First-principles study of the impact of chemical doping and functional groups on the absorption spectra of graphene

The rational design of the electronic band structures and the associated properties (e.g., optical) of advanced materials has remained challenging for crucial applications in optoelectronics, solar desalination, advanced manufacturing technologies, etc. In this work, using first-principles calculations, we studied the prospects of tuning the absorption spectra of graphene via defect engineering, i.e., chemical doping and oxidation. Our computational analysis shows that graphene functionalization with single hydroxyl and carboxylic acid fails to open a band gap in graphene. While single epoxide functionalization successfully opens a bandgap in graphene and increases absorptivity, however, other optical properties such as reflection, transmission, and dielectric constants are significantly altered. Boron and nitrogen dopants lead to p- and n-type doping, respectively, while fluorine dopants or a single-carbon atomic vacancy cannot create a significant bandgap in graphene. By rigorously considering the spin-polarization effect, we find that titanium, zirconium, and hafnium dopants can create a bandgap in graphene via an induced flat band around the Fermi level as well as the collapse of the Dirac cone. In addition, silicon, germanium, and tin dopants are also effective in improving the optical characteristics. Our work is important for future experimental work on graphene for laser and optical processing applications.

Performance analysis of all optical 2 × 1 multiplexer in 2D photonic crystal structure

In optical processing systems, multiplexer is used to design optical devices such as arithmetic logic unit (ALU) and shift register (SR). Through this paper, we investigate the application of nonlinear photonic crystal ring resonator (PhCRR) based on nonlinear Kerr effect for realizing an all optical 2 × 1 multiplexer. The structure consists of two PhCRRs and five optical waveguides using hexagonal lattice silicon (Si) rods with a background of air. Performance of all optical 2 × 1 multiplexer is replicated with the help of finite difference time domain (FDTD) procedure at a wavelength of 1571 nm, and simulations presented an ultra-compact optical structure with ultra-fast switching speed.

All-optical logic gates using a plasmonic MIM waveguide and elliptical ring resonator

All - optical logic gates OR, XOR, AND and NOT based on two - dimensional ( 2D ) plasmonic metal - insulator - metal ( MIM ) coupled with Elliptical Ring Resonator ( ERR ) are presented , simulated and investigated by using the numerical method of the FEM (finite elements method ). The results are compared and validated with the finite difference time domain ( FDTD ) method . The proposed logic gates are achieved with the same structure using the constructive and destructive optical interferences between a control signal and input signal(s). Their characterization was mainly done for two spectral regions , visible and near - infrared . A high intensity contrast ratios (CR) between the logic states ( “1” and “0” ) can be achieved (28 dB ) at these spectral regions . We introduce a new parameter , "gap- threshold ratio ( GTR )", to characterize the gap between the maximum and minimum of the transmitted signal intensity for all logic gates. The suggested value of transmission threshold between logic 0 and logic 1 states is T th =0.2 . A comparison of the two parameters , (CR) and ( GTR ), with previous works shows that the proposed structure gives very good results for all logic gates configurations. The proposed all- optical logic gates configuration can be a key components in optical processing and telecommunication devices .

Optical Logic Gates Using A Graphene Clad Two Surface Plasmonic Polariton Mode Coupler for Optical Processor

An optical pulse controlled 2x2 two mode interference (OTMI) coupler having silicon core surrounded by Graphene clad has been introduced as fundamental gates of optical processing. Considering strong light –matter interactions and nonlinear optical property of graphene, we have shown coupling characteristics depending on additional phase change between the excited surface plasmon polariton modes propagating through the silicon core. By applying optical pulse energy of 0.82 pJ and width of 4 ps on graphene clad, the NOT, AND, and OR logic gates are demonstrated. The coupling length of the proposed device is ~ 3.2 times less than that of already reported optical device based on SPP modes. Our results promise to obtain the development of integrated of optical processor with high speed, compact and low power optical logic gates having high fabrication tolerance.

Super-regular femtosecond laser nanolithography based on dual-interface plasmons coupling

Advances in femtosecond laser-material interaction facilitate the extension of maskless optical processing to the high efficiency and deep-subwavelength scale. Here, a hybrid plasmon lithography technique has been demonstrated by irradiating near-infrared femtosecond laser pulses onto the Si material coated with thin Cr films in a vacuum chamber, and superior nanograting structures are found to deeply penetrate through the thin Cr film into the underlying Si substrate. In stark contrast to the common ripple structures formed on the Si surface, the Cr-layer mediated Si nanograting structures not only exhibit the spatially super-regular arrangements with a deep-subwavelength period of 355 nm but also present the nonsinusoidal sharp-edged groove geometry with a large depth-to-width aspect ratio of 2.1. Theoretical analyses and calculations reveal that the anomalous structure characterizations are physically ascribed to the excitation of dual-interface coupled plasmons in the thin metal layer, which possess the squeezed spatial wavelength and the periodic columnar intensity distributions. Moreover, the further deepening of periodic nanostructures into the Si substrate is also elucidated by the simulation of electric field enhancements at the bottom of shallow grooves under irradiation of subsequent laser pulses. In combination with a wet etching process, the Si nanograting structures can be modified into the smooth and narrow-mouthed V-profiles, whose optical measurements show a near omnidirectional antireflection especially in the visible range of 565–750 nm, which is expected for the design of advanced photonic devices.

Developing Cost-Effective and High-Speed 40 Gbps FSO Systems Incorporating Wavelength and Spatial Diversity Techniques

Free-space optical (FSO) communication systems are being anticipated to offer promising alternatives to existing radio networks in delivering high-speed data access to end-users. Ease of installation, robust features, and cost-effective operation have been the hallmark of FSO systems, and these features will play an obvious role in deciding the ways in which futuristic smart communication models will operate. Despite these arrays of features, FSO links suffer severe performance degradation due to channel-induced impairments caused by atmospheric effects such as rain, haze, and fog. In this work, we have investigated and compared the performance of 40 Gbps FSO links for different channel conditions ranging from clear weather to severe attenuation by incorporating spatial and wavelength diversity as performance booster techniques. The use of an erbium-doped fiber amplifier (EDFA) with FSO links has also been proposed here. Using performance metrics like bit error rate (BER) and eye patterns, it has been found that the use of EDFA not only helps in compensating for the link losses but also aids in realizing an all-optical processing based last-mile access system. The proposed FSO system will be capable of bridging the existing backbone fiber networks with end-users with minimal changes to the existing hardware regime, thereby proving to be extremely cost-effective in sharp contrast to radio-frequency generations which require major infrastructure overhaul.

Active and Quantum Integrated Photonic Elements by Ion Exchange in Glass

Ion exchange in glass has a long history as a simple and effective technology to produce gradient-index structures and has been largely exploited in industry and in research laboratories. In particular, ion-exchanged waveguide technology has served as an excellent platform for theoretical and experimental studies on integrated optical circuits, with successful applications in optical communications, optical processing and optical sensing. It should not be forgotten that the ion-exchange process can be exploited in crystalline materials, too, and several crucial devices, such as optical modulators and frequency doublers, have been fabricated by ion exchange in lithium niobate. Here, however, we are concerned only with glass material, and a brief review is presented of the main aspects of optical waveguides and passive and active integrated optical elements, as directional couplers, waveguide gratings, integrated optical amplifiers and lasers, all fabricated by ion exchange in glass. Then, some promising research activities on ion-exchanged glass integrated photonic devices, and in particular quantum devices (quantum circuits), are analyzed. An emerging type of passive and/or reconfigurable devices for quantum cryptography or even for specific quantum processing tasks are presently gaining an increasing interest in integrated photonics; accordingly, we propose their implementation by using ion-exchanged glass waveguides, also foreseeing their integration with ion-exchanged glass lasers.

Performance Analysis of Optical AND Gate Using T- Shaped Waveguide

Ultra-compact all-optical AND logic gate is realized for optical processing and photonic integrated devices with two-dimensional photonic crystal waveguides based on beam interference principle. The performance of the structure is examined and evaluated by optimized parameters such as refractive index and silicon rod radius. The results obtained from a numerical calculation using the finite-difference time-domain (FDTD) method and plane wave expansion method. The photonic crystal based all-optical AND logic gate has benefits of compact size as 38.88 μ𝑚2, operated at low power levels, high transmission ratio and good time response as 0.124 ps. Associated with conventional semiconductor optical amplifiers, Mach-Zehnder interferometer and photonic crystal ring resonators proposed methodology provides better performance with a high achieved more than 97% transmission ratio, at a wavelength of 1.55 μm.