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Up to now colloidal photonic crystal (CPC) lasers essentially relied on photonic band gap edge effects, as a consequence of the poor passband quality achieved by previously reported engineering methods. In this paper, we demonstrate lasing oscillation in CPCs based on the defect mode passband effect. As defect mode, we introduce a highquality monolayer of silica spheres internally functionalized with laser dyes in a sandwiched CPC structure. This defect layer contains the gain medium for lasing action and at the same time breaks the translational symmetry of the crystal, resulting in a pronounced passband within the photonic band gap. The CPC acts as an optical resonator, effectively ensuring the feedback mechanism. The spectroscopic measurements and theoretical simulations match well and reveal that the relatively low-threshold lasing exhibited by the structure can uniquely be attributed to the efficient coupling of the spontaneous emission of the dye to the defect mode of the CPC. Our work provides a new promising strategy toward applications of functionalized self-assembled CPCs with a planar defect in all-optical switching, optoelectronics, and energy-harvesting, and even in the future generation of electro-optical devices, such as lab-on-a-chip with allintegrated optical spectroscopy techniques.

Exceptional points (EPs) are degeneracies in open wave systems with coalescence of at least two energy levels and their corresponding eigenstates. In higher dimensions, more complex EP physics not found in two-state systems is observed. We consider the emergence and interaction of multiple EPs in a four coupled optical waveguides system by non-Hermitian coupling showing a unique EP formation pattern in a phase diagram. In addition, absolute phase rigidities are computed to show the mixing of the different states in definite parameter regimes. Our results could be potentially important for developing further understanding of EP physics in higher dimensions via generalized paradigm of non-Hermitian coupling for a new generation of parity-time (PT) devices. It is well-known that systems with open boundaries 1 or material loss and gain 2 can be described by Hamiltonians that are non-Hermitian. Non-Hermitian systems have recently attracted great scientific interests, both theoretically and experimentally, in open systems with energy gain or loss. A non-Hermitian Hamiltonian can exhibit many of intriguing phenomena beyond that of a Hermitian system. Consequently, researchers have become increasingly aware of the potential effects and applications reminiscent of the non-Hermitian systems. For instance, a non-Hermitian Hamiltonian have nonorthogonal eigenfunctions with complex eigenvalues where the imaginary part corresponding to decay or growth 2 . At certain points in parameter space fin as exceptional points (EPs) 3 or non-Hermitian

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