We suggest a concept of a tunable graphene-based terahertz (THz) surface emitting laser with diffusion pumping. We employ significant difference in the electronic energy gap of graphene and a typical wide-gap semiconductor, and demonstrate that carriers generated in the semiconductor can be efficiently captured by graphene resulting in population inversion and corresponding THz lasing from graphene. We develop design principles for such a laser and estimate its performance. We predict up to 50 W/cm2 terahertz power output for 100 kW/cm2 pump power at frequency around 10 THz at room temperature.

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The amplification of terahertz plasmons in a pair of parallel active graphene monolayers is studied theoretically. The plasmon wave in a symmetric double-layer graphene structure splits into two branches with a symmetric and an antisymmetric distribution of tangential to graphene component of the electric field across the plane of symmetry of the structure. It is shown that, normalized to the wavelength, the terahertz plasmon amplification factor of the symmetric mode in the double-layer graphene structure could be greater than that in a single graphene layer by four orders of magnitude.

The Dirac plasmon propagation in active pristine graphene with the carrier population inversion created by the diffusion of the photoexcited carriers from a semiconductor substrate is studied theoretically. It is shown that an order of magnitude smaller pump power can be used for the diffusion pumping as compared to direct optical pumping of graphene for obtaining the same plasmon gain in graphene. We find that the field of the amplified plasmons remains strongly confined in the vicinity of graphene similarly to the case of the attenuated plasmons. Remarkably, the diffusion pumping is characterized by low insertion losses due to small photoexcited carrier concentration in the carrier-supplying semiconductor substrate in the region of the plasmon field confined near graphene.

In this Letter we research the space charge limiting current value at which the oscillating virtual cathode is formed in the relativistic electron beam as a function of the external magnetic field guiding the beam electrons. It is shown that the space charge limiting (critical) current decreases with growth of the external magnetic field, and that there is an optimal induction value of the magnetic field at which the critical current for the onset of virtual cathode oscillations in the electron beam is minimum. For the strong external magnetic field the space charge limiting current corresponds to the analytical relation derived under the assumption that the motion of the electron beam is one-dimensional [High Power Microwave Sources. Artech Microwave Library, 1987. Chapter 13]. Such behavior is explained by the characteristic features of the dynamics of electron space charge in the longitudinal and radial directions in the drift space at the different external magnetic fields. House