Propagation of Gaussian Schell-model beams in modulated graded-index media.

The evolution of partially coherent beams in longitudinally modulated graded-index media is studied. The special cases of Gaussian Schell-model beams and parametric modulation, when the modulation period is half the fiber self-imaging period, are examined in detail. We show that the widths of the intensity and coherence of Gaussian Schell-model beams undergo amplification in parametrically modulated parabolic graded-index media. The process is an analog of quantum mechanical parametric amplification and generation of squeezed states. Our work may find application in spatial and temporal imaging of partially coherent beams in fiber-based imaging systems.

Resonance in quantum dot fluorescence in a photonic bandgap liquid crystal host.

Microcavity resonance is demonstrated in nanocrystal quantum dot fluorescence in a one-dimensional (1D) chiral photonic bandgap cholesteric-liquid crystal host under cw excitation. The resonance demonstrates coupling between quantum dot fluorescence and the cholesteric microcavity. Observed at a band edge of a photonic stop band, this resonance has circular polarization due to microcavity chirality with 4.9 times intensity enhancement in comparison with polarization of the opposite handedness. The circular-polarization dissymmetry factor g(e) of this resonance is ~1.3. We also demonstrate photon antibunching of a single quantum dot in a similar glassy cholesteric microcavity. These results are important in cholesteric-laser research, in which so far only dyes were used, as well as for room-temperature single-photon source applications.

Phase-locked laser system for use in atomic coherence experiments.

We describe a phase-coherent laser system designed for use in experiments involving coherently prepared atomic media. We implement a simple technique based on a sample-and-hold circuit together with a reset of the integrating electronics that makes it possible to scan continuously the relative frequency between the lasers of over tens of gigahertz while keeping them phase locked. The system consists of three external-cavity diode lasers operating around 795 nm. A low-power laser serves as a frequency reference for two high-power lasers which are phased locked with an optical phase-locked loop. We measured the residual phase noise of the system to be less than 0.04 rad(2). In order to show the application of the system towards atomic coherence experiments, we used it to implement electromagnetically induced transparency in a rubidium vapor cell and obtained a reduction in the absorption coefficient of 92%.

Honeycomb pattern formation by laser-beam filamentation in atomic sodium vapor.

We have observed transverse pattern formation leading to highly regular structures in both the near and far fields when a near-resonant laser beam propagates without feedback through an atomic sodium vapor. One example is a regular far-field honeycomb pattern, which results from the transformation of the laser beam within the vapor into a stable three-lobed structure with a uniform phase distribution and highly correlated power fluctuations. The predictions of a theoretical model of the filamentation process are in good agreement with these observations.

Direct measurement of a photoconductive receiver's temporal response by dithered-edge sampling.

We present the results of a direct measurement of the temporal response of a terahertz (THz) photoconductive receiver obtained by dithered-edge sampling. The receiver response has structure that accounts for the negative-going leading edge of the pulse shape that is often seen in measurements made with these receivers in a conventional sampling arrangement. We show that the THz pulse shape measured by conventional photoconductive sampling is indeed a cross correlation of the pulse with the measured receiver's response.

Spatiotemporal shaping of terahertz pulses.

We report temporal shaping of few-cycle terahertz pulses, using a slit in a conductive screen as a high-pass filter. The filter's cutoff frequency was tuned by changing the width of the slit; the slope of the cutoff transition was altered by changing the thickness of the screen. We measured the transmission function of the filters, using large-aperture photoconducting antennas to create and detect the incident and transmitted electric field. Our experimental results were in excellent agreement with the performed finite-difference time-domain simulations of the propagation of the pulse through the slit. When the screen thickness was greater than the slit width, the filter was well modeled by a short, planar waveguide. Using a simple transfer function, we accurately describe the sharp cutoff and dispersion of such a filter.

Analysis of antibodies against components of the autonomic nervous system in diabetes mellitus.

Antibodies to autonomic nervous system structures have previously been detected using a complement fixation immunofluorescence test in the sera of patients with insulin-dependent diabetes mellitus (IDDM) and non-insulin dependent diabetes mellitus (NIDDM). These antibodies might play a role in the aetiology of autonomic neuropathy. Sera from 45 IDDM, 40 NIDDM and 52 control subjects were tested by immunofluorescence for antibodies to human sympathetic ganglia, human adrenal medulla and rabbit vagus nerve. The use of human sympathetic ganglia was compared with rabbit tissue for the detection of sympathetic ganglia antibodies; the results for these autonomic nervous system antibodies were also compared with results using an ELISA. There was no relationship between the presence of antibodies detected by ELISA and those detected by immunofluorescence, but of 14 IDDM patients with thyroid antibodies, 12 had autonomic nervous system antibodies detected by either immunofluorescence or ELISA (p < 0.005 compared to patients without thyroid antibodies). To further characterize the autoantigen(s), immunoblotting was performed. An adrenal antigen corresponding to 74 kDa was detected in sera from three patients, only one of whom had antibodies detectable by ELISA and immunofluorescence. One IDDM serum showed specific binding to a vagus nerve antigen corresponding to 33 kDa. No specific binding to sympathetic ganglia antigen was demonstrated. Antibodies against autonomic nervous system antigens are an inconsistent feature of diabetes, and appear more associated with coincidental autoimmunity against other organs such as the thyroid.

Natural modes for the analysis of optical bistability and laser instability.

A self-consistent description is given for the propagation of a weak optical probe beam through a homogeneously broadened two-level medium in the presence of a strong, collinear, near-resonant pump beam. The propagation of the probe beam is given in terms of bichromatic natural modes, which contain frequency components symmetrically displaced from that of the pump beam. The two frequency components have a definite relative amplitude and phase, which are determined solely by the frequency detunings of the optical fields from the atomic resonance and by the intensity of the pump field. A simple method for calculating these natural modes is derived, and their importance in describing nearly degenerate four-wave mixing, optical bistability, and the stability of homogeneously broadened ring lasers is discussed.

Coherent trapping of atomic populations.

It is often desirable in laser spectroscopy and isotope separation to extract as much as possible of an atomic or molecular population that is distributed among a number of ground-state sublevels and low-lying metastable levels. We describe a form of coherent trapping that occurs when multiple resonant laser beams are used to couple the various ground states to a common upper level. This effect prevents the extraction of the entire population. We then study the effect with two dye lasers and an atomic beam and suggest possible ways to maximize the pumping efficiency.