Q uasicrystals1-4 are a class of lattices characterized by a lack of translational symmetry. Nevertheless, the points of the lattice are deterministically arranged, obeying rotational symmetry. Thus, we expect properties that are different from both crystals and glasses. Indeed, naturally occurring electronic quasicrystals (for example, AlPdMn metal alloys) show peculiar electronic, vibrational and physico-chemical properties. Regarding artificial quasicrystals for electromagnetic waves, three-dimensional (3D) structures have recently been realized at GHz frequencies 5 and 2D structures have been reported for the near-infrared region 6-9 . Here, we report on the first fabrication and characterization of 3D quasicrystals for infrared frequencies. Using direct laser writing 10,11 combined with a silicon inversion procedure 12 , we achieve high-quality silicon inverse icosahedral structures. Both polymeric and silicon quasicrystals are characterized by means of electron microscopy and visible-light Laue diffraction. The diffraction patterns of structures with a local five-fold real-space symmetry axis reveal a ten-fold symmetry as required by theory for 3D structures.Quasicrystals are different from both crystals and glasses: crystals have long-range translational symmetry, whereas glasses show only short-range order. Quasicrystals show long-range order but not in a repeating fashion yielding periodicity [1][2][3][4] : although the local arrangements of atoms are fixed in a regular pattern, each atom has a different atom configuration surrounding it. The Laue diffraction pattern of quasicrystals can, for example, show peaks with a five-or ten-fold symmetry axis, whereas crystals can reveal only two-, three-, four-or six-fold symmetries. Quasicrystals can be viewed as a projection of a six-dimensional (6D) crystal to three dimensions 3,4 . Although nature provides us with 3D quasicrystals for electrons 1 , corresponding structures for light need to be fabricated artificially. Here, the projection procedure is not just a Gedanken experiment, but can rather be The 'central atom' is in the centre of the structure. b, The same as a, but the 'central atom' is outside the structure. c, Oblique-incidence overview of a. d, A focused-ion-beam cut of a structure corresponding to a, but oriented along a local two-fold symmetry axis, revealing an 3D structure.used for the actual fabrication. This was first realized in 2005 at microwave frequencies 5 . The subtle but important difference between real atoms in quasicrystals and the dielectric building blocks, 'photonic atoms' , in photonic quasicrystals is that real atoms can 'float' in vacuum via their binding potential. The
We investigate the optical properties of Dibenzoterrylene (DBT) molecules in a spin-coated crystalline film of anthracence. By performing single molecule studies, we show that the dipole moments of the DBT molecules are oriented parallel to the plane of the film. Despite a film thickness of only 20 nm, we observe an exceptional photostability at room temperature and photon count rates around 10 6 per second from a single molecule. These properties together with an emission wavelength around 800 nm make this system attractive for applications in nanophotonics and quantum optics.
Quantum technologies could largely benefit from the control of quantum emitters in sub-micrometric size crystals. These are naturally prone to integration in hybrid devices, including heterostructures and complex photonic devices. Currently available quantum emitters in nanocrystals suffer from spectral instability, preventing their use as single-photon sources for most quantum optics operations. In this work we report on the performances of single-photon emission from organic nanocrystals (average size of hundreds of nm), made of anthracene (Ac) and doped with dibenzoterrylene (DBT) molecules. The source has hours-long photostability with respect to frequency and intensity, both at room and at cryogenic temperature. When cooled to 3 K, the 00-zero phonon line shows linewidth values (50 MHz) close to the lifetime limit. Such optical properties in a nanocrystalline environment recommend the proposed organic nanocrystals as single-photon sources for integrated photonic quantum technologies.
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