Carrier-envelope Phase Research Articles

Coherent control of extreme ultraviolet emission generated through frustrated tunneling ionization

Coherent extreme ultraviolet (EUV) line emissions can be generated from an atom excited through frustrated tunneling ionization (FTI). The phase variation of the EUV emission in a generation medium along the propagation direction is a critical parameter that determines the phase-matching condition of this new light source. Here we show that the EUV emission sensitively depends on the intensity and phase of a driving laser pulse and the target position. Angle-resolved EUV spectra measured at different target positions and the carrier-envelope phases of the laser pulse exhibit an intensity modulation, showing similar behavior to that of a long-trajectory high harmonic radiation. The four-step model developed for the FTI emission accurately describes the coherent control of the EUV emission. These findings are essential ingredients for developing coherent extreme ultraviolet sources with high photon flux and for utilizing the FTI emission in time-resolved spectroscopy.

Improved Carrier-Envelope Phase Determination Method for Few-Cycle Laser Pulses Using High-Order Above-Threshold Ionization

Recent studies indicate that the stereo-ATI carrier-envelope phase meter (CEPM) is an effective method to determine the carrier-envelope phase (CEP) of each and every single few-cycle laser pulse. In this method, a two-dimensional parametric asymmetry plot (PAP), which can be obtained with the measured data in two short time-of-flight intervals, is applied to extract the CEP. Thus, part of the data containing useful CEP information is discarded in the PAP method. In this work, an improved method was developed to effectively exploit most of the experimental data. By this method, we achieve a CEP precision of 57 mrad over the entire 2π range for 5.0 fs laser pulses.

Few-cycle short-wave-infrared light source for strong-field experiments at 200 kHz repetition rate

We present a compact, few-cycle, short-wave infrared light source delivering 13 µJ, carrier-envelope phase (CEP) stable pulses around 2 µm, operating at 200 kHz repetition rate. Starting from an ytterbium fiber amplifier, the seed is produced via white-light generation followed by difference frequency generation, and later amplified in two BiBO nonlinear crystals. A pulse duration of 15.8 fs is measured with the dispersion scan technique, while the CEP stability is assessed via a monolithic spectral interferometry scheme. We demonstrate the potential of the system to drive strong-field experiments by performing high-order harmonic generation in argon gas.

High-order above-threshold ionization from a coherent superposition of states

Quantum theory of high-order above-threshold ionization (HATI) of atoms, prepared in a coherent superposition of bound states, by a strong laser field is formulated. Numerical results are obtained using the solutions of the time-dependent Schr\"odinger equation and the improved strong-field approximation. As an example, coherent superposition of the excited $2s$ and $2p$ states of the He atom is used. Applying a two-level model, it is shown how the relative amplitude and phase of the states in this coherent superposition can be controlled with a weak resonant laser pulse. Numerical results presented for HATI by a strong nonresonant probe laser pulse confirm that the photoelectron spectra and the momentum distributions strongly depend on the relative phase of the states in the prepared coherent superposition. For the case of HATI by a strong resonant laser pulse, we modify our strong-field-approximation theory by applying the two-level model to describe the time evolution of the atomic bound state before the instant of ionization. Contrary to the HATI from a single state by a long laser pulse, the corresponding photoelectron spectra of HATI from the coherent superposition of states depend on the carrier-envelope phase of the resonant ionizing pulse. Therefore, the strong-field ionization from a coherent superposition of states by a resonant laser pulse can be used to determine the carrier-envelope phase for long pulses for which the standard methods such as stereo--above-threshold ionization technique fail. The results strongly depend on the relative amplitude and phase of states in the coherent superposition of states, which can be controlled with a weak resonant laser pulse.

Carrier-envelope-phase measurement of sub-cycle UV pulses using angular photofragment distributions

Carrier-envelope-phase (CEP) of sub-cycle ultraviolet (UV) pulse strongly influences the dynamics of quantum systems, but its characterization is not accessible experimentally. Here we investigate photodissociation of a diatomic molecule from its ground-rovibrational state in a linearly polarized weak sub-cycle UV pulse with a controlled CEP. The angular distribution of photofragments shows an asymmetric profile deviating from the well-known {cos }^{2}- or {sin }^{2}-like ones, which can be identified as a way to imprint CEP. We unveil that such an effect stems from the temporal neighboring rotational excitation by molecular permanent dipole interaction through the joint contributions between counter-rotating and rotating terms. This in turn, opens different pathways in photodissociation dynamics. Given that the temporal excitation between various states with close energies can be manipulated by CEP of sub-cycle UV pulses, our results pave ways for understanding and manipulating electron, nuclear and their joint dynamics with variation of CEP of attosecond pulses.

Few-cycle excitation of atomic coherence: an analytical solution beyond the rotating-wave approximation

Developing an analytical theory for atomic coherence driven by ultrashort laser pulses has proved to be challenging due to the breakdown of the rotating wave approximation (RWA). In this paper, we present an approximate analytical solution that describes a two-level atom under the excitation of a far-off-resonance, few-cycle pulse of arbitrary shape without invoking the RWA. As an example of its applicability, a closed-form solution for Gaussian pulses is explicitly given, and the result is used to analyse the impact of carrier envelope phase on atomic population ratios. Comparisons with numerical solutions validate the accuracy our solution within the scope of the approximation. Finally, we outline an alternative approach that can lead to a more accurate solution by capturing the nonlinear behaviors of the system. The work lays out feasible theoretical paths toward analytically describing two-level atoms driven by ultrashort pulses.

Strong-Field Ionization Amplitudes for Atomic Many-Electron Targets

The strong-field approximation (SFA) has been widely applied in the literature to model the ionization of atoms and molecules by intense laser pulses. A recent re-formulation of the SFA in terms of partial waves and spherical tensor operators helped adopt this approach to account for realistic atomic potentials and pulses of different shape and time structure. This re-formulation also enables one to overcome certain limitations of the original SFA formulation with regard to the representation of the initial-bound and final-continuum wave functions of the emitted electrons. We here show within the framework of Jac, the Jena Atomic Calculator, how the direct SFA ionization amplitude can be readily generated and utilized in order to compute above-threshold ionization (ATI) distributions for many-electron targets and laser pulses of given frequency, intensity, polarization, pulse duration and carrier–envelope phase. Examples are shown for selected ATI energy, angular as well as momentum distributions in the strong-field ionization of atomic krypton. We also briefly discuss how this approach can be extended to incorporate rescattering and high-harmonic processes into the SFA amplitudes.

100-mJ class, sub-two-cycle, carrier-envelope phase-stable dual-chirped optical parametric amplification.

Based on dual-chirped optical parametric amplification (DC-OPA) and type-I BiB3O6 (BiBO) crystals, the generation of >100 mJ, 10.4 fs, 10 Hz, carrier-envelope phase (CEP)-stable laser pulses, which are centered at 1.7 µm, was demonstrated producing a peak power of 10 TW. CEP-dependent high harmonic generation (HHG) was implemented to confirm the sub-two-cycle pulse duration and CEP stabilization of infrared (IR) laser pulses. As far as we know, the obtained pulse energy and peak power represented the highest values for sub-two-cycle CEP-stable IR optical parametric amplification. Additionally, the prospects of achieving high-energy water window isolated attosecond pulses (IAPs) via our developed laser source were discussed.

Селектор импульсов для получения фемтосекундного излучения с управляемой фазой несущей относительно огибающей

In non-linear optical processes, such as obtaining attosecond pulses, it is extremely important to control the carrier envelope phase. To do this, various periodic trains of identical femtosecond pulses with a controlled phase can be created. In addition, since there is no frequency comb offset in such sequences, the process of measuring optical frequencies is greatly simplified. The pulse selector has been developed to obtain a sequence of identical femtosecond pulses with a controlled carrier – envelope phase. The selector makes it possible to obtain a “pure” sequence of identical femtosecond pulses at the modulator output (when every 125th pulse is selected from the original sequence with a repetition rate of 250 MHz) in a fairly wide range of control signal phase tuning. This range is 1.3 degrees. The phase tuning of the pulse selector provides the possibility of obtaining one hundred twenty-five such sequences with a phase tuning discreteness of 2π/125. The simplest way to reduce discreteness is to increase the ratio of the pulse repetition rate of a femtosecond laser to the shift of its frequency comb. The phase characteristic of the pulse selector was obtained by registering the time dependence of the synthesized sequence with a selector phase tuning step of 0.1 degrees. We measured the spectra of the sequences at different phases of the pulse selector, as well as the emission spectrum of the master laser at the minimum transmission of the modulator in the absence of modulation. The spectrum with the maximum amplitude corresponded to the case when identical pulses with the highest amplitude were selected from the original sequence. The difference between these spectra made it possible to isolate the spectrum of a pure sequence of identical femtosecond pulses without taking into account the "background" that occurs due to the modulator has a finite attenuation of –20 dB.

Flexible THz Carrier‐Envelope Phase Shifter Based on Metamaterials

AbstractThe ability to manipulate terahertz (THz) waveform is important for its applications. Despite rapid progress in metamaterials (MMs), the carrier‐envelope phase (CEP) control for THz pulse is still challenging due to its broadband feature. THz scanning tunneling microscope (THz‐STM), as an emerging technique combining both high temporal and spatial resolution, imposes additional requirements. Here, an ultra‐thin and flexible THz CEP shifter with five groups of different MM arrays is proposed. The CEP shifter is based on a specified split‐ring resonator with a pair of cross‐oriented gratings to enhance transmission efficiency. Simulation and experimental results show that the CEP of the THz pulse is modulated by passing through different MM arrays in turn, which can be shifted as high as 2π. The CEP shifter has good tolerance to wide‐angle oblique incidence and structural bending, which is expected to be used as a key component for THz‐STM. The structure of the CEP shifter has many potential applications, including THz wave focusing, beam‐steering, spatial phase modulation, and structured light generation.

Stabilizing the carrier–envelope phase of an amplified Ti:sapphire laser pulse to a noise level of sub-100 mrad

To generate an intense femtosecond pulse with an energy of several mJ or more, the pulse must be sufficiently stretched and compressed using grating-based optics in a chirped pulse amplification system. However, the stability of the carrier–envelope phase (CEP) is readily degraded by movement of the diffraction gratings due to vibration caused by pump lasers and other equipment. By suppressing the vibration and acoustic waves, we reduced the CEP noise to much less than 100 mrad (rms). With feedback control using a wedge pair with a single shot f - 2 f interferometer (1 ms integration time), we were able to stabilize the CEP, achieving a CEP noise of 58 mrad (rms) at a pulse energy of 0.5 mJ and 99 mrad (rms) at a pulse energy of 7 mJ and a repetition rate of 1 kHz. We also analyzed the effects of vibration on the pulse stretcher and compressor using a geometrical optics approach and found that roof mirrors were the most influential.

PHz Electronic Device Design and Simulation for Waveguide-Integrated Carrier-Envelope Phase Detection

Carrier-envelope phase (CEP) detection of ultrashort optical pulses and low-energy waveform field sampling have recently been demonstrated using direct time-domain methods that exploit optical-field photoemission from plasmonic nanoantennas. These devices are compact and integratable solid-state detectors operating at optical frequencies under ambient conditions using low pulse energies (picojoule-level). Potential applications include frequency-comb stabilization, optical time-domain spectroscopy, compact tools for attosecond science and metrology, and petahertz-scale information processing. However, to date these devices have been driven by free-space optical waveforms and their implementation within integrated photonic platforms has yet to be demonstrated. In this work, we design and simulate fully-integrated plasmonic nanoantennas coupled to a SiN-core waveguide for CEP detection. We find that when coupled to realistic on-chip, few-cycle supercontinuum sources, these devices are suitable for direct time-domain CEP detection within integrated photonic platforms. We estimate a signal-to-noise ratio of 30 dB at 50 kHz resolution bandwidth, and address technical details, such as the tuning of the nanoantennas plasmonic resonance, CEP slippage in the waveguide, optical losses, and sensitivity to driving pulse characteristics such as energy and duration. Our results provide the basis for future design and fabrication of time-domain CEP detectors and allow for the development of fully-integrated attosecond science applications, frequency-comb stabilization and light-wave-based PHz electronics.

Probing phonon dynamics with multidimensional high harmonic carrier-envelope-phase spectroscopy

We explore pump-probe high harmonic generation (HHG) from monolayer hexagonal-boron-nitride, where a terahertz pump excites coherent optical phonons that are subsequently probed by an intense infrared pulse that drives HHG. We find, through state-of-the-art ab initio calculations, that the structure of the emission spectrum is attenuated by the presence of coherent phonons and no longer comprises discrete harmonic orders, but rather a continuous emission in the plateau region. The HHG yield strongly oscillates as a function of the pump-probe delay, corresponding to ultrafast changes in the lattice such as specific bond compression or stretching dynamics. We further show that in the regime where the excited phonon period and the pulse duration are of the same order of magnitude, the HHG process becomes sensitive to the carrier-envelope phase (CEP) of the driving field, even though the pulse duration is so long that no such sensitivity is observed in the absence of coherent phonons. The degree of CEP sensitivity versus pump-probe delay is shown to be a highly selective measure for instantaneous structural changes in the lattice, providing an approach for ultrafast multidimensional HHG spectroscopy. Remarkably, the obtained temporal resolution for phonon dynamics is ∼1 femtosecond, which is much shorter than the probe pulse duration because of the inherent subcycle contrast mechanism. Our work paves the way toward routes of probing phonons and ultrafast material structural changes with subcycle temporal resolution and provides a mechanism for controlling the HHG spectrum.

Pulse-to-pulse detection of terahertz radiation emitted from the femtosecond laser ablation process.

Determining the dynamics of electrons and ions emitted from a target material during laser ablation is crucial for desirable control of laser processing. However, these dynamics are still challenging to understand because of a lack of ubiquitous spectroscopic tools to observe tangled-up dynamics appearing at ultrafast timescales. Here by harnessing highly sensitive single-shot terahertz time-domain spectroscopy using an echelon mirror, we investigate pulse-to-pulse temporal profile of terahertz radiation generated from the material surface. We clearly found that the carrier-envelope phase and the electric field amplitude of the terahertz waveform systematically vary between the pre- and post-ablation depending on the laser fluence and irradiated pulse numbers. Our results provide a stepping-stone towards perception of Coulomb explosion occurring throughout the laser ablation process, which is indispensable for future laser processing applications.

Carrier-envelope phase effect on light bullet dynamics

A light bullet (LB) is a wave packet of a few optical cycles that is extremely compressed in space and time, which is formed in the bulk transparent dielectric during femtosecond filamentation under anomalous group velocity dispersion. We demonstrate for the first time that the carrier-envelope phase shift during propagation of a near single-cycle LB causes synchronous oscillations of the spatial, temporal and energy parameters of its core with the period decreasing with increasing carrier wavelength. When analyzing the structure of color centers and induced plasma channels in fluorides, it was experimentally found that LB parameter oscillations lead to a periodic change in the nonlinear optical interaction with the dielectric.

Achieving high molecular alignment and orientation for CH_3F through manipulation of rotational states with varying optical and THz laser pulse parameters

Increasing interest in the fields of high-harmonics generation, laser-induced chemical reactions, and molecular imaging of gaseous targets demands high molecular “alignment” and “orientation” (A&O). In this work, we examine the critical role of different pulse parameters on the field-free A&O dynamics of the CH_3F molecule, and identify experimentally feasible optical and THz range laser parameters that ensure maximal A&O for such molecules. Herein, apart from rotational temperature, we investigate effects of varying pulse parameters such as, pulse duration, intensity, frequency, and carrier envelop phase (CEP). By analyzing the interplay between laser pulse parameters and the resulting rotational population distribution, the origin of specific A&O dynamics was addressed. We could identify two qualitatively different A&O behaviors and revealed their connection with the pulse parameters and the population of excited rotational states. We report here the highest alignment of langle {mathrm{cos}^2theta }rangle = 0.843 and orientation of langle {mathrm{cos}(theta )}rangle = 0.886 for CH_3F molecule at 2 K using a single pulse. Our study should be useful to understand different aspects of laser-induced unidirectional rotation in heteronuclear molecules, and in understanding routes to tune/enhance A&O in laboratory conditions for advanced applications.

Contrast enhancement in near-infrared electro-optic imaging.

Access to subtle ultrafast effects of light-matter interaction often requires highly sensitive field detection schemes. Electro-optic sampling, being an exemplary technique in this regard, lacks high sensitivity in an imaging geometry. We demonstrate a straightforward method to significantly improve the contrast of electric field images in spatially resolved electro-optic sampling. A thin-film polarizer is shown to be an effective tool in enhancing the sensitivity of the electro-optic imaging system, enabling an adjustment of the spectral response. We show a further increase of the signal-to-noise ratio through the direct control of the carrier envelope phase of the imaged field.

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.

Versatile few-cycle high-energy MID-IR OPCPA for nonlinear optics, spectroscopy and imaging

High-power, high-energy, ultrashort, mid-infrared (MID-IR) laser systems operating at high repetition rates are of considerable interest for many science applications, such as coherent vibrational spectroscopy, label-free imaging, time-resolved pump-probe and high-harmonic generation studies. We developed an optical parametric chirped-pulse amplifier (OPCPA) system employing a difference-frequency generation in a lithium gallium sulfide nonlinear crystal in the final amplifier stage, which provides in principle the possibility for passive carrier-envelop-phase (CEP) stability. The OPCPA efficiently down-converts a 1 μm 200 μJ Yb-YAG pump pulse into the MID-IR spectral range generating μJ-level pulses at a repetition rate of 200 kHz. Two modes of operations providing complimentary MID-IR pulse properties are presented. Depending on the envisaged application, one can switch between (a) a wavelength-tunable (4.2–11 μm) source and (b) a broadband source centered at ≈8.5 μm by controlling the group-delay dispersion of the signal pulse. The broadband, high-energy MID-IR pulses have a short pulse duration of 74±2 fs, which corresponds to only ≈3 optical cycles at the central wavelength of 8.5 μm.

Control of frustrated double ionization channels with the carrier-envelope phase of few-cycle linearly polarized laser pulses

Using a three-dimensional classical method, we theoretically explore the frustrated double ionization (FDI) of Ar atoms under few-cycle linearly polarized laser pulses with different carrier-envelope phases (CEPs). Our calculated spectra are similar to the recent experimentally observed two peeks in the electron momentum distribution (Larimian et al 2020 Phys. Rev. Res. 2 013021). The dependence of FDI on laser CEPs is investigated, and the result reveals that the two different pathways of FDI can be controlled by CEPs. The information on which pathway leads to FDI is encoded in the electron-electron energy distribution shortly after recollision and momentum distribution at the ionization time after recollision.