Anil K. Malik

Investigations on terahertz radiation generated by two superposed femtosecond laser pulses

The generation of terahertz (THz) radiation based on tunnel ionization of a gas jet is analytically investigated when two superposed short pulse lasers with finite initial phase difference are focused on to it after passing through an axicon. The phase difference between these two lasers plays an important role for the optimization of rate of ionization, evolution of plasma density, and subsequently the residual current due to dipole oscillations. The directionality of the emitted THz radiation can be controlled by tuning initial phase difference between the two laser pulses. Since a nonuniform plasma is produced during the tunnel ionization, the effect of radial variation in the electron density in the plasma channel is studied on the frequency of the emitted THz radiation and on its power. Higher power THz radiation is obtained for the higher fields of the lasers. With optimum initial phase of the laser envelope and the channel width, the mechanism seems to be much more efficient than some of the other mechanisms.

Strong terahertz radiation by beating of spatial-triangular lasers in a plasma

Resonant excitation of terahertz (THz) radiation by beating of two spatial-triangular laser beams having different frequencies and wave numbers but the same electric fields is proposed, where the ponderomotive force in the transverse direction is realized due to the beating and spatial variation of the lasers’ fields. This gives rise to a stronger transient transverse current due to a sharp gradient in the laser field, and subsequently THz radiation is excited resonantly in the presence of a periodic density structure. The present scheme yields the THz field ∼105 kV/cm and the efficiency ∼10−2 for the laser intensity ∼1014 W/cm2.

Tuning and Focusing of Terahertz Radiation by DC Magnetic Field in a Laser Beating Process

The role of an external DC magnetic held in tuning the frequency and power of terahertz (THz) radiation and obtaining focused radiation is clarified in a beating process using two spatial-triangular lasers, where a nonlinear ponderomotive force acting on the electrons provides oscillating current in longitudinal and transverse directions. The transient transverse current oscillating at the beating frequency produces THz radiations in extraordinary mode, when the beating frequency of lasers matches the upper hybrid frequency for the resonant excitation. Based on the maximum momentum transfer with the application of density ripples, this scheme is realized with high efficiency (>;0.01), where the radiation may attain >;107 V/cm held amplitude. This held can be further enhanced using higher amplitudes of density ripples and lasers fields. The consequences of any deviation from the triangular profile of lasers are also discussed by comparing the results with rounded triangular shape lasers.

Tunable and collimated terahertz radiation generation by femtosecond laser pulses

A mechanism is proposed for the generation of tunable terahertz (THz) radiation under the application of two femtosecond laser pulses and an external magnetic field, where quick tunnel ionization is achieved that leads to higher plasma density evolution and large residual current for the efficient THz radiation generation. With the optimization of magnetic field, phase difference, and amplitudes of lasers’ fields, a THz source can be obtained with tunable frequency and power along with a control on the direction of radiation emission.