Ultrafast Control Research Articles

Passively phase-locked Er:fiber source of single-cycle pulses in the near infrared with electro-optic timing modulation for field-resolved electron control.

A single-cycle light source in the near infrared is demonstrated enabling sensitive applications of ultrafast optical field control of electronic transport. The compact Er:fiber system generates passively phase-locked pulses with broadband spectra covering 150 THz to 350 THz at a duration of 4.2 fs and 40 MHz repetition rate. A second output arm is equipped with an electro-optic modulator (EOM) that switches the arrival time of the pulses by 700 ps at arbitrary frequencies up to 20 MHz, enabling timing modulation of the pump pulse without changing the average intensity. As a benchmark demonstration, we investigate the carrier relaxation dynamics in low-temperature-grown InGaAs (LT-InGaAs) using quantum interference currents (QuICs).

Ultrafast Tunable Broadband Optical Anisotropy in Two-Dimensional ReS2

Anisotropic two-dimensional materials provide promising platforms for polarization-driven optoelectronic and photonic devices. In particular, layered rhenium disulfide (${\mathrm{Re}\mathrm{S}}_{2}$) has recently attracted intensive attention due to its unique linear dichroism. However, control over the optical properties of ${\mathrm{Re}\mathrm{S}}_{2}$ is mostly limited to the band-edge excitons, which significantly limits the application wavelength range. Here, we utilize ultrafast transient absorption microscopy to tune the broadband optical anisotropy of few-layer ${\mathrm{Re}\mathrm{S}}_{2}$. We observe a broad nonexcitonic photoinduced-absorption feature, which exhibits weak anisotropy immediately after pump excitation, but, surprisingly, shows a strong polarization dependence after a few picoseconds, leading to a broadband enhancement in the optical anisotropy. We attribute this to carrier cooling and subsequent anisotropic free-carrier absorption due to anisotropic carrier effective masses. This work provides not only a principle for the ultrafast active control of broadband polarization-sensitive photonic devices but also insights into anisotropic carrier dynamics in two-dimensional materials.

Coherent acoustic modulation and defect-sensitive ultrafast carrier dynamics of Pr0.5Ca0.5MnO3 thin films investigated by time-resolved terahertz spectroscopy

In transition metal oxides, the potential of competing energetics of interacting fundamental entities is best displayed in perovskite manganites via the formation of a variety of exotic phases; however, there are limitations of extreme sensitivity to extrinsic and intrinsic defects and the slightest of structural modulations. Here, we report the effect of oxygen annealing and epitaxial strain on the ultrafast carrier excitation and relaxation mechanism in charge-ordered (CO) manganite Pr0.5Ca0.5MnO3 (PCMO) thin films of 60 and 150 nm thicknesses, both as-grown and oxygen annealed, as investigated by optical pump–terahertz (THz) probe measurements. Transient THz transmittance is negative for both films. Bi-exponential relaxation behavior accompanied with acoustic modulations was observed that varies along with strain and oxygen content of the films. As fitted by the sum of exponentials, the fast relaxation time constant is found to be fluence independent, while the slow relaxation time constant decreases with pump fluence for both films and is less for the annealed film suggesting that the relaxation in PCMO strongly depends on strain and oxygen content. This study on non-equilibrium carrier dynamics depicting the sensitivity of defects and subtle structural modifications is unprecedented in demonstrating the ultrafast control of CO manganites.

Magnetic-field–induced Zeeman excitation mode in paramagnetic NdGaO3single-crystal probed by magneto-terahertz spectroscopy

The magnetic field control of quantum matter enables to study various exotic phenomena which emerge in complex oxide materials. The thin film heterostructures of these systems, potentially relevant in oxide electronics, require a great deal of care in distinguishing the properties of the film from those of the substrate underneath. Here, we unravel a low-energy excitation mode in NdGaO3 crystals which is an important constituent of oxide thin film heterostructures. Employing terahertz (THz) time-domain spectroscopy with control of the external magnetic field, an excitation mode resonating at a frequency of 0.2THz was observed along two key (001) and (110) crystallographic orientations. The mode develops and strengthens with an application of increasing magnetic field and weakens with increasing temperature; all attributes conforming to the predictions for the energetics of Zeeman splitting and intra-level transition within Kramer's doublet ground state. A dominance of the magnetic field control of this mode along (110) compared to that along (001) orientation also further unveils a structural control. NdGaO3crystals are widely used as substrates in oxide thin film heterostructures; hence, while realizing ultrafast control of magnetic order, specifically in oxide thin films, the Kramer crystal field splitting mode should be carefully separated from ordered spin resonance mode.

Spatially selective excitation of spin dynamics in magneto-photonic crystals by spectrally tunable ultrashort laser pulses

Abstract In this work, we tackle the problem of the spatially selective optical excitation of spin dynamics in structures with multiple magnetic layers. The 120 fs circularly polarized laser pulses were used to launch magnetization precession in an all-dielectric magneto-photonic crystals (MPC) formed by magnetic layers sandwiched between and inside two magnetic Bragg mirrors. Optical pump-probe experiments reveal magnetization precession triggered via ultrafast inverse Faraday effect with an amplitude strongly dependent on the pump central wavelength: maxima of the amplitude are achieved for the wavelength tuned at the cavity resonance and at the edge of the photonic bandgap. The optical impact on the spins caused by the inverse Faraday effect and spectrum of this effect are found to correlate mostly to the direct Faraday effect. We show that even though the pump laser pulses propagate along the whole structure tuning their wavelength allows localization of a larger spin precession either in the cavity layer or in the Bragg mirror layers selectively. The results pave the way to the ultrafast optical control of magnetization dynamics at a sub-wavelength scale that is vital for modern magneto-photonics and magnonics.

Inverse Faraday effect in Mott insulators

The inverse Faraday effect (IFE), where a static magnetization is induced by circularly polarized light, offers a promising route to ultrafast control of spin states. Here we study the IFE in Mott insulators using the Floquet theory. We find two distinct IFE behavior governed by the inversion symmetry. In the Mott insulators with inversion symmetry, we find that the effective magnetic field induced by the IFE couples ferromagnetically to the neighboring spins. While for the Mott insulators without inversion symmetry, the effective magnetic field due to IFE couples antiferromagnetically to the neighboring spins. We apply the theory to the spin-orbit coupled single- and multiorbital Hubbard model that is relevant for the Kitaev quantum spin liquid material and demonstrate that the magnetic interactions can be tuned by light.

All-Optical Switching on the Nanometer Scale Excited and Probed with Femtosecond Extreme Ultraviolet Pulses

Ultrafast control of magnetization on the nanometer length scale, in particular all-optical switching, is key to putting ultrafast magnetism on the path toward future technological application in data storage technology. However, magnetization manipulation with light on this length scale is challenging due to the wavelength limitations of optical radiation. Here, we excite transient magnetic gratings in a GdFe alloy with a periodicity of 87 nm by the interference of two coherent femtosecond light pulses in the extreme ultraviolet spectral range. The subsequent ultrafast evolution of the magnetization pattern is probed by diffraction of a third, time-delayed pulse tuned to the Gd N-edge at a wavelength of 8.3 nm. By examining the simultaneously recorded first and second order diffractions and by performing reference real-space measurements with a wide-field magneto-optical microscope with femtosecond time resolution, we can conclusively demonstrate the ultrafast emergence of all-optical switching on the nanometer length scale.

Robust and tunable Weyl phases by coherent infrared phonons in ZrTe5

Ultrafast control of structural and electronic properties of various quantum materials has recently sparked great interest. In particular, photoinduced switching between distinct topological phases has been considered a promising route to realize quantum computers. Here we use first-principles and effective Hamiltonian methods to show that in ZrTe5, lattice distortions corresponding to all three types of zone-center infrared optical phonon modes can drive the system from a topological insulator to a Weyl semimetal. Thus achieved Weyl phases are robust, highly tunable, and one of the cleanest due to the proximity of the Weyl points to the Fermi level and a lack of other carriers. We also find that Berry curvature dipole moment, induced by the dynamical inversion symmetry breaking, gives rise to various nonlinear effects that oscillate with the amplitude of the phonon modes. These nonlinear effects present an ultrafast switch for controlling the Weyltronics-enabled quantum system.

Ultrafast light-induced THz switching in exchange-biased Fe/Pt spintronic heterostructure

The ultrafast optical control of magnetization in spintronic structures enables one to access to the high-speed information processing, approaching the realm of terahertz (THz). Femtosecond visible/near-infrared laser-driven ferromagnetic/nonmagnetic metallic spintronic heterostructures-based THz emitters combine the aspects from the ultrafast photo-induced dynamics and spin-charge inter-conversion mechanisms through the generation of THz electromagnetic pulses. In this Letter, we demonstrate photoexcitation density-dependent induced exchange-bias tunability and THz switching in an annealed Fe/Pt thin-film heterostructure, which otherwise is a widely used conventional spintronic THz emitter. By combining the exchange-bias effect along with THz emission, the photo-induced THz switching is observed without any applied magnetic field. These results pave the way for an all-optical ultrafast mechanism to exchange-bias tuning in spintronic devices for high-density storage, read/write magnetic memory applications.

Thinfilm Hybrid Nanostructures: A Perspective to Subcycle Opto-Electronics and Coherent Control

In this article we present a theoretical investigation of gold-silica-silver nanostructures and their optical properties with respect to ultrafast electronic applications and coherent control by tailored optical fields. We found a remarkable sensitive behavior to the carrier envelope phase (CEP) of the driving laser pulses in the coupling of surface and bulk plasmons leading to a superposition of distinct modes with a time-dependent amplitude structure. Furthermore, we show a rather complex temporal evolution of plasmonic surface modes. Our results suggest the potential for coherent control of the time-dependent resonant coupling between surface and volume modes by tailored laser pulses and foster the field of time-dependent spectroscopy of thinfilm hybrid nanostructures with single layer thickness down to the two-dimensional limit.

Resonant and phonon-assisted ultrafast coherent control of a single hBN color center

Single-photon emitters in solid-state systems are important building blocks for scalable quantum technologies. Recently, quantum light emitters have been discovered in the wide-gap van der Waals insulator hexagonal boron nitride (hBN). These color centers have attracted considerable attention due to their quantum performance at elevated temperatures and wide range of transition energies. Here, we demonstrate coherent state manipulation of a single hBN color center with ultrafast laser pulses and investigate in our joint experiment–theory study the coupling between the electronic system and phonons. We demonstrate that coherent control can be performed not only resonantly on the optical transition giving access to the decoherence but also phonon-assisted, which reveals the internal phonon quantum dynamics. In the case of optical phonons, we measure their decoherence, stemming in part from their anharmonic decay. Dephasing induced by the creation of acoustic phonons manifests as a rapid decrease in the coherent control signal when traveling phonon wave packets are emitted. Furthermore, we demonstrate that the quantum superposition between a phonon-assisted process and resonant excitation causes ultrafast oscillations of the coherent control signal. Our results pave the way for ultrafast phonon quantum state control on the nanoscale and open up a new promising perspective for hybrid quantum technologies.

Light-Induced Magnetization at the Nanoscale.

Triggering and switching magnetic moments is of key importance for applications ranging from spintronics to quantum information. A noninvasive ultrafast control at the nanoscale is, however, an open challenge. Here, we propose a novel laser-based scheme for generating atomic-scale charge current loops within femtoseconds. The associated orbital magnetic moments remain ferromagnetically aligned after the laser pulses have ceased and are localized within an area that is tunable via laser parameters and can be chosen to be well below the diffraction limit of the driving laser field. The scheme relies on tuning the phase, polarization, and intensities of two copropagating Gaussian and vortex laser pulses, allowing us to control the spatial extent, direction, and strength of the atomic-scale charge current loops induced in the irradiated sample upon photon absorption. In the experiment we used He atoms driven by an ultraviolet and infrared vortex-beam laser pulses to generate current-carrying Rydberg states and test for the generated magnetic moments via dichroic effects in photoemission. Abinitio quantum dynamic simulations and analysis confirm the proposed scenario and provide a quantitative estimate of the generated local moments.

High-performance polarization management devices based on thin-film lithium niobate

High-speed polarization management is highly desirable for many applications, such as remote sensing, telecommunication, and medical diagnosis. However, most of the approaches for polarization management rely on bulky optical components that are slow to respond, cumbersome to use, and sometimes with high drive voltages. Here, we overcome these limitations by harnessing photonic integrated circuits based on thin-film lithium niobate platform. We successfully realize a portfolio of thin-film lithium niobate devices for essential polarization management functionalities, including arbitrary polarization generation, fast polarization measurement, polarization scrambling, and automatic polarization control. The present devices feature ultra-fast control speeds, low drive voltages, low optical losses and compact footprints. Using these devices, we achieve high fidelity polarization generation with a polarization extinction ratio up to 41.9 dB and fast polarization scrambling with a scrambling rate up to 65 Mrad s−1, both of which are best results in integrated optics. We also demonstrate the endless polarization state tracking operation in our devices. The demonstrated devices unlock a drastically new level of performance and scales in polarization management devices, leading to a paradigm shift in polarization management.

Ultrafast control of vibrational states of polar molecules with subcycle unipolar pulses

We investigate theoretically the nonresonant excitation of vibrational levels in polar molecules by unipolar radiation pulses of duration much shorter than the characteristic period of the molecule's vibration. We consider several profiles of the potential of the interaction of atoms in a diatomic molecule and derive analytically the probabilities of the molecule's transition to excited vibrational states when driven by subcycle unipolar pulses. It is shown that the excitation efficiency is governed by the electric pulse area so that unipolar half-cycle pulses turn out to be the most efficient ones. We introduce the characteristic scale of the electric pulse area, which serves as a measure of the pulse action on the vibrational states of the molecule. The results are generalized to the interaction of excited vibrational and rotational states and it is shown that the behavior of the vibrational levels' populations versus the electric pulse area as well as the introduced characteristic scale stays valid.

Optical excitation of electromagnons in hexaferrite

Understanding ultrafast magnetization dynamics on the microscopic level is of strong current interest due to the potential for applications in information storage. In recent years, spin-lattice coupling has been recognized as essential for ultrafast magnetization dynamics. Magnetoelectric multiferroics of type II possess intrinsic correlations among magnetic sublattices and electric polarization $(P)$ through spin-lattice coupling, enabling fundamentally coupled dynamics between spins and lattice. Here, we report on ultrafast magnetization dynamics in a room-temperature multiferroic hexaferrite possessing ferrimagnetic (FM) and antiferromagnetic sublattices, revealed by time-resolved resonant x-ray diffraction. A femtosecond above-bandgap excitation triggers a coherent magnon in which the two magnetic sublattices entangle and give rise to a transient modulation of $P$. A microscopic mechanism for triggering the coherent magnon in this FM insulator based on the spin-lattice coupling is proposed. Our finding opens a general pathway for ultrafast control of magnetism.

Omnidirectional Panoramic Video System With Frame-by-Frame Ultrafast Viewpoint Control

This study developsan omnidirectional panoramic video system that can simultaneously observe zoom-in images at dozens of different viewpoints in a <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math notation="LaTeX">$360^{\circ }$</tex-math></inline-formula> field of view by synchronizing hundreds of frames per second video shooting, with a rotation of tens-of-rotation-per-second mirror-rotator. When video shooting a scene with a certain exposure utilizing a mirror-rotator, apparent motion in the captured images becomes too large to observe non-blurred images, owing to its high-speed rotation. To reduce motion blurs caused by the rotation of a mirror-rotator, our system introduces an ultrafast pan-tilt mirror mechanism that can cancel apparent motion in images by executing frame-by-frame viewpoint control, so that the apparent speed in images becomes zero when the camera shutter is open, and it returns to its home position when the shutter is closed. Our system can capture 40 omnidirectionally different view images at 10 fps in real time by synchronizing 400-fps video shooting with frame-by-frame ultrafast pan-tilt mirror control for motion blur reduction when a mirror-rotator rotates at 10 rps. Its performance is verified by experimental results for several omnidirectional real scenes, and our system can capture clear and bright panoramic zoom-in images without decreasing the camera exposure when omnidirectionally observing dozens-of-meters-distance outdoor scenes.

Excitation polarization-independent photo-induced restoration of inversion symmetry in Td-WTe2

Td-WTe2 is a topologically nontrivial material and exhibits a variety of physical properties, such as giant unsaturated magnetoresistance and the unconventional thermoelectric effect, due to its topological nature. It is also known to exhibit ultrafast topological phase transitions that restore its inversion symmetry by intense terahertz and mid-infrared pulses, and these properties demonstrate the possibility of ultrafast control of devices based on topological properties. Recently, a novel photo-induced topological phase transition by using polarization-controlled infrared excitation has been proposed, which is expected to control the material topology by rearranging the atomic orbitals near the Weyl point. To examine this topological phase transition, we experimentally studied the excitation-polarization dependence of the infrared-induced phase dynamics in a thin-layer of Td-WTe2. Time-resolved second harmonic generation (SHG) measurements showed that SHG intensity decreases after the infrared pump regardless of the polarization. Polarization-resolved infrared pump–probe measurements indicated that the polarization-selected excited state relaxes quite rapidly (i.e., within 10–40 fs). Considering these experimental results, we conclude that it is difficult to control the photo-induced phase transition through orbital-selective excitation owing to the rapid loss of carrier distribution created by polarization-selective excitation in thin-layer Td-WTe2 under our experimental condition. These results indicate that the suppression of the electron scattering process is crucial for experimentally realizing the photo-induced phase transition based on the polarization selection rule of the materials.

Exploring far-from-equilibrium ultrafast polarization control in ferroelectric oxides with excited-state neural network quantum molecular dynamics.

Ferroelectric materials exhibit a rich range of complex polar topologies, but their study under far-from-equilibrium optical excitation has been largely unexplored because of the difficulty in modeling the multiple spatiotemporal scales involved quantum-mechanically. To study optical excitation at spatiotemporal scales where these topologies emerge, we have performed multiscale excited-state neural network quantum molecular dynamics simulations that integrate quantum-mechanical description of electronic excitation and billion-atom machine learning molecular dynamics to describe ultrafast polarization control in an archetypal ferroelectric oxide, lead titanate. Far-from-equilibrium quantum simulations reveal a marked photo-induced change in the electronic energy landscape and resulting cross-over from ferroelectric to octahedral tilting topological dynamics within picoseconds. The coupling and frustration of these dynamics, in turn, create topological defects in the form of polar strings. The demonstrated nexus of multiscale quantum simulation and machine learning will boost not only the emerging field of ferroelectric topotronics but also broader optoelectronic applications.

Sub-nanosecond operations on superconducting quantum register based on Ramsey patterns

An ultrafast qubit control concept is proposed and analyzed theoretically to reduce the duration of operations with single and multiple superconducting qubits. It is based on the generation of Ramsey fringes due to unipolar sub-nanosecond control pulses. The interference of waves of qubit states population propagating forward and backward in time play a key role in the concept. The influence of the shape and duration of control pulses on the contrast of the interference pattern is revealed in the frame of Ramsey’s paradigm. Protocols for the observation of Ramsey oscillations and an implementation of various gate operations are developed for flux qubits. We also suggest a notional engineering solution for creating the required sub-nanosecond control pulses with the desired shape and amplitude. It is demonstrated that this makes it possible to control the quantum states of the system with a fidelity of more than 99%.

Picosecond Pulsed-Periodic High-Peak Power Nd:YAG Laser Operationally Controlled by KTP-Based Pockels Cell

Electro-optical modulators are effectively used for ultrafast pulse lasers operation control. The scheme of picosecond pulse-periodic high-peak-power Nd:YAG lasers is composed of an active-passive mode-locked and negative feedback-controlled master oscillator and regenerative amplifier based on common end-diode-pumped Nd:YAG crystal. A double-crystal thermally compensated Pockels cell based on KTP crystals of the Y-cut direction is employed as a key control element. The cell was assembled using a pair of equal-length crystals grown according to high-resistivity technology. The scheme provides output pulses with energy up to 1.6 mJ, a duration of 25 ps at repetition rates tunable from 0 to 200 Hz. The laser operation stages are analyzed in detail. The scheme looks attractive and promising for developing advanced ultrafast laser systems with higher repetition rates, peak and, accordingly, average power levels. The Pockels cell based on KTP crystals expands the line of available fast electro-optical control elements, along with the previously used RTP ones. The factors limiting laser pulse energy and repetition rate are discussed. Parasitic nonlinear conversion in the crystals of the Pockels cell along the axis may play an essential role. The results of comparative measurements of the second and third harmonics made with the Pockels cells based on KTP and RTP crystals of both X-cut and Y-cut directions are presented. The minimum second and third harmonics efficiency levels observed in the Y-cut Pockels cells of the KTP crystal seem to be their important advantage.