JP2016161890A - Optical device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical device using graphene that can have graphene and propagation light efficiently interact with each other.SOLUTION: An optical device comprises: a first waveguide 103 and a second waveguide 104 arranged on a substrate 101 via an insulation layer 102; and a light control unit 105 arranged between the first waveguide 103 and the second waveguide 104. The first waveguide 103 and the second waveguide 104 are disposed such that their incoming/outgoing end 134 and incoming/outgoing end 144 face each other. The light control unit 105 comprises: a dielectric layer 106 formed on the substrate 101; and a carbon layer 107 composed of graphene formed on the dielectric layer 106. The carbon layer 107 is a region sandwiched between the incoming/outgoing end 134 of the first waveguide 103 and the incoming/outgoing end 144 of the second waveguide 104 and is formed on the dielectric layer 106.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an optical device using graphene that enables optical control such as light modulation and optical switch.

  Numerous elements are known as optical semiconductor materials used for elements capable of converting light and electricity. Such optical semiconductor materials can generally be classified into II-VI group semiconductor materials, III-V group semiconductor materials, and IV group semiconductor materials. Among these, in the optical communication field, group IV semiconductor materials typified by silicon / germanium that can be applied to optical application technology (silicon photonics technology) are attracting attention. Optical application technology using Group IV semiconductor materials has excellent advantages such as mass production, miniaturization, and integration. From ultrashort-distance optical communications represented by interchip optical interconnection, A wide range of applications to optical communications is being studied.

  In recent years, in anticipation of further technologies, application technology development of optical communication using carbon materials, particularly carbon nanotubes or graphene called nanocarbon has been actively performed (see Non-Patent Document 1). For example, graphene has advantages in high carrier mobility, optical wavelength independence, high optical nonlinearity, and the like, and can realize an optical device that is different from conventional materials (see Non-Patent Document 2).

  Since graphene is composed of a single layer or several layers of carbon atoms, forming a propagation mode with only graphene has a large loss and many disadvantages. For this reason, in general, graphene is loaded on an optical circuit made of various materials as a base. For example, a structure in which graphene and related elements are formed only in an active region on a dielectric waveguide such as silicon, silicon nitride, quartz, indium phosphide, and gallium arsenide, and the dielectric waveguide is responsible for light propagation in other regions. (Hybrid structure).

  For example, a micro optical circuit using a semiconductor material such as silicon has a very large refractive index difference (Δn = about 2.5) between a waveguide core material and a clad material. Waves (leakage electric field) increase. For this reason, since a high interaction with graphene is obtained, there is a report example applied to a modulator or a light receiver that combines graphene with the above-described minute optical circuit.

M. Liu et al., "A graphene-based broadband optical modulator", Nature, vol.474, pp.64-67, 2011. F. Bonaccorso et al., "Graphene photonics and optoelectronics", NATURE PHOTONICS, vol.4, pp.611-622, 2010. S. A. Mikhailov and K. Ziegler, "New Electromagnetic Mode in Graphene", Physical Review Letters, vol.99, no.1, 016803, 2007.

  As described above, an optical device (such as an optical modulator and a light receiver) in which an optical waveguide system having a high refractive index difference and graphene, including silicon, is integrated, has many remarkable advantages. There is a problem that the interaction between the field and graphene, which is a two-dimensional material, is still insufficient. In order to enhance the interaction between the electric field of graphene and the propagation mode light, it is possible to estimate by the overlapping area. Here, in general, the interaction efficiency obtained by the cross-sectional shapes of the two portions that are allowed to interact with each other is different, and is 100% when the cross-sectional shapes of the two portions completely match.

  As an example, as a result of estimation of the above-described optical device based on mode analysis by an electromagnetic field simulator, a silicon waveguide cross section covered with graphene (graphene thickness: 0.34 nm, silicon waveguide cross section: vertical 200 nm) × 400 nm) In the total electric field in the propagation mode, the proportion interacting with graphene was approximately 0.06 to 0.08%. As described above, the conventional optical device in which the optical waveguide system and the graphene are integrated has a problem that the graphene and the propagating light cannot be efficiently interacted with each other.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to allow graphene and propagating light to interact efficiently in an optical device using graphene. To do.

  The optical device according to the present invention is arranged between the first waveguide and the second waveguide, and the first waveguide and the second waveguide, which are arranged on the substrate so that the respective input and output ends face each other. A light control unit, and the light control unit is a dielectric in a region sandwiched between a dielectric layer formed on the substrate, an input / output end of the first waveguide, and an input / output end of the second waveguide. The carbon layer made of graphene formed on the layer, the first electrode layer formed on the substrate in contact with the lower surface of the dielectric layer, and the carbon layer in a state where the first electrode layer is electrically separated And a second electrode layer formed on the substrate in contact with the upper surface.

  In the optical device, the first electrode layer and the second electrode layer are formed in a region other than the region sandwiched between the input / output end of the first waveguide and the input / output end of the second waveguide.

  In the above optical device, the first waveguide and the second waveguide may be an optical waveguide composed of a core made of silicon. The first waveguide and the second waveguide may be surface plasmon waveguides.

  As described above, according to the present invention, an excellent effect that graphene and propagating light can be efficiently interacted is obtained in an optical device using graphene.

FIG. 1 is a perspective view showing a configuration of an optical device according to Embodiment 1 of the present invention. FIG. 2 is a cross-sectional view showing a partial configuration of the optical device according to Embodiment 1 of the present invention. FIG. 3 is a perspective view showing the configuration of the optical device according to Embodiment 2 of the present invention. FIG. 4 is a perspective view showing the configuration of another optical device according to Embodiment 1 of the present invention. FIG. 5 is a characteristic diagram showing the coupling efficiency of the TE propagation mode with the optical fiber when a spot size conversion unit is provided for connection with the optical fiber. FIG. 6 is a characteristic diagram showing the coupling efficiency of the TM propagation mode with the optical fiber when a spot size conversion unit is provided for connection with the optical fiber.

  Hereinafter, embodiments of the present invention will be described with reference to the drawings.

[Embodiment 1]
First, Embodiment 1 of the present invention will be described with reference to FIGS. FIG. 1 is a perspective view showing a configuration of an optical device according to Embodiment 1 of the present invention. FIG. 2 is a cross-sectional view showing a partial configuration of the optical device according to Embodiment 1 of the present invention.

  This optical device is arranged between the first waveguide 103 and the second waveguide 104, and between the first waveguide 103 and the second waveguide 104, which are arranged on the substrate 101 via the insulating layer 102. A light control unit 105.

  In the first embodiment, the first waveguide 103 includes a core 131 made of silicon and an upper clad 132 formed on the core 131. The second waveguide 104 includes a core 141 made of silicon and an upper clad 142 formed on the core 141. Further, the first waveguide 103 and the second waveguide 104 are arranged such that the respective incident / exit ends 134 and the incident / exit ends 144 face each other.

  The core 131 includes a spot size conversion unit 133 that narrows toward the incident / exit end 134. The core 141 includes a spot size conversion unit 143 that narrows toward the incident / exit end 144. As the spot size conversion units 133 and 143 move away from the input and output ends 134 and 144, the spot size conversion units 133 and 143 are adiabatically (gradually) spread in plan view from the middle to the core 131 and the core 141 (for example, width 400 nm) in the single propagation mode condition. Yes.

  The light control unit 105 includes a dielectric layer 106 formed on the substrate 101 and a carbon layer 107 made of graphene formed on the dielectric layer 106. The carbon layer 107 is formed on the dielectric layer 106 in a region sandwiched between the input / output end 134 of the first waveguide 103 and the input / output end 144 of the second waveguide 104. In the region where the light control unit 105 is disposed, the upper clad 132 and the upper clad 142 are not formed and are in an open state. In the present invention, the region where the carbon layer 107 is disposed is the active region.

  The first electrode layer 108 formed on the substrate 101 (insulating layer 102) in contact with the lower surface of the dielectric layer 106 and the upper surface of the carbon layer 107 are electrically separated from the first electrode layer 108. And a second electrode layer 109 formed on the substrate 101. In Embodiment 1, these are electrically separated from each other by being arranged on the insulating layer 102 made of an insulating material. The first electrode layer 108 and the second electrode layer 109 are formed in a region other than the region sandwiched between the incident / exit end of the first waveguide 103 and the incident / exit end of the second waveguide 104. Note that a voltage can be applied between the first electrode layer 108 and the second electrode layer 109.

For example, the substrate 101 can be made of silicon, and the insulating layer 102 can be made of silicon oxide. The dielectric layer 106 can be made of Al ₂ O ₃ . Here, the thickness of the dielectric layer 106 depends on the dielectric constant of the constituent material, and it is important to set it to an optimum thickness. The thinning of the dielectric layer 106 enables efficient carrier accumulation. On the other hand, the dielectric breakdown threshold varies greatly depending on the growth method. For this reason, when the dielectric layer 106 is made of Al ₂ O ₃ , it is preferably formed by an atomic layer growth method and has a thickness of 10 to 30 nm.

  The lengths of the dielectric layer 106 and the carbon layer 107 in the width direction may be determined in accordance with the widths of the incident / exit end 134 and the incident / exit end 144. For example, if the width of the incident / exit end 134 and the incident / exit end 144 is 100 nm, the lengths of the dielectric layer 106 and the carbon layer 107 in the width direction need only exceed 100 nm. Further, the carbon layer 107 in the propagation direction of signal light (propagation light) may be about 30 μm, for example. The transmission loss difference in graphene (TE polarization loss−TM polarization loss) = 1 dB / μm, and light modulation efficiency greatly exceeding this can be expected in the light control unit 105 using the carbon layer 107. Therefore, the above-mentioned 30 μm is long enough to be quenched at 30 dB.

  Graphene can actively control a metal-semiconductor transition by applying an electric field to a gate electrode (see Non-Patent Document 3). In a state where an electric field is applied perpendicularly to the carbon layer 107 made of graphene and behaves like a metal, light emitted from the incident / exit end 134 of the first waveguide 103 is coupled to the surface of the carbon layer 107. A surface plasmon propagation mode is formed. For this reason, the light is coupled to the input / output end 144 of the second waveguide 104 with a relatively small insertion loss and output.

  On the other hand, when the semiconductor layer behaves without an electric field being applied to the carbon layer 107, the light emitted from the incident / exit end 134 of the first waveguide 103 due to interband absorption is completely transmitted by the carbon layer 107. Therefore, the light does not propagate to the incident / exit end 144 of the second waveguide 104.

  Therefore, by applying a voltage between the first electrode layer 108 and the second electrode layer 109 and applying an electric field to the carbon layer 107, the first waveguide 103 to the second waveguide 104 can be connected at any time. The light output can be switched. According to Embodiment 1, the carbon layer 107 made of undoped graphene absorbs light in the steady state and becomes transparent when a voltage (electric field) is applied. Note that the operating point can be adjusted by intentional chemical doping, electric field doping, or the like for the carbon layer 107.

  In the first embodiment, the spot size converters 133 and 143 are provided in the first waveguide 103 and the second waveguide 104, but this is not a necessary configuration.

  By the way, in manufacturing the optical device described above, it is important not to damage the carbon layer made of graphene. For example, when a silicon oxide film or the like is deposited on graphene, it is known that the graphene is damaged by a film stress on the graphene, an activation gas at the time of deposition, or a plasma atmosphere. If damage is given in this way, the original performance of the graphene material cannot be exhibited, and efficient optical input / output coupling cannot be obtained.

  On the other hand, in the first embodiment, the first waveguide 103 and the second waveguide 104 constituted by the cores 131 and 141 made of silicon are used, and on the insulating layer 102 on which the carbon layer 107 is disposed. The upper clad 132 and the lower clad 142 are formed. Therefore, it is important to form the upper clad 132 and the lower clad 142 without damaging the carbon layer 107.

  For this purpose, an upper clad 132 and a lower clad 142 are formed by selectively depositing silicon oxide by a well-known lift-off method with the light control unit 105 covered with a lift-off mask. By doing so, in the formation of the upper clad 132 and the lower clad 142, the light control unit 105 (carbon layer 107) is protected by the lift-off mask, and no damage is applied.

[Embodiment 2]
Next, Embodiment 2 of the present invention will be described with reference to FIG. FIG. 3 is a perspective view showing the configuration of the optical device according to Embodiment 2 of the present invention. This optical device is arranged between the first waveguide 303 and the second waveguide 304, and between the first waveguide 303 and the second waveguide 304, which are arranged on the substrate 101 via the insulating layer 102. A light control unit 105.

  The first waveguide 303 is a surface plasmon waveguide provided with a waveguide pattern 331 made of a metal such as Cu or Al and guided by a surface plasmon polariton induced on the surface of the waveguide pattern 331. Similarly, the second waveguide 304 is a surface plasmon waveguide provided with a waveguide pattern 341 made of a metal such as Cu or Al, and guided by surface plasmon polariton induced on the surface of the waveguide pattern 341. Further, the first waveguide 303 and the second waveguide 304 are arranged such that the respective incident / exit ends 332 and the incident / exit ends 342 face each other. Here, transmission of light in the surface plasmon waveguide is performed by a light propagation mode via plasmons in which free electrons collectively vibrate on the metal surface and behave as pseudo particles.

  As described above, the second embodiment is characterized by using a surface plasmon waveguide, and the light control unit 105 is the same as in the first embodiment described above, and a detailed description thereof is omitted. Also in the optical device configured in this manner, as in the first embodiment, a voltage is applied between the first electrode layer 108 and the second electrode layer 109 to apply an electric field to the carbon layer 107. The light output from the first waveguide 303 to the second waveguide 304 can be switched at an arbitrary time.

  Although all optical circuits can be configured by surface plasmon propagation, the loss is overwhelmingly larger than that of a silicon core optical waveguide, and it is coupled to an optical fiber at the input / output section with the outside. I can't. Therefore, in the optical waveguide structure of the first embodiment, it is very effective to use a spot size conversion structure at the input / output unit with the outside.

  For example, as shown in FIG. 4, spot size converters 135 and 145 may be provided on the input / output ends 136 and 146 connected to the optical fiber. The spot size conversion unit 135 includes a portion that decreases for a while toward the input / output end 136, and the spot size conversion unit 145 includes a portion that decreases for a while toward the input / output end 146. In addition, both have regions having the same width (thickness) from the input / output end 136 to the input / output end 146.

  In this way, by providing the spot size conversion units 135 and 145 on the input / output ends 136 and 146 connected to the optical fiber, as shown in FIG. 5 and FIG. Coupling efficiency over 300 nm or more (3 dB cut-off) can be obtained for both polarizations of .4, 2.4 dB / facet. The thickness of the upper clad 132 and the upper clad 142 is 1.2 μm. Further, the length of the region having the same width from the input / output end 136 to the input / output end 146 is set to 500 μm, and the length of the region in which the width gradually changes is set to 500 μm. 5 and 6, (a) shows the core width of the input / output ends 136 and 146 is 150 nm, (b) shows the core width of the input / output ends 136 and 146 is 200 nm, and (c) shows the input / output ends. The case where the core width of the ends 136 and 146 is 250 nm is shown.

  By the way, by providing the second core having a refractive index between the clad and the core, it is possible to further suppress the mode mismatch loss and the adiabatic propagation loss. However, in this case, the clad is made thicker, and it is desirable to avoid it as much as possible in consideration of damage to the carbon layer made of graphene.

  As described above, in the present invention, light is propagated from the first waveguide to the second waveguide by the surface plasmon propagation mode in a state of performing a metallic behavior in the carbon layer made of graphene constituting the light control unit. Therefore, in the optical device using graphene, graphene and propagating light can be efficiently interacted.

  The surface plasmon propagation mode is generated by a dense wave in free electrons in the carbon layer, and is localized near the surface of the carbon layer. Therefore, the light after being coupled from the first waveguide is confined in a very small area on the surface of the carbon layer. The thickness of the cross section of this region is about 1/10 with respect to the wavelength. In this way, since a very small region is confined, even when the thickness of the carbon layer is about 0.34 nm, in the case of a silicon waveguide between the carbon layer (graphene) (see Non-Patent Document 2) Interaction will occur more efficiently.

  In addition, according to the present invention, the length of the light control unit in the propagation direction can be remarkably reduced to about 30 μm, so that there is an advantage that the bandwidth is not limited by the CR time constant. Since graphene has very high mobility, optical switching (optical modulation) on the order of several hundreds GHz is possible.

  In the present invention, by combining the propagation mode of a dielectric waveguide such as silicon or the propagation mode of a surface plasmon waveguide with the surface plasmon propagation mode of graphene, Increased graphene interaction could be achieved.

  In an optical integrated device using graphene, a sandwich structure, a stacked structure, etc. are assumed to enhance the interaction. However, since the device needs to be grown again after placing the graphene in the fabrication of these devices, the graphene And the film quality of the crystal regrowth layer is relatively poor. On the other hand, in the invention, since an optical device can be manufactured by a photolithography technique that is generally used without using regrowth, a structure in which various graphenes are integrated is assumed.

  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 ... Board | substrate, 102 ... Insulating layer, 103 ... 1st waveguide, 104 ... 2nd waveguide, 105 ... Light control part, 106 ... Dielectric layer, 107 ... Carbon layer, 108 ... 1st electrode layer, 109 ... 1st 2 electrode layers, 131... Core, 132... Upper clad, 133... Spot size converter, 134... Entrance / exit end, 141... Core, 142.

Claims (4)

A first waveguide and a second waveguide, which are disposed on the substrate with each input / output end facing each other;
A light control unit disposed between the first waveguide and the second waveguide,
The light control unit
A dielectric layer formed on the substrate;
A carbon layer made of graphene formed on the dielectric layer in a region sandwiched between the input and output ends of the first waveguide and the input and output ends of the second waveguide;
A first electrode layer formed on the substrate in contact with a lower surface of the dielectric layer;
An optical device comprising: a second electrode layer formed on the substrate in contact with an upper surface of the carbon layer in a state of being electrically separated from the first electrode layer. The optical device according to claim 1.
The first electrode layer and the second electrode layer are formed in a region other than a region sandwiched between an input / output end of the first waveguide and an input / output end of the second waveguide. device. The optical device according to claim 1 or 2,
The optical device, wherein the first waveguide and the second waveguide are optical waveguides composed of a core made of silicon. The optical device according to claim 1 or 2,
The optical device, wherein the first waveguide and the second waveguide are surface plasmon waveguides.

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