When atoms or molecules interact with intense bichromatic laser light, the relative optical phase between the two colors influences the formation of an electron wave packet in the continuum, further motion of the wave packet, and the phase accumulated by the wave packet on its trajectory. We show how the control over the wave packet motion can be used to generate single subfemtosecond pulses in the extreme ultraviolet spectral region. We also demonstrate control over the phase of the atomic polarizability.
When atoms or molecules interact with intense bichromatic laser light, the relative optical phase between the two colors influences the formation of an electron wave packet in the continuum, further motion of the wave packet, and the phase accumulated by the wave packet on its trajectory. We show how the control over the wave packet motion can be used to generate single subfemtosecond pulses in the extreme ultraviolet spectral region. We also demonstrate control over the phase of the atomic polarizability.
Ultrafast high harmonic beams provide new opportunities for coherently controlling excitation and ionization processes in atoms, molecules, and materials on attosecond time scales by employing multiphoton two-pathway electron-wave-packet quantum interferences. Here we use spectrally tailored and frequency tuned vacuum and extreme ultraviolet harmonic combs, together with two phase-locked infrared laser fields, to show how the total single and double photoionization yields of argon can be coherently modulated by controlling the relative phases of both optical and electronic-wave-packet quantum interferences. This Letter is the first to apply quantum control techniques to double photoionization, which is a fundamental process where a single, high-energy photon ionizes two electrons simultaneously from an atom.
Whenever scientific tools are significantly advanced; new discoveries soon follow. My thesis work relies on advances in ultrafast laser technology that allow precise control over the shape of sub 20-fs laser pulses. This laser system enabled many new experiments that yielded significant advances in attosecond science, atomic physics; extreme nonlinear optics, quantum chemistry, coherent control, molecular physics, ultrafast optical phase modulation, EUV coherent imagines, and EUV radiation sources. By using an automated learning control algorithm coupled to our deformable mirror pulse shaper; we have demonstrated the control of a process on an attosecond time-scale for the first time. We demonstrate the manipulation of the phase of an electron wave packet, allowing us to tailor the EUV spectrum generated by high-order harmonic generation. These optimal fields were discovered by a learning algorithm that both found an unknown optimal solution; but also discovered an unknown phase-matching mechanism that occurs in the interaction of a single atom and light pulse. We control molecular vibrational and rotational coherences with the same learning algorithm; demonstrating selective control in room temperature molecules at atmospheric pressures. Moreover, we demonstrate that modifications to the algorithm can help automatically uncover the physics of the control mechanism found by the learning algorithm. We also demonstrate a new pulse-compression technique using rotational phase modulation, where optical pulses were compressed by an order of magnitude after phase modulation by a molecular rotational coherence and subsequent propagation through a transparent window. This technique will enable efficient pulse compression in the deep-UV. Finally, we measure the coherence of EUV light generated by high-harmonic upconversion of a femtosecond laser. In phase-matched hollow-fiber geometry, the EUV light exhibits the highest inherent spatial coherence of any source in this region of the spectrum. We use this source to demonstrate the coherent image with first table-top EUV radiation. Since this source exhibits full spatial coherence at very short wavelength, this light source represents has smallest inherent effective source-size of any light source yet created. We also demonstrate that the spectrum of an optical field can be determined by measuring the interference of a double-slit pattern.
Metastable states are important actors in the ionisation of atoms and molecules. Sub-femtosecond extreme ultraviolet pulses can coherently populate several transiently bound states at once, thus starting the attosecond clocks which are required to monitor and control ultrafast electronic evolution above the ionisation threshold. Here we illustrate, from a theoretical point of view, the effects coherent superpositions of 1 P o doubly excited states in the helium atom have on channel-resolved photoelectron spectra as well as on the transient absorption spectrum of the atom in the extreme ultraviolet region, when they are created by a single-attosecond pulse in the presence of a strong few-cycle near-infrared/visible pulse which acts as a probe. Interference fringes varying rapidly with the pump-probe time delay are visible in both photoelectron and transient absorption spectra. From such fringes, the wave packet itself can conceivably be reconstructed. Conversely, all observables are modulated by the characteristic beating periods of the wave packet, so that control of partial ionisation yields, branching ratios, and light absorption or amplification can be achieved.
We show experimentally that spectral phase manipulation of ultrashort extreme-uv light pulses can induce and control coherent transient excitation of the He(1s3p) excited state by the nonresonant components of the broadband extreme-ultraviolet light. The spectral phase manipulation of the 15th harmonic of an intense 805nm, 80fs pulse is achieved by propagation of the euv light through a variable optical density of He gas. The acquired spectral phase due to the dispersive interaction of the off-resonance components in the euv pulse with the He(1s3p) resonance enhances and modifies the transient excitation. The temporal evolution of the coherently prepared transient He(1s3p) amplitude is probed by ionization to the continuum with a 400nm, 80fs pulse.
The four-wave mixing produced with two ultrashort phase-locked !-3! laser pulses propagating coherently in a two-level system in the infrared spectral region is shown to depend on the pulses relative phase. The Maxwell-Bloch equations are solved beyond the rotating-wave approximation to account for field frequencies which are largely detuned from the atomic resonance. The relative phase dominating the efficiency of the coupling to the 5! anti-Stokes Raman component is determined by sign of the total ac Stark shift induced in the system, in such a way that the phase influence disappears precisely where the ac Stark effect due to both pulses is compensated. This fundamental quantum interference effect can be the basis for nonlinear ultrafast optical spectroscopy techniques. Phenomena arising from the coherent control of nonlinear interactions are of importance in fields as diverse as optoelectronics and materials research, in high harmonic generation, in photoionization or molecular dissociation, and in biological applications such as spectroscopy and imaging, among others [1, 2, 3, 4, 5, 6, 7]. Optical quantum coherent control is based on the fact that the phases of interfering transition amplitudes in light-matter interactions can be controlled through the optical phase of coherent light sources that drive the interaction, in such a way that the transition rates to final states and the dynamics at various stages of the process can be modified [1]. In a recent paper [8], a theoretical investigation on the quantum coherent control of the optical transient four-wave mixing of two intense phase-locked femtosecond laser pulses of central angular frequencies ω and 3ω propagating in a two-level atom (TLA) was reported. It was shown how the nonlinear (χ (3) ) coupling to the antiStokes Raman field at frequency 5ω depends critically on the initial relative phase φ of the propagating pulses. In Ref. [8], the study was centered to intense pulses in the visible and ultraviolet spectral regions, with frequencies at resonance or lower than the atomic transition. The phenomena observed in [8], however, can be scaled to various laser and material parameters. In the infrared spectral range, for instance, experiments on ultrafast molecular dynamics are frequently performed by help of twocolor pump probe nonlinear spectroscopy techniques. In this type of studies, due to the high intensities inherent to ultrashort (subpicosecond) pulses, nonlinear effects such as stimulated Raman processes may become important and are often utilized as a complementary tool to gain information [9]. In this Letter, we address the conditions for quantum coherent control of transient four-wave mixing interactions with two phase-locked ω-3ω femtosecond laser pulses in the mid-IR spectral region. Our purpose is to reveal the basic physics for a strict two-level medium and to this end the Maxwell-Bloch TLA will be considered. We will study pulses with a duration of 300 fs (with a w 5w
Experimental and theoretical work is presented on the control of wave packet dynamics in atomic and molecular systems. Using sequences of phase-locked optical pulses, the link between optical phase and quantum mechanical phase is explored in the Na atom and Naj dimer, representing a step towards the logical engineering of quantum states in more complicated systems. A novel apparatus, constructed to study and control the vibrational dynamics of vibrational wave packets on the Na ionic potential surface, is described in chapter 3. Theoretical simulations for proposed experiment are presented in chapter 4. Control is achieved by exploiting the phase-evolution of the constituent vibrational quantum states within the wave packet superposition. The phase relationship and the accumulated phase difference between the various components of the wave packet is determined, and a sequence of phase-locked optical pulses is employed to selectively enhance or depopulate specific vibrational states, or sets of vibrational states. The quantum state composition of the resulting wave packet, and the efficiency of the control scheme, is determined by calculating the multi-pulse response of the time-dependent vibrational state populations. In chapter 5, the quantum interference between Rydberg electron wave packets in the Na atom is investigated using pairs of phase-locked wave packets, allowing manipulation of the total orbital angular momentum of Na Rydberg atoms. Initially the wave packet is composed of a superposition of s and d Rydberg series. Exploitation of the difference between the quantum defects of the two series allows specific angular momentum compositions within the resultant wave packet to be engineered. Experimentally, this final quantum state distribution is analysed in the frequency domain using state selective field ionisation, and in the time domain using the optical Ramsey method. The theoretical calculations show how the phase difference between pairs of optical pulses is linked to the corresponding Rydberg frequency spectrum, therefore enabling the control of the quantum state composition of the wave packets.