JP2016061815A - Optical resonator structure - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical resonator structure such that detection sensitivity can be improved by using a photonic crystal by silicon easily manufactured.SOLUTION: Within a band in the vicinity of a band (PBG) end of a photonic band gap of a resonance portion 103, a period and diameter of a first hole portion 131 are set in a state where the optical energy of a target wavelength is available. In addition, within a photonic band gap of reflection portions, 104, 105, periods and diameters of second hole portions 141, 151 are set in a state where the optical energy of a target wavelength is available.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical resonator structure in which an optical resonator is constituted by a core made of silicon.

  Unlike fluorescent biosensors that detect specific molecules using fluorescent labels that use fluorescent molecules as labeling agents, research and development of biosensors that perform molecular detection optically without labeling are used to analyze chemical substances in the human body and detect diseases early. It is advanced from the viewpoint of medical application. As a technique for performing molecular detection optically without labeling, for example, there is measurement using a surface plasmon resonance (SPR) measuring device. The SPR measuring device can detect a change in refractive index / reflectance due to molecular adsorption. Although this technology is widely used in biochemical research, it is difficult to reduce the size. Biosensors that do not require expert analysis and can be used easily at home require downsizing and cost reduction, and, like electronics, function on the chip using LSI (Large Scale Integration) processing technology. An integrated sensor is desirable.

  To date, sensor chips based on optical techniques manufactured using LSI processing technology have been reported. For example, there is a sensor in which a micro ring optical resonator having a diameter of μm is formed on the basis of a silicon optical waveguide obtained by processing a plate-like silicon layer into a thin wire having a rectangular cross section with a width and height of several hundreds of nanometers. Patent Document 1). By selectively adsorbing molecules on the surface of the optical waveguide due to the antigen-antibody reaction, the refractive index changes, and the light can be captured by changing the light transmittance. The change in transmittance is due to the shift of the wavelength at which resonance occurs (resonance wavelength).

  Further, in a photonic crystal (PhC) in which holes are periodically formed in a silicon slab, a point defect having a hole size changed in part is provided to operate as an optical resonator (Non-Patent Document 2). It is possible to capture molecular adsorption.

T. Claes et al., "Experimental characterization of a silicon photonic biosensor consisting of two cascaded ring resonators based on the Vernier-effect and introduction of a curve fitting method for an improved detection limit", Optics Express, vol.18, no. 22, pp.22747-22761,2010. M. R. Lee and P. M. Fauchet, "Nanoscale microcavity sensor for single particle detection", Optics Letters, vol.32, no.22, pp.3284-3286, 2007. J. T. Robinson et al., "On-chip gas detection in silicon optical microcavities", Optics Express, vol.16, no.6, pp.4296-4301,2008.

  However, in the above-described resonator, the power of light is mainly confined in silicon, and the molecules adsorbed on the surface by evanescent waves that ooze out weakly from the waveguide are captured. Therefore, the sensitivity ( The lower limit of the detected concentration is difficult. Here, in order to improve the detection sensitivity, it is effective to increase the Q value (quality factor) of a micro ring optical resonator made of silicon or a resonator using a photonic crystal. However, at present, the Q value of the resonator is improved mainly by ultra-precision machining that reduces the roughness of the waveguide side wall at the atomic level. This processing is not easy, and conventionally, there is a problem that it is not easy to improve the Q value.

  The present invention has been made to solve the above problems, and an object thereof is to improve detection sensitivity by using a photonic crystal made of silicon that can be easily manufactured.

  An optical resonator structure according to the present invention includes a silicon core formed on a clad layer, a resonance part disposed in a part of the core, and two reflection parts formed in the core with the resonance part interposed therebetween. A plurality of first holes that are periodically arranged in a row in the extending direction of the core and formed in a column shape in the normal direction of the cladding layer, and in the extending direction of the core. A plurality of second holes that are periodically arranged in a row and are columnar in the normal direction of the clad layer, and formed in the core of the reflective portion, near the band edge of the photonic band gap of the resonant portion A state where the period and diameter of the first hole are set in a state where there is light energy of the target wavelength in the band, and there is light energy of the target wavelength in the photonic band gap of the reflection unit The period and the diameter of the second hole are set.

  In the optical resonator structure, the core may have a rectangular cross-sectional shape.

  As described above, according to the present invention, it is possible to obtain an excellent effect that detection sensitivity can be improved by using a photonic crystal made of silicon that can be easily manufactured.

FIG. 1 is a configuration diagram showing a configuration of an optical resonator structure according to an embodiment of the present invention. FIG. 2 shows the state of distribution in the core 202 and the hole 203 of light having energy in an energy band larger than the upper end of the photonic band gap and light having energy in an energy band larger than the upper end of the photonic band gap. It is explanatory drawing for demonstrating. FIG. 3 is a characteristic diagram showing the intensity of light after the light guided through the optical waveguide formed by the core 102 passes through the resonance unit 103 and the reflection units 104 and 105. FIG. 4 is a characteristic diagram showing the relationship between the number of first holes 131 in the resonance unit 103 and the Q value. FIG. 5 is a characteristic diagram showing the relationship between the number of first holes 131 in the resonance unit 103 and the S × Q value (FOM).

  Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a configuration diagram showing a configuration of an optical resonator structure according to an embodiment of the present invention. FIG. 1A shows a plane. FIG. 1B shows a cross section of FIG.

  This optical resonator structure includes a core 102 made of silicon formed on a clad layer 101. The core 102 has a rectangular shape in sectional view, and has a width of 400 nm and a thickness of 220 nm in sectional view, for example. In the core 102, a resonance part 103 and two reflection parts 104 and 105 are formed. The reflection unit 104 and the reflection unit 105 are arranged with the resonance unit 103 interposed therebetween.

  A plurality of first holes 131 are formed in the core 102 of the resonance unit 103. The first hole 131 has a columnar shape extending in the normal direction of the cladding layer 101. Further, the first hole portions 131 are periodically arranged in one row in the extending direction of the core 102.

  In addition, a plurality of second hole portions 141 and 151 are also formed in the reflection portions 104 and 105. The second hole portions 141 and 151 are also columnar shapes extending in the normal direction of the cladding layer 101. The second hole portions 141 and 151 are also periodically arranged in one row in the extending direction of the core 102.

  Here, in the embodiment of the present invention, the first hole 131 is in a state where there is energy of light of the target wavelength in the band near the band (PBG) end of the photonic band gap of the resonator 103. Period and diameter are set. In other words, the energy of light of the target wavelength exists in the band near the band edge of the photonic band gap of the photonic crystal determined by the hole diameter of the first hole 131 and the arrangement period thereof. To do. For example, when the core 102 has a thickness of 220 nm and a width of 400 nm, the first hole 131 has a period of 440 nm and a radius of a period × 0.175 (nm).

  Further, the periods and diameters of the second hole portions 141 and 151 are set so that the energy of light having a target wavelength exists within the photonic band gap of the reflecting portions 104 and 105. In other words, the energy of the light of the target wavelength exists in the photonic band gap of the photonic crystal determined by the hole diameters of the second hole portions 141 and 151 and their arrangement period. For example, the second hole portions 141 and 151 have a period of 430 nm and a radius of period × 0.225 (nm).

  By setting each of the above dimensions, an optical resonator structure operating in a wavelength range of 1.55 μm can be obtained. Note that the first hole 131 and the second holes 141 and 151 have different refractive indexes from the surrounding core.

  Here, light having energy in an energy band (Air band) larger than the upper end of the photonic band gap is generated from silicon formed on the cladding layer 201 as shown in the light distribution of FIG. A large number of holes 203 having a smaller refractive index are formed in the core 202. On the other hand, light having energy in a smaller energy band (Dielectric band) than the lower end of the photonic band gap has a refractive index around the hole 203 as shown in the light distribution of FIG. It will be present in the larger core 202.

  With the above-described configuration, the reflection units 104 and 105 function as a mirror, and by including the resonance unit 103, the reflection units 104 and 105 function as a resonator as illustrated in FIG. FIG. 3 is a characteristic diagram showing the intensity of light after the light guided through the optical waveguide formed by the core 102 passes through the resonance unit 103 and the reflection units 104 and 105. In the configuration described above, when the light guided through the optical waveguide by the core 102 propagates, the resonance unit 103 is in a state where there are many inside the holes of the first hole 131. When functioning as a biosensor in this state, the interaction between the detection molecule and light increases, and the detection sensitivity can be improved regardless of the improvement of the Q value.

  In addition, since the resonating unit 103 and the reflecting units 104 and 105 are configured by arranging the holes in a row, the resonating unit 103 and the reflecting units 104 and 105 do not spread in a direction perpendicular to the optical waveguide direction by the core 102, and are compared with a two-dimensional photonic crystal. Can be configured with a small area. Therefore, for example, it is easy to integrate a plurality of optical resonator structures on one chip.

  In addition, in the above-described embodiment, the period and the diameter of the first hole 131 are set so that the energy of the light of the target wavelength exists in the band near the band edge of the photonic band gap of the resonance unit 103. Since it is set, the Q value can be further improved. As a result of the simulation, if the period and the diameter of the first hole 131 are set in a state where the energy of light of the target wavelength is within the band in the range of 0.02 eV from the band edge of the photonic band gap. It turns out to be good. This is because the light staying time in the optical resonator (resonant unit 103) can be increased by the slow light effect in which the group velocity is reduced with the above configuration. In particular, as will be described later, a higher Q value can be obtained by increasing the number of the first holes 131 and increasing the length of the resonance portion 103 in the waveguide direction.

  By the way, the performance index of a sensor can be generally defined by a product “S × Q value” of a detection efficiency S value (amount of change in resonance wavelength with respect to unit refractive index change) and a Q value of a resonator. For example, when a biosensor is configured with the resonator structure in the embodiment, the detection lower limit is proportional to “S × Q value”. Since the Q value is further improved by setting the photonic band gap of the resonating unit 103 described above, the performance of the sensor can be improved.

Here, in the optical resonator structure as described above, in general, the period and the diameter of the hole portion of the resonance portion are set in a state where the energy of the light of the target wavelength is outside the photonic band gap, and the resonance is performed. It is a vessel. However, in the sensor using this configuration, the S × Q value remains at about 2 × 10 ⁴ nm / RIU. As a biosensor, further improvement in detection efficiency is required, and this performance may not satisfy the requirement.

  As is clear from the above definition, first, improvement of the S value can be considered to improve the performance. In order to improve the S value, it is necessary to enhance the interaction between the oozing light (evanescent light) of the optical waveguide light made of Si and the target biopolymer (protein, DNA) or cell. For this reason, a slot structure is conceivable as a structure that can suppress the propagation loss of light while increasing the evanescent light. In fact, it has been reported that the S value is improved by producing a slot ring resonator in which a slot structure is introduced into an Si ring (see Non-Patent Document 3).

  However, the slot structure has a problem of being sensitive to an increase in propagation loss due to manufacturing errors of the slot structure and the like, and is not easy to manufacture.

  On the other hand, as shown in the embodiment, the improvement of the Q value can be realized by optimizing the setting of the period and the diameter of the first hole 131 in the resonating unit 103 and easy to manufacture.

  Next, the number of first hole portions 131 in the resonance portion 103, in other words, the length of the resonance portion 103 including a plurality of first hole portions 131 having a predetermined period determined by the above, and an increase in Q value The relationship will be described.

  When the number of the first holes 131 in the resonating part 103 is increased, the Q value can be increased as shown in FIG. Thus, according to the configuration of the present invention, it is possible to obtain a higher Q value by lengthening the resonance unit 103 including the plurality of first holes 131 having a predetermined period. In particular, as shown in FIG. 4A, it is effective at an optimum edge distance. Note that the edge distance is a connection distance (distance between the centers of the end holes) between the resonance unit 103 and the reflection units 104 and 105. FIG. 4A illustrates an edge distance of 0.3 to 0.4 μm. In (b), the edge distance is 0.5 μm, (c) is 0.1 μm, and (d) is the edge distance is 0.2 μm. By increasing the edge distance, the resonance peak moves to the long wavelength side. By setting the edge distance to an appropriate value, the position of the resonance peak is controlled to function as a resonator in the target light.

  Further, as described above, the performance as a sensor is indicated by “S × Q value”. Therefore, as shown in FIG. 5, when the number of first holes 131 in the resonance portion 103 is increased, the performance as a sensor is increased. Can be improved. As described above, the slow light effect that decreases the group velocity can increase the staying time of light in the optical resonator (resonant unit 103). As a result, the mutual light between the light and the target molecule in the resonator 103 can be increased. This is probably because the time of action increases. In particular, as shown in FIG. 5A, it is effective at an optimum edge distance. 5A shows an edge distance of 0.3 to 0.4 μm, FIG. 5B shows an edge distance of 0.5 μm, and FIG. 5C shows 0.1 μm.

  By the way, the optical resonator structure in the above-described embodiment can be formed (manufactured) by using a well-known SOI (Silicon on Insulator) substrate. For example, the core 102 and each hole may be formed by using the buried insulating layer of the SOI substrate as the cladding layer 101 and patterning the surface silicon layer.

  In the biosensor using the optical resonator structure in the above-described embodiment, the change in the refractive index and the light absorption due to the adsorption of the molecule to be analyzed inside the first hole 131 of the resonance unit 103 is the optical waveguide formed of the core 102. Can be detected as a change in the peak wavelength or intensity of the light spectrum.

  Here, in order to function as a biosensor, it is necessary to adsorb the biomarker to the resonance unit 103. In many cases, the biomarker is a very small amount of molecules present in the liquid or gas to be measured. In order to efficiently adsorb the biomarker to the resonance unit 103, a part other than the resonance unit 103 is covered with a protective film, and the resonance unit 103 is covered. In addition, the sample liquid and the sample gas in which the biomarker is dissolved may be directly brought into contact with each other.

  Specifically, there is a method in which a target liquid or gas is dropped onto the resonance unit 103 and sprayed using a dispenser or the like. In addition, there is a method in which the resonating unit 103 is incorporated in the micro flow channel and the target liquid or gas is transported to the resonating unit 103 by external pressure or capillary force (in the case of liquid). By such a method, the detection molecule can be adsorbed in the first hole 131.

  In the measurement, light is incident from one of the optical resonator structures in which the biomarker is adsorbed in the first hole 131 of the resonance unit 103, and the emitted light is received on the opposite side, and the incident light and the emitted light are compared or output. This is done by comparing before and after sensing of the light (for example, before and after the antigen-antibody reaction). The incident light covers a wavelength region that is sensitive to a change in refractive index before and after biomarker sensing. When the incident light has a single wavelength, the intensity change of the emitted light is observed.

  By changing the intensity change for each incident wavelength by scanning the wavelength of the incident light, the spectral change can be measured. If the outgoing light is split using incident light having a broad wavelength, a similar spectral change can be obtained in a short time. The light source and the light receiver may be selected in accordance with the measurement target and the measurement method in consideration of the stability, heat resistance, measurement time, and the like of the biomaterial.

  As described above, according to the present invention, the period and diameter of the first hole portion are in a state where the energy of light of the target wavelength exists in the band near the band edge of the photonic band gap of the resonance portion. Therefore, detection sensitivity can be improved by using a photonic crystal made of silicon that can be easily manufactured. According to the present invention, a highly sensitive biosensor can be realized for biopolymers (proteins, DNA), cells, and the like.

  The present invention is not limited to the embodiment described above, and many modifications and combinations can be implemented by those having ordinary knowledge in the art within the technical idea of the present invention. It is obvious.

  DESCRIPTION OF SYMBOLS 101 ... Cladding layer, 102 ... Core, 103 ... Resonance part, 104, 105 ... Resonance part, 131 ... 1st hole part, 141, 151 ... 2nd hole part.

Claims (2)

A core made of silicon formed on the cladding layer;
A resonating portion disposed in a part of the core;
Two reflecting parts formed on the core across the resonance part;
A plurality of first holes formed in the core of the resonating portion, arranged in a row periodically in the extending direction of the core and being columnar in the normal direction of the cladding layer;
A plurality of second holes formed in the core of the reflective portion, and arranged in a row periodically in the extending direction of the core to be columnar in the normal direction of the cladding layer,
Within the band near the band edge of the photonic band gap of the resonance part, the period and the diameter of the first hole are set in a state where there is light energy of a target wavelength,
The optical resonator structure, wherein the period and the diameter of the second hole portion are set so that the energy of light having a target wavelength exists within the photonic band gap of the reflecting portion. The optical resonator structure according to claim 1,
An optical resonator structure, wherein the core has a rectangular cross-sectional shape.