Abraham Vázquez-Guardado

Polarization-independent actively tunable colour generation on imprinted plasmonic surfaces.

Structural colour arising from nanostructured metallic surfaces offers many benefits compared to conventional pigmentation based display technologies, such as increased resolution and scalability of their optical response with structure dimensions. However, once these structures are fabricated their optical characteristics remain static, limiting their potential application. Here, by using a specially designed nanostructured plasmonic surface in conjunction with high birefringence liquid crystals, we demonstrate a tunable polarization-independent reflective surface where the colour of the surface is changed as a function of applied voltage. A large range of colour tunability is achieved over previous reports by utilizing an engineered surface which allows full liquid crystal reorientation while maximizing the overlap between plasmonic fields and liquid crystal. In combination with imprinted structures of varying periods, a full range of colours spanning the entire visible spectrum is achieved, paving the way towards dynamic pixels for reflective displays.

Wireless optoelectronic photometers for monitoring neuronal dynamics in the deep brain

Significance Wireless systems for imaging/recording neuronal activity in untethered, freely behaving animals have broad relevance to neuroscience research. Here, we demonstrate a thin, flexible probe that combines light sources and photodetectors into a platform with submillimeter dimensions, capable of direct insertion into targeted regions of the deep brain. This system allows wireless stimulation and recording of fluorescence associated with genetically encoded calcium indicators, with unique capabilities in visualizing neuronal activity. Studies using unconstrained, freely moving animal models in two different behavioral assays demonstrate the robust, reliable operation of these devices and allow comparisons to traditional photometry systems based on fiber-optic tethers to external light sources and detectors. Capabilities for recording neural activity in behaving mammals have greatly expanded our understanding of brain function. Some of the most sophisticated approaches use light delivered by an implanted fiber-optic cable to optically excite genetically encoded calcium indicators and to record the resulting changes in fluorescence. Physical constraints induced by the cables and the bulk, size, and weight of the associated fixtures complicate studies on natural behaviors, including social interactions and movements in environments that include obstacles, housings, and other complex features. Here, we introduce a wireless, injectable fluorescence photometer that integrates a miniaturized light source and a photodetector on a flexible, needle-shaped polymer support, suitable for injection into the deep brain at sites of interest. The ultrathin geometry and compliant mechanics of these probes allow minimally invasive implantation and stable chronic operation. In vivo studies in freely moving animals demonstrate that this technology allows high-fidelity recording of calcium fluorescence in the deep brain, with measurement characteristics that match or exceed those associated with fiber photometry systems. The resulting capabilities in optical recordings of neuronal dynamics in untethered, freely moving animals have potential for widespread applications in neuroscience research.

Unified Electromagnetic-Electronic Design of Light Trapping Silicon Solar Cells

A three-dimensional unified electromagnetic-electronic model is developed in conjunction with a light trapping scheme in order to predict and maximize combined electron-photon harvesting in ultrathin crystalline silicon solar cells. The comparison between a bare and light trapping cell shows significant enhancement in photon absorption and electron collection. The model further demonstrates that in order to achieve high energy conversion efficiency, charge separation must be optimized through control of the doping profile and surface passivation. Despite having a larger number of surface defect states caused by the surface patterning in light trapping cells, we show that the higher charge carrier generation and collection in this design compensates the absorption and recombination losses and ultimately results in an increase in energy conversion efficiency. The fundamental physics behind this specific design approach is validated through its application to a 3 μm thick functional light trapping solar cell which shows 192% efficiency enhancement with respect to the bare cell of same thickness. Such a unified design approach will pave the path towards achieving the well-known Shockley-Queisser (SQ) limit for c-Si in thin-film (<30 μm) geometries.

Multi-spectral infrared spectroscopy for robust plastic identification

The identification and classification of plastics plays an important role in waste management and recycling processes. Present electrical and optical sorting techniques lack the required resolution for accurate identification in a high throughput manner for a diverse set of plastics commonly found in municipal waste. In this work a multi-spectral infrared spectroscopic technique is employed to construct a unique fingerprint library of 12 plastic resin groups that are commonly encountered in municipal waste. We test the proposed method in a blind plastic identification experiment, which shows excellent unbiased identification accuracy. This simple optical technique in combination with the multi-spectral library will enable high throughput and accurate detection of various plastics from recovered solid waste.

Hybrid cavity-coupled plasmonic biosensors for low concentration, label-free and selective biomolecular detection.

Simple optical techniques that can accurately and selectively identify organic and inorganic material in a reproducible manner are of paramount importance in biological sensing applications. In this work, we demonstrate that a nanoimprinted plasmonic pattern with locked-in dimensions supports sharp deterministic hybrid resonances when coupled with an optical cavity suitable for high sensitive surface detection. The surface sensing property of this hybrid system is quantified by precise atomic layer growth of aluminum oxide using the atomic layer deposition technique. The analyte specific sensing ability is demonstrated in the detection of two dissimilar analytes, inorganic amine-coated iron oxide nanoparticles and organic streptavidin protein. Femto to nanomolar detection limits were achieved with the proposed coupled plasmonic system based on the versatile and robust soft nanoimprinting technique, which promises practical low cost biosensors.

Superchiral light generation on achiral nanostructured surfaces

In this work, we demonstrate superchiral light generation based on achiral plasmonic surfaces. At resonance, the symmetric cavity-coupled plasmonic system generates single-sign chiral near-field whose helicity is determined solely by the handedness of the incident light. We elucidate the mechanism for such unique superchiral near field generation and find its origin in coherent and synergetic interactions between plasmonic and photonic cavity modes. The cavity-coupling enhances otherwise weak plasmonic chiral near-field by many folds. Furthermore, the system in a unique way suppresses the far field chirality due to its totally symmetric geometry providing a route for surface-enhanced chiroptic spectroscopy on a single surface.