Carrier-envelope Phase Control Research Articles

Wavefront control of subcycle vortex pulses via carrier-envelope-phase tailoring

The carrier-envelope phase (CEP) of an ultrashort laser pulse is becoming more crucial to specify the temporal characteristic of the pulse’s electric field when the pulse duration becomes shorter and attains the subcycle regime; here, the pulse duration of the intensity envelope is shorter than one cycle period of the carrier field oscillation. When this subcycle pulse involves a structured wavefront as is contained in an optical vortex (OV) pulse, the CEP has an impact on not only the temporal but also the spatial characteristics owing to the spatiotemporal coupling in the structured optical pulse. However, the direct observation of the spatial effect of the CEP control has not yet been demonstrated. In this study, we report on the measurement and control of the spatial wavefront of a subcycle OV pulse by adjusting the CEP. To generate subcycle OV pulses, an optical parametric amplifier delivering subcycle Gaussian pulses and a Sagnac interferometer as a mode converter were integrated and provided an adequate spectral adaptability. The pulse duration of the generated OV pulse was 4.7 fs at a carrier wavelength of 1.54 µm. To confirm the wavefront control with the alteration of the CEP, we developed a novel f-2f interferometer that exhibited spiral fringes originating from the spatial interference between the subcycle OV pulse and the second harmonic of the subcycle Gaussian pulse producing a parabolic wavefront as a reference; this resulted in the successful observation of the rotation of spiral interference fringes during CEP manipulation.

Efficient and Continuous Carrier-Envelope Phase Control for Terahertz Lightwave-Driven Scanning Probe Microscopy.

The fundamental understanding of quantum dynamics in advanced materials requires precise characterization at the limit of spatiotemporal resolution. Ultrafast scanning tunneling microscopy is a powerful tool combining the benefits of picosecond time resolution provided by single-cycle terahertz (THz) pulses and atomic spatial resolution of a scanning tunneling microscope (STM). For the selective excitation of localized electronic states, the transient field profile must be tailored to the energetic structure of the system. Here, we present an advanced THz-STM setup combining multi-MHz repetition rates, strong THz near fields, and continuous carrier-envelope phase (CEP) control of the transient waveform. In particular, we employ frustrated total internal reflection as an efficient and cost-effective method for precise CEP control of single-cycle THz pulses with >60% field transmissivity, high pointing stability, and continuous phase shifting of up to 0.75 π in the far and near field. Efficient THz generation and dispersion management enable peak THz voltages at the tip-sample junction exceeding 20 V at 1 MHz and 1 V at 41 MHz. The system comprises two distinct THz generation arms, which facilitate individual pulse shaping and amplitude modulation. This unique feature enables the flexible implementation of various THz pump-probe schemes, thereby facilitating the study of electronic and excitonic excited-state propagation in nanostructures and low-dimensional materials systems. Scalability of the repetition rate up to 41 MHz, combined with a state-of-the-art low-temperature STM, paves the way toward the investigation of dynamical processes in atomic quantum systems at their native length and time scales.

Carrier-envelope phase on-chip scanner and control of laser beams

The carrier-envelope phase (CEP) is an important property of few-cycle laser pulses, allowing for light field control of electronic processes during laser-matter interactions. Thus, the measurement and control of CEP is essential for applications of few-cycle lasers. Currently, there is no robust method for measuring the non-trivial spatial CEP distribution of few-cycle laser pulses. Here, we demonstrate a compact on-chip, ambient-air, CEP scanning probe with 0.1 µm3 resolution based on optical driving of CEP-sensitive ultrafast currents in a metal−dielectric heterostructure. We successfully apply the probe to obtain a 3D map of spatial changes of CEP in the vicinity of an oscillator beam focus with pulses as weak as 1 nJ. We also demonstrate CEP control in the focal volume with a spatial light modulator so that arbitrary spatial CEP sculpting could be realized.

Sub-cycle pulse revealed with carrier-envelope phase control of soliton self-compression in anti-resonant hollow-core fiber.

The influence of the carrier-envelope phase (CEP) of a pump pulse on the multioctave supercontinuum (SC) generation in a gas-filled anti-resonant hollow-core fiber (AR HCF) by soliton self-compression (SSC) has been explored. We have shown an octave-wide third harmonic generation (THG) in the visible-to-near-infrared range during the pulse compression down to a sub-cycle duration. The CEP of a multi-cycle pump pulse provides control of interference between the third harmonic (TH) and the SC that indicates the coherent synthesis of a sub-cycle pulse with a duration of about 0.4 optical cycles and a peak power of more than 2 GW at the fiber output.

Probing Rashba spin-orbit coupling by subcycle lightwave control of valley polarization

We perform nonperturbative calculations of light-field driven valley-polarization process in monolayer ${\mathrm{MoS}}_{2}$ which has additional Rashba spin-orbit couplings (SOCs). The ultrafast electron dynamics is simulated within the independent particle picture by solving density-matrix equations in the basis of linear combination of atomic orbitals, where tight-binding (TB) models including both intrinsic atomic and Rashba SOCs are used to calculate relevant matrix elements. We demonstrate that the Rashba-type SOCs can be manifested by suboptical-cycle control of valley selectivity excitations, in particular necessary via using few-cycle linearly polarized pulse with controlled carrier-envelope phase (CEP). This procedure will lead to a CEP-dependent valley Hall conductivity (VHC), which exhibits an important phase shift among different Rashba coupling strengths. The additional analysis shows that this phase shift is mainly determined by the ${d}_{z}^{2}$-orbital TB Rashba parameter from Mo atom and originates from contribution of conduction bands to VHC, where the Berry curvature modified by Rashba SOC plays a crucial role. Moreover, we also provide a qualitative interpretation on the Rashba-dependent VHC in terms of suboptical-cycle Landau-Zener-St\"uckelberg interference. Our results suggest a feasible approach for probing Rashba SOCs in hexagonal two-dimensional materials, and might pave the way of achieving more controls in the future valleytronics application.

Carrier envelope phase stabilization with a true common-path collinear ƒ–2ƒ interferometer

A variety of technologies have been so far demonstrated for the carrier-envelope phase (CEP) control of mode-locked oscillators. An instrumental part of these technologies is the configuration of interferometers and the application of feedback control. Of the devices, the collinear ƒ–2ƒ interferometer developed in this work is compact and robust against disturbances because the optical paths of the ƒ and 2ƒ components are collinear within the interferometer. To compensate for the delay time between the ƒ and 2ƒ components, a birefringent time plate made of α-BBO is installed in the interferometer. We achieved an in-loop CEP stability of 27 mrad (rms) using conventional feedback control with an acousto-optic modulator to control the pump power. We believe that the collinear ƒ–2ƒ interferometer offers the best choice as an interferometer for CEP stabilization of the front-end oscillator in the chirped-pulse amplification system.

Carrier-envelope phase control of synthesized waveforms with two acousto-optic programmable dispersive filters.

We demonstrate the scanning and control of the carrier-envelope phases (CEPs) of two adjacent spectral components totally spanning more than one-octave in the short-wave infrared (SWIR) wavelength region by operating two individual acousto-optic programmable dispersive filters (AOPDFs) applied to each of the two spectral components. The total CEP shift of the synthesized sub-cycle pulse composed of the two spectral components is controlled with simultaneous scans of the two CEPs. The resultant error of the controlled CEP was 642 mrad, so that this technique is useful for searching zero CEP of the synthesized pulse with the maximum field amplitude. In addition, we conduct a closed feedback loop to compensate for the CEP fluctuation by using the two AOPDFs together. As a result, we succeed to reduce the rms error of the CEP from 399 mrad to 237 mrad.

Waveform Control of Relativistic Electron Dynamics in Laser-Plasma Acceleration

The interaction of ultraintense laser pulses with an underdense plasma is used in laser-plasma acceleration to create compact sources of ultrashort pulses of relativistic electrons and x rays. The accelerating structure is a plasma wave, or wakefield, that is excited by the laser ponderomotive force, a force that is usually assumed to depend solely on the laser envelope and not on its exact waveform. Here, we use near-single-cycle laser pulses with a controlled carrier-envelope phase to show that the actual waveform of the laser field has a clear impact on the plasma response. The beam pointing of our relativistic electron beam oscillates in phase with the carrier-envelope phase of the laser, at an amplitude of 15 mrad, or 30% of the beam divergence. Numerical simulations explain this observation through asymmetries in the injection and acceleration of the electron beam, which are locked to the carrier-envelope phase. These results imply that we achieve waveform control of relativistic electron dynamics. Our results pave the way to high-precision, subcycle control of electron injection in plasma accelerators, enabling the production of attosecond relativistic electron bunches and x rays.Received 26 May 2021Revised 10 December 2021Accepted 11 January 2022DOI:https://doi.org/10.1103/PhysRevX.12.011036Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasHigh intensity laser-plasma interactionsLaser wakefield accelerationPonderomotive effectsUltrashort pulsesAccelerators & BeamsPlasma PhysicsAtomic, Molecular & Optical

Realization of carrier envelope phase control in an erbium-doped all-fiber comb via an intracavity electrical polarization controller.

We present an erbium-doped all-fiber comb that is realized via intracavity piezoelectric mechanical responses by inserting an electrical polarization controller (EPC) for carrier envelope phase offset (fceo) control and sticking a piezoelectric transducer (PZT) for repetition rate (fr) stabilization. With only three components inside the oscillator, it facilitates an all-fiber comb system with high integration and robustness. Besides fceo control, smart, recoverable, and precise searching of mode-locked states can also be achieved by finely tuning the EPC driving voltages. The fceo signal can be tightly phase-locked due to the 20 kHz 3 dB bandwidth of the EPC device. Compared with the traditional pump feedback mode, the EPC approach exhibits a larger and flatter gain in the near-zero-dispersion zone of the fiber comb, corresponding to less EPC induced intracavity fluctuations. It is also a benefit for reducing cross regulation between fr and fceo in long-term stabilization of the comb.

Optical current generation in graphene: CEP control vs. ω + 2ω control

Abstract The injection of directional currents in solids with strong optical fields has attracted tremendous attention as a route to realize ultrafast electronics based on the quantum-mechanical nature of electrons at femto- to attosecond timescales. Such currents are usually the result of an asymmetric population distribution imprinted by the temporal symmetry of the driving field. Here we compare two experimental schemes that allow control over the amplitude and direction of light-field-driven currents excited in graphene. Both schemes rely on shaping the incident laser field with one parameter only: either the carrier-envelope phase (CEP) of a single laser pulse or the relative phase between pulses oscillating at angular frequencies ω and 2ω, both for comparable laser parameters. We observe that the efficiency in generating a current via two-color-control exceeds that of CEP control by more than two orders of magnitude (7 nA vs. 18 pA), as the ω + 2ω field exhibits significantly more asymmetry in its temporal shape. We support this finding with numerical simulations that clearly show that two-color current control in graphene is superior, even down to single-cycle pulse durations. We expect our results to be relevant to experimentally access fundamental properties of any solid at ultrafast timescales, as well as for the emerging field of petahertz electronics.

Simple carrier-envelope phase control and stabilization scheme for difference frequency generation-based systems.

We report about a setup for carrier-envelope phase (CEP) control and stabilization in passive systems based on difference frequency generation (DFG). The principle of this approach relies on the amplitude to phase modulation transfer in the white-light generation process. A small modulation of the pump laser intensity is used to obtain a DFG output modulated in CEP. This technique is demonstrated in a CEP-stable system pumped by an Yb-doped fiber amplifier. It is first characterized by measuring CEP modulations produced by applying arbitrary waveforms. The CEP actuator is then used for slow drifts correction in a feedback loop. The results show the capability of this simple approach for OPA/OPCPA CEP-stabilized setups.

Identifying observable carrier-envelope phase effects in laser wakefield acceleration with near-single-cycle pulses

Driving laser wakefield acceleration with extremely short, near single-cycle laser pulses is crucial to the realization of an electron source that can operate at kHz-repetition rate while relying on modest laser energy. It is also interesting from a fundamental point of view, as the ponderomotive approximation is no longer valid for such short pulses. Through particle-in-cell simulations, we show how the plasma response becomes asymmetric in the plane of laser polarization, and dependent on the carrier-envelope phase (CEP) of the laser pulse. For the case of self-injection, this in turn strongly affects the initial conditions of injected electrons, causing collective betatron oscillations of the electron beam. As a result, the electron beam pointing, electron energy spectrum, and the direction of emitted betatron radiation become CEP dependent. For injection in a density gradient, the effect on beam pointing is reduced and the electron energy spectrum is CEP independent, as electron injection is mostly longitudinal and mainly determined by the density gradient. Our results highlight the importance of controlling the CEP in this regime for producing stable and reproducible relativistic electron beams and identify how CEP effects may be observed in experiments. In the future, CEP control may become an additional tool to control the energy spectrum or pointing of the accelerated electron beam.

Sub-cycle valleytronics: control of valley polarization using few-cycle linearly polarized pulses

So far, it has been assumed that selective excitation of a desired valley in the Brillouin zone of a hexagonal two-dimensional material has to rely on using circularly polarized fields. We theoretically demonstrate a way to control the valley excitation in hexagonal 2D materials on a few-femtosecond timescale using a few-cycle, linearly polarized pulse with controlled carrier–envelope phase. The valley polarization is mapped onto the strength of the perpendicular harmonic signal of a weak, linearly polarized pulse, which allows to read this information all-optically without destroying the valley state and without relying on the Berry curvature, making our approach potentially applicable to inversion-symmetric materials. We show applicability of this method to hexagonal boron nitride and M o S 2 .

Active control of ultrafast electron dynamics in plasmonic gaps using an applied bias

In this joint experimental and theoretical study we demonstrate coherent control of the optical eld emission and electron transport in plasmonic gaps subjected to intense single-cycle laser pulses. Our results show that a small applied dc bias or an external THz eld allows to modulate and direct the electron photocurrents in the gap of a connected nanoantenna operating as an ultra-fast nanoscale vacuum diode for lightwave electronics. Using Time-Dependent Density Functional Theory calculations we elucidate the main physical mechanisms behind the observed eects and show that an applied dc eld modies the optical eld emission and quiver motion of photoemitted electrons within the gap. The quantum many-body theory reproduces the measured net electron transport in the experimental device which allows us to establish a new paradigm for controlling nanocircuits at Petahertz frequencies. The interaction of intense short laser pulses with matter provides access to the dynamics of electronic excita-tions in a highly nonlinear regime characterised by emission of energetic electron bursts of sub-cycle duration and by generation of high harmonics used to track the evolution of the quantum systems at attosecond time scales [1 4]. For metal surfaces and metal nanoparticles, the coupling of light with collective electronic excitations (plas-mons) allows to engineer enhanced optical elds at the hot spots with characteristic sizes well below the dirac-tion limit [5, 6]. Thus, the optical eld emission regime can be reached for incident eld strengths signicantly smaller than those required for molecular and atomic species in the gas phase [4, 7, 8]. In contrast to electron photoemission via multiphoton absorption, optical eld emission can be seen as an electron tunneling at the metal vacuum interface in a situation where the potential barrier is strongly modied by the instantaneous optical eld. The in-depth studies performed for metallic nanotips and plasmonic nanoparticles revealed that optical eld electron emission can be manipulated at femtosecond time scales via the carrier-envelope phase (CEP) of the driving laser pulse [911]. Using THz elds or an applied bias along with the optical excitation oers additional possibilities [1217] of coherent control. In this context, among the plasmonic nanoobjects that can be applied for light wave electronics [1823], the dimer antenna with a nanoscale gap is very relevant. On the one hand, the coupling between electrons and photons in narrow gaps of dimer antennas leads to light emission originating from inelastic electron tunneling events [2427]. On the other hand, the highly nonlinear optical eld electron emission process [19, 22, 23, 28, 29] as well as optically assisted electron tunneling [3032] allow rectication at optical frequencies and CEP control of the electron transport across the junction [21, 33]. In this Letter, we demonstrate coherent control of the net electric current in a nanocircuit comprising a single bowtie nanoantenna with a 6 nm wide gap as presented in Fig. 1. It follows from our results that, along with the CEP of the incident pulse, the petaherz currents of the optically emitted electrons in the gap of the nanoantenna can be directed and controlled by applying a dc eld two orders of magnitude smaller than the optical eld in the junction. Notably, such a dc bias alone does not trigger any electron ow across the gap, i.e. electron tunneling is impossible for this width of the junction. Our study thus extends the possible application of the static or THz elds beyond the control of the electron (photo)emission from metallic tips [1217] and electron tunneling [3436]. Along with dielectric, semiconductor [3739], graphene-based [18, 40], and tunneling [33] structures, the analogue of the ultrafast rectifying vacuum diode demonstrated here (see also [29]) paves the way towards petaherz electronics [41]. The gold bowtie nanoantenna has been fabricated by electron beam lithography on a silica substrate. The two arms of the antenna are interfaced macroscopically (see Fig. 1b) with a transimpedance amplier which allows for a readout of the optically driven currents via a lock-in scheme. An additional DC bias can be applied across the antenna gap by means of a bias tee. The ultrafast currents are driven by the electric eld transients of single-cycle light pulses of a carrier wavelength of 1250 nm and a duration of 4.2 fs. These pulses are generated by a

Dynamic quantum state holography

We present a pulse-shaper-based holographic technique for the time-resolved and phase-sensitive observation of ultrafast quantum dynamics. The technique combines bichromatic white light polarization pulse shaping with the tomographic reconstruction of photoelectron wave packets. The physical scheme is based on the interference of a probe wave packet from $N+1$ resonance-enhanced multiphoton ionization via the target states and a reference wave packet from $M+1$ multiphoton ionization of the ground state. To create the wave packets, we employ carrier-envelope phase stable bichromatic ($M\ensuremath{\omega}$:$N\ensuremath{\omega}$) pump-probe pulse sequences. The scheme is demonstrated on femtosecond Rydberg wave-packet dynamics in potassium atoms using corotating circularly polarized ($2\ensuremath{\omega}$:$3\ensuremath{\omega}$) pulse sequences. The interference of continuum states with different angular momenta yields a crescent-shaped photoelectron wave packet rotating in the laser polarization plane due to the interplay of the optical phase and the accumulated quantum phase. Carrier-envelope phase control of the rotation provides access to the photoelectron asymmetry, enabling background-free detection of the crescent's angular motion which maps the bound-electron dynamics.

Influence of the Doppler effect of a periodically moving mirror on the carrier-envelope frequency of a pulse train.

We investigate the influence of the optical Doppler effect on the carrier-envelope frequency (CEF) of a mode-locked pulse train. The laser pulses are Doppler-shifted in frequency during reflection off a periodically moving mirror that is driven by an electro-dynamical exciter inside an f-2f interferometer. Depending on the relative movement of the mirror at the instant of reflection, we experimentally demonstrate a CEF shift of the laser pulses up to ±69 kHz, which is sufficient for the carrier-envelope phase control of laser amplifiers with repetition rates of 10kHz and beyond. Using piezoelectric thick films, we show that the scheme also appears to be adaptable to the megahertz repetition rates of typical oscillators. As the phase control is exerted extra-cavity, Doppler-induced CEF modulation is virtually free of any side effects of traditional stabilization schemes that typically act on the pump power. Finally, the Doppler scheme may overcome servo loop bandwidth limitations associated with pump power control.

Phase control and stabilization in attosecond beamline with fast Fourier transform

With the unveiling of molecular and atomic dynamics, scientists crave finer and faster tools to communicate with the microworld. Attosecond pump-probe enjoys its reputation as the fastest camera, hinting ultrafast movements in the delay graph. To employ this camera, the stability and delay control should have very great accuracy comparable to the camera resolution. It is also of significant importance for stabilizing the carrier envelope phase (CEP) in few-cycle laser field. When dealing with a huge quantity of data, conventional Fourier transform algorism is challenging in high-speed control. Here we put forward the efficient calculation method, fast Fourier transform (FFT) algorism in Mach-Zehnder interferometer for arm length locking and <i>f</i>-2<i>f</i> for CEP locking. In the interferometer locking, 532 nm continuous wave laser is used in the Mach-Zehnder interferometer, and the phase of the FFT term corresponding to the delay between the two arms of the interferometer serves as a feedback signal on piezo transducer (PZT) in the delay line to reduce the change of the arm length. In the CEP control experiment, data to be analyzed are the <i>f</i>-2<i>f</i> spectrum interference fringes recorded by the spectrometer. The CEP values are obtained from the first order of FFT module output of the integrated spectrum interference fringes, and a labview program examines the relative phase drift and sends a feedback voltage signal to the PZT through the proportion integration differentiation module to compensate slow CEP drift after the chirped pulse amplification system by changing the insert length of a prism pair. The results show that the root mean square (RMS) of the arm length difference is 1.24 nm (4.1 attosecond for light to travel) per meter in the interferometer locking over 12 h, and the RMS of CEP is 227 mrad under 3 ms integration time in the CEP locking over 20 min. These results are able to meet the requirement of the accuracy for attosecond pulse generation and attosecond pump-probe experiments. We also use FFT to stabilize the CEP and relative time simultaneously in the waveform synthesis for 8 h (Huang P, Fang S, Gao Y, Zhao K, Hou X, Wei Z 2019 <i>Appl. Phys. Lett.</i> <b>115</b> 031102), the phase-locking system results in a CEP stability of 280 mrad and a relative time stability of 110 as at a repetition rate of 1 kHz. These results imply that the FFT is versatile and reliable in ultrafast control.

Control of molecular dissociation by spatially inhomogeneous near fields

Tailored intense laser fields can be used to steer electron and nuclear dynamics in molecules and control chemical reactions. Here we demonstrate that inhomogeneous nanoscopic near fields provide an additional mechanism for controlling light-induced chemical reactions. This is investigated by quantum-dynamical calculations on dissociation of H-2(+) in few-cycle near fields with a controlled carrier-envelope phase. Directional dissociation asymmetry analysis reveals that the spatial asymmetry of the field is critical in controlling and in modifying field-induced dissociation pathways such as zero-photon dissociation, bond softening, and above-threshold dissociation. The degree of impact strongly depends on the maximum length scale of coherent control in a dissociation channel. The results pave the way towards coherent control of molecular reactions in a wide range of molecules in inhomogeneous near fields.

Continuous every-single-shot carrier-envelope phase measurement and control at 100 kHz.

With the emergence of high-repetition-rate few-cycle laser pulse amplifiers aimed at investigating ultrafast dynamics in atomic, molecular, and solid-state science, the need for ever faster carrier-envelope phase (CEP) detection and control has arisen. Here we demonstrate a high-speed, continuous, every-single-shot measurement and fast feedback scheme based on a stereo above-threshold ionization time-of-flight spectrometer capable of detecting the CEP and pulse duration at a repetition rate of up to 400kHz. This scheme is applied to a 100kHz optical parametric chirped pulse amplification few-cycle laser system, demonstrating improved CEP stabilization and allowing for CEP tagging.

Carrier-envelope phase controlled isolated attosecond pulses in the nm wavelength range, based on coherent nonlinear Thomson-backscattering

A proposal for a novel source of isolated attosecond XUV—soft x-ray pulses with a well controlled carrier-envelope phase difference (CEP) is presented in the framework of nonlinear Thomson-backscattering. Based on the analytic solution of the Newton–Lorentz equations, the motion of a relativistic electron is calculated explicitly, for head-on collision with an intense fs laser pulse. By using the received formulas, the collective spectrum and the corresponding temporal shape of the radiation emitted by a mono-energetic electron bunch can be easily computed. For certain suitable and realistic parameters, single-cycle isolated pulses of ca. 20 as length are predicted in the XUV—soft x-ray spectral range, including the 2.33–4.37 nm water window. According to our analysis, the generated almost linearly polarized beam is extremely well collimated around the initial velocity of the electron bunch, with considerable intensity and with its CEP locked to that of the fs laser pulse.