Second-harmonic emission at a frequency that is twice the laser frequency is an important diagnostic for nonlinear laser–plasma interaction. It is forbidden for centrosymmetric materials such as the bulk of sapphire. The symmetry, however, can be broken by dielectric discontinuities as a result of plasma generation inside a solid dielectric. In the present work, we explore the basic characteristics of experimentally observed second-harmonic emission during focusing a femtosecond Bessel beam inside sapphire. We employ three-dimensional particle-in-cell simulations and the Helmholtz wave equation for theoretical investigations. We analyze how the efficiency of second-harmonic generation and its polarization depend on the plasma parameters. We find that the second-harmonic is generated either due to the coalescence of two-surface electromagnetic waves or nonlinear interaction between the transverse electromagnetic wave and the longitudinal electron plasma wave driven by linear mode conversion. Experimental results agree with the theoretical predictions and confirm the existence of over-critical plasma inside the sapphire that is essential for the resonance of plasma waves or excitation of surface plasmons.
The creation of high-energy-density ([Formula: see text] joules per cm3) over-critical plasmas in a large volume has essential applications in the study of warm dense matter, being present in the hot cores of stars and planets. It was recently shown that femtosecond Bessel beams enable creating over-critical plasmas inside sapphire with sub-wavelength radius and several tens of micrometers in length. Here, the dependence of field structure and absorption mechanism on the plasma density transverse profile are investigated by performing self-consistent Particle-In-Cell (PIC) simulations. Two limiting cases are considered: one is a homogeneous step-like profile that can sustain plasmon formation, and the second is an inhomogeneous Gaussian profile, where resonance absorption occurs. Comparing experimental absorption measures to analytical predictions allows determining the plasma parameters used in PIC simulations. The PIC simulation results are in good agreement with experimental diagnostics of total absorption, near-field fluence distribution, and far-field radiation pattern. We show that in each case, an ambipolar field forms at the plasma surface due to the expansion of the hot electrons and that electron sound waves propagate into the over-critical region.
A triboelectric vibration energy harvester under rotational magnetic excitation for wind energy harvesting applications is developed. The triboelectric beam generates electricity using the magnetic impact-induced vibration. The triboelectric energy harvester consists of a clamped-clamped (CC) beam embedded in an outer case. The lower side of the CC-beam acts as an upper electrode that will undergo a contact-separation motion with another lower electrode with a bonded Polydimethylsiloxane (PDMS) insulator. A permanent magnet will be attached to the CC-beam’s upper side and will face another magnet attached to a rotation rigid shaft blade at the same polarity. While the fan rotates, a repulsive magnetic force will be created between the two magnets that will excite the CC-beam, lead to the contact-separation motion, and generation of electricity. A single degree of freedom model is presented and simulated to extract the dynamic behavior and the generated electrical signal. The relationship between the output voltage and excitation rotational frequency is analyzed. The relationship between the output voltage and the distance between magnets and its effect on the bandwidth is also discussed. The effect of the surface charge density on the generated output voltage signal is examined. The rotational wind triboelectric energy harvester can effectively scavenge wind energy.
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