Attosecond transient absorption is one of the promising new techniques being developed to exploit the availability of sub-femtosecond extreme ultraviolet (XUV) pulses to study the dynamics of the electron on its natural time scale. The temporal resolution in a transient absorption setup comes from the control of the relative delay and coherence between pump and probe pulses, while the spectral resolution comes from the characteristic width of the features that are being probed. In this review we focus on transient absorption scenarios where an attosecond pulse of XUV radiation creates a broadband excitation that is subsequently probed by a few cycle infrared (IR) laser. Because the attosecond XUV pulses are locked to the IR field cycle, the exchange of energy in the laser–matter interaction can be studied with unprecedented precision. We focus on the transient absorption by helium atoms of XUV radiation around the first ionization threshold, where we can simultaneoulsy solve the time-dependent Schrodinger equation for the single atom response and the Maxwell wave equation for the collective response of the nonlinear medium. We use a time-domain method that allows us to treat on an equal footing all the different linear and nonlinear processes by which the medium can exchange energy with the fields. We present several simple models, based on a few-level system interacting with a strong IR field, to explain many of the novel features found in attosecond transient absorption spectrograms. These include the presence of light-induced states, which demonstrate the ability to probe the dressed states of the atom. We also present a time-domain interpretation of the resonant pulse propagation features that appear in absorption spectra in dense, macroscopic media. We close by reviewing several recent experimental results that can be explained in terms of the models we discuss. Our aim is to present a road map for understanding future attosecond transient absorption experiments in more complex systems.

http://absimage.aps.org/image/DAMOP14/MWS_DAMOP14-2014-000677.pdf

Theory of strong-field attosecond transient absorption

Non-Hermitian lattice systems with unconventional transport phenomena and topological effects have attracted intensive attention recently. Non-Hermiticity is generally introduced by engineering on-site gain/loss distribution or inducing asymmetric couplings by applying an imaginary gauge field. Here, we extend the concept of non-Hermitian lattices from spatial to frequency dimension and emulate various non-Hermitian transport phenomena arising from asymmetric coupling in synthetic dimension. The non-Hermitian frequency lattice is created by introducing complex gauge potentials through appropriate complex modulations in a slab waveguide. This complex gauge potential can induce asymmetric couplings among spectral modes and give rise to various non-Hermitian transport phenomena such as amplified and decayed frequency diffraction, refraction and non-Hermitian Bloch oscillations. The latter manifest themselves as both power oscillation and asymmetric oscillation patterns, and can be exploited to probe in the bulk the non-Hermitian skin effect. Frequency-domain Bloch oscillations with both exponentially growing oscillation amplitude and energy are also predicted. Our results pave the way towards emulating non-Hermitian transport phenomena and topological effects in synthetic dimension on a photonic platform, with potential applications to spectral manipulation of optical signals and energy harvesting.

/pdf/discrete-diffraction-and-bloch-oscillations-in-non-hermitian-4uy1ba8zbi.pdf

Discrete diffraction and Bloch oscillations in non-Hermitian frequency lattices induced by complex photonic gauge fields

Abstract Tunneling is one of the most fundamental and ubiquitous processes in the quantum world. The question of how long a particle takes to tunnel through a potential barrier has sparked a long-standing debate since the early days of quantum mechanics. Here, we propose and demonstrate a novel scheme to accurately determine the tunneling time of an electron. In this scheme, a weak laser field is used to streak the tunneling current produced by a strong elliptically polarized laser field in an attoclock configuration, allowing us to retrieve the tunneling ionization time relative to the field maximum with a precision of a few attoseconds. This overcomes the difficulties in previous attoclock measurements wherein the Coulomb effect on the photoelectron momentum distribution has to be removed with theoretical models and it requires accurate information of the driving laser fields. We demonstrate that the tunneling time of an electron from an atom is close to zero within our experimental accuracy. Our study represents a straightforward approach toward attosecond time-resolved imaging of electron motion in atoms and molecules. 

/pdf/full-experimental-determination-of-tunneling-time-with-3h84a646.pdf

Full experimental determination of tunneling time with attosecond-scale streaking method

The authors have identified dynamically enhanced Autler-Townes splittings in the photoelectron energy spectrum of atoms in a XUV-pump and XUV-probe protocol.

Enhancing Autler-Townes splittings by ultrafast XUV pulses

Optical bistability in aperiodic multilayer composed of graphene and Thue-Morse lattices

The temporal structures of ultrashort optical pulses are key to the study of ultrafast phenomena. The authors demonstrate an all-optical method for time-domain characterization of an ultrashort optical pulse. Utilizing nonionizing lasers to induce the ac Stark effect in a helium atom, by interrogating the quasienergies of the laser-dressed atom using extreme-ultraviolet attosecond pulses, the waveform of an optical pulse can be precisely diagnosed. Using a nonionizing laser minimizes plasma-induced pulse distortion, and provides a complementary detection scheme for an effective and reliable ``optical oscilloscope''.

Perturbed ac Stark Effect for Attosecond Optical-Waveform Sampling

Interferometry, a key technique in modern precision measurements, has been used for length measurement in engineering metrology and astronomy. An analogous time-domain interferometric technique would represent a significant complement to spatial domain applications and require the manipulation of interference on extreme time and energy scales. Here, we report an all-optical interferometer using laser-driven high order harmonics as attosecond temporal slits. By controlling the phase of the temporal slits with an external field, a time domain interferometer that preserves both attosecond temporal resolution and hundreds of meV energy resolution is implemented. We apply this exceptional temporal resolution to reconstruct the waveform of an arbitrarily polarized optical pulse, and utilize the provided energy resolution to interrogate the abnormal character of the transition dipole near the Cooper minimum in argon. This novel attosecond interferometry paves the way for high precision measurements in the time-energy domain using all-optical approaches.

/pdf/all-optical-attosecond-time-domain-interferometry-388dftvgx0.pdf

All-optical attosecond time domain interferometry.

When an atom or molecule is illuminated by a short laser pulse, the intense electric field couples different electronic states promptly and alters the final products. Detecting such interaction processes in a time-resolved way requires techniques with unprecedented time resolution and is important for applications such as coherent chemical reaction control. Here by solving the time-dependent Schr\"odinger equation, we demonstrate that the time evolution of complex amplitudes of bound electronic states during the interaction with an optical laser pulse can be extracted with high accuracy using attosecond streaking.

Probing laser-driven bound-state dynamics using attosecond streaking spectroscopy

Extreme ultraviolet (EUV) transient absorption spectrum of helium dressed by a moderately intense infrared laser pulse is investigated. Strategies for correct retrieval of the time-dependent quasi-energies of helium with excitation energies covering both singly and doubly excited states are proposed. For long-lived singly excited states, the profound hyperbolic structures due to long lasting dipole can be diminished by convoluting the transient absorption spectrogram with a spectral window, allowing the time-dependent quasi-energies close to 1s2p resonance to be correctly mapped out. For short-lived doubly excited states near 2s2p resonance, the radiation dipole decays rapidly due to autoionization and the transient absorption spectrogram already recovers the main structure of quasi-energies without the convolution operation. The quantum simulation indicates that the convolution operation controls the effective decay speed of the dipole moment, which effectively builds up an instant probe that is essential for mapping time dependent quasi-energies of laser dressed systems.

Mapping time-dependent quasi-energies of laser dressed helium.

Interferometry, as the key technique in modern precision measurements, has been used for length diagnosis in the fields of engineering metrology and astronomy. Analogous interferometric technique for time domain precision measurement is a significant complement to the spatial domain applications and requires the manipulation of the interference on the extreme time and energy scale. Here we present an all-optical interferometry for high precision measurement with attosecond temporal and hundreds of meV energy resolution. The interferometer is based on laser driven high order harmonics which provide a robust sequence of attosecond temporal slits. As applications, we reconstruct the waveform of an arbitrarily polarized optical pulse using the time resolving capability, and capture the abnormal character of the transition dipole near a Cooper minimum in argon using the energy resolution provided. This novel attosecond interferometry opens the possibility for high precision measurement in time energy domain using an all-optical approach.

Wave-front controlled attosecond time domain interferometry

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