290 research outputs found
Ramsey-like measurement of the decoherence rate between Zeeman sub-levels
Two-photon processes that involve different sub-levels of the ground state of
an atom, are highly sensitive to depopulation and decoherence within the ground
state. For example, the spectral width of electromagnetically induced
transparency resonances in type system, are strongly affected by the
ground state depopulation and decoherence rates. We present a direct
measurement of decay rates between hyperfine and Zeeman sub-levels in the
ground state of Rb vapor. Similar to the relaxation-in-the-dark
technique, pumping lasers are used to pre-align the atomic vapor in a well
defined quantum state. The free propagation of the atomic state is monitored
using a Ramsey-like method. Coherence times in the range 1-10 ms were measured
for room temperature atomic vapor. In the range of the experimental parameters
used in this study, the dominant process inducing Zeeman decoherence is the
spin-exchange collisions between rubidium atoms.Comment: 7 pages, 7 figure
Modulation of Negative Index Metamaterials in the Near-IR Range
Optical modulation of the effective refractive properties of a "fishnet"
metamaterial with a Ag/Si/Ag heterostructure is demonstrated in the near-IR
range and the associated fast dynamics of negative refractive index is studied
by pump-probe method. Photo excitation of the amorphous Si layer at visible
wavelength and corresponding modification of its optical parameters is found to
be responsible for the observed modulation of negative refractive index in
near-IR.Comment: 11 figures, 4 figure
Directional excitation without breaking reciprocity
We propose a mechanism for directional excitation without breaking reciprocity. This is achieved by embedding an impedance matched parity-time symmetric potential in a three-port system. The amplitude distribution within the gain and loss regions is strongly influenced by the direction of the incoming field. Consequently, the excitation of the third port is contingent on the direction of incidence while transmission in the main channel is immune. Our design improves the four-port directional coupler scheme, as there is no need to implement an anechoic termination to one of the ports
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Computerized adaptive testing and short form development for child and adolescent oral health patient-reported outcomes measurement.
ObjectivesTo develop computerized adaptive testing (CAT) and short forms of self-report oral health measures that are predictive of both the children's oral health status index (COHSI) and the children's oral health referral recommendation (COHRR) scales, for children and adolescents, ages 8-17.Material and methodsUsing final item calibration parameters (discrimination and difficulty parameters) from the item response theory analysis, we performed post hoc CAT simulation. Items most frequently administered in the simulation were incorporated for possible inclusion in final oral health assessment toolkits, to select the best performing eight items for COHSI and COHRR.ResultsTwo previously identified unidimensional sets of self-report items consisting of 19 items for the COHSI and 22 items for the COHRR were administered through CAT resulting in eight-item short forms for both the COHSI and COHRR. Correlations between the simulated CAT scores and the full item bank representing the latent trait are r = .94 for COHSI and r = .96 for COHRR, respectively, which demonstrated high reliability of the CAT and short form.ConclusionsUsing established rigorous measurement development standards, the CAT and corresponding eight-item short form items for COHSI and COHRR were developed to assess the oral health status of children and adolescents, ages 8-17. These measures demonstrated good psychometric properties and can have clinical utility in oral health screening and evaluation and clinical referral recommendations
Surface Structure of Protonated R-Sapphire (11̅02) Studied by Sum-Frequency Vibrational Spectroscopy
Sum frequency vibrational spectroscopy was used to study the protonated R-plane (1{bar 1}02 ) sapphire surface. The OH stretch vibrational spectra show that the surface is terminated with three hydroxyl moieties, two from AlOH{sub 2} and one from Al{sub 2}OH functional groups. The observed polarization dependence allows determination of the orientations of the three OH species. The results suggest that the protonated sapphire (1{bar 1}02 ) surface differs from an ideal stoichimetric termination in a manner consistent with previous X-ray surface diffraction (crystal truncation rod) studies. However, in order to best explain the observed hydrogenbonding arrangement, surface oxygen spacing determined from the X-ray diffraction study requires modification
Development of toolkits for detecting dental caries and caries experience among children using self-report and parent report.
ObjectivesTo develop child- and parent-reported toolkits for active caries and caries experience in children and adolescents, ages 8-17.MethodsA sample of 398 child/parent dyads recruited from 12 dental practices in Los Angeles County completed a computer-assisted survey that assessed oral health perceptions. In addition, children received a dental examination that identified the presence or absence of active caries and caries experience. A Multiple Adaptive Regression Splines model was used to identify a subset of survey items associated with active caries and caries experience. The splines and coefficients were refined by generalized cross-validation. Sensitivity and specificity for both dependent variables were evaluated.ResultsEleven child self-reported items were identified that had sensitivity of 0.82 and specificity of 0.45 relative to active caries. Twelve parent-reported items had a sensitivity of 0.86 and specificity of 0.50. Seven child self-reported items had a sensitivity of 0.86 and specificity of 0.34, and 11 parent-reported items had a sensitivity of 0.86 and specificity of 0.47 for caries experience.ConclusionsThe survey items identified here are useful in distinguishing children with and without active caries and with and without caries experience. This research presents a path towards using children's and their parents' reports about oral health to screen for clinically determined caries and caries exposure. The items identified in this study can be useful when clinical information is unavailable
Drude Conductivity of Dirac Fermions in Graphene
Electrons moving in graphene behave as massless Dirac fermions, and they
exhibit fascinating low-frequency electrical transport phenomena. Their dynamic
response, however, is little known at frequencies above one terahertz (THz).
Such knowledge is important not only for a deeper understanding of the Dirac
electron quantum transport, but also for graphene applications in ultrahigh
speed THz electronics and IR optoelectronics. In this paper, we report the
first measurement of high-frequency conductivity of graphene from THz to mid-IR
at different carrier concentrations. The conductivity exhibits Drude-like
frequency dependence and increases dramatically at THz frequencies, but its
absolute strength is substantially lower than theoretical predictions. This
anomalous reduction of free electron oscillator strength is corroborated by
corresponding changes in graphene interband transitions, as required by the sum
rule. Our surprising observation indicates that many-body effects and Dirac
fermion-impurity interactions beyond current transport theories are important
for Dirac fermion electrical response in graphene
Electrical Control of Plasmon Resonance with Graphene
Surface plasmon, with its unique capability to concentrate light into
sub-wavelength volume, has enabled great advances in photon science, ranging
from nano-antenna and single-molecule Raman scattering to plasmonic waveguide
and metamaterials. In many applications it is desirable to control the surface
plasmon resonance in situ with electric field. Graphene, with its unique
tunable optical properties, provides an ideal material to integrate with
nanometallic structures for realizing such control. Here we demonstrate
effective modulation of the plasmon resonance in a model system composed of
hybrid graphene-gold nanorod structure. Upon electrical gating the strong
optical transitions in graphene can be switched on and off, which leads to
significant modulation of both the resonance frequency and quality factor of
plasmon resonance in gold nanorods. Hybrid graphene-nanometallic structures, as
exemplified by this combination of graphene and gold nanorod, provide a general
and powerful way for electrical control of plasmon resonances. It holds promise
for novel active optical devices and plasmonic circuits at the deep
subwavelength scale
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