Polarized Pump Pulse Research Articles

Ultrafast anisotropic x-ray scattering in the condensed phase

The advent of x-ray free electron lasers offers new opportunities for x-ray scattering (XRS) studies of ultrafast molecular dynamics in liquids, which have so far been limited to the 100 ps resolution of synchrotrons. In particular, anisotropic XRS induced by photoselection, using a linearly polarized pump pulse, can enhance the contrast of the signal from excited molecules against the diffuse background and allows the probing of their vibrational and rotational dynamics. Here, we present a computational approach for calculating transient scattering intensities, based on molecular dynamics simulations. This is applied to the study of the excited state dynamics of molecular iodine dissolved in n-hexane. We report that at short times the transient XRS patterns reflect the evolving vibrational and rotational dynamics of I2, even when the disordered solvent environment is included. We then use these simulations to derive the anticipated signal-to-noise ratio for a large class of model diatomic systems in solution, indicating that an S/N ⩾1 will be possible from single-shot experiments in weakly scattering solvents.

Optical polarization of localized hole spins inp-doped quantum wells

The initialization of spin polarization in localized hole states is investigated using time-resolved Kerr rotation. We find that the sign of the polarization depends on the magnetic field and the power and the wavelength of the circularly polarized pump pulse. An analysis of the spin dynamics and the spin-initialization process shows that two mechanisms are responsible for spin polarization with opposite sign: the difference of the $g$ factor between the localized holes and the trions, as well as the capturing process of dark excitons by the localized hole states.

Electron spin coherence and effect of spin polarization on electron relaxation dynamics in GaAs

Time-resolved circularly and linearly polarized pump-probe spectroscopy is used to study the evolution of the electron spin coherence and electron relaxation dynamics in bulk GaAs at 9.6 K. In particular, their dependence on photon energy (or electron excess energy) is carefully investigated. The absorption quantum beats which are observed in circularly polarized pump-probe spectroscopy are obtained, reflecting the dephasing of the electron spin coherence. A circularly dichromatic pump-probe model is developed with both the spin-polarization-dependent band-filling and band-gap renormalization effects being taken into account. The model is used to simulate the differential transmission spectra for the collinearly polarized, co-helicity circularly polarized and cross-helicity circularly polarized pump-probe configurations, respectively. It is found that the model simulates well the features of the absorption quantum beats for a spin-dependent thermalized distribution of the photocreated carriers by a circularly polarized pump pulse, such as the variation of the oscillatory amplitude and phase reversal of the absorption quantum beats with photon energy increase. The simulation is in good agreement with our experimental results and reveals the effect of spin polarization on electron relaxation dynamics.

Molecular Orientational Distribution at Interfaces Using Second Harmonic Generation

We present a novel method for obtaining the orientational distribution of molecules at the air/water interface by using a circularly polarized pump pulse that is incident normal to the interface to excite interfacial molecules to their excited electronic states. The second harmonic generation intensities of two linearly polarization combinations (45� -in/ S-out and P-in/P-out) were measured. The ratio of their intensities measured before and after the pump pulse was then used to obtain the mean orientational angle and the orientational distribution utilizing a Gaussian distribution. The mean orientational angle and the orientational distribution of the organic dye molecule, coumarin 314, at the air/water interface were found to be 55 ( 2� with respect to the surface normal and a full width at half-maximum of 16 ( 3� .

Dynamic terahertz polarization in single-walled carbon nanotubes

We have investigated the anisotropic dynamic dielectric response of aligned and well-isolated single-walled carbon nanotubes using optical-pump terahertz (THz)-probe techniques. The polarization anisotropy measurements demonstrate that the THz radiation interacts only with radiation polarized parallel to the nanotubes which have been selectively excited by a polarized pump pulse thus allowing controlled THz polarization to be achieved from unaligned nanotubes.

Detection of local electron and nuclear spin dynamics by time-resolved Kerr microscopy

We implement time-resolved Kerr microscopy (TRKM) system with ∼ 1 μ m spatial resolution for observation of coupled electron–nuclear spin dynamics such as dynamic nuclear polarization (DNP) in a local area of the laser spot. From TRKM measurements, Larmor precession of electron spins injected by right ( σ + ) or left ( σ − ) circular polarized pump pulse is detected. The difference of the precession frequencies between σ + and σ − pumping corresponds to the polarization-dependent nuclear magnetic field caused by DNP. This indicates that we successfully detect coupled electron–nuclear spin dynamics in a local area of the laser spot size of ∼ 1.5 × 1.5 μ m 2 . Furthermore, we show the pump and probe power dependences of DNP in high optical intensity region. All-optical NMR is also demonstrated at arbitrary frequencies by controlling acousto-optical modulator.

Photoinduced transient Faraday rotation in NiO

A non-thermal ultrafast magneto-optical response of antiferromagnetic NiO to irradiation with an intense optical pulse was observed. With temporally overlapping pump and probe pulses, the circularly polarized pump pulse induced rotation of the linear polarization of the probe pulse along with two-photon absorption and cross-phase modulation. The sign of the rotation of the linear polarization reversed with the reversal of the pump helicity. This non-resonant phenomenon is described as a third-order nonlinear optical effect and is ascribed to an inverse Faraday effect: photoinduced magnetization.

Stimulated rotational Raman scattering by a polarization-modulated femtosecond pulse

Transient stimulated rotational Raman scattering induced by a polarization-modulated femtosecond pulse whose polarization varies with time is reported. The peak intensity of the polarization-modulated pulse produced in this study is low compared to that of a pulse without polarization variation, and this leads to the low efficiency of self-phase-modulation and thus the high efficiency of stimulated Raman scattering compared to the case of a pump pulse without polarization modulation. The scattering process is unique in terms of the high peak ratio of the anti-Stokes to Stokes emissions compared to the ratio in the case of the scattering induced by linearly, circularly, and elliptically polarized pump pulses. Under optimum conditions, the ratio approaches 1, which is not realized in the general case of stimulated rotational Raman scattering.

Analytical model of the effective transient optical response of symmetric-top molecules in the presence of a rotational coherence

A nonperturbative analytic solution to the transient effective susceptibility tensor due to the presence of a rotational wave packet in nonrigid, symmetric-top molecules excited by intense, arbitrarily polarized laser pulses is presented. Calculations using the analytic model are carried out for the case of a linear and a symmetric-top molecule, including the effect of vibration-rotation coupling on the wave packet. We also compare the analytic results to a numeric solution of the Schrödinger equation for a linearly polarized pump pulse in order to assess the validity of the impulsive approximation.

Large in-plane spin-dephasing anisotropy in a [0 0 1]-grown two-dimensional electron system

A large anisotropy in the spin dephasing times for the different in-plane directions of [0 0 1] quantum wells has been predicted in calculations by Averkiev and Golub [Phys. Rev. B 60 (1999) 15582] for systems with both Rashba and Dresselhaus spin-orbit fields. Here, we present time-resolved Faraday rotation measurements performed in the Voigt geometry on a high-mobility 2D electron system grown on a [0 0 1] substrate. An out-of-plane spin polarization is created in the sample by a circularly polarized pump pulse. The magnetic field applied in the sample plane forces the spins to precess perpendicular to it. For a magnetic field applied along [1 1 0], the spins precess into the [ 1 1 ¯ 0 ] direction, which is in the sample plane, for a magnetic field along [ 1 1 ¯ 0 ] , the spins precess into the [1 1 0] direction. The experimentally determined spin dephasing times for these two different cases differ by about 60%, demonstrating that the predicted anisotropy is present in our sample.

Increase of spin dephasing times in a 2D electron system with degree of initial spin polarization

We report on time-resolved Faraday/Kerr rotation measurements on a high-mobility 2D electron system. A variable initial spin polarization is created in the sample by a circularly polarized pump pulse, and the spin polarization is tracked by measuring the Faraday/Kerr rotation of a time-delayed probe pulse. By varying the pump intensity, the initial spin polarization is changed from the low-polarization limit to a polarization degree of several percent. The observed spin dephasing time increases from less than 20 ps to more than 200 ps as the initial spin polarization is increased. To exclude sample heating effects, additional measurements with constant pump intensity and variable degree of circular polarization are performed. The results confirm the theoretical prediction by Weng and Wu [Phys. Rev. B 68 (2003) 075312] that the spin dephasing strongly depends on the initial spin polarization degree. The microscopic origin for this is the Hartree–Fock term in the Coulomb interaction, which acts as an effective out-of plane magnetic field.

Optical gratings induced by field-free alignment of molecules

We analyze the alignment of molecules generated by a pair of crossed ultrashort pump pulses of different polarizations by a technique based on the induced time-dependent gratings. Parallel polarizations yield an intensity grating, while perpendicular polarizations induce a polarization grating. We show that both configurations can be interpreted at moderate intensity as an alignment induced by a single polarized pump pulse. The advantage of the perpendicular polarizations is to give a signal of alignment that is free from the plasma contribution. Experiments on femtosecond transient gratings with aligned molecules were performed in ${\mathrm{CO}}_{2}$ at room temperature in a static cell and at $30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in a molecular expansion jet.

Integrable optical-fiber source of polarization-entangled photon pairs in the telecom band

We demonstrate an optical-fiber-based source of polarization-entangled photon pairs with improved quality and efficiency, which has been integrated with off-the-shelf telecom components and is, therefore, well suited for quantum communication applications in the 1550-nm telecom band. Polarization entanglement is produced by simultaneously pumping a loop of standard dispersion-shifted fiber with two orthogonally polarized pump pulses, one propagating in the clockwise and the other in the counterclockwise direction. We characterize this source by investigating two-photon interference between the generated signal-idler photon pairs under various conditions. The experimental parameters are carefully optimized to maximize the generated photon-pair correlation and to minimize contamination of the entangled photon pairs from extraneously scattered background photons that are produced by the pump pulses for two reasons: (i) spontaneous Raman scattering causes uncorrelated photons to be emitted in the signal and idler bands and (ii) broadening of the pump-pulse spectrum due to self-phase modulation causes pump photons to leak into the signal and idler bands. We obtain two-photon interference with visibility $>90%$ without subtracting counts caused by the background photons (only dark counts of the detectors are subtracted), when the mean photon number in the signal (idler) channel is about 0.02/pulse, while no interference is observed in direct detection of either the signal or idler photons.

Wavelength-tunable multicolor Raman soliton generation using an ellipse polarized pump pulse and highly birefringent optical fibers

Wavelength-tunable multicolor optical pulse generation based on the self-frequency shift of a Raman soliton was demonstrated. An ellipse polarized pump source, a highly birefringent (HB) optical fiber, and a highly nonlinear dispersion-shifted fiber were used to generate Raman soliton pulses. The wavelength was tuned by controlling the polarization of the pump source. Raman soliton pulses with six different colors and low wavelength crosstalk under 20 dB have been successfully generated.

Frequency tunable polarization and intermodal modulation instability in high birefringence holey fiber

We present an experimental analysis of polarization and intermodal noise-seeded parametric amplification, in which dispersion is phase matched by group velocity mismatch between either polarization or spatial modes in birefringent holey fiber with elliptical core composed of a triple defect. By injecting quasi-CW intense linearly polarized pump pulses either parallel or at 45 degrees with respect to the fiber polarization axes, we observed the simultaneous generation of polarization or intermodal modulation instability sidebands. Furthermore, by shifting the pump wavelength from 532 to 625 nm, we observed a shift of polarization sidebands from 3 to 8 THz, whereas intermodal sidebands shifted from 33 to 63 THz. These observations are in excellent agreement with the experimental characterization and theoretical estimates of phase and group velocities for the respective fiber modes.

Ultra stable all-fiber telecom-band entangled photon-pair source for turnkey quantum communication applications

We demonstrate a novel alignment-free all-fiber source for generating telecom-band polarization-entangled photon pairs. Polarization entanglement is created by injecting two relatively delayed, orthogonally polarized pump pulses into a piece of dispersion-shifted fiber, where each one independently engages in four-photon scattering, and then removing any distinguishability between the correlated photon-pairs produced by each pulse at the fiber output. Our scheme uses a Michelson-interferometer configuration with Faraday mirrors to achieve practically desirable features such as ultra-stable performance and turnkey operation. Up to 91.7% two-photon-interference visibility is observed without subtracting the accidental coincidences that arise from background photons while operating the source at room temperature.

Optical induction of magnetization and observation of fast spin dynamics in aqueous solutions of copper ions

Optically induced magnetization and its fast dynamics in aqueous solutions of transition-metal ions is studied by the polarization spectroscopy with the pump-probe technique. The fast spin dynamics in solutions at room temperature, which cannot be detected by the conventional magnetic-resonance methods, was observed by an all-optical method. A circularly polarized pump pulse creates the magnetization in the ground state of the copper ions in aqueous solutions of copper sulfate, and the time evolution of the magnetization in the subnanosecond region is monitored through the change of the polarization of the probe pulse. Quantum-beat free-induction decay signals are observed in transverse magnetic fields, whose Fourier transform gives the ESR spectra. The observed concentration dependence of the relaxation rate in no external magnetic field is interpreted by the spin cross relaxation due to the magnetic dipole interaction between the copper ions.

Optical-Fiber Source of Polarization-Entangled Photons in the 1550 nm Telecom Band

We present a fiber-based source of polarization-entangled photons that is well suited for quantum communication applications in the 1550 nm band of standard fiber-optic telecommunications. Polarization entanglement is created by pumping a nonlinear-fiber Sagnac interferometer with two time-delayed orthogonally polarized pump pulses and subsequently removing the time distinguishability by passing the parametrically scattered signal and idler photon pairs through a piece of birefringent fiber. Coincidence detection of the signal and idler photons yields biphoton interference with visibility greater than 90%, while no interference is observed in direct detection of either signal or idler photons. All four Bell states can be prepared with our setup and we demonstrate violations of the Clauser-Horne-Shimony-Holt form of Bell's inequality by up to 10 standard deviations of measurement uncertainty.

Exciton quantum beats in CdMnTe quantum wells

We report on the observation of the teraherz quantum beats in the four level system consisting of the heavy hole exciton states coupled to the magnetic ions via exchange interaction. Such states are prepared in CdMnTe quantum well where the magnetic field is applied at 45° with respect to the sample surface normal. We show that the time-resolved Kerr rotation signal provides the information on the dynamics of the spin transitions in this complex system via circular dichroism (i) which makes the signal oscillate when the spin polarization created by the circularly polarized pump pulse precesses in the magnetic field; the linear dichroism (ii) which gives rise to the signal even in the absence of the net spin polarization, e.g. when the exciton states are excited by the linearly polarized pump pulse. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrafast Time-Resolved Imaging Gate

Ultrafast time-resolved imaging gate provides interesting tools for observation of ultrafast phenomena, three-dimensional shape measurements, and biomedical imaging. In this paper, various time-resolved gating techniques based on electronic methods, coherence of light, or optical nonlinear effects are reviewed with regard to potential of time-resolved imaging. As a highly functional time-resolved imaging gate with femtosecond gating time and light amplification, we present here a femtosecond amplifying optical Kerr gate, which is realized by the optical Kerr effect in an excited state with light amplification and two perpendicularly polarized pump pulses with time delay. Furthermore, to evaluate its potential of practical application, we demonstrate three-dimensional shape measurements of a colored transparent object and a diffusing object with use of the femtosecond amplifying optical Kerr gate.