Angel J. Garcia-Adeva

Anti-stokes laser cooling in bulk erbium-doped materials.

We report the first observation of anti-Stokes laser-induced cooling in the Er3+:KPb2Cl5 crystal and in the Er3+:CNBZn (CdF2-CdCl2-NaF-BaF2-BaCl2-ZnF2) glass. The internal cooling efficiencies have been calculated by using photothermal deflection spectroscopy. Thermal scans acquired with an infrared thermal camera proved the bulk cooling capability of the studied samples. The implications of these results are discussed.

Infrared-to-visible upconversion of Er 3+ ions in GeO 2 –PbO–Nb 2 O 5 glasses

We report the infrared-to-visible upconversion luminescence of Er3+-doped lead–niobium–germanate glasses (GeO2–PbO–Nb2O5) with different Er2O3 concentrations (0.5, 1, 2, and 3 wt.%) under continuous-wave and pulsed-laser excitation in the near-infrared region inside the 4I9/2 level. Intense green emission due to the (2H11/2, 4S3/2)→4I15/2 transitions was observed at room temperature together with a weak red emission corresponding to the 4F9/2→4I15/2 transition. These upconversion emissions are attributed to a two-photon process. The time evolution of the green emission from the 4S3/2 level indicates that energy-transfer upconversion and excited-state absorption are responsible for the upconversion luminescence. The increase of the weak red emission with increasing Er2O3 concentration, together with its temporal behavior under infrared excitation, suggests that for Er2O3 concentrations higher than 1 wt.%, the upconverted red emission is the result of multiphonon relaxation from the 4S3/2 level and energy-transfer processes.

Band gap atlas for photonic crystals having the symmetry of the kagomé and pyrochlore lattices

The interesting photonic band gap maps of two- and three-dimensional photonic crystals with the symmetry of the kagome and pyrochlore lattices are reported. From large complete photonic band gaps in three dimensions to enormous partial gaps in two dimensions for certain polarizations occurring at feature sizes that make these lattices amenable to fabrication, the results described below make this family of photonic crystals very promising for potential applications and very interesting from the fundamental point of view.

Upconversion cooling of Er-doped low-phonon fluorescent solids

We report on a mechanism for laser cooling of fluorescent solids based on infrared-to-visible upconversion often found in rare-earth-doped low-phonon materials. This type of optical cooling presents some advantages with regard to conventional anti-Stokes cooling. Among them, it allows us to obtain cooling in a broader range of frequencies around the barycenter of the infrared emitting band.

The finite element method applied to the study of two-dimensional photonic crystals and resonant cavities

A critical assessment of the finite element (FE) method for studying two-dimensional dielectric photonic crystals is made. Photonic band structures, transmission coefficients, and quality factors of various two-dimensional, periodic and aperiodic, dielectric photonic crystals are calculated by using the FE (real-space) method and the plane wave expansion or the finite difference time domain (FDTD) methods and a comparison is established between those results. It is found that, contrarily to popular belief, the FE method (FEM) not only reproduces extremely well the results obtained with the standard plane wave method with regards to the eigenvalue analysis (photonic band structure and density of states calculations) but it also allows to study very easily the time-harmonic propagation of electromagnetic fields in finite clusters of arbitrary complexity and, thus, to calculate their transmission coefficients in a simple way. Moreover, the advantages of using this real space method in the context of point defect cluster quality factor calculations are also stressed by comparing the results obtained with this method with those obtained with the FDTD one. As a result of this study, FEM comes out as an stable, robust, rigorous, and reliable tool to study light propagation and confinement in both periodic and aperiodic dielectric photonic crystals and clusters.

Anti-Stokes laser cooling in erbium-doped low-phonon materials

Here we report the first experimental evidences of anti-Stokes laser-induced cooling in two different low phonon erbium-doped matrices: a KPb2Cl5 crystal and a fluorochloride glass. The local cooling was detected by using a photothermal deflection technique whereas the bulk cooling was detected by means of a calibrated thermal sensitive camera. The Er3+ ion was excited in the 4I9/2 manifold. It is worthwhile to mention that the cooling was observed in the spectral region where some upconversion processes that initiate at the pumped level occur. Together with the spectroscopic characterization, a short discussion on the experimental and theoretical background of the cooling process including the possible influence of upconversion processes is presented.

Designing integrated circuitry in nanoscale photonic crystals

Current data networks rely on a mix of optical carriers and electronic circuitry. Signals are transported in the optical domain by fibers that support high bandwidths and low losses. Meanwhile, electronic devices distributed along the backbone provide the rest of the essential functions needed for a modern packetswitching network, such as routing, storing, and queuing the data traffic. The performance of such networks, however, is limited by the electronics, which have a maximum speed of only a few gigabits per second. To satisfy the increasing bandwidth requirements of emerging applications and to eliminate this bottleneck, we wish to replace the electronic circuitry with analogous optical systems that provide transparency and high bandwidth while consuming significantly less energy. Furthermore, we imagine a broad range of applications for such optical circuits beyond their use in networks, such as for optical computing or lightweight, highly integrated automotive and aeronautical monitoring. Conventional optical circuits, however, must be large to reduce losses when guiding light around curves. Photonic crystals offer a handy alternative technology that allows us to design nanoscale circuits that can control the flow of light with innovative silicon patterns (see Figure 1). A photonic crystal is an engineered inhomogeneous periodic structure made of two or more materials with very different dielectric constants.1 It supports a complete photonic band gap: a frequency range within which no photons can propagate through the material. Irregularities (‘defects’) introduced in the structure form localized defect states, which allow light to be guided through the structure on a subwavelength scale with minimal losses. By introducing different forms of disorder in a photonic crystal, one can achieve more complex functions. Tailoring this disorder for a particular application, however, is extremely difficult. The traditional design approach starts with a proposed structure, then allows only a few degrees of freedom to vary Figure 1. Schematic view of basic functions that an optical circuit must be able to perform to manage the flow of light in a photonic crystal structure.

Local internal and bulk optical cooling in Nd-doped crystals and nanocrystalline powders revisited

In the present work, we report on infrared thermography measurements in Nd-doped KPb2Cl5 crystal and powder above and below the barycentre of the 4F3/2 level that were performed in order to assess the relative weights of both the direct anti-Stokes absorption processes and those assisted by either excited state absorption or energy transfer upconversion when cooling takes place in the material. As the laser induced temperature changes are usually small, we used a special configuration of the samples that allowed us to obtain differential measurements where an undoped sample acted as a temperature baseline. This method allows us to ascertain whether the recorded temperature changes are optically induced or they are due to some other effect.

Inverse design of novel nanophotonic structures

We report on the design of ultra-wide bandwidth novel passive devices based on photonic crystal (PC) technology for efficiently performing the essential functionalities required by any future photonic integrated circuit (PIC). We utilize various heuristic optimization methods as an inverse design (ID) engine for achieving promising PC systems that outperform previous topologies based on intuition. We also present the application of these techniques for attaining a global optimum solutions. PC topologies proposed throughout this manuscript are constrained in order to fulfill the limitations imposed by lithographic manufacturing techniques. Therefore, these designs are not only interesting from a theoretical point of view but also of great practical importance, since they can be readily manufactured.

Local internal and bulk optical cooling in Nd-doped crystals and nanocrystalline powders

The first experimental demonstration of local internal and bulk optical cooling in samples of Nd-doped KPb2Cl5 crystals and Nd-doped KPb2Cl5 nanocrystalline powders upon laser excitation between the 4F3/2 and 4F5/2 manifolds is reported. The possibility of controlling the dynamics of the bulk optical cooling process by adequately tuning the excitation wavelength is also demonstrated.