563 research outputs found
Fabrication of mirror templates in silica with micron-sized radii of curvature
We present the fabrication of exceptionally small-radius concave microoptics
on fused silica substrates using CO2 laser ablation and subsequent reactive ion
etching. The protocol yields on-axis near-Gaussian depressions with radius of
curvature microns at shallow depth and low surface roughness of 2
angstroms. This geometry is appealing for cavity quantum electrodynamics where
small mode volumes and low scattering losses are desired. We study the optical
performance of the structure within a tunable Fabry-Perot type microcavity,
demonstrate near-coating-limited loss rates (F = 25,000) and small focal
lengths consistent with their geometrical dimensions.Comment: 5 pages, 4 figure
Spin-Polarized Electrons in Monolayer MoS
The optical susceptibility is a local, minimally-invasive and spin-selective
probe of the ground state of a two-dimensional electron gas. We apply this
probe to a gated monolayer of MoS. We demonstrate that the electrons are
spin polarized. Of the four available bands, only two are occupied. These two
bands have the same spin but different valley quantum numbers. We argue that
strong Coulomb interactions are a key aspect of this spontaneous symmetry
breaking. The Bohr radius is so small that even electrons located far apart in
phase space interact, facilitating exchange couplings to align the spins
A catchment scale assessment of patterns and controls of historic 2D river planform adjustment
The supply, transfer and deposition of sediment from channel headwaters to lowland sinks, is a fundamental process governing upland catchment geomorphology, and can begin to be understood by quantifying 2D river planform adjustments over time. This paper presents a catchment scale methodology to quantify historic patterns of 2D channel planform adjustment and considers geomorphic controls on 2D river stability. The methodology is applied to 18 rivers (total length = 24 km) in the upland headwaters of the previously glaciated Wasdale catchment (45 km2), Lake District, northwest England. Planform adjustments were mapped from historic maps and air photographs over six contiguous time windows covering the last 150 yr. A total of 1048 adjustment and stable reaches were mapped. Over the full period of analysis (1860–2010) 32% (8 km) of the channels studied were adjusting. Contrasts were identified between the geomorphic characteristics (slope, catchment area, unit specific stream power, channel width and valley bottom width) of adjusting and stable reaches. The majority of adjustments mapped were observed in third and fourth order channels in the floodplain valley transfer zone, where the channels were laterally unconfined (mean valley bottom widths of 230 ± 180 m), with low sediment continuity. In contrast, lower order channels were typically confined (mean valley bottom widths of 31 ± 43 m) and showed relative 2D lateral stability. Hence, valley bottom width was found to be important in determining the available space for rivers to adjust. Over the full period of analysis 38% of planform adjustments involved combined processes, for example, as bar and bend adjustments. The study demonstrates the importance of stream network hierarchy in determining spatial patterns of historic planform adjustments at the catchment scale. The methodology developed provides a quantitative assessment of planform adjustment patterns and geomorphic controls, which is needed to support the prioritisation of future river management and restoration
Deterministic enhancement of coherent photon generation from a nitrogen-vacancy center in ultrapure diamond
The nitrogen-vacancy (NV) center in diamond has an optically addressable,
highly coherent spin. However, an NV center even in high quality
single-crystalline material is a very poor source of single photons: extraction
out of the high-index diamond is inefficient, the emission of coherent photons
represents just a few per cent of the total emission, and the decay time is
large. In principle, all three problems can be addressed with a resonant
microcavity. In practice, it has proved difficult to implement this concept:
photonic engineering hinges on nano-fabrication yet it is notoriously difficult
to process diamond without degrading the NV centers. We present here a
microcavity scheme which uses minimally processed diamond, thereby preserving
the high quality of the starting material, and a tunable microcavity platform.
We demonstrate a clear change in the lifetime for multiple individual NV
centers on tuning both the cavity frequency and anti-node position, a Purcell
effect. The overall Purcell factor translates to a Purcell
factor for the zero phonon line (ZPL) of and an
increase in the ZPL emission probability from to . By
making a step-change in the NV's optical properties in a deterministic way,
these results pave the way for much enhanced spin-photon and spin-spin
entanglement rates.Comment: 6 pages, 4 figure
Voltage-Controlled Optics of a Quantum Dot
We show how the optical properties of a single semiconductor quantum dot can
be controlled with a small dc voltage applied to a gate electrode. We find that
the transmission spectrum of the neutral exciton exhibits two narrow lines with
eV linewidth. The splitting into two linearly polarized
components arises through an exchange interaction within the exciton. The
exchange interaction can be turned off by choosing a gate voltage where the dot
is occupied with an additional electron. Saturation spectroscopy demonstrates
that the neutral exciton behaves as a two-level system. Our experiments show
that the remaining problem for manipulating excitonic quantum states in this
system is spectral fluctuation on a eV energy scale.Comment: 4 pages, 4 figures; content as publishe
Modeling complex flow structures and drag around a submerged plant of varied posture
Although vegetation is present in many rivers, the bulk of past work concerned with modeling the influence of vegetation on flow has considered vegetation to be morphologically simple and has generally neglected the complexity of natural plants. Here we report on a combined flume and numerical model experiment which incorporates time-averaged plant posture, collected through terrestrial laser scanning, into a computational fluid dynamics model to predict flow around a submerged riparian plant. For three depth-limited flow conditions (Reynolds number = 65,000–110,000), plant dynamics were recorded through high-definition video imagery, and the numerical model was validated against flow velocities collected with an acoustic Doppler velocimeter. The plant morphology shows an 18% reduction in plant height and a 14% increase in plant length, compressing and reducing the volumetric canopy morphology as the Reynolds number increases. Plant shear layer turbulence is dominated by Kelvin-Helmholtz type vortices generated through shear instability, the frequency of which is estimated to be between 0.20 and 0.30 Hz, increasing with Reynolds number. These results demonstrate the significant effect that the complex morphology of natural plants has on in-stream drag, and allow a physically determined, species-dependent drag coefficient to be calculated. Given the importance of vegetation in river corridor management, the approach developed here demonstrates the necessity to account for plant motion when calculating vegetative resistance
Transform-limited single photons from a single quantum dot
A semiconductor quantum dot mimics a two-level atom. Performance as a single
photon source is limited by decoherence and dephasing of the optical
transition. Even with high quality material at low temperature, the optical
linewidths are a factor of two larger than the transform-limit. A major
contributor to the inhomogeneous linewdith is the nuclear spin noise. We show
here that the nuclear spin noise depends on optical excitation, increasing
(decreasing) with increasing resonant laser power for the neutral (charged)
exciton. Based on this observation, we discover regimes where we demonstrate
transform-limited linewidths on both neutral and charged excitons even when the
measurement is performed very slowly
Quantum confined Stark effect in a MoS monolayer van der Waals heterostructure
The optics of dangling-bond-free van der Waals heterostructures containing
transition metal dichalcogenides are dominated by excitons. A crucial property
of a confined exciton is the quantum confined Stark effect (QCSE). Here, such a
heterostructure is used to probe the QCSE by applying a uniform vertical
electric field across a molybdenum disulfide (MoS) monolayer. The
photoluminescence emission energies of the neutral and charged excitons shift
quadratically with the applied electric field provided the electron density
remains constant, demonstrating that the exciton can be polarized. Stark shifts
corresponding to about half the homogeneous linewidth were achieved. Neutral
and charged exciton polarizabilities of (7.8~\pm~1.0)\times
10^{-10}~\tr{D~m~V}^{-1} and (6.4~\pm~0.9)\times 10^{-10}~\tr{D~m~V}^{-1} at
relatively low electron density (8 \times 10^{11}~\tr{cm}^{-2}) have been
extracted, respectively. These values are one order of magnitude lower than the
previously reported values, but in line with theoretical calculations. The
methodology presented here is versatile and can be applied to other
semiconducting layered materials as well
Simple atomic quantum memory suitable for semiconductor quantum dot single photons
Quantum memories matched to single photon sources will form an important
cornerstone of future quantum network technology. We demonstrate such a memory
in warm Rb vapor with on-demand storage and retrieval, based on
electromagnetically induced transparency. With an acceptance bandwidth of
= 0.66~GHz the memory is suitable for single photons emitted by
semiconductor quantum dots. In this regime, vapor cell memories offer an
excellent compromise between storage efficiency, storage time, noise level, and
experimental complexity, and atomic collisions have negligible influence on the
optical coherences. Operation of the memory is demonstrated using attenuated
laser pulses on the single photon level. For 50 ns storage time we measure
\emph{end-to-end efficiency}
of the fiber-coupled memory, with an \emph{total intrinsic efficiency}
. Straightforward technological improvements can
boost the end-to-end-efficiency to ; beyond
that increasing the optical depth and exploiting the Zeeman substructure of the
atoms will allow such a memory to approach near unity efficiency.
In the present memory, the unconditional readout noise level of photons is dominated by atomic fluorescence, and for input pulses
containing on average photons the signal to noise level would
be unity
Cavity-enhanced Raman scattering for in situ alignment and characterization of solid-state microcavities
We report cavity-enhanced Raman scattering from a single-crystal diamond
membrane embedded in a highly miniaturized fully-tunable Fabry-P\'{e}rot
cavity. The Raman intensity is enhanced 58.8-fold compared to the corresponding
confocal measurement. The strong signal amplification results from the Purcell
effect. We show that the cavity-enhanced Raman scattering can be harnessed as a
narrowband, high-intensity, internal light-source. The Raman process can be
triggered in a simple way by using an optical excitation frequency outside the
cavity stopband and is independent of the lateral positioning of the cavity
mode with respect to the diamond membrane. The strong Raman signal emerging
from the cavity output facilitates in situ mode-matching of the cavity mode to
single-mode collection optics; it also represents a simple way of measuring the
dispersion and spatial intensity-profile of the cavity modes. The optimization
of the cavity performance via the strong Raman process is extremely helpful in
achieving efficient cavity-outcoupling of the relatively weak emission of
single color-centers such as nitrogen-vacancy centers in diamond or rare-earth
ions in crystalline hosts with low emitter density
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