Enhanced coherent transition radiation from midinfrared-laser-driven microplasmas

• Published in: Scientific reports Vol. 12; no. 1; pp. 7660 - 12
• Main Authors: Glek, P. B., Zheltikov, A. M.
• Format: Journal Article
• Language: English
• Published: London Nature Publishing Group UK 10-05-2022; Nature Publishing Group; Nature Portfolio
• Subjects: 639/624/400/1119; 639/624/400/385; 639/624/400/561; 639/624/400/584; Acceleration; Electrons; Energy; Energy conversion; Humanities and Social Sciences; Laser radiation; Lasers; Light; multidisciplinary; Radiation; Scaling; Science; Science (multidisciplinary); Terahertz Radiation
• ISSN: 2045-2322; 2045-2322
• DOI: 10.1038/s41598-022-10614-0

We present a particle-in-cell (PIC) analysis of terahertz (THz) radiation by ultrafast plasma currents driven by relativistic-intensity laser pulses. We show that, while the
I
0
λ
0
2
product of the laser intensity
I
0
and the laser wavelength
λ
0
plays the key role in the energy scaling of strong-field laser-plasma THz generation, the THz output energy,
W
THz
, does not follow the
I
0
λ
0
2
scaling. Its behavior as a function of
I
0
and
λ
0
is instead much more complex. Our two- and three-dimensional PIC analysis shows that, for moderate, subrelativistic and weakly relativistic fields,
W
THz
(
I
0
λ
0
2
) can be approximated as (
I
0
λ
0
2
)
α
, with a suitable exponent
α
, as a clear signature of vacuum electron acceleration as a predominant physical mechanism whereby the energy of the laser driver is transferred to THz radiation. For strongly relativistic laser fields, on the other hand,
W
THz
(
I
0
λ
0
2
) closely follows the scaling dictated by the relativistic electron laser ponderomotive potential
F
e
, converging to
W
THz
∝
I
0
1
/
2
λ
0
for very high
I
0
, thus indicating the decisive role of relativistic ponderomotive charge acceleration as a mechanism behind laser-to-THz energy conversion. Analysis of the electron distribution function shows that the temperature
T
e
of hot laser-driven electrons bouncing back and forth between the plasma boundaries displays the same behavior as a function of
I
0
and
λ
0
, altering its scaling from (
I
0
λ
0
2
)
α
to that of
F
e
, converging to
W
THz
∝
I
0
1
/
2
λ
0
for very high
I
0
. These findings provide a clear physical picture of THz generation in relativistic and subrelativistic laser plasmas, suggesting the THz yield
W
THz
resolved as a function of
I
0
and
λ
0
as a meaningful measurable that can serve as a probe for the temperature
T
e
of hot electrons in a vast class of laser–plasma interactions. Specifically, the
α
exponent of the best (
I
0
λ
0
2
)
α
fit of the THz yield suggests a meaningful probe that can help identify the dominant physical mechanisms whereby the energy of the laser field is converted to the energy of plasma electrons.