The properties of the seeded OPG source are attractive for THz generation. The seeded OPG has a narrow bandwidth and is seamlessly tunable. Using a single pump laser for two seeded OPGs provides two independently tunable sources. The output of these can then be mixed in a crystal such as DAST to generate tunable THz. Figure 1 shows the basic idea.
Figure 1. Narrow bandwidth THz generation via DFG in DAST. Two independently tunable seeded OPGs provide the two inputs to the DFG stage. Since both OPG's are pumped with the same laser, there is no timing jitter between the pulses in the DFG crsytal.
Since the two OPG's are independent, the difference frequency is limited only by the DFG crystal. Below in Figure 2 we show tunability from 1.5 to 5 THz.
Figure 2. Measurement of THz output using a DAST crystal. The sharp features are due to water vapor absorption.
In the THz region water vapor poses a significant challenge due to absorption. In Figure 2 above and Figure 3 below, the water features are narrow because the measurement was made at low pressure. At 1 atm these lines broaden further.
Figure 3. A smaller portion of Figure 2 showing the measured transmission and the calculated transmission based on Hitran.
In addition to the approach shown above, we also generate broad bandwidth THz by the optical rectification of fsec pulses. This technique allows us to measure the electric field of the THz pulse, see Figure 4.
Figure 4. THz field measured using electro-optic sampling in ZnTe.
The Fourier transform of the field gives the THz spectrum as shown in Figure 5.
Figure 5. Spectrum of field shown in Figure 4. Note the same water lines measured in Figures 2 and 3 show up here as well.