Mesa-Sidewall Effect on Coherent Terahertz Radiation via Spontaneous Synchronization of Intrinsic Josephson Junctions in ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$

We investigate the effect of the ${\mathrm{Bi}}_2{\mathrm{Sr}}_2{\mathrm{Ca}\mathrm{Cu}}_2{\mathrm{O}}_{8+\delta }$ mesa sidewall on the spontaneous synchronization of intrinsic Josephson junctions and concomitant terahertz radiation. We develop a lithography technique to control the sidewall angles. Using a Fourier interferometer, we characterize the radiation emitted from a series of mesas with different sidewall angles. We determine that a uniform distribution of the oscillation frequencies is essential to produce coherent radiation outside the mesa. It is also noted that in the resonant state, the input excess energy is transferred to the excitation of the coherent state.

Figure 1

(a) SEM image of two mesas fabricated on an identical $\mathrm{Bi}$-2212 crystal base. (b) Cross-section profiles for mesas A and B measured by atomic force microscopy. (c) Temperature dependence of $c$-axis resistance.

Figure 2

(a) Current-voltage characteristics and (b) lock-in signals from a silicon-composite bolometer at $T_b=40.0\mathrm{K}$. Dashed arrows indicate the current-sweep direction. The inset shows the Fourier-transform spectrum measured at the highest peaks of the bolometer output, indicated by arrows in (a),(b).

Figure 3

$T_b$ dependence of IVCs with color-coded bolometer output for mesa A. $T_b$ is stabilized in the range from 20.0 to 50.0 K in steps of 5.0 K. The inset should $f$ tunability versus $T_b$.

Figure 4

Magnified views of returning IVC branches and detected radiation power on a color-coded scale for (a) mesa A and (b) mesa B. Dashed lines represent baselines of IVCs with the assumption that input excess energy is transferred to excitation of the resonant state.