Yibiao Yang

Enhanced normal-direction excitation and emission of dual-emitting quantum dots on a cascaded photonic crystal surface

Large normal-direction excitation and emission of dual-emitting quantum dots (QDs) are essential for practical application of QD sensors based on the ratiometric fluorescence response. We have numerically demonstrated an all-dielectric four-layer cascaded photonic crystal (CPC) structure (alternating TiO2 and SiO2/SU8 layers with two dimensional nanoscale patterns in each layer) which is capable of providing normal-direction high Q-factor leaky modes at excitation wavelengths of QDs and two low Q-factor leaky modes coinciding with the two emission peaks of a dual-emitting QD. Normal-direction excitation and far-field emission of the dual-emitting QDs are enhanced significantly when QDs are distributed on/in the top TiO2 layer of the CPC structure, especially in the spatial distribution areas of the resonant leaky modes. QDs can be positioned differently depending on the applications. Positioning QDs on the top TiO2 layer will improve the signal-to-noise ratios of QD biomedical/chemical/temperature sensors, while embedding QDs in the top TiO2 layer will increase the light extraction from the QD light emitting device, making our CPC a versatile optical coupling structure. Our CPC-QD structure is experimentally feasible and robust against the parameter perturbation in real fabrication.

Enhanced Broadband Electromagnetic Absorption in Silicon Film with Photonic Crystal Surface and Random Gold Grooves Reflector.

We show a hybrid structure consisting of Si film with photonic crystal surface and random triangular gold grooves reflector at the bottom, which is capable of realizing efficient, broad-band, wide-angle optical absorption. It is numerically demonstrated that the enhanced absorption in a broad wavelength range (0.3–9.9 μm) due to the scattering effect of both sides of the structure and the created resonance modes. Larger thickness and period are favored to enhance the absorption in broader wavelength range. Substantial electric field concentrates in the grooves of surface photonic crystal and in the Si film. Our structure is versatile for solar cells, broadband photodetection and stealth coating.

Tunable high reflective bands to improve quantum dot white light-emitting diodes

Quantum dot (QD) light emitting diodes (LEDs), whose narrow emission bandwidth can be tuned by changing the type and size of the QDs, show pure and saturated colors, and thus become a new generation of LEDs. However, the brightness of QD-LEDs should be increased to be used in practical applications. Here, we propose a simple 5-layered hetero-structure (alternating TiO2 and SiO2 layers) on a SiO2 substrate, which can provide multiple high reflection bands (HRBs) corresponding to UV, blue, green and red colors, in which the UV band can be used as excitation and the other bands can generate white light. On the one hand, HRBs can enhance the light extraction efficiency of fluorescence. On the other hand, HRBs can enhance the light–matter interaction (e.g., UV or violet light excitation of fluorescent materials) due to the interference from the multiple reflection and incidence. Thus the HRBs are capable of enhancing the brightness of white QD-LEDs efficiently. Moreover, our structure has fewer layers than one-dimensional photonic crystals, which leads to easier fabrications. Reflection bands of our structure can be adjusted by tuning the thickness of layers according to practical applications. The structure is environmentally friendly and can be used in displays, infrared illumination, optical communication, etc.

Ultra-wide tuning single channel filter based on one-dimensional photonic crystal with an air cavity

By inserting an air cavity into a one-dimensional photonic crystal of LiF/GaSb, a tunable filter covering the whole visible range is proposed. Following consideration of the dispersion of the materials, through modulating the thickness of the air cavity, we demonstrate that a single resonant peak can shift from 416.1 to 667.3 nm in the band gap at normal incidence by means of the transfer matrix method. The research also shows that the transmittance of the channel can be maximized when the number of periodic LiF/GaSb layers on one side of the air defect layer is equal to that of the other side. When adding a period to both sides respectively, the full width at half maximum of the defect mode is reduced by one order of magnitude. This structure will provide a promising approach to fabricate practical tunable filters in the visible region with ultra-wide tuning range.

Tunable dual-channel filter based on the photonic crystal with air defects

We propose a tuning filter containing two channels by inserting a defect layer (Air/Si/Air/Si/Air) into a one-dimensional photonic crystal of Si/SiO2, which is on the symmetry of the defect. Two transmission peaks (1528.98 and 1564.74 nm) appear in the optical communication S-band and C-band, and the transmittance of these two channels is up to 100%. In addition, this design realizes multi-channel filtering to process large dynamic range or multiple independent signals in the near-infrared band by changing the structure. The tuning range will be enlarged, and the channels can be moved in this range through the easy control of air thickness and incident angle.

Spatial remote luminescence enhancement by a half-cylindrical Au groove

Enhancing the luminescence emission from upconversion nanoparticles (UCNPs) is crucial to reduce the limits of biosensing based on UCNPs. Photonic crystal and surface plasmon structures have been used to enhance the emission of phosphor, while the enhancement distance is only limited to the near-field range. For in vitro biodetection, the distance between the UCNP and the substrate is usually in the far-field range. This work proposes the use of a half-cylindrical gold groove to enhance the emission of UCNPs even when the nanoparticles are at a far-field distance from the substrate. Due to scattering, constructive interference and coupling of optical modes in the Au groove, large-area/far-field range resonance modes can be created. Efficient resonance coupling between the optical mode and the excitation/emission of UCNPs can occur in the far-field range, which is able to enhance the absorption, quantum yield, extraction and collection of emission of UCNPs. The enhancement region is not only limited to the near-field range of the structure but also expands to the far-field range of the structure. By comparing different shapes and materials of the grooves, the proposed structure can improve the emission by 8.25 times when the UCNP is 4.4 μm away from the bottom of the groove compared with the bare glass. Besides, our structure is experimentally feasible and will greatly contribute to the development of high sensitivity and high SNR fluorescence biosensors.

Sharp convex gold grooves for fluorescence enhancement in micro/nano fluidic biosensing

The enhancement of biosensing sensitivity based on a quantum dot (QD) is limited by the long distance between the QD and the substrate in in vitro detection, which prevents the development of biosensors. Here an individual sharp convex gold groove is proposed to enhance remote fluorescence by exciting and collecting fluorescence efficiently. The structure shows a higher emission power than other wider gold groove structures when the QD is individually placed at five random positions inside the groove. Compared with bare glass, the total power enhancement factor of our structure is up to 17.0 times, 6.6 times and 6.4 times when the QD is 3.5 μm, 7.6 μm and 9.0 μm away from the bottom of the groove, respectively, due to the scattered emission of the QD and guided resonance modes inside the groove. In addition, the structure is easy to fabricate. The individual sharp convex gold groove is expected to be used as one unit of multi-channels in micro/nano fluidic biosensing. The sample volume could be very small or large according to real applications due to the particular geometric features of our structure.