704 research outputs found
Hiding In Plain Sight
Since the first successful measurements of stellar trigonometric parallax in the 1830s, the study of nearby stars has focused on the highest proper motion stars (mu \u3e 0.18 /yr). Those high proper motion stars have formed the backbone of the last 150 years of study of the Solar Neighborhood and the composition of the Galaxy. Statistically speaking, though, there is a population of stars that will have low proper motions when their space motions have been projected onto the sky. At the same time, over the last twenty years, populations of relatively young stars (less than ~100 Myr), most of them with low proper motions, have been revealed near (\u3c100 \u3epc) the Sun. This dissertation is the result of two related projects: A photometric search for nearby (\u3c25 \u3epc) southern-hemisphere M dwarf stars with low proper motions (mu \u3c 0.18 /yr), and a search for nearby
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
Broadband nanodielectric spectroscopy by means of amplitude modulation electrostatic force microscopy (AM-EFM)
In this work we present a new AFM based approach to measure the local dielectric response of polymer films at the nanoscale by means of Amplitude Modulation Electrostatic Force Microscopy (AM-EFM). The proposed experimental method is based on the measurement of the tipâsample force via the detection of the second harmonic component of the photosensor signal by means of a lock-in amplifier. This approach allows reaching unprecedented broad frequency range (2â3Ă104 Hz) without restrictions on the sample environment. The method was tested on different poly(vinyl acetate) (PVAc) films at several temperatures. Simple analytical models for describing the electric tipâsample interaction semi-quantitatively account for the dependence of the measured local dielectric response on samples with different thicknesses and at several tipâsample distances
Nanodielectric mapping of a model polystyrene-poly(vinyl acetate) blend by electrostatic force microscopy
We present a simple method to quantitatively image the dielectric permittivity of soft materials at nanoscale using electrostatic force microscopy (EFM) by means of the double pass method. The EFM experiments are based on the measurement of the frequency shifts of the oscillating tip biased at two different voltages. A
numerical treatment based on the equivalent charge method allows extracting the values of the dielectric permittivity at each image point. This method can be applied with no restrictions of film thickness and tip radius. This method has been applied to image the morphology and the nanodielectric properties of a model
polymer blend of polystyrene and poly(vinyl acetate)
Nanoscale dielectric properties of insulating thin films: From single point measurements to quantitative images
Dielectric relaxation (DR) has shown to be a very useful technique to study dielectric materials like polymers and other glass formers, giving valuable information about the molecular dynamics of the system at different length and time scales. However, the standard DR techniques have a fundamental limitation: they have no spatial resolution. This is of course not a problem when homogeneous and non-structured systems are analyzed but it becomes an important limitation for studying the local properties of heterogeneous and/or nano-structured materials. To overcome this constrain we have developed a novel approach that allows quantitatively measuring the local dielectric permittivity of thin films at the nanoscale by means of Electrostatic Force Microscopy. The proposed experimental method is based on the detection of the local electric force gradient at different values of the tip-sample distance. The value of the dielectric permittivity is then calculated by fitting the experimental points using the Equivalent Charge Method. Even more interesting, we show how this approach can be extended in order to obtain quantitative dielectric images of insulating thin films with an excellent lateral resolution
Imaging dielectric relaxation in nanostructured polymers by frequency modulation electrostatic force microscopy
We have developed a method for imaging the temperature-frequency dependence of the dynamics of nanostructured polymer films with spatial resolution. This method provides images with dielectric compositional contrast well decoupled from topography. Using frequency-modulation electrostatic-force-microscopy, we probe the local frequency-dependent (0.1â100 Hz) dielectric response through measurement of the amplitude and phase of the force gradient in response to an oscillating applied electric field. When the phase is imaged at fixed frequency, it reveals the spatial variation in dielectric losses, i.e., the spatial variation in molecular/dipolar dynamics, with 40 nm lateral resolution. This is demonstrated by using as a model system; a phase separated polystyrene/polyvinyl-acetate (PVAc) blend. We show that nanoscale dynamic domains of PVAc are clearly identifiable in phase images as those which light-up in a band of temperature, reflecting the variations in the molecular/dipolar dynamics approaching the glass transition temperature of PVAc
The interoceptive hippocampus: mouse brain endocrine receptor expression highlights a dentate gyrus (DG)âcornu ammonis (CA) challengeâsufficiency axis
Funding: The authors received no specific funding for this work.Peer reviewedPublisher PD
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
Measuring dielectric properties at the nanoscale using Electrostatic Force Microscopy
Several electrostatic force microscopy (EFM) - based methods have been recently developed to study the nanoscale dielectric properties of thin insulating layers. Some methods allow measuring quantitatively the static dielectric permittivity whereas some others provide qualitative information about the temperature-frequency dependence of dielectric properties. In this chapter, all these methods are described and illustrated by experiments on pure and nanostructured polymer films. A section is dedicated to EFM probe - sample models and especially to the Equivalent Charge Method (ECM)
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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