A metamaterial structure is provided, including a substrate and a plurality of resonators that are provided on different surfaces of the substrate or different layers of the substrate. The resonators have resonance characteristics different from each other, and the metamaterial structure has a permittivity, a permeability, and a refractive index respectively different from those of the substrate in a predetermined frequency bandwidth.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Korean Patent Application No. 10-2010-093293, filed on Sep. 27, 2010, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.

BACKGROUND

The present disclosure relates to artificial material structures, electromagnetic characteristics of which are controlled, and more particularly, to multi-layered hybrid metamaterial structures.

2. Description of the Related Art

Recently, many studies have been conducted on metamaterials which have electromagnetic characteristics that may be controlled by a new method. Metamaterials are referred to as new materials or structures because their optical characteristics, such as scattering parameter, refractive index, permittivity and permeability, may be arbitrary controlled. A new left-hand rule may be applied to the metamaterial instead of the conventionally well known Fleming's right-hand rule, and actively using the metamaterials, light may be modulated by using electrical variable characteristics. Due to the characteristics of metamaterials, studies have been conducted related to electromagnetic radiation such as radio frequency (RF) waves, micrometer waves, Tera-Hertz (THz) waves, infrared rays, and visible light. In particular, metamaterials are very important in applications to fields such as biophysics, medicine, spectroscopy, imaging, and security. In the case of a split-ring resonator (SRR), in which the characteristics of the metamaterials have been well studied due to their superior resonance characteristics, electrical and magnetic control is possible, and thus, studies on the control of permeability characteristics have been conducted.

SUMMARY OF THE INVENTION

Provided are multi-layered hybrid metamaterial structures that have increased resonance characteristics and may be actively controllable.

According to an aspect of an exemplary embodiment, a metamaterial structure includes: a substrate; and a plurality of resonators that are provided on different surfaces of the substrate or on different layers of the substrate and have resonance characteristics different from each other. The metamaterial structure has a permittivity, a permeability, and a refractive index respectively different from those of the substrate in a predetermined frequency bandwidth. The metamaterial structure may be used as a resonator having a negative refractive index (NIM), as a modulator for RF waves, micrometer waves, THz waves, infrared rays, or visible light, as a switch, as a pause shifter, or as a filter.

The resonators may include first and second resonators having resonance frequencies different from each other. Since the metamaterial structure includes the resonators having resonance frequencies different from each other, an electromagnetic wave may be controlled in a wide bandwidth frequency.

Each of the resonators may be an SRR having at least one gap. A first resonator may be provided on a first layer of the substrate and the second resonator may be provided on a second layer of the substrate, and a gap of the first resonator and a gap of a second resonator may have different sizes. Also, the gaps of the first and second resonators may be oriented in different directions. The polarizing characteristic of an electromagnetic wave that passes through the metamaterial structure may be changed according to the directions in which the gaps are oriented. Since the resonators have gaps oriented in different directions from each other, the resonators may be less dependent on the polarizing direction. Of course, the gap of the resonator provided on the first layer and the gap of the resonator provided on the second layer may be oriented in the same direction.

The substrate may be a single layer substrate including one dielectric material or a multi-layer substrate including at least two different dielectric materials. The dielectric may be formed of an insulating material, an n-type semiconductor material, or a p-type semiconductor material.

The resonators may be provided on opposite surfaces of a single substrate. Also, if the substrate is a multi-layer substrate, at least one resonator may be provided on each of the multiple layers. The multi-layer substrate may be a stacked type in which substrates are stacked after forming the resonators on each of the substrates, or may be formed through sequential processes of deposition and plating.

Power sources may be applied to the resonators. For this purpose, the metamaterial structure may further include electrodes that are electrically connected to the resonators. The layer where the resonators are provided may be an n-type or p-type semiconductor layer doped with an n-type or p-type dopant. The substrate may further include an insulating layer. The insulating layer may insulate between the conductive semiconductor layers or insulate the metamaterial structure from the outside.

Of the resonators, the resonators provided on different layers may be independently electrically controlled.

In a multi-layer substrate, areas of the layers of the substrate may decrease from a lowermost layer to an uppermost layer, and at least a portion of each of the resonators disposed on the layers may be exposed to the outside.

A metamaterial structure according to embodiments described herein may be used as a modulator for RF waves, micrometer waves, THz waves, infrared rays, or visible light. In this case, the substrate may be a transparent material with respect to the corresponding wavelength of an electromagnetic wave or may be thin enough to sufficiently transmit the corresponding electromagnetic wave.

DETAILED DESCRIPTION

Exemplary embodiments will now be described more fully with reference to the accompanying drawings. It will be understood that when an element or layer is referred to as being “on” another element or layer, the element or layer may be directly on another element or layer or intervening elements or layers. In the drawings, the thickness of each of layers or sizes may be exaggerated for convenience of explanation or clarity. Like reference numerals in the drawings denote like elements, and portions nothing to do with the description are omitted.

A metamaterial according to an exemplary embodiment is an artificial structure having electromagnetic characteristics that may not be readily seen in ordinary materials present in nature. The characteristics of a metamaterial are due to its structural components, such as resonators, rather than to the material composition itself, and thus, a metamaterial may be referred to as a metamaterial structure.

FIG. 1is a schematic perspective view of a metamaterial structure10according to an exemplary embodiment.FIG. 2shows first and second resonators15and16provided on opposite surfaces of the metamaterial structure10ofFIG. 1.

Referring toFIG. 1, the metamaterial structure10according to the current embodiment includes a substrate11and the first and second resonators15and16.

The substrate11may be a single layer that includes a single dielectric material or a multi-layer substrate that includes at least two dielectric materials different from each other. The one or more dielectric materials that constitute the substrate11may be an insulating material or a semiconductor material.

The first and second resonators15and16may be formed of a conductive material such as Au, Al, or Cu. The first and second resonators15and16are formed on opposite surfaces of the substrate11and have resonance characteristics different from each other. The different surfaces of the substrate11may be different layers of the substrate11.

FIG. 2(a) shows the first resonator15formed on a first surface of the substrate11, andFIG. 2(b) shows the second resonator16formed on a second surface of the substrate11. Referring toFIG. 2, the first and second resonators15and16may be split-ring resonators (SSRs). That is, the first and second resonators15and16may be formed as ring shaped conductive materials respectively having gaps G1and G2. The first and second resonators15and16may be disposed so that centers of the first and second resonators15and16may be on the same axis.

The electromagnetic characteristics of the metamaterial structure10may be closely related to the resonance characteristics of the first and second resonators15and16. The types and sizes of the first and second resonators15and16may be controlled so that the metamaterial structure10may have a negative permittivity, a negative permeability, or a negative refractive index in a predetermined frequency bandwidth. That is, the metamaterial structure10may be controlled to have metamaterial characteristics, that is, the metamaterial structure10as a whole may be made to have a permittivity, a permeability, and a refractive index that are different from the inherent permittivity, permeability, and refractive index of the substrate11in a predetermined frequency section resonating an electromagnetic wave incident to the first and second resonators15and16.

The first and second resonators15and16may have at least one of outer diameters D1and D2, widths W1and W2, and gaps G1and G2different from each other. For example, as depicted inFIG. 2, the metamaterial structure10according to the current embodiment may have wide bandwidth characteristics including both a high resonance frequency f1of the first resonator15and a low resonance frequency f2of the second resonator16by making the size of the first resonator15smaller than that of the second resonator16. The sizes of the first and second resonators15and16are closely related to a wavelength of the electromagnetic wave to be handled. For example, the first and second resonators15and16may have sizes from a few millimetres to a few tens of millimetres with respect to an electromagnetic wave in a radio frequency (RF) region.

The gap G1of the first resonator15may be oriented in a direction different from that of the gap G2of the second resonator16. The polarizing characteristic or phase of the electromagnetic wave to be resonated is closely related to the orientation directions of the gaps G1and G2, and thus, the polarizing characteristic or phase of the electromagnetic wave to be resonated may be appropriately controlled by making the gaps G1and G2be oriented in different directions. For example, as depicted inFIG. 2, as seen from the center of a ring, the gap G1of the first resonator15may be provided to be oriented in a +Y direction, and the gap G2of the second resonator16may be provided to be oriented in a −Y direction. According to circumstances, as seen from the center of the ring, the orientation direction of the gap G1of the first resonator15may be the same as the orientation direction of the gap G2of the second resonator16, or may be different by 90 degrees.

In the current embodiment, the configuration is that the first and second resonators15and16are respectively formed on the first and second surfaces of the substrate11. However, the first and second resonators15and16are not limited thereto, that is, the plural first and second resonators15and16may be formed on the first and second surfaces of the substrate11, plural of the first and second resonators15and16may be periodically arranged on the first and second surfaces of the substrate11, or the first and second resonators15and16may be formed on the same surface of the substrate11.

Next, resonance characteristics of the metamaterial structure10according to the current embodiment will now be described.

A numerical analysis was performed through a computer simulation with respect to an example of the metamaterial structure10in which the first and second resonators15and16were formed of Au on the first and second surfaces of the substrate11, and the substrate was formed of GaAs having a permittivity ε of 12.9.

FIGS. 3A and 3Bare S-parameter graphs with respect to the first and second resonators15and16, respectively, according to an exemplary embodiment.FIG. 4is a graph showing resonance characteristics of the metamaterial structure10in which the first and second resonators15and16are designed together as inFIGS. 3A and 3B.

Referring toFIG. 3A, the first resonator15is designed to resonate in a relatively high frequency region of approximately 1.25 THz, and referring toFIG. 3B, the second resonator16is designed to resonate in a relatively low frequency region of approximately 0.65 THz.

Referring toFIG. 4, in the metamaterial structure10that includes both the first and second resonators15and16designed as described above, the permeability coefficient S21has a value of −22 dB at approximately 0.63 THz where a resonance is generated, and the antiresonance coefficient S11has a value of −25 dB at approximately 0.68 THz where anti-resonance is generated.

FIG. 5is a schematic perspective view of a configuration of a metamaterial structure20according to a comparative example of the present invention.FIG. 6is a graph showing the resonance characteristics of the metamaterial structure20ofFIG. 5, according to the comparative example.

Referring toFIG. 5, the metamaterial structure20according to the comparative example includes first and second resonators25and26formed on a surface of a substrate21. The metamaterial structure20according to the comparative example is different from the metamaterial structure10according to the embodiment described above in that, in the metamaterial structure10, the first and second resonators15and16are formed on opposite surfaces of the substrate11; however, in the metamaterial structure20, both the first and second resonators25and26are formed on the same surface of the substrate21. Except for this, the metamaterial structure20is substantially the same as the metamaterial structure10in that, for example, the substrate21and the first and second resonators25and26of the metamaterial structure20may have the same sizes and may be respectively formed of the same materials as those of the substrate11and the first and second resonators15and16of the metamaterial structure10.

Referring toFIG. 6, in the metamaterial structure20according to the comparative example, the value of the permeability coefficient S21where a resonance is generated is −18 dB, and the value of antiresonance coefficient S11where an anti-resonance is generated is −11 dB.

When the graphs ofFIG. 4andFIG. 6are compared, it is seen that the metamaterial structure10according to the above-described embodiment has values of the permeability coefficient S21where a resonance is generated and the antiresonance coefficient S11where an anti-resonance is generated that are respectively reinforced by 4 dB and 14 dB with respect to the comparative metamaterial structure20. That is, the metamaterial structure10according to the above-described embodiment has high resonance characteristics.

In the described above embodiment, a metamaterial structure10in which the first and second resonators15and16are SRRs has been explained; however, metamaterial structures described herein are not limited thereto.FIG. 7shows various modified versions of resonators that may generate metamaterial characteristics. As resonators that may be used as metamaterials, besides the modified versions of resonators depicted inFIG. 7, although not shown, various resonator examples are known and may be adapted to the metamaterial structure10of the above-described embodiment.

Furthermore, in the above-described embodiments, the rings of the first and second resonators15and16are formed of a conductive material and the gap is insulated. However, the embodiments are not limited thereto, that is, the first and second resonators15and16may have complementary structures, as compared to the previous embodiments. That is, the rings of the first and second resonators15and16may be formed of a dielectric, and regions of both of the surfaces of the substrate11may be coated with a conductive material except the gaps and the rings of the first and second resonators15and16.

FIG. 8is a schematic cross-sectional view of a configuration of a metamaterial structure30according to another exemplary embodiment.FIG. 9shows first and second resonators35and36provided on opposite surfaces of the metamaterial structure30ofFIG. 8.

Referring toFIGS. 8 and 9, the metamaterial structure30according to the current embodiment includes a substrate31and the first and second resonators35and36formed on opposite surfaces of the substrate31. At this point, bias voltages may be independently applied to each of the first and second resonators35and36from external first and second power sources V1and V2.

The substrate31may include first and second doped semiconductor layers32and34disposed on both surfaces of an insulating layer33. For example, the insulating layer33may be formed of GaAs, which is a semi-insulating material, and the first and second semiconductor layers32and34may be formed by doping GaAs with an n-type dopant or a p-type dopant. According to circumstances, the first and second semiconductor layers32and34may be formed by stacking semiconductor materials having permittivities different from each other on opposite surfaces of the insulating layer33. The first and second semiconductor layers32and34may have thicknesses equal to or different from each other.

The first and second resonators35and36are formed on opposite surfaces of the substrate31and may be formed to have resonance characteristics different from each other.

FIG. 9(a) shows the first resonator35provided on an upper surface of the first semiconductor layer32(refer toFIG. 8), andFIG. 9(b) shows the second resonator36provided on a lower surface of the second semiconductor layer34(refer toFIG. 8). Referring toFIG. 9, the first and second resonators35and36may be SRRs. That is, the first and second resonators35and36may be formed, respectively, of conductive materials35aand36ahaving a ring shape with gaps35band36b. The first and second resonators35and36may be disposed so that the centers of the first and second resonators35and36may be on the same axis. At this point, the first and second resonators35and36may be formed in different sizes (for example, diameters D1and D2) so that the resonance characteristics thereof may be different from each other, or the gaps35bof the first resonator35may be oriented in directions different from that of the gaps36bof the second resonator36. For example, the first resonator35is formed in a relatively small size to have a relatively high resonance frequency f1, and the second resonator36is formed in a relatively large size to have a relatively low resonance frequency f2, and thus, as shown inFIG. 10, the metamaterial structure30according to the current embodiment may have wide bandwidth characteristics including a high resonance frequency f1of the first resonator35and a low resonance frequency f2of the second resonator36.

In the current embodiment, the three gaps35band36bof each of the first and second resonators35and36are exemplary, and as described with reference toFIG. 2, the first and second resonators35and36respectively may have a one or more gaps. Furthermore, as resonators that may be used as metamaterials, the modified versions of resonators depicted inFIG. 7and various resonator examples (not shown) may be adapted as the first and second resonators35and36according to the current embodiment.

Electrodes (not shown) for connecting electrically are provided on each of the first and second semiconductor layers32and34and the first and second resonators35and36. The first power source V1is connected to the conductive materials35aof the first resonator35and the first semiconductor layer32, and the second power source V2is connected to the conductive materials36aof the second resonator36and the second semiconductor layer34. At this point, the conductive materials35aand36aof the first and second resonators35and36are respectively connected to the first and second semiconductor layers32and34via a Schottky contact, and the first and second semiconductor layers32and34are respectively connected to the first and second power sources V1and V2via an Ohmic contact.

Next, operation of the metamaterial structure30according to the current embodiment will now be described. For convenience of explanation, a case when the first and second semiconductor layers32and34are formed of GaAs doped with an n-type dopant will be explained.

When a voltage is not applied to the first resonator35, the conductive materials35aof the first resonator35may be electrically connected by the doping of the first semiconductor layer32. In this case, a resonance is not substantially generated in the first resonator35by an electromagnetic wave incident to the metamaterial structure30. Likewise, when a voltage is not applied to the second resonator36, the conductive materials36aof the second resonator36may be electrically connected by the doping of the second semiconductor layer34, and thus, a resonance is not substantially generated in the second resonator36.

When a negative (−) voltage is applied to the conductive materials35aof the first resonator35, free electrons in a region adjacent to the conductive materials35aof the first semiconductor layer32are pushed away, and thus, the region becomes a depletion region32b. The depletion region32bblocks the current flow between the conductive materials35aof the first resonator35, and thus, a resonance may be generated in the first resonator35by an incident electromagnetic wave. Likewise, when a negative (−) voltage is applied to the conductive materials36aof the second resonator36, free electrons in a region adjacent to the conductive materials36aof the second semiconductor layer34are pushed away, and thus, the region becomes a depletion region34b. The depletion region34bblocks the current flow between the conductive materials36aof the first resonator36, and thus, a resonance may be generated in the first resonator36by an incident electromagnetic wave. Reference numerals32aand34arepresent regions where high concentrations of free electrons are maintained in the first and second semiconductor layers32and34, respectively.

As described above, the characteristics of a metamaterial of the metamaterial structure30may be on/off controlled in real time according to whether a voltage is applied or not. Furthermore, the sizes of the depletion regions32band34bmay be controlled according to the amplitude of the bias voltage applied to the conductive materials35aand36afrom the external first and second power sources V1and V2. Since the sizes of the depletion regions32band34bchange the resistance and capacitance of the first and second resonators35and36, the resonance frequency of an electromagnetic wave may be controlled in real time by controlling the intensity of the applied voltage.

Therefore, the metamaterial structure30according to the current embodiment may be used as an active device, such as an optical modulator, and thus, may control optical characteristics of the metamaterial structure30in real time.

The metamaterial structure30according to the current embodiment has a configuration in which the insulating layer33is interposed between the first and second semiconductor layers32and34; however, the insulating layer33may be omitted.FIG. 11is a schematic cross-sectional view of a modified version of the metamaterial structure ofFIG. 8.

Referring toFIG. 11, in a metamaterial structure30′ according to a modified version, a substrate31′ is formed such that the first and second semiconductor layers32and34directly contact each other. As described above, the depletion regions32band34bare respectively formed in the first and second semiconductor layers32and34according to bias voltages applied from the external first and second power sources V1and V2, and the depletion regions32band34bare formed in regions of the substrate31′ that are close to outer surfaces thereof, where the first and second resonators35and36are formed. Therefore, although the insulating layer33is omitted, there is no substantial difference in the operation of the metamaterial structure30′.

FIG. 12is a schematic cross-sectional view of a configuration of a metamaterial structure40according to another exemplary embodiment.FIG. 13shows first through third resonators45a,45b, and45cprovided on surfaces of the metamaterial structure40ofFIG. 12.

Referring toFIGS. 12 and 13, the metamaterial structure40according to the current embodiment has a structure in which first through third unit metamaterials40a,40b, and40care stacked through combination of first through third substrates41a,41b, and41c. In the first through third unit metamaterials40a,40b, and40c, first through third semiconductor layers43a,43b, and43crespectively are provided on the first through third substrates41a,41b, and41c, and the first through third resonators45a,45b, and45crespectively are formed on the first through third semiconductor layers43a,43b, and43c. Bias voltages may be independently applied to each of the first through third resonators45a,45b, and45cfrom external first through third power sources V1, V2, and V3. Electrodes (not shown) for connecting electrically are provided on each of the first through third semiconductor layers43a,43b, and43cand the first through third resonators45a,45b, and45c.

The first through third substrates41a,41b, and41cmay be formed of the same material or materials having permittivities different from each other. Also, the first through third semiconductor layers43a,43b, and43cmay be formed of the same material or materials having permittivities different from each other, and may be doped with an n-type dopant or a p-type dopant. For example, the first through third substrates41a,41b, and41cmay be formed of GaAs, which is semi-insulating, and the first through third semiconductor layers43a,43b, and43cmay be formed by doping GaAs with an n-type dopant or a p-type dopant. The first through third substrates41a,41b, and41cmay have thicknesses equal to or different from each other, and also, the first through third semiconductor layers43a,43b, and43cmay have thicknesses equal to or different from each other.

The first through third resonators45a,45b, and45crespectively are provided on the first through third semiconductor layers43a,43b, and43c, and may be formed to have resonance characteristics different from each other.

FIGS. 13(a),(b),(c) respectively illustrate the first through third resonators45a,45b, and45c. Referring toFIG. 13, the first through third resonators45a,45b, and45cmay be SRRs. The first through third resonators45a,45b, and45cmay be disposed so that the centers thereof may be on the same axis. The first through third resonators45a,45b, and45cmay be formed to have sizes (for example, diameters D1, D2, and D3) different from each other or the gaps thereof may be oriented in directions different from each other so that the resonance characteristics thereof may be different from each other.

For example, the first resonator45amay be formed in a relatively small size to have a relatively high resonance frequency f1, the second resonator45bmay be formed in a relatively medium size to have a relatively medium size resonance frequency f2, and the third resonator45cmay be formed in a relatively large size to have a relatively low resonance frequency f3, and thus, as depicted inFIG. 14, the metamaterial structure40according to the current embodiment may have wide bandwidth characteristics including the high, medium, and low resonance frequencies f1, f2, and f3of the first through third resonators45a,45b, and45c.

The first substrate41aand the first semiconductor layer43ahave an area smaller than that of the second substrate41band the second semiconductor layer43b, and thus, an outer region of the second semiconductor layer43bmay be exposed to the outside. Since the second resonator45baccording to the current embodiment has an SRR structure, the second resonator45bmay be provided on the exposed outer region of the second semiconductor layer43b. Likewise, the second substrate41band the second semiconductor layer43bhave an area smaller than that of the third substrate41cand the third semiconductor layer43c, and thus, an outer region of the third semiconductor layer43cis exposed to the outside, and the third resonator45cmay be provided on the exposed outer region of the third semiconductor layer43c. The first resonator45ais provided on the uppermost surface of the metamaterial structure40, and thus, is exposed to the outside. In this way, since the first through third resonators45a,45b, and45care exposed to the outside, the first through third resonators45a,45b, and45crespectively may be readily electrically wired to external first through third power sources V1, V2, and V3.

The first power source V1is connected to the first resonator45aand the first semiconductor layer43a, the second power source V2is connected to the second resonator45band the second semiconductor layer43b, and the third power source V3is connected to the third resonator45cand the third semiconductor layer43c. At this point, the first through third resonators45a,45b, and45crespectively are connected to the first through third semiconductor layers43a,43b, and43cvia a Schottky contact, and the first through third semiconductor layers43a,43b, and43crespectively are connected to the first through third power sources V1, V2, and V3via an Ohmic contact.

The operation of the metamaterial structure40according to the current embodiment is substantially the same as that of the metamaterial structure30described with reference toFIGS. 8 through 10except that the metamaterial structure40includes three resonance frequencies to be controlled. That is, when voltages are not applied to the first through third resonators45a,45b, and45c, the first through third resonators45a,45b, and45cmay be electrically connected to each other via the highly doped state of the first through third semiconductor layers43a,43b, and43cby bypassing the gaps, and thus, a resonance is not substantially generated. However, when voltages are applied to the first through third resonators45a,45b, and45c, depletion regions are formed in the first through third semiconductor layers43a,43b, and43cnear the gaps of the first through third resonators45a,45b, and45c, and thus, an electrical flow is effectively blocked, and accordingly, a resonance may be generated by an incident electromagnetic wave.

As described above, the metamaterial characteristics of each of the first through third unit metamaterials40a,40b, and40cmay be controlled in real time according to whether a voltage is applied or not or according to the amplitude of the voltage. Therefore, the metamaterial structure40according to the current embodiment may function as an active device.

In the current embodiment, since the second and third resonators45band45chave an SRR structure, the second and third resonators45band45cmay be respectively provided on the outer regions of the second and third semiconductor layers43band43c; however, the current embodiment is not limited thereto. As resonators that may be used as metamaterials, the modified versions of resonators depicted inFIG. 7and various resonator examples (not shown) may be used as the first through third resonators45a,45b, and45caccording to the current embodiment. In this case, portions of the second and third resonators45band45cmay be exposed on the exposed outer regions of the second and third semiconductor layers43band43c.

Also, in the current embodiment, a case when the metamaterial structure40has three unit metamaterial structures is described as an example. However, the current embodiment is not limited thereto and the metamaterial structure40may include two unit metamaterial structures, or may include to four or more unit metamaterial structures.

FIG. 15is a schematic cross-sectional view of a configuration of a metamaterial structure50according to another exemplary embodiment of.FIG. 16shows first through fourth resonators55a,55b,55c, and55dprovided on surfaces of the metamaterial structure50ofFIG. 15.

Referring toFIGS. 15 and 16, the metamaterial structure50according to the current embodiment has a structure in which first through fourth unit metamaterials50a,50b,50c, and50dare stacked on a substrate51. The metamaterial structure50is formed by sequentially stacking the first through fourth unit metamaterials50a,50b,50c, and50don the substrate51in the order stated. The first through fourth unit metamaterials50a,50b,50c, and50drespectively include first through fourth semiconductor layers53a,53b,53c, and53d, and the first through fourth resonators55a,55b,55c, and55drespectively formed on the first through fourth semiconductor layers53a,53b,53c, and53d. Bias voltages may be independently applied to each of the first through fourth resonators55a,55b,55c, and55dfrom external first through fourth power sources V1, V2, V3, and V4. Electrodes (not shown) for connecting electrically are provided on each of the first through fourth semiconductor layers53a,53b,53c, and53dand the first through fourth resonators55a,55b,55c, and55d.

The substrate51may be formed of an insulating material or a semiconductor material. The first through fourth semiconductor layers53a,53b,53c, and53dmay be formed of the same semiconductor material or semiconductor materials having permittivities different from each other by doping an n-type dopant or a p-type dopant. For example, the substrate51may be formed of semi-insulating GaAs, and the first through fourth semiconductor layers53a,53b,53c, and53dmay be formed by doping GaAs with an n-type dopant or a p-type dopant. The first through fourth semiconductor layers53a,53b,53c, and53dmay have thicknesses equal to or different from each other.

The first through fourth resonators55a,55b,55c, and55drespectively are provided on the first through fourth semiconductor layers53a,53b,53c, and53d, and are formed to have resonance characteristics different from each other. Since the first through fourth semiconductor layers53a,53b,53c, and53drespectively may be understood as different layers of the substrate51in a broad sense, also it may be understood that the first through fourth resonators55a,55b,55c, and55dare formed on the different layers of the substrate51.

FIGS. 16(a),(b),(c), and (d) show the first through fourth resonators55a55b,55c, and55d, respectively. Referring toFIG. 16, the first through fourth resonators55a,55b,55c, and55dmay be SRRs. The first through fourth resonators55a,55b,55c, and55dmay be disposed so that the centers thereof may be on the same axis. The first through fourth resonators55a,55b,55c, and55dmay be formed in sizes (for example, diameters D1, D2, D3, and D4) different from each other or gaps thereof may be oriented in directions different from each other so that the first through fourth resonators55a,55b,55c, and55dmay have resonance characteristics different from each other.

For example, the first through fourth resonators55a,55b,55c, and55dmay be formed to have resonance frequencies f1, f2, f3, and f4different from each other by sequentially differing the sizes of the first through fourth resonators55a,55b,55c, and55d. In this case, as depicted inFIG. 17, the metamaterial structure50according to the current embodiment may have wide bandwidth characteristics that include all of the resonance frequencies f1, f2, f3, and f4of the first through fourth resonators55a,55b,55c, and55d.

The first power source V1is connected to the first resonator55aand the first semiconductor layer53a, the second power source V2is connected to the second resonator55band the second semiconductor layer53b, the third power source V3is connected to the third resonator55cand the third semiconductor layer53c, and the fourth power source V4is connected to the fourth resonator55dand the fourth semiconductor layer53d. At this point, the first through fourth resonators55a,55b,55c, and55drespectively are connected to the first through fourth semiconductor layers53a,53b,53c, and53dvia a Schottky contact, and the first through fourth semiconductor layers53a,53b,53c, and53drespectively are connected to the first through fourth power sources V1, V2, V3, and V4via an Ohmic contact.

The operation of the metamaterial structure50according to the current embodiment is substantially the same as that of the metamaterial structures30and40described with reference toFIGS. 8 through 10andFIGS. 11 through 13except that the metamaterial structure50includes four resonance frequencies to be controlled. That is, when voltages are not applied to the first through fourth resonators55a,55b,55c, and55d, the first through fourth resonators55a,55b,55c, and55dmay be electrically connected to each other via the highly doped state of the first through fourth semiconductor layers53a,53b,53c, and53dby bypassing the gaps, and thus, a resonance is not substantially generated. However, when voltages are applied to the first through fourth resonators55a,55b,55c, and55d, depletion regions are formed in the first through fourth semiconductor layers53a,53b,53c, and53dnear the gaps of the first through fourth resonators55a,55b,55c, and55d, and thus, an electrical flow is effectively blocked, and accordingly, a resonance may be generated by an incident electromagnetic wave.

As described above, the metamaterial characteristics of each of the first through fourth unit metamaterials50a,50b,50c, and50dmay be controlled in real time according to whether a voltage is applied or not or the amplitude of the voltage. Therefore, the metamaterial structure50according to the current embodiment may function as an active device.

Also, in the current embodiment, a case when the first through fourth resonators55a,55b,55c, and55dare SRRs is described as an example. As resonators that may be used as metamaterials, the modified versions of resonators depicted inFIG. 7and various resonator examples (not shown) may be used as the first through fourth resonators55a,55b,55c, and55daccording to the current embodiment. Also, in the current embodiment, the metamaterial structure50having four unit metamaterial structures is described as an example; however, the current embodiment is not limited thereto and the metamaterial structure50may include two or three unit meta-material structures, or may extend to more than five unit metamaterial structures.

The metamaterial structures described in the above embodiments may have one or more of the following advantages.

First, the resonance characteristics may be increased by disposing resonators having resonance frequencies different from each other on each of the layers of the substrate.

Second, resonance frequency and frequency bandwidth may be actively controlled through an electrical control with respect to resonators having resonance characteristics different from each other.

Third, electrical controls with respect to resonators may be performed in each of the layers.

While the metamaterial structures described herein have been described with reference to the embodiments depicted in drawings for better understanding, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the inventive concept as defined by the appended claims.