Formation of Hot Images in Laser Beams through a Self-defocusing Kerr Medium Slab

A nonlinear hot image is usually thought as of a special case of self-focusing, and thus occurs when a laser beam propagates through a slab of self-focusing medium. Here we show theoretically that a hot image may also be formed by a thin slab of self-defocusing medium. The physical origin for this hot image formation is akin to the in-line volume-phase holographic imaging due to the intensity-dependent refractive-index modulation of the self- defocusing medium. NumericM simulations confirm the theoretical prediction and further identify the dependence of the hot image on the beam power, the modulation depth of obscuration and the thickness of self-defocusing medium. The analysis presented here brings new insight into the physics of hot image formation in the high power laser system.     

Polarization properties of SC generated in CaF2

Polarization of SC generated by 80 fs pulses of Ti:Sapphire laser strongly depends on the orientation of CaF₂ plate with respect to the plane of polarization of the pump light. Intensity modulation of the broad blue-shifted wing of SC against crystal rotation for the polarization parallel and orthogonal to the polarization of the pump beam is observed with CaF₂ and Sapphire. Observed modulation is due to evolution of the polarization state along the propagation distance in crystal.     

Transition from Backscattering Speckles to Phase Conjugation in LiNbO3：Fe

We investigate the backscattering for converging beams in iron-doped lithium niobate crystals. Due to the nonlinear properties of the crystal, backscattering exhibits temporally fluctuating speckles that make a transition to the phase conjugate of the incident beam under certain circumstances. Our observation seems to point to a new kind of self-pumped phase conjugation in photorefractive media.     

Detection of scattered light pulses at femto-Joule level by using a picosecond BBO optical parametric amplifier

We report a technique for detecting weak scattered light pulses based on a 532 nm pumped picosecond β-barium-borate collinear optical parametric amplifier. The measured maximum slope gain factor of the amplifier was found to be around 10⁷, and the energy detection limit was of the order of fJ/pulse for the signal of 730 nm and the idler of 1.5 μm at a pumping intensity of 2.83 GW/cm². The linearity of the gain for this amplifier was found to be excellent for a seeding level of lower than 420 fJ per pulse. The maximum gains and the energy detection limits for the scattered light pulses from various scattering targets were found to be poorer than that from the reflective mirror, owing to the degraded beam quality and the depolarization of the scattered light. A reduction of the maximum gain for the scattered light with the increase of the angle of incidence, which causes pulse broadening and reduces the photon flux of the signal, was investigated. The feasibility of detecting weak scattered light in the infrared by using idler-to-signal frequency up-conversion was also demonstrated, in which the infrared seeder located in the idler branch of the amplifier was injected as the seeding beam and was then parametrically up-converted into the visible signal branch, with an even higher gain.     

A precise data processing method for extracting χ from Z-scan technique

We present a precise data processing method, in the Z-scan experiments using a Gaussian beam and trimmed Airy beam, for directly extracting nonlinear refraction from the closed aperture Z-scan trace with the aid of the open aperture Z-scan trace when the materials exhibit nonlinear refraction and nonlinear absorption simultaneously. This method is still applicable when the nonlinear absorption is dominant and the closed aperture Z-scan curve degenerates into a single valley configuration, which is a salient advantage over other methods. In addition, we give gracefully empirical formulas with very high precision, which have good practicability for characterizing the optical nonlinearities of materials by use of the Gaussian beam and trimmed Airy beam Z-scan technique, respectively.     

Radiation force exerted on nanometer size non-resonant Kerr particle by a tightly focused Gaussian beam

We calculate the radiation force that is exerted by a focused continuous-wave Gaussian beam of wavelength λ on a non-absorbing nonlinear particle of radius a ? 50λ/π. The refractive index of the mechanically-rigid particle is proportional to the incident intensity according to the electro-optic Kerr effect. The force consists of two components representing the contributions of the electromagnetic field gradient and the light scattered by the Kerr particle. The focused intensity distribution is determined using expressions for the six electromagnetic components that are corrected to the fifth order in the numerical aperture (NA) of the focusing objective lens. We found that for particles with a < λ/21.28, the trapping force is dominated by the gradient force and the axial trapping force is symmetric about the geometrical focus. The two contributions are comparable with larger particles and the axial trapping force becomes asymmetric with its zero location displaced away from the focus and towards the beam propagation direction. We study the trapping force behavior versus incident beam power, NA, λ, and relative refractive index between the surrounding liquid and the particle. We also examine the confinement of a Kerr particle that exhibits Brownian motion in a focused beam. Numerical results show that the Kerr effect increases the trapping force strength and significantly improves the confinement of Brownian particles.     

Propagation of four-petal Gaussian beams in strongly nonlocal nonlinear media

The propagation of four-petal Gaussian beams in strongly nonlocal nonlinear media has been studied. The analytical solution and the analytical second-order moment beam width are obtained. For the off-waist incident and the waist incident cases, the intensity pattern evolves periodically during propagation in strongly nonlocal nonlinear media. Under the off-waist incident condition, the second-order moment beam width varies periodically during propagation, whatever the input power is. But under the waist incident condition, there exists a critical power. When the input power equals the critical power, the second-order moment beam width remains invariant, otherwise the second-order moment beam width varies periodically. Numerical simulations based on the nonlocal nonlinear Schrödinger equation are carried out for comparison with the theoretical predictions. The results show that the numerical simulations are in good agreement with the analytical results in the case of strong nonlocality.     

Electromagnetic wave scattering from two interacting small spherical particles. Influence of their optical constants, ε and μ

In this research, the influence of the optical constants ε and μ on the scattering patterns of a system consisting of two interacting Rayleigh particles is analyzed. We study specific scattering configurations in which the particles are separated by a fixed distance and where the connecting axis has fixed orientations with respect to the incident electromagnetic field. Multiple scattering effects and how they are affected by the optical properties of the particles are considered.     

Laser Mode-Dependent Size of Plasma Zones Induced by Femtosecond Laser Pulses in Fused Silica

We carry out the numerical simulations of femtosecond laser propagation with TEMoo mode, TEM10 mode and a beam combining both the modes in fused silica. It is found that the transverse size of plasma zones induced by laser pulses with the TEM10 mode is smaller than that induced by the TEM00 mode, while the longitudinal size is almost the same, and the saturated plasma density is higher. The transverse size, the longitudinal size and the ratio of the longitudinal to transverse size, for the beam combining both the modes, all could be reduced at the same time in comparison with the TEM00 mode under the same focusing conditions.     

Induced Diffraction in Phase-Mismatched Second-Harmonic Generation

We show analytically that in phase-mismatched second-harmonic generation, an effective diffraction is induced at the second-harmonic （SH） frequency. Numerical simulation results agree with the analytical predictions. Compared to the case of linear propagation, the effect of the overall diffraction at the SH frequency becomes doubled due to the induced diffraction, which causes an interesting result that the SH beam width will be larger than that of the fundamental field.     

Elliptically polarized Terahertz emission through four-wave mixing in a two-color filament in air

We diagnosed the polarization characteristics of Terahertz emission from a two-color femtosecond laser filament when the polarizations of ω and 2ω beams are orthogonal. We discovered that the THz pulse is elliptically polarized. The generation mechanism could be through four-wave optical rectification inside the filament zone where the inversion symmetry of air is broken.     

Growth and optical properties of In:Nd:LiNbO3 crystals

In, Nd double-doped LiNbO₃ (LN) crystals have been grown for the first time. Their infrared (IR) transmission spectra were measured and discussed to investigate their structure and defects. The optical damage resistance of Nd:In:LiNbO₃ crystals were characterized by straightly observing transmission facula distortion method. The optical damage resistance of In (4.0 mol%):Nd:LiNbO₃ was much higher than that of Nd:LiNbO₃. The defects were discussed in this paper to explain the optical damage resistance in the In:Nd:LiNbO₃ crystals.     

Link between the laws of geometrical optics and the radiative transfer equation in media with a spatially varying refractive index

We proposed in a previous paper [J.-M. Tualle, E. Tinet, Opt. Commun. 228 (2003) 33] a modified radiative transfer equation to describe radiative transfer in a medium with a spatially varying refractive index. The present paper is devoted to the demonstration that this equation perfectly works in the non-absorbing/non-scattering limit, what was contested by Martí-López and coworkers [L. Martí-López, J. Bouza-Domínguez, R.A. Martínez-Celorio, J.C. Hebden, Opt. Commun. 266 (2006) 44]. The assertion that this equation would imply a zero divergence of the rays is also commented.     

Interaction of incoherent white-light solitons

We study numerically the formation and interactions of incoherent white-light solitons in a nonlinear media with logarithmic saturable nonlinearity based on the coherent density approach. We find that not only the formation but also the interactions of incoherent white-light solitons are fundamentally a collective effect, where all the frequencies participate in the process, and self-adjust their respective contributions to the formation and interactions. We find that the interactions of incoherent white-light solitons in logarithmic media are always attractive.     

Theoretical study of saturable Kerr nonlinearity using top-hat beam Z-scan technique

Saturable Kerr nonlinearity is theoretically investigated by use of the top-hat beam Z-scan technique. The saturation intensity changes the nonlinear refractive profile and decreases the sensitivity of the Z-scan measurements, which were quantitatively analyzed. An empirical formula for the saturable Kerr nonlinearity, which gave the relationship between the light intensity and the peak-valley transmittance difference, was accomplished. A high-accuracy method to determine the nonlinear refractive index and the characteristic saturation intensity was proposed.     

Enhanced frequency conversion of nonadiabatic pulses in a double Λ system driven by two pumps with and without carrier beams

We show that nonadiabatic, resonant amplitude- and phase-modulated pulses can be frequency converted with greater efficiency than adiabatic resonant pulses in a double Λ system, interacting with two strong cw beams on one side of the system, and a weak pulsed probe on the other. Indeed, in this double EIT (electromagnetically induced transparency) configuration, conversion efficiencies close to unity, similar to those achieved using highly detuned pulses, can be obtained using highly nonadiabatic resonant pulses. The distance at which the maximum conversion occurs is shorter than in a coherently-prepared Λ system. This counteracts the increased absorption that occurs in the double EIT configuration, so that both produce similar conversion efficiencies. The absorption experienced by matched nonadiabatic pulses in the double EIT system, at all propagation distances, can be overcome by superimposing the nonadiabatic pulses as amplitude modulations on carrier fields. Thus we demonstrate the formation of adiabatons in the double EIT system, and of diabatons in both the coherently-prepared Λ system and the double EIT system. Both the diabatons and adiabatons satisfy pulse-matching conditions. In addition, the asymptotic amplitude of the complementary amplitude modulations is proportional to the ratio of the pump to probe carrier Rabi frequencies, and is the same in each of the configurations.     

Broadband four-wave optical parametric amplification in bulk isotropic media in the ultraviolet

We report on four-wave optical parametric amplification of the ultrashort ultraviolet light pulses in bulk fused silica and CaF₂. Exact phase-matching in these isotropic media is achieved by means of non-collinear interaction with cylindrical beam focusing. Four-wave optical parametric amplifier efficiently operates in the UV spectral range with 1-ps laser pulses, delivering amplified signal energy exceeding 50 μJ using millijoule pump pulses in the visible (527 nm). Results of scanning of the parametric gain profile suggest that broad amplification bandwidth as wide as ∼20 nm (at FWHM) under these experimental settings is achieved, which might support amplification of sub-10-fs ultraviolet pulses with central wavelength around 330 nm. It is also shown experimentally and verified theoretically that the parametric gain profile exposes a distinct inhomogeneity and its bandwidth notably broadens due to effects of self- and cross-phase modulation imposed by the intense pump beam.     

Intense femtosecond pulse shaping using a fused-silica spatial light modulator

We present the design concept of a setup of a pulse shaper to be used for high-power femtosecond lasers. The pulse shaper is constructed from a high-damage threshold fused-silica spatial light modulator and a 4-f optical system based on the design concept to avoid optical damage. We have successfully demonstrated a pulse compression of 20 fs, 5 mJ pulses obtained from a 1 kHz repetition rate Ti:sapphire chirped pulse amplification system at an average power of 5 W.     

Structural properties of C60 in solution

In this paper SANS results on C₆₀ solutions are presented. The data analysis allows to determine the value of the gyration radius of C₆₀ dissolved in CS₂, a solvent capable to guarantee a good contrast for the neutron probe. The study indicates that the gyration radius R_g is independent of concentration.     

Theory of optical bistability in a non-linear quasi-waveguide

Optical bistable behavior in a quasi-waveguide containing non-linear film has been experimentally investigated in several publications in the past years; however, the physical mechanism for optical bistability has not been theoretically explained. In the present letter, we propose a theoretical model and successfully explain the observed optical bistability of both the reflected light and the scattered light (m-lines) in a non-linear quasi-waveguide. The optical bistability in the non-linear quasi-waveguide is due to the scattering-type wavevector mismatch mechanism. Received: 10 July 2002 / Revised version: 10 September 2002 / Published online: 20 December 2002 RID="*" ID="*"Corresponding author. Fax: +86-25/359-5535, E-mail: htwang@nju.edu.cn