Probing the decelerating trajectory of a Raman soliton using temporal reflection.

Temporal reflection is a process where an optical pulse reflects off a moving boundary with different refractive indices across it. In a dispersive medium, this process creates a reflected pulse with a frequency shift that changes its speed. Such frequency shifts depend on the speed of the moving boundary. In this work, we propose and experimentally show that it is possible to probe the trajectory of the boundary by measuring the frequency shifts while changing the initial delay between the incident pulse and the boundary. We demonstrate this effect by reflecting a probe pulse off a short soliton, acting as a moving boundary that decelerates inside a photonic crystal fiber because of intrapulse Raman scattering. We deduce trajectory of the soliton from the measured spectral data for the reflected pulse.

Spatial beam narrowing in multimode graded-index fiber amplifiers: an analytic approach.

Doped and optically pumped graded-index (GRIN) fibers can be used to amplify an optical beam such that its spatial quality is improved at the output end of the fiber compared with that of the unamplified beam. We develop a simple model of the amplification process in such GRIN fiber amplifiers and show that the resulting equations can be solved analytically with suitable approximations. The solution shows that the width of the amplifying beam oscillates but also becomes narrower because of the radial dependence of the optical gain. The main advantage of our simplified approach is that it provides an analytic expression for the damping distance of beam-width oscillations that shows clearly the role played by various physical parameters.

Focusing of partially coherent light by a graded-index lens.

We use coherence theory to study how the focusing of an optical beam by a graded-index (GRIN) lens is affected when the incoming beam is only partially coherent. The Gaussian-Schell model is used to show that the intensity of a partially coherent beam exhibits self-imaging and evolves in a periodic fashion in a GRIN medium with a parabolic index profile. Spatial coherence of the beam affects a single parameter that governs how much the beam is compressed at the focal point. Our results show that the focal spot size depends on the fraction of the beam's diameter over which coherence persists. Focusing ceases to occur, and the beam may even expand at the focal point of a GRIN lens, when this fraction is below 10%.

Electrically induced adiabatic frequency conversion in an integrated lithium niobate ring resonator.

Changing the frequency of light outside the laser cavity is essential for an integrated photonics platform, especially when the optical frequency of the on-chip light source is fixed or challenging to be tuned precisely. Previous on-chip frequency conversion demonstrations of multiple GHz have limitations of tuning the shifted frequency continuously. To achieve continuous on-chip optical frequency conversion, we electrically tune a lithium niobate ring resonator to induce adiabatic frequency conversion. In this work, frequency shifts of up to 14.3 GHz are achieved by adjusting the voltage of an RF control. With this technique, we can dynamically control light in a cavity within its photon lifetime by tuning the refractive index of the ring resonator electrically.

Role of frequency dependence of the nonlinearity on a soliton's evolution in photonic crystal fibers.

We reveal the crucial role played by the frequency dependence of the nonlinear parameter on the evolution of femtosecond solitons inside photonic crystal fibers (PCFs). We show that the conventional approach based on the self-steepening effect is not appropriate when such fibers have two zero-dispersion wavelengths, and several higher-order nonlinear terms must be included for realistic modeling of the nonlinear phenomena in PCFs. These terms affect not only the Raman-induced wavelength shift of a soliton but also impact its shedding of dispersive radiation.

Impact of the boundary's sharpness on temporal reflection in dispersive media.

We investigate the impact of the finite rise time of a spatiotemporal boundary inside a dispersive medium used for reflection and refraction of optical pulses. We develop a matrix approach in the frequency domain for analyzing such spatiotemporal boundaries and use it to show that the frequency range over which reflection can occur is reduced as the rise time increases. We also show that total internal reflection can occur even for boundaries with long rise times. This feature suggests that spatiotemporal waveguides can be realized through cross-phase modulation even when pump pulses have relatively long rise and fall times.

Power optimization for phase quantization with SOAs using the gain extinction ratio.

Phase-sensitive amplifiers (PSAs) can work as M - level phase quantizers when waves generated with specific phase values are allowed to mix coherently in a nonlinear medium. The quality of an M - level phase quantizer depends on the relative powers of the mixing waves and requires their optimization. If the mixing waves also experience gain in the nonlinear medium, such as in semiconductor optical amplifiers (SOAs), this optimization becomes non-trivial. In this paper, we present a general method to optimize phase quantization using a PSA made using an SOA, based on gain extinction ratio (GER), which is an experimentally measurable quantity. We present a simple theory to derive the optimal GER required to achieve an M -level quantization. We further experimentally demonstrate two- and four-level phase quantization schemes with an SOA, operated at the optimized GER, with pump power levels as low as 1 mW.

Distributed feedback lasing based on a negative-index metamaterial waveguide.

This Letter lays the foundation of a new type of distributed feedback (DFB) laser whose optical feedback is due to the evanescent coupling between an active positive-index material (PIM) waveguide and a lossy negative-index metamaterial (NIM) waveguide. Active PIM-NIM coupled-mode equations are presented and solved to characterize the dispersion relation, resonant optical gain, and lasing. The photonic bandgap of this grating-less DFB laser does not depend on a Bragg wavenumber, but depends on the difference between the wavenumbers of the PIM and NIM waveguides; controlling this wavenumber difference allows for single-mode lasing and, ultimately, single-mode broadband lasing.

A time-to-frequency converter for measuring the shape of short optical pulses.

A time-to-frequency converter was constructed using an electro-optic phase modulator as a time lens, allowing the pulse shape in time to be transferred to the frequency domain. We used such a device to record the temporal shape of infrared pulses at a wavelength of 1053 nm (width about 7 ps) and compared these measurements to those made by using both a streak camera and an autocorrelator. This side-by-side comparison illustrates the benefits and limitations of each of the measurement methods. Numerical simulations were used to establish that our time-lens-based system can accurately measure the shape of infrared pulses between 3 ps and 12 ps. We also use our numerical model to determine how such a system can be modified to measure pulses whose width lies in the range of 1-30 ps, a range of interest for the OMEGA-EP laser at the Laboratory for Laser Energetics.

Fraunhofer diffraction and the state of polarization of partially coherent electromagnetic beams.

We generalize the concept of Fraunhofer diffraction to partially coherent electromagnetic beams and show how the state of polarization is affected by a circular aperture. It is illustrated that the far-zone properties of a random beam can be tuned by varying the aperture radius. We find that even an incident beam that is completely unpolarized can sometimes produce a field that is highly polarized.

Degree of polarization in the focal region of a lens.

We examine the 3D distribution of the degree of polarization (DOP) in the focal region of a thin paraxial lens. Analytic expressions for the case of a focused Gaussian-Schell model beam are derived. These show that the DOP satisfies certain spatial symmetry relations. Furthermore, its value varies strongly in the vicinity of the geometrical focus, and its maximum, which need not occur at the focus, can be significantly higher than that of the incident beam.

Graded-index solitons in multimode fibers.

We investigate stability of optical solitons in graded-index (GRIN) fibers by solving an effective nonlinear Schrödinger equation that includes spatial self-imaging effects through a length-dependent nonlinear parameter. We show that this equation can be reduced to the standard NLS equation for optical pulses whose dispersion length is much longer than the self-imaging period of the GRIN fiber. Numerical simulations are used to reveal that fundamental GRIN solitons as short as 100 fs can form and remain stable over distances exceeding 1 km. Higher-order solitons can also form, but they propagate stably over shorter distances. We also discuss the impact of third-order dispersion on a GRIN soliton.

Controlling the degree of polarization of partially coherent electromagnetic beams with lenses.

We show theoretically that the degree of polarization of a partially coherent electromagnetic beam changes dramatically as the beam is being focused. A low numerical aperture lens can considerably enhance the degree of polarization at its geometrical focus. When two identical lenses are employed in a 4f configuration, the degree of polarization of a beam can be tailored by using amplitude masks in the Fourier plane located in the middle of the two lenses. Our findings open up the possibility to control this fundamental property of random beams in a simple manner.

Fourier processing with partially coherent fields.

We describe how Fourier signal processing techniques can be generalized to partially coherent fields. Using standard coherence theory, we first show that focusing of a partially coherent beam by a lens modifies its coherence properties. We then consider a 4f imaging system composed of two lenses and discuss how spatial filtering in the Fourier plane allows one to tune the coherence properties of the beam. This, in turn, provides control over the beam's directionality, spectrum, and degree of polarization.

Dynamics and detection of the Newton-Wigner time delays at interfaces using a swivelling method.

Evanescent waves are ubiquitous at interfaces with optical, seismic or acoustic waves, and also with electron, neutron or atom beams. Newton was the first to suspect that both small time delays and spatial shifts exist during total internal reflection. However, these effects are so tiny that the spatial shifts were only observed in 1947 in optics, whereas the time delay values predicted by the Wigner model in the 10-14 s range in optics had to await femtosecond lasers to be detected with difficulty. The spatial shifts have been isolated in many areas but the time delays, though fundamental, generally remain out of reach, particularly with particles. In textbooks usually both quantities are supposed to be simply linked. Here we report, using swivelling detectors, that the spatial and temporal measurements are intimately intermingled, especially in the so-called cyclical regime. Indeed, while the spatial shift does not depend on the type of detection, the measured time delay can be positive, negative or zero, but controllable. We also discuss how such intricate measurements of spatial and temporal effects allow crucial time penalties to be eliminated in guided soliton propagation, and should be used to unambiguously identify the Newton-Wigner time delays for particles.

Dynamics of soliton cascades in fiber amplifiers.

We study numerically the formation of cascading solitons when femtosecond optical pulses are launched into a fiber amplifier with less energy than required to form a soliton of equal duration. As the pulse is amplified, cascaded fundamental solitons are created at different distances, without soliton fission, as each fundamental soliton moves outside the gain bandwidth through the Raman-induced spectral shifts. As a result, each input pulse creates multiple, temporally separated, ultrashort pulses of different wavelengths at the amplifier output. The number of pulses depends not only on the total gain of the amplifier but also on the width of the input pulse.

Theoretical analysis of hot electron injection from metallic nanotubes into a semiconductor interface.

Metallic nanostructures under optical illumination can generate a non-equilibrium high-energy electron gas (also known as hot electrons) capable of being injected into neighbouring media over a potential barrier at particle boundaries. The nature of this process is highly nanoparticle shape and size dependent. Here, we have derived an analytical expression for the frequency dependent rate of injection of these energetic electrons from a metallic nanotube into a semiconductor layer in contact with its inner boundary. In our derivation, we have considered the quantum mechanical motion of the electron gas confined by the particle boundaries in determining the electron energy spectrum and wave functions. We present a comprehensive theoretical analysis of how different geometric parameters such as the outer to inner radius ratio, length and thickness of a nanotube and illumination frequency affect the hot electron injection and internal quantum efficiency of the nanotube. We reveal that longer nanotubes with thin shells and high inner to outer radius ratios show better performance at visible and infrared frequencies. Our derivations and results provide the much needed theoretical insight for optimization of thin nanotubes for different hot electron based applications.

Stomach specific polymeric low density microballoons as a vector for extended delivery of rabeprazole and amoxicillin for treatment of peptic ulcer.

The study was intended to develop a new intra-gastric floating in situ microballoons system for controlled delivery of rabeprazole sodium and amoxicillin trihydrate for the treatment of peptic ulcer disease. Eudragit S-100 and hydroxypropyl methyl cellulose based low density microballoons systems were fabricated by employing varying concentrations of Eudragit S-100 and hydroxypropyl methyl cellulose, to which varying concentrations of drug was added, and formulated by stirring at various speed and time to optimize the process and formulation variable. The formulation variables like concentration and ratio of polymers significantly affected the in vitro drug release from the prepared floating device. The validation of the gastro-retentive potential of the prepared microballoons was carried out in rabbits by orally administration of microballoons formulation containing radio opaque material. The developed formulations showed improved buoyancy and lower ulcer index as compared to that seen with plain drugs. Ulcer protective efficacies were confirmed in ulcer-bearing mouse model. In conclusion, greater compatibility, higher gastro-retention and higher anti-ulcer activity of the presently fabricated formulations to improve potential of formulation for redefining ulcer treatment are presented here. These learning exposed a targeted and sustained drug delivery potential of prepared microballoons in gastric region for ulcer therapeutic intervention as corroborated by in vitro and in vivo findings and, thus, deserves further attention for improved ulcer treatment.

Design of all-optical, hot-electron current-direction-switching device based on geometrical asymmetry.

We propose a nano-scale current-direction-switching device(CDSD) that operates based on the novel phenomenon of geometrical asymmetry between two hot-electron generating plasmonic nanostructures. The proposed device is easy to fabricate and economical to develop compared to most other existing designs. It also has the ability to function without external wiring in nano or molecular circuitry since it is powered and controlled optically. We consider a such CDSD made of two dissimilar nanorods separated by a thin but finite potential barrier and theoretically derive the frequency-dependent electron/current flow rate. Our analysis takes in to account the quantum dynamics of electrons inside the nanorods under a periodic optical perturbation that are confined by nanorod boundaries, modelled as finite cylindrical potential wells. The influence of design parameters, such as geometric difference between the two nanorods, their volumes and the barrier width on quality parameters such as frequency-sensitivity of the current flow direction, magnitude of the current flow, positive to negative current ratio, and the energy conversion efficiency is discussed by considering a device made of Ag/TiO2/Ag. Theoretical insight and design guidelines presented here are useful for customizing our proposed CDSD for applications such as self-powered logic gates, power supplies, and sensors.