Electronic Kerr Effect Research Articles

In-situ diagnostic of femtosecond laser probe pulses for high resolution ultrafast imaging

Ultrafast imaging is essential in physics and chemistry to investigate the femtosecond dynamics of nonuniform samples or of phenomena with strong spatial variations. It relies on observing the phenomena induced by an ultrashort laser pump pulse using an ultrashort probe pulse at a later time. Recent years have seen the emergence of very successful ultrafast imaging techniques of single non-reproducible events with extremely high frame rate, based on wavelength or spatial frequency encoding. However, further progress in ultrafast imaging towards high spatial resolution is hampered by the lack of characterization of weak probe beams. For pump–probe experiments realized within solids or liquids, because of the difference in group velocities between pump and probe, the determination of the absolute pump–probe delay depends on the sample position. In addition, pulse-front tilt is a widespread issue, unacceptable for ultrafast imaging, but which is conventionally very difficult to evaluate for the low-intensity probe pulses. Here we show that a pump-induced micro-grating generated from the electronic Kerr effect provides a detailed in-situ characterization of a weak probe pulse. It allows solving the two issues of absolute pump–probe delay determination and pulse-front tilt detection. Our approach is valid whatever the transparent medium with non-negligible Kerr index, whatever the probe pulse polarization and wavelength. Because it is nondestructive and fast to perform, this in-situ probe diagnostic can be repeated to calibrate experimental conditions, particularly in the case where complex wavelength, spatial frequency or polarization encoding is used. We anticipate that this technique will enable previously inaccessible spatiotemporal imaging in a number of fields of ultrafast science at the micro- and nanoscale.

Nonlinear thermal and electronic optical properties of graphene in N-methylpyrrolidone at 800 nm with femtosecond laser pulses

Graphene is a useful saturable absorber in a variety of lasers working in mode-locking or Q-switch regimes. The optical performance of chemically synthesized graphene is still not completely characterized. In this study, the saturable absorption and the nonlinear refractive index of graphene flakes in N-methylpyrrolidone, in both liquid and solid phases, have been studied at 800 nm with the z-scan technique using femtosecond laser pulses. The results obtained using a Ti:sapphire laser oscillator in the mode-locking regime (6 fs, 78 MHz) or in the continuous wave shows that the optical properties of graphene have a thermal origin, while at the lower repetition rate and higher energy and intensity of a Ti:sapphire amplifier (95 fs, 1 kHz), it shows the electronic Kerr effect. Solid samples with very high optical densities, equivalent to 60 layers of graphene grown by chemical vapor deposition (CVD), can be fabricated. They show a higher saturation intensity (Is≈100 GW cm–2) than CVD-grown (74 MW cm–2) or epitaxially grown (4 GW cm–2) graphene and intensity-dependent changes in transmission from 25% to 43%. This change in transmission in multilayer solid samples points to a good performance as a saturable absorber in laser cavities.

Effect of –OH functionalization, C2 methylation, and high radiation fields on the non-linear optical response of imidazolium ionic liquids

Considering the impending applications of room temperature ionic liquids (RTILs) in various areas involving high optical and radiation fields, it is pertinent to probe the structure–property correlation of these solvents exposed to such conditions. Herein, femtosecond Z-scan technique (at high pulse repetition rate, 80 MHz) was employed to investigate the non-linear optical response of imidazolium RTILs in 3 scenarios: (1) -OH functionalization, (2) C2 methylation, and (3) influence of high radiation fields. Large negative non-linear refractive values (n 2) were observed in all the RTIL samples and have been attributed predominantly due to the thermal effects. In order to isolate and determine the contribution of electronic Kerr effect, the Z-scan experiments were also carried out at low pulse repetition rate (i.e. 500 Hz) by means of a mechanical chopper. The closed aperture transmittance profile showed the valley-peak pattern, which signifies positive non-linearity. Nonetheless, the variation in the n2 values of the RTILs follows the same trend in low pulse repetition rate as was observed in case of high pulse repetition rate. The trend in the n 2 values clearly showed the decrease in the non-linearity in the first two cases and has been attributed to the weakening of the ion-pair formation, which adversely affects the charge transfer between the ionic moieties via C2 position. However, an increase in the n 2 values was observed in case of ILs irradiated to high radiation doses. This enhancement in the non-linearity has been assigned to the formation of double bond order radiolytic products. These results clearly indicate a strong correlation between the non-linearity and the strength of cation–anion interaction amongst them. Therefore, such information about these solvents may significantly contribute to the fundamental understanding of their structure–property relationships.

Optimal all-optical switching of a microcavity resonance in the telecom range using the electronic Kerr effect.

We have switched GaAs/AlAs and AlGaAs/AlAs planar microcavities that operate in the "Original" (O) telecom band by exploiting the instantaneous electronic Kerr effect. We observe that the resonance frequency reversibly shifts within one picosecond when the nanostructure is pumped with low-energy photons. We investigate experimentally and theoretically the role of several parameters: the material backbone and its electronic bandgap, the quality factor, and the duration of the switch pulse. The magnitude of the frequency shift is reduced when the backbone of the central λ-layer has a greater electronic bandgap compared to the cavity resonance frequency and the frequency of the pump. This observation is caused by the fact that pumping with photon energies near the bandgap resonantly enhances the switched magnitude. We thus find that cavities operating in the telecom O-band are more amenable to ultrafast Kerr switching than those operating at lower frequencies, such as the C-band. Our results indicate that the large bandgap of AlGaAs/AlAs cavity allows to tune both the pump and the probe to the telecom range to perform Kerr switching without detrimental two-photon absorption. We observe that the magnitude of the resonance frequency shift decreases with increasing quality factor of the cavity. Our model shows that the magnitude of the resonance frequency shift depends on the pump pulse duration and is maximized when the duration matches the cavity storage time to within a factor two. In our experiments, we obtain a maximum shift of the cavity resonance relative to the cavity linewidth of 20%. We project that the shift of the cavity resonance can be increased twofold with a pump pulse duration that better matches the cavity storage time. We provide the essential parameter settings for different materials so that the frequency shift of the cavity resonance can be maximized using the electronic Kerr effect.

Model of optical nonlinearity of air in the mid-IR wavelength range

A model of optical nonlinearity of air (atmospheric nitrogen and oxygen) is developed. This model can be used to calculate numerically the propagation of radiation with a wavelength close to . It takes into account the electronic Kerr effect, higher order nonlinearities, ionisation of a gas medium by electron impact, and pulse group-velocity dispersion. The applicability limits of the Drude approximation for calculating the impact-ionisation rate are also considered.

All-optical switching of a microcavity repeated at terahertz rates

We have repeatedly and reproducibly switched a GaAs-AlAs planar microcavity operating in the "original" telecom band by exploiting the virtually instantaneous electronic Kerr effect. We achieve repetition times as fast as 300 fs, thereby breaking the terahertz modulation barrier. The rate of the switching in our experiments is only determined by optics and not by material-related relaxation. Our results offer opportunities for fundamental studies of cavity quantum electrodynamics and optical information processing in the subpicosecond time scale.

Competition between electronic Kerr and free-carrier effects in an ultimate-fast optically switched semiconductor microcavity

We have performed ultrafast pump-probe experiments on a GaAs-AlAs microcavity with a resonance near 1300 nm in the "original" telecom band. We concentrate on ultimate-fast optical switching of the cavity resonance that is measured as a function of pump-pulse energy. We observe that at low pump-pulse energies the switching of the cavity resonance is governed by the instantaneous electronic Kerr effect and is achieved within 300 fs. At high pump-pulse energies the index change induced by free carriers generated in the GaAs start to compete with the electronic Kerr effect and reduce the resonance frequency shift. We have developed an analytic model which predicts this competition in agreement with the experimental data. Our model includes a new term in the intensity-dependent refractive index that considers the effect of the probe pulse intensity, which is resonantly enhanced by the cavity. We calculate the effect of the resonantly enhanced probe light on the refractive index change induced by the electronic Kerr effect for cavities with different quality factors. By exploiting the linear regime where only the electronic Kerr effect is observed, we manage to retrieve the nondegenerate third order nonlinear susceptibility for GaAs from the cavity resonance shift as a function of pump-pulse energy.

Comparison of frequency-resolved optical polarization gating induced by molecular alignment and Kerr effects

We experimentally demonstrated that both the electronic Kerr effect and the molecular alignment in gaseous molecules could be applied as transient gates to diagnose 400 nm target pulses. Their birefringence dissimilarity was clearly visualized by the measured spectrogram and retrieved gate function. In the atomic gas argon, a relatively weak and instantaneous cross phase modulation within the pulse duration was observed, while in the molecular gas N(2), the delayed rotational Raman excitation played a dominative role.

Investigation of nonlinear dynamics in CdS by time-resolved nondegenerate pump–probe system with phase object

The time-resolved nondegenerate pump–probe system with phase object is employed for investigation of nonlinear absorption and refraction dynamics in CdS. The 532 nm laser beam with 21 ps duration is used as the excitation and the laser beams of 600 and 680 nm with 10 ps duration from optical parametric generation are used for probing. The experimental results at both probe wavelengths show free-carrier absorption and large free-carrier refraction along with two-photon absorption and bound electronic optical Kerr effect. By numerically fitting the experimental data based on the nondegenerate pump–probe theory, the nondegenerate two-photon absorption coefficient, the nondegenerate Kerr coefficient, the free-carrier decay time, the free-carrier absorptive cross-section and free-carrier refractive coefficient at different wavelengths are all determined.

Hybrid microspheres for nonlinear Kerr switching devices

Electronic Kerr effect in a polyfluorene derivative is used to reversibly switch near infrared probe beam resonantly coupled to a hybrid polymer-silica microspherical resonator. NIR pumping at 780 nm in pulsed laser regime is used for non-linear switching of the WGM resonances that shift as much as 2 GHz for 50 mW of average pump power, compared to a shift of 250 MHz for the same average pump power at CW regime. The absence of temporal drift and the magnitude of this shift confirm the Kerr nature of the switching, ruling out thermooptical effects.

Ultimate fast optical switching of a planar microcavity in the telecom wavelength range

We have studied a GaAs–AlAs planar microcavity with a resonance near 1300 nm in the telecom range by ultrafast pump-probe reflectivity. By the judicious choice of pump frequency, we observe an ultimate fast and reversible decrease in the resonance frequency by more than half a linewidth due to the instantaneous electronic Kerr effect. The switch-on and switch-off of the cavity is only limited by the cavity storage time of τcav=0.3 ps and not by intrinsic material parameters. Our results pave the way to supraterahertz switching rates for on-chip data modulation and real-time cavity quantum electrodynamics.

Direct measurement of field-free molecular alignment by spatial (de)focusing effects

We directly measure the field-free molecular alignment of various room-temperature molecular gases based on alignment-induced spatial focusing and defocusing effects. By imaging the spatial profile of a time-delayed probe pulse with increased and decreased local intensity at the beam center, the parallel and perpendicular molecular alignments are clearly characterized. Meanwhile, the electronic Kerr effect, weak plasma contribution and field-free molecular alignment impact could be distinguished from the measured signals.

General approach to spatiotemporal modulational instability processes

In this article, we derive the general exact solution of the modulation instability gain. The solution described here is valid for 1-D, 2-D, and 3-D cases considering any temporal response function of the medium and with possible higher order Kerr nonlinearities. In particular, we show that the gain induced by modulation instability is initial condition dependent, while the usual calculations do not lead to such a dependence. Applications for current and high-interest nonlinear propagation problems, such as 1-D optical fiber propagation with delayed Raman response and 2-D filamentation in gases, are investigated in detail. More specifically, we demonstrate that the 2-D model of filamentation based on the balance between higher order Kerr terms leads to a modulation instability window. The impact of both self-steepening and space-time defocusing effects is also highlighted. Finally, we discuss the influence of the finite-time response of the different order electronic Kerr effects on the growth of the expected modulation instability bands.

Femtosecond filamentation in air and higher-order nonlinearities

According to a recent experiment, the instantaneous electronic Kerr effect in air exhibits a strong intensity dependence, the nonlinear refractive index switching sign and crossing over from a self-focusing to a defocusing nonlinearity. A subsequent theoretical work has demonstrated that this has paradigm-changing consequences for the understanding of filamentation in air, so it is important to subject the idea of higher-order nonlinearities to stringent tests. Here we use numerical modeling to propose an experiment capable of discriminating between the standard and the new intensity-dependent Kerr-effect models.

Femtosecond filament induced birefringence in argon and in air: Ultrafast refractive index change

The birefringence of a linearly polarized femtosecond laser filament in gases has been previously established. In this work, we report the time-dependent refractive index measurements of the filament based upon the spectral modulation of a weak probe pulse in air and in argon gas at 1 atmospheric pressure. The polarization dependence of the refractive index modulation induced by the delayed molecular alignment and by the electronic Kerr effect is highlighted. A numerical simulation of the refractive indices, which takes into account the molecular alignment, the electronic Kerr non-linearity and the plasma, is in good agreement with the measurements.

Observation of a fifth-order optical nonlinearity in Bi0.9La0.1Fe0.98Mg0.02O3 ferroelectric thin films

Ferroelectric Bi0.9La0.1Fe0.98Mg0.02O3 thin films with high optical quantity were deposited on quartz substrates by radio frequency magnetron sputtering at 650 °C. The interplay between third- and fifth-order optical nonlinearities is observed by performing Z-scans under femtosecond laser excitation with 1.60 eV photon energy. The measured third-order nonlinearities mainly originate from electronic Kerr effect and two-photon absorption (2PA), while the population redistribution assisted by 2PA leading to an equivalent stepwise χ(3):χ(1) process is the main mechanism of the fifth-order effect.

Identification of competing ultrafast all-optical switching mechanisms in Si woodpile photonic crystals

We present a systematic study of ultrafast all-optical switching of Si photonic bandgap woodpile crystals using broadband tunable nondegenerate pump-probe spectroscopy. At pump-probe coincidence, we investigate the behavior of the differential reflectivity at the blue edge of the stop band for a wide range of pump and probe frequencies. Both dispersive and absorptive features are observed from the probe spectra at coincidence. As the pump frequency is tuned through half the electronic bandgap of Si, the magnitude of both these features increases. For the first time, to the best of our knowledge, we unambiguously identify this dispersive effect with the electronic Kerr effect in photonic crystals and attribute the absorptive features to nondegenerate two photon absorption. The dispersive and absorptive nonlinear coefficients are extracted and are found to agree well with the literature. Finally, we propose a nondegenerate figure of merit, which defines the quality of switching for all nondegenerate optical switching processes.

Kerr and free carrier ultrafast all-optical switching of GaAs/AlAs nanostructures near the three photon edge of GaAs

We performed nondegenerate pump-probe experiments on a GaAs/AlAs photonic cavity structure. We switched the photonic properties using the optical Kerr effect and free carriers excited by three photon absorption. The structure was probed at 1150–1640 nm, in the telecom spectral range below the stop gap. In the measurements we observe surprisingly large nondegenerate electronic Kerr coefficients over a broad wavelength range. We also extracted the three photon absorption coefficient for GaAs at three wavelengths in the near infrared. We conclude that the electronic Kerr effect is so large that the resonance of a moderate Q (Q>1000) photonic cavity could be successfully switched instantaneous.

Surface Plasmon-Mediated Energy Transfer in Heterogap Au−Ag Nanowires

We report the observation of energy transfer from a gold (Au) nanodisk pair to a silver (Ag) nanowire across a 120 nm gap via surface plasmon resonance (SPR) excitation. The enhanced electromagnetic (EM) fields generated by Au SPR excitation induce oscillation of the conduction electrons in the Ag segment, transferring energy to it even though the Ag segment has only weak resonant interactions with the incident EM radiation. The induced Ag SPR produces strong EM fields at the position of the Ag segment, leading to a Raman signal approximately 15 times greater than when the Ag segment is alone (not adjacent to the Au nanodisk pair). The Raman intensity is found to depend nonlinearly on the incident laser intensity for laser power densities of 10 kW/cm(2), which is consistent with the results of EM theory calculations which are not able to account for the factor of 15 enhancement based on a linear mechanism. This suggests that energy transfer from the Au disk pair to the Ag segment involves an enhanced nonlinear polarization mechanism such as can be produced by the electronic Kerr effect or stimulated Raman scattering.

Third-order nonlinear-optical parameters of gold nanoparticles in different matrices

Third-order nonlinear-optical properties of gold nanoparticles embedded in Al 2O 3, ZnO and SiO 2 have been investigated by the Z-scan method at the wavelength of 532 nm using nanosecond Nd 3+:YAG laser radiation. The nonlinear refractive index, nonlinear absorption coefficient, and the real and imaginary parts of the third-order nonlinear susceptibility are deduced. The results of the investigation of nonlinear refraction using the off-axis Z-scan configuration are presented and the mechanisms responsible for the nonlinear response are discussed. The prevailing influence of the electronic Kerr effect over the possible thermo-optical contribution is demonstrated.