We demonstrate a strong coherent backward wave oscillation using forward propagating fields only. This is achieved by applying laser fields to an ultra-dispersive medium with proper chosen detunings to excite a molecular vibrational coherence that corresponds to a backward propagating wave. The physics then has much in common with propagation of ultra-slow light. Applications to coherent scattering and remote sensing are discussed.PACS numbers: 32.80. Qk, 42.65.Dr, 42.50.Hz Quantum coherence [1,2] has been shown to result in many counter-intuitive phenomena. The scattering via a gradient force in gases [3], the forward Brillouin scattering in ultra-dispersive resonant media [4,5], electromagnetically induced transparency [6,7,8,9], slow light [10,11,12,13] In this Letter, we predict strong coherent backward scattering via excitation of quantum coherence between atomic or molecular levels. The developed approach can also be used to control the direction of the signal generated in coherent Raman scattering and other four-wave mixing (FWM) schemes.Let us consider the four-wave mixing in a 3-level atomic medium. The pump and Stokes fields E 1 and E 2 (whose Rabi frequencies are defined as Ω 1 = ℘ 1 E 1 /h and Ω 2 = ℘ 2 E 2 /h, where ℘ 1 and ℘ 2 are the dipole moments of the corresponding transitions) with wave vectors k 1 and k 2 and angular frequencies ν 1 and ν 2 induce a coherence grating in the medium (see Fig. 1 ) given by [2]Let us stress that the ρ cb coherence grating has anIn an ultradispersive medium (see Fig. 2) where fields propagate with a slow group velocity, the two co-propagating fields have wavevectors given bywhere V g is the group velocity of the first wave, ω ab is the frequency of transition between levels a and b, and k 2 = ν 2 /c. Thus these two fields create a coherence grating The field 3 propagating in the same direction will be scattered in the opposite direction because the coherence excited by fields 1 and 2 is propagating in the opposite direction (see Fig.2). Level scheme, double-Λ (b), for implementation of coherent back scattering.in the medium with spatial phase determined by k 1 − k 2 = ω cb /c + (ν 1 − ω ab )/V g which depends strongly on the detuning δ = ν 1 − ω ab . By properly choosing the detuning, δ, one can make k 1 − k 2 negative.After the coherence ρ bc is induced in the medium, a probe field E 3 , with Rabi frequency Ω 3 = ℘ 3 E 3 /h and

We use time-resolved coherent Raman spectroscopy to obtain molecule-specific signals from dipicolinic acid (DPA), which is a marker molecule for bacterial spores. We use femtosecond laser pulses in both visible and UV spectral regions and compare experimental results with theoretical predictions. By exciting vibrational coherence on more than one mode simultaneously, we observe a quantum beat signal that can be used to extract the parameters of molecular motion in DPA. The signal is enhanced when an UV probe pulse is used, because its frequency is near-resonant to the first excited electronic state of the molecule. The capability for unambiguous identification of DPA molecules will lead to a technique for real-time detection of spores.molecular spectroscopy ͉ coherent anti-Stokes Raman scattering ͉ bacterial spores ͉ molecular coherence ͉ nonlinear optics T he historical use of bacterial spores for biological warfare and the recent terrorism attacks are a concern for national security, pointing to the need for rapid analysis and detection of unknown chemical and biological agents. A major component of bacterial spores is dipicolinic acid (DPA) and its various salts such as calcium dipicolinate (CaDPA), which can contribute up to 17% of the dry weight of the spores. This fact motivates our study of DPA because it is a ready-made marker for endospores (1).Although many laser spectroscopic techniques have been successfully applied in chemistry and biology, not all of them are practical for detection purposes. Methods based on fluorescence spectroscopy are not effective because the fluorescence signal does not usually offer adequate selectivity. Although a spontaneous Raman signal can be very selective, and has been successfully used for detection of DPA, it is often very weak and requires long acquisition times.Methods that use coherent Raman spectroscopy are more efficient because quantum coherence can dramatically increase the nonlinear response of the media (2-6). Applications of quantum coherence now constitute a broad range of research such as quantum computing and quantum-state storage (7-9), manipulation of single quanta (10-12), efficient frequency conversion (13,14), and subfemtosecond pulse generation (15,16).Recently, it was suggested that femtosecond coherent anti-Stokes Raman scattering (CARS) can be used for the detection of biomolecules in real time (1) and can improve LIDAR (light detection and ranging) efficiency by several orders of magnitude (17). In the femtosecond adaptive spectroscopic technique for CARS (FAST CARS), the quantum coherence between the vibrational states is maximized before probing the molecules. Several relevant experimental demonstrations have justified this technique (18)(19)(20)(21)(22)(23).In this work, we present the experimentally observed coherent Raman signal from DPA in sodium-hydroxide-buffered water solution (H 2 O͞NaOH) in the visible and UV spectral regions. We also study how the signal depends on concentration of NaDPA (sodium DPA). We demonstrate that the sensiti...

We utilize femtosecond time-resolved coherent anti-Stokes Raman spectroscopy (CARS) to study the vibrational dynamics of methanol-water solutions. We measure the beat wavenumbers between Raman modes and coherence decay rates of C-H stretch modes (at 2835 cm −1 and 2943 cm −1 ) in methanol. We observe a qualitative dependence of the coherence relaxation rate on the concentration of methanol in its water solution. This may be linked to the formation of the recently observed supramolecular structures in methanol-water mixtures.