Energy efficiency of femtosecond laser ablation of refractory metals

We present the results of an experimental study of the ablation energy thresholds and ablated mass for a number of refractory metals (Ti, Zr, Nb, Mo) by femtosecond (τ _0.5 = 45–70 fs) exposed to laser pulses in the ultraviolet — near infrared range (λ = 266, 400, 800 nm) under atmospheric conditions and under vacuum (p ~ 10^–2 Pa). We have analyzed the ablation efficiency (mass yield per unit energy of the acting coherent radiation) and ablation energy thresholds vs. the laser pulse duration and photon energy.     

Generation of radiation on yellow and infrared lines in sodium vapour excited to the 4P level

Generation of the radiation pulses on resonant (589 nm) as well as infrared (2210 nm and 1140 nm) lines was observed in sodium vapour excited by laser light tuned to the 3S–4P transition.     

Formation of defects in lithium fluoride ceramics upon irradiation with femtosecond laser pulses

The interaction between femtosecond laser radiation in the filamentation mode and lithium fluoride optical ceramics is investigated experimentally. It is shown that irradiating optical ceramics based on lithium fluoride with femtosecond laser pulses in the near infrared spectral region effectively produces luminescence centers characteristic of radiation-colored single crystals.     

Plasmon production in a plasma by a Raman process. Threshold of stimulation of the plasmons. Application to a helium plasma

An investigation of the plasmon production by Raman conversion of laser light is reported. The threshold laser power for plasmon amplification is derived from a balance the gain process (Raman conversion) and the loss processes (Landau and collisional dampings). Application to a helium plasma provides threshold values for visible and infrared laser radiation near some He(I) forbidden transitions.     

Ultrafast Broadband Diagnostics of the Filling of the s Band at the Two-Photon Femtosecond Laser Excitation of a Gold Film

JETP Letters - The transmission of near infrared femtosecond laser pulses (wavelength of 800 nm) and supercontinuum radiation (300–750 nm) generated by them through a 50-nm gold film immersed...     

Effect of combined radiation processing on parameters of Si-based MOS transistors

The variation of parameters of Si-based MOS-transistors affected by combined infrared and X- ray photons processing were presented. It was found that X-ray irradiated transistor under incoherent infrared radiation is characterized by lowering of radiation sensitivity. As positive charge and its radiation-induced rate of generation in undergate SiO₂ insulator decreases, we observe a reduction of both ΔU_No and voltage threshold U_th transistor components. Under pulsed laser processing (λ?=?1.06?µm, τ?=?10^?3s) of the X-rays irradiated transistor the energy density of laser beam corresponding to the minimum threshold voltage was established. It was shown that the combined treatment can be effectively used to control the transistor parameters. The model describing the radiation changes of parameters in Si-SiO₂ structure was proposed. The effect of Na⁺, K⁺ and H⁺ ions in undergate insulator on variation of charge state and interface reconstruction under laser irradiation were predicted by proposed model.     

太阳对成像型天基激光告警系统的影响

针对成像型天基激光告警系统中的太阳背景辐射问题,通过理论分析和数值计算研究了太阳与系统探测器不同空间位置关系时直射太阳光和地球反射太阳光对系统成像信噪比、威胁激光检测、探测器自身安全的影响.结果表明:太阳光是天基激光告警系统的主要背景辐射,地球反射太阳光是系统红外探测器成像的主要背景光|反射太阳光辐照在探测器像面上的最大能量密度为0.97 mJ/cm2,反射太阳光背景下,攻击激光成像信噪比在103以上,信标激光信噪比小于1,反射太阳光对攻击激光检测没有影响,对信标激光成像是强背景干扰|直射太阳光辐照探测器是系统不可避免的情况,太阳成像光斑功率密度最大时约20 W/cm2,将导致探测器像元饱和甚至损坏|太阳成像光斑特征与入射激光相似,会造成系统严重虚警.     

Generation of coherent soft-X-ray radiation extending far beyond the titanium L edge

Coherent soft-x-ray radiation up to photon energies of 700 eV is obtained by focusing several-mJ, 10-fs near infrared laser pulses into a He gas jet. The observed nearly constant photon yield over several hundred eVs may be attributed to nonadiabatic self-phase matching, originating from a substantial ionization within a fraction of the optical cycle of the driving laser pulse.     

Diode-pumped singly resonant continuous-wave optical parametric oscillator with wide continuous tuning of the near-infrared idler wave

We demonstrate wide, continuous tuning of the single-frequency idler wave of a cw singly resonant optical parametric oscillator (SRO). The SRO consists of a periodically poled LiNbO(3) crystal for quasi-phase matching in a four-mirror signal-resonant ring cavity. The SRO, excited by 2.25 W of 924-nm radiation from an InGaAs diode laser, generates as much as 200 mW of single-frequency 2.1-mum idler radiation. We tune the idler frequency continuously within a range as large as 56 GHz by changing the wavelength of the diode pump laser. The versatility of this continuously tunable single-frequency infrared source is demonstrated by recording of N(2)O rovibrational absorption lines near 2.1 mum.     

Tunable picosecond infrared pulses generated by stimulated electronic Raman scattering of a mode-locked Ti:Sapphire laser in potassium vapor

A down-conversion to the mid-infrared region by using Stimulated Electronic Raman Scattering (SERS) in potassium vapor is described. The pump radiation is a frequency-doubled regeneratively amplified Ti:Sapphire laser with a pulse duration of 2 ps, pulse energy of 0.2 mJ, and repetition rate of 10 Hz. With the pumping frequency tuned around the potassium 4s-5p transition, nearly transform-limited infrared radiation tunable between 2.2 and 3.4 m has been generated with a peak infrared energy of 12 µJ, corresponding to a quantum efficiency of 17%, and with a pulse duration of 2 ps. The present tuning range could be extended by extending the tuning range of the pump laser. In comparison, intense infrared radiation of 90 µJ energy but with a very narrow tunability around 2.9 m has also been generated by SERS in barium vapor.     

近红外激光照明器均匀性评价技术研究

近红外激光照明是在低照度情况下,用近红外激光作为光源对远距离目标进行主动照明.由于激光的产生机制、匀光装置和照明器采用的光学系统公差、大气湍流等因素,近红外激光光束在目标面上的光强空间分布不均匀,对成像、观测准确度产生较大影响.本文提出采用近红外激光光强功率谱衡量照明近红外激光光强分布均匀性,即采用归一化后光强功率谱所围成的面积作为衡量近红外激光照明均匀性的参量.利用这种方法,对不同的近红外激光照明器,在相同的工作状态下的照明均匀性评估参量进行了研究,结果验证了采用归一化后光强功率谱所围成的面积作为评价参量的合理性.     

Analysis of Inner-Scale Effect on Atmosphere Scintillation for Infrared Laser Beam Propagating on Earth-Space Paths

On Earth-space paths, based on ITU-R C_n² (h) model, by means of a modification of the Rytov method that incorporates an amplitude spatial frequency filter function under strong fluctuation conditions, considering atmospheric turbulence inner-scale, an expression is developed for the scintillation index of a visible or a near and middle infrared laser beam wave that is valid under moderate to strong irradiance fluctuations. The quantitative analyses of scintillation index are done by this analytic model for laser Gaussian collimated beam wave at visible or near and middle infrared wave band. The results agree with theoretic analysis scintillation. This scintillation model can be converted into a Gaussian beam wave model with finite inner-scale on horizontal sight paths of invariable C_n².     

Optical bistability produced by resonant Joule's heating of semiconductors

The thermal hysteresis of Joule's absorption of infrared radiation in then-type semiconductor magnetoplasma is analysed. Such bistability is connected with the temperature dependence of the relaxation time of free electrons, their density being constant. The resonant Joule's absorption bistability of the pumping infrared wave near a Langmuir resonance of free electrons is discussed as well as the hysteresis cross-modulation of an infrared probing wave by means of a pumping microwave. The possibilities of smooth detuning of the hysteresis regime due to a magnetic-field control and low energy thresholds of these regimes are illustrated for InSb thin films in relatively weak magnetic fieldsH1–2 kG.     

超热电子输运背向光辐射的实验研究

在100TW掺钛蓝宝石飞秒激光器上利用光学CCD相机和光学多道分析仪,分别在靶背法线方向测量了超热电子光辐射的空间分布和光谱.测量结果显示：光辐射空间分布图案呈圆环状,而辐射区域有发散角和光强分布,且包含多种辐射成分.光辐射光谱在800nm附近出现尖峰,是激光的基频(ω₀)波,这一现象归因于超热电子束在输运的过程中产生的微束团而引起的相干渡越辐射(CTR).随着激光能量的增加,CTR光谱峰向红光方向移动,基频波红移的主要原因是由于等离子体临界面的迅速膨胀.如果考虑超热电 关键词： 超热电子 光辐射 共振吸收 红移     

Generaton of tunable 16 μm radiation by stimulated hyper-raman effect in strontium vapour

The generation of tunable 16 μm radiation by stimulated hyper-Raman scattering from atomic strontium vapour is reported. The infrared radiation has a linewidth of 0.4 cm⁻¹ which corresponds to that of the nitrogen laser pumped dye laser used as a pump source. Due to relatively low pump intensity the infrared power was only 20 mW. However, the system is easily scalable to a higher conversion efficiency.     

A broadband Schottky point contact mixer for visible laser light and microwave harmonics

Frequency differences of up to 900 GHz between two laser lines near 568 nm were measured by mixing laser light and harmonics of 90 GHz microwave radiation. The Schottky point contact diodes used consisted of tungsten tips onn-GaAs bases with different degrees of doping. The formation of the contacts and the sensitivity and frequency limitations of the device are discussed.     

Intrinsic efficiency and critical power deposition in the e-beam sustained Ar:Xe Laser

Experimental investigations on an e-beam sustained near infrared Ar:Xe laser have been carried out to determine the intrinsic efficiency at optimized conditions. A parametric study at different sustainer currents reveals a maximum output energy depending on current density. Up to 8 bar the optimized laser output power per unit volume increases linearly with 1.1MW/1 bar. Intrinsic efficiencies of up to about 8% are feasible.     

Stimulated Raman scattering of a high-Q liquid-hydrogen droplet in the ultraviolet region

We show that a liquid-hydrogen droplet can achieve high- Q values that exceed 10(9) for whispering-gallery modes in the ultraviolet. We show also that pumping high- Q liquid-hydrogen droplets with ultraviolet laser radiation generates many vibrational and rotational Raman sidebands that cover a broad spectral range from the ultraviolet to the near infrared.     

Visualization of the spatio-temporal structure of a pulsed photoelectron beam formed by femtosecond laser radiation

A method based on an original electron microscope created for investigating photoelectron beams is presented. It ensures a nanometer spatial resolution and picosecond time resolution. Electrons appearing when a metal needle is irradiated by femtosecond laser pulses are transmitted through a dielectric microcapillary and are subjected to a ponderomotive potential created by femtosecond laser radiation focused near the capillary tip. The position-sensitive detection scheme allows for the detection of the spatial profile of a photo-electron beam with a magnification of K ≅ 4 × 10⁴. The time structure of the photoelectron beam is visualized by scanning the delay time between laser pulses irradiating the needle and a laser pulse focused near the capillary tip.     

A matter of scale: from far‐field microscopy to near‐field nanoscopy

Vibrational spectroscopy is a powerful analytical tool which provides chemical information about a sample without a priori knowledge. By combining vibrational spectroscopy with different microscopic techniques, scientists can visualize and characterize the chemical composition of a sample on length scales which cover many orders of magnitude; from far‐field radiation used in microwave astronomy and Fourier transform infrared microscopy, to near‐field scattering used in tip‐enhanced Raman spectroscopy and scanning near‐field optical or infrared microscopy. Here, various modern chemical mapping techniques are reviewed and their advantages and disadvantages are discussed. Also, a basic theoretical background is provided for each technique along with several illustrative examples.