Terahertz frequency sweep oscillator, terahertz spectrometer, oscillation method, and spectroscopic measurement method

Abstract

To provide a terahertz frequency sweep oscillator, a terahertz spectrometer, an oscillation method, and a spectroscopic measurement method capable of sweeping frequencies in the terahertz band.SOLUTION: A terahertz band frequency sweep oscillator 20 includes an oscillator element 21 that oscillates terahertz waves by the AC Josephson effect, a power supply 22 that pulse-drives the oscillator element 21, and a control device 23 that controls the oscillator element 21, and controls the power supply 22 by referring to the relationship between the pulse shape of the power supply 22 and the oscillation frequency and temperature of the oscillator element 21.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a terahertz frequency sweep oscillator, a terahertz spectrometer, an oscillation method, and a spectroscopic measurement method. [Background technology]

[0002] Electromagnetic waves with frequencies on the order of terahertz (THz) (hereinafter referred to as THz waves) are said to have frequencies ranging from 0.1 THz to 10 THz, which is almost equal to the frequencies of intermolecular vibrations and crystal lattice vibrations. Therefore, THz waves are expected to be utilized in a wide range of fields, including non-destructive testing such as material identification, security, medical diagnosis, weather observation, environmental monitoring, astronomy, and high-speed, high-capacity communication.

[0003] Known monochromatic light sources in the terahertz range include resonant tunneling diodes (RTDs) and quantum cascade lasers (QCLs). Furthermore, terahertz spectroscopic techniques include terahertz time-domain spectroscopy (THz-TDS) and Fourier transform infrared spectroscopy (FT-IR) using mercury lamps as light sources.

[0004] In recent years, oscillators equipped with high-temperature superconductors have attracted attention as devices that emit THz waves. For example, Patent Document 1 discloses a THz wave oscillator that utilizes the intrinsic Josephson junction of a high-temperature superconductor. [Prior art documents] [Patent Documents]

[0005] [Patent Document 1] Japanese Patent Publication No. 2019-149418 [Overview of the Initiative] [Problems that the invention aims to solve]

[0006] In the aforementioned RTD and QCL techniques, no frequency sweeping techniques are known. Furthermore, the frequency resolution in typical THz-TDS techniques is only a few gigahertz. Additionally, even when extremely short pulses are irradiated onto an object using THz-TDS techniques, optical property analysis requires special techniques such as spatial property correction and fast Fourier transform (FFT). FT-IR techniques require considerable time for spectrum acquisition.

[0007] The THz wave oscillator element using Bi2212 described in Patent Document 1 can sweep the frequency by voltage, but it requires appropriate control of the element's temperature, and there were limitations to the size of the device and the speed of frequency sweep, such as the need to use an external heater.

[0008] The present invention has been made in view of the above circumstances, and aims to provide a terahertz frequency sweep oscillator, a terahertz spectrometer, an oscillation method, and a spectroscopic measurement method that are capable of sweeping frequencies in the terahertz band. [Means for solving the problem]

[0009] A first aspect of the present invention is a terahertz frequency sweep oscillator comprising an oscillator element that emits terahertz waves by the AC Josephson effect, a power supply that pulse-drives the oscillator element, and a control device that controls the oscillator element, wherein the power supply is controlled by referring to the relationship between the pulse shape generated by the power supply and the oscillation frequency and temperature of the oscillator element.

[0010] In the second embodiment, the relationship between the pulse shape generated by the power supply and the oscillation frequency and temperature of the oscillator is based on pre-acquired data. A third embodiment is the first or second embodiment, wherein the oscillating element is a high-temperature superconductor Bi2Sr2CaCu2O 8+δ It is composed of single crystals.

[0011] A fourth embodiment is one of the first to third embodiments, wherein the pulse shape raises the temperature of the oscillating element to a temperature at which terahertz waves can be oscillated during the rising edge, and during the falling edge, the oscillation frequency changes over time while the temperature of the oscillating element changes. A fifth embodiment is a configuration in which, in any one of the first to fourth embodiments, the frequency at which the oscillation frequency changes while the oscillation element is pulse-driven includes the fundamental resonant frequency of the oscillation element or a lower frequency and a frequency higher than the fundamental resonant frequency.

[0012] The sixth embodiment comprises, in any one of the first to fifth embodiments, a plurality of oscillators connected in parallel to the power supply and having different fundamental resonant frequencies from one another, and a changeover switch for switching the connection between the power supply and the oscillators, wherein the power supply is controlled for each of the oscillators by referring to the relationship between the pulse shape generated by the power supply and the oscillation frequency and temperature of the oscillator.

[0013] The seventh embodiment is a terahertz spectrometer comprising a terahertz frequency sweep oscillator according to any one of the first to sixth embodiments and a terahertz wave detector.

[0014] The eighth aspect is an oscillation method that outputs a predetermined oscillation frequency by pulse-driving the oscillation element using a terahertz band frequency sweep oscillator according to any one of the first to sixth aspects.

[0015] The ninth aspect is a spectroscopic measurement method that includes the steps of changing the oscillation frequency of an oscillation element over time by pulse driving the oscillation element using the terahertz spectrometer of the seventh aspect, and detecting the output of the oscillation element with the detector. [Effects of the Invention]

[0016] According to the present invention, it is possible to provide a terahertz frequency sweep oscillator, a terahertz spectrometer, an oscillation method, and a spectroscopic measurement method that are capable of sweeping frequencies in the terahertz band.

Brief Description of the Drawings

[0017] [Figure 1] It is a configuration diagram illustrating a terahertz band frequency sweep oscillator of the first embodiment. [Figure 2] It is a graph showing an example of the frequency spectrum of an oscillation element. [Figure 3] It is a configuration diagram showing an example of a terahertz band frequency sweep oscillator. [Figure 4] It is a drawing substitute photograph showing an example of an oscillation element. [Figure 5] It is a drawing substitute photograph showing an example of an oscilloscope screen. [Figure 6] It is a drawing substitute photograph showing the frequency measurement result at 1.020 THz in FIG. 5. [Figure 7] It is a drawing substitute photograph showing the frequency measurement result at 0.510 THz in FIG. 5. [Figure 8] It is a drawing substitute photograph of a partially enlarged view of the screen in FIG. 5. [Figure 9] It is a graph explaining the output obtained by pulse driving an oscillation element. [Figure 10] It is a graph showing the spectrum at point A in FIG. 9. [Figure 11] It is a graph showing the spectrum at point B in FIG. 9. [Figure 12] It is a graph showing the spectrum at point C in FIG. 9. [Figure 13] It is a configuration diagram illustrating a terahertz band frequency sweep oscillator of the second embodiment.

Modes for Carrying Out the Invention

[0018] Hereinafter, the present invention will be described based on preferred embodiments.

[0019] Figure 1 illustrates a terahertz frequency sweep oscillator according to the first embodiment. The terahertz frequency sweep oscillator 10 shown in Figure 1 comprises an oscillator element 11 and a power supply 12 for pulse-driving the oscillator element 11. The oscillator element 11 oscillates terahertz waves by the alternating current (AC) Josephson effect.

[0020] The oscillator element 11 may include a superconductor having a multilayer structure of superconducting layers and insulating layers, and a structure in which Josephson junctions are stacked. The material constituting the superconductor may be, for example, a high-temperature superconductor Bi2Sr2CaCu2O 8+δ A single crystal (hereinafter referred to as Bi2212) is an example. In Bi2212, the CuO2 layer functions as a superconducting layer and the Bi2O2 layer functions as an insulating layer, and a superconducting current flows between the superconducting layers through the insulating layer due to the Josephson effect.

[0021] If the oscillator element 11 has a mesa structure, a mesa 11b is formed on the crystal substrate 11a. The crystal substrate 11a and the mesa 11b may be a single crystal. The shape of the mesa 11b formed on the crystal substrate 11a is not particularly limited and can be a square, rectangle, triangle, pentagon or other polygon, or a circle.

[0022] The oscillator element 11 does not have to be a mesa structure, and even if it is a mesa structure, its dimensions and other properties can be set as appropriate. A mesa structure is a portion of the superconducting crystal that forms a plateau shape or a columnar shape. The superconducting crystal may consist of separate single crystals for the crystal substrate 11a and the mesa 11b. The number of mesas 11b formed on the crystal substrate 11a is not particularly limited and may be one or two or more.

[0023] If the oscillator 11 has a mesa structure, a voltage is applied from the power supply 12 to at least the mesa 11b via the wiring 13. Although not specifically shown, a conductive layer (not shown) or the like may be placed between the mesa 11b and the wiring 13. The power supply 12 that drives the mesa 11b is connected via the wiring 13. The area around the oscillator 11 is maintained at the low temperature necessary for the operation of the superconductor.

[0024] Conductive components such as wiring 13 and conductive layers (not shown) can be formed from metals such as gold, silver, and copper. A metal thin-film circuit may be printed on the lower surface of the heat dissipation means 15 (details to be described later) and pressed onto the upper surface of the mesa 11b. To connect the wiring 13 to the mesa 11b on the crystal substrate 11a, a raised portion (not shown) may be formed on the crystal substrate 11a using a suitable material such as CaF2, and the wiring 13 may be connected to a conductive layer (not shown) formed from the mesa 11b up to the raised portion.

[0025] The sides of the mesa 11b may be surrounded by a gas such as helium or a vacuum. The medium in contact with the mesa 11b is preferably a material with low dielectric loss, and specifically, in addition to a vacuum, examples include gases with low boiling points such as helium, hydrogen, and neon, liquids cooled below the boiling point such as nitrogen, resins such as benzocyclobutene (BCB), and heat dissipators such as diamond grease.

[0026] The upper surface of the mesa 11b may be in contact with a vacuum, in contact with the same medium as the sides of the mesa 11b, or in contact with an insulating solid such as high-purity silicon, sapphire, or high-purity alumina ceramics. As will be described in more detail later, the upper surface of the mesa 11b may also be in contact with the heat dissipation means 15.

[0027] The crystal substrate 11a can be fixed on the support substrate 14, and a mesa 11b can be formed on the crystal substrate 11a by processing with dry or wet etching. The sides of the mesa structure may be vertical or inclined, and this can be adjusted by appropriately selecting the processing conditions.

[0028] The material of the support substrate 14 is not particularly limited, but examples include sapphire, diamond, alumina, and copper.

[0029] When a voltage is applied to mesa 11b, terahertz waves are emitted from mesa 11b. The oscillation frequency of the terahertz waves is determined by the resonant frequency, which depends on the width W of mesa 11b. If mesa 11b has a width of 70 μm, the fundamental resonant frequency is 0.500 THz (500 GHz). In this case, a specific example of a mesa structure is one with a length of 400 μm and a height of 2 μm.

[0030] Resonance occurs when the resonator dimension w, such as the width of Mesa 11b, is equal to an integer multiple (m times) of half the wavelength (λ / 2) of the Josephson plasma wave (w = mλ / 2). For example, when c is the speed of light in a vacuum and n is the refractive index of the medium, the frequency f is given by f = mc / 2nw. When m is 1, the fundamental resonant frequency (fundamental mode) is obtained. When m is 2 or greater, the frequencies of higher-order modes are obtained. When the medium is a Bi2212 single crystal, the value of n is approximately 4.2.

[0031] When a voltage is applied perpendicular to the stacking plane of a Josephson junction in a superconductor, the AC Josephson effect occurs. The AC Josephson effect is the phenomenon in which an alternating current flows when a constant voltage is applied between two superconductors separated by an extremely thin insulating layer. Since the frequency of the alternating current is proportional to the voltage applied to each layer of the Josephson junction, non-resonant terahertz waves are generated as electromagnetic waves.

[0032] When non-resonant terahertz waves resonate with the natural resonant frequency of an element structure such as a mesa structure, oscillating currents flow coherently with aligned phases between stacked Josephson junctions, causing resonance. As a result, resonant terahertz waves are radiated to the outside. Resonant terahertz waves have a stronger intensity peak than non-resonant terahertz waves.

[0033] Figure 2 is a graph showing an example of the frequency spectrum of the oscillator element 11. The linewidth varies depending on the bias conditions and is generally reported to be between tens of MHz and hundreds of MHz. Figure 2 is based on measurements using FT-IR, with a measurement resolution of approximately 8 GHz.

[0034] As shown in Figures 3 and 4 of Patent Document 1, it is known that the frequency emitted from an oscillator depends on the ambient temperature and bias voltage.

[0035] As mentioned above, the basic principle of terahertz wave oscillation is the AC Josephson effect, so frequency and voltage are proportional. In Figure 4 of Patent Document 1, when the applied voltage is approximately 6V or higher, the ambient temperature is 25K or lower, and the oscillation frequency is approximately 1.0THz or higher. It is shown that the oscillation frequency tends to decrease when the voltage is low.

[0036] The voltage that can be applied to a Josephson junction has a lower limit determined by temperature. To lower the frequency, the temperature of the crystal needs to be increased. Regarding the relationship between oscillation frequency and oscillation intensity, as mentioned above, it oscillates strongly at the fundamental resonant frequency determined by the mesa width. The intensity increases again at the second resonant frequency. It also oscillates weakly at other frequencies.

[0037] Figure 3 shows an example of a terahertz frequency sweep oscillator 20. A pulse-driven power supply 22 is connected to the oscillator element 21. The power supply 22 is the source of the pulses. At least one of the oscillator element 21 and the power supply 22 is controlled by a control device 23. The oscillation method of the terahertz frequency sweep oscillator 20 is to output a predetermined oscillation frequency by pulse-driving the oscillator element 21 with the power supply 22.

[0038] The oscillator element 21 may be the mesa-structured oscillator element 11 described above. Specifically, an array of 24 mesas, as shown in the embodiment of Figure 4, is used. The mesa-structured oscillator element 11 may be an array, or it may be a single oscillator element 11 rather than an array.

[0039] In the illustrated example, the voltage drop across the oscillator 21 is measured via a differential amplifier 25 connected to channel 3 (CH3) of the oscilloscope 31. A 1Ω resistor 24 is placed between the oscillator 21 and the power supply 22. The current flowing through the oscillator 21 is detected via a differential amplifier 25 connected to channel 2 (CH2) of the oscilloscope 31.

[0040] The illustrated terahertz spectrometer 30 comprises a terahertz frequency sweep oscillator 20 and a terahertz wave detector 33. Examples of the detector 33 include an InSb hot electron bolometer (HEB) and a Schottky barrier diode. The detector 33 is connected to the spectrometer 34.

[0041] The spectroscopic measurement method using the terahertz spectrometer 30 includes the steps of changing the oscillation frequency of the oscillation element 21 over time by pulse driving the oscillation element 21, and detecting the output of the oscillation element 21 with the detector 33.

[0042] The output of detector 33 is connected to channel 4 (CH4) of oscilloscope 31. Channel 1 (CH1) of oscilloscope 31 is connected to power supply 22 and can measure the voltage drop at the point where a 1Ω resistor is applied to the oscillator element 21. A digital oscilloscope was used as oscilloscope 31.

[0043] Figure 5 shows an example of an oscilloscope screen. The horizontal axis represents time (10 μs / div.), and the vertical axis shows the voltage drop across the oscillator 21 (2 V / div.) as line B, the current flowing through the oscillator 21 (400 mA / div.) as line P, the output voltage of HEB (20 mV / div.) as line G, and the voltage drop across the oscillator + 1 Ω (2 V / div.) as line Y.

[0044] The pulse shape in the illustrated example is roughly trapezoidal. The left side of the trapezoid represents a rising voltage, where the voltage increases over time. The right side of the trapezoid represents a falling voltage, where the voltage decreases over time. The voltage corresponding to the top side of the trapezoid is approximately constant along line Y, but shows a slight tendency to decrease over time along line B.

[0045] Surprisingly, a phenomenon was observed in which the oscillation frequency changed over time while a single pulse was applied to the oscillation element 21. Figure 6 shows the frequency measurement results at 1.020 THz (1020 GHz) in Figure 5. Figure 7 shows the frequency measurement results at 0.510 THz (510 GHz) in Figure 5.

[0046] The method for measuring the oscillation frequency is not particularly limited, but in the illustrated example, the spectrometer 34 is configured by an optical system consisting of a detector 33 using HEB and a Michelson interferometer. The Michelson interferometer is configured by arranging an oscillating element 21 that serves as a light source, a fixed mirror (not shown), a moving mirror (not shown), and a detector 33 around a beam splitter (not shown).

[0047] The electromagnetic waves emitted from the oscillating element 21 are split into two by the beam splitter. One electromagnetic wave passes through the beam splitter, heads towards the moving mirror, and then returns to the beam splitter. The other electromagnetic wave is reflected by the beam splitter, heads towards the fixed mirror, and then returns to the beam splitter. The combined electromagnetic waves from both are directed towards the detector 33.

[0048] By moving the moving mirror, electromagnetic waves whose intensity fluctuates due to interference are detected by the detector 33. The output of the detector 33 is input to a computer 32, such as a personal computer (PC), using the cursor measurement function of the oscilloscope 31. By performing a Fourier transform on the input interference pattern in the computer 32, the frequency spectrum can be obtained.

[0049] The terahertz frequency sweep oscillator 20 controls the power supply 22 by referring to the relationship between the pulse shape generated by the power supply 22 and the oscillation frequency and temperature of the oscillation element 21. This allows the terahertz waves emitted from the oscillation element 21 to be frequency swept in the terahertz band.

[0050] Figure 8 shows a partially enlarged view of the oscilloscope 31 screen shown in Figure 5. The relationship between these radiation intensities and frequencies is similar to that of a DC bias (see Figures 3-4 of Patent Document 1). Line G is minus 1 times the output of detector 33 and is swinging in the negative direction.

[0051] By adjusting the pulse voltage, duration, and shape, the temperature change of the Bi2212 crystal can be controlled. By synchronizing the time changes of temperature and voltage, the frequency can be swept. The time required for frequency sweeping is several tens of microseconds; in the illustrated example, the frequency is swept from approximately 1.100 THz to approximately 0.430 THz in about 24 μs.

[0052] The repetition frequency can be set to several kHz. In the illustrated example, the repetition frequency is 1 kHz. Intensity spectra can be acquired at such high speeds. To obtain absorption spectra, intensity spectra can be acquired for both a reference sample and the target sample and compared. The signal-to-noise ratio can be improved by averaging.

[0053] The relationship between the pulse shape generated by the power supply 22 and the oscillation frequency and temperature of the oscillator element 21 may be based on pre-acquired data. The data, programs, etc., necessary for controlling the pulse shape may be stored in the control device 23, or they may be input from an external source via communication. Communication may be wireless or wired.

[0054] The pulse shape applied from the power supply 22 to the oscillator 21 raises the temperature of the oscillator 21 to a temperature at which terahertz waves can be generated during the rising edge, and during the falling edge, the oscillation frequency can change over time as the temperature of the oscillator 21 changes.

[0055] For example, during pulse driving, it is possible that the oscillation frequency changes over time while the temperature of the oscillation element 21 continuously rises, or that the oscillation frequency changes over time while the temperature of the oscillation element 21 continuously falls. It is also possible that the temperature of the oscillation element 21 rises and then begins to fall.

[0056] Due to the short pulse width, the degree of temperature change in the oscillator due to heat transfer with the outside may be low. Also, due to the low pulse voltage, the amount of heat generated by electrical resistance inside the oscillator may be small compared to the heat dissipated to the outside.

[0057] If the pulse voltage is too low, the temperature of the oscillator may not rise to a temperature at which terahertz waves can be generated during the rising edge of the pulse. If the pulse voltage is too high, the temperature of the oscillator may rise too rapidly, exceeding the superconducting transition temperature and preventing terahertz wave generation.

[0058] If an appropriate pulse is selected, a temperature change may occur in the oscillator element during a single pulse drive, across a relatively wide temperature range from the ambient temperature to the superconducting transition temperature. For example, when Bi2212 is used as the oscillator element, this temperature range is below approximately 90K, the superconducting transition temperature.

[0059] Factors contributing to the temperature rise include the conversion of some of the pulse energy into heat through electrical resistance. Factors contributing to the temperature decrease include heat dissipation and exhaust from the oscillating element 21 to the outside. As will be described later, a heat exhaust means 15 (see Figure 1) may be provided for the oscillating element 21.

[0060] As mentioned above, in an oscillator that emits electromagnetic waves using the AC Josephson effect, the oscillation frequency is proportional to the applied voltage. Therefore, by continuously decreasing the applied voltage over time, the oscillation frequency can be continuously decreased over time. During the falling edge of a pulse, the applied voltage decreases gradually, thus allowing the oscillation frequency to be gradually decreased.

[0061] The frequency at which the oscillation frequency changes while the oscillator element is pulse-driven may include the fundamental resonant frequency of the oscillator element or a lower frequency, and a frequency higher than the fundamental resonant frequency. For example, with a and b being different positive integers, the oscillation frequency may change from a frequency of approximately a times the fundamental resonant frequency to a frequency of approximately b times the fundamental resonant frequency. The oscillation frequency may also be varied within a range of 0.01 times or more, 0.05 times or more, 0.1 times or more, 0.2 times or more, 0.3 times or more, or 0.5 times or more of the fundamental resonant frequency of the oscillator element.

[0062] In the embodiment described above, oscillation begins at a frequency approximately twice the fundamental resonant frequency, and the oscillation frequency changes down to a frequency lower than the fundamental resonant frequency. Surprisingly, it was possible to stably oscillate electromagnetic waves over a wide frequency range, not just around the fundamental resonant frequency and its integer multiples.

[0063] Next, the terahertz frequency sweep oscillator 20 of the first embodiment will be described in more detail. In the illustrated example of the terahertz frequency sweep oscillator 10, a crystal substrate 11a is placed on a support substrate 14, and six mesas 11b are formed on the crystal substrate 11a.

[0064] The number of mesas 11b formed on the crystal substrate 11a is not particularly limited and may be one or two or more. By simultaneously applying a pulse voltage to multiple superconducting crystals (mesas 11b, etc.) with the same fundamental resonant frequency, the oscillation intensity can be increased.

[0065] The mesas 11b arranged vertically in Figure 1 are connected in parallel to the power supply 12, which is connected via the wiring 13. The mesas 11b arranged horizontally in Figure 1 are connected in series to the power supply 12, which is connected via the wiring 13, via the crystal substrate 11a. A bias is applied to each mesa 11b in the height direction between the crystal substrate 11a and the wiring 13. Although not specifically shown, when connecting an odd number of mesas 11b (even just one) between the wiring 13, the wiring 13 may be connected to the crystal substrate 11a to connect to the underside of the mesa 11b. For connection to the underside of the mesa 11b, a conductive layer (not shown) to which the wiring 13 is connected may be placed on the crystal substrate 11a.

[0066] The heat dissipation means 15 dissipates heat from the single-crystal mesa 11b from at least the side opposite to the crystal substrate 11a or the support substrate 14. In Figure 1, the heat dissipation means 15 is shown separated from the mesa 11b for easier understanding of the configuration, but it is preferable to bond the heat dissipation means 15 to the side of the mesa 11b opposite to the crystal substrate 11a or the support substrate 14.

[0067] The heat dissipation means 15 may be a plate made of a material with high thermal conductivity. Examples of materials with high thermal conductivity include sapphire and alumina ceramics. As described above, the crystalline substrate 11a is the same superconductor as the mesa 11b. It is also possible to omit the crystalline substrate 11a and place a superconductor with a shape similar to the mesa 11b on the support substrate 14.

[0068] It is preferable to use sapphire, diamond, or copper, or a material with equivalent or superior heat dissipation capabilities, for the support substrate 14. These materials exhibit high thermal conductivity at the operating temperature of the oscillation element 11 (for example, around 40K, which is the temperature reached by small refrigerators such as Stirling coolers), thus enabling efficient heat dissipation from the support substrate 14.

[0069] Next, referring to the graph in Figure 9, we will explain the output obtained by pulse-driving the oscillator. The horizontal axis of all these graphs is time. The first stage, Vb, is the output voltage of the pulse power supply. The second stage, Voltage, is the voltage applied to the oscillator. The third stage, Frequency, is the frequency of the oscillation spectrum measured at the measurement point indicated by the black circle. The fourth stage, Intensity, is the relative value of the oscillation strength.

[0070] The repetition frequency of the pulse power supply's output voltage is 1 kHz. The voltage applied to the oscillator is the output voltage of the pulse power supply minus the voltage drop across the 1 Ω current monitoring resistor. After the pulse voltage is applied, the crystal temperature rises and the resistance decreases, causing the current to increase. The second stage voltage has a different shape from the output voltage of the first stage pulse power supply because the voltage drop across the 1 Ω current monitoring resistor increases due to the increase in current.

[0071] In the third frequency row of Figure 9, as described above, the frequencies of the peak positions in the spectrum measured with a Michelson interferometer spectrometer are plotted. Due to the AC Josephson effect, the oscillation frequency is proportional to the voltage applied to the oscillating element, so once calibrated, the frequency can be determined from the voltage. Calibration is necessary because the height of the mesa is involved. Measuring the frequency takes time, but the voltage can be determined instantaneously.

[0072] In the fourth intensity row of Figure 9, the peak heights of the spectral lines are plotted. For example, the spectra at points A, B, and C in Figure 9 are as shown in Figures 10, 11, and 12, respectively.

[0073] In the specific example described above, a mesa with a width of 70 μm is used, and its fundamental resonant frequency is 0.5 THz. Therefore, it oscillates strongly around 0.5 THz, and the intensity increases around the second resonant frequency of 1 THz. Sufficient intensity can be obtained with an oscillation width of ±30 GHz around 0.5 THz. For this reason, if the absorption spectrum (fingerprint spectrum) of the target substance (e.g., a pathogenic marker substance) is known and sweeping over a wider frequency range is not necessary, the resonant frequency of the mesa can be matched to the "fingerprint" to detect the target substance quickly and with high sensitivity.

[0074] In the method described in Patent Document 1 (Japanese Patent Application No. 2018-32280), it is necessary to change the temperature of the crystal (ambient temperature) when changing the oscillation frequency. In contrast, according to the first embodiment, sweeping in a frequency range that could not be achieved conventionally without changing the ambient temperature can be achieved by voltage alone. Furthermore, high-speed sweeping and a high repetition frequency make it possible to track fingerprint spectra in the terahertz band with a time resolution of about milliseconds.

[0075] For example, if the fingerprint spectrum to be monitored is predetermined, the fundamental resonant frequency of the oscillator can be matched to the fingerprint spectrum. A shorter pulse width allows for a higher repetition frequency, enabling tracking of time changes in sub-milliseconds. An arbitrary waveform generator can be used as the pulse power supply, and any suitable pulse waveform can be used, not limited to trapezoidal or triangular pulses.

[0076] According to the terahertz frequency sweep oscillator and terahertz spectrometer of the first embodiment, the time evolution of the crystal temperature and the voltage applied to the crystal can be synchronized by pulse driving the oscillator element. This makes it possible to provide a technology that enables high-speed sweeping of the frequency of radiated terahertz waves over a wide bandwidth using only pulse voltage, without the need to adjust the crystal temperature in the oscillator element.

[0077] Furthermore, when driving an array of oscillators with different resonant frequencies using pulses, it is possible to provide a technique for frequency sweeping high-intensity terahertz wave radiation over a wide frequency range. In addition, high-speed frequency sweeping and high-speed repetition by pulse driving enable high-speed spectroscopic measurements in the terahertz band.

[0078] In the technology of the first embodiment, pulse driving of the oscillator element can suppress the rise in temperature of the oscillator element. Furthermore, by appropriately controlling the pulse shape, pulse width, etc., it is possible to change the frequency by changing the voltage while simultaneously controlling the temperature of the oscillator element. When the high-temperature superconductor Bi2212 is used as the oscillator element, the oscillation linewidth is tens to hundreds of megahertz, and evaluation is possible with a frequency resolution an order of magnitude higher than that of a normal THz-TDS device.

[0079] Furthermore, the first embodiment of the technique, which involves frequency sweeping monochromatic light, can be processed using a simple optical system and conventional signal processing, making spectrum acquisition easy. The spectral intensity in the terahertz band is 2 to 3 orders of magnitude higher than that of mercury lamps. Moreover, high-speed and highly sensitive detection is possible with a repetition frequency of several kHz. Averaging can also be performed as needed. Since frequency sweeping is possible in tens of microseconds, spectra can be acquired in a short time.

[0080] In pulsed drive, voltage is applied in a non-steady state. Therefore, when the ambient temperature of the oscillator is higher than in conventional technology, voltage can be applied before the crystal temperature rises, which is expected to enable oscillation at higher frequencies.

[0081] Figure 13 illustrates a terahertz frequency sweep oscillator 40 of the second embodiment. The terahertz frequency sweep oscillator 40 has an oscillator element array 41 having a plurality of oscillator elements 41b arranged on a substrate 41a. The power supply 42 can be the same as the power supplies 12 and 22 described above. In the second embodiment, redundant explanations may be omitted for matters that can be done in the same way as in the first embodiment.

[0082] The substrate 41a may be the crystal substrate 11a or the support substrate 14 described above. The oscillator element 41b may be the mesa 11b described above, or it may not be a mesa structure. The substrate 41a and each oscillator element 41b may be integrated as a single crystal.

[0083] Each oscillator element 41b is connected to a power supply 42 via wiring 43a, 43b and a selector switch 44, such that one oscillator element 41b is selected at a time. The selector switch 44 is located between the oscillator element 41b and the power supply 42 in one of the wirings 43a. Although not specifically shown, multiple oscillator elements 41b with the same fundamental resonant frequency may be connected in series between one wiring 43a and the other wiring 43b.

[0084] The fundamental resonant frequency f of the oscillator element 41b ci(i = 1, 2, 3, …) are different. In the illustrated example, an oscillator array 41 including six oscillator elements 41b with subscript i ranging from 1 to 6 is illustrated. Although not particularly illustrated, the number of oscillator elements 41b that can be switched via a switching switch 44 can be any integer of 2 or more. Thereby, the sweep frequency range can be expanded by using only strong oscillations near the fundamental resonance frequency f ci . If necessary, oscillations at integer multiples of 2 or more (higher modes) of the fundamental resonance frequency f ci may be used.

[0085] For example, each oscillator element 41b shares a frequency range with strong oscillation intensity of about ±40 GHz centered on the fundamental resonance frequency f ci . As the frequency range with strong oscillation intensity, the vicinity of the fundamental resonance frequency f ci of each oscillator element 41b is selected as the output and used for spectroscopic measurement and the like. The specific frequency range is not limited to about ±40 GHz and can be appropriately set according to the allowable range of intensity decrease between the fundamental resonance frequencies f ci of each oscillator element 41b.

[0086] The allowable range of intensity decrease may adopt, for example, a criterion that intensity stronger than the minimum value of intensity between the fundamental resonance frequency f ci and its double frequency can be obtained. Alternatively, a criterion that intensity stronger than the minimum value of intensity between the fundamental resonance frequencies f ci , f cj of two oscillator elements 41b with different subscripts i and j can be obtained may be adopted.

[0087] Near the fundamental resonance frequency, the frequency range with strong oscillation intensity may be, for example, a range where intensity of 0.01 times or more of the intensity at the fundamental resonance frequency f ci can be obtained, and further, a range where intensity of 0.1 times or more can be obtained.

[0088] The oscillator elements 41b can be switched using the changeover switch 44, and at the same time, an optimal pulse voltage can be applied to each oscillator element 41b from the power supply 42. The switching of the oscillator elements 41b using the changeover switch 44 can be automated. For automation, for example, the control device 23 (see Figure 3) described above may be used. The pulse voltage may differ for each oscillator element 41b.

[0089] In the DC bias operation described in Patent Document 1, it was necessary to change the ambient temperature (crystal temperature). However, according to the second embodiment, similar to the first embodiment, the crystal temperature of the oscillator element 41b can be changed transiently by pulse voltage. Since it is not necessary to change the ambient temperature, higher speeds are possible.

[0090] Although not specifically shown in the figures, in the second embodiment as well, it is preferable to calibrate the relationship between the applied voltage and the oscillation frequency when the terahertz frequency sweep oscillator 40 is started up with a spectrometer, and it is also possible to automate this calibration. This allows the applied voltage to be used as an indicator of frequency thereafter. This is because measuring the oscillation frequency takes time, but the applied voltage can be determined immediately.

[0091] The time width of each pulse voltage is, for example, 10 μs to 100 μs, and the repetition frequency can be set to several kHz to tens of kHz. Even if the spectrum from each oscillator element 41b is integrated and measured 100 times, it is theoretically possible to sweep from 0.3 THz to 0.9 THz in just a few seconds.

[0092] The spectra emitted from each oscillator element 41b of the oscillator element array 41 can be processed or utilized using a computer such as a personal computer (PC).

[0093] When using an oscillator array 41 in which each oscillator element 41b has a different fundamental resonant frequency, the illustrated example shows the case where the same power supply 42 and changeover switch 44 are used. Although not specifically shown in the illustration, if a different power supply is used for each oscillator element without using a changeover switch, a pulse voltage can be applied to each oscillator element under optimal conditions, and fine adjustment of the oscillation timing and high-speed sweeping are also possible.

[0094] In the second embodiment, the fundamental resonant frequencies f ci Multiple oscillators 41b with different properties are used, and the connection between the power supply 42 and the oscillators 41b is switched via a changeover switch 44.

[0095] Although the present invention has been described above based on preferred embodiments, the present invention is not limited to the embodiments described above, and various modifications are possible without departing from the spirit of the invention. Modifications include adding, substituting, omitting, or otherwise changing the components in each embodiment. It is also possible to combine components used in two or more embodiments as appropriate. [Industrial applicability]

[0096] According to the technology of the present invention or its embodiments, the absorption spectrum (fingerprint spectrum) unique to a substance in the terahertz band can be detected at high speed and with high sensitivity. For this reason, it is conceivable to have applications in various fields, such as medical diagnosis, including the determination of the presence or absence of pathogenic marker substances, and the determination of component content or the presence or absence of impurities in various manufacturing sites. [Explanation of symbols]

[0097] 10, 20, 40…Terahertz frequency sweep oscillator, 11, 21, 41b…Oscillating element, 11a…Crystal substrate, 11b…Mesa, 12, 22, 42…Power supply, 13, 43a, 43b…Wiring, 14…Support substrate, 15…Heat dissipation means, 23…Control device, 24…Resistor, 25…Differential amplifier, 30…Terahertz spectrometer, 31…Oscilloscope, 32…Computer, 33…Detector, 34…Spectrometer, 41…Oscillating element array, 41a…Substrate, 44…Changeover switch.

Claims

1. An oscillator that generates terahertz waves using the AC Josephson effect, A power supply for pulse-driving the aforementioned oscillator element, The system comprises a control device for controlling the oscillator element, A terahertz frequency sweep oscillator that controls the power supply by referring to the relationship between the pulse shape generated by the power supply and the oscillation frequency and temperature of the oscillation element.

2. The terahertz frequency sweep oscillator according to claim 1, wherein the relationship between the pulse shape generated by the power supply and the oscillation frequency and temperature of the oscillation element is data acquired in advance.

3. The aforementioned oscillator element is a high-temperature superconductor Bi ２ Sr ２ CaCu ２ O ８＋δ A terahertz frequency sweep oscillator according to claim 1, comprising a single crystal.

4. The terahertz frequency sweep oscillator according to claim 1, wherein the pulse shape allows the temperature of the oscillator to rise to a temperature at which terahertz waves can be oscillated during the rising edge, and during the falling edge, the oscillation frequency can change over time while the temperature of the oscillator changes.

5. The terahertz band frequency sweep oscillator according to claim 1, wherein the frequency at which the oscillation frequency changes while the oscillator element is pulse-driven includes the fundamental resonant frequency of the oscillator element or a frequency lower thereunder and a frequency higher than the fundamental resonant frequency.

6. Multiple oscillators are connected in parallel to the power supply, and each has a different fundamental resonant frequency. The system includes a changeover switch for switching the connection between the power supply and the oscillator element, The terahertz band frequency sweep oscillator according to claim 1, wherein the power supply is controlled for each of the oscillator elements by referring to the relationship between the pulse shape of the power supply, the oscillation frequency and temperature of the oscillator element.

7. A terahertz spectrometer comprising a terahertz band frequency sweep oscillator according to any one of claims 1 to 6, and a terahertz wave detector.

8. An oscillation method comprising using a terahertz band frequency sweep oscillator according to any one of claims 1 to 6, and outputting a predetermined oscillation frequency by pulse driving the oscillation element.

9. A step of changing the oscillation frequency of the oscillation element over time by pulse driving the oscillation element using the terahertz spectrometer described in claim 7, A spectroscopic measurement method comprising the step of detecting the output of the oscillating element using the detector.

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