Determination of the mid-IR femtosecond surface-damage threshold of germanium

To assess the potential uses of germanium as a nonlinear material in the mid IR we have measured the surface-damage threshold of germanium optical windows using femtosecond pulses at a wavelength of 3.9 μm. By working with a wavelength corresponding to a photon energy of less than half the band-gap energy, free-carrier generation due to one- and two-photon absorption was eliminated. The laser pulses had an energy of 5.5 μJ, a duration of 255 fs, and were focused to a waist size of approximately 100 μm. The multi-shot damage threshold of the germanium windows was estimated to be approximately 94 GW/cm². The data should be of immediate benefit to industrial laser researchers and applications engineers working in the mid-IR region.     

Nonlinear mid-IR radiation in two-frequency semiconductor lasers with a corrugated waveguide

The nonlinear generation of a difference frequency mode in an injection quantum-well semiconductor laser is considered. A laser based on the InGaAs/GaAs/InGaP heterostructure is proposed, which generates two modes in the 1-μm range and the difference mode in a corrugated waveguide in the range from 10 to 20 μm. It is shown that the power of the difference mode produced by a laser with a 100-μm-wide waveguide in the mid-IR range at room temperature can be as high as a few microwatts if the power of the short-wave modes is 10 W.     

Study of the optical properties of germanium films in the mid-IR spectral range

The optical properties of Si-doped germanium films have been investigated. It is shown that Ge films with 1.5 wt % Si have a minimum optical loss. The optical constants of Ge films have been measured at different substrate temperatures. The optimal conditions for producing films with minimum loss are determined.     

Formation of fast multicharged heavy ions under the action of a superintense femtosecond laser pulse on the cleaned surface of a target

It is found that the mean charge of tungsten ions in a solid tungsten target cleaned from the surface layer of hydrocarbon and oxide compounds and exposed to femtosecond laser radiation with an intensity exceeding 10¹⁶ W/cm² attains 22+, while the maximum charge is 29+. The maximum energy of such ions approaches 1 MeV. The corresponding values obtained on a dirty target with the same laser pulse parameters constitute 3+, 5+, and 150 keV. The results of numerical simulation show that such a large maximum charge of ions can be attained owing to the emergence of an electrostatic ambipolar field at the sharp boundary between the plasma and vacuum. The main mechanism of ionization of ions with maximum charges is apparently impact ionization in the presence of an external quasi-static field. In addition, direct above-threshold ionization by this field can also play a significant role. It is also shown that heavy ions in a clean target are accelerated by hot electrons. This leads to the formation of high-energy ions. The effect of recombination on the charge of the ions being detected is analyzed in detail.     

Formation of relief on a metallic target surface under the action of compression-plasma flows

The results of experimental and theoretical investigations of relief formation on the surface of a steel target (grade St 3 steel, GOST (State Standard) 380) during treatment by compression-plasma flows are represented. The density of energy absorbed by the target varied in the range from 15 to 25 J/cm², the pulse duration was 100 μs, and the pulse number was N = 1, 3, 5, 7. The experiment revealed the expansion of boundaries of the central area (the area on which the plasma flow is incident normally to the surface) with increasing pulse number. This is explained by the more uniform surface treatment at a greater pulse number. It is shown that to describe relief formation in the central area there is a need to take into account the pressure of the plasma flow on the target surface, apart from surface tension forces and energy dissipation due to viscosity.     

Hybrid vertical directional coupling between a long range surface plasmon polariton waveguide and a dielectric waveguide

To investigate light coupling between a long range surface plasmon polariton (LRSPP) waveguide and a conventional integrated optical component, a hybrid vertical directional coupler consisting of a LRSPP waveguides and a dielectric waveguide is investigated and fabricated. In the proposed coupler the dielectric waveguide and LRSPP waveguide are vertically configured for dense integration and strong coupling. The characteristics of the even and odd super-modes of the coupler are also analyzed to design the device. The fabricated device exhibits damped sinusoidal behavior along the coupling length due to propagation loss of the LRSPP waveguide. The maximum power transfer of 86% from the LRSPP waveguide to the dielectric waveguide is achieved at the coupling length of 600 μm. The measured characteristics of the device are in relatively good agreement with a theoretical analysis.     

Excitation of terahertz surface polaritons in a cylindrical waveguide by femtosecond laser pulses

Generation of terahertz surface polaritons in homogeneous round cross-section plasma waveguides upon nonlinear optical rectification of femtosecond laser pulses is analyzed theoretically. It is assumed that nonlinear polarization inducing a surface electromagnetic wave is formed at the waveguide boundary in a thin layer of the nonlinear dielectric that surrounds the waveguide. The efficiency of the femtosecond radiation conversion into surface polaritons is studied as a function of the waveguide radius and duration of the exciting laser pulse.     

Laser surface patterning using a Michelson interferometer and femtosecond laser radiation

We report on a simple method to obtain surface gratings using a Michelson interferometer and femtosecond laser radiation. In the optical setup used, two parallel laser beams are generated using a beam splitter and then focused using the same focusing lens. An interference pattern is created in the focal plane of the focusing lens, which can be used to pattern the surface of materials. The main advantage of this method is that the optical paths difference of the interfering beams is independent of the distance between the beams. As a result, the fringes period can be varied without a need for major realignment of the optical system and the time coincidence between the interfering beams can be easily monitored. The potential of the method was demonstrated by patterning surface gratings with different periods on titanium surfaces in air.     

Defect capture under rapid solidification of the melt induced by the action of femtosecond laser pulses and formation of periodic surface structures on a semiconductor surface

A theory of defect-strain instability with formation of periodic surface relief in semiconductors irradiated by ultra-short (τ_p=10^-13 s) powerful laser pulses is developed. The period and time of formation of surface relief are calculated. Regimes of multi-pulse laser ablation leading to formation of either a smooth surface or arrays of surface-relief spikes are pointed out and corresponding experimental results are interpreted from the viewpoint of the developed theory. Received: 4 December 2000 / Accepted: 23 July 2001 / Published online: 11 February 2002     

Optical charging of real surface of germanium over a wide range of temperatures

A study is made of the charging of a real germanium surface under the effect of light. Mechanisms of electron and hole transport from germanium to the oxide layer are discussed. Conclusions are drawn as to the nature of the deep electron and hole traps in the oxide.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 36–41, May, 1979.In conclusion the authors wish to thank V. F. Kiselev for his discussion of the paper and his useful comments.     

Self-organization of ordered nano- and microstructures on the semiconductor surface under the action of laser radiation

Experimental results on the formation of ordered nano- and microstructures on the surface of semi-conductors under the action of laser pulses with various durations and fluences are interpreted from the unified view point of the theory of the defect-deformation (DD) instability of surface relief. A universal linear dependence of the period of the structures on the thickness of the subsurface layer enriched with mobile point defects and formed due to the laser action and occurrence of two scales of modulation of the surface relief are established and described. The structure symmetry and its evolution with an increasing laser fluence and magnitude of external anisotropic stress are described. Similarities with the formation of nanostructures under ion-beam irradiation and electrochemical etching are revealed and discussed within the framework of the DD instability theory.     

Time variation of the intensity of spectral lines due to a plasma generated under the action of intense infrared laser radiation on a germanium target

The time waveforms of the intensity of spectral lines associated with a germanium laser plasma is studied by the emission spectroscopy method. The plasma is generated by irradiating a germanium target by a pulsed-periodic IR radiation from a neodymium laser, and the intensity waveforms are taken at distances of 1 and 7 mm from the target. From the intensity waveforms, the plasma parameters, such as the ion recombination time and the electron temperature and concentration, are determined. The time variation of the emission characteristics and plasma parameters is analyzed.     

Investigation of erythrocyte membrane damage under the action of γ radiation in a wide dose range using electroporation

Human erythrocyte membrane damage under the impact of γ radiation on blood suspension is studied in a wide dose range (2–1000 Gy, irradiation dose rate 2.75 Gy/min). It is shown that the irradiation in the absorbed dose range from 600 Gy and higher results in hemolysis of erythrocytes immediately (or within several hours) after irradiation, and the value of the hemolysis rate constant increases with increasing absorbed dose. For finding hidden membrane damage occurring several hours after irradiation with smaller doses, the suspension was affected by a high voltage pulsed electric field (PEF). It is shown that, for an absorbed dose range from 2 to ～350 Gy, no noticeable increase in the erythrocyte hemolysis rate was observed after the action of PEF on the suspension, as compared to the nonirradiated suspension. This testifies that, in this dose range, the degree of membrane damage is small and practically independent of absorbed dose value. For doses from 400 to ～550 Gy, a noticeable increase in the hemolysis rate after the action of PEF growing with increasing absorbed dose was observed.     

Formation of diffraction-free beams by laser generation of conical radiation in a Kerr medium

Received: 16 June 1998     

Deformation of germanium under the joint action of an electric current and a magnetic field

The specific features in the behavior of deformation characteristics of low-ohmic p-type germanium single crystals subjected to different types of combined plastic deformation and the anisotropy of the electrical resistance of specimens in the longitudinal and transverse directions have been investigated. Both the acceptor and donor actions of dislocations have been observed in the motion of charge carriers along the direction of compression of the specimen. Under conditions of the joint action of a weak magnetic field and a combined plastic deformation, a decrease in the macroplasticity effects has been revealed. Anisotropy of the electrical resistance of p-Ge specimens in the longitudinal and transverse directions has been found. A possible explanation of the observed effect is given.     

Nonlinear evolution equations for surface plasmons for nano-focusing at a Kerr/metallic interface and tapered waveguide

Maxwell's equations for a metallic and nonlinear Kerr interface waveguide at the nanoscale can be approximated to a (1+1) D Nonlinear Schrodinger type model equation (NLSE) with appropriate assumptions and approximations. Theoretically, without losses or perturbations spatial plasmon solitons profiles are easily produced. However, with losses, the amplitude or beam profile is no longer stationary and adiabatic parameters have to be considered to understand propagation. For this model, adiabatic parameters are calculated considering losses resulting in linear differential coupled integral equations with constant definite integral coefficients not dependent on the transverse and longitudinal coordinates. Furthermore, by considering another configuration, a waveguide that is an M–NL–M (metal–nonlinear Kerr–metal) that tapers, the tapering can balance the loss experienced at a non-tapered metal/nonlinear Kerr interface causing attenuation of the beam profile, so these spatial plasmon solitons can be produced. In this paper taking into consideration the (1+1)D NLSE model for a tapered waveguide, we derive a one soliton solution based on He's Semi-Inverse Variational Principle (HPV).