Carrier-envelope Phase Research Articles

MIR laser CEP estimation using machine learning concepts in bulk high harmonic generation

Monitoring the carrier-envelope phase (CEP) is of paramount importance for experiments involving few-cycle intense laser fields. Common measurement techniques include f-2f interferometry or stereo-ATI setups. Here we demonstrate a new concept, both by simulations and by experiments, for CEP estimation in the mid-infrared regime using machine learning (ML) techniques that rely on the observation of the spectrum of high harmonic generation (HHG) in bulk material. Once the ML model is trained, the method provides a way for cheap and compact in-situ CEP tagging. This technique can complement other CEP monitoring methods, can capture the complex correlation between the CEP and the observable HHG spectra, and is readily generalizable for any laser wavelengths.

Theoretical analysis of the phase characteristics in few-cycle laser coherent beam combining

To meet the increasing demand for high-energy (Joule-level) few-cycle pulses in strong-field physics research, coherent beam combining (CBC) presents an effective method to further enhance the power of few-cycle pulse sources. However, due to the broad spectrum and ultrashort pulse duration of few-cycle pulses, achieving efficient beam combining is particularly challenging. In this paper, we employ the angular spectrum (ASM) method to numerically simulate the propagation of few-cycle pulses in a focusing system. By constructing a three-dimensional (2D + 1D) model, we accurately quantify the effects of laser parameters such as carrier-envelope phase (CEP) difference, time delay (TD), dispersion between beams, and combining configurations on the combining efficiency and CEP shift. The study results provide design guidelines for high energy few-cycle laser systems based on coherent beam combining and reveal the possibility to significantly enhance the combining efficiency while maintaining high CEP stability, which will finally provide a reliable light source for advancing the frontier of strong-field physics.

On-chip petahertz electronics for single-shot phase detection

Attosecond science has demonstrated that electrons can be controlled on the sub-cycle time scale of an optical waveform, paving the way towards optical frequency electronics. However, these experiments historically relied on high-energy laser pulses and detection not suitable for microelectronic integration. For practical optical frequency electronics, a system suitable for integration and capable of generating detectable signals with low pulse energies is needed. While current from plasmonic nanoantenna emitters can be driven at optical frequencies, low charge yields have been a significant limitation. In this work we demonstrate that large-scale electrically connected plasmonic nanoantenna networks, when driven in concert, enable charge yields sufficient for single-shot carrier-envelope phase detection at repetition rates exceeding tens of kilohertz. We not only show that limitations in single-shot CEP detection techniques can be overcome, but also demonstrate a flexible approach to optical frequency electronics in general, enabling future applications such as high sensitivity petahertz-bandwidth electric field sampling or logic-circuits.

180 mW, 1 MHz, 15 fs carrier-envelope phase-stable pulse generation via polarization-optimized down-conversion from gas-filled hollow-core fiber

Gas-filled hollow core fibers allow the generation of single-cycle pulses at megahertz repetition rates. When coupled with difference frequency generation, they can be an ideal driver for generating carrier-envelope phase stable, octave-spanning pulses in the short-wavelength infrared. In this work, we investigate the dependence of the polarization state in gas-filled hollow-core fibers (HCF) on the subsequent difference frequency generation stage. We show that by adjusting the input polarization state of light in geometrically symmetric systems, such as hollow-core fibers, one can achieve precise control over the polarization state of the output pulses. This manipulation preserves the temporal characteristics of the generated ultrashort pulses, especially when operating at a near single-cycle regime. We leverage this property to boost the downconversion efficiency of the near single-cycle pulses in a type I difference frequency generation stage. Our technique overcomes the bandwidth and dispersion constraints of the previous methods that rely on broadband waveplates or adjustment of crystal axes relative to the laboratory frame. This advancement is crucial for experiments demanding pure polarization states in the eigenmodes of the laboratory frame.

Higher Harmonic Generation Arising from the Bessel Function Expansions of the Carrier-Envelope-Phase Driven by the Femtosecond Optical Kerr Effect

This paper expands upon previous reports based on the EM Kerr model-based HHG simulation from the carrier-envelope phase (CEP) of the electromagnetic laser light wave based on electronic self-phase modulation (ESPM) from the optical Kerr index. The underpinning mechanism described for generating HHG arises from the CEP of the nonlinear index of refraction, which yields a series of Bessel functions. The envelope of the electric field E(t) of the intense ultrashort light pulses drives the CEP, resulting in many Bessel functions, one of the salient features of HHG. The HHG from the large number of Bessel function modes can produce attosecond pulses and possibly can even produce zeptosecond pulses by Kerr mode-locking among the large number N of Bessel functions.

Solid and hollow plasmonic nanoresonators for carrier envelope phase read-out

The geometry of gold plasmonic nanoantennae was numerically optimized to maximize their sensitivity to the carrier envelope phase (CEP) of the exciting ultra-short laser pulses. Three structure types, triangular, teardrop-shaped and plasmonic lens, were optimized in solid and hollow compositions as well. Hollow / solid singlets results in the largest/intermediate CEP dependent (Q1) – to – CEP independent (Q0) integrated current components’ ratio, while their Q1 was the smallest / intermediate. The largest / intermediate Q12/Q0 CEP sensitivity was achieved via solid / hollow plasmonic lenses due to their large near-field enhancement and Q1, while the Q1/Q0 ratio was smaller than for counterpart singlets.

Carrier-envelope phase effects on the reflectance of diamond films irradiated by ultrashort single-cycle light pulses: A comparison study in attosecond and femtosecond regimes

Diamond films irradiated with few-cycle laser pulses are emerging as a all-optical information processing device with high-speed and low-power-consumption. The possibility to steer, control or switch such few-cycle laser pulses with diamond films is expected to pave the way towards ultrafast all-optical switching. Here, we show theoretically the effect of the carrier-envelope phase (CEP) of the ultrashort single-cycle light pulses on the reflectance of diamond films within the framework of multi-scale theory. We observe clear variations in the reflectance behavior of femtosecond pulses and attosecond pulses. Our most important finding is that the reflectivity difference of attosecond pulses with different waveforms and the same spectrum on diamond films exhibits a perfect quadratic relationship with the CEP. This observation suggests that the nonlinearity of attosecond pulses is different from that of femtosecond pulses. Analysis of these results allows us to gain new insights in the interaction between attosecond light pulses and matter. This opens fascinating avenues for all-optical information processing over ultrashort lengths and timescales.

Modulation and real-time monitoring of the carrier-envelope phase of terahertz pulses based on shaping of near-infrared femtosecond pulses.

We propose a novel, to our knowledge, method for modulating and real-time monitoring of the carrier-envelope phase (CEP) of terahertz (THz) pulses. CEP is an essential parameter in the interaction of THz waves with matter due to the difference in temporal symmetry when the carrier is extended for several cycles. CEP can be continuously modulated at full range with high speed by oscillating the optical path length of the Michelson interferometer under 1 µm, as confirmed by electro-optic (EO) sampling. The proposed method can be combined with a data acquisition method that links the experimental parameters and measurements of individual high-repetition THz pulses to realize robust CEP modulation measurements. As the proposed CEP modulation and monitoring system does not require EO sampling but only extracts CEP dependence, the trend toward ultrafast physical property control and observation using THz pulses will spread to other fields.

Carrier-envelope phase-stabilized ultrashort pulses from a gas-filled multi-pass cell

Few-cycle laser pulses at a high repetition rate with a stable carrier-envelope phase are required for next-generation attosecond time-resolved spectroscopies. One way to generate these pulses is the nonlinear compression of laser pulses via gas-filled hollow-core fibers. Recently, an alternative approach based on multi-pass cells (MPCs) has been shown to be very efficient for post-compression of turn-key, industrial-grade, high average power Yb-doped solid-state laser amplifiers. However, to expand the system for exploring strong-field laser applications, its carrier-envelope phase stability needs to be demonstrated in the compressed pulses. In this Letter, we present the generation of carrier-envelope phase-stabilized 40 fs pulses with 380 μJ energy at 50 kHz by compressing the output of a Yb:KGW amplifier in a gas-filled MPC. Comparable short-term carrier-envelope phase errors of 412 and 435 mrad root mean square were observed from the amplifier and MPC, respectively, indicating that the phase stability of the amplified pulses is well-maintained during pulse compression in the MPC.

Ultra-Nonlinear Subcycle Photoemission of Few-Electron States from Sharp Gold Nanotapers.

The generation of ultrashort electron wavepackets is crucial for the development of ultrafast electron microscopes. Recent studies on Coulomb-correlated few-electron number states, photoemitted from sharp metallic tapers, have shown emission nonlinearities in the multiphoton photoemission regime which scale with the electron number. Here, we study few-electron photoemission from gold nanotapers triggered by few-cycle near-infrared pulses, demonstrating extreme 20th-order nonlinearities for electron triplets. We report interferometric autocorrelation traces of the electron yield that are quenched to a single emission peak with subfemtosecond duration due to these high nonlinearities. The modulation of the emission yield by the carrier-envelope phase suggests that electron emission predominantly occurs during a single half cycle of the driving laser field. When applying a bias voltage to the tip, recollisions in the electron trajectories are suppressed and coherent subcycle electron beams are generated with promising prospects for ultrafast electron microscopy with subcycle time resolution.

Absence of quantum optical coherence in high harmonic generation

The optical phase of the driving field in the process of high harmonic generation and the coherence properties of the harmonics are fundamental concepts in attosecond physics. Here, we consider driving the process by incoherent classical and nonclassical light fields exhibiting an undetermined optical phase. With this, we introduce the notion of quantum optical coherence into high harmonic generation and show that high harmonics can be generated from incoherent radiation despite having a vanishing electric field. We explicitly derive the quantum state of the harmonics when driven by carrier-envelope phase unstable fields and show that the generated harmonics are incoherent and exhibit zero electric field amplitudes. We find that the quantum state of each harmonic is diagonal in its photon number basis, but nevertheless has the exact same photon statistics as the widely considered coherent harmonics. From this, we conclude that assuming coherent harmonic radiation can originate from a preferred ensemble fallacy. These findings have profound implications for attosecond experiments and how to infer the harmonic radiation properties. Published by the American Physical Society 2024

Phase-Sensitive Plasma Nonlinearity Controlled by Ultrashort Pulses

The generation of spectral components sensitive to the carrier-envelope phase of a laser pulse in a thin zinc selenide film has been experimentally demonstrated and confirmed by a numerical simulation. A pump–probe scheme has been implemented so that a pump pulse with a duration of about 1.5 field cycles, a central wavelength of 1.7 μm, and a stabilized carrier-envelope phase induces photoionization in a thin zinc selenide film. The probe pulse is scattered by the plasma, generating new phase-sensitive spectral components at the edges of its spectrum. The theoretical analysis has confirmed plasma nonlinearity as a mechanism for generating these components. The observed effect can be used to characterize the carrier-envelope phase of ultrashort pulses during the generation of high-order harmonics and sequences of attosecond pulses.

Attosecond two-color x-ray free-electron lasers with dual chirp-taper configuration and bunching inheritance

Attosecond x-ray pulses play a crucial role in the study of ultrafast phenomena occurring within inner and valence electrons. To achieve attosecond time-resolution studies and gain control over electronic wave functions, it is crucial to develop techniques capable of generating and synchronizing two-color x-ray pulses at the attosecond scale. In this paper, we present a novel approach for generating attosecond pulse pairs using a dual chirp-taper free-electron laser with bunching inheritance. An electron beam with a sinusoidal energy chirp, introduced by the external laser, passes through the main undulator and afterburner, both with tapers. Two-color x-ray pulses are generated from the main undulator and the afterburner, respectively, with temporal separations of several femtoseconds and energy separations of tens of electron volts. Notably, the afterburner is much shorter than the main undulator due to the bunching inheritance, which reduces the distance between two source points and alleviates the beamline focusing requirements of the two-color pulses. A comprehensive stability analysis is conducted in this paper, considering the individual effects of shot noise from self-amplified spontaneous emission and carrier-envelope phase jitter of the few-cycle laser. The results show that the radiation from the afterburner exhibits excellent stability in the proposed scheme, which is beneficial for x-ray pump-probe experiments. The proposed scheme opens up new possibilities for attosecond science enabled by x-ray attosecond pump-probe techniques and coherent control of ultrafast electronic wave packets in quantum systems. Published by the American Physical Society 2024

Control over the secondary collision of electron in high-order harmonic generation

We investigated the high-order harmonic generation by interacting linearly polarized laser pulses with the atomic target. The temporal evolution of harmonic emission and the underlying mechanisms of rescattering electrons are thoroughly investigated through a combination of quantum analysis and classical trajectory simulations. The manipulation of the carrier-envelope phase (CEP) provides a promising avenue for controlling electron recollisions, revealing a systematic linear relationship between ionization and recombination times across varying CEP values. Moreover, examining phase properties in emitted harmonics during secondary collisions presents intriguing modulations, offering a potential experimental approach to verify the presence of secondary recollisions.

Chirality reversal with the carrier-envelope phase: A next generation QTAIM interpretation

Simulated circularly-polarized 10 fs laser pulses that induce a mixture of excited states are applied to ethane. Additionally, the carrier-envelope phase (CEP) angle ϕ, that quantifies the relationship between the time-varying direction of electric (E)-field and the amplitude envelope was used to manipulate the mechanical and chiral properties of ethane using Next Generation Quantum Theory of Atoms in Molecules (NG-QTAIM). The chirality assignments were reversed from S to R as the CEP angle ϕ was increased from ϕ = 0.0° to ϕ = 180°. A one-to-one mapping between the CEP angle ϕ and NG-QTAIM trajectories was discovered.

Generation of double attosecond pulses with the same intensity and carrier-envelope phase

Generation of double attosecond pulses with the same intensity and carrier-envelope phase

Generation of attosecond electron bunches of tunable duration and density by relativistic vortex lasers in near-critical density plasma

Attosecond electron bunches have wide application prospects in free-electron laser injection, attosecond X/γ-ray generation, ultrafast physics, etc. Nowadays, there is one notable challenge in the generation of high-quality attosecond electron bunch, i.e., how to enhance the electron bunch density. Using theoretical analysis and three-dimensional particle-in-cell simulations, we discovered that a relativistic vortex laser pulse interacting with near-critical density plasma can not only effectively concentrate the attosecond electron bunches to over critical density, but also control the duration and density of the electron bunches by tuning the intensity and carrier-envelope phase of the drive laser. It is demonstrated that this method can efficiently produce attosecond electron bunches with a density up to 300 times of the original plasma density, peak divergence angle of less than 0.5∘, and duration of less than 67 attoseconds. Furthermore, by using near-critical density plasma instead of solid targets, our scheme is potential for the generation of high-repetition-frequency attosecond electron bunches, thus reducing the requirements for experiments, such as the beam alignment or target supporter.

Control of quantum paths in harmonic generation through orthogonal fields specific frequency ratios

By solving a two-dimensional, time-dependent Schrödinger equation, we investigate high-order harmonic generation for the H2+ molecular ion in orthogonally polarized two-color laser pulses. We find that harmonic generation depends on the frequency ratio n=ωyωx . When the wavelength is 800 nm and n = 1.2, the harmonic plateau becomes smoother, and the quantum orbital interference decreases. We change the fundamental wavelength and find that the harmonic spectrum exhibits a supercontinuum structure, and the quantum orbital is controllable. When the wavelength is 1600 nm and 2000 nm, and n = 1.2, we gain a deeper understanding of the physical process of harmonics. We have provided the time-frequency distribution and the probability density of an electron wave packet picture. Next, we analyzed the impact of the carrier-envelope phase on harmonics, and we combined Lissajous figures to continue our analysis. The research results find that when the carrier-envelope phase is 0, 0.5π, π, and 1.5π, the harmonic intensity becomes higher, and all exhibit a supercontinuum structure. We chose certain orders of harmonics, and isolated attosecond pulses can be synthesized.

Vibronic Correlations in Molecular Strong-Field Dynamics.

We investigate the ultrafast vibronic dynamics triggered by intense femtosecond infrared pulses in small molecules. Our study is based on numerical simulations performed with 2D model molecules and analyzed in the perspective of the renowned Lochfrass and bond-softening models. We give a new interpretation of the observed nuclear wave packet dynamics with a focus on the phase of the bond oscillations. Our simulations also reveal intricate features in the field-induced nuclear motion that are not accounted for by existing models. Our analyses assign these features to strong dynamical correlations between the active electron and the nuclei, which significantly depend on the carrier envelope phase of the pulse, even for relatively "long" pulses, which should make them experimentally observable.

Robust Isolated Attosecond Pulse Generation with Self-Compressed Subcycle Drivers from Hollow Capillary Fibers.

High-order harmonic generation (HHG) arising from the nonperturbative interaction of intense light fields with matter constitutes a well-established tabletop source of coherent extreme-ultraviolet and soft X-ray radiation, which is typically emitted as attosecond pulse trains. However, ultrafast applications increasingly demand isolated attosecond pulses (IAPs), which offer great promise for advancing precision control of electron dynamics. Yet, the direct generation of IAPs typically requires the synthesis of near-single-cycle intense driving fields, which is technologically challenging. In this work, we theoretically demonstrate a novel scheme for the straightforward and compact generation of IAPs from multicycle infrared drivers using hollow capillary fibers (HCFs). Starting from a standard, intense multicycle infrared pulse, a light transient is generated by extreme soliton self-compression in a HCF with decreasing pressure and is subsequently used to drive HHG in a gas target. Owing to the subcycle confinement of the HHG process, high-contrast IAPs are continuously emitted almost independently of the carrier-envelope phase (CEP) of the optimally self-compressed drivers. This results in a CEP-robust scheme which is also stable under macroscopic propagation of the high harmonics in a gas target. Our results open the way to a new generation of integrated all-fiber IAP sources, overcoming the efficiency limitations of usual gating techniques for multicycle drivers.