65 research outputs found
Lasing in localized modes of a slow light photonic crystal waveguide
We demonstrate lasing in GaAs photonic crystal waveguides with InAs quantum
dots as gain medium. Structural disorder is present due to fabrication
imperfection and causes multiple scat- tering of light and localization of
light. Lasing modes with varying spatial extend are observed at random
locations along the guide. Lasing frequencies are determined by the local
structure and occur within a narrow frequency band which coincides with the
slow light regime of the waveguide mode. The three-dimensional numerical
simulation reveals that the main loss channel for lasing modes located away
from the waveguide end is out-of-plane scattering by structural disorder.Comment: 8 pages, 4 figure
Niobium superconducting nanowire single-photon detectors
We investigate the performance of superconducting nanowire photon detectors
fabricated from ultra-thin Nb. A direct comparison is made between these
detectors and similar nanowire detectors fabricated from NbN. We find that Nb
detectors are significantly more susceptible than NbN to thermal instability
(latching) at high bias. We show that the devices can be stabilized by reducing
the input resistance of the readout. Nb detectors optimized in this way are
shown to have approximately 2/3 the reset time of similar large-active-area NbN
detectors of the same geometry, with approximately 6% detection efficiency for
single photons at 470 nm
Reset dynamics and latching in niobium superconducting nanowire single-photon detectors
We study the reset dynamics of niobium (Nb) superconducting nanowire
single-photon detectors (SNSPDs) using experimental measurements and numerical
simulations. The numerical simulations of the detection dynamics agree well
with experimental measurements, using independently determined parameters in
the simulations. We find that if the photon-induced hotspot cools too slowly,
the device will latch into a dc resistive state. To avoid latching, the time
for the hotspot to cool must be short compared to the inductive time constant
that governs the resetting of the current in the device after hotspot
formation. From simulations of the energy relaxation process, we find that the
hotspot cooling time is determined primarily by the temperature-dependent
electron-phonon inelastic time. Latching prevents reset and precludes
subsequent photon detection. Fast resetting to the superconducting state is
therefore essential, and we demonstrate experimentally how this is achieved
Graphene Nano-Ribbon Electronics
We have fabricated graphene nano-ribbon field-effect transistor devices and
investigated their electrical properties as a function of ribbon width. Our
experiments show that the resistivity of a ribbon increases as its width
decreases, indicating the impact of edge states. Analysis of temperature
dependent measurements suggests a finite quantum confinement gap opening in
narrow ribbons. The electrical current noise of the graphene ribbon devices at
low frequency is found to be dominated by the 1/f noise.Comment: 6 pages, 7 figure
Curved grating fabrication techniques for concentric-circle grating, surface-emitting semiconductor lasers
We describe the fabrication and operational characteristics of a novel, surface-emitting semiconductor laser that makes use of a concentric-circle grating to both define its resonant cavity and to provide surface emission. A properly fabricated circular grating causes the laser to operate in radially inward- and outward-going circular waves in the waveguide, thus, introducing the circular symmetry needed for the laser to emit a beam with a circular cross-section. The basic circular-grating-resonator concept can be implemented in any materials system; an AlGaAs/GaAs graded-index, separate confinement heterostructure (GRINSCH), single-quantum-well (SQW) semiconductor laser, grown by molecular beam epitaxy (MBE), was used for the experiments discussed here. Each concentric-circle grating was fabricated on the surface of the AlGaAs/GaAs semiconductor laser. The circular pattern was first defined by electron-beam (e-beam) lithography in a layer of polymethylmethacrylate (PMMA) and subsequently etched into the semiconductor surface using chemically-assisted (chlorine) ion-beam etching (CAIBE). We consider issues that affect the fabrication and quality of the gratings. These issues include grating design requirements, data representation of the grating pattern, and e-beam scan method. We provide examples of how these techniques can be implemented and their impact on the resulting laser performance. A comparison is made of the results obtained using two fundamentally different electron-beam writing systems. Circular gratings with period lambda = 0.25 microns and overall diameters ranging from 80 microns to 500 microns were fabricated. We also report our successful demonstration of an optically pumped, concentric-circle grating, semiconductor laser that emits a beam with a far-field divergence angle that is less than one degree. The emission spectrum is quite narrow (less than 0.1 nm) and is centered at wavelength lambda = 0.8175 microns
Understanding of hydrogen silsesquioxane electron resist for sub-5-nm-half-pitch lithography
The authors, demonstrated that 4.5-nm-half-pitch structures could be achieved using electron-beam lithography, followed by salty development. They also hypothesized a development mechanism for hydrogen silsesquioxane, wherein screening of the resist surface charge is crucial in achieving a high initial development rate, which might be a more accurate assessment of developer performance than developer contrast. Finally, they showed that with a high-development-rate process, a short duration development of 15 s was sufficient to resolve high-resolution structures in 15-nm-thick resist, while a longer development degraded the quality of the structures with no improvement in the resolution
Contrast enhancement behavior of hydrogen silsesquioxane in a salty developer
The authors investigated a contrast enhancement behavior of hydrogen silsesquioxane (HSQ) in a salty development system (NaOH/NaCl). Time-resolved analysis of contrast curves and line-grating patterns were carried out to investigate the unique properties of a salty development process. In NaOH developer without salt, the development process was saturated beyond a certain development time. On the other hand, the addition of salt enabled a continuous development, which was not observed in the pure NaOH development. The continuous thinning process enhances the contrast of HSQ in the salty developer, which allows a fast collapsing behavior in HSQ line-grating patterns. During development process, salt seems to have the role of modifying HSQ by breaking network bonds preferentially, leading to a continuous development rate
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