Compound-photonic structures with gain and loss 1 provide a powerful platform for testing various theoretical proposals on non-Hermitian parity-time-symmetric quantum mechanics 2-5 and initiate new possibilities for shaping optical beams and pulses beyond conservative structures. Such structures can be designed as optical analogues of complex parity-timesymmetric potentials with real spectra. However, the beam dynamics can exhibit unique features distinct from conservative systems due to non-trivial wave interference and phase-transition effects. Here, we experimentally realize paritytime-symmetric optics on a chip at the 1,550 nm wavelength in two directly coupled high-Q silica-microtoroid resonators with balanced effective gain and loss. With this composite system, we further implement switchable optical isolation with a non-reciprocal isolation ratio from 28 dB to 18 dB, by breaking time-reversal symmetry with gain-saturated nonlinearity in a large parameter-tunable space. Of importance, our scheme opens a door towards synthesizing novel microscale photonic structures for potential applications in optical isolators, on-chip light control and optical communications.One of the most fundamental postulates in canonical quantum mechanics, formulated by Dirac and von Neumann, mandates that the Hermiticity of each operator be directly associated with a physical observable. As such, the spectrum of a self-adjoint operator is ensured to be real and the total probability (or unitary evolution) is conserved. In 1998, however, Bender and colleagues 2 discovered a wide class of complex non-Hermitian Hamiltonians that can possess entirely real spectra below a certain phase-transition point, provided they satisfy combined parity-time (PT) symmetry. This counterintuitive discovery immediately aroused extensive theoretical interest in extending canonical quantum theory by including non-Hermitian but PT-symmetric operators 2-5 . For instance, a PT-symmetric Hamiltonian operator may contain a complex potential V(x) subject to a spatial-symmetry constraint V(x) ¼ V*(2x). One of the most striking properties of a PT-symmetric operator stems from the appearance of a sharp, symmetrybreaking transition once a non-Hermitian operator crosses a certain critical threshold 2-5 . On crossing that 'exceptional point', the spectrum ceases to be real and starts to become complex. This transition signifies the appearance of a spontaneous PT symmetry breaking from the exact-to the broken-PT phase. Despite much fundamental theoretical success in the development of PT-symmetric quantum mechanics, an experimental observation of pseudo-Hermiticity remains elusive and very challenging in real physical settings. Thanks to the formal equivalence between the quantum-mechanical Schrödinger equation and the paraxial optical diffraction equation, complex PT-symmetric potentials can be easily achieved in optics by spatially modulating the refractive index with properly placed gain and loss in a balanced manner 1 . This analogy immediately spurred theoretica...

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A transition-metal-free approach was disclosed for intermolecular aryl C-N bonds formation between phenols and cyclic anilines via cross-dehydrogenative coupling (CDC) amination that was mediated by visible light, wherein K2S2O8 served as an external oxidant. The salient features of this protocol include circumventing the requirement for prefunctionalized starting materials and achieving single regioselectivity of amination adducts at room temperature.

A novel visible-light-induced trifluoromethylarylation/1,4-aryl shift/desulfonylation cascade reaction using CF3SO2Cl as CF3 source was described. The protocol provides an efficient approach for the synthesis of α-aryl-β-trifluoromethyl amides and/or CF3-containing oxindoles as well as the isoquinolinediones under benign conditions.

The direct intermolecular aryl C–N coupling reaction from precursors without preactivated C–H and N–H bonds has been challenging. Herein, an oxidative system combining a catalytic amount of organic photocatalyst with stoichiometric amount of persulfate was developed to enable the successful cross-dehydrogenative-coupling amination between phenols and acyclic diarylamines in a nonmetallic method. This protocol precludes both coupling partners from prefunctionalization and achieved single regioselectivity of amination products under genuinely simple and benign conditions. Broad scopes of substrates were evaluated with moderate to high efficacy, and the reaction efficiency of electron-deficient phenothiazine and phenol was highly improved. A radical/radical cross-coupling pathway was proposed based on mechanistic studies, wherein a radical chain propagation process was involved.