66 research outputs found
Numerical sampling rules for paraxial regime pulse diffraction calculations
Sampling rules for numerically calculating ultrashort pulse fields are discussed. Such pulses are not monochromatic
but rather have a finite spectral distribution about some central (temporal) frequency. Accordingly,
the diffraction pattern for many spectral components must be considered. From a numerical implementation
viewpoint, one may ask how many of these spectral components are needed to accurately calculate the pulse
field. Using an analytical expression for the Fresnel diffraction from a 1-D slit, we examine this question by
varying the number of contributing spectral components. We show how undersampling the spectral profile produces
erroneous numerical artifacts (aliasing) in the spatial–temporal domain. A guideline, based on graphical
considerations, is proposed that determines appropriate sampling conditions. We show that there is a relationship
between this sampling rule and a diffraction wave that emerges from the aperture edge; comparisons are
drawn with boundary diffraction waves. Numerical results for 2-D square and circular apertures are presented
and discussed, and a potentially time-saving calculation technique that relates pulse distributions in different
z planes is described
Numerical sampling rules for paraxial regime pulse diffraction calculations
Sampling rules for numerically calculating ultrashort pulse fields are discussed. Such pulses are not monochromatic
but rather have a finite spectral distribution about some central (temporal) frequency. Accordingly,
the diffraction pattern for many spectral components must be considered. From a numerical implementation
viewpoint, one may ask how many of these spectral components are needed to accurately calculate the pulse
field. Using an analytical expression for the Fresnel diffraction from a 1-D slit, we examine this question by
varying the number of contributing spectral components. We show how undersampling the spectral profile produces
erroneous numerical artifacts (aliasing) in the spatial–temporal domain. A guideline, based on graphical
considerations, is proposed that determines appropriate sampling conditions. We show that there is a relationship
between this sampling rule and a diffraction wave that emerges from the aperture edge; comparisons are
drawn with boundary diffraction waves. Numerical results for 2-D square and circular apertures are presented
and discussed, and a potentially time-saving calculation technique that relates pulse distributions in different
z planes is described
Intrinsic response time of graphene photodetectors
Graphene-based photodetectors are promising new devices for high-speed
optoelectronic applications. However, despite recent efforts, it is not clear
what determines the ultimate speed limit of these devices. Here, we present
measurements of the intrinsic response time of metal-graphene-metal
photodetectors with monolayer graphene using an optical correlation technique
with ultrashort laser pulses. We obtain a response time of 2.1 ps that is
mainly given by the short lifetime of the photogenerated carriers. This time
translates into a bandwidth of ~262 GHz. Moreover, we investigate the
dependence of the response time on gate voltage and illumination laser power
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