Porous silicon (PSi) on p${}^{++}$-type (111) silicon substrate has been fabricated by electronically etching method in hydrofluoric acid (HF) media from 5 to 110 mA/cm${}^2$ of anodizing current density. The problem of determining the optical properties of (111) PSi is board through implementing a photoacoustic (PA) technique coupled to an electrochemical cell for real-time monitoring of the formation of porous silicon thin films. PA amplitude allows the calculation of the real part of the films refractive index and porosity using the reflectance self-modulation due to the interference effect between the PSi film and the substrate that produces a periodic PA amplitude. The optical properties are studied from specular reflectance measurements fitted through genetic algorithms, transfer matrix method (TMM), and the effective medium theory, where the Maxwell Garnett (MG), Bruggeman (BR), and Looyenga (LLL) models were tested to determine the most suitable for pore geometry and compared with the in situ PA method. It was found that (111) PSi exhibit a branched pore geometry producing optical anisotropy and high scattering films.

This work investigates the changes in the optical response of photonic crystals based on porous silicon (PSi) as a function of temperature. Using the transfer matrix method in combination with thermo-optical properties, we numerically calculate the optical response of two types of photonic crystals: Distributed Bragg Reflectors (DBRs) and Fabry–Perot microcavities (FPMs). The results reveal that the photonic bandgap shifts with increasing temperature and pressure, with the defect mode in the microcavity notably shifting to longer wavelengths as the temperature rises. Additionally, we explore the transformation of PSi into porous quartz (PQz) via thermal oxidation, which preserves the porosity and multilayer structure, while altering the chemical composition. This results in geometrically identical photonic systems with distinct chemical properties, offering enhanced stability. Our simulations show that PSi structures exhibit a redshift in the photonic bandgap due to thermal expansion, while PQz structures remain optically stable even at elevated temperatures. This work highlights the potential of PQz as a robust material for high-temperature photonic applications, with tunable optical properties and stable performance under extreme conditions. The findings emphasize the feasibility of using porous-silicon-based photonic crystals for advanced optical devices in harsh environments.

Human breath contains a myriad of analytes that are associated with human health conditions, making it a noninvasive source of biomarkers. Researchers and engineers are pushing the boundaries of breath analysis for the next generation of diagnostics. Breath analysis with surface enhanced Raman spectroscopy, which allows for the identification of molecular fingerprints in a simple way, represents a powerful platform for the next generation of sensors which are expected to provide tools for personalized healthcare and preventive medicine.

This work proposes an optimization tool based on genetic algorithms for the inverse design of photonic crystals. Based on target reflectance, the algorithm generates a population of chromosomes where the genes represent the thickness of a layer of a photonic crystal. Each layer is independent of another. Therefore, the sequence obtained is a disordered configuration. In the genetic algorithm, two dielectric materials are first selected to generate the population. Throughout the simulation, the chromosomes are evaluated, crossed over, and mutated to find the best-fitted one based on an error function. The target reflectance was a perfect mirror in the visible region. As a result, it was found that obtaining photonic crystal configurations with a specific stop band with disordered arrangements is possible. The genetic information of the best-fitted individuals (layer sequence, optical response, and error) is stored in an h5 format. This method of generating artificial one-dimensional photonic crystal data can be used to train a neural network for solving the problem of the inverse design of any crystal with a specific optical response.

Sebum, a lipid-rich biofluid secreted onto the skin surface, holds promise as a non-invasive biomarker source but poses analytical challenges due to its hydrophobicity. Here, we demonstrate that aluminum foil substrates enable direct, solvent-free sampling and Raman spectroscopic analysis of human sebum, preserving key vibrational signatures of lipids, proteins, and carotenoids. We compared unprocessed sebum with chloroform/methanol and diethyl ether extraction methods in three volunteers, finding that solvent-free sampling yields more intense and chemically rich Raman spectra, while solvent-based methods attenuate signals and introduce compositional bias. Differential spectral analysis revealed that solvent extraction modifies the carbonyl and hydrocarbon stretching regions in a solvent-dependent manner. Our technical note focuses specifically on how sample processing affects the Raman spectral signature of sebum, underscoring the importance of solvent-free approaches and providing a methodological baseline for standardized Raman-based sebum analysis.