Longueur d'onde de Compton | EPFL Graph Search

Quand un photon primaire heurte une particule libre, un photon secondaire est émis dont la longueur d’onde est plus grande que celle du photon primaire, c'est l'effet Compton. La différence de longueur d’onde entre le photon primaire et le photon émis, est proportionnelle à une valeur constante portant le nom de longueur d’onde de Compton, comme l'exprime la relation suivante (voir l'article principal sur la diffusion Compton pour plus d'explications) : :\Delta \lambda = \lambda _C \left( 1 - \cos \theta \right), où :

\Delta \lambda est le décalage entre les longueurs d'onde du photon incident et du photon diffusé ;
\lambda _C est la longueur d'onde de Compton ;
\theta est l'angle de diffusion.

Nous pouvons donc comparer cette constante à un quantum de longueur d'onde. Contrairement à la longueur d'onde de de Broglie, la longueur d'onde de Compton ne correspond pas à une longueur d'onde observable dans une propagation, elle n'e

Effects of multiple scattering on angle-independent structural color in disordered colloidal materials

Sofia Magkiriadou

Disordered packings of colloidal spheres show angle-independent structural color when the particles are on the scale of the wavelength of visible light. Previous work has shown that the positions of the peaks in the reflectance spectra can be predicted accurately from a single-scattering model that accounts for the effective refractive index of the material. This agreement shows that the main color peak arises from short-range correlations between particles. However, the single-scattering model does not quantitatively reproduce the observed color: the main peak in the reflectance spectrum is much broader and the reflectance at low wavelengths is much larger than predicted by the model. We use a combination of experiment and theory to understand these features. We find that one significant contribution to the breadth of the main peak is light that is scattered, totally internally reflected from the boundary of the sample, and then scattered again. The high reflectance at low wavelengths also results from multiple scattering but can be traced to the increase in the scattering cross section of individual particles with decreasing wavelength. Both of these effects tend to reduce the saturation of the structural color, which limits the use of these materials in applications. We show that while the single-scattering model cannot reproduce the observed saturations, it can be used as a design tool to reduce the amount of multiple scattering and increase the color saturation of materials, even in the absence of absorbing components.

AMER PHYSICAL SOC2020

The effects of Prandtl number on the nonlinear dynamics of Kelvin-Helmholtz instability in two dimensions

Jeremy Peter Parker

It is known that the pitchfork bifurcation of Kelvin-Helmholtz instability occurring at minimum gradient Richardson number Ri(m) similar or equal to 1/4 in viscous stratified shear flows can be subcritical or supercritical depending on the value of the Prandtl number, Pr. Here, we study stratified shear flow restricted to two dimensions at finite Reynolds number, continuously forced to have a constant background density gradient and a hyperbolic tangent shear profile, corresponding to the 'Drazin model' base flow. Bifurcation diagrams are produced for fluids with Pr = 0.7 (typical for air), 3 and 7 (typical for water). For Pr = 3 and 7, steady billow-like solutions are found to exist for strongly stable stratification of Ri(m) beyond 1/2. Interestingly, these solutions are not a direct product of a Kelvin-Helmholtz instability, having half the wavelength of the linear instability, and arising through a superharmonic bifurcation. These short-wavelength states can be tracked down to at least Pr approximate to 2.3 and act as instigators of complex dynamics, even in strongly stratified flows. Direct numerical simulations of forced and unforced two-dimensional flows are performed, which support the results of the bifurcation analyses. Perturbations are observed to grow approximately exponentially from random initial conditions where no modal instability is predicted by a linear stability analysis.

2021

Mode interference and UV-induced mode conversion in few-mode fibers

Soham Basu

Resonant and non-resonant effects in few-mode fibers have found use in versatile devices. From sensing perspective, extensive research has been done on fringe sensitivity of nonresonant two-mode interference. Resonant mode converter gratings are promising candidates for the fields of mode division multiplexing and active devices based on few-mode fibers. This thesis explores multi-parameter sensing using two effects of two-mode interference (TMI), namely fringe shift and shift of group-velocity equalization (GVE) wavelength. The following sensing parameters were achieved: a) TMI fringe shift can be used for measuring the effect of changes in temperature and strain on intermodal dispersion. For exposure of the fiber with a scanning laser spot of constant velocity, the change in intermodal dispersion due to such particular exposure can also be measured using TMI fringes. b) GVE wavelength shift can be used for straightforward sensing of temperature and strain, without suffering from any dependency on the fiber length like TMI fringes. Using the shift of GVE wavelength during exposure of the full fiber with a scanning laser spot, the change in core index can be estimated due to a simple model. This allows measurement of small index changes due to irradiation, which is not possible with FBGs due to imperceptible strength of FBGs for small index changes and lower sensitivity. Combining the GVE and FBG resonance wavelengths provides a method for differentiating strain and temperature. The GVE wavelength also has an approximately three times higher temperature sensitivity compared to FBG resonance at similar wavelengths. The strain sensitivity of GVE and FBG resonance wavelengths are of similar magnitude but of opposite sign. TMI fringe shift can also be used to measure the extra phase added between two modes by each mark of laser irradiation. Two methods have been developed for completely characterizing the intermodal dispersion, which were verified experimentally for the LP01-LP02 dispersion. Intermodal and intramodal reflection peaks of two excited modes from an FBG written in the few-mode fiber can be used to approximately estimate the offset of the intermodal dispersion of the pristine fiber. Combining this with TMI phase unwrapping provides estimate of complete intermodal dispersion including offset. Measuring the resonance wavelength for a mode converter written using marks already characterized using TMI provides a precise estimate of the offset of the intermodal dispersion of the pristine fiber. From the estimate of extra phase added by each mark and intermodal dispersion of the pristine fiber, the intermodal dispersion experienced by resonantly interacting modes in an MC can be predicted. Fabrication of broadband mode converter gratings at any desired wavelength is an ongoing field of research. Different strategies were evaluated for making broadband MCs: a) Partial core irradiation with a tightly focussed laser spot was simulated with simple approximate models. In was concluded that the losses might be too high due to poor mode overlaps. b) It was determined using simulations that phase-shifted mode converter gratings can provide broader than 35 nm bandwidth at 20 dB conversion strength using reasonable irradiation parameters. The method was verified by achieving 41 nm bandwidth at 16 dB conversion strength.

EPFL2020

Mode interference and UV-induced mode conversion in few-mode fibers

Soham Basu

EPFL2020