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Results for “OPTICAL MODES”

Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 163 records · Page 9
All sourcesNASA NTRSDOE OSTI
DOE OSTI2018

Stable and Unstable Spatial Modes in a Resonator with a Half-Disk Shape

This work was devoted to the study of stable and unstable optical modes in segment shaped resonators. The size of the studied resonators was much larger than the wavelength of the emitted light, that allowed to use the approaches of geometric optics. It was observed, that with an increase in the δ/R deviation parameter (where δ is the deviation from the center of the half-disk) only a small number of stable spatial modes remain. The possibility of the existence of “sealed” (non-radiating modes) and “open” modes (radiating modes) in resonators of this type was also shown.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Bose–Einstein condensation versus Dicke–Hepp–Lieb transition in an optical cavity

We provide an exact solution for the interplay between Bose–Einstein condensation and the Dicke–Hepp–Lieb self-organization transition of an ideal Bose gas trapped inside a single-mode optical cavity and subject to a transverse laser drive. Based on an effective action approach, we determine the full phase diagram at arbitrary temperature, which features a bi-critical point where the transitions cross. We calculate the dynamically generated band structure of the atoms and the associated suppression of the critical temperature for Bose–Einstein condensation in the phase with a spontaneous periodic density modulation. Moreover, we determine the evolution of the polariton spectrum due to the coupling of the cavity photons and the atomic field near the self-organization transition, which is quite different above or below the Bose–Einstein condensation temperature. At low temperatures, the critical value of the Dicke–Hepp–Lieb transition decreases with temperature and thus thermal fluctuations can enhance the tendency to a periodic arrangement of the atoms. -- Highlights: •Atoms inside a driven cavity can undergo two transitions: self-organization and BEC. •The phase diagram has four phases which coexist at a bi-critical point. •Atom–cavity coupling creates a dynamical lattice for the atoms. •Finite temperature can enhance the tendency towards self-organization. •We calculate the detailed spectrum of the polaritonic excitations.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Optical fibres and fibre tapers with an array of Bragg gratings

The properties of optical fibres with an array of fibre Bragg gratings written directly during the fibre drawing are considered. Such optical fibres offer new possibilities for producing new types of active and passive fibre elements due to the enhanced back-reflected signal of the so-called artificial Rayleigh scattering, and are of interest for modelling physical phenomena associated with the statistics of reflecting centres in optical fibres, such as, e.g., localisation of photons. We studied fibres with an array of Bragg gratings written in photosensitive fibres, ordinary single-mode optical fibre of the SMF-28 type and in fibre with a core doped with erbium ions. To control the spectrum width of the reflected signal, we used chirped phase masks, as well as writing arrays of gratings in tapered optical fibres. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2011

Optical system and method for gas detection and monitoring

A free-space optical path of an optical interferometer is disposed in an environment of interest. A light beam is guided to the optical interferometer using a single-mode optical fiber. The light beam traverses the interferometer's optical path. The light beam guided to the optical path is combined with the light beam at the end of the optical path to define an output light. A temporal history of the output light is recorded.

Polzin, Kurt A.↗
NASA NTRS2001

Radiation Losses Due to Tapering of a Double-Core Optical Waveguide

The theoretical model we designed parameterizes the power losses as a function of .the profile shape for a tapered, single mode, optical dielectric coupler. The focus of this project is to produce a working model that determines the power losses experienced by the fibers when light crosses a taper region. This phenomenon can be examined using coupled mode theory. The optical directional coupler consists of a parallel, dual-channel, waveguide with minimal spacing between the channels to permit energy exchange. Thus, power transfer is essentially a function of the taper profile. To find the fields in the fibers, the approach used was that of solving the Helmholtz equation in cylindrical coordinates involving Bessel and modified Bessel functions depending on the location.

Lyons, Donald R.↗
DOE OSTI2016

Experimental generation of tripartite polarization entangled states of bright optical beams

The multipartite polarization entangled states of bright optical beams directly associating with the spin states of atomic ensembles are one of the essential resources in the future quantum information networks, which can be conveniently utilized to transfer and convert quantum states across a network composed of many atomic nodes. In this letter, we present the experimental demonstration of tripartite polarization entanglement described by Stokes operators of optical field. The tripartite entangled states of light at the frequency resonant with D1 line of Rubidium atoms are transformed into the continuous variable polarization entanglement among three bright optical beams via an optical beam splitter network. The obtained entanglement is confirmed by the extended criterion for polarization entanglement of multipartite quantized optical modes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS2005

Graded-index whispering gallery mode resonators

Whispering gallery mode optical resonators which have spatially-graded refractive indices. In one implementation, the refractive index spatially increases with a distance from an exterior surface of such a resonator towards an interior of the resonator to produce substantially equal spectral separations for different whispering gallery modes. An optical coupler may be used with such a resonator to provide proper optical coupling.

Savchenkov, Anatoliy↗
DOE OSTI2015

Optical phonons in PbTe/CdTe multilayer heterostructures

The infrared reflection spectra of PbTe/CdTe multilayer nanostructures grown by molecular-beam epitaxy are measured in the frequency range of 20–5000 cm{sup −1} at room temperature. The thicknesses and high-frequency dielectric constants of the PbTe and CdTe layers and the frequencies of the transverse optical (TO) phonons in these structures are determined from dispersion analysis of the spectra. It is found that the samples under study are characterized by two TO phonon frequencies, equal to 28 and 47 cm{sup −1}. The first frequency is close to that of TO phonons in bulk PbTe, and the second is assigned to the optical mode in structurally distorted interface layers. The Raman-scattering spectra upon excitation with the radiation of an Ar{sup +} laser at 514.5 nm are measured at room and liquid-nitrogen temperatures. The weak line at 106 cm{sup −1} observed in these spectra is attributed to longitudinal optical phonons in the interface layers.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Numerical investigation on a new designed hollow-core photonic crystal fiber with large modal separation

We investigate the characteristics of optical modes in a new designed hollow-core photonic crystal fiber (PCF) with large modal separation. The large effective refractive index difference between the adjacent second-order vector modes can be maintained higher than 0.001 within the wavelength range from 1 to 2 μm. The maximum value can arrive 0.01199 between HE{sub 21} and TM{sub 01} mode at 2 μm. The fundamental mode of the proposed PCF has flat dispersion in a wide spectral range, which can be applied on wideband supercontinuum generation. The influence on the characteristics by the PCF structure has also been investigated. The increase of the size of the central air hole and the duty cycle affects the flatness of the dispersion curve to some extent. Our simulated results could provide an optimized fiber structure for the experimental study on stable transmission of vector modes.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

Ballistic vs. diffusive heat transfer across nanoscopic films of layered crystals

We use non-equilibrium molecular dynamics to study the heat transfer mechanism across sandwich interfacial structures of Si/n-atomic-layers/Si, with 1 ≤ n ≤ 20 and atomic layers composed of WSe{sub 2} and/or graphene. In the case of WSe{sub 2} sheets, we observe that the thermal resistance of the sandwich structure is increasing almost linearly with the number of WSe{sub 2} sheets, n, indicating a diffusive phonon transport mechanism. By contrast in the case of n graphene layers, the interfacial thermal resistance is more or less independent on the number of layers for 1 ≤ n ≤ 10, and is associated with ballistic phonon transport mechanism. We attribute the diffusive heat transfer mechanism across WSe{sub 2} sheets to abundant low frequency and low group velocity optical modes that carry most of the heat across the interface. By contrast, in graphene, acoustic modes dominate the thermal transport across the interface and render a ballistic heat flow mechanism.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2014

Design for high-power, single-lobe, grating-surface-emitting quantum cascade lasers enabled by plasmon-enhanced absorption of antisymmetric modes

Resonant coupling of the transverse-magnetic polarized (guided) optical mode of a quantum-cascade laser (QCL) to the antisymmetric surface-plasmon modes of 2nd-order distributed-feedback (DFB) metal/semiconductor gratings results in strong antisymmetric-mode absorption. In turn, lasing in the symmetric mode, that is, surface emission in a single-lobe far-field beam pattern, is strongly favored over controllable ranges in grating duty cycle and tooth height. By using core-region characteristics of a published 4.6 μm-emitting QCL, grating-coupled surface-emitting (SE) QCLs are analyzed and optimized for highly efficient single-lobe operation. For infinite-length devices, it is found that when the antisymmetric mode is resonantly absorbed, the symmetric mode has negligible absorption loss (∼0.1 cm{sup −1}) while still being efficiently outcoupled, through the substrate, by the DFB grating. For finite-length devices, 2nd-order distributed Bragg reflector (DBR) gratings are used on both sides of the DFB grating to prevent uncontrolled reflections from cleaved facets. Equations for the threshold-current density and the differential quantum efficiency of SE DFB/DBR QCLs are derived. For 7 mm-long, 8.0 μm-wide, 4.6 μm-emitting devices, with an Ag/InP grating of ∼39% duty cycle, and ∼0.22 μm tooth height, threshold currents as low as 0.45 A are projected. Based on experimentally obtained internal efficiency values from high-performance QCLs, slope efficiencies as high as 3.4 W/A are projected; thus, offering a solution for watt-range, single-lobe CW operation from SE, mid-infrared QCLs.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
NASA NTRS1981

Multiwavelength monolithic integrated fiber optics terminal - An update

This paper serves as an update for the Multiwavelength Monolithic Integrated Fiber Optic Terminal (MMIFOT) being developed by MDAC-St. Louis for NASA's Johnson Space Center. The program objective is to utilize guided wave optical technology to develop a passive optical wavelength multiplexing subsystem with a single mode optical fiber serving as the transmission medium.

Spear-Zino, J. D.↗
DOE OSTI2016

Overlapping double potential wells in a single optical microtube cavity with vernier-scale-like tuning effect

Spatially and temporally overlapping double potential wells are realized in a hybrid optical microtube cavity due to the coexistence of an aggregate of luminescent quantum dots embedded in the tube wall and the cone-shaped tube's geometry. The double potential wells produce two independent sets of optical modes with different sets of mode numbers, indicating phase velocity separation for the modes overlapping at the same frequency. The overlapping mode position can be tuned by modifying the tube cavity, where these mode sets shift with different magnitudes, allowing for a vernier-scale-like tuning effect.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Thermal and Elastic Properties of Ce{sub x}Th{sub 1 –x}O{sub 2} Mixed Oxides: A Self-Consistent Thermodynamic Approach

A self-consistent thermodynamic model of thorium dioxide and its dilute alloy Ce{sub 0.05}Th{sub 0.95}O{sub 2}, which are of great importance for atomic power engineering, has been constructed based on generalized Debye and Einstein models. In terms of this model, the temperature dependences of the bulk modulus, density, and heat capacity of the systems have been obtained. The anharmonicity of the acoustical and optical modes of the crystal lattice and its change in passing from ThO{sub 2} to Ce{sub 0.05}Th{sub 0.95}O{sub 2} has been discussed.

36 MATERIALS SCIENCE↗
DOE OSTI2014

Si{sub 3}N{sub 4} optomechanical crystals in the resolved-sideband regime

We demonstrate sideband-resolved Si{sub 3}N{sub 4} optomechanical crystals supporting 10{sup 5} quality factor optical modes at 980 nm, coupled to ≈4 GHz frequency mechanical modes with quality factors of ≈3000. Optomechanical electromagnetically induced transparency and absorption are observed at room temperature and in atmosphere with intracavity photon numbers in excess of 10{sup 4}.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2015

Exploring the effective photon management by InP nanoparticles: Broadband light absorption enhancement of InP/In{sub 0.53}Ga{sub 0.47}As/InP thin-film photodetectors

High-index dielectric and semiconductor nanoparticles with the characteristics of low absorption loss and strong scattering have attracted more and more attention for improving performance of thin-film photovoltaic devices. In this paper, we focus our attention on InP nanoparticles and study the influence of the substrate and the geometrical configurations on their scattering properties. We demonstrate that, compared with the InP sphere, the InP cylinder has higher coupling efficiency due to the stronger interactions between the optical mode in the nanoparticle and its induced mirror image in the substrate. Moreover, we propose novel thin-film InGaAs photodetectors integrated with the periodically arranged InP nanoparticles on the substrate. Broadband light absorption enhancement is achieved over the wavelength range between 1.0 μm and 1.7 μm. The highest average absorption enhancement of 59.7% is realized for the photodetector with the optimized cylinder InP nanoparticles. These outstanding characteristics attribute to the preferentially forward scattering of single InP nanoparticle along with the effective coupling of incident light into the guided modes through the collective diffraction effect of InP nanoparticles array.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Radiation from laser-microplasma-waveguide interactions in the ultra-intense regime

When a high-contrast ultra-relativistic (>10{sup 20} W/cm{sup 2}) laser beam enters a micro-sized plasma waveguide, the pulse energy is coupled into waveguide modes, which significantly modifies the interaction between the electrons and electromagnetic wave. Electrons pulled out from the walls of the waveguide form a dense helical bunch inside the channel and are efficiently accelerated by the transverse magnetic modes to hundreds of MeV. The asymmetry in the transverse electric and magnetic fields drives strong oscillations, which lead to the emission of bright, well-collimated, hard X-rays. In this paper, we present our study on the underlying physics in the aforementioned process using 3D particle-in-cell simulations. The mechanism of electron acceleration and the dependence of radiation properties on different laser plasma parameters are addressed. An analytic model and basic scalings for X-ray emission are also presented by considering the lowest optical modes in the waveguide, which is adequate to describe the basic phenomenon. In addition, the effects of high-order modes as well as laser polarization are also qualitatively discussed. The considered X-ray source has promising features, potentially making it a competitive candidate for a future tabletop synchrotron source.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗