Two-color laser-induced air plasma filaments are highly effective for generating intense (>1 MV/cm) and ultrashort (sub-100 fs) terahertz (THz) pulses [
Journal of the European Optical Society-Rapid Publications, Volume. 20, Issue 1, 2024042(2024)
Frequency-resolved measurement of two-color air plasma terahertz emission
We investigated the far-field terahertz beam profile generated from an air plasma induced by two-color femtosecond laser pulses. Under our experimental conditions (filament length shorter than the dephasing length between the two-color pulses), using electro-optic sampling in both ZnTe (0.2–2.2 THz) and GaP (0.4–6.8 THz) crystals, and ultra-broadband ABCD technique (1–17.5 THz), we determined that the THz beam exhibits a unimodal beam pattern below 4 THz and a conical one above 6 THz. This experimental finding is consistent with theoretical studies based on the unidirectional pulse propagation equation, which predict the transition of THz emission from a flat-top profile to a conical one due to the destructive interference of THz waves emitted from the plasma filament. Our results also underscore the importance of accounting for experimental artifacts, such as photo-excited losses in silicon resulting in on-axis THz absorption along with the influence of drilled mirrors, in characterizing the complex spatial and frequency-dependent behavior of two-color plasma-induced terahertz emission.
1 Introduction
Two-color laser-induced air plasma filaments are highly effective for generating intense (>1 MV/cm) and ultrashort (sub-100 fs) terahertz (THz) pulses [
THz emission from a bichromatic air plasma has been explained by various models, including the two-dimensional (2D) transverse photocurrent or four-wave mixing, and can be numerically computed by the unidirectional pulse propagation equation [
As the ω and 2ω pulses propagate along the filament, their relative phase changes, and we can define the dephasing length as Ld = λ/[2(nω − n2ω)], where λ = 800 nm and nω,2ω is the refractive index of the plasma filament at ω and 2ω, respectively. Because of this dephasing effect, maximum THz generation is expected for plasma filaments with length Ld about to 22–25 mm at 800 nm [
In this paper, we investigated the transition from unimodal to conical THz emission from two-color air plasma (the situation corresponding to Lf < Ld). For this purpose, we used four different experimental setups (THz camera, electro-optic sampling (EOS) in ZnTe and GaP crystals, air-biased coherent detection (ABCD) technique to properly determine the THz beam shape as a function of the frequency from 0.2 up to 17.5 THz. We paid special attention to experimental artifacts that can mask on-axis THz emission, such as photo-excited losses in silicon wafers and the influence of the hole drilled in the center of the last mirror used to reflect the THz beam, and transmit the laser probe beam onto the detector. As a result, we determined that the THz beam exhibits a unimodal beam pattern below 4 THz and a conical beam pattern above 6 THz.
2 Experimental setups
Investigating the frequency-resolved characterization of broadband THz beam generated by two-color air plasma is challenging since the spectral content of the corresponding THz ultrashort pulses spans nearly above two decades, typically from 0.1 to 100 THz. It is therefore almost impossible to use a single detection system to fully cover this ultra-wide spectral range. For this reason, we employed a set of four different experimental setups to properly characterize the far-field THz emission.
A commercially available THz camera (
2.1 Detection with an incoherent THz camera
For all setups presented in this paper (
Figure 1.(a) Experimental setup: detection with a THz camera. L: plano-convex lens (f = 300 mm); M: off-axis parabolic mirror (f = 150 mm). Inset: picture of the plasma filament (the size of the square is 1 × 1 cm2). (b) Evolution of the THz beam along the Z-axis, as defined in (a).
THz radiation is collimated by an off-axis parabolic mirror M (f = 150 mm) and then focused onto a THz camera (RIGI from Swiss Terahertz, uncooled FPA 160 × 120 microbolometers, 25 μm pixel size) by a second off-axis parabolic mirror M (f = 150 mm), after passing through a 1 mm-thick high-resistivity float zone silicon wafer, placed at Brewster angle, to filter out the remaining intense laser pump light. With this configuration, we carefully checked that the insertion of a high-density polyethylene (HDPE) window in front of the silicon wafer does not change the THz beam imaging, demonstrating that the detection is not affected by the photo-induced carriers in the silicon filter [
2.2 Detection with coherent 2DEOS in ZnTe crystal
The second detection system uses 2DEOS to measure the time-dependent spatial distribution of the THz electric field [
Figure 2.Coherent 2D electro-optic sampling setup using ZnTe detection crystal. (a) Experiment. CH: optical chopper for dynamic subtraction; L1: plano-convex lens (f = 300 mm); CF: ceramic filter; SF: silicon filter; BS: beamsplitter; AN: analyzer; L2: objective lens (f = 50 mm). (b) Left: 2D transverse profile of the THz electric field for a given time delay between the THz and probe pulses (scale: the white bar is 5 mm long); Center: temporal evolution of the THz electric field at the position corresponding to the intersection of the white lines (left image); Right: corresponding amplitude of THz spectrum.
Using 2DEOS with a time-delayed femtosecond laser probe pulse, we map the distribution of the THz electric field onto the spatial profile of the laser probe beam (horizontally-polarized). The intensity of this latter is detected by a 256 × 256 pixels complementary metal oxide semiconductor (CMOS) camera, after passing through the analyzer AN (perpendicularly-oriented towards the probe beam polarization) and the objective lens L2 (f = 50 mm). For a given time delay between the THz and the probe pulses, the system captures a 2D image at 800 nm, representing the THz electric field distribution (
For data analysis, each pixel in the stack of CMOS images provides the temporal evolution of the THz electric field at that position (
2.3 Aperture-scanning-assisted 1DEOS detection in GaP crystal
This detection is based on 1DEOS in GaP crystal and balanced detection. In this experiment, the THz beam travels through dry air, and the residual pump laser is blocked by a high-resistivity silicon wafer placed far away from the plasma to avoid any absorption of the THz beam. After passing through a set of four off-axis parabolic mirror M (f = 150 mm), the THz electric field is measured by a standard electro-optic detection using a 200 μm-thick 〈110〉 GaP crystal optically contacted onto a 3 mm-thick 〈100〉 GaP crystal (
Figure 3.(a) Aperture-scanning-assisted balanced detection in GaP crystal. (a) Experiment. L1: plano-convex lens (f = 300 mm); M: off-axis parabolic mirror (f = 150 mm, diameter = 40 mm); (b) Left: temporal evolution of the THz electric field; Right: corresponding amplitude of THz spectrum.
Without any scanning aperture, a typical temporal evolution of the THz electric field generated with this setup is displayed in
2.4 Aperture-scanning-assisted ABCD technique
This detection is based on the ABCD technique, widely used for air-photonics THz detection [
Figure 4.Aperture-scanning-assisted balanced detection with ABCD system. (a) Experiment. L1: plano-convex lens (f = 300 mm); L2: plano-convex lens (f = 200 mm); M1: off-axis parabolic mirror (f = 100 mm, diameter = 50 mm); M2: off-axis parabolic mirror (f = 150 mm, diameter = 50 mm). (b) Left: temporal evolution of the THz electric field; Right: corresponding amplitude of THz spectrum.
Without any scanning aperture, a typical temporal evolution of the THz electric field generated with this setup is displayed in
3 Experimental results and discussion
In this section, we present the results obtained with the four experimental setups described earlier, followed by a brief conclusion at the end of each sub-section. A comprehensive summary and analysis of all the data will be presented in
3.1 Detection with an incoherent THz camera
From these initial findings, we conclude that the far-field THz emission from two-color air plasma, observed with a standard THz camera, conforms to a conical shape as documented in previous studies [
3.2 Detection with coherent 2D electro-optic sampling (2DEOS) in ZnTe crystal
In this section, we discuss the conclusions drawn from a previously published study in 2020, focusing on the characterization of conical versus Gaussian THz emission from two-color laser-induced air plasma filaments [
When an alumina wafer is positioned before the commonly used silicon wafer, the THz emission exhibits a Gaussian-like profile for frequencies ranging from 0.2 and 2.5 THz (
Figure 5.Coherent 2DEOS in ZnTe crystal. Evolution of THz amplitude as a function of frequency and HOA. (a) Alumina first. (b) Silicon first. Yellow color indicates a higher THz amplitude. Insets: 2D spatial distributions of the THz electric field at 0.37 THz and 1.05 THz (from Ref. [17]).
Conversely, when the silicon filter is placed first, a central dark region becomes evident for frequencies above 0.5 THz (
From these findings, we conclude that below 2.5 THz, the THz beam exhibits an almost unimodal Gaussian spatial distribution when preceded by a large bandgap ceramic filter before the silicon wafer. This highlights the critical importance of the positioning of the silicon wafer: it should be either preceded by another filter or placed sufficiently far away from the filament to prevent photo-excited losses and consequent undesired on-axis THz absorption.
3.3 Aperture-scanning-assisted 1DEOS in GaP crystal
In this experiment, a circular aperture is scanned horizontally across the THz beam, with a 1 mm scan step (
Figure 6.Aperture-scanning-assisted balanced detection in GaP crystal. The laser probe pulse is transmitted by a hole (4 mm in diameter) drilled in the last parabolic mirror used to focus the THz pulse onto the GaP crystal. (a) Evolution of THz amplitude as a function of frequency and pinhole position. (b) Amplitude of the spectra integrated for different spectral windows.
From
In a modified experiment aiming at investigating the influence of the central hole drilled in the last parabolic mirror (not shown in
Figure 7.Aperture-scanning-assisted balanced detection in GaP crystal. The parabolic mirror with a hole has been replaced by a lens followed by an ITO plate. (a) Evolution of THz amplitude as a function of frequency and pinhole position. (b) Amplitude of the spectra integrated for different spectral windows.
From this experiment, two conclusions can be drawn. First, in addition to potential absorption losses caused by the silicon wafer, the use of mirrors with a central hole introduces an experimental artifact in the characterization of THz emission from two-color air plasma. Second, within the effective spectral bandwidth defined for this experiment (0.4–6.8 THz,
3.4 Aperture-scanning-assisted ABCD technique
Building on the findings from
Figure 8.Aperture-scanning-assisted ABCD technique. The drilled mirror has been covered by an ITO plate. (a) Evolution of THz amplitude as a function of frequency and pinhole position. (b) Integrated spectra for different spectral bandwidths. (c) Amplitude of the spectra at three different positions of the pinhole, as indicated by the dashed line in (a). “Center” refers to 0 mm (spectrum at the center of the THz beam), “Ring” refers to −2.5 mm (spectrum along the emission cone emitted at high frequency, above 12 THz), “Edge” refers to −7 mm (spectrum at the edge of the emission cone).
Another prominent feature observed in
In summary, this sub-section underscores the effectiveness of the ABCD technique in spectrally resolving THz beams emitted from two-color air plasma. Within the effective spectral bandwidth defined for this experiment (1–17.5 THz,
4 Conclusion
Based on a comprehensive investigation using four different experimental setups, we explored the spectral dependence of far-field THz emission from a two-color laser-induced air plasma filament. Under our experimental conditions (filament length Lf shorter than the dephasing length Ld between the two-color pulses), we draw the following main findings and conclusions from each experimental approach.
In the experiment using a standard THz camera, we observed a clear conical THz emission, consistent with previous studies. This method did not detect any potential unimodal THz emission due to its incoherent detection nature, primarily sensitive to high THz frequencies (> 4 THz). To enhance the detection of the unimodal component of the THz emission, one should incorporate a low-pass THz filter (cut-off frequency < 4 THz) in front of the camera.
Using coherent 2DEOS in a ZnTe crystal revealed an almost unimodal Gaussian spatial distribution of THz emission below 2.5 THz. We emphasized the importance of minimizing photo-excited losses in silicon to mitigate severe on-axis THz absorption.
The aperture-scanning-assisted 1DEOS in GaP crystal exposed an artifact arising from the use of mirrors with a central hole, affecting the central part of the THz beam. After correcting this artifact, the THz beam exhibited a unimodal pattern within the effective spectral bandwidth of the experiment (0.4–6.8 THz).
Finally, using aperture-scanning-assisted ABCD technique with an ITO plate modification to reject the previous artifact, we clearly distinguished unimodal THz emission below 4 THz and conical emission above 6 THz. These results are consistent with numerical simulations based on the unidirectional pulse propagation equation for short filament lengths (Lf < Ld) [
In conclusion, each experimental configuration provided valuable insights into the spectral characteristics of THz emission from two-color laser-induced air plasma. This study highlights the critical role of experimental setup and artifact mitigation for accurate characterization of THz beams. Future perspectives include investigating the evolution of THz emission with various filament lengths to further enhance experimental understanding and validate available theoretical models.
[1] DJ Cook, RM Hochstrasser. Intense terahertz pulses by four-wave rectification in air.
[2] M Kress, T Löffler, S Eden, M Thomson, HG Roskos. Terahertz-pulse generation by photoionization of air with laser pulses composed of both fundamental and second-harmonic waves.
[3] H Zhong, N Karpowicz, X-C Zhang. Terahertz emission profile from laser-induced air plasma.
[4] HG Roskos, MD Thomson, M Kreß, T Löffler. Broadband THz emission from gas plasmas induced by femtosecond optical pulses: From fundamentals to applications.
[5] TI Oh, YS You, N Jhajj, EW Rosenthal, HM Milchberg, KY Kim. Intense terahertz generation in two-color laser filamentation: energy scaling with terawatt laser systems.
[6] J Zhao, W Liu, S Li, D Lu, Y Zhang, Y Peng, Y Zhu, S Zhuang. Clue to a thorough understanding of terahertz pulse generation by femtosecond laser filamentation.
[7] A Gorodetsky, AD Koulouklidis, M Massaouti, S Tzortzakis. Physics of the conical broadband terahertz emission from two-color laser-induced plasma filaments.
[8] L Bergé, S Skupin, C Köhler, I Babushkin, J Herrmann. 3D Numerical simulations of THz generation by two-color laser filaments.
[9] YS You, TI Oh, KY Kim. Off-axis phase-matched terahertz emission from two-color laser-induced plasma filaments.
[10] AA Ushakov, PA Chizhov, VA Andreeva, NA Panov, DE Shipilo, M Matoba, N Nemoto, N Kanda, K Konishi, VV Bukin, M Kuwata-Gonokami, OG Kosareva, SV Garnov, AB Savelev. Ring and unimodal angular-frequency distribution of THz emission from two-color femtosecond plasma spark.
[11] P Klarskov, AC Strikwerda, K Iwaszczuk, PU Jepsen. Experimental three-dimensional beam profiling and modeling of a terahertz beam generated from a two-color air plasma.
[12] IA Nikolaeva, NR Vrublevskaya, GE Rizaev, DV Pushkarev, DV Mokrousova, DE Shipilo, NA Panov, LV Seleznev, AA Ionin, OG Kosareva, AB Savelev. Terahertz ring beam independent on ω–2ω phase offset in the course of two-color femtosecond filamentation.
[13] M Rasmussen, O Nagy, S Skupin, A Stathopulos, L Berge, P Jepsen, B Zhou. Frequency-resolved characterization of broadband two-color air-plasma terahertz beam profiles.
[14] V Blank, MD Thomson, HG Roskos. Spatio-spectral characteristics of ultra-broadband THz emission from two-colour photo-excited gas plasmas and their impact for nonlinear spectroscopy.
[15] VA Andreeva, OG Kosareva, NA Panov, DE Shipilo, PM Solyankin, MN Esaulkov, P González de Alaiza Martínez, AP Shkurinov, VA Makarov, L Bergé, SL Chin. Ultrabroad Terahertz spectrum generation from an air-based filament plasma.
[16] J Degert, M Tondusson, V Freysz, E Abraham, S Kumar, E Freysz. Ultrafast, broadband and tunable terahertz reflector and neutral density filter based on high resistivity silicon.
[17] CB Sørensen, L Guiramand, J Degert, M Tondusson, E Skovsen, E Freysz, E Abraham. Conical versus Gaussian terahertz emission from two-color laser-induced air plasma filaments.
[18] E Hecht.
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Eiji Hase, Jérôme Degert, Eric Freysz, Takeshi Yasui, Emmanuel Abraham. Frequency-resolved measurement of two-color air plasma terahertz emission[J]. Journal of the European Optical Society-Rapid Publications, 2024, 20(1): 2024042
Category: Research Articles
Received: Jul. 1, 2024
Accepted: Oct. 14, 2024
Published Online: Dec. 16, 2024
The Author Email: Abraham Emmanuel (emmanuel.abraham@u-bordeaux.fr)