Terahertz emission from a metallic surface induced by a femtosecond optic pulse

Femtosecond Laser-Induced Emission of Coherent Terahertz Pulses from Ruthenium Thin Films

We demonstrate emission of electromagnetic pulses with frequencies in the terahertz (THz) range from ruthenium thin films through a second-order nonlinear optical process. Ruthenium deposited on different substrates showed different THz emission properties. We provide evidence that for Ru on glass above a certain power threshold, laser-induced oxidation occurs, resulting in an increased slope of the linear dependence of the THz electric field amplitude on pump power. The THz electric field is mainly polarized parallel to the sample surface, pointing in the same direction everywhere. In contrast to Ru on glass, the electric field amplitude of the THz pulses emitted by Ru on sapphire and on CaF2 shows a simple single linear dependence on pump power, and it is polarized orthogonal to the sample surface. In this case, thermal oxidation in an oven enhances the emission and introduces an additional polarization component along the sample surface. This component also points in the same direction everywhere on the surface, similar to the as-deposited Ru on glass. Although the precise THz generation mechanism remains an open question, our results show a strong correlation between the emission strength and the degree of oxidation. Furthermore, the results highlight the importance of the interfaces, i.e., both the choice of the substrate and the chemical composition of the top surface in THz emission experiments. Knowledge of the state of the sample surface is therefore crucial for the interpretation of THz emission experiments from (nonmagnetic) metal surfaces.

Excitation of high-intensity terahertz surface modes of plasma slab under action of p-polarized two-frequency laser radiation

The excitation of the terahertz (THz) high-intensity surface modes when the two-frequency p-polarized laser radiation interacts with a plasma slab is studied. It was found that the significant amplification of the laser field in the plasma slab occurs when p-polarized laser radiation is incident at the angle of total reflection. It is shown that, under the action of laser radiation ponderomotive forces, the resonant excitation of the THz mode of the plasma slab occurs if the frequency difference of the laser fields coincides with the eigenfrequency of the surface mode. It is established that the giant increase in the energy flux density of the THz mode occurs when p-polarized laser radiation is incident at the angle of total reflection on the near-critical plasma slab with rare electron collisions if the conditions of resonant excitation are satisfied. It is shown that in this case the energy flux density of THz mode can significantly exceed the laser intensity.

Competition of quasi-cylindrical and surface waves excited at the femtosecond pulse effect on the metal

Nonlinear generation of the quasi-cylindrical and surface waves in terahertz frequency domain under exposure of a femtosecond laser pulse focused into a strip on a metal was studied. Competition between generated waves is determined by the value of the product of electron collision frequency and laser pulse duration. Comparison of magnetic field pulses of the quasi-cylindrical and surface waves generated on the surface is given for gold, silver, and aluminum. It is shown that far from the focusing strip the surface wave pulse contains oscillations arising due to frequency dispersion.

Laser-induced anisotropy of electronic pressure and excitation of edge currents inside metal

We show theoretically that anisotropy of the electronic distribution function inside the laser-irradiated metal leads to the formation of edge currents at the timescale of distribution isotropization. When the electronic pressure in the skin layer is anisotropic, the pressure gradient appears to be non-potential force producing a low-frequency magnetic field. In the case of femtosecond laser pumping, the estimated internal magnetic field reaches magnitude up to 1 T even in the non-damaging interaction regime. We demonstrate that this field is localized inside the metal, while just a minor part of its energy is radiated into free space as a sub-terahertz signal.

Free-electron mechanisms of low-frequency radiation generation on metal surfaces

We present a time-domain solution of the equations describing low-frequency radiation generation when a metal surface is irradiated by a non-damaging femtosecond optical pulse of s-polarized radiation. Ponderomotive force, drag current, and temperature gradient along the surface are found to be sources of the terahertz radiation. A comparison of these sources under typical experimental conditions reveals that both the drag current and the thermal gradient give similar contribution to the terahertz signal. The ponderomotive force effect is several times less than these two and exists only during the incident pulse.

Terahertz radiation from bismuth surface induced by femtosecond laser pulses

We report on the first experimental observation of terahertz (THz) wave generation from bismuth mono- and polycrystalline samples irradiated by femtosecond laser pulses. Dependencies of the THz signal on the crystal orientation, optical pulse energy, incidence angle, and polarization are presented and discussed together with features of the sample surfaces. The optical-to-THz conversion efficiency was up to two orders of magnitude higher than for metal at a moderate fluence of ∼1  mJ/cm². We also found nonlinear effects not previously observed using other metal and semiconductor materials: (a) asymmetry of THz response with respect to a half-turn of a sample around its normal, (b) THz polarization control by orientation of the sample surface, and

High-power THz to IR emission by femtosecond laser irradiation of random 2D metallic nanostructures

Terahertz (THz) spectroscopic sensing and imaging has identified its potentials in a number of areas such as standoff security screening at portals, explosive detection at battle fields, bio-medical research, and so on. With these needs, the development of an intense and broadband THz source has been a focus of THz research. In this work, we report an intense (~10 mW) and ultra-broadband (~150 THz) THz to infrared (IR) source with a Gaussian wavefront, emitted from nano-pore-structured metallic thin films with femtosecond laser pulse excitation. The underlying mechanism has been proposed as thermal radiation. In addition, an intense coherent THz signal was generated through the optical rectification process simultaneously with the strong thermal signal. This unique feature opens up new avenues in biomedical research.

Terahertz wave generation from thin metal films excited by asymmetrical optical fields

We experimentally demonstrated terahertz (THz) wave emission from thin metal (gold) films excited by asymmetrical optical fields synthesized using an in-line phase compensator. By driving the electrons in thin metal films asymmetrically, THz wave emission is observed at normal incidence of two-color pump beams. Coherent control of THz wave emission from metal films suggests that a mechanism similar to that of the air-plasma THz source excited by two-color laser fields can be used to describe the generation processes.