Performance limitations of resonant refractive index sensors with low-cost components

Resonant biosensors are attractive for diagnostics because they can detect clinically relevant biomarkers with high sensitivity and in a label-free fashion. Most of the current solutions determine their detection limits in a highly stabilised laboratory environment, which does, however, not apply to real point-of-care applications. Here, we consider the more realistic scenario of low-cost components and an unstabilised environment and consider the related design implications. We find that sensors with lower quality-factor resonances are more fault tolerant, that a filtered LED lightsource is advantageous compared to a diode laser, and that a CMOS camera is preferable to a CCD camera for detection. We exemplify these findings with a guided mode resonance sensor and experimentally determine a limit of detection of 5.8 ± 1.7×10−5 refractive index units (RIU), which is backed up by a model identifying the various noise sources. Our findings will inform the design of high performance, low cost biosensors capable of operating in a real-world environment

Reducing the Surface Area of Black Silicon by Optically Equivalent Structures

Black silicon is a promising low-cost technology to boost the efficiency of solar cells. The large surface area of black silicon, however, imposes challenges such as increased surface recombination and Auger recombination in doped nanostructures. This issue motivates the search for structures with lower surface area but similar optical properties. Here, we identify an approach for reducing the surface area of black silicon, while maintaining optical performance. Specifically, we have demonstrated via simulations that wavelength-scale arrays of nanotapers have similar antireflection properties as black silicon, but with less than half of the surface area. Additionally, we highlight that the light-trapping properties of black silicon are not optimal, and that its performance can be further improved by using nanostructures with controlled scattering properties. We quantify the optical performance of the structures and their surface area, thus identifying optically equivalent structures with reduced surface areas. We believe that these findings will help to boost the efficiency of devices based on black silicon

Paths to light trapping in thin film GaAs solar cells

It is now well established that light trapping is an essential element of thin film solar cell design. Numerous light trapping geometries have already been applied to thin film cells, especially to silicon-based devices. Less attention has been paid to light trapping in GaAs thin film cells, mainly because light trapping is considered less attractive due to the material's direct bandgap and the fact that GaAs suffers from strong surface recombination, which particularly affects etched nanostructures. Here, we study light trapping structures that are implemented in a high-bandgap material on the back of the GaAs active layer, thereby not perturbing the integrity of the GaAs active layer. We study photonic crystal and quasi-random nanostructures both by simulation and by experiment and find that the photonic crystal structures are superior because they exhibit fewer but stronger resonances that are better matched to the narrow wavelength range where GaAs benefits from light trapping. In fact, we show that a 1500 nm thick cell with photonic crystals achieves the same short circuit current as an unpatterned 4000 nm thick cell. These findings are significant because they afford a sizeable reduction in active layer thickness, and therefore a reduction in expensive epitaxial growth time and cost, yet without compromising performance

Spatial resolution effect of light coupling structures

This research project was founded by the National Council for Scientific and Technological Development (CNPq) of Brazil (302397/2014-0), by the National Natural Science Foundation of China (11204386, 11411130117, 11334015), by the Open research project of the State Key Laboratory of Optoelectronic Materials and Technologies, Sun-Yat Sen University of China (OEMT-2015-KF-12, OEMT-2015-KF-13) and by EPSRC of U.K. under grant EP/J01771X/1 (Structured Light). Kezheng Li is also supported by the aboard exchange scholar and international doctoral cooperative project of Sun Yat-sen University.The coupling of light between free space and thin film semiconductors is an essential requirement of modern optoelectronic technology. For monochromatic and single mode devices, high performance grating couplers have been developed that are well understood. For broadband and multimode devices, however, more complex structures, here referred to as "coupling surfaces", are required, which are often difficult to realise technologically. We identify general design rules based on the Fourier properties of the coupling surface and show how they can be used to determine the spatial resolution required for the coupler's fabrication. To our knowledge, this question has not been previously addressed, but it is important for the understanding of diffractive nanostructures and their technological realisation. We exemplify our insights with solar cells and UV photodetectors, where high-performance nanostructures that can be realised cost-effectively are essential.Publisher PDFPeer reviewe