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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 1,729 records · Page 96
All sourcesNASA NTRSDOE OSTI
DOE OSTI2015

Antiferromagentic resonance detected by direct current voltages in MnF{sub 2}/Pt bilayers

We performed coplanar waveguide-based broadband ferromagnetic resonance experiments on the antiferromagnetic insulator MnF{sub 2}, while simultaneously recording the DC voltage arising in a thin platinum film deposited onto the MnF{sub 2}. The antiferromagnetic resonance is clearly reflected in both the transmission through the waveguide as well as the DC voltage in the Pt strip. The DC voltage remains largely unaffected by field reversal and thus presumably stems from microwave rectification and/or heating effects. However, we identify a small magnetic field orientation dependent contribution, compatible with antiferromagnetic spin pumping theory.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Design and measurement of a TE{sub 13} input converter for high order mode gyrotron travelling wave amplifiers

A technique to launch a circular TE{sub 13} mode to interact with the helical electron beam of a gyrotron travelling wave amplifier is proposed and verified by simulation and cold test in this paper. The high order (HOM) TE{sub 13} mode is excited by a broadband Y-type power divider with the aid of a cylindrical waveguide system. Using grooves and convex strips loaded at the lateral planes of the output cylindrical waveguide, the electric fields of the potential competing TE{sub 32} and TE{sub 71} modes are suppressed to allow the transmission of the dominant TE{sub 13} mode. The converter performance for different structural dimensions of grooves and convex strips is studied in detail and excellent results have been achieved. Simulation predicts that the average transmission is ∼−1.8 dB with a 3 dB bandwidth of 7.2 GHz (91.5–98.7 GHz) and port reflection is less than −15 dB. The conversion efficiency to the TE{sub 32} and TE{sub 71} modes are, respectively, under −15 dB and −24 dB in the operating frequency band. Such an HOM converter operating at W-band has been fabricated and cold tested with the radiation boundary. Measurement from the vector network analyzer cold test and microwave simulations show a good reflection performance for the converter.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2016

Long wavelength stimulated emission up to 9.5 μm from HgCdTe quantum well heterostructures

Stimulated emission from waveguide HgCdTe structures with several quantum wells inside waveguide core is demonstrated at wavelengths up to 9.5 μm. Photoluminescence line narrowing down to kT energy, as well as superlinear rise in its intensity evidence the onset of the stimulated emission, which takes place under optical pumping with intensity as small as ∼0.1 kW/cm{sup 2} at 18 K and 1 kW/cm{sup 2} at 80 K. One can conclude that HgCdTe structures potential for long-wavelength lasers is not exhausted.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Ultra-small and broadband polarization splitters based on double-slit interference

An ultra-small and broadband polarization splitter is numerically and experimentally demonstrated based on the double-slit interference in a polymer-film-coated double-slit structure. The hybrid slab waveguide (air-polymer-Au) supports both the transverse-magnetic and transverse-electric modes. The incident beam from the back side can excite these two guided modes of orthogonally polarized states in the hybrid structure. By exploiting the difference slit widths and the large mode birefringence, these two guided modes propagate to the opposite directions along the front metal surface. Moreover, the short interference length broadens the operation bandwidth. Experimentally, a polarization splitter with a lateral dimension of only about 1.6 μm and an operation bandwidth of 50 nm is realized. By designing the double-slit structure in a hybrid strip waveguide, the device dimension can be significant downscaled to about 0.3 × 1.3 μm{sup 2}. Such an ultra-small and broadband polarization splitter may find important applications in the integrated photonic circuits.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Periodic domain inversion in x-cut single-crystal lithium niobate thin film

We report the fabrication of periodically poled domain patterns in x-cut lithium niobate thin-film. Here, thin films on insulator have drawn particular attention due to their intrinsic waveguiding properties offering high mode confinement and smaller devices compared to in-diffused waveguides in bulk material. In contrast to z-cut thin film lithium niobate, the x-cut geometry does not require back electrodes for poling. Further, the x-cut geometry grants direct access to the largest nonlinear and electro-optical tensor element, which overall promises smaller devices. The domain inversion was realized via electric field poling utilizing deposited aluminum top electrodes on a stack of LN thin film/SiO{sub 2} layer/Bulk LN, which were patterned by optical lithography. The periodic domain inversion was verified by non-invasive confocal second harmonic microscopy. Our results show domain patterns in accordance to the electrode mask layout. The second harmonic signatures can be interpreted in terms of spatially, overlapping domain filaments which start their growth on the +z side.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Low absorption loss p-AlGaN superlattice cladding layer for current-injection deep ultraviolet laser diodes

Current injection into AlGaN-based laser diode structures with high aluminum mole fractions for deep ultraviolet emission is investigated. The electrical characteristics of laser diode structures with different p-AlGaN short period superlattice (SPSL) cladding layers with various aluminum mole fractions are compared. The heterostructures contain all elements that are needed for a current-injection laser diode including cladding and waveguide layers as well as an AlGaN quantum well active region emitting near 270 nm. We found that with increasing aluminum content in the p-AlGaN cladding, the diode turn-on voltage increases, while the series resistance slightly decreases. By introducing an SPSL instead of bulk layers, the operating voltage is significantly reduced. A gain guided broad area laser diode structure with transparent p-Al{sub 0.70}Ga{sub 0.30}N waveguide layers and a transparent p-cladding with an average aluminum content of 81% was designed for strong confinement of the transverse optical mode and low optical losses. Using an optimized SPSL, this diode could sustain current densities of more than 4.5 kA/cm{sup 2}.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

On-chip generation and control of the vortex beam

A method to generate and control the amplitude and phase distributions of an optical vortex beam is proposed. By introducing a holographic grating on the top of a dielectric waveguide, the free space vortex beam and the in-plane guiding wave can be converted to each other. This microscale holographic grating is very robust against the variation of geometry parameters. The designed vortex beam generator can produce the target beam with a fidelity up to 0.93, and the working bandwidth is about 175 nm with the fidelity larger than 0.80. In addition, a multiple generator composed of two holographic gratings on two parallel waveguides is studied, which can perform an effective and flexible modulation on the vortex beam by controlling the phase of the input light. Our work opens an available avenue towards the integrated orbital angular momentum devices with multiple degrees of optical freedom, which can be used for optical tweezers, micronano imaging, information processing, and so on.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Controlling slow and fast light and dynamic pulse-splitting with tunable optical gain in a whispering-gallery-mode microcavity

We report controllable manipulation of slow and fast light in a whispering-gallery-mode microtoroid resonator fabricated from Erbium (Er{sup 3+}) doped silica. We observe continuous transition of the coupling between the fiber-taper waveguide and the microresonator from undercoupling to critical coupling and then to overcoupling regimes by increasing the pump power even though the spatial distance between the resonator and the waveguide was kept fixed. This, in turn, enables switching from fast to slow light and vice versa just by increasing the optical gain. An enhancement of delay of two-fold over the passive silica resonator (no optical gain) was observed in the slow light regime. Moreover, we show dynamic pulse splitting and its control in slow/fast light systems using optical gain.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2016

Bias-free spin-wave phase shifter for magnonic logic

A design of a magnonic phase shifter operating without an external bias magnetic field is proposed. The phase shifter uses a localized collective spin wave mode propagating along a domain wall “waveguide” in a dipolarly-coupled magnetic dot array with a chessboard antiferromagnetic (CAFM) ground state. It is demonstrated numerically that the remagnetization of a single magnetic dot adjacent to the domain wall waveguide introduces a controllable phase shift in the propagating spin wave mode without significant change to the mode amplitude. It is also demonstrated that a logic XOR gate can be realized in the same system.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
DOE OSTI2016

Self-consistent non-stationary theory of the gyrotron

For a long time, the gyrotron theory was developed assuming that the transit time of electrons through the interaction space is much shorter than the cavity fill time. Correspondingly, it was assumed that during this transit time, the amplitude of microwave oscillations remains constant. A recent interest to such additional effects as the after-cavity interaction between electrons and the outgoing wave in the output waveguide had stimulated some studies of the beam-wave interaction processes over much longer distances than a regular part of the waveguide which serves as a cavity in gyrotrons. Correspondingly, it turned out that the gyrotron theory free from the assumption about constant amplitude of microwave oscillations during the electron transit time should be developed. The present paper contains some results obtained in the framework of such theory. The main attention is paid to modification of the boundary between the regions of oscillations with constant amplitude and automodulation in the plane of normalized parameters characterizing the external magnetic field and the beam current. It is shown that the theory free from the assumption about the frozen wave amplitude during the electron transit time predicts some widening of the region of automodulation.

70 PLASMA PHYSICS AND FUSION TECHNOLOGY↗
DOE OSTI2021

Thin THz QCL active regions for improved continuous-wave operating temperature

We compare the performance of 10 and 5 μm thick metal–metal waveguide terahertz quantum-cascade laser ridges operating around 2.7 THz and based on a 4-well phonon depopulation active region design. Thanks to reduced heat dissipation and lower thermal resistance, the 5 μm thick material shows an 18 K increase in continuous wave operating temperature compared to the 10 μm material, despite a lower maximum pulsed-mode operating temperature and a larger input power density. A maximum continuous wave operating temperature of 129 K is achieved using the 5 μm thick material and a 15 μm wide ridge waveguide, which lased up to 155 K in the pulsed mode. The use of thin active regions is likely to become increasingly important to address the increasing input power density of emerging 2- and 3-well active region designs that show the highest pulsed operating temperatures.

47 OTHER INSTRUMENTATION↗
DOE OSTI2013

Fabrication-tolerant integrated polarisation splitter based on cascaded Mach–Zehnder interferometers

We report a fabrication-tolerant polarisation splitter based on cascaded Mach – Zehnder interferometers. This configuration enables a factor of 2 – 3 increase (at the 20 dB level) in the tolerance to the phase difference in comparison with a single interferometer. As an example, we present numerical simulation of a splitter with a centre wavelength of 650 nm, based on a planar waveguide from Si{sub 3}N{sub 4}. The permissible channel waveguide width deviation from calculation results (20-dB extinction coefficient bandwidth) is ∼8% (∼30 nm) for the TE polarisation and ∼30% (100 nm) for the TM polarisation. (fiber and integrated optics)

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2017

Regularities of femtosecond filamentation in the case of superposition of Gaussian and annular laser beams

The self-action of high-power femtosecond laser beams with a radiation wavelength of 800 nm (formed by superposition of Gaussian and annular beams) in air is theoretically modelled. A detailed analysis of self-focusing and filamentation of this radiation is performed based on the numerical solution of the spectral equation of the unidirectional propagation of a wave packet with allowance for the nonlinearity of the medium, and by tracing averaged diffraction-optical rays. It is shown that, in terms of the diffraction-beam optics, the outer annulus forms a waveguide, which facilitates self-trapping of the central part of the combined beam, supplying the light energy to the filament. The spatial stability of this waveguide depends on the energy stored in the annulus and on the distance from the latter to the beam axis.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2017

Optimisation of spontaneous four-wave mixing in a ring microcavity

A theory of spontaneous four-wave mixing in a ring microcavity is developed. The rate of emission of biphotons for pulsed and monochromatic pumping with allowance for the dispersion of group velocities is analytically calculated. In the first case, pulses in the form of an increasing exponential are considered, which are optimal for excitation of an individual resonator mode. The behaviour of the group velocity dispersion as a function of the width and height of the waveguide is studied for a specific case of a ring microcavity made of silicon nitride. The results of the numerical calculation are in good agreement with the experimental data. The ring microcavity is made of two types of waveguides: completely etched and half etched. It is found that the latter allow for better control over the parameters in the manufacturing process, making them more predictable. (nonlinear optical phenomena)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2018

Amplified luminescence of heavily doped Al{sub x}Ga{sub 1 – x}N structures under optical pumping

Spectral, temporal, and polarisation characteristics of luminescence of heavily doped Al{sub x}Ga{sub 1 – x}N films on a sapphire substrate are studied under pulsed pumping at the wavelength λ = 266 m. Spectra of spontaneous emission related to donor – acceptor transitions are inhomogeneously broadened with the FWHM of above 0.5 eV and cover the entire visible range. Spectra of radiation emitted from an edge of investigated structure comprise several narrow-band equidistant components, each of them being split to TE and TM modes with mutually orthogonal polarisations. This is related to plane waves propagating inside a plane waveguide along a zigzag path in the conditions of total internal reflection from waveguide surfaces. The optical gains measured for Al{sub 0.5}Ga{sub 0.5}N/AlN at λ ≈ 510 nm, Al{sub 0.74}Ga{sub 0.26}N/AlN at λ ≈ 468 nm, and AlN/Al{sub 0.6}Ga{sub 0.4}N/AlN/Al{sub 2}O{sub 3} at λ ≈ 480 nm were, respectively, ∼70, 20, and 44 cm{sup −1}. The luminescence quantum efficiencies measured for Al{sub 0.74}Ga{sub 0.26}N, Al{sub 0.65}Ga{sub 0.35}N, and Al{sub 0.5}Ga{sub 0.5}N films are, respectively, 0.79, 0.49, and 0.14; the transition cross sections calculated at emission band centres are ∼10{sup −18} cm{sup 2}. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2019

Fluorescence and lasing in an electric-field-induced periodic structure of a cholesteric liquid crystal

Fluorescence and lasing in a structure with an electric-field-induced spatially periodic director modulation in the plane of a planar-oriented layer of a cholesteric liquid crystal (CLC) is experimentally investigated. The thickness of the CLC layer is chosen close to the natural pitch of the cholesteric helix, which corresponds to the second Grandjean zone. The electric-field-induced periodic field of the CLC director leads to spatial modulation of the refractive index and the appearance of optical properties typical of 1D photonic crystals, when light propagating in the plane of the CLC layer experiences Bragg reflection. In polarised light, the induced spatial modulation of the refractive index manifests itself in the form of banded domains oriented in the plane of the CLC layer perpendicular to the original (unperturbed electric field) direction of the director in the centre of the layer. For the electric field strengths that correspond to fundamentally different distributions of the field of a CLC director, the fluorescence spectra of the DCM laser dye are studied both for different geometries (including the waveguide regime) and for different levels of optical pumping. In the range of electrical voltages corresponding to the induction of a spatially periodic photonic structure, multimode lasing in the waveguide regime is detected. It is shown that the mode composition of lasing depends on the electric field, which affects the properties of the distributed feedback. (paper)

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2014

An interpretation of flare-induced and decayless coronal-loop oscillations as interference patterns

We present an alternative model of coronal-loop oscillations, which considers that the waves are trapped in a two-dimensional waveguide formed by the entire arcade of field lines. This differs from the standard one-dimensional model which treats the waves as the resonant oscillations of just the visible bundle of field lines. Within the framework of our two-dimensional model, the two types of oscillations that have been observationally identified, flare-induced waves and 'decayless' oscillations, can both be attributed to MHD fast waves. The two components of the signal differ only because of the duration and spatial extent of the source that creates them. The flare-induced waves are generated by strong localized sources of short duration, while the decayless background can be excited by a continuous, stochastic source. Further, the oscillatory signal arising from a localized, short-duration source can be interpreted as a pattern of interference fringes produced by waves that have traveled diverse routes of various pathlengths through the waveguide. The resulting amplitude of the fringes slowly decays in time with an inverse square root dependence. The details of the interference pattern depend on the shape of the arcade and the spatial variation of the Alfvén speed. The rapid decay of this wave component, which has previously been attributed to physical damping mechanisms that remove energy from resonant oscillations, occurs as a natural consequence of the interference process without the need for local dissipation.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Low-loss millimeter-wave resonators with an improved coupling structure

Millimeter-wave superconducting resonators are a useful tool for studying quantum device coherence in a new frequency domain. However, improving resonators is difficult without a robust and reliable method for coupling millimeter-wave signals to 2D structures. We develop and characterize a tapered transition structure coupling a rectangular waveguide to a planar slotline waveguide with better than 0.5 dB efficiency over 14 GHz, and use it to measure ground-shielded resonators in the W band (75–110 GHz). Having decoupled the resonators from radiative losses, we consistently achieve single-photon quality factors above 10 5 , with a two-level-system loss limit above 10 6 , and verify the effectiveness of oxide removal treatments to reduce loss. These values are 4–5 times higher than those previously reported in the W band, and much closer to typical planar microwave resonators. The improved losses demonstrated by these on-chip millimeter-wave devices shed new light on quantum decoherence in a different frequency regime, offer increased selectivity for high-frequency detectors, and enables new possibilities for hybrid quantum experiments integrating millimeter-wave frequencies.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗