Hossam Galal
This report introduces the Transfer Waveguide (TRANSGUIDE); an ultra-thin flat technology that promises light emitting applications a practical solution to total internal reflection light trapping and diverging emission. By invoking reciprocity, light can be temporarily stored in the form of a virtual-dipole and recovered back again.
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A highly sensitive refractive index sensor based on grating-assisted strip waveguide directional coupler is proposed. The sensor is designed using two coupled asymmetric strip waveguides with a top-loaded grating structure in one of the waveguides. Maximum light couples from one waveguide to the other at the resonance wavelength, and the change in resonance wavelength with the change in refractive index of the medium in the cover region is a measure of the sensitivity. The proposed sensor would be an on-chip device with a high refractive index sensitivity of ~ 104 nm/RIU, and negligible temper…
In this research, we report the experimental evidence of the directional Fano resonances at the scattering of a plane, linearly polarized electromagnetic wave by a homogeneous dielectric sphere with high refractive index and low losses. We observe a typical asymmetric Fano profile for the intensity scattered in, practically, any given direction, while the overall extinction cross section remains Lorentzian. The phenomenon is originated in the interference of the selectively excited electric dipolar and quadrupolar modes. The selectivity of the excitation is achieved by the proper choice of the…
Recently, gradient index cavities, or so-called transformation cavities, designed by conformal transformation optics have been studied to support resonant modes with both high Q-factors and emission directionality. We propose a new design method for transformation cavities to realize a narrower width of the refractive index profile, a great advantage in experimental implementations, without losing the benefits of conformal mapping. We study the resonant modes with both high Q-factor and directional emission in the newly designed transformation cavities, where the refractive index profile is 50…
The possibility of making an optically large (many wavelengths in diameter) object appear invisible has been a subject of many recent studies. Exact invisibility scenarios for large (relative to the wavelength) objects involve (meta)materials with superluminal phase velocity (refractive index less than unity) and/or magnetic response. We introduce a new approximation applicable to certain device geometries in the eikonal limit: piecewise-uniform scaling of the refractive index. This transformation preserves the ray trajectories, but leads to a uniform phase delay. We show how to take advantage…
Controlling the propagation and coupling of light to sub-wavelength antennas is a crucial prerequisite for many nanoscale optical devices. Recently, the main focus of attention has been directed towards high-refractive-index materials such as silicon as an integral part of the antenna design. This development is motivated by the rich spectral properties of individual high-refractive-index nanoparticles. Here, we take advantage of the interference of their magnetic and electric resonances, to achieve remarkably strong lateral directionality. For controlled excitation of a spherical silicon nano…
We present simulations of an efficient high numerical aperture solid immersion metalens concept for fluorescence microscopy. The technique exploits the preferential emission of interfacial dipoles into a high refractive index substrate combined with a metalens and a conventional tube lens for imaging them. We have thus simulated dipole emission and an all-dielectric metasurface on opposite sides of a high refractive index substrate. Our calculations predict dipole collection efficiencies of up to 87 percent. The simulated beam propagation through the imaging system shows excellent performance …
We consider light tunneling inhibition in periodic array of optical couplers due to the specially designed longitudinal and transverse modulation of the refractive index. We show that local out-of-phase longitudinal modulation of refractive index in channels of directional couplers in combination with the global refractive index modulation between adjacent couplers allow simultaneous suppression of both local and global energy tunneling inside each coupler and between adjacent couplers. This enables localization of light in single waveguide despite the remarkable difference of corresponding lo…
High-Index Semiconductor Nanoparticles as Low-Loss Alternatives to Gold for Refractive Index Sensing
高指数半导体纳米颗粒在可见光范围内提供低损耗的折射率传感替代方案，通过多极共振实现方向性散射。
We report the design of highly efficient optical antennas employing a judicious synthesis of metallic and dielectric materials. In the proposed scheme, a pair of metallic coupled nanoparticles permits large enhancements in both excitation strength and radiative decay rates, while a high refractive index dielectric microsphere is employed to efficiently collect light without spoiling the emitter quantum efficiency. Our simulations indicate potential fluorescence rate enhancements of 3 orders of magnitude over the entire optical frequency range.
We analyze, by the finite-difference time-domain numerical methods, several ways to enhance the directional emission from photonic crystal waveguides through the beaming effect recently predicted by Moreno et al. [Phys. Rev. E 69, 121402(R) (2004)], by engineering the surface modes and corrugation of the photonic crystal surface. We demonstrate that the substantial enhancement of the light emission can be achieved by increasing the refractive index of the surface layer. We also measure power of surface modes and reflected power and confirm that the enhancement of the directional emission is re…
We present a rotated polarization directional coupler (RPDC) on a photonic chip. We demonstrate a double-track approach to modify the distribution of refractive index between adjacent tracks and form a single waveguide with arbitrary birefringent optical axis. We construct a RPDC with the two axis-rotated waveguides coupled in a strong regime. The obtained extinction ratios on average are about 16dB and 20dB for the corresponding orthogonal polarizations. We perform the reconstruction of Stokes vector to test the projection performance of our RPDC, and observe the average fidelities up to 98.1…
High-refractive index dielectric nanoparticles may exhibit strong directional forward light scattering at visible and near-infrared wavelengths due to interference of simultaneously excited electric and magnetic dipole resonances. For a spherical high-index dielectric, the so-called first Kerker's condition can be realized, at which the backward scattering practically vanishes for some combination of refractive index and particle size. However, Kerker's condition for spherical particles is only possible at the tail of the scattering resonances, when the particle scatters light weakly. Here we …
X-ray computed tomography reconstruction algorithm for refractive index gradient
本文提出矢量Radon变换及其逆变换，在小角度偏转条件下利用标准圆周扫描重建三维X射线折射率梯度图，并通过实验验证了方法的准确性。
Straight Directional Couplers via Scan-Engineered Index Control
论文提出了一种基于玻璃的直行方向波导耦合器和干涉仪的新设计，利用飞秒激光直接写入技术，在通信波长下实现50:50方向耦合器和不平衡臂马赫-曾德干涉仪，展示了紧凑高密度三维光子集成的潜力。
High Refractive Index (HRI) dielectric particles smaller than the wavelength, isolated or forming a designed ensemble are ideal candidates as new multifunctional elements for building optical devices. Their directionality effects are traditionally analyzed through forward and backward measurements, even if these directions are not suitable for practical purposes. Here we present unambiguous experimental evidence in the microwave range that, for a dimer of HRI spherical particles, a perfect switching effect (perfect means off = null intensity) is observed out of those directions as a consequenc…
Controlling the dispersion and directionality of the emission of nanosources is one of the major goals of nanophotonics research. This control will allow the development of highly efficient nanosources even at the single photon level. One of the ways to achieve this goal is to couple the emission to Bloch modes of periodic structures. Here, we present the first measurements of the directional emission from nanowire photonic crystals by using Fourier microscopy. With this technique we efficiently collect and resolve the directional emission of nanowires within the numerical aperture of a micros…
We study the effect of dynamical tunneling on emission from ray-chaotic microcavities by introducing a suitably designed deformed disk cavity. We focus on its high quality factor modes strongly localized along a stable periodic ray orbit confined by total internal reflection. It is shown that dominant emission originates from the tunneling from the periodic ray orbit to chaotic ones; the latter eventually escape from the cavity refractively, resulting in directional emission that is unexpected from the geometry of the periodic orbit, but fully explained by unstable manifolds of chaotic ray dyn…
High-power and highly directional semiconductor cylinder-lasers based on an optical resonator with deformed cross section are reported. In the favorable directions of the far-field, a power increase of up to three orders of magnitude over the conventional circularly symmetric lasers was obtained. A "bow-tie"-shaped resonance is responsible for the improved performance of the lasers in the higher range of deformations, in contrast to "whispering-gallery"-type modes of circular and weakly deformed lasers. This resonator design, although demonstrated here in midinfrared quantum-cascade lasers, sh…
In this study, we propose the application of non-Hermitian photonic crystals (PCs) with anisotropic emissions. Unlike a ring of exceptional points (EPs) in isotropic non-Hermitian PCs, the EPs of anisotropic non-Hermitian PCs appear as lines symmetrical about the $Γ$ point. The non-Hermitian Hamiltonian indicates that the formation of EPs is related to the non-Hermitian strength. The real spectrum appears in the $Γ$Y direction and has been validated as the complex conjugate medium (CCM) by effective medium theory (EMT). But for the $Γ$X direction, EMT indicates that the effective refractive in…
We evaluate the sensing properties of plasmonic waveguide sensors by calculating their resonant transmission spectra in different regions of the non-Hermitian eigenmode space. We elucidate the pitfalls of using modal dispersion calculations in isolation to predict plasmonic sensor performance, which we address by using a simple model accounting for eigenmode excitation and propagation. Our transmission calculations show that resonant wavelength and spectral width crucially depend on the length of the sensing region, so that no single criterion obtained from modal dispersion calculations alone …
We report on femtosecond laser writing of single mode optical waveguides in chalcogenide Gallium Lanthanum Sulphide (GLS) glass. A multiscan fabrication process was employed to create waveguides with symmetric single mode guidance and low insertion losses at 800 nm wavelength, compatible with Ti:Sapphire ultrafast lasers. μRaman and X-Ray microanalysis were used to elucidate the origin of the laser-induced refractive index change in GLS. Nonlinear refractive index measurements of the waveguides were performed by finding the optical switching parameters of a directional coupler, demonstrating t…
We design coupled optical microcavities and report directional light emission from high-$Q$ modes for a broad range of refractive indices. The system consists of a circular cavity that provides a high-$Q$ mode in form of a whispering gallery mode, whereas an adjacent deformed microcavity plays the role of a waveguide or collimator of the light transmitted from the circular cavity. As a result of this very simple, yet robust, concept we obtain high-$Q$ modes with promising directional emission characteristics. No information about phase space is required, and the proposed scheme can be easily r…
Trapping of microspheres with a single focused laser beam is usually limited to materials with relative refractive indexes slightly larger than one. We show that directional light scattering can be employed to optically trap high-index materials. For this purpose, we propose a material platform to achieve zero backward scattering (ZBS), also known as the first Kerker condition, in a composite media containing spherical inclusions of silica embedded in a SiC microsphere. By tuning the volume filling fraction of inclusions and the microsphere radius, stable trapping can be achieved, provided tha…
We propose to use optical antennas made out of natural hyperbolic material hexagonal boron nitride (hBN), and we demonstrate that this medium is a promising alternative to plasmonic and all-dielectric materials for realizing efficient subwavelength scatterers and metasurfaces based on them. We theoretically show that particles out of hyperbolic medium possess different resonances enabled by the support of high-k waves and their reflection from the particle boundaries. Among those resonances, there are electric quadrupole excitations, which cause magnetic resonance of the particle similar to wh…
We report on experiments with deformed polymer microlasers that have a low refractive index and exhibit unidirectional light emission. We demonstrate that the highly directional emission is due to transport of light rays along the unstable manifold of the chaotic saddle in phase space. Experiments, ray-tracing simulations, and mode calculations show very good agreement.
Optical waveguides in the form of glass fibers are the backbone of global telecommunication networks. In such optical fibers, the light is guided over long distances by continuous total internal reflection which occurs at the interface between the fiber core with a higher refractive index and the lower index cladding. Although this mechanism ensures that no light escapes from the waveguide, it gives rise to an evanescent field in the cladding. While this field is protected from interacting with the environment in standard optical fibers, it is routinely employed in air- or vacuum-clad fibers i…
Ultra-directional and high-efficiency $μ$LEDs via gradient index filled micro-Horn collimators
本研究提出在μLED顶部集成递减折射率多层抗反射涂层与喇叭准直器，利用临界角外光传播，实现±15°锥内十倍光提取提升，总效率约80%，且结构紧凑，适用于高分辨率密集阵列。
Metamaterials offer unprecedented flexibility for manipulating the optical properties of matter, including the ability to access negative index, ultra-high index and chiral optical properties. Recently, metamaterials with near-zero refractive index have drawn much attention. Light inside such materials experiences no spatial phase change and extremely large phase velocity, properties that can be applied for realizing directional emission, tunneling waveguides, large area single mode devices, and electromagnetic cloaks. However, at optical frequencies previously demonstrated zero- or negative-r…
Light switching based on half space invisible states
本文提出通过定制入射光激发半空间非辐射态，利用载流子注入引起的折射率微小变化，在 AlGaAs 与 InP 球形粒子中实现散射光的高效定向切换。
Molybdenum trioxide (MoO$_3$) in-plane anisotropy has increasingly attracted the attention of the scientific community in the last few years. Many of the observed in-plane anisotropic properties stem from the anisotropic refractive index and elastic constants of the material but a comprehensive analysis of these fundamental properties is still lacking. Here we employ Raman and micro-reflectance measurements, using polarized light, to determine the angular dependence of the refractive index of thin MoO$_3$ flakes and we study the directional dependence of the MoO$_3$ Young's modulus using the b…
Optical Fiber-Waveguide Hybrid Architecture for Mid-Infrared Ring Lasers
本文提出并实现了基于氟化物玻璃方向耦合器的光纤-波导混合集成中红外环形激光器，首次实现全光纤芯片集成，输出2716.7纳米、14.2毫瓦稳定激光，为三维光子集成开辟新途径。
The unique optical properties of phase change materials (PCMs) can be exploited to develop efficient reconfigurable photonic devices. Here, we design, model, and compare the performance of programmable 1X2 optical couplers based on: Ge$_2$Sb$_2$Te$_5$, Ge$_2$Sb$_2$Se$_4$Te$_1$, Sb$_2$Se$_3$, and Sb$_2$S$_3$ PCMs. Once programmed, these devices are passive, which can reduce the overall energy consumed compared to thermo-optic or electro-optic reconfigurable devices. Of all the PCMs studied, our ellipsometry refractive index measurements show that Sb$_2$S$_3$ has the lowest absorption in the tel…
Routing Light Emission from Monolayer MoS$_2$ by Mie Resonances of Crystalline Silicon Nanospheres
本研究利用晶体硅纳米球的磁偶极与电偶极 Mie 共振，解析、实验并模拟验证了其对单层 TMDC 激子光致发光方向性的调控。
Scaling behavior of the degree of circular polarization of surface plasmon polariton
本研究揭示表面等离激元极化激元圆偏振度随有效折射率的幂律标度规律，推广至手性发射体耦合方向性，提出增强光-谷相互作用的手性光子平台，为手性纳米光子系统设计提供准则。
Transverse spin angular momentum of light is a key concept in recent nanophotonics to realize unidirectional light transport in waveguides by spin-momentum locking. Herein we theoretically propose subwavelength nanoparticle chain waveguides that efficiently sort optical spins with engineerable spin density distributions. By arranging high-refractive-index nanospheres of different sizes in a zigzag manner, directional optical spin propagation is realized. The origin of the efficient spin transport is revealed by analyzing the dispersion relation and spin angular momentum density distributions. …
In situ Thermal Trimming of Waveguides in a Standard Active Silicon Photonics Platform
本文在标准硅光子平台上制备悬浮加热器结构，实现硅与氮化硅波导的双向原位热折射率修调，验证了开关偏置点修调应用，探究了修调机制并估算了器件长期老化特性。
Optical characterisation of ilmenite by reflectance spectroscopy
针对双向反射光谱法表征钛铁矿光学特性时的n/k提取、K-K约束弱化及强吸收处理问题，结合平板方向半球反射和分层介质发射/吸收信息，改进方法并确定其波长相关内部散射系数与n/k特性，提升了表征准确性。
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