JP2938447B1 - Nonlinear optical semiconductor device - Google Patents

Abstract

Abstract: PROBLEM TO BE SOLVED: To expand an absorption band, flatten an absorption coefficient, reduce a chromatic dispersion value, and suppress a higher-order dispersion in an optically controlled nonlinear optical semiconductor device using transition between subbands. SOLUTION: This is a nonlinear semiconductor device utilizing nonlinear optical response based on intersubband absorption of a semiconductor quantum well layer, wherein a plurality of quantum well layers 21 are stacked via a barrier layer 22 and a tunnel is formed between each well layer 21. The first and second coupled multiple quantum well structures 23 and 24 are stacked on a substrate, and the absorption spectrum in the first coupled multiple quantum well structure 23 is used to flatten the overall absorption spectrum. The quantum well structures 23 and 24 are designed such that the long-wavelength skirt overlaps the short-wavelength skirt of the absorption spectrum in the second coupled multiple quantum well structure 24.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optically controlled semiconductor optical modulator, an optically controlled semiconductor optical gate switch, an optically controlled semiconductor optical routing switch, a wavelength conversion element, The present invention relates to a nonlinear optical semiconductor device such as a mode-locked laser and a tunable dispersion compensator, and more particularly to a nonlinear optical semiconductor device using a nonlinear optical response based on an intersubband transition.

[0002]

2. Description of the Related Art In a backbone transmission system in the coming multimedia era, a large amount of information of several hundred gigabits to several terabits per second flows through an optical fiber. To realize an optical node that can freely process such information, an ultra-high-speed optical switch controlled by ultra-short optical pulses is required. If the purpose is periodic optical demultiplexing, a physical phenomenon in which the internal state returns to the original state in the cycle of demultiplexing can be used. However, in order to switch the signal light on a bit basis, it is necessary to return to the original state within the time corresponding to one slot of the signal.

As a principle of such an ultra-high-speed optical switch capable of operating in a bit mode, use of transition between subbands in a nitride semiconductor quantum well is considered (for example, Japanese Patent Application No. 6-248451, Japanese Patent Application No. No. 7-42014, Japanese Patent Application No. 7-102198, Japanese Patent Application No. 8-31534, or N. Suzuki and N. Iizuka, Japanese Journal of Appl
ied Physics, vol. 36, pp. L1006-L1008, 1997).

The relaxation time between subbands is about three orders of magnitude faster than the relaxation time between bands. In particular, if a quantum well of a nitride semiconductor is used, transition between subbands in the near infrared region, which is important in optical communication, can be realized. In addition, since the interaction between LO phonons and electrons is large, the relaxation time between subbands can be reduced by GaAs.
It can be one order of magnitude faster than the s system.

The relaxation time between subbands in a GaN-based quantum well is as short as about 100 fs at a transition wavelength of 1.55 μm and room temperature.
It is considered that a bit mode optical switching operation with a very high repetition rate of about 1 Tb / s can be performed. In addition, since a semiconductor is used, a small, light, and stable optical switch can be realized, and it goes without saying that mass production is possible. Furthermore, nitride semiconductors are particularly tough semiconductor materials and are resistant to temperature rise.

As described above, the intersubband transition in the nitride semiconductor quantum well is a very attractive device principle for realizing an ultrahigh-speed optical switch. However, the full width at half maximum of the spectrum of the intersubband absorption is usually on the order of 100 nm. Further, the refractive index is large on the long wavelength side of the sub-band absorption peak and is small on the short wavelength side, and large chromatic dispersion occurs near this absorption wavelength band.

On the other hand, the subpicosecond optical pulse has a wide spectral width due to the principle of uncertainty. For example, center wavelength 1.55μ
m, the spectral half-width of a Fourier transform limited Gaussian light pulse having a half-width of 100 fs is about 36 nm. In an optical switch, the wavelengths are often shifted in order to easily separate the control light and the signal light, but in order to avoid overlapping of the spectrum tails, it is necessary to shift both wavelengths by 80 to 100 nm or more.

If the wavelength difference between the control light and the signal light is large, the timing of the simultaneously input control light pulse and signal light pulse gradually shifts inside the device due to the walk-off due to the group velocity dispersion, resulting in lower efficiency. I do. Also, the pulse width of each pulse is widened due to group velocity dispersion. At this time, if there is a higher-order dispersion (dispersion that is not proportional to the wavelength), it is difficult to compensate for the dispersion by a medium having the opposite group velocity dispersion.

If the wavelength difference between the signal light and the control light is large, the two cannot be arranged near the center of the sub-band absorption spectrum, so that the absorption coefficient differs between the long wavelength component and the short wavelength component of the pulse spectrum. Since the spectrum of the light pulse subjected to the wavelength-dependent intensity modulation narrows and the pulse width increases, the waveform is distorted even if dispersion compensation is performed, and the original waveform cannot be restored.

Further, an ultrashort light pulse transmitted through an absorbable saturated medium or a non-linear refractive index medium has a wider spectral width due to wavelength chirp. For a linearly chirped optical pulse, chirp compensation is performed using a prism pair, a diffraction grating pair, an arrayed waveguide diffraction grating, a resonator, an interferometer, etc., so that a Fourier transform limited pulse (spectrum is narrower than the first pulse) is obtained. Pulse compression is possible. However, if the spectrum of the intersubband absorption is not sufficiently wide, the third-order nonlinear susceptibility varies in a complicated manner with respect to the wavelength, so that a clean linear chirp is not applied.
Therefore, even if chirp compensation is performed, it is not possible to obtain an ultrashort pulse limited by Fourier transform.

[0011]

As described above, in the nonlinear optical semiconductor device utilizing the nonlinearity based on the transition between sub-bands, since the absorption band is not sufficiently wide for the spectrum of the ultrashort optical pulse, the optical pulse There is a problem that the spectrum and waveform are complicatedly deformed and cannot be used in multiple stages even if dispersion compensation or chirp compensation is performed. Also, there is a problem that efficiency cannot be increased if there is a walk-off due to large wavelength dispersion.

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and an object of the present invention is to provide an optically controlled nonlinear optical semiconductor device utilizing transition between subbands, in which the absorption band is expanded and the absorption coefficient is increased. An object of the present invention is to achieve flattening, reduction of chromatic dispersion value and suppression of higher-order dispersion, to keep the pulse shape and spectrum change of ultrashort optical pulses within a compensable range, and to enable multistage connection. Another object of the present invention is to reduce the walk-off of light pulses having different wavelengths and to improve the efficiency as a nonlinear optical element.

[0013]

(Structure) In order to solve the above problem, the present invention employs the following structure. That is, the present invention relates to a nonlinear semiconductor device utilizing nonlinear optical response based on intersubband absorption of a semiconductor quantum well layer, in which a plurality of quantum well layers are stacked via a barrier layer and each well layer is tunnel-coupled. The first and second coupled multiple quantum well structures (or superlattice structures) are laminated, and the long wavelength side of the absorption spectrum in the first coupled multiple quantum well structure and the second coupled multiple quantum well structure Are characterized in that each quantum well structure is designed such that the bottoms on the short wavelength side of the absorption spectrum in (1) overlap (that is, the entire absorption spectrum becomes flat). Hereinafter, even if not otherwise specified, a multi-quantum well structure includes a superlattice structure.

In other words, in this structure, the first coupled multiple quantum well structure in which the refractive index increases with the inter-subband absorption in a predetermined wavelength region, the refraction in the predetermined wavelength region with the inter-subband absorption increases And a second coupled multiple quantum well structure having a reduced rate. In the first coupled multiple quantum well structure and the second coupled multiple quantum well structure, by changing at least one of the thickness of the well layer, the composition of the well layer, the thickness of the barrier layer, and the composition of the barrier layer, The absorption center wavelength and wavelength bandwidth can be shifted. The degree of overlap of the absorption spectra of the two coupled multiple quantum well structures can be precisely adjusted by changing two or more of the above parameters. Further, the relative magnitude of absorption of the two coupled multiple quantum well structures can be adjusted by the number of well layers, the arrangement of the coupled multiple quantum well structures in the optical waveguide structure, and the like.

Incidentally, there is usually some fluctuation in the thickness and composition of the layers constituting the actual quantum well. Here, “thickness” and “composition” indicate an average thickness and composition. In addition, a structure in which the thickness or composition is deliberately delicately shifted between adjacent well layers or adjacent barrier layers, or the composition of the well layer or barrier layer changes in the thickness direction (deformed well layer, deformed barrier layer) Is also conceivable. Changing these structural parameters can also be said to be effectively changing the thickness and composition.

Here, as a preferred embodiment of the present invention, the first coupled multiple quantum well structure and the second coupled multiple quantum well structure each include five or more well layers each having a tunnel coupling through a thin barrier layer. That is what you are doing. Further, it is preferable that a plurality of the first coupled multiple quantum well structures and a plurality of the second coupled multiple quantum well structures are alternately stacked.

In another preferred embodiment of the present invention, the wavelength of the light used is all within the intersubband absorption wavelength band of the second coupled multiple quantum well structure, that is, the refractive index increases with the intersubband absorption. The wavelength band is set in the wavelength band that is set.

Also, the first coupled multiple quantum well structure and the second
In addition to the third coupled multiple quantum well structure in which the refractive index decreases in the wavelength region where the refractive index increases with the intersubband absorption of the second coupled multiple quantum well structure. They may be stacked. Similarly, a large number of four or more coupled multiple quantum well structures may be stacked.

Further, a fourth coupled multiple quantum well structure having an absorption center wavelength intermediate between the first coupled multiple quantum well structure and the second coupled multiple quantum well structure, and a second coupled multiple quantum well structure. A fifth coupled multiple quantum well structure having an absorption center wavelength between those is provided between the well structure and the third multiple quantum well layer, and the first coupled fourth quantum well structure and the fourth coupled multiple quantum well structure are provided. A fourth coupled multiple quantum well structure, a second coupled multiple quantum well structure, a second coupled multiple quantum well structure and a fifth coupled multiple quantum well structure, and a fifth coupled multiple quantum well structure and a third coupled multiple quantum well structure. Preferably, each of the coupled multiple quantum well structures is tunnel-coupled via a thin barrier layer.

As the nonlinear optical medium having intersubband absorption, a coupled multiple quantum well made of a nitride semiconductor is preferably used. The average thickness of each of the well layer and the barrier layer constituting the coupled multiple quantum well structure is as follows:
Preferably it is in the range of 4 to 7 monolayers. Furthermore, the quantum well layer made of the nitride semiconductor may be one component of an optical waveguide integrated with a laminated structure made of a material other than the nitride semiconductor formed on another semiconductor substrate. .

The present invention relates to an optical control element (optical gate switch, optical intensity modulator, optical phase modulator, etc.) in which the output power or phase of signal light is changed by control light, and the output destination of signal light by control light pulses. Optical routing elements (digital optical nonlinear switch, interferometer type optical switch, directional coupler type optical switch, etc.) whose wavelength changes, when signal light pulse and pump light pulse are input, wavelength different from signal light and pump light The present invention can be applied to a nonlinear optical semiconductor device such as a wavelength conversion device that outputs an optical pulse, a saturable absorber of a mode-locked laser, and a tunable chromatic dispersion compensating device in which the magnitude of chromatic dispersion with respect to signal light changes according to control light power.

(Function) In the coupled multiple quantum well structure, the subband energy is split by tunnel coupling between the well layers, so that the absorption spectrum is broadened as compared with the multiple quantum well structure in which there is no coupling in each well layer. The uniform inter-subband absorption spectrum of each level has a wide spectral width. Therefore, if the structure is a coupled multiple quantum well structure in which five or more well layers are tunnel-coupled, the inter-subband absorption spectrum is close to the center wavelength. Can be almost flattened. In a superlattice structure with a large number of wells, the split sub-band energy forms a mini-band. However, as long as the barrier layer is not too thin, only about five wells are combined from the viewpoint of spectral flattening. It is enough.

When the barrier layer is very thin and the energy splitting is large due to strong coupling between the wells, it is necessary to further increase the number of wells to be coupled. Also, for convenience of explanation, here, assuming that the absorption spectrum shape is flat, the entire split base subband and the split excitation subband are included, including the case where the number of wells to be coupled is too small to be called a superlattice. The whole is called a base mini-band and an excitation mini-band, respectively.

The absorption between the mini-bands of the coupled multiple quantum well structure or the superlattice structure has a non-uniform spectrum spread.
However, since the time constant of spectrum hole burning (electron-electron scattering time between levels constituting one mini-band) is as short as several tens to several hundreds of femtoseconds, a sufficiently wide light pulse , Saturation of the entire absorption spectrum occurs (this does not mean that the nonlinear absorption spectrum is flattened), and the refractive index also changes over the entire absorption bandwidth.

By laminating two coupled multiple quantum well structures having a flat absorption spectrum so that the tails of the absorption wavelengths overlap well, a wider absorption spectrum can be realized. The absorption spectrum of the coupled multiple quantum well structure can be controlled by changing at least one of the average thickness of the well layer, the average composition of the well layer, the average thickness of the barrier layer, and the average composition of the barrier layer. As described above, a modified well layer or a modified barrier layer may be used.
The degree of coupling between the wells can be controlled by the composition and thickness of the barrier layer. Similarly, by laminating three or more coupled multiple quantum well structures, a wider band can be realized.

In the conventional multiple quantum well structure, the refractive index change rate (dn / d
dλ) is larger than that near the tail of the absorption spectrum. In other words, the refractive index spectrum has poor symmetry on both sides of the maximum point and the minimum point. Therefore, the first multiple quantum well layer whose refractive index increases with intersubband absorption and the second multiple quantum well whose refractive index decreases with intersubband absorption in the predetermined wavelength region. Even if well layers are combined, the symmetry of the refractive index spectrum shape is poor, so that the group velocity dispersion (proportional to d ² n / dλ ² ) of the entire optical waveguide cannot be reduced, and higher-order dispersion also occurs.

On the other hand, in the coupled multiple quantum well structure in which five or more well layers are tunnel-coupled, the anomalous dispersion of the phase velocity in the absorption flat wavelength region is small, and the maximum point of the refractive index spectrum and The symmetry of the refractive index spectrum on both sides of the minimum is improved. Accordingly, the change in the refractive index of the first coupled multiple quantum well structure and the change in the refractive index of the second coupled multiple quantum well structure in a predetermined wavelength range can be offset to some extent, so that the group velocity dispersion of the entire optical waveguide can be reduced, and as a whole, Higher order dispersion can also be suppressed.

Since a plurality of first coupling multiple quantum well structures having a large refractive index and a plurality of second coupling multiple quantum well structures having a small refractive index are alternately stacked at a predetermined wavelength, the influence of nonlinear light absorption is reduced. Even so, the change in the transverse mode shape of the entire optical waveguide can be kept small. Further, when there is a tunnel coupling between the minibands of the two types of coupled multiple quantum well structures, the number of wells is relatively small.
By alternately stacking multiple types of coupled multiple quantum well structures in a multi-period manner, the carrier distribution can be made more uniform than by stacking two types of superlattice layers having a large total film thickness. The relaxation time can also be shortened.

When the coupling between the coupled multiple quantum well structures is weak in the configuration of the present invention, the absorption saturation of each coupled multiple quantum well structure becomes independent, and nonlinear absorption over each absorption wavelength band hardly occurs. In such a case, it is preferable to use only in the absorption band of one coupled multiple quantum well structure. The refractive index of the entire quantum well structure of the present invention is low in the absorption wavelength band of the first coupled multiple quantum well structure and is high in the absorption wavelength band of the second coupled multiple quantum well structure. Therefore, when used within the absorption wavelength band of the second coupled multiple quantum well structure, the overlap between the coupled multiple quantum well structure and the waveguide mode is more pronounced than when used within the absorption wavelength band of the first coupled multiple quantum well structure. Can be large, high efficiency. Even when the broadening of the nonlinear susceptibility cannot be achieved, the effect of the present invention on the linear response (absorption and chromatic dispersion) is not impaired.

If it is desired to increase the bandwidth of the nonlinear optical response, the tunnel coupling between the coupled multiple quantum well structures must be strengthened. To enhance the coupling between the two coupled multiple quantum well structures for both the base and excitation minibands, an intermediate between the first coupled multiple quantum well structure and the second coupled multiple quantum well structure is provided. Preferably, a fourth coupled multiple quantum well structure having an absorption center wavelength is provided to increase the energy overlap of the corresponding miniband of an adjacent coupled multiple quantum well structure.

As described above, when there is coupling between the coupled multiple quantum well structures, for example, when the absorption saturation of the first coupled multiple quantum well structure occurs, the tunneling through the fourth coupled multiple quantum well structure causes 2 from the base miniband of the coupled multiple quantum well structure to the base miniband of the first coupled multiple quantum well structure, and from the excitation miniband of the first coupled multiple quantum well structure to the excitation miniband of the first coupled multiple quantum well structure. Electrons move to the band, and absorption saturation over the entire spectrum width can be realized. If the barrier layer is thin enough,
The time constant of tunneling can be shortened, and a wide band operation can be realized. Note that setting the excitation light on the short wavelength side in the absorption band can make the carrier relax more quickly and uniformly than setting it on the long wavelength side.

However, in this case, the dispersion of the fourth coupled multiple quantum well structure does not cancel out the others. Accordingly, in the structure having the third coupled multiple quantum well structure described above, the fifth coupled intermediate quantum well structure having the absorption center wavelength between the second coupled multiple quantum well structure and the third coupled multiple quantum well structure. If a coupled multiple quantum well structure is provided and tunnel-coupled to each other, the wavelength dispersion of the fourth coupled multiple quantum well structure and the fifth coupled multiple quantum well structure can be offset to some extent, and the center of the entire absorption spectrum can be reduced. The dispersion in the vicinity can be reduced and the higher-order dispersion can be suppressed. Also, the imaginary part of the absorption spectrum and the third-order nonlinear susceptibility can be flattened.

By using a nitride semiconductor for the coupled multiple quantum well structure, the near infrared region (wavelength 1.5) used in optical communication can be used.
A transition between sub-bands such as a 5 μm band and a 1.3 μm band is easily realized. Further, since the relaxation time between sub-bands in the nitride semiconductor is as short as about 100 fs, and the relaxation time in the sub-band and the mini-band is as short as several tens of fs, an ultra-high-speed nonlinear optical response is possible. In addition, the originally uniform absorption wavelength bandwidth can be widened in response to the short intra-subband relaxation time.

When the average thickness of the well layers and barrier layers constituting the coupled multiple quantum well structure is in the range of 4 to 7 monolayers, the wavelength range used in optical communication can be covered, and the absorption spectrum can be appropriately adjusted. Spread and flatness are achieved.

By integrating a coupled multiple quantum well structure made of a nitride semiconductor with a laminated structure made of another material such as InGaAsP formed on another semiconductor substrate, for example, an InP substrate, a passive optical waveguide and an optical waveguide can be obtained. It can be integrated with other optical semiconductor elements such as amplifiers and semiconductor lasers,
Various stable functions can be realized even for ultrafast pulses.

Thus, according to the nonlinear optical semiconductor device of the present invention, broadening and flattening of the absorption spectrum, reduction of chromatic dispersion and suppression of higher-order dispersion, and broadening of the nonlinear susceptibility can be achieved, and dispersion compensation and chirp compensation can be achieved. In addition, the output pulse can be kept as a narrow Fourier-transform limited pulse, and multistage connection is possible. Further, the walk-off of light pulses of different wavelengths can be reduced, and high efficiency can be achieved. Further, it becomes possible to perform switching on the wavelength-multiplexed signal light at a time.

[0037]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The details of the present invention will be described below with reference to the illustrated embodiments. (First Embodiment) FIG. 1 is a diagram schematically showing a configuration of a nonlinear optical semiconductor device (digital optical routing switch) according to a first embodiment of the present invention, and FIGS. It is a figure which shows the structure of XX 'cross section and YY' cross section.

This optical switch is a photonic integrated circuit 1 having, as main components, a Y branch portion 2, an input optical waveguide 3, and an output optical waveguide 4 (4 ₁ , 4 ₂ ) formed on an InP substrate 10. Provided in the form of Although grooves 6 are formed on both sides of the mesas 2, 3 and 4 constituting the optical waveguide, plateaus 5 are formed on both ends of the substrate 1. Further, a nitride semiconductor laminate 7 including a GaN underlayer 18 and an AlN / GaN quantum well structure 17 is directly bonded to a part of the plateau portion 5 and the upper part of the Y branch portion 2. The end face of the mesa waveguide 3,4 _1, 4 _2, the optical fiber 8 are butt connected.

As shown in FIGS. 2 and 3, the mesas 2, 3,
4 and 5 are InGaAsP etching stop layers 11,
Undoped InP cladding layer 12, undoped InGa
AsP bulk light guide layer 13, undoped InP cladding layer 14, InGaAsP etching stop layer 15,
It consists of an undoped InP cladding layer 16. In the portion where the nitride semiconductor laminate 7 is covered, the uppermost InP cladding layer 16 and the etching stop layer 15 are removed by etching, and the nitride semiconductor laminate 7 is directly bonded on the InP cladding layer 14. ing. The etching stop layers 11 and 15 are provided for convenience of processing and are not essential for the operation of the present invention.

In the mesa waveguides 3 and 4 where the nitride semiconductor laminate 7 is not bonded, light is transmitted to the light guide layer 13.
, And propagates to the upper and lower cladding layers 12, 14, 16. In the Y branch 2 to which the nitride semiconductor laminate 7 is bonded, light is emitted from the InP cladding layer 14 and the AlN
Since the light is totally reflected at the interface of the / GaN quantum well structure 17, most of the light propagates in a form confined to the InP side, but a part of the light leaks into the AlN / GaN quantum well structure 17 in the form of an attenuated wave. ing. For this reason, the AlN / GaN quantum well structure 17 is affected by intersubband absorption.

Optical circulators 9 (9 ₁ , 9 ₂ , 9 ₃ ) are connected to the mesa waveguides 3 and 4 via an optical fiber 8. Input signal light is inputted from the left side of FIG via the optical circulator 9 ₃ and the optical fiber 8 and the input optical waveguide 3 in the Y branch portion 2. On the other hand, the control light is transmitted through the optical circulator 9.
₁ or 9 _2, optical fiber 8 through the waveguide 4 ₁ or 4 _2, is input from the right side of the figure Y branch portion 2. The outputs of the control light and the signal light are separated by the optical circulators 9 ₁ , 9 ₂ , 9 ₃ . Both lights are polarized perpendicular to the substrate (T
(M mode).

[0042] If there is no control light, since the signal light is absorbed by intersubband absorption of AlN / GaN quantum well structure portion 17 in the Y branch portion 2, the output optical waveguide 4 _1, 4 ₂ not output. If a strong control light from the optical circulator 9 ₁ is input, occurs absorption saturation of the connected branches to the optical waveguide 4 ₁ Y-branch portion 2, the signal light is outputted to the optical waveguide 4 _1. Conversely, the control light from the optical circulator 9 ₂ when it is input, the signal light is output from the branching of the light guide 4 _2.

FIG. 4 shows an AlN / GaN quantum well structure 1
7 is a view schematically showing a conduction band structure of a part of FIG. The quantum well structure 17 has a monolayer thickness of 4
a first coupled multiple quantum well structure 23 having six monolayers of GaN well layers 21 ₁ stacked with a 1N barrier layer 22 ₁ interposed therebetween; and a four monolayer thick four layer monolayer arranged on both sides of the structure 23. and AlN barrier layer 22 _2, a thickness of 4 - monolayer a
across the lN barrier layer 22 ₁ thickness 6.7 - monolayer of Ga
A second coupling multiple quantum well structure 24 in which the N-well layer 21 ₂ formed by laminating five layers, thickness 4 arranged on both sides of the structure 24
And AlN barrier layer 22 ₂ - monolayer, formed by stacking five cycles the unit laminate structure consisting of.

[0044] Incidentally, coupled multi-quantum well structure 23, 24 AlN barrier layer 22 ₂ disposed between the would be shared by each of the quantum well structure, thus the barrier layer 22 ₂ is the thickness of all four - monolayer Has become. In practice, the thickness of each layer slightly fluctuates due to the problem of crystal growth. For the well layers 21 ₁ and 21 ₂ , about 1 × Si, which is an n-type impurity, is added.
10 ¹⁹ cm ^-3 doped. Here, the barrier layers 22 ₁ ,
22 ₂ may also be doped. In this case, the barrier layer 22
Electrons generated from ₁ and 22 ₂ are also accumulated in the well layers 21 ₁ and 21 ₂ .

When the thickness of the barrier layer is reduced, subband energy splitting occurs due to tunnel coupling between wells. In the case of an AlN / GaN quantum well, this effect becomes significant when the barrier layer is 5 monolayers or less. As a result, in the coupled multiple quantum well structures 23 and 24, a state similar to a kind of mini-band composed of the same number of sub-bands as the number of wells having slightly different energies is formed (hereinafter referred to as a mini-band for convenience). ). The energy width of the mini band is A
It becomes wider as the 1N barrier layer becomes thinner.

In the quantum well structure 17 having the above structure,
Two minibands, a base miniband 25 and an excitation miniband 26, are formed, and a second excitation miniband 27 is also formed near the upper end of the well of the second coupled multiple quantum well structure 24. Usually, the energy band of the mini band on the high energy side is wider. In the excitation miniband 26 _{1 of} the first coupled multiple quantum well structure 23, the energy difference between the lowest energy subband and the highest energy subband is about 98 meV, which is the energy of LO phonon in GaN (88 meV). Greater than.

When there is no light input, almost all of the electrons are distributed in the base miniband 25. AlN barrier layer 22 ₂ between two types of coupled multiple quantum well structures 23 and 24
Is thin, the base minibands 25 of the two types of coupled multiple quantum well structures 23 and 24 are coupled, and the energy distribution is almost flat over the entire quantum well structure 17 due to the internal electric field. However, the first coupled multiple quantum well structure 23 has a wider mini-bandwidth and a slightly higher average energy.

For the excitation miniband 26, the high energy level 26 ₁ is the first coupled multiple quantum well structure 23.
In addition, although the low energy level 26 ₂ tends to be localized in the second coupled multiple quantum well structure 24, there is a slight overlap in energy, so that the two coupled multiple quantum well structures 23 and 24 Tunneling of electrons between the excitation minibands 26 ₁ and 26 ₂ is possible.

FIG. 5 shows (a) the absorption spectrum α and (b) the wavelength dispersion of the refractive index n for the cw light of the quantum well structure 17. The dashed line indicates each coupled multiple quantum well structure 2
The spectra of 3, 24 and the solid line are the spectra of the entire quantum well structure 17. Absorption peak is wavelength 1.561 μm
Nearby. The absorption coefficient is from 1.412 μm to 1.65.
The value of 90% or more of the peak value is maintained over 238 nm between 0 μm, and a flat broadband absorption spectrum is realized. For comparison, a dotted line shows an example of the spectrum of the conventional quantum well only.

In this embodiment, the maximum wavelength of the refractive index in the first coupled multiple quantum well structure 23 and the minimum wavelength of the refractive index in the second coupled multiple quantum well structure 24 are set so as to substantially match (1. (Around 52 μm). As a result, as a whole, the minimum value of the refractive index (1.32 μm
380n between the maximum value (around 1.70 μm)
m, the anomalous dispersion of the phase velocity is suppressed to about half that of the single coupled multiple quantum well, and the group velocity dispersion (d ² n /
dλ ² ) is also suppressed to a small value.

Assuming a Fourier transform limited Gaussian light pulse having a full width at half maximum of 200 fs for both the control light and the signal light, the full width at half maximum of the spectrum is 2.2 THz. Taking into account the deviation from the ideal Fourier transform limited pulse and the bottom of the spectrum, a band five times as large as this is required, and the center wavelength needs to be shifted by about 80 to 100 nm. Here, the control light wavelength is set to 1.55 μm, and the signal light wavelength is set to 1.65 μm.

When a strong ultrashort light pulse resonating with the transition energy between the mini-bands is incident, a large number of electrons in the base mini-band 25 are instantaneously excited by the excitation mini-band 26. Due to the broad spectral width, absorption occurs simultaneously in multiple subbands that make up the miniband. At this time,
Since the control light wavelength depends on the shorter wavelength side of the absorption band of the second coupled multiple quantum well structure 24, the excitation miniband 26 ₂ starts from the lower energy side subband of the base miniband 25.
Is mainly absorbed in the high-energy sub-band. As a result, both of the mini-band 25 and 26 _2,
There are more electrons occupying the high energy subband than the electrons occupying the low energy subband.

The absorption saturation has a time constant (L
(The relaxation time from the excitation mini-band 26 ₂ to the basal mini-band 25 by O-phonon scattering). Usually, the relaxation time in the miniband is mainly governed by electron-electron scattering (thermalization process) and LO phonon scattering. The average time interval of electron-electron scattering of one electron depends on energy and electron distribution, but is about 10 to 50 fs in this system. On the other hand, the time constant of relaxation in the mini-band due to LO phonon scattering is 10 to 30 fs.

Therefore, the relaxation time in the mini-band is about 1/10 of the relaxation time between the mini-bands, and the absorption saturation covers the entire absorption spectrum of the second coupled multiple quantum well structure 24 in addition to the hole burning component. The components appear prominently. As a result, the absorption coefficient of the signal light having the center wavelength of 1.65 μm also decreases. However, the components extending over the entire absorption spectrum are accompanied by a response delay of about the time required for relaxation within the mini-band (several ten fs).

Since there is coupling between the coupled multiple quantum well structures 23 and 24, the second coupled multiple quantum well structure 24
But absorption saturation occurs. However, this effect is smaller than the response in the same coupling multiple quantum well, and the response delay is large. The influence of the absorption saturation of the second coupled multiple quantum well structure 24 on the signal light having the wavelength of 1.65 μm is small. Since the time constant of relaxation between mini-bands is shorter than the half width of the light pulse,
Absorption recovers quickly when the control light pulse disappears.

During absorption saturation caused by the control light pulse, the signal light pulse is output from a predetermined port. Signal light is affected by linear group velocity dispersion with propagation,
Spectrum of signal light pulse (1.62 μm to 1.68 μm)
m) Since the wavelength dependence of the absorption coefficient and the group velocity dispersion are kept small over the entirety, the dispersion can be easily compensated externally. Therefore, the optical switches of the present embodiment can be connected in multiple stages and used without significantly distorting the signal light pulse waveform.

Since the group velocity dispersion is smaller than that in the case where the first coupled multiple quantum well structure 23 is not provided, the walk-off between the control pulse and the signal light pulse can be reduced, and the efficiency of the nonlinear optical response can be increased. . The small dispersion value also has the advantage that the dispersion compensator can be shortened. In addition,
When both wavelengths are arranged in the absorption band of the second coupled multiple quantum well structure 24 as in this embodiment, the refractive index of the quantum well layer can be increased, and the overlap with the guided light mode can be increased. It is advantageous.

(Second Embodiment) FIG. 6 shows a second embodiment of the present invention.
It is a figure which shows typically the structure of the nonlinear optical semiconductor element (optical gate switch array) which concerns on embodiment.

The optical gate switch array of this embodiment is
It comprises a nitride semiconductor nonlinear optical waveguide 102 formed on a sapphire substrate 101 and an array of passive optical waveguides 103 connected to its input and output. In the figure, for convenience 2
Although only a set of optical waveguides is described, a larger number of optical waveguides may be integrated.

An optical splitter 105 for splitting a signal light via an optical fiber 104 is provided on an input side of an optical waveguide constituting the optical gate switch, and an optical multiplexing device for multiplexing the signal light and the control light. The vessel 106 is connected. The full width at half maximum of the light pulse is 500 fs, the control light wavelength is 1.48 μm, and the signal light is multiplexed into five wavelengths in the wavelength range from 1.52 μm to 1.68 μm. Each light pulse is applied to the optical waveguide 10.
Polarization control is performed so that 2,103 propagates in the TM mode.

When no strong control light pulse is incident, the signal light is absorbed by the intersubband absorption of the nonlinear optical waveguide 102. At the port where the strong control light pulse is incident, a signal light pulse wavelength-multiplexed appears on the output optical waveguide due to absorption saturation of the nonlinear optical waveguide 102. The control light that cannot be absorbed by the nonlinear optical waveguide 102 is removed by a wavelength filter 107 connected via an optical fiber 104. Although not shown, a signal light pulse dispersion compensator is connected as needed.

FIG. 7 is a cross-sectional view schematically showing a cross section including the nonlinear optical waveguide 102 of the optical gate switch.
The nonlinear optical waveguide 102 is provided on the sapphire substrate 101,
Undoped Al _0.8 Ga through the GaN underlayer 111
_0.2 N (7 monolayer) / GaN (7 monolayer) superlattice lower cladding layer 112, undoped GaN / In
_0.2 Ga _0.8 N quantum well optical waveguide layer 113, Si-doped A
l _0.8 Ga _0.2 N / GaN quantum well structure 114 (carrier density〜1 × 10 ¹⁹ cm ⁻³ ), undoped Al _0.8
Ga _0.2 N (7 monolayer) / GaN (7 monolayer)
It has a structure in which the superlattice upper cladding layer 115 is sequentially laminated, and the upper cladding layer 115 is processed into a mesa shape. The passive optical waveguide 103 has a layer structure obtained by removing the Si-doped Al _0.8 Ga _0.2 N / GaN quantum well layer 114 from the layer structure of the nonlinear optical waveguide 102.

FIG. 8 is a diagram schematically showing a conduction band structure of a part (a basic unit described later) of the Al _0.8 Ga _0.2 N / GaN quantum well structure 114. Al _0.8 Ga _0.2 N
/ GaN quantum well structure 114 includes a first coupled multiple quantum well structure 121, a fourth coupled multiple quantum well structure 124,
Second coupled multiple quantum well structure 122, fifth coupled multiple quantum well structure 125, third coupled multiple quantum well structure 123
Have 5 monolayers of Al _0.8 Ga _0.2 N between each
A basic unit is a structure sequentially laminated with the barrier layer 126 interposed therebetween.
_It has a structure in which two periods are repeated with the _0.8 Ga _0.2 N barrier layer 127 interposed therebetween.

Each coupled multiple quantum well structure has a Ga
N well layer 128 and Al _0.8 Ga _{0.2 of} 5 monolayer thickness
And N barrier layer 129. The thickness of the well layer and the number of the well layers are set to 5 monolayers and 7 wells in the first coupled multiple quantum well structure 121, and set to 5 in the fourth coupled multiple quantum well structure 124.
5 monolayer, 6 well, second coupled multiple quantum well structure 1
Reference numeral 22 designates 6 monolayers and 6 wells, fifth coupled multiple quantum well structure 125 includes 6.5 monolayers and 5 wells, and third coupled multiple quantum well structure 123 includes 7 monolayers and 5 wells.

Each coupled multiple quantum well structure 121, 122,
The base mini-bands 130 of 123, 124 and 125 are almost flatly spread over the whole by tunneling. The energy of the excitation mini-band 131 is slightly different for each coupling multiple quantum well structure, but since there is an overlap with the adjacent coupling multiple quantum well structure, electrons can move with each other by tunneling.

FIG. 9 shows this Al _0.8 Ga _0.2 N / GaN
7 shows the wavelength dependence of the absorption spectrum α of the quantum well structure 114 and the refractive index change Δn / n. The broken line represents the spectrum of each of the coupled multiple quantum well structures 121, 122, 123, 124, and 125, and the solid line represents the spectrum of the entire quantum well structure 114. Although there are some irregularities, a relatively flat absorption coefficient can be obtained in the wavelength range of 1.45 μm to 1.8 μm,
The phase dispersion anomalous dispersion wavelength range is from around 1.4 μm to 1.9.
It extends to around 5 μm.

In this example, all the coupled multiple quantum well structures have the same barrier layer thickness. However, there is a tendency that the subband interval in the miniband on the short wavelength side is widened and concavities and convexities are increased. In order to reduce this unevenness, the barrier layers of the first and fourth coupled multiple quantum well structures 121 and 124 having thinner wells are made to be monolayer thicker, or the coupled multiple quantum well structure is reduced. The number of wells can be increased.

In such a configuration, the optical non-linearity due to the intersubband absorption and the basic operation principle of the mitigation are substantially the same as those of the first embodiment. It can be used. Further, since the coupling between adjacent coupled multiple quantum well structures is stronger than in the first embodiment, and the overlap of the wavelength region covered by each coupled multiple quantum well structure is larger, the influence of absorption saturation (non-linearity) The optical response extends over a wider wavelength range than in the first embodiment. Therefore, when the control light pulse having the wavelength of 1.48 μm is incident, 1.52 μm
The gate can be opened and closed at a high speed in a lump with respect to the signal light of five channels multiplexed in the wavelength range from μm to 1.68 μm.

Since the group velocity dispersion value is small, the walk-off of wavelength-multiplexed ultrashort optical pulses distributed over a wide wavelength range can be reduced, and the efficiency can be improved.
Note that the present invention is not limited to the above embodiments. Conditions such as the thickness and number of the well layers and the thickness of the barrier layers in the first and second coupled multiple quantum well structures can be appropriately changed as long as the well layers are tunnel-coupled to form a coupled quantum well. Further, the number of layers of each coupled multiple quantum well structure can be appropriately changed according to specifications.

The present embodiment can be variously modified, such as a well structure, an optical waveguide structure, a device configuration, a wavelength arrangement, addition of fine adjustment electrodes, integration, and the like. The material system is not limited to nitride semiconductors, but AlAs / InGaAs.
When a material having a smaller effective mass than a nitride-based semiconductor, such as an s-based or AlAsSb / InGaAs-based material, is used, it is necessary to set the barrier layer to be thicker than in the embodiment. If the barrier layer is thinner than the optimum value, the undulation of the absorption spectrum becomes large, so that it becomes necessary to couple a larger number of wells. In addition, various modifications can be made without departing from the scope of the present invention.

[0071]

As described above in detail, according to the present invention, in an ultrafast nonlinear optical semiconductor device utilizing nonlinear optical response based on subband absorption, a long wavelength absorption spectrum in the first coupled multiple quantum well structure is obtained. By stacking the first and second coupled multiple quantum well structures such that the bottom of the first and second coupled multiple quantum well structures overlap the shorter wavelength of the absorption spectrum in the second coupled multiple quantum well structure, the absorption band can be expanded and the absorption coefficient can be increased. , The chromatic dispersion value can be reduced, and the higher-order dispersion can be suppressed. Then, the change of the pulse shape or spectrum of the ultrashort light pulse can be kept within a range where it can be compensated, so that multi-stage connection becomes possible. Further, it is possible to reduce the walk-off of optical pulses having different wavelengths, and to improve the efficiency as a nonlinear optical element.

[Brief description of the drawings]

FIG. 1 is a diagram schematically showing a schematic configuration of a nonlinear optical semiconductor device according to a first embodiment.

FIG. 2 is a view showing a cross-sectional structure taken along the line XX ′ of the nonlinear optical semiconductor device of FIG. 1;

FIG. 3 is a diagram showing a cross-sectional structure taken along the line YY ′ of the nonlinear optical semiconductor device of FIG. 1;

FIG. 4 is a diagram schematically showing a conduction band structure of a part of the AlN / GaN quantum well structure according to the first embodiment.

FIG. 5 is a diagram for explaining the absorption spectrum and the refractive index dispersion of the AlN / GaN quantum well structure according to the first embodiment.

FIG. 6 is a diagram schematically showing a schematic configuration of a nonlinear optical semiconductor device according to a second embodiment.

FIG. 7 is a view showing a cross-sectional structure of a nonlinear optical waveguide section of the nonlinear optical semiconductor device of FIG. 6;

FIG. 8 is a diagram schematically showing a conduction band structure of a part of the AlGaN / GaN quantum well structure according to the second embodiment.

FIG. 9 is a diagram for explaining an absorption spectrum and a refractive index dispersion of an AlGaN / GaN quantum well structure according to the second embodiment.

[Explanation of symbols]

1 InP optical integrated circuit 2 Y branch 3, 4, 103 passive optical waveguide 5 plateau 6 groove 7 nitride semiconductor 8, 104 optical fiber 9 optical circulator 10 ‥‥ InP substrate 11,15 ‥‥ InGaAsP etching stop layer 12,14,16 ク ラ ッ ド InP cladding layer 13 ‥‥ InGaAsP optical guide layer 17,114 ‥‥ Al (Ga) N / GaN quantum well structure 18,111 {GaN layer 21, 128} GaN well layer 22, 126, 127, 129 {Al (Ga) N barrier layer 23, 121} first coupled multiple quantum well structure 24, 122} second coupling Multiple quantum well structure 25,130 {base mini-band 26,131} excitation mini-band 101 {sapphire substrate 102} nonlinear optical waveguide 105} optical splitter 106 {optical multiplexer 107} wavelength filter 112, 115 {AlGaN cladding layer 113} InGaN optical guide layer 123} third coupled multiple quantum well structure 124 {fourth coupled multiple quantum well structure 125} ５Fifth coupled multiple quantum well structure

Claims (3)

(57) [Claims] 1. A nonlinear optical semiconductor device utilizing a nonlinear optical response based on intersubband absorption of a semiconductor quantum well layer, wherein a plurality of quantum well layers are stacked via a barrier layer, and each well layer is tunnel-coupled. The first and second coupled multiple quantum well structures are stacked, and the longer wavelength side of the absorption spectrum of the first coupled multiple quantum well structure and the shorter absorption spectrum of the second coupled multiple quantum well structure. A nonlinear optical semiconductor device, wherein each quantum well structure is designed such that the skirts on the wavelength side overlap. 2. The first and second coupled multiple quantum well structures include:
2. The nonlinear optical semiconductor device according to claim 1, wherein each of the nonlinear optical semiconductor devices has five or more quantum well layers. 3. The first and second coupled multiple quantum well structures include:
2. The nonlinear optical semiconductor device according to claim 1, wherein each of the nonlinear optical semiconductor devices is made of a nitride semiconductor.