Dolores Golmayo

Self-assembly approach to optical metamaterials

Photonic crystals can be viewed just as a subclass of a larger family of material systems called metamaterials in which the properties largely derive from the structure rather than from the material itself. Opals have only a relatively recent history as photonic bandgap materials and have received a strong thrust from their adequacy as scaffoldings for further templating other materials with photonic applications for instance. The tortuous route from materials to devices might perhaps find reward in the ease and low cost of fabrication of these materials. In this paper we present a review of recent work and work under way in our laboratory tending towards synthesis based on self-assembly to realize metamaterials in the optical range. This comprises the formation of the templates (opals) and subsequent synthesis of guest materials such as semiconductors, metals and insulators. The possibility of further processing allows additional two-dimensional and quasi-two-dimensional patterning for the design of new structures. In this paper we show how the raw matter can be checked for quality and learn how to use its optical properties to evaluate application potential. Issues relating to the optical properties (such as crystalline quality, finite size effects and infiltration with other materials) are examined. We show some examples where opals are used to pattern the growth of other materials with photonic applications (such as metals and semiconductors) and developments leading to both vertical and lateral engineering are shown.

Transmittance photoplethysmography and pulse oximetry with near infrared laser diodes

Photoplethysmography and pulse oximetry are widely used techniques for noninvasive monitoring of heart rate and peripheral oxygen saturation. Over the last years we have been working onto the application of near infrared range wavelengths to replace those of the classical pulse oximeters. The development of laser diodes based sensors, processing algorithms, a calibration procedure, and the comparison with a commercial pulse oximeter, have been followed by a set of experimental studies. The results exposed here demonstrate the applicability of transmittance photoplethysmography and pulse oximetry using laser diodes emitting at specific wavelengths for a wide range of situations.

Electrical and optical properties of undoped InP grown at low temperature by atomic layer molecular beam epitaxy

The electrical and optical properties of undoped InP layers grown at low temperatures by solid source atomic layer molecular beam epitaxy are investigated. Phosphorus surface coverage during epitaxy is controlled by monitoring the evolution of reflection high‐energy electron diffraction pattern during growth. An accurate phosphorus supply by means of a valved cracking phosphorus cell is employed. The relation between phosphorus incorporation and the electronic properties of the epilayers is examined, and it is found that, at a substrate temperature of 340 °C, residual electron concentration increases linearly with phosphorus flux. Residual doping of InP layers grown at 340 °C has been reduced down to 1×1016 cm−3, and Hall mobilities of 3260 cm2/V s at 300 K and 14 830 cm2/V s at 65 K are reported. Low‐temperature photoluminescence of low background doping layers is dominated by near band transitions.

Tuning of spontaneous emission of two-dimensional photonic crystal microcavities by accurate control of slab thickness

We have found a blueshift in the cavity modes confined in two-dimensional photonic crystal microcavities when the thickness of the slab was varied uniformly by accurate dry etching. The shifts in the wavelength of the cavity modes were around 2nm towards shorter wavelengths per nanometer reduced in the thickness of the slab. Three-dimensional plane wave expansion calculations showed that the observed shifts are inside the calculated photonic band gap of the structures. The variation in the energy position of the peaks with the thickness has been analyzed by three-dimensional finite difference time domain calculations for a one missing hole microcavity. This tuning of the emission wavelength with the change in the thickness slab shows the important effect of the third dimension in photonic crystals made out of semiconductor slabs and it can be of interest for its application in the final processed photonic devices like photonic crystal lasers.

Improvement of the Temperature Characteristic of 1.3 µm GaInAsP Laser Diodes with GaInAsP/InP Short-Period Superlattice Barriers

We report on the characteristics of tensile-strained 1.3 µm InGaAsP multi-quantum well lasers with InP/GaInAsP short-period superlattices (SPSLs) in barriers and waveguide. Growth was carried out by all solid source atomic layer molecular beam epitaxy (ALMBE) without growth interruptions. Infinite length threshold current densities are as low as 176 A/cm2 per quantum well for as cleaved broad area lasers. The values for the characteristic temperature T0 are as high as 90 K for cavity lengths of 1200 µm. The improvement in T0 is attributed to the increased effective barrier height by the short-period superlattices.

Qualitative and Quantitative Analysis of Crystallographic Defects Present in 2D Colloidal Sphere Arrays

In this work, we present a study of the typical spontaneous defects present in self-assembled colloidal monolayers grown from polystyrene and silica microspheres. The quality of two-dimensional crystals from different colloidal suspensions of beads around 1 μm in diameter has been studied qualitatively and quantitatively, evaluated in 2D hexagonal arrays at different scales through Fourier analysis of SEM images and optical characterization. The crystallographic defects are identified to better understand their origin and their effects on the crystal quality, as well as to find the best conditions colloidal suspensions must fulfill to achieve optimal quality samples.

Three Regimes of Water Adsorption in Annealed Silica Opals and Optical Assessment

Physisorbed and structurally bound (surface and internal) water in silica opals are distinguished and quantified by thermogravimetry. By controlled dehydroxylation with thermal annealing, we correlate these forms of water with the silica chemistry. In particular, we find that the silica capability to physically adsorb water from ambient moisture exhibits three regimes, associated with the distinct condensation behavior of bonded and unbonded surface silanols. Features in both opal IR absorbance and photonic band gap reproduce the physisorbed water regimes. This allows direct assessment of the water content and its evolution just by routine optical spectroscopy, being a useful tool for local and nondestructive analysis of colloidal silica. Besides, this provides a simple recipe for accurate tuning of the opal photonic band gap (about 10% in position and width) by just selecting the annealing temperature.

Vanadium dioxide thermochromic opals grown by chemical vapour deposition

The semiconductor–metal transition that occurs in vanadium dioxide at 68 °C is the object of increasing interest in modern optics due to its applications in ultrafast optical devices. In a three-dimensional photonic crystal, the fine control of the structure is important for optimizing the switching magnitude and the spatial homogeneity. We report a controlled process to fabricate large-area high quality VO2–SiO2 opals with fine control over the filling volume. The method comprises two stages. The first stage is a chemical vapour deposition synthesis whereby the vanadium pentoxide is grown. The second one is a thermal annealing that allows reducing the vanadium pentoxide to vanadium dioxide. We have carried out a steady-state study of the semiconductor–metal transition, for opals with the Bragg peak around the 1.55 µm spectral region, by means of reflectance spectroscopy. As the temperature increases approaching the phase transition the intensity of the reflectance peak decreases and a small blueshift can be observed at 65 °C. When the phase transition is achieved at 68 °C the intensity of the reflectance peak decreases drastically and the Fabry–Perot oscillations disappear.

Light Emission from Nanocrystalline Si Inverse Opals and Controlled Passivation by Atomic Layer Deposited Al2O3

This work was partially supported by EU FP7 NoE Nanophotonics 4 Energy grant No. 248855; the Spanish MICINN CSD2007-0046 (Nanolight.es), CSD2008-00023 (FUNCOAT), CSD2009-00013 (Imagine), MAT2009-07841 (GLUSFA), TEC2008-06756-C03-02/TEC, CSIC PIF08-016 (Intramural Frontera), MAT2010-18432 and the Comunidad de Madrid S2009/MAT-1756 (PHAMA) projects. F. G. G. was supported by the JAE Postdoctoral Program from the CSIC. M. I. is a Ramon y Cajal researcher.

NIR transmittance pulse oximetry system with laser diodes

A transmittance pulse oximetry system based on near-infrared laser diodes (LD) for monitoring arterial blood hemoglobin oxygen saturation (So2) has been previously reported. In this work we present the results obtained after improvements in the sensor configuration, signal processing algorithm and calibration procedure. The pulse oximetry system also comprises the sensor electronics, and a data acquisition board installed on a handheld personal computer. The two LD chips are mounted on a single metal heat-sink and as photo- detectors are used silicon p-i-n photodiodes with the first amplifier stage situated in their back side. The real time calculation of the parameters related to So2 is carried out through a numeric separation of the pulsatile and non- pulsatile components of the photoplethysmographic signals for both wavelengths and a non-linear filtering. Patients with respiratory failure conditions were monitored as a part of the calibration procedure in order to cover a wide range of So2-values. A calibration curve have been derived through the determination of in vitro arterial So2 with a significant quantity of experimental points ranging from 60 to almost 100%. The obtained results demonstrate that it is possible to apply the proposed system to monitoring a wide range of oxygen saturation levels.