Abstract:
A compact, solid-state THz source based on the driven Josephson vortex lattice in a highly anisotropic superconductor such as Bi 2 Sr 2 CaCu 2 O 8  that allows cw emission at tunable frequency. A second order metallic Bragg grating is used to achieve impedance matching and to induce surface emission of THz-radiation from a Bi 2 Sr 2 CaCu 2 O 8  sample. Steering of the emitted THz beam is accomplished by tuning the Josephson vortex spacing around the grating period using a superimposed magnetic control field.

Description:
CROSS REFERENCE TO RELATED PATENT APPLICATIONS  
       [0001]     This present application claims priority to U.S. Provisional Patent Application No. 60/786,269, filed Mar. 27, 2006 incorporated herein by reference in its entirety. 
     
    
       [0002]     The United States Government has certain rights in this invention pursuant to Contract No. DE-AC02-06CHI1357 between the United States Department of Energy and UChicago Argonne, LLC as operator of Argonne National Laboratories. 
     
    
     BACKGROUND OF THE INVENTION  
       [0003]     Terahertz (THz) science deals with the region of the electromagnetic spectrum that is frequently defined as ranging from 0.3 to 20 THz (10-600 cm −1 , 1 mm-15 μm wavelength). The field of terahertz science is rapidly developing and portends immense promise in fields as diverse as medical diagnostics, space exploration, environmental monitoring, security, manufacturing and pharmaceuticals. This wide range of areas of application derives primarily from two unique properties of THz radiation, 1) its spectral specificity to vibrational and rotational modes of a wide variety of important chemical and biomolecular species, and 2) to its well known penetrating properties through, for example, conventional packaging materials, clothes and plastics. A third property, its wavelength range of 15 μm to 1 mm, also allows for imaging with good spatial resolution.  
         [0004]     The development of sources of THz radiation is being pursued along various conventional approaches, including ultra-fast laser pumped photoconductive switches, pumped gas lasers, optical difference frequency generation and parametric oscillation, frequency doubled diodes, quantum cascade lasers. An alternative approach is based on the superconducting Josephson effect that occurs between layers of superconducting materials that are separated by thin insulating non-superconducting materials. When a voltage difference, U, is applied between the superconducting layers, an alternating electromagnetic wave (referred to as Josephson plasma waves in what follows) arises in the insulating layer whose frequency, v, is given by v=K J  U, where K J =4.84×10 14  Hz/V is the Josephson constant. This translates into a voltage of 2 mV for a frequency of 1 THz. The maximum voltage that can be applied to a superconductor without destroying the superconducting state is limited by the size of the superconducting energy gap, which is a material dependent parameter. The CuO 2 -based high-temperature superconductors display energy gaps of several tens of meV, and the THz-frequency range is accessible with these materials. In particular, highly anisotropic high-temperature superconductors, such as the Bi 2 Sr 2 CaCu 2 O 8  derived superconductors, are composed of superconducting CuO 2  layers separated by insulating layers, thus forming stacks of intrinsic Josephson junctions. The potential of the Josephson effect in these compounds as an efficient source for THz-radiation has been recognized for some time, and extensive numerical simulations indicated high THz-power. However, so far it has proven very difficult to extract THz-radiation power from high-temperature superconducting sources, the major limitations arising from a) the large mismatch of the wavelengths of the Josephson alternating electromagnetic field and of free-space THz-radiation, and from b) the typically very small surface areas from which THz emission can occur.  
       SUMMARY OF THE INVENTION  
       [0005]     The present invention relates to methods and apparatus for overcoming the deficiencies in the prior art superconducting THz sources, namely the large impedance mismatch of the Josephson plasma waves and the free-space THz radiation, as well as the low power output arising from the small size of the emitting surfaces. In one embodiment of the present invention a metallic grating is fabricated on top of the stack of Josephson junctions. The function of this grating is twofold: a) It serves as an electrical contact to supply a voltage and current to the superconductor and b) the period of the metallic grating is designed in such a way that Bragg reflection of the Josephson plasma waves occurs, thereby enabling matching of the wavelengths. The THz radiation will thereby be emitted through the slots in the grating realizing the so-called surface-emitting configuration. This configuration, as opposed to the previously envisioned end-fire configuration, significantly enhances the active surface area, and thereby greatly increases the radiated power. Furthermore, a new capability of THz radiation is opened up by the present invention, namely the steering of a coherent THz beam, which could have great impact on THz imaging applications, for example, in stand-off chemical detection.  
         [0006]     One aspect of the invention relates to a compact, solid-state THz source based on the driven Josephson vortex lattice in a highly anisotropic superconductor such as Bi 2 Sr 2 CaCu 2 O 8  that allows THz emission at a tunable frequency. A second order metallic Bragg grating is used to achieve impedance matching and to induce surface emission of THz-radiation from a Bi 2 Sr 2 CaCu 2 O 8  sample. Steering of the emitted THz beam is accomplished by tuning the Josephson vortex spacing around the grating period using a superimposed magnetic control field.  
         [0007]     These and other objects, advantages, and features of the invention, together with the organization and manner of operation thereof, will become apparent from the following detailed description when taken in conjunction with the accompanying drawings, wherein like elements have like numerals throughout the several drawings described below. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0008]      FIG. 1  illustrates a conventional superconducting THz source in end-fire configuration;  
         [0009]      FIG. 2A  is an illustration of one form of surface-emitting THz source comprising a Bi 2 Sr 2 CaCu 2 O 8  crystal positioned between two Au electrodes; and  FIG. 2B  is another embodiment of a surface-emitting THz source in which the surface of a Bi 2 Sr 2 CaCu 2 O 8  crystal is patterned in such a way as to yield a mesa-shaped stack resting on a larger base and a metal grating positioned on top of the mesa; and  
         [0010]      FIG. 3  graphically illustrates the mechanism of converting the non-radiative THz-Josephson plasma waves into THz radiation. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0011]     In accordance with an embodiment of the invention, a THz emitter includes a stack  101  of superconducting layers  103  that are separated by insulating layers  105  (see  FIGS. 2A and 2B ). In  FIG. 2A  these layers  103 ,  105  are disposed between a top metal layer  115  and a bottom metal layer  113 , generally gold, a gold alloy or any conventional accepted conductor layer. As shown in  FIGS. 2A and 2B , the entire device  100  is quite thin, with the top metal layer  115  having a thickness denoted as  119  and the bottom layer  113  having a thickness denoted as  121 . In one preferred embodiment, each of the top and bottom layers  115 ,  121  have a thickness of about  75  nm each. The superconducting stack  101  has a thickness labeled as  123  (see  FIG. 2A ), and in one preferred embodiment the superconducting stack  101  thickness is less than about 100 nm. The thickness denoted  123  comprises a series of the superconducting layers  103  separated by insulating layer portions  105 . In  FIG. 2A  electrical contact to the superconducting stack  101  is realized through metal layers  119 ,  121  attached to the top and bottom of stack  101 . The top metal layer  119  is patterned into a grating.  FIG. 2B  illustrates an embodiment of the surface-emitting THz source in which the surface of a wide superconducting stack is patterned in such a way as to yield a mesa-shaped stack resting on the wider base. A metal grating is positioned on top of the mesa; and an electrical current, I, is injected through the metal grating, travels down the mesa and into the base, and is extracted through the counter electrode deposited onto the surface of the base next to the mesa. The functioning of the embodiment is identical to that shown in  FIG. 2A . The drawings  2 A and  2 B are not to scale, the spacing between the superconducting layers and the width and period of the slots in the grating in particular are largely exaggerated for clarity.  
         [0012]     In a conventional prior art form of a superconducting THz emitter as shown in  FIG. 1 , the THz-power  135  would radiate from an end face  134  (an end-fire configuration) of the device  100 . The thickness of the superconductor stack  101 , is limited to typically less than about 100 nm due to Joule heating in the device  100 . Therefore, the active surface area is very small, thereby substantially limiting the net radiated power. Furthermore, there is a very large mismatch between the free-space wavelength, about 300 μm at 1 THz, and the Josephson plasma wavelength, set by the vortex spacing, typically less than 10 μm. As a consequence, the prior art devices are inefficient because the THz waves do not emit from the edge surfaces but instead, are back-reflected into the superconductor.  
         [0013]     As described hereinbefore, the device  100  of the present invention is shown schematically in different embodiments in  FIGS. 2A and 2B . THz emission is accomplished by any one of a variety of high temperature superconductors as known in the art. Josephson vortices move (in a uniform direction) parallel to the superconductor layers  103  inducing a voltage. This voltage in turn induces an oscillating current due to the well-known Josephson effect. The input energy is stored in the form of a standing Josephson plasma wave with part of the energy coupled into radiation through a metal grating  131  in the top face  115  rather than the side face  134  of the prior art devices (see  FIG. 1 ).  
         [0014]     In one preferred embodiment, the source of electromagnetic energy is a single crystal superconductor, Bi 2 Sr 2 CaCu 2 O 8 , having the layers  103  and  105 . The superconducting layers  103  are composed of superconducting CuO 2 -planes that are coupled to each other across intervening insulating BiO—SrO layer portions  105  through weak Josephson coupling. When subjected to an applied magnetic field, B, denoted by  107  oriented parallel to the planes, the magnetic field lines will penetrate the material in the form of Josephson vortices  109  that squeeze in between the superconducting layers  103 . The centers of the Josephson vortices  109  reside in the insulating layer portions  105 . A current, I, denoted by  117  flows perpendicular to the planes down the stack of the layers ( 103 ) and also the insulating layers  105  and the current flow generates a Lorentz force that drives the Josephson vortices  109  parallel to the planes in a direction denoted as  111 . Since they move in a rather un-impeded fashion, they travel as a regular lattice at high velocities (see  FIG. 2A ). This coherently moving Josephson vortex  109  lattice induces ac electromagnetic waves and currents. Their frequency, v, is determined by the voltage, U, across the superconductor layers  103  and can, therefore, be tuned by the applied current. A voltage of 2 mV per layer corresponds to 1 THz, and the technologically important terahertz frequency range of 0.1 to 5 THz can easily be scanned. The wavelength, {tilde over (λ)}, is given by the distance between the Josephson vortices  109 , typically several micrometers, and is determined by the applied field. Typical field strengths are several kG, which in an optimized device, can be generated with permanent magnets.  
         [0015]     The layered superconducting structure of the device  100 , as shown in  FIGS. 2A and 2B , exhibits a spectrum of electromagnetic modes, the Josephson plasma waves, that are analogous to the guided modes in a waveguide. If the applied field and current are adjusted in such a way that the velocity of the moving lattice of the Josephson vortices  109  coincides with that of a Josephson plasma mode, resonance occurs; and a large amount of energy is pumped from the moving lattice of the vortices  109  into this plasma wave. It has been estimated that the resulting electro-magnetic power density in the stack  101  at THz-frequencies is very large, about 100 W/cm 2 .  
         [0016]     In preferred form of the present device  100 , the metal grating  131  is patterned onto the top one of the superconducting stack  101  as shown schematically in  FIGS. 2A and 2B . In an exemplary embodiment, the metal grating  131  functions also as an electrical contact. In addition, in one embodiment, the metal grating  131  will induce a strong periodic variation of the dielectric constant at the surface of the superconducting stack  101 . Since, in one embodiment, the stack  101  is thinner than both the plasma wavelength and the screening length (typically about 300 nm at 1 THz), this periodic perturbation affects the propagation of the Josephson plasma waves throughout the entire stack  101 . As a consequence, the Josephson plasma waves can undergo Bragg reflection, similar to Bragg reflection of x-rays or neutrons from crystals. Bragg reflection allows an elegant way of controlling the propagation of electromagnetic waves.  
         [0017]     The principle of operation is illustrated in  FIG. 3 . The lines  203  indicate the dependence (here taken for simplicity as linear) of the frequency of the Josephson plasma waves upon their in-plane wave vector, and the location  205  marks the operation point that has been selected by the applied current and field, ω THz  and k // =2π/a. The shaded area  207  represents electromagnet waves that can travel in free space, the light cone. The large separation of the location  205  from the light cone  207  illustrates the mismatch in wavelengths described above. If a grating with period equal to the Josephson plasma wavelength is introduced, the plasma waves can undergo Bragg reflection upon which their wave vector changes by the grating vector, 2π/a, as shown by the arrows  211  in  FIG. 3 . The metal grating  131  induces a strong periodic modulation of the dielectric constant at the surface which gives rise to Bragg reflection of the plasma waves. If the grating period is chosen equal to the plasma wavelength, then the initial large wave vector of the plasma wave (location  205 ) is transformed into a small wave vector  213  inside the light cone  207 , and the plasma waves become radiative through the top surface  115 .  
         [0018]     As a result, the large wave vector  205  is transferred to the small wave vector  213 , well within the light cone  207 , corresponding to a THz-wave with essentially zero in-plane wave vector. This wave is transferred through the grating into THz-radiation as indicated by the arrows  135  in  FIGS. 2A and 2B , leading to top surface emission as opposed to highly undesirable, conventional edge emission from the sides of the device  100  (see  FIG. 1 ). In one embodiment, an enhancement by more than a factor of 10 3  in active surface area is achieved as compared to the end-fire configuration of the prior art.  
         [0019]     Another aspect of the invention relates to tunability, that is, change in frequency of the emitted THz-radiation. For the stack  101  containing N superconducting layers, the frequency-wave vector relation is not a simple line (as indicated in  FIG. 3 ), but consists of N closely spaced branches, which form a quasi-continuous band of frequencies. The width of this band determines the range over which the frequency of the THz-radiation can be tuned.  
         [0020]     Another aspect of the present invention involves THz beam steering. If the grating period is slightly different from the vortex spacing, then the small wave vector  213  in  FIG. 3  will not be located exactly at zero; instead the THz plasma wave maintains a small in-plane wave vector k // . The emitted THz radiation has the same in-plane wave vector and is emitted at an angle α with respect to the normal given by sin(α)=ck // /ω THz . Therefore, by superimposing a small additional magnetic field onto the applied magnetic field  107 , the vortex spacing; and thus the position of the small wave vector and large wave vector in  FIG. 3  can be modulated. Consequently, the resulting k //  vector and the beam direction can be controlled. With a superimposed ac-field the beam could be scanned back and forth for use in THz imaging applications, for example, for the stand-off chemical detection.  
         [0021]     In one exemplary embodiment, the superconducting stack  101  comprises between about seventy and one hundred separate superconducting layers. In an exemplary embodiment, a dielectric material is provided in communication with the grating  131 .  
         [0022]     Extensive numerical simulations of the distribution of supercurrents and electromagnetic fields in Josephson-coupled superconducting stacks show large power densities of THz-waves. For example, for the preferred embodiment shown in  FIG. 2A , a power density has been obtained of 797 W/cm 2  at a frequency of 3.7 THz residing at the top surface, which translates into an emitted radiation power of 0.12 mW for the device  100 .  
         [0023]     The foregoing description of embodiments of the present invention have been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the present invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the present invention. The embodiments were chosen and described in order to explain the principles of the present invention and its practical application to enable one skilled in the art to utilize the present invention in various embodiments, and with various modifications, as are suited to the particular use contemplated.