Ultrafast Optical Control Research Articles

Ultrafast spin-polarization control of Dirac fermions in topological insulators

Three-dimensional topological insulators (TIs) are characterized by spin-polarized Dirac-cone surface states that are protected from backscattering by time-reversal symmetry. Control of the spin polarization of topological surface states (TSSs) using femtosecond light pulses opens novel perspectives for the generation and manipulation of dissipationless surface spin currents on ultrafast timescales. Using time-, spin-, and angle-resolved spectroscopy, we directly monitor for the first time the ultrafast response of the spin polarization of photoexcited TSSs to circularly-polarized femtosecond pulses of infrared light. We achieve all-optical switching of the transient out-of-plane spin polarization, which relaxes in about 1.2 ps. Our observations establish the feasibility of ultrafast optical control of spin-polarized Dirac fermions in TIs and pave the way for novel optospintronic applications at ultimate speeds.

Ultrafast optical control of magnetization dynamics in polycrystalline bismuth doped iron garnet thin films

Controlling the magnetization dynamics on the femtosecond timescale is of fundamental importance for integrated opto-spintronic devices. For industrial perspectives, it requires to develop simple growth techniques for obtaining large area magneto-optical materials having a high amplitude ultrafast Faraday or Kerr response. Here we report on optical pump probe studies of light induced spin dynamics in high quality bismuth doped iron garnet polycrystalline film prepared by the spin coating method. We demonstrate an ultrafast non-thermal optical control of the spin dynamics using both circularly and linearly polarized pulses.

Writing and reading of an arbitrary optical polarization state in an antiferromagnet

The ability to store arbitrary polarization states of light in an antiferromagnetic material (YMnO3) potentially adds a new degree of freedom to data storage applications. The interaction between light and magnetism is considered a promising route to the development of energy-efficient data storage technologies. To date, however, ultrafast optical magnetization control has been limited to a binary process, whereby light in either of two polarization states generates (writes) or adopts (reads) a magnetic bit carrying either a positive or negative magnetization. Here, we report how the fundamental limitation of just two states can be overcome, allowing an arbitrary optical polarization state to be written magnetically. The effect is demonstrated using a three-sublattice antiferromagnet—hexagonal YMnO3. Its three magnetic oscillation eigenmodes are selectively excited by the three polarization eigenstates of the light. The magnetic oscillation state is then transferred back into the polarization state of an optical probe pulse, thus completing an arbitrary optomagnonic write–read cycle.

Ultrafast optical control of orbital and spin dynamics in a solid-state defect.

Atom-scale defects in semiconductors are promising building blocks for quantum devices, but our understanding of their material-dependent electronic structure, optical interactions, and dissipation mechanisms is lacking. Using picosecond resonant pulses of light, we study the coherent orbital and spin dynamics of a single nitrogen-vacancy center in diamond over time scales spanning six orders of magnitude. We develop a time-domain quantum tomography technique to precisely map the defect's excited-state Hamiltonian and exploit the excited-state dynamics to control its ground-state spin with optical pulses alone. These techniques generalize to other optically addressable nanoscale spin systems and serve as powerful tools to characterize and control spin qubits for future applications in quantum technology.

Switching the vibrational excitation of a polyatomic ion in multi-photon strong field ionization

The multiphoton ionization (MPI) of CH3I has been investigated by angular resolved photoelectron spectroscopy as a function of femtosecond laser excitation intensity. A sudden change in the electron kinetic energy is observed above a specific field strength. The multiphoton excitation at a fixed wavelength of 800nm becomes vibronically resonant due to Stark shifting of intermediate Rydberg state levels. The present letter gives an experimental evidence for ultrafast optical control of the vibrational excitation in a polyatomic ion by adjusting the intensity of a femtosecond laser pulse.

Ultrafast optical control by few photons in engineered fiber

Fast control of a strong optical beam by a few photons is an outstanding challenge that limits the performance of quantum sensors and optical processing devices. We report that a fast and efficient optical gate can be realized in an optical fiber that has been engineered with molecular-scale accuracy. Highly efficient, distributed phase-matched photon-photon interaction was achieved in the fiber with locally controlled, nanometer-scale core variations. A three-photon input was used to manipulate a Watt-scale beam at a speed exceeding 500 gigahertz. In addition to very fast beam control, the results provide a path to developing a new class of sensitive receivers capable of operating at very high rates.

Quantum Localization of Coherent π-Electron Angular Momentum in (P)-2,2'-Biphenol.

Controlling π-electrons with delocalized character is one of the fundamental issues in femtosecond and attosecond chemistry. Localization of π-electron rotation by using laser pulses is expected to play an essential role in nanoscience. The π-electron rotation created at a selected aromatic ring of a single molecule induces a local intense electromagnetic field, which is a new type of ultrafast optical control functioning. We propose a quantum localization of coherent π-electron angular momentum in (P)-2,2'-biphenol, which is a simple, covalently linked chiral aromatic ring chain molecule. The localization considered here consists of sequential two steps: the first step is to localize the π-electron angular momentum at a selected ring of the two benzene rings, and the other is to maintain the localization. Optimal control theory was used for obtaining the optimized electric fields of linearly polarized laser pulses to realize the localization. The optimal electric fields and the resultant coherent electronic dynamics are analyzed.

Ultrafast optical control of group delay of narrow-band terahertz waves

We experimentally demonstrate control over the group delay of narrow-band (quasi continuous wave) terahertz (THz) pulses with constant amplitude based on optical switching of a metasurface characteristic. The near-field coupling between resonant modes of a complementary split ring resonator pair and a rectangular slit show an electromagnetically induced transparency-like (EIT-like) spectral shape in the reflection spectrum of a metasurface. This coupling induces group delay of a narrow-band THz pulse around the resonant frequency of the EIT-like spectrum. By irradiating the metasurface with an optical excitation pulse, the metasurface becomes mirror-like and thus the incident narrow-band THz pulse is reflected without a delay. Remarkably, if we select the appropriate excitation power, only the group delay of the narrow-band THz pulse can be switched while the amplitude is maintained before and after optical excitation.

Optical control of the vibrational excitation of the polyatomic ions via strong field multi-photon ionization

The dynamics of multi-photon ionization of CH3I under strong field has been studied experimentally by femtosecond photoelectron imaging. Ultrafast optical control of the vibrational excitation in a polyatomic ion by strong field multi-photon ionization is experimentally realized. The present work enhances the intensity of the ionization beam from 1.6×1013 to 2.5×1013 W/cm2. In the order of this higher field, a new energy component is observed and attributed. From the photoelectron imaging, photoelectron kinetic energy distributions and the photoelectron angular distributions are obtained. The discussions of the previous letter are mostly based on the photoelectron kinetic energy information, and the present study emphasizes on the trend of the photoelectron angular anisotropy. More detailed dynamics on vibrational optical control is further explored.

Ultrafast optical control using the Kerr nonlinearity in hydrogenated amorphous silicon microcylindrical resonators

Microresonators are ideal systems for probing nonlinear phenomena at low thresholds due to their small mode volumes and high quality (Q) factors. As such, they have found use both for fundamental studies of light-matter interactions as well as for applications in areas ranging from telecommunications to medicine. In particular, semiconductor-based resonators with large Kerr nonlinearities have great potential for high speed, low power all-optical processing. Here we present experiments to characterize the size of the Kerr induced resonance wavelength shifting in a hydrogenated amorphous silicon resonator and demonstrate its potential for ultrafast all-optical modulation and switching. Large wavelength shifts are observed for low pump powers due to the high nonlinearity of the amorphous silicon material and the strong mode confinement in the microcylindrical resonator. The threshold energy for switching is less than a picojoule, representing a significant step towards advantageous low power silicon-based photonic technologies.

Femtosecond Laser Pulse Induced Ultrafast Demagnetization in Fe/GaAs Thin Films

As the magnetic storage density approaches 1 TB/in <formula formulatype="inline" xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex Notation="TeX">$^{2}$</tex></formula> , a grand challenge is looming as how to read/write such a huge amount of data within a reasonable time. In this study, we demonstrate femtosecond demagnetization in 10-nm epitaxially grown Fe thin films by applying low-energy femtosecond laser pulses. We used time-resolved pump-probe Magneto Kerr Effect spectroscopy to record ultrafast laser induced demagnetization and its recovery. The demagnetization time was found to be 70 fs from the time-resolved hysteresis loops. The time scale is much shorter than the phonon thermalization time. Our results show that the demagnetization can be completed before electron-phonon equilibration is achieved, and therefore indicate the feasibility of ultrafast optical control of demagnetization responses for next generation femtosecond-switching magnetic storage devices.

Ultrafast dynamics of orbital-order-induced polarization

Ultrafast photoinduced modulation is demonstrated in the orbital-order-induced polarization in manganite thin films. The evolution of the polarization modulation and the lattice temperature rise after photoexcitation are tracked separately by monitoring the relevant nonlinear optical susceptibility tensor elements. Substantial portion of the polarization disappears within the excitation pulse width (∼120 fs), indicating the purely electronic origin triggered by an interband excitation. We propose oxide heterostructure devices incorporating polar gating layers capable of ultrafast optical control of electronic phases.

Ultrafast universal quantum control of a quantum-dot charge qubit using Landau–Zener–Stückelberg interference

A basic requirement for quantum information processing is the ability to universally control the state of a single qubit on timescales much shorter than the coherence time. Although ultrafast optical control of a single spin has been achieved in quantum dots, scaling up such methods remains a challenge. Here we demonstrate complete control of the quantum-dot charge qubit on the picosecond scale, orders of magnitude faster than the previously measured electrically controlled charge- or spin-based qubits. We observe tunable qubit dynamics in a charge-stability diagram, in a time domain, and in a pulse amplitude space of the driven pulse. The observations are well described by Landau–Zener–Stückelberg interference. These results establish the feasibility of a full set of all-electrical single-qubit operations. Although our experiment is carried out in a solid-state architecture, the technique is independent of the physical encoding of the quantum information and has the potential for wider applications.

Ultrafast manipulation of near field coupling between bright and dark modes in terahertz metamaterial

We demonstrate ultrafast optical control of near field coupling between bright and dark mode resonances in metamaterials. The meta-molecule design consists of two orthogonally twisted resonators tightly coupled through near fields. We place ion implanted silicon patch with ultrafast carrier lifetime inside dark resonator split gap to achieve active control of its fundamental resonance that determines the near field coupling in the meta-molecule. Upon near infrared photoexcitation, we observed ultrafast dynamical transition of near field coupling between bright and dark resonators allowing the meta-molecule to change its state from coupled to decoupled and eventually back to the coupled state.

Ultrafast optical control of magnetization in EuO thin films

All-optical pump-probe detection of magnetization precession has been performed for ferromagnetic EuO thin films at 10 K. We demonstrate that the circularly polarized light can be used to control the magnetization precession on an ultrafast time scale. This takes place within the 100-fs duration of a single laser pulse, through combined contribution from two nonthermal photomagnetic effects, i.e., enhancement of the magnetization and an inverse Faraday effect. From the magnetic field dependencies of the frequency and the Gilbert damping parameter, the intrinsic Gilbert damping coefficient is evaluated to be \ensuremath{\alpha} \ensuremath{\approx} 3 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}3}$.

Non-thermal excitation and control of magnetization in Fe/GaAs film by ultrafast laser pulses

We present our recent study of non-thermal excitation and coherent control of spin reorientation in 10-nm epitaxially grown Fe thin films by low-energy femtosecond laser pulses. The magnetization dynamics and hysteresis curves were recorded by pump-probe differential magnetic Kerr (DMK) spectroscopy using linearly polarized laser beams. A sharp switching in DMK signal is observed when we rotated the pump polarization. This result indicates a non-thermal origin of magnetization excitation and reorientation in Fe films. We reveal that spins can interact coherently with the polarization induced by the pulsed laser field in magnetic metals. Such opto-magnetic interactions are instantaneous and are only limited in time by the properties of laser pulses. Our results suggest the feasibility of ultrafast optical control of both the magnetization and the demagnetization responses in magnetic films.

Spin switching in a Mn-doped quantum dot using the optical Stark effect

In a single quantum dot doped with a single Mn atom, ultrafast optical control can be used to coherently manipulate the Mn spin state. Here we show that due to the optical Stark effect induced by a detuned laser pulse the Mn spin can be efficiently flipped by one during the action of the pulse. We discuss the influence of different pulse envelopes on the flipping process. We then show how this flipping mechanism can be used in a switching protocol to address all Mn spin states selectively and point out the advantage of exploiting the optical Stark effect compared to previously proposed methods.

Ultrafast optical control of entanglement between two quantum-dot spins

The interaction between two quantum bits enables entanglement, the two-particle correlations that are at the heart of quantum information science. In semiconductor quantum dots much work has focused on demonstrating single spin qubit control using optical techniques. However, optical control of entanglement of two spin qubits remains a major challenge for scaling from a single qubit to a full-fledged quantum information platform. Here, we combine advances in vertically-stacked quantum dots with ultrafast laser techniques to achieve optical control of the entangled state of two electron spins. Each electron is in a separate InAs quantum dot, and the spins interact through tunneling, where the tunneling rate determines how rapidly entangling operations can be performed. The two-qubit gate speeds achieved here are over an order of magnitude faster than in other systems. These results demonstrate the viability and advantages of optically controlled quantum dot spins for multi-qubit systems.

Ultrafast optical spin echo in a single quantum dot

Many proposed photonic quantum networks rely on matter qubits to serve as memory elements1,2. The spin of a single electron confined in a semiconductor quantum dot forms a promising matter qubit that may be interfaced with a photonic network3. Ultrafast optical spin control allows gate operations to be performed on the spin within a picosecond timescale4,5,6,7,8,9,10,11,12,13,14, orders of magnitude faster than microwave or electrical control15,16. One obstacle to storing quantum information in a single quantum dot spin is the apparent nanosecond-timescale dephasing due to slow variations in the background nuclear magnetic field15,16,17. Here we use an ultrafast, all-optical spin echo technique to increase the decoherence time of a single quantum dot electron spin from nanoseconds to several microseconds. The ratio of decoherence time to gate time exceeds 105, suggesting strong promise for future photonic quantum information processors18 and repeater networks1,2. An ultrafast, all-optical spin echo technique is used to increase the decoherence time of a single quantum dot electron spin from nanoseconds to several microseconds. The ratio of decoherence time to gate time exceeds 105, suggesting strong promise for future photonic quantum information processors and repeater networks.

PHASE MEASURED AND CONTROLLED TECHNOLOGY BASED ON ITS BASIC AND HARMONIC LASER BEAM IN COHERENT CONTROL

In ultrafast optical coherent control,the existence of the phase distortion and phase inconsistence on fundamental frequency and its second harmonic generation laser wavefront leads to fail to achieve the desired experiment results in most cases.An experimental method in diagnosing such distortion situation was designed to make an effective auto-control system applied sucessfully in the coherent control photocurrent experiments.The retrieval algorithm was used,and the spatial surface structure was calculated based on the relationship between the beam intensity and phase difference,thus the voltage or direction was achieved.In this way,the mobile-mirror moves to less aberration.The mentioned experimental result proves that the system can adjust the two wavefronts being parallel automatically in real time.