Hamad Ullah - Academia.edu (original) (raw)

Papers by Hamad Ullah

Annalen der Physik, 2021

Chiral near‐fields are crucial in the analysis of chiral molecules, circularly polarized luminesc... more Chiral near‐fields are crucial in the analysis of chiral molecules, circularly polarized luminescence, and sensing. Under circularly polarized light illumination, left‐ and right‐handed chiral near‐fields occur around nanostructures. However, most previously reported works require structural chirality based on asymmetric 3D nanostructures. Usually, left‐ and right‐handed chiral near‐fields are difficult to separate in space, leading to low efficiency in applications. Here, achiral U‐shaped groove nanostructure arrays are proposed to generate uniform chiral near‐fields under circularly polarized light illumination. In contrast to vertical parallel plate nanostructures, a bottom layer is added below the two vertical parallel plates to design U‐shaped groove nanostructures that induce magnetic polaritons mode. In the cavity of the achiral U‐shaped groove nanostructure, chiral near‐field is induced by the excitation of electric and magnetic near‐fields with parallel components. These chiral near‐fields strongly depend on the structural parameters of the U‐shaped groove nanostructure. In addition, chiral molecules are introduced into the cavity of the U‐shaped groove nanostructure to utilize it as sensors for chiral molecular detection. Numerical results show that the circular dichroism of the molecules is enhanced remarkably. These findings pave the way toward the development of practical and scalable platforms for new chiroptical applications.

Optics Express, 2020

Circular dichroism (CD) has been widely studied in recent decades because of its wide application... more Circular dichroism (CD) has been widely studied in recent decades because of its wide application in biomedical detection. Nanostructures with different heights (NDH) usually increase the transmission CD effect. To achieve such nanostructures, one needs to repeatedly perform the electron-beam lithography (EBL) method twice or more, layer-by-layer, which is a very complicated process. Here, we propose a method to prepare NDH by combining the EBL and oblique angle deposition (OAD) techniques. L-shaped planar silver nanostructures are prepared using EBL and normal electron beam deposition, and the OAD method is then used to partially cover one arm of the L-shaped nanostructure. Numerical simulations reveal that the height difference in the two arms of the L-shaped NDH (LSNDH) causes a difference in the polarization directions of the left- (LCP) and right-circularly polarized (RCP) incident light, thereby, generating CD effects. A 2D material is used to cover the LSNDH to further increa...

Nanotechnology, 2020

Chirality, which has long been known as an intrinsic property of living organisms, has caught the... more Chirality, which has long been known as an intrinsic property of living organisms, has caught the interest of researchers due to the rapid emergence of chiral metamaterials. The chiroptical response of noble metal nanostructures in visible and near-infrared regions has been widely investigated. Herein, we propose a bilayer Ag metastructure, in which a chiral L-shaped nanostructure at the bottom is coupled with an achiral nanorod acquiring different positions in the top layer with respect to the long and/or short arm of the chiral L-shaped nanostructure at the bottom layer. The metastructure generates a giant circular dichroism (CD) signal resulting from the strong coupling of the multipolar and dipolar resonant modes on the two layers, in the visible and near-infrared regions. With changing the position of the achiral nanorod, an unusual reversal of the CD spectra is observed, along with a fourfold increase in CD intensity in the short wavelength range due to the multipolar resonant modes. The position of the achiral nanorod is tailored by the azimuthal angle of the substrate during the fabrication of the metastructure using the oblique angle deposition method. This study provides insights into the variation of the coupling strength between a chiral L-shaped nanostructure and an achiral nanorod. The results can be useful in designing chiral-achiral composite nanoantennas for sensing devices.

Applied Optics, 2018

Circular dichroism (CD) of nanostructures is in great demand for applications in biological molec... more Circular dichroism (CD) of nanostructures is in great demand for applications in biological molecules, photocurrent devices, and photocatalysis. Planar nanostructures can be prepared in a concise manner, and their CD effects have gained much research interest. In this study, tilted zigzag-shaped nanohole (TZSN) arrays are proposed, and the CD effect is studied by the finite element method. A strong resonance occurs in the gap by tuning the charge distributions between adjacent nanoholes. Meanwhile, the CD effect of TZSN arrays is strongly dependent on the structural parameters of TZSN. Results provide a novel method for tuning the CD effects of nanohole arrays on a film.

Journal of the Optical Society of America B, 2019

Chirality is important in the fields of science and engineering. In comparison to 3D nanostructur... more Chirality is important in the fields of science and engineering. In comparison to 3D nanostructures, planar nanostructures have a great advantage of easy fabrication. However, previous studies on planar chiroptical effects were limited by the mechanism of enhancing the CD effect. In this paper, a narrow gap is designed between two single planar L-shaped nanorods to facilitate gap plasmon coupling and induce the splitting of the quadrupole mode. This splitting can enhance the circular dichroism (CD) effect. Simulated results also proclaim that CD properties are manipulated by geometrical parameters. The structural design and the new mechanism of the CD effect may provide a novel method for chiral manipulations of light and chiral optical devices.

Optical Materials, 2019

At present, the combination of graphene and metal nanostructures is explored for strong light-mat... more At present, the combination of graphene and metal nanostructures is explored for strong light-matter interaction. In this study, the optical properties of a composite nanostructure are studied experimentally by simply coating a monolayer graphene on a sandwich nanostructure (Ag nanoparticles-SiO 2 film-Ag nanoparticles) fabricated via oblique angle deposition. Results indicate that graphene coverage leads to enhanced light absorption in the visible range. The simulation results show that the interaction between graphene and nanoparticles plasmons increase the light absorption originating from the strong localized electric field of graphene. In addition, the light-graphene interaction can be tuned via plasmon resonance by changing the size of nanoparticles and the thickness of the SiO 2 film. The simplicity of the fabrication process and the tunability of plasmon resonance makes the application of graphene-covered nanostructures in diverse areas promising.

Applied Optics, 2019

Circular dichroism (CD) of metasurfaces has been used in biological monitoring, analytical chemis... more Circular dichroism (CD) of metasurfaces has been used in biological monitoring, analytical chemistry, and perfect polarization converters. In this work, a metasurface consisting of nanoholes and tilted nanorods is proposed to achieve the CD effect. Numerical calculations show that electrical current forms between the film and the tilted nanorods under circularly polarized light illumination, and CD effects originate from the coupling between the current oscillations at the film and those on the tilted nanorods. This electrical oscillation mode provides unique coupling mechanisms for the CD effect. In addition, CD is strongly dependent on the structural parameters, and the resonant modes can be tuned by modulating the currents on the film. These results are helpful for designing novel chiral optical structures and provide unique methods for circular polarizers.

Journal of Physics D: Applied Physics, 2019

Annalen der Physik, 2019

Asymmetric transmission (AT) reflects the conversion efficiency of a chiral nanostructure for cir... more Asymmetric transmission (AT) reflects the conversion efficiency of a chiral nanostructure for circularly polarized light and is widely used in polarization and optoelectronic devices. In this study, a new mechanism is proposed to generate AT when a planar chiral nanostructure is illuminated under left‐handed circularly polarized (LCP) and right‐handed circularly polarized (RCP) light illumination. The new mechanism can be achieved by breaking the symmetry of the designed planar chiral nanostructure which give rise to a new transmittance peak and dip at a particular wavelength under RCP and LCP light illumination, respectively. The proposed new mechanism is also capable of actively tuning the generated resonant modes. Besides this, when graphene strips are added to the designed planar chiral nanostructure, similar results are obtained as that from breaking the symmetry of the planar chiral nanostructure. In this case, the generated AT could also be actively tuned by varying the Fermi energies of graphene strips.

EPL (Europhysics Letters), 2019

The circular dichroism (CD) effect characterized by different optical responses between left and ... more The circular dichroism (CD) effect characterized by different optical responses between left and right circularly polarized lights is widely applied for polarization-resolved detection and imaging. The tunable CD effect is of substantial importance in improving the detection sensitivity and imaging resolution. In this paper, we show that planar Z-shaped composite metamaterial embedded with VO2 that exhibits insulator-metal transition (IMT) can enable thermally tunable chirality. Simulated by the finite element method, the tunable CD effect can be achieved by changing the environment temperature to initiate the IMT of VO2 . We also demonstrate that the underlying mechanism of the CD effect generation is the electric multipole oscillation response in the vicinity of the VO2 gap in vertical direction. These findings not only provide a new strategy to change chirality and tune the effect of CD dynamically but also broaden its potential applications in polarization-resolved detection and imaging.

Applied Surface Science, 2019

Artificially engineered chiral plasmonic nanostructures (CPNs) have attracted considerable attent... more Artificially engineered chiral plasmonic nanostructures (CPNs) have attracted considerable attention and have been widely studied in the recent decades because of their distinguishing optical properties. Researchers have focused on noble metal nanostructures, because of their strong chiroptical response in visible and near-infrared regions. In this study, a system of a nanorod coupled with a nanosurface, which were both made of silver, was proposed. Glancing angle deposition (GLAD) method was used to fabricate CPNs. The fabricated CPNs generated a strong circular dichroism (CD) signal under visible and near-infrared light illumination. A high peak was observed at approximately 600 nm, and an increasing trend of the CD intensity with a redshift was confirmed when the area of the nanosurface was increased. The generated CD could be tuned easily by changing the area of the nanosurface with an active control of the vapor deposition angle (glancing angle of the substrate) in the GLAD method.

Annalen der Physik, 2018

Chiral plasmonic (CP) systems are shown to exhibit effectual circular dichroism (CD). A general m... more Chiral plasmonic (CP) systems are shown to exhibit effectual circular dichroism (CD). A general mechanism based on the Jones matrix for predicting the CD signal of CP systems is proposed. Considering the near-field coupling of adjacent layers of trilayer CP systems, the coupling matrix, which shows the relevance of the coupling status to the relative position of the adjacent layers, is analytically derived under the framework of the transfer matrix method. To prove the coupling matrix, a trilayer system is designed, and the optical properties of the relevant system arrays are simulated using the finite-element method. The fitting functions of the simulated CD signals at resonant wavelengths are consistent with the theoretical formulas. The proposed mechanism facilitates an understanding of the underlying physical mechanism of CD and offers a design principle for fabrication of CP systems that are applicable to biosensing.

Optics express, Jan 27, 2017

Plasmonic bending beams, which preserve their spatial shapes while propagating along curved traje... more Plasmonic bending beams, which preserve their spatial shapes while propagating along curved trajectories in metal-dielectric interface, offer important applications in the fields of fiber sensor, optical trapping, and micro-nano manipulation. In this work, circular hole array, as a local point-like sources of surface plasmon polaritons, is designed on the metal film to generate multiple plasmonic bending beams. The electric field intensity of multiple plasmonic bending beams is controlled by polarization angle of input light. In addition, the electric filed intensity of multiple plasmonic bending beams relies on circle hole radius. These findings provide guidance in the design and optimization of plasmonic bending beam generators.