18 research outputs found
Analytical coupled-wave model for photonic crystal quantum cascade lasers
A coupled-wave model is developed for photonic-crystal quantum cascade
lasers. The analytical model provides an efficient analysis of full
three-dimensional large-area device structure, and the validity is confirmed
via simulations and previous experimental results.Comment: 21 pages and 8 figure
Origins and conservation of topological polarization defects in resonant photonic-crystal diffraction
We present a continuative definition of topological charge to depict the
polarization defects on any resonant diffraction orders in photonic crystal
slab regardless they are radiative or evanescent. By using such a generalized
definition, we investigate the origins and conservation of integer polarization
defects across the whole Brollouin zone. We found that these polarization
defects eventually originate from the mode degeneracy that is induced by
lattice coupling as a consequence of momentum space folding, or inter-band
coupling that can be either Hermitian or Non-hermitian. By counting all types
of polarization defects, the total topological charge numbers in a given
diffraction order is a conserved quantity across the whole Brillouin zone that
is determined by lattice geometry only
Topological unidirectional guided resonances emerged from interband coupling
Unidirectional guided resonances (UGRs) are optical modes in photonic crystal
(PhC) slabs that radiate towards one side without the need for mirrors on the
other, represented from a topological perspective by the merged points of
paired, single-sided, half-integer topological charges. In this work, we report
a mechanism to realize UGRs by tuning the interband coupling effect originating
from up-down symmetry breaking. We theoretically demonstrate that a type of
polarization singularity, the circular-polarized states (CPs), emerge from
trivial polarization fields owing to the hybridization of two unperturbed
states. By tuning structural parameters, two half-charges carried by CPs evolve
in momentum space and merge to create UGRs. Our findings show that UGRs are
ubiquitous in PhC slabs, and can systematically be found from our method, thus
paving the way to new possibilities of light manipulation
Observation of topologically enabled unidirectional guided resonances
Unidirectional radiation is important for various optoelectronic applications, such as lasers, grating couplers and optical antennas. However, almost all existing unidirectional emitters rely on the use of materials or structures that forbid outgoing wavesâthat is, mirrors, which are often bulky, lossy and difficult to fabricate. Here we theoretically propose and experimentally demonstrate a class of resonances in photonic crystal slabs that radiate only towards one side of the slab, with no mirror placed on the other side. These resonances, which we name âunidirectional guided resonancesâ, are found to be topological in nature: they emerge when a pair of half-integer topological charges1â3 in the polarization field bounce into each other in momentum space. We experimentally demonstrate unidirectional guided resonances in the telecommunication regime by achieving single-side radiative quality factors as high as 1.6 Ă 105. We further demonstrate their topological nature through far-field polarimetry measurements. Our work represents a characteristic example of applying topological principles4,5 to control optical fields and could lead to energy-efficient grating couplers and antennas for light detection and ranging
Topologically enabled ultrahigh-Q guided resonances robust to out-of-plane scattering
© 2019, The Author(s), under exclusive licence to Springer Nature Limited. Because of their ability to confine light, optical resonators1â3 are of great importance to science and technology, but their performance is often limited by out-of-plane-scattering losses caused by inevitable fabrication imperfections4,5. Here we theoretically propose and experimentally demonstrate a class of guided resonances in photonic crystal slabs, in which out-of-plane-scattering losses are strongly suppressed by their topological nature. These resonances arise when multiple bound states in the continuumâeach carrying a topological charge6âmerge in momentum space and enhance the quality factors Q of all nearby resonances in the same band. Using such resonances in the telecommunication regime, we experimentally achieve quality factors as high as 4.9 Ă 105â12 times higher than those obtained with standard designsâand this enhancement remains robust for all of our samples. Our work paves the way for future explorations of topological photonics in systems with open boundary conditions and for their application to the improvement of optoelectronic devices in photonic integrated circuits