A tunable nano-optofluidic polymer optical filter based on guided-mode resonance

Optical filters with reconfigurable spectral properties are highly desirable in a wide range of applications. We propose and experimentally demonstrate a tunable submicro-optofluidic polymer guided-mode resonance (PGMR) filter. The device is composed of a periodic grating sandwiched between a high index waveguide layer and a low index capping layer, which integrates submicro-fluidic channel arrays and a PGMR filter elegantly. A finite difference time domain (FDTD) method is employed to understand the spectral properties and determine appropriate device parameters. We fabricated the polymer guided-mode resonance filter with a method combining two-beam interference lithography, floating nanofilm transfer and thermal bonding techniques. Experimental results show that our tunable submicro-optofluidic PGMR filters can provide a broad spectral tuning range (13.181 nm), a narrow bandwidth (85%) in the visible region. Such submicro-optofluidic PGMR filters are highly compatible with existing nano/microfluidic technologies and would be valuable for the integrated flexible optical system

Dislocated Double-Layer Metal Gratings: An Efficient Unidirectional Coupler

We propose theoretically and demonstrate experimentally a dislocated double-layer metal grating structure, which operates as a unidirectional coupler capable of launching surface plasmon polaritons in a desired direction under normal illumination. The structure consists of a slanted dielectric grating sandwiched between two gold gratings. The upper gold grating has a nonzero lateral relative displacement with respect to the lower one. Numerical simulations show that a grating structure with 7 periods can convert 49% of normally incident light into surface plasmons with a contrast ratio of 78 between the powers of the surface plasmons launched in two opposite directions. We explain the unidirectional coupling phenomenon by the dislocation-induced interference of the diffracted waves from the upper and lower gold gratings. Furthermore, we developed a simple and cost-effective technique to fabricate the structure via tilted two-beam interference lithography and subsequent shadow deposition of gold. The experimental results demonstrate a coupling efficiency of 36% and a contrast ratio of 43. The relatively simple periodic nature of our structure lends itself to large-scale low-cost fabrication and simple theoretical analysis. Also, unlike the previous unidirectional couplers based on aperiodic structures, the design parameters of our unidirectional coupler can be determined analytically. Therefore, this structure can be an important component for surface-plasmon-based nanophotonic circuits by providing an efficient interface between free-space and surface plasmon waves

Achieving Very Bright Mechanoluminescence from Purely Organic Luminophores with Aggregation-Induced Emission by Crystal Design

Although bright organic mechanoluminescence (ML) has been observed for a few luminophores with aggregation-induced emission (AIE), details of the positive effect of AIE on ML performance remain unclear and a feasible design principle for the AIE-ML compounds has not yet been presented. Herein, an effective strategy for the molecular design of efficient AIE-ML materials is demonstrated, based on tetraphenylethene (TPE) building blocks with formyl substituents, which yield non-centrosymmetric crystal structures with prominent piezoelectric properties for molecular excitation combined with AIE features for intense emission. Following this approach, three AIE-active compounds have been developed and found to show unique ML characteristics. Furthermore, the results of single crystal X-ray analysis and density functional theory (DFT) calculations suggest that the ML performance would probably be further enhanced by creating molecules with larger dipolar moments and enhanced AIE properties