We show that a laser beam which propagates through a cubic-quintic nonlinear optical material may reach, for a given power, a condensed state with a collisional dynamics resembling a liquid drop. We qualitatively describe the analogies between this system and the usual fluids and show them by simulating numerically total reflections of these beams with planar boundaries and localized defects. We use the analogy "liquid light" to stress the connections with the dynamics of quantum fluids, including Bose-Einstein condensates.

The strong radial confinement and the pronounced evanescent field of the guided light in optical nanofibers yield favorable conditions for ultra-sensitive surface spectroscopy of molecules deposited on the fiber. Using the guided mode of the nanofiber for both excitation and fluorescence collection, we present spectroscopic measurements on 3,4,9,10-perylenetetracarboxylic dianhydride molecules (PTCDA) at ambient conditions. Surface coverages as small as 1 per thousand of a compact monolayer still give rise to fluorescence spectra with a good signal to noise ratio. Moreover, we analyze and quantify the self-absorption effects due to reabsorption of the emitted fluorescence light by circumjacent surface-adsorbed molecules distributed along the fiber waist.

We review our recent progress in the production and characterization of tapered optical fibers with a sub-wavelength diameter waist. Such fibers exhibit a pronounced evanescent field and are therefore a useful tool for highly sensitive evanescent wave spectroscopy of adsorbates on the fiber waist or of the medium surrounding. We use a carefully designed flame pulling process that allows us to realize preset fiber diameter profiles. In order to determine the waist diameter and to verify the fiber profile, we employ scanning electron microscope measurements and a novel accurate in situ optical method based on harmonic generation. We use our fibers for linear and non-linear absorption and fluorescence spectroscopy of surface-adsorbed organic molecules and investigate their agglomeration dynamics. Furthermore, we apply our spectroscopic method to quantum dots on the surface of the fiber waist and to caesium vapor surrounding the fiber. Finally, towards dispersive measurements, we present our first results on building and testing a single-fiber bi-modal interferometer.Comment: 13 pages, 18 figures. Accepted for publication in Applied Physics B. Changes according to referee suggestions: changed title, clarification of some points in the text, added references, replacement of Figure 13

The control over the transmission properties of tapered optical fibers (TOFs) is an important requirement for a whole range of applications. Using a carefully designed flame pulling process that allows us to realize preset fiber radius profiles, we fabricate TOFs with a nanofiber waist. We study the spectral transmission properties of these TOFs as a function of the taper profile and the waist length and show how the transmission band of the TOF can be tuned via different fiber profile parameters. Based on these results, we have designed a nanofiber-waist TOF with broadband transmission for surface spectroscopy of organic molecules. Moreover, our method allows us to analyze the loss mechanisms of optical nanofibers.

We report results from studies of the Autler-Townes ͑AT͒ effect observed in sodium molecules from a molecular beam. A relatively weak laser field P couples an initially populated rovibronic level g in the electronic ground state ͑here X 1 ⌺ g + , vЉ =0, JЉ =7͒ to a selected excited rovibronic level e ͑here A 1 ⌺ u + , vЈ = 10, JЈ =8͒, which in turn is coupled by a relatively strong laser field S to a more highly excited level f ͑here 5 1 ⌺ g + , v = 10, J =9͒, a scheme we idealize as a three-state ladder. The AT effect is seen by scanning the frequency of the P field while recording fluorescence from both the e and f levels in separate detection channels. We present qualitative theoretical considerations showing that, when the P field is weak, the ratio of doublet component areas in the excitation spectrum from level f can be used to determine the lifetime of this level. We obtain a value of 17± 3 ns. When the P field is stronger, such that its Rabi frequency is larger than the decay rate of level e, the fraction of f-level population that decays to the intermediate electronic state A 1 ⌺ u + can be deduced from the AT spectrum. When supplemented with values of Franck-Condon and Hönl-London factors, our measurements give a value for the branching ratio ͑the fraction returning to level e͒ of r e = 0.145 with a statistical error of ±0.004. The use of a strong P field on the g-e transition and a weak S field as a probe on the e-f transition results in complex line shapes in the excitation spectrum of level f, not showing the familiar Autler-Townes doublet structure.

A technique for adiabatic control of the population flow through a preselected decaying excited level in a three-level quantum ladder is presented. The population flow through the intermediate or upper level is controlled efficiently and robustly by varying the pulse delay between a pair of partly overlapping coherent laser pulses. The technique is analyzed theoretically and demonstrated in an experiment with Na2 molecules.

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