HIGH-POWER TERAHERTZ OSCILLATOR

Abstract

[Problem]

An object of the present invention is to provide a small-size high-power terahertz oscillator that achieves a stable oscillation in a terahertz frequency band even at a room temperature.

[Means for solving the problem]

The present invention is the high-power terahertz oscillator that has a structure in which a bow-tie antenna is disposed on a substrate, a cavity resonator which includes two cavities is disposed at a power supply portion of the bow-tie antenna and a RTD is disposed along a bottom of a wall of the cavity resonator which defines the two cavities, and stably oscillates waves in the terahertz frequency band at room temperature by using the RTD, the bow-tie antenna and the cavity resonator.

Claims

1-6. (canceled)

7. A high-power terahertz oscillator, comprising: a bow-tie antenna, which includes first and second conductor strips, disposed on a substrate; a cavity resonator, which includes two cavities, disposed at an oscillating portion of said bow-tie antenna; and a resonant tunneling diode, which is cuboid, disposed along a bottom of a wall which defines said two cavities, wherein said first conductor strip of said bow-tie antenna is integrated with said cavity resonator, and wherein an oscillation in a terahertz frequency band is obtained from said resonant tunneling diode, said bow-tie antenna and said cavity resonator, by applying a bias voltage between said second conductor strip of said bow-tie antenna and a side surface or a top surface of said cavity resonator connected to said first conductor strip of said bow-tie antenna.

8. The high-power terahertz oscillator according to claim 7, wherein a bottom surface of said resonant tunneling diode is in contact with a top surface of said second conductor strip of said bow-tie antenna.

9. The high-power terahertz oscillator according to claim 7, wherein said first and second conductor strips of said bow-tie antenna and said cavity resonator are made of a good conductor, and said substrate is made of a semi-insulator.

10. The high-power terahertz oscillator according to claim 8, wherein said first and second conductor strips of said bow-tie antenna and said cavity resonator are made of a good conductor, and said substrate is made of a semi-insulator.

11. The high-power terahertz oscillator according to claim 7, wherein an insulating thin film is disposed between said first and second conductor strips of said bow-tie antenna and is disposed between a bottom of said cavity resonator and said second conductor strip of said bow-tie antenna.

12. The high-power terahertz oscillator according to claim 8, wherein an insulating thin film is disposed between said first and second conductor strips of said bow-tie antenna and is disposed between a bottom of said cavity resonator and said second conductor strip of said bow-tie antenna.

13. The high-power terahertz oscillator according to claim 9, wherein an insulating thin film is disposed between said first and second conductor strips of said bow-tie antenna and is disposed between a bottom of said cavity resonator and said second conductor strip of said bow-tie antenna.

14. The high-power terahertz oscillator according to claim 10, wherein an insulating thin film is disposed between said first and second conductor strips of said bow-tie antenna and is disposed between a bottom of said cavity resonator and said second conductor strip of said bow-tie antenna.

15. The high-power terahertz oscillator according to claim 7, wherein a recess is formed at said second conductor strip of said bow-tie antenna, a conductor bridge extending from said cavity resonator is connected to an upper surface of said substrate in said recess, and a stabilizing resistor is connected between said conductor bridge and a side surface of said second conductor strip of said bow-tie antenna.

16. The high-power terahertz oscillator according to claim 8, wherein a recess is formed at said second conductor strip of said bow-tie antenna, a conductor bridge extending from said cavity resonator is connected to an upper surface of said substrate in said recess, and a stabilizing resistor is connected between said conductor bridge and a side surface of said second conductor strip of said bow-tie antenna.

17. The high-power terahertz oscillator according to claim 9, wherein a recess is formed at said second conductor strip of said bow-tie antenna, a conductor bridge extending from said cavity resonator is connected to an upper surface of said substrate in said recess, and a stabilizing resistor is connected between said conductor bridge and a side surface of said second conductor strip of said bow-tie antenna.

18. The high-power terahertz oscillator according to claim 10, wherein a recess is formed at said second conductor strip of said bow-tie antenna, a conductor bridge extending from said cavity resonator is connected to an upper surface of said substrate in said recess, and a stabilizing resistor is connected between said conductor bridge and a side surface of said second conductor strip of said bow-tie antenna.

19. The high-power terahertz oscillator according to claim 11, wherein a recess is formed at said second conductor strip of said bow-tie antenna, a conductor bridge extending from said cavity resonator is connected to an upper surface of said substrate in said recess, and a stabilizing resistor is connected between said conductor bridge and a side surface of said second conductor strip of said bow-tie antenna.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In the accompanying drawings:

[0017] FIG. 1 is a perspective structure diagram showing one example of a terahertz oscillator including a conventional resonant tunneling diode;

[0018] FIG. 2 is a schematic view showing an example of an output behavior of the terahertz oscillator including the conventional resonant tunneling diode;

[0019] FIG. 3 is a characteristic diagram showing a characteristic example of the resonant tunneling diode;

[0020] FIG. 4 is an equivalent circuit diagram of the terahertz oscillator including the conventional resonant tunneling diode;

[0021] FIG. 5 is a perspective view showing a configuration example of a high-power terahertz oscillator according to the present invention;

[0022] FIG. 6 is a partially detailed diagram of the high-power terahertz oscillator according to the present invention;

[0023] FIG. 7 is a cross-sectional view showing a cross-sectional structure example of the terahertz oscillator according to the present invention, taken along a Y-Y′ direction in FIG. 6;

[0024] FIG. 8 is a cross-sectional view showing a cross-sectional structure example of the terahertz oscillator according to the present invention, taken along a Z-Z′ direction in FIG. 6;

[0025] FIGS. 9A to 9C are a cross-sectional view showing a cross-sectional structure example of the terahertz oscillator according to the present invention, taken along an X-X′ direction in FIG. 6, and a cross-sectional structure example of other modification examples

[0026] FIG. 10 is a schematic view showing a cross-sectional structure example of the resonant tunneling diode;

[0027] FIG. 11 is an equivalent circuit diagram of the terahertz oscillator according to the present invention;

[0028] FIG. 12 is a characteristic diagram showing a characteristic example of the terahertz oscillator according to the present invention;

[0029] FIG. 13 is a cross-sectional view showing another structure example of a cavity resonator;

[0030] FIG. 14 is a perspective view showing another configuration example of the high-power terahertz oscillator according to the present invention; and

[0031] FIG. 15 is a characteristic diagram showing characteristic examples of the terahertz oscillator shown in FIG. 14.

MODE FOR CARRYING OUT THE INVENTION

[0032] In a terahertz oscillator including a resonant tunneling diode device (hereinafter, referred to as “RTD”) having semiconductor nanostructures, the present invention investigates a cause of a low-power output in a conventional slot antenna integrated RTD oscillator, and proposes a small-size terahertz oscillator having a novel structure (the dimension is (width about 1 [mm])×(length about 1 [mm])×(height a few μm)) in which a cavity resonator having two cuboid cavities and a bow-tie antenna (a bow-tie angle θ) including two conductor strips are integrated with the RTD, as a means of resolving the cause of the low-power output. That is, instead of using the conventional slot antenna, the cavity resonator having two cavities, which are a cuboid whose cross-section is a rectangle or a trapezoidal body whose cross-section is a trapezoid, is disposed at a resonant portion of the bow-tie antenna which determines an oscillation frequency. The two cavities of the cavity resonator are defined by a wall hanging from a ceiling. Further, an output radiation of the oscillator is performed by the bow-tie antenna which is spatially separated from the cavity resonator, the cuboid RTD whose cross-section is a rectangle is disposed between a bottom surface portion of the wall of the cavity resonator and one of the conductor strips of the bow-tie antenna, and a bias voltage is applied to the RTD by an upper electrode connected to the cavity resonator and a lower electrode connected to the other of the conductor strips of the bow-tie antenna. In other words, the upper electrode of the RTD is the bottom surface portion of the wall, and the lower electrode of the RTD is a top surface of the bow-tie antenna. A power supply portion is constituted by one side of the bow-tie antenna (the other of the conductor strips) and a part of the cavity resonator (a side surface or a top surface) connected to one of the conductor strips which is opposite to one side of the bow-tie antenna.

[0033] The cavity resonator including the ceiling and the wall is made of good conductor such as gold (Au), copper (Cu), silver (Ag) or platinum (Pt), and the bow-tie antenna including the two conductor strips is also made of the good conductor such as the gold (Au), the copper (Cu), the silver (Ag) or the platinum (Pt).

[0034] Since an area in which a high-frequency current is flown becomes larger in the cavity resonator, an inductance can largely be reduced in comparison with the slot antenna, and the oscillation frequency can be higher without decreasing the capacitance of the RTD. Because a negative differential conductance become larger by increasing a mesa area of the RTD, a high-power output can be obtained in the terahertz oscillator according to the present invention.

[0035] By adopting the oscillator structure of the present invention, the high-power output which is two orders or more of magnitude (about 2 [mW] to about 7 [mW]) in comparison with the output of the conventional structure is confirmed to be obtained at 1 [THz] belong to the terahertz frequency band and the room temperature by a simulation.

[0036] Embodiments of the present invention will be described with reference to the accompanying drawings as follows.

[0037] FIG. 5 is a perspective view of the high-power terahertz oscillator 10 according to the present invention, and the bow-tie antenna 20, which is made of the good conductor such as the gold (Au) and includes the two conductor strips 21 and 22 having a bow-tie angle θ, is disposed on an upper surface of a semi-insulating substrate (SI) substrate 11 including indium phosphide (InP) whose shape is a square having about 1 [mm] length and height is a few μm. The conductor strips 21 and 22 of the bow-tie antenna 20 have line symmetry, a shape of the conductor strips 21 and 22 is substantially a rectangle, inclinations toward both ends are provided with an oscillating portion in which the conductor strips 21 and 22 are opposite, as shown in FIG. 5, and the bow-tie angle θ is formed by both the inclination angles of the conductor strips 21 and 22. The conductor strips 21 and 22 are made of the good conductor such as the gold (Au).

[0038] The RTD 40 and the cavity resonator 30 are disposed at the oscillating portion in which the two conductor strips 21 and 22 of the bow-tie antenna 20 are opposite, and a detailed perspective view is shown in FIG. 6. The Y-Y′ cross-section, the Z-Z′ cross-section and the X-X′ cross-section are shown in FIG. 7, FIG. 8 and FIG. 9A, respectively. The cavity resonator 30 is also made of the good conductor such as the gold (Au). In FIGS. 7 and 8, the conductor strip 21 of the bow-tie antenna 20 and the cavity resonator 30 are separated for easily identifying. In actual, the conductor strip 21 of the bow-tie antenna 20 and the cavity resonator 30 are an integrated structure which is made of the good conductor such as the gold (Au). Since both the conductor strip 21 of the bow-tie antenna 20 and the cavity resonator 30 are made of the good conductor, the conductor strip 21 of the bow-tie antenna 20 and the cavity resonator 30 may be separated and may have a contact structure.

[0039] As shown in FIGS. 5 to 8, the two conductor strips 21 and 22, which constitute the bow-tie antenna 20 and have the bow-tie angle θ, are disposed on the semi-insulating (SI) substrate 11 made of “InP”, and the cavity resonator 30 having the two cuboid cavities 31 and 32 whose cross-section is rectangle is disposed on the respective surfaces of the two conductor strips 21 and 22 of the bow-tie antenna 20. A part of the conductor strip 22 of the bow-tie antenna 20 serves as the power supply portion 22A for applying the bias voltage (DC voltage), and a part of the cavity resonator 30 (the side surface or the top surface) which is opposite to the conductor strip 22 serves as the power supply portion 33 for applying the bias voltage (DC voltage).

[0040] As shown in FIGS. 6 to 8, the two cavities 31 and 32 of the cavity resonator 30 are defined by the plate wall 34 hanging from a center of the ceiling. In the present example, as shown in FIG. 9A, both the front surface and the back surface of the cavities 31 and 32 are open-circuit (opening). The cuboid RTD 40 whose cross-section is a rectangle is disposed between the bottom of the wall 34 of the cavity resonator 30 and the upper surface of the conductor strip 22 of the bow-tie antenna 20. As shown in FIG. 9A, the RTD 40 is extended from the front surface (open-circuit) of the cavities 31 and 32 to the back surface (open-circuit) of the cavities 31 and 32. The bottom surface of the cavity resonator 30 and the conductor strip 22 of the bow-tie antenna 20 are separated by the insulating thin film 12 whose thickness is the same as that of the RTD 40 (about 2 [μm]). The insulating thin film 12 is extended to an outside of the cavity 31, and is downwardly bent at the outside of the cavity 31 so that the conductor strip 21 is electrically insulated from the conductor strip 22. The insulating thin film 12 whose cross-section is an L-shape is made of an insulating material such as silicon dioxide “SiO.sub.2”.

[0041] As shown in FIGS. 6 and 8, for example, a rectangular recess 22B is provided at a proximate portion of the cavity resonator 30 in the conductor strip 22 of the bow-tie antenna 20, and the L-shape conductor bridge 35 extending from the side surface of the cavity resonator 30 is disposed in the recess 22B. A stabilizing resistor 50 whose resistance value is “Rs” and which is made of, for example, “InGaAs” and is disposed on the bottom of the recess 22B, is connected to the conductor bridge 35, and the stabilizing resistor 50 is connected to the facing side surface of the conductor strip 22. The stabilizing resistor 50 suppresses a parasitic oscillation whose frequency is a few GHz, resulting in obtaining the stable terahertz oscillation. The conductor bridge 35 has a function of an inductance “Ls”. Because the opening surface of the cavity resonator 30 and the stabilizing resistor 50 are connected in this structure, the loss is not occurred and then the output power is not limited due to the above connection.

[0042] Here, the shape of the recess 22B is not limited to the rectangle, and may be an ellipse or a circle.

[0043] The bias voltage from one of the power supply portions 22A applying the bias is applied to the conductor strip 22 of the bow-tie antenna 20, and the lower electrode 40ED of the RTD 40 serves as one of the power supply portions 22A via the conductor strip 22. The bias voltage from the other of the power supply portions 33 is applied to the side surface or the top surface of the cavity resonator 30, and the upper electrode 40EU of the RTD 40 serves as the other of the power supply portions 33 via the wall 34 of the cavity resonator 30. The output from the oscillation circuit is radiated between the electrodes (the conductor strips 21 and 22) of the bow-tie antenna 20 via the side surface and the bottom surface of the insulating thin film 12. Since the insulating thin film 12 is sufficiently thin (about 2 [μm]), the connection between the cavity resonator 30 and the bow-tie antenna 20 is small and then the radiation resistance “Ra” of the bow-tie antenna is not almost affected from the cavity resonator 30.

[0044] By implementing such a structure, the oscillation circuit is constituted by the cavity resonator 30 and the RTD 40. The oscillation frequency is determined by the inductance of the cavity resonator 30 and the capacitance of the RTD 40. Since the inductance of the cavity resonator 30 is small, the RTD which has a large mesa area and a large capacitance can be used, and the high-power output can be realized by using the RTD 40 having the large mesa area.

[0045] A configuration example of the RTD 40 is shown in FIG. 10, and an “n.sup.+-GaInAs” layer (400 [nm]), an “n-GaInAs” layer (50 [nnm]), a “u-GaInAs” layer (5 [nm]), an “AlAs” layer (1.5 [nm]), a “GaInAs” layer (4.5 [nm]), an “AlAs” layer (1.5 [nm]), a “u-GaInAs” layer (5 [nm]), an “n-GaInAs” layer (50 [nnm]), and an “n.sup.+-GaInAs” layer (50 [nm]) are successively stacked between the lower electrode 40ED and the upper electrode 40EU, which are a 3-layer structure (Au/Pd/Ti) including the gold (Au), the palladium (Pd) and the titanium (Ti). As shown in FIGS. 7 and 8, the upper electrode 40EU is corresponding to the bottom of the wall 34 of the cavity resonator 30, and the lower electrode 40ED is corresponding to the upper surface of the conductor strip 22 of the bow-tie antenna 20. The lowermost layer in FIG. 10 is the semi-insulating substrate 11 (the “Si—InP” substrate).

[0046] FIG. 11 shows an equivalent circuit of the terahertz oscillator 10 according to the present invention. The radiation resistor “Ra” of the bow-tie antenna 20 is independent of the inductance “Lr” of the cavity resonator 30, and therefore the trade-off relationship between the oscillation frequency and the output power of the terahertz oscillator can be resolved. Thereby, the inductance “Lr” of the cavity resonator 30 can largely be reduced in comparison with that of the slot antenna, and the oscillation frequency can be higher without decreasing the capacitance of the RTD 40. Accordingly, the high-power output can be obtained by the large area RTD device having a large mesa area. In this RTD device, the radiation resistor “Ra” of the bow-tie antenna 20 has no relation to the inductance “Lr” of the cavity resonator 30.

[0047] In a case that the actual structure and material of the RTD 40 are a “GaInAs/AlAs” on the InP substrate 11, the material of the electrodes is the gold (Au), and the material of the insulating thin film is the silicon dioxide (SiO.sub.2), when the mesa area of the RTD 40 is variable, the simulation characteristics of the terahertz oscillator 10 according to the present invention are shown in FIG. 12. In the present example, the width “W” and the length “L” of the cavity resonator 30 are 32 [μm], and the height “H” is 2 [μm]. As is apparent from this simulation result, the frequency band is in a range of 1.0 [THz] to 2.0 {THz}, and the output power whose maximum value is about 8 [mW] is obtained. The bow-tie antenna 20 can vary the radiation impedance by changing the bow-tie angle θ, and the output power is enabled to be maximized by matching the radiation impedance with the components of the RTD 40.

[0048] In the above example, both ends of the cavities 31 and 32 of the cavity resonator 30 are opened (open-circuit). In the example of FIG. 9B, one end of the cavities 30 is closed (short-circuit). Thereby, the standing waves in which the electric field is zero at the closed side (short-circuit side) are distributed, the cavity resonator 30 in FIG. 9B resonates with a higher frequency than the resonance frequency of the cavity resonator in FIG. 9A in which both ends are opened, and the oscillation frequency of the cavity resonator in FIG. 9B becomes higher than that of the cavity resonator in FIG. 9A.

[0049] As shown in FIG. 9C, the structure in which both ends of the cavity resonator 30 are not opened but are closed may be adopted. In this case, the standing waves in which the electric field is zero at both ends are distributed, the cavity resonator 30 or in FIG. 9C resonates with a higher frequency than the resonance frequency of the cavity resonator in FIG. 9B in which only one end is closed, and the oscillation frequency of the cavity resonator 30 in FIG. 9C becomes further higher than that of the cavity resonator in FIG. 9B.

[0050] Further, although the cross-section of the cavities 31 and 32 of the cavity resonator 30 is a rectangle in the above examples, this cross-section may be a trapezoid as shown in FIG. 13.

[0051] Furthermore, in the above examples, the cavities 31 and 32 of the cavity resonator 30 are disposed in parallel with the bow-tie angle θ of the bow-tie antenna 20, that is, in parallel with the conductor strips 21 and 22. However, as shown in FIG. 14, the cavities 31 and 32 of the cavity resonator 30 may be disposed in a right angle direction to the resonant portions of the conductor strips 21 and 22. In the cavity resonator 30 in which both ends are an opening surface, the cuboid RTD 40 is disposed from one of the opening surfaces of the cavity resonator 30 to the other of the opening surfaces of the cavity resonator 30, and both opening surfaces are perpendicular to the cuboid RTD 40. The top portion and the bottom portion of the RTD 40 are connected to the inner top surface and the inner bottom surface of the cavity resonator 30, respectively. The bottom surface of the cavity resonator 30 is separated from other components by the insulating thin film 12 which is inserted between the bottom surface of the cavity resonator 30 and the side surface of the cavity resonator 30.

[0052] In the structure shown in FIG. 14, by using an electromagnetic field simulator (HFSS (Trademark) manufactured by Ansys, Inc.), an admittance constituted by the cavity resonator 30 and the bow-tie antenna 20 is calculated, and the characteristics of the RTD 40 and the oscillation characteristics are analyzed, resulting in obtaining the characteristics shown in FIG. 15. In FIG. 15, the output power [mW] is obtained in a case that the bow-tie angle [deg] and the RTD mesa area are selected as parameters. By optimizing the bow-tie angle, it is expected that the output power which is 2 [mW] or more is obtained in a case that the oscillation frequency is 1.5 [THz].

INDUSTRIAL APPLICABILITY

[0053] By using the high-power terahertz oscillator according to the present invention, a compact chip, which measures absorption spectra of a material whose absorption spectra are existed in a terahertz frequency band, can be realized. It is considered that the high-power terahertz oscillator according to the present invention enables to facilitate a further development in the fields such as the imaging and the analysis in chemistry and medical regions, and a large-capacity wireless communications using the terahertz waves.

EXPLANATION OF REFERENCE NUMERALS

[0054] 1 resonant tunneling diode (RTD) [0055] 2 slot antenna [0056] 3 substrate [0057] 4 lower electrode [0058] 5 upper electrode [0059] 6 stabilizing resistor [0060] 10 high-power terahertz oscillator [0061] 11 substrate (InP) [0062] 12 insulating thin film [0063] 20 bow-tie antenna [0064] 21, 22 conductor strip [0065] 30 cavity resonator [0066] 31, 32 cavity [0067] 34 wall [0068] 35 conductor bridge [0069] 40 resonant tunneling diode (RTD) [0070] 50 stabilizing resistor