Paul A Snow

Vapor sensing using the optical properties of porous silicon Bragg mirrors

Large wavelength shifts have been measured in the reflectivity spectra of Bragg mirrors etched in porous silicon after exposure of the mirrors to vapor from organic solvents. The shift in the Bragg wavelength of the mirror arises from refractive index changes, induced by capillary condensation of the vapor in the mesoporous silicon, in the layers of the mirrors. Modeling of the reflectivity changes shows that the layer liquid volume fraction occurring in the measurements was 0.29 for acetone and 0.33 for chlorobenzene. Time-resolved measurements show that condensation occurs on the time scale of tens of seconds.

Derivatized Porous Silicon Mirrors: Implantable Optical Components with Slow Resorbability

The stability of derivatized mesoporous silicon mirrors in simulated human blood plasma has been assessed. The rate at which they are dissolved in-vivo is predicted to be tunable by surface chemistry over timescales of weeks to years, and high reflectivity can be maintained until the bottom of the multilayer stack starts to corrode. Such biodegradable optical components could be utilized to direct and define optical path lengths for therapeutic treatments and minimally-invasive diagnostics.

Time-resolved sensing of organic vapors in low modulating porous silicon dielectric mirrors

Time-resolved vapor diffusion into low modulating porous silicon dielectric mirrors has uncovered salient information about the spatial concentration distribution of the vapor within the porous matrix. Results indicate that relatively high concentrations of vapor result in instantaneous capillary condensation in the pores and yield a uniform concentration distribution. Results are modeled for both a concentration gradient and uniform filling of the porous matrix and compared to the experimental observations. Experiments with acetone, butanone, and pentanone have shown a linear dependence of the capillary condensation saturation time on the molecular weight of the organic solvents. An indicator of a concentration gradient within the matrix has been identified in the form of the full width at half maximum of the first-order reflectivity peak.

Hypersonic acoustic mirrors and microcavities in porous silicon

Periodic solid state structures exhibit transmission stop bands for waves of certain frequencies. We report the realization and direct measurement of acoustic band gaps in porous silicon multilayer structures which exhibit ∼50 dB stop bands for longitudinal acoustic waves in the gigahertz range. Furthermore, realization of an acoustic microcavity structure in porous silicon is demonstrated.

Elastic properties of porous silicon studied by acoustic transmission spectroscopy

The porosity dependence of the elastic properties of porous silicon in different crystallographic directions is studied. The velocity of longitudinal acoustic waves in porous silicon layers electrochemically etched in (100), (110), and (111) oriented wafers has been measured by acoustic spectroscopy in the gigahertz frequency range. This non-destructive method was used for porous silicon layers with porosity of 25-85% obtaining velocities in the range of about 1 to 7 km s-1. The implication of constant Poisson’s ratio of porous silicon is examined. The effect of velocity dispersion due to multiple scattering is considered. The c11 stiffness constant can be obtained from the velocity measurement in the [100] direction of a cubic crystal. We show that, using the results for velocity in [110] or [111] directions and Keating’s relation, the stiffness constants c12 and c44 can be obtained. The velocity dependence on porosity was fitted as v=v0(1-φ)κ, where v0 is the velocity in bulk silicon, φ is porosity, and...

Mechanisms of luminescence tuning and quenching in porous silicon

Voltage-dependent tuning of photoluminescence (PL) and electroluminescence (EL) spectra of porous silicon layers grown on n+ and n− substrates has been studied using electrolyte contacts. It has been demonstrated that PL and EL tuning occur under accumulation conditions. A mechanism that explains both PL and EL tuning is proposed on the basis of a kinetic; “scheme of squares” that considers incremental electron injection into quantum-confined particles, leading to Auger quenching of photoexcited or electrochemically generated electron-hole pairs. The tuning effects are predicted to occur as a consequence of the dependence of electron accumulation on the energy of the conduction band, which is turn related to particles size. PL and EL spectra have been simulated for an ensemble of particles of different sizes, and it is shown that the model correctly predicts all the experimentally observed characteristics of the voltage tuning.

Hypersonic rugate filters based on porous silicon

Periodic solid state structures exhibit transmission stopbands for waves of certain frequencies. We demonstrate porous silicon based rugate filters with 40 dB rejection first-order stopbands for longitudinal acoustic waves at hypersonic frequencies and the predicted suppression of higher order bands.

All‐optical modulation with ultrafast recovery at low pump energies in passive InGaAs/InGaAsP multiquantum well waveguides

We report on the observation of ultrafast (<1 ps) recovery of all‐optical modulation based on below band gap, optically induced excess absorption using pump pulse energies of less than 4 pJ. At higher pump pulse energies, the hot carriers produced by two‐photon absorption significantly reduce the recovery rate. The increase in absorption recovery times at the higher densities of hot carriers is consistent with a contribution to the nonlinear absorption change from hot electron assisted absorption. The measured hot electron assisted absorption cross section of 2×10−20 m2 is in good agreement with the theoretical value.

Strong modification of photoluminescence in erbium-doped porous silicon microcavities

A microcavity composed of porous silicon multilayer mirrors was electrochemically etched and doped with erbium. Measurements of the reflectivity and photoluminescence spectra are presented. Thermal processing under a nitrogen atmosphere optically activated the erbium ions. Photopumping yielded room temperature emission around 1.54 μm from the erbium-doped samples with the emitted light strongly modified by the microcavity structure. Emission spectra with a peak at 1.536 μm had a full width at half maximum of ∼6 nm.

Optimized light emission from layered porous silicon structures

We discuss in detail the physics of light emission from porous silicon microcavities formed by periodically modulating the porosity to produce multilayered structures. Changing the porosity alters not only the refractive index and absorption but also the luminescence, resulting in a complex interplay of effects that has not yet been addressed in the literature as far as we know. A transfer matrix model is developed that accounts for the dispersion of the refractive index, absorption, and photoluminescence. A multilayer porous silicon mirror is found to emit light almost as well as a conventional distributed feedback microcavity system with a mid-stop-band resonant state.