Shape and polarization distribution of non-circular conical diffraction beams from conjugate cascades

Peculiar non-circularly shaped vector type beams can be obtained naturally by the conical diffraction phenomenon if specific manipulations in wavevector space are performed between optically biaxial crystals arranged in a cascade. We analyze in detail this situation by focusing on the general shapes and the polarization distribution. Both are shown to be correlated to the values of structure parameters introduced in this work. These control parameters depend on the conical diffraction cone aperture angle, on the crystal lengths, and on the magnification values due to x- and y-oriented cylindrical lenses placed between the crystals and coupling common conjugate planes. The local polarization is found to be always linear with the exception of regions where structures composing the pattern intersect or overlap, where elliptical or circular polarization can occur. The way in which the obtained patterns depend on the orientation of individual crystal samples around the common optical axis and on an eventual polarization filtering at various stages of the cascade is discussed as well. Theoretical and experimental findings agree well, as verified for the case of a cascade of two crystals.

Non-circularly shaped conical diffraction

Waves with tailored shape and vectorial non-homogeneous polarization are of much interest due to the many prospects for relevant applications in the classical and quantum domains. Such vector beams can be generated naturally via conical diffraction in optically biaxial crystals. The recent strongly revived attention to this phenomenon is motivated by modern applications such as optical trapping, polarimetry or super-resolution imaging, partly enabled by new configurations increasing the beam complexity, like those with several crystals in cascade. However, up to now all beams generated by conical diffraction conserve at their sharpest plane the underlying circular shape connected with the planar section of light cones. Here we show that a proper manipulation in wave-vector space within a conical diffraction cascade produces vector beams with highly peculiar non-circular forms, leading to an interesting and reconfigurable platform for easily shaping all structured wave properties, increasing complexity and information content. The experimental observations are confirmed by numerical integration of a paraxial model incorporating the effects of the wave-vector space manipulation.

Photocurrent response of MoS₂ field-effect transistor by deep ultraviolet light in atmospheric and N₂ gas environments

Molybdenum disulfide (MoS2), which is one of the representative transition metal dichalcogenides, can be made as an atomically thin layer while preserving its semiconducting characteristics. We fabricated single-, bi-, and multilayer MoS2 field-effect transistor (FET) by the mechanical exfoliation method and studied the effect of deep ultraviolet (DUV) light illumination. The thickness of the MoS2 layers was determined using an optical microscope and further confirmed by Raman spectroscopy and atomic force microscopy. The MoS2 FETs with different number of layers were assessed for DUV-sensitive performances in various environments. The photocurrent response to DUV light becomes larger with increasing numbers of MoS2 layers and is significantly enhanced in N2 gas environment compared with that in atmospheric environment.

Raman fingerprint of doping due to metal adsorbates on graphene

The properties of single-layer graphene are strongly affected by metal adsorbates and clusters on graphene. Here, we study the effect of a thin layer of chromium (Cr) and titanium (Ti) metals on chemical vapor deposition (CVD)-grown graphene by using Raman spectroscopy and transport measurements. The Raman spectra and transport measurements show that both Cr and Ti metals affect the structure as well as the electronic properties of the CVD-grown graphene. The shift of peak frequencies, intensities and widths of the Raman bands are analyzed after the deposition of metal films of different thickness on CVD-grown graphene. The shifts in G and 2D peak positions indicate the doping effect of graphene by Cr and Ti metals. While p-type doping was observed for Cr-coated graphene, n-type doping was observed for Ti-coated graphene. The doping effect is also confirmed by measuring the gate voltage dependent resistivity of graphene. We have also found that annealing in Ar atmosphere induces a p-type doping effect on Cr- or Ti-coated CVD-grown graphene.