JP5445894B2 - Terahertz band electromagnetic wave oscillator with directivity - Google Patents

Description

The present invention relates to a coherent terahertz (10 ¹² Hz) band electromagnetic wave oscillator using a high-temperature superconductor, and in particular, a high-intensity output terahertz band electromagnetic wave oscillator having directivity for oscillating a high-intensity terahertz band electromagnetic wave. About.

  Terahertz electromagnetic waves (for example, 0.1 to several tens of THz, hereinafter referred to as “THZ band” where appropriate) have extremely high transparency compared to currently used electromagnetic waves in the gigahertz band, so that objects can be viewed in detail. In the near future, it will be widely used in physicochemical spectrometers, identification of various molecules, macromolecules, proteins, etc., sophisticated imaging fields, medical and diagnostic equipment, aerospace or defense fields, high-speed communications, etc. It is a promising frequency band with a wide range of applications to be used. This wide range of applicability comes from two unique properties of terahertz electromagnetic waves. That is, the spectrum specificity of terahertz electromagnetic waves for vibration and rotation modes of a wide variety of important chemical or biological substances, and the permeability of terahertz electromagnetic waves that pass through packaging materials, clothing, plastics, and the like.

For this reason, conventionally, as a THz band electromagnetic wave oscillator, a high-power femtosecond (1/10 ¹⁵ ) level laser beam is irradiated onto a semiconductor, and a pulse-like wideband THz is generated by a current induced by the strong electric field. In addition to an optical switch method for generating a band wave and a parametric method for irradiating a nonlinear optical crystal with a high-power laser beam, various methods such as a free electron laser, synchrotron radiation, and photomixing have been known.

In addition, as examples of other known techniques for electromagnetic wave oscillators in the THz region, BSCCO (bismuth / strontium / calcium / copper oxide: Bi ₂ Sr ₂ CaCu ₂ O ₈ , Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ ), Laminate in which intrinsic Josephson junctions of a superconducting layer made of a superconducting single crystal such as TBCCO (thallium, barium, calcium, copper oxide: TL ₂ Ba ₂ Ca ₂ Cu ₃ O ₁₀ ) and an insulating layer are laminated in series. A THz band oscillator using a superconducting single crystal element having a Josephson junction has been proposed (see, for example, Patent Document 1 or Patent Document 2).
Japanese Patent Laid-Open No. 2006-210585 JP 2005-251863 A

  However, all of these conventional THz-band oscillators require a large-scale incidental device despite their low oscillation efficiency (ratio of THz-band electromagnetic wave output to input energy) (several percent or less). It was not practical.

  That is, as described in Patent Document 1 or Patent Document 2, any of the conventional THz band electromagnetic wave oscillation device proposals using Josephson junctions applies a strong magnetic field to a narrow element width of a superconducting element. Therefore, it is necessary to install a large external magnetic field application device as an accessory device, and it is difficult to oscillate and use an electromagnetic wave in a high intensity THz band that is not portable.

  On the other hand, as described above, an electromagnetic wave in the THz band is obtained using a superconducting single crystal element having a laminated Josephson junction in which intrinsic Josephson junctions of a superconducting layer and an insulating layer made of a superconducting single crystal are laminated in series. Although the oscillation itself is widely known, the radiation characteristic of the electromagnetic wave radiated from the superconducting intrinsic Josephson junction element was not known at all. In other words, the radiation characteristics of electromagnetic waves from antennas formed of metal conductors are radiated from intrinsic Josephson junction elements of high-temperature superconductors, although there have been many research examples and engineering has been established. The spatial distribution of electromagnetic wave intensity is different from the directivity of electromagnetic radiation expected by the conventional antenna theory, and its characteristics have not been known so far.

  In the oscillation of electromagnetic waves in the THz band using the intrinsic Josephson junction of a high-temperature superconductor, improvement of the oscillation intensity is extremely important. Conventionally, regarding the intensity distribution of electromagnetic waves radiated from a mesa structure on a rectangular plane with a predetermined shape, it is assumed that the strongest electromagnetic wave is radiated in the horizontal direction of the rectangular plane mesa structure from the mechanism of the electromagnetic wave oscillation. The electromagnetic waves radiated in the direction have been used.

  However, the inventors of the present application have experimentally determined that electromagnetic waves radiated in a direction tilted upward by a predetermined angle from the horizontal direction due to the shape of the rectangular planar mesa structure formed on the single crystal of the superconductor BSCCO structure. It was confirmed that it has the strongest strength. This is a result that could not be predicted in the prior art, and if this result is used, a high-intensity THz band electromagnetic wave can be obtained in a specific direction.

  The present invention has been made in view of various problems of such a conventional THz-band electromagnetic wave oscillation device, and is based on a new experimental confirmation that a high-intensity THz-band electromagnetic wave is obtained in a specific direction. A THz band electromagnetic wave oscillation device (device) having a small and light weight, capable of continuous oscillation with high efficiency, coherent and variable in frequency, and having high directivity and high intensity output. An object is to provide an electromagnetic wave oscillation device.

For this reason, the present invention is a single crystal body of a BSCCO superconductor having a multi-layered intrinsic Josephson junction, and a rectangular planar mesa formed so that the c-axis of the BSCCO superconductor is in the height direction. the structure and oscillating means, the more rectangular planar mesa structure, a terahertz band electromagnetic wave radiation having a directivity which is characterized by utilizing the electromagnetic waves emitted in a predetermined angle direction inclined from the horizontal direction above the rectangular planar mesa structure A device is provided. Here, the predetermined angle is characterized by having a central angle of 60 degrees and a range of 52.5 to 67.5 degrees.

Here, the rectangular planar mesa structure is characterized by being excited by an electromagnetic cavity resonance that does not require an external magnetic field application, and this electromagnetic cavity resonance is a Fabry-Perot cavity resonance.

Then, the said rectangular planar mesa structure, it is the Si hemispherical lens is placed.

By the way, in the present invention, the frequency of the electromagnetic wave radiated to the rectangular planar mesa structure can be changed by changing the width of the rectangular planar mesa structure.

As described above, the directional terahertz band electromagnetic wave oscillation device according to the present invention is a single crystal body of a BSCCO superconductor having a multilayered intrinsic Josephson junction , and the c-axis of the BSCCO superconductor is high. Since the rectangular planar mesa structure formed in the direction is used as the oscillation means, it is not necessary to provide an oscillation means such as a special antenna. In the electromagnetic wave oscillation device, since it is not necessary to provide a magnetic field application device for applying a strong magnetic field to the narrow element width of the superconducting element, an electromagnetic wave in a portable and high intensity terahertz band is generated. It was realized to use.

  Hereinafter, the terahertz band electromagnetic wave oscillation device having directivity according to the present invention will be described in detail. First, a single crystal body having a superconductor BSCCO structure excited by electromagnetic cavity resonance will be described in detail.

  The superconducting Josephson junction has the basic property of forming a high-frequency electromagnetic field having a frequency proportional to the voltage, but this property enables the production of a coherent and modulatable high-frequency oscillator. This attractive possibility has been extensively explored in the past by manufacturing large arrays of artificial Josephson junctions.

A major problem is that all the junctions in the array are synchronized and oscillated in phase, but this can be achieved, for example, by coupling them to the same electronic resonant circuit. Due to the intrinsic Josephson effect in BSCCO (Bismuth / Strontium / Calcium / Copper Oxide: Bi ₂ Sr ₂ CaCu ₂ O ₈ , Bi ₂ Sr ₂ Ca ₂ Cu ₃ O ₁₀ ), in the form of a laminate, ie single crystal It is possible to easily produce a one-dimensional array consisting of a very large number of closely packed identical joints in the form of so-called mesas formed as:

  If all the junctions in the above laminate are forced to vibrate at the same frequency and in phase, there will be a strong coherent electromagnetic emission line whose total power is measured by the square of the number of junctions. It is thought that it is obtained. This synchronization is further facilitated by coupling the junction to an external microwave cavity, or the mesa's own resonant mode. Electromagnetic waves inside large area mesas propagate as Josephson plasma mode.

  The in-plane velocity of this mode is highly dependent on the out-of-plane wave vector, the maximum velocity matches the in-phase mode (all joints vibrate in phase), and the minimum speed vibrates in the adjacent joints out of phase. It corresponds to the reverse phase mode. Furthermore, in all of the above modes, the multiple reflections at the mesa side result in standing wave patterns and Fabry-Perot cavity resonances known as Fiske modes.

  In all but the common mode, on the same surface, the average electric field cancels each other, so that only the common mode generates detectable external electromagnetic waves. A Josephson vortex moving grid is often used to excite the Fiske resonance. However, a triangular vortex lattice structure is advantageous for inductive interaction between vortices, but in this structure, a non-electromagnetic wave-generating reverse phase mode is exclusively excited. A proposal was made that the side effect in a sufficiently narrow mesa, that is, the dynamic action at high drive, would enable stabilization of a vortex lattice with square symmetry that can excite the common mode. No proof of this proposal has been reported yet.

In the present invention, a means is disclosed that makes it possible to efficiently convert an alternating electromagnetic field localized at the Josephson junction into a coherent, deflectable high frequency electromagnetic wave. Our approach is based on the coupling of waves at the junction to the Fabry-Perot cavity mode. This has two effects. That is,
(A) A large number of Josephson junctions will operate synchronously in common mode, which is not primary as observed in uncorrelated junctions, but secondary with the number of junctions Increases available electromagnetic energy.
(B) At resonance, energy is efficiently absorbed into the synchronous mode, enhancing its strength by a factor equal to the quality factor of the cavity. The quality factor of this cavity can be increased and is ultimately limited only by the loss in the cavity.

  Analysis of the basic symmetry of a device with a junction as described above shows that a uniform current flowing perpendicular to the junction cannot excite the resonant cavity mode. Therefore, the prior art related to using a Josephson junction as an electromagnetic wave generation source exclusively considers operation in a magnetic field applied parallel to the junction. Next, so-called Josephson vortices enter between the superconducting layers, breaking the symmetry and exciting a resonance known as Fiske resonance.

In the present invention, by forming a non-uniform coupling between the current and the electromagnetic wave mode, the symmetry is broken and the resonant cavity mode can be excited in a zero applied magnetic field. This can be realized by, for example, the following method, but is not limited thereto.
(A) The BSCCO composition gradient induces a non-uniform critical current density across the width of the mesa. The superconductivity (TC, JC) of BSCCO is highly dependent on its oxygen content. Thus, by using controlled quenching in an oxygen atmosphere, a critical current density is established near one side that is higher than near the opposite side.
(B) For example, an asymmetric cross section of a mesa with a trapezoid rather than a rectangle induces an asymmetric current and an asymmetric reflection coefficient at multiple sides.
(C) An asymmetric critical current distribution can also be established by irradiating one side of the mesa with an electron or proton beam, or intentionally suppressing superconductivity by implanting ions. It is.

  The electromagnetic wave generation and transfer properties of an ideal mesa composed of the same layer with exactly the same critical current modulation can be evaluated for various modulation profiles.

  Currently, continuous mesochromatic THz radiation up to 0.85 THz with high intensity is extracted from the mesa-type intrinsic Josephson junction in the superconductor BSCCO structure having the multi-layered intrinsic Josephson junction as described above. It is known to get. Here, the inventor of the present application has increased the estimated total power to 5 μW when the emission characteristics of the light-emitting superconductor (LES) are 0.63 THz. And by using the Si hemisphere lens together, the power at the detector after the spectrometer is compared to the previous example, which reaches 23 times that of a mercury lamp commonly used for far-infrared spectrum observation. Finally, it is improved by 270 times. The power fluctuation in the oscillation drive for 20 minutes is less than 4%, and the electromagnetic wave oscillation is very stable.

  The THz band electromagnetic wave oscillation device according to the present invention has a simple structure, a simple bias operation in a zero magnetic field, and durability against a large number of heating cycles between room temperature and 20K. It shows a promising THz source.

The Josephson effect that occurs between superconducting electrodes separated by a thin insulating layer (Josephson junction) constitutes a natural converter that converts DC voltage into high-frequency electromagnetic radiation, and 1 mV corresponds to 0.483 THz. To do. And the light emission from a single junction is very weak. However, in the layered high temperature superconductor BSCCO in which the superconducting CuO ₂ layer is separated by the insulating Bi—Sr—O insulating layer, the dense filling of the intrinsic Josephson junction is coupled to each other, so that the electromagnetic wave oscillation ( The intensity of the following (hereinafter referred to as “light emission”) can be made considerably large. Such a LES device has been successfully implemented as a mesa device at Argonne National Laboratory (ANL).

FIG. 1 is a schematic view of a BSCCO light emitting superconductor (LES) provided with a Si hemispherical lens according to the present invention.
In FIG. 1, the applied c-axis current (z direction) excites the fundamental cavity mode (stable half wave) in the mesa width (y direction), and high frequency electromagnetic radiation is emitted from each side. . Polarization and frequency were analyzed by FT-IR employing a Martin Papplet interferometer, and radiation distribution and stability were measured by a modulation technique employing an optical chopper and a Si composite bolometer detector.

By the way, in the BSCCO light emitting superconductor shown in FIG. 1, more than 500 junctions are created to oscillate in the same phase, and the emission frequency is 0.36 to 0.85 THz in inverse proportion to the mesa width ranging from 100 μm to 40 μm. Thus, it is possible to generate continuous wave coherent radiation power up to 0.5 μW. Luminescence lasts up to a temperature of 50K. This light emission does not require application of a magnetic field.
The inventor of the present application is the same method as the ANL mesa device, but with a BSCCO emission superstructure in the form of a mesa with a uniquely prepared 60 μm width, 400 μm length and about 1.9 μm height. The emission characteristics of the conductor (LES) were obtained.

  The mesa and electrical contacts are made on a cleaved BSCCO single crystal by a series of thermal evaporation, photolithography and Ar ion etching steps. Observation with a scanning electron microscope (SEM) shows that the mesa has a trapezoidal shape of 50 μm at the top width and 64 μm at the bottom by the Ar ion etching process. The applied c-axis current (z direction) excites the fundamental cavity mode (stable half-wave) in the mesa width (y direction), and high frequency electromagnetic radiation is emitted as shown in FIG. . Light emission from the side of the mesa is collected with or without a silicon hemisphere lens. Current-voltage characteristics and bias conditions for light emission are described in detail elsewhere.

FIG. 2 employs a Martin-Puplett interferometer, a modified Michelson interferometer using wire gratings at the entrance and exit, and a beam splitter (FARIS-1 spectrometer, JASCO). The spectral characteristics of the luminescence analyzed by FT-IR are shown. The frequency value roughly matches the fundamental cavity resonance f = c / 2nw, where w is the width of the mesa, n≈3.5 is the far-infrared refractive index of the BSCCO c-axis , and c is the speed of light. Means.

FIG. 2 shows the spectral characteristics of the terahertz band electromagnetic wave oscillator having directivity according to the present invention.
Here, in FIG. 2 (a), the light emission from LES is compared with the radiation from the mercury lamp and black body at 1200K, and all are detected by the TGS pyroelectric sensor. The peak intensity at 0.63 THz is 23 times stronger than the radiation from mercury lamps commonly used for infrared spectrum observation. The frequency is mostly determined by the mesa width w with the relationship f = c / 2nw, where n≈3.5 is the refractive index of the sample and c is the speed of light. The observed 10 GHz (FWHM) line width is limited by the resolution of the instrument.
Also, FIG. 2 (b) shows the spectra measured at the orthogonal and parallel settings of the wire grating at the entrance and exit to the Martin Papplet interferometer to the BSCCO crystal layer. The polarization ratio of the emission from LES, defined as the ratio of the emission peak, is 50, but the broad peak near 3 THz due to thermal radiation from the instrument has only a ratio of 1.2.

  As shown in FIG. 2 (a), the light emitted from the LES through the Si hemispherical lens is compared to the radiation from a mercury lamp and black body at 1200K, all detected by a TGS pyroelectric sensor. Each light source is standard for a FARIS-1 spectrometer. Although the sensitivity of the pyroelectric sensor is 1/100 to 1/1000 worse than the Si composite bolometer operating at T = 4.2K, it is suitable for each application because it operates at room temperature and is low cost. . The peak intensity at 0.63 THz is 23 times stronger than the radiation from mercury lamps commonly used for far-infrared spectrum observation. Since the observed 10 GHz (FWHM) line width is considerably limited by the resolution of 7.5 GHz of the FT-IR spectrometer, the actual intensity ratio is judged to be considerably large.

  When the electric field vector is parallel to the wire, the metal wire grid acts as a polarizing filter for electromagnetic waves, and electromagnetic waves below the cutoff frequency are blocked by the grid, but electromagnetic waves pass through the grid for orthogonal configurations. To do.

  FIG. 2 (b) shows the spectra measured at the orthogonal and parallel settings of the wire grating at the entrance and exit for a Martin Papplet interferometer to a layer of BSCCO crystals. Without the Si hemispherical lens, the light emitted toward 52.5 ° <θ <67.5 ° and −7.5 ° <ψ <7.5 ° (see FIG. 1) is collected by the concave mirror and is Detected by a composite bolometer.

The polarization ratio of emission from LES, defined as the ratio of the emission peak, is 50, but the broad peak near 3 THz observed as the product of thermal radiation from the instrument and the spectral sensitivity of FT-IR is ~ 1.2. Have a ratio of Light emission from LES whose electric field vector is orthogonal to the layer of BSCCO crystal is highly polarized.
The distribution characteristics and temporal stability of the emission from the LES were measured by a modulation technique employing an optical chopper that performs 70 Hz modulation, a Si composite bolometer detector, and a lock-in amplifier.

  FIG. 3 shows the distribution characteristics of light emission from a BSCCO light emitting superconductor (LES) without a Si hemisphere lens. Here, the angles θ and ψ are defined in FIG. 3A shows the θ dependency of the emission intensity at ψ = 0 ° with an angular resolution of 4.3 °, and FIG. 3B shows the ψ dependency at θ = 0 °. That is, FIGS. 3A and 3B show the θ dependency of the emission intensity at ψ = 0 ° and the ψ dependency at θ = 0 °, respectively, without the Si hemisphere lens. .

  The distance between the LES and a detector with an effective aperture of 12.7 mm was 170 mm, resulting in an angular resolution of 4.3 °. Since the wavelength of light emission is 470 μm, which is considerably longer than the mesa height of 1.3 μm, the light emission is remarkably expanded to a high θ by diffraction. A fine structure due to diffraction is also seen. Since the mesa has a length of 400 μm, the dispersion of light emission is reduced in the ψ direction. From the radiation distribution due to the sensitivity of the Si composite bolometer detector, it can be understood that the total radiation power is 5 μW or less, which is 0.03% of the input power.

  FIG. 4 shows the light emission time stability of the terahertz band electromagnetic wave oscillator having directivity according to the present invention. As shown in FIG. 4, when the bias to LES is turned on and off, the detected signal changes by ˜20%. An additional 80% offset is caused by thermal radiation from the instrument. In 20 minutes of operation, the power variation with an average time of 0.3 seconds appears to be less than 4%. That is, FIG. 4 shows the stability of light emission from the LES. In this case, the LES is turned on by “ON” and stopped by “OFF” by shifting the bias. In 20 minutes of operation, the power variation with an average time of 0.3 seconds is less than 4%. Even with the Si hemisphere lens, 80% of the detected signal is thermal radiation from ambient equipment with a 3 THz long wave pass filter inserted in front of the detector. However, for various applications, the monochromaticity of LES is excellent, and so the simultaneous use of only a narrow band pass filter will be required.

Although the emission frequency f is mostly determined by the width of the mesas that exemplify cavity resonance, even if it can be continuously adjusted by bias, the adjustable range observed depends on the sample. In order to excite the resonance, the Josephson plasma frequency f = V _jct / Φ ₀ should match the resonance frequency, where Φ ₀ is the flux quantum and V _jct is the voltage per junction. is there. The adjustable range is limited by this condition.

  FIG. 5 shows frequency modulation (another 60 μm mesa sample) with bias voltage. Here, the emission frequency f is almost determined by the width of the mesa, but can also be continuously adjusted by the bias. In this case, Δf / f = 5.7%. The observed linewidth of 10 GHz (FWHM) is considerably limited by the 7.5 GHz resolution of the FT-IR spectrometer. That is, FIG. 5 shows the current best example of frequency modulation with a bias voltage (another 60 μm mesa sample). In this case, the adjustable range Δf is 5.7% of f. The observed linewidth of 10 GHz (FWHM) is considerably limited by the 7.5 GHz resolution of the FT-IR spectrometer.

  To date, great efforts have been made to develop semiconductor diode technology, nonlinear optics, giant pulse current switching, quantum cascade lasers, quantum dots, and the like as sources of THz radiation. Currently, the lowest emission frequency of a quantum cascade laser is approximately 1.6 THz with a continuous wave emission power of .about.0.5 mW. The LES described here is very good in the frequency range of 0.5 THz to 1.5 THz, which is also the range where compact solid sources are most scarce. The results presented here show that LES already meets the requirements for some of the commercial applications. In the future, performance in terms of frequency, intensity, frequency purity, etc. will be significantly improved by optimizing materials and mesa structures, array formation and focus adjustment techniques.

As described above in detail, the directional terahertz-band electromagnetic wave oscillation device according to the present invention is a single crystal of a BSCCO superconductor having a multi-layered intrinsic Josephson junction , and c of the BSCCO superconductor. An electromagnetic wave radiated in a direction inclined at a predetermined angle upward from the horizontal direction of the rectangular planar mesa structure from the rectangular planar mesa structure using a rectangular planar mesa structure formed such that the axis is in the height direction. Is used. Here, the predetermined angle is in the range of 52.5 to 67.5 degrees with the central angle being 60 degrees.

  The present invention relates to a terahertz electromagnetic wave oscillation device having directivity, and includes a physicochemical spectrometer, identification of various molecules, polymers, proteins, and the like, elaborate imaging fields, medical and diagnostic devices, aerospace Or, it has industrial applicability in a wide range of application fields widely used in the defense field, high-speed communication, and the like.

1 shows a schematic diagram of a BSCCO light emitting superconductor (LES) with a Si hemispherical lens according to the present invention. The spectral characteristic of the terahertz band electromagnetic wave oscillation apparatus which has the directivity based on this invention is shown. The distribution characteristic of light emission from a BSCCO light emitting superconductor (LES) without a Si hemispherical lens is shown. 1 shows the light emission time stability of a terahertz band electromagnetic wave oscillation device having directivity according to the present invention. Fig. 5 shows frequency modulation with another bias voltage (another 60 m mesa sample).

Claims (8)

  A single crystal body of a BSCCO superconductor having a multi-layered intrinsic Josephson junction, and a rectangular planar mesa structure formed so that the c-axis of the BSCCO superconductor is in the height direction is used as the oscillation means, A terahertz band electromagnetic wave oscillating device having directivity using electromagnetic waves radiated in a direction inclined at a predetermined angle upward from a horizontal direction of the rectangular planar mesa structure from a rectangular planar mesa structure.   2. The terahertz electromagnetic wave oscillation device with directivity according to claim 1, wherein the predetermined angle is in a range of 52.5 to 67.5 degrees with a central angle of 60 degrees.   3. The terahertz band electromagnetic wave oscillator having directivity according to claim 2, wherein an Si hemispherical lens is disposed on the rectangular planar mesa structure.   4. The terahertz band electromagnetic wave oscillator having directivity according to claim 1, wherein the rectangular planar mesa structure is excited by electromagnetic cavity resonance that does not require application of a magnetic field from the outside. 5.   5. The terahertz band electromagnetic wave oscillator having directivity according to claim 4, wherein the electromagnetic cavity resonance is a Fabry-Perot cavity resonance. 6. The terahertz band electromagnetic wave oscillator having directivity according to claim 5, wherein a width of the rectangular planar mesa structure is determined based on a frequency of an electromagnetic wave radiated to the rectangular planar mesa structure. 6. The terahertz band electromagnetic wave oscillator having directivity according to claim 5, wherein the rectangular planar mesa structure has a width of 40 μm or more and 100 μm or less.   The terahertz band electromagnetic wave oscillation device having directivity according to claim 5, wherein the oscillation frequency can be continuously adjusted by changing a bias voltage applied to the rectangular planar mesa structure.

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