We present a short review of some optical devices based on multilayered porous silicon, which can be easily obtained by varying the formation current during the etching process. These include Bragg reflectors and Fabry–Perot microcavities, which can be adjusted from the visible to the near infrared. The interface roughness, tragic in the case of multilayers, is studied. It can be drastically reduced when changing the electrolyte viscosity. The high reflectivities obtained in this way are measured by Cavity Ring–Down Spectroscopy. Problems occurring when realising thin layers and an efficient way to adjust precisely the optical thicknesses of the thin layers constituting the multilayered structure are also presented. Finally we present a method of calculation of the emission which takes absorption into account and is able to explain the angular dependence of the luminescence.

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Optical properties of multilayered porous silicon

We have measured an enhancement factor of Raman signal up to 30 times using a Fabry-Perot structure made of porous silicon (PS) layers of different porosity. The obtained enhancement was due to the coupling of the laser radiation and Stokes photons of porous silicon with the microcavity mode at the optimal laser beam incidence and scattering angles. Our results provide a way to increase the sensitivity of Raman spectroscopy for studying the species inside porous silicon which can considerably influence the properties of this material and hence of PS based devices.

Raman scattering enhancement in porous silicon microcavity

The experimental spectral dependence of the intensity of the second harmonic (SH) generated in microcavities based on porous silicon photonic crystal demonstrates resonant intensity enhancement (by a factor of ∼2×102) in the vicinity of the cavity mode and at the edges of the photonic band gap. The enhancement is due to the combined effect of pump radiation localization inside the microcavity, multiple SH interference in the photonic crystal, and two-photon resonance of the porous silicon quadratic susceptibility at the SH frequency.

Giant second harmonic generation in microcavities based on porous silicon photonic crystals

Summary form only given. In this paper the electrochemical fabrication of coupled cavities microstructures based on porous silicon photonic crystals with the cavity modes located inside the photonic band gap is reported. The enhancement of the second-harmonic (SH) response of such structures in the vicinity of cavity modes is experimentally observed.

https://ieeexplore.ieee.org/iel5/8015/22145/01031118.pdf

Giant second-harmonic generation in coupled microcavities based on porous silicon photonic crystals

This paper presents a detailed study, based on Fourier transform infrared (FTIR) analysis, on the derivatisation of p + -type porous silicon (PS) via thermal activated hydrosilylation of α-undecene. The α-undecene was used as a model for alkene precursor to calculate the efficiency of the hydrosilylation reaction, determined by comparing the integrated intensity of the SiH x peaks (2200–2000 cm −1 ) of the as-prepared and modified sample, by varying three parameters: the reaction time, the reagent concentration and the sample thickness. This study aims to optimize the PS derivatisation as a function of the thickness, in order to consider the thermal activation as a useful process for modifying PS slab waveguide for sensing application. Moreover two methods using ω-carboxy alkenes (where the carboxylic moiety was an ester or an acid) to obtain a PS surface –COOH-terminated, to be exploited for bioconjugation protocols, were tested and the reactions efficiency compared with the alkene model, i.e. α-undecene. FTIR spectroscopy and contact angle measurements were used to characterize the modified PS samples.

Effect of the reaction time, reagent concentration and sample thickness on the thermal derivatisation of P+ porous silicon with α-undecene and ω-carboxy alkenes : A FTIR investigation

A Fabry–Perot microcavity centred in the infrared around 2000–2200 cm−1 has been realised by controlling the current and the time of a steps anodization procedure of a p+ type silicon wafer in an electrochemical cell with a hydrofluoric acid solution. The interaction of the cavity mode, detectable as a sharp deep in the high reflectivity plateau, with the Si–H vibration modes gives rise to a splitting of the cavity mode dispersion curve (frequency vs wavevector) and to an enhancement of the infrared absorption of the Si–H bands. Both, the dispersion of the coupled modes (cavity/Si–H modes) and their infrared absorption enhancement have been studied.

G Mattei

Papers

Splitting of porous silicon microcavity mode due to the interaction with Si–H vibrations