JP2011014857A - Optical element, photon generator using the same, light generator, optical recorder, and photodetector - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a single photon generator which has both high excitation efficiency and photon extraction efficiency, and has little background noise.SOLUTION: A quantum dot 4 is embedded between an upper semiconductor layer 2 and a lower semiconductor layer 3. An opening 6 with a projection having a structure in which metal projections 11-14 are protruded from the opening is formed on an upper metal light shielding film 5. Excitation light having polarized light in a Y direction emitted from an excitation light source 21 is resonant with an antenna Y formed of the metal projections 13 and 14 and excites the quantum dot 4. Then, light emitted from the quantum dot is combined with an antenna X formed of the projections 11 and 12 and extracted to the outside as a photon of X polarized light.

Description

  The present invention relates to an optical element and a photon generation device, a light generation device, an optical recording device, and a light detection device using the optical element, and in particular, an optical element having at least two antennas that resonate with different wavelengths, respectively. The present invention relates to a photon generation device, a light generation device, an optical recording device, and a photodetection device using the above. Of these devices using optical elements, a photon generator, in particular, relates to a photon generator that can extract single photons generated from electron-hole pairs based on photoexcitation with higher extraction efficiency than conventional devices.

  In optical devices such as a photon generator, a light generator, an optical recording device, and a photo detector using a semiconductor, a metal film is usually used for an electrode or a light shielding film. Since the light is used only for blocking / transmitting light, the opening has a simple shape such as a circle or a rectangle. Accordingly, it has been impossible to preferentially transmit a specific wavelength and a specific polarization with a metal film having an opening.

By the way, quantum information technology that uses the quantum mechanical state of light or matter is expected to provide advanced security technologies such as highly secure cryptographic communications and highly confidential authentication. For example, in quantum cryptography communication, information expressed using quantum mechanical degrees of freedom such as polarization and phase (hereinafter, quantum information) is transmitted on photons to ensure the presence of an eavesdropper on the communication path. It can be detected and a secure private key can be distributed.
A key device for such quantum information technology is a single photon generator that generates photons one by one. Single photons can be generated from single atoms, single ions, etc., but single photon sources based on semiconductor quantum dots are actively studied, especially as devices for generating single photons from semiconductors. ing.
In order to generate a single photon from a quantum dot, it is necessary to generate a pair of electrons and holes in the quantum dot. There are two types of current injection type that inject holes. Among them, the development of the high-quality single-photon generator has been realized that is preceded by the development of the photo-excitation type and the influence of the background noise is relatively small. The photon generator of the present invention is also based on this photoexcited single photon generation technique.

Although it is necessary to use light emission from a single quantum dot to generate a single photon, the density of quantum dots grown by molecular beam epitaxy (MBE) is usually about 1 × 10 ¹⁰ (cm ⁻² ). Because of its large size, light collected from many quantum dots is picked up only by focusing with an objective lens, and a single photon cannot be obtained. As a method for coping with this and isolating a single quantum dot, roughly two types of methods are employed. The first method is a method in which only an island (referred to as a mesa structure) containing about one quantum dot is left and its periphery is etched away. However, in this case, background light caused by defects on the etching side wall and impurities becomes noise, which may reduce the quality of single photons. Another method is a method in which a metal light-shielding film is formed on a quantum dot sample, and an opening for isolating about one quantum dot is formed there (see, for example, Non-Patent Document 1). If this method is used, the influence of background light can be minimized. In addition, since the metal film can be used as an electrode, it is possible to perform electrical control such as finely adjusting the wavelength or electrically injecting electrons by applying a voltage to the quantum dots.

However, there are still issues to be improved. One of them is the problem of photon extraction efficiency. Photons emitted from the quantum dots are usually collected by a high-power lens and then coupled to a fiber or the like. However, since single photon generation by quantum dots is based on spontaneous emission, photons are emitted randomly at all azimuth angles. For this reason, photons emitted in directions other than the lens (for example, in a direction parallel to the substrate surface) cannot be effectively extracted outside. In particular, when the metal opening method is adopted, the possibility that the photon is scattered by the metal is increased, and the extraction efficiency is lowered.
As a structure for improving the photon extraction efficiency from the quantum dots, several methods using a microresonator have been tried. A typical example is shown below.
Non-Patent Document 2 shows a structure in which quantum dots are embedded in a micropillar resonator. This micro pillar resonator can be said to be an advanced version of the mesa structure. InAs quantum dots, which are light emitters, are sandwiched from above and below by a DBR mirror composed of a multilayer film of GaAs and AlAs, and have a structure cut out in a column shape perpendicular to the substrate by dry etching on the side. In such a structure, in principle, one quantum dot can be isolated by cutting a pillar into a thin layer, and light emission from the quantum dot is caused by the vertical cavity effect and total reflection on the side wall. It is also possible to take out efficiently in the upward direction.

Non-Patent Document 3 shows a structure in which quantum dots are embedded in point defects of a two-dimensional photonic crystal. In this structure, since the two-dimensional photonic crystal portion is a forbidden band with respect to the emission wavelength of the quantum dots, it is a microresonator in which light is confined only at the point defect portion. Under such a structure, light emission in the lateral direction (substrate direction) of the quantum dots is strongly suppressed by the band gap effect, so that photons can be efficiently extracted in a direction perpendicular to the substrate.
However, in these structures, as in the case of the mesa structure, background noise due to etching sidewall defects and impurities may become a problem. In addition, since it is not technically easy to attach electrodes to these structures, it is considered unsuitable for electrical control.

In Non-Patent Document 4, a single photon generation device having a structure in which a DBR mirror and a metal micro-aperture are combined is realized. If this structure is employed, the metal opening above the quantum dot can be used as an electrode for electrical control. In addition, a DBR mirror is disposed below the quantum dot, and a resonator is formed between the upper air-semiconductor interface. This allows photons from the quantum dots to be emitted mainly upward. As a result, the extraction efficiency is improved as compared with the apparatus of Non-Patent Document 1, but it is still considered that the transmittance at the minute aperture ultimately limits the extraction efficiency.
The above three documents relate to an example using a semiconductor resonator, but recently, a device using a metal microstructure has been reported. Non-Patent Document 5 employs a structure in which a point light source such as a quantum dot is arranged in the vicinity of a silver nanowire having a diameter of 50 to 100 nm. Since a high-density surface plasmon mode stands at the air-silver interface in such a fine nanowire, spontaneous emission from a point light source preferentially couples to the nanowire plasmon mode, and eventually evanescent. It can be extracted into a dielectric waveguide or fiber through coupling. Non-Patent Document 6 shows that the extraction of light from quantum dots is improved by a micro antenna structure made of aluminum. However, since these structures do not have a structure for isolating a single quantum dot, it is difficult to apply to a high density self-forming semiconductor quantum dot. Also, a method of using such an antenna as an electrode for electrical control is not explicitly shown.

Z. Yuan, et al., Science 295, 102 (2002) M. Pelton, et al., Phys. Rev. Lett. 89, 233602 (2002) D. G. Gevaux, et al., Appl. Phys. Lett. 88, 131101 (2006) A. J. Bennett, et al., Appl. Phys. Lett 86, 181102 (2005) D. E. Chang, et al., Phys. Rev. Lett. 97, 053002 (2006) J. N. Farahani, et al., Phys. Rev. Lett. 95, 017402 (2005)

  The first object of the present invention is to provide a plurality of optical elements capable of transmitting only a specific polarized light at a specific wavelength. A second problem of the present invention is to make it possible to add a new function to an optical device such as a photon generation device, a light generation device, an optical recording device, or a light detection device using such an optical element. is there.

  For example, as described above, the structure for efficiently extracting photons proposed in several documents still has some problems. One of them is that the efficiency of photoexcitation is still low even when the photon extraction efficiency is optimized. In the photoexcited single photon generating element, it is necessary to introduce excitation light as shown in the energy level diagram of FIG. 1 in order to excite the quantum dot from the ground level to the excitation level. That is, the quantum dot of the ground level G is excited by the excitation light to the excitation level E, and after being relaxed to the level S by non-radiative relaxation, a single photon is generated and relaxed to the ground level G. In this case, when only single photons are finally extracted from the quantum dots, it is necessary to cut wavelength components other than single photons with a wavelength filter in order to avoid an increase in background noise due to scattering of excitation light and the like. is there. At this time, if the wavelength of the excitation light and the wavelength of the single photon are too close, it becomes difficult to sufficiently cut the background noise. Therefore, as the excitation light, light having a wavelength sufficiently separated from the single photon (usually 50 nm or more) is preferably used. That is, when the wavelength of the excitation light is λe and the wavelength of the single photon is λs, it is usually desirable that λe <λs−50 nm.

As described above, when the wavelengths of the excitation light and the single photon are greatly different from each other, both of them cannot be resonated simultaneously with one resonator. Therefore, when priority is given to the extraction efficiency of single photons, it is necessary to enter sufficiently strong excitation light to excite the quantum dots, and the excitation efficiency is reduced.
In addition, in a photon generator based on a mesa structure that isolates a single quantum dot by etching, such as a micro-pillar resonator or a photonic crystal resonator, background noise is caused by light emission from impurities trapped in impurities or defects on the side wall. Will become bigger. In addition, attaching electrodes to these structures is not technically easy and is not suitable for electrical control. In that respect, there is an advantage in an aperture-based photon generator that can shield light from other than the quantum dots with a metal film and can also be used as an electrode. However, when the aperture diameter is reduced, the photon extraction efficiency decreases. End up.
As described above, there has never been a photoexcited single photon generating element having both high photoexcitation efficiency and extraction efficiency and having high S / N and electrical controllability.
One of the objects of the present invention is to provide a single photon generator that has both high photoexcitation efficiency and photon extraction efficiency and is less affected by background light.

The optical element of the present invention is provided with at least two metal protrusions projecting into the opening in different directions from the peripheral edge of the opening, and the at least two metal protrusions and gaps formed at respective tips are provided. At least two antennas configured to include each have different resonance wavelengths.
In such an opening with protrusions, the metal protrusion operates as an antenna for the minute light emitting part, and as a result, photons emitted from the minute light emitting part can be efficiently extracted to the outside.
Preferably, the metal protrusion includes a metal protrusion formed in a pair so as to have a gap at a central portion of the opening. Preferably, the metal protrusions formed in a pair include at least two pairs of metal protrusions orthogonal to each other.

  The optical element of the present invention is characterized in that w1 ≠ w2, where w1 and w2 are gaps formed at the tips of the orthogonal metal protrusions, respectively. By designing such an opening with an asymmetric protrusion, the antenna effect can be expressed at different wavelengths depending on the polarization.

  The photon generator according to the present invention includes a light emitting layer including a minute light emitting portion, first and second semiconductor layers stacked with the light emitting layer interposed therebetween, and the minute semiconductor layer formed on the first semiconductor layer. A metal film having an opening on the light emitting portion; and an excitation light source for exciting the minute light emitting portion, wherein the opening has at least two metal protrusions projecting from the peripheral portion of the opening to the inside of the opening in different directions. It is the opening with a protrusion provided with.

  In addition, the photon generator of the present invention includes an excitation light source that generates light having a wavelength that resonates with one of the two antennas configured by the gap between the orthogonal metal protrusion tips. Further, the two antennas configured by the gaps at the tips of the orthogonal metal protrusions resonate with different optical transition levels of the minute light emitting part, respectively. Thereby, high efficiency can be achieved through different antennas in both the light excitation process and the light extraction process.

In the photon generator of the present invention, the size of the minute light emitting part is smaller than the emission wavelength. Preferably, quantum dots are used as the minute light emitting part. Further, as a position where the quantum dots are arranged, preferably, when the emission wavelength of the quantum dots is λ, and the refractive index of the semiconductor layer is n, the edge of the metal film including the projection opening of the light emitting layer The depth d of the above satisfies d <λ / 2n. More preferably, when the emission wavelength of the quantum dots is λ and the refractive index of the semiconductor layer is n, the depth d from the end of the metal film including the opening with the protrusion of the light emitting layer is λ / 20n. <D <λ / 5n is satisfied. In this way, by making the distance between the micro light emitting part (quantum dot) and the projection opening close to an appropriate distance, the light generated from the micro light emitting part is more strongly coupled to the projection opening and the photon extraction efficiency Can be optimized.
Preferably, in the opening with protrusions, the tip of the metal protrusion protruding into the opening is pointed. As a result, the antenna mode is more easily excited in the gap formed at the tip of the metal protrusion, and the light extraction efficiency can be further increased.

Furthermore, according to another embodiment of the present invention, at least a part of the metal film is embedded in the semiconductor layer. As a result, light generated from the quantum dots can be more strongly coupled to the opening with protrusions, and the photon extraction efficiency can be improved.
Furthermore, according to another embodiment of the present invention, an oxide film having an opening is inserted between the metal film and the light emitting layer under the protrusion-equipped opening. Thereby, it is possible to prevent light generated from the quantum dots from escaping in the direction of the substrate surface and to more strongly couple the light to the opening with the protrusion, and thus it is possible to improve the photon extraction efficiency.

Furthermore, according to another embodiment of the present invention, at least one of the two semiconductor layers has a structure in which two types of semiconductors having different refractive indexes are alternately stacked. In this case, since the semiconductor multilayer structure functions as a DBR mirror, an effective Fabry-Perot resonator is formed around the quantum dots, and the light emitted from the quantum dots is strongly coupled to the aperture with protrusions, resulting in the extraction efficiency of photons. Can be further improved.
Furthermore, according to another embodiment of the present invention, the metal film is used as an electrode, and a metal electrode is formed on the second semiconductor layer to form a junction with the second semiconductor layer. Carrier injection is possible.

(Function)
In the opening with protrusions, the metal protrusion protruding inside the opening operates as an effective antenna and plays a role of strengthening the coupling between light and quantum dots. In the present invention, at least two metal protrusions that are asymmetric in length are formed in directions orthogonal to each other, and the protrusion in one direction operates as an antenna for photoexcitation and the other protrusion operates as an antenna for extracting photons. .
By changing the length of the metal protrusion, etc., it becomes possible to independently optimize the resonance wavelengths of the respective antennas for light excitation and light extraction. Thus, a photoexcitation photon generator having both high excitation efficiency and photon extraction efficiency can be configured through the antenna effect.
Furthermore, the metal film with a projection-like opening is expected to have an effect as a light-shielding film that blocks background light emitted from other than the quantum dots.
Another advantage of using such a metal film with an opening with protrusions is that the metal protrusion can be used not only as an antenna but also as an electrode. For example, by efficiently applying an electric field to the quantum dots, the emission wavelength can be finely adjusted through the Stark effect.

  According to the optical element of the present invention, it becomes possible to efficiently extract only a plurality of light beams having a specific wavelength and a specific polarization. And by applying this optical element to an optical device, it becomes possible to exhibit new functions and effects in the optical device. For example, in the photon generation device, the excitation effect and the photon extraction efficiency from the quantum dots can be improved by the antenna effect in the opening with protrusions. In addition, since light is selectively extracted from a limited space near the center of the projection opening, the background light due to impurities can be reduced.

The energy level diagram of the quantum dot used with the single photon generator of this invention. The top view and sectional drawing which show the structure of the single photon generator which is the 1st Embodiment of this invention. The device structure figure and operation | movement explanatory drawing explaining the gap width dependence of the antenna effect in opening with a protrusion. The device structure figure and energy level diagram explaining operation | movement of the single photon generator which is the 1st Embodiment of this invention. The device structure figure and operation | movement simulation figure which show the specific Example of the single photon generator which is the 1st Embodiment of this invention. The top view and sectional drawing which show the Example of the single photon generator which is the 2nd Embodiment of this invention. The top view and sectional drawing which show the structure of the single photon generator which is the 3rd Embodiment of this invention. The top view and sectional drawing which show the structure of the single photon generator which is the 4th Embodiment of this invention. The top view and sectional drawing which show the structure of the single photon generator which is the 5th Embodiment of this invention. The top view and sectional drawing which show the structure of the single photon generator which is the 6th Embodiment of this invention. Sectional drawing explaining operation | movement of the single photon generator which is the 6th Embodiment of this invention. The figure which shows the form from which the metal film which has an opening with a protrusion in this invention differs. The top view and sectional drawing which show the structure of the surface emitting laser using the opening with a single protrusion which is the 7th Embodiment of this invention. The top view and sectional drawing which show the structure of the surface emitting laser using the opening arrangement | sequence with a protrusion which is the 7th Embodiment of this invention. The figure which shows the structure of the wavelength tunable surface emitting laser which is the 8th Embodiment of this invention. The figure explaining operation | movement of the wavelength tunable surface emitting laser which is the 8th Embodiment of this invention. The figure which shows the structure of the wavelength tunable surface emitting laser using the opening array with a protrusion which is the 8th Embodiment of this invention. The optical probe which has an opening with a protrusion in the front-end | tip used in the 9th Embodiment of this invention. The figure which shows the structure of the optical recording device which is the 9th Embodiment of this invention. The transmission spectrum of the opening with a protrusion used for the optical recording device which is the 9th Embodiment of this invention. Diarylethene, which is a recording medium (photochromic material) used in the ninth embodiment of the present invention. The figure which shows operation | movement of the filter which is the 10th Embodiment of this invention. The top view and sectional drawing which show the structure of the photon detection apparatus which is the 11th Embodiment of this invention. The figure explaining the operation | movement of the Example of the wavelength selection photodetector which is the 11th Embodiment of this invention. The figure which shows the structure of the image sensor which is the 12th Embodiment of this invention. The figure explaining operation | movement of the Example of the wavelength selection image sensor which is the 12th Embodiment of this invention. The figure explaining the structure of the color image sensor which is another Example of the 12th Embodiment of this invention.

Next, embodiments of the present invention will be described in detail with reference to the drawings.
(First embodiment)
FIGS. 2A and 2B are a top view and a cross-sectional view showing the structure of the single photon generator according to the first embodiment of the present invention. As shown in FIG. 2, a quantum dot layer 1 having quantum dots 4 is embedded between an i-type upper semiconductor layer 2 and an i-type lower semiconductor layer 3. On the upper semiconductor layer 2, a metal light shielding film 5 is attached. The metal light-shielding film 5 is provided with a projection-shaped opening 6 for taking out photons from the quantum dots 4. Here, unlike the normal circular opening, the protrusion-provided opening 6 has the metal protrusions 11 and 12 left and right (in the X direction) and the metal protrusions 13 and 14 up and down (the Y direction) from the periphery of the opening to the center. B) has an overhanging structure. Further, the quantum dots 4 are located immediately below the gap between these metal protrusions.
In such an opening 6 with protrusions, a pair of metal protrusions facing each other functions as an effective optical antenna. That is, when light is incident on the projection opening, the plasmon antenna mode is excited in the gap between the metal projections, and through this mode, the transmittance of light having a polarization parallel to the antenna is resonantly increased at a specific wavelength. (Hereafter, this is called the antenna effect for the sake of simplicity).

Therefore, in the opening 6 with protrusions in FIG. 2, the metal protrusions 11 and 12 function as antennas for X-polarized light (hereinafter referred to as antenna X), and the metal protrusions 13 and 14 are antennas for Y-polarized light (hereinafter referred to as antenna Y). Function as.
Here, the resonance wavelength λ at which the antenna effect appears depends on the gap width w. In order to show this gap width dependency, in FIG. 3, for the sake of simplicity, an explanation will be given with an example of the projection-equipped opening X60 having only an antenna for X-polarized light, with simulation results. FIGS. 3A and 3B are a top view and a cross-sectional view of a light-emitting element using the projection opening X60 having two metal projections extending in the X direction. When quantum dots emit light with such a structure, an antenna effect appears for X-polarized light parallel to the antenna, and the transmittance increases resonantly at a specific wavelength determined by the structure.

FIG. 3C shows a simulation result of the transmittance spectrum (transmission wavelength dependency of the transmittance) when different gap widths w (w = 60 nm, 80 nm, 100 nm) are assumed. Here, the aperture diameter is fixed at 200 nm. As can be seen from this figure, the resonance of the antenna effect tends to shift to the longer wavelength side as the gap width w of the antenna becomes smaller.
The dependence of the resonance wavelength on the gap width can be understood by considering each metal protrusion as a dipole (dipole) as shown in FIG. In a structure in which two metal protrusions face each other like the protrusion opening X60, a negative (attractive) dipole-dipole interaction occurs between these protrusions. Therefore, as the gap width w becomes smaller, the negative interaction becomes stronger and the antenna mode energy becomes smaller. As a result, the resonance of the antenna effect shifts to the longer wavelength side.
For the above reasons, the resonance wavelength λ of the antenna effect changes depending on the gap width w. On the contrary, if this property is used, the wavelength at which the antenna effect appears can be controlled by designing the gap width w.
The above is the case of an opening with protrusions having two protrusions (one antenna), but the same description applies to the opening 6 with protrusions of FIG. 2 having four protrusions (two antennas). 2 has two sets of antennas, antenna X and antenna Y, the antenna effect is manifested in each of X-polarized light and Y-polarized light. The resonance wavelength at which the antenna effect occurs is a function of the aperture diameter a and the gap widths w1 and w2. Therefore, by optimizing the three parameters a, w1, and w2, the resonance wavelength for X-polarized light and the resonance wavelength for Y-polarized light can be controlled independently.

In the present embodiment, the gap width w2 of the antenna Y is designed to be longer than the gap width w1 of the antenna X (w2> w1). Therefore, the resonance wavelength of the antenna effect differs between X polarized light and Y polarized light. Yes. Here, the X-polarized light is designed to resonate with the wavelength λs of the single photon and the Y-polarized light resonates with the wavelength λe of the excitation light (see the energy level diagram of FIG. 1 for the wavelength, λs, and λe).
In this design, by adjusting the three degrees of freedom of the structural parameters a, w1, and w2, the two degrees of freedom of the desired resonance wavelengths λs and λe are determined. . However, when the gap widths w1 and w2 are too large, the two metal protrusions are not coupled to each other, so that a sufficient antenna effect cannot be obtained. Therefore, the opening 6 with protrusions is designed so that the gap widths w1 and w2 are larger than the size s of the quantum dots and sufficiently smaller than the emission wavelength λs of the quantum dots. Here, s <w1 <w2 <λ / 5.
Further, the distance from the quantum dot 4 to the projection-attached opening 6, that is, the distance d between the lower end of the metal light-shielding film 5 and the quantum dot layer 1 is also an important parameter. In order to couple the light from the quantum dots 4 to the electromagnetic field mode of the protruding opening 9, the distance d needs to be at least d <λ / 2n. Here, n represents the refractive index of the upper semiconductor layer 2.

The smaller the distance d, the stronger the coupling. However, it is known from the literature (DE Chang, et al., Phys. Rev. Lett. 97, 053002) that if d is too small, quantum dots do not shine due to non-radiative relaxation to metal. Therefore, in order to optimize the photon extraction efficiency, it is desirable that λ / 20n <d <λ / 5n.
In the present embodiment, as the metal light-shielding film 5 material, gold, silver, which can induce surface plasmon with a relatively small optical loss in the visible light to near infrared light region in the projection opening 6, Aluminum, copper, or the like is used.
As described above, in the present invention, unlike a conventional surface emitting laser or the like, a micro light source having a wavelength smaller than the emission wavelength is disposed immediately below the gap between the metal protrusions, so that the light from the light source can be converted into an antenna mode with a protrusion opening. It is directly bonded to. The function of such an “antenna for efficiently extracting light from a nanoscale light source” is a feature of the present invention.

Next, the operation of the photoexcitation single photon generator will be described with reference to FIG. First, (1) excitation light having a wavelength λe is generated from an excitation light source, and is incident on the projection opening 6 as Y-polarized light. As a result, the excitation light resonates with the antenna Y, and the quantum dots 4 are excited to the level E. (2) The excited quantum dot transits to the level S by non-radiative relaxation. At this time, since spin relaxation also occurs simultaneously, the spin state in the quantum dot is random. (3) When the quantum dot relaxes from the level S to the level G, it emits a single photon of wavelength λs. At this time, since the spin state in the quantum dot is random, the polarization during light emission is usually random. However, due to the antenna effect of the projection opening, the emitted photon is preferentially coupled to the antenna X, and is extracted as an X-polarized photon upward in the drawing.
In this way, high-efficiency operation is possible by using plasmon resonance of the opening with protrusions in both the excitation process and the light emission process. In addition, since the polarized light of the excitation light and the single photon are orthogonal to each other, they can be easily separated by using the polarization beam splitter (PBS) 20.

Next, an example of the first embodiment will be described with reference to FIG. FIGS. 5A and 5B are a top view and a cross-sectional view showing an outline of the device structure of this embodiment. As the quantum dot 4, a self-formed InGaAs / GaAs quantum dot having a diameter of 20 nm grown on an i-GaAs substrate by molecular beam epitaxy (MBE) (a structure in which an InGaAs quantum dot is embedded in an i-GaAs barrier layer) is used. . Further, the thickness of the upper semiconductor layer 2 was set to 50 nm. It is known that the InGaAs / GaAs quantum dots grown in this way generally emit light at a wavelength near 1 μm.
By patterning a resist on such a quantum dot substrate by electron beam lithography and lifting it off after Au deposition, the opening with protrusions 6 shown in FIGS. 5A and 5B is produced. Here, as the parameters of the projection opening 6, the metal film thickness is 100 nm, the opening diameter is 240 nm, the gap width between the metal projections 11 and 12 is 100 nm, the gap width between the metal projections 13 and 14 is 120 nm, and the metal projection 11 The width of ˜14 was 60 nm.

The structure described here can also be produced by the focused ion beam method (FIB). However, the quantum dots existing immediately below the projection opening are not damaged as much as possible during the process. For this reason, it is desirable to manufacture by a lift-off method.
In order to confirm the performance of the designed photon generator, the efficiency with which photons are extracted to the outside via the antenna of the projection opening 6 when the point light source is placed at the position of the quantum dot was examined by simulation. The transmittance spectrum (dependence of transmittance on the emission wavelength) is expected to be as shown in FIG. Reflecting that the gap width of the antenna is asymmetrical in the vertical and horizontal directions, the resonance wavelength is different between X-polarized light and Y-polarized light. In this case, since the antenna X has a shorter gap width than the antenna Y, the resonance wavelength of X-polarized light is longer than the resonance wavelength of Y-polarized light. Specifically, as shown in FIG. 5C, the X-polarized light resonates near the wavelength λs = 970 nm, and the Y-polarized light resonates near the wavelength λe = 780 nm.

  In this case, the specific photon generation procedure is as follows. (1) Excitation light having a wavelength of λe = 780 nm is generated by a light source such as a Ti sapphire laser, and is incident on the projection-attached opening 6 as Y-polarized light. As a result, the excitation light resonates with the antenna Y, and the sample is excited. (2) When the excitation wavelength is λe = 780 nm, the InGaAs quantum dots 4 are not directly excited, but the surrounding GaAs barrier layer is excited and photocarriers (electrons, holes) are generated. The generated optical carrier is eventually captured by the quantum dots 4 and excitons (electron / hole pairs) are formed. At this time, since spin relaxation also occurs simultaneously, the spin state of the exciton trapped in the quantum dot 6 becomes random. (3) When the quantum dot relaxes from the level S to the level G, it emits a single photon having a wavelength λs = 970 nm. At this time, although the spin state in the quantum dot is random, the emitted photon is preferentially coupled to the antenna X with the projection opening, and is extracted as an X-polarized photon upward in the drawing.

(Second Embodiment)
FIGS. 6A and 6B are a top view and a cross-sectional view showing a second embodiment of the single photon generator of the present invention. In FIG. 6, the same reference numerals are given to the portions corresponding to the portions of FIG. 2 showing the first embodiment, and the overlapping description will be omitted as appropriate (this point will be described with reference to the following embodiments) Is the same). In the present embodiment, in addition to the basic configuration of the first embodiment shown in FIG. 2, the metal protrusions 11 to 14 having pointed tip portions are employed in the opening 6 with protrusions. By sharpening the tip of the metal protrusion in this way, the antenna mode is more easily excited in the gap therebetween, and the light extraction efficiency can be further increased.

(Third embodiment)
FIGS. 7A and 7B are a top view and a cross-sectional view showing a configuration of a single photon generation apparatus according to the third embodiment of the present invention. In the photon generator of this embodiment, in addition to the basic configuration of FIG. 2, a buried metal protrusion 15 is added immediately below the tip ends of the metal protrusions 11 to 14 of the protrusion-equipped opening 6.
As the material of the embedded metal protrusion 15, gold, silver, aluminum, copper, or the like having a relatively small optical loss in the visible light to near infrared light region is used as in the metal protrusions 11 to 14. Such a buried metal protrusion 15 can be produced by forming a depression in the upper semiconductor layer 4 by electron beam lithography and dry etching, and depositing a metal from the depression.
The height h of the buried metal protrusion 15 is set to about d / 2 <h <d with respect to the buried depth d of the quantum dot layer 1. That is, when the depth of the quantum dot layer is d = 50 nm, the height of the embedded metal protrusion 15 is about h = 30 nm. As a result, the antenna effect felt by the quantum dots can be further strengthened, and at the same time, photons emitted from the quantum dots can be prevented from escaping to the substrate side (the lateral direction in the drawing).

(Fourth embodiment)
FIGS. 8A and 8B are a top view and a cross-sectional view showing a configuration of a single photon generation apparatus according to the fourth embodiment of the present invention. In the present embodiment, in addition to the basic configuration of FIG. 2, an oxide film 16 in which an opening is formed below the opening 6 with a protrusion is added in the upper semiconductor layer 2. Such an oxide film 16 can be produced, for example, by heating an AlGaAs layer to about 400 ° C. in a water vapor atmosphere and performing wet oxidation to Al 2 O 3 from the side surface.
The depth h of the oxide film 16 is about d / 2 <h <d with respect to the buried depth d of the quantum dot layer 1. That is, when the quantum dot layer depth is d = 50 nm, the depth of the oxide film 16 is about h = 30 nm.
The oxide film 16 plays a role of confining the light mode in the lateral direction, whereby the light from the quantum dots 4 can be more strongly coupled to the projection opening 6.

(Fifth embodiment)
FIGS. 9A and 9B are a top view and a cross-sectional view showing a configuration of a single photon generation apparatus according to the fifth embodiment of the present invention. The basic configuration of this embodiment is the same as that of the first embodiment shown in FIG. 2, but in this example, a DBR mirror 17 is formed under the lower semiconductor layer 3. This structure acts as a mirror for light having a wavelength emitted from the quantum dots 4, and a resonator is formed between the DBR mirror 17 and the upper end of the upper semiconductor layer 2. As a result, the photons emitted from the quantum dots 4 are more efficiently coupled to the antenna mode of the projection opening 6, and the photons emitted in the direction opposite to the projection opening 6 are also generated by the DBR mirror 17. It reflects and contributes to the transmitted light in the opening 6 with a protrusion. Thus, the use of the DBR mirror 17 can further improve the photon extraction efficiency.
For example, when an InGaAs / GaAs quantum dot is used as the quantum dot 4, a superlattice structure in which about 10 to 20 layers of GaAs and AlAs are alternately stacked can be used as the DBR mirror 17. The DBR mirror 17 may be provided on the metal light shielding film 5 side of the quantum dot layer 1 or may be provided on both sides of the quantum dot layer 1.

(Sixth embodiment)
FIGS. 10A and 10B are a top view and a cross-sectional view showing a configuration of a single photon generation apparatus according to the sixth embodiment of the present invention. The metal light-shielding film with the projection-formed opening 6 of the present invention can be used not only as an antenna for enhancing the coupling between the quantum dots 4 and light but also as an electrode. This embodiment shown in FIG. 10 relates to a Schottky diode type single photon generator using a metal light-shielding film (5) as a Schottky electrode 9. As shown in FIG. 10, an n-type semiconductor layer 7 is provided below the lower semiconductor layer 3, an ohmic electrode 8 is provided on the n-type semiconductor layer 7, and a Schottky electrode 9 is provided on the upper semiconductor layer 2. It is attached. That is, this embodiment has a structure in which the quantum dots 4 are embedded in the ni Schottky diode.
In order to couple the light from the quantum dots 4 to the projection opening 6, the thickness of the upper upper semiconductor layer 2 is about 50 nm. The thickness of the i-type lower semiconductor layer 3 is about 30 nm. Further, a transparent electrode layer 10 is formed between the upper semiconductor layer 2 and the Schottky electrode 9 in order to efficiently apply an electric field to the quantum dots 4 located immediately below the opening. As the transparent electrode layer 10, for example, Ti having a thickness of 5 nm is used. Further, ITO (Indium tin oxide) or the like may be used.
In this case, it is possible to inject electrons from the n-type semiconductor layer 7 to the quantum dots 4 by applying a voltage. Therefore, by combining the electrical injection of electrons by voltage control and the generation of electron-hole pairs by photoexcitation, a state called negatively charged charge excitons can be generated in the quantum dots and emitted.

The operation of the Schottky diode type single photon generator of the present embodiment will be specifically described with reference to FIG. First, by applying an appropriate bias voltage between the ohmic electrode 8 and the Schottky electrode 9, one electron is electrically injected from the n-type semiconductor layer 7 into the quantum dot 4 [FIG. 11 (a)]. In this state, a pair of electrons and holes is further excited in the quantum dots 4 by photoexcitation. As a result, a negatively charged exciton state consisting of two electrons and one hole is generated in the quantum dot 4 [FIG. 11 (b)]. When this charged exciton emits light, one photon is emitted [FIG. 11 (c)]. This photon can be efficiently extracted to the outside through the projection opening 6.
There are two advantages of the single-photon generator using such charged exciton emission. The first point is that an ideal single photon generator without peak splitting can be realized. In neutral excitons, due to the exchange interaction between electron spins and hole spins, an energy difference called fine mutual splitting occurs due to polarization, whereas in charged excitons in which two electron spins are paired, Such splitting does not occur. Secondly, unlike neutral excitons, the charged excitons do not have a spin state that does not emit light, which is called a dark state, and therefore the final emission rate can be increased.
In the above example, negatively charged excitons consisting of two electrons and one hole are generated by electrically injecting electrons. Conversely, by electrically injecting holes, The same effect can be obtained by generating positively charged excitons composed of one electron and two holes. In this case, a p-type semiconductor layer may be used instead of the n-type semiconductor layer 7. Further, an n-type semiconductor layer or a p-type semiconductor layer may be provided on the metal light shielding film 5 side, the metal light shielding film may be used as an ohmic electrode, and a Schottky electrode may be provided on the lower semiconductor layer 3 side.

  The preferred embodiments and examples of the present invention have been described above, but the present invention is not limited to these embodiments and examples, and various modifications can be made without departing from the scope of the present invention. It is a thing. For example, as for the shape of the opening with protrusions, the shape of the opening with protrusions is not limited to one in which two protrusions protrude from the circular opening as shown in FIG. The opening shape is not necessarily circular, and a structure in which four protrusions protrude from the square opening as shown in FIG. 12B may be used instead. In addition, it is not always necessary that one set of protrusions face each other, and as shown in FIG. 12C, the same effect can be expressed by two orthogonal protrusions. In this case, the antenna mode is excited between the tip of the protrusion and the end of the opening. However, compared to such a shape, the antenna mode is more easily induced when the protrusions face each other. Therefore, the shape using four protrusions as shown in FIG. preferable. Further, as shown in FIG. 12D, the protrusions do not necessarily have to be orthogonal to each other.

  As described above, various variations of the aperture shape are conceivable, but it is necessary to provide the projection-shaped aperture with some spatial asymmetry so that the antenna effect appears at two or more different wavelengths depending on the polarization. For example, in the case of FIG. 12A described in the first embodiment and FIG. 12B based on a square opening, w1 and w2 are the gap widths of the upper and lower and left and right protrusion pairs, w1 It is necessary to set ≠ w2. Further, as shown in FIG. 12C, in the case of two openings with metal protrusions, w1 ≠ w2 when the length between the metal protrusion tip and the edge of the opposite opening is defined as w1 and w2. As a result, X and Y polarized light can have different resonance wavelengths. Also, in the case of an opening having three metal protrusions as shown in FIG. 12D, for example, one of the lengths L1, L2, and L3 of the three protrusions is shortened (L1 <L2 = L3). Thus, asymmetry can be created. Furthermore, asymmetry may be created by making the opening shape an ellipse or a rectangle as shown in FIGS.

By the way, in the structure of FIG. 7 shown in the third embodiment, the metal protrusion of the opening is partially embedded in the upper semiconductor layer 4, and the antenna is considered to be modified from a planar structure to a three-dimensional structure. You can also As described above, the antenna according to the present invention can also be configured with a three-dimensional structure such as a metal protrusion having an L-shaped structure or an inclined surface. Therefore, the spatial asymmetry described above can be created not only by the asymmetry of the planar structure such as the gap width but also by the asymmetry of the three-dimensional structure. For example, taking FIG. 7 as an example, the height h of the embedded metal protrusion 15 is changed between the antenna X and the antenna Y, or the embedded metal protrusion 15 is provided only on one antenna side to create asymmetry. Also good.
Further, the semiconductor material of the quantum dots and the semiconductor layer is not limited to InGaAs or GaAs. Further, the quantum dots are not limited to self-forming types. In the above description, the embodiment in which quantum dots are mainly used as the light source has been described. However, it can be replaced with other micro-light emitters that generate photons. For example, donor (or acceptor) impurities in a semiconductor, nanocrystals, nitrogen vacancies in diamond, and the like are listed as candidates. Furthermore, what combined each embodiment is also contained in this invention.

(Seventh embodiment: application to VCSEL)
Up to this point, the embodiments using a micro light emitter such as a quantum dot have been described. However, the projection opening can also be applied to a surface emitting laser (VCSEL). FIG. 13 shows an application example to such a VCSEL.
13A and 13B are a partial plan view of an upper electrode used in the seventh embodiment and a cross-sectional view of the seventh embodiment. In FIG. 13, 31 is an InGaAs active layer, 32 is a DBR mirror made of a laminated film of p-type AlGaAs and GaAs, 33 is a DBR mirror made of a laminated film of n-type AlGaAs and GaAs, and 34 is an n-GaAs substrate. , 35 is polyimide, 36 is an upper electrode, and 37 is a lower electrode. Here, an inversion distribution is formed in the active layer 31 by injecting current from the upper and lower p-type layers and the n-type layer to the InGaAs active layer 31, and the DBR mirror 32 and the DBR mirror 33 Laser oscillation can be generated in the formed resonator. Further, similarly to the metal light-shielding film of the embodiments so far, the upper electrode 36 is provided with the opening 6 with a protrusion. At this time, at least one of the antenna X and the antenna Y in the projection opening 6 is designed to resonate with the laser oscillation frequency. Thereby, a laser beam can be efficiently extracted outside via the antenna effect.
In such a VCSEL structure, the metal film (upper electrode) having the projection-shaped opening 6 serves both as an electrode for current injection and an antenna for light extraction. By using a metal film having an opening with minute protrusions as an electrode, the effective resonator volume can be designed to be small, and high-speed modulation characteristics with a small CR time constant can be obtained.

In FIG. 13, laser light is extracted from one opening with protrusions. However, light extraction efficiency can be further improved by periodically arranging a plurality of openings with protrusions (see Japanese Patent No. 3766238). . FIGS. 14A and 14B are an enlarged top view and a cross-sectional view showing an example of such a VCSEL structure. In FIG. 14, the same reference numerals are given to the same parts as those in FIG. 13, and thus a duplicate description is omitted. In this arrangement, the interval between the adjacent openings with protrusions is determined by laser oscillation. It is designed to be equal to the surface plasmon wavelength λspp at the frequency ω. Specifically, λspp is expressed by the following equation as a function of ω.
λspp = (2πc / ω) √ {(ε _m + ε _s ) ε _m ε _s }
Here, ε _m is the real part of the dielectric constant of the metal material used for the upper electrode, ε _s is the dielectric constant of the p-type semiconductor layer (in this case, the GaAs layer), and c is the speed of light in vacuum. By designing in this way, the transmittance of the projection opening is greatly improved through the surface plasmon mode, and light can be efficiently extracted to the outside.

(Eighth embodiment: wavelength variable VCSEL)
The projection opening according to the present invention is characterized in that the antenna effect can be expressed at two different wavelengths by making the length (gap width) of the antenna asymmetrical in the vertical and horizontal directions. If such characteristics are utilized, application to various wavelength tunable light emitting devices is possible. Here, as an example, a wavelength tunable VCSEL using a MEMS structure as shown in FIG. 15 will be described. FIG. 15A is a perspective view showing the eighth embodiment, FIG. 15B is a plan view of the wavelength variable electrode, and FIG. 15C is an operation explanatory view thereof. In FIG. 15A, the same reference numerals are assigned to the portions corresponding to the portions in FIG. 13, and therefore redundant description is omitted. In this embodiment, the p-type semiconductor is formed on the active layer 31. An upper electrode 36 is provided via the layer 40, and the DBR mirror 32 extends as a cantilever portion 39 on the upper electrode 36. A wavelength tunable electrode 38 is formed on the DBR mirror 32, and the projection opening 6 is formed not at the upper electrode 36 but at the tip of the wavelength tunable electrode 38. Since this wavelength tunable VCSEL can control the laser oscillation wavelength by changing the resonator length, it is possible to switch the emission wavelength between two different wavelengths that undergo antenna resonance through the asymmetric protrusion opening 6. In the following, these two resonance wavelengths are assumed to be λ ₁ and λ ₂ (λ ₁ > λ ₂ ).
Hereinafter, the device structure and operation will be described more specifically. In FIG. 15 (a), the basic arrangement is such that the InGaAs active layer 31 is sandwiched between upper and lower AlGaAs / GaAs DBR mirrors, and light is extracted through the projection opening 6 formed on the uppermost metal film. Is the same as in the seventh embodiment. The difference from the seventh embodiment is that the lower DBR mirror 33 is completely fixed to the GaAs substrate 34, whereas the upper DBR mirror 32 protrudes from the side like a cantilever through a gap. It is fixed in the air in the form. Here, the resonator length (distance between DBR mirrors) when there is no deflection in the cantilever portion 39 is converted to an optical path length (actual distance multiplied by refractive index) to be λ _1/2. Design the device structure as follows. (That is, assuming that the length of the GaAs layer including the active layer is L _GaAs and the length of the _air gap is L _air , n _GaAs L _GaAs + n _air L _air = λ _1/2 where n _GaAs = 3. 3, n _air = 1)
In such a structure, by applying a voltage to the wavelength variable electrode 38, an electrostatic force is generated between the cantilever portion 39 and the upper electrode 36, and the deflection of the cantilever portion 39 can be controlled. it can. Since this corresponds to changing the resonator length effectively, the oscillation wavelength can be changed in a relatively wide range.

16A and 16B are cross-sectional views showing the operation principle of the present embodiment. If no voltage is applied to the wavelength tunable electrode 38 [Fig. 16 (a)], since the electrostatic force is not generated, the resonator length has a value determined by design, in this case oscillates at a wavelength lambda _1. On the other hand, when a voltage is applied to the wavelength variable electrode 38 (FIG. 16 (b)), the cantilever portion 39 bends downward due to electrostatic force, and the resonator length is shortened. L _air is reduced enough to apply a large voltage, by setting an appropriate applied voltage to be a _{n GaAs L GaAs + n air L} air = λ 2/2, can be made to oscillate at a wavelength lambda _2.
As described above, the projection-equipped opening 6 can efficiently extract light having a wavelength λ ₁ as X-polarized light and light having a wavelength λ ₂ as Y-polarized light. By the method as described above, a wavelength tunable VCSEL capable of easily switching the oscillation wavelength between the wavelengths λ ₁ and λ ₂ can be configured.

Moreover, as shown in FIG. 17, the light extraction efficiency can be further increased by periodically arranging the openings with protrusions. The distance between the apertures at this time is, for example, an interval in the X direction: λspp ₁ = λ ₁ √ {(ε _m + ε _s ) ε _m ε _{s in} order to increase the transmittance at two wavelengths λ ₁ and λ _2. }, There is a method of arranging in the Y direction at intervals of λspp ₂ = λ ₂ √ {(ε _m + ε _s ) ε _m ε _s }.

(Ninth embodiment: optical recording apparatus)
So far, the application of a light-emitting device with an asymmetric protrusion with resonance at two different wavelengths has been described, but this structure can also be applied to other applications. As an example, application to an optical recording apparatus will be described here.
With the development of a highly information-oriented society, the amount of stored information required has increased dramatically, and it is thought that the increasing demand will continue in the future. Technically, however, the diffraction limit of the condensing spot size is a problem in optical recording, and the thermal fluctuation limit of magnetization is a problem in magnetic recording. The extension of conventional technologies such as DVD, Blu-ray, HDD, etc. The situation is difficult.
In order to overcome these limitations, there is an increasing expectation for optical recording using a near-field optical probe. The most common near-field optical probe is an aperture-type probe that generates a near field around a small aperture, but a structure that can efficiently generate localized light that is smaller than the wavelength is required for application. Proposed aperture type probes and scattering type probes using localized plasmons in metal microspheres have been proposed.

However, most of these probes that have been proposed in the past are supposed to be used at a single frequency. In fact, the microspheres and plasmon apertures that have been proposed so far exhibit an enhancement effect as an antenna only near a specific frequency. However, it is possible to create a new added value by adopting an antenna structure that resonates at two different wavelengths like the opening with an asymmetric protrusion of the present invention. For example, optical recording apparatuses often use light of different wavelengths for writing and reading. Therefore, an antenna probe that resonates with each of writing and reading can be configured by using the asymmetric protrusion opening of the present invention.
If a technique for locally generating light of two wavelengths with an antenna is used, a new optical recording medium is expected to be adopted. One of them is a photochromic recording material that utilizes changes in the state of an organic molecule between isomers caused by light. Unlike conventional “heat mode” type recording in which light is simply used as a heat source for heating a medium, photochromic recording that directly uses photoreaction does not involve thermal diffusion. Furthermore, since writing using two-photon absorption becomes possible, information recording with high resolution and high sensitivity is expected. A material that requires two wavelengths of light to transition between two states, such as a photochromic material, can also be used for the recording medium. In addition, even in phase change memory magnetic recording or the like, there is a possibility that further increase in capacity can be achieved by using light of a plurality of wavelengths.

In order to solve the above-mentioned problem, in this embodiment, an optical probe 14 having an opening 6 with protrusions in which four metal protrusions protrude as shown in FIG. 18 is used. Here, the gap width w2 of the antenna Y is designed to be longer than the gap width w1 of the antenna X (w2> w1), and therefore the resonance wavelength differs between the X-polarized light and the Y-polarized light. Therefore, antenna resonance can be expressed at two different wavelengths by appropriately selecting the polarization.
An optical recording apparatus as shown in FIG. 19 can be configured by using the opening with projections. In this embodiment, an optical probe 14 having a projection-shaped opening 6 is used, and a writing light source a (Nd: YAG laser) 42 and a writing light source b (Ti: Sapphire laser) 43 are used as two types of writing light sources. Near-field light is generated by condensing the light emitted from the light through a beam-splitter (BS) 46 and a polarization beam-splitter (PBS) 47 at the projection-provided opening at the tip of the optical probe 41, and is generated on the recording medium 45. Write information. Reading is also performed by the detection device 44 via the projection-equipped opening and the beam splitter 46.
Here, the parameters of the openings with protrusions were set to a film thickness of 100 nm, an opening diameter of 200 nm, a gap width between the antennas X of 60 nm, and a gap width between the antennas Y of 140 nm. The transmission spectrum [FDTD (Finite Difference Time Domain) simulation result] of this asymmetric ridge opening is shown in FIG. Reflecting the fact that the gap width of the antenna is asymmetrical in the vertical and horizontal directions, the resonance wavelength differs between X-polarized light and Y-polarized light. For X-polarized light, the wavelength λx = 1070 nm, and for Y-polarized light It can be seen that antenna resonance occurs near the wavelength λy = 760 nm.

  As the optical recording medium, for example, a photochromic material (diarylethene) shown in FIG. 21 can be used. In this material, it is possible to induce a state transition between two isomers A and B by entering light of 380 nm and 532 nm. In addition, since the state transition by two-photon absorption is possible even at a wavelength doubled (760 nm and 1064 nm), in this embodiment, the writing light source a is changed to “B → A” and the writing light source b is changed to “A → B ”can be used for each transition. Since the two-photon transition probability is proportional to the square of the intensity, information storage with higher resolution can be expected.

For reading, since the refractive index differs between the two isomers A and B by about 10%, for example, weak light is incident on the recording medium through the optical probe, and the reflectance is examined by the detection system. Or B can be determined.
The embodiment of the optical recording apparatus shown here is merely an example, and various variations can be considered in the configuration and operation procedure. For example, in FIG. 19, two antenna resonances having different resonance wavelengths are used to transition between two states A and B of the photochromic material. However, these two antenna resonances are used for writing and reading, respectively. Is also possible. Further, the recording medium is not limited to the photochromic material shown here.

(Tenth embodiment: filter)
As described above, the protrusion-equipped opening has been described as a structure that increases the light transmittance or enhances the near field according to the polarization and wavelength. However, from a different viewpoint, it can be considered as a filter that efficiently transmits only a specific wavelength and a specific polarization. FIG. 22 shows an operation example of such a filter 48. Various light of different wavelengths or polarization, when incident on the projection with the opening 6 of the filter 48, only light of polarization Y in a wavelength lambda ₁ light polarization X, and the wavelength lambda ₂ passes.
Such a fine aperture type filter exhibits its power when combined with a photodetector, a sensor array, and the like described later. In addition, the protruding aperture can be scaled in principle, and by increasing the aperture shape, the antenna effect can be exhibited even in the terahertz region. Therefore, it can also be used as a terahertz wave filter with the same configuration. In the case of terahertz waves, the configuration of a polarizing plate, a wavelength filter, and the like is not easy compared to visible light and the like, so that the use of the antenna effect as in the present invention is useful.

(Eleventh embodiment: photodetection device)
The projection opening according to the present invention is also suitable for application to a photodetector. As an eleventh embodiment, FIG. 23 shows an example in which a projection opening is applied to a pin photodiode.
FIG. 23B is a sectional view of the eleventh embodiment, and FIG. 23A is a partial plan view of the upper electrode. Here, light having a wavelength λ ₁ having X polarization or light having a wavelength λ ₂ having Y polarization is enhanced near the metal protrusion by the antenna effect and efficiently absorbed by the i-Si layer 51 therebelow. Electrons and holes generated in the i-Si layer 51 due to absorption are taken out as current from the lower electrode 37 and the upper electrode 36 through the n-Si layer 53 and the p-Si layer 52, respectively. Here, a portion of the p-Si layer 52 immediately below the opening with protrusions protrudes in a columnar shape, and the periphery of the protrusion is surrounded by the SiO ₂ insulating film 54.
Note that, as in the first embodiment, the distance from the protruding opening 6 to the absorption layer (here, the i-Si layer 51), that is, in this case, the thickness d of the p-Si layer 52 is an important parameter. In order to increase the absorption efficiency in the absorption layer by enhancing the electromagnetic field in the vicinity of the metal protrusion, the distance d is preferably at least d <λ / 2n. Here, n represents the refractive index of the p-Si layer 52. If possible, it is more desirable that d <λ / 5n.
Also in this embodiment, the upper electrode 36 having the protrusion-equipped opening 6 also has two roles of an electrode for taking out current and an antenna for taking out light. The advantage of a detector using such a small opening with protrusions is that the area where the metal electrode and the semiconductor are in contact with each other can be reduced, so that the effective capacity is reduced and high speed operation is possible. .

Next, an example of the eleventh embodiment of the present invention will be described with reference to FIG. As an added value brought about by the projection opening according to the present invention, a wavelength / polarized light selection function is newly added. This embodiment utilizes this added value, and here operates as a wavelength selective photodetector that selectively detects only light of a specific wavelength.
First, as shown in FIG. 24A, consider a case where a polarizer X is inserted in front of the detector 55. Here, it is assumed that the polarizer X 56 has a function of a polarizing filter that transmits only X-polarized light in a wide wavelength band. When light having various wavelengths and polarizations is irradiated from the outside to such an arrangement, only X-polarized light passes through the polarizer X and enters the detector 55. In the projection with the opening 6 at this time, since to be limited to the wavelength lambda ₁ of the light to the antenna resonance with respect to the X direction of polarization, only the light of wavelength lambda ₁ is selectively detected. Similarly, as shown in FIG. 24B, when a polarizer Y 57 that passes only Y-polarized light is inserted in front of the detector 55, only Y-polarized light is incident on the detector 55, so that the wavelength λ ₂ Only the light of is selectively detected. As described above, a wavelength selective photodetector that selectively detects only light of wavelength λ ₁ or λ ₂ can be configured by inserting a polarizer.
Note that the photodetector to which the projection opening is applied is not limited to the pin photodiode shown here. The opening with a protrusion can be similarly applied to an avalanche photodiode using an avalanche multiplication phenomenon, and can also be applied to a photodetector having a Schottky junction between a metal electrode and a semiconductor.

(Twelfth embodiment: Image sensor)
Next, application to an image sensor will be described as a twelfth embodiment. FIG. 25 (a) is a diagram showing a basic device configuration, and FIG. 25 (b) is an equivalent circuit diagram thereof. The photosensors are arranged in a two-dimensional array, and each sensor is a photodiode comprising a pn junction provided in a p-type semiconductor layer 64 on an n-type Si substrate, as shown in FIG. 25 (a). 61. An n-type transfer channel 62 is provided adjacent to the photodiode 61, and a transfer electrode 63 is provided thereon via an insulating layer 66. On the insulating layer 66, a light shielding film 67 having a projection-formed opening 6 is formed on each photodiode.
FIG. 25B shows a corresponding CCD image sensor circuit. The electrons photoexcited by the photodiode 61 move to the transfer channel 62 and then are carried to the output unit 70 through the vertical CCD 68 and the horizontal CCD 69 in a bucket relay manner by voltage control of the transfer electrode 63. A two-dimensional image of light can be measured by sequentially detecting the amount of charge generated by each photodiode in this way.
Here, the light-shielding film 67 having the projection-shaped opening 6 functions as a light-shielding film for preventing crosstalk between elements and noise in the transfer unit, and operates as an antenna that efficiently detects light, and has a specific wavelength. It also functions as a “color filter” that filters only the color. In addition, if a projection-shaped opening 6 having an asymmetric shape is used, a polarization selection function can be added. By utilizing the light shielding film with projections 6 having many functions as described above, device elements can be reduced and the sensor can be reduced in size.

Next, an example of the twelfth embodiment will be described with reference to FIG. The present embodiment takes advantage of the features of the light-shielding film with openings with protrusions, and shows the operation as wavelength selective sensing utilizing the degree of freedom of polarization. First, as shown in FIG. 26A, a polarizer X 56 is inserted in front of the image sensor 71. Here, it is assumed that the polarizer X has a function of a polarizing filter that transmits only X-polarized light in a wide wavelength band. When light having various wavelengths and polarization is irradiated from the outside to such an arrangement, only X-polarized light passes through the polarizer X 56 and enters the image sensor 71. At this time, since the antenna resonance with respect to the polarized light in the X direction is limited to the light of wavelength λ _{1 in} the aperture with protrusion, only the light of wavelength λ ₁ is selectively detected. Similarly, as shown in FIG. 26B, a polarizer Y 57 that passes only Y-polarized light is inserted in front of the image sensor 71. In this case, only the light of the Y polarized light to incident on the image sensor 71, only the light of wavelength lambda ₂ is selectively detected. As described above, it is possible to configure an image sensor that selectively detects only light of wavelength λ ₁ or λ ₂ by inserting a polarizer.

Next, another example of the twelfth embodiment will be described with reference to FIG. In this embodiment, a color image sensor is realized by using the feature as a color filter with an opening with protrusions. Here, an opening with protrusions R, an opening with protrusions G, and an opening with protrusions B are provided for each row. The gap width is changed little by little so that each sensor can selectively detect light of different wavelengths. That is, in the opening R with projections, gaps between the metal projection has become a w1, w2, the wavelength lambda ₁ with light and the wavelength lambda ₂ of the X-polarized light only light in the Y-polarized light is transmitted, the opening G with projections, gap between the metal protrusion has a w3, w4, a light and a wavelength lambda ₄ of X-polarized light only light in the Y-polarized light at the wavelength lambda ₃ is transmitted, the protrusion with the opening B, the gap between the metal protrusions w5, It has a w6, the wavelength lambda ₅ with light and the wavelength lambda ₆ of X-polarized light only light in the Y-polarized light is transmitted. Thereby, the color image sensor 72 for detecting a color two-dimensional image can be configured.
A sensor array using such an opening with protrusions is not limited to the visible light region, but can be used in the terahertz region by designing the opening size and the gap width.

DESCRIPTION OF SYMBOLS 1 Quantum dot layer 2 Upper i-type semiconductor layer 3 Lower i-type semiconductor layer 4 Quantum dot 5 Metal light shielding film 6 Protrusion opening 7 n-type semiconductor layer 8 Ohmic electrode 9 Schottky electrode 10 Translucent electrode 11, 12 Metal protrusion 13, 14 Metal protrusion 15 Embedded metal protrusion 16 Oxide film opening 17 n-type DBR mirror 20 Polarization beam splitter (PBS)
21 Excitation light source 31 InGaAs active layer 32, 33 DBR mirror 34 n-GaAs substrate 35 Polyimide 36 Upper electrode 37 Lower electrode 38 Wavelength variable electrode 39 Cantilever 40 p-type semiconductor layer 41 Optical probe 42 Writing light source a
43 Writing light source b
44 Detector 45 Recording medium 46 Beam splitter (BS)
47 Polarized beam splitter (PBS)
48 Filter 51 i-Si layer 52 p-Si layer 53 n-Si layer 54 SiO ₂ insulating film 55 Detector 56 Polarizer X
57 Polarizer Y
60 Opening X with protrusion
61 Photodiode 62 Transfer channel 63 Transfer electrode 64 p-type semiconductor layer 65 n-type Si substrate 66 insulating layer 67 light shielding film 68 vertical CCD
69 Horizontal CCD
70 Output unit 71 Image sensor 72 Color image sensor

Claims (24)

  There is provided a metal film having an opening, and provided with at least two metal protrusions projecting from the peripheral edge of the opening in different directions to the inside of the opening, and formed at the tip of each of the at least two metal protrusions An optical element characterized in that at least two antennas configured to include different gaps have different resonance wavelengths.   The optical element according to claim 1, wherein two of the at least two metal protrusions are orthogonal to each other.   3. The optical element according to claim 1, wherein two of the at least two metal protrusions are opposed to a pair of metal protrusions with a gap therebetween.   4. The optical element according to claim 1, wherein w <b> 1 ≠ w <b> 2, wherein gaps formed at two tips of the at least two metal protrusions are w <b> 1 and w <b> 2, respectively. .   A light emitting layer including a minute light emitting portion, first and second semiconductor layers stacked with the light emitting layer interposed therebetween, and a metal having an opening on the minute light emitting portion formed on the first semiconductor layer A photon generator comprising: a film; and an excitation light source for exciting the minute light emitting unit, wherein the optical element according to any one of claims 1 to 4 is used as the opening.   The at least two antennas each including the at least two metal protrusions and a gap formed at each tip respectively resonate with different optical transition levels of the minute light emitting part. Item 6. The photon generator according to Item 5.   The one of at least two antennas each including the at least two metal protrusions and a gap formed at each tip thereof resonates with light from the excitation light source. The photon generator according to claim 5 or 6.   The photon generator according to any one of claims 5 to 7, wherein a size of the minute light emitting portion is smaller than an emission wavelength.   The photon generator according to any one of claims 5 to 8, wherein a quantum dot is used as the minute light emitting unit.   When the light emission wavelength of the quantum dots is λ and the refractive index of the first semiconductor layer is n, the light emitting layer has a depth satisfying d <λ / 2n from the end of the metal electrode including the projection opening. The photon generator according to claim 5, wherein the photon generator is formed as follows.   When the emission wavelength of the quantum dots is λ and the refractive index of the first semiconductor layer is n, the light-emitting layer is λ / 20n <d <λ / 5n from the end of the metal electrode including the opening with protrusions. The photon generator according to any one of claims 5 to 10, wherein the photon generator is formed to a depth of   The photon generator according to any one of claims 5 to 11, wherein the metal film is formed of a material capable of inducing surface plasmon.   The photon generator according to any one of claims 5 to 12, wherein a tip end of the metal projection protruding inside the opening is pointed.   The photon generator according to claim 5, wherein at least a part of the metal electrode is embedded in the first semiconductor layer.   The photon generator according to any one of claims 5 to 14, wherein an oxide film having an opening is inserted between the metal film and the light emitting layer on the minute light emitting portion.   16. The structure according to claim 5, wherein at least one of the first and second semiconductor layers has a structure in which two kinds of semiconductors having different refractive indexes are alternately stacked. The photon generator according to 1.   At least one of the first and second semiconductor layers includes a p-type layer or an n-type layer, and the metal film is used as an electrode, and a metal electrode that forms a junction with the second semiconductor layer is provided. The photon generator according to claim 5, wherein the photon generator is provided.   18. The optical means for separating light from the minute light emitting part and light to the minute light emitting part is provided on the projection-attached opening. 18. Photon generator.   An active layer, a p-type semiconductor layer and an n-type semiconductor layer stacked with or including the active layer, and the active layer formed in contact with the p-type semiconductor layer or the n-type semiconductor layer And a metal electrode having at least one opening, wherein the optical element according to any one of claims 1 to 4 is used as the opening.   A light source, a light detector, and an optical recording medium are provided, and optical writing or optical reading to or from the optical recording medium is performed via the optical element according to any one of claims 1 to 4. Optical recording device.   21. The optical recording apparatus according to claim 20, wherein the optical recording medium has a photochromic material film.   A photodetection layer, a p-type semiconductor layer and an n-type semiconductor layer stacked in a manner sandwiching or including the photodetection layer, and a metal electrode having at least one opening on the photodetection layer, An optical element according to any one of claims 1 to 4 is used as the opening.   5. The optical element according to claim 1, further comprising: a plurality of photodetecting elements; and a charge transfer unit configured to transfer signal charges detected by the photodetecting elements. A photodetection device for detecting an image pattern by receiving light through a light source.   24. The photodetecting device according to claim 22, wherein a polarizer is disposed above the optical element.

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