The ionization equilibrium of an electron-hole plasma in a highly excited semiconductor is investigated. Special attention is directed to the influence of many-particle effects such as screening and lowering of the ionization energy causing, in particular, the Mott effect ͑density ionization͒. This effect limits the region of existence of excitons and, therefore, of a possible Bose-Einstein condensate at low temperatures. Results for the chemical potential and the degree of ionization are presented for zinc selenide ͑ZnSe͒. A possible window for the occurrence of a Bose-Einstein condensate of excitons is shown, taking into account the Mott effect.
Starting from the Maxwell-Lorentz equations, Poynting's theorem is reconsidered. The energy flux vector is introduced as Se = (E × B)/µ0 instead of E × H, because only by this choice the energy dissipation can be related to the balance of the kinetic energy of the matter subsystem. Conservation of the total energy as the sum of kinetic and electromagnetic energy follows. In our discussion, media and their microscopic nature are represented exactly by their susceptibility functions, which do not necessarily have to be known. On this footing, it can be shown that energy conservation in the propagation of light through bounded media is ensured by Maxwell's boundary conditions alone, even for some frequently used approximations. This is demonstrated for approaches using additional boundary conditions and the dielectric approximation in detail, the latter of which suspected to violate energy conservation for decades.
In display technologies or data processing, planar and subwavelength free-space components suited for flat photonic devices are needed. Metasurfaces, which shape the optical wavefront within hundreds of nanometers, can provide a solution for thin and portable photonic devices, e.g. as CMOS-compatible modules. While conventional electro-optic modulators are inconvenient to operate in free space configurations, its principle can largely be applied to the development of active metasurfaces with the prospect of modulation speeds up to the GHz region. Here, we use this concept to realize fast and continuous modulation of light at low voltage and MHz speed with a lithium niobate metasurface tuned by the linear electro-optic effect. Furthermore, we exploit the resonance in the visible to enhance the modulation of the transmitted light by two orders of magnitude, namely by a factor of 80, compared to the unstructured substrate. This proof-ofconcept work is a first important step towards the use of lithium niobate metasurfaces for free space modulation.
Metal‐oxides are promising candidates to substitute silicon in intra‐chip optical interconnects, as they exhibit great electric field tuning capabilities. The development of crystal ion slicing of thin films from bulk crystals and the advances over epitaxial growth have allowed the integration of metal‐oxides on a single chip. In terms of performance, they possess strong electro‐optic response over broad bandwidths across near‐infrared. However, lattice and thermal expansion coefficient mismatch limits the compatibility with available substrates and other materials, while physical hardness makes high quality nanostructures difficult to implement. Here, a novel concept of electro‐optic (EO) switching is introduced: an adjacent BaTiO3 nanoparticle film to a plasmonic metasurface provides reflection changes up to 0.15% under 4 V of control signal for modulation frequencies up to 20 MHz, in the near‐infrared. The nanoparticle films show EO coefficients (37.04 ± 25.6 pm V−1) comparable to lithium niobate crystals, are deposited uniformly over large scale and on any type of substrate, while retain optical nonlinear properties (e.g. second‐harmonic generation). Photonic nanostructures such as metasurfaces incorporated with nanoparticle films can harness the multifunctional properties of metal‐oxides such as BaTiO3 to form a new family of switchable nano‐devices across the entire visible to near‐infrared part of the spectrum.
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