JP4742382B2 - Optical switching device - Google Patents

Description

  The present invention relates to an optical switching element and a magneto-optical switching element using carbon nanotubes, and more particularly to a control technique for response speed and response sensitivity of these elements.

  With the advent of the era of full-fledged broadband and digital home appliances, the development of elemental technologies for high-capacity and high-speed communication has been desired. In order to cope with a data transfer rate of terabit / second, communication using an optical fiber is indispensable, and it is important to search for an optical device material that can be used in the used wavelength band (1.5 μm).

  Single-walled carbon nanotubes are known to exhibit large nonlinear optical characteristics with an increased optical response reflecting their one-dimensionality. The time response is on the order of 1 ps and shows high-speed optical switching characteristics. The transition of the semiconductor nanotube corresponds to a wavelength (1.5 μm) used in optical communication, and showing a large nonlinearity at that wavelength is a great advantage as an optical switching element. Further, by performing modulation related to light absorption by a magnetic field, application to various fields such as a memory element as well as an optical switching element becomes possible.

  In an optical switching element using carbon nanotubes, it is important to increase the response time. In the optical switching element using carbon nanotubes, the absorption coefficient based on the magnetic field changes depending on the magnitude of the magnetic flux penetrating the cross section of the carbon nanotubes. In a constant magnetic field (magnetic flux density), increasing the diameter of the carbon nanotubes increases the cross-sectional area of the carbon nanotubes, so that the change in absorption due to the magnetic field increases. However, when the diameter of the carbon nanotube is changed, the absorption wavelength is changed, so that there is a problem that the wavelength is deviated from the wavelength band used as the switching element.

  An object of the present invention is to increase the response speed of an optical switching element using carbon nanotubes. Another object of the present invention is to provide a technique for adjusting a modulation of an absorption coefficient with respect to a magnetic field in a magneto-optical element using carbon nanotubes without changing the wavelength band used.

  Regarding light absorption of carbon nanotubes, the first and second peaks with low photon energy (long wavelength) due to carbon nanotubes having semiconducting properties, and high photon energy (short wavelength) due to metallic carbon nanotubes. A third peak is observed. When the diameter of the carbon nanotube is increased, all absorption peaks shift to the low energy side (long wavelength side) and shift from the target wavelength region toward the low energy side wavelength region.

Here, with respect to the optical switching element based on light absorption, the problem can be solved by the following means.
1) In an optical switching element using carbon nanotubes, the response time can be shortened by using the absorption peak of the higher order side (short wavelength side) peak. The shift in the absorption wavelength due to the use of the higher-order peak is adjusted by increasing the diameter of the carbon nanotube.

2) Regarding the magneto-optical switching element.
a) When the diameter of the carbon nanotube is small, since the cross-sectional area is small, the magnetic flux penetrating the carbon nanotube is reduced, and the degree of modulation of light absorption by the magnetic field is reduced. Therefore, when the diameter of the carbon nanotube is increased, the degree of modulation of light absorption by the magnetic field can be changed by increasing the cross-sectional area.
b) When the diameter of the carbon nanotube is increased, the first peak shifts to the low energy side and shifts from the target wavelength region toward the low energy side wavelength region. Therefore, while changing the diameter, the optical magnetic field response characteristic at the second or third peak is used instead of the first peak.

  When the above technique is used, the response speed in optical switching can be increased. The absorption wavelength changing at this time can be adjusted by using not only the primary peak but also the higher order peak.

  In the magneto-optical element, the diameter of the carbon nanotube is increased in order to improve the sensitivity of the magneto-optical response. At this time, the absorption peak shifts to the long wavelength side. Therefore, the absorption wavelength band can be adjusted to a desired wavelength by using the absorption peak on the higher order side. At the same time, high-speed response by optical switching is possible.

  The characteristics of the electronic structure of single-walled carbon nanotubes (hereinafter referred to as “SWCNT”) reflect either the metal or the semiconductor depending on the structure (chirality), reflecting the one-dimensional nature of the structure. The dimensional electronic structure is realized. Such an electronic structure is strongly reflected in the optical transition, and can be observed, for example, as a photon energy dependency of an absorption coefficient α (E) representing the magnitude of light absorption, that is, a light absorption spectrum.

FIG. 1A is a schematic diagram of SWCNT, in which the y axis is taken in the tube axis direction. Assuming that SWCNT is a cylinder obtained by rolling a two-dimensional graphite sheet (graphene), electrons move on the surface of the cylinder. Therefore, in SWCNT, the movement in one direction out of the two-dimensional translational motion of graphite is localized in the tube circumferential direction. The movement in the other direction remains as a free movement in the tube axis direction (y direction), which realizes the one-dimensional electronic state of the nanotube. FIG. 1B shows the band structure and density of states of the semiconductor nanotube, and FIG. 1C shows the band structure of the metal nanotube. Reflecting the one-dimensional nature of motion, the state density has a divergence called van Hove singularity. Considering the parity of the initial state and the final state (π orbital or π ^* orbital and angular momentum quantum number) and the direction of the photoelectric field, arrows A, B, and C in FIGS. What is shown is an acceptable transition. These transitions have a large transition intensity due to the divergence of the state density due to the quasi-one-dimensional nature of SWCNT. In the absorption spectrum of a normal SWCNT sample in which a metal and a semiconductor tube are mixed, an optical transition of a semiconducting SWCNT indicated by a symbol A and a symbol B and an optical transition of a metallic SWCNT indicated by a symbol C are included. Observed.

  Next, a method for generating SWCNT will be briefly described. Carbon nanotubes produced by the arc discharge method or laser ablation method contain catalytic metals, fullerenes, amorphous carbon, and the like, and are therefore purified by oxidation or the like. In the measurement of Raman spectrum, such a sample can be used as it is, but in order to measure the absorption spectrum, it is necessary to prepare a thin film. For example, a thin film containing a large number of SWCNTs is prepared by dispersing in an organic solvent and spraying it on a substrate to evaporate the solvent. Alternatively, it is possible to obtain an oriented sample in which the directions of SWCNTs are aligned by dispersing the polymer in a polymer and stretching it.

  FIG. 2 is a diagram showing a schematic configuration of a method (pump / probe spectroscopy) for measuring an absorption spectrum and an absorption change induced by optical excitation by pump light with respect to a thin film sample containing a large number of SWCNTs. In the pump-probe spectroscopy shown in FIG. 2, the pump light 3 irradiated to the SWCNT and the probe light 5 by the light pulse are irradiated from different directions on the thin film sample 1 made of SWCNT. Light absorption is measured by the probe light 5 and the detector 7. By irradiating the sample 1 with the pump light 3 by the light pulse, the light absorption of the sample 1 changes, and the absorption coefficient for the probe light 5 is modulated. By measuring the change in the emitted light intensity of the probe light 5, the change in the absorption spectrum can be obtained.

  FIG. 3A shows an absorption spectrum measured at 4.2 K when the average diameter of SWCNT is 1.22 nm. As shown in the figure, in the absorption of the SWCNT thin film sample, absorption peaks due to semiconducting SWCNT appear near photon energy 0.7 eV (peak A) and 1.4 eV (peak B), and due to metallic SWCNT. An absorption peak is observed around 2.0 eV (peak C). These absorption peaks correspond to optical transitions indicated by arrows A, B, and C in FIGS.

  FIG. 3B shows a change in the absorption spectrum measured by the probe light immediately after the pump light is incident. It can be seen that the absorption peaks A, B and C are reduced by the pump light.

  Hereinafter, the optical switching element according to the first embodiment of the present invention using the light absorption characteristics will be described with reference to the drawings. In the optical switching element shown in FIG. 3, the inventors have come up with the use of signal light having a wavelength at the peak position on the higher order side of SWCNT.

  FIG. 4 is a diagram showing how the absorption change ΔA in the respective absorption bands of the peaks A, B, and C recovers with time. It can be seen that the change in the absorption A of the semiconducting tube around the photon energy of 0.79 eV is recovered with a relaxation time constant of about 1 ps. On the other hand, the absorption C of the metallic tube has a relaxation time of about 0.2 ps. Thus, it can be seen that the switching speed can be increased by performing optical switching using the higher-order side, for example, the peak C of the metallic tube.

  Next, a modification of the present embodiment will be described. FIG. 9A is a graph showing the SWCNT tube diameter dependence of the absorption spectrum measured at room temperature, and the SWCNT tube diameter was changed to 0.95 nm, 1.20 nm, 1.30 nm, and 1.40 nm. It is a figure which shows the absorption spectrum from photon energy 0.5eV to 2.5eV in the case. It can be seen that as the average tube diameter increases, each of the first peak A, the second peak B, and the third peak C shifts to a high and low energy side (long wavelength side). FIG. 9B shows an example in which the change in the absorption peak position with respect to the change in diameter is calculated. For example, if the wavelength to be used is 1.55 μm used in optical fiber communication, that is, the energy of 0.8 eV, the tube diameter corresponds to about 1 nm in order to use the peak A due to the semiconductor tube. On the other hand, the peak B corresponds to a tube having a diameter of about 2.1 nm and the peak C corresponds to a diameter of about 3.1 nm. That is, by using a tube having a large diameter, an optical switch using a higher-order absorption peak that responds more quickly can be produced without changing the wavelength of the signal light to be used.

  Next, application to a magneto-optical switching element will be described. The conditions for measuring the absorption spectrum in the magnetic field were a magnetic field of 10T, and the magnetic response was measured with a polarization absorption spectrum measuring apparatus (not shown).

  The magneto-optical switching element according to the second embodiment of the present invention will be described below with reference to the drawings. 5 and 7 irradiate the SWCNT with the signal light hν in a direction perpendicular to the extending direction of the SWCNT (polarized light E parallel to the SWCNT), and in FIG. 5, the magnetic field in a direction parallel to the extending direction of the SWCNT. 7 is a diagram illustrating a simplified configuration example of a magneto-optical switching element in which a magnetic field B is applied in a direction perpendicular to the extending direction of SWCNTs in FIG. FIG. 6 is a diagram showing an example of magnetic response characteristics of light absorption in the arrangement of FIG. FIG. 6A shows an absorption spectrum (absorbance) at an optical energy of 0.6 to 1.2 eV when no magnetic field is applied (0T) and when a magnetic field is applied (10T) in the arrangement shown in FIG. The absorption spectrum is changed by the magnetic field. In order to make the change at this time easy to understand, the change due to the magnetic field is shown as a difference spectrum (ΔA = A (10T) −A (0T)) in FIG. 6B. As shown in FIG. 6B, the absorption is greatly reduced at the absorption peak position.

  On the other hand, FIG. 8A shows an absorption spectrum (Absorbance) at an optical energy of 0.6 to 1.2 eV when no magnetic field is applied (0T) and when a magnetic field is applied (10T) in the arrangement shown in FIG. (B) is a figure which shows the displacement ((DELTA) A = A (10T) -A (0T)) by a magnetic field. As shown in FIG. 8, it can be seen that in this arrangement, there is almost no change in absorption even when a magnetic field is applied up to 10T. That is, in the arrangement of FIG. 5, it can be seen that the light absorption is changed by the magnetic field, and the light transmitted through the SWCNT is modulated. This change in light absorption due to the magnetic field is expected to be based on the change in the electronic structure in SWCNT due to the magnetic field. In this way, switching using the magnetic field as the control means for the signal light can be performed by the absorption change based on the magnetic field in the SWCNT.

  Next, a magneto-optical switching element according to a third embodiment of the present invention will be described with reference to the drawings. According to theoretical prediction, the absorption change due to the magnetic field is proportional to the magnitude of the magnetic flux penetrating the SWCNT cross section (magnetic field × cross-sectional area). Therefore, in order to increase the amount of change in absorption due to a magnetic field, it is effective to first increase the diameter of SWCNT. That is, when the diameter of the carbon nanotube is small, since the cross-sectional area is small, the magnetic flux penetrating the carbon nanotube is reduced, and the degree of modulation of light absorption by the magnetic field is reduced. Therefore, when the diameter of the carbon nanotube is increased, the degree of modulation of light absorption by the magnetic field can be increased by increasing the cross-sectional area. This point will be described later with reference to FIGS. 10 to 13 from the viewpoint of keeping the absorption wavelength constant.

  FIG. 9 is a graph showing the SWCNT tube diameter dependence of the absorption spectrum measured at room temperature, when the SWCNT tube diameter is changed to 0.95 nm, 1.20 nm, 1.30 nm, and 1.40 nm. It is a figure which shows the absorption spectrum from the photon energy of 0.5 eV to 2.5 eV. As the average tube diameter increases, each of the first peak A, the second peak B, and the third peak C shifts the absorption band to the low energy side (long wavelength side). I understand. In order to cope with a shift based on a change (increase) in the diameter of SWCNT, the wavelength of the signal light in the magneto-optical switching element can be adjusted to match the peak position according to the change in diameter.

  However, it is generally preferable to keep the wavelength of the signal light constant rather than adjusting the wavelength of the signal light. From this point of view, when the diameter of the SWCNT is increased, the inventor can improve the magnetic response sensitivity and obtain a high-speed response, but the peak position shifts to the longer wavelength side accordingly. Instead of the first peak A that was used, it came up to use the higher order peak, ie the second peak B or the third peak C. According to FIG. 8B, for example, if the wavelength to be used is 1.55 μm used in optical fiber communication, that is, 0.8 eV, the peak A resonates when the tube diameter is about 1 nm. On the other hand, peak B corresponds to a 2.1 nm tube, and peak C corresponds to a tube having a diameter of about 3.1 nm. That is, by using a tube having a large diameter, a magneto-optical switching element using an absorption peak that reacts to a magnetic field with higher sensitivity can be produced without changing the wavelength of the signal light to be used. The results of actual calculation of this state are shown in FIGS.

  10 to 12, when the magnetic field is not applied (B = 0T) when the SWCNT diameter is 1.04 nm, 2.10 nm, and 3.12 nm, and when the magnetic field is applied (B = 10T). It is a figure which shows the peak of absorption. As can be seen from these figures, the peak is split into two when a magnetic field is applied, and the absorption decreases at the wavelength at which the original peak was present. For example, in FIG. 10, it can be seen that the absorption intensity is reduced from about 650 to about 100. Similar peak fluctuations were observed for FIGS. 11 and 12. Further, the larger the diameter of the tube, the larger the energy ΔE at which the peak is split. The larger the splitting energy, the larger the absorption change in an actual device. FIG. 13 is a diagram collectively showing these states. In the A band, the absorption peak fluctuation ΔE at a wavelength of about 0.8 eV when a 10 T magnetic field is applied is 4.7 meV, the B band is 10.5 meV, and the C band is 14.5 meV. In this way, by increasing the diameter and changing the peak at 0.8 eV from the A band to the B band and then the C band, the change in peak energy ΔE when a constant magnetic field is applied is greatly increased. It is possible to increase the absorption change due to the magnetic field. This is because when the SWCNT diameter is small, the magnetic flux penetrating the SWCNT is reduced because the cross-sectional area is small, and the degree of modulation of light absorption by the magnetic field is reduced, whereas the diameter of the carbon nanotube is increased. This is because the cross-sectional area increases and the degree of modulation of light absorption by the magnetic field can be changed.

  As described above, according to the magneto-optical switching element according to the present embodiment, the magnetic response sensitivity of absorption can be improved by increasing the diameter of SWCNT. Furthermore, the energy (wavelength) of the absorption peak changed by increasing the diameter can be adjusted to be constant by using a higher-order band. Furthermore, the optical switching response speed can be increased by using the higher-order peak.

  As described above, when the above-described technique is used, a high-speed optical switching element can be realized because an optical switching response becomes fast. Further, since the magneto-optical response characteristics in a desired wavelength band can be improved, a highly sensitive magneto-optical element using carbon nanotubes can be realized.

  The present invention can be applied to an optical switching element and a magneto-optical switching element using carbon nanotubes.

FIG. 1A is a schematic diagram of SWCNT, in which the y axis is taken in the tube axis direction. FIG. 1B shows the band structure and density of states of the semiconductor nanotube, and FIG. 1C shows the band structure of the metal nanotube. FIG. 3 is a diagram showing a schematic configuration of a measurement system of a method (pump / probe spectroscopy) for measuring an absorption spectrum and absorption change induced by optical excitation by pump light with respect to a sample. FIG. 3A is a spectrum diagram showing the energy dependence of absorption measured at 4.2 K when the average diameter of SWCNT is 1.22 nm. FIG. 3B is a diagram illustrating a change in the absorption spectrum measured by the probe light immediately after the pump light is incident. It is a figure showing a mode that the change ΔA of absorption in each absorption band of peaks A and C recovers in time. A simplified configuration example of a magneto-optical switching element in which signal light hν is irradiated to SWCNT in a direction perpendicular to the extending direction of SWCNT and a magnetic field is applied in a direction parallel to the extending direction of SWCNT is shown. FIG. In the arrangement of FIG. 5, the absorption spectrum (Absorbance) at a light energy of 0.6 to 1.2 eV and the change in absorption due to the magnetic field (ΔA = A) when no magnetic field is applied (0T) and when a magnetic field is applied (10T). It is a figure which shows (10T) -A (0T)). It is a figure which shows the simplified structural example of the magneto-optical switching element which applied the magnetic field to the direction perpendicular | vertical to the extension direction of SWCNT. In the arrangement in FIG. 7, the absorption spectrum (Absorbance) at a light energy of 0.6 to 1.2 eV and the change in absorption due to the magnetic field (ΔA = A) when no magnetic field is applied (0T) and when a magnetic field is applied (10T). It is a figure which shows (10T) -A (0T)). FIG. 9A is a graph showing the SWCNT tube diameter dependence of the absorption spectrum measured at room temperature, and the SWCNT tube diameter was changed to 0.95 nm, 1.20 nm, 1.30 nm, and 1.40 nm. It is a figure which shows the absorption spectrum from photon energy 0.5eV to 2.5eV in the case. FIG. 9B shows the calculated relationship between the absorption peak position and the tube diameter. It is a figure which shows the peak of absorption when a magnetic field is not applied when the diameter of SWCNT is 1.04 nm (B = 0T) and when a magnetic field is applied (B = 10T). It is a figure which shows the absorption peak when not applying a magnetic field when the diameter of SWCNT is 2.10 nm (B = 0T) and when applying a magnetic field (B = 10T). It is a figure which shows the peak of absorption when a magnetic field is not applied when the diameter of SWCNT is 3.12 nm (B = 0T) and when a magnetic field is applied (B = 10T). It is a figure which shows the relationship between the change of absorption based on the diameter of SWCNT, and a magnetic field.

Explanation of symbols

  DESCRIPTION OF SYMBOLS 1 ... Thin film sample which consists of SWCNT, 3 ... Pump light, 5 ... Probe light, 7 ... Detector.

Claims (3)

Light having an optical element composed of carbon nanotubes having nonlinear optical characteristics, a first light source that irradiates the optical element with signal light, and a second light source that irradiates the optical element with control light A switching device,
A plurality of peak absorption wavelength observed at the carbon nanotube in the detection of light absorption responses using the change in the transmitted signal light by the optical absorption when irradiated with the control light to the optical element, the semiconductor properties A first peak having a second peak having a semiconductor position different from the first peak, and a third peak having a metallic characteristic having a peak position different from that of the first and second peaks. The diameter of the carbon nanotube is set so that the peak position of the absorption wavelength of the third peak corresponds to the wavelength of the signal light to be used . 2. The optical switching according to claim 1 , wherein the optical element is formed of a thin film including a large number of SWCNTs formed by dispersing carbon nanotubes in an organic solvent, spraying the carbon nanotubes on a substrate, and evaporating the solvent. apparatus. 2. The optical switching device according to claim 1 , wherein the optical element is composed of an alignment sample in which the directions of SWCNTs are aligned by dispersing carbon nanotubes in a polymer and stretching the carbon nanotubes.

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