Session U13: Precision Measurements and Non Linear Optics

Absolute Optical Frequency Measurements of the Cesium D1 Transitions in a Thermal Atomic Beam using a Femtosecond Laser Frequency Comb

High resolution spectroscopy of the cesium D1 line is performed in a thermal atomic beam using a narrow linewidth CW diode laser offset locked to a single tooth of a femtosecond laser frequency comb. A computer controlled RF synthesizer changes the offset frequency, thus scanning the optical frequency of the probe laser. A photodiode collects the fluorescence and the signal is recorded by a computerized data acquisition system. The 1st order Doppler shift is eliminated by orienting the laser beam in a direction perpendicular to the atomic beam. Optical frequencies for all four pairs of hyperfine components are measured independently from which the D1 line centroid and excited state hyperfine splitting are obtained. The combination of our results with those of recoil-shift atom interferometry experiments can be used to determine a new value for the fine structure constant. This work is partially supported by the National Science Foundation under grant number PHY99-87984.   

Probing the speed of light with radio waves at extremely-low frequencies

The speed of light, a fundamental physical constant and thought to be independent of frequency, is tested here with naturally occurring radio waves from lightning discharges in the atmosphere at extremely-low frequencies. It is shown that the speed of light in the frequency range \mbox{5-50 Hz} is known with an accuracy determined by perturbations of the ionospheric reflection height associated with space weather phenomena, which place an upper limit on the photon rest mass $m_\gamma$\mbox{$\stackrel{\scriptscriptstyle <}{\scriptscriptstyle \sim}$4\cdot 10^{-52}$ kg} to date.\\   

Energy transfer: vibrational control and nonlinear wavepacket interferometry

The time-development of photoexcitations and dynamical formation of entanglement between electronic and vibrational degrees of freedom in molecular aggregates is analyzed by applying method of vibronic 2D wavepackets [ J. Chem. Phys. {\bf 118}, 46 (2003); {\it quant- ph/0412219}], theory of electronic energy transfer, and Jaynes-Cummings model [J. Chem. Phys. {\bf 120}, 11209 (2004); {\it math-ph/0403023}]. Following the ultrafast excitation of donor [ J. Phys. Chem. {\bf 99}, 2568 (1995); {\it chem-ph/9411004}] the population of acceptor rises by small portions each vibrational period, oscillates force and back between donor and acceptor with later damping and partial revivals of this oscillation. The transfer rate gets larger as donor wavepacket approaches the acceptor equilibrium configuration, which is possible at specific energy differences of donor and acceptor and at maximal amount of the vibrational motion along the line that links donor and acceptor equilibria positions. The four-pulse phase-locked nonlinear wavepacket 2D interferograms reflect the shape of the relevant 2D vibronic wavepackets and have maxima at longer delay between excitation pulses for dimers with equal donor-acceptor energy difference compare to dimers with activationless energy configuration [J. Phys. Chem. A. {\bf 108}, 11196 (2004)].   

Visible and near infrared optical nonlinearity of silica nanoaerogels

Silica nanoaerogels have attracted great attention in the last decade because of its specific thermal, electrical, optical, and mechanical properties and many potential applications. However, the nonlinear optical property of silica nanoaerogels has not been investigated until our previous work in 2002. In this work, we report new experimental results of the optical nonlinearity of silica nanoaerogels and discuss the origin of the nonlinearity in the visible to near infrared wavelength range. The high nonlinearity and large nonlinear figure of merit show that silica nanoaerogels are a promising candidate for nonlinear photonic applications.   

Slow and fast light propagation in nonlinear Kerr media.

Sub- and superluminal propagation of light pulse in Kerr materials has been investigated. Group velocities as slow as much less than 1 millimeter per second to as fast as negative several hundreds meters per second can be easily obtained in Kerr medium, which possesses large nonlinear refractive index and long relaxation time, such as Cr doped Alexandrite, Ruby, and GdAlO3. The physical mechanism is the strong highly dispersive coupling between different frequency components of the pulse. The new mechanism of slowing down pulses as well as producing superluminal pulses enlarges the very specific materials to all kinds of nonlinear optical materials.   

Periodicity measurements in liquid crystal lattices

Electrochemical impedance is used to characterize surface corrosion, and for batteries and fuel cells. For most data, the impedance locus is in the first quadrant of the complex plane where the capacitive semi-circle is seen. We use electrochemical impedance to examine the periodicity of water-soluble liquid crystals. Systems examined include DNA, prothrombin, and collagen. A need for specific counter-ions for obtaining or extending the periodicity is demonstrated. Microscopy of dried samples shows crystal arrays including DNA tetrahedra. With optimum periodicity of the liquid crystal, the impedance locus passes through four quadrants of the complex plane. From the capacitive semi-circle in the upper right quadrant, the impedance reverses direction in a counterclockwise manner into all four quadrants at low frequencies (below 8.0 Hz.). The process is first visualized by the inflection to negative DC resistance (-$Z'$). The equivalent circuit is that of a transmission cable with a negative Randle's sub-circuit. The negative resistance is interpreted as an organized electronic state similar to a diode. The reverse impedance assay is a quantitative technique for extending the study of periodic lattices to complex liquid systems.   

Optical Fiber Bragg Grating Response Function and its Temperature Dependence for a Polymer Coating of Temperature Dependent Thermal Expansion Coefficient

In-fiber Bragg gratings are highly sensitive to thermal and strain fields and have been widely used as temperature and strain sensors. Effects of the strain-optic and thermo-optic elements of optical fiber Bragg grating (OFBG) response function are often mixed, inter-competing and complicated to isolate. Moreover, both these elements are influenced by the thermal expansion coefficients, \textit{$\alpha $}, of the fiber core, cladding and the protective coating composite. Polymer protective coatings with large \textit{$\alpha $} have been reported to improve temperature sensitivity ($\sim $ 0.1 nm/\r{ }C) of the response function. However, the results are complicated and less reliable due to non-linearity of \textit{$\alpha $}(${\rm T})$ when using wider temperature range (such as 20 K -- 400 K). This paper reports determination of the OFBG response function for a polymer coating of a large but linear \textit{$\alpha $}(${\rm T})$. The results have been compared with responses from OFBGs coated with materials of non-linear \textit{$\alpha $}(${\rm T})$.   

Electronic energy transfer: vibrational control and nonlinear wavepacket interferometry

The time-development of photoexcitations in molecular aggregates exhibits specific dynamics of electronic states and vibrational wavefunction. We discuss the dynamical formation of entanglement between electronic and vibrational degrees of freedom in molecular aggregates with theory of electronic energy transfer and the method of vibronic 2D wavepackets [ J. Chem. Phys. 118, 46 (2003); quant-ph/0412219]. The vibronic dynamics is also described by applying Jaynes-Cummings model to the electronic energy transfer [J. Chem. Phys. 120, 11209 (2004); math-ph/0403023]. Following the ultrafast excitation of donor [ J. Phys. Chem 99, 2568 (1995); chem-ph/9411004] the population of acceptor rises by small portions per each vibrational period, oscillates force and back between donor and acceptor with later damping and partial revivals of this oscillation. The transfer rate gets larger as donor wavepacket approaches the acceptor equilibrium configuration, which is possible at specific energy differences of donor and acceptor and at maximal amount of the vibrational motion along the line that links donor and acceptor equilibria positions. The four-pulse phase-locked nonlinear wavepacket 2D interferograms reflect the shape of the relevant 2D vibronic wavepackets and have maxima at longer delay between excitation pulses for dimers with equal donor-acceptor energy difference compare to dimers with activationless energy configuration [Cina, Fleming, J. Phys. Chem. A. 108, 11196 (2004)].   

Helix Pre-Tilted Monomer - Polymer Liquid Crystal System for Measurement of Shear Stress Vector

The direction and magnitude of air flow-induced shear stress vector have been measured selective reflection optical response of a pre-tilted shear sensitive cholesteric helix in a monomer-polymer liquid crystal (LC). Optical wavelength, $\lambda $, of the selectively reflected light measured normal to the test surface for a white light incident at $\sim $ 20\r{ } to the normal varies linearly (slope $\sim $ 0.38 nm/Torr for the investigated system of a monomer LC and a polymer LC Vectra A130) with the shear stress measured in terms the of air flow differential pressure, $\Delta $p. This method offers an unique experimental for shear stress vector measurement. Effects of the change in direction of incidence have been discussed from first principles based on a shear stress induced helix tilt and its deformation. It implies from the suggested model that in case of shear stress associated helix deformation, the optical response is likely to reverse on interchanging the directions of the incidence and the reflection for a given shear stress vector. ** Distinguished Research Scientist (Ret.)