Virtual Processes and Superradiance in Spin-Boson Models

We consider spin-boson models composed by a single bosonic mode and an ensemble of $N$ identical two-level atoms. The situation where the coupling between the bosonic mode and the atoms generates real and virtual processes is studied, where the whole system is in thermal equilibrium with a reservoir at temperature $\beta^{-1}$. Phase transitions from ordinary fluorescence to superradiant phase in three different models is investigated. First a model where the coupling between the bosonic mode and the $j-th$ atom is via the pseudo-spin operator $\sigma^{,z}_{(j)}$ is studied. Second, we investigate the generalized Dicke model, introducing different coupling constants between the single mode bosonic field and the environment, $g_{1}$ and $g_{2}$ for rotating and counter-rotating terms, respectively. Finally it is considered a modified version of the generalized Dicke model with intensity-dependent coupling in the rotating terms. In the first model the zero mode contributes to render the canonical entropy a negative quantity for low temperatures. The last two models presents phase transitions, even when only Hamiltonian terms which generates virtual processes are considered

Tuning the polarization states of optical spots at the nanoscale on the poincar´e sphere using a plasmonic nanoantenna

It is shown that the polarization states of optical spots at the nanoscale can be manipulated to various points on the Poincar´e sphere using a plasmonic nanoantenna. Linearly, circularly, and elliptically polarized near-field optical spots at the nanoscale are achieved with various polarization states on the Poincar´e sphere using a plasmonic nanoantenna. A novel plasmonic nanoantenna is illuminated with diffraction-limited linearly polarized light. It is demonstrated that the plasmonic resonances of perpendicular and longitudinal components of the nanoantenna and the angle of incident polarization can be tuned to obtain optical spots beyond the diffraction limit with a desired polarization and handedness

De la clase virtual de emergencia a la planificación de la enseñanza mediadas por tecnologías

En este trabajo se presentan experiencias de formación docente en niveles medio, superior y universitario relacionados con la incorporación de tecnologías de la enseñanza usando aulas virtuales. Se considerará como base para el análisis las actividades desarrolladas durante 2021 en la primera cohorte de la Diplomatura en Tecnologías Aplicadas a la Enseñanza en Entornos Virtuales de la Facultad de Ciencias de la Administración (UNER), describiendo brevemente la metodología implementada, las estrategias utilizadas, los resultados alcanzados y la percepción de los participantes sobre las actividades realizadas. Durante la pandemia los entornos virtuales fueron un aspecto clave para el sostenimiento de las actividades académicas en todos los niveles educativos. Mucho se ha expresado en cuanto a la innovación requerida en este contexto para tratar de mantener el vínculo con los estudiantes y construir espacios virtuales que permitan garantizar un aprendizaje significativo. En concordancia, y con el regreso a la presencialidad, es de interés sostener las potencialidades de las TICs en entornos presenciales e híbridos, enfocando la modalidad virtual ya no como “de emergencia” sino como estrategia planificada que enriquece el aula y las actividades académicas.Red de Universidades con Carreras en Informátic

Controlling Field Asymmetry in Nanoscale Gaps for Second Harmonic Generation

Field hotspots in nanoscale gaps of plasmonic antennas can boost nonlinear processes such as harmonic generation, photoelectron emission, and ultrafast electron transport. Alongside large field enhancement, such phenomena often require control over the field symmetry in the hotspot, which is challenging considering the nanometer length scales. Here, by means of strongly enhanced second harmonic generation, unprecedented control over the field distribution in a hotspot is demonstrated by systematically introducing geometrical asymmetry to the antenna gap. Focused helium ion beam milling of mono-crystalline gold is used to realize asymmetric-gap dimer antennas in which an ultra-sharp tip faces a flat counterpart. This allows to increase both field enhancement and asymmetry while maintaining the bonding antenna mode at the fundamental frequency, thereby enhancing second harmonic radiation to the far-field. Combining these findings with second harmonic radiation patterns as well as quantitative nonlinear simulations, remarkably detailed insight into the mechanism of second harmonic generation at the nanoscale is obtained. These results open new opportunities for the realization of nonreciprocal nanoscale systems.Field hotspots in nanoscale gaps of plasmonic antennas can boost nonlinear processes, which often require high field enhancement combined with an asymmetric field distribution. This work demonstrates unprecedented control over the symmetry of the field distribution by systematically introducing geometrical asymmetry in the antenna gap, thereby strongly enhancing second harmonic generation. These results open new opportunities for the realization of nonreciprocal nanoscale systems.imag