34 research outputs found
Conical refraction of a high-M2 laser beam
We report on experiments with conical refraction of laser beams possessing a high beam propagation parameter M2. With beam propagation parameter values M2=3 and M2=5, unusual Lloyd's distributions with correspondingly three and five dark rings were observed. In order to explain this phenomenon, we extend the dual-cone model of the conical refraction that describes it as a product of interference of two cones that converge and diverge behind the exit facet of the crystal. In the extended model, these converging/diverging cones are represented as the cone-shaped quasi-Gaussian beams possessing the M2 parameter of an original beam. In this formalism, a beam-waist of these cone-shaped beams is proportional to the M2 value and defines the area of their interference which is a width of the Lloyd's ring. Therefore, the number of dark rings in the Lloyd distribution is defined by the M2 value and can be much greater than unity. The results of the numerical simulations within the extended dual-cone model are in excellent agreement with the experiment
Ultralow-noise terahertz detection by p-n junctions in gapped bilayer graphene
Graphene shows a strong promise for detection of terahertz (THz) radiation
due to its high carrier mobility, compatibility with on-chip waveguides and
transistors, and small heat capacitance. At the same time, weak reaction of
graphene's physical properties on the detected radiation can be traced down to
the absence of band gap. Here, we study the effect of electrically-induced band
gap on THz detection in graphene bilayer with split-gate p-n junction. We show
that gap induction leads to simultaneous increase in current and voltage
responsivities. At operating temperatures of ~25 K, the responsivity at 20 meV
band gap is from 3 to 20 times larger than that in the gapless state. The
maximum voltage responsivity of our devices at 0.13 THz illumination exceeds 50
kV/W, while the noise equivalent power falls down to 36 fW/Hz^0.5. These values
set new records for semiconductor-based cryogenic terahertz detectors, and pave
the way for efficient and fast terahertz detection
Zero-bias photodetection in 2d materials via geometric design of contacts
Structural or crystal asymmetry are necessary conditions for emergence of
zero-bias photocurrent in light detectors. Structural asymmetry has been
typically achieved via doping being a technologically complex process.
Here, we propose an alternative approach to achieve zero-bias photocurrent in
2d material flakes exploiting the geometrical non-equivalence of source and
drain contacts. As a prototypical example, we equip a square-shaped flake of
PdSe with mutually orthogonal metal leads. Upon uniform illumination with
linearly-polarized light, the device demonstrates non-zero photocurrent which
flips its sign upon 90 polarization rotation. The origin of zero-bias
photocurrent lies in polarization-dependent lightning-rod effect. It enhances
the electromagnetic field at one contact from the orthogonal pair, and
selectively activates the internal photoeffect at the respective metal-PdSe
Schottky junction. The proposed technology of contact engineering can be
extended to arbitrary 2d materials and detection of both polarized and natural
light
Polarization-resolving graphene-based mid-infrared detector
The ability to resolve the polarization of light with on-chip devices
represents an urgent problem in optoelectronics. The detectors with
polarization resolution demonstrated so far mostly require multiple oriented
detectors or movable external polarizers. Here, we experimentally demonstrate
the feasibility to resolve the polarization of mid-infrared light with a single
chemical-vapor-deposited graphene-channel device with dissimilar metal
contacts. This possibility stems from an unusual dependence of photoresponse at
graphene-metal junctions on gate voltage and polarization angle. Namely, there
exist certain gate voltages providing the polarization-insensitive signal;
operation at these voltages can be used for power calibration of the detector.
At other gate voltages, the detector features very strong polarization
sensitivity, with the ratio of signals for two orthogonal polarizations
reaching ~10. Operation at these voltages can provide information about
polarization angles, after the power calibration. We show that such unusual
gate- and polarization-dependence of photosignal can appear upon competition of
isotropic and anisotropic photovoltage generation pathways and discuss the
possible physical candidates.Comment: 6 pages, 4 figure
Ecologically relevant choanoflagellates collected from hypoxic water masses of the Baltic Sea have untypical mitochondrial cristae
Abstract Background Protist communities inhabiting oxygen depleted waters have so far been characterized through both microscopical observations and sequence based techniques. However, the lack of cultures for abundant taxa severely hampers our knowledge on the morphology, ecology and energy metabolism of hypoxic protists. Cultivation of such protists has been unsuccessful in most cases, and has never yet succeeded for choanoflagellates, even though these small bacterivorous flagellates are known to be ecologically relevant components of aquatic protist communities. Results Quantitative data for choanoflagellates and the vertical distribution of Codosiga spp. at Gotland and Landsort Deep (Baltic Sea) indicate its preference for oxygen-depleted zones. Strains isolated and cultivated from these habitats revealed ultrastructural peculiarities such as mitochondria showing tubular cristae never seen before for choanoflagellates, and the first observation of intracellular prokaryotes in choanoflagellates. Analysis of their partial 28S rRNA gene sequence complements the description of two new species, Codosiga minima n. sp. and C. balthica n. sp. These are closely related with but well separated from C. gracilis (C. balthica and C. minima p-distance to C. gracilis 4.8% and 11.6%, respectively). In phylogenetic analyses the 18S rRNA gene sequences branch off together with environmental sequences from hypoxic habitats resulting in a wide cluster of hypoxic Codosiga relatives so far only known from environmental sequencing approaches. Conclusions Here, we establish the morphological and ultrastructural identity of an environmental choanoflagellate lineage. Data from microscopical observations, supplemented by findings from previous culture-independent methods, indicate that C. balthica is likely an ecologically relevant player of Baltic Sea hypoxic waters. The possession of derived mitochondria could be an adaptation to life in hypoxic environments periodically influenced by small-scale mixing events and changing oxygen content allowing the reduction of oxygen consuming components. In view of the intricacy of isolating and cultivating choanoflagellates, the two new cultured species represent an important advance to the understanding of the ecology of this group, and mechanisms of adaptations to hypoxia in protists in general.</p