In the current work we have investigated the terahertz response of Ca over doped YBaCuO thin films using both time and frequency domain spectroscopy. We have used 5% and 10% Ca concentration and film thicknesses of 500-600 Å. The films were deposited by off-axis DC sputtering on LaAlO₃ substrates, and show a clear c-axis orientation. The phase transition measured with resistivity and inductance methods shows a typical T_C=85 K for the 5% Ca and T_C=77 K for the 10% Ca. THz measurements were performed with two methods: 1. Time domain THz spectroscopy (THz-TDS) using a TeraView system. 2. Frequency domain THz spectroscopy using a submillimeter spectrometer in the Mach-Zehnder arrangement.

For both methods a basic data analysis was performed using the two-fluid model and a variable dielectric function (VDF). The obtained imaginary part of the conductivity was proportional to 1/ω as known from the delta-function response. The real part of the conductivity showed well-known frequency and temperature dependence, where it increases below T_C and reaches maxima at about 50 K. However, a sharp decrease of the real part of the conductivity was observed at about 10 cm^-1. This decrease happens below T_C and becomes dominant as temperature decreases. This feature was observed in both time domain and frequency domain spectroscopy. It was observed on both the 5% and 10% Ca doped samples but it is more dominant on the 10% sample. Moreover, this sharp decrease in σ₁(ω) at 10 cm^-1 was not observed in optimally doped YBCO samples. We would like to emphasize at this point that these values are much smaller than those obtained by Microwave and Tunnelling measurements, arguing for the existence of a complex order parameter in the overdoped regime with an imaginary component of about 1.8 meV. In our presentation the difference between these two observations will be discussed.