JP4063740B2 - Two-dimensional photonic crystal having air bridge structure and manufacturing method thereof - Google Patents

Description

  The present invention relates to a two-dimensional photonic crystal used for a multiplexer / demultiplexer or the like in a field such as wavelength division multiplex communication and a method for manufacturing the same.

  In recent years, photonic crystals, which are optical functional materials having a periodic refractive index distribution, have attracted attention. The photonic crystal has a feature that a band structure is formed with respect to the energy of light and electromagnetic waves due to its periodic refractive index distribution, and an energy region (photonic band gap) where propagation of light and electromagnetic waves is impossible is formed. Have. Note that “light” used in this specification includes electromagnetic waves.

  By introducing an appropriate defect in the photonic crystal, an energy level (defect level) is formed in the photonic band gap. Accordingly, only light having a wavelength corresponding to the energy of the defect level in the wavelength (frequency) range corresponding to the energy in the photonic band gap can exist at the position of the defect. A waveguide is formed by providing this defect in a line shape, and an optical resonator is formed by providing the defect in a dot shape. The wavelength (resonance wavelength) of light that resonates in this point defect depends on its shape and refractive index.

  Various optical devices are being studied using this resonator and waveguide. For example, by arranging this resonator in the vicinity of the waveguide, among the light of various wavelengths propagating in the waveguide, light having a wavelength that matches the resonance wavelength of the resonator is externally transmitted from the waveguide through the resonator. In addition to functioning as a demultiplexer for taking out light, the demultiplexer also functions as a multiplexer that introduces light having the resonance wavelength of the resonator into the waveguide from the outside via the resonator. For example, in the field of optical communication, such a multiplexer / demultiplexer is used for wavelength division multiplexing communication in which light of a plurality of wavelengths is propagated through a single fiber and a separate signal is placed on the light of each wavelength. it can.

  Photonic crystals include one-dimensional crystals, two-dimensional crystals, and three-dimensional crystals. Among these, two-dimensional photonic crystals have an advantage that they are relatively easy to manufacture. As an example, Patent Document 1 discloses a two-dimensional photonic crystal in which substances having a lower refractive index than that material are periodically arranged on a plate material (slab) having a high refractive index. A two-dimensional photonic crystal and an optical multiplexer / demultiplexer in which a waveguide having a defect in a shape and a point-like defect disturbing a periodic arrangement are provided adjacent to the waveguide are described. Here, the periodic arrangement of the low refractive index materials can be formed by periodically forming holes in the slab.

Japanese Patent Laid-Open No. 2001-272555 ([0019] to [0032], FIG. 1)

  In the two-dimensional photonic crystal of Patent Document 1, the slab is disposed in the air. Since the difference in refractive index between the slab and air is large, light is totally reflected at the boundary between the two. Therefore, in this two-dimensional photonic crystal, it is difficult to lose the energy of light in the direction perpendicular to the slab surface. In the direction parallel to the surface of the slab, the loss of light energy is prevented by the photonic band gap.

  As disclosed in Patent Document 1, generally, the slab needs to be considerably thin (in the embodiment, about 0.25 μm), and thus the strength in the thickness direction is low. In particular, in the two-dimensional photonic crystal formed by providing a large number of holes in the slab, the strength in the thickness direction further decreases. When the strength in the thickness direction is low, there arises a problem that, for example, the yield in manufacturing is deteriorated.

  As a configuration for increasing the strength of the two-dimensional photonic crystal, a two-dimensional photonic crystal (hereinafter referred to as “photonic crystal with substrate”) in which the crystal is placed on a substrate can be considered. In this case, the upper surface of the crystal is in contact with air, while the lower surface of the crystal is in contact with the substrate.

  However, when a point-like defect is provided in such a photonic crystal with a substrate, when a point-like defect is provided in a two-dimensional photonic crystal (hereinafter referred to as “substrate-free photonic crystal”) that is air both above and below the slab As a result, the characteristic of a point defect as a resonator is deteriorated. Fig. 1 shows the case of the resonator in the case where the same point-like defects are provided in the substrate-free photonic crystal ((a)) and the photonic crystal with substrate ((b)) having the same shape except for the presence or absence of the substrate. The experimental value of a resonance wavelength spectrum is shown. The half width of the spectrum is wider in (b) than in (a). Therefore, the wavelength resolution of the resonator is lower in the photonic crystal with substrate than in the case of the substrateless photonic crystal. The Q value, which is a value indicating the performance of the resonator, is as small as Q = 280 in (b) compared to Q = 2350 in (a), and the photonic crystal with a substrate loses more from the resonator. The energy of light is great.

  The problem to be solved by the present invention is to provide a two-dimensional photonic crystal having a mechanical strength higher than that of a conventional two-dimensional photonic crystal and having a high resonator performance, and to provide such a two-dimensional photonic crystal. It is to provide a method for suitably producing a nick crystal.

The inventor of the present application considered that there are the following two reasons that the characteristics of a point-like defect as a resonator in a photonic crystal with a substrate are lower than those of a substrateless photonic crystal.
First, since the refractive index of the substrate is higher than that of air in the photonic crystal with a substrate, the effect of confining light in the direction of the lower surface of the crystal is smaller than that of the substrate-free photonic crystal in which the upper and lower sides of the slab are air. It is. Therefore, light leaks from the lower part of the resonator, and the performance as the resonator is lowered.

The second point is that in the photonic crystal with a substrate, the parameters related to the characteristics of the crystal are vertically asymmetrical, which causes an influence due to the symmetry of light existing in the resonator.
In the case of a substrate-free photonic crystal, the light existing in the resonator is a TE wave in which only the electric field vibrates in the direction in the crystal plane. The photonic crystal forms a photonic band gap with respect to the TE wave. Therefore, in a substrate-free photonic crystal, leakage of TE waves in the resonator into the crystal plane is prevented by the photonic band gap. On the other hand, in the photonic crystal with a substrate, due to the asymmetry, the TM wave and the TE wave in which only the magnetic field vibrates in the direction in the crystal plane are combined and exist in the resonator. Since there is no photonic band gap for TM waves, in a photonic crystal with a substrate, TM waves in the resonator easily leak in the in-plane direction, and therefore the performance as a resonator is low.

The inventor of the present application examines a configuration capable of ensuring high mechanical strength without causing deterioration in characteristics caused by these, and invents a two-dimensional photonic crystal having an air bridge structure as described below. It came to. That is, the two-dimensional photonic crystal having an air bridge structure according to the present invention is
a) a slab body;
b) a substrate provided on the lower surface of the main body;
c) a plurality of regions arranged in a lattice pattern with a predetermined period on the main body, different in refractive index from the main body,
d) point-like defects formed by providing the defects of the different refractive index regions in the form of dots,
e) an air bridge space provided in the substrate facing only a part of the range in which the different refractive index region of the main body is provided;
The point-like defect is provided within the range of the part .

Moreover, the method which can manufacture suitably the two-dimensional photonic crystal which has an air bridge structure was invented. That is, the method for producing a two-dimensional photonic crystal having an air bridge structure according to the present invention is a method for producing a two-dimensional photonic crystal from a plate material in which a main body layer and a substrate layer are laminated.
a pore forming step of forming an etchant introduction holes penetrating the a) said body layer,
By introducing an etchant through b) etchant introduction holes, and the air-bridge space formation step of forming a space on the substrate layer by etching the substrate layer around the etchant introduction holes,
c) Formation of a two-dimensional photonic crystal that periodically forms vacancies formed by removing the main body layer by a predetermined size within a predetermined range of the main body layer and forms dot-like defects in the vacancies Process,
I have a,
The space is formed in the substrate layer so as to face only a part of the range in which the holes of the main body layer are formed,
The point defect is provided within the range of the portion;
It is characterized by.

Embodiments and effects of the invention

(1) Two-dimensional photonic crystal having an air bridge structure In the two-dimensional photonic crystal of the present invention, the main body is a slab that is a plate-like body that is sufficiently thinner than the size in the in-plane direction. A substrate is provided on the lower surface of the main body. This substrate is for increasing the strength of the two-dimensional photonic crystal. Here, for convenience, the surface on which the substrate is provided is described as the “lower surface” of the main body. However, the two-dimensional photonic crystal of the present invention is not limited to being used with the substrate on the lower side, and is arranged in any orientation. Can be used.

  In this main body, a plurality of regions (different refractive index regions) having a refractive index different from that of the main body are arranged in a lattice pattern with a predetermined period. Thus, a two-dimensional photonic crystal is formed in which a photonic band gap that does not allow light in a predetermined wavelength band determined by the period to pass in the in-plane direction of the main body is formed. This two-dimensional photonic crystal has the above-described structure, so that total reflection occurs between the main body and the outside of the main body (air) due to the difference in refractive index between the two. Will not leak. Here, there are a triangular lattice, a square lattice, and the like as a lattice for disposing the different refractive index regions. Further, the different refractive index region may be either one having a lower refractive index than that of the main body or one having a higher refractive index than that of the main body, but is desirably formed by periodically making holes in the main body. By making the different refractive index region a hole, the difference in refractive index from the main body can be increased, the formation of the different refractive index region can be facilitated during manufacturing, and the manufacturing method described later can be easily applied. .

  By providing the defects in the different refractive index regions in a dot shape, a dot defect is formed. As this point-like defect, it is possible to use a different refractive index region having a different size or a defect from another different refractive index region. Further, the number of different refractive index regions where defects are provided may be one or a plurality of adjacent regions. When defects in a plurality of adjacent different refractive index regions are provided, the plurality of defects combine to form a single point defect. The point defect functions as an optical resonator in which light having a wavelength determined by its shape resonates. Further, a plurality of point defects may be provided. By making the shapes of the plurality of point defects different from each other, it is possible to resonate light having different wavelengths in each point defect.

  A space is provided in a predetermined range of the substrate facing the point defect. This space may be formed only directly under the point-like defect, but may be formed in a wider range, that is, a range away from the outer edge of the point-like defect by a certain distance. Outside the range where the space is provided, the main body is supported by the substrate. Further, when a plurality of point-like defects are provided, this space may be provided for each point-like defect, or one space facing a plurality of point-like defects may be provided. Since the two-dimensional photonic crystal of the present invention has a bridge-like structure in which a main body bridge is built on a space (air), the structure of the two-dimensional photonic crystal is referred to as an “air bridge structure” below. And the formed space is called “air bridge space”.

  Since the two-dimensional photonic crystal has the air bridge structure as described above, the performance of the optical resonator due to the point-like defect is higher than that of the photonic crystal with a substrate and equal to that of the substrate-free photonic crystal. Can do. The reason is as follows. First, since air exists above and below the main body at the point-like defect position, the difference in refractive index from the outside can be increased even on the substrate side surface of the main body, and the light confinement effect is great. In addition, since both the top and bottom of the point defect are air, the parameters related to the characteristics of the point defect are vertically symmetric at that position. Thereby, only the TE wave is present in the resonator, and the light existing in the resonator can be prevented from leaking in the in-plane direction of the crystal due to the photonic band gap.

  Moreover, since the two-dimensional photonic crystal of the present invention is supported by the substrate outside the range where the air bridge space is provided, the mechanical strength can be ensured. In particular, when the different refractive index region is a vacancy, the mechanical strength of the crystal is lowered by providing the vacancies, so that the configuration of the present invention is beneficial.

  Next, the size of the air bridge space will be examined. Even if the air bridge space is formed only directly under the point defect, the performance of the optical resonator is improved as compared with the case of the photonic crystal with a substrate. An air bridge space is formed from the outer edge of the defect to a region separated by one period or more of the different refractive index region. Thus, by forming the air bridge space over a wider range, it is possible to obtain a high optical resonator performance equivalent to that of the substrate-free photonic crystal. On the other hand, in order to secure the strength of the crystal, it is desirable that the formation range of the air bridge space be a certain size or less. This range varies somewhat depending on the thickness of the main body and the size and material of the different refractive index region, but usually, at least one direction in the surface of the main body has 25 cycles of the different refractive index region outward from the outer edge of the point defect. It is desirable to set the range within minutes. If this range is set to 25 cycles or less even in one direction, it is supported in that direction, so that the strength of the crystal can be secured even if the other direction is longer than that. Further, if the depth of the air bridge space is small, the performance of the optical resonator is improved as compared with the case of the photonic crystal with a substrate, but more desirably, the influence of light leakage to the substrate and vertical asymmetry is exerted. In order to sufficiently eliminate the above, it is desirable to set the period of the different refractive index region to twice or more.

  The two-dimensional photonic crystal of the present invention can be used as a multiplexer / demultiplexer by further providing a waveguide near the point-like defect. This waveguide is formed by providing defects in the different refractive index region in a linear shape. In particular, it is desirable that the different refractive index region be formed in a linear manner, that is, by not providing the different refractive index region. As a result, demultiplexing of the light having the resonance wavelength from the superimposed light having a plurality of wavelengths flowing through the waveguide and extracting the light to the outside of the crystal, or multiplexing the light having the resonance wavelength from the outside of the crystal to the superimposed light of the waveguide. It becomes a vessel.

  When a part of such a waveguide faces the air bridge space, if the waveguide is formed with the same width as the air bridge waveguide part and the other part, the transmission wavelength band of the air bridge waveguide part is reduced. It shifts to a shorter wavelength side than the transmission wavelength band of the other part (“substrate waveguide part”). Since the transmission wavelength band of the entire waveguide is a common part of the transmission wavelength band of the two parts, it is narrower than when there is no air bridge waveguide part. Therefore, it is desirable to make the width of the waveguide in the air bridge waveguide portion larger by a predetermined size than the waveguide width of the substrate waveguide portion. Thereby, since the transmission wavelength band of the air bridge waveguide part is shifted to the long wavelength side, such a narrowing of the transmission wavelength band can be prevented.

  However, due to manufacturing limitations of the two-dimensional photonic crystal, the portion where the waveguide faces the air bridge space does not match the portion where the waveguide is widened, and the waveguide has a narrow waveguide width. There is a possibility that a region having a wide waveguide width can be formed in the substrate waveguide portion. If a region having a narrow waveguide width is formed in the air bridge waveguide portion, the waveguide transmission band in that region remains on the short wavelength side, so that the transmission wavelength band of the entire waveguide cannot be widened. On the other hand, when a region having a wide waveguide width is formed in the substrate waveguide portion, the waveguide transmission band in that region is shifted to the long wavelength side, and the common transmission wavelength band on the short wavelength side is reduced. However, on the short wavelength side, if the light is a mixture of TE waves and TM waves, TM waves leak in the waveguide, but can be guided even outside the common transmission wavelength band. If the wide waveguide area in the substrate waveguide section is at most several periods of the different refractive index area, TM wave leakage can be suppressed to a practically acceptable level and the wide common transmission wavelength band can be used effectively. be able to. Therefore, by design, when the width of the waveguide is set to be a predetermined size wider than the width of the waveguide of the other part up to a position away from the air bridge waveguide portion by a predetermined distance, Even if the position for changing the waveguide width is shifted, it is possible to prevent the common transmission wavelength band of the waveguide from becoming narrow.

(2) Method for Producing Two-dimensional Photonic Crystal Having Air Bridge Structure Next, a method for producing a two-dimensional photonic crystal having an air bridge structure will be described. This method can be suitably used particularly when manufacturing a two-dimensional photonic crystal having a hole of a different refractive index region.

In this method, a plate material formed by laminating a main body layer and a substrate layer is used as a material. A two-dimensional photonic crystal is formed in the main body layer, and an air bridge space is formed in the substrate layer. As such a plate material, for example, a substrate in which the main body layer is made of Si and at least a portion of the substrate layer in which the air bridge space is provided can be made of SiO ₂ can be used. Specifically, a commercially available SOI (Silicon on insulator) substrate in which a SiO ₂ thin film is formed on a thick Si film and a Si thin film is further formed on the SiO ₂ thin film can be used. In the case of an SOI substrate, the Si thin film serves as the main body layer, and the SiO ₂ thin film and the Si thick film serve as the substrate layer. Of the substrate layers, the air bridge space is formed in the SiO ₂ thin film.

  First, holes that penetrate the main body layer are formed in the plate material (hole forming step). Here, these holes are referred to as “etching agent introducing holes”. The holes for introducing the etching agent become a part of the different refractive index region in the completed two-dimensional photonic crystal. However, not all the different refractive index regions are formed in this hole forming step, but only in a part of the main body layer. The holes for introducing the etching agent can be formed using, for example, photolithography or electron beam (EB) drawing and etching techniques.

Next, the etching agent is introduced into the substrate layer through the holes for introducing the etching agent. As a result, the etching agent enters the substrate layer from the etching agent introduction hole, and the substrate layer is etched. Thus, the substrate layer around the holes for introducing the etchant is removed, and an air bridge space is formed at that position (air bridge space forming step). An existing etchant or etching gas can be used as the etchant. For example, when the substrate layer is made of SiO ₂ may be used hydrogen fluoride solution.

  After the etching is completed, holes are formed periodically in the main body layer by removing the main body layer by a predetermined size. By this process, all the different refractive index regions are formed. At the same time, point defects are formed in the main body layer facing the air bridge space. The point-like defect is formed by providing a hole having a diameter different from that of other holes, or by losing the hole. By these operations, a two-dimensional photonic crystal is formed in the main body layer (two-dimensional photonic crystal formation step). The vacancies and dot-like defects can be formed using photolithography, EB drawing and etching techniques, and the like, as in the method described in Patent Document 1. As described above, a two-dimensional photonic crystal in which an air bridge space is formed below the point defect can be manufactured.

  When a waveguide is further provided in the two-dimensional photonic crystal of the present invention, such as when used in a two-dimensional photonic crystal multiplexer / demultiplexer, in the two-dimensional photonic crystal formation step, the vacancy defects are linearized. What is necessary is just to provide. If the waveguide faces the formed air bridge space, the waveguide is formed so that the width of the waveguide of the portion (air bridge waveguide portion) is wider than the other portions as described above. It is desirable to do. Further, as described above, a predetermined distance from the air bridge waveguide portion can be prevented so that the common transmission wavelength band of the waveguide can be prevented from being narrowed even if the position where the waveguide width is changed during manufacturing is shifted. It is desirable that the width of the waveguide be formed by a predetermined size larger than the width of the waveguide in other portions up to a position that is far away.

  According to the manufacturing method of the present invention, a two-dimensional photonic crystal having an air bridge structure can be manufactured easily, that is, at low cost, using an existing technique used in semiconductor manufacturing.

A configuration example of the two-dimensional photonic crystal having the air bridge structure of the present invention is shown in FIG. (a) is a perspective view thereof, (b) is a plan view, (c) is a cross-sectional view in the AA ′ direction (left side) and a cross-sectional view in the BB ′ direction (right side) shown in (b). . The device has a Si substrate layer 13 made of SiO ₂ substrate layer 12 and the Si consists body layer 11, SiO ₂ composed of Si. The SiO ₂ substrate layer 12 and the Si substrate layer 13 are combined to form a substrate that supports the main body layer 11.

The holes 15 are arranged in the main body layer 11 with a triangular lattice pattern. Thereby, since a photonic band gap is formed, light having a wavelength corresponding to the energy in the photonic band gap cannot propagate in the main body layer. Here, the period a of the triangular lattice was 0.41 μm. Further, one point-like defect 16 is formed by missing three holes 15 arranged in a straight line, that is, by not providing three holes. In the SiO ₂ substrate layer 12, a space 17 free from SiO ₂ is provided in a region below the point defect 16. The range in which this space 17 is provided will be described later.

In a region in contact with the SiO ₂ substrate layer 12 below the main body layer 11, a direction perpendicular to the main body layer 11 is caused by a difference in refractive index between Si (refractive index˜3.5) and SiO ₂ (refractive index˜1.5). It is possible to confine light to some extent. However, the confinement is insufficient as compared with the case where the main body layer is in contact with air. If SiO ₂ exists immediately below the point defect 16, light leaks from the point defect 16 to the SiO ₂ substrate layer 12. End up. Further, since vertical asymmetry occurs at the position of the point defect 16, TE wave and TM wave are coupled, and light leaks from the point defect 16 into the surface of the main body layer 11. Therefore, in this embodiment, a space 17 is provided below the point defect 16 to prevent light from leaking from the point defect 16 into the surface of the SiO ₂ substrate layer 12 and the main body layer 11.

With respect to the two-dimensional photonic crystal having the structure as shown in FIG. 2, the Q value of the point defect 16 was obtained from the calculation by the time domain difference method (FDTD method). Here, the space 17 is provided only under the point defect 16 (FIG. 3 (a)), from the point defect 16 to the outside by one grating period ((b)), and to the outside by two periods ((( The Q value was calculated for each case of c)). The depth of the space 17 was 2a. For comparison, the SiO ₂ substrate layer 12 is provided on the entire lower surface of the main body layer 11 including the area immediately below the point defect 16 when the entire upper and lower surfaces of the main body layer 11 are air (substrate-free photonic crystal). Q value was calculated for the case (photonic crystal with substrate). These calculation results are shown in FIG. In FIG. 4, the non-substrate photonic crystal is abbreviated as “no substrate”, and the photonic crystal with substrate is abbreviated as “with substrate”. In the cases (b) and (c), a Q value equivalent to the point defect provided in the substrate-free photonic crystal can be obtained. Therefore, the space 17 may be in a range in which the strength of the main body layer 11 can be ensured in a range wider than (b). As described above, it can be seen that a point-like defect resonator having high performance can be realized by the two-dimensional photonic crystal of this example.

  There are various types of point defects formed in the main body layer 11 other than those shown in FIG. Some examples are shown in FIG. (a) shows a point defect 161 formed by making one of the holes 15 larger in diameter than the other holes. Such a point defect can easily control the resonance wavelength by changing its diameter. (b) shows two identical point defects arranged side by side. One space 172 is provided for these two point-like defects 1621 and 1622. The two point-like defects 1621 and 1622 resonate with light of the same wavelength and function as one resonator. (c) and (d) are obtained by disposing a plurality of different point defects 163 to 165 on one photonic crystal. Each of the point defects 163 to 165 resonates light having different wavelengths. In (c), spaces 173 to 175 are provided facing each point-like defect. On the other hand, in (d), one space 176 common to all of these point-like defects 163 to 165 is provided. Although the space 176 is larger than the spaces 173 to 175, the length L in the in-plane direction perpendicular to the arrangement direction of the point-like defects 163 to 165 is short, so that sufficient strength can be secured.

  FIG. 6 shows an example in which a waveguide is provided in the two-dimensional photonic crystal of the present invention. The waveguide 18 is configured by missing the holes 15 in a linear manner, that is, by not providing the holes 15. In addition, the holes 15 may be simply deleted from the lattice points of the triangular lattice in which the holes 15 are arranged. However, by further shifting the arrangement of the holes 15 and appropriately setting the width of the waveguide 18, The wavelength band of transmitted light can be adjusted. By providing such a waveguide in the vicinity of the point-like defect, the light having the resonance wavelength is demultiplexed from the superposed light having a plurality of wavelengths flowing through the waveguide, and the light having the resonance wavelength is extracted outside the crystal. Thus, the multiplexer / demultiplexer multiplexes the superimposed light of the waveguide.

Next, an embodiment of a method for producing a two-dimensional photonic crystal having an air bridge structure according to the present invention will be described with reference to FIG.
An SOI substrate 21 composed of three layers of a main body layer 11 made of Si, an SiO ₂ substrate layer 12 and an Si substrate layer 13 is prepared. A commercially available SOI substrate 21 can be used. First, two holes 22 for introducing an etching agent are formed in the main body layer 11 ((a)). For example, the etching agent introduction hole 22 is formed by applying an EB resist on the surface of the main body layer 11 and performing EB drawing at the position of the etching agent introduction hole 22 and then using an etching gas (for example, SF ₆ ). It is performed by performing dry etching. Next, the SOI substrate 21 is immersed in the hydrogen fluoride aqueous solution 23 ((b)). The aqueous hydrogen fluoride solution that has entered through the holes 22 for introducing the etchant etches only the SiO ₂ substrate layer 12 without affecting the main body layer 11 and the Si substrate layer 13. As a result, the SiO ₂ substrate layer 12 is etched from the etching agent introduction hole 22 to a certain distance, and the air bridge space 24 is formed. Next, vacancies 25 serving as different refractive index regions are formed in a triangular lattice shape, and point-like defects 26 lacking vacancies are formed so as to be arranged at positions on the air bridge space 24. Here, the position of the SOI substrate 21 is adjusted so that the etching agent introducing hole 22 becomes one of the holes in the different refractive index region. The formation of the holes 25 is also performed by using the EB drawing and dry etching techniques in the same manner as the etching agent introducing holes 22. Thus, the two-dimensional photonic crystal having the air bridge structure of the present invention is completed.

  In the case where a waveguide is further provided, instead of (c-1), as shown in (c-2), it is only necessary to prevent the holes 25 from being formed in the lattice point at the position of the waveguide 27.

FIG. 8 shows a scanning electron microscope (SEM) photograph of the two-dimensional photonic crystal manufactured according to this example. (a) and (b) show the two-dimensional photonic crystal as viewed from above the main body layer 11. In this example, a point defect and a waveguide are formed. In the region 31 that appears discolored, an air bridge space is formed immediately below the main body layer 11. An SEM photograph obtained by photographing a cross section of the SOI substrate 21 in the vicinity of the boundary of the region 31 from obliquely above is shown in FIG. The SiO ₂ substrate layer 12 is etched to form an air bridge space 32. Further, the reason why the SiO ₂ substrate layer 12 is etched obliquely (reference numeral 33) is that the upper portion of the SiO ₂ substrate layer 12 closer to the main body layer is etched farther from the etching agent introducing hole 22; And the crystallinity of SiO ₂ .

  8A and 8B, the positions between the waveguide and the air bridge space are different. In (a), a part of the waveguide passes through the region 31, whereas in (b) the entire waveguide passes outside the region 31. However, there is no difference between (a) and (b) regarding the positional relationship between the waveguide and the point defect. In (b), in order to arrange the point defects on the air bridge space and arrange the waveguide outside the air bridge space, the outer edge of the point defects is arranged at a position substantially in contact with the end of the region 31.

FIG. 9 shows the result of measuring the demultiplexing spectrum of the light demultiplexed from the waveguide into the point defect for the two-dimensional photonic crystals (a) and (b). This figure also shows the measurement results of the demultiplexed spectra of the substrate-free photonic crystal and the substrate-attached photonic crystal. The Q values of the point defects were Q = 1600 in (a) and Q = 900 in (b). In both (a) and (b), a higher Q value was obtained than in the case of the photonic crystal with substrate, and in particular, in (a), a value equivalent to the Q value of the substrate-free photonic crystal was obtained. The Q value in (b) is slightly lower than the Q value of the substrate-free photonic crystal. However, in (b), since the outer edge of the point defect is arranged at a position almost in contact with the end of the region 31, the point defect is This is considered to be due to the fact that some SiO ₂ remains below.

Next, a result of analyzing a waveguide transmission band when a waveguide is provided immediately above the air bridge space in a two-dimensional photonic crystal having an air bridge structure is shown.
First, as shown in FIG. 10A, the widths of the waveguides in the region 41 immediately above the SiO ₂ substrate layer and the region 42 directly above the air bridge space 44 are equal (the width is 1.11 times the period a of the hole 43). ), The band through which light passes through the waveguides in the region 41 and the region 42 is the common band 45 shown in FIG. The vertical axis of (b) is a normalized frequency obtained by multiplying the frequency of light by the period of the holes 43 and dividing by the speed of light, and the horizontal axis indicates the wave number of light. The shaded portion indicates a frequency band that is not preferable for use as a waveguide transmission band because the TE wave is coupled with the TM wave in the waveguide and the TM wave leaks in the in-plane direction.

  On the other hand, as shown in FIG. 11A, when the width of the waveguide in the region 42 is increased to 1.16 times the period a of the hole 43 by adjusting the position of the hole 43, FIG. ), The waveguide transmission band of the region 42 is shifted to the low frequency side, and the common band 46 can be made wider than when the width of the waveguide cannot be expanded.

  Next, when manufacturing a two-dimensional photonic crystal having an air bridge structure, the case where the boundary of the region where the width of the waveguide is widened deviates from the boundary of the air bridge space is examined using the same analysis as described above. . As shown in FIG. 12A, when a narrow waveguide region 51 exists immediately above the air bridge space 50, the light in the band 53 passes through the waveguide in the region 51 as shown in FIG. Since this is not possible, the transmission band 55 of the entire waveguide is narrower than in the case where there is no deviation. On the other hand, when a wide waveguide region 52 exists outside the air bridge space 50 as shown in FIG. 13 (a), the light in the band 54 has a TM wave as shown in FIG. 13 (b). Although it leaks in the in-plane direction of the slab, it can propagate through the entire waveguide. If the region 52 is narrow, the influence of this leakage is small. In other bands, the region 52 has no influence. Therefore, even if the wide waveguide region 52 exists outside the air bridge space 50, the common transmission wavelength band 56 of the waveguide is not narrowed.

Experimental values of resonance wavelength spectra of point defects provided on conventional substrate-free photonic crystals ((a)) and photonic crystals with substrates ((b)). The perspective view, top view, and sectional drawing which show one Example of the two-dimensional photonic crystal which has an air bridge structure of this invention. The figure which shows the air bridge space made into the calculation object of the Q value of a point defect. The graph which shows the result of having calculated the Q value of the point defect in a present Example. The top view which shows the other example of the point defect in a present Example. The top view which shows an example of the waveguide in a present Example. The figure which shows one Example of the manufacturing method of the two-dimensional photonic crystal which has an air bridge structure of this invention. The SEM photograph of the two-dimensional photonic crystal which has the air bridge structure manufactured in the present Example. The graph which shows the demultiplexing spectrum of the two-dimensional photonic crystal which has the air bridge structure manufactured by the present Example, and the conventional two-dimensional photonic crystal. The figure which shows a waveguide transmission zone | band in case the width | variety of a waveguide is uniform. The figure which shows the waveguide transmission zone | band when the width of the waveguide just above an air bridge space is made wide. The figure which shows a waveguide transmission zone | band in the case where the narrow area | region of a waveguide exists just above air bridge space. The figure which shows a waveguide transmission zone | band in the case where the wide area | region of a waveguide exists in the outer side of air bridge space.

Explanation of symbols

11 ... body layer 12 ... SiO ₂ substrate layer 13 ... Si substrate layer 15,25,43 ... vacancies 16,161,1621,1622,163,164,165,26 ... point defect 17,171,172,173, 174, 175, 176 ... Spaces 18, 27 ... Waveguide 21 ... SOI substrate 22 ... Etching agent introduction hole 23 ... Hydrogen fluoride aqueous solution 24, 32, 44, 50 ... Air bridge space

Claims (12)

a) a slab body;
b) a substrate provided on the lower surface of the main body;
c) a plurality of regions arranged in a lattice pattern with a predetermined period on the main body, different in refractive index from the main body,
d) point-like defects formed by providing the defects of the different refractive index regions in the form of dots,
e) an air bridge space provided in the substrate facing only a part of the range in which the different refractive index region of the main body is provided;
A two-dimensional photonic crystal having an air bridge structure , wherein the point-like defects are provided within the range of the part .   2. The two-dimensional photonic crystal having an air bridge structure according to claim 1, wherein the different refractive index region is formed by providing a hole in a main body.   The two-dimensional photonic crystal having an air bridge structure according to claim 1, wherein a plurality of the point defects are provided. The range of the air bridge space is 1 to 25 periods of the period of the different refractive index region outward from the outer edge of the point defect in at least one direction within the surface of the main body. A two-dimensional photonic crystal having the air bridge structure according to any one of. 5. The two-dimensional photonic crystal having an air bridge structure according to claim 1, wherein the depth of the air bridge space is at least twice the period of the different refractive index region.   The two-dimensional structure having an air bridge structure according to any one of claims 1 to 5, further comprising a waveguide in which defects in the different refractive index region are linearly provided in the vicinity of the point defects. Photonic crystal. A part of the waveguide is an air bridge waveguide portion facing the air bridge space, and the width of the waveguide in the air bridge waveguide portion is a predetermined size larger than the width of the waveguide of other portions. The two-dimensional photonic crystal having an air bridge structure according to claim 6, which is wide. In a method for producing a two-dimensional photonic crystal from a plate material in which a main body layer and a substrate layer are laminated,
a pore forming step of forming an etchant introduction holes penetrating the a) said body layer,
By introducing an etchant through b) etchant introduction holes, and the air-bridge space formation step of forming a space on the substrate layer by etching the substrate layer around the etchant introduction holes,
c) Formation of a two-dimensional photonic crystal that periodically forms vacancies formed by removing the main body layer by a predetermined size within a predetermined range of the main body layer and forms dot-like defects in the vacancies Process,
I have a,
The space is formed in the substrate layer so as to face only a part of the range in which the holes of the main body layer are formed,
The point defect is provided within the range of the portion;
A method for producing a two-dimensional photonic crystal having an air bridge structure.   9. The air bridge structure according to claim 8, wherein, in the two-dimensional photonic crystal formation step, a waveguide is formed by providing a linear defect of the vacancy in the vicinity of the point defect. A method for producing a two-dimensional photonic crystal. A part of the waveguide is an air bridge waveguide part facing the space, and the width of the waveguide in the air bridge waveguide part is larger than the width of the other part of the waveguide by a predetermined size. A method for producing a two-dimensional photonic crystal having an air bridge structure according to claim 9, wherein a waveguide is formed on the substrate.
A method for producing crystal. 2 main layer of the plate is made of Si, a layer of at least said space is formed by a substrate layer having an air-bridge structure according to claim 8, characterized in that it consists of SiO ₂ A method for producing a two-dimensional photonic crystal.   The method of manufacturing a two-dimensional photonic crystal having an air bridge structure according to claim 11, wherein the etchant is a hydrogen fluoride solution.

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