Ultralow-loss optical interconnect enabled by topological unidirectional guided resonance.

Grating couplers that interconnect photonic chips to off-chip components are crucial for various optoelectronics applications. Despite numerous efforts in past decades, the existing grating couplers are still far from optimal in energy efficiency and thus hinder photonic integration toward a larger scale. Here, we propose a strategy to achieve ultralow-loss grating couplers by using unidirectional guided resonances (UGRs), suppressing the useless downward radiation with no mirror on the bottom. By engineering the dispersion and apodizing the geometry of grating, we experimentally realize a grating coupler with a record-low loss of -0.34 dB and 1-dB bandwidth exceeding 30 nm at the telecom wavelength of 1550 nm and further demonstrate an optic via with a loss of only -0.94 dB. Given that UGRs ubiquitously exist in a variety of grating geometries, our work sheds light on a systematic method to achieve energy-efficient optical interconnect and paves the way to large-scale photonic integration.

Topological Unidirectional Guided Resonances Emerged from Interband Coupling.

Unidirectional guided resonances (UGRs) are optical modes in photonic crystal slabs that radiate toward one side without the need for mirrors on the other. In this Letter, we report a mechanism to realize UGRs by tuning the interband coupling effect originating from up-down symmetry breaking. We theoretically find that UGRs that reside along high-symmetric lines correspond to phase singularities of far-field radiation, depicted by phase winding numbers as a type of topological indices. We investigate the phase dislocation lines in three-dimensional parameter space and elaborate on the interplay between UGRs and non-Hermitian degeneracies accordingly. Our findings reveal the topological nature of UGRs about their generation, evolution, and annihilation in general parameter spaces, thus paving the way to new possibilities of light manipulation.

Observation of miniaturized bound states in the continuum with ultra-high quality factors.

Light trapping is a constant pursuit in photonics because of its importance in science and technology. Many mechanisms have been explored, including the use of mirrors made of materials or structures that forbid outgoing waves, and bound states in the continuum that are mirror-less but based on topology. Here we report a compound method, combining lateral mirrors and bound states in the continuum in a cooperative way, to achieve a class of on-chip optical cavities that have high quality factors and small modal volumes. Specifically, light is trapped in the transverse direction by the photonic band gap of the lateral hetero-structure and confined in the vertical direction by the constellation of multiple bound states in the continuum. As a result, unlike most bound states in the continuum found in photonic crystal slabs that are de-localized Bloch modes, we achieve light-trapping in all three dimensions and experimentally demonstrate quality factors as high as Q=1.09×106 and modal volumes as low as [Formula: see text] in the telecommunication regime. We further prove the robustness of our method through the statistical study of multiple fabricated devices. Our work provides a new method of light trapping, which can find potential applications in photonic integration, nonlinear optics and quantum computing.

All-pass phase shifting enabled by symmetric topological unidirectional guided resonances.

All-pass phase shifting (APS), which involves a wave propagating at a constant, unitary amplitude but with pure phase variation, is extremely desired in many optoelectronic applications. In this work, we propose a method of realizing APS by out-of-plane excitation of a topologically enabled unidirectional guided resonance (UGR), which resides in a photonic crystal slab with P or C2z symmetries. Briefly, the symmetries and unidirectional features reduce the number of ports to one that simultaneously adds or drops energy. As a result, the phase independently shifts by varying the frequency but the amplitude remains as unitary under plane wave excitation. Theory and simulations confirm our findings. A paradox that the background contribution deviates from Fabry-Perot resonance is clarified from a multi-resonances picture.

Analytical theory of finite-size photonic crystal slabs near the band edge.

An analytical three-dimensional (3D) coupled-wave theory (CWT) for the finite-size photonic crystal slabs (PhCs) has been presented to depict the discretized modes at band-edges residing inside and outside the continuum. Specifically, we derive the CWT equations of slow-varying envelop function of dominant Bloch waves. By combining the trial solutions that are composed of a basis of bulk states with appropriate boundary conditions (B.C.), we analytically solve the equations and discuss the far-field patterns, asymptotic behavior and flatband effect of the finite-size modes, respectively. The proposed method presents a clear picture in physics for the origins of finite-size modes and provides an efficient and comprehensive tool for designing and optimizing PhC devices such as PCSELs.

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 waves-that 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 'unidirectional guided resonances', are found to be topological in nature: they emerge when a pair of half-integer topological charges1-3 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 × 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.

Because of their ability to confine light, optical resonators1-3 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 continuum-each carrying a topological charge6-merge 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 × 105-12 times higher than those obtained with standard designs-and 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.

Demonstration of a thermo-optic phase shifter by utilizing high-Q resonance in high-index-contrast grating.

A thermo-optic phase shifter is proposed and demonstrated by utilizing the high-Q resonance in high-index-contrast grating (HCG). The Q-factor up to ∼12000 is measured in a footprint of 110 µm×300 µm. By heating the HCG with paired metal strip micro-heaters, the optical resonance shifts, which induces phase modulation. A phase shift of ∼1.2π under heating power of ∼32 mW is directly observed and demodulated from the fringes shifting in a Michelson interferometer. The proposed configuration can also be extended to realize high-speed phase shift by adopting electro-optical modulation.

Analytical coupled-wave model for photonic crystal surface-emitting quantum cascade lasers.

An analytical coupled-wave model is developed for surface-emitting photonic-crystal quantum cascade lasers (PhC-QCLs). This model provides an accurate and efficient analysis of full three-dimensional device structure with large-area cavity size. Various laser properties of interest including the band structure, mode frequency, cavity loss, mode intensity profile, and far field pattern (FFP), as well as their dependence on PhC structures and cavity size, are investigated. Comparison with numerical simulations confirms the accuracy and validity of our model. The calculated FFP and polarization profile well explain the previously reported experimental results. In particular, we reveal the possibility of switching the lasing modes and generating single-lobed FFP by properly tuning PhC structures.