One of the major challenges for emerging planar subwavelength micro lens/metasurfaces is the significant chromatic behavior due to phase mismatch of subwavelength phase shifters. In this work, the continuous achromatic micro lens covering the whole visible wavelength is demonstrated for the first time based on relatively low index contrast gratings. Based on the unique chromatic phase shift behavior of polymer nano structure, we have designed and fabricated a broadband continuous subwavelength achromatic microlens that can cover 250 nm of visible bandwidths (from 435 nm to 685 nm) with focal shift less than 5%. Our works represent the first time to design, fabricate and characterize micro scale lens (7 microns in size) promising for compact integrated nanophotonic devices on chip. There are many advantages of using a polymer based micro lens such as easy fabrication on flexible substrate and potential applications including imaging, spectroscopy, lithography, laser fabrication and future integrated wearable devices.
Conventional subwavelength grating concentrating lenses are designed based on calculated phase overlap, wherein the phase change is fixed by the grating thickness, bar-width, and airgap, and therefore the focus. We found that certain concentration effects can still be maintained by changing the grating thickness with the same bar-widths and airgap dimensions. Following that, we discovered the existence of the grating thickness threshold; light concentration intensity spikes upon exceeding this limit. However, the light concentration property does not change continuously with respect to a steady increase in grating thickness. This observation indicates that there exists a concentration mode self-interference effect along the light propagation direction inside the gratings. Our results may provide guidance in designing and fabricating microlenses in a potentially more easy and controllable manner. Such approaches can be utilized in various integrated nanophotonics applications ranging from optical cavities and read/write heads to concentrating photovoltaics.
This work aims to reveal the strong influence of TiO2 nanostructures on the light absorption property of TiO2 and perovskite mixture. Three TiO2 nanostructures, i.e., nanoparticles (S1), ultrapure nanorods (S2), and ultrasmall nanorods (S3), were studied: S1 was selected as a baseline; S2 and S3 were synthesized from S1 by using modified hydrothermal processes. Mesoporous TiO2 thin films were spin-coated from solutions containing these TiO2 nanorods and nanoparticles (S1 as baseline). Organic–inorganic hybrid perovskite CH3NH3PbI3 was then incorporated into these mesoporous TiO2 thin films. Optical absorption results showed that the perovskite mixture with ultrasmall TiO2 nanostructures (S3) has significantly higher optical absorption coefficient. Finite-difference time domain models were built based on three distinct nanostructures of TiO2 and CH3NH3PbI3 mixtures fabricated (S1 to S3) to understand their optical absorption properties. Our work is promising to fabricate TiO2 nanostructures, as a backbone structure, for a series of applications including photovoltaics and photodetection.
We have demonstrated the significant impacts of grating tapered sidewall profile on the subwavelength grating wideband reflector characteristics when taking into account the practical fabrication process. Two different classes of wideband reflectors, referred to as zero-contrast gratings and high-contrast gratings, are numerically investigated in detail and the distinct differences of the impacts due to a grating tapered sidewall are observed. Our works reveal that this tapered sidewall profile plays a critical role in determining the reflection bandwidth, average reflectance, and the band edge. The results could be widely utilized in applications of a variety of nanophotonic devices and their integration, as well as facilitate the design of the fabrication process on how to control the degree of tapered sidewall profile for the integrated subwavelength grating nanophotonic devices.
Scintillators are important functional parts in x-ray and Υ-radiation medical imaging instruments, while the high refractive index of scintillation materials significantly reduced the light yield from the scintillators to the detectors, which limited acquired image quality. In this paper, we reviewed two ways to improve the light yield of scintillators via nano photonic devices based on different scintillation materials and integrated nano structures.