Carrier-envelope Phase Shift Research Articles

Density-dependent carrier-envelope phase shift in attosecond pulse generation from relativistically oscillating mirrors

Density-dependent carrier-envelope phase shift in attosecond pulse generation from relativistically oscillating mirrors

Bubble structure evolution and electron injection controlled by optical cycles in wakefields

The evolution of bubble structure and electron injection in laser wakefield acceleration with different optical cycles is investigated through three-dimensional particle-in-cell simulations. Under fixed transverse and longitudinal ponderomotive force, the effect of optical cycles on the evolution of bubble structure and electron injection is studied by changing the laser wavelength. For a multi-cycle laser, electron acceleration is dominated by the ponderomotive force that produces symmetrical bubble and continuous injection. As the optical cycles decrease, the dominant effect of the electron acceleration can transition from the ponderomotive force to the carrier wave, and the carrier envelope phase shift can cause transverse oscillation of the bubble and periodic electron injection in the direction of laser polarization. The criterion for the dominant acceleration mechanism and the dependence of transition distance on the optical cycles and pulse width are obtained. The results are beneficial for manipulating electron acceleration and betatron radiation generation.

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.

Carrier-Envelope-Phase-Controlled Acceleration of Multicolored Attosecond Electron Bunches in a Millijoule-Laser-Driven Wakefield

Multicolor attosecond electron bunches with tens of megaelectronvolts energy are extremely useful for wide applications, such as multicolor x-ray generation. Here we propose a scheme to generate such electron bunches with a laser wakefield accelerator driven by a millijoule few-cycle laser pulse via carrier-envelope-phase (CEP)-controlled ionization injection. It is found that the rapid CEP shift of the driving pulse in plasma due to plasma dispersion and Gouy phase shift enables the inner shell electrons of background high-$Z$ ions to be ionized periodically. These electrons are then subsequently injected into the wakefield, where each injection lasts for a distance of a few micrometers only, leading to the generation of multicolor electron bunches with narrow peaks in the energy spectrum and a subfemtosecond duration. With typical laser parameters available currently, simulations show that electron bunches with central energy of about $30\phantom{\rule{0.2em}{0ex}}\mathrm{MeV}$, energy spread of about $2\mathrm{%}$, and duration of $460$ attoseconds can be obtained. Such electron sources can be driven by current laser systems operating at kilohertz repetition rates, making them attractive for wide applications.

Polarity reversal of wakefields driven by ultrashort pulse laser.

Using an analytical model and computer simulation, we show that the wakefield driven by an ultrashort laser pulse in high-density plasma periodically reverses its polarity due to the carrier-envelope phase shift of the driver. The wakefield polarity reversal occurs on spatial scales shorter than the typical length considered for electron acceleration with the laser-wakefield mechanism. Consequently, the energies of accelerated electrons are significantly affected. The results obtained are important for the laser-wakefield acceleration under the conditions relevant to present-day high-repetition-rate laser systems.

Sub-cycle Manipulation of Electrons in a Tunnel Junction with Phase-controlled Single-cycle THz Near-fields

By utilizing terahertz scanning tunneling microscopy (THz-STM) with a carrier envelope phase shifter for broadband THz pulses, we could successfully control the near-field-mediated electron dynamics in a tunnel junction with sub-cycle precision. Measurements of the phase-resolved sub-cycle electron tunneling dynamics revealed an unexpected large carrier-envelope phase shift between far-field and near-field single-cycle THz waveforms.

Tailoring Single-Cycle Near Field in a Tunnel Junction with Carrier-Envelope Phase-Controlled Terahertz Electric Fields.

Light-field-driven processes occurring under conditions far beyond the diffraction limit of the light can be manipulated by harnessing spatiotemporally tunable near fields. A tailor-made carrier envelope phase in a tunnel junction formed between nanogap electrodes allows precisely controlled manipulation of these processes. In particular, the characterization and active control of near fields in a tunnel junction are essential for advancing elaborate manipulation of light-field-driven processes at the atomic-scale. Here, we demonstrate that desirable phase-controlled near fields can be produced in a tunnel junction via terahertz scanning tunneling microscopy (THz-STM) with a phase shifter. Measurements of the phase-resolved subcycle electron tunneling dynamics revealed an unexpected large carrier-envelope phase shift between far-field and near-field single-cycle THz waveforms. The phase shift stems from the wavelength-scale feature of the tip-sample configuration. By using a dual-phase double-pulse scheme, the electron tunneling was coherently manipulated over the femtosecond time scale. Our new prescription-in situ tailoring of single-cycle THz near fields in a tunnel junction-will offer unprecedented control of electrons for ultrafast atomic-scale electronics and metrology.

Carrier envelope phase shifter for broadband terahertz pulses.

We demonstrated controlled shifting of the internal phase of broadband terahertz (THz) pulses. The internal phase of an ultrashort pulse is called the carrier envelope phase (CEP), which is an important parameter in the interaction of few-cycle light pulses and matter. Our CEP shifter utilizes the ultra-broadband feature of prism wave plates. We analytically derived the amount of CEP shift achievable by the CEP shifter using Jones matrixes. THz time-domain measurements clearly showed the shift of the CEP, and the results agreed well with the calculated values. The CEP shift was as high as 2π, indicating that any CEP values can be chosen using our CEP shifter.

Electro-optic prism-pair setup for efficient high bandwidth isochronous CEP phase shift or group delay generation

We report the experimental demonstration of an electro-optic prism pair pure carrier-envelope phase (CEP) shifter at low voltage (shift of 1rad for a voltage of 90V, applied to a crystal of 5mm aperture). Validating our mathematical model, the experiments prove that this set-up which uses two rubidium titanyl phosphate (RTP) crystals, can be used either as an efficient high bandwidth CEP shifter without modifying the group delay of an ultrashort pulse (isochronous CEP shifter) or alternatively as a group delay generator with quasi-constant CEP (Pure Group Delay generator). These two configurations which correspond to specific geometries are characterized by spectral interferometry with a 800nm mode-locked Ti:sapphire laser. The results are in very good agreement with the model. In the pure group delay mode, a group delay of 2.3fs is obtained at 1000V/cm without significant CEP shift. In the isochronous mode, a shift of 5.5rad at 1000V/cm is generated without significant delay. The applied voltage is also lowered by a factor of nearly three in this configuration, compared to the case of an RTP rectangular slab of the same total length.

Multi-level effects in the high-order harmonic generation driven by intense frequency-comb laser fields**Project supported by the National Natural Science Foundation of China (Grant Nos. 11374239, 21203144, and 11074199), the Doctoral Fund of Ministry of Education of China (Grant No. 20120201120056), and the Fundamental Research Funds for the Central Universities, China.

High harmonic generation (HHG) driven by intense frequency-comb laser fields can be dramatically enhanced via multiphoton resonance by tuning the carrier-envelope phase (CEP) shift, without increasing the driving intensity. However, the multiphoton-resonant enhancement (MRE) factor in the realistic atomic hydrogen is much smaller than that in a two-level system. To study the deviation, we present a theoretical investigation of the multiphoton resonance dynamics of three-level systems driven by intense frequency-comb laser fields. The many-mode Floquet theorem (MMFT) is employed to provide a nonperturbative and exact treatment of the interaction between the quantum system and the laser fields. The investigations show that the dipole interaction of a two-level system with the third level affects the multiphoton resonance dynamics and enhances the HHG spectra. It is the dipole interaction of the excited level of the two-level system with other levels that results in the smaller MRE factor in the realistic atomic system.

Carrier-envelope phase effects on the spatial coherence of high-order harmonics.

In this work, we report the Carrier-Envelope Phase Effects on the Spatial Coherence of High-order Harmonics driven by phase-stabilized few-cycle mid-infrared laser pulses. The degree of coherence varies with carrier-envelope phase periodically with a period of π. At the same time, as the harmonic orders increase, the extreme points on the curve of coherence degree vs. carrier-envelope phase shift toward the direction of carrier-envelope phase increasing. Through theoretical analysis, we find that the ionization induced frequency chirp plays an important role in the Carrier-Envelope Phase Effects on the Spatial Coherence. This effect suggests a possible method to optimize the spatial coherence of harmonics by tuning the carrier-envelope phase of the driving field.

Efficient isolated attosecond emission in a pre-excited medium using noncarrier-envelope phase-stabilized nanoplasmonic fields

We theoretically investigate the isolated attosecond pulse generation in a pre-excited medium by using local nanoplasmonic fields with few-cycle duration. It is shown that the combined effect of the temporal ionization of the pre-excited medium and the spatial inhomogeneity of the local nanoplasmonic field can dramatically control the quantum path, leading to broadband supercontinuous harmonics with high yield. Moreover, this spatiotemporal combination is robust against the carrier-envelope phase (CEP) shift, and the broadband supercontinuum can be supported by nearly all the CEPs of the local field. It would open a route to obtain efficient isolated attosecond emission with no need for the CEP-stabilized laser technique.

Coherent control and giant enhancement of multiphoton ionization and high-order-harmonic generation driven by intense frequency-comb laser fields: Anab initiotheoretical investigation

We present an ab initio theoretical investigation of the coherent control and significant enhancement of multiphoton ionization (MPI) and high-order-harmonic generation (HHG) of atoms and molecules by means of intense frequency-comb laser fields. We show that the nonperiodically or quasiperiodically time-dependent Schr\odinger equation for the frequency-comb laser excitation problem can be transformed exactly into a time-independent non-Hermitian generalized Floquet matrix eigenvalue problem by means of the many-mode Floquet theory and the complex-scaling transformation. The generalized Floquet Hamiltonian can be optimally discretized and the complex quasienergy eigenvalues and eigenfunctions can be solved accurately and efficiently by means of the generalized pseudospectral method. The procedure is applied to a case study of the resonance-enhanced MPI and HHG of atomic hydrogen driven by an intense frequency-comb laser field. Our study shows that both the MPI and HHG rates can be coherently controlled by tuning the laser parameters such as the pulse-to-pulse carrier-envelope-phase (CEP) shift. In particular, both the MPI and HHG rates exhibit dramatic enhancement by tuning the CEP shift, due to the phenomenon of multiphoton resonances.

Linear Electro Optic Effect for High Repetition Rate Carrier Envelope Phase Control of Ultra Short Laser Pulses

This paper is devoted to analyzing the principle and applications of the linear electro-optic (EO) effect for the control of the carrier-envelope-phase (CEP). We introduce and detail here an original method, which relies on the use of an EO dispersive prism pair in a compressor-like configuration. We show that, by choosing an adequate geometry, it is possible to shift the CEP without changing the group delay (isochronous carrier-envelope-phase shifter) or change the induced group delay without varying the CEP. According to our calculations, when applying an electric field around 400 V/cm to the rubidium titanyle phosphate (RTP) prisms in a double pass configuration (2 × 40 mm total length), one obtains a CEP shift of π rad at 800 nm without inducing a group delay. In contrast, this CEP shift is obtained for an electric field around 1.4 kV/cm in a RTP rectangular slab of the same total length and, in this case, the group delay is of the order of a few fs.

Carrier-envelope phase shift of few-cycle pulses along the focus of lenses and mirrors beyond the nonreshaping pulse approximation: the effect of pulse chirp

The carrier-envelope phase (CEP) shift of few-cycle pulses along a focal region is evaluated without neglecting the pulse reshaping inherent to any focusing process. The CEP shift is then found to strongly depend on the chirp of the focused pulse. Based on this effect, realistic optical systems for focusing few-cycle pulses are proposed to control their focal CEP shift for their phase-sensitive interactions with matter.

On the use of lenses to focus few-cycle pulses with controlled carrier–envelope phase

From a model of focusing with lenses that includes the effects of the lens variable thickness, material dispersion, aperture, spherical and chromatic aberrations, we characterize the conditions under which a lens can focus to few-cycle, transform-limited pulses propagating without distortion along the focal region. A lens also allows to control the carrier–envelope phase shift along the focus. The carrier–envelope phase shift is drastically reduced by focusing with specific focal lengths and input spot sizes, which are of the same order as those typically used in experiments involving focusing for phase-sensitive, light-matter interactions.

Isolated attosecond pulse generation with the stability against the carrier-envelope phase shift and with the high-beam quality from CO gas medium

We theoretically investigate the isolated attosecond pulse generation with a 1300-nm infrared few-cycle laser pulse from the CO medium. It is found that the supercontinuum in the plateau from CO in the microscopic level can be generated for nearly all the carrier-envelop phase(CEP) of the driving pulse. The macroscopic investigation shows that when the molecular axis is parallel to the electric field, the supercontinuum can be phase-matched in broader spectral range than the antiparallel case and achieve a good beam quality with the divergence angle of 0.2 mrad, which benefits for some potential applications of ultrafast detections with high spatial and temporal resolutions.

Coherent sampling of active mode-locked terahertz quantum cascade lasers and frequency synthesis

Researchers demonstrate a terahertz quantum cascade laser operating in a regime of active mode-locking by modulating its bias current with a radiofrequency synthesizer. This technique allows coherent sampling of the terahertz electric field as well as control over the laser's carrier–envelope phase shift.

Robust generation of isolated attosecond pulse against the variation of carrier envelope phase of driving laser pulses

We propose a scheme for generating isolated attosecond pulse (IAP) via high-order harmonic generation in gases using a chirped two-color laser field of multicycle duration. In contrast to previous techniques where the stable carrier-envelope phase (CEP) of the driving laser pulses is a prerequisite for IAP generation, the proposed scheme is robust against the large variation of CEP. We show the generation of IAP with an intensity fluctuation less than $50%$ and an intensity contrast ratio higher than $5:1$ when the CEP shift is as large as $1.35$$\ensuremath{\pi}$.

Carrier-envelope phase dynamics of octave-spanning dispersion-managed Ti: sapphire lasers

The carrier-envelope phase dynamics of few-cycle octave-spanning Ti:sapphire lasers are analyzed based on a numerical one-dimensional dispersion-managed laser model. The dominant contribution to the carrier-envelope phase shift with respect to intracavity energy arises from the asymmetric impact of self-steepening on pulse formation and laser output. We show that this term is larger by a factor of four than the energy-dependent round trip phase and is thus more significant than in the corresponding result for conventional soliton lasers. Frequency shifts due to the Raman effect are studied and found to be of minor impact for octave-spanning lasers.