Applications In Strong-field Physics Research Articles

Signatures of linear Breit-Wheeler pair production in polarized γγ collisions

The polarization characteristics of the linear Breit-Wheeler (LBW) pair-production process in polarized $\ensuremath{\gamma}\ensuremath{\gamma}$ colliders have been investigated via our developed spin-resolved binary collision simulation method. We find that the polarization of $\ensuremath{\gamma}$-photons modifies the kinematics of scattering particles and induces the correlated energy-angle shift of LBW pairs, and the latter's polarization characteristic depends on the helicity configures of scattering particles. We confirm that the polarized $\ensuremath{\gamma}\ensuremath{\gamma}$ collider with an asymmetric setup can be performed with currently achievable laser-driven high-density x rays and high-brilliance $\ensuremath{\gamma}$-photon beams to produce abundant polarized LBW pairs, fulfilling the detection power of polarimetries. Our method and results on the polarized LBW process have plenty of significant applications in strong-field physics, high-energy physics and astrophysics, such as calibrating and monitoring the polarized $\ensuremath{\gamma}\ensuremath{\gamma}$ collider and challenging the current understanding of astrophysical objects through enhancing the opacity of $\ensuremath{\gamma}$-photons to exacerbate the inconsistency between some observations and standard models.

Mid-Infrared Few-Cycle Pulse Generation and Amplification

In the past decade, mid-infrared (MIR) few-cycle lasers have attracted remarkable research efforts for their applications in strong-field physics, MIR spectroscopy, and bio-medical research. Here we present a review of MIR few-cycle pulse generation and amplification in the wavelength range spanning from 2 to ~20 μm. In the first section, a brief introduction on the importance of MIR ultrafast lasers and the corresponding methods of MIR few-cycle pulse generation is provided. In the second section, different nonlinear crystals including emerging non-oxide crystals, such as CdSiP2, ZnGeP2, GaSe, LiGaS2, and BaGa4Se7, as well as new periodically poled crystals such as OP-GaAs and OP-GaP are reviewed. Subsequently, in the third section, the various techniques for MIR few-cycle pulse generation and amplification including optical parametric amplification, optical parametric chirped-pulse amplification, and intra-pulse difference-frequency generation with all sorts of designs, pumped by miscellaneous lasers, and with various MIR output specifications in terms of pulse energy, average power, and pulse width are reviewed. In addition, high-energy MIR single-cycle pulses are ideal tools for isolated attosecond pulse generation, electron dynamic investigation, and tunneling ionization harness. Thus, in the fourth section, examples of state-of-the-art work in the field of MIR single-cycle pulse generation are reviewed and discussed. In the last section, prospects for MIR few-cycle lasers in strong-field physics, high-fidelity molecule detection, and cold tissue ablation applications are provided.

All-optical sampling of few-cycle infrared pulses using tunneling in a solid

Recent developments in ultrafast laser technology have resulted in novel few-cycle sources in the mid-infrared. Accurately characterizing the time-dependent intensities and electric field waveforms of such laser pulses is essential to their applications in strong-field physics and attosecond pulse generation, but this remains a challenge. Recently, it was shown that tunnel ionization can provide an ultrafast temporal “gate” for characterizing high-energy few-cycle laser waveforms capable of ionizing air. Here, we show that tunneling and multiphoton excitation in a dielectric solid can provide a means to measure lower-energy and longer-wavelength pulses, and we apply the technique to characterize microjoule-level near- and mid-infrared pulses. The method lends itself to both all-optical and on-chip detection of laser waveforms, as well as single-shot detection geometries.

Single and double ionization of helium in intense bicircular laser fields

The single and double ionization of helium in intense 2ω:ω bicircular laser fields is analyzed with a classical ensemble method. A high-performance computational cluster is utilized to generate high-resolution data and to probe low-rate events. It is found that both counter- and co-rotating fields support rescattering and non-sequential double ionization (NSDI). The effects of scattering off of the ionic core are evident in the single-ionization energy spectra, and the role of the Coulomb potential is clearly visible in the electron momentum distributions. The dynamics of the double ionization processes are investigated by analyzing ionization phase, electron energy and momenta, and rescattering timing. For corotating fields, rescattering is only possible for ionization near the field minimum, placing limits on the efficiency of NSDI. For counterrotating fields, the electron trajectories that contribute to rescattering are found to fall into only three categories (looping, triangular, or a combination of the two), and the contributions from these trajectories is distinguishable in the resulting double-ion electron momentum distributions. This study explores the full range of field amplitude ratios that contribute to the dynamics of rescattering, and therefore provides an overall view of the limitations of using bicircular fields for applications in strong-field physics.

Tunable, few-cycle, CEP-stable mid-IR optical parametric amplifier for strong field applications

We present a robust, three-stage optical parametric amplifier driven by a Ti:Sapphire ultrafast laser system that implements passive carrier-envelope phase stabilization and directly generates five-cycle mid-IR pulses by dispersion compensation. The source is based on potassium titanyl arsenate crystals and exploits intra-pulse difference-frequency seed generation in the mid-IR. This source will be particularly suited for applications in strong-field physics, such as high order harmonic generation and photoelectron spectroscopy.

Atomic and Molecular Processes in a Strong Bicircular Laser Field

With the development of intense femtosecond laser sources it has become possible to study atomic and molecular processes on their own subfemtosecond time scale. Table-top setups are available that generate intense coherent radiation in the extreme ultraviolet and soft-X-ray regime which have various applications in strong-field physics and attoscience. More recently, the emphasis is moving from the generation of linearly polarized pulses using a linearly polarized driving field to the generation of more complicated elliptically polarized polychromatic ultrashort pulses. The transverse electromagnetic field oscillates in a plane perpendicular to its propagation direction. Therefore, the two dimensions of field polarization plane are available for manipulation and tailoring of these ultrashort pulses. We present a field that allows such a tailoring, the so-called bicircular field. This field is the superposition of two circularly polarized fields with different frequencies that rotate in the same plane in opposite directions. We present results for two processes in a bicircular field: High-order harmonic generation and above-threshold ionization. For a wide range of laser field intensities, we compare high-order harmonic spectra generated by bicircular fields with the spectra generated by a linearly polarized laser field. We also investigate a possibility of introducing spin into attoscience with spin-polarized electrons produced in high-order above-threshold ionization by a bicircular field.

Relativistic single-cycle tunable infrared pulses generated from a tailored plasma density structure

The availability of intense, ultrashort coherent radiation sources in the infrared region of the spectrum is enabling the generation of attosecond X-ray pulses via high harmonic generation, pump-probe experiments in the "molecular fingerprint" region and opening up the area of relativistic-infrared nonlinear optics of plasmas. These applications would benefit from multi-millijoule single-cycle pulses in the mid to long wavelength infrared (LW-IR) region. Here we present a new scheme capable of producing tunable relativistically intense, single-cycle infrared pulses from 5-14$\mu$m with a 1.7% conversion efficiency based on a photon frequency downshifting scheme that uses a tailored plasma density structure. The carrier-envelope phase (CEP) of the LW-IR pulse is locked to that of the drive laser to within a few percent. Such a versatile tunable IR source may meet the demands of many cutting-edge applications in strong-field physics and greatly promote their development.

Weak-field versus Born-Oppenheimer asymptotics in the theory of tunneling ionization of molecules

The weak-field asymptotic theory (WFAT) and the Born-Oppenheimer approximation (BOA) provide two general approaches to the theory of tunneling ionization of molecules enabling one to treat the internuclear motion quantum mechanically on an equal footing with the electronic motion. The approaches are based on the asymptotic expansions in field strength and electron-to-nuclear mass ratio, respectively. The problem is that they yield quite different formulas for the ionization rate, which causes an ambiguity for applications in strong-field physics. A recent experimental observation of the isotope effect in tunneling ionization of hydrogen molecules [X. Wang et al., Phys. Rev. Lett. 117, 083003 (2016)] motivates us to address this theoretical ambiguity. The WFAT and BOA have different but overlapping regions of applicability in the field strength---nuclear mass plane. We link the approaches analytically, in particular showing that the rate formulas in fact coincide in the overlap region where they both apply, and compare their predictions quantitatively.

Integral-equation approach to the weak-field asymptotic theory of tunneling ionization

An integral equation approach to the weak-field asymptotic theory (WFAT) of tunneling ionization is developed. An integral representation for the exact partial amplitudes of ionization into parabolic channels is derived. The WFAT expansion for the ionization rate follows immediately from this relation. Integral representations for the coefficients in the expansion are obtained. The integrals accumulate where the ionizing orbital has large amplitude and are not sensitive to its behavior in the asymptotic region. Hence, these formulas enable one to reliably calculate the WFAT coefficients even if the orbital is represented by an expansion in Gaussian basis, as is usually the case in standard software packages for electronic structure calculations. This development is expected to greatly simplify the implementation of the WFAT for polyatomic molecules, and thus facilitate its growing applications in strong-field physics.

Generalized velocity-gauge form of the light-matter interaction Hamiltonian beyond the dipole approximation

The exact velocity-gauge minimal-coupling Hamiltonian describing the laser-matter interaction is transformed into another form by means of a series of gauge transformations. The Hamiltonian corresponding to this point of view is valid for an arbitrary time- and space-dependent laser field, also known as a nondipole field. In effect, the Hamiltonian represents a generalization of the original velocity-gauge minimal-coupling Hamiltonian in the sense that the particle's (classical) velocity in the laser propagation direction is also explicitly accounted for by a new operator term. Imposing the so-called long-wavelength approximation (LWA) on the field, i.e., assuming the laser wavelength being much larger than the extent of the atomic system, the spatial dependence of the field can be neglected and the interaction Hamiltonian reduces to a simpler form. Nevertheless, the resulting LWA Hamiltonian includes the effect of the magnetic-field component of the laser, which is in clear contrast with the usual dipole approximation Hamiltonian derived by imposing the LWA directly on the initial velocity-gauge minimal-coupling Hamiltonian. As such, the weak-field condition necessary to justify neglecting the magnetic field, and the LWA condition, can be considered independently in this formalism, making it an attractive alternative for a broad range of applications in strong-field physics. We demonstrate that, from a numerical perspective, this form of the light-matter interaction is advantageous compared to its standard velocity-gauge counterpart as it gives rise to faster convergence properties when describing ionization dynamics in superintense fields beyond the dipole approximation.

Weak-field asymptotic theory of tunneling ionization: benchmark analytical results for two-electron atoms

The recently developed many-electron weak-field asymptotic theory of tunneling ionization of atoms and molecules in an external static electric field (Tolstikhin et al 2014, Phys. Rev. A 89, 013421) is extended to the first-order terms in the asymptotic expansion in field. To highlight the results, here we present a simple analytical formula giving the rate of tunneling ionization of two-electron atoms H− and He. Comparison with fully-correlated ab initio calculations available for these systems shows that the first-order theory works quantitatively in a wide range of fields up to the onset of over-the-barrier ionization and hence is expected to find numerous applications in strong-field physics.

Compression of TW class laser pulses in a planar hollow waveguide for applications in strong-field physics

We demonstrate pulse post-compression of a TW class chirped pulse amplification laser employing a gas-filled planar hollow waveguide. A waveguide throughput of 80% is achieved for 50 mJ input pulse energy. Good focusability is found and after compression with chirped mirrors a pulse duration of sub-15 fs is measured in the beam center. Whereas a total energy efficiency of ≈70% should be achievable, our post-compressor currently delivers 20 mJ output pulse energy (≈40% efficiency), mostly limited by apertures of chirped mirrors and vacuum windows. The viability of the planar hollow waveguide compression scheme for applications in strong-field physics is demonstrated by generating high-order harmonics in a pulsed Ar gas cell.

Tunable 1.6–2 μm near infrared few-cycle pulse generation by filamentation

We present results on tunable few-cycle laser pulses generated in the near infrared region obtained by filamentation in a krypton cell combined with group velocity dispersion compensation in fused silica. We obtain a spectral broadening of a factor ∼2–3 over the entire spectral domain studied. The central wavelength is tuned from 1.6 μm to 2 μm via an optical parametric amplification source. In optimum experimental conditions, where the input central wavelength is set to 1.7 μm, 1.8 μm, and 1.9 μm, the incident power to the gas cell exceeded the critical power of Krypton by a factor of ∼4 and the achieved spectral broadening covered ∼300 nm. Using group velocity compensation in bulk fused silica, we obtain near infrared output pulses as short as 2–3 optical cycles with 200 μJ energy per pulse. This near infrared filamentation tunable few-cycle pulse source is an important achievement for strong field physics applications such as attoscience, where wavelength scaling has an important effect.

Generation of high-fidelity few-cycle pulses at 21 μm via cross-polarized wave generation

We demonstrate the generation of temporally clean few-cycle pulses at 2.1 μm by shortening of 6-optical-cycle pulses via cross-polarized wave (XPW) generation in BaF(2), CaF(2) and CVD-Diamond crystals. By combining spectra and single-shot third-order intensity cross-correlation traces in a novel Bayesian pulse retrieval technique, we measured pulse durations of 20 fs, corresponding to 2.8 optical cycles. Our results show that XPW generation in the infrared could provide a high-fidelity source of few-cycle pulses for strong-field physics applications. It could also serve as an injector for high-peak power ultrafast mid-IR wavelength parametric amplifiers.

Generalizing higher-order Bessel-Gauss beams: analytical description and demonstration

We report on a novel class of higher-order Bessel-Gauss beams in which the well-known Bessel-Gauss beam is the fundamental mode and the azimuthally symmetric Laguerre-Gaussian beams are special cases. We find these higher-order Bessel-Gauss beams by superimposing decentered Hermite-Gaussian beams. We show analytically and experimentally that these higher-order Bessel-Gauss beams resemble higher-order eigenmodes of optical resonators consisting of aspheric mirrors. This work is relevant for the many applications of Bessel-Gauss beams in particular the more recently proposed high-intensity Bessel-Gauss enhancement cavities for strong-field physics applications.