JP4805663B2 - Near-field waveguide, near-field waveguide method, and optical shift register - Google Patents

Description

  The present invention relates to a waveguide in which a near field is guided, a near field waveguide method using the waveguide, and an optical shift register using the waveguide.

  As information transmission means, there is a tendency to use light as a medium in order from long-distance communication, and optical communication has spread from backbone optical communication to FTTH (Fiver To The Home). In the future, it is considered that the adoption of information transmission by light will be advanced between devices, between boards, between chips, and within chips. However, it is difficult to confine light in a region below the wavelength (diffraction limit), and a special technique is required to form a waveguide between chips or within a chip.

Conventionally, in order to transmit light, a technique using a near field (a minute dot array), a technique using a plasmon (Patent Document 1), and a technique using an exciton (Patent Document 2) are known.
JP 2003-207667 A JP 2004-157326 A

  However, the transmission efficiency of the near field in the minute dot array decreases exponentially as the near field advances. Plasmon cannot be ignored by metal. Since excitons are fast in energy relaxation, there is a problem of lifetime that can only be transmitted within the relaxation time.

  An object of the present invention is to provide a near-field waveguide having a minute size exceeding the diffraction limit of light, a near-field waveguide method using the waveguide, and an optical shift register using the waveguide.

  A near-field waveguide according to an aspect of the present invention includes a near-field waveguide unit in which quantum dots whose energy levels are quantized are arranged, and a pair of outer-field waveguide units provided outside both ends of the near-field waveguide unit. An electrode, a near-field excitation unit that introduces a near field into a quantum dot at one end of the near-field waveguide unit, and a near-field output unit that outputs a near field from the quantum dot at the other end of the near-field waveguide unit It is characterized by having.

  In the near-field waveguide method according to another aspect of the present invention, electrons are injected into a quantum dot at one end of a near-field waveguide unit in which quantum dots with quantized energy levels are arranged, and the near-field waveguide A near-field is excited in the electron trajectory of a quantum dot into which electrons at one end have been injected, a voltage is applied to the near-field waveguide, and localization is performed along the direction in which the quantum dots are arranged in the near-field waveguide. The near field is moved together with the electron orbit, and the near field moved to the electron orbit of the quantum dot at the other end of the near field waveguide is output.

  An optical shift register according to still another aspect of the present invention includes a near-field waveguide unit in which quantum dots whose energy levels are quantized are arranged, and a pair provided outside both ends of the near-field waveguide unit. A near-field excitation unit that introduces a near field into a quantum dot at one end of the near-field waveguide unit, and a near-field output unit that outputs a near field from the quantum dot at the other end of the near-field waveguide unit; And a near-field readout section that reads out a near-field held by the quantum dots in the near-field waveguide section.

  According to the near-field waveguide and the near-field waveguide method of the present invention, the near-field can be guided with high propagation efficiency, and an optical shift register can be realized.

  Hereinafter, the near-field waveguide and the near-field waveguide method according to the embodiment of the present invention will be described in detail.

  Near-field is light that does not propagate and is localized on the surface of the material ("Near-field light basics" by Otsuka Motoichi and Kobayashi Kiyoshi, Ohmsha, published on January 5, 2003). The near field is not limited by the diffraction limit and can be localized in a region below the wavelength. Since the near-field intensity decreases exponentially as the distance from the surface of the material increases, the near-field existence range is about the wavelength from the material surface. Further, if microspheres of the same size exist, the coupled state of the microspheres is formed by the near field, so that the energy of the near field can be moved. However, as long as the surfaces are not in contact, the propagation efficiency of the near field decreases exponentially.

  When a microsphere whose near field is excited on the surface is moved, naturally, the localized near field is also moved. However, it is not an appropriate method to move the near field using the physical movement of the microsphere. Here, it should be considered that the surface of the material in which the near field is localized is an electron orbit existing in the skin of the material. That is, if the electron orbit on the surface of the microsphere excited by the near field can be separated from the microsphere and only the electron orbit can be moved, the near field moves along with the electron orbit. Therefore, as the near-field waveguide, a material system in which electrons can be injected and the outermost electron trajectories are largely different before and after the injection of electrons may be used. Since the electron orbit before excitation of the near field is a SOMO orbit (anion state), there is no hole, and there is no energy relaxation due to recombination. Therefore, electrons do not have a lifetime like excitons. In addition, although plasmon excitation always has absorption, this proposal can use a material having a small absorption coefficient, and therefore the signal intensity is not greatly attenuated unlike metal.

Next, a substance in which outermost electron trajectories are largely different before and after electron injection will be described. In order to greatly change the state of the electron orbit by injecting electrons as compared with before injection, quantum dots may be used. A quantum dot is a fine particle having a size of the order of nm or less and having discrete energy levels. The quantum dots may be any of metal nanoparticles, semiconductor nanoparticles, and organic molecules. Typical quantum dot materials are nanoparticle materials such as Cu, Au, Ag, Fe, Ni, Co, Zn, Cr, W, Ti, Al, In, Ir, Mn, Mo, Bi, and Pt. III-V group semiconductors such as metals, Si, Ge, Sn, Pb, diamond, GaAs, AlAs, InAs, GaP, InSb, II-VI group semiconductors such as ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, TiO ₂ , Oxides such as ZnO, C ₆₀ , carbon nanotubes, ferrocene, nickelocene, Na ₂ SO ₄ , CH ₃ COONa, CH ₃ COOK, (COONa) ₂ , CuCl, CH ₃ COOAg, MgSO ₄ , CaSO ₄ , (COO) _{2 Ca, ZnSO 4, ZnCl 2} , at least one is used is selected from the group consisting of organic molecules, inorganic molecules such as (COO) ₂ Zn It is.

Since the quantum dot is small in size, the density of states of each energy level is smaller than that of the bulk, and the influence of the injected one electron becomes large. For example, it has been clarified that the Coulomb blockade phenomenon occurs by injection of one electron in nano-sized fine particles ("Introduction to Single Electron Tunneling" by Junji Haruyama, Corona, 2002 first edition). For the sake of simplicity, description will be made by taking Na ⁺ ions and Na atoms as examples of quantum dots. The Na ⁺ ion has a 1S orbit, 2S orbit, and 2P _xyz orbit closed. When one electron is injected here, a 3S orbital is formed so as to cover the 1S, 2S, 2P _xyz orbitals, and becomes Na atoms. When a near field is excited by this Na atom, it is excited by a 3S orbit and the 1S, 2S, and 2P _xyz orbits are electrostatically shielded by a 3S orbit and are hardly affected. Thus, by arranging the Na ⁺ ions, only the atoms become Na neutral atoms when injecting electrons into one end of the Na ⁺ ions will have 3S trajectory to excite near-field to the 3S orbit, Na ⁺ When an electric field is applied in the direction in which ions are arranged, the 3S orbit where the near field is localized moves one after another, and the near field moves. At this time, it seems as if Na atoms are moving.

Electron movement between quantum dots occurs by the tunnel effect. The tunnel effect is a phenomenon that penetrates a barrier when an electron having energy E smaller than V ₀ collides with a barrier having a height of potential V _{0 in} a quantum mechanical system. Occurs when the probability t is not zero, either inside or outside the barrier. If calculated according to the Schroedinger equation, the transmission rate through the barrier, that is, the probability t of the tunnel effect, is expressed by the following equation (for example, “Introduction to Single Electron Tunneling” by Junji Haruyama, Corona, 2002, first edition).

According to this formula, it can be seen that tunneling is more likely to occur as the barrier thickness a decreases.
In addition, when electrons are injected into the quantum dots, Coulomb blockade may occur. There are the following three conditions for the occurrence of coulomb blockade.

Here, k is the Boltzmann constant, T is temperature, R _T is the junction tunneling resistance, R _Q is the resistance quantum (25.8kΩ), Re (Zt ( ω)) is the real part of the external field electromagnetic environment impedance, epsilon is the dielectric Rate, S is the bonding area, and a is the thickness of the tunneling barrier.

First, the barrier needs to be thin enough to allow electrons to tunnel, and then the barrier charging energy (quantum dot energy level increase) is larger than the environmental temperature energy kT (k: Boltzmann constant, T: absolute temperature). It is. Therefore, although there is a condition that the electrostatic capacitance of the barrier is small, the film thickness, which is one of the factors that determine the electrostatic capacitance, is limited to be thin enough to be tunneled as described above. In addition, the influence of the area, that is, the surface area of the quantum dots is decisive, and the size of the quantum dots is inevitably small. From the above, the overall voltage is set so that a voltage equal to or higher than the Coulomb blockade voltage V _c corresponding to the charging energy E _c is applied between the quantum dot in which the injected electron orbit exists and the quantum dot at the movement destination. What is necessary is just to apply. The voltage is divided between a pair of electrodes, between the electrodes and quantum dots, and between each quantum dot and quantum dot. The distance between the quantum dot at the input end and the adjacent electrode is determined by the distance between the quantum dots. by separating the electrode - it can be easily applied to mode voltage V _c - Kuronbukke between quantum dots. Since the SOMO (Single Occupied Molecular Orbital) level is close between the quantum dot and the quantum dot, a large voltage difference is not required. Therefore, if electrons are injected into the quantum dot at the input end, the near field is excited by the electron trajectory, and the near field can be sequentially moved to the quantum dot at the output end. When guiding a plurality of near fields, a plurality of quantum dot arrays may be arranged in parallel.

  Next, an optical shift register to which the above near-field waveguide is applied will be described. In this optical shift register, one quantum dot is used as a memory. The outermost electron cloud in which the near field is excited can be moved between quantum dots or stopped by an external electric field. For example, when eight quantum dots are arranged in series, it can be used as an 8-bit optical shift register. In order to shift the bits, the required external field is applied parallel to the quantum dot array. By controlling the direction of the external field, the bits can be shifted to the right or left. Conventionally known are an optical shift register (Japanese Patent No. 2610287) and an optical inverter circuit (Japanese Patent Laid-Open No. 6-95194), both of which use bistable elements and have a complicated structure. . In contrast, the optical shift register using quantum dots according to the embodiment of the present invention is small in size and can be configured with a simple structure.

  Embodiments of the present invention will be described below with reference to the drawings.

(Example 1)
1A and 1B show the near-field waveguide of this example. FIG. 1A is a plan view, and FIG. 1B is a cross-sectional view. As shown in FIGS. 1A and 1B, Si is processed by electron beam lithography, and cylindrical quantum dots 1 having a diameter of about 10 nm and a height of about 10 nm are arranged in a line at intervals of 2 nm. The near-field waveguide unit 10 was formed. SiO ₂ was formed between the quantum dots 1. The total length of the near-field waveguide 10 is about 5 μm. A pair of electrodes 11 and 12 were formed on the outer side of the quantum dot 1 at one end (input end) and the quantum dot 1 at the other end (output end) of the quantum dot array, respectively, with an interval of 5 nm. As shown in FIG. 1B, an optical fiber probe is disposed as the near-field excitation unit 13 above the quantum dot 1 at the input end, and an optical fiber probe is disposed as the near-field output unit 14 above the quantum dot 1 at the output end. did. Each optical fiber probe has a sharpened tip and a metal film coated on the side of the tip.

  The near-field propagation in the near-field waveguide will be described with reference to FIGS. As shown in FIG. 2A, a voltage is applied between the electrodes 11 and 12, and electrons are injected from the electrode 11 near the input end of the near-field waveguide 10 into the quantum dot 1 at the input end. Next, the near field excitation unit 13 excites the near field in the electron trajectory of the quantum dot 1 at the input end where the electrons are injected. 2B, 2C, and 2D, a voltage is applied to the near-field waveguide 10 by the electrodes 11 and 12, and the localized near-field along the near-field waveguide 10 is changed. It moves sequentially with the electron orbit and finally moves to the quantum dot 1 at the output end. As shown in FIG. 2 (e), the near-field output unit 14 outputs the near-field moved to the electron orbit of the quantum dot 1 at the output end of the near-field waveguide 10.

  As described above, when the size of the quantum dot 1 is about 10 nm, the Coulomb blockade voltage is about 50 mV. No output was observed at the near-field output unit 14 even when the near-field excitation unit 13 continued to excite a near-field having a wavelength of 532 nm without applying a voltage to both ends of the near-field waveguide unit 10. On the other hand, when the near-field excitation unit 13 continues to excite a near-field having a wavelength of 532 nm with a voltage of about −1 V applied to the input-end electrode 11 with respect to the output-end electrode 12, the near-field output unit 14 the near field output was observed.

(Example 2)
FIG. 3 shows a perspective view of the near-field waveguide of this embodiment. On a glass substrate (not shown), a film 20 in which quantum dots 1 made of Au having a diameter of about 4 nm were dispersed in polystyrene was applied to a thickness of about 0.5 μm. As shown in FIG. 3, the plurality of quantum dots 1 do not have to be arranged in a line, and may be an aggregate. Electrodes 21 and 22 were formed by sputtering an Au film on both ends of the film 20. The distance between the electrodes 21 and 22 is about 50 μm. A region between the electrodes 21 and 22 of the film 20 is used as a near-field waveguide. An optical fiber probe was disposed as the near-field excitation unit 23 above the input end, and an optical fiber probe was disposed as the near-field output unit 24 above the output end.

  A near-field output unit 24 is installed at a position A on the near-field waveguide between the electrodes 21 and 22, and a voltage of about 500 V is applied between the electrodes 21 and 22 to excite the near-field by the near-field excitation unit 23. As a result, the near field was able to be observed by the near field output unit 24 installed at the position A. When the application of the voltage between the electrodes 21 and 22 was stopped, the near field could not be observed at the near field output unit 24. Next, when the near-field output unit 24 is installed at a position B off the near-field waveguide between the electrodes 21 and 22, a voltage of about 500 V is applied between the electrodes 21 and 22, and the near-field excitation unit 23 is applied. Even when the near field was excited by the near field, the near field could not be observed at the near field output unit 24.

Instead of quantum dots of Au, Ag, Pb, Ti, TiO 2, Al, Cu, In, Ir, W, Cr, Ni, Fe, Co, Zn, Ge, Sn, Mn, Mo, Bi, Pt, Similar results were observed when quantum dots made of GaAs, AlAs, InAs, GaP, InSb, ZnO, ZnS, CdS, ZnSe, CdSe, ZnTe, and CdTe were used.

(Example 3)
FIG. 4A is a perspective view of the near-field waveguide according to the present embodiment. FIG. 4B is a plan view of a waveguide obtained by combining a near-field waveguide and a pair of planar waveguides according to this embodiment. Electrodes 31 and 32 were formed on the glass substrate 30. Between these electrodes 31 and 32, were coated with a film 40 formed by dispersing the quantum dot 1 made of C ₆₀ in polystyrene. This film 40 is used as a near-field waveguide, and its length is about 2 μm. Electrodes 41 and 42 connected to the electrodes 31 and 32 on the glass substrate 30 were formed at both ends of the film 40. Further, planar waveguides 43 and 44 having a core and a clad and having sharpened tips are provided at both ends of the film 40. Since the planar waveguides 43 and 44 have sharpened tips, a near field is generated at the tips. The planar waveguide 43 at the input end is used as the near-field excitation unit, and the planar waveguide 44 at the output end is used as the near-field output unit. When a near field was excited by the planar waveguide 43 by applying a voltage of about 2 kV between the electrodes 31 and 32, the near field could be observed by the planar waveguide 44.

Instead of C ₆₀ quantum dots, carbon nanotubes, ferrocene, nickelocene, Na ₂ SO ₄ , CH ₃ COONa, CH ₃ COOK, (COONa) ₂ , CuCl, CH ₃ COOAg, MgSO ₄ , CaSO ₄ , (COO) Even when quantum dots made of ₂ Ca, ZnSO ₄ , ZnCl ₂ , (COO) ₂ Zn were used, near-field waveguiding could be confirmed.

Example 4
A near-field waveguide 10 was formed in the same manner as in Example 1 except that eight quantum dots 1 were arranged, and used as an 8-bit shift register. FIG. 5 shows a conceptual diagram of the optical shift register of this embodiment. 6A and 6B schematically show the operation of the optical shift register of this embodiment. Similar to Example 1, Si was processed by electron beam lithography, and eight columnar quantum dots 1 having a diameter of about 10 nm and a height of about 10 nm were arranged in a row at intervals of 2 nm to obtain near-field conduction. A wave portion 10 was formed. SiO ₂ was formed between the quantum dots 1. The total length of the near-field waveguide 10 is about 5 μm. A pair of electrodes 11 and 12 were formed outside the quantum dot 1 at the input end and the quantum dot 1 at the output end of the quantum dot array, respectively, with a spacing of 5 nm. An optical fiber probe is disposed as the near-field excitation unit 13 above the quantum dot 1 at the input end, and an optical fiber probe is disposed as the near-field output unit 14 above the quantum dot 1 at the output end.

  In this optical shift register, the outermost electron cloud and the near field are shifted by applying an electric field in the column direction of the near-field waveguide 10 in synchronization with the input of the near-field, and the near-field is transferred to an arbitrary qubit 1. Remember. Here, a state in which 8-bit information “11011000” is input is shown. As shown in FIG. 6B, this information is read using a fiber probe of a near-field optical microscope as the near-field reading unit 15.

  Information can also be read out by a near-field waveguide connected to each bit.

2 is a plan view and a cross-sectional view of a near-field waveguide according to Embodiment 1. FIG. The figure which shows typically propagation of the near field in the near field waveguide of Example 1. FIG. FIG. 6 is a perspective view of a near-field waveguide according to a second embodiment. The perspective view of the near-field waveguide of Example 3, and the top view of the waveguide which combined the near-field waveguide of Example 3 and a pair of planar waveguide. FIG. 10 is a conceptual diagram of an optical shift register according to a fourth embodiment. FIG. 10 is a diagram schematically illustrating the operation of the optical shift register according to the fourth embodiment.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Quantum dot, 10 ... Near field waveguide part, 11, 12 ... Electrode, 13 ... Near field excitation part, 14 ... Near field output part, 15 ... Near field read-out part, 20 ... Membrane, 21, 22 ... Electrode, DESCRIPTION OF SYMBOLS 23 ... Near field excitation part, 24 ... Near field output part, 30 ... Glass substrate, 31, 32 ... Electrode, 40 ... Film | membrane, 41, 42 ... Electrode, 43, 44 ... Planar waveguide.

Claims (5)

A near-field waveguide composed of quantum dots arranged to move the near-field,
By injecting electrons into a quantum dot at one end of the near-field waveguide by tunnel effect, an outermost electron orbit having different characteristics from that before electron injection is formed, and a voltage is applied to the near-field waveguide to apply proximity. A pair of electrodes provided outside the ends of the near-field waveguide to move the field;
A near-field excitation unit for exciting a near-field in an electron orbit of a quantum dot at one end of the near-field waveguide unit;
A near-field waveguide comprising: a near-field output unit that outputs a near-field from a quantum dot at the other end of the near-field waveguide unit.   The near-field waveguide according to claim 1, wherein the quantum dots are formed of an organic material or an inorganic material having discrete energy levels. The quantum dots are Cu, Au, Ag, Fe, Ni, Co, Zn, Cr, W, Ti, Al, In, Ir, Mn, Mo, Bi, Pt, Si, Ge, Sn, Pb, GaAs, AlAs. , InAs, GaP, InSb, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, TiO ₂ , ZnO, C ₆₀ , carbon nanotubes, ferrocene, nickelocene, Na ₂ SO ₄ , CH ₃ COONa, CH ₃ COOK, (COONa) _2. Proximity according to claim 1, characterized in that it is selected from the group consisting of ₂ , CuCl, CH ₃ COOAg, MgSO ₄ , CaSO ₄ , (COO) ₂ Ca, ZnSO ₄ , ZnCl ₂ , (COO) ₂ Zn. Field waveguide. Electrons are injected into the quantum dot at one end of the near-field waveguide unit composed of quantum dots arranged to move the near-field,
Exciting a near field to an electron orbit of a quantum dot into which electrons at one end of the near field waveguide are injected,
A voltage is applied to the near-field waveguide, and along the direction in which the quantum dots of the near-field waveguide are arranged, the localized near-field is moved together with the electron trajectory,
A near-field waveguide method for outputting a near-field moved to an electron orbit of a quantum dot at the other end of the near-field waveguide unit. A near-field waveguide composed of quantum dots arranged to move the near-field,
A pair provided outside both ends of the near-field waveguide to inject electrons into quantum dots at one end of the near-field waveguide and apply a voltage to the near-field waveguide to move the near-field. Electrodes,
A near-field excitation unit for exciting a near-field in an electron orbit of a quantum dot at one end of the near-field waveguide unit;
A near-field output unit that outputs a near-field from a quantum dot at the other end of the near-field waveguide; and
An optical shift register comprising: a near-field readout unit that reads out a near-field held by a quantum dot in the near-field waveguide unit.

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