13 shared publications
School of Electrical and Electronic Engineering; Nanyang Technological University; Singapore 639798 Singapore
13 shared publications
DSO National Laboratories
11 shared publications
Engineering Physics Department, Faculty of Engineering, Ain Shams University, Abbasiya, Cairo, Egypt
10 shared publications
Université Paris-Est, UPEM, ESYCOM EA 2552, 5 Boulevard Descartes, 77420 Champs sur Marne, France
7 shared publications
School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
(2006 - 2017)
Electromagnetic (EM) absorption plays a foremost important role in the area of energy harvesting, stealth technology, interference shielding and biological imaging etc. Perfect metamaterial/metasurface absorber, which obtains near-unity EM absorption through subwavelength artificial structure, usually suffers from narrow bandwidth. Here, for the first time, we demonstrate a curved water-based metasurface which functions as an active ultra-broadband absorbing material working across the entire Ku, K and Ka bands. Distinct from conventional metallic metamaterial/metasurface, the proposed water-based metasurface acquires broadband absorption property from the dielectric Mie resonance and periodic grating effect, which exhibits an experimental absorptivity of ∼99% and an absorption bandwidth (absorptivity higher than 90%) that covers 71.4% of the central frequency. Furthermore, near-unity absorption is maintained when the soft metasurface is bent into different curvatures, promoting its potential applications on non-planar circumstances.
This paper reports the design, fabrication and preliminary characterization of optofluidic Fabry-Perot micro coupled cavities enabling on-chip refractometry with large dynamic range.
Large dynamic range refractometry is needed in several applications such as in portable food analyzers, where the quality of fruits and beverages are classified based on their Brix number, which indicates the sugar content. The Brix numbers is obtained from the refractive index using standard conversion tables [1-2], where the refractive index varies from 1.333 (0 oB) to 1.49 (80 oB). The conventional design of integrated refractometer is based on Fabry-Perot cavity, in which the wavelength of longitudinal modes shifts with the change in the refractive index of the filling medium [3-4]. Their dynamic range is usually in the order of 10-3, limited by the free spectral range (FSR) of the micro cavity. Therefore, the food analyzers are usually based on volume optics components and the change of the refraction angle with the change in the refractive index that allows for the large dynamic range of sensing.
In this work, we report a novel design based on cascading two coupled Fabry-Perot micro cavities allowing for orders of magnitude increase in the FSR besides the decrease in the linewidth which enhances the resolution as well. Fig. 1 shows the schematic diagram of the new design and the idea of operation. The lengths of the two cavities are slightly different and adjusted such that some modes are suppressed after each allowed mode. The use of Si/Air layers to form the Bragg mirror with thickness of 3.8/3.6 mm allowed us to achieve designs with mode separation of 40 nm around a wavelength of 1550 nm.
Fig. 2 shows a SEM photo of part of the fabricated structures. The fabrication was done using standard MEMS technology in which DRIE process is used to make a 150 mm deep etching in Si to form the Bragg mirrors, fiber grooves, and the microfluidic channels and ports. Test structures in the form of single cavities and mirrors were also fabricated on the same chip.
Fig. 3 shows the measured transmittance of one of the fabricated coupled-cavities together with the measured transmittance of the single cavity corresponding to one of them. The single cavity has a length of 128 mm and the coupled-cavities have lengths L1 = 160 mm and L2 = 128 mm as referred to Fig. 1. The mirrors are composed of two Si layers in both cases. We can see the increase in the FSR achieved in the coupled cavity which is about 3 times as the single cavity and also the reduction in the line-width by less than half. In fact the technology tolerance, especially the over-etching, had a great effect on the fabricated structures. The fabricated mirrors had a wider bandwidth than expected and we could obtain a much larger FSR in the coupled-cavity. In some designs we obtained only one peak in the whole measured band of 140 nm.
This paper presents a new optofluidic detection system to measure the absolute refractive index map of host cells. This new system: cell refractive index tomography, can measure the absolute refractive index map of host cells. During virus infection, cell morphology and the intracellular contents of the host cells are changed. Such changes can be monitored by detecting the refractive index variation. By monitoring of the cell’s refractive index, the detection of virus infection in real-time and label-free is determined.
This paper reports a new design of optofluidic Fabry–Pérot (FP) micro cavity that combines the highest reported quality factor for an on-chip FP resonator that exceeds 3600, and the highest reported sensitivity for an on-chip volume refractometer based on a FP cavity that is about 1000 nm/RIU.
For using the optical resonator as a refractometer, it is convenient to have sharp and highly selective resonance peaks for accurate measurements; thereby the quality factor (Q) of the resonator is preferred to be high. The highest reported Q factor reported by other groups is only 400 . This limitation comes from using straight mirror for the FP, which causes high diffraction loss due to beam expansion after few round trips. Our group has previously reported a cavity employing curved mirrors and a micro-tube in-between holding the analyte . The curvature of the mirrors and the micro-tube achieved better light confinement and hence high Q factors up to 2,800. On the other hand, the sensitivity was only 428 nm/RIU since the analyte doesn’t fill the entire cavity. The highest reported sensitivity by literature was 907 nm/RIU in case of the analyte occupying the whole cavity .
In this work, a novel structure for FP micro-cavity is reported, achieving both high Q factor resonator and high sensitivity refractometer. The proposed structure is schematically depicted in Fig. 1. It employs cylindrical Bragg mirrors forming the FP cavity to confine light in the in-plane direction, while an external cylindrical lens - implemented by a fiber rod lens (FRL) - is used to confine light in the out-of-plane direction before it enters the FP cavity and after it exits to be efficiently collected by the collecting fiber. The cavities are fabricated from silicon by Deep Reactive Ion Etching (DRIE) process, and then capped by a PDMS cover. The FRLs are placed later after micro fabrication. A photo of the chip combining several cavities with different lengths is shown in Fig. 2. The analyte is passed between the mirrors enabling its detection from the shift of resonance peaks of the transmission spectra. The spectra are obtained by recording the output from an optical detector while varying the input light wavelength from a tunable laser in the near infrared band. The spectrum of a cavity of 318 µm physical length filled with DI-water is presented in Fig. 3 showing the narrow line widths of the resonance peaks. The peak has a linewidth of 0.44 nm, which provides a Q factor of 3649.
Mixtures of ethanol and DI-water with different ratios are used as analytes with different refractive indices to exploit the device as a micro-opto-fluidic refractometer, which are plotted in Fig. 4. The sensitivity is obtained from the slope of the linear plot in Fig. 5 of the resonance wavelength shift versus the difference in refractive index between the analytes and the DI-water, which is taken as a reference. The cavity used has a physical length of 256 µm and the obtained sensitivity is 1000 nm/RIU.
Disordered optics is a fascinating area, yet not fully understood, as it increasingly attracts interest due to the virtues of multiple scattering of light in diffusive materials and their numerous applications to imaging through opaque media, to spectroscopy as well as more fundamental optical transport properties including Anderson localization. Among the various kinds of disordered media, Black Silicon is a randomly micro-structured surface exhibiting several outstanding optical and wetting properties, which makes it increasingly used in various applications that will be discussed. On the other hand, complex fluids and especially colloidal suspensions have the specific feature of the time-dependence of the disorder; even in the static equilibrium regime of no flow, complex fluids exhibit Brownian motion of the embedded particles leading to an additional dimension to the disorder. The heterogeneous nature of complex fluids induces specific behavior at the microscale as well as collective effects at larger scales. This holds true not only regarding rheology but also in the optical domain, where one can define an effective refraction index in a medium subject to microscale effects of light scattering. Furthermore, it is also intuitive to think of a possible time-dependence of those properties. This poses the question of the most appropriate length scale and time scale to perform measurements in such media.
Zinc Oxide Nanowires (ZnO-NWs) gained a lot of interest due to their diverse and unique semiconductive, optical, and piezoelectric properties. ZnO-NWs has been used in different applications such as nanogenerator of electricity , chemical sensors , photovoltaic cells  and recently in water purification . In all those applications, performance is directly related to the actual properties of the ZnO-NWs. The latter properties can be investigated in detail using Scanning Electron Microscopy (SEM), X-Ray Diffraction and High Resolution Transmission Electron Microscopy for the purpose of characterization and optimization of the growth process.
However, once ZnO-NWs are integrated within a microfluidic device, there is a need for a much simpler characterization technique for checking in situ the quality and uniformity of ZnO-NWs during and after their growth. In this work, we take advantage of the strong dependence of the ZnO-NWs properties on their dimensions, density and possible contamination, considered here as quality indicators. We also note that those indicators can be obtained by optical measurement of effective thickness (deff), effective refraction index (neff) and light absorption, respectively, of the ZnO-NW array, which is considered here as a thin film layer (Fig.1). We propose herein a simple and time-saving method measuring all those parameters in the same experiment, based on the reflection spectral response in the Ultraviolet (UV), Visible (Vis) and Near-Infra-Red (NIR) range.
The purity of the ZnO growth is observed through absorbance, while, ZnO-NWs density (neff) and height can be obtained from the reflection response, which reveals interference patterns in the ZnO-NWs layer. The optical path (deff x neff) is retrieved by the Free Spectral Range (FSR).
The ZnO-NWs are synthesized using the hydrothermal method . Typical results for different growth times are shown in Fig. 2. A top view of the synthesized NWs array grown over silicon is shown in Fig. 1 together with illustrative schematic of the synthesized NWs cross-section at different positions. Denser and longer NWs are synthesized on the edge of the chip while the density and length decrease moving away from the edge. The neff of the ZnO-NWs layer is in-between air and ZnO refractive indices, based on the ZnO-NWs density. The measured absorbance (normalized to silicon) is shown in Fig. 3 for ZnO growth for 2 hours (2h) at different positions. Absorbance cut-off is observed around 370 nm corresponding to the ZnO bandgap absorption. The ripples correspond to the reflection response of the ZnO thin film where the smallest FSR observed at the edge relates to the denser ZnO-NWs growth (higher neff) compared to the other two regions. Fig. 4 relates to a 3h growth where some residual Zn(OH)x masking the reflection response at the edge in addition to the expanded absorption. Besides density, the purity of the synthesized ZnO can be evaluated by the absorbance cut-off wavelength. However, pure ZnO has cut-off around 370 nm, the non-pure ZnO have higher cut-off (Fig. 5) for the 4h growth where the purest ZnO is synthesized at the center of the chip