Nonlinear Mixing of Two Laser Beams and THz Plasmons Generation in Graphene Coated Optical Fibre

Graphene is a most suitable material for Terahertz (THz) radiation generation. An efficient mechanism of THz surface plasmons (THz SPs) generation in graphene coated optical fibre by nonlinear mixing of two laser beams is proposed. The graphene coated fiber supports THz SPs with plasmon resonance in the THz regime and controllable by thickness of graphene and radius of optical fibre. The laser beams exert a difference frequency ponderomotive force on the electrons of the graphene. This ponderomotive force induces a nonlinear current in graphene which driving the difference frequency THz SPs. The normalized amplitude of THz SPs decreases with frequency as the nonlinear coupling gets weaker. The efficiency of the device is around 0.01% at a laser intensity of 3x1014 W/cm2. This scheme will be useful making the compact THz radiation source and THz plasmon sensor.

Hat-Top Beams for Generating Tunable THz Radiations Using a Medium of Conducting Nanocylinders

There are a large number of studies for terahertz (THz) radiation generation, but tunable THz sources are still a challenge since it is difficult to tune frequency, focus and intensity of the radiation simultaneously. The present work proposes the THz generation by the interaction of two hat-top laser beams with a host medium of argon gas containing graphite nanocylinders, as these beams result in highly nonlinear interaction because of a smooth dip in their peak intensity and a fast rise and fall in the overall intensity pattern. Such an interaction produces nonlinear current (6.7 × 108 A/m2) because of the electron cloud of the nanocylinders, which can be modulated by the laser and medium properties for realizing tunable THz radiation. The orientation of basal planes of nanocylinders is shown to be important for this mechanism, though it may be challenging for the experimentalists. The resonant excitation takes place when the plasmon frequency matches the beating frequency of the laser beams, and in the proposed mechanism one can have longitudinal surface plasmon resonance (~12 THz) and transverse surface plasmon resonance (~40 THz), leading to frequency-tunable THz radiation. The role of height and inter particle distance between the adjacent nanocylinders on the THz field amplitude and the efficiency of the mechanism is uncovered by controlling the aspect ratio in the nanocylinders. For example, reducing the inter particle distance from 180 nm to 60 nm leads to the enhancement of THz field from 1´108 V/m to 5.48´108 V/m. The profile of the emitted THz radiation is investigated in detail under the effect of various parameters in order to prove the practicality of the proposal. The proposed design and mechanism would be attractive for electromagnetic and communication societies which are dealing with millimeter-waves and THz components in addition to its medical application.

Competing processes in generation of the third optical harmonic in air under femtosecond infrared repetitively pulsed excitation

Parametric interaction of electromagnetic and gravitational waves with the radiation generation at the third harmonic wavelength is one of the ways to detect gravitational interaction in a material medium. To implement the effect in question, superstrong fields must be used, but in this case competing nonlinear processes arise, leading to the generation of the third harmonic as a result of laser radiation filamentation. This paper investigates the characteristics of the radiation recorded for femtosecond (250 fs) laser pulses with a wavelength of λ = 1032 nm focused in air. The threshold pump power made it possible to observe the formation of a filament with concomitant generation of narrow-band radiation at the focus of the lens at the third harmonic wavelength λ = 344 nm. The research presents spectral and spatial dependences of ultraviolet radiation (λ = 344 nm) at pumping power of infrared radiation (λ = 1032 nm) of 500 mW. Energy dependences of the third harmonic generation efficiency in the power range from 150 to 1750 mW are obtained.

Detailed study of the GRB 190114C spectral lags in the energy range of 5 keV – 2 MeV

The spectral lags of gamma ray bursts are defined as the difference in the registration time of the same radiation pulse in different energy channels of the recording device. This parameter can characterize both the mechanism of radiation generation by the source and the physical conditions of radiation propagation from the source to the observer. In this paper, the dependence of the arrival time of photons on their energy for the gamma ray burst GRB 190114C is obtained from the data of the Gamma ray Burst Monitor (NaI detectors) of the Fermi Gamma ray Space Telescope. It is shown that this dependence is mainly due to the back edges of the light curve pulses. The spectral lags of the leading edges of the pulses are small and comparable in magnitude to the measurement errors. The observed anomaly in the energy range from 5 to 20 keV is probably related to the quasi-thermal radiation of the source.

Terahertz Radiation Generation Process In The Medium Based On The Array of The Elongated Nanoparticles

We conduct a theoretical study and numerical simulations of terahertz radiation generation in the medium based on armchair-edge nanoribbons and zigzag nanotubes with metallic conductivity. The multicascade mechanism of radiation generation is considered in the task of terahertz radiation generation. The level of the injection current in nanoparticle arrays has been estimated. The task expands to the similar medium where radiation current is generated with the use of infrared radiation stimulated absorption, for example, radiation of a CO2 laser. Numerical results for the effective dielectric function have been obtained with the use of the effective medium-approximation, the Maxwell-Garnett’s theory and the Maxwell-Garnett model with the Clausius-Mossotti correction (geometrical model arrays).