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Frontiers of Optoelectronics

Front. Optoelectron.    2014, Vol. 7 Issue (2) : 121-155     DOI: 10.1007/s12200-013-0371-5
REVIEW ARTICLE |
Investigation of ultra-broadband terahertz time-domain spectroscopy with terahertz wave gas photonics
Xiaofei LU1,Xi-Cheng ZHANG2,*()
1. SunEdison Inc. 501 Pearl Dr. Saint Peters, MO 63376, USA
2. The Institute of Optics, University of Rochester, Rochester, NY 14627-0186, USA
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Abstract

Recently, air plasma, produced by focusing an intense laser beam to ionize atoms or molecules, has been demonstrated to be a promising source of broadband terahertz waves. However, simultaneous broadband and coherent detection of such broadband terahertz waves is still challenging. Electro-optical sampling and photoconductive antennas are the typical approaches for terahertz wave detection. The bandwidth of these detection methods is limited by the phonon resonance or carrier’s lifetime. Unlike solid-state detectors, gaseous sensors have several unique features, such as no phonon resonance, less dispersion, no Fabry-Perot effect, and a continuous renewable nature. The aim of this article is to review the development of a broadband terahertz time-domain spectrometer, which has both a gaseous emitter and sensor mainly based on author’s recent investigation. This spectrometer features high efficiency, perceptive sensitivity, broad bandwidth, adequate signal-to-noise ratio, sufficient dynamic range, and controllable polarization.

The detection of terahertz waves with ambient air has been realized through a third order nonlinear optical process: detecting the second harmonic photon that is produced by mixing one terahertz photon with two fundamental photons. In this review, a systematic investigation of the mechanism of broadband terahertz wave detection was presented first. The dependence of the detection efficiency on probe pulse energy, bias field strength, gas pressure and third order nonlinear susceptibility of gases were experimentally demonstrated with selected gases. Detailed discussions of phase matching and Gouy phase shift were presented by considering the focused condition of Gaussian beams. Furthermore, the bandwidth dependence on probe pulse duration was also demonstrated. Over 240 times enhancement of dynamic range had been accomplished with n-hexane vapor compared to conventional air sensor. Moreover, with sub-20 fs laser pulses delivered from a hollow fiber pulse compressor, an ultra-broad spectrum covering from 0.3 to 70 THz was also showed.

In addition, a balanced detection scheme using a polarization dependent geometry was developed by author to improve signal-to-noise ratio and dynamic range of conventional terahertz air-biased-coherent-detection (ABCD) systems. Utilizing the tensor property of third order nonlinear susceptibility, second harmonic pulses with two orthogonal polarizations was detected by two separated photomultiplier tubes (PMTs). The differential signal from these two PMTs offers a realistic method to reduce correlated laser fluctuation, which circumvents signal-to-noise ratio and dynamic range of conventional terahertz ABCD systems. A factor of two improvement of signal-to-noise ratio was experimentally demonstrated.

This paper also introduces a unique approach to directly produce a broadband elliptically polarized terahertz wave from laser-induced plasma with a pair of double helix electrodes. The theoretical and experimental results demonstrated that velocity mismatch between excitation laser pulses and generated terahertz waves plays a key role in the properties of the elliptically polarized terahertz waves and confirmed that the far-field terahertz emission pattern is associated with a coherent process. The results give insight into the important influence of propagation effects on terahertz wave polarization control and complete the mechanism of terahertz wave generation from laser-induced plasma.

This review provides a critical understanding of broadband terahertz time-domain spectroscopy (THz-TDS) and introduces further guidance for scientific applications of terahertz wave gas photonics.

Keywords terahertz spectroscopy      terahertz detection      ?broadband      gas sensor     
Corresponding Authors: Xi-Cheng ZHANG   
Issue Date: 25 June 2014
 Cite this article:   
Xiaofei LU,Xi-Cheng ZHANG. Investigation of ultra-broadband terahertz time-domain spectroscopy with terahertz wave gas photonics[J]. Front. Optoelectron., 2014, 7(2): 121-155.
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http://journal.hep.com.cn/foe/EN/10.1007/s12200-013-0371-5
http://journal.hep.com.cn/foe/EN/Y2014/V7/I2/121
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Fig.1  Schematic of experimental setup. BS: beam splitter. BBO: type-I beta barium borate. PM: parabolic mirror. PMT: photomultiplier tube. HV: high voltage modulator. Terahertz wave was generated through laser-induced plasma in air. An iris with a diameter of 10 mm was placed at a distance about 50 mm after the plasma. A high-resistivity silicon wafer was used to block the residual pump beam. The terahertz beam and probe beam were focused collinearly in the presence of a modulated bias, resulting in a second harmonic signal, which was detected by PMT
Fig.2  Schematic illustration of gas cell. Both the entrance and exit windows were made of quartz, which is relatively transparent to both fundamental and second harmonic beams. The quartz material has low absorption at the frequency below 5 THz. The electrical field was connected through an electrical feed-through at the top of gas cell and the gases were introduced through the bottom of the cell
Fig.3  Measured second harmonic intensity (I2ω) versus the probe pulse energy (Iω) at a bias field of 7.5 kV/cm, and gas pressure of 756 torr with Xe and SF6 gases. Dots are from measurements and dashed lines are quadratic fits. The deviation of the probe energy dependence above 50 μJ for Xe and 70 μJ for SF6 are consistent with the onset of intensity clamping due to plasma formation
Fig.4  Terahertz (a) waveforms and (b) spectra obtained with different probe pulse energy. Electrodes were placed around the center of plasma. Black arrows are the guides for the absorption frequency, which shows a red shift while increasing the probe pulse energy
Fig.5  Measured second harmonic intensity (I2ω) versus DC bias field (EDC) at gas pressure of (a) 100 torr and (b) 756 torr, with a probe pulse energy of 50 μJ. Dots are from measurements and dashed lines are linear fits
Fig.6  Detected second harmonic intensity (I2ω) verses third order nonlinear susceptibility (χ(3)) of gases. Red dots are experimental data and black dashed line is the quadratic fit. Y-axis is normalized with the reference signal taken with 100 torr nitrogen gas at the same experimental condition. Also, all the χ(3) are normalized with that of nitrogen
Fig.7  Pressure dependence of detected second harmonic intensity from (a) xenon; (b) propane and (c) n-butane gas at different focus condition of terahertz beam. Optical probe beam power was set to be 20 mW and bias field strength is about 8 kV/cm. The Rayleigh length of terahertz beam was controlled by an iris in terahertz beam path. Black (red) dots are from measurements without iris (with iris) condition and dashed lines are fit from analytical expression. Fitted Rayleigh lengths are zT = 0.8 mm and zT = 1.5 mm, respectively
Fig.8  2D plot of detected terahertz spectra with Xe versus pressure at different terahertz focus condition. The terahertz Rayleigh lengths without and with iris were estimated to be 0.8 and 1.5 mm, respectively. The different phase match of each frequency component results in a spectral shift toward high frequency. (a) F number= 2.4; (b) F number= 1.8
Fig.9  Pressure dependence of various frequency components with Xe sensor at (a) F number= 1.8 and (b) F number= 2.4. The probe beam power is 20 mW and bias field strength is about 8 kV/cm. Data were normalized with peak value for clarity
Fig.10  Illustration of Gouy phase shift in an ABCD system. zR and zT are the Rayleigh range of optical probe beam and terahertz beam, respectively
Fig.11  Calculated Gouy phase shift corresponding to (a)ω+ω-Ω and (b) ω+ω+Ω process during ABCD. z is the longitudinal position along beams' propagation direction. Zero position is the focus position. Rayleigh ranges of terahertz beam and optical probe beam are 0.8 and 4.8 mm, respectively. Green, red and blue curves represent the phase change of terahertz waves, optical beams and phase difference between the two, respectively
Fig.12  Measured terahertz (a) waveforms and (b) spectra at various electrodes position along z-axis using ABCD
Fig.13  Extracted phase information from measurements together with a theoretical fit. Rayleigh ranges of optical probe beam and terahertz beam are estimated to be 1.5 and 0.8 mm, respectively
Fig.14  Terahertz waveforms and spectra obtained with (a) and (d) 80 fs; (b) and (e) 50 fs ; (c) and (f) sub-20 fs laser pulses
χ0(3)/χ0(3)(N2) [63]-dk/m-1 [67]IP/eV [71]FOM/FOM(N2)
N2111915.581
Ar1.111215.761.37
CH42.825912.611.66
Kr3.022914.002.43
Xe7.554212.132.69
Tab.1  Properties of a few gases suitable for terahertz wave detection
Fig.15  Calculated trend of (a) and (c) dynamic range (DR); (b) and (d) signal-to-noise ratio (SNR) with respect to (a) and (b) probe pulse energy (Iω); (c) and(d) DC field strength (EDC) in ABCD system
Fig.16  Typical signal output and dark current vs. supply voltage (gain voltage) of a PMT (Courtesy Hamamatsu Photonics K.K.)
Fig.17  Comparison of (a) dynamic range (DR) and (b) signal-to-noise ratio (SNR) with respect to the gain of PMT. Red dots are measured experimental data and dashed black curves are the guide for the eye
Fig.18  Comparison of (a) dynamic range (DR) and (b) signal-to-noise ratio (SNR) with respect to bias field strength. Blacked dashed lines are the guides for the eyes
Fig.19  Schematic of balanced ABCD for terahertz waves. WP: Wollaston prism. PMT: photomultiplier tube. The polarization of each beam and the experimental coordinate are shown inside the gray circle
Fig.20  A circuit diagram of current subtractor. Current outputs from two PMTs (P1 and P2) are sent through an analogy device AD 820, which can provide a output proportional to the subtraction between P1 and P2 (Courtesy Dr. Brian Schulkin)
Fig.21  Each component measured in balanced ABCD. Waveforms are offset vertically for clarity
Fig.22  Comparison of measured dynamic ranges (DRs) between (a) conventional ABCD and (b) balanced ABCD. Background fluctuations with a 500 times magnification are shown in gray circles. Waveforms are normalized with peak values. Waveforms are obtained with a lock-in time constant of 100 ms and a total of 9 scans
Fig.23  Comparison of measured signal-to-noise ratio (SNR) between (a) conventional ABCD and (b) balanced ABCD. Terahertz spectra and noise floors are shown in red and black lines, respectively. Signals were obtained with a lock-in time constant of 100 ms and a total of 9 scans
conventional ABCDbalanced ABCD
signal (S)2Iω2ETHzEbias4A2Iω2ETHzEbias
background fluctuation (ETHz=0)2IωδIω(Ebias)20
noise (ETHz0, ?S?IωδIω)*2IωδIω[(ETHz)2+(Ebias)2+2ETHzEbias]8A2IωδIωETHzEbias
signal-to-noise ratioIωETHzEbiasδIω[(ETHz)2+(Ebias)2+2ETHzEbias]Iω2δIω
dynamic rangeETHzEbiasIωδIω
Tab.2  Theoretical comparison of signal-to-noise ratio and dynamic range between conventional ABCD and balanced ABCD
Fig.24  Schematics of hollow fiber pulse compressor used in our experiment. 35 fs second laser pulses was input into a hollow core fiber placed in a neon-filled chamber. The self-phase modulation of intense laser pulses provides a sufficient bandwidth. Output pulses are compressed down to sub-10 fs by three pairs of chirped mirrors
Fig.25  Measured spectra of output pulses from hollow-core fiber pulse compressor with various (a) input pulse energy and (b) neon gas pressure in the chamber
Fig.26  Experimental setup of terahertz ABCD with few-cycle pulses. SM: spherical mirror. PMT: photomultiplier tube. A 60%–40% ultrafast beam splitter was used to separate laser pulses into optical pump pulses and optical probe pulses. The pump pulses were focused with a spherical mirror with 150 mm effective focal length to ionize ambient air. A 1 mm thick silicon wafer was used to block the residue optical beam. The optical probe beam went through a time delay stage and focused by another spherical mirror into the detection region
Fig.27  Comparison of generated terahertz field strength with various BBO thicknesses. Electrical field strength was measured using a 100 μm GaP crystal
Fig.28  Measured (a) waveforms and (b) spectra of ultra-broadband terahertz waves generated with ultra-short pulses with different input pulse energy
Fig.29  Measured (a) waveform and (b) corresponding spectrum in ABCD with short pulses. Pulse duration is estimated to be around 15 fs
Fig.30  Illustration of double helix electrical field applied on plasma region. Laser pulses are focused to ionize air where a pair of double helix electrodes was positioned. The trajectory of electrons follows the direction of external electrical field, resulting in an elliptically polarized terahertz wave in far field. The laboratory coordinate and definition of handedness are shown in the figure
Fig.31  Temporal evolution of electrical field vector of (a) right-handed and (b) left-handed elliptically polarized and (c) linear polarized terahertz pulses. Simulated temporal evolution of electrical field vector of (d) right-handed and (e) left-handed elliptically polarized and (f) linear polarized terahertz pulses. EXP: experimental results. SIM: simulation results
Fig.32  Measured far-field terahertz waves in x (upper) and y (lower) direction at various electrodes position from pairs of (a) right-handed; (b) left-handed helical and (c) linear electrodes. The calculated far-field terahertz waves in x and y direction at various electrodes position from (d) right-handed and (e) left-handed helical and (f) linear electrodes are also shown for comparison. Black curves show the polarization trajectories of far-field terahertz radiation at position z = -20, -10, 0, 10 mm, respectively. EXP: experimental results. SIM: simulation results
Fig.33  Measured temporal evolution of electrical field vector of (a) right-handed and (b) left-handed circular polarized terahertz pulses with a HDPE Fresnel prism. The phase difference and amplitude ratio between x and y component for (c) right-handed and (d) left-handed circular polarized terahertz pulses in frequency domain; (e) phase difference and (f) amplitude ratio of elliptically polarized terahertz pulses generated from double helix electrodes are shown for comparison. EXP: experimental results. SIM: simulation results
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