Teraherz-wave connector and teraherz-wave integrated circuits, and wave guide and antenna structure

The terahertz-wave connector includes: a 2D-PC slab; lattice points periodically arranged in the 2D-PC slab, the lattice points for diffracting the THz waves in PBG frequencies of photonic band structure of the 2D-PC slab in order to prohibit existence in a plane of the 2D-PC slab; a 2D-PC waveguide disposed in the 2D-PC slab and formed with a line defect of the lattice points; and an adiabatic mode converter disposed at the edge face of the 2D-PC slab to which the 2D-PC waveguide extended, the 2D-PC waveguide extended to the adiabatic mode converter. There is provided also the THz-wave IC to which such a terahertz-wave connector is applied.

This application is based upon and claims the benefit of priority from prior Japanese Patent Application Nos. P2013-41606 filed on Mar. 4, 2013, and P2014-28756 filed on Feb. 18, 2014, the entire contents of which are incorporated herein by reference.

TECHNICAL FIELD

The present invention relates to a terahertz (THz)-wave connector and a THz-wave integrated circuit (IC), and a waveguide and antenna structure. In particular, the present invention relates to: a THz-wave connector and THz-wave IC each which can reduce a connection loss in an interface between two dimensional photonic crystal (2D-PC) slab and the waveguide; and a waveguide having nonreflective structure for controlling an influence of light interference and multiple reflection in a waveguide end, and an antenna structure to which such a waveguide are applied.

BACKGROUND ART

In recent years, for THz wave band (0.1 THz to 10 THz) positioned in intermediate frequencies between electromagnetic waves and light waves, studies of applicabilities of ultra high-speed wireless communications, sensing, imaging, etc. have become active, and there has been expected its practical application. However, since THz-wave systems are composed of large-sized and three-dimensional structured components under the current circumstances, large-sized and expensive configurations are required for such THz-wave systems. In order to miniaturize the whole of such systems, implementation of THz-wave ICs integrating devices is indispensable.

Utilization of technologies of both of a light wave region and an electric wave region can be considered as fundamental technologies of the THz-wave ICs. However, optical components, e.g. lenses, mirrors, are composed of large-sized and three-dimensional structured components, and therefore are not suitable for the integration. Moreover, it is becoming difficult to produce hollow metal waveguides used in the electric wave region due to its fine three-dimensional structure. A waveguide loss in planar metallic-transmission lines is increased as effect of metallic absorption is increased.

As a fundamental technology of THz-wave ICs, there has been studied applicability of a 2D-PC slab where outstanding progress is seen in the light wave region (e.g., refer to Non Patent Literatures 1-3.).

The waveguide for the THz wave band is standardized in a range from WR6 (110 GHz to 170 GHz) to WR1 (0.75 THz to 1.1 THz). Although the cross-sectional size is as small as in a range from 1651 μm×826 μm to 254 μm×127 μm, it needs to be formed by machining and be fixed with a screw at the connection. For example, there is a loss of approximately 0.5 dB in WR3 (220 GHz to 325 GHz) (e.g., refer to Non Patent Literature 4.).

Moreover, there have been also reviewed resonant and waveguiding line defect modes in a two-dimensional electromagnetic band-gap slab structure for millimeter wave frequency bands (e.g., refer to Non Patent Literature 5.).

Moreover, generally in the PC waveguide, since not only the THz wave band, but also a terminal portion of the waveguide has a large refractive index difference between semiconductor and air, there is influence of light interference (Fabry-Pérot resonance) and multiple reflection due to edge face reflection (e.g., refer to Non Patent Literature 6.).

CITATION LIST

SUMMARY OF THE INVENTION

Technical Problem

The waveguide is high-cost, and therefore there is a problem in respect of connection loss. Although metallic-transmission lines have been also proposed as THz-wave transmission lines, there is a problem in respect of absorption loss.

In the Non Patent Literature 5, although input/output propagation loss of two-dimensional electromagnetic band-gap slab structure has been reviewed in W band millimeter wavebands (from 75 GHz to 100 GHz), it is not disclosed regarding detailed structure.

Moreover, a result of influences of a light interference and multiple reflections due to the edge face reflection becomes a cause of a noise and communication band restrictions, etc., and makes use and exact estimation of devices difficult.

The object of the present invention is to provide a THz-wave connector which can reduce the connection loss in the interface between the 2D-PC slab and the waveguide, and a THz-wave IC to which such a THz-wave connector is applied.

Moreover, the object of the present invention is to provide a waveguide having nonreflective structure for controlling the influence of light interference and multiple reflections in the waveguide end, and an antenna structure to which such a waveguide is applied.

Solution to Problem

According to one aspect of the present invention, there is provided a terahertz-wave connector comprising: a 2D-PC slab; lattice points periodically arranged in the 2D-PC slab, the lattice points for diffracting terahertz waves in photonic bandgap frequencies of photonic band structure of the 2D-PC slab in order to prohibit existence in a plane of the 2D-PC slab; a 2D-PC waveguide disposed in the 2D-PC slab and formed with a line defect of the lattice points; and an adiabatic mode converter disposed at an edge face of the 2D-PC slab to which the 2D-PC waveguide extended, the 2D-PC waveguide extended to the adiabatic mode converter.

According to another aspect of the present invention, there is provided a terahertz-wave integrated circuits, wherein the terahertz-wave connector is disposed in at least one side of input and output interfaces of the 2D-PC slab.

According to still another aspect of the present invention, there is provided a waveguide comprising: a 2D-PC slab; lattice points periodically arranged in the 2D-PC slab, the lattice points for diffracting light waves or terahertz waves in photonic bandgap frequencies of photonic band structure of the 2D-PC slab in order to prohibit existence in a plane of the 2D-PC slab; a 2D-PC waveguide disposed in the 2D-PC slab and formed with a line defect of the lattice points; and an adiabatic mode converter disposed at an edge face of the 2D-PC slab to which the 2D-PC waveguide extended, the 2D-PC waveguide extended to the adiabatic mode converter.

According to yet another aspect of the present invention, there is provided an antenna structure comprising: a 2D-PC slab; lattice points periodically arranged in the 2D-PC slab, the lattice points for diffracting light waves or terahertz waves in photonic bandgap frequencies of photonic band structure of the 2D-PC slab in order to prohibit existence in a plane of the 2D-PC slab; a 2D-PC waveguide disposed in the 2D-PC slab and formed with a line defect of the lattice points; and an adiabatic mode converter disposed at an edge face of the 2D-PC slab to which the 2D-PC waveguide extended, the 2D-PC waveguide extended to the adiabatic mode converter.

Advantageous Effects of Invention

According to the present invention, there can be provided the THz-wave connector which can reduce the connection loss in the interface between the 2D-PC slab and the waveguide, and the THz-wave IC to which such a THz-wave connector is applied.

According to the present invention, there can be provided the waveguide having nonreflective structure for controlling the influence of light interference and multiple reflections in the waveguide end, and the antenna structure to which such a waveguide is applied.

DESCRIPTION OF EMBODIMENTS

There will be described embodiments of the present invention, with reference to the drawings. In the following drawings, same blocks or elements are designated by same reference characters to eliminate redundancy and for simplicity. However, it should be known about that the drawings are schematic and are differ from an actual thing. Of course, the part from which the relation and ratio of a mutual size differ also in mutually drawings is included.

The embodiments to be described hereinafter exemplify the apparatus and method for a technical concept or spirit of the present invention; and do not specify dispositions, etc. of each component part as examples mentioned below. The embodiments of the present invention may be changed without departing from the spirit or scope of claims.

A 2D-PC slab12according to basic technology includes structure in which lattice points12A having periodic structure of the same degree as a wavelength of THz waves are formed in the 2D-PC slab12, as shown inFIG. 1. In an example ofFIG. 1, the lattice points12A having the periodic structure have a triangular lattice. Since the 2D-PC slab12according to the basic technology inputs a terahertz (THz) input wave Wismaller than the wavelength (smaller than λ/4) from an edge face and outputs a terahertz (THz) output wave Woas shown inFIG. 1, the coupling loss in input and output interfaces is relatively large. The coupling efficiency in the input and output interfaces is equal to or less than approximately several percent.

As shown inFIG. 2, the THz-wave connector1according to the first embodiment includes: a 2D-PC slab12; lattice points12A periodically arranged in the 2D-PC slab12, the lattice points12A for diffracting the THz waves in photonic bandgap (PBG) frequencies of photonic band structure of the 2D-PC slab12in order to prohibit existence in a plane of the 2D-PC slab12; a 2D-PC waveguide14disposed in the 2D-PC slab12and formed with a line defect of the lattice points12A; and an adiabatic mode converter10disposed at an edge face of the 2D-PC slab12to which the 2D-PC waveguide14extended, the 2D-PC waveguide14extended to the adiabatic mode converter10.

In the THz-wave connector1according to the first embodiment, as shown inFIG. 2, the adiabatic mode converter10, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12. Moreover, the side surface of the tapered shape may have an inclined plane as shown inFIG. 2.

Moreover, the THz-wave connector1according to the embodiment may have protective structure for covering the adiabatic mode converter10with a resin layer38etc., as shown inFIG. 3. As the resin layer38, polymer resins, e.g. an ultraviolet (UV) curing resin or a thermosetting resin, etc. are applicable in the present embodiment, for example.

An extremely low-loss interface between the THz-wave connector1and the waveguide28according to the first embodiment can be achieved by inserting the adiabatic mode converter10formed at an edge face of the 2D-PC slab12into a waveguide line36in the waveguide28, as shown inFIG. 4.

More specifically, according to the THz-wave connector1according to the first embodiment, the extremely low-loss connection to the waveguide28can be achieved by introducing the adiabatic mode converter10into the edge face of the 2D-PC slab12, and controlling an excessive surface wave in the crystal edge face close to the waveguide flange34with devising the crystal edge face structure.

MODIFIED EXAMPLE 1

In the THz-wave connector1according to a modified example 1 in the first embodiment, as shown inFIG. 5, the adiabatic mode converter10A, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12, and the side surface of the tapered shape may have a curved surface. In this case, the curved surface may have a hyperboloid surface or an exponential surface.

MODIFIED EXAMPLE 2

In the THz-wave connector1according to a modified example 2 in the first embodiment, as shown inFIG. 6, the adiabatic mode converter10B, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12, and the side surface of the tapered shape may have a plurality of stepped surfaces.

MODIFIED EXAMPLE 3

In the THz-wave connector1according to a modified example 3 in the first embodiment, as shown inFIG. 7A, the adiabatic mode converter10A, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12, and the side surface of the tapered shape may have a curved surface. The shape shown inFIG. 7Ais set up so that the length of the adiabatic mode converter10A is relatively shorter than that of the shape shown inFIG. 5.

MODIFIED EXAMPLE 4

In the THz-wave connector1according to a modified example 4 in the first embodiment, as shown inFIG. 7B, the adiabatic mode converter10C may have a conical shape so that the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12. In this case, the modified example of conical shape may include not only a trumpet-like shape as shown inFIG. 7Bbut also a simple conical shape.

MODIFIED EXAMPLE 5

In the THz-wave connector1according to a modified example 5 in the first embodiment, as shown inFIG. 8A, the adiabatic mode converter10C may have a quadrangular pyramid shape so that the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12.

MODIFIED EXAMPLE 6

In the THz-wave connector1according to a modified example 6 in the first embodiment, as shown inFIG. 8B, the adiabatic mode converter10C may have a wedge-like shape so that the thickness of the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12.

MODIFIED EXAMPLE 7

In the THz-wave connector1according to a modified example 7 in the first embodiment, as shown inFIG. 9A, the adiabatic mode converter10C may have a plurality of stairs-like shapes so that the thickness of the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12.

MODIFIED EXAMPLE 8

In the THz-wave connector1according to a modified example 8 in the first embodiment, as shown inFIG. 9B, the adiabatic mode converter10C may have a plectrum-like shape so that the thickness of the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12. In this case, the adiabatic mode converter10, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12, and the side surface of the tapered shape has a curved surface.

MODIFIED EXAMPLE 9

In the THz-wave connector1according to a modified example 9 in the first embodiment, as shown inFIG. 9B, the adiabatic mode converter10C may have a wedge-like shape so that the width of the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12. In this case, the adiabatic mode converter10, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12, and the side surface of the tapered shape has a inclined plane.

In the THz-wave connector1according to the first embodiment, the structure of the adiabatic mode converter10is not limited to the structures shown inFIGS. 2-9, but can also use a structure with which any one or more of such structures are combined with each other. For example, a plurality of step shapes may be introduced into the side surface of the quadrangular pyramid shape. Alternatively, the plurality of the step shape may be introduced into the side surface of the conical shape or conical trumpet-like shape.

In the THz-wave connector1according to the first embodiment, the adiabatic mode converter10covered with the resin layer38is inserted into the waveguide line36, as shown inFIG. 10. InFIG. 10, the lengths X-Y of the aperture of the waveguide line36are approximately 0.4 mm and approximately 0.8 mm, for example. Moreover, the thickness Y0of the bottom of the adiabatic mode converter10is approximately 0.2 mm, for example.

The 2D-PC slab12is dielectric plate structure having two-dimensional periodic structure. According to such a design, a Photonic Band Gap (PBG) in which an electromagnetic mode cannot exist appears. Furthermore, the waveguide mode and the resonant mode can be introduced in the PBG by disturbing the periodic structure, and thereby a low-loss waveguide and resonator in a micro region equal to or less than the wavelength size can be achieved.

In this case, the bandwidth of the PBG depends on the refractive index of dielectrics, and has preferable high-refractive index materials.

Materials of the 2D-PC slab12providing the interface with the THz-wave connector1according to the first embodiment may be formed of semiconducting materials.

As the semiconducting materials, the following are applicable. More specifically, silicon (Si), GaAs, InP, GaN, etc. are applicable thereto, and GaInAsP/InP based, GaInAs/GaAs based, GaAlAs/GaAs based or GaInNAs/GaAs based, GaAlInAs/InP based, GaAlInP/GaAs based, GaInN/GaN based materials, etc. are applicable thereto. In particular, high resistivity Si has a high refractive index in the THz wave bands, and therefore there is little material absorption.

In the periodic structure of lattice points12A in the 2D-PC slab12to which the THz-wave connector1according to the first embodiment can be applied, an example of the square lattice is schematically illustrated as shown inFIG. 11A, and an example of the triangular lattice is schematically illustrated as shown inFIG. 11B.

Moreover, in the periodic structure of lattice points12A in the 2D-PC slab12to which the THz-wave connector1according to the first embodiment can be applied, an example of the rectangular lattice is schematically illustrated as shown inFIG. 12A, and an example of the rhombic lattice (face-centered rectangle lattice) is schematically illustrated as shown inFIG. 12B.

Moreover, the lattice points12A of the 2D-PC slab12to which the THz-wave connector1according to the first embodiment can be applied may be provided with a hole shape of any one of the polygonal shape, circular shape, oval shape, or ellipse shape. Moreover, the hole shape of the lattice points12A may pass there through, and may have recessed structure. Furthermore, impurities may be doped at predetermined concentration in the materials composing the 2D-PC slab12.

Moreover, the lattice points12A may be formed as an air hole, or may be filled up with a semiconductor layer differing in the refractive index therefrom, for example. For example, the lattice point may be formed by a GaAs layer filled up with an GaAlAs layer.

Moreover, in the THz-wave connector1according to the first embodiment, it is possible to adapt as the lattice point (hole)12A not only the structure where the hole of air is formed, but the structure where (a part of) the hole is filled up with a low-refractive index (low-dielectric constant) medium.

Moreover, layered structure for sandwiching the top and bottom principal surfaces of the 2D-PC slab12with the low-refractive index medium may be adopted, in the THz-wave connector1according to the first embodiment.

Moreover, layered structure for adding the low-refractive index medium only to the top surface or the bottom surface among the top and bottom principal surfaces of the 2D-PC slab12is also applicable, in the THz-wave connector1according to the first embodiment.

Moreover, a configuration in which the 2D-PC slab12is mounted on a low-refractive index printed circuit board may be adopted, in the THz-wave connector1according to the first embodiment.

Moreover, layered structure for sandwiching the top and bottom principal surfaces of the 2D-PC slab12with a metal may be adopted, in the THz-wave connector1according to the first embodiment.

Moreover, layered structure for adding the metal only to the top surface or the bottom surface among the top and bottom principal surfaces of the 2D-PC slab12is also applicable, in the THz-wave connector1according to the first embodiment. Although the absorption loss due to the metal increases in the THz band, the above-mentioned configuration for laminating the metal may be adopted since the absorption loss is not higher than that of the light wave region.

Moreover, not only the semiconductor materials but also the high-refractive index medium can be applied, as the materials of the 2D-PC slab12. For example, magnesium oxide (MgO) is applicable to the 2D-PC slab12since the refractive index in the THz wave band becomes approximately 3.1 which is high dielectric (insulator).

(Experimental System of Spectroscopy)

FIG. 13shows a photograph example of an experimental system of a spectroscopy using the 2D-PC slab providing input/output interfaces with the THz-wave connector according to the first embodiment. Moreover, a schematic block configuration corresponding toFIG. 13is illustrated as shown inFIG. 14.

As shown inFIGS. 13 and 14, the spectroscopy using the 2D-PC slab12providing the input/output interfaces with the THz-wave connector1according to the first embodiment includes: a millimeter-wave generator (synthesizer)30; a multiplier (×3)242connected to the synthesizer30; a multiplier (×3)241connected to the multiplier242; a waveguide28connected to the multiplier241; a 2D-PC slab12connected to the waveguide28; a waveguide26connected to the 2D-PC slab12; a mixer22connected to the waveguide26; and a spectrum analyzer32connected to the mixer22.

The 2D-PC waveguide14approximately 19 mm in the length having a tapered structure was fabricated as input/output structure to the waveguides26,28using a Si substrate having the resistivity of 3000 ωcm. Moreover, a 2D-PC slab without the 2D-PC waveguide14was also fabricated for a comparison therewith.

The spectroscopic system (FIGS. 13 and 14) composed of the millimeter-wave generator (synthesizer)30, the multipliers241,242, the spectrum analyzer32, and the WR3 waveguides26,28are used; the 2D-PC slab (sample)12providing the input/output interfaces with the fabricated THz-wave connector1according to the first embodiment is connected with the waveguides26,28; the input signal frequency from the multiplier241to the 2D-PC slab (sample)12is varied in a range from 0.28 THz to 0.39 THz; and thereby transmission characteristics were measured by the spectrum analyzer32.

(Transmission Characteristics Depending on the Existence or Nonexistence of THz-wave Connector)

FIG. 15shows a relationship between the transmissivity T and the frequency f, between the 2D-PC slab12and the waveguides26,28depending on the existence or nonexistence of the THz-wave connector1according to the first embodiment. InFIG. 15, the curved line C0corresponds to the case where there is no THz-wave connector1, and the curved line C1corresponds to the case where there is the THz-wave connector1and the case where there is no suitable gap between the THz-wave connector1and the waveguide flange40,34. The structure of the THz-wave connector1has the similar configuration as that ofFIG. 2, and the length of the adiabatic mode converter10(taper length L1) is approximately 3 mm.

As clearly fromFIG. 15, the transmissivity T increases by introducing the THz-wave connector1according to the first embodiment. Note that the transmissivity reduction of approximately 6 dB is observed in specific frequencies of a D0portion on the curved line.

The combined state with the surface wave varies and the frequencies to which the transmissivity is reduced varies by shortening the length of the adiabatic mode converter10(taper length L1). However, a phenomenon in which the transmissivity T becomes lower appears. By providing the suitable gap between the THz-wave connector1and the waveguide flange40,34, as shown inFIG. 22described below, the extremely low-loss transmission characteristics of equal to or less than 0.1 dB are obtained within a range of approximately 23 GHz equivalent to the band fw1ranging from 0.314 THz to 0.337 THz, thereby improving also the phenomenon in which the transmissivity T becomes lower.

(Transmission Characteristics Depending on the Existence or Nonexistence of Gap Between THz-wave Connector and Waveguide Flange)

FIG. 16shows a relationship between the transmissivity T and the frequency f, between the 2D-PC slab12and the waveguide26depending on the existence or nonexistence of a gap between the THz-wave connector1and the waveguide flange40according to the first embodiment. InFIG. 16, the curved, dashed line G0corresponds to the case where there is no gap between the THz-wave connector1and the waveguide flange40(FIG. 17), and the curved, solid line G corresponds to the case where there is a gap between the THz-wave connector1and the waveguide flange40(FIG. 18). In this case, the length of the adiabatic mode converter10(taper length L1) is approximately 3 mm.

In the case of there is no gap between the THz-wave connector1and the waveguide flange40, as shown inFIG. 17, the waveguide flange40contacts with the edge face of the 2D-PC slab12. On the other hand, in the case where there is a gap between the THz-wave connector1and the waveguide flange40, as shown inFIG. 18, the waveguide flange40is disposed to be distanced at the gap distance WGfrom the edge face of the 2D-PC slab12.

In the case of there is no gap between the THz-wave connector1and the waveguide flange40, as shown inFIG. 17, the waveguide flange40contacts with the edge face of 2D-PC slab12. Accordingly, reduction in the transmissivity in the specific frequencies is observed due to excitation of a surface mode of the THz input wave hνi.

On the other hand, in the case where there is a gap between the THz-wave connector1and the waveguide flange40, as shown inFIG. 18, the waveguide flange40is disposed to be distanced at the gap distance WGfrom the edge face of the 2D-PC slab12. Accordingly, the surface mode of the THz input wave hνican be controlled. In particular, it is preferable to be set as the gap distance WG>the wavelength/3.

In the configuration shown inFIGS. 17 and 18, the structure of the THz-wave connector1has the similar configuration as that ofFIG. 2, and the length L1of the adiabatic mode converter10is approximately 3 mm.

In the THz-wave connector1according to the first embodiment, the adiabatic mode converter10is introduced into the edge face of the 2D-PC slab12; the crystal edge face structure is devised; the waveguide flange40is disposed to be distanced at the gap distance WGfrom the edge face of the 2D-PC slab12; and the excessive surface wave is controlled in the crystal edge face close to the waveguide flange40; thereby achieving the extremely low-loss connection with the waveguide26.

MODIFIED EXAMPLE 10

The gap structure shown inFIG. 18may be formed only in a peripheral part of the adiabatic mode converter10.

A schematic plane configuration of a THz-wave connector1according to a modified example 10 of the embodiment is illustrated as shown inFIG. 19.

In the THz-wave connector1according to the modified example 10 of the first embodiment, in order to form a gap area12B which is a bottom surface of a recess structure (recess portion)12C, the recess structure12C having the depth (gap distance) W1and the length W2in the edge face direction is formed in the edge face of the 2D-PC slab12at the base portion of the adiabatic mode converter (protruding portion)10. More specifically, as shown inFIG. 19, in the peripheral part of the base of the adiabatic mode converter10, the gap distance W1may be formed between the edge face of the 2D-PC slab12where the adiabatic mode converter10is disposed and the waveguide flange (40) disposed in the edge face of the 2D-PC slab12, and thereby the edge face of the 2D-PC slab12may be distanced from the waveguide flange.

In the THz-wave connector1according to the modified example 10 of the first embodiment, the adiabatic mode converter10is introduced into the edge face of the 2D-PC slab12, the crystal edge face structure is devised, the waveguide flange40is disposed to be distanced at the gap distance W1from the edge face of the 2D-PC slab12. Thus, the excessive surface wave is controlled in the crystal edge face close to the waveguide flange40, thereby achieving the extremely low-loss connection with the waveguide26. In particular, it is preferable to be set as the gap distance W1>the wavelength/3.

FIG. 20shows a theoretical analysis results of the frequency characteristics of a transmissivity of the THz-wave connector1according to the first embodiment (in the case of where there is no suitable gap between the THz-wave connector and the waveguide flange). InFIG. 20, the band fw indicates a band on the basis of the PBG of the 2D-PC waveguide14. In this case, the taper length L1=4.5 mm.

The THz-wave connector1according to the embodiment can obtain low-loss of equal to or less than 3 dB through the whole of the waveguide band of the 2D-PC waveguide14. In particular, if the Fabry-Perot resonance in the 2D-PC waveguide14can be controlled, the low-loss of equal to or less than 0.3 dB can be obtained.

FIG. 21shows an experimental result of the frequency characteristics of transmissivity T in the 2D-PC slab12to which the THz-wave connector1according to the first embodiment is applied. InFIG. 21, the curved line A corresponds to a configuration of the 2D-PC waveguide+THz-wave connector, and the curved line B correspond to a configuration of 2D-PC waveguide-less+THz-wave connector.

As shown inFIG. 21, since the propagation in the PBG band (0.30 THz to 0.39 THz) is prohibited in the case where there is no 2D-PC waveguide14(curved line B), the transmissivity T (dB) in the PBG band is approximately from −40 dB to −60 dB which is extremely low. On the other hand, in the case where there is the 2D-PC waveguide14(curved line A), the waveguide mode equal to or greater than 0.31 THz which becomes the propagation region appears, extremely low-loss characteristics of equal to or less than approximately 1 dB are obtained in particular in a range from 0.311 THz to 0.325 THz shown with fw2inFIG. 21.

Furthermore,FIG. 22shows an experimental result of the frequency characteristics of the transmissivity of the 2D-PC slab to which the THz-wave connector according to the first embodiment is applied (in the case where there is a suitable gap between the THz-wave connector and the waveguide flange). InFIG. 22, the band fw1is equivalent to a band ranging from 0.314 THz to 0.337 THz. As clearly fromFIG. 22, the extremely low-loss transmission characteristics of equal to or less than 0.1 dB are obtained within the range of approximately 23 GHz equivalent to the band fw1.

(Relationship Between Lattice Constant and Operable Frequencies)

The 2D-PC waveguide is formed by introducing the line defect into the periodic structure of a dielectric plate structure having two-dimensional periodic structure. It is possible to confine electromagnetic waves in the dielectrics due to the PBG effect that an electromagnetic mode in the in-plane direction cannot exist and the total reflection effect in the vertical up-and-down direction to the 2D-PC slab planar. Accordingly, the propagation loss of the 2D-PC waveguide is small.

FIG. 23shows an electromagnetic field simulation result of a relationship between the lattice constant a in the lattice points12A which are periodically arranged in the 2D-PC slab12to which the THz-wave connector according to the first embodiment can be applied, and the waveguide band frequency f of the 2D-PC waveguide14.

As shown inFIG. 23, the operational frequency band can be varied to higher frequency by making the lattice constant small. For example, the operation are possible ranging from approximately 0.9 to approximately 1.1 THz in the lattice constant a=80 μm, ranging from approximately 0.31 THz to approximately 0.38 THz in the lattice constant a=240 μm (experiment structure), and ranging from approximately 0.10 THz to approximately 0.12 THz in the lattice constant a=750 μm.

According to the electromagnetic field simulation result of the relationship between the lattice constant a of the lattice points12A and the PGB frequency which are periodically arranged in the 2D-PC slab12to which the THz-wave connector according to the first embodiment can be applied, the PGB frequency band can be varied to higher frequency by making the lattice constant small. For example, the PGB frequency band appears ranging from approximately 0.9 THz to approximately 1.1 THz in the lattice constant a=80 μm, ranging from approximately 0.30 THz to approximately 0.38 THz in the lattice constant a=240 μm (experiment structure), and ranging from approximately 0.10 THz to approximately 0.13 THz in the lattice constant a=720 μm.

(Relationship Between Propagation Loss and Resistivity of Silicon)

FIG. 24shows an electromagnetic field simulation result of a relationship between the propagation loss and the Si resistivity in the case of using Si as a material of the 2D-PC slab12which the THz-wave connector1according to the first embodiment can be applied.FIG. 24shows a result of calculating the propagation loss (dB/cm) with respect to the Si resistivity (ωcm) by electromagnetic field simulation, in consideration of the absorption loss of Si composing the 2D-PC with the Drude Model (Drude model). In this case, a circular triangular lattice of 144 μm in diameter was arranged with the lattice constant a=240 μm, and the 2D-PC slab having the PBG band ranging from 0.30 THz to 0.39 THz was used for Si of 200 μm in thickness.

FIG. 24proves that the propagation loss becomes not more than 0.2 (dB/cm), in the Si resistivity equal to or greater than 3000 ωcm. The aforementioned value is a small value as compared with a metallic-transmission line and a waveguide to which a metallic absorption loss of 0.3 THz in frequencies affects. More specifically, it proves that the 2D-PC waveguide can sufficiently be applied as the transfer line for the THz-wave IC. In particular, the 2D-PC waveguide14using high resistivity Si is extremely low-loss.

The THz-wave connector1according to the first embodiment is applicable to a THz-wave IC.

FIG. 25shows a schematic bird's-eye view configuration of a THz-wave IC2providing at least one side of input and output interfaces of the 2D-PC slab12with the THz-wave connector1according to the first embodiment. Moreover,FIG. 26shows a configuration of a multi/demultiplexer on the THz-wave IC2to which the THz-wave connector1according to the embodiment is applied. As shown inFIGS. 25-26, the THz-wave IC2includes a first slab area A1, a second slab area A2, and a third slab area A3. The first slab area A1includes multi/demultiplexer formation regions P, Q, R, S, U, V enclosed with dashed lines as shown inFIG. 26, and includes a plurality of non-periodically arranged lattice points12A. The second slab area A2and the third slab area A3include pluralities of periodically arranged lattice points12A.

As shown inFIG. 25, the THz-wave IC2to which the THz-wave connector1according to the first embodiment is applied includes: a 2D-PC slab12; lattice points12A periodically arranged in the 2D-PC slab12, the lattice points12A for diffracting the THz waves in PBG frequencies of photonic band structure of the 2D-PC slab12in order to prohibit existence in a plane of the 2D-PC slab.

A 2D-PC waveguide14disposed in the 2D-PC slab12and formed with a line defect of the lattice points; and an adiabatic mode converter (protruding portion)10disposed at an edge face of the 2D-PC slab12to which the 2D-PC waveguide14extended, the 2D-PC waveguide14extended to the adiabatic mode converter10.

The THz-wave IC2to which the THz-wave connector1according to the first embodiment is applied may include a plurality of transceivers181,182,183,184,185,186, an antenna16, and a PC multi/demultiplexer20, as shown inFIG. 25. In this case, a plurality of the transceivers181,182,183,184,185,186can transmit and receive THz waves having a plurality of different frequencies f1, f2, f3, f4, f5, f6. InFIG. 25, each arrow displayed corresponding to the frequencies f1, f2, f3, f4, f5, f6denotes transmission or reception directions.

As shown inFIG. 26, the multi/demultiplexers which can input/output specific frequencies can be formed by disturbing the periodic structure of the lattice points12A arranged periodically in the 2D-PC slab12. Such multi/demultiplexers are formed in multi/demultiplexer formation regions P, Q, R, S, U, V enclosed with dashed lines as shown inFIGS. 25 and 26. The frequency band of the multi/demultiplexer is adjustable with a method (number of pieces) of filling holes of the lattice points12A, the hole size of the filled surroundings, the shift of the positions of holes, change of the size of surrounding period of holes, etc. For example, if the hole is made smaller, the number of pieces is increased, or the period is made longer, the frequency band applicable will be shifted to a low frequency side. On the other hand, if the hole is made larger, the number of pieces is decreased, or the period is made shorter, the frequency band applicable will be shifted to a high frequency side. More specifically, if the hole is made larger, it will be shifted to the higher-frequency side since the refractive index sensed in the THz waves becomes smaller, but, conversely, if the hole is made smaller, it will be shifted to the lower-frequency side since the refractive index sensed in the THz waves becomes larger.

For example, in the multi/demultiplexer formation region P enclosed with the dashed line, the surrounding hole size is set up larger. In the multi/demultiplexer formation region Q, the size of the surrounding hole12C is set up smaller. A small hole12S is introduced in the multi/demultiplexer formation region V. In the multi/demultiplexer formation region S, as shown with the arrow, two holes are shifted to inside. In the multi/demultiplexer formation region R, as shown with the arrow, two holes are shifted to outside. In the multi/demultiplexer formation region U, a central hole is filled, and thereby the number of pieces is decreased. The above-mentioned configuration of the multi/demultiplexer formation regions is merely one example.

In the adiabatic mode converter (tapered structure)10of the THz-wave connector according to the first embodiment, the refractive index becomes lower adiabatically from the semiconductor having higher refractive index (e.g. approximately 3) to the medium having lower refractive index (e.g. approximately 1). Accordingly, it is possible to significantly reduce an influence of the edge face reflection. Such an adiabatic mode converter10is nonreflective structure which can be integrated/formed collectively in the PC waveguide. Accordingly, the THz-wave connector according to the first embodiment acts a role important in addition to the connection with the waveguide. More specifically, it is not only limited to the connector but also can be applied also as a waveguide of nonreflective structure, or a radiator of nonreflective structure. Moreover, handling frequency bands are not limited to the THz wave band, but a general light waves are also included. In this case, as the PC, the lattice constant a of the lattice points12A is miniaturized, and thereby the operating wavelength may be set as ranging from approximately 1 μm to 2 μm bands, and the lattice constant is set as ranging from approximately 250 nm to approximately 500 nm, etc., for example. Moreover, the diameter and the depth of the lattice points12A are respectively approximately 200 nm and approximately 300 nm, for example. The numerical examples can be appropriately changed according to materials, a wavelength, etc. to compose the 2D-PC slab12. For example, in the 2D-PC slab12to which GaAs/GaAlAs based materials are applied, the wavelength is approximately 200 nm to approximately 400 nm.

The structure of the adiabatic mode converter10in the waveguide3according to the second embodiment is the same as that of the adiabatic mode converters10,10A,10B,10C in the THz-wave connector1according to the first embodiment.

The waveguide3according to the second embodiment includes: a 2D-PC slab12; lattice points12A periodically arranged in the 2D-PC slab12, the lattice points12A for diffracting the light waves or the THz waves in PBG frequencies of photonic band structure of the 2D-PC slab12in order to prohibit existence in a plane of the 2D-PC slab12; a 2D-PC waveguide14A disposed in the 2D-PC slab12and formed with a line defect of the lattice points12A; and an adiabatic mode converter10disposed at an edge face of the 2D-PC slab12to which the 2D-PC waveguide14A extended, the 2D-PC waveguide14extended to the adiabatic mode converter10.

In this case, the waveguiding structure of nonreflective structure which can be integrated/formed collectively in the PC waveguide14A is formed of the 2D-PC waveguide14A disposed in the 2D-PC slab12and formed of the line defect of the lattice points12A, and the adiabatic mode converter10disposed at an edge face of the 2D-PC slab12to which the 2D-PC waveguide14A extended, the 2D-PC waveguide14extended to the adiabatic mode converter10.

Moreover, in the waveguide3according to the second embodiment, in the same manner asFIG. 2, the adiabatic mode converter10, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12. Moreover, the side surface of the tapered shape may have an inclined plane in the same manner asFIG. 2.

Moreover, the waveguide3according to the second embodiment may have protective structure for covering the adiabatic mode converter10with a resin layer38etc., in the same manner asFIG. 3.

Moreover, in the waveguide3according to the embodiment, the adiabatic mode converter10A, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12, and the side surface of the tapered shape may have a curved surface, in the same manner asFIG. 5. In this case, the curved surface may have a hyperboloid surface or an exponential surface.

Moreover, in the waveguide3according to the second embodiment, the adiabatic mode converter10B, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12, and the side surface of the tapered shape may have a plurality of stepped surfaces, in the same manner asFIG. 6.

Moreover, in the waveguide3according to the second embodiment, the adiabatic mode converter10A, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12, and the side surface of the tapered shape may have a curved surface, in the same manner asFIG. 7A.

Moreover, in the waveguide3according to the second embodiment, the adiabatic mode converter10C may have a conical shape so that the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12, in the same manner asFIG. 7B. In this case, the modified example of conical shape may include not only a trumpet-like shape, but also a simple conical shape in the same manner asFIG. 7B.

Moreover, in the waveguide3according to the second embodiment, the adiabatic mode converter10C may have a quadrangular pyramid shape so that the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12, in the same manner asFIG. 8A.

Moreover, in the waveguide3according to the second embodiment, the adiabatic mode converter10C may have a wedge-like shape so that the thickness of the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12, in the same manner asFIG. 8B.

Moreover, in the waveguide3according to the second embodiment, the adiabatic mode converter10C may have a plurality of stairs-like shapes so that the thickness of the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12, in the same manner asFIG. 9A.

Moreover, in the waveguide3according to the second embodiment, the adiabatic mode converter10C may have a plectrum-like shape so that the thickness of the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12, in the same manner asFIG. 9B. In this case, the adiabatic mode converter10, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12, and the side surface of the tapered shape has a curved surface.

Moreover, in the waveguide3according to the second embodiment, the adiabatic mode converter10C may have a wedge-like shape so that the width of the tip part becomes thinner as being distanced from the edge face of 2D-PC slab12, in the same manner asFIG. 9C. In this case, the adiabatic mode converter10, in a planar view of the 2D-PC slab12, may have a tapered shape so that a tip part becomes thinner as being distanced from the edge face of the 2D-PC slab12, and the side surface of the tapered shape has a inclined plane.

Moreover, in the waveguide3according to the second embodiment, the recess structure having the length W2in the edge face direction and the depth (gap distance) W1may be formed in the edge face of the 2D-PC slab12in the base portion, in the adiabatic mode converter10as same as that ofFIG. 19. More specifically, in the edge face of the 2D-PC slab12where the adiabatic mode converter10is disposed, an excessive surface wave in the crystal edge face can be controlled in a peripheral part of the base in the adiabatic mode converter10. In particular, it is preferable to be set as the gap distance W1>the wavelength/3.

Moreover, in the waveguide3according to the second embodiment, the structure of the adiabatic mode converter10is not limited to the above-mentioned structures, but can also use a structure with which any one or more of such structures are combined with each other. For example, a plurality of step shapes may be introduced into the side surface of the quadrangular pyramid shape. Alternatively, the plurality of the step shape may be introduced into the side surface of the conical shape or conical trumpet-like shape.

Moreover, in the waveguide3according to the second embodiment, the lattice point for forming resonant-state may be arranged in any one selected from the group consisting of a square lattice, a rectangular lattice, a face-centered rectangle lattice, and a triangular lattice.

Moreover, the lattice points12A may be provided with any one of the polygonal shape, circular shape, oval shape, or ellipse shape.

Moreover, in the waveguide3according to the second embodiment, the 2D-PC slab12may be formed of a semiconducting material. More specifically, anyone of Si, GaAs, InP, GaN, etc. are applicable to the semiconducting material, and any one of GaInAsP/InP based, GaInAs/GaAs based, GaAlAs/GaAs based or GaInNAs/GaAs based, GaAlInAs/InP based, GaAlInP/GaAs based, GaInN/GaN based materials, etc. are applicable to the semiconducting material. Moreover, the 2D-PC slab12may be formed with silicon having the resistivity equal to or greater than 3000 ωcm.

Since the waveguide3according to the second embodiment has a nonreflective effect in general optical devices using the waveguide3, the waveguide3according to the second embodiment is applicable also to antenna structure140, for example.

FIG. 27shows a schematic bird's-eye view configuration of antenna structure140to which the waveguide3according to the second embodiment is applied.

As shown inFIG. 27, the antenna structure140to which the waveguide3according to the second embodiment includes: a 2D-PC slab12; lattice points12A periodically arranged in the 2D-PC slab12, the lattice points12A for diffracting the light waves or the THz waves in PBG frequencies of photonic band structure of the 2D-PC slab12in order to prohibit existence in a plane of the 2D-PC slab; a 2D-PC waveguide14A disposed in the 2D-PC slab12and formed with a line defect of the lattice points; and an adiabatic mode converter10disposed at an edge face of the 2D-PC slab12to which the 2D-PC waveguide14A extended, the 2D-PC waveguide14A extended to the adiabatic mode converter10.

As shown inFIG. 27, the antenna structure140to which the waveguide3according to the second embodiment is applied includes: an input/output interface60; a PC multi/demultiplexer20; a transmitter18T; a receiver18R; a 2D-PC waveguide14A; and a waveguide3having nonreflective structure used as termination structure of the 2D-PC waveguide14A. The input/output interface60is a coupler from free space, and is composed of a grating coupler consisting of a one-dimensional PC.

(Simulation Result of Reflectance)

FIG. 28Ashows a simulation result showing a relationship between the reflectance R and the frequency f in the PC waveguide14A to which the waveguide3according to the second embodiment is applied.FIG. 28Bshows a schematic bird's-eye view configuration of the waveguide3according to the second embodiment, and the PC waveguide14A to which such a waveguide3is applied. In this case, the waveguide3shown inFIG. 28Bincludes a recess structure (a recessed portion12C having a bottom surface12B) in the edge face of the 2D-PC slab12of the base portion in the adiabatic mode converter10, in the same manner asFIG. 19.

Moreover,FIG. 29Ashows a simulation result showing a relationship between a reflectance R and a frequency f in the PC waveguide in the case of not applying the waveguide of nonreflective structure, as a comparative example.FIG. 29Bshows a schematic bird's-eye view configuration of the PC waveguide14in the case of not applying the waveguide having the nonreflective structure, as the comparative example.

The PC waveguide region is expressed with ΔF (PC) inFIGS. 28A and 29A.

In the PC waveguide14A to which the waveguide3according to the second embodiment is applied, as shown inFIG. 28A, the average reflectance R in ΔF (PC) (0.313 THz to 0.395 THz) is approximately 1.1%. On the other hand, the average reflectance R in ΔF (PC) of the PC waveguide14according to the comparative example is higher as approximately 26%. In addition, the result shown inFIG. 29Ais a result in the one-sided edge face of the PC waveguide14. Accordingly, the actual value becomes larger than the value shown inFIG. 29A.

In the PC waveguide14A to which the waveguide3according to the second embodiment is applied, the average reflectance R in ΔF (PC) is reduced to approximately 1/23 as compared with the case where the waveguide having the nonreflective structure is not applied (no tapered structure). In the PC waveguide14A to which the waveguide3according to the embodiment is applied, the configuration shown inFIG. 28Bmay include an antireflection film formed of a dielectric multilayer etc., in the edge face of the opposite side which does not form the adiabatic mode converter10.

In the adiabatic mode converter10having the tapered structure of the waveguide3according to the second embodiment, the refractive index becomes lower adiabatically in the guiding direction from the semiconductor having higher refractive index (e.g. approximately 3) to the medium having lower refractive index (e.g. approximately 1). Therefore, the waveguide3acts also as a radiator (a kind of radiation antenna) for radiating the light waves or THz waves to free space from the waveguide confined in the PC. Moreover, the waveguide3can operate also as an input mechanism for inputting the light waves or THz waves into the waveguide from the free space in the same manner as a general antennas.

(Result of Transmission Experiment of Waveguide Having Taper)

FIG. 30shows an experimental result showing a relationship between transmission intensity (a. u.) and a frequency f in the PC waveguide14A to which the waveguide3according to the second embodiment is applied (an example of a transmission spectrum).

In the PC waveguide14A to which the waveguide3according to the second embodiment is applied, since the tip part of the PC waveguide14A includes the adiabatic mode converter10having the tapered structure, disorder of the spectrum under the effect of interference of the edge face is significantly reduced, as shown inFIG. 30.

(Simulation Result of Electromagnetic Field Radiation Pattern)

FIG. 31shows a simulation result of three-dimensional electromagnetic field radiation pattern in the PC waveguide14A to which the waveguide3according to the second embodiment is applied.FIG. 32shows a simulation result of a cross-sectional radiation pattern in which directivity is indicating in a taper tip direction DT

In the waveguide3according to the second embodiment, the light waves or the THz waves are radiated directionally from the PC waveguide14A in the taper tip direction. The antenna gain in this case is approximately 10.44 (dBi), for example. In the present embodiment, the dBi is a value which indicates the directive intensity with respect to homogeneous radiation with the dB unit. That is, the increase value of the dB intensity compared with the homogeneous radiation is indicated as a unit. InFIG. 31, the directivity in a range in which the electric power is increased by 3 dB compared with the homogeneous radiation is approximately 40 degrees at one side.

The waveguide3according to the second embodiment acts as an antenna in a wide-band operation without the frequency dependence of the radiation direction, reflecting the lowness of the reflectance R.

(Structure Example of Arrayed Taper)

In a structure example in which the adiabatic mode converter (tapered part) of the waveguide according to the second embodiment is arrayed,FIG. 33Ashows a two array antenna,FIG. 33Bshows an example of three array antenna,FIG. 33Cshows an example of four array antenna, andFIG. 33Dshows another example of the four array antenna.

More specifically, as shown inFIG. 33A, the two array antenna includes two tapered parts10A1,10A2. As shown inFIG. 33B, three array antenna includes three tapered parts10A1,10A2,10A3. As shown inFIG. 33C, the four array antenna includes four tapered parts10A1,10A2,10A3,10A4. As shown inFIG. 33D, another example of the four array antenna also includes four tapered parts10A1,10A2,10A3,10A4.

Furthermore, in a structure example arraying tapered parts in the waveguide3according to the second embodiment,FIG. 34Ashows an example of eight array antenna, andFIG. 34Bshows an example of 24 array antenna. As shown inFIG. 34A, the eight array antenna includes eight tapered parts10A1,10A2,10A3,10A4, . . . ,10A8. Furthermore, as shown inFIG. 34B, the 24 array antenna includes 34 tapered parts10A1,10A2,10A3,10A4, . . . ,10A24.

FIG. 35shows a simulation result of a relationship between the antenna gain (dBi) and the number N of the arrays in structure arraying the adiabatic mode converters10, in the waveguide3according to the second embodiment. The adiabatic mode converters10are arrayed, thereby increasing the aperture area, and improving the radiation directivity, i.e., antenna gain. The maximum intensity is increased in proportion to the number N of the arrays. In this case, the distance D between the arrays may be set as 0<D<λ. Still more preferable, the distance D may be set as λ/8<D<(3/8)λ, and the optimum value is a range of D to λ/4. In this Case, the λ is a wavelength of the light waves or the THz waves radiated or received in the waveguide3according to the embodiment.

InFIGS. 33 and 34, a relationship between the sizes d1, d2, d3, d4of each part of the taper preferable satisfies d1>λ and d4=d2+d3>λ.

In the waveguide3according to the second embodiment, the near field array antenna can be composed by arraying the adiabatic mode converters10(tapered parts).

As explained above, according to the present invention, there can be provided the THz-wave connector which can reduce the connection loss in the interface between the 2D-PC slab and the waveguide, and the THz-wave IC to which such a THz-wave connector is applied.

Moreover, according to the present invention, there can be provided the waveguide having nonreflective structure for controlling the influence of light interference (Fabry-Perot resonance) and multiple reflections in the waveguide end, and the antenna structure to which such a waveguide is applied.

The present invention has been described by the embodiments, as a disclosure including associated description and drawings to be construed as illustrative, not restrictive. This disclosure makes clear a variety of alternative embodiments, working examples, and operational techniques for those skilled in the art.

Such being the case, the present invention covers a variety of embodiments, whether described or not. Therefore, the technical scope of the present invention is determined from the invention specifying items related to the claims reasonable from the above description.