or interval) between an i-th surface and an (i−1)th surface (see  FIG. 12 ). For example, “z 1 ” is a distance from an inner circumferential surface of the first material member E 11  to the reference point (center point) C 1 , and “z 2 ” is a distance from the inner circumferential surface of the first material member E 11  to an outer circumferential surface of the first material member E 11 . In Mathematical Equation 1, “j 1,I ,” denotes an i-th zero position of J 1 (x) which is a primary Bessel function. “Y” of Mathematical Equation 1 may be expressed by Mathematical Equation 2, and “x i ” of Mathematical Equation 2 is determined considering absorption properties and transmission properties of a medium (material member or space between material members), which may be expressed by Mathematical Equation 3. In Mathematical Equation 3, “α 0 ” denotes an absorption coefficient of the medium (material member or space between material members), “δ” denotes a half width at half maximum (HWHM) of an absorption spectrum of the medium, and “T” denotes a time during which light passes through the medium. 
     The widths and intervals of the material members forming the electromagnetic wave focusing device, e.g., z 1 , z 2 , z 3 , z 4 , z 5 , z 6 , z 7 , and z 8  of  FIG. 12 , may be obtained by using Mathematical Equations 1 to 3. In some cases, after z 1  is set to an arbitrary value, the values of z 2 , z 3 , z 4 , z 5 , z 6 , z 7 , and z 8  may be obtained from the set value of z 1  by using Mathematical Equations 1 to 3. Alternatively, after z 2  is set to an arbitrary value, the values of z 3 , z 4 , z 5 , z 6 , z 7 , and z 8  may be obtained from the set value of z 2  by using Mathematical Equations 1 to 3. However, the condition of Mathematical Equation 3 is merely exemplary, and Mathematical Equation 3 may vary according to a material, that is, a material of a medium, that is used. In other words, “x i ” of Mathematical Equation 3 may be a sort of a material coefficient, and the condition of Mathematical Equation 3 may be changed according to the type of a material. For example, the condition of Mathematical Equation 3 may vary according to whether dielectric or semiconductor is used as the medium or a combination of dielectric and semiconductor is used as the medium. 
     During interaction between the electromagnetic waves and the medium, absorption and/or emission of the electromagnet waves may occur on a surface/boundary of the medium (material members). An amplification phenomenon may be generated by an in-phase effect and a phase modulation effect at a surface of a particular material member or in an area adjacent thereto, due to coherent interaction of the absorption and/or emission. As a result, superior light focusing properties may be obtained. To this end, the widths and intervals of the material members forming the medium may be appropriately selected to satisfy the coherence condition with respect to the electromagnetic waves. According to the principle, an electromagnetic wave focusing device having a very small thickness and a small size (width) and also having superior light focusing properties may be manufactured. 
     The electromagnetic wave focusing device may be referred to as a focusing apparatus having a “spatially coherent structure (SCS)” or a “spatially coherent stack (SCS)” structure. 
     In the case of the existing Faraon-type metasurface, lens performance is implemented by arranging numerous amorphous silicon (a-Si) nanoposts on a substrate. Since an absorption coefficient of a nanopost is large in the visible light region, it is difficult to use the nanoposts in the visible light region. Also, since nanoposts having smaller sizes are necessary for use in in shorter wavelength regions, the processing difficulty is increased. In the case of a Pendry superlens, an image is formed in a point area by using a metamaterial having a negative (−) refractive index regardless of a diffraction limit. However, a complicated and difficult process is needed for manufacturing a Pendry superlens. Also, it is almost impossible to manufacture a metamaterial having a negative refractive index in various visible light wavelength regions, and there is a fundamental limitation such as light loss by metal. In the case of near-field scanning optical microscopy (NSOM), although resolution may be increased by using a near field of a metal tip, it is required that a distance between the metal tip and a sample is shorter than a wavelength. Accordingly, there is the difficulty that a user is required to have high-level proficient technique, expensive equipment is necessary, and coupling efficiency is low. In the case of stimulated emission depletion microscopy (STED), there is a limit of being a “functional” high-resolution technology using a fluorescent material. 
     According to various exemplary embodiments, an electromagnetic wave focusing device D 10  having superior properties may be easily implemented by using a combination of simple patterns in a size within about 10 μm. The electromagnetic wave focusing device D 10  may have a small thickness, for example, a thickness equal to or less than a subwavelength, and may be formed of a material having a relatively high absorption coefficient, such as, a-Si. Since the electromagnetic wave focusing device D 10  uses both the absorption and transmission of the electromagnetic waves, even when a material having a high absorption coefficient is employed, the electromagnetic wave focusing device D 10  may exhibit a superior function as a light focusing component (lens). Also, since the width of material patterns forming the electromagnetic wave focusing device D 10  is equal to or greater than about 50 nm or equal to or greater than about 100 nm, the electromagnetic wave focusing device D 10  may be easily manufactured without processing difficulties. Also, the electromagnetic properties may be easily controlled by appropriately determining the width, interval, and thickness of the material patterns forming the electromagnetic wave focusing device D 10 . The electromagnetic wave focusing device D 10  may be operable as a lens or a light focusing component from the visible ray region to an infrared ray (IR) region, or in some cases, even in a microwave region or an ultraviolet (UV) ray region. Also, since the electromagnetic wave focusing device D 10  may be a non-metal structure without using metal, the light loss problem due to metal may not be generated. 
     When the electromagnetic wave focusing device D 10  according to an exemplary embodiment is applied to a camera lens or a microscope lens, high resolving power/super-resolution imaging may be implemented. In the case of a microscope, since a non-fluorescence method using no fluorescent material is used, the microscope may correspond to “true” super-resolution technology. Also, since the interval between the electromagnetic wave focusing device D 10  and an object (sample) to be observed is about several micrometers (μm) or more, high-level proficient technique is not required unlike the NSOM in manipulating the microscope adopting the electromagnetic wave focusing device D 10  and the object (sample) damage problem may not be generated. 
     When the electromagnetic wave focusing device D 10  according to an exemplary embodiment is used, a high resolving power/super-resolution microscope may be implemented overcoming the revolving power limit of the existing optical system. A high resolving power/super-resolution microscope having a resolving power equal to or less than ½ of the wavelength of the incident electromagnetic waves may be implemented. For example, the resolving power of the microscope may be equal to or less than about 200 nm or equal to or less than about 100 nm. The electromagnetic wave focusing device D 10  may have an NA of about 0.3 or more. As the NA increases, resolution may be enhanced. 
     According to another exemplary embodiment, at least two of the material members forming the electromagnetic wave focusing device may have different thicknesses, an example of which is illustrated in  FIG. 13 . 
       FIG. 13  is a cross-sectional view for explaining an electromagnetic wave focusing device according to another exemplary embodiment. The position of a cross-section of  FIG. 13  may correspond to that of the cross-section taken along a line A-A′ of  FIG. 1 . This is also the case with respect to  FIGS. 14 and 15 . 
     Referring to  FIG. 13 , a plurality of material members E 12 , E 22 , and E 32  may be provided around the reference point C 1 . The widths and intervals of the material members E 12 , E 22 , and E 32  may vary with distance from the reference point C 1 . At least two of the material members E 12 , E 22 , and E 32  may have different thicknesses. For example, the thicknesses of the material members E 12 , E 22 , and E 32  may increase with distance from the reference point C 1 . In this case, the thickness of the second material member E 22  may be greater than the thickness of the first material member E 12 , and the thickness of the third material member E 32  may be greater than the thickness of the second material member E 22 . As such, a focusing effect of the electromagnetic waves may be further reinforced by appropriately determining not only the widths and intervals of the material members E 12 , E 22 , and E 32 , but also the thicknesses of the material members E 12 , E 22 , and E 32 . 
     Although  FIG. 13  illustrates a case in which all of the material members E 12 , E 22 , and E 32  having different thicknesses are in a single layer structure, according to another exemplary embodiment, at least one of the material members E 12 , E 22 , and E 32  may have a multilayer structure, and at least two of the material members E 12 , E 22 , and E 32  may have different numbers of layers, an example of which is illustrated in  FIG. 14 . 
       FIG. 14  is a cross-sectional view for explaining an electromagnetic wave focusing device according to another exemplary embodiment. Referring to  FIG. 14 , at least one of a plurality of material members E 13 , E 23 , and E 33  may have a multilayer structure, and at least two of the material members E 13 , E 23 , and E 33  may have different numbers of layers. For example, the first material member E 13  may have a single layer structure, the second material member E 23  may have a double layer structure, and the third material member E 33  may have a triple layer structure. In this state, two layers L 21  and L 22  forming the second material member E 23  may be formed of different materials or the same material. Also, at least two of three layers L 31 , L 32 , and L 33  forming the third material member E 33  may be formed of different materials or the same material. The E 13 , L 21 , and L 31  may be formed of a first material, the L 22  and L 32  may be formed of a second material that is different from the first material, and the L 33  may be formed of a third material that is different from the first and second materials, or formed of the first material. As such, the focusing effect of electromagnetic waves may be controlled by appropriately determining the number of material layers and materials of at least two of the material members E 13 , E 23 , and E 33 . 
     Although  FIGS. 13 and 14  illustrate cases in which the thicknesses of the material members E 12 , E 22 , and E 32 /E 13 , E 23 , and E 33  increase with distance from the reference point C 1 , this is merely exemplary and the thicknesses of the material members E 12 , E 22 , and E 32 /E 13 , E 23 , and E 33  may be variously determined. For example, the thicknesses of the material members may decrease with distance from the reference point C 1 , an example of which is illustrated in  FIG. 15 . 
       FIG. 15  is a cross-sectional view for explaining an electromagnetic wave focusing device according to another exemplary embodiment. Referring to  FIG. 15 , the thicknesses of a plurality of material members E 14 , E 24 , and E 34  may decrease with distance from the reference point C 1 . Also, as necessary, at least two of the material members E 14 , E 24 , and E 34  may have material layers forming each of the material members E 14 , E 24 , and E 34 , the number of the material layers being different from each other. For example, the first material member E 14  may have a triple layer structure, the second material member E 24  may have a double layer structure, and the third material member E 33  may have a single layer structure. 
     Although not illustrated, the thicknesses of material members may increase and then decrease, or may decrease and then increase, with distance from the reference point (center point) C 1 . Also, the material members may have random thicknesses. 
     Although, in the above-described exemplary embodiments, the material members forming the electromagnetic wave focusing device each have a ring shape, the material members may have a shape other than a ring shape. While a ring structure completely surrounds the reference point (center point), another structure other than a ring structure may have only partially surround the reference point (center point). Cases in which the material members have structures other ring structures is illustrated in  FIGS. 16 to 19 . 
       FIG. 16  is a plan view of an electromagnetic wave focusing device D 15  according to another exemplary embodiment. 
     Referring to  FIG. 16 , the electromagnetic wave focusing device D 15  may include a first focusing unit G 15  provided on a surface of a substrate SUB 15 . The first focusing unit G 15  may include a plurality of material members E 15  to E 45  arranged such that each material member is respectively farther from the reference point C 1 . The intervals and/or widths of the material members E 15  to E 45  may vary (increase/decrease) with distance from the reference point C 1 . Each of the material members E 15  to E 45  may have a rectangular shape. For example, the material members E 15  to E 45  may have a rectangular or square shape. 
     The electromagnetic wave focusing device D 15  may further include a second focusing unit G 25 . The second focusing unit G 25  may have a structure that is the same or similar to that of the first focusing unit G 15 . For example, the second focusing unit G 25  and the first focusing unit G 15  may have symmetrical structures with respect to the reference point C 1 . 
     As such, even when the material members E 15  to E 45  of the electromagnetic wave focusing device D 15  have a structure (rectangular structure) other than the ring shape, the electromagnetic waves may be focused and output by the material members E 15  to E 45  at the reference point C 1  or in the vicinity thereof. 
     In the structure of  FIG. 16 , a width of the substrate SUB 15  in a Y-axis direction may be reduced. In other words, the size of the electromagnetic wave focusing device D 15  may be reduced. Also, if desired, one or more other devices (not shown) may be further provided on the substrate SUB 15  at one or both sides of the focusing units G 15  and G 25 . 
     Although  FIG. 16  illustrates a case in which the first and second focusing units G 15  and G 25  are horizontally and symmetrically arranged with respect to the reference point C 1 , according to another exemplary embodiment, the focusing units may be arranged in four directions with respect to the reference point C 1 , an example of which is illustrated in  FIG. 17 . 
     Referring to  FIG. 17 , an electromagnetic wave focusing device D 16  may include the first and second focusing units G 15  and G 25  horizontally arranged in an X-axis direction with respect to the reference point C 1 , and further include third and fourth focusing units G 35  and G 45  horizontally arranged in a Y-axis direction. The first to fourth focusing units G 15  to G 45  may have substantially the same structure, except for the arrangement direction of the material members E 15  to E 45 . Compared to the structure of  FIG. 16 , when the number of the focusing units is increased as illustrated in  FIG. 17 , a focusing effect of the electromagnetic waves may be increased. 
     In the structures of  FIGS. 16 and 17 , the lengths of the material members E 15  to E 45  forming the focusing units G 15  to G 45  may vary, an example of which is illustrated in  FIG. 18 . 
     Referring to  FIG. 18 , a first focusing unit G 17  may include a plurality of material members E 17  to E 47  sequentially arranged with distance from the reference point C 1 . The lengths of the material members E 17  to E 47  may increase with distance from the reference point C 1 . Accordingly, the first focusing unit G 17  may have a structure tapered toward the reference point C 1 . The second to fourth focusing units G 27 , G 37 , and G 47  may have the structure tapered toward the reference point C 1 , similar to the structure of the first focusing unit G 17 . As such, when the focusing units G 17  to G 47  has the structure tapered toward the reference point C 1 , the focusing effect of the electromagnetic waves may be further improved. In  FIG. 18 , SUB 18  and D 17  respectively denote a substrate and an electromagnetic wave focusing device. 
     Each of the material members E 17  to E 47  of  FIG. 18  may be modified to have a curved shape, not a straight shape, an example of which is illustrated in  FIG. 19 . 
     Referring to  FIG. 19 , a first focusing unit G 18  may have a plurality of material members E 18  to E 48 . The material members E 18  to E 48  may have a concavely circular arc shape with respect to the reference point C 1  or a similar shape thereto. The sizes of the material members E 18  to E 48  may increase with distance from the reference point C 1 . Second to fourth focusing units G 28 , G 38 , and G 48  having the same structure as that of the first focusing unit G 18  but a different arrangement direction from that of the first focusing unit G 18  may be further provided. As such, when the material members E 18  to E 48  have a circular arc shape or a similar shape thereto, the focusing effect of the electromagnetic waves may be further improved. 
     Although  FIGS. 16 to 19  illustrate cases in which the focusing units, e.g., G 15  and G 25  of  FIG. 16 , are symmetrically arranged with respect to the reference point C 1 , according to another exemplary embodiment, asymmetrical structure may be possible. For example, in  FIG. 16 , one of the first and second focusing units G 15  and G 25  may be excluded, and in  FIGS. 17 to 19 , one to three focusing units of the first to fourth focusing units, e.g., G 15  to G 45  of  FIG. 17 , may be excluded. Also, although  FIGS. 16 to 19  illustrate a case of using two to four focusing units, the electromagnetic wave focusing device may be formed by using three to five focusing units. 
     According to another exemplary embodiment, the material members forming the electromagnetic wave focusing device may have a structure different from the ring stricture while encompassing the reference point (center point), examples of which are illustrated in  FIGS. 20 and 21 . 
     Referring to  FIG. 20 , a material member E 2  may have an octagonal ring structure. 
     Referring to  FIG. 21 , a material member E 3  may have a rectangular ring structure. 
     Although  FIGS. 20 and 21  illustrate the single material members E 2  and E 3 , as described above in  FIGS. 1 and 2 , a plurality of material members having different sizes may be arranged at different distances with respect to the reference point C 1 , and the intervals and/or widths of the material members may vary with distance from the reference point C 1 . In addition to the modified shapes as illustrated in  FIGS. 20 and 21 , the shape of the material member may be variously changed. 
       FIG. 22  is a cross-sectional view of a structure of an electromagnetic wave focusing device D 19  according to another exemplary embodiment. 
     Referring to  FIG. 22 , the electromagnetic wave focusing device D 19  may include a plurality of material members E 19 , E 29 , and E 39  provided on one surface of a substrate SUB 19 . The material members E 19 , E 29 , and E 39  may be sequentially arranged with distance from an imaginary reference axis CX 1 . The intervals and/or widths of the material members E 19 , E 29 , and E 39  may vary with distance from the reference axis CX 1 . The electromagnetic wave focusing device D 19  may further include a plurality of the intermediate material members N 19 , N 29 , and N 39  filling spaces between the material members E 19 , E 29 , and E 39 . The intermediate material members N 19 , N 29 , and N 39  may be formed of a material having a refractive index different from that of the material of the material members E 19 , E 29 , and E 39 . When the material members E 19 , E 29 , and E 39  is formed of a first material, the intermediate material members N 19 , N 29 , and N 39  may be formed of a second material that is different from the first material. Among the intermediate material members N 19 , N 29 , and N 39 , the first intermediate material member N 19  corresponding to the reference axis CX 1  may have a circular (disk) shape, not a ring shape, when viewed from the above. The other intermediate material members N 29  and N 39  may have a ring shape or a similar shape thereto. The material members E 19 , E 29 , and E 39  may have a ring shape or a similar shape thereto. 
     As illustrated in  FIG. 22 , when the intermediate material members N 19 , N 29 , and N 39  are provided between the material members E 19 , E 29 , and E 39 , a design rule for focusing the electromagnetic waves may be determined by the above-described Mathematical Equations 1 to 3. 
     According to another exemplary embodiment, a cover layer may be further included in the structure of  FIG. 22 , an example of which is illustrated in  FIG. 23 . 
     Referring to  FIG. 23 , an electromagnetic wave focusing device D 20  may further include a cover layer CL 19  covering the material members E 19 , E 29 , and E 39 , and the intermediate material members N 19 , N 29 , and N 39 . The cover layer CL 19  may be formed of a dielectric material or a semiconductor material. The cover layer CL 19  may have a small thickness. For example, the cover layer CL 19  may have a thickness of equal to or less than about 10 nm or equal to or less than about 5 nm. When the cover layer CL 19  has a small thickness, the cover layer CL 19  may not interfere with focusing of electromagnetic waves at the opposite side of the substrate SUB 19 , the electromagnetic waves being input from one side of the substrate SUB 19  and passing through the material members E 19 , E 29 , and E 39 . The cover layer CL 19  may protect the material members E 19 , E 29 , and E 39 , and the intermediate material members N 19 , N 29 , and N 39 . 
       FIG. 24  is a plan view of an electromagnetic wave focusing device D 1   l  according to another exemplary embodiment. 
     Referring to  FIG. 24 , the electromagnetic wave focusing device D 1   l  may include the material members E 10 , E 20 , and E 30  having a ring shape and provided on a substrate (not shown), and may further include a dummy pattern ED 40  outside the material members E 10 , E 20 , and E 30 , surrounding the material members E 10 , E 20 , and E 30 . The material members E 10 , E 20 , and E 30  may have a circular ring shape, whereas the dummy pattern ED 40  may have a circular inner circumference and a rectangular outer circumference. The dummy pattern ED 40  may be formed of the same material as that of the material members E 10 , E 20 , and E 30 . The interval between the dummy pattern ED 40  and the third material member E 30  may be greater than the interval between the second material member E 20  and the third material member E 30 . Also, the interval between the dummy pattern ED 40  and the third material member E 30  may be greater than the width (distance between an inner side and an outer side) of the third material member E 30 . The dummy pattern ED 40  may not affect or may substantially not affect the focusing of electromagnetic waves. However in some cases, a part of the dummy pattern ED 40  may somewhat affect the focusing of electromagnetic waves. 
       FIG. 25  is a plan view of an electromagnetic wave focusing device D 10 ′ according to another exemplary embodiment. 
     Referring to  FIG. 25 , the electromagnetic wave focusing device D 10 ′ may include a plurality of material members E 10 ′, E 20 ′, and E 30 ′ provided on the substrate SUB 10 . The material members E 10 ′, E 20 ′, and E 30 ′ may include, for example, the first material member E 10 ′, the second material member E 20 ′, and the third material member E 30 ′. The first material member E 10 ′ may have a circular structure covering the reference point (center point) C 1 . The second material member E 20 ′ and the third material member E 30 ′ may each have a ring structure. When the electromagnetic wave focusing device D 10  of  FIG. 1  has a structure with an empty center, the electromagnetic wave focusing device D 10 ′ of  FIG. 25  may have a structure with a filled center. As such, the focusing effect of electromagnetic waves may be obtained in the structure with a filled center. 
       FIG. 26  is a plan view of an electromagnetic wave focusing device D 50  according to another exemplary embodiment. 
     Referring to  FIG. 26 , the electromagnetic wave focusing device D 50  may include a plurality of material members E 51 , E 52 , E 53  provided on a substrate SUB 50 . The material members E 51 , E 52 , and E 53  may be sequentially arranged in a direction in which each component is farther from the reference point C 1 . The widths and/or intervals of the material members E 51 , E 52 , and E 53  may decrease with distance from the reference point C 1 . For example, the widths and intervals of the material members E 51 , E 52 , and E 53  may sequentially decrease with distance from the reference point C 1 . 
     The electromagnetic wave focusing devices according to above-described exemplary embodiments may be applied to various optical apparatuses for various purposes. For example, the electromagnetic wave focusing devices may be applied to various imaging apparatuses including microscopes. When the electromagnetic wave focusing device is applied to a lens unit of a microscope, a high resolving power/super-resolution microscope may be implemented. Also, the electromagnetic wave focusing device may be applied to an internet of things (IoT) device on which an integrated photonic chip (IPC) based on silicon or non-silicon is mounted. Also, the electromagnetic wave focusing device, which is mounted on mobile phones or next generation flexible displays, may be applied to cameras or various devices performing mobile healthcare functions. In addition, the electromagnetic wave focusing device may be applied to various optical apparatuses and electronic apparatuses. Also, the structures according to the embodiments (that is, the electromagnetic wave focusing devices) may be applied to various fields using a beam focusing or collimating function, for example, camera lenses, optical zoom lenses, or image sensors of smartphones or wearable devices. In the case of the zoom lens, using a plurality of structures (that is, electromagnetic wave focusing devices) by connecting/arranging the same may be taken into consideration. 
       FIG. 27  illustrates an optical apparatus (microscope) employing an electromagnetic wave focusing device according to an exemplary embodiment. 
     Referring to  FIG. 27 , an objective lens unit  100  may be disposed facing an object  10  to be observed. The objective lens unit  100  may include one of the electromagnetic wave focusing devices according to one or more of the above-described exemplary embodiments. The objective lens unit  100  may be provided with an electromagnetic waves source unit  200  that irradiates electromagnetic waves. The electromagnetic waves source unit  200  may be referred to the “light source unit”. The electromagnetic waves generated by the electromagnetic waves source unit  200  may have a wavelength in a visible light region or an IR region, or in some cases, a wavelength in a microwave region or a UV region. The electromagnetic waves (light) generated by the electromagnetic waves source unit  200  may be focused by the electromagnetic wave focusing device of the objective lens unit  100  and irradiated onto the object  10 . 
     The optical apparatus (microscope) according to the present exemplary embodiment may further include an image providing unit  300  that shows an image of the object  10  obtained through the objective lens unit  100 . The image providing unit  300  may include, for example, a display device. The image providing unit  300  may be electrically connected to the objective lens unit  100 . Also, the image providing unit  300  may be connected to the objective lens unit  100  in wireless communications. 
     The optical apparatus (microscope) may have a high resolving power/super-resolution by the electromagnetic wave focusing device used in the objective lens unit  100 . In detail, the optical apparatus (microscope) may have a resolving power less than ½ of the wavelength of the electromagnetic waves incident upon the electromagnetic wave focusing device. 
     The optical apparatus (microscope) may be configured to obtain an image in a method of scanning the object  10 . In this state, the objective lens unit  100  may scan the object  10  at an interval spaced apart from the object  10  by several micrometers or more. Also, the optical apparatus (microscope) may obtain an image in a non-fluorescent method using no fluorescent material. 
     Although not illustrated in  FIG. 27 , the optical apparatus (microscope) may further include at least one of various elements such as a controller, an operating unit, a data processor, a communicator, a user interface, a memory etc. The optical apparatus may include a memory storing software instructions thereon and a processor which executes the software stored in the memory. 
     In the case of the NSOM, although resolution may be increased by using a near field of a metal tip, it is required that a distance between the metal tip and a sample is shorter than a wavelength. Accordingly, there is the difficulty that a user is required to have high-level proficient technique, expensive equipment is necessary, and coupling efficiency is low. In the case of the STED, there is a limit of being a “functional” high-resolution technology using a fluorescent material 
     In the case of the microscope according to an exemplary embodiment, since a non-fluorescence method using no fluorescent material is used, the microscope may correspond to “true” super-resolution technology. Also, since the interval between the electromagnetic wave focusing device (that is, lens unit) and the object  10  to be observed is about several micrometers (μm) or more, high-level proficient technique is not required unlike the NSOM in manipulating the microscope adopting the electromagnetic wave focusing device and the object (sample) damage problem may not be generated. When the electromagnetic wave focusing device according to an exemplary embodiment is used, a high resolving power/super-resolution microscope may be implemented overcoming the revolving power limit of the existing optical system. A high resolving power/super-resolution microscope having a resolving power equal to or less than ½ of the wavelength of the incident electromagnetic waves may be implemented. For example, the resolving power of the microscope may be equal to or less than about 200 nm or equal to or less than about 100 nm. 
     It should be understood that exemplary embodiments described herein should be considered in a descriptive sense only and not for purposes of limitation. Descriptions of features or aspects within each exemplary embodiment should typically be considered as available for other similar features or aspects in other exemplary embodiments. For example, it may be seen that the structures of the electromagnetic wave focusing devices described with reference to  FIGS. 1 to 3 , and  FIGS. 12 to 26 , may be variously modified. In a detailed example, it may be seen that the intervals and/or widths of the material members forming the electromagnetic wave focusing device may be randomly changed as each material member is farther from the reference point and the shapes of the material members may be variously changed. Also, it may be seen that the structure of the optical apparatus described with reference to  FIG. 27  may be variously changed, and the field of the optical apparatus to which the electromagnetic wave focusing device is applied may be variously changed. While one or more embodiments have been described with reference to the figures, it will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope as defined by the following claims.