Quantum Path Control Research Articles

Macroscopic control of quantum paths in high order harmonics by a weak second harmonic field

The phase matching of quantum paths in high-order harmonic generation is investigated in a two-color laser field. Our results show that the phase-matching properties of the short and long trajectories can be modulated effectively by adding a weak second harmonic field. This scheme can not only achieve the quantum path selection, but more importantly it can achieve on-axis phase matching for both of the two trajectories simultaneously. A bright interference pattern induced by the simultaneously phase-matched two trajectories can be observed by placing a near-field on-axis filter, which has great potential for experimentally characterizing the full single-atom dipole moment.

Efficient control of quantum paths via dual-gas high harmonic generation

The accurate control of the relative phase of multiple distinct sources of radiation produced by high harmonic generation is of central importance in the continued development of coherent extreme UV (XUV) and attosecond sources. Here, we present a novel approach which allows extremely accurate phase control between multiple sources of high harmonic radiation generated within the Rayleigh range of a single-femtosecond laser pulse using a dual-gas, multi-jet array. Fully ionized hydrogen acts as a purely passive medium and allows highly accurate control of the relative phase between each harmonic source. Consequently, this method allows quantum path selection and rapid signal growth via the full coherent superposition of multiple HHG sources (the so-called quasi-phase-matching). Numerical simulations elucidate the complex interplay between the distinct quantum paths observed in our proof-of-principle experiments.

Quantum path control in harmonic generation by temporal shaping of few-optical-cycle pulses in ionizing media

Temporal reshaping of the electric field of few-optical-cycle pulses in a low-density ionizing gas has been used to achieve control of the electron trajectories in the process of high-order harmonic generation. As a result of such a quantum path control mechanism, isolated or multiple attosecond pulses have been produced, depending on the carrier-envelope phase of the driving field. In particular, complete spectral tunability of the harmonic peaks over the whole spectral range has been demonstrated. Experimental results have been interpreted using a nonadiabatic three-dimensional propagation model and a nonadiabatic stationary phase model.

Generation of intense isolated sub-40-as pulses from a coherent superposition state by quantum path control in the multicycle regime

We theoretically investigate high-order harmonic generation and attosecond pulses by numerically solving the three-dimensional time-dependent Schr\"odinger equation from a helium ion in a two-color laser field, which is synthesized by adding a 1600-nm laser pulse to a multicycle 800-nm laser pulse. The numerical results show that the short quantum path selection and broadband continuum spectra are achieved by adjusting the relative phase between two laser pulses, and isolated attosecond pulses can be generated successfully. Compared with the case of He${}^{+}$ ions from the 1$s$ ground state, the emission efficiency of the continuous harmonics and the intensity of the isolated attosecond pulse are enhanced approximately thirteen orders of magnitude by preparing He${}^{+}$ ions in a coherent superposition of the states 1$s$ and 2$s$. Furthermore, the bandwidth of the continuum spectrum is further broadened by increasing the intensity of the 1600-nm laser pulse, and an intense 38-as isolated pulse with a bandwidth of 109 eV is straightforwardly obtained.

Generation of isolated sub-50-as pulses by quantum path control in the multicycle regime

We theoretically propose a method of producing single attosecond pulse by high-order harmonic generation with a two-color multicycle driver laser field. It is shown that this scheme can modulate the quantum paths by adjusting the relative phase. In the case of the short quantum path control, a broadband supercontinuum spectrum with a 93 eV spectral width can be produced which directly generates an isolated 46 as pulse without phase compensation. When the long quantum path is picked out, a harmonic continuum spectrum with a bandwidth of 101 eV appears, an intense isolated 39 as pulse with high-signal-to-noise ratio is generated successfully by superposing some properly selected harmonics, the phases of which are locked. In both cases, our simulations show that the value of the time delay between two lasers can be varied between $\ensuremath{-}0.2$ fs and 0.2 fs to keep the isolated sub-50-as pulse generation. In particular, these results are analyzed by using both classical and three-dimensional quantum-mechanical approaches.

Generation of isolated sub-100-as pulses with 30-fs lasers

We propose a scheme to generate isolated sub-100-as pulses with 30-fs lasers. Our model is based on high-order harmonic generation from a helium atom irradiated by a two-color femtosecond laser field, in which the driving and the control fields are linearly polarized and the polarization angle between them is $\ensuremath{\pi}/4$. By mixing the polarization gating technique and optimization of two-color lasers, we destroy the symmetry of the driving field from two directions and realize quantum path control with multicycle lasers.

Generation of a broadband soft X-ray supercontinuum by quantum path control

数值研究了双色激光脉冲与一维氩原子相互作用的谐波发射功率谱.双色激光脉冲由800 nm和1600 nm的两束激光组成，脉冲持续时间为9.7 fs.计算结果表明，相对相位为π时，谐波截止频率从单色场情形的58 eV大幅度地展宽到316 eV，获得频宽为93 eV的软X射线连续辐射谱.此外，还采用短程的普薛耳－特勒势和长程的软化库仑势进行比对，数值结果表明该连续辐射谱的存在具有普适性.通过半经典的“三步”模型和时频分析小波变换方法对该连续辐射谱进行分析，发现其主要由单一的“短”量子路径生成，直接叠加频率从22

Quantum path selection and control in high-order harmonic generation using a spatially shaped laser beam

We propose a scheme to identify and separate the contributions from the short and long electron trajectories in the high-order harmonic generation in argon atoms. The modification of the diving laser field by using a diaphragm prior to focusing is demonstrated to be an effective way of controlling the short and long path contributions in harmonic emission since different macroscopic phase-matching conditions are satisfied, respectively. We show that the short and the long path harmonic emission are separated by adjusting the laser focusing geometry and modifying the laser field. This offers a way of selectively probing and controlling the electron dynamics in intense laser fields.

Control of quantum paths in the multicycle regime and efficient broadband attosecond pulse generation

We theoretically investigate the broadband isolated attosecond generation in a two-color multicycle laser pulse. It is shown that quantum path selection can be realized and then a sub-100-as isolated attosecond pulse is straightforwardly obtained. By adding an UV attosecond pulse to the synthesized two-color field, the continuous harmonic yield is significantly enhanced by 2 orders and the bandwidth is further broadened, which is beneficial to the efficient generation of a sub-100-as isolated attosecond pulse.

Quantum Path Interferences in High-Order Harmonic Generation

We have investigated the intensity dependence of high-order harmonic generation in argon when the two shortest quantum paths contribute to the harmonic emission. For the first time to our knowledge, experimental conditions were found to clearly observe interference between these two quantum paths that are in excellent agreement with theoretical predictions. This result is a first step towards the direct experimental characterization of the full single-atom dipole moment and demonstrates an unprecedented accuracy of quantum path control on an attosecond time scale.

Quantum-path control in high-order harmonic generation at high photon energies

We show through experiment and calculations how all-optical quasi-phase-matching of high-order harmonic generation can be used to selectively enhance emission from distinct quantum trajectories at high photon energies. Electrons rescattered in a strong field can traverse short and long quantum trajectories that exhibit differing coherence lengths as a result of variations in intensity of the driving laser along the direction of propagation. By varying the separation of the pulses in a counterpropagating pulse train, we selectively enhance either the long or the short quantum trajectory, and observe distinct spectral signatures in each case. This demonstrates a new type of coupling between the coherence of high-order harmonic beams and the attosecond time-scale quantum dynamics inherent in the process.

Quantum path control in few-optical-cycle regime

We theoretically show that selection of a single quantum path in high-order harmonics generation can be realized in a few-optical-cycle regime with two-color schemes. We also demonstrate, in theory as well, the generation of spectrally smooth and ultrabroad extreme ultraviolet supercontinuum in argon gas which can produce single similar to 79 as pulses with currently available ultrafast laser sources. Our finding can be beneficial for generating isolated sub-100 as extreme ultraviolet pulses.

Control of quantum paths of high-order harmonics and attosecond pulse generation in the presence of a static electric field

The time–frequency properties of high-order harmonic generation in the presence of a static electric field are investigated. It is found that the quantum paths contributing to the harmonics can be controlled by adding a static electric field. The highest photon energies of harmonics emitted in the adjacent half-cycles of the laser field are modulated by the static electric field, and then an attosecond pulse train with one burst per optical cycle can be extracted. For the ratio between the laser and the static field of 0.39, the harmonic spectrum is extended to Ip + 9.1Up, and the harmonics above Ip + 0.7Up are emitted almost in phase. The phase-locked harmonics covered by a broad bandwidth are produced, and then a regular attosecond pulse train with a pulse duration of 80 as is generated.

Quasi-phase-matching and quantum-path control of high-harmonic generation using counterpropagating light

Quasi-phase-matching and quantum-path control of high-harmonic generation using counterpropagating light

Control of quantum paths in high-order harmonic generation via a ω+3ω bichromatic laser field

The time-frequency properties of high harmonic generation (HHG) driven by a bichromatic field consisting of a fundamental and a weak third harmonic field are investigated. The selection of an individual quantum path contributing to harmonic generation can be achieved by adjusting the relative phase between the two components of the driving field. The classical trajectory simulation of the strong-field electron dynamics is performed to analyse the physical process. Our calculations show that it is the control of the ionization step that leads to the quantum path selection. This quantum selection can be used to generate regular and strong attosecond pulses.

Control of long electron quantum paths in high-order harmonic generation by phase-stabilized light pulses

In this work we report on the first experimental demonstration of selection of the long electron quantum paths in the process of high-order harmonic generation by phase-stabilized multiple-cycle light pulses. A complete experimental investigation of the role of intensity and carrier-envelope phase of the driving pulses on the spectral characteristics of the long quantum paths is performed. Simulations based on the nonadiabatic saddle-point method and on a complete nonadiabatic three-dimensional model reproduce the main features of the experimental results. The use of phase-stabilized driving pulses allows one to control, on an attosecond temporal scale, the spectral and temporal characteristics associated with the electron quantum paths involved in the harmonic generation process.

Strong field quantum path control using attosecond pulse trains.

We show that attosecond pulse trains have a natural application in the control of strong field processes. In combination with an intense infrared laser field, the pulse train can be used to microscopically select a single quantum path contribution to a process that would otherwise consist of several interfering components. We present calculations that demonstrate this by manipulating the time-frequency properties of high order harmonics at the single atom level. This quantum path selection can also be used to define a high resolution attosecond clock.