Abstract
Photonic crystals (PhC) are spatially ordered structures with lattice parameters comparable to the wavelength of propagating light. Their geometrical and refractive index features lead to an energy band structure for photons, which may allow or forbid the propagation of electromagnetic waves in a limited frequency range. These unique properties have attracted much attention for both theoretical and applied research. Devices such as high-reflection omnidirectional mirrors, low-loss waveguides, and high- and low-reflection coatings have been demonstrated, and several application areas have been explored, from optical communications and color displays to energy harvest and sensors. In this latter area, photonic crystal fibers (PCF) have proven to be very suitable for the development of highly performing sensors, but one-dimensional (1D), two-dimensional (2D) and three-dimensional (3D) PhCs have been successfully employed, too. The working principle of most PhC sensors is based on the fact that any physical phenomenon which affects the periodicity and the refractive index of the PhC structure induces changes in the intensity and spectral characteristics of the reflected, transmitted or diffracted light; thus, optical measurements allow one to sense, for instance, temperature, pressure, strain, chemical parameters, like pH and ionic strength, and the presence of chemical or biological elements. In the present article, after a brief general introduction, we present a review of the state of the art of PhC sensors, with particular reference to our own results in the field of mechanochromic sensors. We believe that PhC sensors based on changes of structural color and mechanochromic effect are able to provide a promising, technologically simple, low-cost platform for further developing devices and functionalities.
Highlights
Photonic bandgap (PBG) crystals, often referred to as photonic crystals, are materials characterized by the periodic modulation of the dielectric constant along one, two, or three directions of space
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On the contrary, when employing chromatic structures, the detection is based on a visual response of the sensor, potentially avoiding any signal transduction; this characteristic could favor the diffusion of these systems as simple and safe devices usable by untrained end-users in different applications fields
Summary
Photonic bandgap (PBG) crystals, often referred to as photonic crystals, are materials characterized by the periodic modulation of the dielectric constant along one, two, or three directions of space. Micromachines 2020, 11, 290 stimuli: chemical, biological and physical These sensors may have an impact on our daily lives, Mraicnrogmianchginferso20m20e, 1n1v, 2i9r0onmental aspects to tumor screening markers or drug delivery, and to3sotfr2u5ctural lhiveeasl,thramngoinnigtofrrionmg. The application of this method leads to Equation (3): it is possible to calculate the reflected wavelength considering the center to-center distance. On the contrary, when employing chromatic structures, the detection is based on a visual response of the sensor, potentially avoiding any signal transduction; this characteristic could favor the diffusion of these systems as simple and safe devices usable by untrained end-users in different applications fields. It can be understood that, to boost the development of colorimetric sensors for different technological applications, it is necessary to create responsive artificial materials characterized by good selectivity, fast response rate, and excellent sensitivity. As a relatively recent trend in the materials science field, the design and fabrication of PhCs with peculiar structural colors has borrowed from nature (e.g., from the examples shown in Figure 2) [26,33,34]
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