Ayed Al Sayem

Negative Refraction with Superior Transmission in Graphene-Hexagonal Boron Nitride (hBN) Multilayer Hyper Crystal

In this article, we have theoretically investigated the performance of graphene-hexagonal Boron Nitride (hBN) multilayer structure (hyper crystal) to demonstrate all angle negative refraction along with superior transmission. hBN, one of the latest natural hyperbolic materials, can be a very strong contender to form a hyper crystal with graphene due to its excellence as a graphene-compatible substrate. Although bare hBN can exhibit negative refraction, the transmission is generally low due to its high reflectivity. Whereas due to graphene’s 2D nature and metallic characteristics in the frequency range where hBN behaves as a type-I hyperbolic material, we have found graphene-hBN hyper-crystals to exhibit all angle negative refraction with superior transmission. Interestingly, superior transmission from the whole structure can be fully controlled by the tunability of graphene without hampering the negative refraction originated mainly from hBN. We have also presented an effective medium description of the hyper crystal in the low-k limit and validated the proposed theory analytically and with full wave simulations. Along with the current extensive research on hybridization of graphene plasmon polaritons with (hyperbolic) hBN phonon polaritons, this work might have some substantial impact on this field of research and can be very useful in applications such as hyper-lensing.

Broad angle negative refraction in lossless all dielectric or semiconductor based asymmetric anisotropic metamaterial

In this article, it has been theoretically shown that broad angle negative refraction is possible with asymmetric anisotropic metamaterials (AAMs) constructed by only dielectrics or lossless semiconductors at the telecommunication and relative wavelength range. Though natural uniaxial materials can exhibit negative refraction, the maximum angle of negative refraction and critical incident angle lie in a very narrow range. This problem can be overcome by our proposed structure. In our structures, negative refraction originates from the highly asymmetric elliptical iso-frequency. This is artificially created by the rotated multilayer sub-wavelength dielectric or semiconductor stack, which acts as an effective AAM. This negative refraction is achieved without using any negative permittivity materials such as metals. As we are using simple dielectrics, fabrication of such structures would be less complex than that of the metal based metamaterials. By considering the time harmonic field incidence, negative refraction has been demonstrated for two dimensional bi-dielectric structures for TM polarization with realistic parameters. Our proposed ideas have been validated by the full wave simulations considering both the effective medium approach and realistic structure model. This device might find some important applications in photonics and optoelectronics.

Tunable slow light with graphene based hyperbolic metamaterial

Slow light has always been a topic of extreme interest for researchers and scientists in order to utilize the full potential of light in switching, memory devices and in quantum optics. However, slow light phenomena and its potential applications in switching and memory devices by tunable graphene based hyperbolic metamaterial have not still been introduced in literature. Here we theoretically propose and numerically analyze a graphene based tunable slow light device from the concept of controlling the group velocity of light in hyperbolic metamaterials. In THz range, graphene-dielectric stack can behave as hyperbolic metamaterials and hyperbolic metamaterials can demonstrate slow light phenomena while being cladded by air. By utilizing the tunability option of graphene, proposed devices can be made tunable, which is the most important criteria to manage the full advantage of slow light. It ultimately paves a fully new way to find novel applications in photonic switch, optical buffers and memory devices. Interestingly, in such graphene based devices, voltage will play a vital role to control the group velocity of light from slow to fast and the carriers are photons instead of electrons.

Control of reflection through epsilon near zero graphene based anisotropic metamaterial

In graphene dielectric multilayer structure, a very interesting phenomena such as epsilon near zero property rises naturally. However, this epsilon near zero property (along with the proper control of chemical potential and gate voltage) of graphene multi-layer stack has not been used so far to make novel photonic switches. In this article, we have shown theoretically that in graphene-dielectric stack, which acts as an anisotropic metamaterial, full control of reflection and so transmission of light at a specific wavelength or frequency can be controlled very simply by external gate voltage. This external voltage tunes the chemical potential and so the parallel permittivity of the anisotropic structure. Our theoretical prediction may pave the way to novel voltage control photonic logic switches, which may play as an intermediate solution before entering into all optical commercial switches (as current silicon and electron based CMOS technology is facing a dead end).

Effect of high k-dielectric as gate oxide on short channel effects of junction-less transistor

In this work, short channel effects such as drain induced barrier lowering, sub-threshold swing, on-current and off-current and on-off ratio have been analyzed for different oxide materials as gate oxide for double gate junction-less transistors by using device simulator. Just like inversion mode MOSFETS, junction-less transistors show better short channel effects when high-k dielectrics are used.

Time dependent force outside a complex magneto-dielectric particle: Abraham-Minkowski controversy

Abraham and Minkowski momenta in material medium have been a long standing dilemma for the last hundred years. It is well-known that the difference between the outside forces of a dielectric particle calculated from Abraham and Minkowski formula is generally difficult to observe due to the cancellation of their time dependent part in the averaging process. However, the missing information can be closely observed from the transient parts of the corresponding formulation of the force densities. Here, in this work, we have theoretically demonstrated the discrepancy between Abraham and Minkowski forces in time domain which clearly indicates their different nature for both absorbing and gain media. In addition, the study broadly supports the fact that the distinction between the two rival force densities is more obvious in a gain medium than in an absorbing one. The results can be applied in case of theoretical and practical optical manipulation of both the force and torque.

Thickness optimization and composition grading effect in heterojunction CIGS Solar Cell

Thin Film Solar Cell is a technology which has a great potential as it has low material consumption and relatively high efficiency to others. Among thin film solar cells CIGS is a promising one. But due to scarcity of gallium and indium its industrial production can be affected. Here in this paper via simulation (using device simulator) we have tried to achieve good acceptable efficiency using very thin layer of CIGS absorber layer with the help of grading and appropriate gallium composition and by optimizing emitter efficiency which is close to the efficiency of CIGS cells having thick absorber layer.

Achieving tetra band performance in simple dielectric and in metamaterial filled patch antennas

In this contribution, we have proposed a robust technique to achieve tetra-band performance in both conventional dielectric (elliptical shape) and also metamaterial loaded (rectangular shape) patch antennas. Incorporation of symmetric slots over the patch in both type of antennas have been shown here by using the concept of additional modified modes and perturbation approach. At first, we have shown that if and only if, elliptical shape patch antenna is properly optimized along with symmetrical slots over metallic patch, it is possible to achieve satisfactory four- band performance without the aid of any complex procedure currently present in literature. Later, we have reported a compact metamaterial loaded tetra band rectangular patch antenna to achieve extremely high gain and directivity performances for all the four bands, which may not be possible with conventional dielectric antennas.

Analytical modeling of threshold voltage of a double gate junction less field effect transistor

In this work a simple analytical accurate model for the threshold voltage of a double gate junction less field effect transistor has been proposed. The derivation of this analytical model has been performed by taking channel doping, oxide thickness, high k dielectric and channel thickness into consideration. Validity of the threshold voltage equation has been verified by device simulation.