High-intensity Optical Pulses Research Articles

Design and performance of a repetition rate controllable and wavelength tunable L + U-band actively mode-locked erbium fiber laser

Mode-locked lasers draw great interest in the laser research field due to their diverse applications and easy achievement of ultra-short and high-intensity optical pulse trains. Actively mode-locked (AML) fiber lasers are more flexible compared to passively mode-locked fiber lasers owing to their ability to control repetition rates and pulse intervals electronically. The design and development of wavelength tunable AML fiber lasers operating beyond the C-band have attracted great attention from the research community. In this paper, a repetition rate controllable and wavelength tunable L+U-band AML erbium fiber laser (AML-EFL) based on a single standard 1480 nm pump laser and Mach–Zehnder modulator (MZM) as an intra-cavity intensity modulator is demonstrated using numerical simulation. The AML-EFL is created using a MZM driven by electrical Gaussian pulses and a tunable optical bandpass filter that is used to tune the laser cavity at any wavelength in the 1565–1645 nm range. The repetition rate of the AML-EFL is controlled from 500 kHz to 1 GHz. Trains of pulses with pulse widths of 103 ns and 50 ps and pulse intervals of 2 µs and 1 ns are successfully generated at repetition rates of 500 kHz and 1 GHz, respectively. A pulse energy of 626 nJ is obtained for a 500 kHz repetition rate. Signal-to-noise ratios of 30 and 32 dB are observed for trains of mode-locked pulses with repetition rates of 500 kHz and 1 GHz, respectively. In addition, slope efficiencies of around 66% for an L-band wavelength of 1582.1 nm and 30% for a U-band wavelength of 1633.6 nm are obtained considering the optimized parameters.

Ultrafast Adsorbate Excitation Probed with Subpicosecond-Resolution X-Ray Absorption Spectroscopy.

We use a pump-probe scheme to measure the time evolution of the C K-edge x-ray absorption spectrum from CO/Ru(0001) after excitation by an ultrashort high-intensity optical laser pulse. Because of the short duration of the x-ray probe pulse and precise control of the pulse delay, the excitation-induced dynamics during the first picosecond after the pump can be resolved with unprecedented time resolution. By comparing with density functional theory spectrum calculations, we find high excitation of the internal stretch and frustrated rotation modes occurring within 200fs of laser excitation, as well as thermalization of the system in the picosecond regime. The ∼100 fs initial excitation of these CO vibrational modes is not readily rationalized by traditional theories of nonadiabatic coupling of adsorbates to metal surfaces, e.g., electronic frictions based on first order electron-phonon coupling or transient population of adsorbate resonances. We suggest that coupling of the adsorbate to nonthermalized electron-hole pairs is responsible for the ultrafast initial excitation of the modes.

Generation of high-intensity ultra-short optical pulses: 2018 Nobel Prize Winners in Physics Gerard Mourou and Donna Strickland

In the early 1980s, French physicist G. Mourou and his Canadian collaborator D. Strickland solved the problem of power drop by dispersing in time and space the processes of amplification and compression: a method of obtaining super-powerful chirped laser pulses (CPA – chirped pulse amplification). The paper presents brief biographical references to Mourou and Strickland. The 2018 Nobel Prize in Physics was awarded “for groundbreaking inventions in the field of laser physics”: Artur Isidorovich Ashkin (Ashkinazi, born 02.09.1922, USA), half of the prize “for the optical tweezers and their application to biological systems”; Gerard Albert Mourou and Donna Theo Strickland (became the third woman to be awarded the Nobel Prize in Physics) (quarterly premium) “for their method of generating high-intensity, ultra-short optical pulses”. Since that time all lasers have been built on a new principle: after the amplifiers place a compressor from diffraction bars. Instead of simply amplifying the pulse, it is first spread out on spectral components spread over time, then they are amplified separately, then again assembled into a single pulse. At each point in time, only a fraction of the pulse is amplified, not the entire pulse, allowing for a much higher peak intensity of laser light flow.

Ultrafast band-gap renormalization and build-up of optical gain in monolayer MoTe2

The dynamics of band-gap renormalization and gain build-up in monolayer MoTe$_2$ is investigated by evaluating the non-equilibrium Dirac-Bloch equations with the incoherent carrier-carrier and carrier-phonon scattering treated via quantum-Boltzmann type scattering equations. For the case where an approximately $300$ fs-long high intensity optical pulse generates charge-carrier densities in the gain regime, the strong Coulomb coupling leads to a relaxation of excited carriers on a few fs time scale. The pump-pulse generation of excited carriers induces a large band-gap renormalization during the time scale of the pulse. Efficient phonon coupling leads to a subsequent carrier thermalization within a few ps, which defines the time scale for the optical gain build-up energetically close to the low-density exciton resonance.

An experimental platform using high-power, high-intensity optical lasers with the hard X-ray free-electron laser at SACLA.

An experimental platform using X-ray free-electron laser (XFEL) pulses with high-intensity optical laser pulses is open for early users' experiments at the SACLA XFEL facility after completion of the commissioning. The combination of the hard XFEL and the high-intensity laser provides capabilities to open new frontiers of laser-based high-energy-density science. During the commissioning phase, characterization of the XFEL and the laser at the platform has been carried out for the combinative utilization as well as the development of instruments and basic diagnostics for user experiments. An overview of the commissioning and the current capabilities of the experimental platform is presented.

A new bound on polymer quantization via an opto-mechanical setup

The existence of a minimal measurable length as a characteristic length in the Planck scale is one of the main features of quantum gravity and has been widely explored in the context. Various different deformations of spacetime have been employed successfully for the purpose. However, polymer quantization approach is a relatively new and dynamic field towards the quantum gravity phenomenology, which emerges from the symmetric sector of the loop quantum gravity. In this article, we extend the standard ideas of polymer quantization to find a new and tighter bound on the polymer deformation parameter. Our protocol relies on an opto-mechanical experimental setup that was originally proposed to explore some interesting phenomena by embedding the minimal length into the standard canonical commutation relation. We extend this scheme to probe the polymer length deformed canonical commutation relation of the center of mass mode of a mechanical oscillator with a mass around the Planck scale. The method utilizes the novelty of exchanging the relevant mechanical information with a high intensity optical pulse inside an optical cavity. We also demonstrate that our proposal is within the reach of the current technologies and, thus, it could uncover a decent realization of quantum gravitational phenomena thorough a simple table-top experiment.

Polypyrrole-Coated Perfluorocarbon Nanoemulsions as a Sono-Photoacoustic Contrast Agent.

A new contrast agent for combined photoacoustic and ultrasound imaging is presented. It has a liquid perfluorocarbon (PFC) core of about 250 nm diameter coated by a 30 nm thin polypyrrole (PPy) doped polymer shell emulsion that represents a broadband absorber covering the visible and near-infrared ranges (peak optical extinction at 1050 nm). When exposed to a sufficiently high intensity optical or acoustic pulse, the droplets vaporize to form microbubbles providing a strong increase in imaging sensitivity and specificity. The threshold for contrast agent activation can further drastically be reduced by up to 2 orders of magnitude if simultaneously exposing them with optical and acoustic pulses. The selection of PFC core liquids with low boiling points (i.e., perfluorohexane (56 °C), perfluoropentane (29 °C), and perfluorobutane (-2 °C)) facilitates activation and reduces the activation threshold of PPy-coated emulsion contrast agents to levels well within clinical safety limits (as low as 0.2 MPa at 1 mJ/cm2). Finally, the potential use of these nanoemulsions as a contrast agent is demonstrated in a series of phantom imaging studies.

Non-equilibrium ultrashort pulse generation strategies in VECSELs

Vertical external cavity surface emitting lasers are ideal testbeds for studying nonlinear many-body systems driven far from equilibrium. The classical laser gain picture fails, however, when a high peak intensity optical pulse of duration shorter than the intrinsic carrier scattering time interacts with electrons in the conduction and holes in the valence band, and the non-equilibrium carrier distributions cannot recover during the presence of the exciting pulse. We present the optimization of ultrashort mode-locked pulses in a vertical external cavity surface emitting laser cavity with a saturable absorber mirror by modelling non-equilibrium quantum dynamics of the electron-hole excitations in the semiconductor quantum-well gain and absorber medium via the semiconductor Bloch equations and treating the field propagation at the level of Maxwell’s wave equation. We introduce a systematic design that predicts the generation of stable mode-locked pulses of duration less than twenty femtoseconds. This factor of five improvement is of interest for mode-locking and ultrafast semiconductor dynamics applications.

Design and performance analysis of optical microring resonator based J − K flip-flop

A simple design of optically pumped J−K flip-flop using a two GaAs–AlGaAs based optical microring resonator (OMRR) is introduced. The nonlinear property of a GaAs–AlGaAs based ring resonator is explored to use it as an optical switch. A high-intensity optical pump pulse is used for the switching operation of the OMRR. A theoretical model of the proposed J–K flip-flop circuit is developed by applying the delay line signal process approach. Z-domain model of the circuit is explored to analyze its performance. Various performance parameters—such as contrast ratio (CR), extinction ratio (ER), on-off ratio (Ton-off), and amplitude modulation (AM)—are determined from the simulation results obtained on MATLAB. The optimum values of CR, ER, Ton-off, and AM are calculated for the proposed model, which confirms the feasibility of the flip-flop.

Radiative trapping in intense laser beams

The dynamics of electrons in counter-propagating, circularly polarized laser beams are shown to exhibit attractors whose ability to trap particles depends on the ratio of the beam intensities and a single parameter describing radiation reaction. Analytical expressions are found for the underlying limit cycles and the parameter range in which they are stable. In high-intensity optical pulses, where radiation reaction strongly modifies the trajectories, the production of collimated gamma-rays and the initiation of non-linear cascades of electron–positron pairs can be optimized by a suitable choice of the intensity ratio.

Metastable electronic states and nonlinear response for high-intensity optical pulses: erratum

Metastable electronic states and nonlinear response for high-intensity optical pulses: erratum

Numerical modeling considerations for an applied nonlinear Schrödinger equation.

A model for nonlinear optical propagation is cast into a split-step numerical framework via a variable stencil-size Crank-Nicolson finite-difference method for the linear step and a choice of two different nonlinear integration schemes for the nonlinear step. The model includes Kerr, Raman scattering, and ionization effects (as well as linear and nonlinear shock, diffraction, and dispersion). We demonstrate the practical importance of numerical effects when interpreting computational studies of high-intensity optical pulse propagation in physical materials. Examples demonstrate the significant error that can arise in discrete, limited precision implementations as one attempts to improve practical operator accuracy through increased operator support size and sampling frequency. We also demonstrate the effect of the method used to obtain the finite-difference operator coefficients defining the equations ultimately used in the discrete model. Smooth, plausible, but incorrect solutions may result from these numerical effects. This implies the necessity of a complete, precise description of all numerical methods when reporting results of computational physics investigations in order to ensure proper interpretation and reproducibility.

Metastable electronic states and nonlinear response for high-intensity optical pulses

In this paper we propose and demonstrate that the ultrafast nonlinear optical response of atoms may be accurately calculated in terms of metastable states obtained as solutions of the stationary Schrodinger equation including the quasi-static applied electric field. We first develop the approach in the context of an exactly soluble one-dimensional atomic model with delta-function potential, as this allows comparison between the exact ultrafast nonlinear optical response and our approximate approach, both in adiabatic approximation and beyond. These ideas are then applied to a three-dimensional hydrogen-like atom and yield similar excellent agreement between the metastable state approach and simulations of the Schrodinger equation for off-resonant excitation. Finally, our approach yields a model for the ultrafast nonlinear optical response with no free parameters. It can potentially replace the light–matter interaction treatment currently used in optical filamentation, and we present a numerical example of application to femtosecond pulse propagation.

Influence of many-body interactions during the ionization of gases by short intense optical pulses.

The excitation of atomic gases by short high-intensity optical pulses leads to significant electron ionization. In dilute systems, the generated distribution of ionized electrons is highly anisotropic, reflecting the quantum mechanical properties of the atomic states involved in the many photon transitions. For higher atomic densities, the Coulomb interaction in the electron-ion system leads to the development of an isotropic electron plasma. To study the ionization process in the presence of the many-body interaction, a fully microscopic model is developed that combines a generalized version of the optical Bloch equations describing the optical excitation with a microscopic description of the many-body interactions. The numerical evaluation shows that the Coulomb interaction significantly modifies the distribution anisotropy already during the excitation process. Whereas a reduced anisotropy is still present after the pulse for low ionization degrees and pressures, it is completely absent for elevated gas densities. An ionization degree is predicted that is significantly enhanced by the many-body interactions.

Characteristics of SBS hundreds picosecond pulse compression and influence of energy on pulse stability in FC-770

A prominent application of stimulated Brillouin scattering (SBS) is pulse sequence compression for obtaining high power and high intensity optical pulse. The characteristics and the stability of the compressed Stokes pulse were investigated using compact two-cell SBS phase-conjugation mirror. FC-770, having low absorption and high breakdown threshold at the wavelength of 1064nm, was used as the SBS medium. Both the Stokes pulse width and the reflectivity were measured as a function of incident pump energy. In our work, a 8ns laser pulse of 1064nm generated by Q-switched Nd: YAG was optimally compressed to 116ps for pump energies of 310mJ, while the energy reflectivity is above 85%. The shortest pulses width obtained exhibit 99ps duration. The stability of the temporal pulse widths after compression was investigated statistically. For non-optimal pump energies pulse widths become larger and the stability is found to decrease.

Nonlinear Photoemission Properties of Carbon Nanotubes: Behavior of High-Density Excitons in One Dimension

This paper describes nonlinear photoemission properties of carbon nanotubes both in terms of experiment and theory. In experiments, we performed photoluminescence excitation spectroscopy where carbon nanotubes were excited with high-intensity optical pulses. We found that, as the intensity of excitation pulses increases, all the photoemission peaks from different structure carbon nanotubes showed clear saturation in intensity. Each peak exhibited a saturation value that was independent of excitation wavelength, indicating that there is an upper limit on the density of excitons. We propose that this behavior is a result of efficient exciton-exciton annihilation through which excitons decay nonradiatively. In order to explain the experimental results, we have developed a model taking into account diffusion-limited limited exciton-exciton annihilation and spontaneous decays of excitons in one dimension. The solution of the model reproduced the experimental results well, from which the exciton densities in carbon nanotubes were estimated. The validity of the model was confirmed by its comparison with Monte Carlo simulations. We also show that the conventional rate equation for exciton-exciton annihilation fails to fit the experimentally observed saturation behaviors, especially at high excitonic densities. Finally, we discuss possible influences of the existence of the upper density limit on some photoelectric applications of carbon nanotubes.

Effects of heat induced by two-photon absorption and free-carrier absorption in silicon-on-insulator nanowaveguides operating as all-optical wavelength converters

We propose a new numerical model to analyze heat induced by two-photon absorption and free-carrier absorption, while high intensity optical pulses propagate along silicon-on-insulator (SOI) nanowaveguides (NWGs). Using this model, we demonstrate that such induced heat causes a shift in the amount of wavelength conversion and hence deteriorates the converter output characteristics for pulses in the picosecond regime. The wavelength shift induced by a pulse with maximum input intensity and full width at half-maximum of I(max)=1.5x10(10) W x cm(-2) and T(FWHM)=30 ps, propagating along a SOI NWG with an effective cross-sectional area of a(eff)=0.15 microm(2), is shown to be Delta lambda(s) approximately 8 pm. We also demonstrate that such a shift can be compensated by tuning the pump intensity down by approximately 6.33%.

Ionization variation of the group velocity dispersion by high-intensity optical pulses

Influence of ionization on the group velocity and group velocity dispersion is studied for the first time. Inversion of the group velocity dispersion of initially normal dispersion medium at given electron number density is predicted.

Electron drift reversal caused by remaining holes in semiconductor superlattices due to effective-mass filtering.

Delayed photoluminescence from a GaAs cladding layer was observed in a GaAs/AlAs short-period super-lattice under high-intensity optical pulse excitation. This phenomenon is explained by a reverse transport process of electrons due to electric-field screening by remaining holes. Because of the influence of effective-mass difference in tunneling, electrons in a superlattice region can immediately be swept out, while holes are left behind. Consequently, a net electric field opposite to the applied field arises when the hole density is sufficiently high, and electrons that are pulled back and reinjected into the GaAs cladding layer recombine with remaining holes, causing the delayed photoluminescence.

Vanishing of Negative Differential Resistance Region due to Electric Field Screening in Wannier-Stark Localization Type Self-Electro-Optic Effect Devices

We report absorption saturation behaviors of the photocurrent versus reverse bias voltage characteristics (i–V characteristics) under high intensity optical pulse excitation in short-period GaAs/AlAs superlattices. While a negative differential resistance (NDR) region in the i–V curve normally appeared due to the Wannier–Stark localization effect under low intensity excitation, vanishing of the NDR was observed under high excitation. Simultaneously, an anomalously stretched photocurrent tail and long-lived photoluminescence were observed. These phenomena are clearly explained by electric field screening caused by remaining photogenerated holes in the superlattice region. Simulation results of carrier transport in thin-barrier superlattices using one-dimensional Green's function for the electric potential explain well the experimental results.