Mechanism and Potential Applications of THz Air Photonics Jianming Dai, Jingle Liu, I-Chen Ho, Nicholas Karpowicz, and X.-C. Zhang Center for Terahertz Research, Rensselaer Polytechnic Institute, Troy, NY 12180, USA Email address: daij3@rpi.edu 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. ©2008 Optical Society of America OCIS codes: (320.7110) Ultrafast nonlinear optics; (190.4380) Nonlinear optics, four-wave mixing; (040.2235) Far infrared or terahertz Recently, THz generation and detection using ambient air or selected gases (i.e., laser-induced gas plasma) as the THz wave emitter and sensor, which is termed as “THz Air Photonics”, has attracted much scientific attention [1-12]. Such an all-air-based THz spectroscopic system provides very high THz intensity and ultra-broad bandwidth covering the entire “THz gap” and beyond [3]. Potential applications include nonlinear spectroscopy and imaging, as well as remote sensing and identification. THz air photonics has already become a common tool in research laboratories for fundamental scientific research, such as nonlinear THz response of different materials [5]. We built up a full quantum mechanical model by numerically solving the time-dependent Schrödinger equation, which accurately describes the formation of the relevant electron wave packets. We have shown that the full THz emission process takes place in two steps: first, a broadband pulse is produced through the asymmetric ionization due to the laser-atom interaction, and then a second step, an “echo” is produced by the interaction of the ionized wave packets with the surrounding gas and plasma [7]. Using the above quantum mechanical model, we found that electrons ionized from an atom or molecule by circularly- or elliptically-polarized femtosecond ω and 2ω pulses exhibit different trajectory orientations as the relative optical phase between the ω and 2ω pulses changes. Experimentally, using a phase compensator [9], we found that, the polarization orientation of the emitted THz waves rotates as the relative optical phase changes, as shown in Figs. 1(a) and 1(b). Similar results have been observed independently by A. Lindenberg’s group [10]. Fig. 1. (a) THz intensity versus THz polarizer angle and the relative optical phase between the ω and 2ω pulses with right-handed circularly-polarized ω pulse and with elliptically-polarized 2ω pulse (with an ellipticity of about 1/11 in terms of optical intensity). (b) THz intensity versus THz polarizer angle and relative optical phase between ω and 2ω pulses when both ω and 2ω beams are right- handed elliptically-polarized with their ellipticities both higher than 0.8 (which means that both of the ω and 2ω beams are close to circular polarization), refer to reference [9]. With short laser pulses (sub-35 fs) from an amplified laser system (Legend Elite, Coherent), we are able to generate and detect much broader THz waves. 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. a308_1.pdf OSA / CLEO/QELS 2010 CMP1.pdf