Zuo Duluo

An Efficient Pulsed CH3OH Terahertz Laser Pumped by a TEA CO2 Laser

An efficient pulsed CH3OH terahertz (THz) laser pumped by a TEA CO2 laser is investigated experimentally. To improve photon conversion efficiency and THz laser energy, two cavity configurations of the TEA CO2 laser, which is external and semi-external, are evaluated. The pump intensities are about 4.7 MW/cm2 and 1.2 MW/cm2, respectively. Higher pump intensity and more stable single lines are obtained in the external cavity. For the 3.8 J pump energy of the 9P(16) transition in the external cavity, the maximum terahertz output energy with 570.5 μm wavelength at 160 Pa is 431 μJ. With a 6 J energy pulse in terms of a semi-external cavity, a 353 μJ terahertz emission (570.5 μm) is produced. The corresponding photon conversion efficiencies are 1.36% and 0.705%, increasing by a factor of about 2.

Terahertz Generation in Nonlinear Crystals with Mid-Infrared CO2 Laser *

The terahertz (THz) generation based on difference frequency generation in nonlinear optical crystals pumped by mid-infrared CO2 laser has been investigated. We present a comprehensive study of the phase-matching conditions in the GaSe, ZnGeP2 and GaAs crystals. A comparison of the characteristics of these crystals as the THz frequency generator is also presented. The investigation of the conversion efficiency shows that GaSe and GaAs are the most promising nonlinear crystals for the efficient and widely tunable THz generation.

A Terahertz Wavemeter Based on a Fabry—Perot Interferometer Composed of Two Identical Ge Etalons

A simple and convenient terahertz wavemeter based on a Fabry—Perot interferometer (FPI) is presented. The interferometer is composed of two identical Ge etalons, which act as high-reflectance mirrors for terahertz waves. The transmission characteristics of the Ge FPI are analyzed using multiple-beam interference theory. The theoretical finesse of the FPI, defined as a ratio of 2π to the phase halfwidth of the transmission fringes, is larger than 12.5. Here, the wavemeter is used to measure the wavelengths of an optically pumped NH3 terahertz laser. The experimental results indicate that the measuring uncertainties are within ±1%. Higher accuracy can be expected if the power or pulse energy of the terahertz source is more stable.

Two Schemes for Generating Efficient Terahertz Waves in Nonlinear Optical Crystals with a Mid-Infrared CO2 Laser

Terahertz wave generation is explored on the basis of difference frequency generation in nonlinear optical crystals with a mid-infrared CO2 laser. The phase-matching angle and the grating period of periodically inverted GaAs in the 100–1000 μm (0.3–3 THz) range are also investigated on the basis of the surface-emitted difference frequency generation. It is found that two schemes of phase-matching-applied collinear phase matching and phase-matching-applied non-collinear phase matching are efficient to obtain THz waves.

An Efficient Photon Conversion Efficiency Ammonia Terahertz Cavity Laser

An efficient ammonia terahertz (THz) cavity laser is reported experimentally. Unlike the past design schemes such as hole couplers and freestanding mesh couplers, in our systems the input and output couplers are fabricated by depositing nickel capacitive metallic meshes on ZnSe and high-resistivity silicon substrates. Thus the couplers not only can be constructed as an F-P oscillator but also can be used as sealed windows that are easier to perform the adjustment of alignment with. To enhance THz laser output energy and photon conversion efficiency, the dominant factors such as pump intensity and gas pressure are investigated experimentally. Finally, a 1.35 mJ terahertz radiation of ammonia laser with 90 μm wavelength (3.33 THz) operating at 1.09 kPa pumped by a 402 mJ TEA CO2 laser with 9R (16) line is generated, and photon conversion efficiencies of 6.5% are achieved.

Time-Resolved Optical Emission Spectroscopy Diagnosis of CO2 Laser-Produced SnO2 Plasma*

The spectral emission and plasma parameters of SnO2 plasmas have been investigated. A planar ceramic SnO2 target was irradiated by a CO2 laser with a full width at half maximum of 80 ns. The temporal behavior of the specific emission lines from the SnO2 plasma was characterized. The intensities of Sn I and Sn II lines first increased, and then decreased with the delay time. The results also showed a faster decay of Sn I atoms than that of Sn II ionic species. The temporal evolutions of the SnO2 plasma parameters (electron temperature and density) were deduced. The measured temperature and density of SnO2 plasma are 4.38 eV to 0.5 eV and 11.38×1017 cm−3 to 1.1×1017 cm−3, for delay times between 0.1 μs and 2.2 μs. We also investigated the effect of the laser pulse energy on SnO2 plasma.

Temperature Characteristics of Cathode Sheath in High-Pressure Volume Discharge Derived from Emanating Shock Wave

The temperature characteristics of cathode sheath in high-pressure volume discharge are investigated experimentally. The cathode sheath temperatures under various discharge conditions are derived from the speeds of the emanating shock waves which are measured by a Mach—Zehnder interferometry. It is found that the cathode sheath temperature and the ratio ΔT3/ΔT1 of temperature rise between cathode sheath and plasma bulk are determined by the specific energy deposition and the breakdown delay time respectively. These results are helpful for discharge stability improving and shock wave reducing.

Pulsed CH3OH terahertz laser radiation pumped by 9P(36) CO2 lasers

An efficient pulsed CH3OH terahertz (THz) laser pumped by a TEA CO2 laser was investigated experimentally. A simple terahertz cavity and a TEA CO2 laser for the optically pumped THz radiation were studied experimentally. To improve THz laser energy and photon conversion efficiency, two different TEA CO2 lasers were developed to pump CH3OH. When CH3OH was pumped by the 9P(36) line with different powers of the CO2 laser, the generation of terahertz radiation with energy as high as 0.307mJ and 23.75mJ were obtained, respectively. The corresponding photon conversion efficiencies were 0.29% and 2.4%. The photon conversion efficiency increases by a factor of about 8. Meanwhile, higher peak power of pump laser effectively improves the photon conversion efficiency. And the optimum THz laser pressure increases with narrower pulse width of pump laser because of increasing absorptive gases molecules of CH3OH with higher peak power of pump laser.

Experimental Research on Plasma Induced by TEA CO2 Laser Propulsion

Results in the air-breathing propulsion experiments with a parabolic light craft and a self-made UV-preionized 100 J TEA CO2 laser device are presented. Air disturbance and the spectrum of the plasma after the interaction of pulsed laser radiation with the light craft were studied. It was found that the focal length of the parabolic light craft had a significant effect on the air-disturbance. Two shock waves were detected for the longer focal length, while only one shock wave detected for the short focal length. The spectrum of the laser-induced plasma, the distribution of the characteristic lines, and the temporal behaviors of the air plasma were studied in detail. The results showed that, the evolution of the laser-induced plasma lasted 20 μs, and the plasma spectrum would reach the maximum intensity at 7 μs.

Theoretical Design of a Wedged Output Coupler Coated by Capacitive Strip-Grating for Optically Pumped Terahertz Lasers

An output coupler for optically pumped terahertz laser consisting of capacitive strip-grating and wedged high-resistivity silicon substrate is designed theoretically and its transmittance performance is also discussed. The etalon effects frequently occurring in previous experiments are effectively suppressed, and thus a flat and accurate transmittance spectrum is obtained in a narrow wavenumber interval of 2 cm−1 near a particular center wavenumber. Furthermore the transmittance sensitivity to the slight shift of substrate thickness is also completely eliminated. The wedged output coupler is easy to fabricate and its substrate may be used repeatedly to meet various transmittance requirements.