29 research outputs found
Influence of the coherence of spectral domain interference of Fano resonance on the degree of polarization of light
We show an intriguing connection between the coherence of spectral domain
interference of two electromagnetic modes in Fano resonance and the resulting
degree of polarization of light. A theoretical treatment is developed by
combining a general electromagnetic model of partially coherent interference of
a spectrally narrow and a broad continuum mode leading to Fano resonance and
the cross-spectral density matrix of the interfering polarized fields of light.
The model suggests a characteristic variation of the degree of polarization
across the region of spectral dip and the peak of Fano resonance as an
exclusive signature of the connection between the degree of polarization and
the coherence of the interfering modes. The predictions of the model is
experimentally verified in the partially polarized Fano resonance spectra from
metal Chalcogenides systems, which emerged due to the interference of a narrow
excitonic mode with the background continuum of scattered light in the
reflectance spectra from the system. The demonstrated connection between
polarization and coherence in the spectral domain Fano-type interference of
electromagnetic modes is fundamentally important in the context of a broad
variety of non-trivial wave phenomena that originate from fine interference
effects, which may also have useful practical implications
Spatial variations of the SrI 4607\AA scattering polarization signals at subgranular scale observed with ZIMPOL at GREGOR telescope
Sr I 4607\AA spectral line shows one of the strongest scattering polarization
signals in the visible solar spectrum. The amplitudes of these signals are
expected to vary at granular spatial scales. This variation can be due to
changes in the magnetic field intensity and orientation (Hanle effect) as well
as due to spatial and temporal variations in the plasma properties. Measuring
the spatial variation of such polarization signal would allow us to study the
properties of the magnetic fields at subgranular region. But, the observations
are challenging since both high spatial resolution and high spectropolarimetric
sensitivity are required at the same time. To the aim of measuring these
spatial variations at granular scale, we carried out a spectro-polarimetric
measurement with the Zurich IMaging POLarimeter (ZIMPOL), at the GREGOR solar
telescope at different limb distances on solar disk. Our results show a spatial
variation of scattering linear polarization signals in Sr I 4607\AA line at the
granular scale at every , starting from 0.2 to 0.8. The correlation
between the polarization signal amplitude and the continuum intensity imply
statistically that the scattering polarization is higher at the granular
regions than in the intergranular lanes.Comment: 4 pages, 3 figures, Proceeding of Third Meeting of the Italian Solar
and Heliospheric Community, OCTOBER 28-31, 2018 - TURI
Probing thermal expansion of graphene and modal dispersion at low-temperature using graphene NEMS resonators
We use suspended graphene electromechanical resonators to study the variation
of resonant frequency as a function of temperature. Measuring the change in
frequency resulting from a change in tension, from 300 K to 30 K, allows us to
extract information about the thermal expansion of monolayer graphene as a
function of temperature, which is critical for strain engineering applications.
We find that thermal expansion of graphene is negative for all temperatures
between 300K and 30K. We also study the dispersion, the variation of resonant
frequency with DC gate voltage, of the electromechanical modes and find
considerable tunability of resonant frequency, desirable for applications like
mass sensing and RF signal processing at room temperature. With lowering of
temperature, we find that the positively dispersing electromechanical modes
evolve to negatively dispersing ones. We quantitatively explain this crossover
and discuss optimal electromechanical properties that are desirable for
temperature compensated sensors.Comment: For supplementary information and high resolution figures please go
to http://www.tifr.res.in/~deshmukh/publication.htm
Carrier Transport in High Mobility InAs Nanowire Junctionless Transistors
Ability to understand and model the performance limits of nanowire
transistors is the key to design of next generation devices. Here, we report
studies on high-mobility junction-less gate-all-around nanowire field effect
transistor with carrier mobility reaching 2000 cm2/V.s at room temperature.
Temperature-dependent transport measurements reveal activated transport at low
temperatures due to surface donors, while at room temperature the transport
shows a diffusive behavior. From the conductivity data, the extracted value of
sound velocity in InAs nanowires is found to be an order less than the bulk.
This low sound velocity is attributed to the extended crystal defects that
ubiquitously appear in these nanowires. Analyzing the temperature-dependent
mobility data, we identify the key scattering mechanisms limiting the carrier
transport in these nanowires. Finally, using these scattering models, we
perform drift-diffusion based transport simulations of a nanowire field-effect
transistor and compare the device performances with experimental measurements.
Our device modeling provides insight into performance limits of InAs nanowire
transistors and can be used as a predictive methodology for nanowire-based
integrated circuits.Comment: 22 pages, 5 Figures, Nano Letter
Facile fabrication of lateral nanowire wrap-gate devices with improved performance
We present a simple fabrication technique for lateral nanowire wrap-gate
devices with high capacitive coupling and field-effect mobility. Our process
uses e-beam lithography with a single resist-spinning step, and does not
require chemical etching. We measure, in the temperature range 1.5-250 K, a
subthreshold slope of 5-54 mV/decade and mobility of 2800-2500 --
significantly larger than previously reported lateral wrap-gate devices. At
depletion, the barrier height due to the gated region is proportional to
applied wrap-gate voltage.Comment: 3 pages, 3 figure