red out and by maintaining the amplitude and phase, the intermediate frequency (IF) remains at the output. This down conversion of the terahertz signal to the IF frequency is the coherent heterodyne detection. This detector can be employed for high sensitivity applications in cryogenic temperatures. 12.8.3 THz PCA To detect the terahertz signal, a photoconductive antenna is a good choice. The working principle of the THz PCA detector is the same as the PCA emitter except for the biasing. The incoming signal electric field creates a voltage which causes the carriers to accelerate. The photocurrent detected with a time delay of t can be explained using Equation 12.2, (12.2) IF signal (~GHz) Telescope THz signal Mixer IF amplifier BACK-END SPECTROMETER Local oscillator FIGURE 12.12 THz heterodyne detection technique. Terahertz Antenna in Future Healthcare Informatics 800 nm, 50 fs, Ti : Sapphire laser 1 KHz Delay Stage Lock in Amplifier detector emitter Balanced photodiode Sample FIGURE 12.13 Schematic diagram of experimental setup for THz generation and detection. The arrival time of the pulse is adjusted by the variable time delay and the THz photocurrent is measured using the ammeter. The photoconductive antenna on the detector side acts as a low pass filter. The experimental setup (THz time domain spectroscopy, THz-TDS) used for the generation and detection of THz-pulsed radi- ation is shown in Figure 12.13. In this setup, the photoconductive antenna is used as an emitter and also as a detector. The ultrashort laser source of 800 nm wavelength with 1 kHz repetition rate and pulse duration of 50fs from the Kerr-lens mode locked Ti: Sapphire laser amplifier is used as the optical input source. Parabolic mirrors are used to focus the generated terahertz beam into the sample. Then, another two parabolic mirrors are used to collimate and focus the detected beam on the detector. A delay is introduced between the optical pulse and terahertz pulse using a motorized delay stage (MDS). At the THz receiver, an optical prob