Evaluation of the device characteristics of Bi2Sr2CaCu2O8+δ terahertz-wave emitters using wet-etching techniques

Nakayama M., Nakagawa S., Yamaguchi T., Minami H., Kadowaki K., Nakao H., Mochiku T., Tsujimoto M., Ishida S., Eisaki H., Kashiwagi T.

To obtain high-performance THz-wave-emitting devices made of single crystals of Bi2Sr2CaCu2O8+δ (Bi2212), a high-temperature superconductor, an understanding of the device characteristics based on crystal characteristics can be a key issue because, in principle, the electrical properties of the intrinsic Josephson junctions (IJJs) constructed in Bi2212 crystals highly depend on crystal conditions, such as carrier concentration, crystal homogeneities, and crystal defects. To evaluate the tendencies of the device characteristics associated with crystal characteristics, we prepared Bi2212 crystals with different Bi/Sr ratios (x=0.05, 0.15, and 0.25) and δ values (annealed under N2 or O2 gas flow conditions). The unit cell parameter c decreased as the Bi/Sr ratio or δ increased. For the same annealing conditions under N2 gas flow, the superconducting transition temperature as well as the size of the hysteresis loop of the current–voltage characteristics and emission characteristics were significantly suppressed for the sample with x=0.25 compared with the corresponding values for the samples with x=0.05 and 0.15. The experimental results clearly indicate that parameters, such as the Bi/Sr ratio and annealing conditions, are crucial factors in determining the electrical characteristics of a device. This information can be a useful guide for the preparation of crystals for IJJ THz-wave devices that can be fine-tuned according to the desired device characteristics.

Miyamoto M., Kobayashi R., Kuwano G., Tsujimoto M., Kakeya I.

Communication using terahertz (∼1012 Hz) electromagnetic waves is critical for developing sixth-generation wireless network infrastructures. Conflictions between stable radiation and frequency modulation of terahertz sources impede the superposing of transmitting signals on carrier waves. The Josephson junctions included in a cuprate superconductor radiate terahertz waves with frequencies proportional to the bias voltages. Thus, the modulation of the bias voltage leads to the modulation of the Josephson plasma emission (JPE) frequency. This study aims to demonstrate the generation of frequency-modulated (FM) terahertz continuous waves from Josephson junctions. A FM bandwidth of up to 40 GHz was achieved when 3 GHz sinusoidal waves were superimposed on 840–890 GHz carrier waves radiated by a JPE. The results verify that the instantaneous JPE frequency follows the gigahertz-modulated bias voltage. The wide-band FM terahertz generation by a monolithic device shows a sharp contrast to the mode-lock frequency comb constructed using highly sophisticated optics on a bench. A further increase of the modulation amplitude facilitates up- or downconversion of frequencies over more than one octave. The obtained FM bandwidth exhibited an improvement of two orders of magnitude in the demodulation signal-to-noise ratio compared with the amplitude-modulated waves. The demonstrated FM JPE stimulates further research on terahertz communication technology and metrology using superconducting devices. Frequency-modulated terahertz continuous waves are generated from Josephson junctions included in a cuprate superconductor. When 3 GHz sinusoidal waves were superimposed on 840–890 GHz carrier waves, the modulation bandwidth reached 40 GHz when a Josephson plasma emission was utilized.

Nakagawa S., Shizu T., Imai T., Nakayama M., Kim J., Minami H., Kadowaki K., Tsujimoto M., Nakao H., Eisaki H., Ishida S., Mochiku T., Hasegawa Y., Kashiwagi T.

We fabricated terahertz (THz) wave emitters from high-temperature superconductor Bi2Sr2CaCu2O8+δ (Bi2212) single crystals annealed under oxygen gas (O2) flow and nitrogen gas (N2) flow conditions. To better understand the annealing effects of the crystal for the device, we evaluated both device properties and a c-axis lattice constant using x-ray diffraction. Compared to the N2-annealed sample, the O2-annealed sample shows higher critical current in the current–voltage characteristics and no clear emission. In addition, multiple hysteresis loops were observed above 75 K. Based on the x-ray diffraction measurements, it is suggested that the presence of multiple hysteresis loops observed in the I–V characteristics of the O2-annealed sample is caused by the existence of layers that have varying levels of oxygen content along the c-axis direction of the crystal. The formation of these layers is attributed to the deposition process of metallic thin films during the device fabrication procedure. This result indicates that the Bi2212 crystal surface of the O2-annealed sample is more sensitive than that of the N2-annealed one. The information is useful for preparing the Bi2212 crystals for THz-wave emitting devices.

Kihlstrom K.J., Reddy K.C., Elghazoly S., Sharma T.E., Koshelev A.E., Welp U., Hao Y., Divan R., Tsujimoto M., Kadowaki K., Kwok W.-., Benseman T.M.

Mesa-shaped structures of the high critical temperature (high-${T}_{c}$) superconductor ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ contain stacked intrinsic Josephson junctions. As such, they are a promising source of coherent radiation in the ``terahertz gap'' range, spanning from approximately 0.3 to 2.0 THz. Technological applications of these devices become far more practical if they can be operated at a cryogenic bath temperature of 77 K or higher. Previous works have reported emission from this type of device at high terahertz power levels at lower operating temperatures, 40--60 K, while at ${T}_{\mathrm{bath}}\ensuremath{\ge}77\phantom{\rule{0.25em}{0ex}}\mathrm{K}$ observed power levels have generally been low. Here we report generation of 130 \ensuremath{\mu}W of coherent power at 0.456 THz from a mesa of ${\mathrm{Bi}}_{2}{\mathrm{Sr}}_{2}{\mathrm{Ca}\mathrm{Cu}}_{2}{\mathrm{O}}_{8+\ensuremath{\delta}}$ doped with 0.16 holes per $\mathrm{Cu}$ atom, at a bath temperature of 77.4 K. We find that the device radiates terahertz power when clearly identifiable cavity modes are excited, and that the frequency and bias voltage corresponding to each of these modes is almost independent of $\mathrm{temperature}$. This is consistent with these modes having terahertz-frequency electric fields with very little dependence on vertical position within the mesa. We also find that the terahertz power radiated from any given mode decreases monotonically as the mesa temperature is increased. On the other hand, the low-frequency modes become inaccessible at low temperatures due to retrapping of the intrinsic Josephson junctions, and the maximum radiation power for the emitting mode is typically achieved at the temperature at which the retrapping voltage reaches the resonance voltage for this mode.

Sun H., Chen S., Wang Y., Sun G., Chen J., Hatano T., Koshelets V.P., Koelle D., Kleiner R., Wang H., Wu P.

A compact cryogenic terahertz emitter is highly desired for applications in terahertz astronomy with a broad frequency range of emissions and relatively high emission power. In this paper, we report on a terahertz emitter based on Bi2Sr2CaCu2O8 + δ (BSCCO) intrinsic Josephson junctions, with a frequency range from 0.15 to 1.01 THz. The emitter is a square gold-BSCCO-gold mesa on a sapphire substrate fabricated by a simple and efficient method. The highest emission power of 5.62 μW at 0.35 THz was observed at 50 K. A record low emission frequency of 0.15 THz was achieved at 85 K, extending the applicability of BSCCO terahertz emitters in the low-frequency range.

Koyama Y., Kitazawa Y., Yukimasa K., Uchida T., Yoshioka T., Fujimoto K., Sato T., Iba J., Sakurai K., Ichikawa T.

We propose a compact, high-power, and high-directivity surface-emitting terahertz (THz) source based on an array of active antennas with integrated patch antennas and resonant-tunneling diodes (RTDs). An array configuration of active antennas, each with an integrated patch antenna and two RTDs and coupled by microstrip lines, enables spatial power combining and improves directivity through coherent oscillation. We confirmed a maximum radiation power of 11.8 mW in a prototype 6 × 6 array at an oscillation frequency of 0.45 THz. Parasitic oscillation at low frequencies was suppressed by use of a bias stabilization circuit consisting of series-connected resistors and capacitors, and the dc to RF efficiency of this device was estimated to be approximately 1%. The radiant intensity of 210 mW/sr and the 3-dB beamwidth of 13° for the measured 6 × 6 array confirmed that directivity was improved by coherent oscillation based on mutual injection locking. The directivity of the patch antenna capable of surface emission can be controlled only by the number of antennas, even if there is no secondary radiator, such as an Si lens. The obtained results suggest that RTDs are promising as practical THz sources for realizing applications of THz imaging and 6G communication.

Krasnov M.M., Novikova N.D., Cattaneo R., Kalenyuk A.A., Krasnov V.M.

Impedance matching and heat management are important factors influencing the performance of terahertz sources. In this work we analyze thermal and radiative properties of such devices based on mesa structures of a layered high-temperature superconductor Bi2Sr2CaCu2O8+δ. Two types of devices are considered containing either a conventional large single crystal or a whisker. We perform numerical simulations for various geometrical configurations and parameters and make a comparison with experimental data for the two types of devices. It is demonstrated that the structure and the geometry of both the superconductor and the electrodes play important roles. In crystal-based devices an overlap between the crystal and the electrode leads to appearance of a large parasitic capacitance, which shunts terahertz emission and prevents impedance matching with open space. The overlap is avoided in whisker-based devices. Furthermore, the whisker and the electrodes form a turnstile (crossed-dipole) antenna facilitating good impedance matching. This leads to more than an order of magnitude enhancement of the radiation power efficiency in whisker-based, compared to crystal-based, devices. These results are in good agreement with presented experimental data.

Cattaneo R., Borodianskyi E.A., Kalenyuk A.A., Krasnov V.M.

Low power efficiency is one of the main problems of THz sources, colloquially known as the THz gap. In this work we present prototypes of THz devices based on whisker-crystals of a hightemperature superconductor Bi2Sr2CaCu2O8+d with a record high radiation power efficiency of 12% at a frequency of 4 THz. We employ various on- and off-chip detection techniques and, in particular, use the radiative cooling phenomenon for accurate evaluation of the emission power. We argue that such devices can be used for creation of tunable, monochromatic, continuous-wave, compact and power-efficient THz sources.

Imai T., Kashiwagi T., Nakagawa S., Nakayama M., Kim J., Kuwano G., Tsujimoto M., Minami H., Kadowaki K.

Okada K., Serita K., Cassar Q., Murakami H., MacGrogan G., Guillet J., Mounaix P., Tonouchi M.

Abstract Imaging with terahertz (THz) waves has been expected as a non-invasive/non-staining visualization tool for breast cancer margins during surgeries. Breast cancer is a generic name for a heterogeneous lesion comprising invasive adenocarcinoma, in situ adenocarcinoma, most frequently in the form of ductal carcinoma in situ (DCIS) and benign tissues. Until now, THz imaging has focused on invasive adenocarcinoma; however, THz analysis of DCIS has hardly been performed. One of the reasons is that the size of an individual DCIS lesion, ranging from 50 to 500 µm, is typically much smaller than that of an invasive carcinoma. This makes it difficult to identify these lesions by THz imaging, which has only a diffraction-limited spatial resolution of several millimeters. To overcome this drawback, we have developed a scanning point terahertz source (SPoTS) microscope with a resolution of 20 µm, in which a near-infrared-pump-laser-induced two-dimensionally-scannable point THz source (φ THz ≈ φ Pump) generated in a GaAs crystal contacts a sample. In this study, utilizing this state-of-the-art microscope, we mainly performed THz near-field transmission imaging of a paraffin-embedded human breast cancer sample containing invasive carcinoma and DCIS, as a preliminary study. Consequently, for the first time, we succeeded in clearly visualizing a DCIS lesion of ∼φ500 µm in the THz images. It was also found that the THz attenuation by DCIS was higher than that by invasive ductal carcinoma. Furthermore, also in a reflection-mode measurement, we successfully obtained a similar outcome to the above transmission-mode one. These results can be caused by the interaction between the THz waves and the cellular density, indicating that SPoTS microscopy may be suitable for DCIS diagnosis.

Delfanazari K., Klemm R.A., Joyce H.J., Ritchie D.A., Kadowaki K.

Current compact emitter and receiver technologies are generally inefficient and impractical at terahertz (THz) frequencies between 0.1 and 10 THz. Hence, a gap exists between mature microwave and developed optical technologies. On-chip, integrated broadly tunable and powerful quantum sources that coherently radiate THz waves between 0.1 and 11 THz (potentially extendable to 15 THz) and with potential output power of >1 mW can be achieved based on quantum tunneling of electron pairs across the stack of intrinsic Josephson junctions (IJJs) naturally present in a single crystal of the layered high-T c superconducting Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO). Such devices have been found to be especially promising solid-state THz sources capable of bridging the entire THz gap, as their wide-frequency tunability range is superior to that obtained from their semiconducting-based rivals, either single resonant-tunneling diodes (RTDs) or THz-quantum cascade lasers (QCLs). Due to the unique electrodynamics of BSCCO, they can also be operated as switching current detectors, paving the way for the realization of on-chip THz-integrated circuits for applications in ultrahigh-speed telecommunications, quantum information, on-chip spectroscopy, and nondestructive sensing, testing, and imaging. This article reviews the history and recent advances in THz sources and detectors based on IJJs with a focus on the application of IJJ THz devices in THz spectroscopy and various types of THz imaging systems such as reflection, transmission, and computed tomography. We show that compact IJJ THz devices with sub-centimeter-sized modules are easy to use in many applications, as they can be regarded as pocket quantum THz torches.

Kleiner R., Wang H.

Electromagnetic radiation at frequencies in the terahertz (THz) range is highly interesting for potential applications in biology or medical diagnostics, high-bandwidth communication, security and defence, nondestructive evaluation, and other fields. However, particularly, the frequency range between 0.5 and 1.5 THz has turned out to be difficult to fill with powerful coherent solid-state sources. Suitably patterned structures made of the high critical temperature superconductor Bi2Sr2CaCu2O8+δ can operate in this frequency range, and the best devices presently reach an output power around 100μW. In this tutorial, we will give an introduction into the basic physics of this type of oscillator and into the challenges that appear. We will explain how the generators are fabricated, which measurement techniques are used for characterization, and which models are used to describe their dynamics. We will also give some examples demonstrating potential applications in the fields of THz imaging and THz spectroscopy.

Kinev N.V., Rudakov K.I., Filippenko L.V., Baryshev A.M., Koshelets V.P.

The flux-flow oscillator (FFO) based on a long Josephson junction has been implemented as a broadband tunable terahertz (THz) source to open space. For this purpose, the transmitting slot antenna has been coupled to the oscillator. Additionally, an elliptical lens with a diameter of 10 mm has been matched to the antenna, forming a narrow output beam of the THz emission. Two designs for the antenna, integrated with the oscillator and developed for operation at different frequency ranges of 0.32–0.55 THz and 0.4–0.7 THz, have been investigated. The FFO has been phase locked to an external reference oscillator by utilizing a harmonic mixer. Its linewidth in the phase-locking regime is determined by the phase noise of the reference oscillator and the number of harmonics used and has been measured to be less than 0.1 MHz. A free-running FFO linewidth from about 2 MHz to several MHz, depending on the operating point, has been obtained. Output emission to open space has been measured by a superconducting integrated spectrometer located in a separate cryostat. The FFO operation as an external source with the achieved emission power and spectral characteristics has demonstrated its applicability for different tasks and purposes where tunable THz sources are required.

Kasagi K., Suzuki S., Asada M.

We proposed and fabricated large-scale arrays of resonant tunneling diode (RTD) oscillators for high-output-power terahertz (THz) sources. The array element is composed of an RTD, a slot resonator, and a dipole array antenna on a dielectric layer stacked on the RTD. In this structure, the output power is radiated in the upward direction of the substrate without a hemispherical silicon lens. The dipole array antenna was designed so that the average output power determined by the variation in the size of the RTD in the array was maximized. The experimental output power was proportional to the element number, and its value was 0.73 mW for an 89-element array at ∼1 THz in a pulsed mode with a repetition rate of 300 Hz and a duty ratio of 10%. Multiple peaks were observed in the oscillation spectra, because the elements were not intentionally coupled with each other. The average output power per element was 9 μW in the array, which was lower than that of the separated single oscillators (21 μW). Possible causes of this difference are discussed.We proposed and fabricated large-scale arrays of resonant tunneling diode (RTD) oscillators for high-output-power terahertz (THz) sources. The array element is composed of an RTD, a slot resonator, and a dipole array antenna on a dielectric layer stacked on the RTD. In this structure, the output power is radiated in the upward direction of the substrate without a hemispherical silicon lens. The dipole array antenna was designed so that the average output power determined by the variation in the size of the RTD in the array was maximized. The experimental output power was proportional to the element number, and its value was 0.73 mW for an 89-element array at ∼1 THz in a pulsed mode with a repetition rate of 300 Hz and a duty ratio of 10%. Multiple peaks were observed in the oscillation spectra, because the elements were not intentionally coupled with each other. The average output power per element was 9 μW in the array, which was lower than that of the separated single oscillators (21 μW). Possible cause...

Shibano Y., Kashiwagi T., Komori Y., Sakamoto K., Tanabe Y., Yamamoto T., Minami H., Klemm R.A., Kadowaki K.

We studied the etching of small crystals of the high-Tc superconductor Bi2Sr2CaCu2O8+δ (Bi2212) with various dilute compositions of hydrochloric and nitric acids. A particular mixture of those acids was chosen to simultaneously fabricate multiple rectangular stand-alone Bi2212 mesa structures from a large, doubly-cleaved and doubly metallic-coated single crystal. The radiation characteristics of these devices were found to be very similar to stand-alone devices fabricated previously using dry-etching techniques. The greatly reduced time and cost of fabrication of stand-alone Bi2212 devices using our wet-etching technique should facilitate the mass production of a large number of identical stand-alone devices from a large single-crystalline Bi2212 substrate.