cthy6 hybrid of frequency and time resolved cars

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Hybrid of Frequency and Time Resolved CARS
Dmitry Pestov, Robert K. Murawski, Ariunbold Gombojav, Xi Wang, Miaochan Zhi, Alexei V. Sokolov, Vladimir A. Sautenkov, Yuri V. Rostovtsev, and Marlan O. Scully
Institute for Quantum Studies and Depts. of Physics and Chemical Engineering, Texas A&M University, College Station, Texas 77843 dmip@neo.tamu.edu

Abstract: We introduce a novel technique that elegantly combines frequency- and time-resolved CARS spectroscopy. The proposed scheme is tested in back-scattered CARS experiments o n a powder of sodium dipicolinate, holding promise for remote/stand-of detection applications.
Ó2007 Optical Society of America
OCIS codes: (300.6230) Spectroscopy, coherent anti-Stokes Raman scattering; (300.6530) Spectroscopy, ultrafast

1. Hybrid CARS: technique and implementation Coherent anti-Stokes Raman Scattering (CARS) is a third-order nonlinear Raman process wherein the interaction of three beams, pump ( p ), Stokes ( s ), and probe ( pr), on a sample gives rise to the anti-Stokes signal at the frequency CARS= p - s + pr [1]. CARS spectroscopy has evolved into a variety of realizations, all of those could be split into two types: frequency-resolved and time-resolved measurements. An advanced technique along the line of the frequency-resolved measurements is multiplex CARS [2,3], sketched out in Fig.1(a). This technique implies the use of a combination of narrowband pump and broadband Stokes pulses ( p = pr) together with the multichannel detection. In multiplex CARS, a significant range of vibrational frequencies is probed simultaneously without loss of spectral resolution. The complete spectral band is recorded at once, which accounts for comprehensiveness and robustness of multiplex CARS spectroscopy. The theme of time-resolved CARS spectroscopy is well presented by femtosecond CARS [4] . I t utilizes ultrashort pulses for preparation and probing (see Fig.1(b)). Since the typical relaxation time of molecular vibrations is on the order of picoseconds, a femtosecond time frame allows one to access the molecular dynamics. Note that delaying the probe automatically takes care of the nonresonant (NR) background due to the instantaneous electronic response. The last one has no memory on this time scale. Its contribution vanishes as fast as the overlap between all the three pulses.
(a) Multiplex CARS
pump/probe Stokes CARS

pump

Frequency

(c)

Hybrid CARS: frequency domain

Stokes

pump

probe

Stokes

pump

Time
Fig.1. Hybrid CARS fundamentals and implementation: (a) Multiplex CARS; (b) Time-resolved CARS; (c) Frequency-related aspects of hybrid CARS; (d) Time-related aspects of hybrid CARS; (e) Setup schematics: DS1,2 are delay stages, L1-5 are lenses, G1,2 are the gratings, M1,2 are alignment mirrors, CCD is a charge-coupled device (Spec-10, Princeton Instruments); (f) 2D spectrogram recorded with NaDPA as a sample: CARS spectrum as a function of the probe pulse delay. Integration time is 1 sec per step; (g) a cross-section of the spectrogram for zero probe delay; (h) a cross-section of the spectrogram for the probe delay 1.6 ps.

probe

(d)

Frequency Hybrid CARS: time domain

CARS

Stokes

Time

probe

(b) Time-resolved CARS (e)

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We suggest to combine t h e advantages of multiplex CARS with the solution for NR background suppression offered by time-resolved CARS spectroscopy (see Fig.1(c,d)). The essence of our hybrid technique is a coherent excitation of a broad band of molecular frequencies by a pair of ultrashort pump and Stokes pulses and subsequent probing of the prepared molecular coherence with an optimally shaped narrowband probe pulse. The probe pulse delay is adjusted to control signal-to-background ratio, or varied in order to maximize the amount of information obtained through the measurement. An implementation of this scheme is schematically shown in Fig.1(e). We utilize a Ti:Sapphire regenerative amplifier (Legend, Coherent: 1 kHz rep.rate, ~1 mJ/pulse) with two evenly pumped OPAs (Opera-VIS/UV and Opera-SFG/UV, Coherent). The output of the first OPA ( p = 733 n m , FWHM~12 nm, 2.1 mJ/pulse) and a small fraction of the amplifier output ( s =803 nm, FWHM~32 nm, 3.6 mJ/pulse) are used as pump and Stokes beams, respectively. The output of the second OPA, the probe beam, is sent through a home-made pulseshaper with an adjustable slit that cuts the bandwidth of the pulse ( pr=578 nm, FWHM~0.7 n m , 0.4 mJ/pulse). As follows, the Stokes and probe pulses pass through delay stages, DS1,2, and then all three beams are focused by a convex 2-inch lens (f = 20 cm) on the rotated sample. The scattered light is collected with a 2-inch achromatic lens (f = 10 cm) and focused onto the entrance slit of the spectrometer (Chromex Spectrograph 250is) with a LN-cooled CCD attached. 2. Experiment and Discussion The powder of sodium dipicolinate (NaDPA) is an easy-to-make substitute for calcium dipicolinate (CaDPA), which is a marker molecule for bacterial spores accounting for over 15% of their dry weight. The spontaneous Raman spectrum for NaDPA in the fingerprint region exhibits four strong Raman lines (1007 cm-1 , 1395 cm-1 , 1442 cm-1 , and 1572 cm-1 ) and resembles the one of CaDPA [5]. A typical CARS trace acquired with NaDPA powder is shown in Fig.1(f). Streak-like vertical lines are the signature of excited NaDPA Raman transitions while the broadband pedestal is the NR background. As expected, the two contributions exhibit different dependence on the probe delay, tpr. The magnitude of the NR background is determined by the overlap of the three laser pulses and therefore follows the probe pulse profile. Relatively long decay time of the Raman transitions under consideration favors their long-lasting presence and makes them stand out when the probe is delayed. The cross-sections of the spectrograms at two different probe delays are given in Fig.1(g) and Fig.1(h). One can see that when all the three pulses are overlapped, tpr = 0, the resonant contribution is severely distorted by the interference with the NR background. Delaying the probe by 1.6 p s improves the signal-tobackground ratio by at least an order of magnitude. We infer that the limitation is imposed by multiple scattering. The absolute frequencies of the observed Raman transitions calculated from the retrieved peak positions and the probe wavelength match to those from spontaneous Raman measurements. With the setup at hand, we have also observed CARS on B.subtilis spores. We estimate the detection speed of the major Raman lines of B.subtilis in the fingerprint region to be on the order of a few seconds and conclude that the technique holds promise for remote/stand-of detection applications. 3. Summary Hybrid frequency- and time-resolved CARS is a novel technique that refines the probing and acquisition of the generated CARS signal, combining multichannel detection with time-resolved background suppression. The versatile implementation of this technique allows to look for a compromise between frequency- and time-resolved CARS, where the ratio between the two is controlled by the width of the pulseshaper slit. The use of a single femtosecond system with one or two OPAs obviates the need for synchronization, common to two-laser systems, and provides excess of pulse energy on a sample even for non-microscopic applications. We have successfully applied the proposed scheme to CARS spectroscopy on NaDPA powder and B.subtilis spores. CARS measurements on dry NaDPA powder demonstrate the potential of the technique. A tenfold improvement in signal-to-background ratio, as compared to multiplex CARS arrangement, is readily achieved even in the presence of multiple scattering. The absolute frequencies of the Raman transitions observed in the CARS experiment match to those from spontaneous Raman measurements. The technique holds promise for remote/standof detection applications. From our first experiments with B.subtilis, the detection speed of its major Raman lines is estimated to be a few seconds. Although these measurements are done with abundance of spores on the sample, further improvements of the detection characteristics are readily expected.
[1] Y.R. Shen, The Principles of Nonlinear Optics (J.Wiley, 1984), Chap. 15. [2] M. Muller and J.M. Schins, J. Phys. Chem. B 106 (2002), pp. 3715-3723. [3] J.-X. Cheng, A. Volkmer, L.D. Book, and X.S. Xie, J. Phys. Chem. B 106 (2002), pp. 8493-8498. [4] A. Materny, T. Chen, M. Schmitt, T. Siebert, A. Vierheilig, V. Engel, W. Kiefer, Appl. Phys. B 71 (2000), pp. 299-317. [5] P. Carmona, Spectrochimica Acta 36A (1980), pp. 705-712.


				
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