Kunwar Pal Singh

Effect of gate length and dielectric thickness on ion and fluid transport in a fluidic nanochannel

We have simulated the effect of gate length and dielectric thickness on ion and fluid transport in a fluidic nanochannel with negative surface charge on its walls. A short gate is unable to induce significant cation enrichment in the nanochannel and ion current is controlled mostly by cation depletion at positive gate potentials. The cation enrichment increases with increasing gate length and/or decreasing dielectric thickness due to higher changes induced in the surface charge density and zeta-potential. Thus, long gates and thin dielectric layers are more effective in controlling ion current. The model without Navier-Stokes equations is unable to correctly predict phenomena such as cation enrichment, increase in channel conductivity, and decreasing electric field. Body force and induced fluid velocity decrease slowly and then rapidly with gate potentials. The effectiveness of ion current control by a gate reduces with increasing surface charge density due to reduced fractional change in zeta-potential.

Effect of surface charge density and electro-osmotic flow on ionic current in a bipolar nanopore fluidic diode

We have simulated bipolar nanopore fluidic diodes for different values of surface charge densities, electrolyte concentrations, and thickness of transition zone. Nanopore enrichment leads to increased nanopore conductivity with the surface charge density at low electrolyte concentrations. Potential drop across the nanopore and electric field inside the nanopore decreases. Forward current and ionic current rectification peaks for a specific value of surface charge density. Even though the electro-osmotic current component remains small as compared to other components, its non-inclusion in the modeling leads to serious errors in the solutions. Significant ion current rectification can be obtained even if transition zone between oppositely charged zones is not narrow. The effect of the surface charge is screened by counterions at higher electrolyte concentrations, which leads to reduced electrolyte polarization and a decrease in the ion current rectification.

Field-effect control of electrokinetic ion transport in a nanofluidic channel

We have simulated field-effect control of electrokinetic ion transport in a fluidic nanochannel with negative surface charge on its walls. A third electrode, known as a gate, is used on the channel walls to modulate its zeta-potential and ion concentration inside it. The ion current is controlled by the gate-induced ion enrichment/depletion and changes of electric field in the vicinity of the gate. There are four regions of ion current control by gate at low electrolyte concentration: decreasing electric field, cation enrichment, quasi-neutrality, and cation depletion as the gate potential changes from negative values to positive values. The effectiveness of ion current control by gate decreases with increasing surface charge density due to change in zeta-potential and overall electro-neutrality condition. The ion current through the nanochannel is also affected by electrolyte concentration. The proposed nanofluidic device could have broad applications in integrated nanofluidic circuits for manipulation o...

Ion current rectification in a fluidic bipolar nanochannel with smooth junction

We have simulated bipolar nanochannel based fluidic diode for different values of junction sharpness. We can obtain significant ion current rectification even for a smooth junction between oppositely charged zones. The rectification increases with junction sharpness due to increase in unipolar character of electrolyte but a sharp junction is not a necessary condition for rectification. The ion current rectification increases with surface charge density due to increase in unipolar character of electrolyte and decrease in reverse ion current. The fluid enters (exits) the nanochannel through the centre from (to) the opposite directions for reverse (forward) bias due to fluid pressure.

Highly focused and efficient terahertz radiation generation by photo-mixing of lasers in plasma in the presence of magnetic field

A mechanism of efficient and highly focused terahertz (THz) radiation generation by photo-mixing of top-hat like lasers with frequencies ω1, ω2 and wave numbers k1, k2 in pre-formed rippled density (corrugated) plasma is proposed. In this mechanism, intensity variation of lasers offers nonlinear ponderomotive force at frequency ω′=ω1−ω2 and wave number k′=k1−k2 which couples with density ripples in the plasma and leads to a strong nonlinear oscillatory current that resonantly excites highly focused and intense THz radiation at frequency ωUH=(ωp2+ωc2) (where ωc is electron cyclotron frequency). The efficiency of emitted THz radiation of the order of 15% is obtained under optimum conditions. It is observed that focus and intensity of emitted radiation can be controlled by selecting a proper profile index of the lasers, ripple parameters, and tuning of external magnetic field.

High-intensity terahertz generation by nonlinear frequency-mixing of lasers in plasma with DC magnetic field

We propose a mechanism of highly focused, tunable and high-intensity terahertz (THz) radiation generation by frequency-mixing of two super-Gaussian lasers with frequencies ω 1 , ω 2 and wave numbers k 1 , k 2 (laser profile index p > 2) in a corrugated plasma in the presence of external static magnetic field

Quasi-monoenergetic GeV electrons from the interaction of two laser pulses with a gas

{B_0}\hat z

Tunable and efficient terahertz radiation generation by photomixing of two super Gaussian laser pulses in a corrugated magnetized plasma

. In this process, a strong nonlinear ponderomotive force is offered to the plasma electrons at frequency ω′ = ω 1 − ω 2 and wave number k ′ = k 1 − k 2 by laser beams. The ponderomotive force results in a strong, controllable nonlinear transverse oscillatory current, which can be optimized by optimizing the external magnetic field, ripple parameters, and laser indexes. This controllable current produces focused and intense THz radiation of tunable frequency and power along with a remarkable efficiency ~25%.

Quasimonoenergic collimated electrons from the ionization of nitrogen by a chirped intense laser pulse

A scheme is proposed for the acceleration of electrons generated during the ionization of a gas by two laser pulses. The electrons created from the ionization of neutral atoms near the rising edge of the pulse do not gain sufficient energy. If a prepulse is used before the main pulse then the prepulse removes electrons from the outer shells, and the main laser pulse interacts with the electrons in the inner shells of high atomic number gases, such as krypton and argon. The electrons are generated close to the peak of the main laser pulse and gain energy in GeV with a small spread in the energy and low emittance angle.

Electron energy enhancement by a circularly polarized laser pulse in vacuum

A scheme of terahertz (THz) radiation generation is investigated by photo-mixing of two super Gaussian laser beams having different frequencies (ω1, ω2) and wave numbers (k→1, k→2) in a performed corrugated plasma embedded with transverse dc magnetic field. Lasers exert a nonlinear ponderomotive force, imparting an oscillatory velocity to plasma electrons that couples with the density corrugations ( n′=nα0eiαz) to generate a strong transient nonlinear current, that resonantly derives THz radiation of frequency ∼ ωh (upper hybrid frequency). The periodicity of density corrugations is suitably chosen to transfer maximum momentum from lasers to THz radiation at phase matching conditions ω=ω1−ω2 and k→=k→1−k→2+α→. The efficiency, power, beam quality, and tunability of the present scheme exhibit high dependency upon the applied transverse dc magnetic field along with q-indices and beam width parameters ( a0) of super Gaussian lasers. In the present scheme, efficiency ∼10−2 is achieved with the optimization of ...