High harmonic generation (HHG) is an ideal probing source. In general, all harmonics are coupled with the corresponding input laser when generated, and for applications, they are separated using additional spectrometers. Herein, we report the angular isolation of relativistic harmonics at a predicted emission angle upon generation and, most importantly, a new phase-matching chain selection rule is derived to generate harmonics. Based on the laser plasma mechanism involving two non-collinear relativistic driving lasers, the nth harmonic carrying the information of both input lasers originates from its adjacent (n – 1)th harmonic coupled with one of the input lasers. Meanwhile, the intensity and emission angle of the generated isolated harmonic are both greatly increased compared with those in the gas scheme. These results are satisfactorily verified by theoretical analysis and three-dimensional particle-in-cell simulations, which have physical significance and are essential for practical applications.

Two collinear laser pulses of finite spot size propagating through a capillary plasma, modeled as a hollow plasma cylinder, are shown to produce beat frequency terahertz (THz) surface plasmons at the inner surface. The evanescent laser fields in the plasma impart oscillatory velocity to electrons and exert a beat ponderomotive force on them. The static component of the ponderomotive force inhibits plasma from filling the vacuum region while the beat frequency component produces a nonlinear current (${\vec J^{{\;\rm NL}}}$) that drives the difference frequency THz surface plasma wave (SPW). Phase matching for the THz surface wave excitation is achieved when the group velocity of the lasers equals the phase velocity of the beat frequency SPW. At laser intensities of ~1014 W/cm2 at 10 μm wavelength, one may attain normalized surface wave amplitude ~ 0.03.

The growing demand for communication systems requires multi-channel solutions. However, the number of antennas in communication systems is dictated by the number of channels in each system which is limited by the available space on a broadcasting platform and by the limited available resources. We have developed a Multi-Channel Phase Control coupler in the VHF frequency range enabling a reduction in the number of antennas to a third of the original setup. The system is based on a phase shifter in a meander stripline geometry connected to each channel individually. The channels are then phase matched simultaneously to a single antenna through a computer-controlled capacitor bank connected to each phase shifter. The system performance shows a low insertion loss of 0.5 dB and a low return loss of −15 dB for the multi-channel setup.