Angular Dependence of the Radiation Power of a Josephson STAR-emitter

Abstract

We calculate the angular dependence of the power of stimulated terahertz amplified radiation (STAR) emitted from a dc voltage applied across a stack of intrinsic Josephson junctions. During coherent emission, we assume a spatially uniform ac Josephson current density in the stack acts as a surface electric current density antenna source, and the cavity features of the stack are contained in a magnetic surface current density source. A superconducting substrate acts as a perfect magnetic conductor with H ||,ac = 0 on its surface. The combined results agree very well with recent experimental observations. Existing Bi2Sr2CaCu2O 8+δ crystals atop perfect electric conductors could have Josephson STAR-emitter power in excess of 5 mW, acceptable for many device applications.PACS numbers: 07.57. Hm, 74.50.+r, 85.25.Cp At present, broad-band terahertz (THz) electromagnetic (EM) waves generated from femtosecond laser pulses and monochromatic THz waves generated by laser mixing, parametric resonance techniques, and quantum cascade lasers, etc., are the most common THz sources [1]. But these techniques are not cost effective in the "THz gap" region 0.1-10 THz required for many important applications. After many years of effort, by application of a static voltage across the intrinsic Josephson junctions in Bi 2 Sr 2 CaCu 2 O 8+δ (BSCCO) mesas, coherent radiation power of 0.5 µW was achieved [2]. The same technique on different samples since led to radiation power of 5 µW and an output efficiency of 3×10 −4 [3]. This is an exciting development, as the "THz gap" may soon be filled.In both experiments, the onset of the intense, coherent THz radiation occurs in or near to the region in the current-voltage (I − V ) characteristic of negative differential resistance (NDR), as for the Gunn effect in Ge [4]. In the absence of more precise information, we assume that the stack of Josephson junctions acts partly as a cavity, in order to amplify the coherent radiation at the fundamental Josephson angular frequency ω J and possibly its harmonics, and partly as a conductor with an ac Josephson current, J ac (x ′ , t) = J(x ′ ) ∞ n=1 a n sin(nω J t), where ω J = 2eV 0 /(Nh) is the Josephson angular frequency, V 0 is the dc voltage applied across the coherent stack of N ≈ 10 3 junctions, e is the electric charge,h is the Planck constant divided by 2π, and a n is the relative amplitude of the nth harmonic of the intrinsic ac Josephson current. The effective radiation sources at the mesa edges are respectively the surface magnetic current M S arising from the electric field in the cavity generated by the non-uniform part of J ac (x ′ , t) [6], and the surface electric current J S arising from the uniform part of J ac (x ′ , t), as sketched in Fig. 1 (a). These sources are obtained from the Faraday and Ampère boundary conditions, respectively [5,6], as used previously [7,8,9,10,11]. We first assume the mesa is suspended in vacuum. In Lorentz gauge, the vector potentials from the respective radiation sources are [5,6],ǫ, inside and outside ...

Three-dimensional numerical analysis of terahertz radiation emitted from intrinsic Josephson junctions with hot spots

“…This kind of "dual" internal modes has already been proposed in a previous theoretical study. 17 However, that study did not calculate the phase dynamics in the IJJs and did not confirm the existence of this mode by numerical calculations.…”

“…[17][18][19] For the interpretation of the experimental results, they assumed two types of radiation sources: the uniform part of ac Josephson current and the nonuniform part of ac Josephson current corresponding to cavity resonant modes. However, they did not calculate the phase dynamics in the IJJs.…”

Output from a Josephson stimulated terahertz amplified radiation emitter

“…The only practical ways to distinguish these models in rectangular mesas is by the observations of the angular dependence of the output power, polarization, and coherence fraction. As indicated previously [23], the antisymmetric cavity model necessarily leads to a maximum in the radiation intensity at θ = 0 it is important to study a different experimental configuration, in which more substantial differences between the output predictions of these two models would be clearly evident. It is also crucial to consider the effects of the superconducting substrate.…”

“…Thus, unlike other radiation sources studied to date, the THz radiation from BSCCO mesas can only be understood completely by this dual-source mechanism. A sketch of a cylindrical mesa with a uniform J S and a nonuniform M S corresponding to the (11) cylindrical cavity mode is shown in As long as rectangular mesas were the only radiation sources under study, it has proved difficult to distinguish the results of these radiation sources, as all of the observed harmonics can be amplified by the cavity features of the mesa [21,23]. The extreme non-linear current-voltage characteristic of each mesa allows for chaotic, nonequilibrium effects to be important, so complications such as soliton kinks [18,19] or layer-dependent kinks [9][10][11][12][13][14] could potentially play important roles in the origin of the radiation.…”

Emission of Terahertz Waves From Stacks of Intrinsic Josephson Junctions

“…The energy for the radiation is supplied by the Josephson oscillations. Several mechanisms of transferring the energy from the Josephson oscillations into the cavity resonance have recently been analyzed theoretically [17], [18], [23]- [26]. The BSCCO resonators used in the present study are mesas fabricated using Argon-ion milling and photolithography techniques from cleaved BSCCO crystals that were grown by a traveling floating zone method [27].…”