Advances in ultrashort laser pulse technology have made it possible to exploit the terahertz (THz) frequency range of the electromagnetic spectrum between the microwave and infrared regions. THz radiation is proving useful to many areas of fundamental research in science, not least for its ability to nondestructively analyze a wide range of materials in detail. Combined with a variety of appropriate techniques, such as spectroscopy, imaging, and high-power sources, THz radiation could ultimately find application in homeland security, quality control of food and agricultural products, environmental monitoring, and communications technology.
One area of special research interest at present is high-field THz physics, which deals with phenomena arising from the interaction of high-power pulses with certain kinds of matter, such as electro-optical materials and semiconductors. These types of investigations usually require field strengths of a few kV/cm to MV/cm, which is below material damage thresholds. Traditional laser-based THz emitters, such as electro-optic crystals, semiconductors, and photoconductive antennas, are subject to low conversion efficiency and material breakdown when irradiated with high-power laser pulses. Consequently, finding high-field THz sources for these schemes is a major challenge.1 Plasma (heated gas), on the other hand, is impervious to the same breakdown. As such, it is naturally a potential high-power THz emitter.
Figure 1. Schematic plot of laser interaction with a nonuniform plasma and the resulting terahertz (THz) emission. n0: Plasma density distribution. L: Inhomogeneous plasma scale.
The key issue with plasma is to find appropriate ways of producing THz emission with high conversion efficiencies. The first experimental observation2 of laser-plasma-produced THz emission was reported in 1993. It was attributed qualitatively to laser-driven electron plasma oscillations. In principle, such oscillations and waves can be produced at a field amplitude as high as 1GV/cm,3 for example, at a plasma density of 1018cm−3, corresponding to a frequency of about 9THz. However, this type of plasma oscillation usually cannot emit electromagnetic waves, as the corresponding plasma current is cancelled by the displacement current. By the same token, it is well known that mode conversion between electron plasma waves and their electromagnetic counterparts can occur in plasma under certain conditions. We recently discovered several means of enabling high-field THz pulses.
First, when a laser pulse propagates obliquely through a tenuous (i.e., very low density) inhomogeneous plasma, THz emission with a frequency near that of the plasma is produced, provided the plasma density increases with distance along the laser propagation direction (see Figure 1).4 When a laser wakefield (a kind of electron plasma wave) is excited in such a nonuniform plasma, its wave vectors evolve with time and space. For given positions, the vectors even become zero, which enables the desired linear mode conversion. Tailoring the plasma density distribution makes it possible to produce broadband THz pulses and control their spectra.
Second, obliquely irradiating a thin, tenuous plasma slab with thickness comparable to the THz wavelength results in emission of two single-cycle THz pulses: one along the laser propagation and another in the specular ‘reflection’ direction5 (see Figure 2). This phenomenon is due to the transient net currents induced at the surface of the plasma vacuum. The central frequencies of the THz pulses can be controlled by either the plasma frequency or the laser pulse duration.
Figure 2. Schematic plot showing laser interaction with a thin plasma slab and the resulting single-cycle THz emission.
Finally, laser field ionization resulting from propagation of a short laser pulse in a gas target can also lead to THz emission. If the leading and trailing edges of the incident laser pulse are highly asymmetric, a high transverse net current is produced.6 Actually, in this case, the transverse current can be generated even in preionized plasma. This results in both forward and backward THz emission. The asymmetric pulse can be a superposition of a laser pulse and its second harmonic, or simply a frequency-chirped pulse. In the latter case, the THz field amplitude is proportional to the incident laser peak amplitude, as shown in Figure 3.
Figure 3. When a laser pulse either positively or negatively chirped interacts with tenuous gas or plasma targets (b), single-cycle THz pulses can also be produced with peak amplitudes that scale linearly with the incident laser amplitude (a). C: Chirping parameter. a0: Peak amplitude.
The speed of the quiver motion of the electrons in the laser fields of these sources approaches that of light in a vacuum, which is why they are known as ‘relativistic’ laser plasmas. They have the potential to deliver THz sources at peak power exceeding a megawatt and field strength exceeding 1MV/cm, using only a few terawatts of laser power. Yet, they can still be made compact (tabletop size) owing to chirped-pulse amplification laser technology. Experiments to test the schemes described here are currently under way.
Zheng-Ming Sheng, Jie Zhang
Department of Physics
Shanghai Jiao Tong University
Laboratory of Optical Physics
Institute of Physics
Chinese Academy of Sciences
Zheng-Ming Sheng is a laser-plasma physicist. His work focuses on the theory and simulation of relativistic laser-plasma interaction as a source of novel particle beams (and radiation sources) and for laser fusion. He is the author of about 150 papers in refereed journals.
Jie Zhang is an experimental laser-plasma physicist. His research focuses on laser-produced plasmas for x-ray lasers, fast electron beams, ion beams, ultrashort laser pulse propagation in air, and plasmas for applications in lidar (light detection and ranging) and laser fusion. He is the author of more than 200 papers in refereed journals. He is currently a professor and the president of Shanghai Jiao Tong University.
6. W.-M. Wang, Z.-M. Sheng, H.-C. Wu, M. Chen, C. Li, J. Zhang, K. Mima, Strong terahertz pulse generation by chirped laser pulses in tenuous gases, Opt. Express 16, pp. 16999, 2008.