We propose a new method of compressing laser pulses to ultrahigh powers based on spatially varying dispersion of an inhomogeneous plasma. Here, compression is achieved when a long, negatively frequency-chirped laser pulse reflects of the density ramp of an over-dense plasma slab. As the density increases longitudinally, high-frequency photons at the leading part of the laser pulse penetrate more deeply into the plasma region than lower-frequency photons, resulting in pulse compression in a similar way to that by a chirped mirror. Proof-of-principle simulations performed using particle-in-cell simulation codes predict compression of a 2.35 ps laser pulse to 10.3 fs—a ratio of 225. As plasma is robust and resistant to damage at high intensities—unlike solid-state gratings commonly used in chirped-pulse amplifcation—the method could be used as a compressor to reach exawatt or zettawatt peak powers [1]. 

Figure 1. A concept of pulse compression in a plasma. The laser pulse compressor based on a density gradient plasma, in which a long-duration, high-energy, negatively frequency-chirped laser pulse reflects from an over-dense plasma slab with an increasing density ramp. High-frequency components at the front of the pulse and low frequency components at the rear part of the pulse reflect at different positions, leading to compression of the laser pulse. 

A novel method is presented for generating radiation using the beat wave of a bi-frequency laser pulse to excite plasma oscillations in a plasma slab that has a density gradient. The plasma wave is localized where it is excited resonantly and becomes a plasma oscillator that produces a beam of radially polarized, terahertz radiation. Particle-in-cell simulations and a theoretical analysis are used to demonstrate its main characteristics, such as its narrow bandwidth. The radiator should have useful applications including driving terahertz-band particle accelerators and for pump-probe experiments [2].

Figure 1. (a) A 3D volumetric contour plot with a 2D projection onto the bottom plane, depicting the radial component of emitted electric field at t =2.6 ps. The plasma density increases linearly from 0 to 0.004 nc over the range x=0 to 140 μm, then remains flat to 160 μm (represented by the solid black line on the front plane); vacuum region ranges from x =−60 to 0 μm. (b) Angular distribution of radiated field Er (GV/m) in vacuum (x =−23.3 μm). (c),(d) Temporal profile of radiated field and its corresponding power spectrum determined at the vacuum side.

A practical configuration for generating narrowband terahertz (THz) pulses based on plasma dipole oscillations (PDOs) is studied using two-dimensional particle-in-cell simulations. In this scheme, two slightly detuned laser pulses collide obliquely in a helium gas. Plasma strips are generated along the paths of the laser pulses by field ionization. The PDO created in the overlap region of the two laser pulses emits a THz pulse with a peak electric field strength of a few gigavolt per meter. An energy conversion efficiency of 0.542x10^3 is achieved for laser pulse intensities 4.82x10^16 W/cm^2, a spot radii of 5 um, and a collision angle of 10.8 deg. A force balance model is extended for the obliquely colliding configuration of the pulses. As the complications, such as generating plasmas separately or aligning the beams with preformed plasma, are eliminated from our new configuration, this makes a future experimental study of PDO more straightforward [3]

Figure 1. PDOs generated with helium gas system. (a) In the first step, two slightly detuned laser pulses collide in underdense plasma. (b) In the second step, plasma electrons are trapped due to ponderomotive potential of beat wave and charge separation occurs to build up dipole field. (c) Finally, after laser pulses pass through, electron blocks are oscillated, and THz pulse can be simultaneously emitted.

Reference

[1] M. S. Hur, et al. "Laser pulse compression by a density gradient plasma for exawatt to zettawatt lasers"  Nature Photonics (2023).

[2] M. Kumar, et al. "Narrowband Terahertz Emission from a Plasma Oscillator Imbedded in a Plasma Density Gradient" Phys. Rev. Lett. 134(1), 015001 (2025).

[3] J. Lee, et al. "Intense narrowband terahertz pulses produced by obliquely colliding laser pulses in helium gas“ Phys. Plasma. 30(4), 043108 (2023).