The high-frequency waves-Terahertz (THz) emission from laser-induced air-plasma is presented. The development of high power and high efficiency THz wave radiation source has significant value. The development of laser technology has led spectroscopic sensing and imaging in the THz region to be widely employed in diverse fields such as material characterization, ultrafast dynamic process studies, and biological science. The emission of THz waves has become one of the most important areas in THz research. In this research, we develop an THz time-domain spectroscopy system(THz- TDS), which is that the input laser pulses are generated from a p-polarized regenerative Ti:sapphire amplifier with a 50- fs pulse duration centered at 800 nm. A 100-μm-thick type-I BBO crystal is employed for frequency doubling the pump pulses and subsequently generating the THz wave. By this setup we investigate that when the terahertz emission from only fundamental wave and with second-harmonic wave mixing, the intensity of the terahertz wave obtained is different. The energy of THz wave generated by only fundamental wave is weak, and that generated by four wave mixing is stronger. The energy intensity of THz wave generated by four wave mixing is much higher than that of THz wave generated by fundamental wave (almost an order of magnitude higher). We believe that the different ionization modes causes the different of terahertz wave energy intensity. In addition, we find that the second harmonic wave grows in a quadratic curve with the increase of pump energy, so we believe that when the pump energy increases, the terahertz wave also grows in a quadratic curve.
In wavefront coding optical system, with the traditional cubic mask plate (CMP) which owns several extension times of depth of focus (DOF), it is difficult to manufacture. With the symmetrical surface type mask plate which can be machine relatively easily, but it presents small multiples of the extension of DOF. In this paper, it makes a presentation of mask plate design by user defined surface(UDS) type where it has an easy mechanical process and several multiples of extension of DOF. It presents the analytic expression by calculation and its DLL data file which is adapted to ZEMAX. It also performs a simulation experiment based on the three mirror Cassegrain(TMC) wavefront system. The experiment results indicate that instead of the traditional mask plate, the UDS surface mask plate can obtain a larger ratio of extension of DOF and increase the valuable types of mask plate surfaces. What is more, it decreases the surface mechanical difficulty compared to the asymmetrical surface mask plate. The type of UDS surface has the unique design and the convenience manufacturing, which is of great value in both application and research.
With the majority needs of the various observation, such as the public security system including security and traffic monitoring system, we can use the optical system, which can not only satisfy the wide field observation, also meet the needs of monitoring. The continuous zoom optical system, which can realize the wide field of monitoring and remain off the accurate detection would meet the requirements.
Wavefront coding, a technique of optical-digital hybrid image, can be used to extend the depth of the field. However, it sacrifices the signal-to-noise ratio (SNR) of system at a certain degree, especially on focus situation. The on-focus modulation transfer function (MTF) of wavefront coding system is much lower than that of generally traditional optical system. And the noise will be amplified in the digital image processing. This paper analyzes characteristics of the SNR of the wavefront coding system in the frequency domain and calculates the rate of noise amplification in the digital processing. It also explains the influence of the image detector noise severely reducing the restored quality of images. In order to reduce noise amplification in the process of image restoration, we propose a modified wiener filter which is more suitable for restoration in consideration of noise suppression. The simulation experiment demonstrates that the modified wiener filter, compared with traditional wiener filter, has much better performance for wavefront coding system and the restored images having much higher SNR in the whole depth of the field.
In this paper we demonstrated a coherent raster-scan imaging system that can acquire phase information based on continuous terahertz imaging. It mixes the terahertz with a Fs-laser by a electro-optic crystal of ZnTe to make a hybrid modulation on the crystal to achieve continuous terahertz detection. In this way, it can not only propagate for a long distance but also achieve phase detection for continuous terahertz imaging. The surface images of objects that are under test can be obtained by the Backward-Wave Oscillator, which the output power is 10mW at 205.994GHz. With the repetition frequency of 80MHz, the output power of the MaiTai is 1.65W and 100fs pulse light at 800nm. The images can achieve diffraction-limited resolution approximately. And the simulated results show that the system can obtain phase imaging of test objects based on continuous terahertz source. The way to get the phase of the signal has significant meaning for coherent detection of continuous terahertz source.
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.
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 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.
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 (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.
The characterization of the emission of terahertz (THz) waves generated by four-wave mixing in the presence of laser-induced air plasma under different pump power is presented. In our experiment, terahertz is detected using THz Air-Biased Coherent Detection (THz-ABCD) method. Red-shift is observed in frequency spectra with enhanced pump power and the bandwidth is narrowing down. Then the localized terahertz radiation along the plasma was studied, by constricting the emission area with a pinhole. The spatio-frequency dependent of THz emission from laser-induced air plasma is observed. These phenomena are explained by intense self-phase modulation of the optical pulse in the plasma. This study reveals that terahertz spectrum can be controlled by changing the pump power to get useful frequency range.