Nonlinear optical effects can be enhanced in tapered optical fibers with diameters less than the wavelength of the propagating light. Here we report on the observation of two-photon absorption using tapered fibers in rubidium vapor at power levels of less than 150 nW. Transit-time broadening produces two-photon absorption spectra with sharp peaks that are very different from conventional line shapes.Tapered optical fibers with small diameters can produce relatively high intensities at low incident power levels due to their small mode area. This has allowed a number of recent demonstrations of nonlinear optical effects at low power levels [1][2][3][4]. Here we describe an experiment in which enhanced two-photon absorption [5,6] was observed in tapered optical fibers in the presence of rubidium vapor. Two-photon absorption was observed at power levels of less than 150 nW. To put this in perspective, these power levels correspond to less than 20 photons on average in the tapered region of the fiber at any given time.Aside from its fundamental interest, two-photon absorption at low intensities may be useful for all-optical switching [2,7-10] or for quantum logic gates based on the Zeno effect [11][12][13]. The tapered optical fibers used in these experiments had diameters of 350 nm, which is less than half the wavelength of the light propagating in the fiber. As a result, atoms moving at thermal velocities pass through the evanescent field of the tapered region in a few nanoseconds, which produces interesting features in the shape of the two-photon absorption lines due to transit-time broadening.Two-photon absorption in a three-level atom is illustrated schematically in Fig. 1. A photon at frequency 1  is detuned from the resonant frequency of the first atomic transition by an amount  , which produces a virtual population of the second atomic state. A second photon at frequency 2  gives a detuning of the sum of the photon energies from the energy of the upper atomic state. We typically held for fixed power levels [5] and is thus considerably smaller than resonant two-photon absorption. We observed both types of two-photon absorption although the off-resonant two-photon absorption required somewhat higher power levels in order to maintain the signal to noise ratio. Tapered optical fibers were fabricated from standard single-mode fiber using the well-known flame brush technique [17]. Our setup was designed to consistently fabricate tapered fibers with diameters of approximately 350 nm over a length of 5 mm. This diameter was chosen to coincide with the optimal combination of mode compression and evanescent power [5,18] for nonlinear interactions using the 5S 1/2 to 5P 3/2 to 5D 5/2 transitions in rubidium, which have wavelengths of 780 and 776 nm, respectively.To heat and pull the fibers to the desired diameter, we used an air and propane flame with a ¼" diameter nozzle with three 1 mm holes in a line perpendicular to the fiber axis. The air and fuel flow were stabilized using regulators in combination with digita...

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We provide a review of recent progress in integrated nonlinear photonics with a focus on emerging applications in all-optical signal processing, ultra-low-power all-optical switching, and quantum information processing.

Low-contrast all-optical Zeno switching has been demonstrated in a Si3N4 microdisk resonator coupled to a hot atomic vapor. The device is based on the suppression of the field build-up within a microcavity due to non-degenerate two-photon absorption. This experiment used one beam in a resonator and one in free-space due to limitations related to device physics. These results suggest that a similar scheme with both beams resonant in the cavity would correspond to input power levels near 20 nW.PACS numbers: 42.65. Pc, 42.82.Et The quantum Zeno effect (QZE) can prevent a randomly occurring process by frequent measurement [1]. It has previously been shown [2][3][4] that this effect could be used to suppress errors in quantum logic gates using strong two-photon absorption (TPA). Recently, this work was extended to show that the QZE has a classical analog that could be used to create a low-loss all-optical switch [5] capable of operating at low powers.Whereas the QZE prevents the buildup of a probability amplitude, the classical Zeno effect suppresses the coherent buildup of the electromagnetic field amplitude within a microresonator. To see how this can be used to create a switch, consider a system in which the resonator is strongly coupled to a two-photon absorbing medium such that two distinct frequencies are required for absorption to take place. With the resonator critically coupled to two waveguides, the presence of a resonant input at either of the two frequencies will result in the light coupling into the resonator and leaving the opposite waveguide. This is due to the destructive interference between the light remaining in the waveguide and the built-up field amplitude in the cavity that couples back to the waveguide. When both frequencies are present in the cavity the TPA prevents the coherent intra-cavity field buildup and the input beams pass by the resonator because there is now insufficient amplitude in the cavity to result in interference.Based on these principles, groups have proposed alloptical Zeno switches employing other dissipative mechanisms, such as saturated absorption in a quantum dot coupled to a photonic crystal cavity [6], inverse Raman scattering (IRS) in a Silicon microdisk [7], IRS in an optical fiber [8] and sum and difference frequency generation in a χ (2) microdisk [9]. More generally, other techniques have recently been investigated to demonstrate all-optical switching with the intent of reducing operating power levels [10][11][12][13][14][15].Here we present experimental progress towards a clas- sical Zeno switch consisting of a Si 3 N 4 microdisk embedded in hot Rubidium (Rb) vapor. A key aspect of this work is the enhanced rate of TPA that can be achieved at low power levels by confining fields to a arXiv:1206.0930v1 [quant-ph]

In electromagnetically induced transparency, the scattering rate of a probe beam is greatly reduced due to destructive interference between two dressed atomic states produced by a strong laser beam. Here we show that a similar reduction in the single-photon scattering rate can be achieved by tuning a probe beam to be halfway between the resonant frequencies of two modes of a cavity. This technique is expected to be useful in enhancing two-photon absorption while reducing losses due to single-photon scattering.