Alexei Koshelev
Physicist Alexei Koshelev (MSD) will discuss his Laboratory-Directed Research and Development (LDRD) sponsored work at the LDRD Seminar Series presentation Tuesday, Feb. 14, 2017.
“Terahertz Devices from Intrinsic Josephson Junctions in Layered Superconductors” will begin at 12:30 p.m. in the Bldg. 203 Auditorium. All are welcome to attend.
Abstract
Electromagnetic waves in the terahertz (THz) frequency range have many potential applications in spectroscopy, medical diagnostics, high-bandwidth communications, security and nondestructive evaluation. Compact solid-state sources which generate powerful continuous-wave coherent radiation in this frequency range are in high demand. Superconducting tunneling junctions are natural voltage-to-frequency converters. Due to the ac Josephson effect, voltage drop across the junction generates oscillating current whose frequency is proportional to the voltage with the universal coefficient 0.483 THz/mV. Layered high-temperature superconductors, such as Bi2Sr2CaCu2O8(Bi-2212), behave as stacks of atomic-size Josephson junctions. Most classical Josephson effects, such as Shapiro steps and Fiske resonances, have been demonstrated in these materials.
Stacks of intrinsic junctions may generate powerful and coherent electromagnetic radiation in the terahertz frequency range provided Josephson oscillations in all junctions are synchronized. A promising way to facilitate synchronization is to excite the cavity resonance mode inside the stack with the frequency set by its lateral size. Such resonant continuous-wave radiation of sizable power has been successfully extracted from mesas fabricated out of Bi-2212 crystals. The observed resonance frequencies are typically in the range 0.4 – 1 THz and scale inversely proportional to the mesa widths. I will discuss scientific issues relevant for excitation of electromagnetic waves by the intrinsic junctions such as mechanisms of coupling to the internal modes, structures and stability of coherent states, and mechanisms of damping. I will review most recent experimental developments including using synchronized mesa arrays, visualization of cavity modes and temperature distribution in the radiating mesas, and new fabrication techniques.
Layered superconductors also support the low-loss Josephson-plasma waves. These waves can be generated either by external electromagnetic waves or locally, by the Josephson oscillations. The possibility of efficient coupling of the Josephson-plasma waves with the free-space electromagnetic waves has been demonstrated recently by studying the transmission through thin Bi-2212 crystal with metallic gratings on the top and bottom. In addition, nonlinearity of the Josephson effect gives possibility of control and manipulation of the Josephson-plasma waves. Such unique electromagnetic properties of layered superconductors provide an attractive platform for designing of THz metamaterials and new ultra-high-frequency electronic devices. I will discuss possible implementations of such devices.
Most experiments so far have been performed using single crystals. Currently, we are growing Bi-2212 thin films using liquid-phase epitaxy. The availability of high-quality films will strongly facilitate fabrication of the intrinsic-junction devices and will enable their commercialization.
Biography
Alexei Koshelev is a scientist in the Materials Science Division. He received his Ph. D. in 1986 from the Landau Institute of Theoretical Physics and was working in the Institute of Solid State Physics (both are in Chernogolovka, Russia). At Argonne he is working on theory of superconductivity including vortex matter, intrinsic Josephson junctions and multiple-band effects.
