Abstract:
The disclosure relates to a detecting system including a terahertz wave source, a detector and a controlling computer. The terahertz wave source includes a terahertz reflection klystron including an electron emission unit, a resonance unit, an output unit. The electron emission unit is configured to emit electrons. The resonance unit includes a resonant cavity communicated with the electron emission unit so that the electron emission unit emit electrons into the resonant cavity. The resonant cavity of the electron emission unit opposite the cavity wall has an output aperture coupled. The output unit is communicated with the resonance unit by the output aperture coupled. The resonance unit generate terahertz wave transmit to the output unit by the output aperture coupled.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application in part of U.S. patent application Ser. No. 15/183,175, Attorney Docket No. US57530, filed on Jun. 15, 2016, entitled “TERA HERTZ REFLEX KLYSTRON,” which claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201510525276.6, filed on Aug. 25, 2015 in the China Intellectual Property Office, disclosure of which is incorporated herein by reference. This application also claims all benefits accruing under 35 U.S.C. §119 from China Patent Application No. 201610386012.1, filed on Jun. 3, 2016 in the China Intellectual Property Office, disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND 
     1. Technical Field 
       [0002]    The present disclosure relates to a terahertz reflex klystron and a detecting system using the same. 
       2. Description of Related Art 
       [0003]    In general, the terahertz (THz) wave refers to an electromagnetic wave whose frequency ranging from 0.3 THz to 3 THz or 0.1 THz to 10 THz. The band of THz wave lies between the infrared wave and the millimeter wave. The THz wave has excellent properties. For example, THz wave has certain ability to penetrate objects, and the photon energy is small. Thus the THz will not cause damage to the objects. At the same time, a lot of material can absorb the THz wave. 
         [0004]    A reflex klystron is used to emit electromagnetic waves. In order to emit THz waves, the feature size of the reflex klystron should be small, and the current density of the electron rejection should be high. A traditional terahertz reflex klystron includes a resonant cavity. The resonant cavity includes two coupling outputting holes located on two opposite side walls. The resonant cavity should have a large width, and the size of the terahertz reflex klystron should be large enough. It is hard to decrease the size of the terahertz reflex klystron, and a micro terahertz reflex klystron array cannot be obtained. 
         [0005]    What is needed, therefore, is a terahertz reflex klystron and a detecting system using the same that overcomes the problems as discussed above. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    Many aspects of the embodiments can be better understood with reference to the following drawings. The components in the drawings are not necessarily drawn to scale, the emphasis instead being placed upon clearly illustrating the principles of the embodiments. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0007]      FIG. 1  is a schematic section view of one embodiment of a detecting system based on terahertz wave. 
           [0008]      FIG. 2  is a schematic section view of one embodiment of a terahertz reflex klystron. 
           [0009]      FIG. 3  is a schematic view of an electron emission unit used in the terahertz reflex klystron of  FIG. 2 . 
           [0010]      FIG. 4  is a scanning electron microscope (SEM) image of a carbon nanotube wire used in the electron emission unit of  FIG. 3 . 
           [0011]      FIG. 5  shows a vertical view of a first grid according to one embodiment. 
           [0012]      FIG. 6  is a functional module view of one embodiment of a controlling computer. 
           [0013]      FIG. 7  is a schematic section view of another embodiment of a detecting system. 
           [0014]      FIG. 8  shows a vertical schematic view of one embodiment of a micro terahertz reflex klystron array. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    The disclosure is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that references to “an” or “one” embodiment in this disclosure are not necessarily to the same embodiment, and such references mean at least one. 
         [0016]    References will now be made to the drawings to describe, in detail, various embodiments of the present detecting system based on terahertz wave. 
         [0017]    Referring to  FIG. 1 , a detecting system  1  of one embodiment based on terahertz wave is provided. The detecting system  1  comprises a terahertz wave source  10 , a detector  18  spaced from the terahertz wave source  10 , and a controlling computer  19  connected to both the terahertz wave source  10  and the detector  18 . 
         [0018]    The detecting system  1  is transmission-type. In use, the object  20  is located between the terahertz wave source  10  and the detector  18 . The terahertz wave  15  is emitted from the terahertz wave source  10 , reaches the object  20 , passes through the object  20 , and received by the detector  18 . The detector  18  obtains the data of the terahertz wave  15  and send the data to the controlling computer  19 . The controlling computer  19  processes the data of the terahertz wave  15  to obtain a result and shows the result to the user. 
         [0019]    Referring to  FIG. 2 , the terahertz wave source  10  comprises a terahertz reflex klystron  10   a.  The terahertz reflex klystron  10   a  includes an electron emission unit  11 , a resonant unit  12  and an output unit  14 . The electron emission unit  11 , the resonant unit  12  and the output unit  14  connect with each other. The resonant unit  12  is located between the electron emission unit  11  and the output unit  14 . The electron emission unit  11  is used to emit electrons. The resonant unit  12  includes a resonant cavity  121  which is connected to the electron emission unit  11 . The electrons are emitted from the electron emission unit  11  and get into the resonant cavity  121 . The resonant cavity  121  includes at least one outputting holes  123 . The output unit  14  and the resonant unit  12  face each other. The output unit  14  and the resonant unit  12  are communicant with each other through the outputting holes  123 . The resonant unit  12  emits terahertz (THz) waves which are transmitted to the output unit  14 . 
         [0020]    The electron emission unit  11  includes an insulating substrate  110 , a cathode  111 , an electron emitter unit  114 , an electron injection layer  113 , an insulating layer  116 , and an electron extraction grid  115 . The cathode  111  is located on the insulating substrate  110 . The electron emitter unit  114  is electrically connected to the cathode  111 . The electron injection layer  113  is located above and insulated from the cathode  111  via the insulating layer  116 . The electron injection layer  113  defines a hollow space  1130 , and the electron emitter unit  114  is located in the hollow space  1130 . The hollow space  1130  defines a first opening, the electron extraction grid  115  covers the first opening. 
         [0021]    A material of the insulating substrate  110  can be silicon, glass, ceramics, plastics, or polymers. A shape and a thickness of insulating base can be selected according to actual needs. The shape of the insulating substrate  110  can be circular, square, or rectangular. In one embodiment, the insulating substrate  110  is square, the length is about 10 mm, and the thickness is about 1 mm. 
         [0022]    The cathode  111  is located on a surface of the insulating substrate  110 . The insulating layer  116  covers the cathode  111 . A first portion of the cathode  111  is exposed to and faces the electron extraction grid  115 , and a second portion of the cathode  111  is covered by the electron injection layer  113 . The electron emitter unit  114  is located on the first portion of the cathode  111  and electrically connected to the cathode  111 . The electron emitter unit  114  faces the electron extraction grid  115 . The first portion of the cathode  111  is exposed out through the hollow space  1130 . 
         [0023]    The cathode  111  is a conductive layer. A material of the cathode  111  can be pure metal, alloy, semiconductor, indium tin oxide, or conductive paste. In one embodiment, the material of the insulating substrate  110  is silicon, and the cathode  111  can be doped silicon. In one embodiment, the material of the cathode  111  is an aluminum film with  20  micrometers. The aluminum film can be deposited on the insulating substrate  110  via magnetron sputtering method. 
         [0024]    A material of the electron injection layer  113  can be silicon, chromium. A thickness of the electron injection layer  113  can be greater than 10 micrometers. In one embodiment, the thickness of the electron injection layer  113  ranges from about 30 micrometers to about 60 micrometers. 
         [0025]    The electron injection layer  113  can have an oblique sidewall around the hollow space  1130 . The hollow space  1130  can be in a shape of inverted funnel and the size of hollow space  1130  is gradually narrowed along a direction away from the cathode  111 . The electron emitter unit  114  can be received in hollow space  1130 . 
         [0026]    The insulating layer  116  located on a surface of the electron injection layer  113 . The insulating layer  116  has two portions, a first portion of the insulating layer  116  is located between the electron injection layer  113  and the cathode  111 , a second portion of the insulating layer  116  is located in the hollow space  1130  and on an inside surface of the electron injection layer  113 . The insulating layer  116  can be resin, plastic, glass, ceramic, oxide, or their mixture. The oxide can be silica, aluminum oxide, or bismuth oxide. In one embodiment, the thickness of insulating layer  116  is about 100 micrometers. The material of the insulating layer  116  is a circular photoresist. In one embodiment, a secondary electron multiply material can be coated on a surface of the second portion of the insulating layer  116 . The secondary electron multiply material can be magnesium oxide, beryllium oxide or diamond. The secondary electron multiply material can improve number of the electrons when the electrons emitted from the electron emitters  1140  hit the side wall of the hollow space  1130 . 
         [0027]    Referring to  FIG. 3 , the electron emitter unit  114  has a tapered shape defining a peak. A height of the electron emitter unit  114  at the central portion is the highest, and the height is gradually decreased along a direction away from the center. Furthermore, the central portion of the electron emitter unit  114  and the center of hollow space  1130  are in a same location. The electron emitter unit  114  includes a plurality of electron emitters  1140 . The plurality of electron emitters  1140  are parallel with each other. The electron emitter  1140  at the center of the electron emitter unit  114  is the highest. The height of the electron emitter unit  114  is gradually decreased along the direction away from the center of the electron emitter unit  114 . 
         [0028]    The material of the electron emitters  1140  can be a carbon nanotube, carbon fiber, or silicon nanofiber. Each of the plurality of electron emitters  1140  includes a first end and a second end, opposite to the first end. The second end is adjacent and electrically connected to the cathode  111 , and the first end extends toward the anode  112 . The first end is configured to emit electrons as an electron emission terminal. The height of the plurality of electron emitter unit  114  is greater than the thickness of the insulating layer  116 . 
         [0029]    The electron emitter unit  114  is spaced from the sidewall of hollow space  1130 . The electron emitter unit  114  defines an emitting surface that is away from the insulating substrate  110 . The emitting surface of the electron emitter unit  114  can be parallel with the sidewall. In detail, a distance between each first end of the electron emitters  1140  and the sidewall of hollow space  1130  is substantially the same. Thus the plurality of first ends and the sidewall have substantially the same distances. The electron emitters  1140  can be carbon nanotubes, carbon fibers, silicon nanowires or silicon tips. Referring to  FIG. 4 , in one embodiment, the electron emitter unit  114  can be a carbon nanotube wire. The carbon nanotube wire includes a plurality of carbon nanotubes parallel with each other or twisted with other. 
         [0030]    Furthermore, an ion bombardment resistance material can be deposited on each of the plurality of electron emitters  1140 . The ion bombardment resistance material can be zirconium carbide, hafnium carbide, or lanthanum hexaboride. The ion bombardment resistance material can protect the plurality of electron emitters  1140  from damage. Thus the lifespan of the electron emitters  1140  can be prolonged. 
         [0031]    The electron emission unit  11  can further include a resistor layer (not shown). The resistor layer is sandwiched between the electron emitter unit  114  and the cathode  111 . The electron emitter unit  114  is electrically connected to the cathode  111 . The resistance of the resistor layer is greater than 10 GΩ to ensure that the cathode  111  can uniformly apply current to the electron emitter unit  114 . The material of the resistor layer can be metallic alloy of nickel, copper, cobalt; the material of the resistor layer can also be metallic alloy, metallic oxide, inorganic composition doped with phosphorus. 
         [0032]    The electron extraction grid  115  is used to leading the electrons emitter from the electron emitter unit  114 . The electron extraction grid  115  is spaced from the electron injection layer  113  and cover the first opening of the hollow space  1130 . While a voltage is applied on the electron extraction grid  115 , the electrons can be extracted from the electron emitter unit  114 . 
         [0033]    The electron extraction grid  115  can be a carbon nanotube composite layer, a carbon nanotube layer, or a graphene layer. An electron transmittance rate of the graphene layer can reach to 98%. Referring to  FIG. 5 , in one embodiment, the electron extraction grid  115  is a carbon nanotube composite layer. The carbon nanotube composite layer has a net structure comprising a carbon nanotube layer  1154  and coating layer  1153 . The carbon nanotube composite structure defines a plurality of apertures  1152  to let the electrons pass through. A size of the aperture  1152  can range from about 1 nanometer to about 200 micrometers, particularly, it is ranged from 10 nanometers to 10 millimeters. 
         [0034]    The carbon nanotube layer  1154  can be a patterned carbon nanotube layer and defines the plurality of holes  1155 . The holes  1155  can be dispersed uniformly. The holes  1155  extend throughout the carbon nanotube layer  1154  along the thickness direction thereof. The holes  1155  can be defined by several adjacent carbon nanotubes, or a gap defined by two substantially parallel carbon nanotubes and extending along the axial direction of the carbon nanotubes. The coating layer  1153  is coated on the plurality of carbon nanotubes in the carbon nanotube layer. After the coating layer formed, the size of the holes  1155  decreases to form the apertures  1152 . The coating layer  1153  is used to protect the carbon nanotube layer  1154 . A material of the coating layer  1153  can be silicon, silicon dioxide, silicon oxide, or aluminum oxide. A thickness of the coating layer  1153  ranges from 1 nanometer to 100 micrometers, particularly, it ranges from 5 nanometers to 100 nanometers. 
         [0035]    The resonant unit  12  includes a resonant cavity frame  128 , an insulating support  126 , a first grid electrode  124 , a second grid electrode  125 , at least one outputting hole  123 , a reflective room  122  and a reflective electrode  127 . The resonant cavity frame  128  defines a resonant cavity  121 . The resonant cavity frame  128  is located on and above the electron injection layer  113 . The resonant cavity frame  128  defines a bottom opening (not labeled) and a top opening (not labeled). The first opening, the bottom opening, and the top opening are running through with each other. The bottom opening is located above the first opening. The bottom opening and the first opening are aligned with each other. The insulating support  126  is located around the bottom opening. The first grid electrode  124  is located above and parallel with the electron extraction grid  115 . The first grid electrode  124  is supported by the insulating support  126  separated from the electron extraction grid  115 . 
         [0036]    A material of the resonant cavity frame  128  can be silicon or chromium. A width of the resonant cavity  121  can be in a range of 70 micrometers to 300 micrometers. An inside wall of the resonant cavity frame  128  is coated by metal, such as copper, aluminum, and other conductive material. In one embodiment, the resonant cavity frame  128  has a tube structure defines the resonant cavity  121 . A diameter of the resonant cavity  121  is 300 micrometers, the output frequency. 
         [0037]    The resonant cavity frame  128  includes a bottom wall and a top wall. The bottom wall is located on the electron extraction grid  115 . The top wall is located above the bottom wall. The bottom opening is defined by the bottom wall. The top opening is defined by the top wall. The at least one outputting hole  123  is located in the top wall. The second grid electrode  125  covers the top opening. The electron extraction grid  115 , the first grid electrode  124  and the second grid electrode  125  are arranged in that order and overlapped with each other. 
         [0038]    The at least one outputting hole  123  is located around the top opening. In some embodiments, the at least one outputting hole includes a plurality of outputting holes arranged orderly, the plurality of outputting holes are arranged uniformly in a circle, and a center of the circle is a center of the top opening. In the embodiment, a number of the outputting hole  123  is four, and the four outputting holes  123  are arranged in symmetry. 
         [0039]    The reflective room  122  includes a reflective electrode  127  located therein. The reflective electrode  127  is located above and faces the second grid electrode  125 . The reflective room  122  covers the top opening and open to the top opening. When a voltage is applied on the reflective electrode  127 , the reflective electrode  127  is used to reflect electrons passing through the second grid electrode  125 . A voltage of the reflective electrode  127  is lower than a voltage of the second grid electrode  125 . And, a speed of the electrons getting into the reflective room  122  is decreased by a retarding field between the reflective electrode  127  and the second grid electrode  125 . 
         [0040]    The output unit  14  includes a wave guide  140 , an absorber  141  and a lens  142 . The wave guide  140  defines a guide room, the absorber  141  is located on a surface of the wave guide  140  and in the guide room. The lens  142  is located at one end of the wave guide and covers an exit of the guide room. 
         [0041]    In work of the terahertz reflex klystron  10   a,  the cathode  111 , the electron extraction grid  115 , the first grid electrode  124 , the second grid electrode  125 , the reflective electrode  127  are separately applied voltage. The electrons are emitted by the electron emitter unit  114  and extracted out the first opening by the electron extraction grid  115 , and, pass through the first grid electrode  124 . The electrons can be accelerated by the first grid electrode  124  and the second grid electrode  125  to form an electron beam with enough current density. The electron beam can pass through the first grid electrode  124 , the resonant cavity  121 , and the second grid electrode  125 . Thus the electron beam will be modulated by a microwave field in the resonant cavity  121 . After the electron beam passes through the second grid electrode  125 , the electron beam will be reflected by the reflective electrode  127 . All the electrons will be reflected by the retarding field in the reflective room  122 . Thus the electron beam will be modulated on density in the retarding field and reflected to the resonant cavity  121 . Therefore, the electrons will oscillate in the resonant cavity  121 . After the electron beam is modulated on density, it will pass through the outputting hole  123  be transferred out into the guide room of the output unit  14 . And, then the terahertz will be formed and output from the lens. 
         [0042]    The terahertz reflex klystron  10   a  has following advantages. The at least one outputting hole  123  is located on the top wall of the resonant cavity frame  128 , a width of the resonant cavity frame  128  can be small, and as such, the terahertz reflex klystron  10   a  can have a small size. Further, because the electron emitter structure has a shape of a cone, and the electron emitter in the central portion is highest. Thus the shielding effect can be reduced. In addition, the through hole of the electron extraction grid  115  is in the shape of inverted funnel. Thus the electrons can be focused by the through hole, and the current emission density can be improved. 
         [0043]    Furthermore, the terahertz wave source  10  can include a moving controlling device (not shown) configured to allow the terahertz reflex klystron  10   a  to move or swing. Thus, the terahertz wave source  10  can scan the object  20 . 
         [0044]    The structure of the detector  18  is not limited and can be selected according to need. The detector  18  can be a photoconductivity switching, electro-optical crystal, bolometer, pyroelectric detector, thermal expansion detector, and frequency mixing and frequency difference detector. 
         [0045]    Referring to  FIG. 6 , the controlling computer  19  includes a processing module  191 , a memory module  192  connected to the processing module  191 , a data acquisition module  193  connected to the processing module  191 , and an emission controlling module  194  connected to the processing module  191 . The emission controlling module  194  is configured to control the terahertz wave source  10  to emit the terahertz wave  15 . The data acquisition module  193  is configured to control the detector  18  to obtain the data of the terahertz wave  15 . The memory module  192  is stored with the standard terahertz wave spectrum. The processing module  191  is configured to identify the object  20  by comparing the data of the terahertz wave  15  with the standard terahertz wave spectrum. The controlling computer  19  can further include a display module and a communication module. The controlling computer  19  can show the identifying result to the user by the display module or send the identifying result to a mobile device, such as mobile phone, by the communication module. 
         [0046]    Referring to  FIG. 7 , a detecting system  1 A of another embodiment based on terahertz wave is provided. The detecting system  1 A comprises a terahertz wave source  10 , two detectors  18  spaced from the terahertz wave source  10 , and a controlling computer  19  connected to both the terahertz wave source  10  and the two detectors  18 . 
         [0047]    The detecting system  1 A is reflection-type. In use, the object  20  is located adjacent to the outputting surface of the terahertz wave source  10 . The terahertz wave  15  is emitted from the terahertz wave source  10 , reaches the object  20 , reflected by the object  20 , and received by the two detectors  18 . The two detectors  18  obtain the data of the terahertz wave  15  and send the data to the controlling computer  19 . The controlling computer  19  processes the data of the terahertz wave  15  to obtain a result and shows the result to the user. 
         [0048]    In one embodiment, the two detectors  18  are located on opposite sides of the terahertz wave source  10 . The angle between a receiving surface of the detector  18  and an outputting surface of the terahertz wave source  10  is defined as α. The angle α is greater than 90 degrees and less than 180 degrees. The angle α can be in a range from about 120 degrees to about 160 degrees. The two detectors  18  cane be located anywhere as long as the terahertz wave  15  reflected by the object  20  can be received by the two detectors  18 . 
         [0049]    Referring to  FIG. 8 , the terahertz wave source  10  includes a substrate (not shown), a plurality of first electrodes  16  located on the substrate, a plurality of second electrode  17  located on the substrate, and a plurality of terahertz reflex klystrons  10   a  located on the substrate. 
         [0050]    The plurality of first electrodes  16  are parallel with each other. The plurality of second electrode  17  are parallel with each other. The plurality of first electrodes  16  and the plurality of second electrode  17  are perpendicular with each other to form a grid structure. The grid structure includes a plurality of cells. Each cell is defined by adjacent first electrodes  16  and adjacent second electrode  17 . Each terahertz reflex klystrons  10   a  is located in one of the plurality of cells and electrically connected to one of the plurality of first electrodes  16  and one of the plurality of second electrodes  17 . The plurality of terahertz reflex klystrons  10   a  that are on the same row are connected to the same one of the plurality of first electrodes  16 . The plurality of terahertz reflex klystrons  10   a  that are on the same column are connected to the same one of the plurality of second electrodes  17 . 
         [0051]    The detector based on terahertz wave has low cost and can be widely applied to security detecting, medical detecting or integrated circuit (IC) detecting. 
         [0052]    It is to be understood that the above-described embodiments are intended to illustrate rather than limit the disclosure. Any elements described in accordance with any embodiments is understood that they can be used in addition or substituted in other embodiments. Embodiments can also be used together. Variations may be made to the embodiments without departing from the spirit of the disclosure. The above-described embodiments illustrate the scope of the disclosure but do not restrict the scope of the disclosure. 
         [0053]    Depending on the embodiment, certain of the steps of methods described may be removed, others may be added, and the sequence of steps may be altered. It is also to be understood that the description and the claims drawn to a method may include some indication in reference to certain steps. However, the indication used is only to be viewed for identification purposes and not as a suggestion as to an order for the steps.