Optical pumping field emission type light-to-current converter

Abstract: An optical pumping field emission type light-to-current converter is constructed by joining an optical wave guide material (1) through which light is propagated, a conductive transparent film (2), a semiconductor material or an insulating material (3), and a conductor material (4) to each other. The optical wave guide material may have a needle-like shape (6) which is obtained by sharpening the light emission guide side thereof. Thereby is obtained an optical pumping field emission type light-to-current converter having a simplified structure high sensitivity and high-speed response on the order of picoseconds (10 -12 seconds) due to a large field emission current.

Description: The present invention relates to an optical detector which is generally used in optical communication, an optical information apparatus, optical measurement, an analysis apparatus and the like. Heretofore, the devices which have been most generally used as light-to-current converters are photomultipliers, charged couple devices called "CCDs" for short, and also photodiodes. With respect to the photomultipliers, it can be given as the advantage thereof that the photomultiplier responds to even very weak light and hence the amplification factor thereof is large, while it can be given as the disadvantage thereof that the response speed thereof is low, i.e., on the order of microseconds, the conversion current thereof is small, i.e., on the order of microampere, and the correlation between the light intensity and the current amount varies to some degree whereby both dark current and noise are large. In addition, the CCD does not directly convert light into current amount, but temporarily converts light into electric charges and then converts the electric charges into current in the stage of reading the electric charges. For this reason, the CCD has the advantage that the correlation between the light intensity and the current is extremely high, while it has also the disadvantage that the response speed thereof is low due to the two stages of the conversion process. Also, the mechanism thereof is complicated. Further, with respect to the photodiode, it is an advantage that the band width is wide, i.e., 20 GHz, while it is given as a disadvantage that the range of the operation temperatures is narrow, i.e., in the range of 10 to 40°C. US Patent No. 4,227,078 discloses a photosensor featuring a photodiode as a light sensitive element, comprising an optical waveguide, a conductive transparent film over the emission side of the waveguide, a layer of semiconductor approximately 2µm in thickness on the emission side of the transparent film, and a metallic electrode on the emission side of the semiconductor. A semiconductor junction for the photodiode is formed either between the semiconductor and the transparent film, or between the semiconductor and an additional semiconductor layer. US Patent No. 4,232,219 discloses a similar device to that disclosed in US 4,227,078, further comprising color filters and corresponding photodiode elements to provide a color sensitive sensor. It is an object of the present invention to provide an optical pumping field emission type light-to-current converter in which by utilising the field emission, the amount of field emission current is increased to realise a response speed on the order of picoseconds (10 -12 seconds) and also the structure thereof is simplified. An optical pumping field emission type light-to-current converter according to the present invention utilises the field emission in order to solve the above-mentioned problems associated with the prior art. The basic principles will hereinafter be described. When applying a negative bias voltage to the sharp tip of a needle-like structure made of a conductive material, a high electric field is generated at the sharp tip of the needle-like structure to reduce the potential barrier against the electrons which are trapped in the surface of the sharp tip of the needle-like structure. As a result, there is increased probability that the electrons in the surface are emitted to the outside on the basis of Heisenberg's uncertainty principle. This phenomenon is called the field emission. In the needle-like structure of the conductor made of metal or the like, the amount of emission current varies greatly depending on the electric field strength at the sharp tip of the needle-like structure, and does not respond to light which is made incident on the sharp tip of the needle-like structure. However, in a semiconductor, in particular, a semiconductor material which has electrical characteristics close to an insulator, or an insulating material, when making light incident on the sharp tip of the needle-like structure, the optical conductivity is increased momentarily so that the amount of field emission current becomes extremely large. The device to which such principles are applied is the optical pumping field emission type light-to-current converter of this invention, as defined in claim 1. Embodiments of the invention will now be described by way of further example only and with reference to the accompanying drawings, in which:- Fig. 1 is a schematic view showing the structure of an optical pumping field emission type light-to-current converter according to a first embodiment of the present invention; and Fig. 2 is a schematic view showing the structure of an optical pumping field emission type light-to-current converter according to a second embodiment of the present invention. The present invention increases, by utilising the field emission, the amount of field emission current and realises a response speed on the order of picoseconds, and also provides an optical pumping field emission type light-to-current converter having a simplified structure. Fig. 1 is a schematic view showing the structure of an optical pumping field emission type light-to-current converter according to a first embodiment of the present invention. In Fig. 1, reference numeral 1 designates a glass series fibre acting as an optical wave guide material. The end of the light emission side is coated with a conductive transparent film 2. In this connection, in the present embodiment, an ITO (In 2 O 3 :Sn) film with about 0.1 µm thickness is formed on the end of the light emission side of the optical wave guide material 1 by the electron beam evaporation system (EB evaporation system). While the ITO film is made by utilising the EB evaporation system, it should be noted that the ITO film may also be formed by utilising the sputtering method, the metal fog method, the spray method or the like. In addition, the conductive transparent film 2 is coated with a semiconductor material or an insulating material. In the present embodiment, an SiO 2 film is employed as an insulating film 3, and the TEOS (Tetra Ethylortho Silicate) - CVD method is employed as the method of forming the same. It is required for the SiO 2 film to have the thickness with which the height of the needle-like structure of the tip of the glass fibre, i.e., the height of about 10 µm, is thoroughly covered. For this reason, an SiO 2 film of about 20 µm thickness is deposited thereon. Thereafter, the surface of the SiO 2 film thus formed is polished in such a way that the distance between the surface of the SiO 2 film and the tip of the needle-like structure having the conductive transparent film formed thereon becomes equal to or smaller than 0.1 µm. One end of the glass fibre 1 having the needle-like structure with about 10 µm height is coated with the ITO (In 2 O 3 :Sb) film with 0.1 µm thickness as the conductive transparent film 2 and the SiO 2 film as the insulating film 3 in this order, and also the surface of the SiO 2 film is polished to be flattened. A conductor material 4 is joined to the surface of the SiO 2 film, and then a bias voltage 5 is applied across the conductive transparent film 2 formed on the glass fibre and the conductor material 4 in such a way that the electric potential of the conductive transparent film becomes negative, whereby the resultant device of interest can be operated as the optical circuit incorporated optical pumping field emission type light-to-current converter. A second embodiment of the present invention will hereinbelow be described with reference to the drawings. While an object of the present embodiment is, similarly to the above-mentioned first embodiment, to provide an optical pumping field emission type light-to-current converter, in which the tip of the conductive material is sharpened to the needle-like structure so that the electrons in the surface of the sharp tip can be emitted with higher probability. Fig. 2 is a schematic view showing the structure of an optical pumping field emission type light-to-current converter according to the second embodiment of the present invention. In Fig. 2, reference numeral 1 designates a glass series fibre as an optical wave guide material one end of which has a plurality of sharpened needle-like structures 6. As for the method of forming the needle-like structures 6, the surface is formed to be irregularly uneven by utilising the blasting method wherein grinding stones used to rough a surface are blasted at the surface, or the polishing method to form the needle-like structures by the mechanical method. In this connection, the height of each of the needle-like structures is equal to or smaller than about 10 µm. Alternately, in addition to the mechanical process, the needle-like structures each with about several hundreds µm height may be formed by utilising the method employing the RIE (Reactive Ion Etching) system or the wet etching method. One end of the glass fibre, including the needle-like structures formed as described above, is coated with the conductive transparent film 2. In this connection, in the present embodiment, an ITO (In 2 O 3 :Sn) film with about 0.1 µm thickness is formed as the conductive transparent film 2 by utilising the electron beam evaporation system (EB evaporation system). While the ITO film is formed by utilising the EB evaporation system, it should be noted that the ITO film may also be formed by utilising the sputtering method, the metal fog method or the spray method. Then, the conductive transparent film 2 is further coated with a semiconductor material or an insulating material. In the present embodiment, an SiO 2 film is employed as an insulating film 3, and the TEOS (Tetra Ethylortho Silicate) - CVD method is employed as the method of forming the same. It is required for the SiO 2 film to have the thickness with which the height of the needle-like structures of the tip of the glass fibre, i.e., the height of about 10 µm, is thoroughly covered. For this reason, a SiO 2 film of about 20 µm thickness is deposited thereon. Thereafter, the surface of the SiO 2 film thus formed is polished in such a way that the distance between the surface of the SiO 2 film and the tip of each of the needle-like structures having the conductive transparent film becomes equal to or smaller than 0.1 µm. One end of the glass fibre 1 having the needle-like structures each with about 10 µm height is coated with the ITO (In 2 O 3 :Sn) film with 0.1 µm thickness as the conductive transparent film 2 and the SiO 2 film as the insulating film 3 in this order, and also the surface of the SiO 2 film is polished to be flattened. A conductive material 4 is joined to the surface of the SiO 2 film, and then a bias voltage 5 is applied across the conductive transparent film 2 formed on the glass fibre and the conductor material 4 in such a way that the electric potential of the conductive transparent film 2 becomes negative, whereby the resultant device of interest can be operated as the optical circuit incorporated optical pumping field emission type light-to-current converter. When light is made incident on the other end of the grass fibre having no needle-like structure of Fig. 2, and then the tips of the needle-like structures are irradiated with the incident light, the optical conductivity is momentarily increased under the structure of Fig. 2 so that the field emission current becomes extremely large. As set forth hereinabove, an optical pumping field emission type light-to-current converter can be constructed by joining a sharpened optical wave guide material, a conductive transparent film, a semiconductor material or an insulating material, and a conductor material to each other, so that the field emission current becomes extremely large. For this reason, a response speed which is equal to or higher than picoseconds (10 -12 seconds) becomes possible. In addition, the amount of conversion current depends on the shape and the number of needles of the needle-like structure with which the field emission is provided, and a bias voltage, and hence the conversion current of 10 millampere or more can be expected. In addition, the structure thereof can be made extremely simple, and also a scaling down (miniaturisation) can be made. Also, due to the scaling down, the optical pumping field emission type light-to-current converter is excellent in mechanical strength. Further, the operating temperatures thereof can be extended from the very low temperatures to about 400°C, thereby realising low noise. The optical pumping field emission type light-to-current converter can be such that the sharpened needle-like structure which is formed at one end of the optical wave guide material, the conductive transparent film contacting with the optical wave guide material, and the semiconductor material or the insulating material contacting with the conductive transparent film are made of diamond, SiO 2 or GaAs. The optical pumping field emission type light-to-current converter can be such that the sharpened needle-like structure which is formed at one end of the optical wave guide material, the conductive transparent film contacting with the optical wave guide material and the semiconductor material or the insulating material contacting with the conductive transparent film, a thin film acting as an optical filter is inserted between the sharpened needle-like structure which is formed at the one end of the optical wave guide material and the conductive transparent film contacting with the optical wave guide material.

Claims: An optical pumping field emission type light-to-current converter comprising an optical wave guide material (1) having a fiber shape with a sharpened needle-like structure (6) at its tip through which light is propagated having a proximal receiving face and a distal emission face, a conductive transparent film (2) disposed over the emission face of the wave guide material (1), a semiconductor material or an insulating material (3) disposed on the distal side of the transparent film (2), a conductor material (4) disposed on the distal side of the semiconductor material or insulating material (3), and voltage applying means (5) electrically connected to the conductive transparent film (2) and the conductor (4) for applying a voltage therebetween, wherein the distance between the distal surface of the semiconductor material or insulating material (3) and the tip of the needle-like structure having the conductive transparent film (2) formed thereon is equal to or less than 0.1 µm. An optical pumping field emission type light-to-current converter according to claim 1, wherein the emission face of the wave guide material (1) comprises one or a plurality of sharpened needle-like structures (6) and the tip of a or each needle-like structure (6) is closer to the distal surface of the semiconductor material or insulator material (3) than the base of a or each needle-like structure (6). An optical pumping field emission type light-to-current converter according to claim 2, wherein the height of a sharpened needle-like structure (6) which is formed at the distal end of the optical wave guide material (1) is in the range of 1 to 500 µm, and the interval of the needle-like structures (6) where a plurality of needle-like structures is formed is in the range of 0.1 to 500 µm. An optical pumping field emission type light-to-current converter according to claim 2, wherein the radius of curvature of a sharpened needle-like structure (6) which is formed at the distal end of the optical wave guide material (1) is equal to or smaller than 1 µm. An optical pumping field emission type light-to-current converter according to claim 2, wherein the conductive transparent film (2) contacting with a sharpened needle-like structure (6) which is formed at the distal end of the optical wave guide material (1) is either a transparent material containing therein Sn 2 O 3 , In 2 O 3 or ZnO, or a material through which light having a specific wavelength penetrates. An optical pumping field emission type light-to-current converter according to claim 5, wherein the thickness of the conductive transparent film (2) contacting with a sharpened needle-like structure (6) which is formed at the distal end of the optical wave guide material (1) is in the range of 0.001 to 1 µm. An optical pumping field emission type light-to-current converter according to claim 5, wherein a sharpened needle-like structure (6) which is formed at the distal end of the optical wave guide material (1), and the conductive transparent film (2) contacting therewith are covered with either a semiconductor material or an insulating material (3). An optical pumping field emission type light-to-current converter according to claim 2, wherein a sharpened needle-like structure (6) which is formed at the distal end of the optical wave guide material (1), the conductive transparent film (2) contacting with the optical wave guide material (1), and the semiconductor material or the insulating material (3) contacting with the conductive transparent film (2) are made of diamond, SiO 2 or GaAs. An optical pumping field emission type light-to-current converter according to claim 8, wherein the interval among the sharpened needle-like structures (6) which are formed at the distal end of the optical wave guide material (1), the conductive transparent film (2) contacting with the optical wave guide material (1), a tip of the semiconductor material or the insulating material (3) contacting with the conductive transparent film (2), and the conductor material (4) is in the range of 0.001 to 0.1 µm. An optical pumping field emission type light-to-current converter according to claim 2, wherein in the structure of a sharpened needle-like structure (6) which is formed at the distal end of the optical wave guide material (1), the conductive transparent film (2) contacting with the optical wave guide material (1) and the semiconductor material or the insulating material (3) contacting with the conductive transparent film (2), a thin film acting as an optical filter is inserted between the needle-like structure (6) which is formed at the distal end of the optical wave guide material (1) and the conductive transparent film (2) contacting with the optical wave guide material (1).

Inventors: NAKAJIMA KUNIO, NISHIKAWA OSAMU, NAKAJIMA, KUNIO, NISHIKAWA, OSAMU
Applicants: SEIKO INSTR INC, SEIKO INSTRUMENTS INC.
Application year: 1998
Main classification: H01J1
IPC codes: G02B6, H01L31, G01J1, H01J1