Detection of Broadband Terahertz Waves with a Laser-Induced Plasma in Gases

Center for Terahertz Research, Rensselaer Polytechnic Institute, Troy, New York 12180, USA.
Physical Review Letters (Impact Factor: 7.51). 10/2006; 97(10):103903. DOI: 10.1103/PHYSREVLETT.97.103903
Source: PubMed


We report the experimental results and theoretical analysis of broadband detection of terahertz (THz) waves via electric-field-induced second-harmonic generation in laser-induced air plasma with ultrashort laser pulses. By introducing the second-harmonic component of the white light in the laser-induced plasma as a local oscillator, coherent detection of broadband THz waves with ambient air is demonstrated for the first time. Our results show that, depending on the probe intensity, detection of THz waves in air can be categorized as incoherent, hybrid, and coherent detection. Coherent detection is achieved only when the tunnel ionization process dominates in gases.

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Available from: Jianming Dai, Jan 15, 2014
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    • "Fig. 2 shows a typical THz waveform and its spectrum with bandwidth exceeding 35 THz. Compared to our previous results [2] [3], the usable bandwidth, which is only limited by the laser pulse duration, has been significantly increased. "
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    ABSTRACT: We present experimental and theoretical investigations on the THz wave generation and detection using ambient air or selected gases as the THz emitter and sensor, as well as the potential applications of THz air photonics.
    Full-text · Article · May 2010
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    • "Since the E 2ω (t) ∝ E THz (t-τ) E ω (t) E ω (t), it follows that the intensity of the measured second-harmonic signal by the photon-multiplier-tube (PMT) is proportional to the intensity of the THz wave: I 2ω (t) ∝ I THz (t-τ), the measurement predicted in Eqn. (1) is incoherent, therefore the phase information is lost.[10] One of the most valuable properties of THz time-domain spectroscopy is coherent detection. "
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    ABSTRACT: Historically, THz technologies were mainly used within the astronomy community for studying cosmic far-infrared radiation background, and by the laser fusion community for the diagnostics of plasmas. Since the first demonstration of THz wave time-domain spectroscopy in the late 80's, there has been a series of significant advances (particularly in recent years) as more intense THz sources and more sensitive detectors provide new opportunities for understanding the basic science in the THz frequency range. Now, the region of the electromagnetic spectrum from 0.3 to 10 THz (1 mm – 30 μm wavelength) is a frontier for research in physics, chemistry, biology, materials science and medicine. Ambient air, when excited with intense femtosecond laser beams, exhibits a remarkable ability to generate and detect pulsed THz waves through an optical nonlinear process. The use of air (or selected gases) as a broadband THz wave emitter and THz wave sensor provides superior bandwidth (0.5 -20 THz at 10% bandwidth), sensitivity (heterodyne), resolution (<MHz), and the standoff sensing capability in atmosphere which was heretofore considered impossible due to water vapor attenuation. However, research into the basic science and engineering of THz waves in laser-induced air plasma, especially the application of wide-band and high-field THz waves with standoff capability, is just beginning. Our proposed instrumentation development explores this new area, with an emphasis on broadband spectroscopy, remote sensing and nonlinear effect.
    Preview · Article · Jan 2010
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    • "The current results, in combination with THz air-breakdown-coherent-detection (THz-ABCD) in which ambient air is used to detect THz waves through nonlinear four-wave mixing optical processes [12], may enable standoff sensing and identification by allowing the sending of optical beams rather than terahertz beams to generate and detect THz waves remotely, utilizing the lower attenuation at the visible range (<0.01 dB/km) from air. "
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    ABSTRACT: We report standoff THz wave generation from femtosecond laser-induced plasma in ambient air through nonlinear optical processes similar to four-wave-mixing at a distance of about 17 meters by remotely focusing optical beams far way. ©2008 Optical Society of America OCIS codes: (320.7110) Ultrafast nonlinear optics; (190.4380) Nonlinear optics, four-wave mixing; (040.2235) THz Terahertz (THz) wave generation, from laser-induced gas plasma with excitation by femtosecond pulses at both 800 () and 400 nm (2) through a mechanism similar to the four-wave-mixing optical nonlinear process, has attracted greater attention due to remarkable THz beam quality and directionality, bandwidth, THz electric field, as well as its potential applications in nonlinear THz spectroscopy and remote sensing and imaging [1-10]. Recently, we have verified the feasibility of remote generation of broadband, intense THz waves using air or laser-induced air plasma as the emitting medium by sending the two optical pulses ( and 2) 116 meters away and focusing them locally, using a phase compensator for the compensation of the temporal walk-off between the  and 2 pulses when they travel in ambient air [11]. Here we report the true remote generation of broadband, intense THz waves by focusing the optical pulses remotely and creating plasma at a distance of 17 meters (which is the limit imposed by the length of our laboratory room). Experimentally, a desktop Ti:sapphire regenerative amplifier, generating ~800 J, ~100 fs pulses at a repetition rate of 1 kHz with a central wavelength at 800 nm (Spectra-Physics Hurricane-I), is used as a laser source. Using a similar phase compensator as described in reference [11], we are able to compensate for the temporal walk-off between the  and 2 pulses and optimize the emitted THz signal by changing the relative phase between the two pulses. Figure 1 shows the schematic diagram of the experimental setup. A negatively pre-chirped  pulse from the amplified laser system generates its second harmonic 2 after passing through the BBO crystal inside the phase compensator. Right after the phase compensator, the residual  beam and the 2 beam are collimated with the 2 pulse leading with a time delay adjusted by the phase compensator. The collimated beams are expanded by a convex spherical mirror M1 and then focused by an 8-inch convex spherical mirror M2, respectively, creating a plasma at a certain distance. The generated THz wave is collimated and re-focused by a pair of off-axis parabolic mirrors and detected by a pyroelectric detector or through electric-optic (EO) sampling with a 1-mm thick ZnTe crystal.
    Full-text · Article · Jul 2009
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