Radially-polarized Laser Research Articles

High fidelity amplification of radially-polarized laser in few-mode polarization-maintaining fiber amplifier

A high-power 20/400-μm few-mode polarization-maintaining (PM) fiber amplifier with a radially polarized output beam is firstly demonstrated and investigated. The reconstructed beam profiles and mode contents of the output laser were obtained using a numerical method. Experimentally, a home-made fiberized mode convertor was employed to convert the LP01 mode to be TM01 mode (radially polarized beam) with an average power of 2 W. In addition, the mode purity and polarization purity were 98.76%, and 98.5%, respectively. Based on a few-mode PM fiber amplifier, instead of the commonly used non-PM fiber amplifier, the radially polarized beam was scaled up to 192.1 W, while preserving the donut-shaped intensity distribution. The corresponding mode purity and polarization purity were measured to be approximately 97% and 98.2%, respectively, which indicates the polarization characteristic of the TM01 mode was preserved well. The beam quality factor (M2) was measured to be ∼2.2. Additionally, long-term stability in terms of both output power and polarization state were also verified experimentally. To the best our knowledge, this is the highest output power from a few-mode PM fiber amplifier laser system, while maintaining a high mode purity and a high polarization purity for radially polarized beam.

All-fiber hectowatt radially polarized laser system.

A high-power all-fiber radially polarized laser system is demonstrated, in which an integrated nanograting mode convertor (S-wave plate) is used for the generation of radially polarized beam. Experimentally, a 1-W radially polarized beam was used as the seed laser, whose mode purity and mode extinction ratio (MER) were 96.5% and 98.3%, respectively. A single-stage few-mode fiber amplifier was employed to boost the 1-W seed laser to an average power of 113.2 W, when the pump power was 160 W. The corresponding slope efficiency and beam quality factor (M2) were approximately 72% and 2.3%, respectively. Moreover, the mode purity and MER of the amplified radially polarized laser were measured to be 95.7% and 97%, respectively. To the best of our knowledge, this is the highest output power from an all-fiber radially polarized laser system without obvious degradations of the mode purity and MER.

Radially Polarized Laser-Induced Electron Acceleration in Vacuum

Enhanced electron acceleration due to tightly focused (TF) short-intense radially polarized (RP) Gaussian laser pulse in vacuum has been investigated. An axicon optical lens is utilized to accomplish a TF laser beam. The TF-RP laser fields are truncated to fifth order in diffraction angle. The innate and attractive symmetry of the tightly focused RP laser pulse imposes the trapping of electrons in the route of the laser propagation throughout laser–electron interaction. The applied axial magnetic field further ensures the enhancement in ponderomotive strength, pushing the electron in the expediting phase up to longer distances. Utilizing the optimum parameters of RP laser and magnetic field, noteworthy augmentation in the electron acceleration up to GeV range is obtained. The electron energy gain variations are plotted and analyzed for initial laser intensity, initial phase, electron initial energy and applied magnetic field parameters.

Relativistic Attosecond Electron Pulses from a Photocathode Radio-Frequency Gun

The interaction of light and electrons has been one of the important frontiers for generating ultrashort electron pulses in free-electron lasers, ultrafast science, and dynamical analysis of matter. However, the generation of a relativistic attosecond electron beam remains a challenge. In a photocathode radio-frequency (rf) gun, this work identifies a regime for obtaining an isolated or a train of relativistic attosecond electron pulses. A photoelectron beam is generated from the cathode under the illumination of a driving laser pulse in the electron gun, which is subsequently microbunched to optical wavelength scale by the field of a synchronized radially polarized laser (RPL) pulse that is focused near the cathode surface. The rf field in the gun cavity simultaneously accelerates the electron pulse to several MeV. The rf field causes a velocity differential inside the electron beam as it passes into the region near the focal point of the RPL (microbunching is processed), and the microbunch at the head is substantially faster than that at the tail. Using this knowledge, the spacing between consecutive microbunches, referred to as bunch spacing, may be controlled across a wide range by regulating the velocity difference, which can be accomplished by tuning the phase and amplitude of the rf field. Numerical simulations show that a train of attosecond bunches with tunable bunch spacing can be generated using realistic laser parameters corresponding to current GW-power-level laser systems, and an isolated attosecond pulse can be obtained when the driving laser pulse is about 50 fs. This regime may result in opportunities in ultrafast electron diffraction and microscopy, free-electron lasers, and other applications that require high-energy electrons with the temporal structure of single-cycle light.

Focused fields of ultrashort radially polarized laser pulses having low-order spatiotemporal couplings

We present a detailed study of the fields and propagation characteristics around the focus of ultrashort radially-polarized laser beams (RPLBs) having low-order spatio-temporal couplings (STCs). The three STCs considered are the focusing of the different frequencies to different positions along the longitudinal coordinate, the focusing of the frequencies to different positions along one transverse coordinate, and the beam waist or Rayleigh range having a power-law frequency dependence. The STCs considered are deemed low-order because they are generally linear in frequency. The combination of a low-order vector beam, ultrashort pulse duration, and the three different STCs shows promise for exotic applications in dielectric or charged particle manipulation and potentially other strong-field phenomena. The STCs presented are all developed in a standard frequency-domain model where each case involves a different chromatic term. We present the results unique to the vector nature of the RPLBs and compare them to the linearly polarized cases, opening up opportunities for control of the electric field around the focus with the additional element of polarization.

Interaction of Ultraintense Radially-Polarized Laser Pulses with Plasma Mirrors

We present experimental results of vacuum laser acceleration (VLA) of electrons using radially polarized laser pulses interacting with a plasma mirror. Tightly focused radially polarized laser pulses have been proposed for electron acceleration because of their strong longitudinal electric field, making them ideal for VLA. However, experimental results have been limited until now because injecting electrons into the laser field has remained a considerable challenge. Here, we demonstrate experimentally that using a plasma mirror as an injector solves this problem and permits to inject electrons at the ideal phase of the laser, resulting in the acceleration of electrons along the laser propagation direction while reducing the electron beam divergence compared to the linear polarization case. We obtain electron bunches with few-MeV energies and a 200 pC charge, thus demonstrating for the first time electron acceleration to relativistic energies using a radially polarized laser. High-harmonic generation from the plasma surface is also measured and provides additional insight into the injection of electrons into the laser field upon its reflection on the plasma mirror. Detailed comparisons between experimental results and full 3D simulations unravel the complex physics of electron injection and acceleration in this new regime: we find that electrons are injected into the radially polarized pulse in the form of two spatially-separated bunches emitted from the p-polarized regions of the focus. Finally, we leverage on the insight brought by this study to propose and validate a more optimal experimental configuration that can lead to extremely peaked electron angular distributions and higher energy beams.

Electron Acceleration in Direct Laser-Solid Interactions Far Beyond the Ponderomotive Limit

In laser-solid interactions, electrons may be generated and subsequently accelerated to energies of the order-of-magnitude of the ponderomotive limit, with the underlying process dominated by direct laser acceleration. Breaking this limit, realized here by a radially-polarized laser pulse incident upon a wire target, can be associated with several novel effects. Three-dimensional Particle-In-Cell simulations show a relativistic intense laser pulse can extract electrons from the wire and inject them into the accelerating field. Anti-dephasing, resulting from collective plasma effects, are shown here to enhance the accelerated electron energy by two orders of magnitude compared to the ponderomotive limit. It is demonstrated that ultra-short radially polarized pulses produce super-ponderomotive electrons more efficiently than pulses of the linear and circular polarization varieties.

Electron acceleration by a radially-polarized laser pulse in a plasma micro-channel: erratum.

Three of the headings of Table 1, which have been switched by mistake in our paper, are corrected here. The rest of the paper, including all results and conclusions, remain intact.

Electron acceleration by a radially-polarized laser pulse in a plasma micro-channel

Encouraged by recent advances in radially-polarized laser technology, simulations have been performed of electron acceleration by a tightly-focused, ultra-short pulse in a parabolic plasma micro-channel. Milli-joule laser pulses, generated at kHz repetition rates, are shown to produce electron bunches of MeV energy, pC charge, low emittance and low divergence. The pivotal role played by the channel length in controlling the process is demonstrated, and the roles of direct and wakefield acceleration are distinguished.

Approximate fields of an ultra-short, tightly-focused, radially-polarized laser pulse in an under-dense plasma: a Bessel-Bessel light bullet

Provided the intensity is not too high (for example, with I << 1018 W/cm2, for a wavelength of 1 μm), response of an under-dense plasma to the fields of a laser pulse can still be considered linear, and inhomogeneous wave equations for the vector and scalar potentials A and Φ, respectively, may be derived from Maxwell’s equations. A rigorous, but approximate, solution to the wave equation satisfied by a one-component, azimuthally symmetric, vector potential is developed using a Fourier transform method. It is found that an ultra-short and tightly-focused, radially-polarized laser pulse, described by this vector potential, propagates in the plasma like a laser bullet. The pulse is termed a Bessel-Bessel bullet because, to leading order in a power-series expansion, the vector potential, from which the pulse fields E and B are derived, is expressed in terms of a Bessel function of the first kind J0 and a spherical Bessel function of the first kind j0.

Electromagnetic fields of an ultra-short tightly-focused radially-polarized laser pulse

Fully analytic expressions, for the electric and magnetic fields of an ultrashort and tightly focused laser pulse of the radially polarized category, are presented to lowest order of approximation. The fields are derived from scalar and vector potentials, along the lines of our earlier work for a similar pulse of the linearly polarized variety. A systematic program is also described from which the fields may be obtained to any desired accuracy, analytically or numerically.

Fabrication of Ion-Shaped Anisotropic Nanoparticles and their Orientational Imaging by Second-Harmonic Generation Microscopy.

Ion beam shaping is a novel and powerful tool to engineer nanocomposites with effective three-dimensional (3D) architectures. In particular, this technique offers the possibility to precisely control the size, shape and 3D orientation of metallic nanoparticles at the nanometer scale while keeping the particle volume constant. Here, we use swift heavy ions of xenon for irradiation in order to successfully fabricate nanocomposites consisting of anisotropic gold nanoparticle that are oriented in 3D and embedded in silica matrix. Furthermore, we investigate individual nanorods using a nonlinear optical microscope based on second-harmonic generation (SHG). A tightly focused linearly or radially-polarized laser beam is used to excite nanorods with different orientations. We demonstrate high sensitivity of the SHG response for these polarizations to the orientation of the nanorods. The SHG measurements are in excellent agreement with the results of numerical modeling based on the boundary element method.

Multi-GeV electron acceleration by a periodic frequency chirped radially polarized laser pulse in vacuum

Linear and periodic effects of frequency chirp on electron acceleration by radially polarized (RP) laser pulse in vacuum have been investigated. A frequency chirp influences the electron dynamics, betatron resonance, and energy gain by electron during interaction with the RP laser pulse and ensures effective electron acceleration with high energy gain (~GeV). The electron energy gain with a periodic frequency chirped laser pulse is about twice as high as with a linear chirp. Our observations reveal electron energy gain of about 10.5 GeV with a periodic chirped RP petawatt laser pulse in vacuum.

Electron injection for enhanced energy gain by a radially polarized laser pulse in vacuum in the presence of magnetic wiggler

We present a scheme of electron injection for enhanced electron energy gain by using a radially polarized (RP) laser pulse in vacuum under the influence of magnetic wiggler. The inherent symmetry of an RP laser pulse enforces the trapping and acceleration of electrons in the direction of propagation of laser pulse during laser electron interaction. A magnetic wiggler encircles the trajectory of accelerated electron and improves the strength of v→×B→ force which supports the retaining of betatron resonance for longer duration and leads to enhance electron acceleration. Four times higher electron energy is observed with a RP laser pulse of peak intensity 8.5×1020 W/cm2 in the presence of magnetic wiggler of 10.69 kG than that in the absence of magnetic wiggler. We have also analyzed the electron injection for enhanced energy gain and observe that the electron energy gain is relatively higher with a sideway injection than that of axial injection of electron. Injection angle δ is optimized and found that at δ=10° to the direction of propagation of laser pulse, maximum energy is obtained.

MeV femtosecond electron pulses from direct-field acceleration in low density atomic gases

Using three-dimensional particle-in-cell (3DPIC) simulations, we show that few-MeV electrons can be produced by tightly focusing few-cycle radially-polarized laser pulses in a low-density atomic gas. In particular, it is observed that for the few-TW laser power needed to reach relativistic electron energies, longitudinal attosecond microbunching occurs naturally, resulting in femtosecond structures with high-contrast attosecond density modulations. The 3DPIC simulations show that in the relativistic regime the leading pulse of these attosecond substructures survives to propagation over extended distances, suggesting that it could be delivered to a distant target, with the help of a properly designed transport beamline.

Sensitiveness of axial magnetic field on electron acceleration by a radially polarized laser pulse in vacuum

We examine the electron acceleration by a radially polarized (RP) laser pulse in vacuum under influence of an intense axial magnetic field. The electron while interaction with a RP laser pulse gets accelerated with high energy gain. The attained energy gain further enhanced up-to the order of GeV with an intense RP laser pulse. We observe a significant enhancement in energy gain in the presence of an intense axial magnetic field in the direction of propagation of laser pulse. The presence of axial magnetic field improves the strength of v→×B→ force which supports the retaining of betatron resonance for longer durations. This improves the electron acceleration with an enhanced energy gain up to 5.2GeV. It is noticed that the axial magnetic field is sensitive to electron acceleration, small change in magnetic field leads to enhance electron energy gain significantly. Our results also show relatively smaller scattering of the electrons in the presence of axial magnetic field.

Proton acceleration by radially polarized chirped laser pulses

Within the framework of plane-wave angular spectrum analysis of electromagnetic fields, a solution for the field of a tightly focused radially polarized (RP) chirped laser pulse is presented. With this solution, direct laser acceleration of protons by this kind of RP laser pulses is investigated numerically. It is found that a RP laser pulse with proper negative frequency chirps can lead to efficient proton acceleration, reaching sub-GeV at the laser intensity of ${10}^{22}\text{ }\text{ }\mathrm{W}/{\mathrm{cm}}^{2}$ from its injection energy of 45 MeV.

Two-color-laser-driven direct electron acceleration in infinite vacuum

We propose a direct electron acceleration scheme that uses a two-color pulsed radially polarized laser beam. The two-color scheme achieves electron acceleration exceeding 90% of the theoretical energy gain limit, over twice of what is possible with a one-color pulsed beam of equal total energy and pulse duration. The scheme succeeds by exploiting the Gouy phase shift to cause an acceleration-favoring interference of fields only as the electron enters its effectively final accelerating cycle. Optimization conditions and power scaling characteristics are discussed.

Direct acceleration of an electron in infinite vacuum by a pulsed radially-polarized laser beam

We study the direct acceleration of a free electron in infinite vacuum along the axis of a pulsed radially-polarized laser beam. We find that net energy transfer from laser pulse to electron is maximized with the tightest focusing. We show that the net energy gain of an electron initially moving at a relativistic velocity may exceed more than half the theoretical limit of energy transfer, which is not possible with an initially stationary electron in the parameter space studied. We determine and analyze the power scaling of maximum energy gain, extending our study to include a relatively unexplored regime of low powers and revealing that substantial acceleration is already possible without the use of petawatt peak-power laser technology.

Observation of discrete localized surface plasmon (LSP) states in isolated nanoscaled Ag-particles synthesized via photochemical reduction route

Metallic Ag-nanoparticles have been synthesized by catalyst free chemical and photochemical reduction processes. During photochemical synthesis, the samples were illuminated independently by unpolarized and radially-polarized continuous laser beams (A = 632.8 nm). The nanoparticles were characterized by bright-field transmission electron microscopy (TEM), linear absorption spectroscopy and fluorescence excitation/emission spectroscopy. Compared to other growth-conditions, sample prepared under radially polarized beam essentially displays well-separated, spherical nanoparticles with a narrow size distribution around average size of 6 nm. In addition, an outstanding enhancement in the fluorescence as large as 39% has been achieved along with observation of distinct localized surface plasmon states. Though the sample produced under unpolarized light could exhibit discrete plasmon states, better resolved states were observed in case of use of radially-polarized beam. It is expected that the radially polarized light could induce resonance energy transfer and hence could control growth conditions giving rise to absolutely unclustered Ag-nanoparticles. Detecting such localized surface plasmon states and understanding conditions of high quantum yield would be promising for single molecule fluorescence, superresolution microscopy and other nanoscopic applications.