Polarized Pump Pulse Research Articles

Ultrafast enhancement and optical control of magnetization in ferromagnet/semiconductor layered structures via superdiffusive spin transports

While several mechanisms of ultrafast demagnetization were still being debated, a new superdiffusive spin transport (SST) was proposed as a new mechanism of ultrafast demagnetization in layered structures, and was demonstrated in ferromagnet/normal metal and ferromagnet/normal metal/ferromagnet layered structures. Transient magnetization enhancement in Fe layer was observed as the evidence of SST in a Ni/Ru/Fe layered structure when the Ni layer was photo-excited. However, it was not observed repeatedly elsewhere. Here, we explore possible SST in a new ferromagnet/semiconductor layered structure without magnetic couplings, and observe transient magnetization enhancement in directly photo-excited L10-MnGa ferromagnetic layer of a L10-MnGa/GaAs layered structure, being very convincing evidence of SST because the magnetization enhancement occurred impossibly in a sole photo-excited L10-MnGa layer if there were no SST from GaAs layer. It is more fascinating that we fulfill ultrafast optical manipulations of transient magnetization in L10-MnGa layer using circularly polarized pump pulses (CPPP). Right-handed CPPP can enhance transient magnetization extremely over linearly polarized pump pulses, while left-handed CPPP can weaken one vastly. A modified three-temperature model is developed to take account of SST effect and is used to reproduce our experimental results well. Our results exhibit high efficient and ultrafast bidirectional magnetic modulations in ferromagnet/semiconductor hybrid structures which have potential applications in ultrafast magneto-optical modulations and optical manipulations of ferromagnet/semiconductor hybrid spintronic devices.

Interference of optical phonons in diamond studied using femtosecond pulses of polarized near-infrared light

The phononic interference of optical phonons in diamond is studied via transient transmittance change measurements using the double pump–probe protocol with polarized pump pulses. With orthogonal-polarized pulses, the phonon amplitude is enhanced/suppressed if the second pump pulse is irradiated by a half-integer/integer multiple of the phonon period. This is contrary to the usual phononic interference (enhancement at integer multiples of the phonon period) observed when using parallel-polarized pulses. These observed results are well explained using a quantum-mechanical model in calculations involving the Raman tensor of the diamond lattice structure. The amplitude of the optical phonons generated via the off-diagonal term of Raman tensor is proportional to the product of the x- and y−components of the optical electric field. The phase of the phonon oscillations is dependent on the angle between the crystal orientation and the polarization of the pump pulse.

Orbital selective dynamics in Fe-pnictides triggered by polarized pump pulse excitations

Quantum materials display exotic behaviours related to the interplay between temperature-driven phase transitions. Here, we study the electron dynamics in one such material, CaFe2As2, a parent Fe-based superconductor, employing time- and angle-resolved photoemission spectroscopy. CaFe2As2 exhibits concomitant transition to spin density wave state and tetragonal to orthorhombic structure below 170 K. The Fermi surface of this material consists of three hole pockets , β and around the Γ-point and two electron pockets around the X-point. The hole pockets have d xy , d yz and d zx orbital symmetries. The β band constituted by d xz /d yz orbitals exhibits a gap across the magnetic phase transition. We discover that polarized pump pulses can induce excitations of electrons of a selected symmetry. More specifically, while s-polarized light (polarization vector perpendicular to the xz plane) excites electrons corresponding to all the three hole bands, p-polarized light excites electrons essentially from , bands which are responsible for magnetic order. Interestingly, within the magnetically ordered phase, the excitation due to the p-polarized pump pulses occur at a time scale of 50 fs, which is significantly faster than the excitation induced by s-polarized light (∼200 fs). These results suggest that the relaxation of different ordered phases occurs at different time scales and this method can be used to achieve selective excitations to disentangle complexity in the study of quantum materials.

Photoinduced anomalous Hall effect in the interacting Haldane model: Targeting topological states with pump pulses

We investigate the nonequilibrium dynamics of the spinless Haldane model with nearest-neighbor interactions on the honeycomb lattice by employing an unbiased numerical method. In this system, a first-order transition from the Chern insulator (CI) at weak coupling to the charge-density-wave (CDW) phase at strong coupling can be characterized by a level crossing of the lowest energy levels. Here we show that adiabatically following the eigenstates across this level crossing, their Chern numbers are preserved, leading to the identification of a topologically-nontrivial low-energy excited state in the CDW regime. By promoting a resonant energy excitation via an ultrafast circularly polarized pump pulse, we find that the system acquires a non-vanishing Hall response as a result of the large overlap enhancement between the time-dependent wave-function and the topologically non-trivial excited state. This is suggestive of a photoinduced topological phase transition via unitary dynamics, despite a proper definition of the Chern number remaining elusive for an out-of-equilibrium interacting system. We contrast these results with more common quench protocols, where such features are largely absent in the dynamics even if the post-quench Hamiltonian displays a topologically nontrivial ground state.

Observation of evanescent spin waves in the magnetic dipole regime

We observed spin-wave transmission through an air gap that works as a prohibited region. The spin waves were excited by circularly polarized pump pulses via the inverse Faraday effect, and their spatial propagation was detected through the Faraday effect of probe pulses using a pump-probe imaging technique. The experimentally observed spin-wave transmission was reproduced using numerical calculations with a Green's function method and micromagnetic simulation. We found that the amplitude of the spin waves decays exponentially in the air gap, which indicates the existence of evanescent spin waves in the magnetic dipole regime. This finding will pave the way for controllable amplitudes and phases of spin waves propagating through an artificial magnonic crystal.

Measurement of asymmetric electron cloud in cluster nano-plasma

A nanocluster ionized by a femtosecond pulse of wavelength larger than the cluster size is known to set up an electron cloud that oscillates with respect to the ion cloud. Most theoretical frameworks on understanding cluster ionization are built on this dipole oscillation model. Here, we probe this dipole by pump-probe absorption spectroscopy. A linearly polarized pump pulse sets up the dipole and the polarization of the probe pulse that comes after a prescribed time is varied, and the differential changes in the absorption are measured. The polarization dependence of the probe pulse absorption gives a direct measure of the asymmetry and ellipticity in the dielectric permittivity. Our results show that orthogonally polarized pump-probe pulses absorb about 18% less compared to the parallel case at the peak of the linear plasma resonance. A dipole excitation model compared to the experimental observations is used to decipher the asymmetry in the transient electron cloud.

Molecular rotation in 3 dimensions at an air/water interface using femtosecond time resolved sum frequency generation.

This paper presents the first study of the rotations of rigid molecules in 3 dimensions at the air/water interface, using the femtosecond time resolved sum frequency generation (SFG) technique. For the purpose of this research, the aromatic dye molecule C153 was chosen as an example of a molecule having two functional groups that are SFG active, one being the hydrophilic -C=O group and the other the hydrophobic -CF3 group. From polarized SFG measurements, the orientations of the two chromophores with respect to the surface normal were obtained. On combining these results with the known relative orientation of the two chromophores in the molecule yields the absolute orientation of C153 at the air/water interface. It was found that the -CF3 axis projected towards the bulk air at an angle of 59° with respect to the interface normal and the -C=O group projected towards the bulk water at an angle of 144°. In order to observe the rotational motions of C153 at the air/water interface, the approach was used to perturb the ground electronic state equilibrium orientational distribution using a polarized resonant pump pulse, which preferentially excites ground state molecules that have their electronic S0 → S1 transition moment aligned closely to the electric field of the incident pump pulse. As a consequence of the photoselection perturbation, the orientational distribution of the remaining ground state molecules was not the equilibrium distribution. Similarly, the orientational distribution of the excited state molecules that were created by the polarized pump pulse was not in their final equilibrium orientational distribution. The rotational motions of the interfacial molecules towards equilibrium were obtained from time dependent measurements of the intensities of the SFG signal generated by the simultaneous incidence at the air/water interface of a visible probe pulse plus an IR probe pulse. In this way, the recovery times to achieve the orientational equilibrium of the two chromophores including the orientation of the normal of the C153 plane with respect to the interface were obtained. The photo-selection process shifts the average orientation angle of the hydrophilic -C=O group by an increase of 4° ± 0.6° with a rotational recovery time constant of 130 ± 20 ps, which is the time to return to an orientational equilibrium distribution. The hydrophobic -CF3 group undergoes a shift that increases its angle by 8° ± 1.5° with a rotational recovery time constant of 210 ± 38 ps. We find that the orientational change of the molecular normal is 4° ± 0.5° and has a rotational recovery time constant of 125 ± 26 ps. The interface-specific time-dependent polarized measurements allowed us to monitor the orientational motions of molecules at interfaces, both in 3 dimensions and in real time.

Nuclear-spin polarization of atoms by chirped laser pulses: application to the muonium

We theoretically show that the use of chirped laser pulses with an appropriate bandwidth is more favorable in achieving a higher degree of nuclear-spin polarization of atoms/ions. The physical origin of this is identified as multiple pump-dump processes during the single chirped laser pulse. As a specific example, we consider the case of muonium (μ+e−, lifetime 2.2 μs). By numerically solving a set of density matrix equations, we find that a chirped 1 ns pulse with a narrow bandwidth instead of a broad bandwidth efficiently induces a resonant pump and following dump processes within a single laser pulse, thereby transferring the angular momentum of the circularly polarized pump pulse to the atoms, which leads to the higher degree of nuclear-spin polarization. For muoniums at rest, a single chirped 1 ns pulse with a 4 GHz bandwidth and peak intensity of 2×106 W/cm2 leads to 43% of nuclear-spin polarization, which is to be compared with 33% of nuclear-spin polarization by the transform-limited 1 ps pulse or chirped 1 ns pulse with a 400 GHz bandwidth for both. We also find that an introduction of the chirp does not help in the presence of significant Doppler broadening. This means that the use of chirped laser pulses is beneficial for the atomic beam at the cross-beam geometry, where the influence of Doppler broadening is negligible, and similarly, if a slow muonium beam is realized in the future to nearly eliminate Doppler broadening, then chirped laser pulses would be useful to attain a higher degree of nuclear-spin polarization.

Dominant role of inverse Cotton-Mouton effect in ultrafast stimulation of magnetization precession in undoped yttrium iron garnet films by 400-nm laser pulses

Magnetization ($\mathbit{M}$) precessions driven by ultrafast laser-induced nonthermal effects are observed in undoped yttrium iron garnet (YIG) films of (100) and (111) orientations using pump-probe time-resolved magneto-optical Kerr effect. The $\mathbit{M}$ precessions show a strong dependence on the polarization direction of linearly polarized pump pulses of 400 nm. In contrast, we can barely observe any $\mathbit{M}$ precession using circularly polarized pump pulses, which indicates that the inverse Faraday effect is negligible. For the case of linear pump polarization, a phenomenological model is introduced, based on the modulation of $\mathbit{M}$ via a modulation of fourth-rank susceptibility tensors by a laser pulse. This allows one to distinguish the contributions of the inverse Cotton-Mouton effect (ICME) from those of the photoinduced magnetic anisotropy (PMA). Using the formula derived from the phenomenological model, we perform the fitting of the polarization-direction-dependent precession phase and amplitude in (100)- and (111)-oriented YIG films. The fitting results reveal that the $\mathbit{M}$-precession excitation originates from a combination effect of ICME and PMA, but the ICME plays the dominant role.

Collisional dynamics in laser-induced plasmas: evidence for electron-impact excitation

We have experimentally investigated the collisional dynamics in femtosecond-laser-induced plasmas and presented the evidence for electron-impact excitation through enhanced high-order harmonic (HH) generation. The measurements were carried out by using an elliptically polarized pump pulse to induce the underdense plasmas and by using a time-delayed linearly polarized probe pulse to drive the HH generation from the plasmas. We found that the rise time of this enhanced HH generation was insensitive to the ellipticity degree (ED) of pump pulse but sensitive to its laser intensity (LI). With further comparison between physical scenarios and qualitative analysis, we demonstrated that the atomic excitation causing the HH enhancement should be attributed to the electron-impact excitation, i.e., the excitation from the collision between neutral atoms and electrons during the lifetime of the underdense plasma.

Ultrafast polarization modulation of laser pulses at terahertz frequencies via optical Kerr effect

Polarized light has already been widely used for photography and display technologies. Magneto-optical Faraday effect, i.e., the light polarization rotates in the magnetic field applied to the material in the direction of light propagation, plays a crucial role in the interaction between light and spin. Faraday effect allow us to understand the nature of magnetization in condensed materials. As an effect opposite to the Faraday effect, the magnetization can be induced in a transparent medium exposed to a circularly polarized electromagnetic wave, which is called inverse Faraday effect. Knowledge of the mechanism provides the opportunities of modulation devices in photonics, ultrafast opto-magnetism and magnonics. In this paper, we experimentally demonstrate a proof-of-concept ultrafast polarization modulation by employing circularly polarized light to demonstrate a strengthened terahertz (THz) frequency Kerr modulation signal, at room temperature. By using the transient pumpprobe spectroscopy with the reflected geometry, we are able to demonstrate the feasibility of such an ultrafast magneto-optical polarization modulation at 0.19 THz in a paramagnetic Li:NaTb (WO<sub>4</sub>)<sub>2</sub> crystal with a thickness of 3 mm. The time-resolved modulation signal is explained by the interaction between two counter-propagating laser pulses (central photon energy of 1.55 eV) within the crystal via the optical Kerr effect. We find that the amplitude of the modulation increases with the pump fluence increasing, while the modulation frequency is dependent neither on the pump fluence nor on polarization of pump beam. However, it can further be found that the phase and amplitude of the transient Kerr modulation are strongly dependent on the helicity of the circularly polarized pump pulses. Indeed, these oscillating signals may be mistaken for spin excitation modes. The present findings allow us to get an insight into the transient magneto-optical dynamical process in transparent medium. In addition, the polarization modulation of ultrashort laser pulse on a picosecond time scale will facilitate all-optical data processing, as well as the polarization-dependent ultrafast dynamics in various material systems, which span from condensed matter to molecular spectroscopy. In this regard, our experimental results provide a possibility for designing novel all-optical (magneto-optical) modulators operating at THz clock frequencies. The magneto-optical polarization response modulated at THz frequencies may have new possibilities for designing all-optical devices, such as ultrafast modulators.

Ab Initio Calculations of Ultrashort Carrier Dynamics in Two-Dimensional Materials: Valley Depolarization in Single-Layer WSe2.

In single-layer WSe2, a paradigmatic semiconducting transition metal dichalcogenide, a circularly polarized laser field can selectively excite electronic transitions in one of the inequivalent K± valleys. Such selective valley population corresponds to a pseudospin polarization. This can be used as a degree of freedom in a "valleytronic" device provided that the time scale for its depolarization is sufficiently large. Yet, the mechanism behind the valley depolarization still remains heavily debated. Recent time-dependent Kerr experiments have provided an accurate way to visualize the valley dynamics by measuring the rotation of a linearly polarized probe pulse applied after a circularly polarized pump pulse. We present here a clear, accurate and parameter-free description of the valley dynamics. By using an atomistic, ab initio approach, we fully disclose the elemental mechanisms that dictate the depolarization effects. Our results are in excellent agreement with recent time-dependent Kerr experiments. We explain the Kerr dynamics and its temperature dependence in terms of electron-phonon-mediated processes that induce spin-flip intervalley transitions.

Terahertz radiation from accelerating charge carriers in graphene under ultrafast photoexcitation

We study the generation of THz radiation from the acceleration of ultrafast photoexcited charge carriers in graphene in the presence of a DC electric field. Our model is based on calculating the transient current density from the time-dependent distribution function which is determined using the Boltzmann transport equation within a relaxation time approximation. We include the time-dependent generation of carriers by the pump pulse by solving for the carrier generation rate using the Bloch equations in the rotating wave approximation (RWA). The linearly polarized pump pulse generates an anisotropic distribution of photoexcited carriers in the $k_x-k_y$ plane. The collision integral in the Boltzmann equation includes a term that leads to the \textit{thermalization} of carriers via carrier-carrier scattering to an effective temperature above the lattice temperature, as well as a \textit{cooling} term which leads to energy relaxation via inelastic carrier-phonon scattering. The radiated signal is proportional to the time derivative of the transient current density. In spite of the fact that the magnitude of the velocity is the same for all the carriers in graphene, there is still emitted radiation from the photoexcited charge carriers with frequency components in the THz range due to a change in the \textit{direction} of velocity of the photoexcited carriers in the external electric field as well as \textit{cooling} of the photoexcited carriers on a sub-picosecond time scale.

Large polarization-dependent exciton optical Stark effect in lead iodide perovskites.

A strong interaction of a semiconductor with a below-bandgap laser pulse causes a blue-shift of the bandgap transition energy, known as the optical Stark effect. The energy shift persists only during the pulse duration with an instantaneous response time. The optical Stark effect has practical relevance for applications, including quantum information processing and communication, and passively mode-locked femtosecond lasers. Here we demonstrate that solution-processable lead-halide perovskites exhibit a large optical Stark effect that is easily resolved at room temperature resulting from the sharp excitonic feature near the bandedge. We also demonstrate that a polarized pump pulse selectively shifts one spin state producing a spin splitting of the degenerate excitonic states. Such selective spin manipulation is an important prerequisite for spintronic applications. Our result implies that such hybrid semiconductors may have great potential for optoelectronic applications beyond photovoltaics.

Femtosecond non-resonance double-pulse spectroscopy of molecular librations in ortho-dichlorobenzene

The first experimental results on selective detection of the collective (libration) response in a liquid using a scheme of non-resonance double-pulse spectroscopy based on detecting the ultrafast optical Kerr effect (OKE) in ortho-dichlorobenzene at room temperature are presented. Nonrelevant vibrational and rotational responses are dumped and the relevant libration molecular responses are amplified by varying the duration, delay, and relative intensities of linearly polarized pump pulses.

Significant photoinduced Kerr rotation achieved in semiconductor microcavities

Giant Kerr rotation and ellipticity are observed and investigated in an asymmetric planar microcavity with a quantum well in the active region. Rotation angle of the polarization plane as well as ellipticity were determined from time- and frequency-resolved measurements of the Stokes vector components of reflected light. It was found that in a small range of the cavity mode detunings the polarized pump pulse creates a large splitting of the lower polariton branch while leaving its linewidth almost the same. This fact gives a possibility to observe at such detunings the Kerr rotation angle and ellipticity, close to their extremes. A theoretical analysis shows that the decisive role in reaching extreme polarization rotation angles is played by the structure asymmetry. Comprehensive analysis of the polarization state of the light in this regime shows that both renormalization of the exciton energy and the saturation of the excitonic resonance contribute to the observed optical nonlinearities.

Phase offset determining of the central frequency laser pulse with the duration of a few cycles with respect to envelope

A method for the phase offset determining of the central frequency laser pulse with the duration of a few cycles with respect to envelope is proposed. The method is based on generating the difference frequency radiation by mutually orthogonal linearly polarized laser pulses propagating in the GaSe crystal with regular periodical domain structure. The dependence of the difference frequency radiation wavelength corresponding to the maximum of the spectrum on the phase difference between the mutually orthogonal linearly polarized pump pulses is obtained. The dependences of normalized spectral densities of the difference frequency radiation pulses on the phase differences between the pump pulses are obtained. This paper addresses the case, where the durations of the interacting pulses are equal to 30 fs, the central wavelengths are 1.98 μm and the difference frequency radiation wavelength is in the range of 8–12 μm. As shown, this method could be applied for phase locking of two femtosecond fiber optic lasers.

Optical spin pumping and magnetization switching in ferromagnets

Motivated by recent experiments on all-optical magnetization switching (AOS), we analyze theoretically optical spin pumping in ferromagnetic metals. It is shown that spin polarizations, induced by linearly and circularly polarized pump pulses, can be comparable in magnitude. We assume that the photoinduced spin polarization influences ultrafast light-induced magnetization dynamics in ferromagnets. In particular, it plays a crucial role in AOS. We analyze the light-induced magnetization dynamics in a system of localized and itinerant spins coupled by s-d exchange interaction, assuming that light heats up the itinerant electrons and, in addition, changes their spin polarization. When the pumped spin polarization has an appropriate sign and magnitude, it triggers magnetization switching. We find that temporal evolution of the magnetization is determined by a complicated interplay between heating, the photoinduced spin polarization, and spin relaxation.

Ultrafast optical manipulation of atomic motion in multilayer Ge-Sb-Te phase change materials

Phase change random access memory devices have evolved dramatically with the recent development of superlattice structure of Ge-Sb-Te material (GST-SL) in terms of its low power consumption. The phase change in GST-SL is mainly characterized by the displacement of Ge atoms. Here we examine a new phase change method, that is the manipulation of Ge-Te bonds using linearly-polarized femtosecond near-infrared optical pulses. As a result, we found that the p -polarized pump pulse is more effective in inducing the reversible and irreversible displacement of Ge atoms along [111] direction in the local structure. This structural change would be induced by the anisotropic carrier-phonon interaction along the [111] direction created by the p -polarized pulse.

Unidirectional Rotation of Molecules Measured by the Rotational Doppler Effect

A pair of linearly polarized pump pulses induce field-free unidirectional molecular rotation, which is detected by a delayed circularly polarized probe. The polarization and spectrum of the probe are modified by the interaction with the molecules, in accordance with the Rotational Doppler Effect.