Generation and detection of pulsed terahertz waves in gas: from elongated plasmas to microplasmas

doi:10.1007/s12200-018-0819-8

Front. Optoelectron.

2018, Vol. 11

Issue (3) : 209-244 https://doi.org/10.1007/s12200-018-0819-8

REVIEW ARTICLE

Generation and detection of pulsed terahertz waves in gas: from elongated plasmas to microplasmas

Fabrizio BUCCHERI¹, Pingjie HUANG², Xi-Cheng ZHANG^1,³(

)

¹. The Institute of Optics, University of Rochester, Rochester, NY 14627, USA
². State Key Laboratory of Industrial Control Technology, College of Control Science and Engineering, Zhejiang University, Hangzhou 310027, China
³. The Beijing Advanced Innovation Center for Imaging Technology, Capital Normal University, Beijing 100037, China

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Abstract

The past two decades have seen an exponential growth of interest in one of the least explored region of the electromagnetic spectrum, the terahertz (THz) frequency band, ranging from to 0.1 to 10 THz. Once only the realm of astrophysicists studying the background radiation of the universe, THz waves have become little by little relevant in the most diverse fields, such as medical imaging, industrial inspection, remote sensing, fundamental science, and so on. Remarkably, THz wave radiation can be generated and detected by using ambient air as the source and the sensor. This is accomplished by creating plasma under the illumination of intense femtosecond laser fields. The integration of such a plasma source and sensor in THz time-domain techniques allows spectral measurements covering the whole THz gap (0.1 to 10 THz), further increasing the impact of this scientific tool in the study of the four states of matter.

In this review, the authors introduce a new paradigm for implementing THz plasma techniques. Specifically, we replaced the use of elongated plasmas, ranging from few mm to several cm, with sub-mm plasmas, which will be referred to as microplasmas, obtained by focusing ultrafast laser pulses with high numerical aperture optics (NA from 0.1 to 0.9).

The experimental study of the THz emission and detection from laser-induced plasmas of submillimeter size are presented. Regarding the microplasma source, one of the interesting phenomena is that the main direction of THz wave emission is almost orthogonal to the laser propagation direction, unlike that of elongated plasmas. Perhaps the most important achievement is the demonstration that laser pulse energies lower than 1 mJ are sufficient to generate measurable THz pulses from ambient air, thus reducing the required laser energy requirement of two orders of magnitude compared to the state of art. This significant decrease in the required laser energy will make plasma-based THz techniques more accessible to the scientific community, as well as opening new potential industrial applications.

Finally, experimental observations of THz radiation detection with microplasmas are also presented. As fully coherent detection was not achieved in this work, the results presented herein are to be considered a first step to understand the peculiarities involved in using the microplasma as a THz sensor.

Keywords terahertz waves Terahertz Air Photonics generation and detection elongated plasmas microplasmas

Corresponding Author(s): Xi-Cheng ZHANG

Just Accepted Date: 22 June 2018 Online First Date: 07 August 2018 Issue Date: 31 August 2018

Cite this article:

Fabrizio BUCCHERI,Pingjie HUANG,Xi-Cheng ZHANG. Generation and detection of pulsed terahertz waves in gas: from elongated plasmas to microplasmas[J]. Front. Optoelectron., 2018, 11(3): 209-244.

URL:

https://academic.hep.com.cn/foe/EN/10.1007/s12200-018-0819-8
https://academic.hep.com.cn/foe/EN/Y2018/V11/I3/209

Fig.1 Electromagnetic spectrum. The terahertz frequency band bridges electronics and optics (modified from http://www.physik.uni-kl.de/en/beigang/forschungsprojekte/)

Fig.2 Typical measured THz waveform (a) and its spectrum (b) obtained as the magnitude of its Fourier transform. The oscillatory tail after the main pulse (Dt>1), corresponding to absorption features in the spectrum, is an effect of the propagation of the pulse through ambient air

Fig.3 Example of THz time domain spectroscopy (a). A reference trace (solid black) and a sample trace (solid red) are acquired without and with the sample inserted in the THz path respectively. Compared to the reference trace, the sample trace has: a reduced amplitude, indicating the presence of absorption; a time shift, indicating the addition in the THz path of a material with refractive index greater than one; several oscillations in its tail, indicating the presence of resonant absorption mechanisms. (b) Fourier transform of the reference and sample traces (modified from http://www.physik.uni-kl.de/en/beigang/forschungsprojekte/)

Fig.4 (a) ABCD setup sketch: the laser probe (w) and the THz beam are collinearly focused through two electrodes generating a signal at the second harmonic 2w. A high voltage square wave biases the electrodes allowing the coherent measurement of THz radiation. (b) Measured second harmonic intensity as a function of the third order nonlinear susceptibility of the gas employed. All values of nonlinear susceptibility are normalized with respect to nitrogen (Reprinted with permission from Ref. [¹⁷], copyright 2012, Elsevier)

Fig.5 (a) THz-REEF geometry. The THz pulse is focused on the plasma generated by the laser probe beam, collinearly with the laser propagation direction. The fluorescence emitted by the plasma can be collected by any angle. (b) Electrons accelerate in the THz field and collide with neighboring molecules. (c) Plasma fluorescence intensity spectrum with (red) and without (black) THz field. The fluorescence lines are all equally enhanced. (Reprinted with permission from Ref. [¹¹⁶], copyright 2011, IEEE)

Fig.6 (Top) Time resolved plasma fluorescence intensity for the cases of antiparallel (blue), symmetric (red) and parallel (black) electron drift velocities. (Bottom) The subtraction of the parallel trace from the antiparallel one reveals the THz waveform (Reprinted with permission from Ref. [¹⁷], copyright 2012, Elsevier)

Fig.7 Acoustic plasma emission measured with a high frequency microphone placed 10 mm away from the plasma with (red) and without (black) THz illumination. The inset shows the setup for THz Enhanced Acoustics measurements (Reprinted with permission from Ref. [¹⁷], copyright 2012, Elsevier)

Tab.1 Advantages and disadvantages of THz Air photonics

Tab.2 Comparison between elongated plasma and microplasma

Fig.8 THz waves are emitted by the ambient air microplasma obtained by focusing the laser excitation through a high NA objective. A high resistivity silicon wafer (filter) is inserted in the THz path in order to block the pump beam. The waveforms are retrieved with electro-optic sampling. The THz generation portion of the setup can be rotated about the position of the microplasma in order to study the angle-dependent emission from the source. The inset is a picture of the microplasma created by focusing laser pulses with energy of 65 mJ through a 0.85 NA air-immersion objective as seen through a UV bandpass filter. The laser excitation propagates from right to left. The plasma is imaged from the side with a commercial iCCD camera. The fluorescence profile is Gaussian. The FWHM for the longitudinal and the transverse fluorescence intensity profile is (36.7±8.7) mm and (28.5±8.7) mm respectively. List of abbreviations: HWP, half wave plate; OBJ, objective; OAPM, off-axis parabolic mirror; POL, THz polarizer

Fig.9 (a) Density plot representing the coherent angle-dependent emission from a microplasma generated with laser pulse energy of 65 mJ. The plot is obtained through spline interpolation of ten THz waveforms recorded at different detection angles in 10° intervals starting from 0°. Each waveform is normalized to the highest value of THz field recorded in the set. Dt is the time delay between the pump and the probe beam. (b) THz pulse energy as a function of detection angle. The pulse energy is extracted from the THz waveforms displayed in (a)

Fig.10 Measured THz waveforms at detection angle of 80 degrees for a laser pulse energy of 65 mJ (top) and of 660 nJ (bottom). For clarity, the plots are offset and the waveform measured at 660 nJ is magnified 600 times

Fig.11 (a) Measured THz spectral amplitudes with 〈110〉 -cut ZnTe crystals of different thicknesses: 1 mm (red curve), 0.22 mm (black curve). The black dashed curve is the measured experimental noise. (b) Density plot representing the angle-dependent spectral emission from a microplasma generated with laser pulse energy of 65 mJ. The plot is obtained through spline interpolation of the Fourier transform of ten THz waveforms recorded at different detection angles in 10° intervals starting from 0°. All the spectra are normalized to one to show how the spectrum does not change appreciably with detection angle. However, by doing so the reader could be lead to believe that there is a strong emission for detection angle close to 0°. This is not the case as the amplitude of the spectra measured at 0° and 10° are more than one order of magnitude lower than those measured at larger angles

Fig.12 Peak amplitude of measured THz waveforms as a function of the azimuthal angle HWP placed in the laser beam path right before the microscope objective. The amplitude does not change upon linear rotations of the laser polarization

Fig.13 (a) THz waveforms measured when the laser beam has the following polarizations: linear (blue), circular (red), vortex (black), radial (magenta), azimuthal (green). (b) Comparison of the peak amplitudes obtained in the cases mentioned above

Fig.14 Parametric plot representing the measured polarization of the THz radiation in the case of p-polarized (red) and s-polarized (blue) laser beam. The polarization state of the collected THz wave does not change with the polarization of the laser beam

Fig.15 (a) THz peak power as a function of laser pulse energy for a detection angle of 80°. The laser source is Spectra Physics Hurricane (800 nm, 100 fs, 0.7 mJ, 1 kHz) and the microplasma is created with the 0.85 NA microscope objective. The dots are the experimental data, while the solid line is a quadratic fit. (b) THz peak power as a function of laser pulse energy for a detection angle of 80° (red). Fluorescence intensity integrated from 200 to 1000 nm as a function of laser pulse energy (gray).The laser source is Coherent Libra (800 nm, 50 fs, 50 fs, 1 kHz) and the microplasma is created with the 0.77 NA aspheric lens. The dots are the experimental data, while the solid line is a quadratic fit

Fig.16 THz pulse energy as a function of detection angle for microplasmas obtained with three different achromatic lenses: 0.77 NA (red); 0.68 NA (blue); 0.40 NA (black)

Tab.3 Summary of the measurements of THz emission from microplasmas obtained with different focusing NA

Fig.17 (a) Experimental setup employing a parabolic reflector for the collection of the THz radiation emitted from the microplasma. (b) Diagram showing the transverse profile of the collimated THz beam exiting the parabolic reflector. The beam is radially polarized. (c) Zoom on the inside of the parabolic reflector showing how the THz radiation is collected. List of abbreviations: HWP, half wave plate; OBJ, objective; FM, flat mirror, OAPM, off-axis parabolic mirror; EOC, electro-optic crystal; EO DET, electro-optic detection

Fig.18 Comparison of THz waveforms obtained with a laser energy of 65 mJ with the following generation schemes: microplasma collected with the parabolic reflector (red); microplasma collected with the off-axis parabolic mirror (blue); two-color elongated plasma (black); one-color elongated plasma (green)

Fig.19 (a) Comparison of the spectrum obtained with a microplasma generated with 65 mJ laser energy (blue) with two-color elongated plasma generated with 102 mJ laser energy (gray). (b) Comparison of the spectrum obtained with a microplasma generated with 65 mJ laser energy (blue) with one-color elongated plasma generated with 214 mJ laser energy (green)

Fig.20 Experimental arrangement for the generation of the two-color microplasma

Fig.21 (a) Visible picture of the conical laser beam exiting the reflecting objective. The laser travels from left to right. The picture is obtained with a long exposure and by slowly moving a lens tissue along the laser propagation axis so to scatter light into the camera. (b) Transverse beam profile of the laser beam captured by a CCD camera at different distances from the focal plane z = 0

Fig.22 (a) THz pulse energy as a function of detection angle in the one-color (red) and two-color (blue) cases. In the one-color case the azimuthal angle of the b-BBO crystal is rotated so to minimize the SH emission, while in the two-color case so to maximize the THz wave emission. (b) Ratio of the THz peak fields in the two-color and one-color case as a function of detection angle (red). The dashed black line signaling a ratio equals to one is inserted as a reference

Fig.23 (a) THz peak amplitude as a function of the relative phase between FB and SH for the one-color (red) and two-color (blue) cases for different detection angles. The curves are offset for clarity. (b), (c) and (d) shows more clearly the experimental data for angle of 90°, 60°, and 30° respectively. Dots are experimental data, while blue line is the fitting of the two-color data with a sine function

Fig.24 Density plot representing the angle-dependent spectral emission from a microplasma generated with the reflective objective and laser pulse energy of 90 mJ. The plot is obtained through spline interpolation of the Fourier transform of ten THz waveforms recorded at different detection angles in 10° intervals starting from 0° obtained in the one-color (a) and two-color (b) cases. (c) Spectral amplitude measured at a detection angle of 60° in the one-color (red) and two-color (blue) cases. All the spectra are normalized to one to show how the spectrum does not change appreciably with detection angle. However, by doing so the reader could be lead to believe that there is a strong emission for detection angle close to 0°. This is not the case as the amplitude of the spectra measured at 0° and 10° are more than one order of magnitude lower than those measured at larger angles

Fig.25 Visual representation of the action of the ponderomotive force. The picture shows in red the intensity profile of a focused laser beam along the propagation axis and any radial direction. Charged particles close to the focal volume are pushed toward region of lower intensity

Fig.26 Longitudinal currents from which THz radiation originates is formed in a three steps process. (a) Electrons and ions are created at the front of the laser pulse; (b) Ions can be considered still due to their mass, while electrons are pushed backward from them by the ponderomotive force. The spatial separation between ions and electrons creates a net charge density behind the ionization front which acts as an effective dipole; (c) After the laser pulse leaves the plasma the charges are brought back together by the restoring force due to Coulomb attraction

Fig.27 Theoretical radiation pattern at a frequency of 1.5 THz of plasmas of lengths (a) 4 mm, (b) 400 mm and (c) 40 mm

Fig.28 Comparison of the radiation patterns calculated with numerical simulation and those measured experimentally. (a) one-color scheme, 40 mm plasma length: experiment (red solid), simulation (dashed, magenta); (b) one-color scheme, 72 mm plasma length (0.77 NA lens), red (simulation, dashed line, experiment, dots); 78 mm plasma length (0.68 NA lens), blue (simulation, dashed line, experiment, dots); 120 mm plasma length (0.40 NA lens), black (simulation, dashed line, experiment, dots); (c) two-color scheme, 40 μm plasma length (0.45 NA Schwarzschild reflective objective): one-color experiment (red, solid); one-color simulation (magenta, dashed); two-color experiment (blue, solid)

Fig.29 (a) Interaction geometries. Top: in copropagation geometry the THz (blue) and optical (red) pulses travel in the same direction. Dt is the time delay between the two. Bottom: in counter-propagation geometry the THz and pulses travel in opposite direction. In this case Dt defines the position along the optical propagation axis at which the THz and optical pulses meet. (b) Plasma fluorescence intensity enhancement as a function of Dt in copropagation (orange) and counter-propagation (blue) geometries. Both curves are normalized to one

Fig.30 Fluorescence intensity spectrum when no THz is applied (black) and for peak THz fields of 67 kV/cm (blue) and 90 kV/cm (red). The THz illuminates the plasma in counter-propagation geometry. All the emission lines belong to N₂ 2⁺ system. The numbers in parenthesis are the upper-lower vibrational levels of the transitions

Fig.31 (a) Experimental setup. The optical (red) and THz (blue) pulses travel in opposite direction. The plasma is imaged from the side with an iCCD camera through a narrowband filter centered at 337 nm. (b) Plasma fluorescence cross-sections with (blue) and without (dashed blue) THz illumination. The red curve is the spatially resolved fluorescence enhancement calculated by subtracting the fluorescence profile with and without THz illumination. (c) Spatially resolved fluorescence enhancement traces for different values of Dt. As Dt increases, the onset of the enhancement moves along the plasma toward the direction which the optical pulse comes from. (d) Plasma fluorescence intensity enhancement as a function of Dt in counter-propagation (blue) geometry. The colored points represent the area underneath the curves of (c) of the corresponding color and letter

Fig.32 Plasma fluorescence intensity enhancement as a function of Dt from plasmas obtained with the following optic components: 4 inch EFL plano-convex lens (blue); 2 inch EFL plano-convex lens (red); 1 inch EFL plano-convex lens (green); 0.14 NA microscope objective (purple). The plots are offset for clarity. Each trace in counter-propagation geometry is plotted together with the one in copropagation geometry obtained with the 4 inch EFL plano-convex lens (orange). Each curve is normalized to one

Fig.33 (a) and (b) Fluorescence intensity spectrum when no THz is applied (red) and peak THz field of 90 kV/cm (blue) for different values of laser energy. The microplasmas are obtained with (a) 1 inch EFL plano-convex lens and (b) the 0.14 NA microscope objective. (c) and (d) Plasma fluorescence intensity enhancement as a function of Dt for different values of laser energy. All curves are normalized to one. The microplasmas are obtained with (c) 1 inch EFL plano-convex lens (d) the 0.14 NA microscope objective

Fig.34 (a) Relative fluorescence enhancement as a function of laser energy for microplasmas obtained with 1 inch EFL plano-convex lens (red) and 0.14 NA microscope objective (green). (b) REEF figure of merit as a function of laser energy for microplasmas obtained with 1 inch EFL plano-convex lens (red) and 0.14 NA microscope objective (green)

Tab.4 Comparison of the REEF traces obtained in copropagation and counter-propagation geometry through the REEF figure of merit (FOM)

Fig.35 Simulation of the REEF trace in copropagation geometry. The plot shows: the simulated plasma fluorescence intensity enhancement (black); the simulated THz waveform (red); the intensity as calculated by squaring the THz waveform (blue); the derivative of the plasma fluorescence intensity enhancement (fuchsia circles). The plasma fluorescence intensity enhancement curve and its derivative have been shifted of an amount t_φ = 300 fs so to show the overlap of the second with the THz intensity curve

Fig.36 Experimental (solid line) and simulated (dashed line) plasma fluorescence intensity enhancement as a function of Dt in counter-propagation geometry for the following focusing conditions (a) 4 inches EFL PC lens; (b) 2 inches EFL PC lens; (c) 1 inches EFL PC lens; (d) 0.14 NA microscope objective

Fig.37 (a) Experimental plasma fluorescence intensity enhancement as a function of Dt in counter-propagation geometry for 2 inches EFL lens case (solid red) and its derivative (dashed red). The black dots represented the sampling points of the experimental data used to construct an interpolated curve of the measured data. The derivative of the interpolated curve is shown as a solid black curve. The curves are offset for clarity. (b) The derivative of the interpolated curve (solid black) is compared to the square of the experimental THz waveform measured with electro-optic sampling (solid blue)

Fig.38 Experimental (solid line) and numerical fitted (dashed line) plasma fluorescence intensity enhancement as a function of Dt in counter-propagation geometry for the following focusing conditions (a) 2 inches EFL PC lens; (c) 1 inches EFL PC lens; (e) 0.14 NA microscope objective. The respective electron densities are plotted in (b), (d) and (f). The solid lines represent the integration of the plasma fluorescence intensity along the radial dimension as measured with the iCCD camera, whereas the dashed line are the numerically evaluated plasma electron densities producing the curves plotted in (a), (c) and (e)

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