The intrinsic broadband and ultrafast photoresponse of graphene has been extensively studied in recent years, promising the new generation of photodetectors covering the unprecedentedly broad spectrum from THz to near-infrared. It has been demonstrated that the broadband and ultrafast photocurrent generation takes place at the graphene-metal interface with contribution from both photo-thermoelectric and photovoltaic effects, stemming from the efficient generation of hot carriers. Although the hot carrier lifetime is of key importance for their efficient extraction, the dynamics of carrier cooling is still far from being completely understood. So far, two fundamentally different scattering mechanisms have been suggested to dominate in graphene: the momentum-conserved collisions with the high-energy optical phonons, and the disorder-driven supercollisions with the acoustic phonons. However, the co-existing relaxation via both optical and acoustic phonons has not been considered, hindering the interpretation of different experiments within a single physical model. In our work, we discuss the non-uniform graphene properties in the graphene-metal photodetectors, and demonstrate that different cooling mechanisms equally contribute to the process due to the presence of the photocurrentgenerating interface defect. Noting the overlooked role of the metal contact in cooling dynamics, we show that the purity of graphene employed for photodetection is of less importance for the relaxation dynamics compared to the contact area in terms of introduced system disorder. Further, we show that the transient photo-thermoelectric response, so far attributed exclusively to supercollisions, can be predicted by considering the contribution from both relaxation pathways: normal and supercollision scattering of hot carriers.
Typical many-wavelength scale of the optical fiber-integrated photonic elements (for example, ring resonators, Bragg reflectors, Mach-Zehnder interferometers, etc.) has been an insuperable obstacle for the realization of truly integrated photonic circuits that would have the dimensions compliant with the semiconductor industry standards. Doped graphene however, promises the deeply subwavelength size of the plasmonic-based optical elements due to the very short plasmon wavelength. In this work, we propose a design of the ultra-compact fiber-integrated optical switch based on the graphene-functionalized plasmonic nano-cavity for ultrafast light modulation. Presence of graphene allows to actively control the plasmonic resonance in the cavity via the electrostatic doping, so that properly tuned Fermi level in graphene results in a strong constructive (destructive) Fano interference between the propagating mode in the fiber and the graphene plasmonic mode in the nano-cavity, increasing (zeroing) the transmission efficiency at given frequency. The nano-cavity effectively works as a plasmonic Fabry-Perot resonator, significantly enhancing the coupling efficiency as well as the interference strength. Due to the strong confinement of graphene plasmons, the active volume of the switch can be as small as 10–3λ0–3, making it possible to build an optical circuit with a very high density of elements. Furthermore, sharp profile of the Fano resonance provides a fast switching speed even with small variation of doping. Therefore, proposed design requires very low driving voltage of ~1V, while providing the modulation depth of at least 0.5.
For modern multimedia devices, such as digital photo and video cameras, compact size of optical system is becoming more and more important. This work is dedicated to zoom lens design and second-order derivative optimization method of designed system with using of computer program which allows to design a small-size high-quality system. For lens designing we are using the third order aberration theory and an equation set describing interrelations of system parameters, such as minimum length of system, focal length of each component, their shifts and distances between them. Also equations include conditions of uninterrupted focusing on CCD. Zoom lens consists of three components, two of which are movable. First component is immovable. Number of lenses in each component (two or three) is determined automatically in according to conditions of minimum aberrations and manufacturability. Designed system contains only second-order surfaces with eccentricities. Complete correcting of aberrations consists of two phases. First, the best constructive parameters (radii of curvature, thicknesses and eccentricities) are determined for designed system. The second-order derivative method is used for that. The main feature of this method is the using of Hesse matrix of wave aberration function. Second, for selected surfaces aspherical coefficients are determined. Wave aberration's dependence of constructive parameters and aspherical coefficients is used.