# Ultra-broadband THz emission from one- and two-Color laser-induced microplasmas at high repetition rate

Etienne Cardi, Stefan Skupin, Luc Bergé, D.R. Bacon, Tatsunosuke Hanano, Takumi Fukuda, Julien Madéo, Keshav M. Dani, Sukhdeep Dhillon, Juliette Mangeney, et al.

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## To cite this version:

Etienne Cardi, Stefan Skupin, Luc Bergé, D.R. Bacon, Tatsunosuke Hanano, et al., Ultra-broadband THz emission from one- and two-Color laser-induced microplasmas at high repetition rate. IRMMW-THz 2025 - 50th International Conference on Infrared, Millimeter and Terahertz Waves, Aug 2025, Helsinki, Finland. pp.1-2, 10.1109/IRMMW-THz61557.2025.11319695. hal-05457557

HAL Id: hal-05457557

https://hal.science/hal-05457557v1

Submitted on 14 Jan 2026

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# Ultra-broadband THz Emission from One- and Two-Color Laser-Induced Microplasmas at High Repetition Rate

E. Cardi<fnref n="1" />, S. Skupin<fnref n="2" />, L. Bergé<fnref n="3" />, D.R. Bacon<fnref n="4" />, T. Hanano<fnref n="4" />, T. Fukuda<fnref n="4" />, J. Madéo<fnref n="4" />, K. Dani<fnref n="4" />, S. Dhillon<fnref n="1" />,
J. Mangeney<fnref n="1" /> and J. Tignon<fnref n="1" />

*Abstract*—We report on the generation of intense and broadband THz radiation from laser-induced microplasmas in air, driven by femtosecond fiber lasers operating at high repetition rates. We investigate both one-color and two-color plasma generation schemes. In the one-color case, we examine the angular emission pattern and spectral content of the THz radiation as a function of the laser focusing geometry. In the two-color configuration, we demonstrate efficient THz generation with a spectral content extending up to 30 THz using a compact Yb-doped fiber femtosecond laser system with a repetition rate of 200 kHz. These results offer a promising path toward practical implementation of plasma-based THz sources for broadband spectroscopy.

## I. INTRODUCTION

The generation of intense, broadband terahertz (THz) radiation has become a central objective in ultrafast optics, driven by the growing demand for spectroscopy of complex materials, high-resolution imaging, and nonlinear THz physics. Among the various generation mechanisms, laser-induced plasmas in gases offer a unique combination of high THz field strength, ultrabroad spectral coverage, sub-cycle temporal resolution, and atmospheric operation. However, most demonstrations of THz emission from plasma sources rely on low-repetition-rate (1 kHz) Ti:Sa lasers, which limit the practicality of these systems for real-world applications due to long acquisition times and low signal-to-noise ratio. In this work, we explore the generation of THz radiation from laser-induced microplasmas in ambient air using a compact Ytterbium (Yb)-doped amplified fiber femtosecond laser system operating at 200-kHz high repetition rate [1]. We investigate both one-color and two-color plasma excitation schemes to evaluate the scalability, efficiency, and spectral performance of plasma-based THz sources under conditions compatible with high-throughput spectroscopy.

## II. THZ GENERATION FROM ONE-COLOR MICROPLASMA

We first investigate the emission of THz radiation from laser-induced one-color microplasmas generated by a tightly focused fundamental (1030 nm) 23-fs pulse with 50 µJ energy in ambient air. Figure 1(a-c) shows the angular distribution of the amplitude of the THz signal measured with a pyroelectric detector for three different pump-beam waist radii (1 µm, 3 µm, and 5 µm according to Gaussian beam propagation in vacuum). A clear narrowing of the emission pattern is observed as the waist radius and thus the Rayleigh length increases, which in turn increases the plasma length [see Fig. 2(c)]. This observation is in agreement with predictions based on

ponderomotive (transition-Cerenkov) current models [2, 3].

The dependence of the THz output power and conversion efficiency as a function of pulse energy is presented in Fig. 1(d), showing a nonlinear increase (quasi-parabolic [3]) increase in the emitted power and a maximum conversion efficiency exceeding 2 × 10⁻⁵ at 45 µJ. A schematic representation of the THz emission geometry is shown in Fig. 1(e), illustrating the forward-conical nature of the emitted radiation in this configuration.

<figure><img src="image_3.png" /><figcaption>Fig. 1. One-color plasma THz angular emission and energy dependence. (a-c) Angular dependence of THz emission for different predicted pump-beam waist radii (in vacuum): (a) 1 µm, (b) 3 µm, and (c) 5 µm. Solid blue and orange curves correspond to the fitted emission pattern before and after convolution by the experimental response function of a f/1-off-axis parabolic mirror collection system and agree with theoretical predictions from the transition-Cerenkov model [2,3]. (d) THz power (red, left axis) showing a quasi-quadratic growth and conversion efficiency (blue, right axis) as a function of laser pulse energy. (e) Scheme of THz forward-conical emission.</figcaption></figure>

To understand the influence of focusing on plasma geometry and THz generation, we measured the plasma profile for different beam waists. Figures 2(a) and 2(b) show the plasma dimensions measured in the near-IR for predicted pump-beam waist radii (in vacuum) of 1 µm and 10 µm, respectively. The longitudinal extent of the plasma increases significantly with the pump-beam waist radius, as quantified in Fig. 2(c). We note that the measured plasma dimensions significantly exceed the predicted beam waist, due to a combination of optical aberrations, plasma defocusing and expansion - the latter effects being dominant. Although we use theoretical waist values in the discussion, a direct measurement in one case (5 µm vs. 1 µm

<footnote n="1">¹Laboratoire de Physique de l'École Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université Paris Cité, 75005 Paris, France.</footnote>

<footnote n="2">²Université de Lyon 1 - CNRS, Institut Lumière Matière, UMR 5306, 69622 Villeurbanne.</footnote>

<footnote n="3">³Centre des Lasers Intenses et Applications, Université de Bordeaux, CNRS, CEA, 33405 Talence, France.</footnote>

<footnote n="4">⁴Femtosecond Spectroscopy Unit, Okinawa Institute of Science and Technology Graduate University, Onna, Okinawa, Japan 904-0495.</footnote>
predicted) confirms the presence of aberrations.

A THz time-domain trace and the frequency-domain spectra obtained from electro-optic sampling (EOS) are shown in Fig. 2(d-e), highlighting the bandwidth reduction and amplitude drop as the focusing becomes looser. The resulting THz beam profile, measured at the focus of an f/1 off-axis parabolic mirror, is shown in Fig. 2(f), with a measured FWHM around 325–375 µm. These results show that tight focusing conditions (going along with higher plasma density) tend to favor high-frequency THz emission in one-color plasma schemes.

<figure><img src="image_4.png" /><figcaption>**Fig. 2.** (a–b) Measured images of the generated microplasma for two different beam waist radii (in vacuum) of (a) 1 µm and (b) 10 µm, showing a transition from compact to elongated plasma shapes. (c) Dependence of the plasma longitudinal (orange dots) and transverse (blue dots) dimensions (1/e² widths) on the laser waist radius. (d) THz time-domain scan (waist radius 1 µm). (e) THz spectra obtained by time-domain spectroscopy. THz spectra obtained by time-domain spectroscopy for different laser waists (in vacuum), showing that tighter focusing leads to broader spectral bandwidth and higher amplitude. (f) THz beam profile measured at the focal plane of an f/1 off-axis parabolic mirror using a microbolometer camera. The values of horizontal and vertical full width at half maximum (FWHM) values are indicated.</figcaption></figure>

### III. THZ GENERATION FROM TWO-COLOR MICROPLASMA

In the second part of this work, we demonstrate THz pulse generation using a two-color scheme (fundamental and second harmonic), which enables significantly broader and stronger THz emission [4] using the same laser system. The generation of THz radiation from two-color plasma usually relies on the creation of a net directional photocurrent driven by the asymmetry of the combined fundamental and second harmonic laser fields during ionization. In previous work, e.g. [5,6], the THz emission is generated from an extended plasma filament produced via self-focusing of femtosecond pulses in air (mm or sub-mm long). In contrast, our approach is based on the formation of a tightly confined microplasma by geometric focusing with waist radius ~1–5 µm. This configuration enables the use of compact µJ-class systems that still produce broadband THz emission and with the potential to access MHz repetition rate. This also allows highly localized THz emission with potential for integration and high spatial resolution. Using a 30-µm-thick GaSe detection crystal tilted at 0°, we recorded a spectrum extending up to ~30 THz, as shown in Fig. 3. The THz spectrum produced by the 2-color plasma is measured by collecting the signal at 0° (the detector is oriented at 0° with a f/1 off-axis parabolic mirror, meaning collecting angles range from -36° to +23°). For the sake of comparison, the 1-color plasma THz spectra are measured by collecting the signal at 0°

in Fig. 3 (a) and 90° in Fig. 3 (b).

To our knowledge, this is the first demonstration of such ultra-broadband THz emission from a two-color micro-plasma source operating at several hundred kHz. Compared to previous works that rely on low-repetition Ti:Sa systems, our approach dramatically reduces acquisition times and improves signal-to-noise ratio. These results open the way for practical applications of plasma-based THz sources in field-resolved spectroscopy of complex materials.

<figure><img src="image_5.png" /><figcaption>**Fig. 3.** Measured THz spectrum generated by two-color microplasma in ambient air (fundamental at 1030 nm, second harmonic at 515 nm, predicted pump-beam waist radius 1.5 µm). Detection was performed using a 30 µm-thick GaSe crystal tilted at 0° to optimize phase matching: orange solid line in (a) and (b). The spectrum extends to 32 THz demonstrating the broadband nature of the two-color generation mechanism. Blue solid lines show the 1-color THz spectra measured (a) at 0° and (b) 90°. For all measurements: 10 scans, 300ms time constant, 400 data points, 25mn overall acquisition time.</figcaption></figure>

### IV. SUMMARY

In summary, we have demonstrated that laser-induced microplasmas in ambient air can serve as efficient sources of broadband THz radiation when driven by high-repetition-rate Yb-based femtosecond lasers. In the one-color configuration, we retrieved THz signatures comparable to Ref. [3] and investigated the influence of laser focusing conditions on angular emission patterns, plasma geometry, and spectral content. In the two-color regime, we achieved ultra-broadband THz emission extending up to 30 THz with significantly enhanced field amplitude, enabled by a compact, high-repetition-rate laser system. These results establish a promising route for practical field-resolved THz spectroscopy with reduced acquisition times and improved signal-to-noise ratio.

### REFERENCES

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[2] C. D Amico et al., “Forward THz radiation emission by femtosecond filamentation in gases: theory and experiment,” *New J. Phys.*, vol. 10, p. 013015 (2008).

[3] F. Buccheri and X.-C. Zhang, “Terahertz emission from laser-induced microplasma in ambient air,” *Optica*, vol. 2, no. 4, pp. 366–371 (2015).

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