Third harmonic generation from graphene lying on different substrates: optical-phonon resonances and interference effects.

Graphene is a nonlinear material which can be used as a saturable absorber, frequency mixer and frequency multiplier. We theoretically study the third harmonic generation from graphene lying on different dielectric (dispersionless or polar) substrates, metalized or non-metalized on the back side. We show that the third harmonic intensity emitted from graphene lying on a substrate, can be increased by orders of magnitude as compared to the isolated graphene, due the LO-phonon resonances in a polar dielectric or due to the interference effects in the substrates metalized on the back side. In some frequency intervals, the presence of the polar dielectric substrate compensates the strongly decreasing with ω frequency dependence of the third-order conductivity of graphene making the response almost frequency independent. Our results can be used for the development of graphene based frequency multipliers operating in microwave through infrared frequencies.

Nonlinear electromagnetic response of a uniform electron gas.

The linear electromagnetic response of a uniform electron gas to a longitudinal electric field is determined, within the self-consistent-field theory, by the linear polarizability and the Lindhard dielectric function. Using the same approach, we derive analytical expressions for the second- and third-order nonlinear polarizabilities of the three-, two-, and one-dimensional homogeneous electron gases with the parabolic electron energy dispersion. The results are valid both for degenerate (Fermi) and nondegenerate (Boltzmann) electron gases. A resonant enhancement of the second- and third-harmonics generation due to a combination of the single-particle and collective (plasma) resonances is predicted.

Coherent nonlinear optical response of graphene.

We investigate the nonlinear optical properties of graphene flakes using four-wave mixing. The corresponding third-order optical susceptibility is found to be remarkably large and only weakly dependent on the wavelength in the near-infrared frequency range. The magnitude of the response is in good agreement with our calculations based on the nonlinear quantum response theory.

[Pharmacokinetics of essentiale N].

The optimum time of exposure of the Essentiale N hepatoprotector with an erythrocyte-leukocyte suspension has been determined in vitro using the blood of healthy donors. Variation of the drug distribution in the blood depending on the exposure time was studied by the method of UV absorption spectroscopy. It is established that the optimum exposure of Essentiale N with an erythrocyte-leukocyte suspension is about 10 min. This corresponds to a minimum concentration of Essentiale N in the supernatant phase, which is evidence for the maximum saturation of the cell composition by the preparation. At the same time, the degree of hemolysis does not exceed the normal physiological level.

Nonlinear electromagnetic response of graphene: frequency multiplication and the self-consistent-field effects.

Graphene is a recently discovered carbon-based material with unique physical properties. This is a monolayer of graphite, and the two-dimensional electrons and holes in it are described by the effective Dirac equation with a vanishing effective mass. As a consequence, the electromagnetic response of graphene is predicted to be strongly nonlinear. We develop a quasi-classical kinetic theory of the nonlinear electromagnetic response of graphene, taking into account the self-consistent-field effects. The response of the system to both harmonic and pulse excitation is considered. The frequency multiplication effect, resulting from the nonlinearity of the electromagnetic response, is studied under realistic experimental conditions. The frequency upconversion efficiency is analyzed as a function of the applied electric field and parameters of the samples. Possible applications of graphene in terahertz electronics are discussed.

New electromagnetic mode in graphene.

A new, weakly damped, transverse electromagnetic mode is predicted in graphene. The mode frequency omega lies in the window 1.667<[see text]omega/micro < 2, where micro is the chemical potential, and can be tuned from radio waves to the infrared by changing the density of charge carriers through a gate voltage.

New type of B-periodic magneto-oscillations in a two-dimensional electron system induced by microwave irradiation.

We observe a new type of magneto-oscillations in the photovoltage and the longitudinal resistance of a two-dimensional electron system. The oscillations are induced by microwave radiation and are periodic in magnetic field. The period is determined by the microwave frequency, the electron density, and the distance between potential probes. The phenomenon is accounted for by interference of coherently excited edge magnetoplasmons in the contact regions and offers perspectives for developing new tunable microwave and terahertz detection schemes and spectroscopic techniques.

Observation of retardation effects in the spectrum of two-dimensional plasmons.

Retardation effects, theoretically predicted more than 35 years ago, are observed in the spectrum of two-dimensional plasmons in high-electron-mobility GaAs/AlGaAs quantum wells. In zero magnetic field, a strong reduction of the resonant plasma frequency is observed due to the hybridization of the plasma and light modes. In a perpendicular magnetic field B, hybrid cyclotron-plasmon modes appear with a very unusual dependence of the frequency on B field. Experimental results are in excellent agreement with the theory.