AbstractｲWe present recent results on experimental measurements and theoretical simulations for terahertz wave generation (quantum mechanical simulation) and detection (four-wave-mixing) with ambient air and selected gases. We demonstrate standoff terahertz wave generation for potential applications of terahertz air photonics in remote sensing and identification. I. INTRODUCTION AND BACKGROUND ntense terahertz (THz) wave generation from gas plasma, induced by a femtosecond pulse (800 nm, Z) and its second harmonic (400 nm, 2Z), is attracting more attention due to the remarkable bandwidth, intensity, and directionality it provides, as well as its potential applications in nonlinear THz spectroscopy and remote sensing and identification [1-13]. This THz source has already become a common tool in research laboratories for fundamental scientific research, such as nonlinear THz responses of different materials [14]. Detection of broadband THz waves using ambient air or selected gases as the sensor has been recently reported [15,16]. The bandwidth of a THz spectrometer using air or selected gases as both the THz emitter and the sensor can be over 30 THz which is only limited by the optical pulse duration [17]. Here we present our most recent theoretical investigation and experimental results in THz air photonics using air or selected gases as both the THz wave emitter and sensor. We also demonstrated the feasibility of THz generation at standoff distances in ambient air. II. RESULTS The basic mechanism for THz wave generation, as proposed by different research groups, was treated as four-wave-mixing (FWM) or semi-classic asymmetric transient current model. Both of these models partially explain the experimental results [1, 4, 5], however, neither of them can explain all of the experimental phenomena. Recently, we built up a full quantum mechanical model by numerically solving the time-dependent Schrödinger equation [18] (TDSE), 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 ｳHFKRｴ LV SURGXFHG E\ WKH LQWHUDFWLRQ RI WKH LRQL]HG wave packets with the surrounding gas and plasma [ 10]. Fig. 1 illustrates the physical picture of the process for THz generation from gas plasma, as well as the electron density distributions for argon subjected to an intense optical field composed of fundamental and second harmonic pulses with phase differences oI ʌ DQG ʌ Fig. 1. (a) Illustration of the process by which the terahertz radiation is emitted. High-intensity laser light composed of fundamental and second harmonic frequency components (Z and 2Z) interacts with the atom, resulting in tunnel ionization. Some of the wave packets formed in the ionization process are accelerated away from the atom and propagate outward (along the laser polarization axis), resulting in a net terahertz dipole moment, which radiates (:). The wave packets then interact with their surroundings, emitting bremsstrahlung, which adds coherently, resulting in a second source of terahertz radiation. (b) and (c) Calculated electron density distributions for argon subjected to an intense optical field composed of fundamental and VHFRQG KDUPRQLF SXOVHV ZLWK SKDVH GLIIHUHQFHV RI ʌ DQG ʌ UHVXOWLQJ in minimal and maximal asymmetry, respectively. The ground state of the atom is removed for clarity. In order to further test this quantum mechanical model, we use elliptically- or circularly-SRODUL]HG Ȧ DQG Ȧ SXOVHV DV WKH optical excitation. As a result of the simulation using this model, we found that electrons ionized from an atom or molecule by circularly- or elliptically-SRODUL]HG IHPWRVHFRQG Ȧ DQG Ȧ pulses exhibit different trajectory orientations as the relative phase between the two pulses changes, resulting in a polarization change of the emitted THz waves. Fig. 2 shows the simulation results with circularly-polarized optical Ȧ DQG Ȧ pulses. To experimentally verify the simulation results, we use an Jianming Dai, Nicholas Karpowicz, and X.-C. Zhang* Center for Terahertz Research, Rensselaer Polytechnic Institute, Troy, New York 12180, USA Physics and Potential Applications of Terahertz Air Photonics I         