arXiv subjects

C. Pedersen

Publications and source records attributed to C. Pedersen.

4 recordsLinked to original sources

arXiv2022

Energetic electron precipitation driven by electromagnetic ion cyclotron waves from ELFIN's low altitude perspective

We review comprehensive observations of electromagnetic ion cyclotron (EMIC) wave-driven energetic electron precipitation using data from the energetic electron detector on the Electron Losses and Fields InvestigatioN (ELFIN) mission, two polar-orbiting low-altitude spinning CubeSats, measuring 50-5000 keV electrons with good pitch-angle and energy resolution. EMIC wave-driven precipitation exhibits a distinct signature in energy-spectrograms of the precipitating-to-trapped flux ratio: peaks at 0.5 MeV which are abrupt (bursty) with significant substructure (occasionally down to sub-second timescale). Multiple ELFIN passes over the same MLT sector allow us to study the spatial and temporal evolution of the EMIC wave - electron interaction region. Using two years of ELFIN data, we assemble a statistical database of 50 events of strong EMIC wave-driven precipitation. Most reside at L=5-7 at dusk, while a smaller subset exists at L=8-12 at post-midnight. The energies of the peak-precipitation ratio and of the half-peak precipitation ratio (our proxy for the minimum resonance energy) exhibit an L-shell dependence in good agreement with theoretical estimates based on prior statistical observations of EMIC wave power spectra. The precipitation ratio's spectral shape for the most intense events has an exponential falloff away from the peak (i.e., on either side of 1.45 MeV). It too agrees well with quasi-linear diffusion theory based on prior statistics of wave spectra. Sub-MeV electron precipitation observed concurrently with strong EMIC wave-driven 1MeV precipitation has a spectral shape that is consistent with efficient pitch-angle scattering down to 200-300 keV by much less intense higher frequency EMIC waves. These results confirm the critical role of EMIC waves in driving relativistic electron losses. Nonlinear effects may abound and require further investigation.

physics.space-ph↗

arXiv2021

A silicon source of heralded single photons at 2 $\mu$m

Mid infrared integrated quantum photonics is a promising platform for applications in sensing and metrology. However, there are only few examples of on-chip single photon sources at these wavelengths. These have limited performances with respect to their C-band counterparts. In this work, we demonstrate a new approach to generate heralded single photons in the mid infrared on a silicon chip. By using a standard C-band pump, the inter-modal spontaneous four wave mixing enables the generation of the herald idler at 1259.7 nm and the heralded signal at 2015 nm. The idler photon is easily detected with a common infrared single photon detector while the signal photon is upconverted to the visible before its detection. In this way, we are able to operate a mid infrared source without the need of mid infrared detectors and laser sources. By measuring a heralded $g^{(2)}$ of $0.23 \, \pm \, 0.08$ we demonstrate the single photon behaviour of the source as well as the feasibility of multi-photon coincidence measurements beyond 2 $\mu$m with our setup. The source exhibits a high intrinsic heralding efficiency of $(59 \, \pm \,5)\%$, a maximum coincidence to accidental ratio of $40.4 \, \pm \, 0.9$ and a generation probability of $\left( 0.72 \, \pm \, 0.10 \right)$ W$^{-2}$.

quant-ph↗

arXiv2020

Film deposition of a self-propelled droplet on a cone with slip

We study the dynamic wetting of a self-propelled viscous droplet using the time-dependent lubrication equation on a conical-shaped substrate for different cone radii, cone angles and slip lengths. The droplet velocity is found to increase with the cone angle and the slip length, but decrease with the cone radius. We show that a film is formed at the receding part of the droplet, much like the classical Landau-Levich-Derjaguin (LLD) film. The film thickness $h_f$ is found to decrease with the slip length $\lambda$. By using the approach of matching asymptotic profiles in the film region and the quasi-static droplet, we obtain the same film thickness as the results from the lubrication approach for all slip lengths. We identify two scaling laws for the asymptotic regimes: $h_fh''_o \sim Ca^{2/3}$ for $\lambda\ll h_f$ and $h_f h''^{3}_o\sim (Ca/\lambda)^2$ for $\lambda\gg h_f$, here $1/h''_o$ is a characteristic length at the receding contact line and $Ca$ is the capillary number. We compare the position and the shape of the droplet predicted from our continuum theory with molecular dynamics simulations, which are in close agreement. Our results show that manipulating the droplet size, the cone angle and the slip length provides different schemes for guiding droplet motion and coating the substrate with a film.

physics.flu-dyn↗

arXiv2019

SHG (532 nm)-induced spontaneous parametric downconversion noise in 1064 nm-pumped IR upconversion detectors

As a novel technique for infrared detection, frequency upconversion has been successfully deployed in many applications. However, investigations into the noise properties of upconversion detectors (UCDs) have also received considerable attention. In this letter, we present a new noise source - second harmonic generation (SHG)-induced spontaneous parametric downconversion (SPDC) - experimentally and theoretically shown to exist in short-wavelength-pumped UCDs. We investigate the noise properties of two UCDs based on single-pass 1064 nm-pumped periodically poled LiNbO$_{3}$ bulk crystals. One UCD is designed to detect signals in the telecom band and the other in the mid-infrared regime. Our experimental demonstration and theoretical analysis reveal the basic properties of this newly discovered UCD noise source, including its dependence on crystal temperature and pump power. Furthermore, the principle behind the generation of this noise source can also be applied to other UCDs, which utilize nonlinear crystals either in waveguide form or with different bulk materials. This study may also aid in developing methods to suppress the newly identified noise in future UCD designs.

physics.optics↗