Self-diffraction Experiments Research Articles

Setting the photoelectron clock through molecular alignment

The interaction of strong laser fields with matter intrinsically provides a powerful tool for imaging transient dynamics with an extremely high spatiotemporal resolution. Here, we study strong-field ionisation of laser-aligned molecules, and show a full real-time picture of the photoelectron dynamics in the combined action of the laser field and the molecular interaction. We demonstrate that the molecule has a dramatic impact on the overall strong-field dynamics: it sets the clock for the emission of electrons with a given rescattering kinetic energy. This result represents a benchmark for the seminal statements of molecular-frame strong-field physics and has strong impact on the interpretation of self-diffraction experiments. Furthermore, the resulting encoding of the time-energy relation in molecular-frame photoelectron momentum distributions shows the way of probing the molecular potential in real-time, and accessing a deeper understanding of electron transport during strong-field interactions.

Ultrafast optical phase modulation with metallic nanoparticles in ion-implanted bilayersilica

The nonlinear optical response of metallic-nanoparticle-containing composites was studiedwith picosecond and femtosecond pulses. Two different types of nanocompositeswere prepared by an ion-implantation process, one containing Au nanoparticles(NPs) and the other Ag NPs. In order to measure the optical nonlinearities, weused a picosecond self-diffraction experiment and the femtosecond time-resolvedoptical Kerr gate technique. In both cases, electronic polarization and saturatedabsorption were identified as the physical mechanisms responsible for the picosecondthird-order nonlinear response for a near-resonant 532 nm excitation. In contrast,a purely electronic nonlinearity was detected at 830 nm with non-resonant 80 fspulses. Regarding the nonlinear optical refractive behavior, the Au nanocompositepresented a self-defocusing effect, while the Ag one presented the opposite, that is, aself-focusing response. But, when evaluating the simultaneous contributions when thesamples are tested as a multilayer sample (silica–Au NPs–silica–Ag NPs–silica), wewere able to obtain optical phase modulation of ultra-short laser pulses, as aresult of a significant optical Kerr effect present in these nanocomposites. Thisallowed us to implement an ultrafast all-optical phase modulator device by using acombination of two different metallic ion-implanted silica samples. This control ofthe optical phase is a consequence of the separate excitation of the nonlinearrefracting phenomena exhibited by the separate Au and Ag nanocomposites.

H-PDLC/Clay Nanocomposites

Holographic polymer-dispersed liquid crystals (H-PDLCs) filled with layer-structured montmorillonite (MMT) clay were fabricated by wave mixing of two coherent argon-ion laser beams. The effects of MMT doping were investigated by comparing a pristine sodium-type MMT and an organophilic MMT (OMMT) via modification. In order to gain insights into the complex formation of the ternary nematic/polymer/clay nanocomposites, wide-angle X-ray diffraction experiments were carried out on pure liquid crystal, MMT clay, and clay dispersed in the liquid crystal in the mesophase as well as isotropic state. Self-diffraction experiments in the Raman–Nath regime revealed that the first-order diffraction efficiency was enhanced in the H-PDLCs consisting of pristine MMT nanoparticles and yet suppressed by the incorporation of OMMT. Holographic nature of the polymer-matrix nanocomposites was confirmed by optical polarizing microscopy. The results of this study may open up great promise held by such ternary polymer-matrix nanocomposites for photonic applications.

Thermo-optic effect and optical third order nonlinearity in nc-Si embedded in a silicon-nitride film

Using a self-diffraction experiment with 7ns pulses at 532nm we studied a silicon nitride film containing silicon nanoclusters (nc-Si) of 3.1+/-0.37 nm mean size. The sample was prepared by remote plasma-enhanced chemical vapor deposition (RPECVD), and we found that its nonlinearity consists of a combination of electronic and thermal contributions. By varying the repetition rate of the laser, we discriminated the responsible mechanisms for the nonlinear response. Using this procedure we determined a total /chi((3))1111/ = 3.3x10(-10)esu, n2 = 2.7x10(-16) m(2)/W, beta = 1x10(-9) m/W and dn/dT =1x10(-4) degrees C(-1) for our sample. We also show results for the optical Kerr effect using 80 fs pulses at 820 nm. The purely electronic nonlinearity measured is characterized by /chi((3))1111/=9.5 x10(-11) esu.

Optical third-order nonlinearity by nanosecond and picosecond pulses in Cu nanoparticles in ion-implanted silica

We report a study of the Kerr effect and nonlinear optical absorption in a high-purity silica sample containing Cu nanoparticles prepared by ion implantation. With a vectorial self-diffraction experiment, we measured the tensorial components of the third-order nonlinear optical susceptibility response at 532 nm with pulses of 7 ns and 26 ps. We identified thermal effect as the main mechanism responsible for the nonlinear refraction in the nanosecond regime and electronic polarization for the picosecond regime. We observed saturable optical absorption in both regimes studied. We also measured the ablation threshold for this material, finding that the contribution of the linear optical absorption to the ablation threshold in each case can be different due to the presence of hot electrons.

Thermal Diffusion Shock Waves

The Ludwig-Soret effect or thermal diffusion, which refers to the separation of liquid mixtures in a temperature gradient, is governed by a nonlinear, partial differential equation in space and time. It is shown here that the solution to the nonlinear differential equation for a binary mixture predicts the existence of shock waves completely analogous to fluid shocks and obeys an expression for the shock velocity that is an exact analogue of the Rankine-Hugoniot relations. Direct measurements of the time dependent, spatial absorption profile of a suspension of nanometer sized particles subjected to a sinusoidal temperature field generated by a pair of continuous laser beams, as well as self-diffraction experiments, show motion of the particles in agreement with the predictions of nonlinear theory.

Dynamic self-diffraction studies in some dye-doped gelatins

Some phenothiazine derivative dyes and triphenylmethane dyes doped into water-swollen gelatin matrices were studied with respect to their ability of thick hologram formation with a low power conventional HeNe laser source. The highest light diffraction efficiencies measured in a self-diffraction experiment were obtained for the well known methylene blue dye but also for other dyes related to it: Giemsa and May-Grunwald stains. For the first time to our knowledge hologram formation abilities were found for crystal violet and brilliant blue dyes in water-swollen gelatin.

Squeezed light generation by means of cascaded second-order nonlinearity

This paper shows that the experimental configuration for self-diffraction experiments can be applied to squeezed light (SL) generation. Nonlinear processes arising from cascading of the second order χ (2) nonlinearity are responsible for the pump waves squeezing far from the perfect phase matching points for second harmonic and sum frequency generation. The operation of squeezing is performed by interference of co-travelling waves which depending on phase matching conditions for the cascaded processes, are affected either by self-phase modulation (SPM) (as in the nonlinear Mach-Zehnder interferometer) or by cross-phase modulation (CPM) (as in the type-II phase matched second harmonic generation scheme). Since large values of induced third order nonlinearities χ ind (3) are manifested in the presence of cascaded χ (2), this scheme of SL generation may be of great practical interest. Diffracted waves may also be used to create squeezed vacuum.

Femtosecond coherent spectroscopy of bulk ZnSe and ZnCdSe/ZnSe quantum wells.

Femtosecond two beam, self-diffraction experiments are performed in ZnSe and ZnCdSe/ZnSe quantum wells. Degenerate wave mixing signals from the third to the thirteenth order are easily visible to the naked eye, implying that nonlinear effects in this system may already be in the nonperturbative regime. Spectrally resolved data indicate that the nonlinearities originate entirely from the excitonic resonance. In addition, the existence of four-wave mixing signals in bulk ZnSe at negative time delay suggests that exciton-exciton interaction plays an important role. Exciton-phonon interaction rates are deduced as a function of temperature in ZnSe and ZnCdSe/ZnSe quantum wells.

Material characterization by holographic methods

After an introduction to the theoretical fundamentals, different holographic techniques for material characterization are presented: Isotropic and anisotropic diffraction measurements, interferometric techniques, holographic microphotometry, isotropic and anisotropic holographic scattering and self-diffraction experiments, beam-coupling topography and neutron holography.

Ultrafast chi /sup (3)/-related processes in semiconductor doped glasses

Experimental results are described for exploiting the fast and large nonlinear susceptibility of semiconductor-doped glasses (SDGs). Self-diffraction experiments were performed using incoherent broadband lasers with femtosecond time resolution yielding new information on the time response of the SDG nonlinearity. The use of backward nondegenerate wave-mixing to demonstrate yellow-to-green frequency conversion with femtosecond resolution is reported. In addition, results are presented which demonstrate the use of the SDG as a picosecond all optical gate at a 100-MHz repetition rate. >

Nonlinear Diffraction of CW-Laserbeams by Spatial Selfphase-Modulation in Nematic Liquid Crystals

Abstract Optical nonlinearities in nematic liquid crystals lead to strong self-defocussing and nonlinear diffraction effects with low power laser intensities of some hundred Watts/cm2. We describe these diffraction effects by expanding the transversally modulated intensity behind the sample into gaussian beams. Using in addition a simple model of light-induced reorientation of nematic liquid crystals as nonlinear mechanism, we get good agreement between theory and self-diffraction experiments, which have been done in the liquid crystal 5CB.

Transport length, quantum efficiency, and trap density measurement in Bi12SiO2

We report the measurement of diffusion transport length LD, quantum efficiency φ for photoelectron generation, and the Debye screening length and associated trap density NA in an undoped Bi12SiO20 (BSO) photorefractive crystal. The method is based on the optical erasing of holographically recorded gratings in the crystal. The measurement of the diffraction efficiency evolution is considerably facilitated profitting from polarization properties of anisotropic diffraction in BSO-type crystals in a self-diffraction experiment. Data processing is carried out using simple linear regression techniques for experimental conditions in two limiting situations: far from and well into trap saturation conditions. We prove that optical erasure closely verifies the decay law predicted by theory even if a highly contrasted pattern of light is used for recording. We measured NA=6.3×1016 cm−3, agreeing in order of magnitude, with already reported values, but found out LD=0.1 μm, which is too much lower than data in literature for an electron-based charge transport material as this one.

Ultrafast processes in silicon studied by transient gratings

Light induced grating measurements on monocrystalline silicon have been performed with excitation by 25 ps pulses both at 532 and 1064 nm. 1300 nm radiation was used as probe pulse. Self-diffraction experiments with parallel and orthogonal polarization of the beams allow us to determine coherence time and duration of the pulses, respectively. By measuring the grating decay time at various grating periods both the ambipolar diffusion constant and linear and surface recombination times can be obtained. Furthermore, it was demonstrated that parameter measurements in silicon can be performed using the diffraction "in reflection."

Laser induced damage, nonlinear absorption, and doubling efficiency of LiO3

Laser-induced damage thresholds of single crystal LilO <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> have been studied using picosecond pulses at 1.06 μm and 0.53 μm. These thresholds depend on wavelength, on crystal orientation, and on the number of times the sample has been irradiated. In addition, the doubling efficiency at high irradiance levels was observed to be a decreasing function of irradiance beyond a critical value. We present evidence to show that this results from the onset of optical parametric down conversion. In separate nonlinear transmission studies, reversible nonlinear transmission of 1.06 μm light was measured, and in self-diffraction experiments, both reversible and irreversible optically-induced complex index of refraction changes at 0.53 μm were observed.