Carrier-envelope Phase Research Articles

Broadband optical frequency comb covering spectral regions at UV, VIS, and NIR

Herein, we present a compact, robust, self-referenced optical frequency comb (OFC) ranging from ultraviolet (UV) to near-infrared (NIR) light. The OFC system employs an all-polarization-maintained fiber figure-9 fiber oscillation as a seed laser, which is separated into two branches at the same repetition rate of approximately 100 MHz. One branch is adopted to implement frequency stabilization. Herewith, the repetition rate and the carrier-envelope phase are separately stabilized while the residual phase noises are reduced to 504 μrad and 1.12 rad (ranging from 1 Hz to 3 MHz), corresponding to fractional frequency stabilities of 1.91 × 10−12 and 1.44 × 10−17, respectively. The other branch is used to implement supercontinuum generation and quadruplicate frequency nonlinear processes, thus achieving an OFC system in the ranges of 266, 532, and 600–2200 nm. The precise frequency transferring characteristics of this OFC system exhibit great potential for precision measurement applications and optical frequency synchronization.

Carrier-Envelope Phase-Controlled Residual Current in Semiconductors

With the purpose of achieving current control by using intense laser field manipulation, we investigate the effect of carrier-envelope phase (CEP) on residual current in SiO2 crystals. By solving semiconductor Bloch equations, we found that the CEP can strongly influence the carrier population of the conduction band, which means that it can act as a simple, but useful, tool to control residual current. That is, the resultant asymmetric distribution in the first Brillouin zone gave rise to non-zero residual current. Additionally, we further consider the two-color laser scheme to achieve better control of residual current, showing that asymmetric two-color laser fields can induce the maximum residual current.

Highly CEP-stable optical parametric amplifier at 2 µm with a few-cycle duration and 100 kHz repetition rate.

We develop a BiB3O6 (BiBO)-based optical parametric amplifier in the spectral region around 2 µm using a Yb:KGW amplifier operating at 100 kHz. The two-stage degenerate optical parametric amplification results in a typical output energy of 30 µJ after compression, spectrum covering 1.7-2.5 µm range, and a pulse duration fully compressible down to 16.4 fs, corresponding to 2.3 cycles. Due to the inline difference frequency generation of the seed pulses, the carrier envelope phase (CEP) is passively stabilized without feedback over 11 hours at the level below 100 mrad including a long-term drift. Short-term statistical analysis in the spectral domain further shows a behavior qualitatively different from that of parametric fluorescence, indicating high degree of suppression of optical parametric fluorescence. The high phase stability together with the few-cycle pulse duration is promising for the investigation of high-field phenomena such as subcycle spectroscopy in solids or high harmonics generation.

Transfer learning, alternative approaches, and visualization of a convolutional neural network for retrieval of the internuclear distance in a molecule from photoelectron momentum distributions

We investigate the application of deep learning to the retrieval of the internuclear distance in the two-dimensional ${\mathrm{H}}_{2}^{+}$ molecule from the momentum distribution of photoelectrons produced by strong-field ionization. We study the effect of the carrier-envelope phase on the prediction of the internuclear distance with a convolutional neural network and investigate the possibility of reconstruction of the internuclear distance from one-dimensional momentum distributions. We apply the transfer learning technique to make our convolutional neural network applicable to distributions obtained for parameters outside the ranges of the training data. The convolutional neural network is compared with alternative approaches to this problem, including the direct comparison of momentum distributions, support-vector machines, and decision trees. These alternative methods are found to possess very limited transferability. Finally, we use the occlusion-sensitivity technique to extract the features that allow a neural network to make its decisions.

Effect of stacking configuration on high harmonic generation from bilayer hexagonal boron nitride.

High harmonic generation from bilayer h-BN materials with different stacking configurations is theoretically investigated by solving the extended multiband semiconductor Bloch equations in strong laser fields. We find that the harmonic intensity of AA'-stacking bilayer h-BN is one order of magnitude higher than that of AA-stacking bilayer h-BN in high energy region. The theoretical analysis shows that with broken mirror symmetry in AA'-stacking, electrons have much more opportunities to transit between each layer. The enhancement in harmonic efficiency originates from additional transition channels of the carriers. Moreover, the harmonic emission can be dynamically manipulated by controlling the carrier envelope phase of the driving laser and the enhanced harmonics can be utilized to achieve single intense attosecond pulse.

Mid-infrared pulse generation using multi-plate white-light generation and optical parametric amplification in LiGaS2 crystals

We demonstrate intense mid-infrared pulse generation with a pulse energy of up to 6.2 μJ and a tunable wavelength range of 5.3–7.4 μm. This light source is based on white-light generation by multi-plate pulse compression of the output of a commercial Yb:KGW laser pulse followed by intra-pulse difference frequency generation (DFG) and optical parametric amplification in LiGaS2 crystals. Due to the use of intra-pulse DFG, we were able to generate carrier-envelope phase (CEP)-stable mid-infrared optical pulses with a CEP standard deviation of 114 mrad, corresponding to a timing fluctuation of 360 attoseconds during the 5-hour-long measurement.

Polarization and phase control of electron injection and acceleration in the plasma by a self-steepening laser pulse

We describe an interplay between two injection mechanism of background electrons into an evolving plasma bubble behind an intense laser pulse: one due to the overall bubble expansion, and another due to its periodic undulation. The two mechanisms occur simultaneously when an intense laser pulse propagating inside a plasma forms a shock-like steepened front. Periodic undulations of the plasma bubble along the laser propagation path can either inhibit or conspire with electron injection due to bubble expansion. We show that carrier-envelope-phase (CEP) controlled plasma bubble undulation induced by the self-steepening laser pulse produces a unique electron injector—expanding phase-controlled undulating bubble (EPUB). The longitudinal structure of the electron bunch injected by the EPUB can be controlled by laser polarization and power, resulting in high-charge (multiple nano-Coulombs) high-current (tens of kilo-amperes) electron beams with ultra-short (femtosecond-scale) temporal structure. Generation of high-energy betatron radiation with polarization- and CEP-controlled energy spectrum and angular distribution is analyzed as a promising application of EPUB-produced beams.

Interferometric carrier-envelope phase stabilization for ultrashort pulses in the mid-infrared.

We demonstrate an active carrier-envelope phase (CEP) stabilization scheme for optical waveforms generated by difference-frequency mixing of two spectrally detuned and phase-correlated pulses. By performing ellipsometry with spectrally overlapping parts of two co-propagating near-infrared generation pulse trains, we stabilize their relative timing to 18 as. Consequently, we can lock the CEP of the generated mid-infrared (MIR) pulses with a remaining phase jitter below 30 mrad. To validate this technique, we employ these MIR pulses for high-harmonic generation in a bulk semiconductor. Our compact, low-cost, and inherently drift-free concept could bring long-term CEP stability to the broad class of passively phase-locked OPA and OPCPA systems operating in a wide range of spectral windows, pulse energies, and repetition rates.

Nonlinear field-control of terahertz waves in random media for spatiotemporal focusing.

Controlling the transmission of broadband optical pulses in scattering media is a critical open challenge in photonics. To date, wavefront shaping techniques at optical frequencies have been successfully applied to control the spatial properties of multiple-scattered light. However, a fundamental restriction in achieving an equivalent degree of control over the temporal properties of a broadband pulse is the limited availability of experimental techniques to detect the coherent properties (i.e., the spectral amplitude and absolute phase) of the transmitted field. Terahertz experimental frameworks, on the contrary, enable measuring the field dynamics of broadband pulses at ultrafast (sub-cycle) time scales directly. In this work, we provide a theoretical/numerical demonstration that, within this context, complex scattering can be used to achieve spatio-temporal control of instantaneous fields and manipulate the temporal properties of single-cycle pulses by solely acting on spatial degrees of freedom of the illuminating field. As direct application scenarios, we demonstrate spatio-temporal focusing, chirp compensation, and control of the carrier-envelope-phase (CEP) of a CP-stable, transform-limited THz pulse.

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.

Investigate the electron dynamics of harmonic minimum from the bichromatic periodic potential

The electron dynamics of harmonic minimum from the bichromatic periodic potential is investigated. It is found that a significant minimum appears on the second plateau of harmonic spectrum driven by two-color field. Both quasi-classical and quantum calculations are performed and the results imply that the competitive electron transition paths are the most important cause of the minimum. Furthermore, the electron dynamics of transition paths is clearly revealed by changing the carrier envelope phase and wavelength. Finally, the paths are effectively manipulated and the optimal harmonic spectrum is obtained by three-color field scheme.

Influence of chirp and carrier-envelope phase on noninteger high-harmonic generation

High harmonic generation (HHG) is a versatile technique for probing ultrafast electron dynamics. While HHG is sensitive to the electronic properties of the target, HHG also depends on the waveform of the laser pulse. As is well known, (peak) positions, $\omega$, in the high-harmonic spectrum can shift when the carrier envelope phase (CEP), $\varphi$ is varied. We derive formulae describing the corresponding parametric dependencies of CEP shifts; in particular, we have a transparent result for the (peak) shift, $d\omega/d\varphi = {-} 2 \bar{\mathfrak f}' \omega/\omega_0$, where $\omega_0$ describes the fundamental frequency and $\bar{\mathfrak f}'$ characterizes the chirp of the driving laser pulse. We compare the analytical formula to full-fledged numerical simulations finding only 17 % average relative absolute deviation in $d\omega/d\varphi$. Our analytical result is fully consistent with experimental observations.

A novel scheme for ultrashort terahertz pulse generation over a gapless wide spectral range: Raman-resonance-enhanced four-wave mixing

Ultrashort energetic terahertz (THz) pulses have created an exciting new area of research on light interactions with matter. For material studies in small laboratories, widely tunable femtosecond THz pulses with peak field strength close to MV cm−1 are desired. Currently, they can be largely acquired by optical rectification and difference frequency generation in crystals without inversion symmetry. We describe in this paper a novel scheme of THz pulse generation with no frequency tuning gap based on Raman-resonance-enhanced four-wave mixing in centrosymmetric media, particularly diamond. We show that we could generate highly stable, few-cycle pulses with near-Gaussian spatial and temporal profiles and carrier frequency tunable from 5 to >20 THz. They had a stable and controllable carrier-envelop phase and carried ~15 nJ energy per pulse at 10 THz (with a peak field strength of ~1 MV cm−1 at focus) from a 0.5-mm-thick diamond. The measured THz pulse characteristics agreed well with theoretical predictions. Other merits of the scheme are discussed, including the possibility of improving the THz output energy to a much higher level.

Forward-backward electron–proton asymmetry from a two-photon crossing of diabatic states of H[formula omitted] in linearly polarized intense laser field

Controlling spatial–temporal overlap between nuclear wave packets propagating on the coupled electronic states with opposite parity by intense laser field leads to a π periodic modulation in the symmetry breaking of the coupled forward–backward electron and proton motions. The most probable directionality of electron and proton moving simultaneously in opposite directions with total electron–proton asymmetry parameter Ae−ptot=0.62 occurs at fixed carrier-envelope phases of ϑ=15° and 195°. Analysis of the time evolution of the nuclear and the electronic-nuclear wave packets on the field-dressed potentials gives rise to newly insightful images of the nonlinear nonperturbative processes for the asymmetrical localization.

Carrier-Envelope Phase Controlling of Ion Momentum Distributions in Strong Field Double Ionization of Methyl Iodide

In strong field ionization of methyl iodide initiated by elliptically polarized few-cycle pulses, a significant correlation was observed between the carrier-envelope phases (CEPs) of the laser and the preferred ejection direction of methyl cation arising from dissociative double ionization. This was attributed to the carrier-envelope phase dependent double ionization yields of methyl iodide. This observation provides a new way for monitoring the absolute CEPs of few-cycle pulses by observing the ion momentum distributions.

Carrier-envelope-phase-dependent below-threshold harmonic generation in few-cycle mid-infrared laser fields.

We theoretically study the dependence of below-threshold harmonic generation (BTHG) of atoms on the carrier-envelope phase (CEP) driven by few-cycle mid-infrared laser pulses. The BTHG spectra can be accurately and efficiently calculated by solving the three-dimensional time-dependent Schrödinger equation using the time-dependent generalized pseudospectral method. We present the BTHG spectra as a function of the laser-field CEP. CEP-dependent enhancement or suppression occurred at low laser field intensities owing to the changes in the resonant effects associated with multiple quantum trajectories. However, the BTHG of atoms driven by high laser intensities is insensitive to the CEP. The synchrosqueezing time-frequency transform of the BTHG and extended semiclassical analysis are performed to elucidate the underlying physical mechanism.

Coherent control of the photoinduced transition in a strongly correlated material

The use of intense tailored light fields is the perfect tool to achieve ultrafast control of electronic properties in quantum materials. Among them, Mott insulators are materials in which strong electron-electron interactions drive the material into an insulating phase. When shining a Mott insulator with a strong laser pulse, the electric field may induce the creation of doublon-hole pairs, triggering a photoinduced transition into a metallic state. In this paper, we take advantage of the threshold character of this photoinduced transition and we propose a setup that consists of a midinfrared laser pulse and a train of short pulses separated by a half period of the midinfrared with alternating phases. By varying the time delay between the two pulses and the internal carrier envelope phase of the short pulses, we achieve control of the phase transition, which leaves its fingerprint at its high harmonic spectrum.

High harmonic generation near a bow-tie nanostructure: sensitivity to carrier envelope phase and plasmonic inhomogeneity

High harmonic generation (HHG) from atoms near a plasmonic nanostructure interacting with a relatively low intensity driving laser field is a promising candidate for table top attosecond pulse source. The effect of carrier envelope phase (CEP) of the few cycle driving pulse on inhomogeneous high harmonics generation is well studied in literature, for example, the harmonic cut-off can be efficiently controlled by tuning the CEP. Here, we show selective enhancements of harmonic spectra due to half-cycle cutoff (HCO) which is highly sensitive to the CEP, in both spatially homogeneous and inhomogeneous driving laser fields. Essentially the selective enhancement of spectral structures results from contributions of both short and long trajectories in certain HCO regions. Compared to the homogeneous HHG in the presence of inhomogeneity, these enhanced groups eventually merge to the background with the increase of the strength of inhomogeneity. This limits the maximum possible tunability of selective enhancement. Further, near cut-off harmonics can be a good candidate to produce isolated attosecond pulses, with substantial control via CEP of the driving laser pulse along with the strength of inhomogeneity.

Photoelectron momentum distributions in the strong-field ionization of atomic hydrogen by few-cycle elliptically polarized optical pulses

We investigate the strong-field ionization of atomic hydrogen in a few-cycle elliptically polarized infrared pulse by solving the time-dependent Schr\"odinger equation. The dependence of the photoelectron momentum distribution on the pulse intensity, ellipticity, length, envelope, and carrier envelope phase is analyzed. In particular, we explain the variation of the electron offset angle with asymptotic electron energy through the combined action of the field and the Coulomb potential, and demonstrate that low-ellipticity pulses make it possible to access the electron release time.

Reconstructing the Semiconductor Band Structure by Deep Learning

High-order harmonic generation (HHG), the nonlinear upconversion of coherent radiation resulting from the interaction of a strong and short laser pulse with atoms, molecules and solids, represents one of the most prominent examples of laser–matter interaction. In solid HHG, the characteristics of the generated coherent radiation are dominated by the band structure of the material, which configures one of the key properties of semiconductors and dielectrics. Here, we combine an all-optical method and deep learning to reconstruct the band structure of semiconductors. Our method builds up an artificial neural network based on the sensitivity of the HHG spectrum to the carrier-envelope phase (CEP) of a few-cycle pulse. We analyze the accuracy of the band structure reconstruction depending on the predicted parameters and propose a prelearning method to solve the problem of the low accuracy of some parameters. Once the network is trained with the mapping between the CEP-dependent HHG and the band structure, we can directly predict it from experimental HHG spectra. Our scheme provides an innovative way to study the structural properties of new materials.