Optical control of the anisotropy of the orientation of molecules in a liquid

The control of the anisotropy of the orientation of molecules in pure 1,2-dichlorobenzene, C₆H₄Cl₂ at room temperature is demonstrated experimentally. To accomplish the optical control, the medium is exposed to non-resonant excitation with two successive linearly polarized laser pulses with a duration of 60 fs. The state of transient anisotropy is probed with the third pulse by detecting the ultrafast optical Kerr effect via optical heterodyne detection and synchronous demodulation. It is shown that variations in the two parameters, the delay time between two pump pulses, and the angle between the polarization directions of the pump pulses ensure the control of the anisotropy of the orientation of molecules in the subpicosecond region. Original Russian Text ? V.G. Nikiforov, G.M. Safiullin, A.G. Shmelev, A.V. Leont’ev, V.S. Lobkov, 2007, published in Pis’ma v Zhurnal éksperimental’noĭ i Teoreticheskoĭ Fiziki, 2007, Vol. 86, No. 10, pp. 757–761.     

Femtosecond laser control of intramolecular vibrations in a liquid

Optical control of coherent intramolecular oscillations in chloroform CHCl₃ and dimethyl sulfoxide (CH₃)₂SO is attained experimentally under normal conditions by means of femtosecond polarization spectroscopy. Nonresonant excitation of the medium is accomplished by a sequence of two linearly polarized laser pulses. The state of the medium is probed by the third pulse via the optical Kerr effect. We show that control over the vibrational dynamics of molecules on a sub-picosecond scale can be achieved by varying the delay between the excitation pulses and their relative intensity.     

Orientation of the AD-1 azo-dye molecules in solid nanostructured films upon the two-photon excitation

The orientation of the AD-1 azo-dye molecules in solid nanostructured films is studied upon the nonlinear excitation using femtosecond laser pulses. A significant dichroism resulting from the irradiation is due to the two-photon absorption. The optical anisotropy induced by the two-photon absorption is proven to be related only to the reorientation of the azo-dye molecules and is not caused by alternative effects (e.g., photobleaching).     

KBe2BO3F2飞秒光参量放大中的群速匹配

提出了KBe₂BO₃F₂ （KBBF）飞秒光参量放大中三波群速完全匹配的方法.对于Ⅰ类相位匹配方式采用非共线结构及倾斜抽运光脉冲波面,对于Ⅱ类相位匹配方式采用共线结构同时倾斜抽运光和信号光脉冲波面,可实现参量带宽最大时三波群速的完全匹配,从而获得更高增益,更窄脉宽的参量光. 关键词： 群速匹配 相位匹配 脉冲波面倾斜 飞秒光参量放大     

Experimental observation of tunable wavelength down- and up-conversions of ultra-short pulses in a periodically poled LiNbO3 waveguide

All-optical tunable wavelength conversion of ps-pulses is proposed and experimentally demonstrated by use of cascaded sum- and difference frequency generation (cSFG/DFG) in a periodically poled LiNbO₃ (PPLN) waveguide. The amplified spontaneous emission (ASE) noise is effectively suppressed by employing two tunable filters. As a result, tunable wavelength down- and up-conversions are simultaneously observed. The temporal evolutions of the signal, pump, control and idler waves propagating along the PPLN waveguide are simulated. From the temporal waveforms and optical spectra in the simulations, it is interesting to find that continuous wave (CW) pump and control are evolved into optical pulses during the cSFG/DFG nonlinear interactions. Moreover, some new sidebands in the output pump spectrum are observed both in the simulations and experiments.     

Interference of dissociating wave packets in the I2 molecule driven by femtosecond laser pulses

The interference between two dissociating wave packets of the I2 molecule driven by femtosecond laser pulses is theoretically studied by using the time-dependent quantum wave packet method.Both the internuclear distanceand velocity-dependent density functions are calculated and discussed.It is demonstrated that the interference pattern is determined by the phase difference and the delay time between two pump pulses.With two identical pulses with a delay time of 305 fs and a FWHM of 20 fs,more interference fringes can be observed,while with two pump pulses with a delay time of 80 fs and a FWHM of 20 fs,only a few interference fringes can be observed.     

Determination of the amplitudes and phases of the scattering matrix upon photodissociation of molecules

The photodissociation of RbI molecules via the second excited state under the action of circularly polarized radiation with a wavelength of 266 nm is studied. By using sub-Doppler Faraday spectroscopy, the angular distributions of the orientation of spins of Rb(5²S_1/2) atoms formed are studied and the anisotropy parameters describing these angular distributions are determined. Based on the anisotropy parameters, the ratio between the amplitudes of the inelastic scattering matrix elements corresponding to the two possible photodissociation channels is determined to be r_Ω=1/r_Ω=0=0.71±0.15, while the phase difference for these matrix elements is found to amount to Δφ = 173° ± 29°.     

Theory and experiment for atomic optical filter based on optical anisotropy in rubidium

An atomic optical filter based on optical anisotropy induced by another left-circularly polarized pump field is theoretically and experimentally investigated in the 5S_1/2–5P_3/2–5D_3/2 ladder-type system of rubidium vapor. The filter displays a single peak transmission of 14.4% with 396 MHz bandwidth narrower than Doppler width. Furthermore, the variation of peak transmission versus pump intensity, pump detuning and cell temperature are also given. The theoretical and experimental results are in good agreement, which can enhance understanding of operation mechanism for this category of filter. The narrow-bandwidth filter is useful for free-space optical systems and laser spectroscopy.     

Study of femtosecond dynamics in vibrationally excited free bis(trifluoromethyl)ketene and metal carbonyl molecules induced by CO bond resonant excitation

The dynamics of relaxation processes in free bis(trifluoromethyl)ketene (CF₃)₂CCO and Fe(CO)₅ and Cr(CO)₆ metal carbonyl molecules after multiphonon excitation of the C=C=O and C=O vibrations by femtosecond laser infrared radiation was studied. The temporal and spectral dependences of the relaxation of the excited vibrational states were measured. Kinetics with a characteristic decay time of about 5 ps was obtained for (CF₃)₂CCO molecules. Its behavior is interpreted as a manifestation of the intramolecular relaxation of the excited vibration states of the resonance mode. Kinetic curves with characteristic times of about 250–500 fs were observed for Fe(CO)₅ and Cr(CO)₆. The behavior of these curves depends on the mutual orientation of the polarizations of the pump and probe pulses.     

Concept of Photorefractive Connection Module by Photorefractive Four-Wave Mixing with Extraordinary Writing Beams and an Ordinary Reading Beam

We propose a new optical network device photorefractive connection module (PRCM) which operates as optical switch, amplifier and signal distributor controlled by parallel optical signals. Simple optical control bus systems can be realized by cascade connection of PRCMs. PRCM branches off a desired channel from the spatial multiplexed optical bus line by appropriate setting of the control beam pattern. PRCM uses cross polarized four wave mixing (CPFWM) with extraordinary polarized writing beams and an ordinary polarized reading beam to achieve a high connection gain to the next PRCM stage. We analyze the phase matching angle of CPFWM in which the optical paths of two pump beams are slightly different. The phase conjugate reflectivity indicating a branching ratio of optical signal is derived and calculated in consideration of the phase mismatching Δk. The optimum pump ratio and the grating vector orientation for the largest phase conjugate reflectivity and signal amplification factor are discussed for optical design of PRCM. Since the measured signal beam power after passing through the BaTiO₃ crystal is three or four times higher than its incident power, PRCM has a sufficient connection gain for optical bus and interconnection systems.     

Preparation and the optical nonlinearity of surface chemistry improved titania nanoparticles in poly(methyl methacrylate)-titania hybrid thin films

With 800-nm, 120-fs laser pulses, optical nonlinearity has been studied in a series of thin films containing poly(methyl methacrylate) (PMMA), filled with surfactant acetylacetone (Acac) capped TiO₂ nanoparticles, which were synthesized by a simple in situ sol-gel/polymerization process, assisted by spin coating and multi-step baking. The resulting nanohybrid thin films have highly optical transparency and demonstrate a unique nonlinear optical (NLO) response. The highest nonlinear refractive index (n₂) is observed up to 6.55 × 10⁻² cm² GW⁻¹ in the nanohybrid thin film of 60 wt% Ti(OBu)₄ in PMMA, with a negligible two-photon absorption (TPA), as confirmed by the Z-scan technique. The titanium precursor loading combined with the nature of the capping molecules are used to influence the ability of nanoparticles to nonlinear optical response. Indeed, the ligands at the nanoparticles’ surface can not only control the extent of the interaction between the organic molecules and the embedded nanoparticles but also influence the optical nonlinearities of nanoparticles.     

Local-field anisotropy of a light wave in quasi-two-dimensional soft-matter objects

Experimental values of the Lorentz tensor components L _j for uniaxial quasi-two dimensional “soft matter” objects on substrates (bilayer lipid membranes, multilayer Langmuir films, smectics A, hexatic smectics B, submicron films of discotics Col _hd , micron anisotropic films of liquid-crystal comblike polymers and macromolecular polymers, submicron films of conjugated conductive polymers), freely suspended submicron films of smectics A, and uniaxially stretched micron films of conjugated conductive polymers have been determined using dispersion of refractive indices in the visible range. The dependences of the components L _j on the type of orientation (axial, planar) of uniaxial molecules (structural units of the film) with respect to the optical axis of the film, the film thickness, the substrate type, the chemical structure of molecules, and their long-range orientational order are established. It is revealed that the smectic A-hexatic B phase transition and two-dimensional crystallization of the smectic layer lead to changes in the components L _j due to the change in the orientational ordering of molecules as a result of the relation between the orientational and hexatic order parameters. All the above objects are characterized by isotropization of the Lorentz tensor L and the local-field tensor f with a simultaneous decrease in the birefringence of the sample and in the anisotropy of the molecular polarizability due to the change in the electronic structure of molecules. The correction for the anisotropy of the local-field tensor f to the orientational order parameter or the anisotropy of the molecular polarizability increases. The existing model approaches to calculating the components L _j for the objects under consideration are compared with the experimental data.     

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.     

High‐power HgGa2S4 optical parametric oscillator pumped at 1064 nm and operating at 100 Hz

The defect chalcopyrite crystal HgGa₂S₄ has been employed in a 1064‐nm pumped optical parametric oscillator operating at 100 Hz, to generate ∼5 ns long idler pulses near 4 µm with energies as high as 6.1 mJ and average power of 610 mW. At crystal dimensions comparable to those available for the commercial AgGaS₂ crystal, operation of the 1064‐nm pumped HgGa₂S₄ OPO is characterized by much lower pump threshold and higher conversion efficiency, with the most important consequence that such a device might become practical at pump levels sufficiently lower than the optical damage threshold.     

Coherent control of the lattice dynamics of bismuth near the lindemann stability limit

The relative contributions of the anharmonicity of the lattice potential and the nonequilibrium concentration of charge carriers to the time dependence of the coherent A _1g phonon frequency in bismuth excited by high-energy ultrashort laser pulses are studied by the coherent control method. The contributions are separated by the pump-probe method in which excitation is performed by two pulses with a controlled time delay. It is shown that, depending on the relative delay between the pump pulses, both correlation and anticorrelation between the amplitude and the initial frequency of oscillations are observed while the chirp and the initial frequency of the coherent phonon are anticorrelated. In addition, it has been found that the contributions of the lattice and the electronic subsystem are always anticorrelated. Therefore, a certain phase can be assigned to an electronic excitation and it may be suggested that the time dependence of the phonon frequency is determined not only by instantaneous values of the lattice and electronic response but also by the phase relations between them.     

Effect of a pulsed magnetic field on the nonlinear dynamics of vortexlike domain walls in magnetic films

The Landau-Lifshitz equation is numerically solved to study the nonlinear dynamic behavior of domain walls with the 2D vortexlike magnetization distribution in magnetically uniaxial films that have in-plane anisotropy and are exposed to a pulsed magnetic field. It is shown that a pulsed magnetic field H _p may induce transitions between various steady wall motions that differ in magnetization distribution. Solitary rectangular pulses, as well as a regular train of rectangular pulses, may be used to control the period of nonlinear dynamic transformations of the wall internal structure and the related period of variation of the wall velocity.     

Optical second harmonic generation and its photoinduced dynamics in ferroelectric semiconductor Sn<Subscript>2</Subscript>P<Subscript>2</Subscript>S<Subscript>6</Subscript>

By means of optical pump–probe technique, the ultrafast dynamics of nonlinear optical response of the ferroelectric semiconductor Sn₂P₂S₆ crystal excited with a femtosecond laser pulse has been investigated. It has been shown that, under the action of femtosecond pulses, change in optical second harmonic generation occurs in the sample, which can be due to screening of existing electric polarization.     

Generation of tunable picosecond light pulses covering the frequency range between 2700 and 32,000 cm−1

The travelling-wave parametric process allows the generation of intense picosecond pulses tunable between 2700 and 17,000 cm⁻¹. Angle- and temperature tuning was investigated using LiNbO₃ crystals of different cuts. Various factors determining the bandwidth of the new pulses are evaluated and experimental spectra are presented. Frequency doubling readily leads to light pulses up to 32,000 cm⁻¹. The jitter of two parametric systems operating in parallel is less than 1 ps for pump pulses of 7 ps.     

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.