High-intensity Ultrashort Pulses Research Articles

Consideration of non-phase-matched nonlinear effects in the design of quasi-phase-matching crystals for optical parametric oscillators.

Femtosecond optical parametric oscillators (OPOs) are widely used in ultrafast nonlinear frequency conversion and quantum information. However, conventional OPOs based on quasi-phase-matching (QPM) crystals have many parasitic non-phase-matched processes which decrease the conversion efficiency. Here, we propose nine-wave coupled equations (NWCEs) to simulate all phase-matched and non-phase-matched interactions in QPM crystals to improve conventional three-wave coupled equations (TWCEs), especially for the situation of high intensity ultrashort pulses and complexly structured crystals. We discuss how to design the poling period of QPM crystal to maximize the conversion efficiency of signal light for a given OPO system. The simulation reveals that the OPO based on chirped periodically poled lithium niobate (CPPLN) with a certain chirp rate has higher signal wave conversion efficiency than that of a PPLN, and demonstrates that NWCEs illustrate more details of the pulse evolution in the OPO cavity. Starting from a CPPLN, an aperiodically poled lithium niobate (APPLN) design is available by modifying the domain lengths of the crystal and optimizing the OPO output power via dynamical optimization algorithm. The results show that by using a properly designed APPLN crystal, a 1600 nm OPO, when pumped by a femtosecond laser with 1030 nm central wavelength, 150 femtosecond pulse duration and 5 GW/cm2 power intensity at the focus, can achieve very efficient output with a signal light conversion efficiency of 50.6%, which is higher than that of PPLN (25.2%) and CPPLN (40.2%). The scheme in this paper will provide a reference for the design of nonlinear QPM crystals of OPOs and will help to understand the complex nonlinear dynamical behavior in OPO cavities.

Application and validation of a viscosity approach to the existence of nanogratings in oxide glasses

Nanogratings are self-organized and sub-wavelength birefringent structures that are formed upon the action of high intensity ultrashort light pulses in the bulk of a transparent material. They have found interest in optics/photonics, microfluidics, optical data storage or sensing applications. However, the ability to successfully imprint 3-dimensional (3D) nanogratings in silicate glasses is a strong function of the glass composition. In this work, we investigate the role of glass viscosity on the ability to induce these nanogratings. We first study the nanogratings formation window in an energy-repetition rate laser parameter landscape for five common oxide glasses: SiO2 (Suprasil), GeO2, and Schott glasses AF32, Borofloat, and BK7. Secondly, and based on previous work, we define a domain of existence of the nanogratings using viscosity-based arguments. The lower limit corresponds to a temperature at which the viscosity is ∼106.6 Pa⋅s, where nanocavitation of the glass occurs, forming the nanopores that compose the nanogratings. An upper temperature limit, set for a viscosity value of ∼103.0 Pa⋅s, relates to either collapse or growth of the nanopores, resulting in the erasure of the nanopores, hence the nanogratings. The experimental results agree with the predictions made by this viscosity approach and literature data. This work opens the door to future glass viscosity engineering to maximize 3D nanogratings imprinting.

Noncollinear phase-matching geometries in ultra-broadband quasi-parametric amplification

Optical parametric chirped pulse amplification (OPCPA) shows great potential in producing ultrashort high-intensity pulses because of its large gain bandwidth. Quasi-parametric chirped pulse amplification (QPCPA) may further extend the bandwidth. However, behavior of QPCPA at a limited pump intensity (e.g., ≤ 5 GW/cm2 in a nanosecond pumped QPCPA) has not yet been investigated fully. We discuss detailedly the ultra-broadband amplification and the noncollinear phase-matching geometry in QPCPA, model and develop a novel noncollinear geometry in QPCPA, namely triple-wavelength phase-matching geometry, which provides two additional phase-matching points around the phase-matching point at the central wavelength. Our analysis demonstrates that the triple-wavelength phase-matching geometry can support stable, ultra-broadband amplification in QPCPA. The numerical simulation results show that ultrashort pulse with a pulse duration of 7.92 fs can be achieved in QPCPA when the pump intensity is limited to 5 GW/cm2, calculated using the nonlinear coefficient of YCa4O(BO3)3.

Study of ultrafast Rabi flopping in colloidal quantum dots at room temperature

The interaction between high-intensity ultrashort optical pulses and materials has led to a number of fascinating optical phenomena, including Rabi flopping and self-induced transparency. Until now, there have been few reports on ultrashort coherent pulse propagation and reshaping in semiconductor materials. Here we investigate Rabi flopping and Rabi splitting in colloidal quantum dots with Fabry-Perot cavity of SU8/Si. The Rabi flopping phenomenon is monitored via the pump-probe differential reflection spectroscopy. A high excitation power reshapes the temporal oscillations so that the fast Fourier transform spectra display several peaks. The photoluminescence spectrum by continuous-wave excitation splits under a proper incident angle, and the splitted photoluminescence spectrum is generally consistent with the amplitude of differential reflectivity as function of wavelength. These results demonstrate that both of the temporal oscillations and the splitting of the continuous-wave excited photoluminescence spectra are due to strong coupling between colloidal quantum dots and the Fabry-Perot cavity.

On complex wave solutions depicted by the variable coefficients coupled Hirota equation

In this paper, the variable coefficients coupled Hirota equation is investigated by virtue of the unified method. Playing an important role in optics, the coupled Hirota equation describes the transmission process when high intensity ultra-short pulses pass through an optical glass fiber. Two types of complex wave solutions, polynomial function and rational function solutions, are gained. The polynomial function solutions incorporate complex solitary, complex soliton and complex elliptic wave solutions, while the rational function solutions involve complex periodic and complex soliton wave solutions. Furthermore, by choosing different values of the parameters, we illustrate the dynamical behaviors and the physical signification of these solutions.

Measuring the frequency chirp of extreme-ultraviolet free-electron laser pulses by transient absorption spectroscopy

High-intensity ultrashort pulses at extreme ultraviolet (XUV) and x-ray photon energies, delivered by state-of-the-art free-electron lasers (FELs), are revolutionizing the field of ultrafast spectroscopy. For crossing the next frontiers of research, precise, reliable and practical photonic tools for the spectro-temporal characterization of the pulses are becoming steadily more important. Here, we experimentally demonstrate a technique for the direct measurement of the frequency chirp of extreme-ultraviolet free-electron laser pulses based on fundamental nonlinear optics. It is implemented in XUV-only pump-probe transient-absorption geometry and provides in-situ information on the time-energy structure of FEL pulses. Using a rate-equation model for the time-dependent absorbance changes of an ionized neon target, we show how the frequency chirp can be directly extracted and quantified from measured data. Since the method does not rely on an additional external field, we expect a widespread implementation at FELs benefiting multiple science fields by in-situ on-target measurement and optimization of FEL-pulse properties.

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.

Control of the half-cycle harmonic emission process for generating the intense and ultrashort single attosecond pulses (SAPs)

Abstract In this paper, the half-cycle harmonic generation process has been controlled by using the asymmetric inhomogeneous chirped pulse combined with the ultraviolet (UV) pulse. It is found that by properly optimizing the chirps and chirp delays of the fundamental two-color pulse, the optimal negative and positive half-cycle laser profiles for the harmonic cutoff extension can be obtained. Further, with the introduction of the negative and positive inhomogeneous effect, respectively, the harmonic cutoff from the negative and positive half-cycle laser profiles can be further improved. Next, with the assistance of the UV pulse, the harmonic intensity can be enhanced due to the UV resonance ionization. Moreover, the single and double UV photon resonance ionizations are much better for generating the higher harmonic intensity. As a result, the stronger and broader harmonic plateaus with the larger harmonic cutoff can be obtained, which can support the generation of the high-intensity ultrashort attosecond pulses with the pulse durations of sub-45 as.

Laser-based synchrotron X-ray radiation experimental scaling.

We review the results obtained in several experimental campaigns with the INRS high-power laser system and determine the X-ray emission scaling from synchrotron radiation produced during laser wakefield acceleration (LWFA) of electrons. The physical processes affecting the generation of intense and stable X-ray beams during the propagation phase of the high-intensity ultrashort pulse in the gas jet target are discussed. We successfully produced stable propagation in the gas jet target of a relativistic laser pulse through self-guiding on length larger than the dephasing and depletion lengths, generating very intense beams of hard X-rays with up to 200 TW on target. The experimental scaling law obtained for the photon yield in the 10-40 keV range is presented and the level of X-ray emission at the 1 PW laser peak power level, now available at several laser facilities, is estimated.

Femtosecond supercontinuum generation in scattering media

We report our results on the effect of random multiple scattering on supercontinuum (SC) generation by intense light pulses. Spectral measurements are performed on the SC generated when femtosecond laser pulses propagate in water containing randomly suspended microspheres. We observed that the SC emission in the presence of scatterers is significantly suppressed in the blue side of the spectrum in comparison to that observed in neat water. We attribute this to the dissipation of incident energy, strong scattering of the shorter wavelengths, and stretching of the pulse temporally. This reasoning is supported by our experiments in neat water where we alter the shape of the incident femtosecond pulse by means of an acousto-optic programmable dispersive filter, and find similar suppression in the blue side with the lengthening of the input pulse. We believe our study will help in understanding the physics governing applications that involve transmission of high-intensity ultrashort pulses through long distances in scattering media like the atmosphere and sea water.

Mono-energetic heavy ion acceleration from laser plasma based composite nano-accelerator

High intensity ultrashort pulse driven ion acceleration is an important area of research due to its richness in underlying physics and potential technological applications. A strong impediment towards employing such sources, in practice, lies in the inherent challenge of producing mono-energetic ions with fine energy control. We demonstrate consistent, highly reproducible, charge-independent mono-energetic acceleration of heavy gold ions to sub-MeV energies with a reasonably high flux of 9 × 1010 ions per shot from the gold-carbon nano-composite target irradiated at moderate laser intensities of 1018–1019 W/cm2. The experimental results establish that the thermal pressure of the expanding hot plasma, instead of the “hot” electrons, is responsible for the acceleration of the mono-energetic heavy Au ions. Our results pave the way for a simple and new way of producing mono-energetic heavy ions. This may find potential applications in advanced particle accelerators, isochoric heating of matter, surface science, swift heavy ion sources, and basic plasma physics.

Dielectric properties of isolated adrenal chromaffin cells determined by microfluidic impedance spectroscopy

Knowledge of the dielectric properties of biological cells plays an important role in numerical models aimed at understanding how high intensity ultrashort nanosecond electric pulses affect the plasma membrane and the membranes of intracellular organelles. To this end, using electrical impedance spectroscopy, the dielectric properties of isolated, neuroendocrine adrenal chromaffin cells were obtained. Measured impedance data of the cell suspension, acquired between 1kHz and 20MHz, were fit into a combination of constant phase element and Cole-Cole models from which the effect of electrode polarization was extracted. The dielectric spectrum of each cell suspension was fit into a Maxwell-Wagner mixture model and the Clausius-Mossotti factor was obtained. Lastly, to extract the cellular dielectric parameters, the cell dielectric data were fit into a granular cell model representative of a chromaffin cell, which was based on the inclusion of secretory granules in the cytoplasm. Chromaffin cell parameters determined from this study were the cell and secretory granule membrane specific capacitance (1.22 and 7.10μF/cm2, respectively), the cytoplasmic conductivity, which excludes and includes the effect of intracellular membranous structures (1.14 and 0.49S/m, respectively), and the secretory granule milieu conductivity (0.35S/m). These measurements will be crucial for incorporating into numerical models aimed at understanding the differential poration effect of nanosecond electric pulses on chromaffin cell membranes.

PHz-Wide Spectral Interference Through Coherent Plasma-Induced Fission of Higher-Order Solitons.

We identify a novel regime of soliton-plasma interactions in which high-intensity ultrashort pulses of intermediate soliton order undergo coherent plasma-induced fission. Experimental results obtained in gas-filled hollow-core photonic crystal fiber are supported by rigorous numerical simulations. In the anomalous dispersion regime, the cumulative blueshift of higher-order input solitons with ionizing intensities results in pulse splitting before the ultimate self-compression point, leading to the generation of robust pulse pairs with PHz bandwidths. The novel dynamics closes the gap between plasma-induced adiabatic soliton compression and modulational instability.

A split-step method to include electron–electron collisions via Monte Carlo in multiple rate equation simulations

A split-step numerical method for calculating ultrafast free-electron dynamics in dielectrics is introduced. The two split steps, independently programmed in C++11 and FORTRAN 2003, are interfaced via the presented open source wrapper. The first step solves a deterministic extended multi-rate equation for the ionization, electron–phonon collisions, and single photon absorption by free-carriers. The second step is stochastic and models electron–electron collisions using Monte-Carlo techniques. This combination of deterministic and stochastic approaches is a unique and efficient method of calculating the nonlinear dynamics of 3D materials exposed to high intensity ultrashort pulses. Results from simulations solving the proposed model demonstrate how electron–electron scattering relaxes the non-equilibrium electron distribution on the femtosecond time scale.

Bright–dark vector soliton solutions for a generalized coupled Hirota system in the optical glass fiber

Studied in this paper are the bright–dark vector soliton solutions for a generalized coupled Hirota system which describes the propagation for the high-intensity ultrashort pulses in the optical glass fiber. Beyond the existing bilinear forms, using an auxiliary function, we obtain the improved bilinear forms and bright–dark soliton solutions under two integrable constraints through the Hirota method and symbolic computation. With the help the analytic and graphic analysis, we study the soliton properties including the amplitudes, velocities and phase shifts, and show that the interactions for the bright–dark two solitons are elastic. For the bright–dark one soliton, parametric conditions that the dark component is “black” or “gray” are obtained. For the bright–dark two solitons, we find that the bright component is affected by the dark component background parameters during such an interaction, while the dark component is not affected by the bright component background parameters. Velocities for the bright–dark two solitons are inversely proportional to the higher-order effect parameter, but amplitudes and phase shifts are independent of it. Besides, the bound-state bright–dark two solitons are also presented.

Spatio-spectral structures in high-order harmonic beams generated with Terawatt 10-fs pulses.

A large international effort is nowadays devoted to increase the energy of the extreme ultraviolet pulses by using high-peak power ultrashort fundamental pulses (Terawatt level). Using such fundamental pulses brings specific constraints that need to be addressed. Here we study high-order harmonic generation in gases with 10 fs pulses at Terawatt peak power and demonstrate that extreme ultraviolet beams can be highly structured and complex in various conditions. We use a single-shot spatially resolved spectral detection and demonstrate direct observation of the spatio-temporal coupling occurring in the generating medium. Clear and reproducible complex spatio-spectral structures are observed in the far field. Similar structures are reproduced with simulations and we show that they are intimately associated to the high nonlinearity of high-order harmonic generation. Those findings are of prime importance for the generation of high-energy attosecond pulses and reveal important issues for their applications.

Self-focusing of the high intensity ultra short laser pulse propagating through relativistic magnetized plasma

In this paper, evolution of the spot size of the ultra short intense laser beam propagating in underdense magnetized cold plasma, taking into account the nonlinearity up to third order and the relativistic effect, has been studied. The plasma embedded in a constant external magnetic field that is set in the plane perpendicular to the electric field vector of the laser beam with different directions. The paraxial wave equation in plasma has been used and the source dependent expansion (SDE) method is employed to solve the equation. Using continuity equation and equation of motion for plasma electrons in the electric field of laser beam a set of equations for the evolution of laser beam structure in plasma is found. Results show that imposing the external magnetic field enhances self-focusing property of the laser beam. Taking into account the relativistic effect increases the effect of the external magnetic field on self-focusing of the laser beam. Increasing the angle between the laser beam magnetic field and external magnetic field will decrease the self-focusing property.

Passive energy jitter reduction in the cascaded third harmonic generation process

In free electron laser (FEL) systems with ultraviolet (UV) laser driven injectors, a highly stable UV source generated through cascaded third harmonic generation (THG) from an infrared (IR) source is a key element in guaranteeing the acceptable current jitter at the undulator. In this letter, the negative slope of the THG efficiency for high intensity ultrashort IR pulses is revealed to be a passive stabilization mechanism for energy jitter reduction in UV. A reduction of 2.5 times the energy jitter in UV is demonstrated in the experiment and simulations show that the energy jitter in UV can be reduced by more than one order of magnitude if the energy jitter in IR is less than 3%, with proper design of the THG efficiency curve, fulfilling the challenging requirement for UV laser stability in a broad scope of applications such as the photoinjector of x-ray FELs.

On the stable propagation of high-intensity ultrashort light pulses

The spatiotemporal dynamics of high-intensity femtosecond laser pulses is studied within a rigorous physical model. The pulse propagation is described by the nonlinear envelope equation. The propagation and the material equations are solved self-consistently at realistic physical conditions. Self-compression of the pulse is found. At certain conditions, transversal width, peak intensity, time duration, and spatiotemporal shape of the pulse remain stable within given propagation range.

Measurement of nonlinear refractive index and ionization rates in air using a wavefront sensor

A wavefront sensor has been used to measure the Kerr nonlinear focal shift of a high intensity ultrashort pulse beam in a focusing beam geometry while accounting for the effects of plasma-defocusing. It is shown that plasma-defocusing plays a major role in the nonlinear focusing dynamics and that measurements of Kerr nonlinearity and ionization are coupled. Furthermore, this coupled effect leads to a novel way that measures the laser ionization rates in air under atmospheric conditions as well as Kerr nonlinearity. The measured nonlinear index n₂ compares well with values found in the literature and the measured ionization rates could be successfully benchmarked to the model developed by Perelomov, Popov, and Terentev (PPT model) [Sov. Phys. JETP 50, 1393 (1966)].