Coherent anti-Stokes Raman scattering in isolated air-guided modes of a hollow-core photonic-crystal fiber

Abstract

Hollow-core photonic-crystal fibers are shown to offer the unique possibility of coherent excitation and probing of Raman-active vibrations in molecules by isolated air-guided modes of electromagnetic radiation. A 3-cm section of a hollow photonic-crystal fiber is used to prepare isolated air-guided modes of pump and probe fields for a coherent excitation of 2331-cm −1 Q-branch vibrations of molecular nitrogen in the gas filling the fiber core, enhancing coherent anti-Stokes Raman scattering through these vibrations by a factor of 15 relative to the regime of tight focusing.Coherent Raman excitations is one of the most important pathways of laser-matter interactions. Coherent Raman processes are at the heart of many efficient spectroscopic [1,2] and microscopic [3] techniques, methods of frequency conversion [4] and time-resolved studies [5,6], as well as lasercooling [7] and quantum-control [8] schemes. Multiple Raman sidebands produced through high-order stimulated Raman scattering allow few-field-cycle pulses to be synthesized [9]. The potential of nonlinear Raman techniques has been recently enhanced by coherent control approaches [10].Many interesting possibilities of coherent Raman processes are associated with waveguide regimes of Raman excitation [11,12]. Hollow fibers [13,14] and Bragg planar waveguides [15] have been shown to improve the efficiency of coherent anti-Stokes Raman scattering (CARS) in the gas phase. Large nonlinear interaction lengths make hollow fibers filled with a Raman-active gas efficient generators of sub-4-fs pulses [9].Air-guided modes in standard hollow fibers are, however, leaky, with the magnitude of losses scaling as 2 / a 3 with the fiber inner radius a and the radiation wavelength [16], which dictates the choice of hollow fibers with a ϳ 50-300 m for nonlinear-optical experiments. Such large-a fibers are essentially multimode, with many guided modes typically contributing to nonlinear-optical interactions [14,17]. This multimode nature of nonlinear-optical processes complicates a precise calibration of the coherent Raman signal as a function of the concentration of Ramanactive species, leads to unwanted interference phenomena in spectroscopic and time-resolved coherant Raman measurements, and should be one of the factors limiting the duration of ultrashort pulses synthesized in hollow fibers.The number of air-guided modes can be radically reduced by using hollow-core photonic-crystal fibers (PCFs) [18,19]. Such fibers guide light due to the high reflectivity of a twodimensionally periodic (photonic-crystal) cladding (the inset in Fig. 1) within photonic band gaps. Low-loss guiding in a few or even a single air-guided mode can be implemented under these conditions in a hollow core with a typical diameter of 10-20 m [18][19][20]. Hollow PCFs with such core diameters have been recently demonstrated to enhance nonlinear-optical processes, including stimulated Raman scattering [21], four-wave mixing (FWM) [22], and selfphase modulation [23]. Air-guided modes in hollow PCFs can sup...

Enhanced spontaneous Raman scattering and gas composition analysis using a photonic crystal fiber

“…High-order, nongrazing modes have high loss and thus are of limited use for enhancement of nonlinear interactions. Most classic hollow-capillary, nonlinear optics enhancement experiments that have been reported describe output signals that required phase matching and consequently did not benefit from the collection of a large number of waveguide modes or the collection of a large solid-angular output [5][6][7][8][9]11,12]. Thus these experiments are not strongly affected by the requirement of grazing incidence.…”

“…It is noted that classical hollow capillary tubes have also been used for the enhancement of nonlinear optical interactions [5][6][7][8][9][10][11][12][13]. These capillaries work best with rays that propagate at grazing incidence, i.e., when only the fundamental EH 11 or low-order waveguide mode propagates [14][15][16][17][18].…”

Experimental demonstration of nanoCARS: coherent anti‐Stokes Raman scattering in mesoporous silica aerogels infiltrated with condensed‐ and gas‐phase Raman‐active materials

“…14 Generally, the ability of CARS spectroscopy to detect small amounts of Raman-active species in the gas or condensed phase makes this technique particularly promising for sensing applications and ecological monitoring. 7,8 Waveguide regimes of CARS, 15,16 implemented with the use of planar film waveguides, 17 hollow 18,19 and photonic-crystal 20 fibers, and also planar photonic bandgap waveguides, 21 have been shown to enhance the potential of CARS to sense a broad variety of Ramanactive molecules, including biological species. Conceptually, waveguide CARS is an impressive example of how the conventional concept of a Raman cell can be modified for a higher sensitivity through geometry-improved phase matching and high local fields.…”

New Tools for Life Science Research Based on Fiber-Optic-Linked Raman and Resonance Raman Spectroscopy

“…We have proposed the use of hollowcore photonic crystal fibers (PCF) as excitation fibers for Raman spectroscopy (72). This follow from several recent reports describing the advantages of hollow-core PCF in a variety of non-linear optical applications (13)(14)(15).…”