Determination of nonlinear optical characteristics of transparent materials using single laser pulses

A new method of express analysis of nonlinear optical properties of materials by determining their nonlinear refraction and nonlinear absorption upon irradiation by a single laser pulse is proposed. The nonlinear optical characteristics of media are determined based on an analysis of variations in the spatial distribution of radiation focused by a cylindrical lens and passed through the medium. The method proposed is tested by the example of optical nonlinearities of CS₂.     

Third-order nonlinear optical response in double-walled carbon nanotubes

Resonant behavior and magnitudes of third-order nonlinear optical susceptibilities in double-walled carbon nanotubes (DWNTs) have been investigated by means of femtosecond pump-probe spectroscopy with different pump-photon energies. With the selective excitation of the E₂₂ exciton transition of the inner tubes labeled by the chiral vector indices (7,5) and (7,6), the imaginary part of nonlinear susceptibility Imχ⁽³⁾ has shown the resonant enhancement compared with the case of the nonresonant excitation of the specific tube. The nonlinear response signal at the E₂₂ transition energy of the (8,7) tube has been also enhanced for the excitation of the G-band phonon sideband of its E₂₂ transition. This result is consistent with the phonon-mediated nonlinear optical process observed for the E₂₂ transitions in single-walled carbon nanotubes (SWNTs). It has been also found that the values of the figure of merit Im χ⁽³⁾/α (α: absorption coefficient) of the inner tubes in DWNTs are smaller than those of the corresponding SWNTs, which is interpreted in terms of decay time shortening due to the energy relaxation between the inner and outer tubes.     

Influence of optical field emission on the nonlinear photoelectric effect induced by ultrashort laser pulses

A decrease in the order of nonlinearity of photoelectron emission from a metal cathode was found experimentally with mode-locked laser pulses at sufficiently high intensities. This phenomenon is interpreted as due to the theoretically predicted appearance of optical field emission.     

Waves in nonlinear lattices: ultrashort optical pulses and Bose-Einstein condensates

The nonlinear Schr?dinger equation i (partial differential)(z)A(z,x,t)+(inverted Delta)(2)(x,t)A+[1+m(kappax)]|A|2A=0 models the propagation of ultrashort laser pulses in a planar waveguide for which the Kerr nonlinearity varies along the transverse coordinate x, and also the evolution of 2D Bose-Einstein condensates in which the scattering length varies in one dimension. Stability of bound states depends on the value of kappa=beamwidth/lattice period. Wide (kappa>1) and kappa=O(1) bound states centered at a maximum of m(x) are unstable, as they violate the slope condition. Bound states centered at a minimum of m(x) violate the spectral condition, resulting in a drift instability. Thus, a nonlinear lattice can only stabilize narrow bound states centered at a maximum of m(x). Even in that case, the stability region is so small that these bound states are "mathematically stable" but "physically unstable."     

Vortices in ultrashort laser pulses

The propagation of optical vortices nested in broadband femtosecond laser beams was studied both numerically and experimentally. Based on the nonlinear Schrödinger equation, the dynamics of different multiple-vortex configurations with varying topological charge were modelled in self-focussing and self-defocussing Kerr media. We find a similar behavior in both cases regarding the vortex–vortex interaction. However, the collapsing background beam alters the propagation for a positive nonlinearity. Regimes of regular and possibly stable multiple filamentation were identified this way. Experiments include measurements on pairs of filaments generated in a vortex beam on an astigmatic Gaussian background with argon gas as the nonlinear medium. Spectral broadening of these filaments leads to a supercontinuum which spans from the visible range into the infrared. Recompression yields <19 fs pulses. Further optimization may lead to much better recompression.     

Reaching the nonlinear regime of Raman amplification of ultrashort laser pulses

The intensity of a subpicosecond laser pulse was amplified by a factor of up to 1000 using the Raman backscatter interaction in a 2 mm long gas jet plasma. The process of Raman amplification reached the nonlinear regime, with the intensity of the amplified pulse exceeding that of the pump pulse by more than an order of magnitude. Features unique to the nonlinear regime such as gain saturation, bandwidth broadening, and pulse shortening were observed. Simulation and theory are in qualitative agreement with the measurements.     

Measurement of ultrashort laser pulses

A proposal for measuring the pulse shape of ultrashort laser pulses is described. The proposed method consists of a Gamo triple intensity-interferometer to measure the triple-correlation function of the light pulses. A subsequent signal processing algorithm is used to reconstruct the true shape of the light pulses out of their triple-correlation function. The suggested method is quite insensitive to noise, as shown by means of computer simulations.     

Microablation with ultrashort laser pulses

This paper reports the micromachining results of different materials (Al, Si, InP and fused silica) using a Ti : sapphire laser at the wavelength of 800 and 267 nm with variable pulse lengths in the range from 100 fs to 10 ps. The hole arrays with a diameter up to a few μm through microdrilling are presented. We discussed how an effective suppression of the thermal diffusion inside the ablated materials and an effective microablation could be realized. If the laser fluence is taken only slightly above the threshold, a hole array can be drilled with diameters even smaller than the wavelength of the laser. Some examples are presented in the paper.     

Remote sensing of the atmosphere using ultrashort laser pulses

Theoretical and experimental studies were performed on the propagation of ultrashort optical terawatt pulses through the atmosphere. Propagation simulations of intense sub-picosecond pulses show that non-linear processes, such as white light generation, can be initiated at a chosen distance by selecting an appropriate group velocity dispersion. With this technique, a white light continuum was generated in the atmosphere whose spectral distribution was characterised in the visible and near infra-red. Applications of this novel light source for atmospheric remote sensing were investigated, combining lidar and time-resolved broadband absorption spectroscopy techniques. Measurements were performed on the oxygen molecule and water vapour. A comparison between the experimental results and the tabulated spectroscopic data led to an excellent correlation with measurements made on water vapour whereas observations on the oxygen showed discrepancy. This study demonstrates that the remote generation of a white light source represents a new way to access the range-resolved multi-trace gas analysis in the atmosphere. Received: 8 December 1999 / Revised version: 18 May 2000 / Published online: 16 August 2000     

Anisotropic dewetting polydimethylsiloxane surface fabricated using ultrashort laser pulses

Anisotropic dewetting polydimethylsiloxane(PDMS) surfaces, which consist of groove-like micro/nanostructures(so-called hierarchical structures), are fabricated using an ultrashort pulsed laser. The contact angles(CAs) are measured parallel to the microgrooves, which are always larger than those measured perpendicular to the microgrooves, exhibiting a superhydrophobic anisotropy of approximately 4° on these fabricated PDMS surfaces at optimized parameters. These pulsed-laser irradiated surfaces exhibit enhanced hydrophobicity with CAs that increase from 116° to 156° while preserving the anisotropic dewetting. Additionally, the wettability of the surfaces with different morphologies is investigated. The temporal evolution of the wettability of the pulsed-laser irradiated PDMS surface is also observed within the first few hours after pulsed laser irradiation.     

Depolarization of white light generated by ultrashort laser pulses in optical media

We report measurements of the extinction ratio (ER) of white light generated upon irradiation of BK7 glass by ultrashort (36 fs) laser pulses with incident power approximately 10(3) times larger than the critical power for self-focusing. At low incident powers, the continuum is symmetric about the incident laser wavelength; at high powers it becomes broader and distinctly asymmetric towards the blue side. We observe that ER degrades by 100-fold after the onset of multiphoton-induced free-electron generation (at incident intensity approximately 2 TW cm-2), which also corresponds to the onset of asymmetry in white-light spectra.     

Interaction of ultrashort elliptically polarized laser pulses with nonlinear helical photonic metamaterial

The effect of nonlinear properties of a material with a periodic structural cell (three-dimensional spiral) on the specificity of transmission and reflection of elliptically polarized laser pulses normally incident on the metamaterial is studied using the finite-difference time-domain method. An analysis of the hodograph of electric-field strength vector showed that an increase in the peak intensity of a linearly polarized laser pulse incident on a sample leads to an increase in the orthogonal component of the electric-field strength vector in the pulse transmitted through the medium. When pulses containing few electric-field periods are incident on a metamaterial, the latter demonstrates radically different optical properties for right- and left-handed circularly polarized light passing through the medium. It was shown that an increase in the intensity of a right-handed (left-handed) circularly polarized ultrashort pulse, incident on a sample composed of a rather large number of right-handed (left-handed) spirals made of nonlinearmaterial, widens the frequency range within which the incident light is almost entirely reflected from the medium.     

Processing of transparent materials using visible nanosecond laser pulses

Laser micromachining of transparent materials is an intensively studied research area from the point of view of microoptical element fabrication. One of the most promising indirect processing methods is the laser-induced back-side dry etching (LIBDE). During this method, transparent targets are contacted with solid thin layers, which absorb and transform the pulse energy resulting in etching. The applicability of LIBDE technology for processing of fused silica using a visible nanosecond dye laser (λ=500 nm, FWHM=11 ns) and a 100-nm-thick aluminium absorbing layer was investigated. The applied fluence was varied in the range of 0–3050 mJ/cm²; the illuminated area was 0.1 mm². The threshold fluence of the LIBDE etching of fused silica was found to be approximately 540 mJ/cm². The chemical composition of the surface layers on and around the etched holes was investigated by field-emission scanning electron microscopy and energy-dispersive X-ray spectrometry. It was found that on average 0.4±0.3 at. % aluminium is built into the upper ∼1-μm-thick volume of the illuminated fused silica, while the aluminium content fell below the detection limit in the case of the original surface. Our experiments proved that the LIBDE procedure is suitable for microprocessing of transparent materials using visible nanosecond laser light. PACS 42.62.-b; 61.80.Ba; 81.16.Rf; 81.65.Cf     

Microdeposition of metal and oxide structures using ultrashort laser pulses

2 O₃ on glass and silicon substrates is performed. The superior quality of the results allows the direct, one-step fabrication of binary-amplitude and multilevel optical diffractive structures. Received: 4 February 1998/Accepted: 9 February 1998     

Ultrafast optical switching of an ionized medium by interfering ultrashort laser pulses

Multibeam ionization of gas and condensed-phase media with interfering ultrashort laser pulses can help to substantially increase the maximum field intensity and the density of free electrons attainable in the focus of the laser field relative to the regime of single-beam ionization. Multibeam ionization schemes offer new solutions for laser micro- and nanomachining, micro- and nanosurgery, spectral and temporal transformation of ultrashort light pulses, as well as remote sensing of the atmosphere. Subfemtosecond changes in the local refractive index induced in the regime of multibeam tunneling ionization enable the high-speed switching of optical signals.     

Molecular wakes for ultrashort laser pulses

The molecular wake-assisted interaction between two collinear femotosecond laser pulses is investigated in air,which leads to the generation of a controllable 1.8 mJ super-continuum pulse with an elongated self-guided channel due to the cross-phase modulation of the impulsively aligned diatomic molecules in air. For two parallel launched femtosecond laser pulses with a certain spatial separation,controllable attraction and repulsion of the pulses are observed due to the counter-balance among molecular wakes,Kerr and plasma effects,where the molecular wakes show a longer interaction distance than the others to control the propagation of the intense ultrashort laser pulses.     

Third-harmonic and sum-frequency generation in a quadratically nonlinear polymer by time-ordered ultrashort laser pulses

Third-harmonic and sum-frequency generation in quadratically nonlinear azopolymer films is experimentally studied using femtosecond chromium forsterite laser pulses. A noncollinear geometry of sum-frequency and third-harmonic generation developed and implemented in this work allows the influence of the time ordering of ultrashort laser pump pulses on nonlinear-optical phenomena to be experimentally observed. Femtosecond laser pulses induce transitions of azopolymer molecules to an electronically excited state and produce vibrational wave packets, leading to an asymmetry in the dependence of the efficiency of second-and third-order nonlinear-optical processes on the delay time between the pump pulses.     

Memory in nonlinear ionization of transparent solids

We demonstrate a shot-to-shot reduction in the threshold laser intensity for ionization of bulk glasses illuminated by intense femtosecond pulses. For SiO2 the threshold change serves as positive feedback reenforcing the process that produced it. This constitutes a memory in nonlinear ionization of the material. The threshold change saturates with the number of pulses incident at a given spot. Irrespective of the pulse energy, the magnitude of the saturated threshold change is constant (approximately 20%). However, the number of shots required to reach saturation does depend on the pulse energy. Recognition of a memory in ionization is vital to understand multishot optical or electrical breakdown phenomena in dielectrics.     

飞秒激光在晶体中传输的级联非线性过程

系统地回顾了有关级联过程导致的各种非线性效应的工作,指出了级联效应可导致不同光谱成分间出现能量转移现象,且二阶级联过程中产生的白光还能够提供种子光进而得到基于参量放大过程的宽带锥形辐射。在此基础上,进一步研究了入射的基频波长对于超连续白光能量转移效果及产生倍频效率的影响以及二阶级联过程引起的偏振调制现象,指出当入射波长越靠近相位匹配波长时倍频效率越高,发现二阶级联效应还可以引起显著的基频光偏振调制。     

飞秒激光在晶体中传输的级联非线性过程

系统地回顾了有关级联过程导致的各种非线性效应的工作,指出了级联效应可导致不同光谱成分间出现能量转移现象,且二阶级联过程中产生的白光还能够提供种子光进而得到基于参量放大过程的宽带锥形辐射。在此基础上,进一步研究了入射的基频波长对于超连续白光能量转移效果及产生倍频效率的影响以及二阶级联过程引起的偏振调制现象,指出当入射波长越靠近相位匹配波长时倍频效率越高,发现二阶级联效应还可以引起显著的基频光偏振调制。