Intense widely-controlled terahertz radiation from laser-driven wires N. Bukharskii and Ph. Korneev National Research Nuclear University MEPhI 31 Kashirskoe shosse 115409 Moscow Russian Federation

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Intense widely-controlled terahertz radiation from laser-driven wires

N. Bukharskii and Ph. Korneev∗

National Research Nuclear University MEPhI, 31 Kashirskoe shosse, 115409 Moscow, Russian Federation

(*ph.korneev@gmail.com)

(Dated: October 26, 2022)

Irradiation of a thin metallic wire with an intense femtosecond laser pulse creates a strong discharge wave that

travels as a narrow pulse along the wire surface. The travelling discharge efﬁciently emits secondary radiation

with spectral characteristics mostly deﬁned by the wire geometry. Several exemplary designs are considered

in the context of generation of intense terahertz radiation with controllable characteristics for various scientiﬁc

and technological applications. The proposed setup beneﬁts by its robustness, versatility and high conversion

efﬁciency of laser energy to terahertz radiation, which reaches several percent.

INTRODUCTION

Much research in recent years has been devoted to the de-

velopment of technology for generating terahertz (THz) radi-

ation, i.e. electromagnetic radiation with the frequencies be-

tween 100 GHz and 10 −30 THz [1–3]. The ever-increasing

attention to this topic stems from numerous possible appli-

cations of THz radiation in both fundamental science and

technology. Many of these applications belong to biologi-

cal and medical science, which is not surprising considering

the unique properties of THz waves. Unlike X-ray, they do

not cause harm to biological tissues as THz frequencies are

too low to ionize bio-molecules, and at the same time a large

portion of the vibrational, rotational and oscillating molecular

degrees of freedom are excited in THz range. These factors,

along with the lower scattering loss in bio-tissues in compar-

ison to infrared or visible light, make THz radiation an ideal

candidate for medical imaging and spectroscopy of biological

tissues [4, 5]. One area of particular interest here is cancer de-

tection and treatment with THz radiation [6–11]. In this case,

THz waves may be used to detect and manipulate a molecular

resonance of cancer DNA, which can be observed at approx-

imately 1.65 THz and appears due to chemical and structural

alterations that biomolecules undergo in cancer cells [12–15].

However, THz imaging can also be applied outside of the

medical sciences domain, for example, in security-related ap-

plications [16, 17]. Due to high penetration of THz radiation

into dry, nonmetallic and nonpolar materials it can be used to

image individual inner areas where the absorption is high, for

instance areas with water content [18], or it can help to iden-

tify the distribution of defects in materials with low absorption

such as foams [19]. Another potential ﬁeld of applications for

THz radiation is related to studying and manipulation of ma-

terial properties. In contrast to visible light, its photons do

not carry excessive energy, allowing for the direct coupling

into excitation states of interest and opening path for a vast

range of perspective studies [20]. Finally, it is worth mention-

ing the possibility of using THz radiation for increasing the

bandwidth of wireless communications systems, allowing for

a faster transmission of a larger amount of data [21, 22].

∗Also at P.N. Lebedev Physical Institute of RAS, 53 Leninskii Prospekt,

119991 Moscow, Russian Federation.

Over the course of history of THz science, various tech-

niques for obtaining THz radiation have been developed.

Among them there are photoconductive antennas, optical rec-

tiﬁcation and laser-plasma interaction schemes, as well as a

number of methods based on topological insulators, spintronic

materials and metasurfaces [23]. As many potential applica-

tions require strong THz ﬁelds, achievable intensity in THz

range often becomes one of the key parameters in the de-

velopment of new THz sources. In this context, methods

involving relativistic laser-produced plasma may be prefer-

able, as THz radiation output from laser plasmas does not

experience saturation for very high intensities, and, in addi-

tion, there is no risk of damaging the medium that is used for

generating THz radiation. A comprehensive review of exist-

ing plasma-based techniques, which generally rely on laser-

excited plasma waves, electron emission or transport, can

be found in Ref. [24]. Obtaining high conversion efﬁciency

and the desired properties of THz radiation with plasma-

based methods requires modiﬁcation of laser-plasma interac-

tion conditions. One of the possible ways of their modiﬁcation

involves optimization of the target geometry and the irradia-

tion scheme. An example of such a scheme are straight laser-

driven metallic wires. Under appropriate conditions they may

be used for generation of THz radiation, as was demonstrated

in a number of recent numerical and experimental studies [25–

31]. The models describing THz radiation are usually based

on electron current excitation along or near the wire. A rather

efﬁcient way to create a powerful and localized electric cur-

rent in a wire is to excite a discharge pulse under short intense

laser irradiation [32–34]. In this work, we show that modiﬁ-

cation of the wire geometry by shaping it as a curved periodic

structure proposes wide possibilities for control of the gen-

erated radiation. Some beneﬁts of using a curved wire have

already been discussed in Ref. [33], where it was shown that

it is possible to obtain high-intensity THz radiation with con-

trollable spectrum and a maximum of radiated power in the

wave zone along the coil axis. However, certain conditions

are needed to ensure that the discharge wave continues oscil-

lating in the coil loop and emits THz radiation instead of fast

grounding along the stalk. In particular, the gap between the

coil ends has to be sufﬁciently small to short-circuit the dis-

charge electric pulse after its ﬁrst round along the coil. These

conditions might require certain laser beam parameters and

use of high-accuracy target fabrication technologies. In this

work it is shown, that use of shaped extended wire as a THz

arXiv:2210.14166v2 [physics.optics] 27 Oct 2022

J⃗

~100 μm

sine

wire

fs laser

J⃗

fs laser

triangle

wire

square

wire

FIG. 1. Sketch of the proposed targets: ’sine’ wire (top), ’triangle’

wire (middle) and ’square’ wire (bottom). The targets are irradiated

on the open end by an intense femtosecond laser pulse. The general

propagation direction of the laser-induced discharge pulse is shown

with black arrows - it propagates from the irradiated end of the wire

to the opposite one along the wire surface.

antenna possesses both robustness and simplicity, provides ex-

cellent control and allows for a very high intensity THz radi-

ation attainable with a very high efﬁciency. We demonstrate

this considering three types of wire proﬁles, namely ’sine’-

shaped (hereafter simply ’sine’), triangle-shaped and square-

shaped targets, irradiated by an intense femtosecond pulse on

one of the open ends, see Fig. 1.

I. DISCHARGE PULSE FORMATION AND SCALINGS

The amplitude of the discharge pulse and its duration de-

pends on the laser pulse parameters. For this setup short laser

pulses are required, i.e. those with duration τbeing short

compared to L/c, where cis the light velocity, Lis a char-

acteristic size [35]. As it is demonstrated below, roughly the

expected discharge pulse duration is similar to the laser pulse

duration, and the discharge pulse intensity is proportional to

the laser pulse intensity, assuming the interaction conditions

are the same. For more certain description, the process of

the discharge pulse formation was studied numerically with

the Particle-in-Cell (PIC) code Smilei [36]. Simulations were

performed in a reduced 2D setup with a simple straight wire

target. This simple setup allows for studying the process of the

discharge pulse formation and propagation for various param-

eters of the laser driver. For all performed simulations the tar-

get presented a 40 µm×1µmrectangle positioned at the cen-

tre of the simulation box with the size of 48.7µm×12.2µm

and contained 3072 ×768 cells. The size of one cell in both

dimensions was ≈15.9 nm, with 10 particles of each kind

per cell, the time resolution was 1.8·10−2fs. The target con-

sisted of ions with atomic number Z=79, which corresponds

to gold, with the mass M=5·103mp, where mpis the mass

of a proton. Though the ions do not noticeably move on the

considered time scale, their mass was increased in order to

provide qualitatively the same ion dynamics as in the case of

the target with a more realistic size ∼(5−10)greater than the

size of the target in this parametric study. The density of ions

at the start of the simulation was set to ni=5.9·1022 cm−3,

which is the solid-state ion density for gold. Initially, the de-

gree of ionisation of the target as well as its electron density

were set to zero, and the ﬁeld ionization model implemented

in Smilei was employed to calculate the values of the afore-

mentioned parameters on each step. The laser pulse was intro-

duced into the simulation box from the lower edge of the box

and irradiated the target at angle of 45◦to its surface. Such an

angle was chosen for better absorption of the laser energy as

the laser pulse propagates some distance along the target to-

gether with the induced discharge wave. Three different laser

pulse durations were considered: τlas.= [12.5,25,50]fs, with

the given values corresponding to the Full Width at Half Max-

imum (FWHM) size of the temporal proﬁle. For each dura-

tion ﬁve different values of maximum intensity in focus were

taken: Imax = [2·1019,1020,1021,1022,1023]W/cm2.

Results of one of the performed simulations (with Imax =

1022 W/cm2and τlas.=12.5 fs) are presented in Fig. 2, (a1-

a4). From the presented plots of the Bzcomponent of the elec-

tromagnetic ﬁeld one can see that the laser irradiates the wire

on the left end, see Fig. 2, (a1). The laser pulse is then par-

tially reﬂected and partially absorbed by the target, see Fig. 2,

(a2). Substantial part of the laser energy is converted into the

energy of a discharge pulse that continues to autonomously

propagate along the wire to the right, in the direction of its

opposite end, even when most of the laser pulse leaves the

simulation box, see Fig. 2, (a3) and (a4). The excited dis-

charge wave is mono-polar, which is consistent with the re-

sults obtained in other works, see for example [30, 32]. For

ultra-short laser drivers considered in this study the wave is

well-localized on the scale of the wire, i.e. it has a form of

a distinct short electromagnetic pulse. Amplitude of this dis-

charge pulse max(Bz)and its duration at FWHM τd.p.depend

on the parameters of the laser driver. Results of the performed

parametric scan are summarized in Fig. 2, (b) and (c).

As can be seen in Fig. 2, (b), dependence of the ampli-

tude of the discharge wave max(Bz)for all the considered

laser durations appears to closely follow the power-law depen-

dence max(Bz)∼(Imax)k, with k≈0.6±0.05. In the simplest

consideration the expected value of kis 0.5, as the magnetic

y [ m]

(a1)

(b)

t= 27 fs (a2) t= 54 fs

0 5 10 15 20 25 30 35 40 45

x [ m]

y [ m]

(a3) t= 81 fs

0 5 10 15 20 25 30 35 40 45

x [ m]

(a4) t= 135 fs

[×105T]

1019 1020 1021 1022 1023

Maximum laser intensity [W/cm2]

103

104

105

max(Bz) [T]

Bmax∼(Imax)0.65

Bmax∼(Imax)0.57

Bmax∼(Imax)0.58

las.= 12.5 fs

las.= 25 fs

las.= 50 fs

0 10 20 30 40 50 60

Laser duration (FWHM) [fs]

Discharge pulse duration (FWHM) [fs]

τd.p.∼τlas.

Ilas.= 2 1019 W/cm2

Ilas.= 1020 W/cm2

Ilas.= 1021 W/cm2

Ilas.= 1022 W/cm2

Ilas.= 1023 W/cm2

(c)

FIG. 2. (a1-a4) Results on 2D PIC simulation for a straight wire target irradiated by a laser pulse with maximum intensity Imax =1022 W/cm2

and FWHM duration τlas.=12.5 fs: Bzelectromagnetic ﬁeld component at time moments 27 fs, 54 fs, 81 fs and 135 fs, respectively. (b)

Dependence of the amplitude of the discharge wave ﬁeld component Bzestimated at point x≈30 µm near the wire surface on the maximum

laser intensity at the focus spot; the data for different laser pulse durations is shown with markers of different colors. The points appear to

closely follow the power-laws represented by solid lines. (c) Dependence of the duration of the discharge pulse at FWHM on the laser duration

at FWHM for various intensities of the laser driver, shown with markers of different color. The points appear to closely follow the linear

dependence represented by the black solid line.

and electric ﬁeld amplitudes are proportional to the square

of the magnetic and electric ﬁeld energy densities, which in

turn are directly proportional to the laser energy and con-

sequently its intensity. The obtained result, however, sug-

gests that the laser-to-discharge-pulse conversion efﬁciency is

somewhat higher than expected, which can be explained by

the reduced effects of dissipation processes such as ionization

at high laser intensities. Comparison of the results for dif-

ferent laser-pulse durations also shows that the amplitude in-

creases with the increase of laser duration, although the differ-

ences are more pronounced at low intensities. Such behaviour

can be attributed to the higher laser energy which is delivered

to the target if the laser pulse duration is increased while its

intensity remains constant. As Fig. 2 (c) shows, the duration

of the discharge pulse almost linearly depends on the duration

of the laser pulse, implying that the former can be directly

controlled by adjusting the latter. The propagation velocity

of the discharge pulse in the considered range of parameters

shows no dependence on the intensity and duration of the laser

driver and constitutes ≈(0.97 −0.98)c. The performed nu-

merical simulations indicate that the discharge pulse is formed

in a broad range of laser intensities, with parameters of the in-

duced discharge pulse being determined by the parameters of

the laser driver. According to the subsequent analysis detailed

below, when such discharge pulses are propagating along an

extended curved wire, they can radiate powerful THz waves

with the properties deﬁned by the wire geometry.

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Intensewidely-controlledterahertzradiationfromlaser-drivenwiresN.BukharskiiandPh.KorneevNationalResearchNuclearUniversityMEPhI,31Kashirskoeshosse,115409Moscow,RussianFederation(*ph.korneev@gmail.com)(Dated:October26,2022)Irradiationofathinmetallicwirewithanintensefemtosecondlaserpulsecreatesastrong...

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Intense widely-controlled terahertz radiation from laser-driven wires N. Bukharskii and Ph. Korneev National Research Nuclear University MEPhI 31 Kashirskoe shosse 115409 Moscow Russian Federation

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