12 research outputs found
Maximal Spontaneous Photon Emission and Energy Loss from Free Electrons
Free electron radiation such as Cerenkov, Smith--Purcell, and transition
radiation can be greatly affected by structured optical environments, as has
been demonstrated in a variety of polaritonic, photonic-crystal, and
metamaterial systems. However, the amount of radiation that can ultimately be
extracted from free electrons near an arbitrary material structure has remained
elusive. Here we derive a fundamental upper limit to the spontaneous photon
emission and energy loss of free electrons, regardless of geometry, which
illuminates the effects of material properties and electron velocities. We
obtain experimental evidence for our theory with quantitative measurements of
Smith--Purcell radiation. Our framework allows us to make two predictions. One
is a new regime of radiation operation---at subwavelength separations, slower
(nonrelativistic) electrons can achieve stronger radiation than fast
(relativistic) electrons. The second is a divergence of the emission
probability in the limit of lossless materials. We further reveal that such
divergences can be approached by coupling free electrons to photonic bound
states in the continuum (BICs). Our findings suggest that compact and efficient
free-electron radiation sources from microwaves to the soft X-ray regime may be
achievable without requiring ultrahigh accelerating voltages.Comment: 7 pages, 4 figure
Aromatic Borozene
Based on our comprehensive theoretical investigation and known experimental
results for small boron clusters, we predict the existence of a novel aromatic
inorganic molecule, B12H6. This molecule, which we refer to as borozene, has
remarkably similar properties to the well-known benzene. Borozene is planar,
possesses a large first excitation energy, D3h symmetry, and more importantly
is aromatic. Furthermore, the calculated anisotropy of the magnetic
susceptibility of borozene is three times larger in absolute value than for
benzene. Finally, we can show that borozene molecules may be fused together to
give larger aromatic compounds with even larger anisotropic susceptibilities.Comment: 4 pages, 4 figures and 1 tabl