JP3467467B2 - Light control element - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light control element using a photonic crystal. INDUSTRIAL APPLICABILITY The present invention is particularly suitable for a broadband optical control element used in an ultrahigh-speed time division multiplexing optical communication system.

[0002]

2. Description of the Related Art Up to now, there have been reports on an all-optical / optical control element aiming at application to an ultrahigh-speed time division multiplex optical communication system. In these elements, information of an optical signal is controlled by using another light. The basic principle is the third-order nonlinear optical effect that changes the optical characteristics of the nonlinear medium in the element by the control light. They are classified into a virtual excitation type (non-resonant type) without actual carrier excitation and a real excitation type (resonant type) with carrier excitation. In the former case, the high speed property is excellent, but the nonlinear optical effect is small. Also. In the latter case, it is known that high-speed response is difficult although the nonlinear optical effect is relatively large. Such an example can be found in, for example, “Ultra High-speed Optical Device” (Kyoritsu Shuppan), pages 157-158.

[0003]

SUMMARY OF THE INVENTION An object of the present invention is to improve the performance of the resonance excitation type all-optical / optical control element to a practical level. In order to apply a resonance type all-optical / optical control element to an ultrahigh-speed communication system, it is essential to increase the nonlinear optical effect and speed up the response. The magnitude of the nonlinear optical effect is directly related to the input power (that is, power consumption) of the control light when the signal light is controlled by the control light. Although a resonance type all-optical / optical control element can obtain a larger nonlinear optical effect than a non-resonant type, a larger nonlinear optical effect is required to perform a switching operation at a practical level. For example, Tajima et al. Reported an all-optical switch operation in a Mach-Zehnder type optical switch using a GaAs-based bulk as a nonlinear waveguide. Input power 6pJ
This control light realizes the π phase shift necessary for the switch operation. An example of this is the Japanese Journal of Applied Physics No. 3, 1994, 1.
Pp. 44-150 (Jpn. J. Appl. Phy
s. Vol. 33, (1994), pp. 144-15
0). As a result, at high speed repetition, for example, 100 GHz repetition, the power consumption inside the element is 0.6 W. 10 elements and optical fiber coupling
If dB, the required input power is several W, which is a large value that cannot be said to be a practical level. In order to solve the above problems, the present invention provides an optical control element with reduced power consumption of control light, for practical use of an all-optical switch.

[0004]

According to the present invention, an incident surface on which an optical signal light and a control light having a wavelength different from the incident light are incident on one end surface of the photonic member, and the photonic member is provided. The other end face has an emission surface, the photonic member has a photonic crystal region and a region in which quantum dots are arranged in a high dielectric portion of the photonic crystal, and in the wavelength range of the control light. There is provided an optical control element characterized by setting an energy level of a quantum dot and a dielectric band of a photonic crystal, and setting an air band of the photonic crystal in a wavelength range of the signal light. With this structure, it is possible to increase the nonlinear optical effect of the resonance-type all-light / light-control element.

The photonic crystal refers to a crystal in which substances having different dielectric constants are periodically arranged. Since the substances having different dielectric constants are periodically arranged, they form a band structure for light. The "photonic member" according to the invention of the present application has a region of such a photonic crystal and a region in which quantum dots are arranged in a high dielectric portion having a high dielectric constant in such a photonic crystal. Therefore, in the "photonic member" according to the present invention, a crystal region having a low dielectric constant in a desired photonic crystal and a crystal region having a high dielectric constant containing quantum dots are periodically arranged. It will be.

Generally, in a photonic crystal, the distribution of the electric field of light is different in the upper and lower bands with the band gap sandwiched. In the lower band, the light electric field concentrates in a region having a higher dielectric constant of the photonic crystal, and in the upper band, the electric field of light concentrates in a region lower than that in the photonic crystal. These bands are called a dielectric band and an air band, respectively. As is clear from the principle of the photonic crystal, the characteristics can be made more remarkable by increasing the difference in dielectric constant between the constituent materials in the high dielectric constant region and the low dielectric constant region. Therefore,
As the periodically arranged materials having different dielectric constants, a semiconductor material having a dielectric constant ε of about 10 and a dielectric constant ε of about 1 are used.
The air is often used. The degree of localization of the optical field depends on the band gap with the lowest energy (this is
(It is called a band gap).

The principle of the present invention will be described with reference to FIG. FIG. 1 illustrates the three sides together. Figure 1 (a)
The region shows the dispersion of the photonic crystal and the change of the dielectric constant of the dielectric part, the region (b) shows the optical field intensity distribution in the photonic crystal, and the region (c) shows the absorption characteristics and the nonlinear coefficient of the quantum dot. It is a figure which shows change. The energy levels corresponding to A and B in the region (a) of FIG. 1 are drawn corresponding to each of the diagrams in each of the diagrams (b) and (c).

The region (a) of FIG. 1 shows the dispersion of a two-dimensional triangular air rod photonic crystal composed of air (in this case, permittivity ε = 1) and a dielectric and the permittivity of the dielectric part. An example of the change of is shown. In the photonic member, as shown in FIG. 1B, dielectric layer regions 26 and air regions (specifically, hole regions) 25 are periodically arranged. And in the photonic member of the present invention,
Quantum dots 26 are embedded in the dielectric region 26.

In the graph of FIG. 1A, the horizontal axis represents the wave number vector and the vertical axis represents the frequency. Curves 21 and 22 are characteristics in the air band, and curves 23 and 24 are characteristics in the dielectric band. In the figure, the solid line and the broken line are dispersions when the dielectric constants of the dielectric parts are different. The solid line shows the characteristic when the permittivity is ε, and the dotted line shows the characteristic when the permittivity changes to ε-Δ. In this example, as the dielectric constant changes,
It shows that the wave vector caused a change of Δk.

Further, in the area (b) of FIG. 1, (a) of FIG.
The electric power distribution inside the photonic member in A and B shown by ◯ in the region is shown in the portions shown by arrows. The upper row shows the characteristics of the air band, and the lower row shows the characteristics of the dielectric band.
The thick solid line characteristic curve 29 shows the intensity distribution of the optical electric field. The vertical axis represents the optical field intensity. The hatched portion in the region (b) of FIG. 1 indicates the photonic crystal dielectric portion 26, and the small ellipse therein is the quantum dot embedded in the photonic crystal dielectric portion, which is the feature of the present invention as described above. 26 is shown. Further, in the invention of the present application, in accordance with the gist thereof, the wavelengths of the control light and the signal light are respectively a dielectric band,
Set the structure of the photonic crystal as set in the air band.

FIG. 1 (c) shows absorption of quantum dots and 3
The following shows the change of the real part of the nonlinear susceptibility. Figure 1 (c)
The dotted line characteristic on the left side of indicates the signal light absorption characteristic. That is, an absorption peak occurs at the level above the dielectric band. The solid line characteristic on the left side of FIG. 1C shows the nonlinear susceptibility corresponding to the change of the coefficient with respect to the dielectric constant or the refractive index. This corresponds to a plus or minus nonlinear change centered on the position of 0. In the present invention, a non-linear change is used. That is, the change that contributes to the air band in the nonlinear change characteristic of FIG.
The wavelength range of absorption of the quantum dots is the wavelength range of control light, that is, A in FIG.
The energy level of the quantum dot is set to match the (dielectric band). Generally, the absorption peak including the non-uniform spread of the quantum dots and the control light are spaced by 10 me
V or less is often used.

In summary according to the configuration of FIG. 1, the optical control operation of the present invention is performed as follows. As shown in (c) of FIG. 1, the energy level of the quantum dot is set to match the energy of the control light (designated as B in (a) of FIG. 1). Therefore, the quantum dots are resonantly excited by the predetermined control light. As a result, as illustrated in FIG. 1C, the nonlinear susceptibility of the quantum dot is changed. In this way, the dielectric constant of the dielectric portion including the quantum dots of the photonic crystal changes. A solid line and a broken line shown in FIG. 1A schematically show this change. That is, the characteristics of the solid line have changed to those of the dotted line. Δk in the figure is a normalized phase change corresponding to this change in the dielectric constant. This phase change makes it possible to control the signal light (this signal light is indicated by A in FIG. 1A).

In the present invention, quantum dots are used as the nonlinear material for the following reasons. In general, of the two bands sandwiching the band gap of the photonic crystal, the band on the high energy side has an air band-like property and the band on the low energy side has a dielectric band-like property. In the present invention, the resonance excitation of the control light whose wavelength is set in the dielectric band is used to change the dispersion of the air band. Therefore,
If bulk crystal, quantum well, etc. are used as the nonlinear material,
The air band on the higher energy side than the dielectric band has large absorption. Therefore, the signal light undergoes a large absorption loss.

On the other hand, as shown in FIG. 1C, the quantum dots have a delta function-like density of states. Therefore, the signal light can propagate without being absorbed by the quantum dots, even though the signal light is set to a higher energy side than the control light set to the energy level of the quantum dots. This example relates to the basic configuration of the present invention.

Another typical form of the present invention is that an input surface on which an optical signal light and a control light having a wavelength different from this are incident on one end surface of the photonic member, and the photonic member. The other end surface has an output surface of the optical signal light and the control light, an optical waveguide optically connected to the input surface of the photonic member, and the photonic member is a photonic crystal. A region and a region in which the quantum dots are arranged in the high dielectric portion of the photonic crystal, and the energy level of the quantum dots and the dielectric band of the photonic crystal are set in the wavelength region of the control light, and the signal An optical control element is characterized in that an air band of a photonic crystal is set in the wavelength range of light. By further providing the photonic member according to the present invention with an optical waveguide that is optically connected, various applications are facilitated. For example, a Mach-Zender element described later is an example.

Still another embodiment of the present invention is characterized in that at least a pair of side surfaces of the photonic member parallel to the light traveling direction are surrounded by another material having a different effective refractive index to form an optical waveguide structure. It is a light control element. Furthermore, an optical waveguide structure is formed by surrounding at least a pair of side surfaces of the photonic member, which intersect the surface of the substrate of the light control element and are parallel to the traveling direction of light, with another material having a different effective refractive index. It is useful. The control light and the signal light are efficiently guided by this optical waveguide structure.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION In order to describe more concrete embodiments and application forms of the present invention, details relating to the embodiments of the present invention will be listed and explained in advance.

First, as the photonic crystal according to the present invention, it is sufficient to use a usual photonic crystal. Photonic crystals include one-dimensional, two-dimensional, and three-dimensional crystal structures. A typical example of the one-dimensional crystal is one in which dielectric thin films are laminated in multiple layers. Thus
A periodic structure with a dielectric constant is formed in the stacking direction. A typical example of the two-dimensional crystal is one in which periodic holes are opened in a semiconductor crystal. A periodic structure of permittivity is formed in the plane. As a semiconductor crystal, silicon, a compound semiconductor material such as GaAs, or III-V such as InP
Group compound semiconductor materials can be used. Furthermore, as for the three-dimensional crystal, for example, it is known that the in-plane periodic structure is periodically shifted and laminated. This example is, for example, what is called autocloning. In this structure, a periodic structure having a dielectric constant in all directions is possible.

As the quantum dots themselves, usual ones can be used. For example, this is one of the self-organized growth methods.
It can be created using an i-Kranstanow method, a Volmer-Weber method, or the like. The size thereof is also the usual one. For example, by supplying InAs on GaAs, for example, about 2 monolayers, a height of 3 nm-10 nm,
Quantum dots having a density of about 10 ⁹ -10 ¹¹ / cm ² can be produced.

Various properties relating to the photonic member of the present invention can be set by adjusting the rod period, air rod / dielectric rod selection, propagation direction selection, polarization selection, photonic crystal lattice selection, and combinations thereof. In this way, it is possible to set the wavelength range of the control light and the signal light and the band structure of the photonic crystal. That is, the position and size of the band gap can be adjusted by changing the pitch and size of the periodic holes. Furthermore, the position and size of the band gap can also be adjusted by selecting, for example, a square lattice, a hexagonal lattice, or a honeycomb type. The various characteristics are set, adjusted, or selected according to the characteristics required for the light control element of the present invention.

Further, in the example of FIG. 1, the brilliant zone end of the photonic band is set in the wavelength range of the signal light and the control light in the light control element. Further, in addition, it is preferable that, in the light control element, a region where the group velocity of the photonic band is small is set in the wavelength region of the signal light and the control light. The adjustment of the brilliant zone end of the photonic band can adjust the position of the band gap and the brilliant zone end by changing the pitch and size of the periodic holes. Generally, the group velocity is small at the edge of the Brilliant zone. In the present invention, the group velocity is preferably in a region smaller than the characteristic of the bulk crystal. For example, it is a typical application example of the present invention.
The phase change, which is the main characteristic of Mach-Zehnder type optical switches and the like, is inversely proportional to the group velocity of light. Therefore, for this purpose,
The smaller this group velocity, the more preferable. The group velocity cannot be adjusted with a bulk crystal, but the photonic member of the present invention can be adjusted in this way.

When the present invention is applied, it is possible to control the signal light extremely efficiently by using the effects based on the configurations (a) to (h) below. The details will be described below. Incidentally, the incident system of the signal light to the photonic member and the control system of the control light may be those of the conventional light / light control element. (A) First, it is the effect of using the quantum dots. Since the quantum dots have a large optical nonlinearity due to the concentration of the density of states, the effect of increasing the phase change, which is the object of the present invention, can be enhanced. (B) Quantum dots have a delta function-like density of states.
Therefore, when the propagation loss of the signal light is suppressed to a constant level, the detuning width between the signal light and the control light can be reduced. The non-linear optical effect brought about by the quantum dots increases as the detuning width decreases ((c) in FIG. 1), so that the non-linear optical effect received by the signal light can be increased.

Next, the increase in the optical nonlinear effect caused by the photonic crystal will be described. FIG. 2, FIG. 3, and FIG. 4 exemplify calculated values of the wave number dispersion of the photonic crystal, the group velocity dispersion, and the phase change induced by the refractive index change of the photonic crystal dielectric portion, respectively. In each figure, the horizontal axis represents energy (E) and the vertical axis represents the respective characteristic values.

Here, a two-dimensional triangular air rod composed of air (dielectric constant ε = 1) and a dielectric is shown as a photonic crystal. The direction of the wave vector (light propagation direction) is the Γ-X direction, and the period of the air rod is 0.27 μm. For comparison, the case of a bulk structure is also shown. The characteristics in the case of the bulk structure are shown by dotted lines in FIGS. 2, 3 and 4 and are indicated by “homo”.

By referring to FIG. 4, it will be understood that the change in the wave number of the signal light is largely secured by the change in the dielectric constant. (C) Further, in the present invention, it is preferable to use a region having a small group velocity or a brilliant zone end for signal light and control light. This will be easily understood from the characteristic of FIG. As shown in FIG. 4, the phase change induced by the change in the refractive index of the photonic crystal dielectric part can be increased. 2 compared to the case of bulk structure
It is possible to increase more than double. (D) Furthermore, by using a region having a small group velocity for the signal light, the interaction between the signal light and the photonic crystal can be increased. (E) Further, by setting the control light in the dielectric band, it becomes possible to concentrate the electric field of the control light in the photonic crystal dielectric portion including the quantum dots, and as a result, the quantum dots can be efficiently used. It can be excited ((b) of FIG. 1). (F) Further, by setting the signal light in the air band, it becomes possible to reduce the electric field of the signal light in the photonic crystal dielectric portion including the quantum dots. The absorption loss can be reduced ((b) of FIG. 1). Therefore, when securing a constant signal light, the detuning width of the signal light and the control light can be reduced. The non-linear optical effect brought about by the quantum dots increases as the detuning width decreases ((c) in FIG. 1), so that the non-linear optical effect received by the signal light can be increased. (G) In the present invention, in order to control the signal light more efficiently, the polarization of the signal light and the control light can be made different in the light control element.

This principle will be described using the wave number / frequency dispersion characteristics of FIG. The example in the figure shows two orthogonal polarization modes T of a two-dimensional triangular array air rod photonic crystal.
It is the dispersion for E and TM. The horizontal axis represents the wave number and the vertical axis represents the frequency. The solid line shows the characteristics for TE polarization and the dotted line shows the characteristics for TM polarization.

In the application of the present invention, when different polarizations are not used, the white dot area of the TM mode dielectric band and the black dot area of the air band indicated by the dotted line in FIG. 5 are the control light and the signal light, respectively. Equivalent to using. Figure 5
In this case, the control light: TM / signal light: TM is indicated.

On the other hand, when different polarizations are used, the TE mode and the white dot area of the dielectric band are used as the control light, and the black dot area of the TM mode air band is used as the signal light. As a result, the detuning width of the control light and the signal light can be reduced, so that the non-linear effect on the signal light can be increased for the above reason. (H) Further, the degree of concentration of the electric field on the photonic crystal dielectric portion tends to increase as the band gap increases. Therefore, the quantum dots can be excited more efficiently by using the TE mode dielectric band. As described above, by using different polarized light, the degree of freedom in design is increased, and the nonlinear effect can be enhanced more effectively.

Next, the advantage of using quantum dots will be shown from another aspect. (I) By using the quantum dots, the quality of the nonlinearity inducing portion can be improved. The repeating interval of the periodic structure of the photonic crystal is submicron. In the present invention, photocarriers are generated in the high dielectric portion of the photonic crystal by resonance excitation with control light. The interface between the high-dielectric part and the low-dielectric part (including the case of air as the low-dielectric part) is
Since it usually has many defects, it becomes a recombination center of photocarriers when this dielectric portion is composed of a bulk crystal or a quantum well. Therefore, the generated photocarriers disappear at the interface, and a sufficient nonlinear effect cannot be obtained. On the other hand, the quantum dots are sufficiently smaller than the repeating interval of the periodic structure of the photonic crystal, and the photocarriers generated inside the quantum dots do not reach the interface due to the confinement effect of the quantum dots. Therefore, the photo carrier generated by the control light contributes to effectively control the signal light. (J) Further, in the present invention, in the light control element, it is preferable to set the high-order energy level of the quantum dot to the wavelength of the control light.

The relaxation rate of the carriers in the quantum dot is higher in the higher energy levels than in the first excitation level.
This is because radiative recombination is the main process in the carrier relaxation at the first excitation level, but in the higher-order excitation level, in addition to the radiative recombination process, there is a faster intra-band radiationless relaxation process. Because it becomes dominant. Usually, the absorption peak including the non-uniform spread of the quantum dot and the control light are spaced by 10 meV.
The following are often used:

Another mode of the present invention can provide a waveguide type optical control element. That is, this mode has an input end for signal light and control light, an optical waveguide structure for guiding the signal light and control light, and an output end for signal light and control light. Is.

Further, in order to improve the performance of the above-mentioned waveguide type light control element, it is preferable to set the photonic band of the photonic crystal so that the group velocities of the signal light and the control light are substantially matched. Inside the photonic crystal, the group velocity of light differs depending on the band type and energy as shown in FIG. In order to control the signal light on which the broadband signal is superimposed in the waveguide by the control light, it is necessary to make the group velocities of the signal light and the control light match.

In order to switch a particular pulse in the signal light pulse train, the control light pulse must act only on that particular signal light pulse. Both the signal light pulse train and the control light pulse propagate in the nonlinear waveguide. Therefore, it is necessary that the delay time of both caused by the propagation speeds of both is within one pulse of the signal light.

A basic example of how to construct the control light, the signal light, the dot energy, and the photonic crystal structure in order to match the group velocities of the signal light and the control light in this way will be described below. To do. Set the wavelengths of control light and signal light. (2) The material, size, etc. of the quantum dot are designed so that the absorption peak of the quantum dot coincides with the wavelength of the control light within 10 meV. (3) The photonic member is designed so that the dielectric band of the photonic member and the vicinity of the band end of the air band match the control light and the signal light set in the item (2) within 10 meV. Further, the energy at the band ends of the dielectric band and the air band is adjusted by adjusting the pitch of the holes of the photonic member, the size of the holes, the rod cycle, the air rod /
Dielectric rod selection, propagation direction selection, polarization selection, photonic crystal lattice (square lattice, hexagonal lattice, etc.) selection,
And by combining these, adjustment and setting can be performed. (4) From the areas adjusted in the item (3), an area in which the group velocities of the signal light and the control light match is set within the required tolerance. FIG. 3 illustrates an example of such group velocity dispersion. (5) Furthermore, in accordance with the request for the characteristics of the light control element, the steps (3) to (4) can be repeated to set a more appropriate structure.

Further, as the optical waveguide structure of the present invention, a refractive index guide type optical waveguide structure as illustrated in FIG. 6 is preferable. This structure is a structure in which the photonic member 61 is surrounded by another material 62 whose effective refractive index is different from that of the photonic member 61, that is, a so-called cladding layer. Incident light 63 enters the photonic member 61. This structure can provide a waveguide type optical control element. That is, in this embodiment, one end surface of the photonic member has an incident surface on which the optical signal light and control light having a wavelength different from this are incident, and the other end surface of the photonic member has an emission surface. And the photonic member has a photonic crystal region and a region in which quantum dots are arranged in a high dielectric portion of the photonic crystal,
And the energy level of the quantum dot and the dielectric band of the photonic crystal are set in the wavelength range of the control light, and the air band of the photonic crystal is set in the wavelength range of the signal light, and for the signal light and the control light. Has an input end and an optical waveguide structure for guiding signal light and control light, and an output end for signal light and control light, and surrounds the photonic member with another material having a different effective refractive index. Therefore, it is an optical waveguide structure.

It is preferable to use self-assembled quantum dots as the quantum dots used in the present invention. That is, in this embodiment, one end surface of the photonic member has an incident surface on which the optical signal light and control light having a wavelength different from this are incident, and the other end surface of the photonic member has an emission surface. And the photonic member has a photonic crystal region and a region in which quantum dots are arranged in the high dielectric portion of the photonic crystal, and the energy level of the quantum dots in the wavelength range of the control light and The dielectric band of the photonic crystal is set, the air band of the photonic crystal is set in the wavelength range of the signal light, and the quantum dots are self-assembled quantum dots.

As a material for the photonic crystal and quantum dots used in the inventions of the present application, it is useful to use a compound semiconductor in the photonic crystal dielectric portion and quantum dots.

Further, as is apparent from the above description, the light control element of the present invention has a great effect on the phase modulation of the signal light by the control light. Therefore, in the light control elements of the inventions of the present application, it is preferable and extremely useful that the basic operation is to modulate the phase of the signal light by the control light.

Next, an example in which the light control element of the present invention is applied to a Mach-Zehnder type all-optical switch will be described. FIG. 7 shows an overview of the Mach-Zehnder type all-optical switch seen from above. 8 and 9 are enlarged perspective views of the input waveguide section and the non-linear waveguide section, respectively, and FIGS. 10 and 11 are ab section in FIG. 8 and a cd section in FIG. 9, respectively. The sectional view shown is shown.

First, the outline of the structure of the device of the present invention will be described. In the device of this example, as shown in FIG. 7, a nonlinear waveguide part having two nonlinear waveguides 71 and 72 is arranged at the center of the device, and three optical input ends 73, 74, 75 and 3 are provided on both sides of the nonlinear waveguide part.
an input waveguide section 78 having a dB coupler waveguide structure 80,
Two output ends 76, 77 and 3 dB coupler waveguide structure 81
Is a Mach-Zehnder interferometer structure including an output waveguide section 79 having

Next, the operating principle of this device will be described. The light incident from the signal input end 74 of the input waveguide section is demultiplexed by the 3 dB coupler 80 of the input waveguide section, passes through different nonlinear waveguides 72, respectively, and then enters the 3d section of the output waveguide section.
The signals are multiplexed by the B coupler 81. When there is no control light input from the control light input terminal 73, the phases of the two lights to be combined are the same, so that the combined light is equivalent to the light of the output waveguide section 2.
The light is emitted from the upper port of the two output terminals 76 and 77.

On the other hand, when the control light is input from the control light input end 73, the control light passes through the upper nonlinear waveguide 71, so that the refractive index of the upper nonlinear waveguide 71 is changed. When the signal light is made incident in this state, a difference occurs in the phases of the two lights multiplexed by the 3 dB coupler 81 in the output waveguide section. When the phase difference between the two becomes π, the multiplexed signal light is emitted from the lower port 77 of the two output ends of the output waveguide section. In this way, the switch operation for switching the output end of the signal light by the control light can be performed.

Next, the detailed structure of the device of this example will be described.
The guidelines for the device structure design here are (1) the dispersion of the photonic crystal and the matching of the energy levels of the quantum dots, and (2) the single propagation mode. The basic structure of the output waveguide section 79 is similar to that of the input waveguide section 78. As described above, FIGS. 8 and 9 are enlarged perspective views of the input waveguide portion and the non-linear waveguide portion, and FIGS.
8A and 8B are cross-sectional views showing an ab cross section in FIG. 8 and a cd cross section in FIG. 9, respectively. Description will be given with reference to these figures.

In both FIGS. 8 and 9, the region 1 to which batching is applied is an optical waveguide portion (propagation portion) having a film thickness of 3 μm, a width of 1 μm, and a length of 10 mm. It has a photonic crystal structure with holes formed. This photonic crystal is a two-dimensional triangular array air rod composed of air (dielectric constant ε = 1) and a dielectric. The propagation direction of the optical waveguide is in the Γ-X direction, the period of the air rod is 0.27 μm, and the ratio of air in the photonic crystal (air filling factor) is 58%. The dispersion of TM mode in the Γ-X direction of this photonic crystal is
It corresponds to FIG. The optical waveguide is made of GaAl with a thickness of 1 μm.
An As lower clad (GaAs mixed crystal ratio = 0.8) 2 is formed on the undoped GaAs substrate 3, and a polyimide clad 4 is formed on both sides of the optical waveguide portion.

The details of the optical waveguide portion are shown in the sectional view of FIG. The optical waveguide is composed of a GaAlAs upper clad (GaAs mixed crystal ratio = 0.85) 6 with a film thickness of 1 μm and a G with a film thickness of 1 μm.
It is composed of an aAs optical waveguide 7 and a GaAlAs lower clad (GaAs mixed crystal ratio = 0.85) 8 having a film thickness of 1 μm. It is sufficient to use a customary structure for such an optical waveguide. With the above structure, the transverse mode of light propagating through the waveguide in both the input / output waveguide section and the nonlinear waveguide section becomes a single mode in both the vertical and horizontal directions.

The propagating light is confined in the GaAs optical waveguide 7 by about 80% in the vertical direction and by about 90% in the GaAs optical waveguide 7 in the horizontal direction. Here, in the nonlinear waveguide portion, the quantum dots 9 made of InAs in the high dielectric portion of the photonic crystal, which is a feature of the present invention, are arranged. Here areal density 1 × ^{10 10} cm ^{- 2} 100 quantum dots
Thirty layers were laminated at an interval of nm. The absorption peak wavelength of the quantum dots was 1.25 μm. In order to compare and evaluate the characteristics of this device, a sample without a photonic crystal or quantum dot as a nonlinear waveguide, that is, an optical waveguide having the same structure as the optical waveguide of the input / output waveguide in the nonlinear waveguide A comparative sample with was prepared.

Next, a procedure for manufacturing this device will be described. Here, the input / output waveguide section and the nonlinear waveguide section are individually manufactured and hybrid-mounted. Of course, these may be manufactured monolithically.

First, a method of manufacturing the input / output waveguide section and the non-linear waveguide section of the comparative sample will be described. MB for crystal growth
The E crystal growth method was used.

GaAlA is formed on the semi-insulating GaAs substrate 3.
s Lower cladding (GaAs mixed crystal ratio = 0.8, film thickness 1μ
m) 2, GaAlAs lower clad (GaAs mixed crystal ratio =
0.1, film thickness 1 μm 8, GaAs optical waveguide (film thickness 1 μm
m) 7, GaAlAs upper cladding (GaAs mixed crystal ratio =
(6) having a thickness of 0.1 μm and a thickness of 1 μm were sequentially grown. The growth temperature and arsenic pressure were 560 ° C. and 1 × 10 ⁻⁵ Torr.
Wet etching with a sulfuric acid-based solution was used for forming the shape of the optical waveguide. After applying the polyimide film by the spin coating method, the back surface of the wafer was polished to a film thickness of 150 μm. After that, it was cleaved to a desired size to prepare an input / output waveguide section and a non-linear waveguide section of a comparative sample.

Next, a method of forming the nonlinear waveguide portion of the present invention will be described. The difference from the method of forming the input / output waveguide section will be shown below. Customary waveguides may follow customary methods.

In the crystal growth, the GaAlAs lower cladding is used.
(GaAs mixed crystal ratio = 0.8, film thickness 1 μm) 2 was grown,
The optical waveguide part 1 including the quantum dots made of InAs is formed.
It was InAs quantum dots with growth temperature of 480 ℃
It grows up to a volume of 2.0 ML and grows in GaAlAs (upper and lower
Embedded) and GaAs (optical waveguide) 100 nm
It is. Then, this process was repeated 30 times. Waveguide shape
After formation, dry etching using chlorine-based gas
A photonic crystal structure was formed. Dry etching pattern
The mask for forming the tongue has two layers of Ni and electron beam resist.
The film is used and the main dry etching conditions are a substrate temperature of 50.
℃, acceleration voltage 500V, gas pressure 1 × 10 ^-FiveWith Torr
did.

Next, the optical measurement method for measuring the optical characteristics and the measurement results will be described. For the measurement, control light having a pulse width of 1 ps is input from the control light input end of the device shown in FIG. 7, and continuous light is input from the signal light input end. Then, the output from the signal light output end was observed with a synchro scan streak camera. Based on this observation, the switching light power (π shift) was defined as the control light input that maximizes the output modulation. As the light source, the photonic crystal of this experiment, and for the measurement of the sample containing the quantum dots, the output light of the CW and mode-locked titanium sapphire laser was converted by an optical parametric oscillator to a wavelength of 1.1 μm-1.4 μm. Light was used. A CW and mode-locked titanium sapphire laser was used as the light source of the comparative sample containing no photonic crystal or quantum dot. The wavelength of this laser was 880 nm-920 nm.

In the Mach-Zehnder device of the present invention, the control light of TM polarization is 1.25 μm, and the wavelength is 1.15.
When signal light of μm and TM polarization is used, the power for the π shift control light is 1200 fJ and the switch speed is 1 n.
s characteristics were obtained. The optical power shown here is a substantial switch power obtained by separately measuring the coupling efficiency of the optical waveguide, the reflection at the element incident end, and the reflection at the connection end, and subtracting them. Further, when the control light having the wavelength of 1.21 μm and the TE polarization, and the signal light having the wavelength of 1.15 μm and the TM polarization are used, the power of the π shift control light is 200 fJ and the switching speed is 1 ns. . It is considered that the improvement in the power consumption of the control light is due to the effect described with reference to FIG. 5 and the anisotropy in the light absorption of the quantum dots.

In the comparative element, the wavelength is 0.88 μm.
When m, TE-polarized control light, and wavelength of 0.92 μm and TE-polarized signal light were used, the characteristics of the π shift control light were 6200 fJ and the switch speed was 1 ns.

From the above three types of measurement results, it will be fully understood that the control light power can be significantly reduced by the structure of the present invention in which the fatonic crystal and the quantum dots are combined.

The effect of improving the response speed by exciting the higher-order energy levels of the quantum dots was examined as follows. The element structure and the manufacturing method are basically the same as those of the above-described sample. Here, in order to reduce the energy level of the quantum dots, heat treatment was performed at 780 ° C. for 30 minutes after MBE growth. As a result, the lowest excitation energy level of the quantum dots changed from 1.25 μm to 1.30 μm,
The excitation level became almost 1.25 μm.

Using this device, the same optical evaluation as described above was performed. This device has an extremely high-speed response with a π-shift control optical power of 400 fJ and a switch speed of 20 ps for TE-polarized control light with a wavelength of 1.24 μm and TM-polarized signal light with a wavelength of 1.20 μm. It could be confirmed.

In this example, an all-optical switch using a Mach-Zehnder interferometer is shown. The present invention can efficiently increase the phase change induced by light. Therefore, it is needless to say that the present invention can be applied to an element using another phase change, such as a directional coupler type optical modulator, or an optical control element using an etalon.

[0059]

INDUSTRIAL APPLICABILITY The present invention can reduce the electric power of the control light used for controlling the signal light in the all-optical / optical control element to a practical level.

[Brief description of drawings]

FIG. 1 is an explanatory diagram for explaining the principle of the invention.

FIG. 2 is a diagram showing an example of a relationship between light energy and wave number dispersion in a photonic crystal.

FIG. 3 is a diagram showing an example of a relationship between light energy and group velocity dispersion in a photonic crystal.

FIG. 4 is a diagram showing an example of a relationship between light energy and a change in wave number induced by a change in dielectric constant in a photonic crystal.

FIG. 5 is a conceptual diagram showing an example of light / light control using different polarizations.

FIG. 6 is a perspective view showing an example of a refractive index guide type optical waveguide using a photonic crystal.

FIG. 7 is a plan view showing an example of a Mach-Zehnder type all-optical switch.

FIG. 8 is a perspective view of an input / output unit of an example of a Mach-Zehnder type all-optical switch.

FIG. 9 is a perspective view of a nonlinear waveguide section of an example of a Mach-Zehnder type all-optical switch.

10 is a cross-sectional view taken along line ab of FIG. 8 showing an input / output unit of an example of a Mach-Zehnder type all-optical switch.

11 is a sectional view taken along line cd of FIG. 9 showing a non-linear waveguide portion of an example of a Mach-Zehnder type all-optical switch.

[Explanation of symbols]

1: optical waveguide part, 2: lower clad, 3: substrate, 4: clad, 5: photonic crystal air hole, 6: upper clad, 7: optical waveguide, 8: lower clad, 9: quantum dot, 25: air , 26: Dielectric, 27: Air band, 28: Dielectric band, 29: Distribution curve of optical electric field intensity, 61: Photonic crystal, 62: Cladding region, 6
3: incident light, 71, 72: non-linear waveguide, 73: control light input end, 74: signal light input end, 75: light input end, 76,
Reference numeral 77: signal light output end, 78: input waveguide portion, 79: output waveguide portion, 80, 81: 3 dB coupler.

─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shigeru Kawamoto 5-7-1, Shiba, Minato-ku, Tokyo NEC Corporation (72) Inventor Kiyoshi Asakawa 5-7-1, Shiba, Minato-ku, Tokyo NEC Incorporated (72) Inventor Niclas Carlson, 5-5 Tokodai, Tsukuba City, Ibaraki Prefecture Technical Research Association, Femtosecond Technology Research Organization (56) Reference JP 10-90634 (JP, A) (58) Survey Areas (Int.Cl. ⁷ , DB name) G02F 1/29-7/00

Claims (5)

(57) [Claims] 1. An input surface on which optical signal light and control light having a wavelength different from this are incident on one end surface of the photonic member, and the optical signal light on the other end surface of the photonic member. The photonic member has an output surface, and the photonic member has a photonic crystal region and a region in which quantum dots are arranged in a high-dielectric part of the photonic crystal, and the photonic member has a quantum dot in the wavelength range of the control light. An optical control element characterized in that an energy level and a dielectric band of a photonic crystal are set, and an air band of the photonic crystal is set in a wavelength range of the signal light. 2. An input surface on which an optical signal light and a control light having a wavelength different from this are incident on one end surface of the photonic member, and the optical signal light on the other end surface of the photonic member. An output surface, and an optical waveguide optically connected to the input surface of the photonic member, wherein the photonic member includes a photonic crystal region and a high dielectric portion of the photonic crystal. It has a region where dots are arranged, and sets the energy level of the quantum dot and the dielectric band of the photonic crystal in the wavelength range of the control light, and sets the air band of the photonic crystal in the wavelength range of the signal light. A light control element characterized by the above. 3. The optical waveguide structure according to claim 1, wherein at least a pair of side surfaces of the photonic member parallel to the light traveling direction are surrounded by another material having a different effective refractive index to form an optical waveguide structure. 2. The light control element according to 2. 4. The polarization according to claim 1, wherein the polarization of the signal light and the polarization of the control light are different from each other.
The light control element according to any one of 1. 5. The respective group velocities of the signal light and the control light are equal to each other in delay time within 1 pulse of the signal light when the signal light and the control light pass through the photonic member. The photonic band of the photonic member is set so that the light control element according to any one of claims 1 to 4.