Device and method for observing a fluorescent sample

A method for observing a fluorescent sample, the sample comprising a fluorescent agent that emits fluorescence light, in a fluorescence spectral band, when it is illuminated by excitation light, in an excitation spectral band, the method comprising:

• a) placing the sample against a holder;
• b) illuminating the sample, with a light source, in the excitation spectral band, the light emitted by the light source propagating along a propagation axis;
• c) detecting fluorescence light, in the fluorescence spectral band, with an image sensor;
• wherein
• the holder comprises a thin layer formed from a first material, of a first refractive index, the thin layer lying in a holder plane perpendicular to the propagation axis, the holder plane extending along a longitudinal axis, the thin layer comprising:
• first inclusions formed from a second material, of a second refractive index different from the first refractive index, the first inclusions defining a first periodic pattern, the first inclusions being distributed with a first period, parallel to the longitudinal axis, the first inclusions forming a first photonic crystal that is resonant in the excitation spectral band;
• second inclusions formed from a third material, of a third refractive index different from the first refractive index, and forming a second periodic pattern, the second inclusions being distributed with a second period, parallel to the longitudinal axis, the second period being strictly longer than the first period, the second inclusions forming a second photonic crystal that is resonant in the fluorescence spectral band;
• at least two second photonic crystals being placed on either side of a first photonic crystal;
• such that:
• part of the light emitted by the light source, at the excitation wavelength, is confined in the thin layer, by the first photonic crystal, resulting in a formation and an amplification of a light wave that propagates at an interface between the thin layer and the sample, at the excitation wavelength;
• the fluorescence light propagates in the thin layer, through a first photonic crystal, while being reflected by the second photonic crystals placed on either side of the first photonic crystal, resulting in an amplification of the fluorescence light.