dispersion by semiconductor devices based on the electron energy bandgap-the forbidden energies separating allowed energy bands. An example of photonic crystals with simple periodic arrays formed from dielectric spheres is shown in Figure 1. According to Bragg's equation, [3] the wavelength of the photons scattered from the crystal lattice can be calculated by the following formula (Equation (1)where d is the diameter of the sphere, m is the Bragg reflection order, and θ is the angle between the normal and incident light. The value of n a is defined as the weighted sum of sphere portion refractive indices and the gap portion (Equation (2)) [3] ∑ φ = n n i i a 2 2(2)where n is the refractive index of different components inside photonic crystals and φ i is the volume fraction of each i portion. For the close-packed structure, φ i of the sphere portion is 0.74. Photon propagation can be precisely controlled by designing a photonic crystal with a specific photonic bandgap. Hence, a future trend in the design of photonic crystals may lie in the modification and realization of spontaneous emission by light emitters integrated with these crystals. [4][5][6] Spontaneous emission refers to an optical process in which a quantum mechanical system in an excited state returns to a lower-energy or ground state and releases energy in the form of a photon. The quantum system could be an atom, molecule or nanocrystal. The photoluminescence (PL) produced by spontaneous emission plays a crucial role in conventional modern technologies used in daily lives, such as television screens (cathode ray tubes), plasma display panels, and fluorescence tubes.While spontaneous emission has enabled the progress of several technologies, uncontrolled spontaneous emission can limit the performance of photonic devices in many applications. One such limitation in device performance in light-emitting diodes (LEDs) occurs when an excessive number of photons generated from spontaneous emission are confined or trapped within the device. This shortcoming is also observed in laser operation when photon emission fails to couple with lasing processes, resulting in energy loss and noise in the signal output. Consequently, precise control over the propagation of spontaneous emission is critical. Due to their ability to manipulate light The modulation of luminescence is essential because unwanted spontaneousemission modes have a negative effect on the performance of luminescencebased photonic devices. Photonic crystals are promising materials for the control of light emission because of the variation in the local density of optical modes within them. They have been widely investigated for the manipulation of the emission intensity and lifetime of light emitters. Several groups have achieved greatly enhanced emission by depositing emitters on the surface of photonic crystals. Herein, the different modulating effects of photonic crystal dimensions, light-emitter positions, photonic crystal structure type, and the refractive index of photonic crystal building bl...

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Lanthanide‐doped nanophosphors are promising in anti‐counterfeiting and security printing applications. These nanophosphors can be incorporated as transparent inks that fluoresce by upconverting near‐infrared illumination into visible light to allow easy verification of documents. However, these inks typically exhibit a single luminescent color, low emission efficiency, and low print resolutions. Tunable resonator‐upconverted emission (TRUE) is achieved by placing upconversion nanoparticles (UCNPs) within plasmonic nanoresonators. A range of TRUE colors are obtained from a single‐UCNP species self‐assembled within size‐tuned gap‐plasmon resonances in Al nanodisk arrays. The luminescence intensities are enhanced by two orders of magnitude through emission and absorption enhancements. The enhanced emissive and plasmonic colors are simultaneously employed to generate TRUE color prints that exhibit one appearance under ambient white light, and a multicolored luminescence appearance that is revealed under near‐infrared excitation. The printed color and luminescent images are of ultrahigh resolutions (≈50 000 dpi), and enable multiple colors from a single excitation source for increased level of security.

Advanced nanoscale synthetic techniques provide a versatile platform for programmable control over the size, morphology, and composition of nanocrystals doped with lanthanide ions. Characteristic upconversion luminescence features originating from the 4f–4f optical transitions of lanthanides can be achieved through predesigned energy transfer pathways, enabling wide applications ranging from ultrasensitive biological detection to advanced spectroscopic instrumentation with high spatiotemporal resolution. Here, we review recent scientific and technological discoveries that have prompted the realization of these peculiar functions of lanthanide-doped upconversion nanocrystals and discuss the mechanistic studies of energy transfer involved in upconversion processes. These advanced schemes include cross relaxation-mediated depletion, multipulse sequential pumping, and nanostructural configuration design. Our emphasis is placed on disruptive technologies such as super-resolution microscopy, optogenetics, nanolasing, and optical anticounterfeiting, which take full advantage of the upconversion nanophenomena in relation to lanthanide-doped nanocrystals.