# Frontiers of Optoelectronics

 Front. Optoelectron.    2019, Vol. 12 Issue (2) : 117-147     https://doi.org/10.1007/s12200-018-0860-7
 REVIEW ARTICLE
Terahertz wave generation from ring-Airy beam induced plasmas and remote detection by terahertz-radiation-enhanced-emission-of-fluorescence: a review
Kang LIU1, Pingjie HUANG2, Xi-Cheng ZHANG1,3()
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 With the increasing demands for remote spectroscopy in many fields ranging from homeland security to environmental monitoring, terahertz (THz) spectroscopy has drawn a significant amount of attention because of its capability to acquire chemical spectral signatures non-invasively. However, advanced THz remote sensing techniques are obstructed by quite a few factors, such as THz waves being strongly absorbed by water vapor in the ambient air, difficulty to generate intense broadband coherent THz source remotely, and hard to transmit THz waveform information remotely without losing the signal to noise ratio, etc. In this review, after introducing different THz air-photonics techniques to overcome the difficulties of THz remote sensing, we focus mainly on theoretical and experimental methods to improve THz generation and detection performance for the purpose of remote sensing through tailoring the generation and detection media, air-plasma.For the THz generation part, auto-focusing ring-Airy beam was introduced to enhance the THz wave generation yield from two-color laser induced air plasma. By artificially modulated exotic wave packets, it is exhibited that abruptly auto-focusing beam induced air-plasma can give an up to 5.3-time-enhanced THz wave pulse energy compared to normal Gaussian beam induced plasma under the same conditions. At the same time, a red shift on the THz emission spectrum is also observed. A simulation using an interference model to qualitatively describe these behaviors has be developed.For the THz detection part, the results of THz remote sensing at 30 m using THz-radiation-enhanced-emission-of-fluorescence (THz-REEF) technique are demonstrated, which greatly improved from the 10 m demonstration last reported. The THz-REEF technique in the counter-propagation geometry was explored, which is proved to be more practical for stand-off detections than co-propagation geometry. We found that in the counter-propagating geometry the maximum amplitude of the REEF signal is comparable to that in the co-propagating case, whereas the time resolved REEF trace significantly changes. By performing the study with different plasmas, we observed that in the counter-propagating geometry the shape of the REEF trace depends strongly on the plasma length and electron density. A new theoretical model suggesting that the densest volume of the plasma does not contribute to the fluorescence enhancement is proposed to reproduce the experimental measurements.Our results further the understanding of the THz-plasma interaction and highlight the potential of THz-REEF technique in the plasma detection applications. Corresponding Authors: Xi-Cheng ZHANG Just Accepted Date: 29 November 2018   Online First Date: 22 January 2019    Issue Date: 03 July 2019
 Cite this article: Kang LIU,Pingjie HUANG,Xi-Cheng ZHANG. Terahertz wave generation from ring-Airy beam induced plasmas and remote detection by terahertz-radiation-enhanced-emission-of-fluorescence: a review[J]. Front. Optoelectron., 2019, 12(2): 117-147. URL: http://journal.hep.com.cn/foe/EN/10.1007/s12200-018-0860-7 http://journal.hep.com.cn/foe/EN/Y2019/V12/I2/117
 Fig.1  Experimental setup for measuring the THz radiation pattern from a laser-induced filamentation (Reprinted from Ref. [46]) Fig.2  Schematic demonstration of experimental setup for THz generation from two-color laser induced air plasma. An intense femto-second laser beam $ω$ is focused by a lens to generate plasma in the air, $β$-BBO is used to generate $2 ω$ Fig.3  (a) Inside the dashed line is the in-line PC (phase compensator). (b) Schematic illustration of the PC incorporated with a wedge pair: DM used to separate or recombine $ω$ and $2ω$ beams; HWP used to control the polarization of the $2ω$ beam. DWP, dual-wavelength waveplate; BP, birefringent plate (α-BBO); QW, quartz wedges; Fs, femtosecond; HR, highly reflect; DM, dichroic mirror (Reprinted from Ref. [53] ©2011 IEEE) Fig.4  Schematic of THz emission from a long, two-color laser-induced filament. The phase difference between 800 nm (dashed red curves) and 400 nm (solid blue curves) pulses along the filament results in a periodic oscillation of microscopic current amplitude and polarity. The resulting far-field THz radiation is determined by interference between the waves emitted from the local sources along the filament. P1, P2 and P3 are respectively the optical path along the different directions as shown in the figure, $θ$ is the angle between P3 and P1 (Reprinted from Ref. [57]) Fig.5  (a) Basic concept of THz-ABCD: electrodes are placed at the geometric focus of collinearly focused THz and optical probe beams with a variable time delay. Second harmonic light is induced from the THz field and the local bias field Ebias. Modulating Ebias allows for heterodyne detection for enhanced sensitivity. (b) Measured second harmonic intensity vs third order nonlinear susceptibility $χ(3)$. All $χ (3)$ are normalized with respect to nitrogen (Reprinted from Ref. [66] ©2011 IEEE) Fig.6  Envisioned scheme for THz stand-off generation and detection. Two dual-color pulses are focused close to the target under investigation creating a plasma emitter and a plasma sensor. Inset shows an absorbance spectrum of 4A-DNT retrieved through THz-REEF (Reprinted from Ref. [71]) Fig.7  (a) Experimental set-up for observing the ring-Airy beam induced air plasma; (b) fluorescence false color images of ring-Airy beam plasmas (top 5) with pulse energy from 0.25 to 0.65 mJ, and Gaussian beam plasma (bottom) with pulse energy 0.65 mJ. The Gaussian plasma image intensity has been reduced 25 times to reach similar visibility of the ring-Airy beam plasmas Fig.8  Experimental set-up. SLM: spatial light modulator, OD: opaque disk, on a transparent glass slide, L: lens, PM: parabolic mirror, M: mirror. Inset: Schematic comparison between the radial intensity distribution of the ring-Airy and the Gaussian beam; zoom-in on the beam area prior and post to the glass slide: case 1 is when using the Gaussian beam, the glass slide is shifted so the OD is removed from the beam path, case 2 is when using the ring-Airy beam, the OD is moved back into the center of the beam to block the unwanted part Fig.9  (a) Typical THz waveform generated from the ring-Airy beam induced plasma pumped with a pulse energy of 0.65 mJ; (b) corresponding spectrum Fig.10  THz emission peak amplitude versus the b-BBO distance from the focus. Blue dots: experimental data; blue solid curve: FWM model fitting result; black dashed curve: nonlinear fitting envelope taking into account the SHG efficiency change as the BBO is moving toward the focus; the insets show two sample THz waveforms having opposite polarities Fig.11  Emitted THz wave peak amplitude as a function of total pump pulse energy (800 and 400 nm). Red dots: experimental data from ring-Airy beam; red dashed line: fitting of the ring-Airy data with the FWM model. The error bars are the measurements standard deviation of each point Fig.12  (a) Fluorescence image comparison between an ring-Airy beam plasma and a Gaussian plasma (intensity reduced by 20 times), both generated with pump pulse energy 0.5 mJ; (b) measured THz waveforms and (c) their corresponding normalized spectra generated by the two plasmas in (a) Fig.13  Comparison between THz emission peak amplitudes from ring-Airy beam and Gaussian beam versus different pump pulse energy Fig.14  Schematic representation of two-color filament THz generation interference model (Reprinted from Ref. [104]) Fig.15  (a) Normalized simulations of THz radiation spectrums without and (b) with the 3-mm-thick ZnTe detection bandwidth limitation; the inset in (a) is the enlarged version of the blue area (0−4 THz). Red solid curves: emission from ring-Airy plasma; black dashed curves: emission from Gaussian plasma Fig.16   Spectrum of air interacting with a 220 fs laser pulse. The lines marked by 1 are assigned to the first negative band system of $N 2+( B3 ∑u+− X 2∑ g+ transition)$ and those marked by 2 are assigned to the second positive band systemof $N2( C3Πu−B3 Πgtransition)$ respectively. In the transitions (v–v′), v and v′ denote the vibrational levels of upper and lower electronic states, respectively (Reprinted from Ref. [105]) Fig.17  (a) Schematics of the interaction between the THz wave and laser induced plasma. (b) Measured fluorescence spectra versus THz field as td = −1 ps. Major fluorescence lines are labeled. (c) Measured quadratic THz field dependence of 357 nm fluorescence emission line as td = −1 ps. Inset: Theisotropic emission pattern of THz-REEF (Reprinted from Ref. [60]) Fig.18  (a) Time-resolved THz-REEF $ΔFL (t d)$ and THz field ETHz(td). (b) Time-resolved $ΔFL( td )$, ETHz(td), $dΔ FL (t d)/ dt d$, and $ETHz2( td)$ on therising edge in the expanded scale of (a). All curves are normalized and offset forclarity (Reprinted from Ref. [60]) Fig.19  Schematic figure of THz-REEF remote sensing experimental setup. DWP: dual wave plate; OPM: off-axis parabolic mirror; PMT: photo multiplier tube detector Fig.20  Terahertz wave assisted electron impact ionization of high-lying states in plasma. (a) High-lying states can be ionized by a series of collisions with energetic electrons; (b) interaction between the terahertz pulse and the asymmetric photoelectron velocity distributions generated by two-color field ionization (Reprinted from Ref. [61]) Fig.21  Measured time-dependent REEF at a relative optical phase change of $Δ φω, 2ω=±mπ/2$. The terahertz wave enhanced fluorescence shows significantdependence on the initial electron velocity distribution, which is determined by $Δ φω ,2ω$ (Reprinted from Ref. [61]) Fig.22  Typical two-color THz-REEF signal. Blue and green: two THz-REEF traces with different relative phase $Δφω,2ω$. Red: the difference between blue andgreen curves, that gives the measured THz waveform Fig.23  THz waveforms measured by two-color THz-REEF technique at different detection distances, 0.1, 7, 14, 20, 30 m respectively Fig.24  Depiction of the co-propagation and counter-propagation interaction geometries. (a) and (c) are showing the plasma fluorescence intensity enhancement as a function of$Δt$ in (a) co-propagating and (c) counter-propagating geometriesmeasured by PMT. Both curves are normalized to 1. (b) In co-propagating geometry the THz (blue) and the NIR (red) pulses travel in the same direction. (d) In counter-propagating geometry the THz and NIR pulses travel in opposite directions. $Δt$is the time delay between the two pulses. The special cases of $Δt$ = t1, t2 in co-propagation or $Δt=t ′1 , t′2$ in counter-propagation are respectively denoted in (a,b) and (c,d) Fig.25  Schematic demonstrations of plasmas of different lengths being generated by (a) 4 inch EFL, (b) 2 inch EFL, and (c) 1 inch EFL plano-convex lenses. (d) Time-resolved counter-propagation THz-REEF measurements of the plasmas generated in (a)−(c) cases Fig.26  (a) Time-resolved counter-propagation THz-REEF intensity for 2 inch EFL lens case (solid red) and its derivative (solid black). The black dots represent the data sampling rate used for the derivative in order to reduce the noise of the result. The curves are offset for clarity. (b) Derivative of REEF signal curve (solid black) is compared to the square of the experimental THz waveform measured with electro-optic sampling (solid blue) Fig.27  Time-resolved counter-propagation THz-REEF measurements of a micro-plasma with different THz pulse peak field strengths. The plasma is generated by a 2 inch EFL lens Fig.28  Micro-plasma fluorescence emission spectra with different values of plasma excitation laser pulse energy. Red is when no THz pulse is applied and blue is when a THz pulse with peak field of 90 kV/cm is applied. The microplasmas are obtained with a 1 inch EFL plano-convex lens Fig.29  Time-resolved counter-propagation two color THz-REEF measurements with different phases $Δϕ$ between$ω$and$2ω$pulses. The plasma is generated with (a) 4 inch EFL lens and (b) 2 inch EFL lens. (c) Plasma fluorescence intensity modulation as a function of the relative phase $Δϕ$ Fig.30  Experimental (solid line) and numerical fitted (dashed line) time resolved counter-propagation THz-REEF intensity for the following focusing conditions: (a) 2 inch EFL lens; (c) 1 inch EFL lens; in (b), (d), the solid shaded areas represent the integration of the plasma fluorescence intensity along the radial dimension as measured with the iCCD camera, and the dashed lines are the numerically evaluated plasma effective electron densities producing the dashed curves plotted in (a) and (c)