The complex transmission coefficient for millimeter and submillimeter waves incident on a ${\mathrm{Ba}}_{0.6}$${\mathrm{K}}_{0.4}$${\mathrm{BiO}}_3$ thin film (82 nm) has been measured over a frequency range of 200–1200 GHz at temperatures above and below ${\mathit{T}}_{\mathit{c}}$ using coherent time-domain spectroscopy. We observe a dramatic change in both the magnitude and phase of the terahertz transmission in the superconducting state caused by a rapid carrier condensation. Both the real (${\mathrm{\sigma }}_1$) and imaginary (${\mathrm{\sigma }}_2$) parts of the complex conductivity are determined directly from the amplitude and phase of the transmitted electric field without the need for a Kramers-Krönig analysis. By fitting ${\mathrm{\sigma }}_2$ in the framework of BCS theory, a superconducting gap 2Δ(0)=6.9 meV=3.8${\mathit{k}}_{\mathit{B}}$${\mathit{T}}_{\mathit{c}}$ is obtained. Below ${\mathit{T}}_{\mathit{c}}$, the ${\mathrm{\sigma }}_1$ is rapidly enhanced for ω/2π<500 GHz, which is attributed to the BCS coherence effects. However, the conductivity exhibits monotonic temperature dependence and no clear ${\mathrm{\sigma }}_1$(T) peak is observed throughout the frequency range measured. The high-frequency penetration depth (∼600 nm) is also extracted and discussed. Our results are consistent with a picture of BCS moderate coupling superconductivity in an intermediate to dirty limit.

• Received 29 December 1994

DOI:https://doi.org/10.1103/PhysRevB.52.3607