Generation and manipulation of multi-cycle terahertz pulses via optical rectification in poled lithium niobate

摘要

We demonstrate control of terahertz (THz) waves developing novel devices in the THz regime: THz pulse shapers. THz technology is a relatively unexplored subject, yet the importance of THz wave manipulation cannot be emphasized enough considering its potential application to THz imaging systems, ultrafast optical signal processing, ultrahigh-speed computing, quantum information science, nanotechnology, and chemical reaction dynamics among other areas. THz time-domain spectroscopy (THz-TDS) can assess the performance of the THz pulse shapers monitoring time-dependent THz wave propagation. THz-TDS permits precise measurements not only of the amplitude but also of the phase of THz waves, thus a comprehensive assessment of the THz devices can be achieved. The phase sensitivity is also vital to many applications such as high-contrast THz imaging and quantum control of semiconductor nanostructures. We develop arbitrary THz pulse generators synthesizing THz waveforms via optical rectification in pre-engineered domain structures of poled nonlinear crystals using femtosecond lasers. The terahertz waveforms coincide with the crystal domain structures. The one dimensional nonlinear wave equation simulates the experimental results with a good qualitative agreement. The ratio of the domain length to the optical pulse length in the crystal turns out to be the crucial limiting factor to generating optimum terahertz fields and preventing waveform distortion. Optical pulse shaping techniques is integrated into the THz pulse generators to extend the scope of THz pulse shaping control. Continuously tunable narrowband THz pulses are generated in a fanned-out periodically-poled lithium niobate crystal. We measure the free induction decay of rotational transitions in gas-phase HCl molecules using the narrow-band THz pulses. The shape of the multi-cycle THz pulses is controlled by adjusting the relative time delay and intensity between the two optical pulses.