High-intensity Optical Fields Research Articles

Photosoliton modes induced by cross–phase modulation in optical media conveying bright-dark vector solitons

It is well known that high-intensity optical fields with soliton features, both bright and dark solitons, can trap and reshape photon fields of lower intensities into optical solitons by means of cross-phase modulation in optical materials with Kerr nonlinearity. In general the induced photon fields are not arbitrarily given, they are well-defined quantum states |ℓ, m〉 of a discrete spectrum which we refer to as quantum photosoliton modes. In this study we propose a scheme for the generation of quantum photosoliton states, by considering linear photon field propagating coupled to a vector high-intensity optical field with coexisting mutually transparent bright and dark solitons. It is found that features of the probe spectrum, specifically the nature, number and degeneracy of the associate quantum modes, depend on the strengths of couplings of the linear photon field to the bright and dark soliton components of the vector-soliton pump. The competition created by these simultaneous couplings leads to a rich variety of photosoliton modes, including modes that are replica of the soliton pump or translation modes of the two components of the vector-soliton pump. For the context where the optical pump consists of a single bright and a single dark soliton structures, it is found that photosoliton modes are all multiple degenerate in addition to being replica of some other modes. However, when the optical pump is a periodic multiplex of bright-dark solitons, photosoliton quantum states retain their identities (i.e. become non degenerate) but some of them are still either replica or translation modes of components of the optical pump.

Localization-limited exciton oscillator strength in colloidal CdSe nanoplatelets revealed by the optically induced stark effect

2D materials are considered for applications that require strong light-matter interaction because of the apparently giant oscillator strength of the exciton transitions in the absorbance spectrum. Nevertheless, the effective oscillator strengths of these transitions have been scarcely reported, nor is there a consistent interpretation of the obtained values. Here, we analyse the transition dipole moment and the ensuing oscillator strength of the exciton transition in 2D CdSe nanoplatelets by means of the optically induced Stark effect (OSE). Intriguingly, we find that the exciton absorption line reacts to a high intensity optical field as a transition with an oscillator strength FStark that is 50 times smaller than expected based on the linear absorption coefficient. We propose that the pronounced exciton absorption line should be seen as the sum of multiple, low oscillator strength transitions, rather than a single high oscillator strength one, a feat we assign to strong exciton center-of-mass localization. Within the quantum mechanical description of excitons, this 50-fold difference between both oscillator strengths corresponds to the ratio between the coherence area of the exciton’s center of mass and the total area, which yields a coherence area of a mere 6.1 nm2. Since we find that the coherence area increases with reducing temperature, we conclude that thermal effects, related to lattice vibrations, contribute to exciton localization. In further support of this localization model, we show that FStark is independent of the nanoplatelet area, correctly predicts the radiative lifetime, and lines up for strongly confined quantum dot systems.

Few-cycle optical-field-induced photoemission from biased surfaces: An exact quantum theory

Photoemission driven by ultrafast optical fields enables spatiotemporal control of electron motion with extremely high precision. Here, we present a quantum model for ultrafast photoelectron emission from a dc-biased surface induced by laser pulses of arbitrary duration, ranging from subcycle to continuous wave, by solving the time-dependent Schr\"odinger equation exactly. The single formulation is valid from photon-driven electron emission in low intensity optical fields to field-driven emission in high intensity optical fields. We find the emitted charge per pulse oscillatorily increases with pulse repetition rate, due to varying coherent interaction of neighboring laser pulses. For a well-separated single pulse, our results recover the experimentally observed vanishing carrier-envelope phase sensitivity in the optical-field regime. We also find that applying a large dc field to the photoemitter is able to greatly enhance the photoemission current and in the meantime substantially shorten the current pulse.

Soliton lattices originating from excitons interacting with high-intensity fields in finite molecular crystals

The effects of long-range intermolecular interactions on characteristic features of soliton bound states, consisting of localized excitons and polaritons in molecular crystals interacting with a high-intensity optical field, are investigated. Analytical solutions to the resulting modified nonlinear Schr\"odinger equation are obtained in terms of elliptic-type bright- and dark-soliton structures, which are assumed to correspond to periodic trains of pulse solitons and kink solitons respectively. Long-range intermolecular interactions are shown to renormalize the exciton-polariton interaction strength, hence generating a significantly huge increase in amplitudes of the bright solitons but a decrease in amplitudes of dark solitons. Results suggest that long-range intermolecular interactions hold relevant roles, both qualitatively and quantitatively, in the formation of amplitude and phase modulated strongly nonlinear exciton-polariton solitary-wave patterns, as well as in the energy transfer along molecular crystals interacting with high-intensity optical fields.

Femtosecond Laser‐Induced Formation of Gold‐Rich Nanoalloys from the Aqueous Mixture of Gold‐Silver Ions

The synthesis of gold‐silver (AuAg) nanoalloys of various compositions has been performed by direct irradiation of highly intense femtosecond laser pulse in the presence of polyvinylpyrrolidone (PVP). The mixture of Au and Ag ions of low concentration was simply introduced into a glass vial and subjected to femtosecond laser pulses for several minutes. The AuAg nanoalloys of 2‐3 nm with reasonably narrow size distribution were formed, and the position of the surface plasmon resonance (SPR) increased monotonically with an increase in the gold molar fraction in the ion solutions. The high resolution transmission electron microscope (HRTEM) images exhibited the absence of core‐shell structures, and the energy dispersive X‐ray spectroscopy (EDX) analysis confirmed that the particles were Au‐rich alloys even for the samples with large fraction of Ag+ ions fed in the solution mixture. The formation mechanism of the alloy nanoparticles in the high intensity optical field was also discussed.

Population inversion in the recombination of optically-ionized plasmas

The possibility of using a high-intensity optical field in conjunction with a gas target to produce a highly ionized plasma filament suitable for recombination XUV lasers in both transient and quasi-steady-state regimes is examined. A distinction is made between low Z ions which can be stripped to the desired ionization state at nonrelativistic intensities and higher Z ions which require relativistic intensities to produce the desired ionization. In the nonrelativistic case (E/sub i/<500 eV), it is shown that electron thermal conduction is extremely effective in cooling approximately 10- mu m diameter filaments imbedded in cold background plasma. In the relativistic case, self-focusing of the ionizing laser radiation may lead to a very small diameter electron-cavitated filaments which will undergo a space-charge-driven expansion (Coulomb explosion) on the time scale of an ion plasma period, resulting in the emission of extremely high currents of moderate energy (E approximately=1/8 Zm/sub e/ c/sup 2/) ions. The implications of such filamentation for the scaling of the present type of recombination laser to short wavelengths are discussed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Three-frequency interactions of high-intensity light waves in media with quadratic and cubic nonlinearities

It is shown that in phase-matched parametric processes and second harmonic generation in noncentrosymmetric media in high-intensity optical fields, an important part may be played by the cubic nonlinearity of the medium that results in disruption of the optimal phase relationship between the interacting waves. As a result, the maximum efficiency of nonlinear conversion of the optical frequencies of the plane waves differ from 100% and the spatial beat frequency also varies. Frequency doubling is particularly sensitive to the influence of the cubic nonlinearity. Numerical calculations of the efficiency of nonlinear processes are given for KPD and LiNbO3 crystals.

Spatially resolved infrared absorption measurements: application of an optical Stark effect

A new technique for obtaining point measurements in gases by infrared absorption is described. The method is based on a recently discovered quadratic Stark effect on vibrational and rotational molecular transitions that is induced by high-intensity optical fields. Detectivity calculations for CO, an important combustion gas, are presented together with a proof-of-principle demonstration of Stark-induced absorption in ammonia.

Observation of an Optical Stark Effect on Vibrational and Rotational Transitions

Vibrational and rotational Raman transitions are shown to be shifted to lower frequencies in the presence of a nonresonant, high-intensity optical field. Experimental results using coherent anti-Stokes Raman spectroscopy for hydrogen and nitrogen are presented. The magnitude of the observed shift is in agreement with a calculation which considers the coupling of the optical field to the internuclear separation via the molecular electronic polarizability. The magnitude of the shift is predicted to be proportional to the optical field intensity.