Pulse front adaptive optics: a new method for control of ultrashort laser pulses

Dephasingless laser wakefield acceleration in the bubble regime

Laser wakefield accelerators (LWFAs) have electric fields that are orders of magnitude larger than those of conventional accelerators, promising an attractive, small-scale alternative for next-generation light sources and lepton colliders. The maximum energy gain in a single-stage LWFA is limited by dephasing, which occurs when the trapped particles outrun the accelerating phase of the wakefield. Here, we demonstrate that a single space-time structured laser pulse can be used for ionization injection and electron acceleration over many dephasing lengths in the bubble regime. Simulations of a dephasingless laser wakefield accelerator driven by a 6.2-J laser pulse show 25 pC of injected charge accelerated over 20 dephasing lengths (1.3 cm) to a maximum energy of 2.1 GeV. The space-time structured laser pulse features an ultrashort, programmable-trajectory focus. Accelerating the focus, reducing the focused spot-size variation, and mitigating unwanted self-focusing stabilize the electron acceleration, which improves beam quality and leads to projected energy gains of 125 GeV in a single, sub-meter stage driven by a 500-J pulse.

Programmable-trajectory ultrafast flying focus pulses

"Flying focus" techniques produce laser pulses with dynamic focal points that travel distances much greater than a Rayleigh length. The implementation of these techniques in laser-based applications requires the design of optical configurations that can both extend the focal range and structure the radial group delay. This article describes a method for designing optical configurations that produce ultrashort flying focus pulses with programmable-trajectory focal points. The method is illustrated by several examples that employ an axiparabola for extending the focal range and either a reflective echelon or a deformable mirror-spatial light modulator pair for structuring the radial group delay. The latter configuration enables rapid exploration and optimization of flying foci, which could be ideal for experiments.

Dynamic Phase and Polarization Modulation Using Two-Beam Parallel Coding for Optical Storage in Transparent Materials

In this paper, we propose and experimentally demonstrate a parallel coding and two-beam combining approach for the simultaneous implementation of dynamically generating holographic patterns at their arbitrary linear polarization states. Two orthogonal input beams are parallelly and independently encoded with the same target image information but there is different amplitude information by using two-phase computer-generated holograms (CGH) on two Liquid-Crystal-on-Silicon-Spatial-Light Modulators (LCOS SLMs). Two modulated beams are then considered as two polarization components and are spatially superposed to form the target polarization state. The final linear vector beam is created by the spatial superposition of the two base beams, capable of controlling the vector angle through the phase depth of the phase-only CGHs. Meanwhile, the combined holographic patterns can be freely encoded by the holograms of two vector components. Thus, this allows us to tailor the optical fields endowed with arbitrary holographic patterns and the linear polarization states at the same time. This method provides a more promising approach for laser data writing generation systems in the next-generation optical data storage technology in transparent materials.

Simulating spatiotemporal dynamics of ultra-intense ultrashort lasers through imperfect grating compressors

The upcoming 100 Petawatt (PW) laser is going to provide a possibility to experimentally study vacuum physics. Pulse compression and beam focusing, which can be affected by the spatiotemporal coupling, are two key processes of generating a 100 PW laser and then determine whether its physical objective can be achieved or not. We improved our previous model of the spatiotemporal coupling where only the grating wavefront error and the output optical field of a common compressor configuration were included, and in the improved model, the grating amplitude modulation, the spatio-spectral clipping, and the optical field inside the compressor were added. By using it, we theoretically investigated the spatiotemporal dynamics of an ultra-intense ultrashort laser passing through an imperfect grating compressor for different cases, especially the spatio-temporal/spectral coupling and the on-target intensity variation induced by the phase and amplitude modulation at different grating positions in two different compressor configurations. This study is of importance for both engineering development and physical application of the upcoming Exawatt-class laser.

Investigating group-velocity-tunable propagation-invariant optical wave-packets

The group-velocity of the propagation-invariant optical wave-packet generated by the conical superposition can be controlled by introducing well-designed arbitrarily-axisymmetric pulse-front deformation, which permits realizing superluminal, subluminal, accelerating, decelerating, and even nearly-programmable group-velocities. To better understand the tunability of the group-velocity, the generation methods of this propagation-invariant optical wave-packet and the mechanisms of the tunable group-velocity in both the physical and Fourier spaces are investigated. We also have studied the relationship with the recently-reported space–time wave-packet, and this group-velocity-tunable propagation-invariant optical wave-packet should be a subset of the space–time wave-packet.

Investigating focus elongation using a spatial light modulator for high-throughput ultrafast-laser-induced selective etching in fused silica

Ultrafast-laser-induced selective chemical etching is an enabling microfabrication technology compatible with optical materials such as fused silica. The technique offers unparalleled three-dimensional manufacturing freedom and feature resolution but can be limited by long laser inscription times and widely varying etching selectivity depending on the laser irradiation parameters used. In this paper, we aim to overcome these limitations by employing beam shaping via a spatial light modulator to generate a vortex laser focus with controllable depth-of-focus (DOF), from diffraction limited to several hundreds of microns. We present the results of a thorough parameter-space investigation of laser irradiation parameters, documenting the observed influence on etching selectivity and focus elongation in the polarization-insensitive writing regime, and show that etching selectivity greater than 800 is maintained irrespective of the DOF. To demonstrate high-throughput laser writing with an elongated DOF, geometric shapes are fabricated with a 12-fold reduction in writing time compared to writing with a phase-unmodulated Gaussian focus.

Survey of spatio-temporal couplings throughout high-power ultrashort lasers

The investigation of spatio-temporal couplings (STCs) of broadband light beams is becoming a key topic for the optimization as well as applications of ultrashort laser systems. This calls for accurate measurements of STCs. Yet, it is only recently that such complete spatio-temporal or spatio-spectral characterization has become possible, and it has so far mostly been implemented at the output of the laser systems, where experiments take place. In this survey, we present for the first time STC measurements at different stages of a collection of high-power ultrashort laser systems, all based on the chirped-pulse amplification (CPA) technique, but with very different output characteristics. This measurement campaign reveals spatio-temporal effects with various sources, and motivates the expanded use of STC characterization throughout CPA laser chains, as well as in a wider range of types of ultrafast laser systems. In this way knowledge will be gained not only about potential defects, but also about the fundamental dynamics and operating regimes of advanced ultrashort laser systems.

Achromatic optical system with diffractive-refractive hybrid lenses for multifocusing of ultrashort pulse beams

We report an achromatic cascade optical system for multifocusing ultrashort pulse beams with a diffractive beam splitter. Distortion compensation requires the removal of pulse front distortions from arrayed pulses, which originate from beam-radius-dependent group delay dispersions. The inclusion of hybrid diffractive-refractive lenses can effectively manage system dispersions. Simple design formulas are derived using the ray-matrix analysis and the designed system is evaluated using 20-fs pulses. We confirm that the hybridized system can remove not only chromatic aberrations but also pulse front distortions, hence improving the system spatio-temporal focusing resolutions. The proposed pulse delivery technique enhances the practicality of materials processing with ultrashort pulses.

Three Dimensional Alternating-Phase Focusing for Dielectric-Laser Electron Accelerators

The concept of dielectric-laser acceleration (DLA) provides the highest gradients among breakdown-limited (nonplasma) particle accelerators and thus the potential of miniaturization. The implementation of a fully scalable electron accelerator on a microchip by two-dimensional alternating phase focusing (APF), which relies on homogeneous laser fields and external magnetic focusing in the third direction, was recently proposed. In this Letter, we generalize the APF for DLA scheme to 3D, such that stable beam transport and acceleration is attained without any external equipment, while the structures can still be fabricated by entirely two-dimensional lithographic techniques. In the new scheme, we obtain significantly higher accelerating gradients at given incident laser field by additionally exploiting the new horizontal edge. This enables ultralow injection energies of about 2.5 keV (β=0.1) and bulky high voltage equipment as used in previous DLA experiments can be omitted. DLAs have applications in ultrafast time-resolved electron microscopy and diffraction. Our findings are crucial for the miniaturization of the entire setup and pave the way towards integration of DLAs in optical fiber driven endoscopes, e.g., for medical purposes.

Adaptive optics in laser processing

Adaptive optics are becoming a valuable tool for laser processing, providing enhanced functionality and flexibility for a range of systems. Using a single adaptive element, it is possible to correct for aberrations introduced when focusing inside the workpiece, tailor the focal intensity distribution for the particular fabrication task and/or provide parallelisation to reduce processing times. This is particularly promising for applications using ultrafast lasers for three-dimensional fabrication. We review recent developments in adaptive laser processing, including methods and applications, before discussing prospects for the future.

Complex spatiotemporal coupling distortion pre-compensation with double-compressors for an ultra-intense femtosecond laser

In an ultra-intense femtosecond chirped-pulse amplification laser, the imperfect diffraction wave-fronts of the second and the third gratings of the compressor, where spatio-spectral coupling exists, could introduce a complex spatiotemporal coupling distortion (STCD) and degrade the pulsed beam in both near- and far-fields. Here, we propose a method of double-compressors for pre-compensation. By inserting a scaled down compressor (small compressor) with a deformable retro-reflection mirror into the beam-line, the frequency-dependent wave-front distortion, i.e., the complex STCD, could be removed. We simulate the results in two different ultra-intense femtosecond lasers with 80 and 400 nm bandwidths for comparison, and near ideal focused peak intensities could be obtained in both cases. Meanwhile, the influences of several miss-matching effects, which might appear in engineering, are also analyzed and discussed for applications.

Optical Control of the Topology of Laser-Plasma Accelerators

We propose a twisted plasma accelerator capable of generating relativistic electron vortex beams with helical current profiles. The angular momentum of these vortex bunches is quantized, dominates their transverse motion, and results in spiraling particle trajectories around the twisted wakefield. We focus on a laser wakefield acceleration scenario, driven by a laser beam with a helical spatiotemporal intensity profile, also known as a light spring. We find that these light springs can rotate as they excite the twisted plasma wakefield, providing a new mechanism to control the twisted wakefield phase velocity and enhance energy gain and trapping efficiency beyond planar wakefields.

Single-shot real-time detection technique for pulse-front tilt and curvature of femtosecond pulsed beams with multiple-slit spatiotemporal interferometry

Spatiotemporal coupling (STC) of femtosecond pulsed beams could significantly reduce the focal-spot intensity of ultra-intense lasers. We theoretically present a very simple method for single-shot real-time detecting pulse-front tilt, curvature, or tilt and curvature (PFT, PFC or PFT&PFC) by using multiple-slit spatiotemporal interferometry (MSTI). An unknown input pulsed beam is spatially cut by a high-density multiple-slit and changed into a series of spatially separated sub-pulses. By only measuring the spatial distribution of the interference pattern in the far-field, PFT, PFC, or PFT&PFC can be detected. Comparing with recent methods, no reference pulses or beams, no temporal or spatial scanning, and no temporal or spectral measurement is required. The single-shot and spatial-only measurement will greatly simplify the real-time detection of PFT and PFC.

Simulating ultra-intense femtosecond lasers in the 3-dimensional space-time domain

Femtosecond petawatt (fs-PW) lasers, with femtosecond pulses and sub-meter-sized beams, could be easily distorted by spatiotemporal coupling (STC). In 2016, a femtosecond terawatt pulsed beam was experimentally reconstructed in the 3-dimensional (3D) space-time domain for the first time, and showing STC induced distortions. Referring to recently developed laser techniques, traditional first-order STCs can be controlled and then removed. However, the complex STC induced by wavefront errors in a meter-sized grating compressor, where the spatial and spectral coordinates of beams and pulses are coupled, would introduce a non-negligible and complicated distortion. Herein, we theoretically simulated this complex STC in the 3D space-time/spectrum domain and presented its evolution with various factors, which opens a new perspective to analyze CPA lasers in the 3D domain.

Magnetic Fluid Deformable Mirror with a Two-Layer Layout of Actuators †

In this paper, a new type of magnetic fluid deformable mirror (MFDM) with a two-layer layout of actuators is proposed to improve the correction performance for full-order aberrations with a high spatial resolution. The shape of the magnetic fluid surface is controlled by the combined magnetic field generated by the Maxwell coil and the two-layer array of miniature coils. The upper-layer actuators which have a small size and high density are used to compensate for small-amplitude high-order aberrations and the lower-layer actuators which have a big size and low density are used to correct large-amplitude low-order aberrations. The analytical model of this deformable mirror is established and the aberration correction performance is verified by the experimental results. As a new kind of wavefront corrector, the MFDM has major advantages such as large stroke, low cost, and easy scalability and fabrication.

Zinc selenide-based large aperture photo-controlled deformable mirror

Realization of large aperture deformable mirrors with a large density of actuators is important in many applications, and photo-controlled deformable mirrors (PCDMs) represent an innovation. Herein we show that PCDMs are scalable realizing a 2-inch aperture device based on a polycrystalline zinc selenide (ZnSe) as the photoconductive substrate and a thin polymeric reflective membrane. ZnSe is electrically characterized and analyzed through a model that we previously introduced. The PCDM is then optically tested, demonstrating its capabilities in adaptive optics.

Pulse front adaptive optics in two-photon microscopy

Adaptive optics has been extensively studied for the correction of phase front aberrations in optical systems. In systems using ultrafast lasers, distortions can also exist in the pulse front (contour of constant intensity in space and time), but until now their correction has been mostly unexplored due to technological limitations. In this Letter, we apply newly developed pulse front adaptive optics, for the first time to our knowledge, to practical compensation of a two-photon fluorescence microscope. With adaptive correction of the system-induced pulse front distortion, improvements beyond conventional phase correction are demonstrated.