Terahertz wave generation from laser induced air plasma has a close relevance with the applied external electric field. In this paper, by applying a high-voltage DC electric filed onto the laser induced air plasma, we demonstrated that the amplitude of generated terahertz wave increases linearly with the external electric field strength, the terahertz amplitude and power are quadratic in the lower power region and the terahertz amplitude is linear with the pump light power in the high power region. Moreover, the polarization direction of the generated terahertz wave changes synchronously with the direction of applied external electric field. The results proved the effect of external electric field on the power and polarization of terahertz wave generation from laser induced air plasma.
The methods of terahertz (THz) wave generation has attracted considerable attention in recent years because it involves a complex plasma process. The theories used to explain the intrinsic mechanism of laser ionization of air to produce THz wave are not particularly numerous, further exploration of its mechanism in the air is necessary. We report the THz wave generation from single color scheme modulated by pre-ionized air plasma via an orthogonal pumping geometry. It is found that the amplitude of the THz signal generated by the pump beam tends to decrease gradually with the increase of the modulation power. We believe that the ponderomotive force plays an important role in the process of the interaction between the pump beam and pre-ionization beam. The hydrostatic state of the electrostatic separation field caused by modulation beam will directly affect the generation efficiency of the THz wave. Our results contribute to further understanding of the theoretical mechanism and expanding of the practical applications of THz wave generation and modulation.
Terahertz wave generation in a filament pumped by a two-color femtosecond laser has been widely used as a source for nonlinear terahertz spectroscopy. Various optical lasers and gases were explored to further optimize the source with broader radiation bandwidth and higher output pulse energy, whereas few attempts were made for this purpose with longer pump wavelengths. We examine the optical excitation-wavelength scaling behavior of terahertz waves generated from two-color laser filament in air, with particular focus on the terahertz wave frequency bandwidth and polarization. Terahertz wave generation from the photocurrent surge driven by lasers with relatively longer wavelengths yields the shift to higher central frequency with rapid increase in the output energy with increasing pump power, which is theoretically validated by considering the optical wavelength-dependent ionization rate. Furthermore, we find the terahertz polarization undergoes a continuous rotation as a function of the laser wavelength. Our results contribute to the understanding of terahertz wave generation in a femtosecond laser filament and suggest a practical way to control the polarization of terahertz pulses for potential applications.
Terahertz wave generation from laser induced air plasma is widely used due to its high electric field and broad frequency bandwidth. For further understanding of the mechanism of the terahertz wave generation in laser-induced plasma as well as the terahertz modulator base on pre-formed air plasma, the generation of terahertz radiation using an effective wavelength scaling mechanism is examined when two-color laser fields are mixed in pre-formed plasma created by synchronized 800nm laser pulse. In our experiment, the effect of preformed plasma is investigated using an orthogonal pumping geometry. With a preformed plasma, both the modulation depth of terahertz radiation energy and the change of terahertz radiation polarization increases with increasing excitation laser wavelength. We found that the terahertz modulation depth and terahertz polarization changes increase as a function of the energy of the 800nm-prepulse. Some possible reasons are discussed. We attribute the terahertz polarization rotation to additional relative phase of the two-color fields introduced by the preformed air plasma. This provides a practical way to control the polarization and energy of terahertz pulses for potential applications.
Terahertz (THz) wave generation from laser induced air plasma is widely used due to its high electric field and broad frequency bandwidth. The most popular and efficient laser-plasma scheme used for THz generation is the two-color scheme, in which a laser pulse at the fundamental frequency is supplemented by its second harmonic that is obtained with the use of a nonlinear crystal. The type-I β-barium borate (BBO) crystal plays a very important role in second harmonic generation. In this research, we investigate the THz generation efficiency with changing the thicknesses of the BBO crystals. Moreover, the wavelength of the excitation laser is tunable from 1200 nm to 1600 nm. The THz generation efficiency is characterized by rotating the BBO crystal with the same tilt angle, changing laser wavelength with invariant pump power. And we also record the two orthogonal components of THz electric field by rotating the ZnTe crystal. We think that the thickness of BBO crystal affects the phase difference between the two components along the ordinary axis and extraordinary axis, resulting in the change of the polarization state of the fundamental wave. Meanwhile, the frequency doubling efficiency of BBO has an impact on the power ratio of the two-color laser. This provides a practical way to control the polarization of THz pulses for potential applications.
Molecular imaging techniques are becoming increasingly important in biomedical research and potentially in clinical practice. We present a continuous-terahertz (THz)-wave molecular imaging system for biomedical applications, in which an infrared (IR) laser is integrated into a 0.2-THz reflection-mode continuous-THz-wave imaging system to induce surface plasmon polaritons on the nanoparticles and further improve the intensity of the reflected signal from the water around the nanoparticles. A strong and rapid increment of the reflected THz signal in the nanoparticle solution upon the IR laser irradiation is demonstrated, using either gold or silver nanoparticles. This low-cost, simple, and stable continuous-THz-wave molecular imaging system is suitable for miniaturization and practical imaging applications; in particular, it shows great promise for cancer diagnosis and nanoparticle drug-delivery monitoring.
Intense Terahertz waves generated from air-induced plasma and serving as broadband THz source provide a promising broadband source for innovative technology. Terahertz generation in selected gases has attracted more and more researchers’ interests in recent years. In this research, the THz emission from different atoms is described, such as nitrogen, argon and helium in Michelson. The THz radiation is detected by a Golay Cell equipped with a 6-mm-diameter diamond-inputting window. It can be seen in the first time that when the pump power lies at a stable level, the THz generation created by the femtosecond laser focusing on the nitrogen is higher than which focusing on the helium, and lower than that produced in the argon gas environment. We believe that the THz intensity is Ar > N > Ne because of its atomic mass, which is Ar > N > Ne as well. It is clear that the Gas molecular decides the release of free electrons ionized from ultra short femtosecond laser through the electronic dynamic analysis. The higher the gas mass is, the stronger the terahertz emission will be. We further explore the THz emission at the different laser power levels, and the experimental results can be commendably quadratic fitted. It can be inferred that THz emission under different gas medium environment still complies with the law of four-wave mixing (FWM) process and has nothing to do with the gas environment: the radiation energy is proportional to the quadratic of incident laser power.
We report intense (~10 mW), ultra-broadband (~150 THz wide), terahertz-to-infrared, Gaussian-wavefront emission from nanopore-structured metallic thin films under femtosecond laser pulse irradiation. The proposed underlying mechanism is thermal radiation. The nanostructures of the metal film are produced by random holes in the substrate. Under pulse-train femtosecond laser irradiation, we found dramatically enhanced optical absorption, with an absorptivity that was equal to as much as 95% of the metallic surface nanostructure, due to both an antireflection mechanism and dissipation of excited surface plasmon polaritons into the metal surface.
Terahertz wave generation from air plasma induced by ultrashort laser pulses has been widely studied in the past decade. We report the study of terahertz wave generation from the laser induced plasma where there is a preformed air plasma. We found that the power of the terahertz wave generated by the main pump pulse decreases in the presence of the preformed plasma. The amount of the power drop increases with the power of pulse that generates preformed plasma. The result confirms the key role of tunneling ionization in the terahertz generation mechanism.
THz wave generation in laser-included plasma has attracted considerable attention recently and the intense THz waves generated from air-induced plasma, serving as broadband THz source for sensing and imaging applications, has attracted more and more researchers’ interests in recent years. In our experiment, terahertz is detected using THz Air-Biased Coherent Detection (THz-ABCD) method. This method can achieve the third-order nonlinear susceptibility tensor to produce field-induced optical second harmonic photons. In the THz-ABCD system, red-shift is observed in frequency spectra with enhanced pump power and decreased bandwidth. After changing the probe power, the red-shift in frequency spectral can also observed with enhanced probe power, but the bandwidth is broadening as the prober power increasing. We further explore of these phenomena through intense self-phase modulation of the optical pulse in the plasma and the collision behavior. This study reveals that we can control THz intensity and bandwidth by changing pump power and probe power in the ABCD system.
Terahertz (THz) emission from laser-induced air plasma is a well known and widely used phenomenon. We report that when two laser beams from the laser creating two plasma filaments interact with each other, THz absorption is observed. We believe that a change in the refractive index of the plasma causes the THz-wave absorption. The following experimental results reveal that the THz absorption becomes more pronounced with increasing pump power and that the gas species surrounding the femtosecond laser filament can also influence the THz absorption rate.
Terahertz wave which can provide innovative sensing and imaging techniques can obtain spectroscopic
information unavailable at other wavelengths. The terahertz air-biased-coherent-detection (ABCD) method can achieve
the third-order nonlinear susceptibility tensor to produce field-induced optical second harmonic photons. Therefore, the intense terahertz wave generated and detected by the laser-induced air plasma provides a promising ultra-broadband terahertz source and sensor for spectroscopy and imaging technique. Aiming at that purpose, an understanding of the
frequency spectrum characterization of terahertz pulse is crucial. In this work, we investigated the variation of the THz
pulse bandwidth measured through the third harmonic generation using the coherent detection scheme, by increasing the
optical probe pulse power and biased electric field. A bandwidth broadening of the measured THz pulse is observed by
increasing either the probe pulse power or the bias voltage strength. We speculate that a pulse shape change of the probe beam and a saturation effect during the second-harmonic generation might cause the bandwidth broaden with probe
power. To further investigate the mechanism, we fixed the power of probe laser at 150mW and changed the bias voltage.
The results show that the frequency spectrum width becomes wider gradually with the increasing of the bias voltage. A theoretical explaination shows that the bandwidth broadening with bias field might be introduced by a pulse shape
change of the bias field induced second harmonic wave. This study reveals that we can control THz intensity and
bandwidth by changing probe power and bias voltage in the ABCD system.