JP3063115B2 - Nonlinear optical material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Summary] With regard to nonlinear optical materials, an object of the present invention is to provide a material exhibiting a large nonlinear optical material. And two or more quantum wells in the valence band, and in each quantum well, when the depth of the valence band well is shallow, the depth of the corresponding conduction band well is deep, and When the depth of the valence band well is deep, the depth of the corresponding conduction band well is configured to be shallow, especially when there are three or more quantum wells, the depth of the central well is adjacent. The nonlinear optical material is characterized by being formed shallower or deeper than the well, or being formed such that the position of the conduction band well and the position of the valence band well are shifted from each other.

[Industrial applications]

The present invention relates to a nonlinear optical material exhibiting large nonlinear optical characteristics.

Optical communication has been put into practical use because of the need to process a large amount of information quickly, but the optical deflector, optical switch,
Lithium niobate (LiNbO ₃ ) for optical components such as modulators
And materials showing a nonlinear optical material such as lithium tantalate (LiTaO ₃ ) are used.

However, in order to increase the speed of optical communication, it is necessary to lower the applied voltage of the directional coupler or the optical modulator currently used or to reduce the size of the electrode, For this purpose, the second-order nonlinear optical characteristic is LiNbO ₃
It is necessary to configure the optical element using a material larger than the above.

In addition, in order to realize high-speed optical communication, instead of using the electro-optic effect (Pockels effect) in which the refractive index of a material changes in proportion to an electric field as in the related art, it is proportional to the intensity of input light. It is necessary to develop an optical communication device using the optical Kerr effect in which the refractive index changes.

[Conventional technology]

As mentioned earlier, inorganic materials such as LiNbO ₃ and LiTaO ₃ have been used as constituents of various optical elements because of their non-linear optical properties. It is known to exhibit optical characteristics [eg, Eiichi Hanamura, Applied Physics, Vol. 56, No. 10 (198
7), 1348] However, no optical element has been practically used because the structure of the quantum well has not been optimized.

[Problems to be solved by the invention]

It is known that a quantum well structure exhibits a large nonlinear optical characteristic, but since a practical quantum well structure has not been proposed, it has not been put to practical use as an optical element.

Therefore, an object of the present invention is to optimize the wave function by a method such as forming a quantum well in a quantum well, and to increase the nonlinear optical effect.

[Means for solving the problem]

A first configuration for solving the above problem is characterized by having a quantum well structure in which the well widths of the conduction band and the valence band are different, and a second configuration includes the first and second configurations. And a third quantum well provided between the first and second quantum wells, the conduction band of the third quantum well having a depth of the first and second quantum wells. When the depth of the valence band of the third quantum well is smaller than the depth of the well of the conduction band of the second quantum well, the depth of the valence band of the third quantum well is changed to the depth of the valence band of the first and second quantum wells. And the depth of the conduction band well of the third quantum well is deeper than the depth of the conduction band wells of the first and second quantum wells. A depth of a valence band well is smaller than a depth of a valence band well of the first and second quantum wells; and In the nonlinear optical material, a plurality of quantum wells are respectively formed in the conduction band and the valence band, and the position of the end of at least one of the quantum wells formed in the conduction band is determined by the quantum well formed in the valence band. The fourth configuration is characterized in that the quantum well structure of the first to third configurations is periodically repeated to form a quantum line. Is configured as a feature.

[Action]

The magnitude of polarization of the optical material can be expressed by the following equations (1) and (2).

Here, β and χ ₂ are the electro-optic effect (Pockels effect)
And a coefficient that governs the generation of the second harmonic, which is a frequency multiplication function.

Also, gamma and chi ₃ are coefficients governing the optical Kerr effect.

By the way, the second-order nonlinear molecular polarizability β is expressed as follows: The transition dipole moment is the expected value of the electron position in the μ _ge conduction band (excited state), and the dipole moment in the μ _e valence band (ground state) is the expected value of the electron position. When indicating the dipole moment is the expected value of the position in mu _g, Therefore, to increase the value of β, the overlap of the wave function between the valence band and the conduction band is increased,
In addition, it is necessary to increase the distance between the valence band and the conduction band in the opposite direction, and since both have a reciprocal relationship,
As shown in FIG. 1, it is necessary to simultaneously consider and optimize the overlap and deviation of the wave functions.

Next, to increase the third-order nonlinear molecular polarizability γ,
The purpose is to increase the overlap between the wave functions of the conduction band and the valence band to increase the transition dipole moment, and to increase the difference between the wave functions of the conduction band and the valence band.

〔Example〕

The material used in the present invention is gallium arsenide (GaA).
s), inorganic materials such as mixed crystals of III-V group semiconductors such as indium phosphide (InP) and II-VI group semiconductors such as cadmium telluride (CdTe), and organic materials in which donor groups and acceptor groups are added to polydiacetylene. Materials can be used.

Example 1; (When the well width of the valence band is narrower than the conduction band) In FIG. 2, FIG. 2A shows the energy level when the well width of the valence band is narrower than the conduction band. FIG. 7B schematically shows the wave functions of the conduction band and the valence band in this case.

FIG. 3 shows that when the well width of the valence band is W and the well width of the conduction band is ML, the value of ML is fixed at 300, 600 and 800, and the γ (secondary) when the value of W is changed is shown in FIG. The value of γ increases as the ML (well width of the conduction band) increases, but a local maximum exists in the middle, and the value of ML is equal to the value of W. Then, the value of γ becomes 0.

Embodiment 2; (When Three Quantum Wells Exist in Quantum Well) FIG. 4 shows the energy levels when three quantum wells exist in the quantum well, and FIGS. (B) schematically shows the wave functions of the conduction band and the valence band in this case. In the center well, the depth of the conduction band well is small, while the depth of the valence band well is Deepening.

This state can be realized, for example, by epitaxially growing three types of III-V compound layers having different compositions in the thickness of 10 to 50 molecular layers between AlGaAs layers.

Fig. 6 shows the ratio between the wave functions of the center and the left and right wells in the conduction band of Fig. 5 (A / B in the figure) and the ratio of the center and left and right wave functions in the valence band (C / D in the figure). Assuming that the values are equal and R, while the widths of the individual wells are equal and indicate a value of W, when W is fixed and R is changed for three cases where the well widths W are 190, 150 and 50, respectively. It is the value of γ calculated.

From the figure, it can be seen that there is a maximum value of γ near R ≒ 2, which decreases as the value increases, and that the value of γ increases as the well width W decreases.

Example 3: FIG. 7 shows that two more quantum wells are provided in the quantum well,
The case where the well of the conduction band is deep and the well of the valence band is shallow is shown.

This state is possible in a compound semiconductor, and also in polydiacetylene to which a donor and an acceptor are added.

Example 4 FIG. 2 or FIG. 3 was obtained by repeating molecular layer epitaxy and repeating the growth of 10 to 50 molecular layers while changing the type and composition of the III-V and II-VI group materials. Quantum wells with energy levels of
By etching this linearly, a quantum line in which a quantum well is formed in a one-dimensional system can be produced.

〔The invention's effect〕

By implementing the present invention, a material made of an inorganic material or an organic material and having a large nonlinear optical characteristic can be produced, and the use of this material makes it possible to improve the performance of optical communication.

[Brief description of the drawings]

1 is an energy level diagram of a non-linear material according to the present invention, FIG. 2 is an energy level diagram and a wave function of another non-linear material according to the present invention, and FIG. FIG. 4 is an energy level diagram of another nonlinear material according to the present invention, FIG. 5 is a wave function corresponding to the energy level diagram of FIG. 4, and FIG. FIG. 7 is an energy level diagram and a wave function of another nonlinear material according to the present invention.

Continuation of the front page (56) References JP-A-2-96720 (JP, A) JP-A-63-225236 (JP, A) JP-A-2-87125 (JP, A) Physical Review B vol. 38, No. 6, p. p. 4056 -4066 (58) Field surveyed (Int. Cl. ⁷ , DB name) G02F 1/355 JICST file (JOIS)

Claims (4)

(57) [Claims] 1. A nonlinear optical material having a quantum well structure in which the conduction band and the valence band have different quantum well structures. 2. The semiconductor device according to claim 2, further comprising a first quantum well, a third quantum well provided between said first quantum well, and a conduction band of said third quantum well. If the depth of the wells is shallower than the depth of the conduction band wells of the first and second quantum wells,
The depth of the valence band well of the third quantum well is greater than the depth of the valence band well of the first and second quantum wells; If the depth is greater than the depth of the conduction band wells of the first and second quantum wells,
A non-linear optical material, wherein a depth of a valence band well of the third quantum well is smaller than a depth of a valence band well of the first and second quantum wells. 3. The nonlinear optical material according to claim 1, wherein a plurality of quantum wells are formed in the conduction band and the valence band, respectively, and at least one of the ends of the quantum well formed in the conduction band is A nonlinear optical material, which is shifted from a position of an edge of a quantum well formed in the valence band. 4. A nonlinear optical material, wherein the quantum well structure according to claim 1 is periodically repeated to form a quantum line.