62,428 research outputs found
Expression of the angular dependence of the quantum efficiency for a thin multi-alkali photocathode and its optical properties
The dependence of the quantum efficiency on the angle and polarization of the
incident photon needs to be formulated for a precise description of the
response of photomultiplier tubes. A simplified one-step model of photoelectron
emission was derived from Spicer's three-step model, and it enabled the
formulation of the dependence of the quantum efficiency in the visible range
for thin multi-alkali (NaKSbCs) photocathodes. The expression of the quantum
efficiency was proved by a measurement of the photocurrent for linearly
polarized light at various incident angles. Meanwhile, the measurement revealed
the complex refractive indices and thicknesses both of the stratified
photocathode and antireflection coating. It is indicated that the angular
dependence of the quantum efficiency is dictated by the optical properties of
the photocathode, which are discussed in detail on the basis of the obtained
parameters
Quantum efficiency of single-photon sources in the cavity-QED strong-coupling regime
We calculate the integrated-pulse quantum efficiency of single-photon sources
in the cavity quantum electrodynamics (QED) strong-coupling regime. An
analytical expression for the quantum efficiency is obtained in the
Weisskopf-Wigner approximation. Optimal conditions for a high quantum
efficiency and a temporally localized photon emission rate are examined. We
show the condition under which the earlier result of Law and Kimble [J. Mod.
Opt. 44, 2067 (1997)] can be used as the first approximation to our result.Comment: 8 pages, 3 figures, final version, tex file uploade
Photoluminescence quantum efficiency of dense silicon nanocrystal ensembles in SiO2
The photoluminescence decay characteristics of silicon nanocrystals in dense ensembles fabricated by ion implantation into silicon dioxide are observed to vary in proportion to the calculated local density of optical states. A comparison of the experimental 1/e photoluminescence decay rates to the expected spontaneous emission rate modification yields values for the internal quantum efficiency and the intrinsic radiative decay rate of silicon nanocrystals. A photoluminescence quantum efficiency as high as 59%±9% is found for nanocrystals emitting at 750 nm at low excitation power. A power dependent nonradiative decay mechanism reduces the quantum efficiency at high pump intensity
Simultaneous readout of two charge qubits
We consider a system of two solid state charge qubits, coupled to a single
read-out device, consisting of a single-electron transistor (SET). The
conductance of each tunnel junction is influenced by its neighboring qubit, and
thus the current through the transistor is determined by the qubits' state. The
full counting statistics of the electrons passing the transistor is calculated,
and we discuss qubit dephasing, as well as the quantum efficiency of the
readout. The current measurement is then compared to readout using real-time
detection of the SET island's charge state. For the latter method we show that
the quantum efficiency is always unity. Comparing the two methods a simple
geometrical interpretation of the quantum efficiency of the current measurement
appears. Finally, we note that full quantum efficiency in some cases can be
achieved measuring the average charge of the SET island, in addition to the
average current.Comment: 11 pages with 5 figure
Quantum efficiency measurement of single photon detectors using photon pairs generated in optical fibers
Using the correlated signal and idler photon pairs generated in a dispersion
shifted fiber by a pulsed pump, we measure the quantum efficiency of a
InGaAs/InP avalanche photodiode-based single photon detector. Since the
collection efficiency of photon pairs is a key parameter to correctly deduce
the quantum efficiency, we carefully characterize the collection efficiency by
studying correlation dependence of photon pairs upon the spectra of pump,
signal and idler photons. This study allows us to obtain quantum efficiency of
the single photon detector by using photon pairs with various kinds of
bandwidths.Comment: 21pages, 6figures, 4tables, accepted for publication in J. Opt. Soc.
Am.
Direct observation of non-classical photon statistics in parametric downconversion
We employ a high quantum efficiency photon number counter to determine the
photon number distribution of the output field from a parametric downconverter.
The raw photocount data directly demonstrates that the source is nonclassical
by forty standard deviations, and correcting for the quantum efficiency yields
a direct observation of oscillations in the photon number distribution
Twin-photon techniques for photo-detector calibration
The aim of this review paper is to enlighten some recent progresses in
quantum optical metrology in the part of quantum efficiency measurements of
photo-detectors performed with bi-photon states. The intrinsic correlated
nature of entangled photons from Spontaneous Parametric Down Conversion
phenomenon has opened wide horizons to a new approach for the absolute
measurement of photo-detector quantum efficiency, outgoing the requirement for
conventional standards of optical radiation; in particular the simultaneous
feature of the creation of conjugated photons led to a well known technique of
coincidence measurement, deeply understood and implemented for standard uses.
On the other hand, based on manipulation of entanglement developed for Quantum
Information protocols implementations, a new method has been proposed for
quantum efficiency measurement, exploiting polarisation entanglement in
addition to energy-momentum and time ones, that is based on conditioned
polarisation state manipulation. In this review, after a general discussion on
absolute photo-detector calibration, we compare these different methods, in
order to give an accurate operational sketch of the absolute quantum efficiency
measurement state of the art
Characterisation of the Medipix3 detector for 60 and 80 keV electrons
In this paper we report quantitative measurements of the imaging performance for the current generation of hybrid pixel detector, Medipix3, used as a direct electron detector. We have measured the modulation transfer function and detective quantum efficiency at beam energies of 60 and 80 keV. In single pixel mode, energy threshold values can be chosen to maximize either the modulation transfer function or the detective quantum efficiency, obtaining values near to, or exceeding those for a theoretical detector with square pixels. The Medipix3 charge summing mode delivers simultaneous, high values of both modulation transfer function and detective quantum efficiency. We have also characterized the detector response to single electron events and describe an empirical model that predicts the detector modulation transfer function and detective quantum efficiency based on energy threshold. Exemplifying our findings we demonstrate the Medipix3 imaging performance recording a fully exposed electron diffraction pattern at 24-bit depth together with images in single pixel and charge summing modes. Our findings highlight that for transmission electron microscopy performed at low energies (energies <100 keV) thick hybrid pixel detectors provide an advantageous architecture for direct electron imaging
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