Optical solitary wave beams in nematic liquid crystals -Nematicons- when propagating in orientation-modulated planar cells with a non-symmetric layout evolve along different paths when launched from opposite sides, thereby exhibiting nonreciprocal behavior with non-overlapping trajectories. We model such solitary-waveguides in cells with non-homogeneously oriented molecular director, with anchoring linearly changing along the sample length or across its width. We report on numerical experiments about generation and propagation of nematicons injected from the opposite two ends of the cell and investigate the trajectories of forward and backward propagating self-confined beams. The obtained non-specular transmission suggests a diode-like response as a backward propagating beam does not reach the forward propagating input port.
In the domain of Earth Explorer satellites nanoengineered nonlinear crystals can optimize UV tunable solid-state laser converters. Lightweight sources can be based on Lithium Niobate (LN) domain engineering by electric field poling and guided wave interactions.
In this Communication we report the preliminary experimental results and the very first demonstration of UltraViolet second-harmonic generation by first-order quasi-phase-matching in a surface-periodically-poled proton-exchanged LN waveguide. The pump source was a Ti-Sapphire laser with a tunability range of 700- 980 nm and a 40 GHz linewidth. We have measured UV continuous-wave light at 390 nm by means of a lock-in amplifier and of a photodiode with enhanced response in the UV. Measured conversion efficiency was about 1%W-1cm-2. QPM experiments show good agreement with theory and pave the way for a future implementation of the technique in materials less prone to photorefractive damage and wider transparency in the UV, such as Lithium Tantalate.
Using self-induced vortex-like defects in the nematic liquid crystal layer of a light valve with photo-sensible wall, we demonstrate the realization of programable optical vortices lattices with arbitrary configuration in space. On each lattice site, every matter vortex acts as a photonic spin-to-orbital momentum coupler and an array of circularly polarized input beams is converted into an output array of vortex beams with topological charges consistent with the vortex matter lattice. The vortex arrangements are explained the basis of light-induced matter defects and topological rules.
This paper reports two configurations of Bragg reflectors based on liquid crystals confined between two small glass plates. Both approaches employ the efficient electro-optic effect in liquid crystals, which allows tunability of the reflectors by using low voltages. The molecular reorientation induced by an applied electric field implies a refractive index modulation seen by polarized light propagating into the liquid crystal. We show design criteria and profile optimization of the electrodes to induce a liquid crystal refractive index periodic modulation, providing a wavelength selective propagation of confined light in the liquid crystal. The two proposed device configurations differ for the top-bottom electrode configuration in one case and coplanar electrodes in the other case. Modeling of both configurations has been carried by calculating the applied electric field distribution and its interaction with the liquid crystal elastic properties taking into account the boundary conditions due to the alignment layer on the inner faces of the glass substrates. The calculated performance in terms of high wavelength selectivity and ultrawide spectral tuning range indicate that the two designed structures can be proposed for both optical filtering and to produce novel low power integrated distributed feedback resonators in dense wavelength division multiplexed fiber optic systems.
Simultaneous generation of the 2nd and 3rd harmonic, excited by linearly frequency- chirped fundamental pulse in quasiphase
matched grating with linearly varying inverse domain sizes has been studied numerically with taking into account
effects of the group velocity mismatches and dispersions. Mechanisms of efficient nonlinear pulse compressions and
conversion efficiencies of the generated harmonics have been analyzed.
The study of optical solitons and light filaments steering in liquid crystals requires utilization of particular
cells designed for top view investigation and realized with an input interface which enables both to control the
molecular director configuration and to prevent light scattering. Up to now, the director orientation imposed by
this additional interface has been only estimated by experimental observations. In this paper, we report on the
design and characterization of liquid crystal cells for investigation of optical spatial solitons as well as on a simple
model describing the configuration of the molecular director orientation under the anchoring action of multiple
interfaces. The model is based on the elastic continuum theory and only strong anchoring is considered for
boundary conditions. Controlling of the director orientation at the input interface, as well as in the bulk, allows
to obtain configurations that can produce distinct optical phenomena in a light beam propagating inside the cell.
For a particular director configuration, it is possible to produce two waves: the extraordinary and the ordinary
one. With a different director configuration, the extraordinary wave only is obtained, which propagates inside
the cell at an angle of more than 7° with respect to the impinging wave vector direction. Under this peculiar
configuration and by applying an external voltage, it is possible to have a good control of the propagation
direction of the optical spatial soliton.
We discuss Bloch oscillations in waveguide arrays created in a nematic liquid crystalline layer. Bloch oscillations can originate from the specific distribution of refractive index, where a linear gradient is added to the transverse periodicity. Light can oscillate periodically in the transverse direction as it propagates, returning to its initial spatial position and profile after each full cycle. To introduce a spatially periodic refractive index modulation in nematic liquid crystalline waveguides a set of comb-shaped transparent ITO electrodes is placed on one of the glass surfaces. The applied bias allows tuning the structure from light confinement in one dimension, i.e. planar waveguiding, to bidimensional confinement. In the proposed geometry, the thickness of the liquid crystal layer changes linearly as a function of the transverse coordinate. In this way, both linear and nonlinear effective index changes are introduced in each waveguide.
Integration of near infrared (NIR) photodetectors on a silicon substrate is a key step for the fabrication of an all silicon based NIR transceiver. To this extent, polycrystalline germanium (poly-Ge) technology is attractive due to the low deposition temperature and cost. Poly-Ge detectors demonstrated broad response, covering the whole NIR spectrum to 1.55 micron, fast, subnanosecond, speed and excellent versatility. In this work we present our recent results on the integration of a poly-Ge photodetector on a SOI silicon waveguide. The use of a waveguide for light in-coupling is appealing for telecom applications where signal is transported on an optical fibre, but, at the same time, it allows to increase detector responsivity. In fact, in this device the light is absorbed into the thin sensitive layer of the poly-Ge/Si heterojunction in a distributed way, during propagation. This releases the strong constraint of the absorption length being smaller than photocarrier collection length typical of normal incidence photodetectors. In the paper, both design issues and experimental results are reported.
We report on the integration of Ge p-i-n heterojunction photodiodes on Si substrates. The crucial role of interface defects at the Ge/Si interface on the performance of photodetectors is analyzed and taken into account in the design of the devices. We have designed and fabricated high performance p-i-n Ge photodiodes for the near infrared. Pure Ge is grown by ultra-high-vacuum CVD followed by a cyclic thermal annealing and ion implantation. Devices are fabricated using standard photolithography. The photodiodes exhibit maximum responsivity of 0.8 A/W at 1.3 micrometers and 0.7 A/W at 1.55 micrometers , reverse dark currents in the 20 mA/cm2 range at 1V and response times as short as 520 ps. Our devices are the first p-i-n Ge on Si photodetectors fabricated by CVD and exhibit high performances for a wide range of applications.
We present a technology for the integration of high performance near-infrared Ge P-I-N photodetectors on Si for Si microphotonics. High quality Ge epilayers were grown on Si by a two-step ultrahigh-vacuum / chemical-vapor-deposition (UHV/CVD) process. Two-step UHV/CVD allows the epitaxial growth of Ge on Si without islanding. Threading-dislocations in Ge epilayers were reduced by cyclic thermal annealing. The reduction of threading-dislocations can be understood in terms of thermal stress induced dislocation glide and reactions. We found that sessile threading-dislocations are not permanent and can be removed.
We numerically investigate efficient frequency doubling of near infrared light in a coupled system of buried and surface waveguides obtained by Reverse Proton Exchange in z-cut Lithium Niobate. For a monomode TE surface guide at 1.32 micrometer and a highly multimode TM (buried) guide at 666 nm and exploiting the d31 nonlinear tensor element, for planar structures we calculated conversion efficiencies as high as 14% micrometer/W cm, with a weak dependence on temperature. Noticeably, this geometry features the physical separation of the harmonics at the output.
We describe a technique for the simultaneous measurement of all the modal birefringences in a (chi) (2) optical guide through surface emitting second harmonic generation (SESHG), which we applied to multilayer AlGaAs waveguides at 1319 nm, both before and after selective AlAs oxidation. By end-fire coupling linearly-polarized laser pulses into ridge waveguides, both forward- and back-propagating eigenpolarizations were excited due to Fresnel reflection at the output facet. Several TE-TM pairs of counterpropagating modes then interact through the quadratic nonlinearity, giving rise to interference of SESHG fields. With a single image acquisition of the SESHG far field by a CCD camera, we could evaluate the modal birefringences between all the excited TE- TM mode pairs at the fundamental frequency. This simple approach led us to estimate form-birefringence of our multilayer quadratic waveguides with the high accuracy required by optimized phase-matched interactions in parametric generators and oscillators. This technique is a valuable complement to standard m-line effective index evaluation, and a versatile one-shot tool for waveguide diagnostics.
Nowadays refractive-index engineering has become a challenging area for experimentalists in semiconductor integrated optics, whereas design constraints are often more strict than both standard technology tolerances and model accuracies. In fact, it is crucial to non-destructively evaluate thicknesses and refractive indices of a multilayer waveguide independently, and to this aim we resorted to X-ray reflectometry and effective index measurements on MBE-grown AlGaAs waveguides, respectively. With the first technique interference effects (Kiessig fringes) arise, which are related to layer thicknesses. By standard data processing, thickness accuracies of +/- 0.05 nm are readily achieved. Effective index measurements were performed at several wavelengths on both slab and rib waveguides, through grating-assisted distributed coupling with both photoresist and etched gratings. Effective indices were determined with an absolute precision as good as 1/2000, adequate for phase matching in parametric devices. Merging thickness and effective index evaluations, the refractive indices of the constituent layers were determined with unprecedented accuracies, in substantial agreement with existing models.
We have observed, for the first to the best of our knowledge, time 2+1 dimensional spatial solitons with an Argon-ion beam in planar cells containing an undoped nematic liquid crystal. The cells provided planar anchoring for the liquid crystal molecules, with transparent electrodes for applying an external voltage across the 75µm-thick crystal. The voltage allowed to pre-orientate the director field m the prmcipal plane contammg the lmearly polarized optical field vector thereby elimmatmg the Fréedericks threshold In this regime, powers low enough to prevent thermal effects could be employed to obtam transverse localization through a local light induced mdex mcrease, trappmg the beam into a spatial soliton Self trapping as-well-as guidance of a copolarized probe at 633nm, were demonstrated at biases of 1v with 3.9 mW powers m the green at 514nm. A beam orthogonally polarized with respect to the director tilt-plane failed to produce self localization thereby confirming the reorientational nature of the phenomenon.
We report the fabrication of fast heterojunction Ge/Si photodetectors which, to the best of our knowledge, exhibit the highest near infrared responsivity at normal incidence reported to date. Such performances are related to the quality of the epitaxial Ge film grown by a two-step UHV-CVD process followed by cyclic thermal annealing. We have measured a fast (FWHM equals 850 ps at 1.3 micrometers ) and efficient (R equals 0.55 A/W at 1.3 micrometers and 0.25 A/W at 1.55 micrometers ) photoresponse. Our technology makes these devices suitable for integration with other electronic and optoelectronic components on Si chips. In the paper we discuss processing technology, material quality, device fabrication and performance measurements.
We report on a novel solid state wavelength meter in the near infrared. The device is an array of six photodetectors based on polycrystalline germanium film evaporated on a silicon substrate and each element is a wavelength sensitive detector. We describe the design, the fabrication and the characterization of such device and we demonstrate its capability in the measurement of the wavelength of quasi- monochromatic light beams.
We report on the fabrication of a detector array for the near infrared on silicon substrate. Thermally evaporated polycrystalline germanium is used as the active layer in the device which consists of 16 pixel with dot-pitch of about 100 micron; the single pixel has a metal-semiconductor-metal structure. We demonstrate a responsivity of 16 mA/W at 1.3 micron and extending down to 1.55 micron. At the same wavelength an operation speed in the nanoseconds range is demonstrated. The overall fabrication process, including substrate cleaning and preparation, requires temperatures lower than 350 degrees Celsius being fully compatible with silicon electronics.
Since the early work of Kelley in 1965, self-focusing in transparent dielectric materials was recognized as one of the mechanisms of optical induced damage in laser materials. Recently the experimental demonstration of stable solitary waves in Quadratic materials has shown that the catastrophic collapse of laser beams can be arrested in materials with a pure second order response. New ways to solve the self- focusing damage mechanism in laser materials can result from such an experimental discovery. Indeed choosing host laser materials with a quadratic nonlinearity can considerably delay the appearance of the laser beam collapse. We present theoretical considerations and numerical calculations on materials design concepts which can in practice resolve the self-focusing collapse in the presence of a quadratic nonlinearity.
Based on cascading of second-order nonlinearities we propose novel schemes for all-optical signal processing exploiting the tensorial nature of the susceptibility. Waves of the same frequency and orthogonal polarizations can interact via the generated second-harmonic, with an indirect energy transfer and the consequent possibility of gain and transistor action, polarization switching, amplitude and phase modulation.
Photoelectron spectroscopy of coordinatively unsaturated organometallic anions can provide a means to probe the ground and low lying excited states of the corresponding neutral radicals. We report results for the early 3D transition metal monocarbonyls VCO and CrCO, and for the late metal complexes FeCO, CoCO and NiCO. Each spectrum displays a transition to the ground state of the neutral complex, and to an excited state whose spin multiplicity differs by two from that of the ground state. For a given complex, these states share nominally the same electron configuration but differ in the spin coupling of the metal 4s electron. There is a reversal in the state ordering as one proceeds across the transition series, from a high spin ground state for VCO (6(Sigma) +) and CrCO (7(Sigma) +) to a low spin ground state for FeCO (3(Sigma) -), CoCO (2(Delta) ) and NiCO (+1)(Sigma) )+). The measured state splittings and vibrational frequencies provide insight into the factors that determine the ordering and bonding properties of these states. Recent results for the linear H-M-CO isomers of Fe and Co are also reported.
We review a number of nonlinear guided-wave effects using gratings in indium antimonide planar
structures. In particular, we report on the experimental demonstrations of guided-wave optical bistability
and limiting in an input distributed coupler; butterfly bistability of an input-output grating-coupler
combination; all-optical tuning of a distributed feedback reflector; and enhancement of bistability effects
by the use of a counter-propagating feedback beam.
A pump and probe experiment is described which allows to study input coupling efficiency of a CdS grating on top of a linear glass waveguide as a function of input power for wavelengths shorter than CdS band gap wavelength. A monotonous decrease of coupling efficiency is found by increasing the pump power. Reference is made also to results obtained for wavelengths larger than the band gap wavelength.
We report on the optical characterization of poly-4BCMU slab waveguides, and on the fabrication of efficient
grating guided-wave couplers using this material. Thin film samples were obtained, by spin coating,
from a cyclopentanone solution. Grating couplers were used to measure the film refractive indices and
thicknesses. Waveguide attenuation was obtained from measurements of the variation in scattering with propagation
distance in the visible and near infrared. These parameters allowed us to design, fabricate and test a
grating coupler with a coupling efficiency of 45% at 1 .064 pm.
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