Harmonic frequency mixing in Bi2Sr2CaCu2O8+X intrinsic Josephson junctions

In a c-axis junction stack $(\mathrm{10 \mu m\times 10 \mu m}$ in $\mathrm{a-b}$ plane) patterned on a ${\mathrm{Bi}}_{\mathrm{2}}{\mathrm{Sr}}_{\mathrm{2}}{\mathrm{CaCu}}_{\mathrm{2}}{\mathrm{O}}_{\text{8+X}}$ single crystal, we performed harmonic mixings between a 100 GHz signal and up to the 98th harmonic of a local oscillator at about 1 GHz. The dependence of $P_{\mathrm{IF}}$ on $\mathrm{V,}$ where $P_{\mathrm{IF}}$ is the mixing output at the intermediate frequency and $V$ is the dc voltage across the junction stack, was strongly affected by the local oscillator power levels. For low values of local oscillator power, the optimum operation point where $P_{\mathrm{IF}}$ became maximum was around zero bias, and typical multi-branch structure of $P_{\mathrm{IF}}$ vs $V$ curves were observed. For high values of local oscillator power, the optimum operation point moved to the high-voltage region. In both cases, interesting oscillations occurred on the $P_{\mathrm{IF}}$ vs $V$ curves. Our experimental results led us to believe that intrinsic Josephson junctions can be good candidates for high frequency applications and the harmonic mixing may be a useful probe to investigate plasma phenomena in the layered superconductors.