Method for observing a fluorescent sample, the sample comprising a fluorescent agent that emits fluorescence light, in a fluorescence spectral band ( λ f), when it is illuminated by excitation light, in an excitation spectral band ( λ e), the method comprising the following steps: a) placing the sample against a holder; b) illuminating the sample, with a light source, in the excitation spectral band ( λ e), 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; the method being such that the holder is placed between the light source and the image sensor, the method being characterized in that the holder comprises a thin layer formed from a first material, of a first refractive index (n 1), the thin layer lying in a holder plane (P 20) 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 (n 2) different from the first refractive index (n 1), the first inclusions defining a first periodic pattern, the first inclusions being distributed with a first period (Pix), parallel to the longitudinal axis, the first inclusions forming a first photonic crystal that is resonant in the excitation spectral band ( λ e); - second inclusions formed from a third material, of a third refractive index (n 3) different from the first refractive index, and forming a second periodic pattern, the second inclusions being distributed with a second period (P 2X), 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 ( λ f); - at least two second photonic crystals being placed on either side of a first photonic crystal; such that: - some of the light emitted by the light source, at the excitation wavelength, is confined in the thin layer, under the effect of the first photonic crystal, resulting in an amplification of a light wave that propagates at an interface between the thin layer and the sample; - 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. a) placing the sample against a holder; b) illuminating the sample, with a light source, in the excitation spectral band ( λ e), 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; - first inclusions formed from a second material, of a second refractive index (n 2) different from the first refractive index (n 1), the first inclusions defining a first periodic pattern, the first inclusions being distributed with a first period (Pix), parallel to the longitudinal axis, the first inclusions forming a first photonic crystal that is resonant in the excitation spectral band ( λ e); - second inclusions formed from a third material, of a third refractive index (n 3) different from the first refractive index, and forming a second periodic pattern, the second inclusions being distributed with a second period (P 2X), 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 ( λ f); - at least two second photonic crystals being placed on either side of a first photonic crystal; - some of the light emitted by the light source, at the excitation wavelength, is confined in the thin layer, under the effect of the first photonic crystal, resulting in an amplification of a light wave that propagates at an interface between the thin layer and the sample; - 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.