All-optical modulation and detection using a gain medium in a pulse shaper.

We demonstrate all-optical modulation and ultrafast detection using an on-resonance optical gain medium, combined with spectral splitting in a Fourier transform pulse shaper. Multiple spectral channels of one optical beam can be independently modulated in time by another beam, allowing high-rate modulation and multiplexing without requiring ultrafast response from the gain medium. For detection of sub-picosecond signals we demonstrate a method of ultrafast signal detection (temporal imaging with no spatial resolution) that utilizes the spatio-temporal tilt of an optical pulse in a pulse shaper. The proposed methods can find applications in optical information technology and ultrafast imaging.

Theoretical description of bifacial optical nanomaterials.

In general, optical nanomaterials composed of noncentrosymmetric nanoscatterers are bifacial, meaning that two counter-propagating waves inside the material behave differently. Thus far a practical theory for the description of such materials has been missing. Herein, we present a theory that connects the design of the bifacial nanomaterial's "atoms" with the refractive index and wave impedance of the medium. We also introduce generalized Fresnel coefficients and investigate the role of electromagnetic multipoles on the bifaciality. We find that in any material two counter-propagating waves must experience the same refractive index, but their impedances can differ. The model is demonstrated in practice by the design of a nanomaterial slab with one of its facets being optically reflective, while the other being totally non-reflective.

Dependence of resonant light transmission properties of a subwavelength slit on structural parameters.

We perform a systematic study of the resonant transmission of visible and near-infrared (NIR) light through a single subwavelength slit in a gold film when the parameters defining the structure are varied. We further examine the optical properties of a related nanostructure, a cross with subwavelength sized features. Focused ion beam (FIB) milling was used to fabricate nanoslits and crosses with linewidths ranging from 26 nm to 85 nm. The dimensions of the structure are found to affect strongly the transmittance spectrum. For example, as the slit becomes narrower the resonance is observed to both sharpen and shift significantly. Our observations are in good agreement with our earlier numerical calculations on the optical properties of nanoslits.

Dipole-dipole interaction between molecules mediated by a chain of silver nanoparticles.

We study the dipole-dipole coupling between two fluorescent molecules in the presence of a chain of metallic nanoparticles. We analyze the spectral behavior of the coupling strength and its dependence on the molecular orientation. Our results show that for certain resonant wavelengths the coupling strength between the molecules is greatly enhanced and is strongly polarization sensitive. We also demonstrate how metallic nanoparticles can be utilized in implementing a polarization-sensitive coupler.

Rotational frequency shifts in partially coherent optical fields.

We study the frequency shifts taking place when a random, stationary optical field rotates with respect to an observer. The field is expanded in terms of fully coherent Laguerre-Gaussian basis modes, for which the rotational frequency shifts have been studied previously. We demonstrate the formalism by considering the spectrum of a Gaussian Schell-model field, and show that for a spatially highly incoherent field, significant spectral changes can be expected.

Supercontinuum generation by nanosecond dual-wavelength pumping in microstructured optical fibers.

We study experimentally the spectral evolution of supercontinua in two different microstructured fibers that are pumped with nanosecond pulses from dual-wavelength sources of either 1064/532 nm or 946/473 nm output. The experimental findings are compared with simulations based on numerically solving the nonlinear Schrödinger equation. The role of cascaded cross-phase modulation processes and the group-delay properties of the fiber are emphasized and demonstrated to determine the extent of the broadening of the continua to the visible wavelengths.

Connection between electric and magnetic coherence in free electromagnetic fields.

We introduce quantitative measures for the description of the electric and magnetic coherence in a stationary, random electromagnetic field at two points, in a volume, and in the Fourier space. These quantities are applied to free electromagnetic fields, and several theorems regarding the relationship between the two types of coherences in such fields are established. Fields which are statistically homogeneous, and those which, in addition, are statistically isotropic are considered separately. Furthermore, the connection between the electric and magnetic coherence is exemplified for some specific statistically homogeneous fields.

Spectral analysis of resonant transmission of light through a single sub-wavelength slit.

We analyze the spectral properties of resonant transmission of light through a sub-wavelength slit in a metal film. We show that the enhanced transmission can be understood in terms of interfering surface-wave-like modes propagating in the slit. We characterize the effect of geometrical and material properties of the slit on the transmission spectrum. Furthermore, we show that the wavelength of the transmission resonance strongly depends on the surrounding medium. This effect may be utilized in sensors, imaging, and the detection of, e.g. biomolecules.

Measurement of anomalous dispersion in microstructured fibers using spectral modulation.

We report on a simple technique to measure the anomalous dispersion of small-core microstructured fibers using short optical pulses. The method relies on the spectral modulation resulting from the evolution of the input pulse into a propagating soliton wave. The technique allows for a direct measurement of the dispersion at the desired wavelength from a single pulse. The measurement error is estimated to be less than 10%.

Enhanced bandwidth of supercontinuum generated in microstructured fibers.

Enhancement of the bandwidth of supercontinuum generated in microstructured fibers with a tailored dispersion profile is demonstrated experimentally. The fibers are designed to have two zero-dispersion wavelengths separated by more than 700 nm, which results in an amplification of two dispersive waves at visible and infrared wavelengths. The underlying physics behind the broad continuum formation is discussed and analyzed in detail. The experimental observations are confirmed through numerical simulations.

Universality of electromagnetic-field correlations within homogeneous and isotropic sources.

We investigate the structure of second-order correlations in electromagnetic fields produced by statistically stationary, homogeneous, and isotropic current distributions. We show that the coherence properties of such fields within a low-loss or nondissipative medium do not depend on the source characteristics, but are solely determined by the propagation properties, and that the degree of coherence of the field is given by the sinc law. Our analysis reproduces the known results for blackbody fields, but it applies to a wider class of sources, not necessarily in thermal equilibrium. We discuss the physics behind the universal behavior of the correlations by comparing the results with those obtained by an electromagnetic plane-wave model.

Heating and phase-space decompression of evanescent-wave cooled atoms by multiple photon reabsorption.

We show that multiple reabsorption of resonance-frequency photons in a cloud of evanescent-wave cooled atoms can have a significant influence on the cooling efficiency and maximum value of the atomic phase-space density.

Degree of polarization for optical near fields.

We investigate an extension to the concept of degree of polarization that applies to arbitrary electromagnetic fields, i.e., fields whose wave fronts are not necessarily planar. The approach makes use of generalized spectral Stokes parameters that appear as coefficients, when the full 3 x 3 spectral coherence matrix is expanded in terms of the Gell-Mann matrices. By defining the degree of polarization in terms of these parameters in a manner analogous to the conventional planar-field case, we are led to a formula that consists of scalar invariants of the spectral coherence matrix only. We show that attractive physical insight is gained by expressing the three-dimensional degree of polarization explicitly with the help of the correlations between the three orthogonal spectral components of the electric field. Furthermore, we discuss the fundamental differences in characterizing the polarization state of a field by employing either the two- or the three-dimensional coherence-matrix formalism. The extension of the concept of the degree of polarization to include electromagnetic fields having structures of arbitrary form is expected to be particularly useful, for example, in near-field optics.

Spectral broadening of femtosecond pulses into continuum radiation in microstructured fibers.

We report on the influence of the choice of the pump wavelength relative to the zero-dispersion wavelength for continuum generation in microstructured fibers. Different nonlinear mechanisms are observed depending on whether the pump is located in the normal or anomalous dispersion region. Raman scattering and the wavelength dependence of the group delay of the fiber are found to play an important role in the process. We give an experimental and numerical analysis of the observed phenomena and find a good agreement between the two.

Pulsed standing-wave mirror for neutral atoms and molecules

Reflection of neutral atoms and molecules by a pulsed standing wave with a duration on the order of nanoseconds is studied. It is shown that, with a suitable choice of the laser parameter values, each period of the standing-wave pattern functions as an independent mirror, thus providing a novel way to manipulate large samples of neutral gas-phase particles even with a single laser pulse. At moderate field intensities, the pulsed standing-wave mirror would be directly applicable, e.g., for the manipulation of buffer-gas cooled molecules.

Conoscopic interferometry of surface-acoustic-wave substrate crystals.

Conoscopic interferometry is applied for determining the crystal orientation of lithium niobate and other commonly employed substrate wafers for integrated-optic and surface-acoustic-wave devices. The method is particularly applicable for detecting the orientation of the optic axes of the strongly birefringent niobate but is less sensitive for lithium tantalate or quartz. Conoscopic interference is a low-cost and easy-to-use method that is especially suitable for laboratory usage.

Solid-state dye laser with modified poly(methyl methacrylate)-doped active elements.

Laser generation with modified poly(methyl methacrylate) (MPMMA)-doped matrices with several different types of Rhodamine-based dyes was obtained. Pumping with a frequency-doubled Q-switched Nd:YAG laser was used. During the experiments, high conversion efficiency was achieved. The strong nonlinear dependence of the operating lifetime and the conversion efficiency of material tested on the pump-pulse-repetition rate was observed. Possible mechanisms responsible for the conversionefficiency drop and the useful lifetime of the material are discussed.

Detection of OH in flames by using polarization spectroscopy.

The potentials of polarization spectroscopy in combustion studies are reported. This well-established technique of nonlinear laser spectroscopy is applied to the detection of OH molecules in a premixed acetylene-oxygen and a propane-air flame. The polarization spectrum is recorded in the A (2)Sigma-X(2)Pi(0, 0) band. The spatial distribution of OH is measured in the flames. The saturation of the signal as a function of the laser pulse intensity is also investigated.