Elastic Photonic Crystal Captures Fingerprints

Rubbery inverse opal films that change color under pressure promise diverse applications from biometrics and mechanical sensing to children’s toys and tamperproof packaging. Developed by researchers at the University of Toronto and at the European Laboratory for Non-Linear Spectroscopy in Florence, Italy, and reported in the March issue of Nature Materials, the photonic crystals also may find a place in tunable LEDs and lasers and in electrical sensors.

To produce the films, explained André C. Arsenault, a doctoral candidate in the lab of Geoffrey A. Ozin at the university, they infiltrate the space between packed 300- to 900 nm-diameter SiO₂ spheres with a photopolymerizable material and etch away the spheres with a dilute HF solution. The resulting foamlike inverse opal — which they have created in sheets several square centimeters in area and have transferred onto glass, metal and plastic substrates — displays a blueshift in its photonic stopgap when compressed. The effect is reversible, and the films can be subjected to thousands of stress cycles without significantly impairing their optical quality.

In demonstrations of potential applications of the elastic photonic crystal, the investigators collected color images of the back side of a sample as it was compressed by an index finger, revealing both the line ridges of the fingerprint and the pressure distribution on the fingertip. They also incorporated PbS colloidal quantum dots into the elastic material and found that compressing the film altered the dots’ photoluminescence spectra.