Qi-Ye Wen

The terahertz (THz) region of the electromagnetic spectrum, spanning the range between 0.1 THz and 10 THz,has experienced a renaissance due to technological developments in sources and detectors. The terahertz time domain spectroscopy (THz-TDS) system is by far one of the most important methods to generate and detect the THz wave. In physics, THz-TDS is a spectroscopic technique in which the properties of a material are probed with short pulses of terahertz radiation. The generation and detection schemes are sensitive to the sample material’s effect on both the amplitude and the phase of the terahertz radiation.THz-TDS has numerous applications in medical diagnose,homeland security, scientific imaging (chemistry,biochemistry, astronomy), communications, and manufacturing.1

In this special section, Dr. Wei-Wei Liu, a professor with the Institute of Modern Optics of Nankai University,was invited to introduce the state-of-art of THz-TDS technologies.

More recent advances towards cost-efficient and compact THz-TDS systems are based on mode-lockedfiber laserssources emitting at a center wavelength of 1550 nm.The first paper entitled “Polarization-Dependent Optimization of Fiber-Coupled Terahertz Time-Domain Spectroscopy System” studies the output performance of a fiber-coupled THz-TDS system by changing the polarization of the pump laser. The experimental result shows that not only the THz signal power but also the temporal waveform varies with the rotation of the exciting pulse polarization. The maximum output power of the emitter is obtained when the polarization of the pump pulse is perpendicular to the edge of the metal electrodes. This investigation results give a prompt that polarization optimization is one crucial factor needing to be taken into account in fabricating and applying a fiber-coupled THz-TDS system.

Due to the ability to measure the polarization state of THz radiation, two-dimensional THz-TDS have great applications in studying the anisotropic responses of materials, characterization, and identification of chemicals and biological materials, etc. Jia-Yu Zhao and Prof. Liu, in the second paper entitled “Two-Dimensional Terahertz Time-Domain Spectroscopy and Its Applications” review the developing progress of two-dimensional terahertz time-domain spectroscopy (THz-TDS) and its diverse applications, including analyzing the polarization of THz radiation from a laser-induced plasma source, studying the corresponding physical mechanism, and characterizing the optical properties of crystals. This review is timely and helpful to those who work in this area.

As the guest editor of this issue, I would like to express my sincere thanks to Prof. Wei-Wei Liu and his co-authors for their contributions to this special section. I also appreciate the assistance of the editorial staffs for their great efforts on making this special section to be published in time.