Research at modern light sources continues to improve our knowledge of the natural world, from the subtle workings of life to matter under extreme conditions. Free-electron lasers, for instance, have enabled the characterization of biomolecular structures with sub-ångström spatial resolution, and paved the way to controlling the molecular functions. On the other hand, attosecond temporal resolution is necessary to broaden our scope of the ultrafast world. Here we discuss attosecond pulse generation beyond present capabilities. Furthermore, we review three recently proposed methods of generating attosecond x-ray pulses. These novel methods exploit the coherent radiation of microbunched electrons in undulators and the tailoring of the emitted wavefronts. The computed pulse energy outperforms pre-existing technologies by three orders of magnitude. Specifically, our simulations of the proposed Soft X-ray Laser at MAX IV (Lund, Sweden) show that a pulse duration of 50–100 as and a pulse energy up to 5 J is feasible with the novel methods. In addition, the methods feature pulse shape control, enable the incorporation of orbital angular momentum, and can be used in combination with modern compact free-electron laser setups.

Strong-field few-cycle terahertz (THz) pulses are an invaluable tool for engineering highly nonequilibrium states of matter. A scheme is proposed to generate quasi-half-cycle GV/m-scale THz pulses with a multikilohertz repetition rate. It makes use of coherent spontaneous emission from a prebunched electron beam traversing an optimally tapered undulator. The scheme is the further development of the slippage control in free-electron lasers [T. Tanaka, Phys. Rev. Lett. 114, 044801 (2015)]. An explicit condition for the spectral amplitude of undulator radiation and a phase condition for the electron density distribution, required for the generation of desired pulses, are presented. The amplitude condition is met by proper undulator tapering, and a generic optimal undulator profile is found analytically. In order to meet the phase condition, the distance between the adjacent bunches is varied according to the instantaneous resonant undulator wavelength. A 3D analytical theory is complemented by a detailed numerical study based on a direct solution to the 3D wave equation.

Resonant optical antennas supporting plasmon polaritons (SPPs)—collective excitations of electrons coupled to electromagnetic fields in a medium—are relevant to sensing, photovoltaics, and light‐emitting devices, among others. Due to the SPP dispersion, a conventional antenna of fixed geometry, exhibiting a narrow SPP resonance, cannot simultaneously operate in two different spectral bands. In contrast, here it is demonstrated that in metallic disks, separated by a nanometric spacer, the hybridized antibonding SPP mode stays in the visible range, while the bonding one can be pushed down to the mid‐infrared range. Such an SPP dimer can sense two materials of nanoscale volumes, whose fingerprint central frequencies differ by a factor of 5. Additionally, the mid‐infrared SPP resonance can be tuned by employing a phase‐change material (VO₂) as a spacer. The dielectric constant of the phase‐change material is controlled by heating the material at the frequency of the antibonding optical mode. These findings open the door to a new class of optoelectronic devices able to operate in significantly different frequency ranges in the linear regime, and with the same polarization of the illuminating wave.

We study the operation regime of a hybrid planar free-electron maser (FEM) amplifier near the magnetoresonant value of the uniform longitudinal (guide) magnetic field. Using analytical expressions for individual test electron trajectories and normal frequencies of their three-dimensional oscillations in the magnetostatic field of the hybrid planar FEM, an analytical condition of chaotization of motion is established and shown to be given by the Chirikov resonance-overlap criterion applied to the normal undulator and cyclotron frequencies with respect to the coupling induced by the undulator magnetic field. It is also shown analytically that, in spite of the well-known drop for the exact magnetoresonance, the gain attains its maximal value in the zone of regular dynamics slightly above the magnetoresonant value of the guide magnetic field. Under the condition of undulator resonance, it is practically independent of the amplitude of the undulator magnetic field and the wavelength of amplified signal. To account for space-charge effects, we propose a theoretical model of a weakly relativistic FEM, which accommodates not only potential but also rotational parts of the nonradiated electromagnetic field of a moving charged particle. It turns out that the rotational part of nonradiated field diminishes the defocusing influence of the potential part on the beam bunching. Numeric simulation of the nonlinear stage of amplification is fulfilled, taking into consideration adiabatic entrance of the electron beam to the interaction region and initial electron velocity spread. We find that nonradiated field and initial electron velocity spread do not influence essentially the efficiency of hybrid planar FEM amplification if parameters of the beam-microwave interaction correspond to the operational regime in the zone of regular dynamics near the magnetoresonance

The interaction of waveguide modes and consequences on laser operation are studied numerically in a THz free electron laser (FEL) driven by short electron bunches. The considered FEL cavity configuration is represented by a parallel-plate waveguide extending over the complete distance between cylindrical cavity mirrors with energy out-coupling through a rectangular slit in one of the mirrors. We describe the multimode FEL cavity desynchronization dynamics and predict strong enhancement in FEL power at special cavity configurations, when modes are degenerate. Furthermore, we predict that simultaneous excitation of several waveguide modes can occur but do not negatively influence the lasing process. DOI: http://dx.doi.org/10.1103/PhysRevSTAB.17.050703 Published by the American Physical Society under the terms of the Creative Commons Attribution 3.0 License. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Published by the American Physical Society