Abstract-High-power terahertz emission from a plasma penetrated by counterstreaming different-size electron beams

Physics of Plasmas

V. V. Annenkov, E. A. Berendeev,  I. V. Timofeev,  E. P. Volchok

A part of the simulation box near its left edge.

https://aip.scitation.org/doi/abs/10.1063/1.5048245

It is found that multi-cycle pulses of high-power coherent terahertz radiation can be efficiently generated in a plasma by counterstreaming long-pulse electron beams driving potential plasma waves via the two-stream instability. Instead of the well-known three-wave interaction processes between oblique beam-driven modes, we propose to generate electromagnetic radiation near the doubled plasma frequency due to the novel and much more efficient mechanism based on the head-on collision of longitudinal plasma waves with mismatching potential profiles. It is shown that this radiation mechanism can be implemented experimentally either by the collision of low-density electron beams with different transverse sizes or by the counter injection of denser equal-size beams unstable against filamentation perturbations. Particle-in-cell simulations for kiloampere electron beams capable of focusing into millimeter-scale sizes demonstrate the possibility to reach the high efficiency of beams-to-THz power conversion (3%–7%), which opens the path to gigawatt-class THz sources with a narrow spectral line.

Abstract-Generation of twisted terahertz radiation through plasma in the presence of a static electric field

Hassan Sobhani, Sahar Feili, Majid Taraz,

http://iopscience.iop.org/article/10.1088/1555-6611/aacfa2/meta

The generation of twisted terahertz radiation through plasma is surveyed by employing a static electric field. By propagating two coaxial Laguerre–Gaussian lasers in plasma, the electron density distribution is perturbed. The static electric field imposes a velocity on the perturbed electron density. This current density can emit radiation at the terahertz frequency. We consider a plasma density distribution to produce terahetrz radiation with a Laguerre–Gaussian envelope. Three possibilities can be predicted for the orbital angular momentum of the twisted terahertz, which is expressed by a new selection rule. By choosing a suitable plasma distribution, only one of the orbital angular momentums can be produced.

Abstract-Enhancing terahertz generation from a two-color plasma using OPA waste light

Shayne A. Sorenson, Clayton D. Moss, Steven K. Kauwe, Jacob D. Bagley, Jeremy A. Johnson

https://arxiv.org/abs/1804.01178

We show experimentally that the terahertz (THz) emission of a plasma, generated in air by a two-color laser pulse (containing a near IR frequency and its second harmonic), can be enhanced by the addition of an 800-nm pulse. We observed enhancements of the THz electric field by a factor of up to 30. This provides a widely accessible means for researchers using optical parametric amplifiers (OPA) to increase their THz yields by simply adding the residual pump beam of the OPA to the plasma generating beam. We investigate the dependence of the THz electric field enhancement factor on the powers of the two-color beam as well as the 800 nm enhancement beam. Numerical calculations using the well-known photocurrent model are in excellent agreement with the experimental observations.

Abstract-Two color laser self focusing and terahertz generation in multi-ion species plasma

Nafis Ahmada,  Saleh T. Mahmouda, Gunjan Purohitb,  Firoz Khan

https://www.sciencedirect.com/science/article/pii/S0030402617317904


An analytical formalism of self focusing of two co-propagating laser beams, fundamental (ω₁)$\left({}_{\mathrm{}}\right)$ and close to second harmonic (ω₂≈2ω₁)$\left({}_{\mathrm{}}\mathrm{}{}_{\mathrm{}}\right)$, in a multi-ion species plasma and resonant terahertz radiation generation has been developed. The nonlinearity arises due to ponderomotive force on electrons by both the beams and subsequent ambipolar redistribution of the plasma. The ions of higher charged state are strongly depleted from the axial region due to space charge field effects and electron charge is largely compensated by the ions of lower charge. When the power of the strong beam equals the critical power for its self-guiding, the plasma provides an oscillatory waveguide for the weak beam; the weak beam shrinks and rebounds when its frequency is lower than that of the strong beam, and it swallows and rebounds when the frequency is higher. When both beams are strong the nonlinearities have cumulative effect on both the beams. The cross coupling of the beams gives rise to nonlinear current giving rise to terahertz radiation at ω=ω₂−2ω₁${}_{\mathrm{}}\mathrm{}{}_{\mathrm{}}$ frequency. An axial density ripple aids phase matching and gives rise to higher power conversion efficiency. Self focusing leads to enhancement in conversion efficiency.

Abstract-Study of Terahertz Radiation Generation by Two Laser Beams in an Axial Magnetized Rippled Density Plasma

Ayoob Hematizadeh  ;  Seyed Masud Jazayeri

http://ieeexplore.ieee.org/document/7945274/

This paper presents a scheme to achieve terahertz radiation by the beating of top-hat lasers in a rippled density collisional magnetized plasma. The nonlinear current at terahertz frequency arises on account of nonlinear ponderomotive force as a result of beating of the two lasers. A uniform static magnetic field which is considered parallel to the direction of lasers leads to, depending on the phase-matching conditions, propagation of right-hand circularly polarized (RCP) or left-hand circularly polarized (LCP) waves in plasma. It is found that, terahertz amplitude of RCP wave for high values of beating frequency is slightly larger than LCP wave. The contribution of magnetic field, laser index and collision frequency are discussed for the efficient terahertz radiation generation. With the optimization of these parameters, the efficiency of order of 25 percent can be achieved in the present scheme.

Abstract-Terahertz Acoustics in Hot Dense Laser Plasmas

Amitava Adak, A. P. L. Robinson, Prashant Kumar Singh, Gourab Chatterjee, Amit D. Lad, John Pasley, and G. Ravindra Kumar

Phys. Rev. Lett. 114, 115001 – Published 17 March 2015

http://journals.aps.org/prl/abstract/10.1103/PhysRevLett.114.115001

We present a hitherto unobserved facet of hydrodynamics, namely the generation of an ultrahigh frequency acoustic disturbance in the terahertz frequency range, whose origins are purely hydrodynamic in nature. The disturbance is caused by differential flow velocities down a density gradient in a plasma created by a 30 fs, 800 nm high-intensity laser (∼5×1016  W/cm2). The picosecond scale observations enable us to capture these high frequency oscillations (1.9±0.6  THz) which are generated as a consequence of the rapid heating of the medium by the laser. Adoption of two complementary techniques, namely pump-probe reflectometry and pump-probe Doppler spectrometry provides unambiguous identification of this terahertz acoustic disturbance. Hydrodynamic simulations well reproduce the observations, offering insight into this process.

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Abstract-Terahertz Plasmonics: Good Results and Great Expectations

Otsuji, T. Shur, M.
Research Institute of Electrical Communication,, Tohoku University, Sendai, 980-8577, JAPAN 
http://ieeexplore.ieee.org/xpl/abstractAuthors.jsp?arnumber=6954554&sortType%3Dasc_p_Sequence%26filter%3DAND%28p_IS_Number%3A6954433%29

The terahertz (THz) range is the next frontier of electronics and optoelectronics with potential applications ranging from imaging, space communications, computing, quality control, and homeland security to biotechnology and medicine. At THz frequencies, the electron inertia becomes important, providing delay between the applied voltage and electron velocity and current. When the electron collisions with impurities and lattice vibrations are infrequent, this delay leads to oscillations of the electronic density (called plasma waves) with the transistor channels serving as resonant cavities for the plasma waves. In the collision-dominated regime, the plasma waves are overdamped but still play a role by dramatically changing the electron distribution in the device channels at THz frequencies. The resonant regime can be used to generate THz radiation. Both resonant and overdamped plasma waves enable other THz electronic devices, such as detectors, mixers, and phase shifters. Periodic (symmetrical and asymmetric) plasmonic structures are especially promising for generation and detection of THz radiation. In this article, we review the state of the art of the plasma-wave electronics for silicon, III-V, III-N, and graphene semiconductor devices and project future performance of plasma-wave THz devices.

L.L. Raja Receives $1.4 M DoD Grant to Expand Plasma Technology

Wednesday, August 27, 2014

Written by Monica Kortsha

http://www.ae.utexas.edu/news/features/757-plasma-dod-raja

Plasma, the fourth state of matter, is created when gaseous molecules are ripped apart into constitutive ions and electrons. Plasma is what makes lightning flash, and electric sparks fly. But it’s also what’s in fluorescent lights and buzzing neon signs.

“Even though it sounds exotic, plasmas are everywhere,” says Dr. Laxminarayan Raja, a plasma expert and researcher at UT’s ASE/EM Department. “Fluorescent lamps, neon signs, plasma TVs, even the sun is a plasma ball.”

Raja is working to expand plasma technology even further by helping design a new class of plasma-based materials that respond to terahertz (THz) waves, a largely untapped region of the electromagnetic spectrum with potential applications in imaging and sensor technology. His research is funded by a $1.4 million five year Multidisciplinary University Research Initiative (MURI)grant from the U.S. Department of Defense and is conducted in partnership with Stanford University, Tufts University, Pennsylvania State University, the University of Washington and the University of California, Los Angeles. The total grant amount between all involved universities is over $7 million.

On the electromagnetic spectrum, THz waves are the midpoint between wavelengths primarily used for industrial purposes, such as microwaves, and those used for imaging purposes, such as infrared. This position endows THz waves with unique properties that are particularly useful for imaging and sensing. However, the position also puts THz waves out of range of typical electromagnetic wave generating, guiding and detecting equipment. This lack of technology even has a name: the “terahertz gap.” 

An illustration of an electromagnetic wave interacting with collimated plasma jets acting as a plasma-based metamaterial. The metamaterial transmits and reflects part of the wave energy and depending on the array geometry and density of individual plasma jets,  exotic properties such as a negative index of refraction can be realized. 

Raja and colleagues are working to address the gap by creating a new class of metamaterials and photonic crystals with integrated plasma elements that can be turned on and off on demand. When the plasma is on, it selectively interacts with terahertz spectrum of an electromagnetic wave. When it is off, no interaction occurs. These metamaterials and photonic crystals, which are constructed of repeating sub-millimeter sized metals and dielectrics elements, can serve to both produce the plasma while maximizing interactions with THz waves from an electromagnetic source.

It’s a novel application in plasma science, but at a general level, the plasma-containing materials are very similar in structure to the repeating rows of plasma-containing cells that make up a plasma television. However, the plasma Raja and colleagues are studying is significantly different from the kind that produces different colored pixels for a TV. It’s much more energetic – which raises the density of the plasma and prevents a THz wave from simply passing through – and the plasma must be contained in large arrays of the metamaterial or photonic crystal structure, with each element in the array being the size of a THz wavelength (less than one millimeter) or smaller so they can collectively manipulate the wave.

While researchers are working to create a metamaterials environment conducive to plasma/THz interaction, finding out what the interactions are and how they can be usefully harnessed is all part of the research, said Raja.

“The goal is to develop fundamental scientific understanding of all different aspects of these waves interacting with plasma metamaterials and plasma photonic crystals,” Raja said. “This understanding is crucial to the development of practical devices of commercial and military interest in the future.”

Raja’s main research focus at UT has been on realistically modeling plasma behavior in aerospace and related applications, such as using plasma-based thrusters to propel space vehicles. For this new research project, he’s transforming his simulation tools to predict how the plasma-based metamaterials and photonic crystals and THz waves will interact. The modeling is a key aspect of the research, enabling hypothesis and scenarios to be tested computationally to aid in the understanding of laboratory experiments conducted by collaborators.

“We have been developing simulation technology for the computational modeling of plasmas for over 10 years now at UT-Austin and all this experience will be brought to bear on the problem,” Raja said.

The research is just getting started, but by the end of the five year grant Raja says he hopes to have developed predictive simulations that can help guide the development of this new area of research, and lead to more information on how THz waves, an elusive part of the electromagnetic spectrum caught between well known ones, can be manipulated.

“We are certain that in the process of developing these tools we will have really pushed the state of the art in the computational modeling and simulations of this particular phenomena,” Raja said.

For more information on Raja's current research, visit his website.