Direct high-power laser acceleration of ions for medical applications

FDTD modeling of excitation-balanced, mJ-level pulse amplifiers in Yb-doped double-clad optical fibers

A finite-difference time-domain (FDTD) simulation of Yb-doped cladding-pumped, mJ-level, excitation-balanced fiber pulse amplifiers (EBFAs) is presented. In EBFAs, two pumps, one above (anti-Stokes pump, or ASP) and one below (Stokes pump, or SP) the signal wavelength, are utilized to reduce the net thermal energy generated due to the quantum defect. From the results of the FDTD simulation, detailed analyses on the fiber length optimization, excited Yb3+ population evolution, pump and signal power evolution, optical-to-optical (o-o) conversion efficiency, wall plug efficiency, as well as thermal energy generation are performed. For example, with an ASP at 990 nm and a SP at 975 nm, only 2.3 µJ of thermal energy is produced when generating a 2 mJ output pulse at 985 nm, whereas a pulse amplifier with only SP pumping rendering the same 2 mJ output gives more than 10 times the thermal energy. In the meantime, the system maintains an o-o efficiency of 8.43% and wall plug efficiency of 6.6%. The results here indicate the feasibility of the power-scaling of excitation-balanced laser systems, and the FDTD model will be beneficial for the design and optimization of such systems. The first half of this paper presents the FDTD model and provides an example calculation outlining the modeling procedure. The remaining half details the impact of varying laser parameters on system performance. These include pumping and input signal energies, repetition rates, and selection of the ASP, SP, and signal wavelengths. The results presented herein can also be extended to excitation balancing in other solid-state laser systems, such as Yb:YAG and Tm:YAG lasers.

Application of vector beams for enhanced high-order harmonics generation in laser-induced plasmas

High-order harmonics driven by phase- and polarization-structured femtosecond pulses are unique sources of the extreme ultraviolet vortex and vector beams, which have various applications. Here, we report the generation of intense high-order harmonics during propagation of the polarization-structured vector beams (radially polarized beam, azimuthally polarized beam, and their superposition) through the laser-induced plasmas (In, C, CdS, Zns, Ag₂S). Low-order harmonics became stronger with radially polarized and azimuthally polarized driving beams compared with the linearly polarized beams, which is explained on the basis of phase matching and specific properties of vector beams. Contrary to that, the resonance-enhanced harmonic generated in the indium plasma in the case of radially polarized and azimuthally polarized beams was twice weaker compared with the harmonic generated by the LP beam due to modification in the resonant transition selection rules leading to a decrease of the oscillator strength of ionic transitions. Harmonic cut-off and intensity in the case of superposition of the radially and azimuthally polarized beams were lesser compared with the cases of the individual (radially polarized and azimuthally polarized) beams.

Ultralow-loss geometric phase and polarization shaping by ultrafast laser writing in silica glass

Polarization and geometric phase shaping via a space-variant anisotropy has attracted considerable interest for fabrication of flat optical elements and generation of vector beams with applications in various areas of science and technology. Among the methods for anisotropy patterning, imprinting of self-assembled nanograting structures in silica glass by femtosecond laser writing is promising for the fabrication of space-variant birefringent optics with high thermal and chemical durability and high optical damage threshold. However, a drawback is the optical loss due to the light scattering by nanograting structures, which has limited the application. Here, we report a new type of ultrafast laser-induced modification in silica glass, which consists of randomly distributed nanopores elongated in the direction perpendicular to the polarization, providing controllable birefringent structures with transmittance as high as 99% in the visible and near-infrared ranges and >90% in the UV range down to 330 nm. The observed anisotropic nanoporous silica structures are fundamentally different from the femtosecond laser-induced nanogratings and conventional nanoporous silica. A mechanism of nanocavitation via interstitial oxygen generation mediated by multiphoton and avanlanche defect ionization is proposed. We demonstrate ultralow-loss geometrical phase optical elements, including geometrical phase prism and lens, and a vector beam convertor in silica glass.

Collisionless shock acceleration of quasimonoenergetic ions in ultrarelativistic regime

Collisionless shock acceleration of carbon ions (C^{6+}) is investigated in the ultrarelativistic regime of laser-plasma interaction by accounting for the radiation reaction force and the pair production in particle-in-cell simulations. Both radiation reaction force and pair-plasma formation tend to slow down the shock velocity, reducing the energy of the accelerated ions, albeit extending the timescales of the acceleration process. The slab plasma target achieves a lower energy spread while the target with a tailored density profile yields higher ion acceleration energies.

Generation of arbitrary axisymmetrically polarized pulses by using the combination of 4-f spatial light modulator and common-path optical system

We proposed and constructed a system to realize broadband generation of arbitrary axisymmetrically polarized (AP) pulses with spatial complex amplitude modulation. This system employs the combination of a spatial light modulator in the 4-f configuration (4-f SLM), and a space variant wave plate as a common path interferometer. The 4-f SLM and the common path interferometer offer compensation for spatial dispersion with respect to wavelength and stability to perturbation, respectively. We experimentally demonstrated the various AP pulses generation by applying modulations of fundamental and higher-order Laguerre-Gauss modes, whose radial indices were, respectively, p = 0 and 1, with high purity, which showed that we were able to generate arbitral AP pulses with spatial complex amplitude modulation. This technique is expected to be applied in both classical and quantum communications with higher-order modes.

Laser-Induced Linear-Field Particle Acceleration in Free Space

Linear-field particle acceleration in free space (which is distinct from geometries like the linac that requires components in the vicinity of the particle) has been studied for over 20 years, and its ability to eventually produce high-quality, high energy multi-particle bunches has remained a subject of great interest. Arguments can certainly be made that linear-field particle acceleration in free space is very doubtful given that first-order electron-photon interactions are forbidden in free space. Nevertheless, we chose to develop an accurate and truly predictive theoretical formalism to explore this remote possibility when intense, few-cycle electromagnetic pulses are used in a computational experiment. The formalism includes exact treatment of Maxwell’s equations and exact treatment of the interaction among the multiple individual particles at near and far field. Several surprising results emerge. We find that electrons interacting with intense laser pulses in free space are capable of gaining substantial amounts of energy that scale linearly with the field amplitude. For example, 30 keV electrons (2.5% energy spread) are accelerated to 61 MeV (0.5% spread) and to 205 MeV (0.25% spread) using 250 mJ and 2.5 J lasers respectively. These findings carry important implications for our understanding of ultrafast electron-photon interactions in strong fields.

Short pulse laser beam beyond paraxial approximation

Nonparaxial perturbative equations are derived from the scalar wave equation by taking into account spatiotemporal couplings. General solutions are obtained in Fourier space and further transformed back in direct space. They depend on parameters that can be used to match various boundary conditions and the perturbative expansion of any nonparaxial exact solutions. This parametrization is used to study the sensitivity of direct electron acceleration off an ultrashort tightly focused laser pulse to nonparaxial corrections of radially polarized electromagnetic fields.

Laser-Driven Ion Acceleration from Plasma Micro-Channel Targets

Efficient energy boost of the laser-accelerated ions is critical for their applications in biomedical and hadron research. Achiev-able energies continue to rise, with currently highest energies, allowing access to medical therapy energy windows. Here, a new regime of simultaneous acceleration of ~100 MeV protons and multi-100 MeV carbon-ions from plasma micro-channel targets is proposed by using a ~10²⁰ W/cm² modest intensity laser pulse. It is found that two trains of overdense electron bunches are dragged out from the micro-channel and effectively accelerated by the longitudinal electric-field excited in the plasma channel. With the optimized channel size, these “superponderomotive” energetic electrons can be focused on the front surface of the attached plastic substrate. The much intense sheath electric-field is formed on the rear side, leading to up to ~10-fold ionic energy increase compared to the simple planar geometry. The analytical prediction of the optimal channel size and ion maximum energies is derived, which shows good agreement with the particle-in-cell simulations.

Electron dynamics controlled via self-interaction

The dynamics of an electron in a strong laser field can be significantly altered by radiation reaction. This usually results in a strongly damped motion, with the electron losing a large fraction of its initial energy. Here we show that the electron dynamics in a bichromatic laser pulse can be indirectly controlled by a comparatively small radiation reaction force through its interplay with the Lorentz force. By changing the relative phase between the two frequency components of the bichromatic laser field, an ultrarelativistic electron bunch colliding head-on with the laser pulse can be deflected in a controlled way, with the deflection angle being independent of the initial electron energy. The effect is predicted to be observable with laser powers and intensities close to those of current state-of-the-art petawatt laser systems.

Axial separation of orthogonally polarized focal field components due to a radially polarized beam

In this paper, we investigate the field distribution in the focal volume of an aberrated radially polarized beam. Using two different forms of the vectorial diffraction theory, we show that the presence of defocus in the beam displaces both the axially and the radially polarized fields parallel to the optical axis of the focusing lens, while the presence of spherical aberration primarily shifts the longitudinally polarized field only. This facilitates axial separation of the two orthogonally polarized field components, resulting in a significant boost to the ratio of the peak longitudinally polarized field to the peak laterally polarized field in the focal plane. We further show that with an appropriate combination of oppositely signed defocus and spherical aberration, the energy density in the focal volume due to the longitudinally polarized field can be caused to peak at the focal plane. The results obtained are expected to be beneficial to the applications requiring a stronger longitudinally polarized focal field relative to the laterally polarized focal field component.

Dense monoenergetic proton beams from chirped laser-plasma interaction

Interaction of a frequency-chirped laser pulse with single protons and a hydrogen gas target is studied analytically and by means of particle-in-cell simulations, respectively. The feasibility of generating ultraintense (10(7) particles per bunch) and phase-space collimated beams of protons (energy spread of about 1%) is demonstrated. Phase synchronization of the protons and the laser field, guaranteed by the appropriate chirping of the laser pulse, allows the particles to gain sufficient kinetic energy (around 250 MeV) required for such applications as hadron cancer therapy, from state-of-the-art laser systems of intensities of the order of 10(21) W/cm(2).

Intense high-quality medical proton beams via laser fields

Simulations based on the coupled relativistic equations of motion show that protons stemming from laser-plasma processes can be efficiently post-accelerated employing single and crossed pulsed laser beams focused to spot radii on the order of the laser wavelength. We demonstrate that the crossed beams produce quasi-monoenergetic accelerated protons with kinetic energies exceeding 200 MeV, small energy spreads of about 1% and high densities as required for hadron cancer therapy. To our knowledge, this is the first scheme allowing for this important application based on an all-optical set-up.

Differences in the kinetics of gamma-H2AX fluorescence decay after exposure to low and high LET radiation

PURPOSE

In order to obtain more insight into heavy ion tumour therapy, some features of the underlying molecular mechanisms controlling the cellular response to high linear energy transfer (LET) radiation are currently analysed.

MATERIALS AND METHODS

We analysed the decay of the integrated fluorescence intensity of gamma-H2AX (phosphorylated histone H2AX) which is thought to reflect the repair kinetics of radiation-induced DNA double-strand breaks (DSB) using Laser-Scanning-Cytometry. Asynchronous human HeLa cells were irradiated with a single dose of either 1.89 Gy of 55 MeV carbon ions or 5 Gy of 70 kV X-rays.

RESULTS

Measurements of the gamma-H2AX-intensities from 15-60 min resulted in a 16 % decrease for carbon ions and in a 43 % decrease for X-rays. After 21 h, the decrease was 77 % for carbon ions and 85 % for X-rays. The corresponding time-effect relationship was fitted by a bi-exponential function showing a fast and a slow component with identical half-life values for both radiation qualities being 24 +/- 4 min and 13.9 +/- 0.7 h, respectively. Apparent differences in the kinetics following high and low LET irradiation could completely be attributed to quantitative differences in their contributions, with the slow component being responsible for 47 % of the repair after exposure to X-rays as compared to 80 % after carbon ion irradiation.

CONCLUSION

gamma-H2AX loss kinetics follows a bi-exponential decline with two definite decay times independent of LET. The higher contribution of the slow component determined for carbon ion exposure is thought to reflect the increased amount of complex DSB induced by high LET radiation.

Ionization dynamics versus laser intensity in laser-driven multiply charged ions

A sensitive method is put forward to determine the intensity of ultrastrong and short laser pulses via multiply charged ions. For guiding this experimentally challenging task, the laser-induced dynamics of these ions is calculated using both the classical relativistic and quantum Dirac equations. The resulting ionization yields and angular distributions are then evaluated to most sensitively deduce the applied maximal laser pulse intensity.