Avner Fleischer

Generation of bright phase-matched circularly-polarized extreme ultraviolet high harmonics

Circularly-polarized extreme UV and X-ray radiation provides valuable access to the structural, electronic and magnetic properties of materials. To date, this capability was available only at large-scale X-ray facilities such as synchrotrons. Here we demonstrate the first bright, phase-matched, extreme UV circularly-polarized high harmonics and use this new light source for magnetic circular dichroism measurements at the M-shell absorption edges of Co. We show that phase matching of circularly-polarized harmonics is unique and robust, producing a photon flux comparable to the linearly polarized high harmonic sources that have been used very successfully for ultrafast element-selective magneto-optic experiments. This work thus represents a critical advance that makes possible element-specific imaging and spectroscopy of multiple elements simultaneously in magnetic and other chiral media with very high spatial and temporal resolution, using tabletop-scale setups.

Adiabatic theorem for non-Hermitian time-dependent open systems

In the conventional quantum mechanics i.e., Hermitian quantum mechanics the adiabatic theorem for systems subjected to time-periodic fields holds only for bound systems and not for open ones where ionization and dissociation take placeD. W. Hone, R. Ketzmerik, and W. Kohn, Phys. Rev. A 56, 4045 1997. Here with the help of the t, t formalism combined with the complex scaling method we derive an adiabatic theorem for open systems and provide an analytical criterion for the validity of the adiabatic limit. The use of the complex scaling transformation plays a key role in our derivation. As a numerical example we apply the adiabatic theorem we derived to a one-dimensional model Hamiltonian of Xe atom which interacts with strong, monochromatic sine-square laser pulses. We show that the generation of odd-order harmonics and the absence of hyper-Raman lines, even when the pulses are extremely short, can be explained with the help of the adiabatic theorem we derived. DOI: 10.1103/PhysRevA.72.032103 I. MOTIVATION When matter is exposed to intense laser fields, high harmonics HH’s of the incident radiation may be produced. Usually, only odd harmonics are obtained even when the laser pulses are short for theoretical and experimental work which demonstrates this see 1,2, respectively. Since the duration of the pulse in time is inversely proportional to its width in energy space, one may find this result surprising, as one may expect to obtain also a large distribution of frequencies in the scattered field. Why are only odd harmonics obtained even when the laser pulses are short? For cw lasers and symmetric field-free potential using the non-Hermitian Floquet theory it was proved that only odd harmonics are obtained when the dynamics is controlled by a single-resonance Floquet quasienergy QE state 3,4. When laser pulses are used it was argued that this proof still holds since usually the populated resonance states are associated with very different lifetimes and the dynamics is controlled by the resonance state which has the longest lifetime. However, this argument may hold only when the duration of the laser pulses is large enough to enable the decay of the short-lived resonances. Indeed numerical simulations showed that the harmonic generation spectra HGS as obtained from a single non-Hermitian complex-scaled resonance Floquet state is in remarkable agreement with the results obtained from conventional i.e., Hermitian timedependent simulations 5. The question that is addressed in this work is whether an analytical criterion for the shape and duration of the laser pulse for which the system is controlled by a singleresonance Floquet state can be given. It is obvious that the question regarding the possibility of the population of a single-resonance state is connected with the question regarding the degree of adiabaticity of the process. The question is therefore under which conditions can a short laser pulse be defined as an adiabatic one. The answer to this question is important not only to harmonic generation HG studies but also for other, more general studies where lasers are used to

Calculations of time-dependent observables in non-hermitian quantum mechanics : The problem and a possible solution

The solutions of the time independent Schrodinger equation for non- Hermitian (NH) Hamiltonians have been extensively studied and calcu- lated in many different fields of physics by using L 2 methods that origi- nally have been developed for the calculations of bound states. The exist- ing non-Hermitian formalism breaks down when dealing with wavepack- ets(WP). An open question is how time dependent expectation values can be calculated when the Hamiltonian is NH ? Using the F-product for- malism, which was recently proposed, (J. Phys. Chem., 107, 7181 (2003)) we calculate the time dependent expectation values of different observable quantities for a simple well known study test case model Hamiltonian. We carry out a comparison between these results with those obtained from conventional(i.e., Hermitian) quantum mechanics (QM) calculations. The remarkable agreement between these results emphasizes the fact that in the NH-QM, unlike standard QM, there is no need to split the entire space into two regions; i.e., the interaction region and its surrounding. Our re- sults open a door for new type of WP propagation calculations within the NH-QM formalism that until now were impossible. In particular our work is relevant to the many different fields in physics and chemistry where complex absorbing potentials are introduced in order to reduce the prop- agation calculations into a restricted region in space where the artificial reflections from the edge of the numerical grid/box are avoided.

Helicity-Selective Phase-Matching and Quasi-Phase matching of Circularly Polarized High-Order Harmonics: Towards Chiral Attosecond Pulses

Author(s): Kfir, O; Grychtol, P; Turgut, E; Knut, R; Zusin, D; Fleischer, A; Bordo, E; Fan, T; Popmintchev, D; Popmintchev, T; Kapteyn, H; Murnane, M; Cohen, O | Abstract:

Sparsity-based super-resolved coherent diffraction imaging of one-dimensional objects

Phase-retrieval problems of one-dimensional (1D) signals are known to suffer from ambiguity that hampers their recovery from measurements of their Fourier magnitude, even when their support (a region that confines the signal) is known. Here we demonstrate sparsity-based coherent diffraction imaging of 1D objects using extreme-ultraviolet radiation produced from high harmonic generation. Using sparsity as prior information removes the ambiguity in many cases and enhances the resolution beyond the physical limit of the microscope. Our approach may be used in a variety of problems, such as diagnostics of defects in microelectronic chips. Importantly, this is the first demonstration of sparsity-based 1D phase retrieval from actual experiments, hence it paves the way for greatly improving the performance of Fourier-based measurement systems where 1D signals are inherent, such as diagnostics of ultrashort laser pulses, deciphering the complex time-dependent response functions (for example, time-dependent permittivity and permeability) from spectral measurements and vice versa.

In-line production of a bi-circular field for generation of helically polarized high-order harmonics

The recent demonstration of bright circularly polarized high-order harmonics of a bi-circular pump field gave rise to new opportunities in ultrafast chiral science. In previous works, the required nontrivial bi-circular pump field was produced using a relatively complicated and sensitive Mach-Zehnder-like interferometer. We propose a compact and stable in-line apparatus for converting a quasi-monochromatic linearly polarized ultrashort driving laser field into a bi-circular field and employ it for generation of helically polarized high-harmonics. Furthermore, utilizing the apparatus for a spectroscopic spin-mixing measurement, we identify the photon spins of the bi-circular weak component field that are annihilated during the high harmonics process.

Amplification of high-order harmonics using weak perturbative high-frequency radiation

Focusing intense linearly polarized monochromatic IR laser pulses into a gas of atoms can lead to the emission of high-energy photons with frequencies extending into the extreme ultraviolet XUV and x-ray region by high-order harmonic generation HHG. The HHG phenomenon stands as one of the most promising methods of producing short attosecond as pulses 1. The contamination of the strong IR field with a second 2–4 or more 5–7 weak XUV fields has a dramatic effect on the dynamical behavior of the electrons, and had drawn a lot of attention in recent years. On the basis of the three-step recollision model 8–10, it had been argued that the role of the XUV field is to switch the initial step in the generation of high-order harmonics from tunnel ionization to the more efficient XUV single-photon ionization. This might explain the improved macroscopic HHG signal obtained in experiments: the XUV-assisted ionization increases the number of atoms that participate in the HHG process and improves phase matching 11. The effect at the single-atom level, however, is less clear. It has been shown that the XUV photons control the timing of ionization, and preferentially select certain quantum paths of the electron 12. While this effect may lead to the enhancement of the low-order harmonics in the plateau, it cannot account for the large enhancement in the cutoff and beyond Fig. 1. A three-step-model classical analysis of HHG suggests that the contribution of the XUV field to the kinetic energy of the returning electron is negligible. The kinetic energy of a classical free electron of charge e and mass m, driven by a linearly polarized strong IR fundamental field of frequency , amplitude 1 , and polarization ek E1t =ek1 cost is Ekt =p 2 t / 2m. pt =e 1 /sint � sinti is the momentum of the electron, and it has been assumed that the electron is freed at time ti with zero momentum. The addition of a weak harmonic XUV field of frequency q where q is a large integer and amplitude q

Sparsity-based super-resolution coherent diffractive imaging of (practically) 1D images using extreme UV radiation

We demonstrate experimentally sparsity-based super-resolution of coherent diffraction imaging (CDI) with extreme UV radiation. We also present the first experimental CDI of a practically one-dimensional object, overcoming the well-known ambiguity problem in one-dimensional phase retrieval.

Weak-field multiphoton femtosecond coherent control in the single-cycle regime

Weak-field coherent phase control of atomic non-resonant multiphoton excitation induced by shaped femtosecond pulses is studied theoretically in the single-cycle regime. The carrier-envelope phase (CEP) of the pulse, which in the multi-cycle regime does not play any control role, is shown here to be a new effective control parameter that its effect is highly sensitive to the spectral position of the ultrabroad spectrum. Rationally chosen position of the ultrabroadband spectrum coherently induces several groups of multiphoton transitions from the ground state to the excited state of the system: transitions involving only absorbed photons as well as Raman transitions involving both absorbed and emitted photons. The intra-group interference is controlled by the relative spectral phase of the different frequency components of the pulse, while the inter-group interference is controlled jointly by the CEP and the relative spectral phase. Specifically, non-resonant two- and three-photon excitation is studied in a simple model system within the perturbative frequency-domain framework. The developed intuition is then applied to weak-field multiphoton excitation of atomic cesium (Cs), where the simplified model is verified by non-perturbative numerical solution of the time-dependent Schrödinger equation. We expect this work to serve as a basis for a new line of femtosecond coherent control experiments.

Self-phase modulation spectral broadening in two-dimensional spatial solitons: toward three-dimensional spatiotemporal pulse-train solitons

We demonstrate self-phase modulation (SPM) spectral broadening in two-dimensional solitons in homogeneous media using two different schemes. In the active mode, a train of pulses are collectively trapped and form a spatial soliton through a photorefractive, slowly responding, and electronically controlled self-focusing nonlinearity, and each pulse experiences spectral broadening by the fast SPM nonlinearity. In the passive mode, the pulse-train beam is guided in a waveguide that is optically induced by a continuous-wave thermal spatial soliton. The soliton formation increased the normalized spectral broadening factor from 0.5% up to 197%. This experiment presents significant progress toward the experimental demonstration of three-dimensional spatiotemporal pulse-train solitons.