Broadly Tunable Subterahertz Emission from Internal Branches of the Current-Voltage Characteristics of Superconducting ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ Single Crystals

Continuous, coherent subterahertz radiation arises when a dc voltage is applied across a stack of the many intrinsic Josephson junctions in a ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{CaCu}}_2{\mathrm{O}}_{8+\delta }$ single crystal. The active junctions produce an equal number of $I\mathrm{\text{−}}V$ characteristic branches. Each branch radiates at a slightly tunable frequency obeying the Josephson relation. The overall output is broadly tunable and nearly independent of heating effects and internal cavity frequencies. Amplification by a surrounding external cavity to allow for the development of a useful high-power source is proposed.

Figure 1

$R1$ structure, $c$-axis resistivity, outermost-branch IVC, emission output, and frequency spectra. (a) Scanning ion microscope image. (b) Atomic force microscope image along $A\mathrm{\text{−}}B$ in (a). (c) Outer-branch IVC. Inset: $R_c(T)$. (d) Plot of $I(V_{\mathrm{out}})$ corresponding to (c). Open circles: $T_{\mathrm{top}}=T_{\mathrm{onset}}=84.4\mathrm{K}$. (e) Spectral intensities from points $A–E$ in (d).

Figure 2

Emission from IVC points at the color-coded frequencies was observed at the filled diamonds. No radiation was detected at the open diamonds. Arrows indicate the numbers $N_{\mathrm{fit}}$ of resistive junctions from fits to the Josephson relation. (a) $R1$. (b) $R2$. Insets: Full IVCs.

Figure 3

Emission frequencies $f$ and intensities from $R1$, and $f$ versus resistive junction numbers $N_{\mathrm{fit}}$ for $R1$ and $R2$. (a) $f(V)$ plots for 11 $R1$ IVC branches. Dashed lines: Fits to $f=f_J=(2e/h)V/N_{\mathrm{fit}}$. Inset: $f(V/N_{\mathrm{fit}})$ plot for all $R1$ data. Dotted line: $f=f_J$. (b),(c) $f(N_{\mathrm{fit}})$ ranges for $R1$ and $R2$, with dotted eye guides. (d) Peak intensity on a logarithmic scale versus $f$ for $R1$. Calculated $f_{m,p}^c$ ranges are given at the top. $N_{\mathrm{fit}}$ values are color-coded.

Figure 4

Emission frequencies and intensities from $R2$. (a) Plots of $f(V)$ for 8 branches in the $R2$ IVCs. Dashed lines: Fits to $f=f_J=(2e/h)V/N_{\mathrm{fit}}$. Inset: Plot of $f(V/N_{\mathrm{fit}})$ for all $R2$ data. Dotted line: $f=f_J$. (b) Peak intensity on a logarithmic scale versus $f$ for $R2$. Calculated $f_{m,p}^c$ ranges are given at the top. $N_{\mathrm{fit}}$ values are color-coded.