Infrared Seeded Parametric Four-Wave Mixing for Sensitive Detection of Molecules

Abstract

We have developed a sensitive resonant four-wave mixing technique based on two-photon parametric four-wave mixing with the addition of a phase matched "seeder" field. Generation of the seeder field via the same four-wave mixing process in a high pressure cell enables automatic phase matching to be achieved in a low pressure sample cell. This arrangement facilitates sensitive detection of complex molecular spectra by simply tuning the pump laser. We demonstrate the technique with the detection of nitric oxide down to concentrations more than 4 orders of magnitude below the capability of parametric four-wave mixing alone, with an estimated detection threshold of 10 12 molecules͞cm 3 . [S0031-9007(97) PACS numbers: 42.65. Ky, 33.20.Lg, 39.30. + w Trace species detection using electronic transitions in the visible and ultraviolet is advantageous due to the strong resonance enhancement of the interaction cross section. Detection of molecules in the ultraviolet can be achieved with many different techniques, the suitability of which is application dependent. Pulsed lasers offer a great deal of versatility, providing broad tunability over the visible and ultraviolet as well as high spectral intensities suitable for nonlinear techniques. Resonance enhanced degenerate four-wave mixing is one such technique which has received much attention due to its high sensitivity and highly collimated coherent signal output [1]. Coherent techniques are particularly applicable to remote sensing applications where there are restrictions to optical access. Currently all nonlinear diagnostic techniques that generate a coherent signal beam require the use of multiple beams satisfying complicated phase matching geometries, and many are susceptible to spectral interference from other molecular species.In this Letter we demonstrate a new resonance enhanced nonlinear technique useful for detecting molecular species in trace amounts, which only requires a single beam, is automatically phase matched, and provides excellent species discrimination. The technique, named seeded parametric four-wave mixing (SPFWM), is based on two-photon resonant parametric four-wave mixing (PFWM) [2,3], with the addition of an extra "seeder" field. Both techniques generate coherent signals at wavelengths different from that of the pump, uniquely determined by the interacting species (see Fig. 1) and which travel collinearly with the pump beam. The addition of a phase matched seeder field to the PFWM process serves to increase the sensitivity by many orders of magnitude. The seeder field is simply provided by PFWM in a high gain (high pressure) cell containing the molecule of interest; the signal fields travel collinearly with the pump, and the unwanted component is removed using a filter. Therefore, SPFWM will automatically provide a phase matched seeder field over the entire extent of a molecular spectrum and so is suitable for use with tunable pulsed dye lasers for detecting complex molecular spectra. We demonstrate the potential of SPFWM using a recently rep...

Sensitive detection of nitric oxide using seeded parametric four-wave mixing

“…28 The sensitivity of SPFWM was also demonstrated with the detection of trace quantities of sodium in a flame, displaying an absolute sensitivity of 5ϫ10 9 atoms/cm 3 . 19 In this paper we discuss the suitability of SPFWM for trace detection of nitric oxide and for molecular diagnostic applications. The theory is described in detail and the comparison with experiment is presented.…”

“…17,18 It has also been supported by direct LIF detection of excited state population transfer which indicated that population transfer is almost entirely suppressed when strong parametric fields are present. 19 This cancellation of population transfer is in distinct contrast to the saturation of population transfer common to DFWM 10 and TC-LIGS and other resonant absorption based techniques. Population saturation results in optical bleaching which effectively limits the signal ͑or degrades it in the case of laser induced grating spectroscopy͒.…”

“…We have recently developed a new and simple means of introducing an extra field into the FWM process. 19,27,28 This is accomplished by cascading two PFWM media, such that a high density, high gain medium can be used to generate the extra seeder field for use in the second low density medium. This simple arrangement which we call seeded parametric four-wave mixing ͑SPFWM͒ provides automatic phase matching of the seeder field in the low density medium over a broad laser tuning range, facilitating the detection of complex molecular spectra.…”

“…This simple arrangement which we call seeded parametric four-wave mixing ͑SPFWM͒ provides automatic phase matching of the seeder field in the low density medium over a broad laser tuning range, facilitating the detection of complex molecular spectra. SPFWM has been demonstrated with the detection of trace amounts of NO in a cell 27 as well as the detection of nascent NO generated in a flame. 28 The sensitivity of SPFWM was also demonstrated with the detection of trace quantities of sodium in a flame, displaying an absolute sensitivity of 5ϫ10 9 atoms/cm 3 .…”

“…SPFWM has been demonstrated with the detection of trace amounts of NO in a cell 27 as well as the detection of nascent NO generated in a flame. 28 The sensitivity of SPFWM was also demonstrated with the detection of trace quantities of sodium in a flame, displaying an absolute sensitivity of 5ϫ10 9 atoms/cm 3 . 19 In this paper we discuss the suitability of SPFWM for trace detection of nitric oxide and for molecular diagnostic applications.…”