Control of circularly polarized THz wave from intrinsic Josephson junctions by local heating

Abstract

This paper reports a practical method of generating circularly polarized terahertz (THz) waves from intrinsic Josephson junctions (IJJs) and controlling their polarization states by external local heating. We theoretically find that a mesa-structure IJJ whose geometry is almost square can emit circularly polarized THz waves by local heating of the mesa. Moreover, we demonstrate that the polarization states of the THz waves change dramatically with the local heating position. Our results indicate that the use of local heating can provide a high level of controllability of the THz emissions and significantly extend the range of applications of IJJ-based THz emitters.Terahertz (THz) electromagnetic (EM) waves have potential applications in a wide range of fields, e.g. nondestructive inspection of materials, medical diagnosis, bio-sensing, and high-speed wireless communication [1]. Toward the realization of a practical THz emitter, various THz sources, such as quantum-cascade lasers and resonant-tunneling diodes, have been studied so far [2,3]. Since the first observation of intense and continuous THz wave emitted from a Bi 2 Sr 2 CaCu 2 O 8+δ (Bi2212) single crystal by Ozyuzer et al. [4], considerable attention has been paid to high-T c cuprate superconductors. A layered structure of Bi2212 single crystals composed of superconducting layers and insulating layers forms a stack of the Josephson junctions whose thicknesses are on the atomic scale ∼ 1.5 nm. Stacks of these natural Josephson junctions, which are referred to as intrinsic Josephson junctions (IJJs), can generate an AC Josephson current in the THz frequency range by application of a voltage, and the THz wave emission is attributed to the generation of this current. Intense THz emissions have been reported for IJJs fabricated in a mesa geometry, which itself behaves as a cavity resonator, and thus, a number of studies on such IJJ mesas have been carried out both experimentally [4][5][6][7][8][9][10][11][12][13][14][15] and theoretically [16][17][18][19][20][21][22][23][24][25][26]. However, in the previous studies, little attention was given to the state of the polarization of the THz waves. Control of the polarization is a fundamental issue for technological applications of THz waves. In particular, the ability to generate and control circularly polarized THz waves would open the way to various applications such as circular dichroism of proteins [27].In this paper, we present a method of generating and controlling circularly polarized THz waves from IJJ mesas by using external local heating. We consider a IJJ mesa whose geometry is almost square and whose temperature distribution is controlled by laser heating (see Fig. 1). We numerically investigate the emission power and the polarization of the THz waves from the mesa by solving the sine-Gordon and the Maxwell equations simultaneously. Interestingly, both the ellipticity and handedness of the THz waves from the IJJ mesa strongly depend on the position of the laser heating. Figure 1 shows a schematic fig...

Integrated, Portable, Tunable, and Coherent Terahertz Sources and Sensitive Detectors Based on Layered Superconductors

“…Controlling of the polarization is of especial importance in many applications of THz waves, for example in THz imaging, sensing, spectroscopy, high-speed and wireless telecommunication, etc [73][74][75][76]. The IJJ THz sources can be an excellent THz source for these purposes because (i) they have a unique ultra-wideband tunability and (ii) their emission intensity can be (internally and externally) thermally controlled [77][78][79][80].…”

“…IJJ THz devices with a rectangular cavity are known to emit linearly polarized light [26]. Asai et al first theoretically demonstrated that circular polarized (CP) THz waves can be emitted from IJJ THz emitters that are locally heated by an external heat source, such as a laser [79]. This is because, in a square cavity, the laser would induce an inhomogeneity in the critical current density, that results in the excitation of the EM cavity TM (1,0) and TM (0,1) modes, whose polarizations are orthogonal to each other, so CP can be generated by a mixture of those two excited modes.…”

Improved excitation mode selectivity of high-Tc superconducting terahertz emitters

“…The IJJ-THz emitters based upon superconducting Bi2212 single crystals have intensively been developed both experimentally and theoretically. [67][68][69][70][71][72][73][74][75][76][77][78][79][80][81][82][83][84][85][86] So far, the best performance has been achieved within the frequency range from 0.3 to 11 THz, [11][12][13]34,35,39 with the maximum output power of $30 lW, [31][32][33][34]39 and with a spectral width of the coherent and continuous emission of THz radiation that is generally less than 0.5 GHz, and under appropriate conditions, it was reported to be narrower than 23 MHz and possibly even 6 MHz. [23][24][25] Furthermore, a synchronized operation of an array of three such emitters was reported to yield the greatly enhanced power of 610 lW.…”

Monolithic Superconducting Emitter of Tunable Circularly Polarized Terahertz Radiation

“…Waves emitted from rectangular IJJ-based sources have been known to be linearly polarized [2]. However, recent numerical studies [14,25,26] have proposed the possibility of realizing circularly polarized emissions from IJJ devices. This paper reports the first experimental demonstration of the generation of circularly polarized sub-THz waves from a monolithic source-a superconducting IJJ stack-wherein the ellipticity and evolution direction are tuned by mesa shape and bias conditions.…”

“…Compared to these devices, IJJ-based sources can cover wider frequency ranges, which are unattainable by other THz sources [4]. Furthermore, the emission intensity of IJJ-based devices can also be thermally controlled, both internally [12,13] and externally [14,15]. By applying a direct current (DC) voltage (𝑉 𝑏 ) across the c-axis of IJJs stacked in a mesa form, a high frequency (HF) current is generated along with emission of sub-terahertz radiation at the Josephson frequency given by 𝑓 𝐽 = (2𝑒𝑉 𝑏 /ℎ𝑁), where ℎ, 𝑒, and 𝑁 denote the Planck constant, elementary charge, and number of contributing IJJs, respectively.…”