Generation of harmonics of femtosecond radiation from the surface of aluminum targets

The results of a study of the generation of harmonics from a laser plasma resulting from the interaction of radiation of femtosecond duration (λ=1.06 μm, t=475 fs, and I～2×10¹⁷ W cm^?2) with aluminum targets are presented. The observed frequency shift of harmonics to the short-wavelength region (1.6 and 5.1 nm for the second and fifth harmonics, respectively) is determined by a collisionless absorption resulting from an anomalous skin effect. The efficiencies of conversion into the second and fifth harmonics in an s-polarized pumping field were lower than the conversion efficiencies in a p-polarized pumping field by a factor of eight and a factor of two, respectively (for intensities I<10¹⁷ W cm^?2). With a further increase in the pumping intensity, these values decreased to 0.8 and 0.5, respectively. The mechanisms of such behavior of the conversion process are considered.     

Second-and third-harmonic generation by carbon nanotubes irradiated with femtosecond laser pulses

Femtosecond pulses of a Cr:forsterite laser are used to study second-and third-harmonic generation in a layer of single-wall carbon nanotubes produced by low-velocity spraying. The harmonic amplitude in our experiments scales as (I _p)ⁿ as a function of the pump intensity I _p, with n=2 and 3 for the second and third harmonics, respectively. This scaling law holds up to pump intensities on the order of 10¹²W/cm². The ratio of the maximum signal to the averaged background in the spectra of the second and third harmonics is estimated as 50 and 30, respectively. The second and third harmonics produced by a linearly polarized pump field are also linearly polarized, with their polarization vectors oriented along the polarization direction of the pump field. The capabilities of nonlinear-optical methods for structural and morphological analysis of carbon nanotubes are discussed, as well as ways to create solid-state carbon-nanotube generators of optical harmonic.     

Study of boron and molybdenum plasmas on multipulse interaction of femtosecond radiation with a target

Laser plasmas generated by femtosecond radiation on the surface of boron and molybdenum targets are investigated by the shadowgraph method. The modes of single-pulse and multipulse interaction of laser radiation with a target are compared. The occurrence of plasma bullets is discussed, which were observed on both single-pulse and multipulse interaction with the same area of a target. The wavefront velocities of expanding boron and molybdenum plasmas were measured to be 5 × 10⁴ and 6 × 10³ m s^?1, respectively. The electron density measured by interferometry using a time delay of 800 ps in a boron plasma excited by 795-nm radiation with an intensity of 10¹⁶ W cm^?2 amounted to 8 × 10¹⁹ cm^?3. The correlation between some specific features of the plasma and the generation of the 3/2 harmonic, observed on multipulse interaction of femtosecond radiation with a boron target, is discussed.     

Generation of narrowband tunable VUV radiation at the Lyman-α wavelength

Tunable narrowband VUV radiation has been generated at the Lyman-α wavelength $\text{λ =}\text{1216}\text{\&}\text{A}\text{ring}$; by frequency tripling in krypton the frequency-doubled output of a powerful dye laser system which is excited with the second harmonic of a Nd-YAG laser. 5 ns long UV dye laser pulses ($\text{λ =}\text{3646}\text{A}\text{?}$) of 1.8 MW peak power yielded VUV light pulses of 2.2 W (5.4 × 19⁹ photons/pulse). The bandwidth of the dye laser radiation could be narrowed to 8.7 × 10^-3cm^-1 (4.6 × 10^-3 Å). The expected bandwidth of the VUV is less than 5.2 × 10^-2cm^-1 (7 × 10^-4 Å). The tunable VUV radiation is used for the recording of the absorption spectra of the Lyman-α resonance transitions in atomic hydrogen and deuterium with doppler-limited resolution.     

Microspectral analysis of dentine with femtosecond laser induced plasma

Microspectral analysis of dentine plasma produced by femtosecond laser radiation with intensities of I ～ 10¹³?10¹⁵ W/cm² in ambient atmosphere has been measured. C, O, Ca, Zn, Na, and Cu spectral lines were identified. The X-ray radiation with energies E > 30 keV has been observed upon laser beam intensities of I ～ 5 × 10¹⁵ W/cm².     

Ellipticity dependence of atomic and molecular high harmonic generation

High harmonic generation is compared in the dependence on the ellipticity of the fundamental laser radiation for an atomic and a molecular system. In particular argon and nitrogen are compared employing molecular beams and intense ( 3×10¹⁴ W/cm²) and ultrashort (80 fs) 800 nm laser pulses. It turns out that for all the harmonics under investigation (H5, H13 and H21) the harmonic yield decreases slower with the ellipticity for the molecule than for the atom. This indicates differences in atomic and molecular high harmonic generation. Received 24 April 2002 Published online 24 September 2002     

The nonlinear optical coefficient,phasematching, and optical damage in the chalcopyrite AgGaSe2

We have made a direct measurement of the absolute nonlinear coefficient for AgGaSe₂ by phasematched second harmonic generation. The measured value is d₃₆ = (3.24 ± 0.50) × 10^?11 m/V. For CO and Co₂ lasers the observed phasematching angles for second harmonic generation and frequency mixing are within one degree of the values calculated by fitting the reported index of refraction data to Sellmeier equations. At 1.06 μm the optical damage threshold depends on the number of incident laser pulses. For 1000 pulses damage occurs at 11 MW/cm². The samples were cut from crack-free single crystal boules with absorption coefficient smaller than 0.1 cm^? at 10.6 μm.     

Generation of the third harmonic of near IR femtosecond laser radiation tightly focused into the bulk of a transparent dielectric in the regime of plasma formation

The generation of the third harmonic of femtosecond laser radiation with a wavelength of 1.24 μm tightly focused into the bulk of fused silica was simulated numerically for the regime with light intensity of 3 × 10¹³ W/cm², which is extreme for solid bodies. The efficiency of third harmonic generation (THG) was found to be restricted to 0.1% in the regime of plasma formation. This is determined by two competitive processes: a decrease of THG efficiency due to an increase of wave detuning and an increase of THG efficiency due to a growth of focusing asymmetry. In an isotropic medium, determination of the threshold of plasma formation by use of the third harmonic signal is shown to be a more sensitive method as compared with the standard scheme of nonlinear transmittance detection.     

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.     

Third-harmonic generation in polyvinylpirrolidone-based metallic complexes

The third-order nonlinear susceptibilities of the aqueous solutions of polyvinylpirrolidone doped with iron and zinc were studied by the method of third-harmonic generation with 1064-nm Nd:YAG laser radiation. The efficiencies of transformation into the third harmonic in these metallic complexes were 8×10^?7 and 5×10^?7, respectively. To determine the role of the Kerr effect in the transformation process, the nonlinear refractive index of the iron-doped polyvinylpirrolidone solution was measured by the Z-scan method and a value of n ₂=?6.7×10^?13 cgs unit was obtained.     

Energy of a shock wave generated in different metals under irradiation by a high-power laser pulse

The energies of a shock wave generated in different metals under irradiation by a high-power laser beam were determined experimentally. The experiments were performed with the use of targets prepared from a number of metals, such as aluminum, copper, silver and lead (which belong to different periods of the periodic table) under irradiation by pulses of the first and third harmonics of the PALS iodine laser at a radiation intensity of approximately 10¹⁴ W/cm². It was found that, for heavy metals, like for light solid materials, the fraction of laser radiation energy converted into the energy of a shock wave under irradiation by a laser pulse of the third harmonic considerably (by a factor of 2–3) exceeds the fraction of laser radiation energy converted under irradiation by a laser pulse of the first harmonic. The influence of radiation processes on the efficiency of conversion of the laser energy into the energy of the shock wave was analyzed.     

High harmonic generation in a laser plasma

The results of investigation of high harmonics of radiation of a Ti:sapphire laser propagating through a laser plasma generated on the surface of different targets are presented. For most of the targets, the intensity distribution of the high harmonics generated is found to form a plateaulike pattern similar to that observed in the case of gas jets. The generation of high harmonics (up to the 65th harmonic, λ = 12.24 nm) is caused by the interaction of femtosecond laser radiation with ions. The conversion efficiency in the plateau region varies from 10^?7 to 8 × 10^?5 depending on the target. The main restriction on the conversion efficiency and the peak intensity of the harmonics generated is caused by the self-defocusing of the femtosecond radiation due to free charge carriers formed as a result of tunnel ionization.     

强激光场中离子HD+光解离几率的相干控制

对含时薛定谔方程用短时传播子的对称分割法求得了非微扰的数值解,计算了强超短脉冲基频激光(波长306.7nm)与其三倍频激光作用下的离子HD⁺光解离的相干控制参量大小设该离子的初态为电子振动.基态其中的相干激发是共振的.二束光之间的相对相位变化从0到360°在基频和倍频激光强度各为5×10¹³W/cm²和5.09×10⁸W/cm²情形下,发现相对相位为π时,光解离几率达到最大。     

Quenching kinetics and small signal gain spectrum of the ZnI photodissociation laser

By photodissociation ZnI₂ with 193 nm (ArF) laser radiation, the rate constants for quenching of the upper and lower energy levels of the ZnI (B → X) laser by ZnI₂ have been measured to be (1.7 ± 0.2) × 10^-9 and (1.4 ± 0.4) × 10^-9 cm³ s^-1, respectively. Although the former rate constant was found to be laser intensity-dependent for I ? 10⁵ W cm^-2, the ZnI(B) state radiative lifetime was determined to be 26 ± 4 ns. Also, the small signal gain coefficient, g₀, of this molecular laser has a peak value of ? 15% cm^-1 at λ ? 602 nm and exceeds 5% cm^-1 for 591 nm ≤ λ ≤ 608 nm for a potential tuning range of at least 170 Å.     

Spectra of secondary emission during generation of optical harmonics in globular photonic crystals

Secondary emission spectra of globular silica photonic crystals when their surfaces were exposed to laser pulses 250 fs long at a power density to 1 TW/cm² have been studied. Optical harmonics and plasma emission were detected in this case. For the opal matrix containing pores filled with air, in the reflection mode, the third optical harmonic with a conversion efficiency of ～10% arises. The highest conversion efficiency for exciting radiation with wavelengths of 1026 or 513 nm is implemented when the frequencies of the exciting radiation or the second harmonic are near the stop band edge. In globular photonic crystals filled with sodium nitrite or barium titanate ferroelectrics, the second optical harmonic is observed. The exciting radiation conversion efficiency to the second optical harmonic was a few percent and depended on the frequency of exciting radiation and photonic crystal globule diameters. It is found that the plasma emission intensity increases with the exciting radiation power density. The dependences of the intensity of the second and third optical harmonics on the pump intensity are constructed for various photonic crystal globule diameters.     

Spectroscopic Studies of Indium Plasma Produced by Fundamental (1,064 nm) and Second (532 nm) Harmonics of an Nd:YAG Laser

In this paper, we present spectroscopic studies of a laser-induced indium (In) plasma produced by fundamental (1,064 nm) and second (532 nm) harmonics of an Nd:YAG laser along with the characteristics determined by plasma parameters. The electron temperature is determined using four lines of neutral indium at 260.17, 271.02, 275.38, and 325.85 nm, in view of the Boltzmann plot method. The temperature varies from 6,470 K at 0.05 nm to 4,990 K at about 2 mm from the target surface for the fundamental wavelength and from 6,250 to 4,880 K for the second harmonic. The electron density is ±300 calculated using the Stark broadened profiles recorded at laser pulse energy 130 mJ (for fundamental) and 72 mJ (for second harmonic) as 5:8·10¹⁶ and 6:9·10¹⁶ cm^?3, respectively. These values decrease to 3:5·10¹⁵ and 4:9·10¹⁵ over a distance of 2 mm from the target surface, respectively. Moreover, we study the variation of N _e as a function of laser irradiance as well as its spatial variation from the target surface.     

Terahertz emission dependence on the intensity ratio of 400–800 nm in generating terahertz waves from two-color laser-induced gas plasma

A transient photocurrent model is used to explain terahertz emission from gas plasma irradiated by a laser pulse and the second harmonic. By introducing the second harmonic, 400 nm, the corresponding terahertz emission is greatly enhanced. The exact dependence of terahertz emission on the intensity ratio of 400–800 nm is studied for the case with total intensity of 5.00 × 10¹⁴ W/cm². Results show the emission reaches the maximum at about the case for energy distribution of I_ω = 4.00 × 10¹⁴ W/cm², I_2ω = 1.00 × 10¹⁴ W/cm².     

A study of the nonlinear refractive index in amorphous Ge–As–S layers and nanocomposites with gold nanoparticles

Nonlinear refractive indices of simple and composite chalcogenide glasses with gold nanoparticles are measured by the Z-scan method using a femtosecond Ti:sapphire laser with a central wavelength of 800 nm and a pulse duration of 40 fs. It is shown that introduction of nanoparticles into thin layers of amorphous GeS₂, As₃₀Ge₂₀S₅₀, and As₃Ge₃₅S₆₂ leads to a decrease in the total nonlinear refractive index due to additional absorption of femtosecond laser radiation without efficient excitation of localized plasmons outside the resonance region. For example, the nonlinear refractive index decreases with addition of gold nanoparticles from 16.1 × 10^–12 to 13.0 × 10^–12 cm²/W for GeS₂, from 3.9 × 10^–12 to 3.2 × 10^–12 cm²/W for As₃₀Ge₂₀S₅₀, and from 5.8 × 10^–12 to 4.7 × 10^–12 cm²/W for As₃Ge₃₅S₆₂.     

Deposition of nanoparticles during laser ablation of nanoparticle-containing targets

We analyze the morphology of ablated nanoparticles after their laser-induced deposition on various substrates. We show that, at moderate laser intensity of the 210 ps pulses on the surface of nanoparticle-containing materials (<5×10⁹ W?cm^?2), the deposited material remains approximately the same as the initial nanoparticles. We compare these deposited nanoparticles with the debris obtained by the laser ablation of bulk material of the same origin as nanoparticles at different intensities of laser radiation. The presence of nanoparticles in laser plumes allowed for analyzing high-order nonlinear optical properties of nanoparticles. The efficient high-order harmonic generation was achieved during propagation of femtosecond pulses through such plasmas.     

Ionenlaser hoher Leistung

Investigations on ion lasers with large bored tubes (7 ... 15 mm I.D.) without additional axial magnetic field are performed. An axial magnetic field is shown to be not necessary to achieve high laser power. By absence of additional magnetic fields the laser construction is considerably simplified. Experimental criterions for maximum laser power are derived by means of a previously published theoretical paper. 120 W total power summed up over the 4p-4s Argon II transitions in the visible part of the spectrum, and 1.5 W ultraviolet power in Argon (3638 Å, 3511 Å) and Krypton (3507 Å) are obtained in continuous mode operation. High inversion densities of 7 · 10⁹ cm^?3 give rise to non-resonant laser oscillations. By multipass amplification the spontaneous emission is amplified up to 20 W/cm², having a beam divergence of about 10^?4 rad. All results are pointing out the influence of radiation trapping effects on the laser power to be smaller than estimated and measured by other authors using conditions deviating from our optimum conditions for maximum laser power.