Abnormal refraction and reflection refers to the phenomenon in which light does not follow its traditional laws of propagation and instead is subject to refraction and reflection at abnormal angles that satisfy a generalization of Snell’s law. Metasurfaces can realize this phenomenon through appropriate selection of materials and structural design, and they have a wide range of potential applications in the military, communications, scientific, and biomedical fields. This paper summarizes the current state of research on abnormal refractive and reflective metasurfaces and their application scenarios. It discusses types of abnormal refractive and reflective metasurfaces based on their tuning modes (active and passive), their applications in different wavelength bands, and their future development. The technical obstacles that arise with existing metasurface technology are summarized, and prospects for future development and applications of abnormal refractive and reflective metasurfaces are discussed.
The luminescence thermal enhancement
(LTE), in lanthanide-doped
upconverting nanocrystals (UCNCs), has attracted extensive attention
for its applications in anticounterfeiting, temperature sensing, and
so on. Some mechanisms for LTE were proposed, including thermally
induced water desorption from the surface of UCNCs, but conclusive
evidence is absent for any of the mechanisms. Here, vacuumizing is
employed in the experiment for the LTE in NaYF4:Ho3+/Yb3+ UCNCs, and significantly promoted LTE is
observed. The 300–450–300 K temperature cycling experiment
in vacuum shows continuous luminescence enhancement without luminescence
intensity recovery as in the cooling process in moisture air, indicating
the effect of water desorption because water readsorption is blocked
in vacuum. It is found that the Ho3+ 5I6 level can be deexcited by direct long-range energy transfer to water
adsorbed at the surface of nanocrystals due to resonant coupling between
the Ho3+ 5I6 → 5I7 transition and the −OH vibration of water. As a result,
water desorption suppresses the 5I6 depopulation
and thus enhances the luminescence of Ho3+. Finally, the
regulation of upconversion LTE is realized by adjusting the size of
nanocrystals and Ho3+ or Yb3+ concentrations.
Our findings indicate that the vacuumizing technique is an effective
method to distinguish water-desorption-induced LTE. This work deepens
the understanding of LTE and offers insights into the regulation of
LTE.
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