JP3022730B2 - Optical structure - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical signal processing element used in the field of optoelectronics, which can process and control light as a signal medium, or a material of these elements, or a return light of a semiconductor laser or an optical amplifier. The present invention relates to an optical structure having a non-reciprocal optical effect useful as a material for blocking light and the like.

[0002]

2. Description of the Related Art In recent years, in the field of communication technology and computer technology, in order to enable large-capacity and ultra-high-speed signal processing, optoelectronic technology for replacing a part or all of a signal medium from a conventional electric signal to an optical signal has been developed. It is developing rapidly. With the development of such optoelectronic technology, it is necessary to be able to freely control and process the flow of optical signals as in the case of conventional electronics using electric signals. That is, there is a need for techniques such as switching light paths and converting wavelengths, such as light exchange and light modulation.

[0003] In general, when the light intensity is less than a certain value, the light does not interact with each other even in a substance as well as in a vacuum, and exhibits only a linear behavior that the output intensity is proportional to the input intensity. Such a response of light is called a linear response, but in a situation where only this linear response occurs, the light cannot be processed and controlled. However, in the case of light exceeding a certain intensity, the effect of a change in the state of a substance due to this light returns to the light itself, or a change in the substance causes an interaction with another light that is different, thereby causing a nonlinear effect. Input / output characteristics. Such a response is called a non-linear response. In addition, light input / output characteristics can be changed by applying a physical quantity such as an electromagnetic field to a substance. This phenomenon can also be broadly called a nonlinear response.

Various types of non-linear optical elements utilizing this non-linear property have been proposed so far as having functions such as light exchange and light modulation. In the control of an optical signal, it may be necessary to transmit only light from one direction and block light in the opposite direction.However, an element having such a function is required to be a material such as the Faraday effect. Or by using the magnetic properties.

[0005]

As described above, in the optoelectronics technology, an element for freely processing and controlling an optical signal is required. One of the functions of such an element is that an optical signal from one direction is required. There is a case where an element having a rectifier property to light that transmits only light and blocks light in the opposite direction is required. This element is used to block return light in order to stabilize the oscillation mode of a semiconductor laser or the operation of an optical amplifier, or to rectify light in an integrated circuit that mixes optical and electrical signals, so-called OEIC. Is useful for However, the conventional techniques have the following problems.

That is, when a substance has no magnetic properties at all, the light transmittance is the same regardless of whether the light enters from the front or back of the substance within the range of linear response, which is a so-called reciprocity. There is a principle of sex. Therefore, in order to realize a non-reciprocal element exhibiting a non-reciprocal response contrary to this principle, in principle, it is conceivable to use some kind of non-linear effect or use a substance having magnetic properties. . However, regarding the former, at present, no effective method for producing a non-reciprocal element using the nonlinear effect has been known. As for the latter, there are optical isolators that use the Faraday effect, which is one of the magnetic effects, to rotate the polarization of light and realize a nonreciprocal effect. In such a situation, it is impossible to produce a fine device as used in a semiconductor laser, and it cannot be monolithically fabricated as an optical isolator of a semiconductor laser. That is, as for the non-reciprocal element as an optical element to be replaced with a highly integrated electronic element, a method of manufacturing the non-reciprocal element has not been known so far.

Accordingly, the present invention overcomes the limitations of the above-described conventional techniques, realizes a novel non-reciprocal optical element as a configuration based on a new theoretical consideration, and realizes a novel optical element such as OEIC. It is an object to enable an optical rectifier, an optical isolator, and the like that can be used.

[0008]

In Means for Solving the Problems] The present invention, as to solve the above problems, in the thin film having an optically active portion having a non-linear optical properties comprising a substance such as a dielectric or semiconductor, the film surface of the thin-film A material layer different from the material constituting the surface defect or the optically active portion, or a single or plural delta doping layer, or even a mixed state is inserted at a position that is asymmetric with respect to the vertical axis direction. An optical structure having non-reciprocal optical characteristics (claims 1 to 3).
2) is provided.

Further, the present invention provides the above thin film,
An optical structure with non-reciprocal optical characteristics in which one or more potential walls are provided for the movement of electrons, holes, or excitons, which are composite particles of these, at asymmetric positions on the front and back of the optically active part. (Claims 3 and 4) are provided. Still further, according to the present invention, in the thin film, the axial structure of the thin film is asymmetrical, and the spatial distribution of the internal photoelectric field due to the light incident into the film in the axial direction of the thin film is such that light is incident from the front. And an optical structure having non-reciprocal optical characteristics (claim 5).

That is, in the present invention, as the above-mentioned structure, a structure in which the amplitude and spatial distribution of the internal photoelectric field are significantly different depending on the incidence from the back and the incidence from the front, and a remarkable structure utilizing an efficient nonlinearity The object has been achieved by providing an ultrafine element having a nonreciprocal optical effect.

[0011]

First, the principle for realizing an optical structure having a nonreciprocal optical effect will be described. Generally, the motion of light is determined by the following Maxwell equation.

[0012]

(Equation 1)

Here, the function 感 is a susceptibility indicating how much polarization is induced with respect to the external field, χ ⁽¹⁾ ,
χ ⁽²⁾ and χ ⁽³⁾ are the primary, secondary and tertiary susceptibilities, respectively. Normally, a susceptibility of second order or higher is called a non-linear susceptibility. Ω, ω _i , ω _j , ω _k are the frequencies of the different types of incident light. Here, if the light intensity is equal to or less than a certain value, there is a substantial contribution only up to the first term on the right side, and higher-order terms may be ignored. In this case, the induced polarization is proportional to the optical field and only the linear response described above occurs. On the other hand, if the light intensity increases, contributions from the second item onward occur, and a non-linear response occurs. The terms after the second item are called nonlinear polarization, and the magnitude of the nonlinear effect is determined by the magnitude of the nonlinear polarization. However, in a substance having ordinary inversion symmetry, the second item becomes zero due to this symmetry, and the maximum nonlinear effect starts from the third order.

Equation (2) shows that the magnitude of the nonlinear polarization is determined by the nonlinear susceptibility and the magnitude of the internal photoelectric field. The magnitude of the former nonlinear susceptibility and the energy spectrum are determined only by the electronic state inherent to the substance without external perturbation. To obtain a large nonlinearity, use the energy near resonance to the excitation energy of the electronic system. There is a need. However, the magnitude of the susceptibility does not depend on how light enters. On the other hand, as will be described later, the internal electric field depends on whether light is incident from the back side or from the front side when the material has an internal structure that is asymmetric with respect to the inversion of its front and back.
Their amplitude and spatial distribution are generally different. Therefore, a substance having a structure in which the internal photoelectric field due to light in the energy region near the resonance at which the nonlinear susceptibility becomes large has a significant asymmetry with respect to the incidence of the front and back will exhibit significant nonreciprocity due to the nonlinear effect. In the present invention, this fact is positively utilized to solve the problems of the prior art as described above.

Regarding a structure showing a remarkable asymmetry of the internal photoelectric field with respect to incidence from the front and back, there has been no idea of realizing a non-reciprocal element in the past, and no method has been known so far. . Therefore, in the present invention, a completely new structure for producing an ultra-fine size structure which has a remarkable asymmetry of the internal optical electric field with respect to incidence from the front and back and which can be used in an ultra-high-integrated circuit is provided. Provide a way.

That is, first, a reference (Phys. Rev. B48)
As described in (1993) 7960 H. Ishihara & K. Cho), in general, in a cohesive substance having a high atomic density such as a dielectric or a semiconductor, the energy excited by light causes the mutual energy between atoms (molecules) to change. It propagates through the medium by action. Therefore, the induced polarization at one point is not determined only by the optical field at that point, but is also affected by the optical fields at all other points. If this is written into the equation, for example, the induced polarization at the point r in the range of the linear response is the susceptibility where the induced polarization at the point r is a function of the coordinates r and r 'as shown in the first term on the right side of the equation (2). Through which the shape is affected by the photoelectric field at all other points r '. Such a relationship between the internal photoelectric field and the induced polarization is called a non-local relationship, such a substance is called a non-local medium, and a response occurring in such a substance is called a non-local response. In such a non-local medium, the induced polarization and the internal optical electric field are connected to each other in a non-local relationship, so that each of them performs a consistent motion with each other. As a result of this movement, a wave in which pure light and a polarized wave are coupled exists inside the medium. Such a coupled wave of polarized waves and light is called polaritons. The internal electric field E appearing in the Maxwell equation is an internal electric field due to polaritons in a nonlocal medium.

Considering an ultra-small system in which the wave function of the electron system inside the material is small enough to maintain coherence, for example, an ultra-thin film having a thickness of several tens to several thousand angstroms, The motion of the polarized wave in the direction perpendicular to the film surface is subjected to quantum confinement, and its wave function has a microscopic spatial distribution. On the other hand, since the internal electric field performs a motion consistent with the induced polarization, this spatial distribution also has a micro-scale. Thus, the internal optical electric field of incident light having energy that resonates with the excitation energy of the electronic system, i.e., the internal optical electric field of polaritons, is short, which is significantly different from the wavelength of light in a vacuum, usually on the order of thousands angstroms to microns. Has a wavelength. For this reason, even in an ultra-thin film, polaritons cause Fabry-Perot interference.

As described above, polaritons have been described as being coupled waves of polarized waves and light. However, the kinetic energy of a polariton in the direction perpendicular to the electron-based film, that is, the kinetic energy of polarized waves, When a structure that becomes a wall is formed by inserting a surface defect or a monoatomic layer film of a different material, the polarization wave component of the polariton is reflected by the wall, and the polariton itself is partially reflected. That is, in such a system, the polaritons exhibit a complex interference effect due to multiple scattering including not only reflection on the film surface of the thin film but also internal surface defects and reflection on a monoatomic layer film. If this surface defect or monoatomic layer film is formed at a position different from the center of the thin film, the way of interference differs between the case where light enters from the back and the case where light enters from the front, and the amplitude and spatial distribution of the internal photoelectric field Will be different. Even in such a case, if the incident light intensity is such that only a linear response occurs, the nonreciprocal effect does not appear due to the principle of reciprocity, but if the intensity is such that the nonlinear effect appears, as described above, Since the magnitude of the nonlinear response depends on the magnitude and the spatial distribution of the internal photoelectric field, a non-reciprocal effect appears in which the transmittance differs between incident from the back and incident from the front. In particular, in the case of incident light having energy that resonates with the excitation energy of the electron system of the substance, the nonlinear susceptibility originally increases resonantly and a large nonlinear effect appears, so that the asymmetric structure described in the above example is used. By making the film with a large difference in the magnitude and spatial distribution of the internal electric field between the front and the back, it has a large nonreciprocal effect and can be used in ultra-high integrated circuits, or a semiconductor laser Thus, an optical element having a fine structure that can be monolithically formed on such an element is realized, and the above-mentioned problem is solved.

The non-reciprocal optical structure according to the present invention comprises:
As described above, by using the principle newly discovered by the present inventor, since a non-reciprocal optical effect is configured to be exhibited in a fine structure, an element having an optical rectification function is superposed. An optical isolator can be monolithically used in a highly integrated optoelectronic circuit or in a semiconductor laser or optical amplifier so as not to hinder integration.

Hereinafter, the present invention will be described in more detail with reference to Examples.

[0021]

EXAMPLE 1 FIG. 1 is a perspective schematic view showing a non-reciprocal optical device according to the first embodiment of the present invention. An optically active thin film (1) made of a semiconductor, a dielectric, or the like has a nonlinear optical effect. FIG. 1 shows the incident light (2) carrying the signal, the incident light (2) carrying the signal, and the output light (3) passing through the incident light (2) carrying the signal, as well as the incident light (4) coming from behind and conforming to the signal. Is shown in A surface defect (5) or a monoatomic layer film or a delta doping of a different material becomes a potential wall for the movement of the electron system in a direction perpendicular to the film surface.
It is disposed at a position other than the center of the thin film. D in the figure is the thickness of the thin film (1) from several tens angstroms to several tens
A value of about 00 angstroms, and c is the distance from the surface of the thin film (1) to the surface defect (5). This thin film (1)
Are set such that the incident light (2) from the table (1) is transmitted but the incident light (4) from the back is not transmitted. Hereinafter, how these are set will be described using the optical response of the exciton system as an example.

First, the internal photoelectric field is calculated according to the linear response theory. The model of the theoretical calculation is as follows. That is, it is assumed that the thin film is made of a discrete lattice, and that each lattice point has an atom having one excitation level.
There is an interaction between each atom. It is also assumed that the electrons excited by each atom remain within that atom, and that the interaction between atoms occurs through dipole interaction. Assuming that a thin film is formed by such a discrete lattice, the 0th layer and the 1st layer are arranged in the atomic layer parallel to the film surface in order from the surface in the direction perpendicular to the film.
Layer, second layer,. . . Name it as the (N + 1) th layer. The optically active film itself is from the first layer to the Nth layer, and a boundary condition is assumed such that the amplitude of the polarization wave is zero in the 0th and (N + 1) th layers. Further, it is set so that only one of the first to Nth layers has a different atomic excitation energy. Although the internal photoelectric field and the linear and nonlinear response spectra are calculated using such a model, the following Hamiltonian is used as a starting point because quantum mechanical calculation is required because the model is a micro model.

[0023]

(Equation 2)

[0024] However, ε ₀ is the excitation energy, t is the energy of the interaction between atoms in each atom, b ⁺ _l is the quantum-mechanical operator to make the excitation to the l th layer, b _l is the l-th layer Is the quantum mechanical operator that extinguishes the excitation, and δ is the shift in the excitation energy of the atoms in the c-th atomic layer. This model is based on excitons, which are composite particles of electrons and holes, and is based on Frenkel excitons, which have no internal degree of freedom among excitons. However, the arguments developed here by this model are independent of the particularity of this model, and apply to other types of excitons. The layer (c-th layer) whose excitation energy is different from the other layers by δ, represented by the third term on the right side of the above equation (3), causes the polarization wave (exciton in this model) to move in the direction perpendicular to the film. On the other hand, it serves as a potential wall, and its origin can be various things such as a plane defect, a heterogeneous substance inserted in a plane, or a delta-doped layer.

The quantity to be calculated is the internal optical field, which is a non-local field as expressed by the above equations (1) and (2).
Obtained by solving the Maxwell equation. However,
Expressions (1) and (2) are expressions based on a continuum model, and therefore, here, those rewritten as discrete lattice expressions are used. In order to specifically calculate the susceptibility appearing in the equation based on the above model, the eigen energies and eigenfunctions of the ground state and excited state of this model must be obtained. (Depending on the model based on the Wannier exciton) (J. Phys. Soc. Jpn Vol.
60 (1991) 3920H. Ishihara & K. Cho). Next, it is a method of calculating the susceptibility from the obtained eigen energy and eigen function of the ground state and the excited state and solving the nonlocal Maxwell equation, but this is a simple method without a layer having different excitation energies. This is based on the model for thin films, but is described in the literature (J. Phy
s. Soc. Jpn. 55 (1986) 4113. K. Cho), and the calculation is performed based on that method. As a result of this calculation, a specific expression of the internal photoelectric field in the thin film is obtained.

FIGS. 2 (a), 2 (b) and 2 (c) show the spatial distribution in the thin film of the intensity I _j of the internal photoelectric field E _j in the thin film by numerical calculation based on this result. Here, the horizontal axis of (a), (b) and (c) in the figure is the number of each layer by a discrete lattice numbered from the front to the back of the thin film, and the vertical axis is 1 for the incident light intensity. The internal electric field light intensity in each layer at the time. In the figure, a curve (1) representing the value of the intensity of the internal photoelectric field and a position (2) of the potential wall with respect to the movement of the polarization wave, that is, a position of c in FIG. 1 are shown.
Also, the position (3) of the front 0th layer and the position (4) of the back 71st layer are shown. As the substance, a parameter was used assuming CuCl having a thickness of about 70 molecular layers (about 378 angstroms), and c was set to 50. The magnitude of the potential was 100 meV. In fact, the magnitude of this potential can be set variously depending on the kind and number of layers of a different material layer to be inserted, the kind of the substance to be doped by delta doping, or the selection of a plane defect.

The amount of 100 meV is within the range of common sense as the magnitude of the potential realized in this way. FIG. 2A shows a case where there is no potential wall. In contrast, it can be seen that the internal photoelectric field is significantly modulated as shown in FIGS. 2B and 2C when the potential wall is introduced. FIGS. 2B and 2C respectively show the spatial distribution of the internal photoelectric field intensity when light is incident from the front and when light is incident from the back. The value of the internal photoelectric field strength due to the incident light (4) on the back surface of FIG. 2B is equal to that of the outgoing light (3) at the front surface of FIG. It can be seen from the principle that the transmittances of the two are equal, but on the other hand, the location dependence and the amplitude in each case of the internal photoelectric field are shown in FIGS. 2 (b) and 2 (c).
It can be seen that they are significantly different. This is because, as described above, the manner of interference of the exciton polaritons differs between the two cases.

As described above, by optimally setting the magnitude and position of the potential of the potential wall, the amplitude and spatial distribution of the internal photoelectric field are significantly different between the incidence of light from behind and the incidence of light from the front. can do. Next, when the amplitude and spatial distribution of the internal electric field are significantly different due to the incidence of light from the back and the incidence of light from the front, the transmittance becomes different in both cases due to the nonlinear effect. This will be further described.

If the magnitude of the potential exceeds 100 meV, the effect of interference appearing in the spectrum of the exciton polariton becomes the same as in the case of an almost infinite potential. A non-local nonlinear Maxwell equation in which the equations (1) and (2) are rewritten into a discrete lattice model based on a model substantially similar to the above linear response is solved. Since the potential is infinite, a single thin film is 2
Considering the same situation where the electron wave function is zero at the boundary, the Maxwell equation is solved in each film, and the solution may be connected under the Maxwell boundary condition. The method of calculating the nonlinear response to solve the nonlocal Maxwell equation in the case of a single film is described in Phys. Rev.
B48 (1993) 7960.H. Ishihara and K. Cho (Solid)
State Commun. 89 837 (1994) .H. Ishihara and K. Ch.
o), and apply the same calculation method and model here.

FIG. 3 (a) shows the results obtained.
(B). 3 (a) and 3 (b) are transmittance spectra when a nonlinear effect is included. FIG. 3 (a) shows a case where light enters from the front, and FIG. 3 (b) shows a case where light enters from the back. The horizontal axis represents incident light energy and the vertical axis represents transmittance. The incident light intensity was 430 kW / cm ² . Material parameters are the same as in FIG.

Referring to FIGS. 3A and 3B, in the case of incidence from the back, since the amplitude of the internal photoelectric field is large, the nonlinear effect is greater than that of incidence from the front. It can be seen that remarkable non-reciprocity appears due to different transmittance. For this reason, for example, in the case of energy around 3.2029 eV (arrow in the figure), the former case is transmitted, but in the latter case, the transmittance is reduced due to the non-linear effect and the light is not transmitted.

As is apparent from the above results, a structure that becomes a potential wall against the motion of the polarization wave due to a plane defect or a planar insertion of a foreign substance is disposed in the thin film at an asymmetric position with respect to the front and the back. It can be seen that a remarkable non-reciprocal effect can be obtained with a thin film having an extremely small thickness of several hundred angstroms if the magnitude and position of the potential are appropriately set. This embodiment exemplifies a non-reciprocal optical element which transmits light incident from the front but does not transmit light incident from the back based on such a principle. Embodiment 2 FIG. 4 is a schematic perspective view showing a non-reciprocal optical element according to a second embodiment of the present invention. An optically active thin film (1) made of a semiconductor, a dielectric, or the like has a nonlinear optical effect. FIG. 4 shows an incident light (2) carrying a signal, an output light (3) through which the incident light (2) carrying a signal has passed, and an incident light coming from the back and having a signal incident from the front. A structure in which a plurality of surface defects or monoatomic layer films of different materials, or delta doping, etc., together with light (4), become potential walls for the movement of the electron system in the direction perpendicular to the film surface. (5) is shown, whereby the entire thin film has an asymmetric structure on both sides. In the figure, d is the thickness of the thin film 1 and is a value from several tens of angstroms to several thousand angstroms, and c1 to c
n is the distance from the surface of the thin film 1 to each structure (5).
The d and the type of the structure (5) and c1 to cn are set so that the incident light (2) from the front of the thin film is transmitted but the incident light (4) from the back is not transmitted.

In this embodiment, the multiple interference of the asymmetric polariton described in the first embodiment is caused by a plurality of potential walls, and the light is incident from the front and from the back rather than from a single potential wall. The difference between the amplitude of the internal photoelectric field and the case is made remarkable. As a result, with respect to the light incident from the front, the decrease in the transmitted light intensity is more suppressed as compared with the case of the single potential wall of the first embodiment. It becomes possible to cut off even better than in the case of one single potential wall. Embodiment 3 FIG. 5 is a schematic perspective view showing a non-reciprocal optical element according to a third embodiment of the present invention. An optically active thin film (1) made of a semiconductor, a dielectric, or the like has a nonlinear optical effect. FIG. 5 shows an incident light (2) carrying a signal, an output light (3) transmitted through the incident light (2) carrying a signal, and an incident light coming from the back and conforming to a signal. (4) The direction perpendicular to the film surface of the electron system due to surface defects, monoatomic layer film of different materials, or delta doping, etc., together with the output light (6) transmitted by the incident light (4) carrying the signal. Structure (5) is shown, which is disposed at a position other than the center of the thin film so as to become a potential wall for the movement of the thin film. In the figure, d is the thickness of the thin film 1 and is a value from several tens angstroms to several thousand angstroms, and c is the distance from the surface of the thin film 1 to the structure 5. The intensity of the transmitted light (3) of the incident light (2) from the surface of this film increases non-linearly with the increase of the incident intensity and has optical bistable characteristics.
The type d and the type of the structure (5) and c are set so that the intensity of the transmitted light (6) of the incident light (4) from the back is simply proportional to the intensity of the incident light.

The function of the device according to this embodiment will be described in detail. First, it is pointed out that the optical bistable function and its function appear nonreciprocally. In general, light in a Fabry-Perot resonator due to a non-linear medium is reflected by a film surface and causes multiple interference by reciprocating in the medium several times. FIG. 6 is a conceptual diagram of the Fabry-Perot resonator. In the drawing, incident light (2), reflected light (3), transmitted light (4), and internal light (5) are shown while the optically active thin film medium (1) has a nonlinear effect. .
D represents the length of the resonator. Light that has entered the medium is multiple-reflected in the resonator as shown in the figure. As a result, reflected light and transmitted light include those returned as a result of multiple reflection, and the multiple interference effect occurs.

In such a resonator, as the incident light intensity is gradually increased, the refractive index changes due to the non-linear effect, and the wavelength region that changes so as to further increase the internal electric field due to multiple interference is It exists depending on the resonator length. As the internal electric field increases, the nonlinear effect further increases, and the refractive index changes so as to further increase the internal electric field. In this way, the feedback mechanism works, and the output light intensity can take two values in a certain incident light intensity region. FIG. 7 is a conceptual diagram of the input / output characteristics in this case. In the figure, the horizontal axis is the input light intensity, and the vertical axis is the output light intensity. The arrow indicates the direction when the input light intensity is changed. In particular, the output light intensity between A and B in the figure shows a low output when the intensity is increased from the low input light intensity. Further, when the intensity is reduced from the high input light intensity, two values of high output are shown. Needless to say, when the output is low, the transmission is low, and when the output is high, the transmission is high. That is, in the bistable region, when the intensity is gradually increased from a low intensity, the light does not transmit much, but when the intensity is gradually decreased from the high intensity, a phenomenon that light is transmitted well occurs. Such an optical bistable phenomenon has been well known in the past, and various applications have been considered in optoelectronic technologies such as an optical memory.

Since the optical bistability phenomenon is an important factor that causes multiple interference of light in a resonator, this resonator generally has a number of light wavelengths so that the light can cause interference. It is believed that double the length is required. Therefore, when considering light having a wavelength of several thousand angstroms or more, a resonator having a length of several microns or more is required. However, when light having energy near the resonance energy of the electron system of the non-local medium is used as described in the first embodiment, the internal photoelectric field is polaritons, and has a much shorter wavelength. For example, the wavelength is about 500 angstroms or less in the case of the energy of the lowest exciton level of CuCl. Therefore, even in an extremely thin thin film like the device of the first embodiment, the internal light causes interference. In fact, the spatial distribution of the internal photoelectric field according to the calculation example used in the description of the first embodiment shows that the wavelength is short. Therefore, in the case of this embodiment, although it is an ultra-thin film, a bistable characteristic appears.

Further, in this embodiment, d and the structure (5)
By appropriately setting the type of light and c, the intensity of the light incident from the table is fed back along with the increase in the intensity due to the interference effect between the film surface on the front side and 5, and bistable occurs. However, for incident from the back, since the type of d and the type of the structure (5) and c are set so that the internal electric field does not increase, no nonlinear effect appears even if the incident light intensity is increased, and therefore, bistable. No characteristics appear.
That is, in this embodiment, the optical bistability appears non-reciprocally. Embodiment 4 FIG. 8 is a schematic perspective view showing a non-reciprocal optical element according to a fourth embodiment of the present invention. An optically active thin film (1) made of a semiconductor, a dielectric, or the like has a nonlinear optical effect.
FIG. 8 shows the incident light (2) carrying a signal, the output light (3) transmitted through the incident light (2) carrying the signal, and the incident light (3) coming from the back and conforming to the signal. 4),
Along with the output light (6) transmitted by the incident light (4) carrying the signal, a plurality of surface defects or monoatomic layer films of different materials,
Alternatively, there is shown a structure (5) in which the potential of the electron system in the direction perpendicular to the film surface becomes a potential wall by delta doping or the like, whereby the entire thin film has an asymmetric structure with respect to the front and back surfaces. ing. In the figure, d is the thickness of the thin film 1 and is a value of about several tens of angstroms to several thousand angstroms, and c1 to cn are the distances from the surface of the thin film 1 to each structure 5. The transmitted light intensity of the incident light (3) from the front of this film increases non-linearly with an increase in the incident intensity and has optical bistable characteristics, but the transmitted light intensity of the incident light (6) from the back is the incident light. D and the type of structure (5) and c1 to c so that they are only proportional to the strength
n is set.

In this embodiment, the multiple interference of the asymmetric polariton described in the first embodiment is caused by a plurality of potential walls, and the light is incident from the front and from the back rather than the single potential wall. The difference between the case and the amplitude of the internal electric field is set to be remarkable. As a result, with respect to the transmitted light of the light incident from the table, the optical bistability characteristic appears with a smaller incident light intensity as compared with the case of the single potential wall of the third embodiment. Embodiment 5 FIG. 9 is a conceptual perspective view showing a non-reciprocal optical element according to a fifth embodiment of the present invention. An optically active thin film (1) made of a semiconductor, a dielectric, or the like has a nonlinear optical effect. FIG. 8 shows the incident light (2) carrying a signal, the output light (3) transmitted through the incident light (2) carrying the signal, and the incident light coming from the back and conforming to the signal. (4),
The incident light (4), which carries the signal, together with the output light (6) that has passed through, is affected by surface defects, monoatomic layer film of different material, or delta doping, etc., and the movement of the electron system in the direction perpendicular to the film surface. Structure (5) is shown, which is disposed at a position other than the center of the thin film so as to become a potential wall. D in the figure is the thickness of the thin film 1 from several tens of angstroms to several tens
A value on the order of 00 angstroms, and c is the distance from the surface of the thin film 1 to the structure 5. The intensity of the transmitted light (3) of the incident light (2) from the front of the film decreases non-linearly with the increase of the incident light intensity, and has optical bistable characteristics. The d and the type of the structure (5) and c are set so that the intensity of the light (6) is simply proportional to the intensity of the incident light.

The function of the device according to this embodiment will be described in detail. First, it is pointed out that an optical bistable function of a type in which the intensity of transmitted light decreases as the intensity of incident light increases and that the function appears nonreciprocally. . In the third embodiment, the optical bistable function in which the transmitted light intensity increases as the incident light intensity increases has been described. However, in the ultrathin film as in this embodiment, the type of light in which the transmitted light intensity decreases specifically Bistable characteristics appear. This is due to the following mechanism. In other words, when light having energy that resonates with the excited state of electrons is incident on a nonlocal medium, the spatial distribution of the internal optical electric field corresponds to the induced polarization. In accordance with each energy, the component of the internal photoelectric field corresponding to the wave function of the eigenmode of the polarized wave confined therein is selectively increased according to each energy. FIG. 10 is an energy spectrum of the component of the internal photoelectric field intensity. Where the component of the internal photoelectric field is the internal photoelectric field E _j at location j

[0040]

(Equation 3)

Each component when expanded as shown in FIG. In Expression (4), n is a number that is an index of each component (each mode). In the figure, the horizontal axis is the incident light energy, and the vertical axis is the magnitude | F _n | ² of the expansion coefficient of the internal photoelectric field. ｜ F
₁ | ² is the lowest mode, that is, the square of the absolute value of the distribution value of the spatial pattern whose node is 0, and | F ₂ | ² is the second mode, that is, 2 of the absolute value of the distribution value of the spatial pattern whose node is 1.
It is a power. The parameter is CuC with a thickness of 42 molecular layers.
1 A thin film was used. Now, it can be seen that the lowest mode and the second mode are appropriately mixed around the energy indicated by the arrow in the figure. In such a situation, when the incident light intensity is increased, the ratio between | F ₁ | ² and | F ₂ | ² changes, and | F ₁ | ² is dominant at first, but gradually | F
₂ | The ratio of ² may increase. The lowest mode has an almost symmetrical spatial distribution in the thin film, but the second mode has an antisymmetrical spatial distribution, so if the proportion of the second mode increases, the spatial distribution of the internal optical electric field itself will increase. Becomes asymmetric. FIG. 11 shows how the spatial distribution of the internal optical electric field changes with the change in the intensity of the incident light.
Shows the result of actual calculation by the model described in FIG. In the figure, the horizontal axis represents the layer number j in the thin film, and the vertical axis represents the electric field intensity in each layer.

<1><2><3> indicate that the case corresponds to a case where the incident light intensity increases in order.
The parameters are the same as in FIG. As can be seen from this figure, in the situation where the ratio of a specific mode changes with the change in the incident light intensity, the amplitude on the back surface opposite to the incident surface decreases as the incident light intensity increases. A phenomenon occurs in which the transmitted light intensity decreases. Actually, the electric field intensity on the back surface when the incident light intensity of <3> in the figure is the largest is the smallest, despite the fact that the internal photoelectric field itself is on average in the film. ing. This phenomenon is due to the fact that the polarization field is subject to quantum mechanical confinement,
It occurs specifically in the case of an ultrathin film in which a specific component of the internal photoelectric field increases in resonance. FIG. 12 shows the result of actually calculating the input / output characteristics under the conditions where such a phenomenon occurs, using the same model as that in the description of the first embodiment. The parameters are the same as in FIG. It can be seen that, contrary to the schematic diagram of FIG. 7, an optical bistable phenomenon occurs in which the output light intensity decreases as the incident light intensity increases.

By setting the incident light energy and d and the type of the structure (5) and c appropriately in the same manner as in the case of the third embodiment, the output light intensity decreases as the incident light intensity increases. A non-reciprocal situation can be created where optical bistable phenomena occur for front-side incidence but not for back-side incidence. This example illustrates an element so fabricated. Embodiment 6 FIG. 13 is a conceptual perspective view showing a non-reciprocal optical element according to a sixth embodiment of the present invention. An optically active thin film (1) made of a semiconductor, a dielectric, or the like has a nonlinear optical effect. FIG. 13 shows the incident light (2) carrying a signal, the output light (3) transmitted through the incident light (2) carrying the signal, and the incident light (3) coming from the back and conforming to the signal. 4),
Along with the output light (6) transmitted by the incident light (4) carrying the signal, a plurality of surface defects or monoatomic layer films of different materials,
Alternatively, there is shown a structure (5) in which the potential of the electron system in the direction perpendicular to the film surface becomes a potential wall by delta doping or the like, whereby the entire thin film has an asymmetric structure with respect to the front and back surfaces. ing. In the figure, d is the thickness of the thin film 1 and is a value of about several tens of angstroms to several thousand angstroms, and c1 to cn are the distances from the surface of the thin film 1 to each structure (5). The intensity of the transmitted light (3) of the incident light (2) from the front of the film decreases non-linearly with the increase of the incident intensity and has optical bistable characteristics, but the transmitted light of the incident light (4) from the back. The d, the type of the structure (5), and c1 to cn are set so that the intensity of (6) is simply proportional to the intensity of the incident light.

In this embodiment, the multiple interference of the asymmetric polariton described in the first embodiment is caused by a plurality of potential walls, and the case where light is incident from the front and the case where light is incident from the back rather than the case of a single potential wall. And the difference between the amplitudes of the internal photoelectric fields is remarkable. As a result, with respect to the transmitted light of the light incident from the table, the optical bistability characteristic appears with a smaller incident light intensity as compared with the case of the single potential wall of the fifth embodiment. Embodiment 7 FIG. 14 is a schematic perspective view showing a non-reciprocal optical element according to a seventh embodiment of the present invention. An optically active thin film (1) made of a semiconductor, a dielectric, or the like has a nonlinear optical effect. FIG. 14 shows the incident light (2) carrying a signal, the output light (3) transmitted through the incident light (2) carrying the signal, and the incident light (3) coming from the back and conforming to the signal. 4),
The output light (6) through which the incident light (4) carrying the signal has passed, the control light (7) incident from the front for controlling the signal light, and the control light (7) incident from the back to control the signal light ( Along with 8), a position other than the center of the thin film so that it becomes a potential wall for the movement of the electron system in a direction perpendicular to the film surface due to a plane defect, a monoatomic layer film of a different material, or delta doping. 5 shows a structure (5) provided in the first embodiment.
In the figure, d is the thickness of the thin film 1 and is a value of about several tens angstroms to several thousand angstroms, and c is the distance from the surface of the thin film 1 to the structure (5). In the device according to this embodiment, the control light (7) incident from the front increases the amplitude of the internal optical electric field, and the control light (8) incident from the back does not increase the amplitude of d and the structure (5). Type and c
Is set. Thus, in the device according to this embodiment, the control light (7) incident from the front can nonlinearly control the signal light (3), but the control light (8) incident from the back does not control the signal light (4). .

A number of proposals have been made to use weak light as control light and control weak signal light by their interaction in a nonlinear medium. Here, the control means, for example, that the signal light that has been transmitted so far is not transmitted by the input of the control light, or the signal light that has not been transmitted is transmitted by the input of the control light. That is, the input / output characteristics of the signal light are changed nonlinearly. For the phenomenon that the input and output characteristics of weak light change due to the incidence of strong light in a nonlocal medium, see Phys. Rev. B48 (1993) 7960.H. Ishihara and K.
Cho) (SolidState Commun. 89 (1994) 837 .H. Ishihara
and K. Cho) for a detailed example. The magnitude of the nonlinear effect of the strong control light (7) also depends on the magnitude of its internal photoelectric field as described in the first embodiment, and the magnitude of the interaction with the weak incident light (2) is , The greater the internal photoelectric field of the control light (7), the greater the magnitude. That is, the signal light (3) can be better controlled as the internal photoelectric field of the control light (7) is larger. In this embodiment, the magnitude of the internal photoelectric field of the control light is determined between the control light (7) incident from the table and the control light (8) incident from the back in the same manner as described in the first embodiment. Significant asymmetry is provided. Accordingly, in the device according to this embodiment, when the control light (7) is incident from the front, the control light changes the input / output characteristics of the signal light, but when the control light (8) is incident from the back, the signal is changed. Does not change the light input / output characteristics. Embodiment 8 FIG. 15 is a schematic perspective view showing a non-reciprocal optical element according to an eighth embodiment of the present invention. An optically active thin film (1) made of a semiconductor, a dielectric, or the like has a nonlinear optical effect. FIG. 15 shows the incident light (2) carrying the signal, the output light (3) transmitted through the incident light (2) carrying the signal, and the incident light (3) coming from the back and conforming to the signal. 4),
The output light (6) through which the incident light (4) carrying the signal has passed, the control light (7) incident from the front for controlling the signal light, and the control light (7) incident from the back to control the signal light ( 8),
In addition, a structure (5) is shown in which a plurality of surface defects or monoatomic layer films of different kinds of materials, or a delta doping or the like, are used as potential walls for the movement of the electron system in a direction perpendicular to the film surface. As a result, the entire thin film has an asymmetric structure with respect to the front and back sides. D in the figure is thin film 1
Is a value of about several tens angstroms to several thousand angstroms, and c1 to cn are distances from the surface of the thin film (1) to each structure (5). In the device according to this embodiment, the control light (7) incident from the front increases the amplitude of the internal photoelectric field, and the control light (8) incident from the back does not increase the amplitude of d and the structure (5). Type and c
1 to cn are set. Thus, in the device according to this embodiment, the control light (7) incident from the front can control the signal light (3) nonlinearly, but the control light (8) incident from the back controls the signal light (4). do not do.

In this embodiment, the multiple interference of the asymmetric polariton described in the first embodiment is caused by a plurality of potential walls. And the difference between the amplitudes of the internal photoelectric fields is remarkable. As a result, the degree of non-reciprocity between the effect of the control light incident from the front and the effect of the control light incident from the back is greater than in the case of the seventh embodiment. Embodiment 9 FIG. 16 is a schematic perspective view showing a semiconductor laser in which an optical isolator using a non-reciprocal optical structure according to a ninth embodiment of the present invention is provided. The semiconductor laser (1), the clad layer (2), the active layer (3), the surface coat layer (4) provided by selective growth by MBE, and the examples 1 to 6 also provided by selective growth by MBE. A non-reciprocal optical structure (5) is shown. The non-reciprocal optical structure transmits light from the inside of the laser, that is, the light from the active layer side, but has the same energy as the oscillation energy or has energy that is not so far away from the outside, that is, light from the air side. Is not transmitted. With such a structure of the semiconductor laser, return light or light from another light source can be prevented from being incident on the semiconductor laser, and deterioration of characteristics of the semiconductor laser can be prevented.

[0047]

As described above, according to the present invention, an optical structure exhibiting effective non-reciprocal input / output characteristics can be realized. It can be used as an optical rectifier in a highly integrated optoelectronic circuit, or as an optical isolator that can be monolithically disposed to prevent deterioration of characteristics of a semiconductor laser or an optical amplifier.

[Brief description of the drawings]

FIG. 1 is a conceptual perspective view of a first embodiment.

FIG. 2 is a spatial distribution diagram of an internal electric field of a CuCl thin film.

FIG. 3 is a non-linear transmittance diagram when light is incident from both the front and back of a CuCl thin film.

FIG. 4 is a perspective conceptual view as a second embodiment.

FIG. 5 is a conceptual perspective view of a third embodiment.

FIG. 6 is a conceptual diagram of a Fabry-Perot optical resonator.

FIG. 7 is a conceptual diagram of optical bistable input / output characteristics.

FIG. 8 is a conceptual perspective view of a fourth embodiment.

FIG. 9 is a conceptual perspective view of a fifth embodiment.

FIG. 10 is an energy spectrum diagram of the intensity of the expansion coefficient F of the internal photoelectric field.

FIG. 11 is an incident light intensity dependence correlation diagram of the spatial distribution of the internal photoelectric field intensity in the CuCl thin film.

FIG. 12 is an optical bistable input / output characteristic diagram of a CuCl thin film.

FIG. 13 is a perspective conceptual view as a sixth embodiment.

FIG. 14 is a perspective conceptual view as a seventh embodiment.

FIG. 15 is a conceptual perspective view of an eighth embodiment.

FIG. 16 is a conceptual perspective view of a ninth embodiment.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References Optical Engineering, Vol. 31 No. Pp. 328-334 (February 1992). Phys. Soc. Jpn, Vol. 60 p. 3920 (1991) (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/35-3/02 G02B 27/28 H01S 5/50-5/50 630 JICST file (JOIS)

Claims (11)

(57) [Claims] 1. A thin film having an optically active portion having non-linear optical characteristics, wherein a surface defect or an optically active portion is formed at a position which is asymmetric with respect to an axial direction perpendicular to the film surface of the thin film. An optical structure having a non-reciprocal optical characteristic in which a material layer different from a material or a delta doping layer is inserted. 2. The optical structure according to claim 1, wherein a material layer different from a material constituting a plurality of surface defects or optically active portions, or a delta doping layer is inserted alone or in a mixture. 3. A thin film having an optically active portion having non-linear optical characteristics , wherein electrons, holes or composite particles thereof are located at positions which are not symmetric with respect to an axial direction perpendicular to the film surface of the thin film. An optical structure characterized by having a potential wall for the movement of excitons and the like, and having non-reciprocal optical characteristics. 4. The optical structure according to claim 3, wherein a potential wall is provided at a plurality of positions of the thin film for the movement of electrons, holes or excitons which are composite particles thereof. 5. The thin film having an optically active portion having non-linear optical characteristics has an asymmetric structure in the axial direction, and the spatial distribution in the axial direction of the thin film of the internal photoelectric field due to light incident on the film is expressed by It differs between when incident from the rear and when incident from the back,
An optical structure having non-reciprocal optical characteristics. 6. The optical structure according to claim 1, wherein the optical structure has optical bistable characteristics only for incident light from one of the front surface and the back surface. Optical structure. 7. The optical structure according to claim 1, wherein the intensity of incident light is increased only for incident light from one of the front surface and the back surface. An optical structure characterized in that the intensity of transmitted light increases nonlinearly. 8. The optical structure according to claim 1, wherein the intensity of incident light is increased only for incident light from one of the front surface and the back surface. An optical structure characterized in that transmitted light intensity decreases nonlinearly. 9. The optical structure according to claim 1, wherein when two types of light are incident, one of the light is incident on one of the front surface and the back surface. An optical structure having non-reciprocal optical characteristics for controlling the input / output characteristics of the other light. 10. An optical isolator having the optical structure according to claim 1. 11. A semiconductor laser device according to claim 10, wherein the optical isolator for blocking external light from entering the laser is provided on an end face.