By combination of two special methods, i.e., persistent spectral hole burning and laser assisted nanoparticle preparation, the dephasing time $T_2$ of surface plasmon excitation in silver nanoparticles was systematically investigated. A strong dependence of $T_2$ on the plasmon energy is found which reflects the relevance of interband damping and makes necessary a precise control of the particle shape when measuring $T_2$. The influence of the reduced dimension on the dephasing dynamics was observed as a decrease of $T_2$ with shrinking particle size. In addition, for silver nanoparticles on quartz substrates, a considerable amount of chemical interface damping was observed.

We describe a new technique to determine the homogeneous linewidths of surface plasmon resonances of metal nanoparticles and thus measure the decay time of this collective electron excitation. The method is based on spectral hole burning and has been applied to supported oblate Ag particles with radii of 7.5 nm. From the experimental results and a theoretical model of hole burning the linewidth of 260 meV corresponding to a decay time of 4.8 fs was extracted. This value is shorter than expected for damping by bulk electron scattering. We conclude that additional damping mechanisms have been observed and reflect confinement of the electrons in nanoparticles with sizes below 10 nm.

Advanced nanophotonics penetrates into other areas of science and technology, ranging from applied physics to biology, which results in many fascinating cross-disciplinary applications. It has been recently demonstrated that suitably engineered light-matter interactions at the nanoscale can overcome the limitations of today’s terahertz (THz) photoconductive antennas, making them one step closer to many practical implications. Here, we push forward this concept by comprehensive numerical optimization and experimental investigation of a log-periodic THz photoconductive antenna coupled to a silver nanoantenna array. We shed light on the operation principles of the resulting hybrid THz antenna, providing an approach to boost its performance. By tailoring the size of silver nanoantennas and their arrangement, we obtain an enhancement of optical-to-THz conversion efficiency 2-fold larger compared with previously reported results for similar structures, and the strongest enhancement is around 1 THz, a frequency range barely achievable by other compact THz sources. We also propose a cost-effective fabrication procedure to realize such hybrid THz antennas with optimized plasmonic nanostructures via thermal dewetting process, which does not require any post processing and makes the proposed solution very attractive for applications.

A theory of selective reflection and transmission is developed for a system consisting of a thin layer of a dilute atomic vapor sandwiched between two transparent solids with parallel interfaces. Strong effects of spatial dispersion due to the atomic motion and electronic quenching on gas-solid interfaces are accounted for. It is shown that both even and odd Doppler-free resonances may occur in selective reflection, depending upon the thickness of the vapor layer. It is also found that the amplitude of selective reflection is a result of the interference between reflections from the two boundaries of the vapor-solid interfaces and, hence, may be greatly enhanced under certain conditions.