We show for the first time that a planar metamaterial, an array of coupled metal split-ring resonators with a unit cell lacking mirror symmetry, exhibits asymmetric transmission of terahertz radiation (0.25-2.5 THz) propagating through it in opposite directions. This intriguing effect, that is compatible with Lorentz reciprocity and time-reversal, depends on a directional difference in conversion efficiency of the incident circularly polarized wave into one of opposite handedness, that is only possible in lossy low-symmetry planar chiral metamaterials. We show that asymmetric transmission is linked to excitation of enantiomerically sensitive plasmons, these are induced charge-field excitations that depend on the mutual handedness of incident wave and metamaterial pattern. Various bands of positive, negative and zero phase and group velocities have been identified indicating the opportunity to develop polarization sensitive negative index and slow light media based on such metamaterials.In contrast to three-dimensionally chiral structures (e.g. helices), planar chiral patterns (e.g. flat spirals) have the intriguing property that their sense of twist is reversed for observation from opposite sides. Not only human observers, but also circularly polarized waves incident on opposite sides of a planar chiral structure, see materials of opposite handedness. It has recently been discovered that planar chiral metamaterial patterns can show different levels of total transmission for circularly polarized waves of the same handedness propagating in opposite directions. The effect, which has been detected in microwave [1,2,3,4] and photonic [5,6] metamaterials and plasmonic nanostructures [7], is known as asymmetric transmission. Such asymmetric transmission phenomenon has not yet been observed for terahertz radiation. The terahertz spectral region has tremendous technological importance since many biological materials and substances have molecular vibration frequencies in this regime, making it highly attractive for sensing, material characterization, spectroscopy and biomedical imaging. In spite of intense research activity in this domain over the past decade terahertz radiation has proved to be extremely challenging to detect, measure, propagate and manipulate since electronic and magnetic responses of natural materials die out at these frequencies, thus earning the name of the socalled "terahertz gap". Recently, terahertz metamaterials [8,9,10,11,12,13,14,15,16,17,18] have shown potential for use in the terahertz gap with their fascinating novel properties but the region still suffers from a severe shortage of devices needed for fully exploiting the attractive potential applications of terahertz radiation.In this Letter we report the first experimental observation of asymmetric transmission in the terahertz do- main. We demonstrate a new type of polarization sensitive terahertz metamaterial device showing directionally asymmetric transmission of circularly polarized waves between 0.25 and 2.5 THz. The phenomenon resembl...

Via terahertz (THz) coherent transients and THz time-domain spectroscopy, we have measured the far-wing absorption line profile of the ensembles of collision-broadened ground state rotational lines of methyl bromide, methyl chloride, and methyl fluoride vapors out to more than 200 line widths from resonance, corresponding to frequency offsets as much as 5 times the resonant frequency. On these far wings the measured absorption is approximately an order of magnitude less than that predicted by the van Vleck−Weisskopf theory. Our observations show that as the offset frequency is increased, a transition occurs from the regime of the van Vleck−Weisskopf theory to the regime of the Lorentz theory. These measurements are fit to a new molecular response theory which explicitly includes the molecular orientation time during a collision. Due to the broad bandwidth of the THz pulses, we demonstrate the validity of this molecular response theory for the far-wing absorption of methyl fluoride, chloride, and bromide. The excellent theoretical fit to our measurements encompassing the frequency range over which this transition occurs indicates a molecular response time on the order of 200 fs. These measurements also permit determination of the line-width dependence on the rotational quantum number J.

We present what is to our knowledge the first comprehensive far-infrared absorption measurement of flames. These measurements, covering the region of 7-88 wave numbers (0.2-2.65 THz) are only now made possible by the technique of terahertz time-domain spectroscopy. We observe a large number of absorption lines-including those of water, CH, and NH(3)-in a stationary, premixed, propane-air flame. The absorption strength permits the determination of species concentration along the beam path. The f lame temperature is determined by comparison of the relative strengths of the water vapor lines.

The frequency-dependent absorption coefficient of CHCl 3 , CCl 4 , and their mixtures are measured by pulsed terahertz time domain transmission spectroscopy. The absorbance spectrum for neat CHCl 3 is shown to compare well with existing experimental data including coverage of the previously difficult to access 0.3-0.9 THz range. Furthermore, fitted curves to the absorbance spectra of the liquid mixtures, based on mole fraction weighted sums of the absorption coefficients of pure CHCl 3 and CCl 4 , indicate the presence of a bulk dipole reducing mechanism, possibly due to clustering of CHCl 3 molecules about CCl 4 . An algebraic extension of the mole fraction weighted fits allows discrimination between relative strengths of the various bimolecular absorption processes. The integrated absorption coefficient for collisionally induced absorption of CHCl 3 -CCl 4 collisions was found to be less than that for CHCl 3 -CHCl 3 collisions by 2.6 ( 0.4 THz cm -1 (integrated absorption coefficient units). Finally, a new procedure for applying Mori's third-order continued fraction to a description of absorption line shapes in liquids is presented. Attempts to fit the observed absorption spectra to the line shape derived from the third-order truncation of Mori's continued fraction were unsuccessful. However, a constrained sum of Mori line shapes was found to fit the low and middle frequency portions of the spectrum reasonably well. This problematic behavior of the Mori analysis may not only exemplify nonexponential relaxation of the intermolecular torques, a known problem associated with the third-order truncation, but also the existence of two (or more) types of motion (i.e., translations, rotations, and possibly collective motions) causing relaxation of the dipolar correlation function. This improvement in the closeness of the Mori absorbance line shape fit to the experimentally determined data illustrates the possibility of straightforward extraction of dynamical properties of liquids from absorbance spectra. This theory provides an analytical, yet limited, alternative to the more complicated but more comprehensive determination of dynamical properties obtained through molecular dynamics simulations.