The anomalous thermal radiation of metals produced by ultrashort laser pulses

The spectral-time characteristics of the secondary radiation produced in silver and tungsten by picosecond laser pules of varying duration and fluence (energy density) have been studied theoretically and experimentally. It is established that the secondary radiation is due to heating. In silver, however, it is not usual and does not correspond, for example, to grey body radiation. This radiation — the anomalous thermal radiation — is defined by the following mechanism. When the electron and phonon subsystems in a metal are heated (including also the nonisothermal process), there appears the glow with the continuous spectral distribution and the intensity exceeding the radiation intensity of the grey body whose temperature is equal to the temperature of the ion or electron subsystem of a metal. This anomaly is either due to overheat of the electron subsystem with respect to the ion subsystem or due to recombination of electrons and therm-ions in a microlayer above the metal surface.     

The anomalous thermal radiation from metals produced by ultrashort laser pulses. I

The spectral-time characteristics of the secondary radiation induced in silver and tungsten by picosecond laser pulses of varying duration and fluence have been studied theoretically and experimentally. It is established that the secondary radiation is due to heating, but in silver, nevertheless, it is not usual and does not correspond, for example, to grey-body radiation. This radiation—the anomalous thermal radiation proceeds by the following mechanism: When the electron and phonon subsystems in a metal are heated (including also the nonisothermal process), there appears a glow with a continuous spectral distribution and an intensity exceeding the radiation intensity of the grey body whose temperature is equal to the temperature of the ion or electron subsystem of a metal. This anomaly is either due to overheating of the electron subsystem with respect to the ion subsystem or due to recombination of electrons and thermions in a microlayer above the metal surface.     

Nonlinear absorption of CO2 laser radiation by nonequilibrium carriers in germanium

Nonlinear absorption of CO₂ laser pulses at the multi-MW level has been studied in n-type germanium at room temperature. This nonlinear absorption limits the level of CO₂ laser intensity that can be transmitted through an optical grade germanium crystal. The observed nonlinearity may be interpreted in terms of nonequilibrium electron-hole pairs generated by hot electrons created by the intense laser field. The number of nonequilibrium carriers generated is measured by photoconductivity experiments, and the results are correlated to the absorption measurements.     

Absorption of CO laser radiation by NO

Absorption of CO i.r. laser radiation by NO has been studied over the temperature range 300°–4000°K using a grating-tunable CO laser in conjunction with a room-temperature absorption cell and a shock tube. The CO laser line with strongest absorption at elevated temperatures was determined to be the V = 7 → 6, J = 12 → 13 line at 1935.4817 cm^-1, which is nearly coincident with the ²Π_{$\text{3}\text{2}$}V = 0 → 1, J = 37/2 → 39/2 transition in NO. The absorption cell measurements (300°K) were used to infer the position of the NO absorption line (a Λ-doublet at 1935.492 and 1935.497 cm^-1) as well as collision-broadening parameters in pure NO and NO dilute in foreign gases: 2γ° (collision-broadened full width at half maximum in cm^-1 atm^-1 at 300°K) = 0.110, NO-NO; 0.072, NO in Ar; 0.069, NO in Kr; 0.109, NO in N₂. Calculations of the NO absorption coefficient at 1935.4817 cm^-1 are presented for a range of conditions applicable to current studies in combustion and NO_x kinetics. Shock tube measurements (630°–4000°K) supporting these calculations are also reported.     

Multiphoton absorption of broad-band CO2 laser radiation by SF6

Increase of the emission bandwidth of a high-pressure CO₂ laser up to 1.5 cm^–1 increases the multiphoton absorption cross-section of SF₆. Comparison with the previously found [9] increased absorption for shorter pulses suggests that this is also a bandwidth effect. Spectral structures as narrow as 1 cm^–1 above the 10^th absorption step are invoked to explain the observations. The temperature effect, which disappears in the broad-band case, confirms this view.     

Self-focussing of CO2 laser radiation in SF6

Self-focussing of high-power TEA CO₂ laser pulses for a number of 10 m P-band lines is reported in SF₆ molecular gas. Application of this effect to estimating the intensity-dependent refractive index of the gas is demonstrated.     

Energy mechanism during machining process by high-power continuous CO2 laser

The high-power continuous CO₂ laser (4 KW) can provide an energy capable of causing melting or even, with special treatment of surface, vaporizing an XC42 - iron sample. During the laser-metal interaction, the energetic machining mechanism takes place according to the following assumptions: Laser energy absorbed by metal is maximal for a p-polarization. The melting front precedes the laser beam. The beam interacts with a preheated surface whose temperature is near the melting point. In such conditions one finds that mean average absorptive power (A), calculated through Maxwell's equations at fusion temperature, is around 25%, which enables us to calculate the laser energy absorbed by the metal. The available thermal models provide a lot of information concerning thermal diffusion but are unable to describe the physical process of the groove. Hence practical information required for industrial applications cannot be obtained. So in this work we have established a model able to calculate the characteristic parameters of the groove (or cut) as a function of laser energy and beam impact diameter (D). This model is based on writing down the balance of exchanged heat during the time of laser-material interaction ((t). This new procedure makes it possible to determine the machining parameters (laser power P, impact diameter D, and machining speed V) which one has to use during the machining process in order to implement an optimum groove (or cut) with predetermined characteristics (width L_s, and groove depth P_r).     

超短脉冲激光辐照硅膜升温效应的模拟研究

考虑到载流子和晶格的热容、热导率、弛豫时间等热力学参数随温度非线性变化因素的影响,利用有限差分算法,数值求解了半导体材料自相关热传导模型,讨论了2 m厚硅膜在波长和脉宽分别为775 nm和500 fs的脉冲激光辐照下的升温规律。数值结果表明：硅膜前表面载流子温度在激光辐照过程的0.69 ps时刻达到最大值,在4.8 ps时刻以后硅膜基本趋于总体热平衡;载流子热容的快速变化以及单光子吸收和由载流子浓度变化而引起的载流子能流变化是导致载流子温度变化的主要原因;超短脉冲激光辐照时,激光作用时间短,各扩散项及传导项不起主要作用,因此硅膜内各主要计算参数与硅膜的厚度及基体材料类型等的关系不大。     

Interaction of CO2 laser radiation with glasses

An illustration of an interaction of pulsed multimode TEA CO₂ laser radiation, through or without a mask, as well as of a laser scanning process of a frequency Q-modulated cw CO₂ laser beam on glass surfaces has been shown. As an object of investigation glass articles with composition as a standard industrial potassium-boron silicate glass, we have used. A complex of investigations shows that the laser treatment leads to qualitative constant changes (well defined peeling structure) depending on the time surface treatment, defocusing and the pulse length of the laser output.     

RIBLLⅡ与CSRe中束流控制系统的设计

介绍了兰州重离子加速器冷却存储环（HIRFL-CSR）的实验环CSRe以及次级束线RIBLLⅡ中束流控制系统的设计。该系统主要采用了Java,COM,Oracle,ARM,DSP,FPGA等技术实现了对磁铁电源的实时、同步控制,已达到对束流的控制。该系统已经运行于现场的束流调试中,并在RIBLLⅡ的束流调试中运行正常、性能稳定。     

Multiwavelength Q-switched CO2 laser with continuous discharge

A low-pressure (20 mbar) CO₂ laser allows to extract pulses at several selected wavelengths simultaneously from the same active medium. We demonstrated this, using an industrial laser modified by a Q-switch and a resonator with two branches. In one branch the wavelengths are spatially separated, whereas in the other they oscillate in one common transverse mode. We designed a multi-wavelength resonator which requires a single additional reflector compared to usual laser cavities. It provided tunable oscillation at six wavelength simultaneously.     

Peculiarities of the ignition of propane-air premixed flows by CO2 laser radiation

Results of an experimental study of the ignition dynamics of propane-air premixed flows under pulsed-periodic laser irradiation are reported. Ignition of homogeneous flows emanating into free atmosphere is considered. A comparative analysis of experimental and predicted delay times for the ignition for a medium at rest is given. This work was supported by the President of the Russian Federation (Grant No. NSh-8597. 2006.1), by the Russian Foundation for Basic Research (Grant No. 05-01-00560) and by the Presidium of the Russian Academy of Sciences (under the Program “Fundamental Problems in Magnetoplasma Aerodynamics”).     

Multiple absorption and dissociation of trifluoromethylhypofluorite under the effect of pulsed CO2 laser radiation

Translated from Zhurnal Prikladnoi Spektroskopii, Vol. 54, No. 5, pp. 753–758, May, 1991.     

Absorption of pulsed CO2 laser radiation by air and carbon dioxide

An optoacoustical method is used in experimental study of nonlinearity in absorption of pulsed CO₂ laser radiation by air with CO₂ and H₂O added and pure CO₂. The laser pulse consists of frequencies related to several rotational transitions, generated simultaneously. Nonlinearity in absorption was detected for laser radiation levels above 0.2 MW/cm².Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 10, pp. 49–51, October, 1982.     

Third harmonic generation of CO2 laser radiation in AgGaSe2 crystal

Generation of third harmonic of CO₂ laser radiation has been obtained in a type-II, ϑ=57° cut 9 mm thick AgGaSe₂ crystal for the first time by sum-frequency-mixing of the fundamental with its second harmonic, the latter being obtained using another type-I, ϑ=55° cut 11 mm thick AgGaSe₂ crystal. The energy conversion efficiencies obtained for second harmonic and third harmonic generations are 6.3% and 2.4% respectively with the input fundamental pump power density of 5.9 MW/cm² only. The wavelength of the fundamental CO₂ laser radiation used for the generation of harmonics is 10.6 μm, P(20) line. A compact TEA CO₂ laser source has been built in the laboratory.