Wavefront Rotation Research Articles

Photon emission and radiation reaction effects in surface plasma waves in ultra-high intensities

Manipulating and harnessing plasmonic phenomena in the ultra-relativistic regime reveal promising prospects for the use of surface plasma waves (SPW) to create high-energy particle and radiation sources in the next generation of multi-petawatt lasers. Indeed, relativistic high-charge electron bunches can be produced by SPW excited by ultra-high intensity femtosecond lasers impinging on a periodically modulated solid-density target. In this regime, there is good evidence that SPW excitation survives and that the produced electron bunches experience strong acceleration, thus emitting large amounts of electromagnetic radiation. Therefore, extending the study to ultra-high laser intensities (I>1021 W/cm2), the use of a resonant grating for SPW generation represents an interesting alternative to light sources, as the energy lost by electrons due to radiation emission is transferred to high-energy γ photons. In addition, we show that using a laser with wavefront rotation coupled with a tailored blazed grating improves photon emission in the ultra-relativistic regime of interaction.

Wavefront rotator with near-zero mean polarization change

A K-mirror is a device that rotates the wavefront of an incident optical field. It has recently gained prominence over the Dove prism, another commonly used wavefront rotator, due to the fact that while a K-mirror has several controls for adjusting the internal reflections, a Dove prism is made of a single glass element with no additional control. Thus, one can obtain much lower angular deviations of transmitting wavefronts using a K-mirror than with a Dove prism. However, the accompanying polarization changes in the transmitted field due to rotation persist even in the commercially available K-mirrors. A recent theoretical work [Appl. Opt. 61, 8302 (2022)APOPAI0003-693510.1364/AO.472543] shows that it is possible to optimize the base angle of a K-mirror for a given refractive index such that the accompanying polarization changes are minimum. In contrast, we show in this paper that by optimizing the refractive index it is possible to design a K-mirror at any given base angle and with any given value for the mean polarization change, including near-zero values. Furthermore, we experimentally demonstrate a K-mirror with an order-of-magnitude lower mean polarization change than that of the commercially available K-mirrors. This can have important practical implications for OAM-based applications that require precise wavefront rotation control.

Quantifying polarization changes induced by rotating Dove prisms and K-mirrors.

Dove prisms and K-mirrors are devices extensively used for rotating the wavefront of an optical field. These devices have several applications, including the measurement of orbital angular momentum, microscopy, beam steering, and pattern recognition. However, the wavefront rotation achieved through these devices is always accompanied by polarization changes in the incident field, which is an undesirable feature in many of these applications. Although the polarization changes induced by a Dove prism have been explored to quite some extent, no such study is available for a K-mirror. In this paper, we theoretically and experimentally investigate polarization changes induced in the transmitted field by a rotating K-mirror. For quantifying such polarization changes, we define a quantity, mean polarization change D, which ranges from 0 to π. We find that K-mirrors can reduce D to about 0.03π for any incident state of polarization; however, reducing D to the same extent with a Dove prism is practically unviable. Therefore, K-mirrors are better alternatives to Dove prisms in applications in which the polarization changes accompanying wavefront rotation need to be minimum.

Mitigation of atmospheric turbulence effect by light beams carrying self-rotating wavefront.

We propose an approach against the turbulence-induced degradation by using laser beam with self-rotating wavefront. Such laser beam, generated by the coherent combination of vortex beams with different helical charges and central angular frequencies, can introduce coupling of its wavefront in spatial and temporal domain, that is, periodic wavefront rotation. When the wavefront rotation is faster than the airflow, the laser beam can travel through the inhomogeneity and anisotropy of air in the azimuthal direction within the time interval of airflow. The wavefront distortion caused by the turbulent atmosphere is therefore rotated and gradually smoothed as the laser beam travels. After the laser propagating through the turbulent atmosphere, the total wavefront distortion becomes centrosymmetric with lower peak-to-valley (PV) value. Such smoothed wavefront distortion can dramatically eliminate the turbulence-induced degradation of laser beams, especially beam centroid drift. We believe that this approach can lead to new trend in remote sensing, free-space optical communication, lidar, etc.

Generating Isolated Attosecond X-Ray Pulses by Wavefront Control in a Seeded Free-Electron Laser

We proposed a simple method based on the seeded free-electron laser (FEL) to generate fully coherent X-ray pulses with durations at dozens of attosecond level. The echo-enabled harmonic generation technique is utilized to generate the fully coherent laser pulse covering the water-window range. A wavefront rotation laser is adopted as the seed to tailor the longitudinal contour of the radiation pulse. Due to the sensitivity of seeded FEL to external lasers, this method can effectively inhibit the bunching of the adjacent regions while preserving an isolated bunching in the middle. Sending such an electron beam into a short undulator, simulation results show that ultrashort X-ray pulses with peak power of GW level and pulse duration as short as 86 attoseconds can be generated. The proposed scheme can make it possible to study the electronic dynamic of the valence electrons of which the time scale is about 100 attoseconds and may open up a new frontier of ultrafast science.

Non-resonant waveguiding-spiralizer for generating acoustic orbital angular momentum with arbitrary number of channels

Acoustic orbital angular momentum (a-OAM) has important application values in several fields such as high-capacity acoustic communication or manipulation of objects. In this paper, a non-resonant waveguiding-spiralizer based on the spiral structure was proposed for generating a-OAM and achieved the wavefront rotation with any channels. Compared with active a-OAM generating devices, these waveguiding-spiral structures were compact enough that could be easily fabricated through three-dimensional (3D) printing technology. While compared with other passive a-OAM generating devices, the principle of wavefront rotation realized by the proposed device was geometrical aisles guiding, which belongs to the non-resonant structure. Therefore, it could effectively reduce the energy loss of sound wave, and avoid the narrow-band feature caused by the resonant structure. Most importantly, the working frequency could be adjusted to any working frequency bands by geometric scaling, and it could work in a variety of media or harsh conditions. In addition, the waveguiding-spiralizer could be designed for any number of channels to realize the multiplexing communication.

Optimal generation and isolation of attosecond pulses in an overdriven ionized medium.

We identify optimal conditions for the generation and isolation of attosecond pulses in an overdriven ionized medium. In a high-pressure and highly ionized gas, the spatiotemporal wavefront rotation of a driving laser can be optimized, leading to complete spatial separation of successive attosecond bursts in the far field. The resulting isolated attosecond pulses (IAPs) are much more divergent such that they are spatially separated from the driving laser in the far field. We show that the time delay of near-field harmonic emission along the radial distance determines the divergence of the attosecond burst in the far field. The generated IAPs are phase matched upon propagation in the second half of the gas medium. Validity of the generation scheme is tested at different carrier-envelope phases for a few-cycle laser pulse and by synthesizing the fundamental and its second harmonic field for a long-duration pulse.

Effect of wavefront rotation on the photoionization process by ultrafast laser spatiotemporal focusing

In this paper, we theoretically studied the propagation of the first-order spatiotemporal coupled ultrafast pulse in fused silica glass by using the Cartesian nonlinear Schrödinger equation coupled with the electron density rate equation. We found that wavefront rotation, one parameter of the shaped pulse, may play an integral role to adjust the asymmetrical modification under different initial coupling conditions. During the focusing process, the pulse wavefront generates or rotates the pulse front through angular dispersion, and this process directly causes the change of inclined direction of the pulse front tilt at the focal plane. The spatiotemporal coupling change could make a difference in the asymmetric distributions of intensity, fluence, and electron density at the plane of interaction, and further affect the photoionization process. Therefore, the research on the influence of wavefront rotation on the pulse front tilt under the initial pulse incidence condition helps us to understand the temporal and spatial evolution of ultrafast laser pulses. Based on our numerical simulation, the possible mechanism of nonreciprocal direct-writing phenomena is revisited by taking into account the effect of wavefront rotation.

Ultrashort high energy electron bunches from tunable surface plasma waves driven with laser wavefront rotation.

We propose to use ultrahigh intensity laser pulses with wave-front rotation (WFR) to produce short, ultraintense surface plasma waves (SPW) on grating targets for electron acceleration. Combining a smart grating design with optimal WFR conditions identified through simple analytical modeling and particle-in-cell simulation allows us to decrease the SPW duration (down to a few optical cycles) and increase its peak amplitude. In the relativistic regime, for Iλ_{0}^{2}=3.4×10^{19}W/cm^{2}μm^{2}, such SPW are found to accelerate high charge (few 10s of pC), high energy (up to 70 MeV), and ultrashort (few fs) electron bunches.

Analysis of angular dispersion induced by wavefront rotation in nanosecond optical parametric chirped pulse amplification.

Angular dispersion observed in a nanosecond optical parametric chirped-pulse amplification (ns-OPCPA) amplifier adopted in the frontend of a multi-PW laser was analyzed. The theory on the angular dispersion, extended by including the wavefront rotation and the pulse front tilt of a strongly chirped laser pulse, revealed that the wavefront rotation is a major contributor to the angular dispersion, as compared to the pulse front tilt, in a ns-OPCPA amplifier. It was also shown that the wavefront rotation could be introduced by the phase mismatch and the noncollinear propagation angle in the noncollinear ns-OPCPA amplifier. The theoretical prediction was experimentally verified by measuring the angular dispersion of the ns-OPCPA frontend installed in the 20-fs, 4-PW Ti:Sapphire laser. We emphasize the importance of the proper characterization and control of the angular dispersion in the ns-OPCPA amplifier since the focus intensity of an ultrahigh power laser could be significantly reduced due to the spatiotemporal effect even for small induced angular dispersion.

Spatiotemporal manipulation on focusing and propagation of surface plasmon polariton pulses.

Surface plasmon polariton (SPP) provides an important platform for the design of various nanophotonic devices. However, it is still a big challenge to achieve spatiotemporal manipulation of SPP under both spatially nanoscale and temporally ultrafast conditions. Here, we propose a method of spatiotemporal manipulation of SPP pulse in a plasmonic focusing structure illuminated by a dispersed femtosecond light. Based on dispersion effect of SPP pulse, we achieve the functions of dynamically controlled wavefront rotation in SPP focusing and redirection in SPP propagation within femtosecond range. The influences of structural parameters on the spatiotemporal properties of SPP pulse are numerically studied, and an analytical model is built to explain the results. The spatiotemporal coupling of modulated SPP pulses to dielectric waveguides is also investigated, demonstrating an ultrafast turning of propagation direction. This work has great potential in applications such as on-chip ultrafast photonic information processing, ultrafast beam shaping and attosecond pulse generation.

Correction of wavefront rotation between interferometer and shack-hartmann sensor using bending modes

In some active optics system, the influence function, which is the surface deformation ability of an actuator, is measured by directly detecting the mirror surface using an interferometer and the surface correction of the primary mirror is done using the Shack-Hartmann (S-H) sensor. However, if the wavefronts of the interferometer and the S-H sensor have an obvious rotation, a large error will be introduced to the correction force calculations. In this paper, bending modes are used to detect this wavefront rotation. Bending modes are a series of orthonormal stiffness-increasing modes calculated using the influence functions. Two methods, optimum search and top-line (a line passes through the surface center and the top of surface deformation peaks) detection, are developed and tested in simulation using the experimental data of a 620-mm active optics system. The detection errors of the two methods are 0.286° and 0.085° in the simulation, respectively. The rotation detection method is then tested on this 620-mm system. The simulation and experimental results show that top-line detection is a suitable method for the detection of wavefront rotation.

Time-resolved imaging of mode-conversion process of terahertz transients in subwavelength waveguides

We studied the mode-conversion process of terahertz pulses from a planar subwavelength waveguide to a tilted rectangular subwavelength waveguide. An unusual wavefront rotation, which led to an extra conversion time, was observed using a time-resolved imaging technique. We simulated the mode conversion process by a finite-difference time-domain method, and the results agreed well with the experiments. According to the simulations, the conversion time was demonstrated to become longer as the tilt angle or width of the rectangular waveguide increased. This work provides the possibility to optimize the future high-speed communications and terahertz integrated platforms.

Isolated attosecond pulse in the water window from many-cycle laser-driven plasma mirrors without pulse engineering

High-order harmonic generation from relativistic laser-driven plasma mirrors is an attractive route to produce highly energetic attosecond pulses in the extreme ultraviolet to x-ray regime. To achieve an isolated attosecond pulse (IAP) driven by many-cycle intense laser pulses, pulse engineering techniques such as polarization modulation and wavefront rotation, are usually needed. Here we show that it is possible to generate an IAP without pulse engineering. Through particle-in-cell simulations, it is found that plasma mirrors can be rapidly heated and deformed in a relatively long preplasma regime. Intense IAP in the high-frequency spectral region is given rise once when the mirror parameters are suitable. The results may offer a new route to generate a bright IAP source for various applications such as bio-imaging and electronic dynamic studies.

Few-Cycle Surface Plasmon Polariton Generation by Rotating Wavefront Pulses

A concept for the efficient generation of surface plasmon polaritons (SPPs) with ultrashort duration close to the single-cycle limit is presented. The scheme is based on grating coupling and laser pulses with wavefront rotation (WFR), so that the resonance condition for SPP excitation is satisfied only for a time window shorter than the driving pulse. The feasibility and robustness of the technique is investigated by means of simulations with realistic parameters. In optimal conditions, we find that a 29.5 fs pulse with an 800 nm wavelength can excite SPPs with <4 fs duration (∼1.5 laser cycles) and a peak field amplitude ∼2.7 times the peak value for the laser pulse.

Direct Writing with Tilted-Front Femtosecond Pulses

Shaping light fields in both space and time provides new degrees of freedom to manipulate light-matter interaction on the ultrafast timescale. Through this exploitation of the light field, a greater appreciation of spatio-temporal couplings in focusing has been gained, shedding light on previously unexplored parameters of the femtosecond light pulse, including pulse front tilt and wavefront rotation. Here, we directly investigate the effect of major spatio-temporal couplings on light-matter interaction and reveal unambiguously that in transparent media, pulse front tilt gives rise to the directional asymmetry of the ultrafast laser writing. We demonstrate that the laser pulse with a tilted intensity front deposits energy more efficiently when writing along the tilt than when writing against, producing either an isotropic damage-like or a birefringent nanograting structure. The directional asymmetry in the ultrafast laser writing is qualitatively described in terms of the interaction of a void trapped within the focal volume by the gradient force from the tilted intensity front and the thermocapillary force caused by the gradient of temperature. The observed instantaneous transition from the damage-like to nanograting modification after a finite writing length in a transparent dielectric is phenomenologically described in terms of the first-order phase transition.

Attosecond lighthouse above 100 eV from high-harmonic generation of mid-infrared pulses

In this paper, we numerically investigate the possibility to obtain a lighthouse emission for the attosecond pulses produced by high-order harmonics of a strong mid-infrared fundamental pulse without any optical element inserted in the path of the generating beam. The parameters of the driving pulse, focusing geometry, gas medium and detection configuration are currently experimentally feasible. Here, we study in detail the specific propagation conditions of the laser beam, and describe the exact mechanism of the sensitive space-time variation of the medium’s refractive index that lead to the dynamic wavefront rotation. This basic requirement for the lighthouse phenomenon is transmitted to the harmonic bursts, which are emitted with different divergence in successive optical half-cycles, thus can be detected in the far field at increasing distances from the optical axis. In this configuration, spectral filtering of the harmonics is not necessary, therefore the total harmonic pulse power might be used in further pump-probe experiments.

Dynamic wavefront rotation in the attosecond lighthouse

Attosecond pulses propagating in different directions, generated in a rotating wavefront of a driving laser field, can provide a source of multiple isolated attosecond pulses. Clear spatial separation of the attosecond pulses is attained if the divergence of the individual attosecond pulse is smaller than their angular separation, which is limited by the bandwidth of the driving laser pulse. Here we demonstrate both experimentally and numerically that an additional wavefront rotation is imposed during the propagation of the driving laser pulse in a highly ionizing medium. This dynamic wavefront rotation enables the generation of the isolated attosecond pulse even in the case when the conditions derived from a linear diffraction theory do not permit the angular separation. The described nonlinear phenomenon has its roots in the half-cycle ionization events, and may open up new ways to study strong field processes in highly ionizing media.

Cnoidal waves, solitons and vortices in the flow of polaritons

We theoretically have investigated the properties of the phonon-polariton waves propagating in Kerr-type dielectric medium, and examined the conditions of appearance of the spatial solitons, cnoidal waves and polariton vortices. It is shown that the polariton flow can propagate in the form of spatial soliton, or can be split into several flows in the form of filaments depending on the field density and parameters of the nonlinear medium. In the polariton flow the vortices may appear at the presence of local inhomogeneities of medium refractive index causing the wavefront rotation.

Full characterization of an attosecond pulse generated using an infrared driver.

The physics of attosecond pulse generation requires using infrared driving wavelength to reach the soft X-rays. However, with longer driving wavelength, the harmonic conversion efficiency drops significantly. It makes the conventional attosecond pulse measurement using streaking very difficult due to the low photoionization cross section in the soft X-rays region. In-situ measurement was developed for precisely this purpose. We use in-situ measurement to characterize, in both space and time, an attosecond pulse produced by ultrafast wavefront rotation of a 1.8 μm fundamental beam. We confirm what models suggest – that each beamlet is an isolated attosecond pulse in the time domain. We get almost constant flat wavefront curvature through the whole photon energy range. The measurement method is scalable to the soft X-ray spectral region.