RESUMEN
Photonic time-crystals (PTCs) are spatially homogeneous media whose electromagnetic susceptibility varies periodically in time, causing temporal reflections and refractions for any wave propagating within the medium. The time-reflected and time-refracted waves interfere, giving rise to Floquet modes with momentum bands separated by momentum gaps (rather than energy bands and energy gaps, as in photonic crystals). Here, we present a study on the emission of radiation by free electrons in PTCs. We show that a free electron moving in a PTC spontaneously emits radiation, and when associated with momentum-gap modes, the electron emission process is exponentially amplified by the modulation of the refractive index. Moreover, under strong electron-photon coupling, the quantum formulation reveals that the spontaneous emission into the PTC bandgap experiences destructive quantum interference with the emission of the electron into the PTC band modes, leading to suppression of the interdependent emission. Free-electron physics in PTCs offers a platform for studying a plethora of exciting phenomena, such as radiating dipoles moving at relativistic speeds and highly efficient quantum interactions with free electrons.
RESUMEN
We study the time reflection and time refraction of waves caused by a spatial interface with a medium undergoing a sudden temporal change in permittivity. We show that monochromatic waves are transformed into a pulse by the permittivity change, and that time reflection is enhanced at the vicinity of the critical angle for total internal reflection. In this regime, we find that the evanescent field is transformed into a propagating pulse by the sudden change in permittivity. These effects display enhancement of the time reflection and high sensitivity near the critical angle, paving the way to experiments on time reflection and photonic time crystals at optical frequencies.
RESUMEN
We study the propagation of electromagnetic waves in disordered photonic time crystals: spatially homogenous media whose refractive index changes randomly in time. We find that the group velocity of a pulse propagating in such media decreases exponentially, eventually coming to a complete stop, while experiencing exponential growth in intensity. These effects greatly depend on the Floquet band structure of the photonic time crystal, with the strongest sensitivity to disorder occurring in superluminal modes. Finally, we analyze the ensemble statistics and find them to coincide with those of Anderson localization, exhibiting single parameter scaling.
RESUMEN
We find that waves propagating in a 1D medium that is homogeneous in its linear properties but spatially disordered in its nonlinear coefficients undergo diffusive transport, instead of being Anderson localized as always occurs for linear disordered media. Specifically, electromagnetic waves in a multilayer structure with random nonlinear coefficients exhibit diffusion with features fundamentally different from the traditional diffusion in linear noninteracting systems. This unique transport, which stems from the nonlinear interaction between the waves and the disordered medium, displays anomalous statistical behavior where the fields in multiple different realizations converge to the same intensity value as they penetrate deeper into the medium.
RESUMEN
BACKGROUND: Research on learners' reactions to failure finds negative emotions may present an obstacle for learning; a painful experience of failure may result in disengagement and avoidance. However, research on styles of emotion regulation and learning from failure is scarce. Self-determination theory's (SDT) conception of adaptive and maladaptive emotion regulation differentiates among three styles of regulation: integrative emotion regulation (IER), suppressive emotion regulation (SER) and amotivated emotion regulation. AIMS: Two studies were conducted, one cross-sectional and one longitudinal, to test the associations between IER and learning from failure among adolescents. SAMPLE: Study 1 comprised 184 adolescents (mean age = 16.55; SD = 1.2). Study 2 comprised 565 adolescents (8-12 graders). The main analysis was based on 192 adolescents' perceptions of failing math grades. METHOD: Study 1 surveyed adolescents on their emotion regulation styles, adaptive and maladaptive coping practices when dealing with failure and tendency to learn from failure. Study 2 was longitudinal and focused on failure in math. We approached participants twice, before and after the math test. CONCLUSIONS: In both studies, IER was related to adaptive coping practices and the tendency to learn from failure. In Study 2, adaptive coping practices mediated relations between IER and learning from failure in math and learning from failure mediated relations between IER and future engagement. These findings suggest that styles of emotion regulation play an important role in learning from failure.
RESUMEN
Photonic time crystals (PTCs), materials with a dielectric permittivity that is modulated periodically in time, offer new concepts in light manipulation. We study theoretically the emission of light from a radiation source placed inside a PTC and find that radiation corresponding to the momentum bandgap is exponentially amplified, whether initiated by a macroscopic source, an atom, or vacuum fluctuations, drawing the amplification energy from the modulation. The radiation linewidth becomes narrower with time, eventually becoming monochromatic in the middle of the bandgap, which enables us to propose the concept of nonresonant tunable PTC laser. Finally, we find that the spontaneous decay rate of an atom embedded in a PTC vanishes at the band edge because of the low density of photonic states.