M.B. de la Mora

Fluorescence tuning of confined molecules in porous silicon mirrors

Emission signal from fluorescent molecules (fluorescein-5-maleimide) in a porous silicon mirror is enhanced by tuning the pore size and reflectance spectrum of the porous silicon multilayer structure. This is achieved when the reflectance spectrum of the silicon mirror overlaps the fluorescent excitation and emission wavelengths of the fluorescent molecule, and chemical linkers assure the molecular confinement.

Anomalous patterned scattering spectra of one-dimensional porous silicon photonic crystals

Far-field secondary emission spectra of one-dimensional periodic photonic structures based on porous silicon show characteristic co-focal rings centered close to the structure plane normal. The rings appear when the frequency of picosecond excitation laser pulses is tuned to the edges of the fourth photonic band gap. They can be clearly distinguished from the typical reflected and transmitted light in the oblique incidence geometry. The rings number is dependent on the excitation frequency and the incidence angle. We explain these anomalous spectral features of porous silicon structures by the spectral filtering of light elastically scattered inside the photonic structure by the narrow photonic bands. The elastic scattering of light due to the photonic disorder in the structure causes the appearance of secondary waves propagating in any direction. But only those waves which fall into the allowed photonic bands penetrate through the whole structure and move through its front or back surfaces. The observed patterned secondary emission is an example of efficient photonic engineering by simple means of multilayer porous silicon structures.

The bifoil photodyne: a photonic crystal oscillator

Optical tweezers is an example how to use light to generate a physical force. They have been used to levitate viruses, bacteria, cells, and sub cellular organisms. Nonetheless it would be beneficial to use such force to develop a new kind of applications. However the radiation pressure usually is small to think in moving larger objects. Currently, there is some research investigating novel photonic working principles to generate a higher force. Here, we studied theoretically and experimentally the induction of electromagnetic forces in one-dimensional photonic crystals when light impinges on the off-axis direction. The photonic structure consists of a micro-cavity like structure formed of two one-dimensional photonic crystals made of free-standing porous silicon, separated by a variable air gap and the working wavelength is 633 nm. We show experimental evidence of this force when the photonic structure is capable of making auto-oscillations and forced-oscillations. We measured peak displacements and velocities ranging from 2 up to 35 microns and 0.4 up to 2.1 mm/s with a power of 13 mW. Recent evidence showed that giant resonant light forces could induce average velocity values of 0.45 mm/s in microspheres embedded in water with 43 mW light power.

Laser ablation efficiency during the production of Ag nanoparticles in ethanol at a low pulse repetition rate (1–10 Hz)

We studied the effect of the repetition rate of laser pulses (RRLP) in the range from 1–10 Hz in the production of silver nanoparticles (Ag-NPs) by laser ablation in ethanol. Laser pulses with a duration of 7 ns, a wavelength of 1064 nm and an energy of 60 mJ were used to ablate a 99.99% pure silver target immersed in 10 ml of ethanol. Transmittance analysis and atomic absorption spectroscopy were used to study the silver concentration in the colloidal solutions. The ablation process was studied by measuring the transmission of the laser pulses through the colloid. It is shown that for a fixed number of laser pulses (NLP) the ablation efficiency, in terms of the ablated silver mass per laser pulse, increases with the RRLP. This result contradicts what had previously been established in the literature.

A photonic self-oscillator based on porous silicon

Abstract We induced mechanical self-oscillations in a microcavity structure made of porous silicon onedimensional photonic crystals (PSi-1DPC) with an air gap. The electromagnetic force generated within the whole photonic structure, by light with a wavelength of 633 nm, is enough to overcome energy losses and sustain selfoscillations. From these mechano-optical measurements we estimated the stiffness and Young’s modulus of porous silicon and compared the results with values reported elsewhere and with values estimated herein by a mechanical method.We obtained good agreement between all values.

Heat transfer in photonic mirrors

The use of secondary mirrors in solar energy concentration is common. However, high concentrated solar radiation heats these mirrors thereby degrading their physical properties. In particular, aluminum mirrors melt because of high temperature due to storage by high radiative heat transfer. In contradistinction photonic crystals could present “perfect reflection” and they can be fabricated using porous silicon which has a higher melting point than aluminum (porous silicon has a melting point higher than 900 K). Porous silicon is a nanostructured semiconductor material which can be fabricated with different porosities and refractive indices. Multilayers of alternating periodic refractive index conform the structure of these photonic crystals. The light that propagates in these structures interacts with its periodic refractive index that generates wavelength gaps of forbidden transmission and so these multilayers conform a mirror. Even these photonic structures are heated when they are exposed to high concentrated solar radiation. In this work we experimentally analyze this heating process and model it using an effective medium approach to explain the increasing temperature behavior.