Efficiency of optical-to-acoustic energy conversion upon the interaction of a pulsed laser radiation with a liquid: I. Calculation of the efficiency upon acoustooptic interaction

Relationships for the efficiency of conversion of laser radiation energy to acoustic energy for the acoustooptic (thermal) mechanism of interaction are derived. The cases of short and long laser pulses interacting with the rigid and free boundaries of a heavily absorbing liquid are considered. The efficiency is numerically calculated for the situation when the radiation of a transverse-excitation atmospheric (TEA) CO₂ laser interacts with water with regard for the temperature variation of the volume thermal expansion coefficient of the latter.     

基于液晶/聚合物光栅的高转化效率有机半导体激光器

采用有机半导体发光材料聚[2-甲氧基-5-(2-乙基己氧基)-1,4-苯乙炔]作为增益介质,低官能度光敏单体制备的液晶/聚合物光栅作为外部反馈谐振腔,制备出参数可独立控制的分离式结构的有机半导体激光器.液晶/聚合物光栅中液晶分子的取向影响光栅折射率调制量,从而影响光栅的反馈能力,最终影响激光器出射激光的性能.通过研究发现决定液晶分子取向的主要有两种与光栅周期有关的作用力,利用这一原理制备不同周期的光栅,光栅周期小于450 nm时,相分离出的液晶分子取向由光栅矢量方向变为光栅沟槽方向,此时光栅的折射率调制量增加,光反馈能力增强.采用周期为395 nm的液晶/聚合物光栅制备二级布拉格散射的有机半导体激光器,相较于大周期光栅(593 nm)制备的激光器,激光阈值由0.70μJ/pulse降低至0.18μJ/pulse,转化效率由2.5%提高到6.4%,且出射激光垂直于基板表面发射,有利于后续的处理及应用.     

The measurement of ultraweak liquid surface wave by laser interferometry

We describe a simple experiment on the interference of refraction light by ultraweak liquid surface wave at low frequency. The surface wave profile is generated by electrically driven vertical oscillations exciters. The high visibility and stable laser interference fringes were gathered experimentally. The theoretical light intensity distribution agrees well with the observations. In particular, we show the relation between the light intensity distribution and the characteristic features of the surface wave. In addition, we can measure the amplitude and wavelength of the ultraweak liquid surface wave at low frequency by this means.     

Adaptative lensing driven by the radiation pressure of a continuous-wave laser wave upon a near-critical liquid liquid interface

The bending of a liquid interface by the radiation pressure of a cw laser wave is generally weak. To strongly enhance the coupling, we investigate lensing in a near-critical phase-separated liquid mixture. By continuously tuning the softness of the meniscus by varying the temperature, we observed huge stationary interface deformations at low beam power that led to an important variation of the resultant soft lensing. We also illustrate the crucial role played by temperature in this process to demonstrate its potential use in lens adaptation.     

Investigation of the population inversion in a He-Ne laser discharge by heterodyne holographic interferometry

Heterodyne holographic interferometry was used for end-on investigation of a He-Ne laser discharge. By tuning an actively stabilized dye laser over the 633 nm neon atomic line the phaseshift was obtained as function of frequency, thus enabling the determination of spatially resolved population inversion.     

Production of heavy atomic clusters upon interaction of laser radiation with matter

The results of an investigation of the formation of Pb, Th, and U clusters over a broad range of numbers of atoms (from a few atoms to macroparticles) upon interaction of high-power pulsed laser radiation with matter are presented. Clusters of fissionable elements are studied for the first time. A setup for determining the yield of clusters and the number of atoms in them, which is based on the use of several different methods (laser resonance fluorescence, time-of-flight measurements, and counting the number of tracks of fission fragments from the cluster nuclei), is described. The dependence of the yield of clusters with various numbers of atoms on the conditions for their formation is discussed. Zh. éksp. Teor. Fiz. 112, 78–88 (July 1997)     

Time-dependent interaction of pulsed CO2 laser radiation with liquids

Many industrial applications of high-power lasers involve the creation by vaporization of a keyhole in a solid target. This structure is unstable with respect to collapse and can be maintained only by achieving a balance between pressure terms from laser vaporization and those of surface tension and hydrostatics. In addition, liquid flow and plasma effects also modulate the laser beam intensity resulting in a complex feedback system in coupling laser radiation into the target. In this paper, we report data obtained on the time dependence of structures associated with laser drilling of an absorbing liquid. These data, obtained at low incident laser intensities in the absence of plasma effects, show the complex nature of the laser-keyhole interaction even in a two-phase system. Some results of experiments carried out in low gravity are also presented.     

Interaction of pulsed laser radiation with a polycarbonate-based composite

Abstract—The interaction of pulsed laser radiation with foil-clad polycarbonate containing diffraction gratings is studied. It is found that the laser beam reflected from diffraction gratings is selffocused in polycarbonate bulk and becomes a seed for changing the polycarbonate structure, which leads to the glass transition and growth of ordered structures with symmetry features, which gives rise to the light dispersion effect in composites. A probable formation mechanism of ordered structures in polycarbonate during irradiation is proposed.     

Theoretical study of thermoelectronic laser energy conversion: II. Characteristics of optical discharge

The dependences of the parameters of a continuous optical discharge in an interelectrode space of a thermoelectronic laser energy converter on the gas pressure, electron emission current, and discharge current are studied. The current-voltage characteristics of the discharge are calculated.     

Higher harmonic generation in laser plasma upon interaction of femtosecond laser pulses with doubly and singly charged ions under conditions of resonance frequency conversion of laser radiation

The higher harmonic generation (HHG) in plasmas formed on the surface of different solid targets (Pb, Ag, Ge, In, Au, Cr, Pt, V, Mn, Ga, Bi, Sb, Cu, Al, Ca, and Sn) by laser ablation is studied. The possibility of HHG upon interaction of femtosecond radiation with doubly charged ions is studied on the example of Mn plasma. This process allowed generation of higher harmonics of up to the 101st order. The results of investigation of individual enhanced harmonics in Sn, Sb, In, and Cr plasmas are presented. Comparison of the conditions of HHG in weakly and strongly excited plasmas showed that the latter is preferable for efficient frequency conversion. It is demonstrated that the use of laser plasma spectroscopy with high time resolution allows one to determine the optimal conditions for generation of harmonics with a plateaulike distribution in the far-UV spectral region.     

Optical interferometry method for the study of density variation in the nanosecond pulsed Nd:YAG laser interaction with aluminum

An interferometer-based technique has been used to determine the change in the refractive index induced by a Q-switched Nd–YAG laser (λ=532 nm and pulse duration 6 ns) on an aluminum target under atmospheric air pressure. The average density changes in front of the target irradiated by two laser pulse energy were observed for different focusing conditions. An Abel inversion technique is also presented for calculating three-dimensional density distributions in cylindrical coordinates.     

Improving the time-averaged thermal efficiency of a laser beam by acoustooptic correction of the directional pattern

Applicability of the acoustooptic method for raising the time-averaged thermal efficiency of laser radiation is substantiated theoretically and confirmed experimentally. The effect produced by laser radiation on materials being processed (laser cutting, welding, engraving, etc.) has a threshold in light intensity. Importantly, a beam with the most frequently used normal (Gaussian) angular distribution of intensity is not optimal from the technological viewpoint. A method proposed for its optimization is based on acoustooptic refraction, i.e., fast nonlinear scanning of the initial beam around its central position, which improves (at certain values of the parameters) the time-averaged angular distribution of the beam intensity. In the experiment, the thermal efficiency of laser radiation is raised by several times.     

High X-ray conversion efficiency with target irradiation by a frequency tripled Nd : Glass laser

High conversion efficiency of laser energy into X-rays from a laser irradiated target is of great interest for a variety of dynamical (pulsed) studies, e.g.: radiography of laser-imploded targets, structure determination by diffraction and absorption fine-structure, and X-ray laser pumping. We report here on a frequency tripled Nd : glass laser used to irradiate targets of various materials at ～5 x 10¹⁴W/cm². We find conversion efficiencies of between 1% and 0.1% (with respect to the incident laser energy) for individual X-ray lines between 1.8 and 7.8 keV. These efficiencies are more than an order of magnitude higher than whose achieved with 1.06 μm lasers.     

Generation of an electrical signal upon the interaction of laser radiation with water surface

A new physical effect lying in the generation of an electrical signal (ES) upon the interaction of IR laser radiation with the water surface at a laser fluences lower than the plasma formation threshold is studied. An ES amplitude exceeding 15 V is detected. A one-to-one relationship between the observed effect and the bulk explosive boiling of water is established, and a qualitative interpretation is proposed. In the case of irradiation of an open surface, the ES is generated due to bulk explosive boiling accompanied by the evacuation and splashing of the surface layer, the destruction of the double electric layer on the surface, and the spread of an electrified vapor-drop mixture (balloelectric effect). When the surface is covered with a transparent plate, ES generation can be caused by the charge separation upon the detachment of the water surface from the plate by a vapor bubble resulting from boiling and the displacement of the charged water surface upon the expansion and contraction of the bubble.     

Measurement of the a.c. impedance of aluminium samples by holographic interferometry

In the present investigation, holographic interferometry was utilized for the first time to measure the alternating current (a.c.) impedance of aluminium samples during the initial stage of anodization processes in aqueous solution without any physical contact. The anodization process (oxidation) of the aluminium samples was carried out chemically in different sulphuric acid concentrations (0.5–3.125% H₂SO₄) at room temperature. In the mean time, a method of holographic interferometric was used to measure the thickness of anodization (oxide film) of the aluminium samples in aqueous solutions. Along with the holographic measurement, a mathematical model was derived in order to correlate the a.c. impedance of the aluminium samples in solutions to the thickness of the oxide film of the aluminium samples which forms due to the chemical oxidation. The thickness of the oxide film of the aluminium samples was measured by the real-time holographic interferometry. Consequently, holographic interferometry is found very useful for surface finish industries especially for monitoring the early stage of anodization processes of metals, in which the thickness of the anodized film as well as the a.c. impedance of the aluminium samples can be determined in situ. In addition, a comparison was made between the a.c. impedance values obtained from the holographic interferometry measurements and from measurements of electrochemical impedance spectroscopy. The comparison indicates that there is good agreement between the data from both techniques.     

A scheme for recording a fast process at nanosecond scale by using digital holographic interferometry with continuous wave laser

A scheme for recording fast process at nanosecond scale by using digital holographic interferometry with continuous wave (CW) laser is described and demonstrated experimentally, which employs delayed-time fibers and angular multiplexing technique and can realize the variable temporal resolution at nanosecond scale and different measured depths of object field at certain temporal resolution. The actual delay-time is controlled by two delayed-time fibers with different lengths. The object field information in two different states can be simultaneously recorded in a composite hologram. This scheme is also suitable for recording fast process at picosecond scale, by using an electro-optic modulator.     

Extremum conditions for the radiation pulse energy and efficiency of a pulse gas-discharge laser

The case of incomplete similarity for which the processes in pulse gas-discharge lasers (PGL) are only similar in time is examined theoretically. It is shown that this kind of similarity is realized in PGL upon attainment of maximum efficiency. Theoretical computations are compared with experimental results on copper vapor PGL.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 8, pp. 17–21, August, 1987.The author is grateful to V. K. Konyukhov, A. A. Isaev, and G. Yu. Lemmerman for attention and useful discussions of the results.     

Generation of high-energy negative hydrogen ions upon the interaction of superintense femtosecond laser radiation with a solid target

The formation of a high-energy (～35 keV) beam of negative hydrogen ions was observed in the expanding femtosecond laser plasma produced at the surface of a solid target by radiation with an intensity of up to 2× 10¹⁶ W/cm². The energy spectra of the H⁺ and H^?-ions show a high degree of correlation.     

Enhancement of measurement sensitivity in determination of changes in the shape of a wavefront by holographic lateral-shearing interferometry

A method for increasing measurement sensitivity in the visualization of dynamics of changes in wavefront shape with the use of holographic lateral-shearing interferometry is considered. At the first stage of this method, a photograph of a reference shear interferogram obtained with tuning to closely spaced fringes is recorded under linear conditions. After chemical treatment, the photograph of the reference shear interferogram is installed in exactly its initial position. At the second stage, a series of photographs of lateral-shear interferograms of the wavefront under study is recorded under nonlinear conditions using the photograph of the reference interferogram. The photographs of the lateral-shear interferograms are optically processed by two coherent beams with selection of the highest diffraction orders, which makes it possible to visualize the dynamic component of the wavefront deformation with an increase in the measurement sensitivity (the static component is excluded in this case). The effect of the degree of nonlinearity of photograph recording on the range in which the measurement sensitivity can be controlled during the optical processing of a pair of matched photographs or one double-exposure photograph of interferograms recorded at different values of a lateral shift is analyzed.