DOI: 10.1007/s00340-005-2062-4 Appl. Phys. B (2005) Lasers and Optics Applied Physics B a.m. zheltikov 1, ✉ m.n. shneider 2 r.b. miles 2 Radar return enhanced by a grating of species-selective multiphoton ionization as a probe for trace impurities in the atmosphere 1 Physics Department, International Laser Center, M.V. Lomonosov Moscow State University, Moscow 119992, Russia 2 Department of Mechanical and Aerospace Engineering, Princeton University, Princeton, NJ 08544-5263, USA Received: 5 October 2005 © Springer-Verlag 2005 ABSTRACT Two-color time-ordered dyads of short laser pulses induce a spatially periodic modulation of the refractive index of the atmosphere through resonance-enhanced multiphoton ion- ization (REMPI), enhancing the back reflection of radio waves. The carrier frequencies and the timing of laser pulses in a dyad provide a unique code for selectively accessing the manifold of energy levels of impurity molecules, inducing REMPI grat- ings of the refractive index only in the presence of impurity molecules, thus suggesting an attractive strategy for the radar- based stand-off detection of trace impurities in the atmosphere. PACS 42.70.Qs; 42.65.Wi 1 Introduction Remote sensing of low concentrations of impurity molecules in the atmosphere has always been considered as one of the most important problems in applied science. Recent terrorist attacks and ever-growing threats of using chemical and bacteriological agents put the problem of remote sens- ing in the context of national security, making it one of the strategic issues for the protection of the civilian population in the war of nations against terror. Light detection and ranging (LIDAR) [1] is a powerful technique for the remote sensing of trace impurities in the atmosphere. LIDAR capabilities have been recently substantially enhanced through the use of high- intensity ultra-short laser pulses [2], which can propagate over large distances in the atmosphere in the regime of filamen- tation [3, 4], providing valuable information on the contam- ination level of atmospheric air [5]. Another promising ap- proach to the stand-off detection of trace impurities in the atmosphere, as shown by recent theoretical studies [6, 7], can be based on coherent anti-Stokes Raman scattering (CARS) and superradiance phenomena combined with the methods of ultra-fast nonlinear spectroscopy [8] using adaptively shaped ultra-short pulses [9] and coherence-controlled population transfer [10–12]. In this work, we show that species-selective resonance- enhanced multiphoton ionization (REMPI), induced by two- ✉ Fax: +7-095-939-5174, E-mail: zheltikov@top.phys.msu.su color time-ordered dyads of short laser pulses and detected by back-reflected radio waves, offers an attractive strategy for the radar-based stand-off detection of trace impurities in the atmosphere. The carrier frequencies and the timing of laser pulses in a dyad provide a unique code for selectively access- ing the manifold of energy levels of impurity molecules, in- ducing REMPI-seeded ionization only in the presence of im- purity molecules. Chirped trains of such pulse dyads can then induce a grating of the refractive index through the REMPI- seeded ionization process, radically increasing the return of probing radar radiation and improving the sensitivity of stand- off impurity detection. The approach proposed here is thus based on a species-selective REMPI of trace impurities in the atmosphere using optimized sequences of laser pulses (Fig. 1). Time separations between the pulses in the sequence are chosen in such a way as to induce a periodic modulation of the refractive index of the gas through species-selective REMPI in order to enhance the return of radio waves and im- prove the sensitivity of radar detection of impurities in the atmosphere. In what follows, we consider in greater detail all the ingredients of this approach. 2 Species-selective resonance-enhanced multiphoton ionization of impurities We start with the REMPI process, which is used in our approach for a selective ionization of impurity molecules, providing a change in the refractive index of the gas. As a generic example of two-color species-selective REMPI, we consider a sequential ionization of OClO molecules by UV pump pulses and a time-delayed visible probe field, experi- mentally demonstrated by Zewail’s group [13]. As shown in Fig. 2, a UV pulse with a central wavelength of 329 nm serves to excite a group of vibrational levels in the A 2 A 2 excited state of OClO through a one-photon transition from the X 2 B 1 ground state of this molecule. The probe pulse with a cen- tral wavelength of 620 nm then ionizes excited-state OClO molecules through a four-photon transition from the A 2 A 2 state. Multiphoton ionization of OClO by the UV pump field alone, which would create an unwanted background in our ar- rangement, can be readily minimized by the adjustment of the intensities of the pump and probe pulses. In experiments [13],